Steel sheet and method for manufacturing same

Abstract

Provided are: a steel sheet having excellent Charpy toughness of HAZ that occurs when heat-welding is applied, excellent strength at 1 / 2 the thickness of a base material, and excellent Charpy toughness after strain aging treatment; and a method for manufacturing same.　The steel sheet has a composition in which the amounts of C, Si, Mn, P, S, Al, Nb, Ti, B, N, and O are within predetermined ranges, Ceq defined by equation (1) is 0.350-0.450%, and the mass % ratio (Ti / N) of Ti to N is 2.00-4.50, and has a steel structure in which the area percentage of a bainite phase at the 1 / 2 position of the plate thickness is at least 80%, and the average grain size of bainite phase grains surrounded by grain boundaries having an orientation difference between adjacent grains of at least 15° is 30 μm or less. Equation (1): Ceq=C+Mn / 6+Cu / 15+Ni / 15+Cr / 5+Mo / 5+V / 5

Description

Steel plate and its manufacturing method

[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 a weld heat affected zone (hereinafter referred to as HAZ) after large heat input welding. The steel plate of the present invention can be suitably used for large structures such as ships, marine structures, cryogenic storage tanks, and architectural and civil engineering structures.

[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 in welding work, and steel materials with excellent toughness in the welds are now required, even when welding work is performed using high-heat-input welding.

[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 thick steel plate base material generally tends to decrease with increasing strength and thickness of the steel plate. Therefore, for example, Patent Documents 1 and 2 disclose techniques that attempt to solve this problem using controlled rolling or controlled cooling methods. Patent Document 3 also discloses a technique for ensuring the Charpy toughness of the base material by refining the crystalline structure of the steel plate through measures such as the application of direct quenching and tempering techniques.

[0005] On the other hand, in the HAZ formed by large heat input welding, the grain refinement effect achieved by the various controlled rolling and cooling processes described above is lost, and therefore it is necessary to ensure the Charpy toughness of the HAZ by adjusting the chemical composition, which is independent of the manufacturing process. Among these, widely known countermeasures include a technique for suppressing the coarsening of austenite grains by finely dispersing TiN, which is relatively stable in the high temperature range during welding, in the steel, and a technique for dispersing Ti oxides, which are stable at higher temperatures, as described in Patent Document 4.

[0006] Patent Document 5 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, and fixes solute N, which has a negative effect on Charpy toughness, as nitrides during cooling after welding, thereby achieving high toughness in the HAZ.

[0007] Japanese Patent Laid-Open No. 57-134518 Japanese Patent Laid-Open No. 59-83722 Japanese Patent Laid-Open No. 63-223125 Japanese Patent Laid-Open No. 57-051243 Japanese Patent Laid-Open No. 2005-2476

[0008] However, the above-mentioned technology utilizing TiN to refine austenite grains has the following problems: when high heat input welding is performed, the weld heat-affected zone is heated to the TiN melting temperature range, which causes TiN to decompose, eliminating the effect of suppressing austenite grain coarsening; and the solute Ti and N produced by the decomposition of TiN embrittle the base structure of the steel, resulting in a significant decrease in the toughness of the weld heat-affected zone. Furthermore, the technology utilizing Ti oxides also has the problem of difficulty in finely and uniformly dispersing the specified oxides in the steel sheet. Furthermore, the above-mentioned technology has difficulty in achieving desired values ​​for the strength at the half-thickness position of the base metal and the Charpy toughness at the half-thickness position after strain aging. Thus, the conventional technology has not yet been considered sufficient as a technology for producing a steel sheet that has excellent Charpy toughness in the HAZ generated during high heat input welding, as well as excellent strength at the half-thickness position of the base metal and Charpy toughness after strain aging. In view of the above circumstances, the present invention aims to provide a steel plate that exhibits excellent Charpy toughness in the HAZ formed during high heat input welding, and also exhibits excellent strength at half the plate thickness position of the base metal and excellent Charpy toughness after strain aging treatment, as well as a method for manufacturing such a steel plate. Here, "excellent Charpy toughness in the HAZ formed during high heat input welding (HAZ toughness)" refers to an average vE-40°C ≥ 53J in a welded joint fabricated using the resulting steel plate with a welding heat input of 500 kJ / cm. Furthermore, "excellent strength at half the plate thickness position of the base metal" refers to a yield strength (YS) of 460 MPa or more when a tensile test is conducted at half the plate thickness position in accordance with JIS Z 2241 (2022). Furthermore, the term "excellent Charpy toughness (base material toughness) after strain aging treatment" in a base material means that, in accordance with the provisions of JIS Z 2242 (2023), a Charpy fracture transition temperature (vTrs) at the half-thickness position after strain aging treatment is performed by applying a 5 to 10% strain at room temperature and then holding at 200 to 250°C for 1 to 2 hours is vTrs≦−40°C.

[0009] In order to solve the above-mentioned problems, the present inventors conducted extensive research into a steel plate that has excellent toughness (base material toughness) while maintaining the strength of the base material and has excellent HAZ toughness in high-heat-input welds, and a manufacturing method for reliably obtaining such a steel plate. As a result, the following findings were obtained. Specifically, the inventors discovered that by controlling the amounts of C, Si, Mn, Al, Nb, Ti, B, N, and O in the steel plate within predetermined ranges, the formation of island martensite (MA) in the HAZ can be effectively suppressed, and a decrease in toughness can be prevented. Furthermore, in order to achieve a yield strength of 460 MPa or more at the half-thickness position, it is effective to control the carbon equivalent (Ceq) to 0.350% or more, which results in the steel structure at the half-thickness position being entirely bainite, thereby improving strength and achieving high strength. Furthermore, in order to achieve excellent toughness of the base material (base material toughness) after strain aging at the half-thickness position of the steel plate, it is effective to control the steel structure at the half-thickness position so that the grain size of bainite surrounded by grain boundaries with a misorientation of 15° or more with adjacent grains is an average of 30 μm or less. This improves toughness due to the grain size refinement effect, and achieves a Charpy fracture transition temperature vTrs≦−40°C after strain aging at the half-thickness position, where the properties are lowest.

