steel tube, sheet steel and production method

专利摘要:
HIGH-RESISTANCE STEEL TUBE HAVING EXCELLENT DUCTILITY AND TENACITY AT LOW TEMPERATURE, HIGH-RESISTANCE STEEL SHEET AND METHOD TO PRODUCE STEEL SHEET High-strength steel tube excellent in deformation capacity and low temperature toughness and the steel tube obtained by a base steel sheet in the form of a tube being welded, in which the base steel sheet contains, in mass%, C: 0.010 to 0.080%, Si: 0.01 to 0.50%, Mn: 1, 2 to 2.8%, S: 0.0001 to 0.0050%, Ti: 0.003 to 0.030%, B: 0.0003 to 0.005%, N: 0.0010 to 0.008%, O: 0.0001 to 0 , 0080%, one or more elements between Cr, Cu, and Ni, P: limited to 0.050% or less, Al: limited to 0.020% or less, Mo: limited to 0.03% or less, a Ceq obtained by ( Expression 1) below 0.30 to 0.53 and a Pcm obtained by (Expression 2) below 0.10 to 0.20, and a balance being composed of iron and inevitable impurities, and a metallic structure of the steel plate base contains 27 to 90% due to polygonal ferrite area and a hard phase composed of u m or both of bainite and martensite as your balance.
公开号:BR112014015715B1
申请号:R112014015715-4
申请日:2012-12-27
公开日:2021-03-16
发明作者:Taishi Fujishiro；Shinya Sakamoto；Takuya Hara；Yoshio Terada
申请人:Nippon Steel Corporation；
IPC主号:

专利说明:

