Steel plate and method for producing the 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

Technical Field

[0001] The present invention relates to a steel plate, particularly a steel plate applicable to high-heat-input welding, and a method for manufacturing the same. Specifically, the present invention relates to a steel plate having excellent toughness in the heat-affected zone (hereinafter referred to as HAZ) after high-heat-input welding. Further, the steel plate of the present invention can be suitably used for large structures such as ships, offshore structures, low-temperature storage tanks, and building and civil structures.

Background Art

[0002] Steel structures used in fields such as ships, offshore structures, buildings, and steel pipes are generally finished into structures of a desired shape by welding. Therefore, from the viewpoint of ensuring safety, in addition to ensuring the strength and toughness of the steel materials used, it is required that the toughness of the welded parts is also excellent.

[0003] Furthermore, in recent years, ships and steel structures have been increasingly enlarged, and the steel materials used have been actively advanced in terms of high strength and increased thickness. Along with this, high-efficiency and high-heat-input welding methods such as submerged arc welding, electrogas welding, and electroslag welding have been applied to welding construction. Even when welding construction is performed by high-heat-input welding, steel materials with excellent toughness in the welded parts are required.

[0004] However, in the above-described steel materials, particularly high-strength steel plates or thick steel plates, cases where it is difficult to achieve both the mechanical properties (particularly Charpy toughness) of the base metal and the Charpy toughness of the HAZ are often found. Since the Charpy toughness of the thick steel plate base metal generally tends to decrease with an increase in the strength and thickness of the steel plate, for example, as described in Patent Document 1 and Patent Document 2, techniques for solving the problem by controlled rolling or controlled cooling methods are disclosed. Further, Patent Document 3 discloses a technique for ensuring the Charpy toughness of the base metal by refining the crystal structure of the steel plate by applying measures such as direct quenching-tempering technology.

[0005] On the other hand, in the HAZ formed by large heat input welding, the grain refinement effect by the various controlled rolling and cooling processes described above disappears. Therefore, it is necessary to ensure the Charpy toughness of the HAZ by adjusting the chemical composition without depending on the manufacturing process. Among others, as a widely known countermeasure, there are disclosed techniques such as suppressing the coarsening of austenite grains by finely dispersing relatively stable TiN in steel in the high-temperature region during welding, and techniques such as dispersing more stable Ti oxides at higher temperatures as described in Patent Document 4.

[0006] Patent Document 5 discloses a method of 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 particularly has a high diffusion rate in the high-temperature region. It can fix the dissolved N that has an adverse effect on the Charpy toughness as a nitride during cooling during welding, and thus achieve high toughness of the HAZ.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, in the above technology that utilizes TiN for the refinement of austenite, when performing high heat input welding, since the heat affected zone of the weld is heated up to the melting temperature range of TiN, TiN decomposes and the effect of suppressing austenite grain coarsening disappears, or the base structure of the steel becomes embrittled due to the dissolved Ti and dissolved N generated by the decomposition of TiN, resulting in a significant decrease in the toughness of the heat affected zone of the weld. Also, the technology that utilizes titanium oxides has the problem that it is difficult to finely and uniformly disperse a predetermined oxide in the steel sheet. Further, in the above technology, it is difficult to achieve desired values for the strength at the 1 / 2 thickness position of the base material and the Charpy toughness at the 1 / 2 thickness position after strain aging treatment. Thus, in the conventional technology, as a technology for steel sheets that are excellent in the Charpy toughness of the HAZ generated when performing high heat input welding and are also excellent in the strength at the 1 / 2 thickness position of the base material and the Charpy toughness after strain aging treatment, it cannot be said to be sufficient yet. Therefore, in view of the above situation, the present invention particularly aims to provide a steel sheet that is excellent in the Charpy toughness of the HAZ (HAZ toughness) generated when performing high heat input welding and is also excellent in the strength at the 1 / 2 thickness position of the base material and the Charpy toughness after strain aging treatment, and a method for manufacturing such a steel sheet. Here, being excellent in the Charpy toughness (HAZ toughness) of the HAZ generated when performing high heat input welding means that, on average, vE - 40°C ≥ 53 J in a welded joint fabricated using the obtained steel sheet with a welding heat input of 500 kJ / cm. Also, being excellent in the strength at the 1 / 2 thickness position of the base material means that a tensile test is conducted in accordance with the provisions of JIS Z 2241(2022) at the 1 / 2 thickness position, and the yield strength (YS) is 460 MPa or more. Also, being excellent in the Charpy toughness (base material toughness) after strain aging treatment in the base material means that, in accordance with the provisions of JIS Z 2242(2023), after applying a strain of 5 - 10% at room temperature and then performing a strain aging treatment of holding at 200 - 250°C for 1 - 2 hours, the Charpy fracture transition temperature (vTrs) at the 1 / 2 thickness position is vTrs ≤ - 40°C.

Means for Solving the Problems

[0009] In order to solve the above problems, the present inventors have intensively studied a steel sheet having excellent toughness (base metal toughness) while ensuring the strength of the base metal and excellent HAZ toughness of the high heat input welded joint, and a manufacturing method for stably obtaining the steel sheet. As a result, the following findings were obtained. Specifically, it was found that by controlling the amounts of C, Si, Mn, Al, Nb, Ti, B, N, and O in the steel sheet to within a predetermined range, the formation of island-shaped martensite (MA) in the HAZ can be effectively suppressed, and a decrease in toughness can be prevented. Also, in order to achieve a yield strength of 460 MPa or more at the 1 / 2 thickness position of the steel sheet, it is effective to control the carbon equivalent (Ceq) to 0.350% or more. By making the steel structure at the 1 / 2 thickness position entirely bainite in this way, the strength is improved and high strength is achieved. Also, in order to achieve excellent toughness (base metal toughness) of the base metal after strain aging treatment at the 1 / 2 thickness position of the above steel sheet, it is effective to control the crystal grain size of bainite surrounded by grain boundaries with an orientation difference of 15° or more from adjacent crystal grains to an average of 30 μm or less for the steel structure at the 1 / 2 thickness position. As a result, the toughness is improved due to the grain size refinement effect, and the Charpy fracture transition temperature after strain aging treatment at the 1 / 2 thickness position where the characteristics are the lowest achieves vTrs ≦ -40°C.