[0010] To obtain the above-described base material toughness, it is effective to heat the steel material to 950 to 1150°C in the casting process. This allows the austenite grain size to be refined, resulting in a finer average grain size of the final bainite crystal grains. Furthermore, it is effective to control the cumulative reduction rate in the hot rolling process to 15.0% or more and the average reduction rate / pass to 5.0% or more when the temperature at the half-thickness position is in the austenite recrystallization temperature range, and to control the cumulative reduction rate in the austenite non-recrystallization temperature range to 50.0% 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 bainite surrounded by grain boundaries with an orientation difference of 15° or more from adjacent grains is an average of 30 μm or less, thereby achieving excellent toughness. Furthermore, in order to obtain a yield strength of 460 MPa or more at the half-thickness position, the hot rolling process is controlled so that the rolling end temperature at the half-thickness position is (Ar point + 100)°C or more. This control reduces the precipitation of B nitrides at the half-thickness position as much as possible, allowing solute B to remain until the accelerated cooling, thereby 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%, it contains C: 0.020 to 0.080%, Si: 0.02 to 0.09%, Mn: 1.00 to 2.50%, P: 0.020% or less, S: 0.010% or less, Al: 0.010 to 0.100%, Nb: 0.005 to 0.040%, Ti: 0.010 to 0.030%, B: 0.0001 to 0.0020%, N: 0.0010 to 0.0100%, and O: 0.0100% or less, and Ceq defined by formula (1) is 0.350 to 0.450%, and the mass% ratio of Ti to N (Ti / N) is 2.00 to 4.50, a steel plate having a composition with the balance consisting of Fe and unavoidable impurities, a structure at the 1 / 2 position of the plate thickness consisting of 80% or more bainite phase in terms of area ratio and other remaining structures, and an average grain size of 30 μm or less of the bainite phase crystal grains surrounded by grain boundaries having a misorientation of 15° or more with adjacent crystal grains, a plate thickness of 50 mm or more, a yield strength of 460 MPa or more at the 1 / 2 position of the plate thickness, a Charpy fracture appearance transition temperature (vTrs) at the 1 / 2 position of the plate thickness after strain aging treatment in which 5 to 10% strain is imparted at room temperature and then held at 200 to 250°C for 1 to 2 hours, vTrs≦-40°C, and a welded joint produced with a welding heat input of 500 kJ / cm has an average Charpy absorbed energy at -40°C of 53 J or more in the HAZ of the weld. 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 the above [1], which contains, in addition to the chemical composition, one or more groups selected from the following group A, group B, and group C, in mass %.Group A: one or more selected from Cu: 1.00% or less, Ni: 2.00% or less, and Cr: 1.00% or less; Group B: one or more selected from Mo: 0.30% or less, V: 0.30% or less, W: 0.30% or less, Co: 0.30% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM (rare earth metal): 0.0200% or less; Group C: one or more selected from Sn: 0.20% or less, As: 0.10% or less, and Bi: 0.20% or less. [3] A method for producing a steel plate according to [1] or [2] above, comprising heating a steel material having the chemical composition to a heating temperature of 950 to 1150°C, and then, setting a rolling start temperature at a plate thickness 1 / 2 position: (Ar3 point + 200)°C or higher, a cumulative reduction rate when the temperature at the half thickness position is in the austenite recrystallization temperature range: 15.0% or more, and an average reduction rate per pass: 5.0% or more; a cumulative reduction rate when the temperature at the half thickness position is in the austenite non-recrystallization temperature range: 50.0% or more; and further, rolling is performed under conditions such that the rolling end temperature at the half thickness position is (Ar3 point + 100)°C or more; subsequently, cooling start temperature at the half thickness position is (Ar3 point + 100)°C or more; and an average cooling rate when the temperature at the half thickness position is in the temperature range of 700 to 500°C: 5.0°C / s or more; and cooling stop temperature at the half thickness position is 500°C or less. [4] In the cooling, the average cooling rate when the temperature at the plate thickness 1 / 2 position is in the temperature range of 700 to 600 ° C. is 5.5 ° C. / s or more. The method for manufacturing a steel plate according to [3].

[0012] According to the present invention, the steel structure is a bainite-based structure, and the average grain size of the bainite grains surrounded by grain boundaries with a misorientation of 15° or more with adjacent grains is 30 μm or less. This provides excellent toughness (base material toughness) even after strain aging, a process that has traditionally been problematic in terms of reducing the toughness of base materials. Furthermore, by controlling the chemical composition within a constant range, excellent toughness is achieved in the HAZ of a joint after high heat input welding (HAZ toughness). Furthermore, according to the present invention, excellent strength is achieved at the 1 / 2 thickness position of the base material by controlling the carbon equivalent (Ceq) to 0.350% or more. Furthermore, according to the manufacturing method of the present invention, a steel plate having the above-mentioned properties can be manufactured by optimizing the rolling conditions under specific chemical composition conditions.

[0013] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments. The steel sheet of the present invention contains, in mass%, C: 0.020 to 0.080%, Si: 0.02 to 0.09%, Mn: 1.00 to 2.50%, P: 0.020% or less, S: 0.010% or less, Al: 0.010 to 0.100%, Nb: 0.005 to 0.040%, Ti: 0.010 to 0.030%, B: 0.0001 to 0.0020%, N: 0.0010 to 0.0100%, and O: 0.0100% or less, and has a component composition in which Ceq defined by formula (1) is 0.350 to 0.450%, the mass% ratio of Ti to N (Ti / N) is 2.00 to 4.50, and the balance is Fe and unavoidable impurities, and The steel has a steel structure in which the bainite phase accounts for 80% or more and the remainder is composed of other structures, and the bainite phase crystal grains are surrounded by grain boundaries with a misorientation of 15° or more with adjacent crystal grains, with an average grain size of 30 μm or less; the plate thickness is 50 mm or more; the yield strength at 1 / 2 the plate thickness position is 460 MPa or more; the Charpy fracture transition temperature (vTrs) at 1 / 2 the plate thickness position after strain aging treatment in which 5 to 10% strain is imparted at room temperature and then held at 200 to 250°C for 1 to 2 hours is vTrs≦−40°C; and in a welded joint produced with a welding heat input of 500 kJ / cm, the Charpy absorbed energy at −40°C in the HAZ of the weld is 53 J or more on average. 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).