Technical Field
[001] The present invention relates to a high strength steel tube and a high strength steel sheet which are excellent in deformability and low temperature toughness and are particularly suitable for a pipe for transporting crude oil and natural gas, and a steel plate production method.
[002] This application is based on, and claims the priority benefit of, Japanese Patent Application No. 2011-287752 and Japanese Patent Application No. 2011-287699, registered on December 28, 2011, the complete contents of which are incorporated here as reference. Background Technique
[003] In recent years, as a means of long-distance transportation of crude oil and natural gas, a pipeline has become increasingly important. Under such circumstances, in order to improve the efficiency of transporting crude oil and natural gas, an increase in the internal pressure of a steel sheet for piping was examined. As a result, a high reinforcement of a steel pipe was required for a high strength pipe. In addition, a steel pipe for high-strength piping must also have toughness in the heat-affected zone (ZAC), toughness of the base material (resistance to breaking capacity), deformability, etc. Therefore, steel sheets and steel tubes have been proposed that are mainly composed of bainite and martensite and have fine ferrite formed there (for example, Patent Documents 1 to 3). However, these are steel sheets and high-strength steel tubes with X100 specification (tensile strength 70 MPa or more) from the American Petroleum Institute (API).
[004] In addition, it was also necessary to improve the performance of high-strength steel tubes with API X70 specification (a tensile strength of 570 MPa or more) and API X80 specification (a tensile strength of 625 MPa or more), which are used in practice as a pipe material for trucks. In this regard, a method has been proposed in which the ZAC of a steel tube having a base material with fine ferrite formed in bainite is heat treated to increase the deformability and toughness at low temperature (for example, Patent Document 4) . In addition, a method has been proposed in which in a steel plate base structure having a component that does not easily cause the transformation of ferrite and improves the low temperature toughness in a ZAC, 20 to 90% polygonal ferrite is formed to increase the low temperature toughness of the steel sheet to be a base material (for example, Patent Document5).
[005] A method of also forming ferrite and improving properties such as the tenacity of the base material and deformability based on the steel sheet and steel tube that achieve both strength and toughness and are composed mainly of bainite and martensite has been proposed. However, recently, it was necessary to increase the thickness of steel tubes for high strength tubes with specification X70 or greater of API specification, (which will be referred to hereinafter as X70), and also with specification X80 or greater of API specification, (which will be referred to hereinafter as X80), so that transport efficiency can be improved and development in deep waters can be conducted. Thus, demands for low temperature toughness and deformability of thick steel tubes for high strength tubing are growing.
[006] In addition, drilling regions for crude oil and natural gas are expected to extend to extremely cold locations, such as the Arctic Circle, in the future, so it is predicted that steel pipes for thick high-strength piping need to have guaranteed low temperature toughness at -40 ° C or less and also at -60 ° C or less. Particularly, when the steel tube is produced, a thick steel sheet is shaped into a tube by a UO, JCO, or folding cylinder step, and then portions of edges are connected from top to end, and the portion of seam is welded by arc welding, but when the thickness of the plate increases, the heat input by welding becomes a high heat input and the grain size in the heat affected zone (heat affected zone to be referred to as ZAC) increases , and therefore a decrease in low temperature toughness becomes a critical problem.
[007] To combat the above, as a technique to improve the low temperature toughness of the ZAC of the steel pipe for high strength thick pipe, a method has been proposed in which the amount of C is extremely reduced to have a fundamental structure of bainite (for example, Patent Documents 6 and 7). In addition, a method has been proposed in which the structure of a ZAC is thinned by the use of intragranular transformation (for example, Patent Documents 8 to 10). A method has also been proposed in which martensite-austenite constituents (martensite-austenite constituent (hereinafter “MA”)), which are detrimental to toughness, are controlled by optimizing connection elements based on a bainite structure having an orientation relationship specified crystal (for example, Patent Document 11), and a method has been proposed to make ZAC thin using intragranular bainite based on bainite also on a thick steel plate with increased hardening capacity (for example, Patent 12 and 13).
[008] The above methods are extremely effective for improving low temperature toughness in the HAZ. However, recently, demands for an increase in the thickness and tenacity at low temperature of steel pipes for high strength pipes are also increasing, and therefore the tenacity of ZAC under the condition of an extra thickness such as a thickness of 20 mm or more and an extremely low temperature such as -60 ° C or less were requested. Prior Art Documents Patent Documents
[009] Patent Document 1: Japanese Laid-open Patent Publication n ° 2003-293078
[0010] Patent Document 2: Japanese Laid-open Patent Publication n ° 2003-306749
[0011] Patent Document 3: Japanese Laid-open Patent Publication n ° 2005-146407
[0012] Patent Document 4: Japanese Laid-open Patent Publication No. 2004-131799
[0013] Patent Document 5: Japanese Laid-open Patent Publication n ° 2009-270197
[0014] Patent Document 6: Patent Publication No. 3602471
[0015] Patent Document 7: Japanese Laid-open Patent Publication n ° 2000-345239
[0016] Patent Document 8: Japanese Laid-open Patent Publication n ° Hei 08-325635
[0017] Patent Document 9: Japanese Laid-open Patent Publication n ° 2001-355039
[0018] Patent Document 10: Japanese Laid-open Patent Publication No. 2003-138340
[0019] Patent Document 11: Japanese Laid-open Patent Publication No. 2007-239049
[0020] Patent Document 12: Japanese Laid-open Patent Publication n ° 2008-163456
[0021] Patent Document 13: Japanese Laid-open Patent Publication No. 2009-149917 Summary of the Invention Problems to be solved by the invention
[0022] To improve the deformability, in a steel plate to be a base material and a steel tube, it is effective to make a composite structure and soft ferrite and hard bainite and martensite. In addition, to improve the toughness of the base material, it is effective to make a thin metal structure composed of ferrite + bainite with fine ferrite formed therein.
[0023] Meanwhile, to improve the toughness in the ZAC, it is effective that the equivalent carbon Ceq and the fracture parameter Pcm are controlled, B, Mo etc., are also added in order to increase the hardening capacity, the formation of ferrite crude that is formed in the grain boundaries is suppressed, and a thin metal structure is formed mainly composed of transformed intragranular structures formed by the use of Ti oxides. Thus, in view of the ease of formation of ferrite, a composition of suitable chemical components for the toughness of ZAC and a composition of chemical components suitable for the toughness of the base material are opposed to each other.
[0024] As a method to solve this, it was proposed a method for forming ferrite by rolling at low temperature of a steel having B and Mo added in compound and having high hardening capacity in a hot rolling step. However, the addition of Mo increases the cost of the alloy and the low temperature lamination increases the load on the production equipment, so that productivity also decreases. Thus, to guarantee both low temperature toughness and deformability, a high alloy cost and a high production cost are required, resulting in the fact that it is extremely difficult to mass-produce cheap steel sheet and pipe. high-strength, high-quality steel that meets these conditions.
[0025] In addition, to improve the efficiency of transporting a pipe for crude oil, natural gas, and the like, as described above, a high reinforcement and an increase in the thickness of the steel pipe for high strength pipes are effective. However, when the thickness of the steel tube is increased as above to achieve an increase in the internal pressure of the steel tube, it becomes difficult to guarantee the toughness in the ZAC at low temperature. When, in particular, a thick material having a thickness of 20 mm or more is welded by arc welding, the heat input becomes a high heat input, the grain size of the ZAC increases, and the amount of MA also increases, so that it is extremely difficult to guarantee toughness at extremely low temperatures of -40 ° C and also - 60 ° C. Then, the conventional methods described above are good enough to guarantee toughness in the ZAC under the condition of an extra thickness such as a thickness of 20 mm or more and an extremely low temperature such as -60 ° C or less.
[0026] The present invention is made considering such circumstances, and the equivalent carbon Ceq and the fracture parameter Pcm are controlled, and B is also added to increase the hardening capacity in order to suppress the formation of ferrite in a ZAC. In addition, in the present invention, on a high-strength steel plate to be a base material, polygonal ferrite is formed to improve the deformability and tenacity at low temperature by controlling the cooling conditions to be carried out after hot rolling without require low temperature lamination. The present invention has an objective of providing a high strength steel sheet excellent in low temperature deformability and toughness, a high strength steel tube using that high strength steel sheet as the base material, and a method of producing the sheet steel while reducing, in particular, the cost of the alloy and the production cost of the high-strength steel sheet.
[0027] Incidentally, in the present invention, ferrite that is not extended in the rolling direction and has an aspect ratio of 4 or less is referred to as polygonal ferrite. Here, the aspect ratio is a value for the length of the ferrite grain divided by the width. Means for solving problems
[0028] Conventionally, B and Mo were both added, and a Ceq which is the index of hardening capacity and the fracture parameter Pcm which is the index of welding capacity were controlled to be in optimum ranges to improve the toughness of the ZAC . So, in order to improve the toughness of the base material and the deformability, a low temperature rolling in a hot rolling step was required, resulting in the fact that it was difficult to mass produce and inexpensively a steel sheet for high-pressure piping. strength and a steel tube that are excellent at low temperature toughness and deformability and, in particular, that are thick.
[0029] In the present invention, the amount of Mo added is limited and a chemical composition is made with a high hardening capacity, in order to suppress the crude ferrite in the grain boundaries in a ZAC. In addition, by optimizing the cooling conditions to be carried out after hot rolling on the steel sheet having such a chemical composition, a structure composed of fine soft polygonal ferrite material and hard bainite and martensite can be made on the steel sheet to be the base material even if the load in the hot rolling step is decreased.
[0030] The essence of the present invention is as follows. [1]
[0031] A steel tube of high strength excellent in deformability and tenacity at low temperature which is a steel tube obtained by a base steel sheet shaped into a tube being welded, in which the base steel sheet contains, in% by weight, C: 0.010 to 0.080%, Si: 0.01 to 0.50%, Mn: 1.2 to 2.8%, S: 0.0001 to 0.0050%, Ti: 0.003 to 0.030%, B: 0.0003 to 0.005%, N: 0.0010 to 0.008%, O: 0.0001 to 0.0080%, one or more elements between Cr, Cu, and Ni, P: limited to 0.050% or less, Al: limited to 0.020% or less, Mo: limited to 0.03% or less, a Ceq obtained by (Expression 1) below 0.30 to 0.53 and a Pcm obtained by (Expression2) below 0.10 at 0.20, and a balance being composed of iron and the inevitable impurities, and the metallic structure of the base steel plate contains 27 to 90% due to polygonal ferrite area and a hard phase composed of one or both between bainite and martensite as a balance. Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo) / 5 ••• (Expression 1) Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + 5B ••• (Expression 2)
[0032] Na (Expression 1) and (Expression 2) above, C, Si, Mn, Ni, Cu, Cr, Mo, and B denote the contents of the respective elements [mass%]. Ni, Cu, Cr, and Mo are calculated as 0 when the respective levels are 0%. [two]
[0033] The high strength steel tube excellent in deformability and low temperature toughness according to item [1], in which the base steel layer also contains one or two or more elements between, in mass%, W: 0 , 01 to 0.50%, V: 0.010 to 0.100%, Nb: 0.001 to 0.200%, Zr: 0.0001 to 0.0500%, Ta: 0.0001 to 0.0500%, Mg: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0050%, Rare Earths: 0.0001 to 0.0050%, Y: 0.0001 to 0.0050%, Hf: 0.0001 to 0.0050%, and Re: 0.0001 to 0.0050%, Ceq is obtained by (Expression 1 ') below instead of (Expression 1) above, and Pcm is obtained by (Expression 2') below in place of (Expression 2 ). Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 ••• (Expression 1 ') Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B ••• (Expression 2 ')
[0034] Na (Expression 1 ') and Na (Expression 2') above, C, Si, Mn, Ni, Cu, Cr, Mo, V, and B denote the contents of the respective elements [mass%]. Ni, Cu, Cr, and Mo are calculated as 0 when the respective levels are 0%. V is calculated as 0 when the content is 0% and the content is less than 0.010% by weight. [3]
[0035] The high strength steel tube excellent in deformability and tenacity at low temperature according to item [1], in which in the base steel plate, in mass%, the C content is 0010 at 0.060% and the content of Al is 0.008% or less, the temperature of initiation of transformation y / α in a zone affected by the heat that is obtained by (Expression 3) below is 500 to 600 ° C, and the transformed intragranular structures are contained in a grain of previous austenite in the zone affected by the heat. Transformation start temperature y / α = -2500Ceq2 + 1560Ceq + 370 ••• (Expression 3) [4]
[0036] The steel tube of high strength excellent in deformability and tenacity at low temperature according to item [3], in which the constituent martensite-austenite in the zone affected by heat is 2.5% or less in fraction of area. [5]
[0037] The high strength steel tube excellent in deformability and tenacity at low temperature according to item [3], in which the high angle grain size of a metallic structure in the zone affected by heat is 80 μm or less. [6]
[0038] The high strength steel tube excellent in deformability and low temperature toughness according to item [3], in which the thickness of the base steel plate is 20 to 40 mm. [7]
[0039] The high strength steel tube excellent in deformability and tenacity at low temperature according to item [3], in which the tensile strength of the base steel plate is 500 to 800 MPa when the circumference direction of the steel tube is set as the direction of pull. [8]
[0040] The high strength steel tube excellent in deformability and tenacity at low temperature according to item [3], in which the base steel plate also contains one or two or more elements between, in mass%, W: 0 , 01 to 0.50%, V: 0.010 to 0.100%, Nb: 0.001 to 0.200%, Zr: 0.0001 to 0.0500%, Ta: 0.0001 to 0.0500%, Mg: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0050%, Rare Earths: 0.0001 to 0.0050%, Y: 0.0001 to 0.0050%, Hf: 0.0001 to 0.0050%, and Re: 0.0001 to 0.0050%, Ceq is obtained by (Expression 1 ') below instead of (Expression 1) above, and Pcm is obtained by (Expression 2') instead of (Expression 2). Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 ••• (Expression 1 ') Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B ••• (Expression 2 ')
[0041] Na (Expression 1 ') and Na (Expression 2') above, C, Si, Mn, Ni, Cu, Cr, Mo, V, and B denote the contents of the respective elements [mass%]. Ni, Cu, Cr, and Mo are calculated as 0 when the respective levels are 0%. V is calculated as 0 when the content is 0% and the content is less than 0.010% by weight. [9]
[0042] The high strength steel tube excellent in deformability and tenacity at low temperature according to item [8], in which the martensite-austenite constituent in the area affected by heat is 2.5% or less in the area fraction. [10]
[0043] The high strength steel tube excellent in deformability and low temperature toughness according to item [8], in which the high angle grain size of a metal structure in the heat affected zone is 80 μm or less. [11]
[0044] The high strength steel tube excellent in deformability and low temperature toughness according to item [8], in which the thickness of the base steel plate is 20 to 40 mm. [12]
[0045] The high strength steel pipe excellent in deformability and low temperature toughness according to item [8], in which the tensile strength of the base steel plate is 500 to 800 MPa when the direction of the circumference of the steel pipe is set as the direction of traction. [13]
[0046] A sheet of high strength steel excellent in deformability and tenacity at low temperature includes, in mass%, C: 0.010 to 0.080%; Si: 0.01 to 0.50%; Mn: 1.2 to 2.8%; S: 0.0001 to 0.0050%; Ti: 0.003 to 0.030%; B: 0.0003 to 0.005%; N: 0.0010 to 0.008%; O: 0.0001 to 0.0080%; one or more elements between Cr, Cu, and Ni; P: limited to 0.050% or less; Al: limited to 0.020% or less; Mo: limited to 0.03% or less; a Ceq obtained by (Expression 1) below 0.30 to 0.53; and a Pcm obtained by (Expression 2) below 0.10 to 0.20; and the balance being composed of iron and the inevitable impurities, in which the metallic structure contains 27 to 90% due to the area of the polygonal ferrite and a hard phase composed of one or both between bainite and martensite as its balance. Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo) / 5 ••• (Expression 1) Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + 5B ••• (Expression 2)
[0047] Na (Expression 1) and (Expression 2) above, C, Si, Mn, Ni, Cu, Cr, Mo, and B denote the contents of the respective elements [mass%]. Ni, Cu, Cr, and Mo are calculated as 0 when the respective levels are 0%. [14]
[0048] The steel sheet of high strength excellent in deformability and tenacity at low temperature according to item [13] also includes: one or two or more elements between, in mass%, W: 0.01 to 0.50% ; V: 0.010 to 0.100%; Nb: 0.001 to 0.200%; Zr: 0.0001 to 0.0500%; Ta: 0.0001 to 0.0500%; Mg: 0.0001 to 0.0100%; Ca: 0.0001 to 0.0050%; Rare Earths: 0.0001 to 0.0050%; Y: 0.0001 to 0.0050%; Hf: 0.0001 to 0.0050%; and Re: 0.0001 to 0.0050%, in which Ceq is obtained by (Expression 1 ') below in linking from (Expression 1) above, and Pcm is obtained by (Expression 2') below in place of ( Expression 2). Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 ••• (Expression 1 ') Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B ••• (Expression 2 ')
[0049] Na (Expression 1 ') and Na (Expression 2') above, C, Si, Mn, Ni, Cu, Cr, Mo, V, and B denote the contents of the respective elements [mass%]. Ni, Cu, Cr, and Mo are calculated as 0 when the respective levels are 0%. V is calculated as 0 when the content is 0% and the content is less than 0.010% by weight. [15]
[0050] The steel sheet of high resistance excellent in deformability and tenacity at low temperature according to item [13], in which, in mass%, the C content is 0.010 q 0.060% and the Al content is 0.008% or less, and the temperature of the start of transformation y / α in a zone affected by the heat that is obtained by (Expression 3) below is 500 to 600 ° C. Transformation start temperature y / α = -2500Ceq2 + 1560Ceq + 370 ••• (Expression 3) [16]
[0051] The steel sheet of high strength excellent in deformability and tenacity at low temperature according to item [15] also includes: one or two or more elements between, in mass%, W: 0.01 to 0.50% ; V: 0.010 to 0.100%; Nb: 0.001 to 0.200%; Zr: 0.0001 to 0.0500%; Ta: 0.0001 to 0.0500%; Mg: 0.0001 to 0.0100%; Ca: 0.0001 to 0.0050%; Rare Earths: 0.0001 to 0.0050%; Y: 0.0001 to 0.0050%; Hf: 0.0001 to 0.0050%; and Re: 0.0001 to 0.0050%, in which Ceq is obtained by (Expression 1 ') below instead of (Expression 1) above, and Pcm is obtained by (Expression 2') below in place of ( Expression 2). Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 ••• (Expression 1 ') Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B ••• (Expression 2 ')
[0052] Na (Expression 1 ') and Na (Expression 2') above, C, Si, Mn, Ni, Cu, Cr, Mo, V, and B denote the contents of the respective elements [mass%]. Ni, Cu, Cr, and Mo are calculated as 0 when the respective levels are 0%. V is calculated as 0 when the content is 0% and the content is less than 0.010% by weight. [17]
[0053] A method of producing a sheet of high strength steel excellent in deformability and tenacity at low temperature includes: in a steel sheet containing, in mass%, C: 0.010 to 0.080%, Si: 0.01 to 0.50%, Mn: 1.2 to 2.8%, S: 0.0001 to 0.0050%, Ti: 0.003 to 0.030%, B: 0.0003 to 0.005%, N: 0.0010 0.008% , O: 0.0001 to 0.0080%, one or more elements between Cr, Cu, and Ni, P: limited to 0.050% or less, Al: limited to 0.020% or less, Mo: limited to 0.03% or less, a Ceq obtained by (Expression 1) below 0.30 to 0.53 and a Pcm obtained by (Expression 2) below 0.10 to 0.20, and the balance being composed of iron and the inevitable impurities , perform heating at 950 ° C or more; perform cooling at an average cooling rate of less than 10 ° C / s; and then perform accelerated cooling at a cooling rate of 10 ° C / s or more to a temperature of Bs or less obtained by (Expression 4) below from a temperature of Ar3 -100 ° C to Ar3 - 10 ° C . Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo) / 5 ••• (Expression 1) Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + 5B ••• (Expression 2) Bs (° C) = 830 - 270C - 90Mn - 37Ni - 70Cr - 83Mo ••• (Expression 4)
[0054] Nas (Expression 1), (Expression 2), and (Expression 5) above, C, Si, Mn, Ni, Cu, Cr, Mo, and B denote the contents of the respective elements [mass%]. Ni, Cu, Cr, and Mo are calculated as 0 when the respective levels are 0%. [18]
[0055] Method of production of high-strength steel sheet excellent in deformability and tenacity at low temperature according to item [17], in which the hot rolling step is performed in the non-recrystallized y rolling region at a temperature start of lamination adjusted for Ar3 to Ar3 + 100 ° C and a reduction ratio adjusted to 1.5 or more. [19]
[0056] Method of production of high-strength steel sheet excellent in deformability and tenacity at low temperature according to item [15], in which the steel plate also contains one or two or more elements, in% by mass, W : 0.01 to 0.50%, V: 0.010 to 0.100%, Nb: 0.001 to 0.200%, Zr: 0.0001 to 0.0500%, Ta: 0.0001 to 0.0500%, Mg: 0, 0001 to 0.0100%, Ca: 0.0001 to 0.0050%, Rare Earths: 0.0001 0.0050%, Y: 0.0001 to 0.0050%, Hf: 0.0001 to 0.0050% , and Re: 0.0001 to 0.0050%, Ceq is obtained by (Expression 1 ') below instead of (Expression 1) above, and Pcm is obtained by (Expression 2') below in place of (Expression 2 ). Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 ••• (Expression 1 ') Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B ••• (Expression 2 ')
[0057] Nas (Expression 1 ') and (Expression 2') above, C, Si, Mn, Ni, Cu, Cr, Mo, V, and B denote the contents of the respective elements [mass%]. Ni, Cu, Cr, and Mo are calculated as 0 when the respective levels are 0%. V is calculated as 0 when the content is 0% and the content is less than 0.010% by weight. [20]
[0058] Production method of high-strength steel sheet excellent in deformability and tenacity at low temperature according to item [19], in which in the hot rolling stage, lamination is carried out in a non-recrystallized y region at a lamination start temperature set for Ars to Ars + 100 ° C and a reduction ratio set to 1.5 or more. Effect of the Invention
[0059] In accordance with the present invention, it becomes possible to suppress the formation of ferrite in the rough grain boundaries in a ZAC and to form polygonal ferrite in a steel sheet which is a base material without requiring low temperature lamination in a hot rolling step. Therefore, it is possible to provide a high strength steel sheet having improved strength and toughness in the HAZ and having extremely excellent deformability and low temperature toughness as a portion of the base material, and a high strength steel tube using this as a base material. [Brief description of the drawings]
[0060] [FIG. 1] FIG. 1 is a view showing the relationship between the hot working temperature and the area ratio of the polygonal ferrite;
[0061] [FIG. 2] FIG. 2 is a view showing the relationship between the temperature of the beginning of the accelerated cooling and the area ratio of the polygonal ferrite;
[0062] [FIG. 3] FIG. 3 is a view showing the relationship between the polygonal ferrite area ratio and the deformability and strength;
[0063] [FIG. 4] FIG. 4 is a view showing the relationship between the polygonal ferrite area ratio and the low temperature toughness of the base material;
[0064] [FIG. 5] FIG. 5 is a view showing the relationship between Ceq and the start temperature of transformation y / a;
[0065] [FIG. 6] FIG. 6 is a view showing the relationship between the temperature of initiation of transformation y / a and the size of the high angle grain;
[0066] [FIG. 7] FIG. 7 is a view showing the relationship between the high angle grain size and the energy absorbed Charpy at -60 ° C;
[0067] [FIG. 8] FIG. 8 is an optical microphotograph of a base structure in a high-strength steel tube of the present invention;
[0068] [FIG. 9] FIG. 9 is a schematic view of the base structure in the high-strength steel tube of the present invention;
[0069] [FIG. 10] FIG. 10 is a schematic view of a ZAC structure in the high-strength steel tube of the present invention;
[0070] [FIG. 11] FIG. 11 is a photograph showing the metallic structure of a ZAC when the temperature at the start of transformation y / a is greater than 600 ° C;
[0071] [FIG. 12] FIG. 12 is a photograph showing the metallic structure of the ZAC when the temperature of initiation of transformation y / a is 500 to 600 ° C;
[0072] [FIG. 13] FIG. 13 is a photograph showing the metallic structure of the ZAC when the fraction of area of M-A is 2.2%; and
[0073] [FIG. 14] FIG. 14 is a photograph showing the metallic structure of the ZAC when the area fraction of M-A is 3.0%. Mode for carrying out the invention
[0074] Hereinafter, an embodiment of the present invention will be explained. Initially, the discoveries of the present inventors that lead to the complement of the present invention will be explained.
[0075] Generally, making crystal grains fine is effective for improving toughness at low temperature and particularly for guaranteeing toughness at low temperatures of -40 ° C, and also - 60 ° C. In a metal structure of a ZAC, in particular, to make crystal grains thin, the suppression of ferrite in the rough grain boundaries is extremely effective. However, it has been found that a composition of chemical components with a high hardening capacity having the effect of suppressing ferrite in grain boundaries in a ZAC makes it difficult to form fine polygonal ferrite that improves the deformability and toughness at low temperature of a material base.
[0076] Thus, the present inventors turned their attention to a method of producing a high-strength steel sheet that does not form ferrite by a thermal history of a ZAC determined according to the thermal input in the welding and the thickness of the sheet. of a steel tube but is capable of forming polygonal ferrite in a hot rolling stage. However, as described above, a composition of chemical components with high hardening capacity that contains Mo and B to be originally added to produce a high-strength steel plate composed mainly of bainite and martensite structure makes it difficult to form polygonal ferrite in a base structure. of a high-strength steel plate.
[0077] Mo has been known to be an element that greatly increases the hardening capacity by compound addition with B. That is, it is indicated that m Mo-B steel added in compound has an effect of also delaying the transformation of ferrite compared to a steel with added B not containing Mo that has the same Ceq. The present inventors initially examined the relationship between rolling conditions in a temperature region where the metal structure is composed of austenite and no recrystallization occurs, that is, and, a non-recrystallized y region and the formation of ferrite in relation to steel with Mo-B added in composite and to steel with B added with increased hardening capacity by elements other than Mo.
[0078] Initially, as steel with added B with increased hardening capacity by elements other than Mo, a steel, containing, in mass%, C: 0.010 to 0.080%, Si: 0.01 to 0.50%, Mn: 1.2 to 2.8%, S: 0.0001 to 0.0050%, Ti: 0.003 to 0.030%, B: 0.0003 to 0.005%, N: 0.0010 to 0.008%, and O: 0.0001 to 0.0080%, and containing one or more elements between Cr, Cu, and Ni, and containing P: limited to 0.050% or less, Al: limited to 0.020% or less, and Mo: limited to 0, 03% or less, and having a Ceq which is the hardening capacity index of 0.30 to 0.53 and a fracture parameter Pcm which is the welding capacity index of 0.10 to 0.20, and containing a balance composed of iron and the inevitable impurities was melted and cast to produce a steel plate.
[0079] Next, each specimen having a height of 12 mm and having a diameter of 8 mm was cut from the obtained steel plates and was subjected to a work / heat treatment simulating a hot rolling. As work / heat treatment, the specimen was subjected to work once at a reduction rate of 1.5, subjected to cooling at 0.2 ° C / s corresponding to air cooling, and also subjected to accelerated cooling to 15 ° C / s corresponding to water cooling. Incidentally, the temperature at which the work is applied (working temperature) has been adjusted to temperature Ar3 or higher to prevent the formation of worked and extended ferrite (worked ferrite) and low temperature lamination to decrease productivity. The temperature of transformation Ar3 in cooling was obtained from a thermal expansion curve.
[0080] After the work / heat treatment, the polygonal ferrite area ratio of the specimens was measured. Incidentally, ferrite that is not extended in the rolling direction and has an aspect ratio of 1 to 4 has been adjusted as a polygonal ferrite.
[0081] The present inventors adjusted the temperature at which the accelerated cooling to 15 ° C / s, corresponding to the water cooling, is initiated (temperature of beginning of the accelerated cooling) to Ar3 - 70 ° C and examined the permission condition of the formation of polygonal ferrite while changing the working temperature described above. The results are shown in FIG. 1. Incidentally, FIG. 1 is one in which the polygonal ferrite area ratio is plotted in relation to the difference between the working temperature and Ar3, and “O” indicates the result of the steel with Mo-B added in composite and “O” indicates the result of the steel with B added with increased hardening capacity by elements other than Mo. As shown in FIG, 1, it is noted that the steel with Mo-B added in compound, while the start rolling temperature of the work / heat treatment described above is adjusted to Ar3 + 60 ° C or less and the rolling at low temperature (lamination introducing tension) at a reduction ratio of 1.5 or more is performed, 27% or more due to polygonal ferrite area can be obtained. That is, in the case of steel with Mo-B added in compound, the working temperature is strictly controlled as well as the rolling needs to be carried out at low temperature. On the other hand, in steel with added B with increased hardening capacity by elements other than Mo, it is seen that 27% or more due to the polygonal ferrite area is formed regardless of the working temperature.
[0082] In addition, they examined the relationship between the temperature of onset of accelerated cooling after hot rolling and the area ratio of the polygonal ferrite, the relationship between the area ratio of the polygonal ferrite and the deformability, and the relationship between the polygonal ferrite area ratio and low temperature toughness. As for the hot rolling, the reheat temperature was adjusted to 1050 ° C, the number of passes was adjusted to 20 to 33, the rolling was finished at an Ar3 temperature or higher, the air cooling was performed and then the cooling to water was run as accelerated cooling. Incidentally, the reduction ratio of the non-recrystallized Y region was adjusted to 1.5 or more, air cooling was performed, and then water cooling (accelerated cooling) was started from various temperatures.
[0083] The polygonal ferrite area ratio of each steel sheet obtained by the hot rolling described above was measured using an optical microscope, and the steel sheet was subjected to a tensile test and a tear test by weight drop (drop weight drop test to be referred to as DWTT) to assess tensile properties and low temperature toughness.
[0084] The tensile property was evaluated using an API specification specimen, the tensile strength, the yield strength, and the ratio of the yield strength (YS) to the tensile strength (TS) (YS / TS to be referred to as yield stress) were found, and thus the area ratio of the polygonal ferrite capable of achieving both strength and deformability was examined.
[0085] In addition, the DWTT was performed at -60 ° C, the shear area (Shear area will be referred to as SA) of a fracture was discovered, and the low temperature toughness was assessed.
[0086] The relationship between the temperature of the beginning of the accelerated cooling and the area ratio of the polygonal ferrite is shown in FIG. 2. In FIG. 2, “O” indicates the result of steel with added B and “O” indicates the result of steel with Mo-B added in compound. Note from FIG. 2 that while the accelerated cooling start temperature after hot rolling is adjusted to Ar3 - 100 ° C to Ar3 - 10 ° C in steel with added B with increased hardening capacity by elements other than Mo, the ratio of area of polygonal ferrite in the steel plate becomes 27 to 90%. That is, while air cooling is carried out to a temperature in the range of Ar3 - 100 ° C to Ar3 - 10 ° C from temperature Ar3 or higher after the hot rolling is finished, 27 to 90% due to area of polygonal ferrite can be formed.
[0087] In addition, for steel with added B with increased hardening capacity by elements other than Mo, the relationship between the polygonal ferrite area ratio and tensile strength, and the ratio between the area ratio of the polygonal ferrite and the yield stress are shown in FIG. 3. "O" indicates the relationship between the polygonal ferrite area ratio and the tensile strength. Note from FIG. 3 that while the polygonal ferrite area ratio becomes 27% or more, the yield stress becomes 80% or less while the polygonal ferrite area ratio becomes 50% or more, an extremely good deformability, which is a yield stress of 70% or less can be obtained.
[0088] In addition, it can be seen from FIG. 3 that to guarantee the tensile strength of 570 MPa or more corresponding to the X70, it is necessary to make the area ratio of the polygonal ferrite become 90% or less. In addition, to guarantee the tensile strength of 625 MPa or more corresponding to the X80, the area ratio of the polygonal ferrite is preferably made to become 75% or less. In addition, to ensure more stable the tensile strength of 625 MPa or more corresponding to the X80. the area ratio of the polygonal ferrite is most preferably made to become 0% or less, and another preferable value is 60% or less.
[0089] That is, it is noted from FIG. 3 that the polygonal ferrite area ratio becomes 27 to 90%, and therefore the balance between deformability and strength improves.
[0090] Furthermore, the relationship between the polygonal ferrite area ratio and the AS shear area at -60 ° C is shown in FIG. 4. Note from FIG. 4 that to obtain 85% or more of the shear area, the area ratio of the polygonal ferrite need only be 20%.
[0091] As above, the present inventors have found that to form sufficient polygonal ferrite in steel with added B so that the low temperature toughness and deformability in the ZAC and the base material can be improved, as a third element to be added with o B, it is important to use connection elements other than Mo. The present inventors performed another detailed examination and obtained the following findings to complete the present invention.
[0092] To form polygonal ferrite in steel with B added to increase the toughness of the base material and the deformability, the effect of the third element to be added with B to improve the hardening capacity is important. So, to guarantee the toughness in the ZAC, a composition with chemical components with increased hardening capacity needs to be made. However, when the compound addition of B and Mo is applied to increase the hardening capacity, the rolling conditions need to be strictly controlled to cause a problem that the cost of production increases and the cost of bonding increases. Thus, it is necessary to make a composition of chemical components in which elements for improving the hardening capacity other than Mo are selected as the third element to be added with B.
[0093] In addition, to improve the hardening capacity, the Ceq which is the hardening capacity index is dropped in the range of 0.30 - 0.53, and also as an element to improve the hardening capacity, elements such as Mn, Cr, Ni and Cu are selected in addition to C.
[0094] In addition, there is no need to perform what is called a stress introducing lamination to form polygonal ferrite after hot rolling. Here, the stress introducing lamination means the hot lamination to be carried out on the condition that the lamination start temperature is Ars + 60 ° C or less and the reduction ratio is 1.5 or more. In the present invention, the polygonal ferrite that improves the deformability and tenacity at low temperature can be formed only by controlling the cooling conditions to be performed after the hot rolling without performing the stress introducing rolling. The temperature of the start of accelerated cooling after hot rolling is adjusted to Ar3 - 100 ° C to Ar3 - 10 ° C, thus making it possible to make the polygonal ferrite area ratio of the steel sheet become 27 to 90%. Incidentally, cooling to the accelerated cooling start temperature can be performed by air cooling, or it can also be performed by slow cooling at an average cooling rate of less than 10 ° C / s.
[0095] In addition, after hot rolling, slow cooling is performed to the accelerated cooling start temperature described above to form polygonal ferrite in this way, and then accelerated cooling is performed at an average cooling rate of 10 °. C / s or more to improve the resistance through the transformation of bainite and the transformation of martensite. In addition, to ensure resistance, the accelerated cooling must be stopped at a temperature of formation of bainite Bs or less.
[0096] Furthermore, in order to improve the toughness in the ZAC at extremely low temperatures of -40 ° C and also -60 ° C, a decrease in M-A is necessary, as a second hard phase and to make the crystal grains fine. However, in a thick material having a thickness of 20mm or more, the heat input in the welding becomes a high heat input, the grain size of the ZAC increases, and MB being the second hard phase detrimental to the toughness also increases. Therefore, it is extremely difficult to guarantee the toughness in the ZAC at extremely low temperatures of -40 ° C and also -60 ° C. Thus, the present inventors hereinafter turned their attention to a method of suppressing the formation of M-A in welding and also preventing the formation of crude ferrite in the grain contours. In addition, the present inventors have turned their attention to a method in which the intragranular transformation that begins in the oxides is promoted and also the hardening capacity is increased, and thus transformed intragranular structures to be formed at the time of welding are made thin, the size high-angle grain of a metallic structure composed of bainite and the transformed intragranular structure is decreased, and the low temperature toughness in the ZAC is improved.Incidentally, the transformed intragranular structure in this modality means intragranular ferrite or intragranular bainite formed in a shape of petal (in a radial pattern) starting from a finely dispersed inclusion.
[0097] Thus, the present inventors have now examined the conditions of the components that affect the temperature at which the transformed intragranular structure is formed in the ZAC.
[0098] Initially a steel containing, in% by mass, C: 0.010 to 0.060%, Si: 0.01 to 0.50%, Mn: 1.2 to 2.8%, S: 0.0001 to 0, 0050%, Ti: 0.003 to 0.030%, B: 0.0003 to 0.005%, N: 0.0010 to 0.008%, and O: 0.0001 to 0.0080%, and containing one or more elements between Cr, Cu and Ni, and containing P: limited to 0.050% or less, Al: limited to 0.008% or less, and M: limited to 0.03% or less, and having a Ceq which is the hardening capacity index of 0, 30 to 0.53 and a fracture parameter Pcm which is the welding capacity index from 0.10 to 0.20, and having a balance composed of iron and the inevitable impurities it was melted and cast to produce a steel plate.
[0099] Next, a specimen having a length of 10 mm and having a diameter of 3 mm was cut from the obtained steel plate and was subjected to a heat treatment simulating a ZAC of a welding zone to measure the temperature of start of the y / α transformation of bainite and the transformed intragranular structure by measuring the thermal expansion. The relationship between Ceq and the start temperature of the transformation y / α at that time is shown in FIG. 5.
[00100] In addition, a 12 mm square specimen having a length of 120 mm was cut from the steel plate and was subjected to the heat treatment described above simulating the ZAC of the welding zone to then measure the size of the high grain. angle of the metallic structure composed of bainite and intragranular structure transformed by an EBSP method (Electron Backscatter Diffraction Pattern). Incidentally, an interface having an angular difference of 15 ° or more between crystal grains was defined as a high angle grain outline and the maximum grain size of the crystal grains surrounded by the high angle grain outline was defined as the size high angle grain grain for ZAC toughness (effective crystal grain size). The results are shown in FIG. 6. Incidentally, the grain size means the radius of a circle having the same area as that of the crystal grain.
[00101] In addition, a 12 mm square specimen having a length of 120 mm was cut from the steel plate and then subjected to a Charpy impact test to measure the absorbed energy at -60 ° C. The results are shown in FIG. 7.
[00102] As shown in FIG. 5, it can be seen that with the increase in Ceq, the temperature of the start of transformation y / a decreases. That is, by increasing the curing capacity, the temperature of initiation of transformation y / a of the transformed intragranular structure can be decreased.
[00103] As shown in FIG. 6, it is noted that with the decrease in the temperature of initiation of transformation y / a, the high angle grain size of the metallic structure composed of bainite and intragraular transformed structure decreases, but when the temperature of initiation of transformation y / a decreases to less than 500 ° C, the high angle grain size increases. This is conceivable because the decrease in the grain size of the intragranular transformed structure to be formed contributes greatly to an effect of thinning the crystal grains obtained by decreasing the temperature at the beginning of the transformation y / a, but when the temperature at the beginning of the transformation y / α becomes very low, no intragranular transformed structure is obtained and a composite structure consisting mainly of bainite and martensite is formed, resulting in the fact that the crystal grains become crude. The effect of making the thin structure obtained by decreasing the temperature at the start of the y / α transformation conceivably results from the fact that as the transformation is caused at a lower temperature, the degree of subcooling increases, the frequency of core formation for intragranular transformation it increases, and intragranular transformation is promoted.
[00104] FIG. 7 is a view showing the relationship between the high angle grain size of the metallic structure composed of bainite and transformed intragranular structure and the energy absorbed Charpy at - 60 ° C. As shown in FIG. 7, it is noted that when the high angle grain size decreases, the absorbed energy Charpy at -60 ° C increases, and when the high angle grain size is 80 μm or less, the energy absorbed at -60 ° C becomes 50 J or more. That is, it is noted that by making the metallic structure composed of bainite and the transformed intragranular structure thin, excellent toughness can be obtained even at an extremely low temperature of -60 ° C.
[00105] As above, the present inventors have discovered a method in which the formation of MA in welding is suppressed, the hardening capacity of the steel is increased to avoid the formation of crude ferrite in the grain boundary, the intraganular transformation is initiated starting if from an inclusion, and also the intragranular transformation is promoted by controlling the temperature of the start of the y / α transformation, in order to reduce the high angle grain size of the metallic structure composed of bainite and the intragranular transformed structure to improve the toughness the low temperature in the ZAC.
[00106] To suppress the formation of M-A in welding, it is effective to decrease the C content and limit the Mo content. MA is formed as a result of the ZAC exposed to high temperature by welding being transformed into an austenite phase, and in a process where the transformation progresses during subsequent cooling, the concentration of C in an untransformed austenite phase progresses, and the austenite phase is stabilized. Therefore, by decreasing the amount of C, the concentration of C for the untransformed austenite phase is suppressed, resulting in the fact that the formation of M-A is suppressed. In addition, by decreasing the amount of C and also limiting the amount of Mo that contribute to the formation of M-A as above, the formation of M-A can also be suppressed.
[00107] In addition, to promote intragranular transformation, decreasing the amount of Al and adding an adequate amount of Ti are effective. When Ti oxides are finely dispersed, they act effectively as formation nuclei for intragranular transformation. However, when Al is added in large quantities, the formation of Ti oxides to act as forming nuclei for intragranular transformation is inhibited, so that, in the present invention, a suitable amount of Ti is added and the amount of Al is added. decreased.
[00108] In addition, to suppress the formation of crude ferrite in the grain boundary that deteriorates the low temperature toughness in the ZAC, it is extremely effective to increase the hardening capacity by adding an adequate amount of B.
[00109] In addition, it is noted that it is extremely important to reduce the high angle grain size of the structure composed of bainite and the transformed intragranular structure, decreasing the temperature of the start of the y / α transformation. Thus, the temperature at the start of the y / α transformation is decreased by the use of elements other than Mo to increase M-A.
[00110] In addition, by adding one or two or more elements between Mn, Cr, Cu, and Ni, the hardening capacity is increased to decrease the temperature of the start of the y / α transformation. Then, due to the thin intragranular structures transformed at low temperature, the metallic structure in the ZAC composed of fine grains made of bainite and transformed intragranular structures, thus making it possible to increase the toughness at low temperature in the ZAC.
[00111] That is, when compared to steels using the transformed intragranular structure that has been reported so far, by decreasing the C content and limiting the Mo content, the formation of M-A is also decreased. Then, by adding Mn and one or two or more elements between Cr, Cu, and Ni, the hardening capacity is increased, the temperature of initiation of the y / α transformation of the transformed intragranular structure is decreased, and the effective size of the crystal grain in relation to the toughness in the ZAC is also decreased. (Chemical composition)
[00112] The chemical composition of a high-strength steel tube and a high-strength steel plate in the present invention will be explained below. Incidentally, in relation to the chemical composition,% means% by mass. (C: 0.01 to 0.080%)
[00113] C is an element to improve the resistance of steel. To form a hard phase made of one or both of bainite and martensite in the metal structure, 0.01% or more of C must be contained. In addition, in the present invention, to achieve both high strength and high toughness, the C content is adjusted to 0.00% or less. In addition, to suppress the formation of M-A in the metal structure of the ZAC in particular to achieve both high strength and high toughness, the C content is adjusted to 0.060% or less. Incidentally, in view of the balance between strength and toughness, the C content is preferably adjusted to 0.02 to 0070%, and more preferably adjusted to 0.02 to 0.050% when the toughness in the ZAC is also considered. (Si: 0.01 to 0.50%)
[00114] Si is a useful element for deoxidation and improvement of resistance. To sufficiently perform deoxidation, 0.01% or more of Si must be contained in the steel. On the other hand, when a content greater than 0.50% of Si is contained in the steel, there is a risk that the tenacity of the ZAC will deteriorate, so that the upper limit of the Si content is adjusted to 0.50%. Incidentally, in view of the balance between strength and toughness and making deoxidation more efficient, the Si content is preferably adjusted to 0.05 to 0.3%, and more preferably adjusted to 0.1 to 0.25%. (Mn: 1.2 to 2.8% or less)