[0010] And, in order to obtain the above-mentioned base metal toughness, it is effective to heat the steel material at 950 to 1150 °C in the casting process. As a result, the austenite grain size can be refined and the average grain size of the final bainite crystal grains can be made fine. Further, in the hot rolling process, when the temperature at the 1 / 2 plate thickness position is in the austenite recrystallization temperature range, the cumulative reduction ratio: 15.0% or more and the average value of the reduction ratio per pass: 5.0% or more, and it is effective to control the cumulative reduction ratio in the austenite non-recrystallization temperature range to 50.0% or more. Thereby, sufficient rolling stress can be applied to the 1 / 2 plate thickness position, and for the steel structure at the 1 / 2 plate thickness position, the crystal grain size of bainite surrounded by grain boundaries with an orientation difference of 15° or more from adjacent crystal grains can be controlled to be 30 μm or less on average, achieving excellent toughness. Also, in order to obtain a yield strength of 460 MPa or more at the 1 / 2 plate thickness position, in the hot rolling process, it is controlled so that the final rolling temperature at the 1 / 2 plate thickness position is (Ar3 point + 100) °C or more. By this control, the precipitation of B nitride at the 1 / 2 plate thickness position can be reduced as much as possible, and the solution B can be left until accelerated cooling to enhance the hardenability. Thereby, the desired strength can be achieved without containing a large amount of alloying elements that have an adverse effect on HAZ toughness.

[0011] The present invention has been completed by further study based on the above findings, and the gist of the present invention is as follows. [1] By 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 containing, and Ceq defined by formula (1) being 0.350 to 0.450%, the mass% ratio of Ti to N (Ti / N) being 2.00 or more and 4.50 or less, having a component composition in which the balance consists of Fe and unavoidable impurities, at the 1 / 2 thickness position, in terms of area ratio, consisting of a bainite phase: 80% or more and the balance of other tissues, and having an average grain size of bainite phase crystal grains surrounded by grain boundaries with an orientation difference of 15° or more from adjacent crystal grains: 30 μm or less, the plate thickness being 50 mm or more, the yield strength at the 1 / 2 thickness position being 460 MPa or more, after applying a strain aging treatment of holding at 200 to 250 °C for 1 to 2 hours after applying a 5 to 10% strain at room temperature, the Charpy fracture transition temperature (vTrs) at the 1 / 2 thickness position is vTrs ≤ -40 °C, a steel plate having a -40 °C Charpy absorbed energy of an average of 53 J or more in the HAZ of the welded part in a welded joint fabricated with a welding heat input of 500 kJ / cm. 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 (% by mass) of each element, and for elements not contained, the content is taken as 0 (zero). [2] In addition to the above component composition, containing, in terms of mass%, one group or two or more groups selected from the following Group A, Group B, and Group C, the steel plate according to [1] above. Group A: One or more selected from Cu: 1.00% or less, Ni: 2.00% or less, 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, Bi: 0.20% or less [3] The method for manufacturing a steel sheet according to [1] or [2], wherein a steel material having the above composition is heated to a heating temperature of 950 to 1150 °C, then, the rolling start temperature at the 1 / 2 thickness position of the plate: (Ar3 point + 200) °C or higher, when the temperature at the 1 / 2 thickness position of the plate is in the austenite recrystallization temperature range, the cumulative reduction ratio: 15.0% or higher, and the average reduction ratio per pass: 5.0% or higher, when the temperature at the 1 / 2 thickness position of the plate is in the austenite non-recrystallization temperature range, the cumulative reduction ratio: 50.0% or higher, further, rolling is carried out under the condition that the rolling end temperature at the 1 / 2 thickness position of the plate: (Ar3 point + 100) °C or higher, then, the cooling start temperature at the 1 / 2 thickness position of the plate: (Ar3 point + 100) °C or higher, and when the temperature at the 1 / 2 thickness position of the plate is in the temperature range of 700 to 500 °C, the average cooling rate: 5.0 °C / s or higher, and cooling is performed under the condition that the cooling stop temperature at the 1 / 2 thickness position of the plate: 500 °C or lower, a method for manufacturing a steel sheet. [4] In the above cooling, further, when the temperature at the 1 / 2 thickness position of the plate is in the temperature range of 700 to 600 °C, the average cooling rate: 5.5 °C / s or higher, the method for manufacturing a steel sheet according to [3] above.

Advantages of the Invention

[0012] According to the present invention, the steel structure is mainly composed of bainite, and the average grain size of the bainite crystal grains surrounded by grain boundaries with an orientation difference of 15° or more from adjacent crystal grains is 30 μm or less. Thereby, excellent toughness (base metal toughness) is obtained even after the strain aging treatment, which has been a problem of the reduction of the toughness of the conventional base metal. Further, by controlling the component composition within a certain range, excellent toughness (HAZ toughness) is obtained in the HAZ of the joint after large heat input welding. Further, according to the present invention, by controlling the carbon equivalent (Ceq) to 0.350% or more, etc., the strength at the 1 / 2 thickness position of the base metal is excellent. Further, according to the manufacturing method of the present invention, a steel sheet having the above characteristics can be manufactured by optimizing the rolling conditions under specific component composition conditions.

Embodiments for Carrying Out the Invention

[0013] Hereinafter, embodiments of the present invention will be described. It should be noted that the present invention is not limited to the following embodiments. The steel plate 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 Ceq defined by formula (1) of 0.350 to 0.450%, a mass % ratio of Ti to N (Ti / N) of 2.00 or more and 4.50 or less, and the balance consists of Fe and inevitable impurities. At the 1 / 2 plate thickness position, in terms of area ratio, it has a structure composed of a bainite phase: 80% or more and the remaining other tissues, and the average grain size of the bainite phase grains surrounded by grain boundaries with an orientation difference of 15° or more from adjacent grains: 30 μm or less. It has a steel structure, the plate thickness is 50 mm or more, the yield strength at the 1 / 2 plate thickness position is 460 MPa or more, and after applying a strain aging treatment of holding at 200 to 250 °C for 1 to 2 hours after applying a 5 to 10% strain at room temperature, the Charpy fracture transition temperature (vTrs) at the 1 / 2 plate thickness position is vTrs ≦ -40 °C, and in a welded joint prepared with a welding heat input of 500 kJ / cm, the -40 °C Charpy absorption energy in the HAZ of the welded part 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 (% by mass) of each element, and for elements not contained, the content is taken as 0 (zero).

