hot rolled steel sheet

专利摘要:
HOT LAMINATED STEEL SHEET AND METHOD FOR ITS PRODUCTION. A hot-rolled steel sheet according to the present invention is a steel sheet containing predetermined components and satisfying OS / Ca 0.8, N 14/48 x Ti "0" (zero). It is a high-strength steel plate for an excellent spiral tube in low temperature toughness in which the fraction of pro-eutectoid ferrite is 3% or more and 20 % or less, and the other is a low temperature transformation phase in a microstructure at a depth of half the thickness of the plate from the plate surface, the average crystal grain size of the entire microstructure is 2.5109> m or less, the area of the average crystal grain size is 9 109> m or less, the standard deviation of the average grain size area is 2.3 109> m or less, and the reflected x-ray intensity ratio {211} / {111} in one direction {211} and in one direction {111} with respect to a plane parallel to the surface of the steel sheet at a depth of half the thickness of the steel sheet from the surface of the steel sheet is 1.1 or more. The steel plate is so high (...).
公开号:BR112012033496B1
申请号:R112012033496-4
申请日:2011-06-30
公开日:2020-06-30
发明作者:Tatsuo Yokoi；Hiroshi Abe；Osamu Yoshida；Yasuhiro Miyatani；Shinichi Araki；Osamu Kawano
申请人:Nippon Steel Corporation；
IPC主号:

专利说明:

[0001] [0001] The present invention relates to a high-strength hot-rolled steel sheet for spiral pipe excellent in low temperature toughness, and a method for its production; BACKGROUND OF THE TECHNIQUE
[0002] [0002] In recent years, areas of development of energy sources such as crude oil, natural gas, have progressed to areas whose natural environment is more severe, such as cold regions such as the North Sea, Siberia, North America, and oceans deep as the North Sea, Gulf of Mexico, Black Sea, Mediterranean Sea, Indian Ocean. In addition, the development of natural gas increases from the point of view of global environmental consideration, and at the same time, a high operating pressure is required from the point of view of the economic efficiency of a piping system. The properties required for a pipeline corresponding to changes in these environmental conditions become more and more improved and diversified. They can, broadly speaking, be classified into (a) thickness / high strength, (b) high toughness, (c) low carbon equivalent (Ceq) according to the improvement of the welding capacity in the field, (d) high severity of corrosion resistance, (e) requirements for high deformation properties on frozen ground, in an area subject to earthquakes. In addition, these properties are generally required in a composite manner according to your usage environment.
[0003] [0003] Furthermore, development in a distant location, in areas of harsh natural environment that are left in their natural state from the point of view of profitability, begins to commence based on the recent increase in demand for crude oil and natural gas . In particular, high toughness capable of being used in cold regions in addition to thickening, high strength to improve transport efficiency are strongly needed for a pipeline in which crude oil and natural gas are transported over a long distance, and it is a technical problem to enable these required properties. LIST OF QUOTES PATENT LITERATURE
[0004] [0004] Patent Literature 1: Japanese Patent No. 3846729 (Japanese National Publication of International Patent Application No. 2005-503483)
[0005] [0005] Patent Literature 2: Japanese Laid-open Patent Publication n ° 2004-315957
[0006] [0006] Patent Literature 3: Japanese Laid-open Patent Publication n ° 2008-240151
[0007] [0007] Patent Literature 4: Japanese Laid-open Patent Publication n ° 2005-281838 NON-PATENT LITERATURE
[0008] [0008] Non-Patent Literature 1: Nippon Steel Technical Report No. 380 2004, Page 70 SUMMARY OF THE INVENTION TECHNICAL PROBLEM
[0009] [0009] The ductile fracture rate (SA) in a DWTT (Drop Weight Tear Test) test that assesses the stopping property of brittle fracture propagation that is incorporated in the specifications as a low temperature toughness index for each project is a value measured according to the API standard, and the value is generally known to decrease as the thickness and strength increase. In particular, a state of stress at the tip of a notch in a specimen transitions from flat stress to flat tension and the degree of triaxial stress is increased as the thickness of the plate increases, and when the thickness of the plate exceeds 16 mm, its effect also becomes noticeable. It is known that it is effective to reinforce controlled lamination, that is, to increase the lamination reduction ratio at a non-recrystallization region temperature in austenite as a means of improving SA.
[0010] [00010] A high impact absorbing energy is necessary from the point of view of preventing the ductile fracture that progresses when the internal pressure is high and the speed of fracture propagation becomes faster than the speed of the reduced pressure wave after the overflow such as a steel pipe for natural gas piping. The occurrence of separation improves SA in appearance, but decreases the energy of absorption and is therefore not preferable. In addition, customers who incorporate "no separation" in the specifications tend to increase. Consequently, it is a technical tendency to satisfy marketing needs to allow both the improvement of SA and the elimination of separation.
[0011] [00011] On the other hand, steel pipes for pipes are classified into seamless steel pipes, UOE steel pipe, welded steel pipe with electrical resistance, and spiral pipe, depending on their manufacturing process, and they are selected from according to usage, size, etc. All of the above tubes, except the seamless steel tube, have characteristics in which a sheet or strip sheet is molded into a tube state, and is subsequently welded by welding to be a product like a steel tube. (hereinafter referred to as "tube"). In addition, these welded steel tubes can be classified depending on whether the hot-rolled steel sheet (hereinafter also called "hot-rolled coil") is used or whether a sheet is used as a material, and the former are the welded steel with electrical resistance and the spiral steel pipe, and the last one is the UOE steel pipe for high strength, large diameter and thickening uses. However, the welded steel tube with previous electrical resistance and the spiral steel tube using the hot coil as material are advantageous from the point of view of their cost and delivery time, and therefore the requirements to make them high strength, large diameter and increasing thickness increase.
[0012] [00012] A big difference between the welded steel tube with electrical resistance and the spiral steel tube whose material is the hot coil exists in its tubalization method. In the welded steel pipe with previous electrical resistance, the longitudinal direction of a pipe and the rolling direction are equivalent, and the circumferential direction of the pipe is equivalent to the direction of the rolling width, just as with the UOE steel pipe. On the other hand, the spiral steel tube is made in such a way that the welding line becomes a spiral state, and the rolling direction and the longitudinal direction of the pipe, and the rolling width direction and the circumferential direction of the pipe do not necessarily equate. It is important that almost all the properties that are incorporated in the specifications as a tube refer to the circumferential direction of the tube, and it is an R direction of the hot coil in the case of the spiral tube. The direction R means a direction corresponding to the circumferential direction of the steel tube when it is made from a spiral steel tube. It is determined by the diameter of the tube at the time of tubing, but it is generally in the 30 ° to 45 ° direction in relation to the rolling direction. The hot rolled coil is generally good in both strength and toughness in the width direction and is therefore desirable because the circumferential direction of the welded steel tube with electrical resistance is the direction of the rolling width. However, the circumferential direction of the spiral steel tube is the R direction of the hot rolled coil, and it tends to an angle in relation to the rolling direction, and therefore both strength and toughness are decreased. Consequently, it is necessary to increase the strength to approximately 70 MPa to 90 MPa when it is converted in the direction of the lamination width even if it is the same API X80 steel tube (YS: 550 MPa, TS: 620 MPa to 827 MPa), and therefore, the hot coil for the spiral steel tube needs to have a more severe resistance-toughness balance.
[0013] [00013] A method of producing a high strength steel tube corresponding to the X120 standard, the UOE tube is described in Non-Patent Document 1.
[0014] [00014] However, the technology mentioned above considers that a thick plate (thick plate) is used as a material, and is achieved by using a method of Interrupted Direct Tempering (IDQ) being a characteristic of a plate production process, and with a high cooling rate and a low cooling stop temperature to allow for both high strength and thickening. In particular, it is a feature in which the tempering reinforcement (reinforcement of the structure) is used to guarantee the resistance.
[0015] [00015] An example of the respective processes for producing a plate is shown in Figure 1. Here, in a heating process, the plate is reheated. Heating is carried out at low temperature to refine the grain of the heated austenite because it is not necessary to consider reinforcing precipitation.
[0016] [00016] The reinforcement of the controlled lamination to improve the toughness, that is, the increase in the lamination reduction ratio in the temperature of the austenite recrystallization region is able to be programmed as required because your laminator is not of the in-line type, but a chair reversible laminator. Consequently, the desired toughness can be obtained as long as the controlled lamination start temperature is managed.
[0017] [00017] In addition, it is common for a finishing laminator to be a cooling equipment that is kept off at a distance in a plate production process, and there is a time interval of approximately 40 seconds from the moment of finishing the lamination until the moment of cooling start. Therefore, the texture orientation is weakened and the occurrence of separation is also suppressed due to recrystallization and the transformation of diffusive ferrite into austenite. In addition, recently, Accelerated Cooling (ACC) by high power cooling equipment has become common in the plate plate process, and the occurrence of separation tends to be suppressed from the point of view of the cooling rate.
[0018] [00018] An example of the respective processes for producing a hot rolled coil being the material of the welded steel tube with electrical resistance and the spiral steel tube which are objectives of the present invention is illustrated in Figure 2. Here, the configuration of the steel is adjusted to a steel component targeted in a refining process. Central segregation is reduced by electromagnetic agitation and smooth reduction of the casting in a continuous lingo process. In a plate reheating process, Nb suppresses the recrystallization of austenite and obtains the increase in precipitation by the precipitates that is made in the solution. In a roughing lamination process, the lamination is carried out in a region of austenite recrystallization temperature and the re-crystallized austenite grain is refined. In a finishing lamination process, the lamina is executed in a non-recrystallization temperature region of austenite, and an α grain after transformation is refined by a controlled lamination effect. In a winding process, the reinforcement of NbC precipitation is obtained by winding at an appropriate temperature.
[0019] [00019] In the production of the hot coil, the coiling process is a characteristic of the process, and it is difficult to coil a thick material at a low temperature due to the capacity restriction of the equipment of a winding device (winder). Therefore, it is impossible to perform the low temperature cooling stop necessary to reinforce the quench. Consequently, it is difficult to guarantee strength by strengthening by strengthening the temper. In addition, it costs a lot of equipment to accelerate the cooling rate in a central portion as quickly as the thick layer production process as for sheet thickness of 16 mm or more, at the cooling rate after lamination.
[0020] [00020] In addition, there is the case where the roughing laminator includes the reversible laminator of a chair, but it is common for the finishing laminator to be an in-line laminator with six to seven chairs. In addition, there are a lot of restrictions because the temperature, the lamination reduction ratio, and the speed are inevitably determined by their mass flow. In addition, the thickness of the raw bar that changes from the raw lamination to the finishing lamination is also restricted by a trimmer and a gap between cylinders of the F1 chair, and it is impossible to adjust the lamination reduction ratio in the temperature of the region of recrystallization as a thick plate process (thick plate).
[0021] [00021] In Patent Document 1, an invention is described in which Ca-Si is added at the time of refining to make a spherical inclusion state, V which has a miniaturizing effect of the crystal grain is added in addition to the reinforcing elements of Nb, Ti, Mo, Ni, and low temperature rolling and low temperature winding are combined to ensure strength, as a technology that allows both high strength and thickening and low temperature toughness in the hot coil for pipeline.
[0022] [00022] However, the temperature of the finishing laminate is relatively low such as 790 ° C to 830 ° C in this technology, and therefore there is fear in the decrease of the energy absorption caused by the occurrence of the separation, and in the stability of the operation because the lamination load becomes high due to the low temperature lamination.
[0023] [00023] In Patent Document 2, there is a description allowing both high strength and low temperature toughness by increasing the suppression of hardness in a welded portion by limiting the PCM value, and making the microstructure a single bainitic ferrite phase, and also limiting the Nb precipitation ratio to allow high strength and low temperature toughness as the technology that allows resistance, low temperature toughness, and excellent field welding capability in the hot coil for the welded steel pipe with resistance. However, low temperature lamination is practically necessary also in this technology to obtain a fine structure and there is fear in the reduction of the absorption energy caused by the occurrence of the separation, and as for the stability of the operation because the lamination load becomes high caused by the lamination at low temperature.
[0024] [00024] In Patent Document 3, a technology is described in which the texture is controlled by limiting the lower limit of the rolling rate after lamination on the hot coil for the welded steel tube with electrical resistance and the steel tube in spiral to reduce separation. In Patent Document 3, a technology is described in which the texture is controlled by limiting the lower limit of the cooling rate after lamination on the hot coil to welded steel pipe with electrical resistance and the spiral steel pipe to reduce the separation. However, it is necessary to not only suppress the separation but also to control the lamination process in order to improve the microstructure itself to allow both the strength of the X80 and the toughness in the sheet thickness of 16 mm or more. In addition, there are currently a number of technical obstacles from the point of view of the shape of the steel sheet, passing capacity and ease of biting the winder mandrel to guarantee the cooling rate in the central portion of the sheet thickness when the thickness of the plate is 16 mm or more.