[00115] Mn is an inexpensive element, and it is an important element to increase Ceq which is the index of hardening capacity, decrease the temperature of initiation of the y / α transformation of the bainite and the intragranular transformed structure, and decrease the size of the high angle grain to increase toughness in ZAC. In addition, although it is added with B, Mn makes it possible to form polygonal ferrite in the base material without performing low temperature lamination and improving the toughness of the base material. To ensure strength and toughness, 1.2% or more of Mn must be contained in the steel.
[00116] On the other hand, when Mn is added excessively, the start temperature of transformation y / α decreases excessively, no intragranular transformed structure can be obtained, the grain size increases, and the toughness in the ZAC is impaired, so that the upper limit is adjusted to 2.8%. In addition, in view of the productivity when melting steel, the upper limit of Mn is preferably adjusted to 2.5%, and more preferably to 2.2%. (S: 0.0001 to 0.0050%)
[00117] S is an impurity, and when more than 0.0050% of S is contained in the steel, crude sulfides are formed and the toughness decreases, so that the S content is adjusted to 0.0050 or less. Incidentally, to also suppress the decrease in toughness, the S content is preferably adjusted to 0.003% or less, and more preferably adjusted to 0.0025% or less. In addition, when Ti oxides are finely dispersed in the steel plate, MnS precipitates, the intragranular transformation is caused, and the toughness of the base steel plates and the ZAC improves. To achieve this effect, 0.0001% or more of S must be contained in the steel. Thus, the S content is adjusted to 0.0001 to 0.0050%. (Ti: 0.003 to 0.030%)
[00118] Ti is an important element to form nitrides of Ti to contribute to the reduction of the crystal grain size of the base steel plate and the ZAC. Therefore, 0.003% or more of Ti must be contained in the steel. To also decrease the size of the crystal grain of the ZAC, the Ti content is preferably 0.005% or more, and more preferably 0.008% or more.
[00119] On the other hand, when Ti is excessively contained in steel, crude inclusions are formed to impair toughness, so that the upper limit of Ti is adjusted to 0.030%. In addition, to disperse Ti oxides more finely, the Ti content is preferably 0.028% or less, and more preferably 0.025% or less.
[00120] When Ti oxides are finely dispersed, they act effectively to form nuclei for intragranular transformation. Incidentally, when the oxygen content is large when Ti is added, crude Ti oxides are formed, so that at the time of steel production, deoxidation is preferably performed by Si and Mn to decrease the oxygen content in the steel. In this case, oxides of Al are more likely to be formed than oxides of Ti, so it is not preferred that an excessive amount of Al should be added to the steel for deoxidation. (B: 0.0003 to 0.005%)
[00121] B significantly increases the hardening capacity, and is an important element to suppress the formation of crude ferrite in the grain contours in the ZAC. To achieve this effect, 0.0003% or more of B must be contained in the steel. In addition, in order to more safely increase the curing capacity, the B content is preferably 0.0005% or more.
[00122] On the other hand, when B is excessively added to steel, crude BN is formed and the toughness of the ZAC in particular decreases, so that the upper limit of the B content is adjusted to 0.005%. (N: 0.010 to 0.008%)
[00123] N forms TiN and suppresses the hardening of the austenite grains when the plate is reheated and suppresses the hardening of the austenite grains in the ZAC to improve the low temperature toughness of the base material and in the ZAC. The minimum amount required for this is 0.0010%.
[00124] On the other hand, when N is excessively contained, BN is formed to impair the effect of improving the hardening capacity of B, resulting in the fact that crude ferrite is formed at the grain boundaries to impair the toughness in the ZAC, or Crude BN is formed to impair toughness in the ZAC. Therefore, the upper limit of N is set to 0.008%. Incidentally, to obtain the effect stably by the addition of N, the N content is preferably adjusted to 0.0020 to 0.007%. (O: 0.0001 to 0.0080%)
[00125] O is an impurity, and the upper limit of the O content needs to be adjusted to 0.0080% to avoid decreasing the toughness caused by the formation of inclusions.
[00126] On the other hand, to form Ti oxides to contribute to the intragranular transformation, the O content that remains in the steel at the time of casting is adjusted to 0.0001% or more.
[00127] Incidentally, when the balance between the guarantee of toughness and the formation of Ti oxides is considered, the O content is preferably adjusted to 0.0010 to 0.0050%. (P: 0.050% or less)
[00128] P is an impurity, and when more than 0.050% of P is contained in the steel, the toughness of the base steel plate decreases significantly. Thus, the P content is limited to 0.050% or less. To improve the toughness of ZAC, the P content is preferably limited to 0.020% or less. Incidentally, the lower limit value of the P content is not adjusted in particular, but adjusting the lower limit value to less than 0.0001% is economically disadvantageous, so that value is preferably adjusted as a lower limit value. (Al: 0.020% or less)
[00129] Al is a deoxidizing element, but to increase the toughness of the steel sheet and the ZAC by suppressing the formation of inclusions, the upper limit of Al needs to be adjusted to 0.020%. The Al content is limited as above, thus making it possible to finely disperse Ti oxides that contribute to the intragranular transformation. In particular, to form sufficiently high Ti oxides that contribute to the intragranular transformation, the upper limit of Al is adjusted to 0.008%. For finely dispersing Ti oxides, the upper limit of Al is preferably 0.005% and, to obtain more stably Ti oxides, the upper limit of Al is more preferably 0.003%. Incidentally, the lower limit value of the Al content is not adjusted in particular, but it can be greater than 0%. (Mo: 0.03% or less)
[00130] Mo significantly increases the hardening capacity by adding compound with B in particular, and is an effective element to achieve high strength of the base steel plate and improve the toughness of the ZAC, but the addition of Mo makes the formation of ferrite difficult polygonal on the base steel plate, so as to create a risk that the low temperature toughness and deformability of the base material cannot be sufficiently guaranteed. Therefore, to improve the toughness and deformability of the base material, the amount of Mo is limited to 0.03% or less. In addition, Mo is an expensive element, and it is preferable that Mo is not added in view of the cost of connection. (Cr, Cu, and Ni)
[00131] In addition, the high strength steel tube and the high strength steel sheet of the present invention contain one or more elements between Cr, Cu, and Ni in addition to the elements described above. Cr forms polygonal ferrite in the base steel plate without performing low temperature lamination even if added with B, and is an element capable of improving the toughness of the base material. In addition, Cr is an inexpensive element and is an important element for increasing Ceq which is an index of hardening capacity, decreasing the start temperature of transformation y / α, and decreasing the high angle grain size to increase toughness of the ZAC. In addition, Cu and Ni are effective elements that increase strength without impairing toughness, and increase Ceq which is an index of hardening capacity and improves toughness in ZAC. In addition, Cu and Ni form polygonal ferrite in the base material without performing low temperature lamination even if added with B, and improve the toughness of the base material. In addition, Cu and Ni are elements that decrease the temperature at the start of the y / α transformation to decrease the high angle grain size. Incidentally, Cu and Ni are preferably composed to suppress the occurrence of surface flaws.
[00132] As will be described later, the contents of Cr, Cu and Ni are limited in order to make the Ceq to be obtained by (Expression 1) (or (Expression 1 ')) become 0.30 to 0.53 and are limited so that the Pcm to be obtained by (Expression 2) (or (Expression 2 ')) becomes 0m10 to 0.20. In addition, in order to suppress the formation of MA in the metallic structure of the ZAC in particular to achieve both high strength and high toughness, the contents of Cr, Cu and Ni are limited in order to make the start temperature of the transformation y / α to be obtained by (Expression 3) becomes 500 to 600 ° C.
[00133] In addition, for the high-strength steel pipe and high-strength steel plate of the present invention, one or two or more elements between W, V, Nb, Zr, and Ta can also be added as an element to improve strength and toughness, in addition to the elements described above. In addition, these elements can be considered as impurities because they do not cause adverse effects particularly when their levels are each lower than the preferable lower limit. (W, V, Nb, Zr, Ta, Mg, Ca, Rare Earths, Y, Hf, and Re)
[00134] Furthermore, in the present invention, one or two or more elements between W, V, Nb, Zr, Ta, Mg, Ca, Rare Earths, Y, Hf, and Re can also be contained as an element to improve the strength and toughness, in addition to the elements described above. In addition, these elements can be considered as impurities because they do not cause adverse effects particularly when their levels are less than the preferable lower limit.
[00135] W, V, Nb, Zr, and Ta each form carbides and nitrides and are elements to improve the strength of steel by reinforcing precipitation, and one or two or more of them may also be contained. To effectively increase the resistance, the lower limit of the amount of W is preferably set to 0.01%, the lower limit of the amount of V is preferably set to 0.010%, the lower limit of the amount of Nb is preferably set to 0.001%, and the lower limits of the amount of Zr and the amount of Ta are both preferably set to 0.0001%.
[00136] On the other hand, when W is added excessively, the resistance sometimes increases excessively due to the improvement of the hardening capacity to impair the toughness, so that the upper limit of the amount of W is preferably adjusted to 0.50%. In addition, when V, Nb, Zr, and Ta are added excessively, carbides and nitrides sometimes become crude to impair toughness, so the upper limit on the amount of V is preferably set to 0.100%, the upper limit on the amount of Nb is preferably set to 0.200%, and the upper limits of the amount of Zr and the amount of Ta are both preferably set to 0.0500%.
[00137] Mg, Ca, Rare Lands, Y, Hf, and Re are each an element to control the shape of inclusions to achieve improved toughness, and one or two or more of these elements may also be contained.
[00138] Mg is an element that has the effect of making fine oxides and suppressing the formation of sulfides. Particularly, Mg oxides act as nuclei for the formation of intragranular transformation, and have the effect of making fine oxides and suppressing the formation of sulfides. Particularly, fine Mg oxides act as nuclei for the formation of intragranular transformation, and have the effect of suppressing the growth of grain size as fixing particles. To achieve these effects, 0.0001% or more of Mg is preferably contained. On the other hand, when more than 0.0100% and Mg is contained, crude oxides are sometimes formed to decrease the toughness of the ZAC, so that the upper limit of the amount of Mg is preferably adjusted to 0.0100%.
[00139] Ca and Terras Raras are useful elements to control the shape of the
[00140] sulfides to suppress the formation of MnS extended in the rolling direction and to improve the properties of the steel material in the direction of the plate thickness, particularly resistance to lamellar tearing. To achieve these effects, the lower limits for the amount of Ca and the amount of Rare Earths are both preferably set to 0.0001% or more. On the other hand, when the amount of Ca and the amount of Rare Earths exceeds 0.0050%, the oxides increase, the fine oxides containing Ti decrease, and the occurrence of intragranular transformation is sometimes inhibited, so that the amount of Ca and the amount of Rare Earths is, each, preferably 0.0050% or less.
[00141] Y, Hf, and Re are also elements that have effects similar to those of Ca and Rare Earths, and when they are added excessively, the occurrence of intragranular transformation is sometimes inhibited. Therefore, the preferable ranges for the amount of Y, the amount of Hf and the amount of Re are each 0.0001 to 0.0050%.
[00142] In addition, the balance different from the elements described above is practically composed of Fe, and elements that do not impair the functions and effects of the present invention such as unavoidable impurities can be added in insignificant amounts. (Ceq equivalent carbon)
[00143] In the present invention, to guarantee the low temperature toughness of the steel sheet and the ZAC, the equivalent carbon Ceq of (Expression 1) below which is calculated from the respective contents [% by mass] of C, Mn, Ni, Cu, Cr, Mo, and V being the elements that contribute to the improvement of the hardening capacity, it is adjusted to 0.30 to 0.53. The equivalent carbon Ceq has been known to correlate with the maximum hardness of the welding zone and is a value to be the index of hardening capacity and welding capacity. Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 ••• (Expression 1)
[00144] Here, C, Mn, Ni, Cu, Cr, and Mo na (Expression 1) above denote the contents of the respective elements [mass%]. Ni, Cu, Cr, and Mo are calculated as 0 when the respective levels are 0%.
[00145] Incidentally, in the present invention, when V is also contained, Ceq is obtained by (Expression 1 ') below in place of (Expression 1) above. Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 ••• (Expression 1 ’)
[00146] Here, C, Mn, Ni, Cu, Cr, Mo, and V na (Expression 1 ') above denote the contents of the respective elements [mass%]. Ni, Cu, Cr, and Mo are calculated as 0 when the respective levels are 0%. V is calculated as 0 when the content is less than 0.010% by weight. (Pcm fracture parameter)
[00147] In addition, to guarantee the low temperature toughness of the steel sheet and the ZAC, the fracture parameter Pcm of (Expression 2) below which is calculated from the [% by mass] contents of C, Si, Mn , Cu, Cr, Ni, Mo, V, and B are adjusted to 0.10 to 0.20. The fracture parameter Pcm was known as a coefficient that allows the assumption of the fracture sensitivity at low temperature in welding, and is a value to be an index of hardening capacity and welding capacity. Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + 5B ••• (Expression 2)
[00148] Here, C, Si, Mn, Ni, Cu, Cr, Mo, and B na (Expression 2) above denote the contents of the respective elements [mass%]. Ni, Cu, Cr, and Mo are calculated as 0 when the respective levels are 0%.
[00149] Incidentally, in the present invention, when V is also contained, the Pcm is obtained by (Expression 2 ') below in binding from (Expression 2) above. Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B ••• (Expression 2 ’)
[00150] Here, C, Si, Mn, Ni, Cu, Cr, Mo, V, and B na (Expression 2 ') above denote the contents of the respective elements [mass%]. Ni, Cu, Cr, and Mo are calculated as 0 when the respective levels are 0%. V is calculated as 0 when the content is less than 0.010% by weight.
[00151] In addition, in order to have a good toughness of the ZAC at an extremely low temperature of -60 ° C in particular, it is necessary to make the microstructure in which the high angle grain size of the metallic structure composed of bainite and intragranular transformed structure either 80 μm or less. To achieve this, in addition to the Ceq and Pcm limitation, the start temperature of the y / α transformation in the ZAC obtained by (Expression 3) below is also adjusted to 500 to 600 ° C. Transformation start temperature y / α = -2500Ceq2 + 1560Ceq + 370 ••• (Expression 3) (Metal structure)
[00152] The metal structure of the base steel sheet of the high-strength steel tube of the present invention and the metal structure of the high-strength steel sheet of the present invention are each composed mainly of polygonal ferrite and contain a hard phase as your balance. Here, FIG. 8 is a photograph showing the metallic structure of the base steel plate. FIG. 9 is a schematic view to explain the metallic structure of the base steel plate. Polygonal ferrite is a ferrite to be formed at a relatively high temperature during air cooling after hot rolling. Polygonal ferrite has an aspect ratio of 1 to 4, and is distinguished from laminated ferrite to be extended (worked ferrite) and acicular ferrite (acicular ferrite) and Widmanstatten ferrite which are formed at a relatively low temperature during accelerated cooling after hot rolling. Here, the aspect ratio is a value for the length of a ferrite grain divided by the width.
[00153] In addition, the polygonal ferrite is observed on the white part, and the massive structure does not contain precipitates such as crude cementite and martensite-austenite constituents (to be referred to as M-A) in a grain with an optical microscope.
[00154] In addition, the hard phase described above is a structure composed of one or both of bainite and martensite. Incidentally, in the structure to be observed with an optical microscope, as different balance of polygonal ferrite, bainite, and martensite, retained austenite, and MA are sometimes contained. The fraction of M-A in the base material is desirably 8.0% or less.
[00155] As shown in FIG. 9, in the metallic structure of the base steel plate, in contrast to white, round and solid polygonal ferrite 1, a hard phase 2 such as bainite appears in the form of a lath or in the form of a plate, for example, and MA 3 appears outside of polygonal ferrite 1.
[00156] The polygonal ferrite area ratio in the steel plate is made to become 27% or more. As described above, in the steel sheet having the chemical composition with increased hardening capacity, polygonal ferrite is formed and the balance is composed of hard phase of bainite and martensite, and thus the balance between strength and deformability is improved. When the polygonal ferrite area fraction is 27% or more, the yield stress which is the deformability index (YS / TS) becomes 80% or less, and when the polygonal ferrite area ratio is 50% or more , the yield stress becomes 70% or less, resulting in the fact that a good deformability cannot be obtained.
[00157] On the other hand, to guarantee the resistance, it is necessary to make the polygonal ferrite area ratio become 90% or less. As shown in FIG. 3, the polygonal ferrite area ratio is made to become 90% or less, thus making it possible to guarantee the tensile strength corresponding to X70 or greater. In addition, to increase the strength to ensure the tensile strength corresponding to X80 or greater, the polygonal ferrite area ratio is preferably made to become 80% or less. In addition, to more stably guarantee the tensile strength corresponding to X80, the polygonal ferrite area ratio is more preferably 70% or less, and the preferable value of the area ratio is 60% or less.
[00158] In addition, the polygonal ferrite area ratio is made to become 27 to 90%, thus improving the balance between the strength and toughness of the steel sheet. The area ratio of the polygonal ferrite is made 20% or more, and thus, as shown in FIG. 4, the toughness lowers the steel sheet temperature significantly improves to make it possible to make the shear area in the DWTT at -60 ° C become 85% or more.
[00159] In addition, in the metallic structure of the steel plate, a different balance of polygonal ferrite is the hard phase composed of one or both of bainite and martensite. The area ratio of the hard phase becomes 10 to 73% because the area ratio of the polygonal ferrite is 27 to 90%.
[00160] Bainite is defined as a structure in which retained austenite carbides and M-A are positioned between slab-shaped, plate-like and solid bainitic ferrites. Martensite is a structure composed of ferrite and lath-shaped or in the form of a plate in which the carbon is dissolved solid in a supersaturated manner, where no carbide precipitates. Retained austenite is that austenite formed at high temperature that is not transformed y / a to be retained at room temperature.
[00161] Incidentally, the thickness of the high-strength steel sheet of the present invention is not limited, but it is particularly effective when the thickness of the sheet is 20 to 40 mm. Similarly, the thickness of the base steel sheet of the high strength steel tube of the present invention is not limited, but is particularly effective when the thickness of the sheet is 20 to 40 mm. (Metal structure of ZAC)
[00162] Furthermore, in order to obtain good toughness in the ZAC at an extremely low temperature of -60 ° C in particular in the high strength steel tube of the present invention, it is important that the metal structure in a previous austenite grain in the ZAC contains the transformed intragranular structure.
[00163] FIG. 10 (a) and FIG. 10 (b) are schematic views to explain the structure of the ZAC in the high strength steel tube of the present invention and are seen to explain the transformed intragranular structure. FIG. 10 (a) shows the state in which no transformed intragranular structure 12 is contained in a previous austenite grain and FIG. 10 (b) shows the state in which the transformed intragranular structures 12 are contained in a previous austenite grain. As will be described later, the high strength steel tube of the present invention is produced in such a way that, for example, the high strength steel sheet (base material) is shaped into a tube, the top portions are welded, and the tube is expanded. On that occasion, the range from the weld metal to a predetermined distance becomes the ZAC.
[00164] In FIG. 10 (a) and FIG. 10 (b), each symbol 11 denotes a previous austenite grain outline, and the region surrounded by this y 11 grain outline corresponds to the interior of the previous austenite grain. The previous austenite grain contour is an austenite grain contour formed when the base structure exposed to high temperature by welding is transformed and austenite. The interior of the previous austenite grain is y / α transformed during the cooling process after welding to be the structure containing the transformed intragranular structures 12.
[00165] FIG. 10 (a) and FIG. 10 (b) each show the metallic structure of two previous austenite grains G1 and G2 that are in contact with each other in the ZAC. The metal structure shown in FIG. 10 (a) and FIG. 10 (b) can be observed in such a way that the ZAC is etched with nital or similar to be enlarged 100 times to 500 times by using an optical microscope or a scanning electron microscope.
[00166] In the high-strength steel tube of the present invention, particularly Al is set to 0.005% or less, thus making it possible to finely disperse Ti oxides in the steel and form the intragranular transformed structure in the previous austenite grain in the ZAC starting from the oxide of Ti (an inclusion).
[00167] Here, as shown in FIG. 10 (a), in the state in which the intragranular transformed structures 12 are not contained in grain y, grains of bainite and grains of martensite 14 to be formed in the previous austenite grain are not divided and the crystal grain sizes in the grain of previous austenite does not decrease.
[00168] In contrast to this, as shown in FIG. 10 (b), in the previous austenite grain in the ZAC, a state is made in which the Ti 12 oxides are finely dispersed because the amount of Al is decreased and the appropriate amount of Ti is added. (Incidentally, Ti 12 oxides are extremely thin).
[00169] Here, the metallic structure of the base material heated to a region y by welding is transformed into austenite, and in a process of the austenite being cooled, ferrite or bainitic ferrite using Ti 12 oxide finely dispersed in the steel as a core is formed in a radial (petal-shaped) pattern. Petal-shaped ferrite is called intragranular ferrite, and petal-shaped bainite is called intragranular bainite. In the present invention, intragranular ferrite and intragranular bainite are collectively referred to as an intragranular transformed structure 13. Intragranular transformed structures 13 have different crystal orientations than those of bainite grains and martensite grains to be obtained normally denoted by the symbol 14, in order to divide these grains of bainite and grains of martensite 14, resulting in the fact that the crystal grain sizes in the previous austenite grain decrease.
[00170] As shown in FIG. 10 (b), in the high strength steel tube of the present invention, the transformed intragranular structures 13 divide structures of crude bainite and structures of crude martensite (the grains of bainite and grains of martensite 14) in the previous austenite grain, so thin the entire ZAC structure. Incidentally, in FIG. 10 (b), only in grain y G1 on one side, the state is shown in which the grains of bainite and grains of martensite 14 are divided by the transformed intragranular structures 13, but in the previous austenite grain G2 also on the other side, the state in which the transformed intragranular structures 13 are formed to divide the bainite grains and the martensite grains 14 is made similarly.
That is, in the high strength steel tube of the present invention in which Al is limited to 0.008% or less, a suitable amount of Ti is added, and thus Ti oxides are finely dispersed, many intragranular transformed structures are formed in ZAC prior austenite grains to split raw bainite (or crude martensite) be formed in prior austenite grains, so that the entire ZAC structure is thinned and the ZAC tenacity improves.
[00172] As above, many intragranular transformed structures are desirably formed so that the high angle grain size of the metal structure in the ZAC can become 80 μm or less. To also increase the toughness in the ZAC at an extremely low temperature, the high angle grain size of the metal structure in the ZAC is preferably made to become 70 μm or less, and also preferably made to become 60 μm or less. Incidentally, as described above, the high-angle grain size of a crystal grain in relation to the interface having an angular difference of 15 ° or more as a grain boundary and is measured by an EBSP (Electron Back Scatter Diffraction Pattern) method . In the metallic structure of ZAC, the interface having an angular difference (the grain boundary) is defined as the high angle grain boundary, and the maximum grain size of the crystal grain sizes surrounded by the high strength grain boundary is defined as the effective high angle grain size for ZAC toughness (an effective crystal grain size).
[00173] In the present invention, the temperature of initiation of transformation y / a in the ZAC obtained by (Expression 3) above is adjusted to 500 to 600 ° C, and thus the formation of transformed intragranular structures is promoted and the grain size of high angle of the metallic structure in the ZAC becomes 80 μm or less. Here, FIG. 11 is a photograph showing the metallic structure of the ZAC when the temperature of initiation of transformation y / α is 500 to 600 ° C. At each location indicated by the arrow in the drawings, there is Ti oxide to be the forming nucleus for the transformed intragranular structure. It is conceivable that the metal structures in FIG. 11 and FIG. 12 are substantially the same in the amount of Al, in the amount of Ti and in the amount of oxygen although the temperatures at the beginning of the transformation y / α are different, so that they are also the same in the dispersion state of Ti oxides. However, as shown in FIG. 11, when the start temperature of transformation y / α is greater than 600 ° C, the number of intragranular transformed structures to be formed is decreased and the high-angle grain size exceeds 80 μm. Incidentally, when the start temperature of transformation y / α decreases to less than 500 ° C, the high-angle grain size similarly exceeds 80 μm. In contrast to this, as shown in FIG. 12, when the temperature of initiation of transformation y / α is 500 to 600 ° C, intragranular transformation is promoted to form many transformed intragranular structures, so that the high-angle grain size becomes 80 μm or less.
[00174] Furthermore, in the high-strength steel pipe of the present invention, M-A is a structure detrimental to the toughness of ZAC. Therefore, M-A in the ZAC is made to become 2.5% or less in the fraction of area. Incidentally, in order to obtain stably good toughness of the ZAC, MA is preferably made to become 2.2% or less in fraction of area, and to obtain a better toughness in the ZAC, MA is preferably made to become 1.7% or less, and more preferably made to become 1.3% or less.
[00175] Here, FIG. 13 is a photograph showing the metallic structure of the ZAC when the fraction of area of M-A is 2.2%. In addition, FIG. 14 is a photograph showing the metallic structure of the ZAC when the area fraction of M-A is 3.0%. In FIG. 13 and FIG. 14, M-A appears as a white part. As shown in FIG. 13, when the fraction of area of M-A is 2.2%, vTrs (transition temperature of the appearance of the fracture) becomes -65 ° C and toughness lowers temperature to -60 ° C or less is guaranteed. In contrast to this, as shown in FIG. 14, when the fraction of area M-A is 3.0%, vTrs (fracture appearance transition temperature) becomes -55 ° C and low temperature toughness at -60 ° C or less is no longer guaranteed.
[00176] Incidentally, in FIG. 13 and FIG. 14, the measurement of the M-A fraction is performed based on the area fractions obtained when the metal structure of the ZAC is observed at magnifications of 500 times using an optical microscope.
[00177] Incidentally, when the tensile strength of the base steel plate is 500 to 800 MPa under the condition that the circumference direction of the high strength steel tube in the present invention is adjusted to the tensile direction, the effect of the present invention can be used more. (Production method)
[00178] Next, the production methods of the high-strength steel sheet and the high-strength steel tube of the present invention will be explained. Initially, a steel plate made up of the chemical composition described above is heated to 950 ° C or more, is subjected to hot rolling at temperature Ar3 or higher, and then cooled slowly to be subjected to accelerated cooling to the temperature of Bs or lowest obtained by (Expression 4) below from a temperature of Ar3 - 100 ° C to Ar3 - 10 ° C at an average cooling rate of 10 ° C / s or more. Bs (° C) = 830 - 270C - 90Mn - 37Ni - 70Cr - 83Mo ••• (Expression 4)
[00179] Here, C, Mn, Ni, Cr, and Mo na (Expression 4) above denote the contents of the respective elements [mass%]. Ni, Cu, Cr, and Mo are calculated as 0 when their levels are 0%.
[00180] The chemical composition described above containing B is made, and therefore the hardening capacity is increased to suppress the formation of ferrite in the ZAC, but polygonal ferrite to improve the deformability and tenacity at low temperature can be formed in the steel plate. high strength to be the base material. In accordance with the present invention in particular, selecting elements for improving the curing capacity other than Mo as a third element to be added with B, the low temperature lamination which applies a load to an unnecessary lamination step, and just by adjusting the temperature of the beginning of the accelerated cooling to be carried out after hot rolling to Ar3 - 100 ° C to Ar3 - 10 ° C, the area ratio of the polygonal ferrite in the steel plate becomes 27 to 90% .
[00181] In the production method, a steel made of the chemical composition described above is initially melted in the steel production step and then casted to form a steel plate. In the steel production stage, a steel has Si and Mn added to it to undergo weak deoxidation, and then has Ti added to it to melt in order to have the chemical composition described above, and then it is cast to form the steel plate. The melting and casting of steel can be carried out by common methods, but continuous casting is preferable in view of productivity. Then the steel sheet is reheated for hot rolling.
[00182] The reheat temperature in hot rolling is adjusted to 950 ° C or more. This is because hot rolling is carried out at a temperature at which the steel structure becomes a single austenite phase, that is, an austenite region to decrease the sizes of the crystal grains of the base steel plate.
[00183] The upper limit of the heating temperature is not particularly prescribed, but to suppress an increase in the effective size of the crystal grain, the heating temperature is preferably adjusted to 1250 ° C or less. Incidentally, to increase the area ratio of the polygonal ferrite, the upper limit of the heating temperature is preferably set to 1100 ° C or less, and more preferably set to 1050 ° C or less.
[00184] Next, the heated steel plate is subjected to hot rolling through a plurality of passes while controlling the temperature and the reduction ratio, and after completion, it is air-cooled and subjected to accelerated cooling. To decrease the crystal grain size of the base steel plate, the rate of reduction of the hot rolling in a recrystallization region of more than 900 ° C is preferably adjusted to 2.0 or more. The reduction ratio in the recrystallization region means the ratio of the thickness of the steel plate sheet to the thickness of the plate at 900 ° C. In addition, the hot lamination needs to be finished at the temperature of Ar3 or more in which the structure of the base material becomes a single austenite phase. When hot rolling is carried out at a temperature lower than the temperature of Ar3, productivity decreases. In addition, worked ferrite having an aspect ratio of more than 4 is formed, the fracture form called separation is formed, and the energy absorbed in a Charpy impact test decreases.
[00185] Incidentally, in the present invention, it is also possible that at the end of the hot lamination step, the lamination start temperature is adjusted for Ar3 to Ar3 + 100 ° C and the non-recrystallized y lamination region to be performed in a region of y not recrystallized at 900 ° C or less is performed. In that case, when productivity is considered, the lamination start temperature is preferably adjusted to Ar3 + 60 to Ar3 + 100 ° C. To decrease the effective crystal grain size of the base steel plate, the ratio of reduction to hot rolling in the non-recrystallized y region is preferably adjusted to 2.5 or more, and to also decrease the effective grain size, the reduction ratio is preferably adjusted to 3.0 or more. Incidentally, in the present invention, the ratio of reduction of the non-recrystallized y lamination region is the ratio of the thickness of the sheet at 900 ° C divided by the thickness of the sheet obtained after the hot rolling is finished.
[00186] Incidentally, the upper limits of the reduction ratios in the non-recrystallized y region and in the recrystallization region are not prescribed, but when the thickness of the plate before hot rolling and the thickness of the plate after hot rolling are considered, the upper limits are usually 12.0 or less.
[00187] In the present invention, as the third element to be added with B, selecting elements other than Mo that improve the hardening capacity is extremely important. This is because in steel with Mo-B added compound whose effect of hardening capacity is greatly improved by the compound addition of B and Mo, the transformation of ferrite is significantly delayed.
[00188] Then, while the different elements of Mo are selected to increase the hardening capacity in this way, the formation of ferrite in the grain contour in the ZAC is suppressed and the formation of polygonal ferrite in the base material is facilitated. At this time, the connection elements other than Mo are used to bring the Ceq which is the index of hardening capacity to the range of 0.30 to 0.53. To achieve this, in addition to C, elements such as Mn, Cr, Ni, and Cu can also be selected.
[00189] To form polygonal ferrite in the base material, the start temperature of the hot rolling is adjusted to a low temperature of Ar3 + 60 ° C or less, and no stress introducing rolling (rolling at low temperature) at a ratio reduction of 1.5 or more is required. However, accelerated cooling after hot rolling needs to start in the range of Ar3 - 100 ° C to Ar3 - 10 ° C. Therefore, the area ratio of the polygonal ferrite of the steel sheet to be the base material becomes 27 to 90%. Incidentally, the temperature of initiation of accelerated cooling is preferably adjusted to be in the range of Ar3 - 70 ° C to Ar3 - 20 ° C.
[00190] Incidentally, prior to lamination in the non-recrystallized y region mentioned above, recrystallization lamination can also be performed. The recrystallization lamination is the lamination in the recrystallization region at over 900 ° C and the non-recrystallization lamination is the lamination in a non-recrystallized region at 900 ° C or less. The recrystallization lamination can also be started immediately after the steel plate is extracted from a heating furnace, so that its starting temperature is not particularly prescribed. In addition, lamination with a plurality of passes can also be performed while controlling the temperature and the reduction ratio.
[00191] Furthermore, in order to decrease the effective crystal grain size of the steel plate, the reduction ratios in the recrystallization lamination and in the non-recrystallized y region are preferably adjusted to 1.5 or more.
[00192] In addition, after hot rolling is finished, slow cooling is performed and then accelerated cooling is performed. In order to form an area ratio of 27 to 90% polygonal ferrite, slow cooling needs to be performed 3x until a temperature less than Ar3 after the lamination in the non-recrystallized y region is finished. Thus, it is necessary to start the accelerated cooling described above at a temperature in the range of Ar3 - 100 ° C to Ar3 - 10 ° C.
[00193] In addition, to suppress the perlite, crude cementite, and crude MA formations to ensure tensile strength and toughness, the average cooling rate of the accelerated cooling mentioned above needs to be adjusted to 10 ° C / s or more . As above, slow cooling is performed up to the start temperature of accelerated cooling to form polygonal ferrite, and then accelerated cooling is performed, thus making it possible to cause the transformation of bainite and the transformation of martensite and improve strength and toughness. Incidentally, the average cooling rate of the accelerated cooling is preferably adjusted to 20 ° C / s or more.
[00194] Incidentally, a certain period of air cooling usually exists until accelerated cooling is started after hot rolling. Cooling (slow cooling) down to the accelerated cooling start temperature after hot rolling can also be performed during this air cooling period. This cooling is defined as slow cooling at an average cooling rate of less than 10 ° C / s. As above, cooling until accelerated cooling is initiated is defined as slow cooling (at an average cooling rate of less than 10 ° C / s), thus making it possible to effectively form polygonal ferrite.
[00195] Here, each cooling rate is defined as the average rate in the center of the thickness of the steel plate and each temperature is defined as the average temperature of the steel plate.
[00196] In addition, in relation to the accelerated cooling described above, to guarantee resistance by suppressing the formation of perlite, crude cementite and crude MA and by the formation of a hard phase composed of one or both between bainite and martensite, the temperature accelerated cooling interruption must be set to Bs or less, which is obtained by (Expression 4) below. Incidentally, Bs is the starting temperature of the transformation of bainite and has been known to be decreased by the addition of the linkers as shown in (Expression 4) below. While the accelerated cooling is carried out until the temperature Bs or lower, the bainite can be formed. Bs (° C) = 830 - 270C - 90Mn - 37Ni - 70Cr - 83Mo ••• (Expression 4)
[00197] In addition, the lower limit of the accelerated cooling interruption temperature is prescribed, and the accelerated cooling can also be carried out up to room temperature. However, when hydrogen productivity and defects are considered, the cut-off temperature is preferably 150 ° C or more.
[00198] Furthermore, the high strength steel tube of the present invention can be produced in a way that the high strength steel sheet produced by the method described above is used as the base material to be shaped into a tube by a UO, JCO or folding step, the top portions are arc welded internally and externally, and then the tube is expanded.
[00199] In relation to the arc welding described above, submerged arc welding is preferably employed in view of the toughness of a weld metal and the productivity. When the high strength steel sheet having a thickness of 20 to 40 mm is particularly used as a base material to produce a welded steel tube, the thermal input of the submerged arc welding to be carried out internally and externally is preferably adjusted to 3, 0 to 10.0 kJ / mm. While the thermal input is in this range, in the steel tube of the present invention having the chemical composition described above, the high angle grain size of the ZAC, which is the effective size of the crystal grain, becomes 80 μm or less and a excellent low temperature toughness can be obtained.
[00200] In addition, when submerged arc welding is performed internally and externally for each pass, the heat input of the internal weld and the heat input of the external weld do not have to be the same under conditions, and the heat inputs can also be somewhat different.
[00201] After arc welding, the tube can also be expanded to improve the circularity of the steel uncle. When the circularity of the steel tube is increased by the expansion of the tube, the steel tube must be deformed to a plastic region, so that the expansion ratio is preferably adjusted to 0.7% or more. Incidentally, the expansion ratio which is the value of the difference between the outer circumferential length of the steel tube after expansion and the outer circumferential length of the steel tube before expansion divided by the outer circumferential length of the steel tube before expansion is expressed percentage basis. When the expansion ratio is adjusted to more than 2%, there is a risk that the base material and the welding zone will decrease in toughness due to plastic deformation. Thus, the expansion ratio is preferably adjusted to 0.7 to 2.0%.
[00202] In addition, in the welding zone and in the ZAC of the steel tube obtained, a heat treatment can also be carried out. When the welding zone and the HAZ are heated to a temperature of 300 to 600 ° C in particular, crude M-A formed along the previous austenite grain contour is decomposed into bainite and fine cementite, so that the toughness improves. Incidentally, when the heating temperature is less than 300 ° C, the decomposition of crude MA sometimes becomes insufficient to make it impossible to obtain a sufficiently improved toughness effect, so that the lower limit is preferably adjusted to 300 ° C or more. On the other hand, when the welding zone is heated to more than 600 ° C, precipitates are sometimes formed to deteriorate the toughness of the weld metal, so that the upper limit is preferably adjusted to 60-0 ° C or less. When M-A formed in ZAC is decomposed into bainite and cementite, the resultant is made having the same shape as M-A and containing fine white precipitates and can be distinguished from M-A by observation with an SEM.
[00203] Regarding heat treatment, the welding zone and the ZAC can be heated externally by a torch, or they can also be subjected to high frequency heating. The tube can be cooled immediately after its outer surface reaches the temperature of the heat treatment, but it is preferably maintained for 1 to 600 s to promote the decomposition of the M-A. However, when equipment cost and productivity are considered, the retention time is preferably set to 300 s or less.
[00204] The high strength steel plate according to the present invention has a steel component with high hardening capacity in which B is added, the additive amount of Mo is limited and, in addition, the equivalent carbon Ceq and the parameter of Pcm fracture fail in the ranges described above. In addition, the metal structure is a structure composed of one or both of bainite and martensite. Therefore, the metallic structure makes it possible to suppress the formation of crude ferrite in the grain boundaries and improves the low temperature toughness in the base material. In addition, the metallic structure is a structure composed of polygonal ferrite and bainite and / or martensite, so that the yield stress can be suppressed and an excellent deformability can be obtained.
[00205] In addition, the high strength steel tube according to the present invention uses the high strength steel sheet described above as the base material, so that the toughness of the base material, toughness of the ZAC, and the deformability at a temperature extremely low can all be improved. In relation to the chemical composition of the high-strength steel plate to be the base material, C is decreased and also Mo is limited, and thus the formation of M-A harmful to low temperature toughness is decreased. In addition, by decreasing the Al content and adding an adequate amount of Tim, the intragranular transformation is promoted, and by adding an adequate amount of B, the hardening capacity is increased and the formation of crude ferrite from the grain outlines and suppressed. In addition, by the addition of one or more elements between Cr, Cu, and Ni, the hardening capacity is increased, and by the transformed intragranular structures transformed at low temperature, the metallic structure in the ZAC is composed of fine grains made of bainite and transformed intragranular structure.
[00206] In addition, even if the high-strength steel tube according to the present invention is particularly 20 mm or more thick and even 30 mm or more thick, an excellent low temperature toughness of ZAC at temperatures extremely low temperatures of -40 ° C and also -60 ° C can be guaranteed. Therefore, the high-strength steel pipe according to the present invention can be applied as a steel pipe for oil pipelines, particularly a thick steel pipe for high-resistance oil pipelines.
[00207] In addition, according to the production method of the high-strength steel sheet according to the present invention, controlling the cooling conditions to be performed after hot rolling makes it possible to form polygonal ferrite without requiring low temperature rolling. in the hot rolling stage. Therefore, it becomes possible to produce the high strength steel sheet having improved ZAC strength and toughness and having extremely excellent low temperature toughness and deformability as a base material portion.
[00208] Furthermore, according to the production method of the high-strength steel sheet according to the present invention, it is possible to make a chemical composition that is capable of sufficiently guaranteeing the hardening capacity even though the additive amount of Mo, which it is expensive, be limited, and form polygonal ferrite without performing lamination at low temperature that was conventionally performed. Therefore, it becomes possible to eliminate the cost of connection and the cost of production.
[00209] In addition, according to the production method of the high strength steel pipe according to the present invention, a steel plate is used having a chemical composition that contributes sufficiently to the hardening capacity, so that when the steel plate steel is welded to produce the steel pipe, formation of crude ferrite in the grain boundary for the ZAC can be suppressed and excellent low temperature toughness can be guaranteed. Example
[00210] Hereinafter the effect of the present invention will be explained by Examples, but the present invention is not limited by the conditions used for the examples below. (Example 1)
[00211] Steels having the chemical compositions shown in Table 1 were cast to form steel plates each having a thickness of 240 to 300 mm by continuous casting according to a common method. Each thickness of the steel plate at that time is shown in Table 2.
[00212] Next, these plates were heated to the reheat temperature shown in Table 2 and were then subjected to hot rolling under the conditions shown in Table 2 and were cooled to produce steel sheets, each having the final thickness shown in Table 2. Incidentally, the lamination conditions of the lamination in the non-recrystallized y region, which is the final stage of the hot lamination of this example, are shown in Table 2.
[00213] In addition, in this example, the steel sheets obtained after hot rolling were cooled slowly to the accelerated cooling start temperature shown in Table 2 (at an average cooling rate of less than 10 ° C / s) , and then were cooled by water cooling under the accelerated cooling conditions shown in Table 2. In addition, Ar3 of each type of steel was obtained so that each specimen having a height of 12 mm and having a diameter of 8 mm was cut from the steel plates produced and was subjected to a work / heat treatment simulating a hot rolling to then be subjected to the measurement of thermal expansion.
[00214] Incidentally, in relation to the chemical compositions and production conditions shown in Table 1 and Table 2, an underline is added to each numerical value that deviates from the ranges of the present invention. In addition, each numerical value of the rolling start temperature in the non-recrystallized y region and each numerical value of the accelerated cooling start temperature means the difference from Ar3. Table 1
Table 2