[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 0.080% C is an element that increases the hardenability of steel and is necessary to achieve the desired strength. In the present invention, the C content is set to 0.020% or more to achieve this effect. On the other hand, if the C content exceeds 0.080%, the amount of MA (Martensite-Austenite Constituent: Island Martensite), which serves as the fracture initiation point, increases, reducing the toughness of the base material. In addition, in the heat-affected zone (HAZ), the coarsening and transformation of austenite due to high heat input welding and the generation of MA significantly reduce the HAZ toughness. For this reason, the C content is set to the range of 0.020 to 0.080%. The lower limit of the C content is preferably 0.025% or more, and more preferably 0.030% or more. The upper limit of the C content is preferably 0.075% or less, and more preferably 0.070% or less.

[0016] Si: 0.02 to 0.09% Si is an element that suppresses the formation of coarse carbides and increases the toughness of the HAZ, and is necessary for achieving a desired HAZ toughness value. It is also an essential component for ensuring the strength of the base metal and for deoxidation. In the present invention, the Si content is set to 0.02% or more to achieve the above effects. On the other hand, if the Si content exceeds 0.09%, not only will the surface quality of the steel be impaired, but the toughness of the HAZ will be significantly reduced due to the formation of MA caused by high heat input welding. For this reason, the Si content is set to the range of 0.02 to 0.09%. The lower limit of the Si content is preferably 0.03% or more, and the upper limit is preferably 0.07% 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 for achieving a desired HAZ toughness value. To achieve this effect, 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 perspectives, the Mn content is set to 2.50% or less. The lower limit of the Mn content is preferably 1.40% or more, and the upper limit is preferably 2.30% 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. To maintain good toughness in thick steel plates with a plate thickness of 50 mm or more, the P content is limited to 0.020% or less and the S content to 0.010% or less. The P content is preferably 0.015% or less, more preferably 0.010% or less. The S content is preferably 0.007% or less, more preferably 0.004% or less. The lower limits of the P and S contents are not particularly limited. However, excessive reductions in P and S increase manufacturing costs, so the P and S contents are preferably set to P: 0.001% or more and S: 0.0005% or more, respectively. More preferably, P: 0.003% or more and S: 0.0010% or more.

[0019] Al: 0.010 to 0.100% Al acts as a deoxidizer, reducing oxide-based inclusions and improving the toughness of the HAZ. 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 the toughness of the HAZ. For this reason, the Al content is set to the range of 0.010 to 0.100%. The lower limit of the Al content is preferably 0.020% or more. The upper limit of the Al content is preferably 0.070% or less, and more preferably 0.050% or less.

[0020] Nb: 0.005 to 0.040% Nb is an element that improves the hardenability of steel and also has the effect of improving the strength and toughness of the base material. To achieve this effect, the Nb content is set to 0.005% or more. The Nb content is preferably set to 0.010% or more, and more preferably set to 0.015% or more. On the other hand, if the Nb content exceeds 0.040%, the strength increases excessively, reducing the toughness of the base material. In addition, MA is generated in the weld heat-affected zone, reducing the HAZ toughness. Therefore, the upper limit of the Nb content is set to 0.040%. From the viewpoint of improving the toughness of the HAZ, the Nb content is preferably set to 0.035% or less, and more preferably set to 0.030% or less.

[0021] Ti: 0.010 to 0.030% Ti precipitates as TiN during steel solidification, suppressing austenite coarsening in the HAZ and serving as ferrite transformation nuclei, thereby contributing to increased HAZ toughness. To ensure the necessary amount of TiN and achieve good HAZ toughness, Ti is contained in an amount of 0.010% or more. The Ti content is preferably 0.012% or more, and more preferably 0.014% or more. On the other hand, a Ti content exceeding 0.030% results in the formation of large amounts of TiN and the coarsening of TiN particles, 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, thereby reducing toughness. Therefore, the Ti content is limited to 0.030% or less. The upper limit of the Ti content is preferably 0.025%. From the viewpoint of improving toughness, the content is more preferably 0.020% or less.

[0022] B: 0.0001 to 0.0020% B is an element that significantly improves hardenability even with trace amounts. Therefore, it can improve the strength of the steel plate (base material). Furthermore, by contributing to improved hardenability in the HAZ, B suppresses the formation and growth of coarse ferrite structures. It also combines with N to form precipitates, acting as transformation nuclei and contributing to the refinement of the structure, thereby improving HAZ toughness. To achieve this effect, the B content is set to 0.0001% or more. On the other hand, if the B content exceeds 0.0020%, coarse Fe-B carbides may be formed. These coarse Fe-B carbides may act as fracture initiation sites, significantly reducing HAZ toughness. Furthermore, they may also reduce base material toughness. Therefore, the B content is set to 0.0020% or less. The B content is more preferably 0.0018% or less, even more preferably 0.0015% or less, and even more preferably 0.0012% or less.

[0023] N: 0.0010 to 0.0100% N combines with Al in the steel, adjusts the grain size during rolling, and strengthens the steel. To achieve this effect, the N content must be 0.0010% or more. Furthermore, if the N content is less than 0.0010%, the HAZ toughness decreases. On the other hand, if the N content exceeds 0.0100%, the toughness of the base material and the HAZ toughness decrease. For this reason, the N content is set to the range of 0.0010 to 0.0100%. The preferred lower limit of the N content is 0.0020% or more, and the preferred upper limit of the N content is 0.0070% or less.

[0024] 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 become the initiation point of 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 0.0050% or less, and more preferably 0.0040% 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. In addition, excessive reduction of O leads to an increase in refining costs, so from a cost perspective, the O content is preferably 0.0010% or more, and more preferably 0.0020% or more.

[0025] In the present invention, each element is contained within the above range, and the carbon equivalent (Ceq) (%) defined by formula (1) satisfies the following range: 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).

[0026] Ceq: 0.350 to 0.450% In order to improve the hardenability of the steel plate and increase its strength, the value of Ceq represented by formula (1) is adjusted to 0.350% or more. This increases the hardenability of the steel plate, and the steel structure becomes bainite throughout, even at the half-thickness position where the cooling rate is slower than at other positions, thereby achieving an improved strength. On the other hand, if the Ceq value exceeds 0.450%, the hardenability becomes excessive, the strength of the matrix structure increases excessively, and the amount of MA (island martensite), which serves as the initiation point for brittle fracture, increases, reducing the toughness of the base material and the HAZ toughness. For this reason, Ceq is set to 0.450% or less. The Ceq value is preferably 0.370% or more, and preferably 0.435% or less.

