Steel plate with excellent toughness in the area affected by heat

专利摘要:
Provided is a steel plate which has a higher average toughness, a predetermined chemical composition and an A value of 5.0 to 25.0, where the A value is expressed by: A = 10,000x [B] x (0.4 + 30x [Ti] -82x [N]), and contains oxides and titanium nitride.
公开号:BE1021426B1
申请号:E2012/0753
申请日:2012-11-08
公开日:2015-11-19
发明作者:Nako Hidenori；Hatano Hitoshi；Okazaki Yoshitomi；Ibano Akira；Deura Tetsushi；Shimamoto Masaki；Sugitani Takashi
申请人:Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.)；
IPC主号:

专利说明:

"Steel plate with excellent toughness in the area affected by heat"
FIELD OF THE INVENTION
The present invention relates to steel plates (thick steel plates) applied to welded structures such as bridges, multi-story buildings and ships. The present invention more specifically relates to steel plates that are excellent in toughness in a heat-affected zone (hereinafter also referred to as ZAC) after high heat-input welding.
BACKGROUND OF THE INVENTION
With increasing sizes of welded structures such as bridges, multi-storey buildings and ships, steel plates having a thickness of 50 mm or more must be increasingly applied to welded structures and this requires inevitably the welding of steel plates having a thickness of 50 mm or more. In these circumstances, high heat welding is required for greater efficiency of the welding procedure.
The heat affected area during high heat welding is maintained in an austenite phase (y) at high temperature for a long time due to heating, and then slowly cooled. Therefore, the heat-affected zone suffers from coarser structures, such as austenitic grains that have grown during heating and large ferrite grains (a) formed during the cooling process, and this causes the area affected by the heat to have insufficient toughness during welding with high heat input. To avoid this, a technique should be developed which allows the toughness in the heat-affected zone (hereinafter also referred to as ZAC toughness) to be maintained at a high level when welding with a high heat input.
To ensure the ZAC toughness, techniques using inclusion particles such as oxides, nitrides and sulfides have been proposed. These inclusion particles block the growth of the austenitic grains, induce the formation of intragranular ferrite and thus help the steel to have a finer structure. Japanese Unexamined Patent Application Publication (JP-A) No. 2001-98340 and JP-A No. 2004-218010 disclose techniques of this class in which particles of titanium nitride are precipitated and dispersed as particles for blocking the growth of austenitic grains and this prevents magnification of the austenitic grains in the heat-affected area during high heat-input welding and protects the steel by preventing it from having insufficient ZAC toughness. However, these techniques have the disadvantage of not providing a stable ZAC toughness and are not applicable to increasing welding heat input in recent technologies because titanium nitride particles are likely to disappear with heat input. increasing welding.
By cons, JP-A No. 2001-20031, JP-A No. 2007-247005, JP-A No. 2008-223062 and JP-A No. 2009-179844 propose techniques using oxide inclusions as particles for block the growth of austenitic grains because oxide inclusions are stable at high temperatures. However, these techniques are still insufficient to be applied to high heat-input welding and should be further improved because oxide inclusions are fewer in number than titanium-containing nitride inclusions and may not provide blocking effects. enough.
Specifically, JP-A No. 2001-20031 discloses that the presence of oxides containing a rare earth element (REM) and / or Zr allows the steel to have good properties in the heat-affected zone. . However, this technique may not always give good properties in the heat-affected area during high heat-welding because the heat input therein is still at a low level. JP-A No. 2007-247005 discloses a technique using oxides containing a REM and / or Zr as in JP-A No. 2001-20031 and evaluates the energy absorbed in the Charpy test (Charpy impact resistance) as ZAC toughness. However, this technique is still insufficient for the reliability of the material (steel) because the Charpy energy absorbed measured in this technique is indicated as an average of multiple specimens, but the minimum of these multiple Charpy energy absorbed specimens should be maintained at a high level for the reliability of the material. JP-A No. 2008-223062 discloses a technique using oxide inclusions and inclusions containing titanium as particles to block the growth of the austenitic grains so as to obtain satisfactory ZAC toughness. However, the use of titanium-containing inclusions has limitations due to increasing heat input in recent technologies and calls are forthcoming to immediately establish a fully utilizing oxide inclusions technique to improve ZAC toughness when welding with high heat input. The present inventors have proposed a technique using the growth-blocking effects of the austenitic grains of the fine oxide inclusions in JP-A No. 2009-179844. However, this technique requires complicated control over the determination of alloy element quantities based on the dissolved oxygen and dissolved sulfur amounts because this technique resorts to the suppression of re-precipitation of fine manganese sulphides. in combination with the growth blocking effects of the austenitic grains of the fine oxide inclusions.
Techniques allowing a steel to have a finer structure due to intragranular ferrite formation induced by inclusion particles have also been proposed. Typically, JP-A No. H07-252586 has proposed a technique using manganese sulfide (MnS) and multicomponent oxides containing titanium and / or REM. Independently, the present inventors have proposed in JP-A No. 2008-223081 a technique for controlling the shape of inclusions to improve the formation of intragranular ferrite. These techniques are constructed on the assumption that inclusions with low interfacial energy relative to intragranular ferrite are effective for intragranular ferrite formation. However, these techniques are still insufficient to guarantee a satisfactory ZAC toughness during high heat-input welding because the interfacial energy between intragranular ferrite and austenite also contributes significantly to the formation of intragranular ferrite, and a simple reduction in Interfacial energy between intragranular ferrite and inclusions may fail to provide sufficient formation of intragranular ferrite.