[00215] The microstructures of the central portions of the plate thickness produced as above were observed with an optical microscope to measure their polygonal ferrite area ratios and their hard phase area ratios composed of bainite and martensite as their balance.
[00216] In addition, for each of the plates, a specimen cut by pressing having a notch provided in parallel to the direction of the thickness of the plate was manufactured with the direction of the width of the plate adjusted as the longitudinal direction based on the API, 5L3, ASTM, and E436, and was subjected to a weight loss burst test (DWTT). The DWTT was performed at -60 ° C and each shear area (SA) was obtained and each low temperature toughness was evaluated. In addition, each tensile property was evaluated in such a way that the specimen of the API specification was used to be subjected to a tensile test to obtain tensile strength. In addition, based on the result obtained by the tensile test, each yield stress (yield strength / tensile strength) was calculated to assess the deformability.
[00217] Incidentally, the tensile property was evaluated as good in the case of X70 or greater (the tensile strength of 570 MPa or more), the deformability was evaluated as good in the case of yield stress of 80% or less, and low temperature toughness was assessed as good in the case of SA of 85% or more.
[00218] The results are shown in Table 3. Table 3


[00219] Products Nos 1, 3 to 7, 9 to 13, 15, and 16 shown in Table 3 are examples of the present invention, where polygonal ferrite having an aspect ratio of 1 to 4 is 27 90% by area . These are excellent steel sheets in deformability and tenacity at low temperature, in which the tensile strengths of X70 or more and also X80 or more are satisfied, the polygonal ferrite area ratio is 27% or more, the yield stress is 80% or less, and SA in DWTT is 85% or more. In addition, they are excellent steel sheets in deformability and low temperature toughness, in which the polygonal ferrite area ratio is 50% or more and the yield strength is 70% or less.
[00220] These steel sheets were each formed into a tube in a UO stage, the top portions of each tube were internally and externally submerged arc, the tubes were each expanded, and the steel tubes were produced. Of these steel tubes, each metallic structure was the same as that of each of the steel sheets, the tensile strength was the same as that of each of the 5 to 20 Mpa steel sheets, and each low temperature toughness was the same as that of each of the steel sheets. Each yield stress of the steel tubes became greater than that of each of the steel sheets by 6 to 17% due to the hardening work when the tube was formed, but was able to be suppressed to 72 to 85%, which was less than 93% which is the maximum yield stress prescribed in X79 and X80 of the API specification, in relation also to the deformability, good results were likely to be obtained.
[00221] On the other hand, product no. 2 shown in Table 3 is an example in which the temperature of initiation of accelerated cooling is low, the area ratio of the polygonal ferrite increases excessively, and the resistance decreases to less than X70.
[00222] Products Nos 8 and 14 are each an example in which the temperature of the beginning of accelerated cooling is high, the polygonal ferrite area ratio decreases, the yield stress increases, and the shear area decreases.
[00223] In addition, products Nos 17 to 19 shown in Table 3 are each a comparative example, in which the chemical composition is outside the range of the present invention. Product No. 17 is an example in which the amount of B is small, the polygonal ferrite increases, and the tensile strength decreases. Products 18 and 19 are each an example in which the amount of Mo is large, the polygonal ferrite decreases, and the deformability and toughness at low temperature decreases. (Example 2)
[00224] The oxygen concentration in the addition of Ti was adjusted to be in the range of 0.001 to 0.003%, and steels having chemical compositions shown in Table 4 and Table 5 were melted in a steel production stage and were then casted for form steel plates. The melting and casting of steel can be carried out by common methods, or they can also be carried out by continuous casting for the sake of productivity. In this example, the melting and casting of the steel were performed by continuous casting.
[00225] Next, the steel plates obtained were heated to 950 ° C or more for hot rolling to then be hot rolled to 700 ° C or more, and then they were water-cooled at an average cooling rate of 10 ° C / s or more to produce steel sheets. Incidentally, in the hot lamination of this example, the lamination reduction ratio in a recrystallized region was adjusted to be in the range of 0 q 3 and the lamination reduction ratio in a non-recrystallized region was adjusted to be in the range of 2 to 9.
[00226] Incidentally, in relation to the chemical compositions shown in Table 4 and Table 5, an underscore is added to each numerical value that deviates from the ranges of the present invention. Table 4