[0025] [00025] In Patent Document 4, a technology is described in which the microstructure is made of a single ferrite-bainitic phase, the stable resistance is obtained by fine precipitates such as Nb, V, and the toughness is guaranteed by the definition of the average size of grain in the hot coil for the steel tube welded with electrical resistance.
[0026] [00026] However, it is intended for a thin plate whose thickness is at most half an inch (12.7 mm) because it is for the welded steel tube with electrical resistance, and there is no description on a method of production of the microstructure to obtain the toughness when the plate thickness is 16 mm or more and to obtain the grain size range. In addition, a use in which another severe resistance-toughness balance is necessary so that the hot coil for the spiral steel pipe as well as the welded steel pipe with electrical resistance is not considered.
[0027] [00027] Consequently, an objective of the present invention is to provide a hot-rolled steel sheet for a spiral tube from the point of view of transport efficiency, field welding capacity, etc. having both high tenacity capable of being used in an area where a severe fracture resistance property is required (particularly in a cold region) and strength of API5L-X80 or more. To achieve the above, an objective of the present invention is to provide a high-strength hot-rolled steel sheet (hot-rolled coil) for a spiral tube and a method capable of producing the hot-rolled steel sheet on a cheap basis. and stable in which the ductile fracture rate (SA) of DWTT and a test temperature of -20 ° C is 85% or more, the separation index where the decrease in absorption energy does not occur practically caused by the occurrence of separation is set to 0.06 mm / mm ² or less, the absorption energy in the event of separation is 240 J or more, also the API5L-X80 standard (the tensile strength is approximately 710 MPa to 740 MPa or more) when the thickness of the plate is 16 mm or more from the point of view of high resistance, are clarified. SOLUTION TO THE PROBLEM
[0028] (1) Chapa de aço laminada a quente satisfaz: C = 0,02% a 0,08%;Si = 0,05% a 0,5%;Mn = 1% a 2%;Nb = 0,03% a 0,12%;Ti = 0,005% a 0,05% em % em massa; e a porção remanescente é feita de Fe e os inevitáveis elementos impureza, em que uma fração de ferrita pró-eutectoide é 3% ou mais e 20% ou menos, e os outros são uma fase de transformação a baixa e perlita de 1% ou menos em uma microestrutura a uma profundidade de metade da espessura da chapa a partir da superfície da chapa, o tamanho médio de grão de cristal de toda a microestrutura é de 1 µm ou mais e 2,5 µm ou menos, a área d tamanho médio de grão é 3 µm ou mais e 9 µm ou menos, o desvio padrão da área do tamanho médio de grão é 0,8 µm ou mais e 2,3 µm ou menos, e a razão de intensidade de raio-x refletido {211}/ {111} em uma direção {211} e e=m uma direção {111} em relação a um plano em paralelo à superfície da chapa de aço à profundidade de metade da espessura a partir da superfície da chapa de aço é 1,1 ou mais.[00028] The present inventors studied hard to solve the problems presented above and, as a result, found that SA is strongly related to a crystalline system of a microstructure in a central portion in the direction of the thickness of the steel sheet, the energy of absorption is related to a pro-eutectoid ferrite fraction of the microstructure, the Si is related to the reflected x-ray intensity of the portion to design the present invention. The summary of the present invention is as follows. (1) Hot rolled steel sheet meets: C = 0.02% to 0.08%; Si = 0.05% to 0.5%; Mn = 1% to 2%; Nb = 0.03% to 0.12%; Ti = 0.005% to 0.05% by weight%; and the remaining portion is made of Fe and the inevitable impurity elements, in which a fraction of pro-eutectoid ferrite is 3% or more and 20% or less, and the others are a low and perlite transformation phase of 1% or less in a microstructure at a depth of half the thickness of the plate from the plate surface, the average crystal grain size of the entire microstructure is 1 µm or more and 2.5 µm or less, the area of the average size of grain is 3 µm or more and 9 µm or less, the standard deviation of the average grain size area is 0.8 µm or more and 2.3 µm or less, and the reflected x-ray intensity ratio {211 } / {111} in a direction {211} and e = m a direction {111} with respect to a plane parallel to the surface of the steel sheet at a depth of half the thickness from the surface of the steel sheet is 1.1 or more.
[0029] (2) Chapa de aço laminada a quente de acordo com o item (1) , também contém: P ≦0,03%;S ≦ 0,005%;O ≦ 0,003%;Al = 0,005% a 0,1%;N = 0,0015% a 0,006%;Ca = 0,0005% a 0,003%;V ≦ 0,15% ("0" (zero)% não está incluído)Mo ≦ 0,3% ("0" (zero)% não está incluído) em % em massa, e satisfaz:O < S/Ca < 0,8 N - 14/ 48 x Ti ≧ "0" (zero)%. (3) Chapa de aço laminada a quente de acordo com o item (2) , também contém um ou dois ou mais tipos de elementos entre: Cr = 0,05% a 0,3%;Cu = 0,05% a 0.3%;Ni = 0,05% a 0,3%;B = 0,0002% a 0,003% em massa. (4) Chapa de aço laminada a quente conforme qualquer um dos itens (1) a (3), também contém: REM = 0,0005% a 0,02% em massa. (5) Chapa de aço laminada a quente conforme qualquer um dos itens (1) a (4), em que a dureza máxima na porção de segregação central da chapa de aço é 300 Hv ou menos, e a largura da faixa de segregação da dureza média de um material base + 50 Hv ou m ais é 200 µm ou menos. (6) Um método de produção de uma chapa de aço laminada a quente inclui: aquecer uma placa lingotada produzida e lingotada para obter a chapa de aço laminada a quente satisfazendo:C = 0,02% a 0,08%;Si = 0,05% a 0,5%;Mn = 1% a 2%;Nb = 0,03% a 0,12%;Ti = 0,005% a 0,05% em massa; ea porção remanescente é feita de Fe e dos inevitáveis elementos impureza, até uma temperatura SRT ou mais descoberta por uma expressão (1) e 1260°C ou menos; eexecutar a laminação a quente para produzir a chapa de aço laminada a quente, em que quando uma tensão acumulada efetiva (ε_eff) descoberta por uma expressão (2) é usada, a laminação a quente é executada de forma que a tensão acumulada efetiva da laminação de desbaste é 0,4 ou mais, a tensão acumulada efetiva da laminação de acabamento é 0,9 ou mais, e o produto da tensão acumulada efetiva na laminação de desbaste pela tensão acumulada efetiva na laminação de acabamento é 0,38 ou mais;resfriar a chapa de aço a uma taxa de resfriamento de2°C/s ou mais e 50°C/s ou menos em uma porção central da espessura da chapa em uma região de temperatura de até 650°C após a lami-nação a quente ter terminado a uma temperatura do ponto de transformação Ar3 ou mais; ebobinar a chapa de aço em uma região de temperatura de 520°C ou mais e 620°C ou menos, em queSRT (°C) = 6670/ (2,26 - log [%Nb] [%C]) - 273 ... (1) aqui, [%Nb] e [%C] representam respectivamente os teores (% em massa) de Nb e C na chapa de aço, em que E_eff=Σεi(t,T)... (2) aqui,E_i(t, T) = εi0/ exp{(t/ τ_R)^2/3},τ_R = τ0 • exp(Q/RT),τ0 = 8,46 x 10^-6,Q = 183200 J,R = 8.314 J/K • mol,em que t representa o tempo acumulado até imediatamente antes da laminação de acabamento em um passe no caso da lamina-ção de desbaste, e representa o tempo acumulado até imediatamente antes do resfriamento no caso da laminação de acabamento, e T representa a temperatura de laminação no passe.[00029] Here, the "inevitable impurity element" means impurities that are not added intentionally, but inevitably mixed in a raw material or during the production process and unable to exclude even if their exclusion is attempted. (2) Hot-rolled steel sheet according to item (1), also contains: P ≦ 0.03%; S ≦ 0.005%; O ≦ 0.003%; Al = 0.005% to 0.1%; N = 0.0015% to 0.006%; Ca = 0.0005% to 0.003%; V ≦ 0.15% ("0" (zero)% is not included) Mo ≦ 0.3% ("0" (zero)% is not included) in mass%, and satisfies: O <S / Ca <0.8 N - 14/48 x Ti ≧ "0" (zero)%. (3) Hot-rolled steel plate according to item (2), also contains one or two or more types of elements between: Cr = 0.05% to 0.3%; Cu = 0.05% to 0.3%; Ni = 0.05% to 0.3%; B = 0.0002% to 0.003% by weight. (4) Hot-rolled steel plate according to any of items (1) to (3), also contains: REM = 0.0005% to 0.02% by weight. (5) Hot-rolled steel sheet according to any of items (1) to (4), where the maximum hardness in the central segregation portion of the steel sheet is 300 Hv or less, and the width of the segregation band of the The average hardness of a base material + 50 Hv or more is 200 µm or less. (6) A method of producing a hot-rolled steel sheet includes: heat a cast and produced cast plate to obtain the hot rolled steel plate satisfying: C = 0.02% to 0.08%; Si = 0.05% to 0.5%; Mn = 1% to 2%; Nb = 0.03% to 0.12%; Ti = 0.005% to 0.05% by weight; and the remaining portion is made up of Fe and the inevitable impurity elements, up to an SRT temperature or more discovered by an expression (1) and 1260 ° C or less; and perform hot rolling to produce hot rolled steel sheet, where when an effective accumulated stress (ε _eff ) discovered by an expression (2) is used, the hot rolling is performed in such a way that the effective accumulated stress of the roughing lamination is 0.4 or more, the effective cumulative stress of the finishing lamination is 0.9 or more, and the product of the effective cumulative stress in the roughing lamination by the effective cumulative stress in the finishing lamination is 0.38 or more ; cool the steel sheet at a cooling rate of 2 ° C / s or more and 50 ° C / s or less in a central portion of the sheet thickness in a temperature region of up to 650 ° C after the hot lamination has ended at a temperature of the Ar3 transformation point or more; and winding the steel sheet in a temperature region of 520 ° C or more and 620 ° C or less, where SRT (° C) = 6670 / (2.26 - log [% Nb] [% C]) - 273 ... (1) here, [% Nb] and [% C] represent the levels (% by mass) ) of Nb and C on the steel plate, And _eff = Σεi (t, T) ... (2) here, E _i (t, T) = εi0 / exp {(t / τ _R ) ^2/3 }, τ _R = τ0 • exp (Q / RT), τ0 = 8.46 x 10 ^-6 , Q = 183200 J, R = 8,314 J / K • mol, where t represents the time accumulated until just before finishing lamination in a pass in the case of roughing lamination, and represents the accumulated time until just before cooling in the case of finishing lamination, and T represents the lamination temperature in the pass.
[0030] [00030] Here, the "effective accumulated stress" is a refining index of the crystal grains effective for improving toughness. That is, it refers to the number of generation sites for a new crystal grain and the growth speed of a recrystallized grain, and the number of generation sites increases and the growth of the grain is suppressed as its value is more high.
[0031] [00031] The "accumulated effective tension of the roughing lamination" is defined to be the effective tension accumulated until just before the finishing lamination, that is, immediately before the lamination in a non-recrystallization region. The "effective accumulated stress of the finishing lamination" is a numerical value in which the tension immediately before cooling after the end of the lamination, that is, just before the transformation from y to a is discovered using the expression (2).
[0032] (7) O método de produção da chapa de aço laminada a quente conforme o item (6), em que o resfriamento é executado entre os respectivos passes de laminação da laminação a quente no momento da laminação a quente. (8) O método de produção da chapa de aço laminada a quente conforme item (5) o (6), onde quando a placa lingotada para obtenção da chapa de aço laminada a quente é lingotada continuamente, o lingotamento é executado enquanto se agita aço fundido por uma agitação eletromagnética induzida e controlando-se a quantidade de redução de laminação do lingotamento contínuo para ser compatível com o encolhimento da solidificação em uma posição de solidificação final da placa lingotada. [00032] "Hot lamination" is a plastic process in which the thickness of the sheet is reduced by lamination by passing a material between cylinders in a region of austenite temperature to conform it to a predetermined shape. (7) The method of production of the hot-rolled steel plate according to item (6), in which cooling is performed between the respective hot-rolling lamination passes at the time of the hot-rolling. (8) The hot rolled steel sheet production method according to item (5) or (6), where when the cast plate to obtain the hot rolled steel plate is cast continuously, the casting is performed while stirring steel fused by an induced electromagnetic stirring and controlling the amount of lamination reduction of the continuous casting to be compatible with the solidification shrinkage in a final solidification position of the casting plate.