[0014] First, the reasons for limiting the component composition of the steel plate in the present invention will be explained. In this specification, “%” regarding the component composition means “% by mass” unless otherwise specified.

[0015] C: 0.020 - 0.080% 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 the above effect, the C content is set to 0.020% or more. On the other hand, when the C content exceeds 0.080%, the amount of MA (Martensite - Austenite constituent: island - shaped martensite) that becomes the fracture origin increases, and the toughness of the base material decreases. In addition, in the heat - affected zone of welding, due to high - heat - input welding, austenite coarsens and transforms, or MA is generated, resulting in a significant decrease in HAZ toughness. Therefore, the C content is in the range of 0.020 - 0.080%. Regarding the lower limit, the preferred C content is 0.025% or more, and more preferably 0.030% or more. Regarding the upper limit, the preferred C content is 0.075% or less, and more preferably 0.070% or less.

[0016] Si: 0.02 - 0.09% Si is an element that has the effect of suppressing the formation of coarse carbides and increasing the toughness of the HAZ, and is necessary to achieve the desired HAZ toughness value. It is also a component necessary for ensuring the strength of the base material and deoxidation. In the present invention, in order to obtain the above effect, the Si content is set to 0.02% or more. On the other hand, when the Si content exceeds 0.09%, not only the surface properties of the steel are impaired, but also MA is generated due to high - heat - input welding, resulting in a significant decrease in the toughness of the HAZ. Therefore, the Si content is in the range of 0.02 - 0.09%. Regarding the lower limit, the preferred Si content is 0.03% or more, and regarding the upper limit, the preferred Si content is 0.07% or less.

[0017] Mn: 1.00 - 2.50% Mn is an element that has the effect of increasing the hardenability of steel, suppressing the formation of coarse carbides, contributing to ensuring the strength of the base material, and increasing the toughness of the HAZ, and is necessary to achieve the desired HAZ toughness value. In order to obtain the above effect, the Mn content is set to 1.00% or more. On the one hand, when the Mn content is high, the strength increases excessively, which not only reduces the toughness of the base metal but also reduces the toughness of the HAZ. From these perspectives, the Mn content should be 2.50% or less. Regarding the lower limit, the preferred Mn content is 1.40% or more, and regarding the upper limit, the preferred Mn content is 2.30% or less.

[0018] P: 0.020% or less, S: 0.010% or less P and S are inevitable impurities in steel. When their contents increase, the toughness of the base metal decreases. In thick steel plates with a thickness of 50 mm or more, in order to maintain good toughness, the P content should be suppressed to 0.020% or less and the S content should be suppressed to 0.010% or less. Regarding the P content, 0.015% or less is preferred, and 0.010% or less is more preferred. Regarding the S content, 0.007% or less is preferred, and 0.004% or less is more preferred. Note that the lower limits of the P content and the S content are not particularly limited. However, excessive reduction of P and S leads to an increase in manufacturing costs. Therefore, the P content and the S content are preferably 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 - 0.100% Al acts as a deoxidizer and has the effect of reducing oxide-based inclusions and improving the toughness of the HAZ. To obtain this effect, it is necessary to set the Al content to 0.010% or more. On the other hand, when the Al content exceeds 0.100%, conversely, the oxide-based inclusions increase, the cleanliness decreases, and the toughness of the HAZ decreases. Therefore, the Al content should be in the range of 0.010 - 0.100%. Regarding the lower limit, the preferred Al content is 0.020% or more. Regarding the upper limit, the preferred Al content is 0.070% or less, and more preferably 0.050% or less.

[0020] Nb: 0.005 - 0.040% Nb is an element that enhances the hardenability of steel and also has the effect of improving the strength and toughness of the base metal. To obtain the above effects, the Nb content is set to 0.005% or more. Incidentally, the Nb content is preferably 0.010% or more, and more preferably 0.015% or more. On the other hand, when the Nb content exceeds 0.040%, the strength increases excessively, so in addition to the decrease in the toughness of the base metal, MA is generated in the heat-affected zone of welding, reducing the HAZ toughness. Therefore, the upper limit of the Nb content is 0.040%. From the perspective of improving the toughness of the HAZ, the Nb content is preferably 0.035% or less, and more preferably 0.030% or less.

[0021] Ti: 0.010 - 0.030% Ti is an element that contributes to the toughening of the HAZ by precipitating as TiN during the solidification of steel, suppressing the coarsening of austenite in the HAZ, and serving as a ferrite transformation nucleus. In order to ensure a necessary amount of TiN and obtain good HAZ toughness, Ti is contained at 0.010% or more. Incidentally, the Ti content is more preferably 0.012% or more, and even more preferably 0.014% or more. On the other hand, when Ti is contained in an amount exceeding 0.030%, problems such as excessive generation of TiN and coarsening of TiN particles occur, and the expected effect cannot be obtained. Therefore, instead, the toughness of the HAZ decreases. Also, as the solid solution Ti increases, the hardenability increases excessively and the strength increases excessively, resulting in a decrease in toughness. Therefore, the Ti content is set to 0.030% or less. Incidentally, the upper limit of the Ti content is preferably 0.025%. Also, from the perspective of improving toughness, it is more preferably 0.020% or less.

[0022] B: 0.0001 - 0.0020% B is an element that has the effect of significantly improving hardenability even with a small addition. Therefore, the strength of the steel plate (base material) can be improved. Also, by contributing to the improvement of hardenability in the HAZ, it suppresses the formation and growth of coarse ferrite grains, and acts as a transformation nucleus by combining with N to form precipitates, thereby contributing to the refinement of the structure and improving the HAZ toughness. To obtain the above effects, the B content is set to 0.0001% or more. On the other hand, when the B content exceeds 0.0020%, there is a risk of forming coarse Fe-B-based carbides. Such coarse Fe-B-based carbides serve as initiation points for fracture, significantly reducing the HAZ toughness. Also, the toughness of the base material may decrease. Therefore, the B content is set to 0.0020% or less. Note that the B content is more preferably 0.0018% or less, still more preferably 0.0015% or less, and even more preferably 0.0012% or less.