Pcm = C + Si / 30 + (Mn + Cu + Cr) 20 + Ni / 60 + Mo / 15 + V / 10 + 5B Transformation temperature y / α = -2500Ceq2 + 1500 Ceq + 370 Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 Pcm = C + Si / 30 + (Mn + Cu + Cr) 20 + Ni / 60 + Mo / 15 + V / 10 + 5B Value in white in the component means that this component is not added
[00227] The underline in the table means that the numerical value is outside the range of the present invention.
[00228] Next, a 12 mm square steel piece having a length of 120 m was cut from each of the steel sheets obtained and was subjected to a simulated thermal cycle treatment at the ZAC simulating a ZAC that was arc welded submerged while varying the thickness of the sheet in the range of 20 to 40 mm and changing the heat input of the weld in the range of 3.0 to 10 kJ / mm. Heating can be performed by dielectric heating, induction heating, or high frequency heating, and cooling can be performed by water, He gas, nitrogen gas, etc. In this example, the heating of the simulated thermal cycle treatment in the ZAC was carried out by heating to 1400 ° C, and the cooling was carried out by He gas or nitrogen gas.
[00229] Incidentally, in products 6, 10, 11, 12, 18, and 19 shown in Table 6, the quenching with the heat treatment temperatures shown in Table 6 set to maximum was performed after the simulated thermal cycle treatment in ZAC.
[00230] From each of the steel parts that was subjected to the simulated thermal cycle treatment at the ZAC, a specimen for observation of the microstructure was taken and was etched and then had its structure observed using an optical microscope or an SEM to measure the fraction of MA area in the ZAC. Here, an intragranular transformed structure was defined as ferrite or bainite shaped like a petal starting from an inclusion.
[00231] In addition, by an EBSP method, an interface having an angular difference of 15 ° was defined as the high angle grain contour and the high angle grain size was measured. In addition, from each piece of steel that was subjected to the simulated thermal cycle treatment at the ZAC, a Charpy impact specimen with V-notch was removed and subjected to a Charpy impact test at - 60 ° C. The energy absorbed Charpy was measured based on the JIS Z 2242.
[00232] The results of the above are shown in Table 6.
[00233] Incidentally, in Table 6, the tensile strength of the base steel sheet is the tensile strength obtained when the direction of the width of the steel sheet is set to the tensile direction and the tensile strength of the base steel tube. is the tensile strength obtained when the direction of the circumference of the steel tube is set as the tensile direction. In addition, the yield stress of the steel tube is the yield stress (ratio of yield strength and tensile strength) obtained when the longitudinal direction (rolling direction) of the steel tube is set as the tensile direction. In each of products 1 to 24 shown in Table 6, the strength was X70 or greater (the tensile strength was 570 MPa or more). In addition, also in each product No. 1 to 24 shown in Table 6, the yield stress was 72 to 85%, which was less than 93%.
[00234] In addition, regarding the evaluation of low temperature toughness in the ZAC, the low temperature toughness in the ZAC was evaluated as good in the case of the absorbed energy Charpy (vE-60) of 50 J or more. Table 6