The present inventors have also proposed in JP-A No. 2009-138255 a technique utilizing oxysulfide-induced intragranular ferrite formation to provide satisfactory ZAC toughness. However, even this technique does not guarantee sufficient ZAC toughness at high heat-input welding because this technique instead requires the dispersion of relatively large oxysulfide particles having sizes of 2 μm or more in a specific numerical density. Specifically, the technique disclosed in JP-A No. H07-252586 is intended to be applied to low heat-input welding, while the techniques disclosed in JP-A No. 2008-223081 and JP-A No. 2009 -138255 are still subject to improvements in terms of Charpy's minimum energy consumption, while the average Charpy energy consumption specified in these techniques is high.
The present inventors have further proposed in JP-A No. 2010-168644 and JP-A No. 2011-219797 oxide dispersion techniques having controlled structures to provide satisfactory ZAC toughness. These techniques allow steel plates to have excellent toughness in a heat affected zone but are still subject to improvements in the manufacture of such steel plates.
The technique disclosed in JP-A No. 2010-168644 controls the amount of Ca based on the amount of dissolved oxygen prior to the addition of Ca to give oxides having predetermined dimensions. However, according to this technique, the addition of Ca should be done within 3 to 20 minutes after the addition of Ti and this may be a greater burden for the operators. The technique disclosed in JP-A No. 2011-219797 is still subject to improvements in productivity because the material should be maintained for a period of 40 minutes to 90 minutes between the addition of Ca and the beginning of the casting. .
SUMMARY OF THE INVENTION
Technical problem
The present invention has been made on the basis of these circumstances and an object of the present invention is to provide a steel plate (thick steel plate) which exhibits satisfactory ZAC toughness not only in its average but also at its minimum. and which therefore has excellent toughness in the heat affected zone and can be manufactured with satisfactory productivity.
Solution to the Problem The invention provides, in one aspect, a steel plate which comprises carbon (C) in a content of 0.03% to 0.12%; silicon (Si) in a content of 0% to 0.25%; manganese (Mn) in a content of 1.0% to 2.0%; phosphorus (P) in a content greater than 0% and less than or equal to 0.03%; sulfur (S) in a content greater than 0% and less than or equal to 0.015%; aluminum (Al) in a content of 0.001% to 0.05%; titanium (Ti) in a content of 0.010% to 0.050%; at least one rare earth element (REM) in a content of 0.0003% to 0.02%; zirconium (Zr) in a content of 0.0003% to 0.02%; calcium (Ca) in a content of 0.0005% to 0.010%; nitrogen (N) in a content of 0.002% to 0.010%; and boron (B) in a content of 0.0005% to 0.0050%, in percent by weight. The steel plate further comprises iron and unavoidable impurities. The steel plate has a value A expressed by expression (1) of 5.0 or higher and 25.0 or less, expression (1) expressed as follows:
A = 104x [B] x (0.4 + 30x | Ti] -82x [N]) (1) where [B], [Ti] and [N] are respectively (percent by mass) of B , Ti and N of the steel plate; the steel plate contains oxides and titanium nitride, each of the oxides contains constituent elements other than oxygen in contents satisfying the following conditions: 2% <Ti <40%, 5% <AI <30%, 5% <Ca <40%, 5% <REM <50%, 2% <Zr30% and 1.5 <REM / Zr, in percentage by weight, where Ti, Al, Ca, REM and Zr are respectively contents ( in percent by mass) to Ti, Al, Ca, at least one rare earth element and Zr, based on the total amount of elements, other than oxygen, constituting each oxide. With respect to oxides, oxide particles having an equivalent circle diameter of less than 2 μm are present in a numerical density of 300 or more per square millimeter and oxide particles having an equivalent circle diameter of 2 μm or more are present in a numerical density of 100 or less per square millimeter. With respect to titanium nitride, titanium nitride particles having an equivalent circle diameter of 1 μηι or more are present in a numerical density of 5 or less per square millimeter.
As used herein, the term "equivalent circle diameter" refers to the size of an oxide particle or titanium nitride particle and refers to the diameter of a supposed circle having an equal area. to that of the oxide particle or the titanium nitride particle. The equivalent circle diameter can be determined by transmission electron microscopy (TEM) or scanning electron microscope (SEM) observation.
The steel plate preferably further comprises at least one member selected from the group consisting of nickel (Ni) in a content of 0.05% to 1.50%; copper (Cu) in a content of 0.05% to 1.50%; chromium (Cr) in a content of 0.05% to 1.50%; and molybdenum (Mo) in an amount of from 0.05% to 1.50%, in mass%, in which the element contents satisfy the following condition: [Ni] + [Cu] + [Cr] + [Mo] ] <2.5% where [Ni], [Cu], [Cr] and [Mo] are the contents (in percentage by mass) respectively in Ni, Cu, Cr and Mo.