[0023] N: 0.0010 - 0.0100% N combines with Al in the steel, adjusts the crystal grain size during rolling, and strengthens the steel. To obtain this effect, the N content needs to be 0.0010% or more. Also, when the N content is less than 0.0010%, the HAZ toughness decreases. On the other hand, when the N content exceeds 0.0100%, the toughness of the base material and the HAZ toughness decrease. Therefore, the N content is in the range of 0.0010 - 0.0100%. Note that the preferred N content for the lower limit is 0.0020% or more, and the preferred N content for the upper limit is 0.0070% or less.

[0024] O: 0.0100% or less O (oxygen) is an element contained as an inevitable impurity, but since it is an element that should be particularly reduced, its content is specified. O forms oxides, serves as the initiation point for brittle fracture, and has an adverse effect of reducing the toughness of the base material and the 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 one hand, the lower limit of the O content is not particularly limited and may be 0.0000%, but usually, O is inevitably contained in the steel as an impurity. Also, excessively reducing O will lead to an increase in refining costs. From the perspective of cost, it is preferable that the O content is 0.0010% or more, and more preferably 0.0020% or more.

[0025] In the present invention, each element is within the above range, and is contained so that the 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 ··· formula (1) Here, C, Mn, Cu, Ni, Cr, Mo, and V in formula (1) represent the content (% by mass) of each element, and for elements not contained, the content is set to 0 (zero).

[0026] Ceq: 0.350 - 0.450% In order to improve the hardenability of the steel plate and increase the strength, the value of Ceq represented by formula (1) is adjusted to 0.350% or more. As a result, the hardenability of the steel plate increases, and even at the 1 / 2 plate thickness position where the cooling rate is lower than that of other positions, the steel structure becomes entirely bainite, and the strength improvement effect can be obtained. On the other hand, when the value of Ceq exceeds 0.450%, the hardenability becomes excessive, the strength of the base structure rises excessively, and the amount of MA (island martensite), which is the starting point of brittle fracture, increases, reducing the base metal toughness and HAZ toughness. Therefore, Ceq is 0.450% or less. The value of Ceq is preferably 0.370% or more and preferably 0.435% or less.

[0027] Ti / N: 2.00 or more and 4.50 or less When Ti / N is less than 2.00, the amount of dissolved N that does not form TiN increases, reducing the toughness of the HAZ. Therefore, Ti / N should be 2.00 or more. Preferably, Ti / N is 2.10 or more, and more preferably 2.20 or more. Also, when Ti / N exceeds 4.50, TiN coarsens, reducing the toughness of the HAZ. Therefore, the upper limit of Ti / N is 4.50. Also, from the perspective of improving HAZ toughness, preferably Ti / N is 4.40 or less, and more preferably 4.30 or less. Note that for Ti / N, each element is the content (mass%) in the steel.

[0028] The basic component composition of the steel sheet of the present invention contains the above and the balance consists of Fe and inevitable impurities. By having this basic component composition and setting the value of Ceq represented by the above formula (1) to 0.350% or more and 0.450% or less, the steel sheet of the present invention can obtain the desired properties. In addition, examples of inevitable 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 group or two or more groups selected from the following Group A, Group B, and Group C. Note that each component of Cu, Ni, Cr, Mo, V, W, Co, Ca, Mg, REM, Sn, As, and Bi can be contained as necessary, so 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 increases the hardenability of steel. Cu contributes to improving the strength after rolling. At the same time, Cu can be contained to improve functions such as toughness, high-temperature strength, or weather resistance. In order to obtain the above effects of Cu, the Cu content is preferably 0.01% or more. On the one hand, when the Cu content exceeds 1.00%, it will cause deterioration of weldability, decrease in toughness, and increase in alloy cost. Therefore, when containing Cu, the Cu content should be 1.00% or less, preferably 0.80% or less. More preferably, the Cu content should be 0.05% or more.

[0032] Ni: 2.00% or less Ni is an element that enhances the hardenability of steel. Ni contributes to the improvement of strength after rolling. At the same time, Ni can be contained to improve functions such as toughness, high-temperature strength, or weather resistance. To obtain the above effects of Ni, it is preferable that the Ni content is 0.05% or more. On the one hand, when the Ni content exceeds 2.00%, it will cause deterioration of weldability, decrease in toughness, and increase in alloy cost. Therefore, when containing Ni, the Ni content should be 2.00% or less, preferably 1.80% or less. More preferably, the Ni content should be 0.20% or more.

[0033] Cr: 1.00% or less Cr is an element that enhances the hardenability of steel. Cr contributes to the improvement of strength after rolling. At the same time, Cr can be contained to improve functions such as toughness, high-temperature strength, or weather resistance. To obtain the above effects of Cr, it is preferable that the Cr content is 0.01% or more. On the one hand, when the Cr content exceeds 1.00%, it will cause deterioration of weldability, decrease in toughness, and increase in alloy cost. Therefore, when containing Cr, the Cr content should be 1.00% or less, preferably 0.50% or less. More preferably, the Cr content should be 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 enhances the hardenability of steel. Mo contributes to the improvement of strength after rolling. At the same time, Mo can be contained to improve functions such as toughness, high-temperature strength, or weather resistance. To obtain the above effects of Mo, the Mo content is preferably 0.01% or more. On the other hand, when the Mo content exceeds 0.30%, it causes deterioration of weldability, reduction of toughness, and increase of alloy cost. Therefore, when containing Mo, the Mo content should be 0.30% or less, and 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 that precipitates as V(CN). This effect is exerted by setting the V content to 0.01% or more. Therefore, the V content is preferably 0.01% or more. On the other hand, when the V content exceeds 0.30%, the toughness may decrease. Therefore, when containing V, the V content should be 0.30% or less, and preferably 0.20% or less. The V content is more preferably 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 plate. To obtain this effect, the W content is preferably 0.01% or more. On the other hand, when the W content exceeds 0.30%, it causes deterioration of weldability and increase of alloy cost. Therefore, when containing W, the W content should be 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 the steel plate. To obtain this effect, the Co content is preferably 0.01% or more. On the one hand, when the Co content exceeds 0.30%, it will lead to deterioration of weldability and an increase in 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 heat-affected zone of the weld and improves toughness. To obtain this effect, when Ca is contained, the Ca content is preferably 0.0005% or more, and more preferably 0.0020% or more. On the one hand, when the Ca content exceeds 0.0100%, it will form coarse inclusions and reduce toughness. Therefore, when Ca is contained, the Ca content is preferably 0.0100% or less, and more preferably 0.0050% or less.