[00235] Products Nos 1 to 19 shown in Table 6 are examples of the present invention, in which the metal structure of ZAC is a microstructure composed of bainite and transformed intragranular structure having a high angle grain size of 80 μm or less in which MA and crushed ferrite in the grain boundaries are suppressed and transformed intragranular structures are confirmed. Regarding the energy absorbed by Charpy, the value of 50 J or more is displayed even at an extremely low temperature of -60 ° C in all cases.
[00236] On the other hand, in the products 20 to 24 shown in Table 6, the chemical composition of the base steel plate or the start temperature of transformation y / α is outside the range of the present invention, and these are comparative examples.
[00237] Product No. 20 is an example in which the amount of B is small and the hardening capacity decreases and, as a result, ferrite is formed in the grain boundary, the high angle grain size increases, and the toughness in the ZAC it decreases.
[00238] Product No. 21 is an example in which the amount of Al is large and no formation of intragranular structures transformed by Ti oxides is obtained, and as a result the high angle grain size increases and the toughness in the ZAC decreases .
[00239] Furthermore, product No. 22 shown in Table 6 is an example in which the chemical composition is in the range of Patent Document 7, the ferrite in the grain boundary is suppressed, and the transformed intragranular structures are obtained, but the additive amount of Mo is excessive and thus the area fraction of MA that is the hard phase exceeds 2.5% and the absorbed energy decreases.
[00240] In products no. 23 and 24 the chemical composition used in Patent Document 8 is applied, the ferrite in the grain boundary is suppressed, and the MA area fraction is low, and product no. 23 is an example in at which the temperature of initiation of transformation y / α is high and the intragranular transformed structures are few compared to the present invention, the size of the high-angle grain increases, and the toughness of the ZAC decreases. The product No. 24 is an example in which the temperature of initiation of transformation y / α is low, so that no intragranular transformed structure is obtained and the toughness in the ZAC decreases.