[0033] [00033] "Induced electromagnetic agitation" is a technology in which a eddy current is induced in molten steel by being an electrical conductor by a moving AC magnetic field created by electromagnetic agitation equipment in a mold in a non-solidified portion in a ingot plate, and the molten steel itself is agitated by the electromagnetic force generated between eddy current and the mobile magnetic field to avoid central segregation concentrated in a continuous casting process.
[0034] (9) O método de produção da chapa de aço laminada a quente conforme o item (6), em que e a chapa de aço laminada a quente é aquela na qual uma fração de ferrita pró-eutectoide é 3% ou mais e 20% ou menos, e as outras são uma fase de transformação a baixa temperatura e perlita de 1% ou menos em uma microestrutura a uma profundidade de metade da espessura de uma chapa de aço a partir da superfície da chapa de aço, o tamanho médio de grão de cristal de toda a microestrutura é 1 µm ou mais e 2,5 µm ou menos, a área do tamanho médio de grão é 3 µm ou mais, e 9 µm ou menos, o desvio padrão da área do tamanho médio de grão é 0,8 µm ou mais e 2,3 µm ou menos, e razão de intensidade de raio-x refletido {211}/ {111} em uma direção {211} em uma direção {111} em relação ao plano em paralelo à superfície da chapa de aço à profundidade de metade da espessura da chapa a partir da superfície da chapa é 1,1 ou mais. (10) O método de produção da chapa de aço laminada a quente conforme o item (6), em que a chapa de aço laminada a quente também contém: P ≦ 0,03%;S ≦ 0,005%;O ≦ 0,003%;Al = 0,005% to 0.1%;N = 0,0015% to 0.006%;Ca = 0,0005% to 0.003%;V ≦ 0,15% ("0" (zero)% não esta incluído);Mo ≦ 0,3% ("0" (zero)% não está incluído) em % em massa,e satisfaz:O < S/Ca < 0,8 N - 14/ 48 x Ti ≧ "0" (zero)%. (11) O método de produção da chapa de aço laminada a quente conforme o item (10), em que a chapa de aço laminada a quente também contém um ou dois ou mais tipos de elementos entre: Cr = 0,05% a 0,3%;,Cu = 0,05% a 0,3%;Ni = 0,05% a 0,3%;B = 0,0002% a 0,003% em % em massa.[00034] The "final solidification position" means the position where the continuous casting plate completes solidification in all thicknesses (9) The hot-rolled steel plate production method according to item (6), in which the hot-rolled steel plate is one in which a fraction of pro-eutectoid ferrite is 3% or more and 20% or less, and the others are a low temperature and perlite transformation phase of 1% or less in a microstructure at a depth of half the thickness of a steel sheet from the surface of the steel sheet, the average grain size crystal of the entire microstructure is 1 µm or more and 2.5 µm or less, the average grain size area is 3 µm or more, and 9 µm or less, the standard deviation of the average grain size area is 0 , 8 µm or more and 2.3 µm or less, and reflected x-ray intensity ratio {211} / {111} in one direction {211} in one direction {111} with respect to the plane parallel to the surface of the steel plate at a depth of half the thickness of the plate from the surface of the plate is 1.1 or more. (10) The method of producing the hot-rolled steel sheet according to item (6), in which the hot-rolled steel sheet also contains: P ≦ 0.03%; S ≦ 0.005%; O ≦ 0.003%; Al = 0.005% to 0.1%; N = 0.0015% to 0.006%; Ca = 0.0005% to 0.003%; V ≦ 0.15% ("0" (zero)% is not included); Mo ≦ 0.3% ("0" (zero)% is not included) in mass%, and satisfies: O <S / Ca <0.8 N - 14/48 x Ti ≧ "0" (zero)%. (11) The method of producing the hot-rolled steel sheet according to item (10), in which the hot-rolled steel sheet also contains one or two or more types of elements between: Cr = 0.05% to 0.3% ;, Cu = 0.05% to 0.3%; Ni = 0.05% to 0.3%; B = 0.0002% to 0.003% in mass%.
[0035] [00035] It becomes possible to produce a high-strength spiral pipe that is API5L-X80 or more in a sheet thickness of 16 mm or more in a cold region where severe fracture resistance properties are required by using a hot-rolled steel plate according to the present invention for a welded steel tube with electrical resistance or for a spiral steel tube. In addition, it is possible to obtain a hot coil for the spiral steel tube in a cheap and stable manner by a production method according to the present invention. BRIEF DESCRIPTION OF THE DRAWINGS
[0036] [00036] Figure 1 is a process flow shining an example of the respective processes for producing a plate.
[0037] [00037] Figure 2 is a process flow illustrating an example of the respective production processes, the hot rolled coil being the material of a welded steel tube with electrical resistance of the present invention.
[0038] [00038] Figure 3 is a conceptual diagram illustrating the position where a micro sample is collected from a DWTT specimen.
[0039] [00039] Figure 4 is a view representing an SA (-20 ° C) of a microstructure by a relationship between the area of the average grain size and the average grain size of the microstructure.
[0040] [00040] Figure 5 is a view illustrating the relationship between the standard deviation of the average grain size of the microstructure and the dispersion (ASA) of the SA (-20 °).
[0041] [00041] Figure 6 is a view representing the relationship between the x-ray intensity ratio reflected in a central portion in the direction of the thickness of the steel sheet and an S.I ..
[0042] [00042] Figure 7 is a view representing the relationship between a pro-eutectoid ferrite fraction (%) and the energy absorbed Charpy from the microstructure.
[0043] [00043] Figure 8 is a view representing SA and S.l. of the microstructure by a relationship between the segregation portion of the highest hardness (Hv) and the segregation width.
[0044] [00044] Figure 9 is a view representing the relationship between the effective accumulated stress of the thinning and the area of the average grain size.
[0045] [00045] Figure 10 is a view representing the relationship between the effective accumulated finishing tension and the average grain size.
[0046] [00046] Figure 11A is a flow of properties represented by the relation of an effective accumulated tension (ε _eff ) of a roughing rolling mill to the total number of hours (roughing rolling passes program) from the extraction according to the Model 1.
[0047] [00047] Figure 11B is a flow of properties representing the relationship between the effective accumulated tension (ε _eff ) of the roughing rolling mill and the total number of hours (roughing rolling program) from the extraction according to Model 2.
[0048] [00048] Figure 11C is a flow of properties representing the relation of the effective accumulated tension (ε _eff ) of the roughing rolling mill to the total number of hours (roughing rolling program) from the extraction according to Model 3.
[0049] [00049] Figure 11D is a flow of properties that represents the ratio of the effective accumulated stress (ε _eff ) of the roughing rolling mill to the total number of hours (roughing rolling program) from the extraction of pattern 4. DESCRIPTION OF THE MODALITIES
[0050] [00050] Initially, the present inventors observed in detail a fracture surface of a hot-rolled steel sheet produced by a hot coil production process for the ductile fracture rate SA (-20 ° C) at -20 ° DWTT C and hot rolled steel sheet separations, considering allowing a hot rolled steel sheet excellent in strength and toughness in the assumption of use for a spiral pipe.
[0051] [00051] As a result, models of occurrence of separations are examined in detail as to the fracture surface on which the separations occur notably although an SA of 100% is obtained in appearance. As a result, they found that the models can be classified into two types of (1) separations occurring positions are not in the central portion of the plate thickness and a portion of separations occur, and (2) the separations occur in the central portion of the plate. plate thickness. Note that when separations are quantified as a separation index (hereinafter SI), the contribution of the model (2) is small, and it is found that the separations are at a level of no problem from a practical point of view since the model (1 ) can be suppressed in most cases.
[0052] [00052] When the model (1) is examined in detail, it is verified by an observation of a SEM of a cross section that the separations are mainly separated in places considered to be a crystal grain edge. That is, it appears that the crystal orientation of each crystal grain refers to the cause of the generation of the model's separations (1).
[0053] [00053] Furthermore, when the model (2) is examined in detail, it is estimated that the separation is the same as that of a so-called pseudo crack as a result of observing the separation that is perpendicular to both the generated fracture surface from the vicinity of the center of the plate thickness to the direction of the plate thickness of the specimen by using a SEM. That is, it is verified that an inclusion such as crude MnS, etc., is a starting point of fracture when the amount of addition of S is limited or when Ca is not added is not necessarily observed in a place considered to be a point of departure. In addition, it is also verified that the crack and the portion where elements such as Mn are thickened caused by central segregation, are equivalent. That is, the possibility that the central segregation responds by causing the separation of the model (2) to a certain extent is strongly suggested.
[0054] [00054] In general, the occurrence of separation is considered to be preferable for low temperature toughness because it decreases the transition temperature. However, when an unstable ductile fracture resistance property is important, such as in a gas pipeline, the energy of the upper layer needs to be improved to improve the unstable ductile fracture resistance property, and it is necessary to decrease the transition temperature while the occurrence of separation is suppressed to allow the above.
[0055] [00055] Consequently, an investigation assuming the case of API5L-X80 as an example and performed to investigate the relationship between the ductile fracture rate SA (-20 ° C) at -20 ° C of the DWTT, the separation, and the microstructure of the steel sheet, grain size, texture, and central segregation. As a result, the following facts are verified:
[0056] [00056] When cast steels having components shown in Table 1 are cast continuously, REM (rare earth elements) is added to change the degree of central segregation of a plate, and the casting of the plate is performed at two levels of execution or non-execution "induced electromagnetic stirring + smoothed reduction" in which the casting is performed while stirring the molten steel by the induced electromagnetic stirring and the reduction is smoothed while controlling the amount of lamination reduction to match the solidification shrinkage to a position final solidification of a cast plate.
[0057] [00057] In addition, the lamination conditions and the cooling conditions when the obtained ingot plate is hot rolled are changed variously to make the size of the crystal grain and the microstructure as a steel plate product change. In particular, the effects of a pass schedule in the recrystallization temperature region are studied in detail. Note that the thickness of the steel sheet product is 18.4 mm.
[0058] [00058] A sample is collected from a position 10 m from the tail of the obtained coil product, and several specimens are cut from the sample. A tensile test is performed by cutting a No. 5 specimen described in JIS Z 2201 from the R direction according to a method of JIS Z 2241. The DWTT (Drop Weight Tear Test) is performed by producing a specimen in which a strip-type specimen with dimensions of 300 mm L x 75 mm W x plate thickness (t) mm is cut in the R direction, and a pressure notch is performed on it.
[0059] [00059] After the DWTT test is performed, its ductile fracture rate (SA (-20 ° C)) is measured and the separation index (hereinafter Sl) is measured to digitize the degree of separation generated on a fracture surface . SI is defined to be a value in which the length of the entire separation (Σ _i li: l _i is, each, separation length) that is parallel to the surface of the plate is divided by an area of cross section (thickness plate x (75 - notch depth)).
[0060] [00060] In addition, a micro sample, as shown in Figure 3, is cut to investigate the crystal grain size, texture, microstructure, and central segregation of each specimen in the DWTT test.
[0061] [00061] Initially, EBSP-OIM ™ (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) is used to measure the size of the crystal grain and the microstructure from the cut micro sample. The sample is polished using abrasive colloidal silica for 30 minutes to 60 minutes, and an EBSP measurement is performed under 400-fold magnification measurement conditions, 160 µm x 256 µm in area, a 0.5 metering pass µm.
[0062] [00062] The EBSP-OIM ™ method consists of equipment and software that radiates electron beams to a highly tilted sample in a scanning electron microscope (SEM), a Kikuchi model formed by backscattering is photographed by a high-resolution camera sensitivity and the image is processed by a computer to measure the orientation of the crystal at the point of irradiation within a short period of time. In the EBSP method, it is possible to perform a quantitative analysis of a fine structure and crystal orientation of the surface of a main sample, and the area for its analysis is an area capable of being observed by SEM, and it is possible to analyze with a resolution of 20 nm at least although it depends on SEM resolution. The analysis is performed for several hours by mapping an area to be analyzed by tens of thousands of points in the same network interval. It is possible to see a crystal orientation distribution and a crystal grain size within the same polycrystalline material. In the present invention, the difference in orientation of the crystal grain is defined to be 15 ° being a floor value of the high angle slope of the grain edge which is generally recognized as the crystal grain edge, the grain is viewed from the mapped image. to find out the average crystal grain size. Although it is described in detail later, the average grain size (total sum of grain sizes / number of crystal grains) when the distribution number is discovered for each grain size of the crystal grain is set to be the "number average grain size ", and the average grain size (grain size corresponding to the average area) when the distribution is discovered in which the number of distributions for each crystal grain size is multiplied by an average grain size area is set to be the "average grain size area". The "average grain size number", the "average grain size area", and a "standard deviation" of the average grain size are values obtained by EBSP-OIM ™.