[0040] Mg: 0.0100% or less Mg, like Ca, refines the structure of the heat-affected zone of the weld and improves toughness. To obtain this effect, when Mg is contained, the Mg content is preferably 0.0005% or more, and more preferably 0.0020% or more. On the one hand, when the Mg content exceeds 0.0100%, it will form coarse inclusions and reduce 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 REM, like Ca, refines the structure of the heat-affected zone of the weld and improves toughness. To obtain this effect, when REM is contained, the REM content is preferably 0.0005% or more, and more preferably 0.0015% or more. On the one hand, when the REM content exceeds 0.0200%, it will form coarse inclusions and reduce toughness. Therefore, when REM is contained, the REM content is preferably 0.0200% or less, and more preferably 0.0100% or less. Here, REM refers to the elements of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids 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.

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

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

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

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

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

[0047] [Area ratio of bainite phase at half of the plate thickness: 80% or more] In the steel plate of the present invention, the area ratio of the bainite phase at half of the plate thickness needs to be 80% or more. When the area fraction of the bainite phase is less than 80%, the fraction of the hard structure decreases and the strength decreases. Therefore, the area ratio of the bainite phase at half of the plate thickness is set to 80% or more. In order to further increase the strength, it is preferably 85% or more, and more preferably 90% or more. The area ratio of the bainite phase may be 100%. As the remainder other than the bainite phase, ferrite may be contained in an area ratio of 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 with an orientation difference of 15° or more from adjacent grains at half of the plate thickness: 30 μm or less] In the steel plate of the present invention, the average grain size of the bainite phase grains surrounded by grain boundaries with an orientation difference of 15° or more from adjacent grains at half of the plate thickness needs to be 30 μm or less. When this average grain size exceeds 30 μm, the fracture surface unit of the cleavage fracture surface of the grains increases with the coarsening of the grain size, so the toughness decreases. Therefore, the average grain size of the bainite phase grains surrounded by grain boundaries with an orientation difference of 15° or more from adjacent grains at half of the plate thickness is set to 30 μm or less. In order to further increase the toughness, it is preferably 25 μm or less, and more preferably 20 μm or less. Although the lower limit of the average particle size is not particularly defined, from the viewpoint of rolling efficiency, the average particle size is preferably 5 μm or more, and 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 in order to ensure rigidity when the steel plate is used for large steel structures. The plate thickness is preferably 60 mm or more, and more preferably 65 mm or more. Further, from the viewpoint of production efficiency, the plate thickness is preferably 100 mm or less, and more preferably 90 mm or less.

[0050] <Strength> Yield strength at the 1 / 2 plate 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 Φ14 mm JIS 14A tensile test piece taken from the 1 / 2 plate 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 usually use a high-strength and thick steel plate base material that can withstand this bending stress. When the yield strength is less than 460 MPa, it is not suitable for use. Therefore, the steel plate of the present invention has a yield strength at the 1 / 2 plate thickness position of 460 MPa or more. The upper limit value is not particularly limited, but the yield strength at the 1 / 2 plate thickness position may be 550 MPa or less.

[0051] <Base metal toughness: Charpy fracture transition temperature after strain aging treatment> vTrs at the 1 / 2 plate thickness position: -40 °C or lower In order to ensure the brittle fracture resistance of the hull while maintaining workability, it is necessary to set vTrs at the mid-thickness position of the steel plate after strain aging treatment to -40°C or lower. When vTrs is higher than -40°C, the hull parts subjected to processing are more likely to brittlely fracture. Therefore, for the steel plate of the present invention, vTrs at the mid-thickness position of the steel plate after strain aging treatment is set to -40°C or lower. Further, in order to enhance the brittle fracture resistance, it is preferable to set vTrs to -45°C or lower, and more preferably to -50°C or lower. Although the lower limit of vTrs is not particularly defined, from the viewpoint of rolling efficiency, it is preferable that the above vTrs is -80°C or higher, and more preferably -70°C or higher. vTrs can be evaluated by collecting a Charpy test piece of JIS No. 4 from the mid-thickness position of the steel plate after strain aging treatment and conducting 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 the welded joint obtained by welding with a heat input of 500 kJ / cm: average 53 J or more In order to ensure the brittle fracture resistance of the heat-affected zone of the weld, it is necessary to make the -40°C Charpy absorbed energy in the HAZ of the welded joint obtained by welding with a heat input of 500 kJ / cm average 53 J or more. The -40°C Charpy absorbed energy in the HAZ is preferably an average of 64 J or more, and more preferably 75 J or more. HAZ toughness can be evaluated by collecting test pieces from joints obtained by high heat input welding.

[0053] As described above, by adjusting the above component composition and improving the toughness by refining the steel structure at the 1 / 2 position of the steel plate, a steel plate having excellent toughness in joints after high-strength and high heat input welding with a plate thickness of 50 mm or more can be obtained.

[0054] Next, the manufacturing method of the steel plate in the present invention will be described.

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

[0056] Hereinafter, each step will be described in detail. The temperature in each step refers to the temperature at the center of the thickness (1 / 2 thickness position) and the center in the plate width direction of the steel material and the hot-rolled sheet, unless otherwise specified. For example, the temperature distribution within the cross-section of the steel sheet can be calculated by heat transfer analysis and corrected by the surface temperature of the steel sheet.

[0057] <Heating step> First, a steel material having the above-described component composition is heated to a heating temperature of 950 to 1150°C. [Heating temperature of steel material: 950 to 1150°C] When the heating temperature of the steel material is less than 950°C, the heating temperature is too low, the deformation resistance becomes high, and the load on the hot rolling mill increases. Therefore, it becomes difficult to carry out the subsequent hot rolling process. Further, when the heating temperature of the steel material is less than 950°C, the added elements are not sufficiently dissolved and the strength decreases. Therefore, the heating temperature of the steel material is set to 950°C or higher. On the one hand, at a high temperature where the heating temperature of the steel material exceeds 1150°C, the austenite grains coarsen. As a result, the toughness decreases. Furthermore, oxidation becomes significant, increasing the oxidation loss and potentially reducing the yield. For these reasons, the heating temperature of the steel material is set to 1150°C or lower. Incidentally, the heating temperature of the steel material is preferably 1000°C or higher, more preferably 1030°C or higher. The above heating temperature is preferably 1130°C or lower, more preferably 1100°C or lower.