权利要求:
Claims (10)
[0001]
1. Steel tube obtained by the base steel sheet formed into a tube shape being welded, in which the base steel sheet consists, in% by mass, of: C: 0.010 to 0.080%, Si: 0.01 to 0, 50%, Mn: 1.2 to 2.8%, S: 0.0001 to 0.0050%, Ti: 0.003 to 0.030%, B: 0.0003 to 0.005%, N: 0.0010 to 0.008%, O: 0.0001 to 0.0080%, one or more elements among Cr, Cu, and Ni, P: limited to 0.050% or less, Al: limited to 0.020% or less, Mo: limited to 0.03% or smaller, a Ceq obtained by (Expression 1) below 0.30 to 0.53 and a Pcm obtained by (Expression 2) below 0.10 to 0.20, and the balance being composed of iron and the inevitable impurities, characterized by the fact that the metallic structure of the base steel plate contains 27 to 90% due to the area of polygonal ferrite and a hard phase composed of one or both of bainite and martensite as its balance; Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo) / 5 ••• (Expression 1) Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + 5B ••• (Expression 2) In (Expression 1) and in (Expression 2) above, C, Si, Mn, Ni, Cu, Cr, Mo, and B denote the contents of the respective elements [ % in large scale]; Ni, Cu, Cr, and Mo are calculated as 0 when the respective contents are 0%, where the base steel sheet also optionally contains, in mass%, one or two or more elements within W: 0.01 to 0 , 50%, V: 0.010 to 0.100%, Nb: 0.001 to 0.200%, Zr: 0.0001 to 0.0500%, Ta: 0.0001 to 0.0500%, Mg: 0.0001 to 0.0100% , Ca: 0.0001 to 0.0050%, Rare Earths: 0.0001 to 0.0050%, Y: 0.0001 to 0.0050%, Hf: 0.0001 to 0.0050%, and Re: 0 .0001 at 0.0050%, Ceq is obtained by (Expression 1 ') below, instead of (Expression 1) above, and Pcm is obtained by (Expression 2') below, instead of (Expression 2); Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 ••• (Expression 1 ') Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B ••• (Expression 2 ') in (Expression 1') and in (Expression 2 ') above, C, Si, Mn, Ni, Cu, Cr, Mo , V, and B denote the contents of the respective elements [mass%]; Ni, Cu, Cr, and Mo are calculated as 0 when the respective levels are 0%; V is calculated as 0 when the content is 0% and the content is less than 0.010% by weight.
[0002]
2. Steel tube, according to claim 1, characterized by the fact that: of the base steel sheet, in% by mass, the C content is 0.010 to 0.060% and the Al content is 0.008% or lower, the temperature of start of transformation y / α in a zone affected by the heat that is obtained by (Expression 3) below is 500 to 600 ° C, and transformed intragranular structures are contained in a previous austenite grain in the zone affected by heat; Transformation start temperature y / α = -2500Ceq2 + 1560Ceq + 370 ••• (Expression 3).
[0003]
3. Steel tube, according to claim 2, characterized by the fact that: the martensite-austenite constituent in the zone affected by heat is 2.5% or less in fraction of area.
[0004]
4. Steel pipe according to claim 2, characterized by the fact that: the high angle grain size of a metal structure in the zone affected by heat is 80 μm or smaller.
[0005]
5. Steel tube according to claim 1 or 2, characterized by the fact that: the thickness of the base steel plate is 20 to 40 mm.
[0006]
6. Steel pipe according to claim 1 or 2, characterized by the fact that: the tensile strength of the base steel plate is 500 to 800 MPa when the direction of the circumference of the steel pipe is adjusted to the direction of traction.
[0007]
7. Steel sheet consisting, in% by mass, of: C: 0.010 to 0.080%; Si: 0.01 to 0.50%; Mn: 1.2 to 2.8%; S: 0.0001 to 0.0050%; Ti: 0.003 to 0.030%; B: 0.0003 to 0.005%; N: 0.0010 to 0.008%; O: 0.0001 to 0.0080%; one or more elements among Cr, Cu, and Ni; P: limited to 0.050% or less; Al: limited to 0.020% or less; Mo: limited to 0.03% or less; a Ceq obtained by (Expression 1) below 0.30 to 0.53; and a Pcm obtained by (Expression 2) below 0.10 to 0.20; and the balance being composed of iron and the inevitable impurities; characterized by the fact that the metallic structure contains 27 to 90% due to the polygonal ferrite area and a hard phase composed of one or both of bainite and martensite as its balance; Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo) / 5 ••• (Expression 1) Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + 5B ••• (Expression 2) in (Expression 1) and (Expression 2) above, C, Si, Mn, Ni, Cu, Cr, Mo, and B denote the contents of the respective elements [ % in large scale]; Ni, Cu, Cr, and Mo are calculated as 0 when their respective contents are 0%, where the high strength steel sheet excellent in deformability and tenacity at low temperature optionally also comprises one or two or more elements, in% in mass, between W: 0.01 to 0.50%; V: 0.010 to 0.100%; Nb: 0.001 to 0.200%; Zr: 0.0001 to 0.0500%; Ta: 0.0001 to 0.0500%; Mg: 0.0001 to 0.0100%; Ca: 0.0001 to 0.0050%; Rare Earths: 0.0001 to 0.0050%; Y: 0.0001 to 0.0050%; Hf: 0.0001 to 0.0050%; and Re: 0.0001 to 0.0050%, where Ceq is obtained by (Expression 1 ') below instead of (Expression 1) above, and Pcm is obtained by (Expression 2') below in place of (Expression 2), Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 ••• (Expression 1 ') Pcm = C + Si / 30 + (Mn + Cu + Cr ) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B ••• (Expression 2 ') in (Expression 1') and (Expression 2 ') above, C, Si, Mn, Ni, Cu, Cr, Mo, V, and B denote the contents of the respective elements [mass%]; Ni, Cu, Cr, and Mo are calculated as 0 when the respective levels are 0%; V is calculated as 0 when the content is 0% and the content is less than 0.010% by weight.
[0008]
8. Steel sheet, according to claim 7, characterized by the fact that: in% by mass, the C content is 0.010 to 0.060% and the Al content is 0.008% or less, and the starting temperature of the y / α transformation in a heat-affected zone that is obtained by (Expression 3) below is 500 to 600 ° C, transformation start temperature y / α = -2500Ceq2 + 1560Ceq + 370 ••• (Expression 3).
[0009]
9. Method of producing a steel plate, characterized by the fact that it comprises: a steel plate consisting, in% by mass, of: C: 0.010 to 0.080%, Si: 0.01 to 0.50%, Mn: 1.2 to 2.8%, S: 0.0001 to 0.0050%, Ti: 0.003 to 0.030%, B: 0.0003 to 0.005%, N: 0.0010 to 0.008%, O: 0 .0001 to 0.0080%, one or more elements among Cr, Cu, and Ni, P: limited to 0.050% or less, Al: limited to 0.020% or less, Mo: limited to 0.03% or less, one Ceq obtained by (Expression 1) below 0.30 to 0.53 and a Pcm obtained by (Expression 2) below 0.10 to 0.20, in which the steel plate optionally still contains one or two or more elements , in mass%, among W: 0.01 to 0.50%; V: 0.010 to 0.100%; Nb: 0.001 to 0.200%; Zr: 0.0001 to 0.0500%; Ta: 0.0001 to 0.0500%; Mg: 0.0001 to 0.0100%; Ca: 0.0001 to 0.0050%; Rare Earths: 0.0001 to 0.0050%; Y: 0.0001 to 0.0050%; Hf: 0.0001 to 0.0050%; and Re: 0.0001 to 0.0050%, where Ceq is obtained by (Expression 1 ') below instead of (Expression 1) above, and Pcm is obtained by (Expression 2') below in place of (Expression 2), Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 ••• (Expression 1 ') Pcm = C + Si / 30 + (Mn + Cu + Cr ) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B ••• (Expression 2 ') in (Expression 1') and (Expression 2 ') above, C, Si, Mn, Ni, Cu, Cr, Mo, V, and B denote the contents of the respective elements [mass%]; Ni, Cu, Cr, and Mo are calculated as 0 when the respective levels are 0%; V is calculated as 0 when the content is 0% and the content is less than 0.010% by weight, and the balance being composed of iron and the inevitable impurities, perform the heating at 950 ° C or higher; perform the hot rolling step at temperature Ar3 or higher; perform cooling at an average cooling rate of less than 10 ° C / s; and then perform accelerated cooling at a cooling rate of 10 ° C / s or greater to the temperature of Bs or less obtained by (Expression 4) below from a temperature of Ar3 - 100 ° C to Ar3 - 10 ° C , Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo) / 5 ••• (Expression 1) Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + 5B ••• (Expression 2) Bs (° C) = 830 - 270C - 90Mn - 37Ni - 70Cr - 83Mo ••• (Expression 4) nas (Expression 1), (Expression 2), and (Expression 4) above, C, Si, Mn, Ni, Cu, Cr, Mo, and B denote the contents of the respective elements [mass%]; Ni, Cu, Cr, and Mo are calculated as 0 when the respective levels are 0%.
[0010]
10. Method, according to claim 9, characterized by the fact that: in the hot lamination stage, lamination is performed in the non-recrystallized region y at a lamination start temperature adjusted for Ar3 to Ar3 + 100 ° C and a reduction ratio adjusted to 1.5 or greater.