[0063] [00063] In addition, a fraction of volume of pro-eutectoid ferrite is discovered as to the microstructure by a method of Average Kernel Disorientation (KAM) without the one equipped with EBSP-OIM ™. In the KAM method, a calculation is performed for each pixel in which the difference in orientation between pixels of six adjacent pixels (first approximation) of a certain regular hexagon of a measurement data, or 12 pixels (second approximation) on the outside of the six pixels, also 18 pixels (third approximation) also outside the 12 pixels are averaged, and the value is adjusted to be a central pixel value.
[0064] [00064] It is possible to create a map representing a change in orientation within a grain by performing this calculation so as not to exceed the grain border. That is, this map represents the stress distribution based on the change in local orientation within the grain. Note that, as a condition of analysis of the present invention, a condition that calculates the difference in orientation between adjacent pixels in the EBSP-OIM ™ is adjusted to be the third approximation and the one whose difference in orientation becomes 5 ° or less is shown. Here, pro-eutectoid ferrite means polygonal ferrite. In the present invention, pro-eutectoid ferrite is defined as a planar fraction of a pixel whose third approximation of the difference in orientation is calculated to be 1 ° or less.
[0065] [00065] The polygonal pro-eutectoid ferrite that transforms at a high temperature is generated as a diffusion transformation, and therefore the displacement density is small and the tension within the grain is small and therefore the difference within the grain crystal orientation is small. Consequently, the volume fraction of the polygonal ferrite obtained by observation under an optical microscope and the area fraction of an area obtained by the third approximation of the 1st orientation difference measured by the KAM method are approximately equivalent if compared with the various research results performed by the inventors up to that time.
[0066] [00066] In addition, the reflected X-ray surface intensity ratio is measured to obtain crystal orientation information. The reflected x-ray surface intensity ratio (hereinafter surface intensity ratio) means the ratio of the reflected x-ray surface intensities in a {211} direction and in a {111} direction (hereinafter represented by {211 }, {111} when it is not particularly specified with respect to a surface parallel to a steel sheet in a central portion of the steel sheet (a portion at a depth of half the thickness of the sheet from the sheet surface), that is, it is a value defined as {211} / {111}. It is a value to be measured using x-ray by a method described in ASTM Standards Designation 81-63. A RINT1500 x-ray measuring device produced by Rigaku Corporation is used as the measurement equipment of the present experiment. Measurement is performed at a measurement speed of 40 times / minute, Mo-Kα is used as an x-ray source, under a tube voltage condition of 60 kV, a 200 mA tube current, and Zr-Kβ is used as a fil tro. An open-angle goniometer is used as a goniometer, the step width is 0.010 °, the slits are a 1 ° divergence slit, a 1 ° dispersion slit, and a slit that receives 0.15 mm light.
[0067] [00067] Next, the Mn concentration distribution of the steel plate is measured by an EPMA () Electronic Probe Microanalyzer) or a CMA (Computer Assisted Microanalyzer) capable of performing image processing of a measurement result by EPMA regarding the quantification of central segregation.
[0068] [00068] At that time, the numerical value of the maximum amount of Mn segregation changes depending on the diameter of the ERPMA probe (or CMA). The present inventors have found that Mn segregation is capable of being adequately evaluated by adjusting the probe diameter to 2 µm. Note that the amount of Mn segregation becomes large in appearance when there is an inclusion such as MnS, and therefore the evaluation is performed excluding the inclusion value when the inclusion appears.
[0069] [00069] In the present invention, the maximum amount of Mn secretion is defined as the maximum amount of Mn (% by weight) in the central segregation portion between a concentration of Mn in which the central segregation portion of the plate steel, that is, at least an area of at least 1 mm in the direction of the sheet thickness, 3 mm in the direction of the sheet width of a central portion of a steel sheet cross section is measured by the measurement method as defined above , and the average value in the direction of the plate width in each direction position of the plate thickness is adjusted to be the concentration of Mn.
[0070] [00070] It is also possible to define the central segregation portion by hardness by measuring the central Mn segregation portion measured as defined above by using a Vickers microhardness tester. For example, an area of 1 mm in the direction of the plate thickness, 3 mm in the direction of the plate width is measured at 25 gx 15 seconds in a 50 µm spacing centered on the central segregation portion by using the Vickers microhardness tester , and the maximum hardness between the average values of the Vickers micro hardness in the direction of the steel plate width in each direction of the plate thickness is defined as the maximum hardness of the central segregation portion. An average hardness in which the maximum hardness in the central segregation portion is excluded from the average hardness of each direction position of the sheet thickness is also averaged and defined as the average hardness of the base material. It is possible to define the area whose hardness becomes the average hardness of the base material + 50 Hv or more as the central segregation portion.
[0071] [00071] SA (-20 ° C) under a condition in which the tensile strength is within a range under a condition in which the tensile strength is within a range of 710 MPa to 740 MPa is represented in Figure 4 by a relationship between the "average grain size number" and the "average grain size area". It is found that SA (-20 ° C) ≧ 85% when the "average grain size number" is 2.5 µm or less and the "average grain size area" is 9 µm or less.
[0072] [00072] Furthermore, it is also found that SA (-20 ° C) also improves even in a similar microstructure by performing "addition of REM + induced electromagnetic agitation + smoothed reduction".
[0073] [00073] In this test, a fragile fracture surface caused by fragile fractures estimated to be generated immediately under a pressure notch in the DWTT test specimen changes once on a ductile fracture surface, but a pseudo crack perpendicular to both the surface of fracture generated in the vicinity of the center of the plate thickness as to the direction of the thickness of the plate specimen becomes a starting point of the fragile fracture surface again when the fracture surface is observed in detail. That is, it turns out that the effects of central segregation on SA (-20 ° C). That is, it appears that there are effects of reducing SI and increasing absorption energy by reducing central segregation.
[0074] [00074] Note that all SA values (-20 ° C) are average values from two samples, and some of the specimens do not satisfy SA (-20 ° C) ≧ 85%. Consequently, the relationship between the difference (ASA) of the two SA samples (-20 ° C) and the standard deviation of the average grain size area obtained by the EBSP-OIM ™ defined above. The results are shown in Figure 5. It appears that when the "standard deviation" of the average grain size area is 2.3 µm or less, the ∆SA (-20 ° C) becomes 20% or less, and dispersion of toughness is suppressed within that range. When ∆SA (-20 ° C) is 20% or less, the minimum value of SA (-20 ° C) is suppressed to approximately 75% and is within a practically permissible range to guarantee SA (-20 ° C) ≧ 85% as an average value.
[0075] [00075] The relationship between the surface intensity ratio and S.l. is shown in Figure 6. It appears that S.l. stabilizes at a very low level to be a value of 0.03 or less when the surface intensity ratio is 1.1 or more. That is, it turns out that it is possible to suppress the separation to a level of virtually no problem if the surface intensity ratio is controlled to be 1.1 or more. Most desirably, [it is possible to make p S. I. at 0.02 or less by controlling the surface intensity ratio at 1.2 or more.
[0076] [00076] Furthermore, an obvious trend is recognized in which the energy of the upper layer in the DWTT test improves by suppressing the separation. That is, when the surface intensity ratio {211} / {111} becomes 1.1 or more, the occurrence of the separation is suppressed, the SI stabilizes at the low level of 0.03 or less, and the decrease in energy of the upper layer being an index of the resistance to unstable ductile fracture resulting from the occurrence of the separation is suppressed, and energy of 1000 J or more can be obtained.
[0077] [00077] Note that it is preferable to adjust the surface intensity ratio to 0.9 or less from the point of view of suppression of the planar plastic anisotropy.
[0078] [00078] The separation is the result of the plastic anisotropy of crystallographic colonies {111} and {100} distributed in a banded state, and is considered to occur on a surface of the edge of these adjacent colonies. It is proven that {111} among these crystallographic colonies it also develops in particular by a lamination in the region of two phases a (ferrite) + γ (austenite) and less than the temperature of the Ar ₃ transformation point. On the other hand, when the lamination is carried out at a temperature of non-recrystallization of the region γ of the temperature of the transformation point Ar ₃ or more, a Cu-type texture being a lamination texture representative of an FCC metal is strongly formed, and is It is known that the texture in which {111} develops is formed after the transformation from γ to α. It is, therefore, possible to prevent the occurrence of separation by suppressing the development of these textures.
[0079] [00079] Next, a V-notch Charpy test is performed to investigate the relationship between the absorption energy and the microstructure, a micro sample is cut from the vicinity of the fracture surface, and the relationship between the absorption energy (vE (-20 ° C)) and the pro-eutectoid ferrite fraction is investigated. Note that a Charpy impact test is performed by cutting a specimen described in JIS Z 2202 from the R direction towards the center of the plate thickness according to a JIS Z 2242 method. eutectoid is a value obtained by the EBSP-OIOM ™ method defined above. The relationship between the pro-eutectoid ferrite fraction under a condition in which the tensile strength is within the range of 710 MPa to 740 MPa and vE (-20 ° C) is shown in Figure 7.
[0080] [00080] There is a good correlation between the fraction of pro-eutectoid ferrite and vE (-20 ° C), and it turns out that a target value for vE (-20 ° C) is 240 J can be obtained when the fraction of pro-eutectoid ferrite is 3% or more.
[0081] [00081] A result in which a central segregation effect added to SA (-20 ° C) and SI is also investigated in detail is shown in Figure 8. The central segregation portion means a layer that is easy to solidify and segregate such as C, P, Mn, Nb, Ti that exist in a central portion of the steel sheet cross section, and the central Mn segregation defined above is also included. It appears that when the hardness (Vickers Hv hardness) of the central segregation portion is a maximum hardness ≦ 300 Hv, and a width (length in the direction of the plate width) of the segregation range of the medium hardness of the base material + 50 Hv or more is 200 µm or less, SA (-20 ° C) ≧ 85%, SI ≦ 0.03 mm- ² , and tan-to SA (-20 ° C) when Sl passes the desired values.
[0082] [00082] Hot-rolled steel sheet used in the present invention is a steel sheet containing the following chemical components in% by mass, and the remainder is made up of Fe and the inevitable impurity elements. C = 0.02% to 0.08%, Si = 0.05% to 0.5%, Mn = 1% to 2%, Nb = 0.03% to 0.12%, Ti = 0.005% to 0.05%, P ≦ 0.03%, S ≦ 0.005%, O ≦ 0.003%, Al = 0.005% to 0.1%, N = 0.0015% to 0.006%, Ca = 0.0005% to 0.003%, V ≦ 0.15% (excluding "0" (zero)%), Mo ≦ 0.3% (excluding "0" (zero)%), are contained, and O <S / Ca <0.8 N - 14/48 x Ti ≧ "0" (zero)%
[0083] [00083] At that time, the hot rolled steel sheet may also contain one or two or more types of the following elements in percentage by mass. Cr = 0.05% to 0.3%, Cu = 0.05% to 0.3%, Ni = 0.05% to 0.3%, B = 0.0002% to 0.003%
[0084] [00084] Subsequently, the reasons for limiting the chemical components of the hot-rolled steel sheet in the present invention are described.
[0085] [00085] C is the element necessary to obtain the desired resistance of API5L-X80 or more and the microstructure. Note that it is impossible to obtain the necessary strength if its content is less than 0.02%, and when it is added in more than 0.06%, many carbides are formed to be the starting points for fractures, and as a result not only toughness in particular, the absorption energy is reduced but also the welding capacity in the field deteriorates notably. Consequently, the amount of C addition is adjusted to be 0.02% more and 0.06% or less. In addition, it is desirable to be 0.05% or less to obtain homogeneous resistance regardless of the cooling rate in a cooling after lamination.
[0086] [00086] Si has a carbide precipitation suppression effect to be the starting point of fracture, and therefore it is added by 0.05% or more, but the weldability in the field deteriorates when it is added by more 0.05%. When general versatility is considered from the point of view of field weldability, it is desirable to be 0.3% or less. In addition, there is a fear that the model scale in a state of tiger stripes is generated and may cause deformation of the surface when it is added by more than 0.15%, and therefore it is desirable to adjust its upper limit to 0, 15%.
[0087] [00087] Mn is a reinforcement element of the solid solution, and therefore is added as needed. However, a rigid segregation strip to form the starting point of the separation is formed because it secretes into the center of a cast plate at a casting time. Consequently, there is a great possibility that the maximum amount of Mn segregation exceeds 2% if it is added by more than 2% even in the way the casting is performed. As a result, the SI becomes worse, and the requirements of the present invention are not met. It is desirable to add 1.8% or less to reduce the SI while considering the variation in the maximum amount of Mn segregation.
