Steel plate and method for producing the same

Abstract

To provide a steel plate capable of achieving superior Charpy toughness in a HAZ produced by high heat-input welding, together with superior strength at a 1 / 2 thickness position of a base material and superior Charpy toughness after strain aging treatment, and to provide a method for producing the steel plate.SOLUTION: A steel plate has a component composition containing predetermined amounts of C, Si, Mn, P, S, Al, Ti, N, and O, with Ceq being 0.300-0.500%, Ti / N being 2.00-5.00, and the balance being Fe and inevitable impurities, and has, at a 1 / 2 thickness position, a steel microstructure in which ferrite+bainite phases account for 80% or more in area ratio, an average grain size of ferrite and bainite phases surrounded by grain boundaries having an orientation difference of 15° or more is 30 μm or less, and an area ratio of MA in the bainite phase is 15% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel plate, particularly a steel plate applicable to large heat input welding, and a manufacturing method thereof. Specifically, the present invention relates to a steel plate having excellent toughness in the heat affected zone (hereinafter referred to as HAZ) after large heat input welding. The steel plate of the present invention is suitable for use in large structures such as ships, marine structures, cryogenic storage tanks, and architectural and civil engineering structures. [Background technology]

[0002] Steel structures used in fields such as ships, marine structures, buildings, and steel pipes are generally finished into structures of desired shapes by welding. Therefore, from the perspective of ensuring safety, these structures are required to ensure the strength and toughness of the steel materials used, as well as excellent toughness of the welds.

[0003] Furthermore, in recent years, ships and steel structures have become increasingly larger, and the steel materials used are being made stronger and thicker. Accordingly, highly efficient, high-heat-input welding methods such as submerged arc welding, electrogas welding, and electroslag welding have come to be used for welding. This high-heat-input welding requires steel materials with excellent toughness in the welded joints, even when performed.

[0004] However, in the above-mentioned steel materials, particularly high-strength steel plates or thick steel plates, there are often cases where it is difficult to achieve both the mechanical properties (particularly Charpy toughness) of the base material and the Charpy toughness of the HAZ. The Charpy toughness of the base material of thick steel plates generally tends to decrease with increasing strength and thickness of the steel plate, and therefore, as described in Patent Documents 1 and 2, for example, techniques have been disclosed that attempt to solve this problem using controlled rolling or controlled cooling methods.

[0005] On the other hand, in the HAZ formed by high heat input welding, the grain refinement effect achieved by the various controlled rolling and cooling processes described above is lost, so it is necessary to ensure the Charpy toughness of the HAZ by adjusting the chemical composition, which is independent of the manufacturing process. One widely known countermeasure is to suppress the coarsening of austenite grains by finely dispersing TiN, which is relatively stable at high temperatures during welding, in the steel. Another example is the technology described in Patent Document 3, which disperses Ti oxide, which is stable at higher temperatures.

[0006] Patent Document 4 discloses a method for preventing a decrease in the Charpy toughness of the HAZ by adding an appropriate amount of B to steel. B is a nitride-forming element and has a fast diffusion rate, particularly in high-temperature regions, so that solute N, which has a negative effect on Charpy toughness, is fixed as a nitride during cooling after welding, thereby achieving high toughness in the HAZ. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 57-134518 [Patent Document 2] Japanese Patent Application Publication No. 59-83722 [Patent Document 3] Japanese Patent Application Publication No. 57-051243 [Patent Document 4] Japanese Patent Application Laid-Open No. 2005-2476 Summary of the Invention [Problem to be solved by the invention]

[0008] As described above, various technologies have been established to improve HAZ toughness, including controlled rolling, controlled cooling to reduce carbon equivalents, and inclusion utilization. However, in TMCP manufacturing, further improving the Charpy toughness of the HAZ while maintaining the strength at half-thickness of the thick steel plate requires reducing the C content and adding other alloying elements, but this increases alloy costs and reduces economic viability. Furthermore, with these technologies, it is difficult to achieve the desired Charpy toughness after strain aging at half-thickness of the base material, as is currently required. This is because solid solution elements become trapped at dislocations after strain aging, increasing the yield stress. As described above, conventional techniques have not yet been sufficient to produce steel plates that have excellent Charpy toughness in the HAZ that occurs during high heat input welding, as well as excellent strength at the half-thickness position of the base metal and excellent Charpy toughness after strain aging treatment. In view of the above circumstances, an object of the present invention is to provide a steel plate that has excellent Charpy toughness in the HAZ that occurs particularly when large heat input welding is performed, and that has excellent strength at the half-thickness position of the base metal and excellent Charpy toughness after strain aging treatment, and a method for manufacturing such a steel plate. Here, "excellent strength at half the plate thickness position of the base material" means that a tensile test is conducted on a test piece taken from half the plate thickness position of the steel plate in accordance with the provisions of JIS Z 2241 (2022), and the yield strength (YS) is 355 MPa or more. "Excellent Charpy toughness after strain aging treatment at half the plate thickness of the base material" means that the Charpy fracture transition temperature vTrs is -40°C or less when a Charpy test conforming to JIS Z 2242 (2023) is conducted on a test piece taken from half the plate thickness of the steel plate after strain aging treatment in which a 5 to 10% strain is applied at room temperature and then held at 200 to 250°C for 1 to 2 hours. Excellent Charpy toughness in the HAZ that occurs when large heat input welding is performed means that the steel plate is given a V-groove with a groove angle of 20° and groove gap of 10 mm, and a welding wire is used to create a joint using electrogas welding (EGW) with a current of 398 A, voltage of 44 V, welding speed of 21 mm / min, and welding heat input of 500 kJ / cm.The test piece surface is located 1 mm deep from the surface of the joint, and a notch is made at a position where the weld metal and base material are 50% each on the fusion zone (FL).A Charpy impact test is performed three times on an NK U4 impact test piece at a test temperature of -40°C, and the average absorbed energy vE-40°C is 53 J or more. [Means for solving the problem]

[0009] In order to solve the above problems, the present inventors have conducted extensive research into a steel plate that has excellent toughness (base material toughness) while ensuring the strength of the base material and that has excellent HAZ toughness in large heat input welds, and a manufacturing method for stably obtaining such a steel plate. As a result, the following findings were obtained. Specifically, they discovered that by controlling the amounts of C, Si, Mn, Al, Ti, and N in the steel sheet within a predetermined range, it is possible to effectively suppress the formation of island martensite (MA: martensite-austenite constituent) in the HAZ and prevent a decrease in toughness. Furthermore, by controlling the carbon equivalent (Ceq) to 0.300% or more, high strength can be achieved by ensuring that the area ratio of the ferrite and bainite phases (ferrite + bainite phase) is 80% or more in the steel structure at half the thickness. Here, the bainite phase refers to the bainite portion of the multi-phase structure. Furthermore, in order to achieve excellent toughness of the base material (base material toughness) in the above steel plate, it is effective to set the average grain size of ferrite and bainite surrounded by grain boundaries with a misorientation of 15° or more in the steel structure at the half-thickness position to 30 μm or less, and to set the area fraction of MA in the bainite phase to 15% or less, thereby improving toughness.