[0027] Ti / N: 2.00 or more and 4.50 or less If Ti / N is less than 2.00, the amount of solute N that does not become TiN increases, reducing HAZ toughness. Therefore, Ti / N is set to 2.00 or more. Ti / N is preferably set to 2.10 or more, and more preferably set to 2.20 or more. Furthermore, if Ti / N exceeds 4.50, TiN becomes coarse and reduces HAZ toughness. Therefore, the upper limit of Ti / N is set to 4.50. Furthermore, from the viewpoint of improving HAZ toughness, Ti / N is preferably set to 4.40 or less, and more preferably set to 4.30 or less. Note that the content (mass%) of each element in Ti / N is the content in steel of the element.

[0028] The basic composition of the steel sheet of the present invention is the above, with the balance being Fe and unavoidable impurities. The steel sheet of the present invention can obtain the desired properties by having this basic composition and adjusting the value of Ceq represented by the above formula (1) to 0.350% or more and 0.450% or less. Other unavoidable impurities include Zn, Pb, and Sb.

[0029] 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 more elements selected from the below-described group A, group B, and group C. Note that, since each of the components Cu, Ni, Cr, Mo, V, W, Co, Ca, Mg, REM, Sn, As, and Bi can be contained as necessary, these components may be 0%.

[0030] Group A: one or more selected from Cu: 1.00% or less, Ni: 2.00% or less, and Cr: 1.00% or less

[0031] Cu: 1.00% or less Cu is an element that improves the hardenability of steel. Cu contributes to improving strength after rolling. In addition, Cu can be added to improve functions such as toughness, high-temperature strength, and weather resistance. In order to obtain the above effects of Cu, the Cu content is preferably 0.01% or more. On the other hand, a Cu content exceeding 1.00% leads to deterioration of weldability, a decrease in toughness, and an increase in alloy cost. Therefore, when Cu is contained, the Cu content is 1.00% or less, and preferably 0.80% or less. The Cu content is more preferably 0.05% or more.

[0032] Ni: 2.00% or less Ni is an element that improves the hardenability of steel. Ni contributes to improving strength after rolling. In addition, Ni can be added to improve functions such as toughness, high-temperature strength, and weather resistance. In order 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 2.00%, it will lead to deterioration of weldability, reduction of toughness, and increase in alloy cost. Therefore, when Ni is contained, the Ni content is preferably 2.00% or less, and more preferably 1.80% or less. The Ni content is more preferably 0.20% or more.

[0033] 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, a Cr content exceeding 1.00% leads to deterioration of weldability, a decrease in toughness, and an increase in alloy cost. Therefore, when Cr is contained, the Cr content is preferably 1.00% or less, and more preferably 0.50% or less. The Cr content is more preferably 0.05% or more.

[0034] Group B: one or more selected from Mo: 0.30% or less, V: 0.30% or less, W: 0.30% or less, Co: 0.30% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0200% or less

[0035] Mo: 0.30% or less Mo is an element that improves the hardenability of steel. Mo contributes to improving 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 0.01% or more. On the other hand, if the Mo content exceeds 0.30%, it will deteriorate weldability, reduce toughness, and increase alloy costs. Therefore, when Mo is contained, the Mo content is preferably 0.30% or less, and more preferably 0.20% or less. The Mo content is more preferably 0.03% or more.

[0036] V: 0.30% or less V is an element that improves the strength of steel by precipitation strengthening, whereby V precipitates as V(CN). This effect is achieved by setting the V content to 0.01% or more. Therefore, the V content is preferably set to 0.01% or more. On the other hand, if the V content exceeds 0.30%, toughness may decrease. Therefore, when V is contained, the V content is set to 0.30% or less, and preferably 0.20% or less. The V content is more preferably set to 0.03% or more.

[0037] W: 0.30% or less W is an element that has the effect of improving the strength of the steel sheet. To obtain this effect, the W content is preferably 0.01% or more. On the other hand, if the W content exceeds 0.30%, it will lead to deterioration of weldability and an increase in alloy costs. Therefore, when W is contained, the W content is set to 0.30% or less, and preferably 0.20% or less. The W content is more preferably 0.03% or more.

[0038] Co: 0.30% or less Co is an element that has the effect of improving the strength of steel sheet. To obtain this effect, the Co content is preferably 0.01% or more. On the other hand, if the Co content exceeds 0.30%, it will deteriorate the weldability and increase the alloy cost. Therefore, when Co is contained, the Co content is preferably 0.30% or less, and more preferably 0.20% or less. The Co content is more preferably 0.03% or more.

[0039] Ca: 0.0100% or less Ca refines the structure of the weld heat affected zone and improves toughness. To achieve this effect, when Ca is contained, the Ca content is preferably 0.0005% or more, more preferably 0.0020% or more. On the other hand, when the Ca content exceeds 0.0100%, coarse inclusions are formed, reducing toughness. Therefore, when Ca is contained, the Ca content is preferably 0.0100% or less, more preferably 0.0050% or less.

[0040] Mg: 0.0100% or less Like Ca, Mg refines the structure of the weld heat affected zone and improves toughness. To achieve this effect, when Mg is contained, the Mg content is preferably 0.0005% or more, and more preferably 0.0020% or more. On the other hand, when the Mg content exceeds 0.0100%, coarse inclusions are formed, reducing toughness. Therefore, when Mg is contained, the Mg content is preferably 0.0100% or less, and more preferably 0.0050% or less.

[0041] REM: 0.0200% 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, more preferably 0.0015% or more. On the other hand, a REM content exceeding 0.0200% forms coarse inclusions, reducing toughness. Therefore, when REM is contained, the REM content is preferably 0.0200% or less, more preferably 0.0100% or less. Here, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanoid elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content refers to the total content of one or more elements selected from the above-mentioned REM elements.

[0042] Group C: One or more selected from Sn: 0.20% or less, As: 0.10% or less, and Bi: 0.20% or less

[0043] Sn: 0.20% or less Sn is an element that has the effect of improving the strength of steel sheet. To achieve this effect, the Sn content is preferably 0.01% or more, more preferably 0.02% or more. On the other hand, if the Sn content exceeds 0.20%, it will lead to deterioration of weldability and an increase in alloy cost. Therefore, when Sn is contained, the Sn content is set to 0.20% or less, preferably 0.10% or less.