The steel plate may further contain at least one member selected from the group consisting of niobium (Nb) in a content of 0.002% to 0.10%; and vanadium (V) in a content of 0.002% to 0.10%, in percent by weight.
The present invention can provide a steel plate which has a satisfactory average and minimum ZAC toughness, has satisfactory toughness in the heat-affected zone during welding not only at low or medium heat input but also at the same time. welding with high heat input and can be manufactured with satisfactory productivity.
DETAILED DESCRIPTION OF THE FORMS OF REALIZATION
FAVORITE
The present inventors have carried out investigations to enable a steel plate to have a more satisfactory ZAC toughness during high heat-input welding even when the steel plate has been manufactured under such conditions of manufacture as to give relatively high productivity. They therefore found that the steel plate can have satisfactory productivity and excellent ZAC toughness during high heat-input welding by allowing oxides to induce intragranular ferrite formation and preventing the formation of large particles of titanium nitride and coarse grain-grain ferrite (allotriomorphous ferrite) both of which negatively affect the ZAC toughness. Specifically, the present inventors have found that adequate control of the chemical oxide compositions ensures the formation of intragranular ferrite and prevents the formation of large particles of titanium nitride, which particles of titanium nitride are formed on oxides in a steel melted in usual techniques; adequate control of the chemical composition of the steel prevents the formation of coarse ferrite at the grain boundaries; and the resulting steel plate has a satisfactory ZAC toughness when welding with high heat input.
More specifically, the present inventors have found that a satisfactory ZAC tenacity can be obtained by dispersing, with respect to the above-mentioned oxides, oxide particles having an equivalent circle diameter of less than 2 μm in a numerical density of 300 or more per square millimeter and limiting oxide particles having an equivalent circle diameter of 2pm or more to a numerical density of 100 or less per square millimeter.
The present invention has been made on the basis of these findings. The constitutive conditions are specified in this document for the reasons mentioned below.
Oxides having specific chemical compositions and having an equivalent circle diameter of less than 2 μm present in a numerical density of 300 or more per square millimeter.
Specifically, the steel plate contains oxides containing elements other than oxygen in contents satisfying the following conditions: 2% <Ti <40%, 5% <AI <30%, 5% <Ca <40 %, 5% <REM <50%, 2% <Zr <30%, and 1.5 <REM / Zr, in percent by weight, and for oxides, oxide particles having a circle diameter equivalent less than 2 μm are present in a numerical density of 300 or more per square millimeter.
Controlling the oxide particles to have an equivalent circle diameter of less than 2 μm promotes the formation of intragranular ferrite to thereby improve the ZAC toughness. Oxide particles having an equivalent circle diameter of 2 μm or more can reduce the barrier energy during the formation of large titanium nitride particles and this can cause a greater amount of large particles of titanium nitride. Oxides having chemical compositions not satisfying the conditions: 2% <Ti <40%, 5% <AI <30%, 5% <Ca <40%, 5% <REM <50%, 2% <Zr <30 %, and 1.5 <REM / Zr, in percent by weight, can prevent sufficient formation of intragranular ferrite. Among these conditions, controlling the ratio (in weight percent) of REM to Zr in the oxides at 1.5 or more can reduce the amount of large titanium nitride particles formed on the oxide surface in molten steel. . Oxide particles having an equivalent circle diameter of less than 2 μm, if present in a numerical density of less than 300 per square millimeter, may not sufficiently induce intragranular ferrite formation, and this may lead to intragranular ferrite to be formed in insufficient quantity and may not provide sufficient ZAC toughness.
Oxide particles having an equivalent circle diameter of 2 μιτι or more present in numerical density of 100 or less per square millimeter
For oxides having chemical compositions meeting the conditions specified above, oxide particles having an equivalent circle diameter of 2 μm or more promote brittle fracture, negatively affect the ZAC toughness and are preferably reduced to a minimum quantity. To avoid this, it is specified here that oxide particles having an equivalent circle diameter of 2 μm or more are present in a numerical density of 100 or less per square millimeter.
Titanium nitride particles having an equivalent circle diameter of 1 μm or more present in numerical density of 5 or less per square millimeter
Titanium nitride particles having an equivalent circle diameter of 1 μm or more, if present in a numerical density of more than 5 per square millimeter, can promote brittle fracture and negatively affect the ZAC toughness. Such titanium nitride particles have parallelepiped rectangular shapes, have a much higher hardness than steel and thus remarkably negatively affect ZAC toughness due to stress concentration. To avoid this, large particles of titanium nitride are controlled more strictly than large oxide particles.
Manufacturing process
The steel plate according to the present invention satisfies the above conditions. Specifically, the steel plate contains oxides and titanium nitride, the oxides comprising constituents other than oxygen in contents satisfying the following conditions:% <Ti <40%, 5% <AI <30 %, 5% <Ca <40%, 5% <REM <50%, 2% <Zr <30% and 1.5 <REM / Zr in percent by weight, in which, for oxides, the particles of oxide having an equivalent circle diameter of less than 2 μιτι are present in a numerical density of 300 or more per square millimeter, and oxide particles having an equivalent circle diameter of 2 μm or more are present in a digital density of 100 or less per square millimeter, and in the case of titanium nitride, large titanium nitride particles having an equivalent circle diameter of 1 μm or greater are present in a numerical density of 5 or less per square millimeter. The manufacture of the steel plate can be carried out under the following manufacturing conditions.