[0010] To obtain the above-mentioned toughness of the base material, it is effective to heat the steel material to 950 to 1200°C during the casting process. This refines the austenite grain size and the final average grain size of the ferrite and bainite crystal grains. Here, bainite refers to individual grains, not the entire structure. In addition, in the hot rolling process, it is effective to control the cumulative reduction rate when the temperature at the 1 / 2 position in the plate thickness is in the austenite recrystallization temperature range to 15.0% or more and the average value of the reduction rate / pass to 5.0% or more, and to control the cumulative reduction rate in the austenite non-recrystallization temperature range to 50.0% or more and the average value of the reduction rate / pass to 4.5% or more. This allows sufficient rolling stress to be applied to the half-thickness position, and the steel structure at the half-thickness position can be controlled so that the grain size of ferrite and bainite surrounded by grain boundaries with an orientation difference of 15° or more is an average of 30 μm or less. Furthermore, in the tempering step after cooling, it is effective to reheat the material to 400° C. or higher and the Ac1 point or lower, and then hold the material at that heating temperature so that the tempering parameter (TP) becomes 19,000 or lower. This allows MA, which is likely to become the starting point of brittle fracture, to be decomposed while maintaining strength, and after strain aging treatment, the amount of solute elements that become fixed to dislocations can be reduced, thereby achieving the desired toughness. Furthermore, to obtain high strength at the half-thickness position, the hot rolling process is controlled so that the rolling end temperature at the half-thickness position is Ar3 point + 100°C or higher. This control reduces the precipitation of B nitrides at the half-thickness position as much as possible, and allows solute B to remain until the accelerated cooling, improving hardenability. This makes it possible to achieve the desired strength without including a large amount of alloying elements that adversely affect HAZ toughness.

[0011] The present invention was completed based on the above findings and further investigations, and the gist of the present invention is as follows. [1] In mass%, C: 0.020~0.150%, Si: 0.02 to 0.50% Mn: 1.00~2.50%, P: 0.020% or less, S: 0.010% or less, Al: 0.010 to 0.100%, Ti: 0.005 to 0.050%, N: 0.0010 to 0.0100%, and O: 0.0100% or less and Ceq defined by formula (1) is 0.300 to 0.500%, The mass% ratio of Ti to N (Ti / N) is 2.00 or more and 5.00 or less, The balance has a composition consisting of Fe and unavoidable impurities, At the 1 / 2 plate thickness position, The total area ratio of the ferrite phase and the bainite phase is 80% or more, The average grain size of the ferrite and bainite phase crystal grains surrounded by grain boundaries with a misorientation of 15° or more is 30 μm or less, A steel plate having a steel structure in which the area ratio of island martensite in the bainite phase is 15% or less. Ceq=C+Mn / 6+Cu / 15+Ni / 15+Cr / 5+Mo / 5+V / 5...Formula (1) Here, C, Mn, Cu, Ni, Cr, Mo, and V in formula (1) represent the content (mass%) of each element, and the content of elements that are not contained is set to 0 (zero). [2] The steel sheet according to [1] above, which contains, in addition to the above-mentioned chemical composition, one or two of the following groups A and B, in mass %: Group A: One or more selected from Nb: 0.050% or less, Cu: 1.00% or less, Ni: 2.50% or less, Cr: 1.00% or less, and B: 0.0050% or less Group B: One or more selected from Mo: 0.500% or less, V: 0.500% or less, W: 0.500% or less, Co: 0.500% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM (rare earth metals): 0.0100% or less [3] The steel sheet according to the above [1] or [2], wherein AcR defined by the formula (2) is 0.01 to 1.00%. AcR=(Ca-(0.18+130×Ca)×O) / (1.25×S)...Equation (2) Here, S, O, and Ca in formula (2) represent the content (mass%) of each element, and the content of elements that are not contained is set to 0 (zero). [4] A method for producing a steel sheet according to any one of [1] to [3], A steel material having the above-described composition is heated to a heating temperature of 950 to 1200°C, Next, the rolling start temperature at the plate thickness 1 / 2 position: Ar3 point + 200 ° C or higher, When the temperature at the plate thickness 1 / 2 position is in the austenite recrystallization temperature range, the cumulative reduction rate is 15.0% or more, and the average value of the reduction rate / pass is 5.0% or more; When the temperature at the plate thickness 1 / 2 position is in the austenite non-recrystallization temperature range, the cumulative reduction rate is 50.0% or more, and the average value of the reduction rate / pass is 4.5% or more; Furthermore, rolling is carried out under the condition that the rolling end temperature at the 1 / 2 position of the plate thickness is Ar3 point + 100 ° C or higher, Next, the cooling start temperature at the plate thickness 1 / 2 position: Ar3 point + 100 ° C or more, And the average cooling rate when the temperature at the plate thickness 1 / 2 position is in the temperature range of 700 to 500 ° C: 5.0 ° C / s or more, And cooling stop temperature at the plate thickness 1 / 2 position: cooling is performed at 500 ° C or less, Then, after cooling, the mixture is reheated to a temperature of 400°C or higher and Ac1 point or lower, After maintaining the maximum temperature at 400°C or higher and Ac1 point or lower so that the TP defined by formula (3) is 10,000 or higher and 19,000 or lower, A manufacturing method of steel plate in which the plate is cooled until the temperature at half the plate thickness reaches room temperature. TP=(T+273)×(21.3-5.8×C+log(t))...Equation (3) Here, in formula (3), C represents the content (mass%), T represents the maximum temperature (°C) reached at the 1 / 2 thickness position during reheating, and t represents the holding time (h) at the maximum temperature reached. [5] The method for manufacturing a steel plate according to [4] above, wherein the average cooling rate when the temperature at the half-thickness position is in the temperature range of 700 to 600°C during the cooling is 5.5°C / s or more. [Effects of the Invention]

[0012] According to the present invention, the steel structure at the half-thickness position has an area ratio of 80% or more of the total ferrite and bainite phases, the average grain size of the ferrite and bainite crystal grains surrounded by grain boundaries with a misorientation of 15° or more is 30 μm or less, and the area ratio of MA in the bainite phase is 15% or less. This provides excellent toughness (base material toughness) even after strain aging, which has traditionally been a problem in reducing the toughness of base materials. Furthermore, by controlling the chemical composition within a constant range, excellent toughness (HAZ toughness) is achieved in the HAZ of joints after high heat input welding. Furthermore, according to the present invention, the strength of the base material at the half-thickness position is excellent by controlling the carbon equivalent (Ceq) to 0.300% or more. Here, the bainite structure is the name of a type of microstructure. Furthermore, according to the manufacturing method of the present invention, a steel sheet having the above-mentioned properties can be manufactured by optimizing the manufacturing conditions under specific component composition conditions. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described. Note that the present invention is not limited to the following embodiments. do not have. The steel sheet of the present invention has a component composition containing, in mass%, C: 0.020 to 0.150%, Si: 0.02 to 0.50%, Mn: 1.00 to 2.50%, P: 0.020% or less, S: 0.010% or less, Al: 0.010 to 0.100%, Ti: 0.005 to 0.050%, N: 0.0010 to 0.0100%, and O: 0.0100% or less, and having a Ceq defined by formula (1) of 0.300 to 0.500%, a mass% ratio of Ti to N (Ti / N) of 2.00 to 5.00, and the balance being Fe and unavoidable impurities; At the halfway point in the plate thickness, the steel structure has an area ratio of the total of the ferrite phase and the bainite phase (ferrite + bainite phase): 80% or more, the average grain size of the ferrite and bainite phase crystal grains surrounded by grain boundaries with a misorientation of 15° or more: 30 μm or less, and the area ratio of island martensite (MA) in the bainite phase is 15% or less. Ceq=C+Mn / 6+Cu / 15+Ni / 15+Cr / 5+Mo / 5+V / 5...Equation (1) Here, C, Mn, Cu, Ni, Cr, Mo, and V in formula (1) represent the content (mass%) of each element, and the content of elements that are not contained is set to 0 (zero).

[0014] First, the reasons for limiting the chemical composition of the steel sheet in the present invention will be explained. In this specification, "%" regarding chemical composition means "mass %" unless otherwise specified.

[0015] C: 0.020 to 0.150% C is an element that has the effect of increasing the hardenability of steel and is necessary to achieve the desired strength. In the present invention, in order to obtain this effect, the C content is set to 0.020% or more. On the other hand, if the C content exceeds 0.150%, the amount of MA generated, which serves as the fracture initiation point, increases, reducing the toughness of the base material. Additionally, in the weld heat-affected zone, coarsening and transformation of austenite due to high heat input welding, and the generation of MA, significantly reduces the HAZ toughness. Therefore, the C content is set to the range of 0.020 to 0.150%. The C content is preferably 0.025% or more, and more preferably 0.030% or more. The C content is preferably 0.120% or less, and more preferably 0.100% or less.