[0058] <Hot rolling process> Next, the steel material heated in the heating process is subjected to rolling (hot rolling) under the conditions that the rolling start temperature at the 1 / 2 thickness position of the plate: (Ar3 point + 200)°C or higher, the cumulative reduction ratio when the temperature at the 1 / 2 thickness position of the plate is in the austenite recrystallization temperature range: 15.0% or higher, and the average value of the reduction ratio per pass: 5.0% or higher, the cumulative reduction ratio when the temperature at the 1 / 2 thickness position of the plate is in the austenite non-recrystallization temperature range: 50.0% or higher, and the rolling end temperature at the 1 / 2 thickness position of the plate: (Ar3 point + 100)°C or higher.

[0059] [Rolling start temperature: (Ar3 point + 200)°C or higher] When hot rolling the steel material heated in the above heating process, if the temperature at the 1 / 2 thickness position of the plate when starting hot rolling is less than (Ar3 point + 200)°C, recrystallization does not occur sufficiently in the hot-rolled sheet after the hot rolling process. Therefore, the austenite grain size does not decrease, and the toughness decreases. Therefore, the rolling start temperature is set to (Ar3 point + 200)°C or higher. From the perspective of ensuring the time for hot rolling in the non-recrystallized region described later, the rolling start temperature is preferably (Ar3 point + 210)°C or higher, more preferably (Ar3 point + 220)°C or higher. The upper limit value is not particularly limited, but the rolling start temperature is preferably (Ar3 point + 280)°C or lower.

[0060] Incidentally, the Ar3 point (°C) can be obtained according to the following formula (2). Ar3 point (°C) = 910 - 273×C - 74×Mn - 57×Ni - 16×Cr - 9×Mo - 5×Cu ··· Formula (2) Here, in Formula (2), each element symbol represents the content (mass %) of the element in the steel, and for elements not contained, it is 0 mass %.

[0061] [Cumulative reduction ratio in the austenite recrystallization temperature range: 15.0% or more] Hot rolling is performed such that the cumulative reduction ratio when the temperature at the 1 / 2 thickness position of the plate is in the austenite recrystallization temperature range is 15.0% or more. If the cumulative reduction ratio in this temperature range is less than 15.0%, the grain refinement of austenite is insufficient, the toughness is not improved, and as a result, the Charpy fracture transition temperature (vTrs) at the 1 / 2 thickness position after strain aging treatment: -40 °C or lower cannot be achieved. The cumulative reduction ratio in this temperature range is preferably 20.0% or more, and more preferably 25.0% or more. The upper limit of the cumulative reduction ratio in this temperature range is not particularly limited, but since the effect of the above-mentioned austenite grain refinement saturates, the cumulative reduction ratio in this temperature range is preferably 70.0% or less, and more preferably 65.0% or less. Here, the cumulative reduction ratio is "[100 × (thickness of the plate before the first pass (immediately before) (mm) - thickness of the plate after the final pass (immediately after) (mm))] / thickness of the plate before the first pass (immediately before) (mm)" within the pass where the temperature at the 1 / 2 thickness position is in the austenite recrystallization temperature range. Also, when the temperature at the 1 / 2 thickness position is in the austenite recrystallization temperature range, it means that in each pass, the temperature at the 1 / 2 thickness position before the start of rolling (immediately before the start of rolling) is in the austenite recrystallization temperature range.

[0062] [Average value of reduction ratio per pass (reduction ratio / pass) in the austenite recrystallization temperature range: 5.0% or more] When the temperature at the center of the plate thickness (at the 1 / 2 position of the plate thickness) is in the austenite recrystallization temperature range, hot rolling is performed such that the average value of the reduction ratio per pass (reduction ratio / pass) is 5.0% or more. If the average value of the reduction ratio / pass in this temperature range is less than 5.0%, the grain refinement of austenite is insufficient, the toughness is not improved, and as a result, the Charpy fracture transition temperature (vTrs) at the 1 / 2 position of the plate thickness after strain aging treatment: -40 °C or less cannot be achieved. The average value of the reduction ratio / pass in this temperature range is preferably 5.5% or more, and more preferably 6.0% or more. The upper limit of the average value of the reduction ratio / pass in this temperature range is not particularly limited, 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 ratios in each pass within the pass where 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 of 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 ratio in the austenite non-recrystallization temperature range: 50.0% or more] Furthermore, when the temperature at the 1 / 2 position of the plate thickness is in the austenite non-recrystallization temperature range, hot rolling is performed such that the cumulative reduction ratio is 50.0% or more. By setting the cumulative reduction ratio in this temperature range to 50.0% or more, sufficient processing strain can be imparted to austenite, the number of nucleation sites increases during bainite transformation, the grain size is refined, and the toughness at the 1 / 2 position of the plate thickness can be improved. On the other hand, if the cumulative reduction ratio in this temperature range is less than 50.0%, austenite is not sufficiently stretched in the rolling direction, the number of nucleation sites during bainite transformation decreases, and as a result, coarse bainite is generated and the grain size cannot be refined. As a result, the Charpy fracture transition temperature (vTrs) at the 1 / 2 position of the base material of the steel plate after strain aging treatment: -40 °C or less cannot be achieved. Therefore, the cumulative reduction ratio in this temperature range shall be 50.0% or more. The cumulative reduction ratio is preferably 55.0% or more, and more preferably 60.0% or more. The upper limit of the cumulative reduction ratio in this temperature range is not particularly limited. From the viewpoint of not inhibiting the rolling efficiency, the cumulative reduction ratio in this temperature range is preferably 75.0% or less, and more preferably 70.0% or less. Here, the cumulative reduction ratio is "[100×(thickness of the plate before the first pass (immediately before) (mm) - thickness of the plate after the final pass (immediately after) (mm))] / thickness of the plate before the first pass (immediately before) (mm)" within the pass where the temperature at the 1 / 2 thickness position of the plate is in the non-recrystallization temperature range of austenite. Also, when the temperature at the 1 / 2 thickness position of the plate is in the non-recrystallization temperature range of austenite, it suffices that the temperature at the 1 / 2 thickness position is in the non-recrystallization temperature range of austenite before the start of rolling (immediately before the start of rolling) in each pass.