[0088] [00088] P is an impurity, and it is more desirable that its content is low. It segregates up to the central portion of a continuous steel cutting bar when it is contained in more than 0.03% to incur a fracture at the grain edge and noticeably decrease tenacity at low temperature, and therefore it is adjusted to be 0.03% or less.
[0089] [00089] In addition, P adversely affects a pipe and the welding capacity in the field, and therefore it is desirable to be 0.015% or less considering the above.
[0090] [00090] S not only incurs fractures at the time of hot rolling, but also deteriorates the toughness at low temperature if the amount of addition is too large, and therefore its content is adjusted to be 0.005% or less. In addition, S secretes in the vicinity of the center of the continuous casting steel bar as MnS, forms MnS extended after rolling to be the starting point of the brittle fracture, and becomes the cause of the occurrence of a pseudo separation (treated as the separation in the present invention) such as fracture due to sheet lamination failure. In addition, it is desirable to be 0.001% or less in consideration of the sour-resistant property.
[0091] [00091] O is an impurity, and its upper limit is limited to 0.003% or less to suppress the aggregation of oxides. And to improve the fracture resistance property induced by hydrogen. It is desirable to adjust the value of the upper limit of the amount of O to 0.002% or less to suppress the generation of the oxide and to improve the base material and the toughness of HAZ.
[0092] [00092] Al is a deoxidizing element, and is added by -0.005% or more to obtain its effect. On the other hand, the effect is saturated if the amount of addition exceeds 0.1%. In addition, a cumulative Al oxide agglomerate is found when it exceeds 0.03%, and therefore it is desirable to be 0.03% or less. When severe low temperature toughness is also required, it is preferable to adjust the upper limit of the amount of Al to 0.017% or less.
[0093] [00093] Nb is one of the most important elements in the present invention. Nb has the suppression of recovery, recrystallization and grain growth effects of austenite during lamination or after lamination due to the dragging effect in a solid solution state and / or a fixing effect such as precipitated carbonitrides, grain refining of the average crystal grain size after transformation, and improved low temperature toughness. Furthermore, it generates fine carbides and contributes to the improvement of resistance by reinforcing precipitation in a winding process, which is a characteristic of the hot coil production process. Note that it is necessary to add it at least 0.05% or more to obtain these effects as defined above. On the other hand, not only does the saturation effect, but the solid solution in a heating process before the hot rolling process also becomes difficult, forms a crude carbonitride to be the starting point of fracture, and there is a possibility in which the low temperature toughness and sour-resistant property deteriorates if it is added by more than 0.12%.
[0094] [00094] Ti is one of the most important elements in the present invention. The Ti starts to precipitate as a nitride at a high temperature immediately after the solidification of the cast plate obtained by continuous casting or conventional casting. The precipitate containing Ti nitride is stable at high temperature and has the fixation effect without being completely dissolved-solid in a subsequent heating of the plate, suppress the hardening of the austenite grain during the heating of the plate, miniaturizes the microstructure to improve toughness at low temperature. The addition of Ti of at least 0.05% or more is necessary to obtain the effects as defined above. On the other hand, the effect will saturate if it is added by more than 0.02%. In addition, when the amount of Ti addition exceeds a stoichiometric component with N (N - 14/48 x Ti ≦ "0" (zero)%), the remaining Ti binds to C. and there is a possibility that the property resistance and tenacity are decreased.
[0095] [00095] Ca is an element that generates Cas sulfide, suppresses the generation of MnS that extends in the lamination direction, and contributes notably to the improvement of toughness at low temperature. When the amount of Ca addition is less than 0.0005%, the effects cannot be obtained, and therefore the lower limit value is adjusted to be 0.0005% or more. On the other hand, when the amount of Ca addition exceeds 0.003%, Ca oxide accumulates and similarly has the possibility of being the starting point of the fragile fracture, and therefore an upper limit is adjusted to be 0.003% or less.
[0096] [00096] In the present invention, Ca is added to form CaS, and therefore S is fixed. Therefore, the S / Ca ratio is an important index. It is stoichiometrically adjusted to be S / 16 = Ca / 20 from the atomic weights of S and Ca. That is, when the ratio of S / Ca is 0.8 or more, MnS is generated, and extended MnS is formed at the time of lamination. As a result, low temperature toughness deteriorates. Consequently, the S / Ca ratio is adjusted to be less than 0.8.
[0097] [00097] N forms Ti nitride as mentioned above, suppresses the hardening of the austenite grain during the reheating of the plate, the refining of the austenite grain size in a last controlled rolling, and the refining of the average grain size after transformation to improve toughness at low temperature. Note that the effects cannot be achieved when their content is less than 0.0015%. On the other hand, ductility is decreased due to aging, and the molding capacity when it is tubulated is decreased if it is contained in more than 0.006%. When the N content is less than a stoichiometric component with Ti (N - 14/48 x Ti ≦ "0" (zero)%), the remaining N binds to C, and there is a possibility that the HIC resistance property and toughness are decreased.
[0098] [00098] The reasons for adding V, Mo, Cr, Ni, Cu are described below. The main objective to also add these elements in addition to the basic components is to expand the thickness of the produced sheet and allow the improvement of properties such as the strength and toughness of the base material without damaging the excellent characteristics of the steel according to the present invention.
[0099] [00099] V generates a fine carbonitride in the winding process, being the characteristic of the hot coil production process, and contributes to the improvement of resistance by reinforcing its precipitation. Note that the effect is saturated if it is added by more than 0.15%. In addition, it is desirable to be less than 0.1% because there is a likelihood that the welding capacity in the field will be decreased if added by 0.1% or more. In addition, it is effective in a very small amount, but it is desirable to add 0.02% or more.
[0100] [000100] Mo has the effect of improving the quenching property and increasing the strength. In addition, Mo has effects that strongly suppress the recrystallization of austenite at the time of controlled rolling, miniaturizing the austenite structure, and improving low temperature toughness together with Nb. Note that the effects saturate if more than 0.3% are added. In addition, there is a likelihood that ductility is decreased and the molding capacity when it is piped is decreased if 0.2% or more is added, and therefore it is desirable to be less than 0.2%. In addition, it is effective in a very small amount, but it is desirable to add 0.02% or more.
[0101] [000101] Cr has an effect of increasing resistance. Note that the effect will saturate if it is added by more than 0.3%. In addition, there is a likelihood that the welding capacity in the field is decreased if it is added by 0.15% or more, and it is desirable to be less than 0.15%. In addition, the effect cannot be expected if less than 0.05% is added, and therefore it is desirable that 0.05% or more be added.
[0102] [000102] Cu has the effects of improving corrosion resistance and the fracture resistance property induced by hydrogen. Note that the effects saturate if more than 0.3% are added. In addition, there is a likelihood that fragile fractures occur at the time of hot rolling and causes surface failure if 0.2% or more is added, and therefore it is desirable to be less than 0.2%. In addition, the effects are not expected if less than 0.05% is added, and therefore it is desirable to add more than 0.05%.
[0103] [000103] Ni rarely forms a cured structure detrimental to low temperature toughness and sour-resistant property in a laminated structure (in particular, in the central segregation range of the plate) compared to Mn, Cr, Mo and therefore there is a effect of improving strength without deteriorating low temperature toughness and welding ability in the field. Note that the effects saturate if more than 0.3% are added. In addition, there is the effect of preventing a fragile Cu fracture, and therefore 1/3 or more of the amount of Cu is added as a target. The effects cannot be expected if less than 0.05% are added and, therefore, the lower limit is adjusted to be 0.05%.
[0104] [000104] B has the effect of improving the quenching property and making it easy to obtain a continuous cooling transformation structure. In addition, B increases the effect of improving the tempering property of Mo and has the effect of synergistically increasing the tempering property together with Nb. Consequently, it is added as needed. Note that it is insufficient to obtain the effects if less than 0.0002% is added, and plaque fractures occur if more than 0.003% are added.
[0105] [000105] REM has effects that uniformly disperse fine oxides in a molten steel by reforming an alumina-based inclusion, also helping the oxides to become nuclei for the generation of equiaxial crystal. Note that the effects cannot be obtained if it is added in less than 0.0005%, and when it is added in more than 0.02%, these oxides are generated in bulk, in groups, gross inclusions are generated, the toughness the low temperature of the weld seam is deteriorated, and the weldability in the field is adversely affected. In addition, it is an element that changes the shape of a non-metallic inclusion that becomes the starting point for fractures and deteriorates the sour-resistant property to be harmless.
[0106] [000106] In the following, the microstructure, etc., of the steel sheet of the present invention is described in detail.
[0107] [000107] The microstructure of the steel plate needs to be as follows to achieve the objective resistance, low temperature toughness, etc .: the pro-eutectoid ferrite fraction is 3% or more and 20% or less and the other is a low temperature transformation product in the microstructure at a depth of half the thickness of a steel plate, the number of the average crystal grain size of the entire microstructure is 2.5 µm or less, the area of the average grain size is 9 µm or less, and its standard deviation is 2.3 µm or less.
[0108] [000108] A large temperature deviation occurs between the front and rear surfaces of a sheet and the center of the sheet thickness when the sheet thickness is 16 mm or more, and a temperature history at each position of the sheet thickness at the beginning at the end of the lamination directly affects the formation of the microstructure, etc. In particular, a triaxial stress intensity is the highest in the central portion of the plate thickness, and the starting point for fractures is the central portion of the plate thickness. In addition, there is the best correlation between the microstructure, etc. and materials such as SA, and therefore the microstructure, etc. half the thickness is adjusted to be a representation of the entire thickness of the plate.
[0109] [000109] Here, the difference between the average crystal grain size number and the average crystal grain size area is mentioned. These numerical values can both be obtained using the EBSP-OIM ™ method mentioned above. In both the grain edge is defined to be 15 ° being a base value for a high angle slope of the grain edge that is generally recognized as the crystal grain edge, and the area surrounded by the grain edge is the crystal grain. The size distribution of the measured grains is plotted on a histogram, and its average value is the "average crystal grain size number" defined in the present invention. On the other hand, a histogram is drawn in which an average area is weighed (find a product) up to the numerical value for each side step of the histogram above, and its average value is the "average grain size area" defined in the present invention. This value makes observation under an optical microscope, etc., even approximate an impression of the microstructure that can be seen with the naked eye and a comparison method and an interception method defined in the JIS.
[0110] [000110] Here, the microstructure of the hot coil for the spiral pipe to be the objective of the present invention is classified into a very fine grain structure corresponding to the "pro-eutectoid ferrite" defined in the present invention and the others, ie , the "low temperature transformation phase" in which its grain size is relatively rough, refers to a grain size of the previous austenite, and is expected to transform to be massive, when it is observed in detail. In other words, the "average crystal grain size number" mainly represents the grain size of the "pro-eutectoid ferrite", and the "average grain size area" represents the grain size of the "transformation phase at low temperature". In addition, the "standard deviation" is an index that represents the difference in grain size between these.
[0111] [000111] According to a result of the detailed studies of the present inventors, an understanding in which tenacity improves as the crystal grain is refined in a relationship between the "crystal grain" and the "tenacity" that was considered not it is a versatile rule, but it is the relationship that takes effect only when the microstructure can be considered as approximately a single phase such as ferrite or bainite. In the event that API-X80 grade high strength steel is the object of the present invention, the microstructure inevitably becomes the microstructure in which the "pro-eutectoid ferrite" and the "low temperature transformation phase" are mixed, and therefore the average average crystal grain size represents only the "average grain size area", that is, the grain size of the "low temperature transformation phase" and is not suitable.
[0112] [000112] Furthermore, in the crack fracture, the weakest connection model is proposed. For example, in the case of a crack fracture, not only in the vicinity of a fracture tip, but also the total of a plastic region is capable of being a starting point for fracture occurrence. When this is defined as a process zone, the total fracture can be incurred if the weakest unit between the process zone fractures. In this case, a floor value (in this case, the "average grain size number" and the "average grain size area") that defines the lower limit of weakness becomes necessary in each of the "ferrite" pro-eutectoid "and the" low temperature transformation phase "although a question about which day is the weakest unit is put aside. In addition, dispersion of these is also important, and their "standard deviations" have to be defined to obtain stable toughness.
[0113] [000113] In the present invention, it is preferable that the number of the average crystal grain size is 3 µm or more, and the standard deviation is 0.8 µm or more in consideration of the difficulty of the operation. These floor values in the present invention are as follows: the number of the average crystal grain size is 1 µm or more and 2.5 µm or less, the area of the average grain size is 3 µm or more and 9 µm or less , and the standard deviation is 0.8 µm or more and 2.3 µm or less.