[0016] Si: 0.02 to 0.50% Silicon is an element that suppresses the formation of coarse carbides and enhances the toughness of the HAZ, and is necessary for achieving the desired HAZ toughness value. Silicon is also an essential component for ensuring the strength of the base metal and for deoxidation. In the present invention, in order to achieve the above effects, the Si content is set to 0.02% or more. On the other hand, if the Si content exceeds 0.50%, not only will the surface quality of the steel be impaired, but the toughness of the HAZ will be significantly reduced due to the generation of MA caused by high heat input welding. Therefore, the Si content is set to the range of 0.02 to 0.50%. The Si content is preferably 0.03% or more, and more preferably 0.05% or more. The Si content is preferably 0.40% or less, and more preferably 0.30% or less.

[0017] Mn: 1.00 to 2.50% Mn is an element that increases the hardenability of steel, suppresses the formation of coarse carbides, contributes to ensuring the strength of the base material, and has the effect of increasing the toughness of the HAZ, and is necessary to achieve the desired HAZ toughness value. To achieve these effects, the Mn content is set to 1.00% or more. On the other hand, if the Mn content is too high, the strength increases excessively, which reduces the toughness of the base material and also reduces the toughness of the HAZ. From these viewpoints, the Mn content is set to 2.50% or less. The Mn content is preferably 1.20% or more, and more preferably 1.40% or more. The Mn content is preferably 2.35% or less, and more preferably 2.15% or less.

[0018] P: 0.020% or less, S: 0.010% or less P and S are unavoidable impurities in steel. High contents of these elements reduce the toughness of the base material. In particular, P concentrates in MA in the HAZ structure, and by inhibiting pearlite transformation, P promotes the formation and coarsening of MA, thereby reducing HAZ toughness. To obtain good toughness in the base material and HAZ, the P content should be limited to 0.020% or less, and the S content should be limited to 0.010% or less. The P content is preferably 0.015% or less, and more preferably 0.010% or less. The S content is preferably 0.008% or less, and more preferably 0.006% or less. There are no particular lower limits for the P and S contents. However, since excessive reduction in P and S increases production costs, the P and S contents are preferably set to P: 0.001% or more and S: 0.001% or more, respectively. More preferably, P: 0.002% or more and S: 0.002% or more.

[0019] Al: 0.010 to 0.100% Al acts as a deoxidizer, reducing oxide-based inclusions and improving HAZ toughness. To achieve this effect, the Al content must be 0.010% or more. On the other hand, if the Al content exceeds 0.100%, the oxide-based inclusions increase, reducing cleanliness and HAZ toughness. For this reason, the Al content is set to a range of 0.010 to 0.100%. The lower limit of the Al content is preferably 0.020% or more, and more preferably 0.030% or more. The upper limit of the Al content is preferably 0.080% or less, and more preferably 0.060% or less.

[0020] Ti: 0.005 to 0.050% Ti is an element that precipitates as TiN during steel solidification, suppresses austenite grain coarsening in the HAZ, and acts as a ferrite transformation nucleus, thereby contributing to high HAZ toughness. To ensure the necessary amount of TiN and obtain good HAZ toughness, the Ti content must be 0.005% or more. The Ti content is preferably 0.007% or more, and more preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.050%, a large amount of TiN is generated and the TiN particles become coarse, preventing the expected effects from being achieved. This actually reduces the toughness of the HAZ. Furthermore, an increase in solute Ti excessively increases hardenability and strength, resulting in a decrease in base material toughness. Therefore, the Ti content is set to 0.050% or less. The upper limit of the Ti content is preferably 0.040%. Furthermore, from the viewpoint of improving base material toughness, the Ti content is more preferably set to 0.030% or less.

[0021] N: 0.0010 to 0.0100% Nitrogen (N) precipitates as TiN during the solidification of steel, suppressing the coarsening of austenite grains in the HAZ and acting as ferrite transformation nuclei, thereby contributing to high HAZ toughness. To ensure the necessary amount of TiN and obtain good HAZ toughness, the N content must be 0.0010% or more. On the other hand, if the N content exceeds 0.0100%, coarse nitrides are formed, which become the starting point of brittle fracture, thereby reducing HAZ toughness. Therefore, the N content is set to the range of 0.0010 to 0.0100%. The N content is preferably 0.0015% or more, more preferably 0.0020% or more. The N content is preferably 0.0085% or less, more preferably 0.0070% or less.

[0022] O: 0.0100% or less O (oxygen) is an element contained as an unavoidable impurity, but its content is specified because it is an element that should be particularly reduced. O forms oxides, which act as initiation points for brittle fracture and have adverse effects such as reducing the toughness of the base material and HAZ toughness. Therefore, the O content is limited to 0.0100% or less. The O content is preferably 0.0075% or less, and more preferably 0.0050% or less. On the other hand, the lower limit of the O content is not particularly limited and may be 0.0000%, but O is usually inevitably contained in steel as an impurity. Moreover, excessive reduction of O leads to an increase in refining costs, so from the viewpoint of cost, the O content is preferably 0.0010% or more, and more preferably 0.0015% or more.

[0023] In the present invention, each element is contained within the above range, and Ceq (carbon equivalent) (%) defined by formula (1) satisfies the following range. Ceq=C+Mn / 6+Cu / 15+Ni / 15+Cr / 5+Mo / 5+V / 5...Equation (1) Here, C, Mn, Cu, Ni, Cr, Mo, and V in formula (1) represent the content (mass%) of each element, and the content of elements that are not contained is set to 0 (zero).

[0024] Ceq: 0.300~0.500% To improve the hardenability of steel sheets and increase their strength, the value of Ceq expressed by formula (1) is adjusted to 0.300% or more. This increases the hardenability of the steel sheets, allowing the desired strength to be obtained even at the half-thickness position where the cooling rate is slower than at other positions. On the other hand, if the Ceq value exceeds 0.500%, the hardenability becomes excessive, the strength of the matrix increases excessively, and the amount of MA (island martensite), which is the starting point of brittle fracture, increases, reducing the toughness of the base material and the HAZ. Therefore, Ceq is set to 0.500% or less. The Ceq value is preferably 0.315% or more, and more preferably 0.330% or more. The Ceq value is also preferably 0.475% or less, and more preferably 0.450% or less.

[0025] Ti / N: 2.00 or more and 5.00 or less If the Ti / N ratio is less than 2.00, the amount of solute N that does not become TiN increases, reducing HAZ toughness. Therefore, the Ti / N ratio is set to 2.00 or more. The Ti / N ratio is preferably set to 2.10 or more, and more preferably set to 2.20 or more. On the other hand, if Ti / N exceeds 5.00, TiN becomes coarse and reduces HAZ toughness. Therefore, the upper limit of Ti / N is set to 5.00. Furthermore, from the viewpoint of improving HAZ toughness, Ti / N is preferably set to 4.75 or less, and more preferably set to 4.50 or less. Note that the Ti / N ratio is expressed as the content (mass%) of each element in the steel.

[0026] The basic composition of the steel sheet of the present invention contains the above, with the balance being Fe and unavoidable impurities.

[0027] In the present invention, in order to further improve the properties, in addition to the basic component composition, it is possible to contain, as necessary, one or two elements selected from the later-described group A and group B. Note that since each of the elements Nb, Cu, Ni, Cr, B, Mo, V, W, Co, Ca, Mg, and REM can be contained as necessary, the content of these elements may be 0%.