Specifically, the manufacture can be carried out as follows. During the production of ingots, the amount of dissolved oxygen in the molten steel is controlled at the level of 0.002% to 0.01% by weight by deoxidation typically with Mn and Si; Al, Ti, (REM and Zr) and Ca are added in this order, the ratio | REM] / [Zr] of the mass of REM [REM] to the mass of Zr [Zr] is controlled at 1.8 or higher , and the adding procedure is controlled so that the time interval t1 of the addition of REM or Zr to the addition of Ca is 10 minutes or more. During casting, cooling at temperatures of the order of 1500 ° C to 1450 ° C is carried out for a time t2 of 300 seconds or less. Then, the reasons why the manufacturing conditions are specified will be described in detail below.
Controlling the amount of dissolved oxygen in the molten steel at the level of 0.002% to 0.01% by deoxidation with Mn and Si Dissolved oxygen, if present in an amount of less than 0.002%, may contribute to the formation of oxides in a necessary amount, which oxides have chemical compositions suitable for inducing the formation of intragranular ferrite. Dissolved oxygen, if present in an amount greater than 0.01%, may be responsible for larger quantities of large oxide particles having an equivalent circle diameter of 2 μm or more, affecting therefore negatively the ZAC toughness.
Control of the [REM] / [Zr] ratio of the mass of REM added [REM] to the added Zr mass [Zr] at 1.8 or higher;
Control of the time interval t1 from the addition of REM or Zr to the addition of Ca to 10 minutes or more
The oxides specified in the present invention promote the formation of intragranular ferrite and cause little formation of large particles of titanium nitride. Especially for the control of the ratio (in percentage by mass) of REM to Zr in the oxides at the level of 1.5 or more, the ratio [REMJ / [Zr] of the mass of REM added [REM] to the mass of Zr added [Zr] should be controlled at 1.8 or higher; and the reaction for the formation of REM or Zr oxides should be able to proceed sufficiently before the addition of Ca as a strongly deoxidizing element. Specifically, the time interval t1 of the addition of REM or Zr to the addition of Ca is controlled at 10 minutes or more to give oxides that have a ratio REM / Zr of 1.5 or more and are present in a predetermined numerical density. If the time interval t1 of the addition of REM or Zr to the addition of Ca is less than 10 minutes, oxides with a REM / Zr ratio of 1.5 or more may be formed in an insufficient amount.
Al, Ti, (REM and Zr) and Ca are added in this order during ingot production. This is because the respective elements, if added in a different order than the order specified above, may not contribute to the formation of the necessary quantities of oxides having suitable chemical compositions of a kind to induce intragranular ferrite formation. Among these elements, calcium has an extremely strong deoxidizing activity, acts as a strongly deoxidizing element and, if it is added before titanium (Ti) and aluminum (Al), is combined with oxygen, and this reduces significantly the amount of oxygen to combine with Ti and Al.
Control of the cooling time t2 at temperatures of the order of 1500 ° C. to 1450 ° C. at 300 seconds or less during casting
Cooling at temperatures of the order of 1500 ° C. to 1450 ° C. during casting, if it is carried out for a time t2 greater than 300 seconds, can be at the origin of the formation of large particles of nitride of titanium due to the segregation of chemical compositions during solidification, thus negatively affecting the ZAC toughness.
Chemical composition
Next, the chemical composition of the steel plate according to the present invention will be described. The steel plate should have contents of respective chemical compositions (elements) in appropriate ranges so as to have good properties of the base metal (steel) and the heat-affected zone, even when it satisfies conditions other than those mentioned above, such as the dispersion of oxides. The steel plate therefore has contents of respective chemical compositions in the ranges mentioned below. With respect to the elements, the contents of Al, Ca, Ti and other oxide-forming elements in the steel plate are indicated as total contents including the element content of the oxides, as evident from their operation and effects. advantageous. The contents (/) of the following chemical compositions are all indicated in percentage by mass.
Carbon content (C): 0.03% to 0.12% The carbon element (C) is essential to help the steel plate to have satisfactory strength. Carbon, if it is present in a content of less than 0.03%, can not contribute to a resistance at the necessary level. However, the carbon content is preferably controlled at 0.12% or less because the carbon, if present in excessively high content, may be responsible for the formation of martensite-austenite (MA) component hard in a large amount that will negatively affect the toughness of the base metal. The carbon content is preferably 0.04% or more and 0.10% or less.
Silicon content (Si); 0% to 0.25% The silicon (Si) element is not essential but is useful for providing necessary strength due to solid solution hardening. However, the Si content is preferably controlled to 0.25% or less because silicon, if added in excess, may invite a greater amount of martensite-austenite (MA) hard component into the area affected by the heat, thus causing the deterioration of the ZAC toughness. The Si content is preferably 0.21% or less and more preferably 0.18% or less.