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EP2799567B1|2019-07-03|

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EP2799567A1|2014-11-05|

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BR112014015715A2|2017-06-13|

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WO2013100106A1|2013-07-04|

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KR101603461B1|2016-03-14|

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JPWO2013100106A1|2015-05-11|

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JP5590253B2|2014-09-17|

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KR20140095103A|2014-07-31|

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EP2799567A4|2015-08-12|

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BR112014015715A8|2017-07-04|

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法律状态:
2018-03-13| B06T| Formal requirements before examination|

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2018-08-14| B06A| Notification to applicant to reply to the report for non-patentability or inadequacy of the application according art. 36 industrial patent law|

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2019-03-19| B06A| Notification to applicant to reply to the report for non-patentability or inadequacy of the application according art. 36 industrial patent law|

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2019-08-13| B25D| Requested change of name of applicant approved|Owner name: NIPPON STEEL CORPORATION (JP) |

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2020-02-04| B06A| Notification to applicant to reply to the report for non-patentability or inadequacy of the application according art. 36 industrial patent law|

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2021-01-12| B09A| Decision: intention to grant|

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2021-03-16| B16A| Patent or certificate of addition of invention granted|Free format text: PRAZO DE VALIDADE: 20 (VINTE) ANOS CONTADOS A PARTIR DE 27/12/2012, OBSERVADAS AS CONDICOES LEGAIS. |

优先权:

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申请号 | 申请日 | 专利标题

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JP2011-287752|2011-12-28|

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JP2011287699|2011-12-28|

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JP2011287752|2011-12-28|

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JP2011-287699|2011-12-28|

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PCT/JP2012/083994|WO2013100106A1|2011-12-28|2012-12-27|High strength steel pipe having excellent ductility and low temperature toughness, high strength steel sheet, and method for producing steel sheet|

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