[0028] Group A: One or more selected from Nb: 0.050% or less, Cu: 1.00% or less, Ni: 2.50% or less, Cr: 1.00% or less, and B: 0.0050% or less

[0029] Nb: 0.050% or less Nb has the effect of increasing the hardenability of steel and expanding the non-recrystallization temperature range during rolling in the austenite region (hot rolling). To obtain these effects, the Nb content is preferably 0.002% or more. On the other hand, if the Nb content exceeds 0.050%, coarse NbC precipitates, which may lead to a decrease in toughness. Therefore, when Nb is contained, the Nb content is preferably 0.050% or less, more preferably 0.035% or less. The Nb content is more preferably 0.005% or more.

[0030] Cu:1.00% or less Cu is an element that improves the hardenability of steel. Cu contributes to improving the strength after rolling. In addition, Cu can be added to improve functions such as toughness, high-temperature strength, and weather resistance. To obtain the above effects of Cu, the Cu content is preferably 0.01% or more. On the other hand, if the Cu content exceeds 1.00%, it will lead to deterioration of weldability, reduction of toughness, and increase in alloy cost. Therefore, when Cu is contained, the Cu content is preferably 1.00% or less, more preferably 0.75% or less. The Cu content is more preferably 0.05% or more, and even more preferably 0.10% or more.

[0031] Ni: 2.50% or less Ni is an element that improves the hardenability of steel. Ni contributes to improving the strength after rolling. In addition, Ni can be added to improve functions such as toughness, high-temperature strength, and weather resistance. To obtain the above effects of Ni, the Ni content is preferably 0.05% or more. On the other hand, if the Ni content exceeds 2.50%, it will lead to deterioration of weldability, reduction of toughness, and increase in alloy cost. Therefore, when Ni is contained, the Ni content is set to 2.50% or less, preferably 2.00% or less. The Ni content is more preferably 0.10% or more, and further preferably 0.15% or more.

[0032] Cr:1.00% or less Cr is an element that improves the hardenability of steel. Cr contributes to improving strength after rolling. Cr can also be added to improve functions such as toughness, high-temperature strength, and weather resistance. To obtain the above effects of Cr, the Cr content is preferably 0.01% or more. On the other hand, if the Cr content exceeds 1.00%, it will lead to deterioration of weldability, reduction of toughness, and increase in alloy cost. Therefore, if Cr is contained, the Cr content should be 1.00% or less, preferably 0.75% or less. The Cr content is more preferably 0.05% or more.

[0033] B: 0.0050% or less B is an element that improves the hardenability of steel even in small amounts. However, if the B content exceeds 0.0050%, it reduces HAZ toughness. Therefore, when B is contained, the B content is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less. In order to improve the strength of the steel sheet, the B content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0008% or more.

[0034] Group B: One or more selected from Mo: 0.500% or less, V: 0.500% or less, W: 0.500% or less, Co: 0.500% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0100% or less

[0035] Mo: 0.500% or less Mo is an element that improves the hardenability of steel. Mo contributes to improving the strength after rolling. In addition, Mo can be added to improve functions such as toughness, high-temperature strength, and weather resistance. To obtain the above effects of Mo, the Mo content is preferably 0.005% or more. On the other hand, if the Mo content exceeds 0.500%, it will lead to deterioration of weldability, reduction of toughness, and increase in alloy cost. Therefore, when Mo is contained, the Mo content is set to 0.500% or less, preferably 0.300% or less. The Mo content is more preferably 0.010% or more.

[0036] V:0.500% or less V is an element that improves the strength of steel by precipitation strengthening, where it precipitates as V(CN). This effect is exerted by setting the V content to 0.005% or more. Therefore, the V content is preferably set to 0.005% or more. On the other hand, if the V content exceeds 0.500%, the HAZ toughness may decrease. Therefore, when V is contained, the V content is preferably 0.500% or less, more preferably 0.300% or less. The V content is more preferably 0.010% or more.

[0037] W: 0.500% or less W is an element that has the effect of improving the strength of the steel sheet, and in order to obtain this effect, the W content is preferably 0.005% or more. On the other hand, if the W content exceeds 0.500%, it will deteriorate the weldability and increase the alloy cost. Therefore, if W is contained, the W content is preferably 0.500% or less, more preferably 0.300% or less. The W content is more preferably 0.010% or more.

[0038] Co:0.500% or less Co is an element that has the effect of improving the strength of the steel sheet, and in order to obtain this effect, the Co content is preferably 0.005% or more. On the other hand, if the Co content exceeds 0.500%, it will deteriorate the weldability and increase the alloy cost. Therefore, if Co is contained, the Co content is preferably 0.500% or less, more preferably 0.300% or less. The Co content is more preferably 0.010% or more.

[0039] Ca:0.0100% or less Ca refines the structure of the weld heat affected zone and improves toughness. To obtain this effect, when Ca is contained, the Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, if the Ca content exceeds 0.0100%, coarse inclusions are formed, which reduces the toughness of the base material. Therefore, when Ca is contained, the Ca content is set to 0.0100% or less, and preferably 0.0075% or less.

[0040] Mg: 0.0100% or less Like Ca, Mg refines the structure of the weld heat affected zone and improves toughness. To obtain this effect, when Mg is contained, the Mg content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, if the Mg content exceeds 0.0100%, coarse inclusions are formed, which reduces the toughness of the base material. Therefore, when Mg is contained, the Mg content is set to 0.0100% or less, and preferably 0.0075% or less.

[0041] REM: 0.0100% or less Like Ca, REM refines the structure of the weld heat affected zone and improves toughness. To achieve this effect, when REM is contained, the REM content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, if the REM content exceeds 0.0100%, coarse inclusions are formed, reducing the toughness of the base material. Therefore, when REM is contained, the REM content is set to 0.0100% or less, and preferably 0.0075% or less. Here, REM refers to scandium (Sc), atomic number 21, yttrium (Y), atomic number 39, and the lanthanide elements ranging from lanthanum (La), atomic number 57, to lutetium (Lu), atomic number 71. The REM content is the total content of one or more elements selected from the above REM elements.

[0042] AcR: 0.01% or more and 1.00% or less AcR=(Ca-(0.18+130×Ca)×O) / (1.25×S)...Equation (2) Here, S, O, and Ca in formula (2) represent the content (mass%) of each element, and the content of elements that are not contained is set to 0 (zero). AcR is an index for controlling the precipitation morphology of Ca inclusions in the HAZ structure, and by properly precipitating the inclusions and using them as ferrite nuclei, the structure after transformation is refined and HAZ toughness is improved. To achieve this effect, AcR is preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, if AcR exceeds 1.00%, precipitates optimal for ferrite formation nuclei will not be precipitated, so AcR is set to 1.00% or less, and preferably 0.80% or less.

[0043] <Steel structure at 1 / 2 plate thickness> The steel structure of the present invention is a structure in which, at the half-thickness position, the total area ratio of the ferrite phase and the bainite phase (ferrite + bainite phase) is 80% or more, the average grain size of the ferrite and bainite phase crystal grains surrounded by grain boundaries with a misorientation of 15° or more is 30 μm or less, and the area ratio of island martensite (MA) in the bainite phase is 15% or less. [Area ratio of ferrite + bainite phase at 1 / 2 thickness: 80% or more] The steel sheet of the present invention must have an area ratio of ferrite + bainite phase of 80% or more at the half-thickness position. If the area ratio of ferrite + bainite phase is less than 80%, the fraction of the remaining high-hardness structure increases, which may reduce toughness. Furthermore, if the area ratio of ferrite + bainite phase is less than 80%, strength will decrease. Therefore, the area ratio of the ferrite + bainite phase at the half-thickness position is set to 80% or more. In order to further increase strength and toughness, the area ratio is preferably set to 82% or more, and more preferably 84% or more. The upper limit is not particularly limited, but the area ratio of the ferrite+bainite phase may be 100% or less, or may be 98% or less. The remainder may consist of a remaining structure. The area ratio of the remaining structure is 20% or less, preferably 18% or less, and more preferably 16% or less. There is no particular lower limit for the remaining structure, but the area ratio of the remaining structure may be 0% or more, or 2% or more.