[0044] As: 0.10% or less As refines the structure of the weld heat affected zone and improves HAZ toughness. To achieve this effect, the As content is preferably 0.001% or more, more preferably 0.002% or more. On the other hand, if the As content exceeds 0.10%, coarse inclusions are formed, reducing HAZ toughness. Therefore, when As is contained, the As content is set to 0.10% or less, preferably 0.05% or less.

[0045] Bi: 0.20% or less Bi refines the structure of the weld heat affected zone and improves HAZ toughness. To achieve this effect, the Bi content is preferably 0.001% or more, and more preferably 0.002% or more. On the other hand, if the Bi content exceeds 0.20%, coarse inclusions are formed, reducing HAZ toughness. Therefore, when Bi is contained, the Bi content is set to 0.20% or less, and preferably 0.10% or less.

[0046] <Steel structure at half-plate thickness position> In the present invention, the steel structure at half-plate thickness position is a structure containing 80% or more of a bainite phase in terms of area ratio, and the bainite phase crystal grains are surrounded by grain boundaries having an orientation difference of 15° or more with adjacent crystal grains and have an average grain size of 30 μm or less.

[0047] [Area fraction of bainite phase at half thickness position: 80% or more] The steel plate of the present invention needs to have an area fraction of bainite phase at half thickness position of 80% or more. If the area fraction of bainite phase is less than 80%, the fraction of hard structures decreases, resulting in decreased strength. Therefore, the area fraction of bainite phase at half thickness position is set to 80% or more. To further increase strength, the above area fraction is preferably 85% or more, and more preferably 90% or more. The area fraction of bainite phase may be 100%. The remainder other than the bainite phase may be ferrite, and the area fraction may be 20% or less, preferably 15% or less, and more preferably 10% or less. The remainder may be 0%.

[0048] [Average grain size of bainite phase grains surrounded by grain boundaries at half-thickness position with a misorientation of 15° or more with adjacent grains: 30 μm or less] In the steel sheet of the present invention, the average grain size of bainite phase grains surrounded by grain boundaries at half-thickness position with a misorientation of 15° or more with adjacent grains must be 30 μm or less. If this average grain size exceeds 30 μm, the grain size coarsens, resulting in a larger fracture surface unit of the cleavage fracture surface of the grain, and thus a decrease in toughness. Therefore, the average grain size of bainite phase grains surrounded by grain boundaries at half-thickness position with a misorientation of 15° or more with adjacent grains is set to 30 μm or less. To further improve toughness, the average grain size is preferably 25 μm or less, more preferably 20 μm or less. While there is no particular lower limit for the average grain size, from the viewpoint of rolling efficiency, the average grain size is preferably 5 μm or more, more preferably 10 μm or more.

[0049] <Plate Thickness> Plate Thickness: 50 mm or More The plate thickness of the steel plate of the present invention is 50 mm or more to ensure rigidity when the steel plate is used in a large steel structure. The plate thickness is preferably 60 mm or more, and more preferably 65 mm or more. Furthermore, from the viewpoint of manufacturing efficiency, the plate thickness is preferably 100 mm or less, and more preferably 90 mm or less.

[0050] <Strength> Yield strength at half thickness position: 460 MPa or more The steel plate of the present invention has a yield strength of 460 MPa or more, measured using a 14 mm Φ JIS 14A tensile test piece taken from the half thickness position. The use of the steel plate of the present invention is not particularly limited. For example, ships such as container ships and bulk carriers typically use high-strength, thick steel plate base materials that can withstand this bending stress, and steel plates with a yield strength of less than 460 MPa are not suitable for use. Therefore, the steel plate of the present invention has a yield strength of 460 MPa or more at the half thickness position. The upper limit is not particularly limited, but the yield strength at the half thickness position may be 550 MPa or less.

[0051] <Base material toughness: Charpy fracture transition temperature after strain aging treatment> vTrs at 1 / 2 thickness position: -40°C or less In order to ensure the brittle fracture resistance of the hull while maintaining workability, it is necessary to set vTrs at 1 / 2 thickness position of the steel plate after strain aging treatment to -40°C or less. If vTrs is higher than -40°C, the worked hull portion is more likely to undergo brittle fracture. Therefore, in the steel plate of the present invention, vTrs at 1 / 2 thickness position of the steel plate after strain aging treatment is set to -40°C or less. In order to further improve the brittle fracture resistance, vTrs is preferably set to -45°C or less, and more preferably vTrs is set to -50°C or less. Note that there is no particular lower limit for vTrs, but from the viewpoint of rolling efficiency, the vTrs is preferably set to -80°C or more, and more preferably -70°C or more. The vTrs can be evaluated by taking a JIS No. 4 impact test piece from the 1 / 2 position of the steel plate after strain aging treatment and carrying out a Charpy test in accordance with the provisions of JIS Z 2242 (2023).

[0052] <HAZ toughness> -40°C Charpy absorbed energy in the HAZ of a welded joint obtained by welding with a welding heat input of 500 kJ / cm: average of 53 J or more In order to ensure brittle fracture resistance in the weld heat affected zone, it is necessary to make the -40°C Charpy absorbed energy in the HAZ of a welded joint obtained by welding with a welding heat input of 500 kJ / cm an average of 53 J or more. The average -40°C Charpy absorbed energy in the HAZ is preferably 64 J or more, and more preferably 75 J or more. HAZ toughness can be evaluated by taking test specimens from joints obtained by high heat input welding.

[0053] As explained above, by adjusting the component composition and improving toughness by refining the steel structure at the 1 / 2 position of the steel plate, it is possible to obtain a steel plate having high strength at a plate thickness of 50 mm or more and excellent toughness in a joint after large heat input welding.

[0054] Next, a method for producing a steel sheet according to the present invention will be described.