[0114] [000114] Pro-eutectoid ferrite is the microstructure relatively full of ductility, and the energy of absorption increases as the volume fraction increases due to the effect of ductility. A pro-eutectoid ferrite of 3% or more is required to obtain the desired absorption energy, but not only the saturation effect but also the decrease in resistance becomes noticeable when it exceeds 20%.
[0115] [000115] Consequently, it is necessary for the pro-eutectoid ferrite to be 3% or more and 20% or less. Note that the existence of pro-eutectoid ferrite is effective in reducing the steel pipe's performance ratio after tubing. In particular, recently, a design is mainly carried out by a Tension Based Design, and it is desired to decrease the yield strength after tubalization. It is desired that the pro-eutectoid ferrite is contained in at least 3% or more in the volume fraction to make the yield ratio after tubalization the desired 0.93 or less. In addition, there are notable effects on increasing absorption energy and suppressing separation by controlling the pro-eutectoid ferrite content by 20% or less. It is estimated because the pseudo crack fracture at one edge between the pro-eutectoid ferrite and the low temperature transformation phase that propagates at an edge between the pro-eutectoid ferrite and the low temperature transformation phase is suppressed.
[0116] [000116] The separation that is estimated not to be affected by the central segregation at the center of the plate thickness between the separations arises from a plastic anisotropy of the crystallography colonies of {111} and {100} distributed in a strip state, and is considered that separation occurs on an edge surface of these adjacent colonies. Consequently, the reflected x-ray intensity ratio {211} / {111} between the {211} plane and the {111} plane that are parallel to the surface portion of the central portion of the plate thickness is used as an index, and it is possible to suppress the separation to a level at which the plastic anisotropy of the crystallographic colonies is capable of suppressing the separation when the value of the reflected x-ray intensity ratio is 1.1 or more.
[0117] [000117] The central segregation generated at the time of casting the plate adversely affects the propagation of fragile fractures in the DWTT test, and also promotes the occurrence of separation. The DWTT test is a test method that assesses how the propagation of fragile fractures generated from the pressure notch portion is delayed by a plastic deformation that forms a ductile fracture surface at the time of testing, but a banded structure rigid generated as a result of central segregation it is difficult to be plastically deformed and therefore the propagation of fragile fractures is accelerated. In addition, the central segregation generates the pseudo crack to be the starting point of the separation. Consequently, the central segregation, in particular, the central segregation of Mn has to be reduced as much as possible to improve the SA of the DWTT being the low temperature toughness index while suppressing the occurrence of the separation. However, it is possible to suppress the occurrence of separation while ensuring the AS when the maximum hardness of the central segregation portion is 300 Hv or less, the width of the segregation range of the hardness of the base material + 50 Hv or more is 200 µm or less . In addition, the width of the structure in a state of rigid strip in the direction of the thickness of the sheet is also desirable to be narrower, and the occurrence of foot separation also suppressed when the thickness of the segregation strip whose concentration of Mn is 1.8% or more is 140 µm or less in the direction of the plate thickness.
[0118] [000118] In order to obtain the strength of the steel plate, there is the case where the resistance is insufficient because it contains only the low temperature transformation phase whose hardness is relatively high within the microstructure defined above. In this case, it is important that the precipitates containing Nb in nanometer size be densely dispersed to allow the reinforcement of the precipitation of the entire microstructure. The nanometer-sized precipitate compositions are mainly Nb, but it is allowed to contain Ti, V, Mo, Cr forming the carbonitride. In addition, the winding temperature range is adjusted to be 520 ° C to 620 ° C so that the precipitates contribute adequately to the reinforcement.
[0119] [000119] Note that when the cooling rate on an exit table is fast, on the order of 20 ° C / s or more in the center of the plate thickness and the winding temperature is 500 ° C or less, the volume fraction of the pro-eutectoid ferrite ≦ 20%, and it is possible to guarantee the resistance of grade X80 by reinforcing the structure of the transformation phase at low temperature even if it is an under-aging state in which the precipitates containing Nb of nanometer size do not present sufficient capacity to reinforce precipitation.
[0120] [000120] It is necessary that a microstructure containing crude carbide such as cementite is not contained to improve the absorption energy, being an index of the necessary fragile fracture interruption performance when a natural gas pipeline is assumed. That is, the low temperature transformation phase in the present invention does not contain the microstructure containing the crude carbide such as cementite.
[0121] [000121] Here, the transformation phase at low temperature is represented by a microstructure that appears when it is very cooled than a state of equilibrium at the moment of cooling or after winding on the exit table. For example, it is the microstructure corresponding to a continuous cooling transformation (Zw) structure described in The Iron and Steel Institute of Japan, Society of basic research, Bainite Research Committee / Edition; Recent Study relating to Bainite structure and transformation action of low-carbon steel - the Final Report of Bainite Research Committee (1994, The Iron and Steel Institute of Japan).
[0122] [000122] That is, it is defined that the microstructure of the continuous cooling transformation structure (Zw) is a microstructure made mainly of bainitic ferrite (α ^or _B ), granular bainitic ferrite (α _B ), quasi-polygonal ferrite (α _q ), also contains a small amount of retained austenite (γ _r ), Martensite-austenite (MA) as described in the reference document defined above, pg. 125 to pg. 12 as an observation frame under an optical microscope. An internal structure of αq does not appear by caustication as polygonal ferrite (PF), but a form of α _q is acicular, and is apparently distinct from PF. Here, a peripheral length of an objectified crystal grain is set to be lq, and the equivalent circle diameter is set to be dq, so a grain that satisfies the condition in which the ratio (lq / dq) is lq / dq ≧ 3.5 is α _q .
[0123] [000123] In addition, it is necessary to adjust the number of the crystal grain size is 2.5 µm or less, the area of the average grain size is 9 µm or less, and the standard deviation is 2.3 µm or less than total microstructure including the above to improve low temperature toughness. This is because the crystal grain size having the direct relationship with the fracture surface unit considered to be the main affector of crack fracture propagation in the brittle fracture is refined, and the low temperature toughness improves.
[0124] [000124] The reasons for the production method of the present invention are described in detail below.
[0125] [000125] In the present invention, the production method prior to the continuous casting process is not particularly limited. That is, a refining process by a steel converter is carried out by passing through a preliminary treatment of cast iron such as dephosphorization of the molten steel and desulfurization of the molten steel after the casting of the pig iron from a vat furnace, if otherwise a process of dissolving a cold iron source such as scrap by an electric oven, etc., and subsequently a component adjustment is carried out so as to be a component content targeted by several secondary refineries, then it is cast by a method such as normal continuous casting, a casting by the conventional method, and, in addition, thin plate casting.
[0126] [000126] Note that a countermeasure against segregation such as a reduction of non-solidified lamination is performed by a segment of the continuous casting to reduce the central segregation at the time of casting the plate. Otherwise, it is necessary to suppress the width in the direction of the thickness of the central segregation plate, making the thickness of the ingot plate thin.
[0127] [000127] Initially, an inclusion based on AI ₂ O ₃ is reformed into a fine oxide containing REM by the addition of REEM, the oxide is uniformly dispersed in a molten steel, electromagnetic agitation is performed to decrease the degree of overheating of the steel melted, so as to effectively use the finely dispersed oxide as a core of an axial crystal generation, and the fine equiaxial crystals are generated in the ingot plate to suppress Mn segregation.
[0128] [000128] Next, a smooth reduction at the time of final solidification in the continuous casting is optimal. Smooth reduction at the time of final solidification is a flow of concentrated molten steel to the non-solidifying portion in the central portion generated by the movement of the concentrated molten steel resulting from the solidification shrinkage, etc., to compensate for the degree of solidification shrinkage and is executed by deletion. It is therefore possible to reduce central segregation.
[0129] [000129] Concretely, REM is added within the scope of the present invention, when the molten steel is cast into a condition in which the stirring flow rate of the molten steel by the electromagnetic stirring induced in a position 10 m under a mold from the mold meniscus is 30 cm / s to 100 cm / s, continuous casting is performed at a reduction speed represented by the product of the casting speed (m / min) and an adjusted reduction gradient (mm / m) is within the range from 0.7 mm / min to 1.1 mm / min and a facility whose cylinder pitch in a position corresponding to the end of solidification is 250 mm to 360 mm in which the ratio of the central solid phase becomes 0.3 to 0 , 7.
[0130] [000130] In the case of a slab obtained by continuous casting or a thin slab, the slab can be directly transferred to a strip laminator in a high temperature slab state, otherwise it can be hot rolled after being cooled to room temperature and reheated by a heating oven. Note that when direct lamination of the plate (HCR: Hot Load Lamination) is performed, it is desirable to cool to less than the temperature of the Ar ₃ transformation point to break the cast structure by transformations from γ to α to γ and make it small the grain size of the austenite at the time of reheating the plate. It is more desirable to cool to less than the temperature of the Ar ₁ transformation point.
[0131] [000131] At the time of hot rolling, the plate reheat temperature (SRT) is adjusted to be the temperature calculated by Expression (1) below or more. SRT (° C) = 6670 / (2.26 -log [% Nb] [% C]) - 273 ... (1) where [% Nb], [% C] respectively represent the contents (% by mass) of Nb, C in a steel material. This expression must represent the temperature of the NbC solution by a product of the NbC solubility. When the temperature of the plate reheat is lower than that temperature, a crude Nb carbonitride generated at the time of plate production is not sufficiently dissolved, and not just the refining effect of the crystal grains due to the suppression of recovery, recrystallization and growth of the austenite grain due to Nb in the last rolling process, and delay in the transformation of γ / α cannot be obtained, but also the effect of generating a fine carbide and improving the strength due to the increased precipitation in the winding process being the feature of the hot rolled coil production process cannot be obtained. Note that when heating is carried out at less than 1100 ° C, the amount of flaking is small and there is a possibility that the inclusion in the surface layer of the plate cannot be removed together with the scale by a subsequent flaking, and therefore is It is desirable that the plate reheat temperature is 1100 ° C or more.
[0132] [000132] On the other hand, when the plate's reheat temperature is greater than 1260 ° C, the austenite's grain size becomes raw, the previous austenite's grain in a later controlled lamination becomes raw, the average size of crystal grain after transformation also becomes crude, and the effect of improving toughness at low temperature cannot be expected. It is more desirable to be 1230 ° C or less.
[0133] [000133] Regarding the heating time of the plate, the plate is retained for 20 minutes or more after reaching the corresponding temperature to completely dissolve the Nb carbonitride. When the time is less than 20 minutes, the crude Nb carbonitride generated at the time of producing the plate is not completely dissolved, and the refining effect of the crystal grain due to the suspension of the recovery, recrystallization and growth of the austenite grain during the moment of hot rolling, and the delay of transformation γ / α, the effect that generates the fine carbide and improves the resistance due to the increase of precipitation in the winding process cannot be obtained.
[0134] [000134] The subsequent hot rolling process generally consists of a rough rolling process made up of several laminator steps including a reversible laminator and a finishing laminating process in which six steps or seven laminator steps are arranged in line. In general, the roughing lamination process has advantages in which the number of passes and the amount of lamination reduction in each pass can be freely adjusted, but each interpass time is long and there is a possibility in which recovery and recrystallization between passes continue. On the other hand, the finishing lamination process is of the in-line type, and therefore the number of passes is the same number as the number of laminators, but each interpass time is short, and there is a characteristic in which an effect controlled rolling mill is easy to obtain. Consequently, a process program that takes full advantage of the characteristics of these rolling processes is necessary in addition to the steel components to allow excellent low temperature toughness.
[0135] [000135] Furthermore, when the thickness of the sheet product exceeds 16 mm, and the bite span of a first finishing laminator is limited by the restriction of the equipment, etc., it is impossible to improve the toughness being the requirement of the present invention for the gain of the lamination reduction ratio of a non-recrystallization temperature region only by the finishing lamination process, and therefore the roughing lamination process is effectively used, and it is very important to refine the grain size of the recrystallization austenite in the region of recrystallization lamination immediately before lamination in the non-recrystallization region.
[0136] [000136] The present invention is aimed at a plate whose thickness is 16 mm or more, and a principle of the present invention is how to refine the grain size of the recrystallization austenite. However, unlike finishing lamination in which: a multi-seat in-line laminator is used in which the rolling tension, the rolling temperature and the interpass time, which are metallographically important items, are determined if a passing program, a temperature start of rolling, and rolling speed are determined; in addition, continuous rolling is performed, roughing rolling is a combination of single chair laminators, and its flexibility of operation is great, but on the contrary, combinations of the optimum pass program, rolling start temperature, and lamination speed that refines the aforementioned recrystallized austenite grain size exists in countless numbers, and the present inventors have applied themselves to quantify the method that makes the present invention possible.