[0044] [Average grain size of ferrite and bainite grains surrounded by grain boundaries with a misorientation of 15° or more at the half-thickness position: 30 μm or less] In the steel sheet of the present invention, the average grain size of the ferrite and bainite phase crystal grains surrounded by grain boundaries with a misorientation of 15° or more at the half-thickness position must be 30 μm or less. If this average grain size exceeds 30 μm, the fracture surface unit of the cleavage fracture surface of the crystal grains becomes larger due to the coarsening of the grain size, thereby reducing the toughness of the base material. Therefore, the average grain size of the ferrite and bainite phase crystal grains surrounded by grain boundaries with a misorientation of 15° or more at the half-thickness position is set to 30 μm or less. To further improve toughness, the above average grain size is preferably 25 μm or less, and more preferably 20 μm or less. Although there is no particular lower limit for the average particle size, from the viewpoint of rolling efficiency, the average particle size is preferably 5 μm or more, and more preferably 10 μm or more.

[0045] [Area ratio of martensite islands (MA) in the bainite phase at the 1 / 2 position in the plate thickness: 15% or less] In the steel plate of the present invention, the area fraction of MA in the bainite phase at the half-thickness position must be 15% or less. That is, when the area fraction of bainite at the half-thickness position is 100%, the area fraction of MA in the bainite phase must be 15% or less. If the area fraction of MA exceeds 15%, brittle fracture originating from MA becomes more likely to occur, reducing the toughness of the base material. Therefore, the area fraction of MA in the bainite phase at the half-thickness position is set to 15% or less. To further improve the toughness of the base material, the area fraction is preferably set to 13% or less, and more preferably set to 12% or less. On the other hand, by setting the area fraction of MA in the bainite phase at the half-thickness position to 1% or more, the effect of increasing strength due to the fine dispersion of MA, a hard phase, can be more easily obtained. Therefore, it is preferable that the area fraction of MA in the bainite phase at the half-thickness position be set to 1% or more. The above area ratio is more preferably 2% or more, and even more preferably 3% or more.

[0046] Next, a method for producing a steel sheet according to the present invention will be described.

[0047] The steel sheet of the present invention is produced by subjecting a steel material having the above-mentioned chemical composition to heating, hot rolling, cooling and tempering under the specific conditions described below. In the method for producing a steel sheet of the present invention, a steel material having the above-described composition is heated to a heating temperature of 950 to 1200°C, Next, the rolling start temperature at the plate thickness 1 / 2 position: Ar3 point + 200 ° C or higher, The cumulative reduction rate when the temperature at the plate thickness 1 / 2 position is in the austenite recrystallization temperature range is 15.0% or more, and the average value of the reduction rate / pass is 5.0% or more; When the temperature at the plate thickness 1 / 2 position is in the austenite non-recrystallization temperature range, the cumulative reduction rate is 50.0% or more, and the average value of the reduction rate / pass is 4.5% or more; Furthermore, rolling is carried out under the condition that the rolling end temperature at the 1 / 2 position of the plate thickness is Ar3 point + 100 ° C or higher, Next, the cooling start temperature at the plate thickness 1 / 2 position: Ar3 point + 100 ° C or more, And the average cooling rate when the temperature at the plate thickness 1 / 2 position is in the temperature range of 700 to 500 ° C: 5.0 ° C / s or more, And cooling stop temperature at the plate thickness 1 / 2 position: cooling is performed at 500 ° C or less, Next, after cooling, the plate is reheated to a heating temperature of 400°C or higher and Ac1 point or lower, and then held at the maximum temperature of 400°C or higher and Ac1 point or lower so that the TP defined by equation (3) is 10,000 or higher and 19,000 or lower, and then cooled (air-cooled) until the temperature at the 1 / 2 position in the plate thickness reaches room temperature. TP=(T+273)×(21.3-5.8×C+log(t))...Equation (3) Here, C in formula (3) represents the content (mass%), T is the maximum temperature (°C) reached at the 1 / 2 thickness position during reheating, and t is the holding time (h) at the maximum temperature.

[0048] Each process is explained in detail below. Unless otherwise specified, the temperature in each process refers to the temperature at the center of the thickness (half the thickness position) and the center in the width direction of the steel material and hot-rolled plate. For example, the temperature distribution in the cross section of the steel plate can be calculated by heat transfer analysis and the result corrected by the surface temperature of the steel plate.

[0049] <Heating process> First, in the heating step, the steel material having the above-mentioned composition is heated to a heating temperature of 950 to 1200°C. [Heating temperature of steel material: 950~1200℃] If the heating temperature of the steel material is less than 950°C, the heating temperature is too low, resulting in high deformation resistance and an increased load on the hot rolling mill. This makes it difficult to perform rolling in the subsequent hot rolling process. Furthermore, if the heating temperature of the steel material is less than 950°C, the additive elements will not dissolve sufficiently, resulting in a decrease in strength. Therefore, the heating temperature of the steel material is set to 950°C or higher. On the other hand, if the heating temperature of the steel material is higher than 1200°C, the austenite grains will coarsen, resulting in a decrease in toughness. Furthermore, oxidation will become more severe, increasing oxidation loss and potentially reducing yield. For these reasons, the heating temperature of the steel material is set to 1200°C or lower. The heating temperature of the steel material is preferably 1000°C or higher, more preferably 1025°C or higher. The heating temperature is preferably 1180°C or lower, more preferably 1150°C or lower.

[0050] <Hot rolling process> Next, in the hot rolling process, the rolling start temperature at the plate thickness 1 / 2 position is Ar3 point + 200 ° C. or higher, When the temperature at the plate thickness 1 / 2 position is in the austenite recrystallization temperature range, the cumulative reduction rate is 15.0% or more, and the average value of the reduction rate / pass is 5.0% or more; When the temperature at the plate thickness 1 / 2 position is in the austenite non-recrystallization temperature range, the cumulative reduction rate is 50.0% or more, and the average value of the reduction rate / pass is 4.5% or more; Furthermore, rolling is performed under the condition that the rolling end temperature at the half thickness position is Ar3+100°C or higher.

[0051] [Rolling start temperature: Ar3 point + 200℃ or higher] When hot rolling the steel material heated in the above-mentioned heating process, if the temperature at the half-thickness position at the start of hot rolling is less than Ar3 point + 200°C, recrystallization does not occur sufficiently in the hot-rolled sheet after the hot rolling process is completed. As a result, the austenite grain size does not become smaller, and toughness decreases. Therefore, the rolling start temperature is set to Ar3 point + 200°C or higher. From the viewpoint of ensuring time to perform hot rolling in the non-recrystallized region described below, the rolling start temperature is preferably Ar3 point + 225°C or higher, and more preferably Ar3 point + 250°C or higher. The upper limit of the rolling start temperature may be determined according to the heating temperature of the steel material described above. That is, the rolling start temperature is preferably 1180°C or lower, and more preferably 1150°C or lower.

[0052] The Ar3 point (°C) can be determined according to the following formula (4). Ar3 point (℃) = 910-273×C-74×Mn-57×Ni-16×Cr-9×Mo-5×Cu...Formula (4) In formula (4), each element symbol represents the content (mass %) of the element in the steel, and elements that are not contained are represented as 0 mass %.