[0055] The steel plate of the present invention is manufactured by heating, hot rolling, and cooling a steel material having the above-mentioned chemical composition under the specific conditions described below. In the method for manufacturing a steel plate of the present invention, the steel material having the above-mentioned chemical composition is heated to a heating temperature of 950 to 1150°C, and then subjected to rolling at a rolling start temperature at half thickness position of (Ar3 point + 200)°C or higher, a cumulative reduction rate when the temperature at half thickness position is in the austenite recrystallization temperature range of 15.0% or higher, and an average reduction rate per pass of 5.0% or higher, and a rolling start temperature when the temperature at half thickness position is in the austenite non-recrystallization temperature range of 15.0% or higher. Rolling (hot rolling) is carried out under conditions of a cumulative reduction rate of 50.0% or more and a rolling end temperature at the 1 / 2 plate thickness position of (Ar3 point + 100)°C or more, and then cooling is carried out under conditions of a cooling start temperature at the 1 / 2 plate thickness position of (Ar3 point + 100)°C or more, an average cooling rate when the temperature at the 1 / 2 plate thickness position is in the temperature range of 700 to 500°C: 5.0°C / s or more, and a cooling end temperature at the 1 / 2 plate thickness position of 500°C or less.

[0056] Each step will be described in detail below. Unless otherwise specified, the temperature in each step refers to the temperature at the thickness center (half thickness position) and width center of the steel material and the hot-rolled sheet. For example, the temperature distribution in the cross section of the steel sheet can be calculated by heat transfer analysis and the result corrected by the surface temperature of the steel sheet.

[0057] <Heating Process> First, a steel material having the above-described composition is heated to a heating temperature of 950 to 1150°C. [Heating Temperature of Steel Material: 950 to 1150°C] 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 increased load on the hot rolling mill. This makes it difficult to perform the subsequent hot rolling process. Furthermore, if the heating temperature of the steel material is less than 950°C, the additive elements will not fully dissolve, resulting in reduced 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 1150°C, austenite grains will coarsen, resulting in reduced toughness. Furthermore, oxidation will become severe, increasing oxidation loss and potentially reducing yield. For these reasons, the heating temperature of the steel material is set to 1150°C or lower. The heating temperature of the steel material is preferably 1000°C or higher, and more preferably 1030°C or higher. The heating temperature is preferably 1130°C or less, and more preferably 1100°C or less.

[0058] <Hot Rolling Step> Next, the steel material heated in the heating step is subjected to rolling (hot rolling) under conditions of a rolling start temperature at the 1 / 2 thickness position: (Ar point + 200)°C or higher, a cumulative reduction rate when the temperature at the 1 / 2 thickness position is in the austenite recrystallization temperature range: 15.0% or higher and an average rolling reduction rate per pass: 5.0% or higher, a cumulative reduction rate when the temperature at the 1 / 2 thickness position is in the austenite non-recrystallization temperature range: 50.0% or higher, and a rolling end temperature at the 1 / 2 thickness position: (Ar point + 100)°C or higher.

[0059] [Rolling start temperature: (Ar3 point + 200)°C or higher] When hot rolling the steel material heated in the heating process described above, if the temperature at the 1 / 2 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 set to (Ar3 point + 210)°C or higher, and more preferably to (Ar3 point + 220)°C or higher. Although there is no particular upper limit, the rolling start temperature is set to (Ar ３ It is preferable to set the temperature at or below the temperature point +280°C.

[0060] The Ar3 point (°C) can be determined according to the following formula (2): Ar3 point (°C) = 910 - 273 x C - 74 x Mn - 57 x Ni - 16 x Cr - 9 x Mo - 5 x Cu Formula (2) In formula (2), each element symbol represents the content (mass%) of that element in the steel, and elements that are not contained are considered to be 0 mass%.

[0061] [Cumulative Reduction in Austenite Recrystallization Temperature Region: 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 region. If the cumulative reduction in this temperature region is less than 15.0%, austenite grain refinement is insufficient, toughness is not improved, and as a result, a Charpy fracture transition temperature (vTrs) of −40° C. or less at the half thickness position after strain aging is not achieved. The cumulative reduction in this temperature region is preferably 20.0% or more, more preferably 25.0% or more. While there are no particular limitations on the upper limit of the cumulative reduction in this temperature region, the effect of austenite grain refinement saturates, so the cumulative reduction in this temperature region is preferably 70.0% or less, more preferably 65.0% or less. The cumulative reduction ratio here is, within a pass where the temperature at the half thickness position is in the austenite recrystallization temperature range, "[100 x (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)." When the temperature at the half thickness position is in the austenite recrystallization temperature range, this means that in each pass, the temperature at the half thickness position before (just before) the start of rolling is in the austenite recrystallization temperature range.

[0062] [Average rolling reduction per pass (reduction / pass) in the austenite recrystallization temperature range: 5.0% or more] Hot rolling is performed with an average rolling reduction per pass (reduction / pass) of 5.0% or more when the temperature at the plate thickness center (half-thickness position) is in the austenite recrystallization temperature range. If the average rolling reduction / pass in this temperature range is less than 5.0%, austenite grain refinement is insufficient, toughness is not improved, and as a result, a Charpy fracture transition temperature (vTrs) of −40° C. or less at the half-thickness position after strain aging is not achieved. The average rolling reduction / pass in this temperature range is preferably 5.5% or more, more preferably 6.0% or more. There is no particular upper limit to the average rolling reduction / pass 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 ratio / pass is the average value of the reduction ratio in each pass within a pass in which the temperature at the 1 / 2 position of 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)".

[0063] [Cumulative Reduction in Austenite Non-Recrystallization Temperature Region: 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 region. By using a cumulative reduction of 50.0% or more in this temperature region, sufficient working strain can be imparted to austenite, increasing the number of nucleation sites during bainite transformation, thereby refining the grain size and improving the toughness at the half-thickness position. On the other hand, if the cumulative reduction in this temperature region is less than 50.0%, the austenite does not elongate sufficiently in the rolling direction, reducing the number of nucleation sites during bainite transformation. This results in the formation of coarse bainite, preventing grain refinement. As a result, the Charpy fracture transition temperature (vTrs) of −40°C or less at the half-thickness position of the base material of the steel sheet after strain aging is not achieved. Therefore, the cumulative reduction in this temperature region is set to 50.0% or more. The cumulative reduction is preferably 55.0% or more, 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 rolling efficiency, the cumulative reduction in this temperature range is preferably 75.0% or less, more preferably 70.0% or less. Note that the cumulative reduction here is "[100 x (plate thickness before (immediately before) the first pass (mm) - plate thickness after (immediately after) the final pass) (mm))] / plate thickness before (immediately before) the first pass (mm)" in a pass where the temperature at the plate thickness half position is in the austenite non-recrystallization temperature range. In addition, when the temperature at the plate thickness half position is in the austenite non-recrystallization temperature range, it is sufficient that the temperature at the plate thickness half position is in the austenite non-recrystallization temperature range before (immediately before) the start of rolling in each pass.