Manganese content (Mn): 1.0% to 2.0% The manganese element (Mn) is useful in helping the steel plate to have satisfactory strength. The manganese is preferably present in a content of 1.0% or more to advantageously have such effects. However, the Mn content is preferably controlled to 2.0% or less because the manganese, if present in excessively high content of more than 2.0%, can cause the heat-affected zone to have excessively high resistance and thus have insufficient ZAC toughness. The Mn content is preferably 1.4% or more and 1.8% or less.
Phosphorus content (P): greater than 0% and less than or equal to 0,03% The phosphorus element (P) is an element of impurity which can often be at the origin of fracture at the grain boundaries and affects negatively the tenacity. To avoid this, the phosphorus content is preferably reduced to a minimum. The phosphorus content is controlled to 0.03% or less and is preferably 0.02% or less, to provide satisfactory ZAC toughness. However, it is difficult industrially to reduce the phosphorus content of steel to zero percent.
Sulfur content (S): greater than 0% and less than or equal to 0.015% The sulfur element (S) forms manganese sulphide at primary austenite grain boundaries in the heat affected zone and adversely affects ZAC toughness. To avoid this, the sulfur content is preferably reduced to a minimum. The sulfur content is preferably 0.015% or less and more preferably 0.010% or less to provide satisfactory ZAC toughness. However, it is difficult industrially to reduce to the sulfur content of the steel at zero percent.
Aluminum content (Al): 0.004% to 0.05% The aluminum element (Al) forms oxides which induce the formation of intragranular ferrite. Aluminum, if present in a content of less than 0.004%, may not yield oxides having predetermined dimensions and may cause the formation of large particles of titanium nitride. However, the Al content is preferably controlled to 0.05% or less because aluminum, if present in excessively high content, can cause the formation of large oxide particles and adversely affect the ZAC toughness. The Al content is preferably 0.007% or more and 0.04% or less.
Titanium content (Ti): 0.010% to 0.050% The titanium element (Ti), when added with REM, Zr and Ca, helps the oxides to be finely dispersed, which oxides have the function of promoting the formation of ferrite intragranular. To advantageously present these effects, titanium is present in a content of 0.010% or more. However, the Ti content is preferably controlled to 0.050% or less because titanium, if present in excessively high content, can cause the formation of a large amount of large titanium nitride particles and adversely affect the negatively ZAC toughness. The Ti content is preferably 0.012% or more and 0.035% or less, and more preferably 0.025% or less.
EMR content: 0.0003% to 0.02% and Zr content: 0.0003% to 0.02%
The rare earth elements (REM) and the zirconium element (Zr), when added after the addition of Ti and before the addition of Ca, form oxides contributing to the formation of intragranular ferrite. These elements present the effects more satisfactorily as their content increases and REM and Zr, when they are each present in a content of 0.0003% or more, may have the effects advantageously. However, the levels of REM and Zr are preferably each controlled at 0.02% or less because REM and Zr, when present in excess, can cause the oxides to be coarse and may adversely affect the ZAC toughness. The levels of REM and Zr are more preferably each 0.0005% or more and 0.015% or less.
Calcium (Ca) content: 0.0005% to 0.010% The calcium (Ca) element, when added 10 minutes or more after the addition of Ti, REM and Zr, forms oxides which contribute to the formation of intragranular ferrite and prevent the formation of large particles of titanium nitride. Calcium, when present in a content of 0.0005% or more, may advantageously have these effects. However, the Ca content is preferably controlled to 0.010% or less because calcium, if present in excessively high content, can cause the formation of large oxide particles and negatively affect the ZAC toughness. The Ca content is preferably 0.0008% or more and 0.008% or less.
Nitrogen Content (N): 0.002% to 0.010% Nitrogen (N) is useful for the formation of titanium nitride particles to provide satisfactory toughness in the heat affected zone. Nitrogen, when present in a content of 0.002% or more, can give the desired titanium nitride particles. However, the nitrogen content is preferably controlled to 0.010% or less because nitrogen, if present at an excessively high level, can promote the formation of large titanium nitride particles. The nitrogen content is preferably 0.003% or more and 0.008% or less.
Boron content (B): 0.0005% to 0.005%
If coarse ferrite at the grain boundaries is formed in the heat affected zone, the heat affected zone may have a limited ZAC toughness (may not have a satisfactory ZAC toughness) even when intragranular ferrite is formed and that the formation of large particles of titanium nitride is prevented. The boron element (B) advantageously prevents the formation of coarse ferrite at the grain boundaries and thus improves the ZAC toughness. Boron exhibits these effects more satisfactorily as its content increases and, when present in a content of 0.0005% or more, may advantageously have the effects. However, the boron content is controlled preferably to 0.005% or less because boron, if present in excessively high content, can promote the formation of coarse bainitic packets from the primary austenite grain boundaries and can negatively affect the ZAC toughness. The boron content is preferably 0.0010% or more and 0.004% or less, and more preferably 0.0015% or more.
The steel plate according to the present invention essentially contains the aforementioned elements, the remainder comprising iron and unavoidable impurities. The steel plate may contain, as unavoidable impurities, Sn, As, Pb and other elements that are typically introduced into the steel from raw materials, building materials and manufacturing facilities. The steel plate can actually positively contain some of the elements mentioned below in order to have other improved properties depending on the types of chemical compositions (elements) to be added.