[0053] [Cumulative reduction in the austenite recrystallization temperature range: 15.0% or more] Hot rolling is performed with a cumulative reduction of 15.0% or more when the temperature at the half thickness position is in the austenite recrystallization temperature range. If the cumulative reduction in this temperature range is less than 15.0%, the austenite grains are not sufficiently refined and toughness is not improved. The cumulative reduction in this temperature range is preferably 20.0% or more, and more preferably 25.0% or more. Although there is no particular upper limit to the cumulative rolling reduction in this temperature range, the cumulative rolling reduction in this temperature range is preferably 70.0% or less, and more preferably 65.0% or less, because the effect of austenite grain refinement described above reaches saturation. The cumulative rolling reduction here is, within a pass where the temperature at the 1 / 2 thickness position is in the austenite recrystallization temperature range, "[100 × (plate thickness before (just before) the first pass (mm) - plate thickness after (just after) the final pass) (mm))] / plate thickness before (just before) the first pass (mm)." The temperature at the half thickness position being in the austenite recrystallization temperature range means that in each pass, the temperature at the half thickness position is in the austenite recrystallization temperature range before the start of rolling (immediately before the start of rolling).

[0054] [Average reduction rate / pass in the austenite recrystallization temperature range: 5.0% or more] Hot rolling is performed with an average rolling reduction / pass value of 5.0% or more when the temperature at the center of the plate thickness (at half the plate thickness position) is in the austenite recrystallization temperature range. If the average rolling reduction / pass value in this temperature range is less than 5.0%, the austenite grain refinement is insufficient and toughness is not improved. The average rolling reduction / pass value in this temperature range is preferably 5.5% or more, and more preferably 6.0% or more. There is no particular upper limit to the average value of the rolling reduction / passes in this temperature range, but from the viewpoint of production efficiency, it is preferably 7.0% or less, and more preferably 6.5% or less. Here, the average value of the reduction rate / pass is the average value of the reduction rate in each pass within a pass in which the temperature at the 1 / 2 position in the plate thickness is in the austenite recrystallization temperature range. Regarding the reduction ratio in each pass, the reduction ratio in the Nth pass is "100 × [(plate thickness after the (N-1)th pass and before the Nth pass) (mm) - (plate thickness after the Nth pass and before the (N+1)th pass) (mm)] / (plate thickness after the (N-1)th pass and before the Nth pass) (mm)".

[0055] [Cumulative reduction in the austenite non-recrystallization temperature range: 50.0% or more] Furthermore, hot rolling is performed with a cumulative reduction of 50.0% or more when the temperature at the half-thickness position is in the austenite non-recrystallization temperature range. By setting the cumulative reduction in this temperature range to 50.0% or more, sufficient processing strain can be imparted to the austenite, increasing the number of nucleation sites during ferrite transformation and bainite transformation, thereby refining the grain size and improving the toughness at the half-thickness position. On the other hand, if the cumulative reduction rate in this temperature range is less than 50.0%, the austenite is not sufficiently elongated in the rolling direction, and the number of nucleation sites during ferrite transformation and bainite transformation decreases. As a result, coarse ferrite and bainite are formed, making it impossible to refine the grain size, and the toughness does not improve. Therefore, the cumulative rolling reduction in this temperature range is set to 50.0% or more, preferably 55.0% or more, and more preferably 60.0% or more. There is no particular upper limit to the cumulative reduction in this temperature range. From the viewpoint of not impairing the rolling efficiency, the cumulative reduction in this temperature range is preferably 75.0% or less, and more preferably 70.0% or less. The cumulative rolling reduction here is, within a pass where the temperature at the 1 / 2 thickness position is in the austenite non-recrystallization temperature range, "[100 × (plate thickness before (just before) the first pass (mm) - plate thickness after (just after) the final pass) (mm))] / plate thickness before (just before) the first pass (mm)." Furthermore, when the temperature at the half thickness position is in the austenite non-recrystallization temperature range, it is sufficient that the temperature at the half thickness position is in the austenite non-recrystallization temperature range before the start of rolling (immediately before the start of rolling) in each pass.

[0056] [Average reduction rate / pass in the austenite non-recrystallization temperature range: 4.5% or more] Hot rolling is performed with an average rolling reduction / pass value of 4.5% or more when the temperature at the center of the plate thickness (at the half-plate thickness position) is in the austenite non-recrystallization temperature range. If the average rolling reduction / pass value in this temperature range is less than 4.5%, the austenite grains will not be elongated sufficiently, resulting in insufficient grain refinement of ferrite and bainite, and toughness will not be improved. The average rolling reduction / pass value in this temperature range is preferably 4.7% or more, and more preferably 5.0% or more. There is no particular upper limit to the average value of the rolling reduction / passes in this temperature range, but from the viewpoint of production efficiency, it is preferably 7.5% or less, and more preferably 7.0% or less. Here, the average value of the reduction rate / pass is the average value of the reduction rate in each pass within a pass in which the temperature at the 1 / 2 position in the plate thickness is in the austenite non-recrystallization temperature range. Regarding the reduction ratio in each pass, the reduction ratio in the Nth pass is "100 × [(plate thickness after the (N-1)th pass and before the Nth pass) (mm) - (plate thickness after the Nth pass and before the (N+1)th pass) (mm)] / (plate thickness after the (N-1)th pass and before the Nth pass) (mm)".

[0057] [Rolling end temperature at 1 / 2 thickness position: Ar3 point + 100°C or higher] The hot rolling process must be completed when the rolling end temperature at the half-thickness position is at least Ar3 point + 100°C. If the temperature of the steel sheet during hot rolling falls below Ar3 point + 100°C, B nitrides precipitate, and the solute B necessary to improve hardenability is insufficient during accelerated cooling, resulting in a decrease in strength. Furthermore, the lower the temperature, the greater the deformation resistance, which causes problems such as a greater load on the hot rolling mill. Note that the rolling end temperature is preferably at least Ar3 point + 110°C, from the viewpoint of setting the cooling start temperature in the subsequent process to at least Ar3 point + 100°C. There is no particular upper limit to the rolling end temperature, but from the viewpoint of rolling efficiency, the rolling end temperature is preferably Ar3 point + 150°C or less, and more preferably Ar3 point + 125°C or less.

[0058] <Cooling process> Next, in the cooling process, the hot-rolled sheet that has been hot-rolled in the hot rolling process is cooled under the following conditions: a cooling start temperature at the 1 / 2 thickness position: Ar3 point + 100°C or higher; an average cooling rate when the temperature at the 1 / 2 thickness position is in the temperature range of 700 to 500°C: 5.0°C / s or higher; and a cooling stop temperature at the 1 / 2 thickness position: 500°C or lower.

[0059] [Cooling start temperature: Ar3 points +100℃ or higher] For the hot-rolled sheet obtained through the above-mentioned hot rolling process, cooling must be initiated at a temperature at the half-thickness position that is at least Ar3 point + 100°C. If the cooling start temperature is below Ar3 point + 100°C, B nitrides will precipitate, and the solute B necessary to improve hardenability will be insufficient during accelerated cooling, resulting in a decrease in strength. Therefore, the cooling start temperature is set to Ar3 point + 100°C or higher. The cooling start temperature is preferably set to Ar3 point + 110°C or higher, and more preferably to Ar3 point + 125°C or lower.

[0060] [Average cooling rate in the temperature range of 700-500°C: 5.0°C / s or more] If the average cooling rate is less than 5.0°C / s when the temperature at the 1 / 2 position in the plate thickness direction is 700 to 500°C, the hardenability will be reduced due to slow cooling, and the strength will be reduced. The temperature range that defines this average cooling rate is set to a temperature range of 700 to 500°C, from the viewpoint of obtaining an effect in which transformation of most of the austenite structure occurs and which contributes greatly to the properties of the steel sheet. That is, in the present invention, the average cooling rate when the temperature at the half thickness position is in the 700 to 500°C temperature range is set to 5.0°C / s or more. This improves hardenability and strength. The average cooling rate in this temperature range is preferably 5.5°C / s or more. The upper limit of the average cooling rate is not particularly limited, but because excessive rapid cooling increases the cooling cost and may result in an excessive increase in hardness and a decrease in toughness, the average cooling rate is preferably 20.0°C / s or less, and more preferably 10.0°C / s or less.