[0137] [000137] Consequently, indexes are adjusted in which the passing program, the rolling start temperature and the rolling speed, more specifically, the temperature, the interpass time, the rolling tension are uniformly evaluated. That is, the present inventors have found that an effective accumulated stress (ε _eff .) Calculated by the expression (2) below is used, and thus conditions at the time of rolling the thick steel sheet whose sheet thickness is 16 mm or more can be uniformly represented. E _eff = Σεi (t, T) ... (2) on here, E _i (t, T) = ε _io / exp {(t / τ _R ) ^2/3 }, τ _R = τ ₀ • exp (Q / RT) τ ₀ = 8.46 x 10 ^-6 , Q = 183200 J, R = 8,314 J / K • mol, where "t" represents the time accumulated until just before finishing lamination in the corresponding pass in the case of roughing lamination, and represents an accumulated time until just before cooling in case of finishing lamination, and "T" represents temperature of lamination in the corresponding pass.
[0138] [000138] The relationship between the effective accumulated roughing stress and the average grain size area is shown in Figure 9, and the relationship between the effective cumulative stress and the average grain size number is represented in Figure 10, That is, as is obvious from Figure 9, the recrystallization austenite immediately before lamination in the non-recrystallization region is refined and the desired toughness can be obtained when the effective accumulated tension (ε _eff ) of the raw lamination is 0.4 or more . The effective accumulated stress (ε _eff ) of the roughing rolling mill is desirable to be 0.6 or less from the point of view of roughing rolling durability caused by the rolling weight load on the roughing rolling mill.
[0139] [000139] The ratio of the effective accumulated tension (ε _eff ) of the roughing rolling mill to the total number of hours from the extraction (roughing rolling pass program) is represented in each of Figs. 11A to Fig 11 D, the roughing lamination patterns are different and the lamination time, the temperature of a roughing bar, the effective accumulated tension are each different. Figure 11A represents model 1, Figure 11B represents model 2, Figure 11C represents model 3, and Figure 11D represents model 4, respectively. In Figure 11A to Figure 11D, R1, R2, R4 represent roughing mill passes. Only R2 is the reversible laminator, and therefore lamination an odd number of times such as R2-1 to R29 is performed. The ε _eff introduced in each pass attenuated by a function of an accumulated time t and a rolling temperature T according to the expression (2) defined above, and the effective accumulated tension (ε _eff ) is obtained by the addition of each ε _eff .
[0140] [000140] In the present invention the ε _eff is adjusted to be 0.4 or more as stated above. In model 1 (comparative example), productivity (the total number of hours from extraction) is thought to be more important than ε _eff , and in model 3 (comparative example), ε _eff is thought to be more important than productivity. In model 2 (comparative example), when the temperature is expected to drop, an initial phase of the rolling pass is performed, it takes a long time until the temperature is lowered because the raw bar is thick, and productivity is decreased. On the other hand, when the wait is performed in a position where the raw bar is thin, it is possible to cool the raw bar within a short period of time, but the effective tension accumulated up to that moment attenuates, and the effective accumulated tension culms one whole becomes less than 0.4 which is defined in the present invention. In model 4 (example of the present invention), productivity and ε _eff are both allowed, and the ε _eff defined in the present invention is adjusted as an index in the roughing lamination, and thus it is possible to optimize both productivity and stress accumulated.
[0141] [000141] Lamination in the recrystallization temperature region in the roughing lamination process is performed, but the lamination reduction ratio in each lamination reduction pass is not limited in the present invention. Note that sufficient tension required for recrystallization is not introduced, grain growth resulting only from the migration of the grain edges occurs, a raw grain is generated, and there is a possibility that low temperature toughness deteriorates if the reduction ratio lamination in each pass of the rough lamination is 10% or less, and therefore it is desirable to perform the lamination with the lamination reduction ratio of more than 10% in each lamination reduction pass in the recrystallization temperature region. Similarly, when the lamination reduction ratio in each lamination reduction pass in the recrystallization temperature region is 25% or more, the displacement cell wall is formed by repeating the introduction of the displacement and recovery during the moment of lamination particularly in a low temperature region in a subsequent step, and a dynamic recrystallization that changes from an edge of a sub-grain to an edge of a high-angle grain occurs in a short period of time in a structure in which a grain whose displacement density is high and a grain whose displacement density is not high are mixed as a microstructure, whose main body is the dynamic recrystallization grains, is the dynamic recrystallization grain, and therefore they grow in relatively raw grains before lamination in the recrystallization region, the grains are generated by the subsequent lamination in the non-recrystallization region, and there is a possibility that at low temperature enamel deteriorates, so it is desirable to adjust the lamination reduction ratio on each lamination reduction pass at the recrystallization temperature to be less than 25%. In addition, the waiting time can be performed until the temperature is lowered to the non-recrystallization temperature region, or cooling by a cooling device can be performed. The latter is capable of reducing waiting time, and is therefore more desirable from the point of view of productivity.
[0142] [000142] On the other hand, as it is obvious from the relationship between the effective accumulated tension of the finishing lamination and the number of the average grain size represented in Figure 10, it is possible to obtain the desired toughness by a controlled lamination effect in the finishing lamination to be the lamination in the non-recrystallization region when the cumulative effective tension of the finishing lamination is 0.9 or more.
[0143] [000143] Here, the effective accumulated tension of the finishing laminate is desirable to be 1.2 or less from a viewpoint of durability of the finishing laminator resulting from the weight of the laminating load in the finishing laminate.
[0144] [000144] In this finishing lamination process, the lamination reduction ratio in each lamination reduction pass is not limited in the present invention. When laminating in the non-recrystallization temperature region, the waiting time is performed until the temperature is decreased until the non-recrystallization temperature region as required, or cooling by the cooling equipment can be performed as needed between the chairs. roughing / finishing lamination, when the temperature at the end of the roughing lamination does not reach the non-recrystallization temperature region. The latter is more desirable because it is possible to reduce the waiting time, and therefore not only does productivity improve, but also the growth of the recrystallization grain is suppressed, and low temperature toughness can be improved.
[0145] [000145] Note that when the total reduction ratio of the finishing lamination exceeds 85%, the displacement density of the ferrite transformation increases by excessive lamination, the amount of pro-eutectoid ferrite generation increases greatly in the microstructure. In addition, the reinforcement of Nb precipitation becomes over-aged to decrease the resistance caused by the transformation of ferrite at high temperature, and there is fear that the texture anisotropy after transformation will become noticeable, resulting from a rotation of the crystal to increase the plastic anisotropy and the decrease in the absorption energy caused by the occurrence of the separation is incurred, and therefore the total lamination reduction ratio of the non-recrystallization temperature region is adjusted to be 85% or less.
[0146] [000146] The ratio of reduction of lamination in the final chair is desirable to be less than 15% from the point of view of the precision of plate shape.
[0147] [000147] Furthermore, it appears that when a product of the effective accumulated tension of the roughing lamination and the effective accumulated tension of the finishing lamination is 0.38 or more, aiming at their synergistic effect, it becomes a necessary and sufficient condition obtain the desired tenacity. The product defined above is desirable to be 0.72 or less from the point of view of the laminator's durability caused by the weight of the rolling load in the roughing and finishing laminations. Here, the effective accumulated tension of the roughing lamination is the size of the crystal grain of the recrystallization austenite, that is, it is one of the indices that determine the size of the crystal grain (medium and grain size area) of the steel plate. The accumulated effective finishing tension is an index in a reduction ratio of accumulated lamination in the non-recrystallization region (there is a correlation with the displacement density before transformation), and it is also the index that determines the size of the crystal grain ( number of the average grain size) of the steel plate. It is necessary to define the lower limit values for each of these effective accumulated stresses, and when the product is 0.38 or less, the desired crystal grain size cannot be obtained.
[0148] [000148] Here, the non-recrystallization temperature region is capable of being estimated from the relationship between the Nb content and the upper non-recrystallization temperature described, for example, in Figure 2 of the Thermomechanical Processing of Microalloyed Austenite Pg. 129; The Effect of Microalloy Concentration on The Recrystallization of Austenite During Hot Deformation (1982 The Metallurgical Society of AIME).
[0149] [000149] In addition, a single or several rough bars are connected between the rough lamination and the finishing lamination. And finishing lamination can be carried out continuously. At that time, the raw bar can be wound once in a coil state, it is stored in a cover having a thermal insulating function as needed, and is rewound again to perform the connection.
[0150] [000150] The finishing temperature of the finishing lamination is adjusted to end at the temperature of the transformation point Ar ₃ or more. In particular, when the temperature becomes lower than the temperature of the Ar ₃ transformation point on the central side of the plate thickness than on the 1/2 t plate thickness, the effect of the {111} and {100} crystallographic colonies distributed in a strip state increases, the value of the reflected x-ray intensity ratio {211} / {111} of the plane {211} and of the plane {111} becomes less than 1.1, the plastic anisotropy of the colonies crystallographic properties become noticeable, excellent separation occurs on the surface of the ductile fracture, the absorption energy is noticeably reduced and therefore the final temperature of the finishing lamination is adjusted to end at the temperature of the Are ₃ transformation point or more in the plate thickness. 1/2 t. it is more desirable if it is 830 ° C or more, then the occurrence of the separation can be suppressed to some extent. In addition, it is desirable to adjust the temperature of the plate surface to that of the Ar ₃ or more transformation point. On the other hand, when it exceeds 870 ° C, the displacement density to be the transformation core decreases due to the recovery between passes, the grain refining effect is lost, and there is a fear that the low temperature toughness will deteriorate. Consequently, it is desirable to terminate the lamination within the temperature range of 830 ° C to 870 ° C.
[0151] [000151] Here, the temperature of the Ar3 transformation point is simply represented by the relationship with the steel components, for example, by the following calculation expression. Air ₃ = 910 - 310 x% C + 25 x% Si - 80 x% Mneq It is observed that, Mneq = Mn + Cr + Cu + Mo + Ni / 2 + 10 (Nb- 0.02) otherwise, Mneq = Mn + Cr + Cu + Mo + Ni / 2 + 10 (Nb - 0.02) +1: in the case where B is added.
[0152] [000152] After the finishing lamination is finished, the cooling begins. The temperature of the cooling start is not particularly limited, but when cooling is started from less than the temperature of the Ar ₃ transformation point, the average grain size of crystal is crude caused by the growth of the grain, and there is fear of the decrease of the resistance, and therefore the temperature of beginning of the cooling is desirable to be the temperature of the transformation point Ar ₃ or more.
[0153] [000153] The cooling rate in a temperature region from the start of cooling to 650 ° C is adjusted to be 2 ° C / s or more and 50 ° C / s or less. When it exceeds 650 ° C, the precipitation of Nb that reinforces the pro-eutectoid ferrite becomes over-aged to decrease resistance. When the cooling rate is less than 2 ° C / s, the average size of the crystal grain is gross caused by the growth of the grain, and there is a fear that the resistance will be decreased. On the other hand, when the cooling rate exceeds 50 ° C / s, it is feared by the plate's warping caused by the thermal stress, and therefore it is adjusted to be 50 ° C / s or less.
[0154] [000154] The cooling rate in a temperature region of 650 ° C until the moment of winding is sufficient an air cooling rate or the equivalent cooling rate. Note that it is desirable that the average cooling rate of 650 ° C until the time of winding is 5 ° C / s or more so that precipitates do not suffer from over-aging caused by harshness to take advantage of the Nb precipitation reinforcement effect to the maximum.
[0155] [000155] After cooling, the coiling process being the characteristic of the hot coil production process is effectively used. The cooling stop temperature and the winding temperature are adjusted to be in the temperature region of 520 ° C or more and 620 ° C or less. When cooling is stopped at more than 620 ° C and winding is performed after this, the Nb precipitates become over-aged and the increase in precipitation is not fully expressed. In addition, the crude carbonitride containing Nb, etc., is formed to be the starting point for fractures, and there is a possibility that the ductile fracture's stopping capacity, low temperature toughness, and sour-resistant property deteriorated. On the other hand, when the cooling is finished below 520 ° C and the winding is carried out, the precipitated fine carbides of Nb, etc., which are extremely effective in obtaining the desired resistance cannot be obtained, and as a result the desired resistance cannot be obtained. Consequently, the temperature region to stop cooling and start winding is adjusted to be 520 ° C or more and 620 ° C or less. EXAMPLE
[0156] [000156] Hereinafter the present invention is also described by examples.
[0157] [000157] Steels A to K having chemical components represented in Table 2 are produced by a steel converter, and secondary refining is performed by CAS or RH. A deoxidation process is performed in the secondary refining process. These steels are directly laminated or reheated after continuous casting, are reduced by rolling to have a plate thickness of 18.4 mm by the finishing lamination subsequent to the roughing lamination, and rolled after cooling on the exit table. Note that the chemical components in the table are represented in% by mass.