At least one member selected from the group consisting of nickel (Ni) in a content of 0.05% to 1.50%, copper (Cu) in a content of 0.05% to 1.50%, chromium (Cr) in a content of 0.05% to 1.50% and molybdenum (Mo) in a content of 0.05% to 1.50%
The nickel (Ni), copper (Cu), chromium (Cr) and molybdenum (Mo) elements are each effective in helping the steel plate to have greater strength and exhibit this effect more satisfactorily as their content increases. increases. These elements are preferably each present in a content of 0.05% or more. However, the contents of these elements are preferably each controlled at 1.50% or less because these elements, if present in excessively high content, may invite high strength and impair ZAC toughness. The contents of these elements are each more preferably 0.10% or more and 1.20% or less.
At least one member selected from the group consisting of niobium (Nb) in a content of 0.002% to 0.10% and vanadium (V) in a content of 0.002% to 0.10%
The niobium (Nb) and vanadium (V) elements precipitate as carbonitrides to prevent the austenitic grains from becoming coarse and are thus effective in providing satisfactory toughness of the base metal. These elements exhibit the effects more satisfactorily as their contents increase and are preferably each present in a content of 0.002% or more to advantageously present the effects. However, the contents of these elements are preferably each controlled at 0.10% or less because the elements, if present in excess, can cause the heat-affected zone to have a coarse structure, thus impairing the toughness. ZAC. The contents of these elements are each more preferably 0.005% or more and 0.08% or less.
Settings
The steel plate according to the present invention has a chemical composition satisfying the above conditions and, in addition, has an A value of 5.0 or higher and 25.0 or less (5.0 <A <25, 0), the value A being expressed by the expression (1):
Value A = 104x [B] x (0.4 + 30x [Ti] -82x [N]) (1) where [B], [Ti] and [N] are percentages by mass respectively in B , Ti and N. The suppression of coarse grain-grain ferrite formation requires not only the addition of boron but also the presence of free boron. The value A determined according to the expression (1) is an index for the amount of free boron to be controlled. A steel plate having an A value of less than 5.0 may suffer from coarse grain ferrite formation. A steel plate having an A value greater than 25.0 may suffer from insufficient ZAC toughness due to the promotion of coarse bainite formation at the austenitic grain boundaries.
The coefficients and ranges of the respective elements in the expression (1) for the determination of the A value were determined experimentally on the basis of the following idea.
Boron is present mainly as free boron and boron nitride. The amount of free boron is therefore roughly determined on the basis of the amount of boron added and the amount of boron nitride. When the heat affected zone is heated to elevated temperatures, the titanium nitride melts to form free nitrogen and the free nitrogen is combined with boron to form boron nitride during the subsequent cooling process. On the basis of this, a linear regression equation of [Ti] and [N] was computationally determined using the Thermo-Wedge software for thermodynamic calculations to derive expression (1). The linear regression equation of [Ti] and [N] represents the amount of free nitrogen when the heat-affected zone is heated to a high temperature (1400 ° C). Next, the proportionality coefficient was determined by comparison with an experimentally determined free amount of free boron assuming that the amount of boron nitride formed during the cooling of the heat-affected zone is proportional to the product of the quantity. free nitrogen and the amount of boron added. The resulting expression representing the amount of boron nitride is subtracted from the amount of boron added to give expression (1): A value = 104x [B] x (0.4 + 30x | Ti] -82x [N] ) (1)
As described in the chemical compositions, the steel plate may additionally contain at least one member selected from the group consisting of Ni, Cu, Cr and Mo. In this case, the contents (in percent by weight) of these elements satisfy the following condition: (Ni] + [Cu] + [Cr] + | Mo] <2.5% In other words, the total content of Ni, Cu, Cr and Mo is less than 2 , 5%.
Large titanium nitride particles are formed by segregation upon solidification in a liquid phase enriched with titanium and nitrogen in the process of solidification of the molten steel. These elements, if added to a total content of more than 2.5%, can lower the solidification temperature and the liquid phase can remain at a low temperature at which the force pushing for the formation of large particles of nitride titanium becomes too big. Therefore, a larger amount of large titanium nitride particles can be formed.
The present invention relates to steel plates. The term "steel plate" refers generally to a steel sheet having a thickness of 3.0 mm or more. On the other hand, the steel plate according to the present invention is preferably a steel plate which has a thickness of 50 mm or more and which will be welded. Accordingly, the steel sheets to which the present invention is applied are preferably, but not exclusively, steel sheets having a thickness of 50 mm or more. It should be noted, however, that these are merely preferred embodiments and are not intended to avoid the application of the present invention to steel plates (or steel sheets) having a thickness of less than 50 mm.
Examples
The present invention will be illustrated in more detail with reference to several experimental examples below. It should be noted, however, that the examples are never construed as limiting the scope of the present invention and may be modified or changed without departing from the scope and spirit of the present invention and all of these are incorporated herein by reference. in the context of the present invention.