[0061] [Average cooling rate in the temperature range of 700-600°C: 5.5°C / s or more (optimal conditions)] When the temperature at the half thickness position is 700 to 600°C, the average cooling rate is preferably 5.5°C / s or more. If the average cooling rate in this temperature range is less than 5.5°C / s, the generation of MA in the bainite phase at the half thickness position may be insufficient, resulting in an insufficient effect of increasing strength. The above average cooling rate is preferably 6.0°C / s or more, and more preferably 6.5°C / s or more.

[0062] The average cooling rate in the temperature range below 500°C is not particularly specified because it does not have a significant effect on the formation of ferrite and bainite, but is preferably 0.5 to 1.0°C / s from the viewpoint of production efficiency.

[0063] Here, the average cooling rate from 700 to 500°C is "(700°C - 500°C) / cooling time (s) from 700°C to 500°C." The average cooling rate from 700 to 600°C is "(700°C - 600°C) / cooling time (s) from 700°C to 600°C."

[0064] [Cooling stop temperature: 500℃ or less] Cooling must be continued until the temperature at the half-thickness position reaches 500°C or less. If the cooling stop temperature exceeds 500°C, the strength will decrease due to transformation at high temperatures. The cooling stop temperature is preferably 480°C or less, and more preferably 460°C or less. On the other hand, although there is no lower limit for the cooling stop temperature, if the cooling stop temperature is too low, the flatness of the steel sheet decreases. Therefore, the cooling stop temperature is preferably 150°C or higher, and more preferably 175°C or higher.

[0065] <Tempering process> Next, after cooling, the plate is reheated to a heating temperature (reheating temperature) of 400°C or higher and Ac1 point or lower, and then held at the maximum temperature of 400°C or higher and Ac1 point or lower so that the TP defined by equation (3) is 10,000 or higher and 19,000 or lower, and then cooled (air-cooled) until the temperature at the 1 / 2 plate thickness position reaches room temperature (25°C).

[0066] [Reheating temperature: 400°C or higher and Ac1 point or lower] If the reheating temperature of the steel sheet during the tempering process is less than 400°C, the temperature is too low to obtain the effects of decomposing MA and relaxing dislocation density, and toughness is not improved. Therefore, the heating temperature of the steel sheet is set to 400°C or higher. On the other hand, if the heating temperature of the steel sheet is higher than the Ac1 point, the dislocation density decreases excessively, resulting in a decrease in strength. For this reason, the heating temperature of the steel sheet is set to the Ac1 point or lower. The heating temperature of the steel sheet is preferably 420°C or higher, and more preferably 440°C or higher. The heating temperature is preferably the Ac1 point -20°C or lower, and more preferably the Ac1 point -40°C or lower.

[0067] The Ac1 point (°C) can be determined according to the following formula (5). Ac1 point (℃) = 751.0 - 26.6 × C + 17.6 × Si - 11.6 × Mn - 23.0 × Cu - 23.0 × Ni + 24.1 × Cr + 22.5 × Mo - 39.7 × V - 5.7 × Ti + 233 × Nb - 169.0 × Al - 895.0 × B (5) In formula (5), each element symbol represents the content (mass %) of the element in the steel, and elements that are not contained are represented as 0 mass %.

[0068] [TP: 10,000 or more and 19,000 or less] If the tempering parameter (TP) is less than 10,000, the decomposition of MA, which is likely to be the initiation point of brittle fracture, is insufficient, making brittle fracture more likely, and the effect of reducing the amount of solute elements that adhere to dislocations after strain aging treatment is also reduced, resulting in a decrease in base material toughness. For these reasons, the heating holding time is determined relative to the heating temperature (maximum temperature reached) so that TP is 10,000 or higher. Note that TP is preferably 11,000 or higher, and more preferably 11,500 or higher. If the TP exceeds 19,000, the dislocation density decreases excessively, resulting in a decrease in strength. For this reason, the heating holding time is determined relative to the heating temperature (maximum temperature reached) so that the TP is 19,000 or less. The TP is preferably 18,000 or less, and more preferably 17,500 or less.

[0069] TP can be calculated according to the following formula (3). TP=(T+273)×(21.3-5.8×C+log(t))...Equation (3) Here, C in formula (3) represents the content (mass%), T is the maximum temperature (°C) reached at the 1 / 2 thickness position during reheating, and t is the holding time (h) at the maximum temperature. The maximum temperature T is set to 400° C. or higher and Ac1 point or lower, as described above. [Example]

[0070] Next, the present invention will be described in detail based on examples. Note that the following examples are preferred examples of the present invention, and the present invention is not limited to these examples.

[0071] Table 1 shows the chemical compositions of the test steels, and Table 2 shows the manufacturing conditions. Molten steels (steel symbols: A to CA) with the chemical compositions shown in Table 1 were melted in a converter and made into steel materials by continuous casting. Then, heating, hot rolling, and cooling were carried out in this order under the manufacturing conditions shown in Table 2 to manufacture steel plates (manufacturing numbers: 1 to 97) with thicknesses of 50 to 100 mm. Note that blank spaces in Table 1 indicate that no element was intentionally added, and include not only cases where no element was contained (0%), but also cases where an element was unavoidably contained, in which case the upper limit value was not exceeded.

[0072] [Table 1-1]

[0073] [Table 1-2]

[0074] [Table 2-1]

[0075] [Table 2-2]

[0076] Each of the obtained steel plates was evaluated for (1) base metal strength, (2) base metal toughness, (3) HAZ toughness, and (4) base metal microstructure by the methods described below.

[0077] (1) Strength of the base material A Φ14 JIS No. 14A test piece was taken from the obtained steel plate at a position halfway through the plate thickness so that the longitudinal axis of the test piece was perpendicular to the rolling direction and parallel to the plate width direction, and a tensile test was carried out in accordance with the provisions of JIS Z 2241 (2022) to measure the yield strength (YS). Here, a yield strength of 355 MPa or more was evaluated as high strength.

[0078] (2) Toughness of the base material The toughness of the base material was evaluated by measuring the Charpy fracture transition temperature at half the plate thickness after strain aging treatment, which involved applying a 5-10% strain at room temperature (25°C) and then holding the specimen at 200-250°C for 1-2 hours. JIS No. 4 impact test specimens were taken from the obtained steel plate at a position halfway through the plate thickness so that the longitudinal axis of the test specimen was parallel to the rolling direction, and a notch was machined parallel to the plate width direction. Charpy tests were performed in the range of 0°C to -80°C in accordance with the provisions of JIS Z 2242 (2023), and the fracture surface area at each test temperature was measured to determine the Charpy fracture surface transition temperature (vTrs). Three specimens were tested at each test temperature, and vTrs was calculated from the average brittle fracture surface area area of ​​the three specimens. Here, examples with vTrs≦−40° C. were evaluated as having excellent Charpy toughness (toughness of base material).

[0079] (3) HAZ toughness Test specimens were prepared from steel plates, with a 20° groove angle and a 10 mm groove gap. A joint was fabricated using electrogas welding (EGW) with a commercially available low-temperature steel welding wire (Kobe Steel, Ltd., DW-S460LG, wire diameter: 1.6 mm) at a current of 398 A, a voltage of 44 V, a welding speed of 21 mm / min, and a heat input of 500 kJ / cm. The specimen surface was defined as a position 1 mm deep from the surface of the resulting joint. A notch was created at the fusion zone (FL) where 50% of the weld metal and 50% of the base metal were welded. NK U4 impact test specimens were then prepared. Charpy impact tests were performed on the specimens at a test temperature of −40°C. The average absorbed energy (vE-40°C) of three specimens run under the same conditions (unit: J) was used to evaluate the HAZ toughness. Examples with a vE-40°C ≥ 53 J were evaluated as having excellent HAZ toughness.