[0064] [Rolling End Temperature at Half 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 (Ar3 point + 100)°C or higher. If the temperature of the steel sheet during hot rolling is lower than (Ar3 point + 100)°C, B nitrides precipitate, and the solute B necessary for improving hardenability is insufficient during accelerated cooling, resulting in a decrease in strength. Furthermore, the lower the temperature, the higher the deformation resistance, which causes problems such as increased load on the hot rolling mill. Note that the rolling end temperature is preferably (Ar3 point + 110)°C or higher, from the viewpoint of setting the cooling start temperature in the subsequent process to Ar3 point (°C) or higher. While there are no particular limitations on the upper limit of the rolling end temperature, from the viewpoint of rolling efficiency, the rolling end temperature is preferably (Ar3 point + 150)°C or lower, and more preferably (Ar3 point + 130)°C or lower.

[0065] <Cooling step> Next, the hot-rolled sheet that has been hot-rolled in the hot rolling step is cooled under the following conditions: a cooling start temperature at the half-thickness position: (Ar point + 100)°C or higher; an average cooling rate when the temperature at the half-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 half-thickness position: 500°C or lower.

[0066] [Cooling start temperature: (Ar point + 100)°C or higher] For the hot-rolled sheet obtained through the above-mentioned hot rolling process, cooling must be started at a temperature of (Ar point + 100)°C or higher at the half-thickness position. If the cooling start temperature is lower than (Ar 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 (Ar point + 100)°C or higher. The cooling start temperature is preferably set to (Ar point + 110)°C or higher, and preferably to (Ar point + 130)°C or lower.

[0067] [Average Cooling Rate in the Temperature Range of 700 to 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 half-thickness position is 700 to 500°C, a large amount of ferrite will be generated in the steel due to slow cooling, making it impossible to increase the area fraction of the bainite phase. As a result, strength will decrease. The temperature range that specifies this average cooling rate is the 700 to 500°C temperature range, from the viewpoint of obtaining an effect in which the transformation of most of the austenite structure occurs and significantly contributing 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 5.0°C / s or more. This will have the effect of increasing the fraction of bainite and improving 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 from the viewpoint of avoiding an increase in cooling costs due to excessive rapid cooling, the average cooling rate is preferably 20°C / s or less, and more preferably 10°C / s or less.

[0068] The average cooling rate outside the above temperature range of 700 to 500°C is not particularly specified because it does not have a significant effect on the formation of bainite, but from the viewpoint of production efficiency, it is preferably set to 0.5 to 1.0°C / s.

[0069] Here, the average cooling rate in the temperature range of 700 to 500°C is "(700 (°C) - 500 (°C)) / cooling time (s) from 700°C to 500°C".

[0070] [Average cooling rate in the temperature range of 700 to 600°C: 5.5°C / s or more] (Preferred condition) By setting the average cooling rate at 1 / 2 the thickness position at 700 to 600°C to 5.5°C / s or more, it is possible to suppress an increase in the amount of ferrite generated in the steel and further improve strength, even when ferrite transformation in the high temperature range is promoted by the introduction of a deformation band associated with strong compression in the non-recrystallized region. It is more preferable that the average cooling rate in this temperature range is 6.0°C / s or more.

[0071] Here, the average cooling rate in the temperature range of 700 to 600°C is "(700 (°C) - 600 (°C)) / cooling time (s) from 700°C to 600°C".

[0072] [Cooling stop temperature: 500°C or less] Cooling must be performed until the temperature at the half-thickness position reaches 500°C or less. If the cooling stop temperature exceeds 500°C, a large amount of ferrite is generated in the steel, making it impossible to increase the area ratio of the bainite phase. As a result, strength decreases. The cooling stop temperature is preferably 450°C or less, and more preferably 400°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 250°C or more, and more preferably 300°C or more.

[0073] 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.

[0074] Tables 1-1 and 1-2 show the chemical compositions of the test steels, and Tables 2-1 and 2-2 show the manufacturing conditions. Molten steels (steel symbols: A to CJ) with the chemical compositions shown in Tables 1-1 and 1-2 were melted in a converter and continuously cast into steel materials. Steel plates (production numbers: 1 to 97) with thicknesses of 50 to 100 mm were then manufactured by heating, hot rolling, and cooling, in that order, under the manufacturing conditions shown in Tables 2-1 and 2-2. Note that blank spaces in Table 1 indicate that no element was intentionally added. This includes not only cases where no element is contained (0%), but also cases where the element is inevitably contained, provided that the preferred lower limit is not exceeded. Note that, like the blank spaces, the B content of Steel BM and Steel CG marked with "-" may also refer to cases where the element is not intentionally added and is not contained (0%), or cases where the element is inevitably contained at less than the lower limit (0.0001% by mass) of the B content specified in the present invention.

[0075]

[0076]

[0077]

[0078]

[0079] Each of the obtained steel plates was evaluated for (1) strength of the base material, (2) toughness of the base material, (3) HAZ toughness, and (4) steel structure of the base material by the methods described below.

[0080] (1) Strength of base material A Φ14 JIS 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, examples with a yield strength of 460 MPa or more were evaluated as having high strength.

[0081] (2) Toughness of Base Metal Toughness was evaluated by the Charpy fracture transition temperature at 1 / 2 the thickness of the plate after strain aging. JIS No. 4 impact test specimens were taken from the 1 / 2 thickness of the obtained steel plate 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). The Charpy fracture surface ratio at each test temperature was measured, and the Charpy fracture surface transition temperature (vTrs) was calculated. Three specimens were tested per test temperature, and vTrs was calculated from the average of the brittle fracture surface ratios of the three specimens. Here, examples with vTrs ≦-40°C were evaluated as having excellent Charpy toughness (toughness of the base metal).