In the experimental examples of this document, the steel sheets were manufactured in the following manner. Initially, sheets having the chemical compositions listed in Tables 1 and 2 were made by melting using a vacuum melting furnace (vacuum induction melting furnace, VIF, 150 kg), the molten metals were cast. in slabs (sectional shape: 150 mm by 250 mm) and the slabs were hot-rolled to give hot-rolled steel sheets having a thickness of 80 mm. Specifically, slabs were heated at 1100 ° C for 3 hours prior to hot rolling, were hot rolled to a finish rolling temperature of 780 ° C or higher, and cooled to a quench temperature. cooling temperature of 450 ° C at an average cooling temperature of 6 ° C per second.
Controlled manufacturing conditions for hot-rolled steel sheets (steel plates) are given in Tables 3 and 4. The controlled conditions are the amount of dissolved oxygen (Of) (in percentage by mass) in the molten steel before the addition of Al (Ti); the order of the additions of Al, Ti, REM, Zr and Ca; the time interval t1 of the addition of REM or Zr to the addition of Ca; the ratio [REM] / [Zr] (mass) of the amount of REM added [REM] to the amount of Zr added [Zr] (the ratio is indicated as "REM / Zr" in the tables); and the cooling time t2 at temperatures of the order of 1500 ° C to 1450 ° C in casting.
EMRs, as shown in Tables 1 and 2, were added as a misch metal containing about 50% Ce and about 25% La in percent by weight. The symbol in Tables 1 and 2 indicates that an element in question has not been added.
The addition orders of Al, Ti, REM, Zr and Ca in Tables 3 and 4 are indicated as "appropriate" when the elements have been added in the order Al, Ti, (REM and Zr) and Ca and as "inappropriate" when the elements have been added in another order. ч
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Table 3
Table 4
Hot-rolled steel plates (steel plates) manufactured under these conditions were examined to determine the numerical density N1 of oxide particles having an equivalent circle diameter of less than 2 μm, the numerical density N 2 of oxide having an equivalent circle diameter of 2 μm or more, the numerical density N3 of titanium nitride particles having an equivalent circle diameter of 1 μm or more and the ZAC toughness according to the following measurement methods. The results of these measurements are shown in Tables 5 and 6.
Measuring the numerical density of oxide particles having an equivalent circle diameter of less than 2 μm
Specimens were cut from the respective steel plates to a depth of one quarter of the thickness from the surface of the plate so that the central axis of each specimen passed to the depth of a quarter of thickness. A cross-section of each specimen parallel to the direction of rolling and the direction of depth was observed with a field-effect scanning electron microscope "SUPRA35 (trade name)" (also hereinafter referred to as FE-SEM) provided by Carl Zeiss AG. The observation was carried out at a magnification of 5000x in a field of vision of 0.0024 mm 2 in twenty points. Respective oxide areas in the viewing fields of view were measured by image analysis from which the equivalent circle diameters of the oxides were calculated. The fact that the oxides have chemical compositions satisfying the above conditions has been verified with an energy dispersive X-ray spectrometer (EDX). Measurement of the chemical compositions with EDX was carried out at an acceleration voltage of 15 kV for a duration of 100 seconds. The number of oxide particles having an equivalent circle diameter of less than 2 μm has been determined in terms of numerical density (N1) per square millimeter. However, oxide particle data having an equivalent circle diameter of 0.2 μm or less were excluded from the data to be analyzed because of insufficient reliability of EDX.
Measuring the numerical density of oxide particles having an equivalent circle diameter of 2 μm or more
Specimens were cut from the respective steel plates to a depth of one quarter of the thickness from the surface of the plate so that the central axis of each specimen passed to the depth of a quarter of thickness. A cross-section of each specimen parallel to the direction of rolling and the direction of the depth was observed with FE-SEM. The observation was carried out at a magnification of 1000x in a field of vision of 0.06 mm2 in twenty points. Respective oxide areas in the viewing fields of view were measured by image analysis from which the equivalent circle diameters of the oxides were calculated. The fact that the oxides have chemical compositions satisfying the above conditions has been verified with an energy dispersive X-ray spectrometer (EDX). Measurement of the chemical compositions with EDX was carried out at an acceleration voltage of 15 kV for a duration of 100 seconds. The number of oxide particles having an equivalent circle diameter of 2 μm or more has been determined in terms of numerical density (N 2) per square millimeter.
Measuring the numerical density of titanium nitrides having an equivalent circle diameter of 1 μm or more
Specimens were cut from the respective steel plates to a depth of one quarter of the thickness from the surface of the plate so that the central axis of each specimen passed to the depth of a quarter of thickness. Images were taken in 20 fields of view of a cross-section of each specimen parallel to the direction of rolling and the direction of depth with an optical microscope at 200x magnification, the number of large titanium nitride particles having a Equivalent circle diameter of 1 μm or more was counted and converted to a numerical density (N3) per square millimeter. A measured area of the images was 0.148 mm 2 per field of view and was 2.96 mm 2 per sample. Titanium nitride has been identified on the basis of shape and color. Specifically, angular and light orange inclusions have been considered as titanium nitride particles. The equivalent circle diameters of the titanium nitride particles were calculated using analysis software. Large particles of titanium nitride form in the oxides.
In this case, the oxides surrounded by titanium nitride particles were excluded from the measurement of equivalent circle diameters.