[0158] [000158] Detailed production conditions are shown in Table 3. Here, a "component" represents a symbol for each ingot plate represented in Table 2, an "electromagnetic agitation + smoothed reduction" represents the presence / absence of "electromagnetic agitation" and the "smoothed reduction" performed at the time of continuous casting to reduce central segregation, the "heating temperature" means the actual performance of the plate heating temperature, the "solution temperature" means the temperature calculated by the expression: SRT (° C) = 6670 / (2.26 - log [% Nb] [% C] - 273,
[0159] [000159] the "retention time" means the retention time in the actual performance of the plate heating temperature, the "gross effective accumulated tension" means the effective accumulated tension of the lamination performed by the roughing lamination calculated by the expression (2 ) hereinafter, "bar cooling" means presence / absence of cooling between lamination chairs aimed at being properly performed according to the lamination conditions, the "effective accumulated tension of finishing" means the effective accumulated tension of the lamination performed by finishing lamination calculated by the expression (2) below, the "roughing and finishing product" means the product of each effective accumulated tension of the lamination performed by the finishing lamination and the roughing lamination. The accumulated effective voltage (ε _eff. ) Is calculated by the expression (2) below. E _eff = Σε _i (t, T) ε _i (t, T) = ε _i0 / exp {(t / τ _R ) ^2/3 } τ _R = τ ₀ • exp (Q / RT) τ ₀ = 8.46 x 10 ^-6 Q = 183200 JR = 8,314 J / K • mol ... (2),
[0160] [000160] An "FT" means the finishing temperature of the finishing laminate, the "Ar ₃ transformation point temperature 'means the calculated Ar ₃ transformation point temperature, the" cooling rate up to 650 ° C "means the average cooling rate when the temperature region of the starting temperature up to 650 ° C is passed, and "CT" means the winding temperature.
[0161] [000161] Steel materials obtained as defined above are shown in Table 4. Investigation methods are illustrated below.
[0162] [000162] The tensile test is performed by cutting the specimen No. 5 described in JIS Z 2201 from the R direction, according to the method of JIS Z 2241. The Charpy impact test is performed by cutting the specimen described in JIS Z 2202 from the direction R of the center of the plate thickness, according to the method of JIS Z 2242.
[0163] [000163] The DWTT test (Drop Weight Tear Test) is performed by cutting a test piece of strip 300 mm long x 75, wide x plate thickness (t) mm from the R direction, and producing the specimen in which the 5 mm pressure notch is made for cutting the specimen strip.
[0164] [000164] Next, EBSP-OIM ™ (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) is initially used to measure the size of the crystal grain and the microstructure of the micro samples cut from each specimen of the DWTT after the test as shown in Figure 3 above. The sample is polished with abrasive colloidal silica for 30 minutes to 60 minutes, and the EBSP measurement is performed under the 400-magnification measurement conditions, ares of 160 µm x 256 µm, the measuring step of 0.5 µm .
[0165] [000165] In addition, the volume fraction of the pro-eutectoid ferrite is discovered in terms of microstructure by the Mean Kernel Disorientation (KAM) method being equipped with EBSP-OIM ™.
[0166] [000166] In addition, regarding the measurements of the maximum amount of Mn segregation, the distribution of the Mn concentration of the plate product is measured by an EPMA (Micro Probe Micro Analyzer), or a CMA (Micro Analyzer with Computer Aid) capable of image processing of result measurement by EPMA. The probe diameter is adjusted to be 2 µm, and the measurement range is in the area of at least 1 mm in the direction of the plate thickness of the central segregation portion of the center of the plate product.
[0167] [000167] In the central segregation portion of Mn measured as mentioned above, the area of 1 mm in the direction of the plate thickness, 3 mm in the direction of the plate width is measured by the Vickers hardness tester at 25 gx 15 s with centering of the 50 µm pitch spacing in the central segregation portion. The average value in the direction of the plate width at each position in the direction of the plate thickness is adjusted as the average hardness of the base material, and the average value in the direction of the plate width of the maximum hardness of the central segregation portion between the hardness is defined as the maximum hardness.
[0168] [000168] In Table 4, the "microstructure" means the microstructure at 1/2 t of the microsample cut from each DWTT specimen after the test. The "maximum amount of Mn segregation" between them is the value measured by the method defined above in the corresponding sample, the "fraction of volume of the pro-eutectoid ferrite" means the value measured by the KAM method of EBSP-OIM ™, the "number mean grain size "," mean grain size area ", and" standard deviation "similarly mean the results of the EBSP-OIM ™ measurement.
[0169] [000169] The result of the "tensile test" represents the result of the JIS specimen No. 5 in the R direction, "SA (-20 ° C)" is the ductile fracture rate in the DWTT test at -20 ° C, the "separation index" similarly represents the fracture surface separation index in the DWTT test at -20 ° C, the "absorption energy vE -20 ° C" is the absorption energy obtained at -20 ° C in the Charpy impact test.
[0170] [000170] The steels according to the present invention are seven steels having the steel numbers 1, 2, 3, 12, 13, 14, and 15. They have characteristics in which a predetermined quantity of steel components is contained, the fraction of pro-eutectoid ferrite is 3% or more and 20% or less and the other is a low temperature transformation phase in the microstructure, the average number of crystal grain size of the entire microstructure is 2.5 µm or less, and its standard deviation is 2.3 µm or less, and the reflected x-ray intensity ratio (211} / {111} of the {211} plane and the {111} plane that are parallel to the portion plate surface core thickness is 1.1 or more.High strength hot rolled steel sheet for spiral tube having tensile strength equivalent to grade X80 and excellent low temperature toughness is obtained as material before tubing.
[0171] [000171] Steels other than those above are outside the range of the present invention according to the reasons described below.
[0172] [000172] In steel number 4, the heating temperature is outside the range of the present invention, and therefore the tensile strength equivalent to grade X80 cannot be obtained, and also the SA (-20 ° C) is low because the Nb solution is insufficient.
[0173] [000173] In steel number 5, the heat retention time is outside the range of the present invention, and therefore the tensile strength equivalent to grade X80 cannot be obtained, and furthermore SA (-20 ° C) is low because the Nb solution is insufficient.
[0174] [000174] In steel number 6, the effective accumulated roughing stress is outside the range of the present invention, and therefore the desired microstructure cannot be obtained and the SA (-20 ° C) is low.
[0175] [000175] In steel number 7, the effective accumulated finishing tension is outside the range of the present invention, and therefore the desired microstructure cannot be obtained, and the SA (-20 ° C) is low.
[0176] [000176] In steel number 8, the product of the effective accumulated roughing tension and the effective accumulated finishing tension is outside the range of the present invention, and therefore the desired microstructure cannot be obtained, and the SA (-20 ° C ) is low.
[0177] [000177] In steel number 9, the temperature of the finishing lamination is the transformation point Ar ₃ or less to incur the lamination in the two-phase region, and therefore the surface intensity ratio is outside the range of the present invention, and the occurrence of the separation is remarkable.
[0178] [000178] In steel number 10, the cooling rate is outside the range of the present invention and therefore the growth of the grain occurs during cooling, the desired microstructure cannot be obtained, and the SA (-20 ° C) is low .
[0179] [000179] In steel number 11, the CT is outside the range of the present invention, and therefore a sufficient precipitation-strengthening effect cannot be obtained, and the tensile strength equivalent to grade X80 cannot be obtained as the material.
[0180] [000180] In steel number 16, the C content is outside the range of the present invention, and therefore the desired microstructure cannot be obtained and vE (-20 ° C) is low.
[0181] [000181] In steel number 17, the Nb content is outside the range of the present invention, and therefore not only the sufficient effect of reinforcing precipitation cannot be obtained, and the tensile strength equivalent to grade X80 cannot be obtained as the material, but also the desired microstructure cannot be obtained and the vE (-20 ° C) is low because a sufficient controlled lamination effect cannot be obtained.
[0182] [000182] In steel number 18, the S / Ca ratio is outside the range of claim 1 of the present invention, and therefore inclusion such as MnS becomes the starting point for brittle fracture, and SA (-20 ° C ) is low.
[0183] [000183] In steel number 19, the Ti content is outside the range of the present invention, and therefore the grain size of the heated austenite becomes crude, the desired microstructure cannot be obtained, and the SA (-20 ° C) is low.
[0184] [000184] In steel number 20, the N * is outside the range of the present invention, and therefore the SA (-20 ° C) is low.
[0185] [000185] In steel number 21, the Mn content is outside the range of the present invention, and therefore the SA (-20 ° C) is low, the occurrence of separation is notable, and the vE (-20 ° C) is low.
[0186] [000186] The present configuration must be considered in all respects as illustrative and not restrictive, and any changes that come within the meaning and equivalence range of the claims are, therefore, considered to be included in them. The invention can be configured in other specific ways without leaving its spirit or its essential characteristics. INDUSTRIAL APPLICABILITY
[0187] [000187] The present invention is capable of being used for the production of a hot-rolled steel plate used for a welded steel pipe with electrical resistance and a spiral steel pipe in the steel industry. In particular, it is possible to use for the production of a high-strength spiral pipe having the standard API5L-X80 or more in a thickness of 16 mm or more also for use in cold regions where a more severe fracture resistance property is required.

权利要求:
Claims (2)
[0001]
Hot-rolled steel plate suitable for the production of API5L steel tubes - grade X80, characterized by the fact that it presents tensile strength according to JIS Z 2241, measured in a specimen No. 5 described in JIS Z 2201 in direction R, 710 MPa or more, in which the steel plate consisting of% mass: C = 0.02% to 0.06%; Si = 0.05% to 0.5%; Mn = 1% to 2%; Nb = 0.05% to 0.12%; Ti = 0.005% to 0.02%; P ≦ 0.03%; S ≦ 0.005%; O ≦ 0.003%; Al = 0.005% to 0.1%; N = 0.0015% to 0.006%; Ca = 0.0005% to 0.003%; V ≦ 0.15%, where "0", or zero,%, is not included; Mo ≦ 0.3%, where "0" or zero,%, is not included; N -14/48 x Ti ≧ "0" (zero)% by mass%; 0 <S / Ca <0.8; optionally, which further comprises one type or two or more types between: Cr = 0.05% to 0.3%; Cu = 0.05% to 0.3%; Ni = 0.05% to 0.3%; B = 0.0002% to 0.003% in mass%; and optionally, which further comprises: REM = 0.0005% to 0.02% in mass%; and the remaining portion composed of Fe and inevitable elements of impurity, and the sheet thickness is 16 mm or more, the fraction of polygonal pro-eutectoid ferrite being 3% or more and 20% or less, and the rest of the phases is a low temperature transformation phase comprising bainitic ferrite, granular bainitic ferrite, quasi-polygonal ferrite, austenite retained and sea-tensite-austenite and 1% or less of perlite in a microstructure at a depth of half the thickness of a sheet steel surface, the average number of crystal grain size of the entire microstructure which is the average size grain (that is, the sum of the grain sizes / the number of crystal grains) when a numerical distribution is found for each grain size of the crystal grain measured by an electron dispersion diffraction pattern method is 1 µm or more and 2.5 µm or less, the average grain size area which is the average grain size (that is, the grain size corresponding to an average area) when a distribution is found in which the numerical distribution of each crystal grain size is multiplied by the area of the average grain size measured by the electron dispersion diffraction standard method is 3 µm or more and 9 µm or less, the standard deviation of the average grain size of the microstructure area is 0.8 µm or more and 2.3 µm or less and the reflected X-ray intensity ratio {211} / {111} in a direction {211} and in a direction {111} relative to a plane parallel to the surface of the plate steel in the depth of half the thickness of the steel plate surface is 1.1 or more.
[0002]
Steel sheet according to claim 1, characterized by the fact that a maximum hardness in a segregation portion close to a center of the hot-rolled steel sheet is 300 Hv or less and a segregation width of medium hardness of a base material + 50 Hv or more is 200 µm or less.

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法律状态:
2019-09-17| B06F| Objections, documents and/or translations needed after an examination request according art. 34 industrial property law|

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2019-10-01| B06U| Preliminary requirement: requests with searches performed by other patent offices: suspension of the patent application procedure|

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2019-10-15| B25D| Requested change of name of applicant approved|Owner name: NIPPON STEEL CORPORATION (JP) |

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2020-04-14| B09A| Decision: intention to grant|

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2020-06-30| B16A| Patent or certificate of addition of invention granted|Free format text: PRAZO DE VALIDADE: 20 (VINTE) ANOS CONTADOS A PARTIR DE 30/06/2011, 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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JP2010149702|2010-06-30|

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JP2010-149702|2010-06-30|

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PCT/JP2011/065014|WO2012002481A1|2010-06-30|2011-06-30|Hot-rolled steel sheet and method for producing same|

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