[0064] [Rolling end temperature at the 1 / 2 thickness position: (Ar3 point + 100) °C or higher] The hot rolling process needs to end with the rolling end temperature at the 1 / 2 thickness position being (Ar3 point + 100) °C or higher. When the temperature of the steel plate in hot rolling becomes less than (Ar3 point + 100) °C, B nitride precipitates, and the solid solution B necessary for enhancing hardenability becomes insufficient during accelerated cooling, resulting in a decrease in strength. Furthermore, since the deformation resistance increases as the temperature decreases, problems such as an increase in the load on the hot rolling mill occur. Note that from the viewpoint of setting the cooling start temperature in the subsequent process to be Ar3 point (°C) or higher, the above rolling end temperature is preferably (Ar3 point + 110) °C or higher. The upper limit of the above rolling end temperature is not particularly limited, but 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 process> Next, the hot-rolled sheet hot-rolled in the hot rolling process is cooled under the conditions that the cooling start temperature at the 1 / 2 thickness position is: (Ar3 point + 100) °C or higher, the average cooling rate when the temperature at the 1 / 2 thickness position is in the temperature range of 700 to 500 °C is: 5.0 °C / s or higher, and the cooling stop temperature at the 1 / 2 thickness position is: 500 °C or lower.

[0066] [Cooling start temperature: (Ar3 point + 100) °C or higher] For the hot-rolled sheet obtained through the above hot rolling process, it is necessary to start cooling at a temperature of (Ar3 point + 100) °C or higher as the temperature at the 1 / 2 thickness position. If the cooling start temperature is lower than (Ar3 point + 100) °C, B nitride precipitates and the solid solution B necessary to enhance hardenability becomes insufficient during accelerated cooling, resulting in a decrease in strength. Therefore, the above cooling start temperature is set to (Ar3 point + 100) °C or higher. The cooling start temperature is preferably (Ar3 point + 110) °C or higher, and preferably (Ar3 point + 130) °C or lower.

[0067] [Average cooling rate in the temperature range of 700 to 500 °C: 5.0 °C / s or higher] When the average cooling rate when the temperature at the 1 / 2 thickness position is in the range of 700 to 500 °C is less than 5.0 °C / s, a large amount of ferrite is generated in the steel due to slow cooling, so the area ratio of the bainite phase cannot be increased. As a result, the strength decreases. The temperature range defining this average cooling rate is set to the temperature range of 700 to 500 °C from the viewpoint of causing the transformation of most austenite structures and obtaining an effect that greatly contributes to the properties of the steel sheet. That is, in the present invention, the average cooling rate when the 1 / 2 thickness position is in the temperature range of 700 to 500 °C is set to 5.0 °C / s or higher. Thereby, the effect of improving the fraction of bainite can be obtained, and the strength is improved. The average cooling rate in this temperature range is preferably 5.5 °C / s or higher. Note that the upper limit of the average cooling rate is not particularly limited, but from the viewpoint of avoiding an increase in cooling cost due to excessive rapid cooling, the average cooling rate is preferably 20 °C / s or lower, and more preferably 10 °C / s or lower.

[0068] The average cooling rate outside the temperature range of 700 to 500 °C mentioned above is not particularly specified because it does not significantly affect the formation of bainite. However, from the perspective of production efficiency, it is preferably 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 to 5.5 °C / s or more when the temperature at the 1 / 2 thickness position of the plate is 700 to 600 °C, even when ferrite transformation in the high-temperature region is promoted by the introduction of deformation bands due to heavy reduction in the non-recrystallized region, an increase in the amount of ferrite formed in the steel can be suppressed, and the strength can be further improved, which is preferable. The average cooling rate in this temperature range is more preferably 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 needs to be carried out until the temperature at the 1 / 2 thickness position of the plate becomes 500 °C or less. If the cooling stop temperature exceeds 500 °C, a large amount of ferrite will be formed in the steel, so the area ratio of the bainite phase cannot be increased. As a result, the strength decreases. The cooling stop temperature is preferably 450 °C or less, and more preferably 400 °C or less. On the other hand, the lower limit of the cooling stop temperature is not limited, but if the cooling stop temperature is too low, the flatness of the steel plate will decrease. Therefore, the cooling stop temperature is preferably 250 °C or more, and more preferably 300 °C or more.

Examples

[0073] Next, the present invention will be specifically described based on examples. Note that the following examples show a preferred example of the present invention, and the present invention is not limited to these examples.

[0074] The components of the test steels are shown in Tables 1-1 and 1-2, and the manufacturing conditions are shown in Tables 2-1 and 2-2. Molten steels (steel symbols: A to CJ) with each component composition shown in Tables 1-1 and 1-2 were melted in a converter and made into steel materials by continuous casting. Thereafter, heating, hot rolling, and cooling were performed in this order under the manufacturing conditions shown in Tables 2-1 and 2-2 to produce steel plates with a thickness of 50 to 100 mm (manufacturing Nos.: 1 to 97). Note that the blanks in Table 1 indicate that elements are not intentionally added, including not only the case where the element is not contained (0%) but also the case where the element is unavoidably contained, provided that the lower limit value is not exceeded in the latter case. Regarding the amount of B in steels BM and CG: the "-" also means that it is not intentionally added, including not only the case where the element is not contained (0%) but also the case where it is unavoidably contained in an amount less than the lower limit value (0.0001 mass%) of the amount of B defined in the present invention.

[0075]

Table 1-1

[0076]

Table 1-2

[0077]

Table 2-1

[0078]

Table 2-2

[0079] For each of the obtained steel plates, (1) the strength of the base metal, (2) the toughness of the base metal, (3) the toughness of the HAZ, and (4) the steel structure of the base metal were evaluated by the methods described below.

[0080] (1) Strength of the base metal From the position of half the plate thickness of the obtained steel plate, a JIS 14A test piece with a diameter of Φ14 was sampled so that the longitudinal axis direction of the test piece was perpendicular to the rolling direction and parallel to the plate width direction, and a tensile test was conducted 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 the base metal The toughness was evaluated by the Charpy fracture transition temperature at the position of half the plate thickness after strain aging treatment. From the position of half the plate thickness of the obtained steel plate, a JIS No. 4 impact test piece was sampled so that the longitudinal axis direction of the test piece was parallel to the rolling direction, notches were machined in the direction parallel to the plate width direction, and a Charpy test was carried out in the range of 0 °C to -80 °C in accordance with the provisions of JIS Z 2242 (2023). The Charpy fracture ratio at each test temperature was measured, and the Charpy fracture transition temperature (vTrs) was determined. Note that 3 tests were conducted for each test temperature, and vTrs was calculated from the average value of the brittle fracture ratios of the 3 specimens. Here, examples with vTrs ≤ -40 °C were evaluated as having excellent Charpy toughness (toughness of the base metal).