ZAC toughness assessment
Weld joint specimens were sampled from the respective steel plates treated to form a V-groove edge and subjected to electrogas arc welding with a heat input of 50 kJ / mm. Each of the three specimens of Charpy notched impact test specimens were sampled from these specimens. The Charpy test specimens were Japanese industry standard (JIS) Z 2242 V-notch specimens and had a notch in a heat-affected zone near a weld seam (bond) at a depth of 20 cm. a quarter of the thickness from the surface of each steel plate. Charpy assay specimens were Charpy assayed at -40 ° C for absorbed energy (vE.40) and an average and a minimum of absorbed energy of the three specimens were determined. Based on the measurement results, a sample with an average vE-40 of greater than 180 J and a minimum vE.40 of more than 120 J was evaluated as having satisfactory ZAC toughness.
Each of the three Charpy test specimens were independently prepared from weld joint specimens by the above procedure, except that the weld joint specimens were subjected to electrogas arc welding with heat of 60 kJ / mm. The Charpy test specimens were tested by Charpy under the above conditions. The absorbed energy (vE.40) of the three specimens was measured and an average was determined. Based on the measurement results, a sample with an average vE.40 of more than 120 J was evaluated as having satisfactory ZAC toughness.
Table 5
Table 6
The results demonstrate the following. Samples Nos. 1 to 30 are examples satisfying the conditions specified in the present invention. They had appropriately controlled chemical compositions, suitably dispersed titanium nitride oxides and particles, and had a satisfactory ZAC (average and minimum) toughness when welding with a heat input of 50 kJ / mm and also satisfactory ZAC toughness (average) when welding with a heat input of 60 kJ / mm. Specifically, samples Nos. 1 to 30 can be used as steel plates with excellent toughness in a heat affected zone.
In contrast, samples 31 to 52 are comparative examples not satisfying at least one of the conditions specified in the present invention. They did not meet a criterion for any of the ZAC toughness (average and minimum) when welding with a heat input of 50 kJ / mm and the ZAC toughness (average) when welding with a contribution of heat of 60 kJ / mm.

权利要求:
Claims (3)
[1]
A steel plate comprising: carbon (C) in a content of 0.03% to 0.12%; silicon (Si) in a content of 0% to 0.25%; manganese (Mn) in a content of 1.0% to 2.0%; phosphorus (P) in a content greater than 0% and less than or equal to 0.03%; sulfur (S) in a content greater than 0% and less than or equal to 0.015%; aluminum (Al) in a content of 0.004% to 0.05% titanium (Ti) in a content of 0.010% to 0.050%; at least one rare earth element (REM) in a content of 0.0003% to 0.02%; zirconium (Zr) in a content of 0.0003% to 0.02%; calcium (Ca) in a content of 0.0005% to 0.010%; nitrogen (N) in a content of 0.002% to 0.010%; and boron (B) in a content of 0.0005% to 0.0050%, in weight percent, wherein: the steel plate further comprises iron and unavoidable impurities; the steel plate has a value A expressed by the expression (1) of 5.0 or higher and 25.0 or less, expression (1) expressed as follows: Value A = 104x [B] x (0, 4 + 30x [Ti] -82x [N]) (1) where [B], [Ti] and [N] are percentages (by mass percentage) respectively of B, Ti and N in the steel plate; the steel plate contains oxides and titanium nitride, where each of the oxides contains constituent elements other than oxygen in contents satisfying the following conditions: 2% <Ti <40%, 5% <AI <30% , 5% <Ca <40%, 5% <REM <50%, 2% <Zr <30% and 1.5 <REM / Zr, where Ti, Al, Ca, REM and Zr are contents (in percentages mass) respectively to Ti, Al, Ca, at least one rare earth element and Zr, based on the total amount of elements, other than oxygen, constituting each oxide; with respect to oxides, oxide particles having an equivalent circle diameter of less than 2 μm are present in a numerical density of 300 or more per millimeter and oxide particles having an equivalent circle diameter of 2 μm or more are present in a numerical density of 100 or less per square millimeter, and in the case of titanium nitride, titanium nitride particles having an equivalent circle diameter of 1 μm or more are present in a numerical density of 5. or less per square millimeter.
[2]
The steel plate of claim 1, further comprising at least one member selected from the group consisting of: nickel (Ni) in a content of 0.05% to 1.50%; copper (Cu) in a content of 0.05% to 1.50%; chromium (Cr) in a content of 0.05% to 1.50%; and molybdenum (Mo) in an amount of from 0.05% to 1.50%, in percent by weight, in which the contents of these elements satisfy the following condition: [Ni] + [Cu] + [Cr] + [ Mo] <2.5% where [Ni], [Cu], [Cr] and [Mo] are the contents (in percentage by mass) respectively in Ni, Cu, Cr and Mo.
[3]
The steel plate of any one of claims 1 and 2, further comprising at least one member selected from the group consisting of: niobium (Nb) in a content of 0.002% to 0.10%; and vanadium (V) in a content of 0.002% to 0.10%, in percent by weight.

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法律状态:
2021-08-11| MM| Lapsed because of non-payment of the annual fee|Effective date: 20201130 |

优先权:

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申请号 | 申请日 | 专利标题

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JP2011248879|2011-11-14|

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JP2011248879|2011-11-14|

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