[0082] (3) HAZ Toughness A test specimen for joints taken from the steel plate was given a V-groove with a groove angle of 20° and a groove gap of 10 mm. A joint was then prepared using commercially available low-temperature steel welding wire by electrogas welding (EGW) at a current of 398 A, a voltage of 44 V, a welding speed of 21 mm / min, and a welding heat input of 500 kJ / cm. The test specimen surface was defined as a position 1 mm deep from the surface of the resulting joint. A notch was created at a position on the fusion line (FL) where the weld metal and base metal were 50% each. NK U4 impact test specimens were then taken. A Charpy impact test was performed on the taken impact test specimens at a test temperature of -40°C. The average absorbed energy (vE-40°C, unit: J) obtained by performing the same tests on three impact test specimens taken from the above test specimen for joints under the same conditions was defined as the HAZ toughness. Here, examples with an average vE-40°C ≥ 53J were evaluated as having excellent HAZ toughness.

[0083] (4) Steel Structure of Base Material [Steel Structure] The steel structure at the half-thickness position was observed as follows. A sample was taken from the width center and half-thickness position of the steel plate so that the plane perpendicular to the rolling direction served as the observation surface. The sample dimensions were 20 mm in the thickness direction, 10 mm in the direction parallel to the rolling direction, and 1 mm in the width direction. The surface perpendicular to the rolling direction of this sample was mirror-polished, and then an optical microscope photograph of the metal structure revealed by nital etching was taken. The photographed area was at the half-thickness position, 200x magnification, and the photographed area was 250 μm × 300 μm. The structure revealed as white chunks in the photograph was designated as ferrite, and the remainder was designated as bainite phase. The area ratio of the bainite phase was evaluated by calculating the area ratio through image analysis. [Average grain size of bainite phase crystal grains] A sample was taken from the width center and half-thickness position of the steel plate so that the plane perpendicular to the rolling direction served as the observation surface. The sample dimensions were 20 mm in the thickness direction, 10 mm in the direction parallel to the rolling direction, and 1 mm in the direction perpendicular to the rolling direction. After mirror polishing the surface of this sample perpendicular to the rolling direction, EBSP analysis was performed at the 1 / 2 position of the sheet thickness under the following conditions. From the obtained crystal orientation map, the circle-equivalent diameter of the structure surrounded by high-angle grain boundaries with an orientation difference of 15° or more with adjacent crystal grains was determined, and the average value of the circle-equivalent diameter in the following analysis region was taken as the average effective crystal grain size. (EBSP conditions) Acceleration voltage: 20 KV, probe current: 50 nA, beam diameter: 50 nm, analysis region: 1 mm x 1 mm region at the 1 / 2 position of the sheet thickness, step size: 0.4 μm

[0084] The test results are shown in Tables 3-1 and 3-2.

[0085]

[0086]

[0087] In the case of the invention examples (Production Nos. 1, 12 to 54, 96, and 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 of 460 MPa or more at the half-thickness position, a Charpy fracture appearance transition temperature of -40°C or less at the half-thickness position after strain aging, and an average Charpy absorbed energy of 53 J or more at -40°C in the HAZ of the welded joint obtained by high heat input welding with a welding heat input of 500 kJ / cm.

[0088] On the other hand, in the case of the comparative examples (Production Nos. 2 to 11, 55 to 95), one or more of the component composition and production conditions were outside the range of the present invention, and the desired component composition and / or the desired steel structure at the half-thickness position could not be obtained, and the target values ​​could not be achieved for at least one of the strength of the base material, the toughness of the base material, and the HAZ toughness.

Claims

1. The steel contains, by mass%, C: 0.020-0.080%, Si: 0.02-0.09%, Mn: 1.00-2.50%, P: 0.020% or less, S: 0.010% or less, Al: 0.010-0.100%, Nb: 0.005-0.040%, Ti: 0.010-0.030%, B: 0.0001-0.0020%, N: 0.0010-0.0100%, and O: 0.0100% or less, and the Ceq defined by formula (1) is 0.350-0.450%, and the mass percentage ratio of Ti to N (Ti / N) is 2.00 or more and 4.50 or less, a steel plate having a composition with the balance consisting of Fe and unavoidable impurities, a structure consisting of bainite phase: 80% or more in terms of area ratio and other remaining structures at the 1 / 2 sheet thickness position, and an average grain size of bainite phase crystal grains: 30 μm or less, surrounded by grain boundaries having an orientation difference of 15° or more with adjacent crystal grains, a sheet thickness of 50 mm or more, a yield strength at the 1 / 2 sheet thickness position of 460 MPa or more, a Charpy fracture transition temperature (vTrs) at the 1 / 2 sheet thickness position after strain aging treatment in which 5-10% strain is applied at room temperature and then held at 200-250°C for 1-2 hours is vTrs≦-40°C, and a welded joint produced with a welding heat input of 500 kJ / cm has an average Charpy absorbed energy at -40°C in the HAZ of the weld of 53 J or more. 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 an element that is not contained is 0 (zero).

2. The steel sheet according to claim 1, further comprising, in addition to the above-mentioned chemical composition, one or more of the following groups A, B, and C, in terms of mass%, Group A: one or more selected from Cu: 1.00% or less, Ni: 2.00% or less, and Cr: 1.00% or less Group B: one or more selected from Mo: 0.30% or less, V: 0.30% or less, W: 0.30% or less, Co: 0.30% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM (rare earth metal): 0.0200% or less Group C: one or more selected from Sn: 0.20% or less, As: 0.10% or less, and Bi: 0.20% or less 3. A method for manufacturing a steel plate according to claim 1 or 2, comprising heating a steel material having the above-mentioned chemical composition to a heating temperature of 950 to 1150°C, followed by rolling under conditions such that: the rolling start temperature at the 1 / 2 thickness position is (Ar3 point + 200)°C or higher; the cumulative reduction when the temperature at the 1 / 2 thickness position is in the austenite recrystallization temperature range is 15.0% or higher and the average rolling reduction per pass is 5.0% or higher; the cumulative reduction when the temperature at the 1 / 2 thickness position is in the austenite non-recrystallization temperature range is 50.0% or higher; and the rolling end temperature at the 1 / 2 thickness position is (Ar3 point + 100)°C or higher; and and an average cooling rate when the temperature at the 1 / 2 position of the plate thickness is in the temperature range of 700 to 500°C: 5.0°C / s or more, and a cooling stop temperature at the 1 / 2 position of the plate thickness: 500°C or less.

4. A method for manufacturing a steel plate according to claim 3, further comprising the step of: in the cooling step, the average cooling rate when the temperature at the 1 / 2 position of the plate thickness is in the temperature range of 700 to 600°C is 5.5°C / s or more.

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