[0080] (4) Steel structure of base material [Steel structure] The steel structure at the 1 / 2 plate thickness position was observed as follows. A sample was taken from the center of the steel plate width and at half the plate thickness so that the plane perpendicular to the rolling direction was the observation surface. The sample dimensions were 20 mm in the plate thickness direction, 10 mm parallel to the rolling direction, and 1 mm in the plate width direction. The surface perpendicular to the plate width direction of this sample was mirror-polished, and then an optical microscope photograph was taken of the metallographic structure revealed by nital etching. The photograph was taken at half the plate thickness position, with a magnification of 200x and a photographing area of ​​250 μm × 300 μm. The area ratios of the ferrite phase, bainite phase, and other remaining phases were evaluated by image analysis from the obtained photographs. Note that the area ratio of MA in the bainite phase is included in the area ratio of the bainite phase.

[0081] [Average grain size of ferrite and bainite phases] A sample was taken from the center of the steel plate width and at half the plate thickness, so that the plane perpendicular to the rolling direction served as the observation surface. The sample dimensions were 20 mm in the plate thickness direction, 10 mm parallel to the rolling direction, and 1 mm in the plate width direction. After mirror polishing the surface perpendicular to the plate width direction of this sample, EBSP analysis was performed at the half-plate thickness position under the following conditions. From the obtained crystal orientation map, the circle-equivalent diameter of the structure surrounded by high-angle grain boundaries with a misorientation of 15° or more with adjacent crystal grains was determined, and the average value of the circle-equivalent diameter in the analysis region below was used as the average effective crystal grain size (average grain size of the ferrite and bainite phase crystal grains). (EBSP conditions) Acceleration voltage: 20 kV, Probe current: 50 nA Beam diameter: 50nm Analysis area: 1mm x 1mm area at 1 / 2 the plate thickness Step size: 0.4μm

[0082] [Area fraction of martensite islands (MA) in the bainite phase] A sample was taken from the center of the steel plate width and at half the plate thickness, so that the plane perpendicular to the rolling direction was the observation surface. The sample dimensions were 20 mm in the plate thickness direction, 10 mm in the rolling direction, and 1 mm in the plate width direction. The surface perpendicular to the plate width direction of this sample was mirror-polished. Next, after performing Revera corrosion, the sample was observed using a scanning electron microscope at 1000x magnification in a field of view of 100 μm × 100 μm, and images of the structure were taken to identify the MA. The images from five fields of view were analyzed using an image analyzer to determine the area ratio of MA when the area ratio of the bainite phase was taken as 100%.

[0083] Table 3 shows the results of these tests.

[0084] [Table 3-1]

[0085] [Table 3-2]

[0086] In the case of the invention examples (production Nos. 1, 16 to 51, and 87 to 97), the chemical composition and manufacturing conditions were within the ranges of the invention, and the steel structure at the half-thickness position described above was also obtained. As a result, steel plates were obtained that had a yield strength at the half-thickness position of 355 MPa or more, a Charpy fracture appearance transition temperature at the half-thickness position after strain aging of -40°C or less, and an average Charpy absorbed energy at -40°C of 53 J or more in the HAZ of the welded joint obtained by high heat input welding with a welding heat input of 500 kJ / cm.

[0087] On the other hand, in the case of the comparative examples (production Nos. 2 to 15, 52 to 86), one or more of the component composition and manufacturing conditions were outside the range of the present invention, and therefore the target values ​​for at least one of the base material strength, base material toughness, and HAZ toughness could not be achieved.

Claims

1. In mass%, C: 0.020-0.150%, Si: 0.02-0.50%, Mn: 1.00-2.50%, P: 0.020% or less, S: 0.010% or less, Al: 0.010-0.100%, Ti: 0.005 to 0.050%, N: 0.0010 to 0.0100%, and O: 0.0100% or less and Ceq defined by formula (1) is 0.300 to 0.500%, The mass% ratio of Ti to N (Ti / N) is 2.00 or more and 5.00 or less, The balance has a composition consisting of Fe and unavoidable impurities, At the 1 / 2 plate thickness position, The total area ratio of the ferrite phase and the bainite phase is 80% or more, The average grain size of the ferrite phase and the bainite phase crystal grains surrounded by grain boundaries with a misorientation of 15° or more is 30 μm or less, A steel plate having a steel structure in which the area ratio of island martensite in the bainite phase is 15% or less. Ceq=C+Mn / 6+Cu / 15+Ni / 15+Cr / 5+Mo / 5+V / 5...Formula (1) Here, C, Mn, Cu, Ni, Cr, Mo, and V in formula (1) represent the content (mass%) of each element, and the content of elements that are not contained is set to 0 (zero).

2. The steel sheet according to claim 1, further comprising, in addition to the chemical composition, one or two of the following groups A and B, in mass %: Group A: One or more selected from Nb: 0.050% or less, Cu: 1.00% or less, Ni: 2.50% or less, Cr: 1.00% or less, and B: 0.0050% or less Group B: one or more selected from Mo: 0.500% or less, V: 0.500% or less, W: 0.500% or less, Co: 0.500% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM (rare earth metals): 0.0100% or less

3. The steel sheet according to claim 1, wherein AcR defined by formula (2) is 0.01 to 1.00%. AcR=(Ca-(0.18+130×Ca)×O) / (1.25×S)...Formula (2) Here, S, O, and Ca in formula (2) represent the content (mass %) of each element, and the content of elements that are not contained is set to 0 (zero).

4. The steel sheet according to claim 2, wherein AcR defined by formula (2) is 0.01 to 1.00%. AcR=(Ca-(0.18+130×Ca)×O) / (1.25×S)...Formula (2) Here, S, O, and Ca in formula (2) represent the content (mass %) of each element, and the content of elements that are not contained is set to 0 (zero).

5. The method for producing a steel sheet according to any one of claims 1 to 4, A steel material having the above-described composition is heated to a heating temperature of 950 to 1200°C, Next, the rolling start temperature at the plate thickness 1 / 2 position: Ar ３ point +200°C or higher, When the temperature at the plate thickness 1 / 2 position is in the austenite recrystallization temperature range, the cumulative reduction rate is 15.0% or more, and the average value of the reduction rate / pass is 5.0% or more; When the temperature at the plate thickness 1 / 2 position is in the austenite non-recrystallization temperature range, the cumulative reduction is 50.0% or more, and the average value of the reduction rate / pass is 4.5% or more; Furthermore, the rolling end temperature at the plate thickness 1 / 2 position: Ar ３ Rolling is carried out under conditions where the temperature is 100°C or higher than the rolling point, Next, the cooling start temperature at the plate thickness 1 / 2 position: Ar ３ point +100°C or higher, And the average cooling rate when the temperature at the plate thickness 1 / 2 position is in the temperature range of 700 to 500 ° C.: 5.0 ° C. / s or more, And cooling stop temperature at the plate thickness 1 / 2 position: cooling is performed at 500 ° C or less, Then, after cooling, the temperature is increased to 400°C or higher and Ac １ After reheating to a temperature below this point, The temperature is 400°C or higher and Ac is 10,000 or higher and 19,000 or lower in order to obtain the T.P. defined by the formula (3). １ After holding at the maximum temperature below the A method for manufacturing steel plate, in which the plate is cooled until the temperature at the half-thickness position reaches room temperature. T. P. =(T+273)×(21.3-5.8×C+log(t))...Formula (3) Here, C in formula (3) represents the content (mass%), T is the maximum temperature (°C) at the plate thickness 1 / 2 position during reheating, and t is the holding time (h) at the maximum temperature.

6. The method for manufacturing a steel plate according to claim 5, wherein the cooling has an average cooling rate of 5.5°C / s or more when the temperature at a plate thickness half position is in a temperature range of 700 to 600°C.

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