[0082] (3) HAZ toughness V-groove machining with a groove angle of 20° and a groove gap of 10 mm was performed on the joint test plate sampled from the steel plate. Using a commercially available welding wire for low-temperature steel, a joint was fabricated by electro-gas welding (EGW) with 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 position 1 mm deep from the surface of the obtained joint was defined as the test piece surface layer, and a notch was provided at the position where the weld metal and the base metal were each 50% on the fusion zone (FL), and an NK U4 impact test piece was sampled. The sampled impact test pieces were subjected to a Charpy impact test at a test temperature of -40 °C. For the 3 impact test pieces sampled from the above joint test plate, the average value vE-40 °C (unit: J) of the absorbed energies obtained under the same conditions was taken as the toughness of the HAZ. Here, examples with an average vE-40 °C ≥ 53 J were evaluated as having excellent HAZ toughness.

[0083] (4) Steel microstructure of the base material [Steel microstructure] The steel microstructure at the 1 / 2 thickness position of the plate was observed as follows. Samples were taken from the center of the width of the steel plate at the 1 / 2 thickness position such that the plane perpendicular to the rolling direction was the observation plane. The dimensions of the sample were 20 mm in the thickness direction, 10 mm in the direction parallel to the rolling direction, and 1 mm in the plate width direction. After mirror-polishing the plane perpendicular to the rolling direction of this sample, an optical microscope photograph of the metallographic structure revealed by nital etching was taken. The photographing part was at the 1 / 2 thickness position, the magnification was 200 times, and the photographing range was 250 μm × 300 μm. From the obtained photograph, the area ratio of the bainite phase was evaluated by calculating the area ratio by image analysis, taking the tissue appearing as white lumps as ferrite and the remaining part as the bainite phase. [Average grain diameter of the bainite phase grains] Samples were taken from the center of the width of the steel plate at the 1 / 2 thickness position such that the plane perpendicular to the rolling direction was the observation plane. The dimensions of the sample 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 plane perpendicular to the rolling direction of this sample, EBSP analysis was performed for the 1 / 2 thickness position under the following conditions. From the obtained crystal orientation map, the equivalent circle diameter of the tissue surrounded by large-angle grain boundaries with an orientation difference of 15° or more from adjacent grains was determined, and the average value of the equivalent circle diameters in the following analysis region was taken as the average effective crystal grain diameter. (EBSP conditions) ·Acceleration voltage: 20 KV, irradiation current: 50 nA ·Beam diameter: 50 nm ·Analysis region: 1 mm × 1 mm region at the 1 / 2 thickness position ·Step size: 0.4 μm

[0084] These test results are shown in Table 3-1 and Table 3-2.

[0085]

Table 3-1

[0086]

Table 3-2

[0087] In the case of the examples of the present invention (Production Nos. 1, 12 to 54, 96, 97), the component composition and production conditions were within the scope of the present invention, and the steel structure at the 1 / 2 thickness position of the plate as described above was also obtained. As a result, the yield strength at the 1 / 2 thickness position was 460 MPa or more, the Charpy fracture transition temperature at the 1 / 2 thickness position after strain aging treatment was -40 °C or less, and a steel plate was obtained in which the -40 °C Charpy absorption energy in the HAZ of the welded joint obtained by high heat input welding with a heat input of 500 kJ / cm was 53 J or more on average.

[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 scope of the present invention, and the desired component composition and / or the desired steel structure at the 1 / 2 thickness position could not be obtained, and the target value could not be achieved in at least any one of the strength of the base material, the toughness of the base material, and the toughness of the HAZ.

Claims

1. In mass percent, 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 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%, The mass% ratio of Ti to N (Ti / N) is 2.00 or more and 4.50 or less, The balance is Fe and unavoidable impurities, The steel has a structure in which, at a position at 1 / 2 of the plate thickness, the structure is composed of 80% or more of a bainite phase by area ratio and the remaining structure, and the average grain size of the bainite phase crystal grains surrounded by grain boundaries having an orientation difference of 15° or more with adjacent crystal grains is 30 μm or less, The plate thickness is 50 mm or more, The yield strength at the plate thickness 1 / 2 position is 460 MPa or more, The Charpy fracture transition temperature (vTrs) at the sheet thickness 1 / 2 position after strain aging treatment in which 5 to 10% strain is applied at room temperature and then held at 200 to 250 ° C for 1 to 2 hours is vTrs ≦ −40 ° C, A steel plate having an average Charpy absorbed energy of 53 J or more at -40°C in the HAZ of a welded joint produced with a welding heat input of 500 kJ / cm. 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 set to 0 (zero).

2. The steel sheet according to claim 1, further comprising, in addition to the above-mentioned composition, one or more of the following groups A, B and C, in mass %. Group A: one or more selected from Cu: 1.00% or less, Ni: 2.00% or less, 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 metals): 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. The method for producing a steel sheet according to claim 1 or 2, A steel material having the above-mentioned composition is heated to a heating temperature of 950 to 1150°C, Next, the rolling start temperature at the plate thickness 1 / 2 position: (Ar 3 point +200) ° C. or higher, The cumulative reduction ratio when the temperature at the sheet thickness 1 / 2 position is in the austenite recrystallization temperature range is 15.0% or more, and the average reduction ratio per pass is 5.0% or more; The cumulative reduction ratio when the temperature at the plate thickness 1 / 2 position is in the austenite non-recrystallization temperature range: 50.0% or more; Furthermore, the rolling end temperature at the plate thickness 1 / 2 position: (Ar 3 The rolling is carried out under conditions where the rolling temperature is equal to or higher than the rolling point + 100°C. 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 is performed under conditions such that the cooling stop temperature at the 1 / 2 plate thickness position is 500°C or less.

4. The method for producing a steel sheet according to claim 3, wherein the cooling further comprises an average cooling rate of 5.5°C / s or more when the temperature at a 1 / 2 thickness position is in a temperature range of 700 to 600°C.

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