Thick steel plate and method for manufacturing the same

Abstract

A steel sheet containing C: 0.04 to 0.16 mass%, Si: 0.10 to 0.50 mass%, Mn: 0.60 to 1.60 mass%, P: 0.005 to 0.030 mass%, Al: 0.015 to 0.050 mass%, Ti: 0.005 to 0.020 mass%, Ca: 0.0005 to 0.0025 mass%, O: 0.0008 to 0.0025 mass%, B: 0.0005 to 0.0020 mass%, N: 0.0030 to 0.0060 mass%, the balance consisting of Fe and unavoidable impurities, satisfying (1) and (2) formulae, a metal structure containing ferrite and pearlite in a total of 90% or more in terms of area ratio, the balance being island-like martensite and / or bainite, a difference between upper yield point and lower yield point being 30 MPa or less, 3[Si] + 6.3[P] + 1.4[Al] ≥ 0.63 (1), [Ca] / [O] ≥ 0.2 (2).

Description

TECHNICAL FIELD

[0001] The present application relates to a thick steel plate and a method for manufacturing the same, for example, to a thick steel plate which can be used for a storage tank and the like of a ship which can carry liquefied petroleum gas (LPG) and liquid ammonia. BACKGROUND

[0002] In response to environmental regulations in recent years, ammonia, which is a carbon (C) -free fuel, has attracted attention as a next-generation fuel. The use of ammonia as a fuel is currently limited, but it is expected to increase in the future. On the other hand, ships are generally used for about 20 years after construction, and therefore, in the shipbuilding industry, there is an increasing demand for construction of LPG transport ships of a specification that can carry ammonia in anticipation of an increase in the amount of ammonia used in the future.

[0003] In the international regulation IGC Code related to the structure and equipment of a liquefied gas transport ship, special requirements are prescribed for a gas transport ship that carries ammonia, and from the viewpoint of preventing stress corrosion cracking, an upper limit of the yield strength is newly prescribed for a steel plate for a storage tank, which is not prescribed for a steel plate for a storage tank of a general LPG transport ship. For example, in the standard for a steel plate for a storage tank of a general LPG transport ship, i.e., cryogenic steel KL33, an upper limit value and a lower limit value of 440 MPa ≤ tensile stress (TS) ≤ 560 MPa are prescribed with respect to the tensile stress (TS). However, only the lower limit value of 325 MPa ≤ yield strength (YP) is prescribed with respect to the yield strength. In contrast, with respect to a steel plate for a storage tank that carries ammonia, the upper limit value of yield strength (YP) ≤ 440 MPa is additionally prescribed.

[0004] In addition, in order to liquefy propane gas (PG), cooling to -42°C or lower is required, and thus, in order to ensure the safety of a storage tank structure that stores LPG, a prescribed low-temperature toughness is also required. Furthermore, a heat-affected zone (HAZ) generated when a steel plate for a storage tank is welded is also required to have a prescribed toughness at low temperatures.

[0005] As a steel plate for a storage tank in response to such a requirement, for example, a thick steel plate disclosed in Patent Documents 1 and 2 is known, and a thick steel plate having a yield strength in the range of 325 to 440 MPa is disclosed. In the invention of Patent Document 1, ensuring low-temperature toughness and a low yield strength ratio is a subject, and in the invention of Patent Document 2, further ensuring HAZ toughness is a subject. In the thick steel plates of Patent Documents 1 and 2, in order to achieve a low yield strength ratio, by dividing the ferrite grain diameter in the structure of the quarter portion of the plate thickness into a plurality of numerical ranges, and by taking the proportion of the number of each of the numerical ranges as one of the requirements, these subjects are solved.

[0006] PRIOR ART DOCUMENTS

[0007] PATENT DOCUMENTS

[0008] Patent Literature 1: Japanese Patent Application Laid-Open No. 2019-214751

[0009] Patent Literature 2: Japanese Patent Application Laid-Open No. 2019-214752 SUMMARY

[0010] PROBLEMS TO BE SOLVED BY THE INVENTION

[0011] When a storage tank is manufactured by welding a thick steel sheet, it is desirable to apply a large heat input welding of 7 kJ / mm or more from the viewpoint of productivity. In this case, not only the thick steel sheet itself needs to have excellent low-temperature toughness, but also the large heat input welding characteristics, that is, the low-temperature toughness of a heat affected zone (HAZ) after the large heat input welding, also needs to be excellent.

[0012] However, the steel sheets described in Patent Literatures 1 and 2 can not have excellent large heat input welding characteristics. In addition, if an attempt is made to adjust the chemical composition and the manufacturing method using the existing method to obtain both the low-temperature toughness and the large heat input welding characteristics, there is a problem in that it is difficult to satisfy both the range of the tensile stress (440 MPa or more and 560 MPa or less) and the range of the yield strength (325 MPa or more and 440 MPa or less) required for a steel sheet for an ammonia-loaded storage tank.

[0013] The present invention has been made in view of such circumstances, and aims to provide a steel sheet satisfying both the range of the tensile stress and the range of the yield strength required for a steel sheet for an ammonia-loaded storage tank, and also having excellent low-temperature toughness and large heat input welding characteristics, and a manufacturing method thereof.

[0014] MEANS FOR SOLVING THE PROBLEMS

[0015] Mode 1 of the present invention is a steel sheet containing C: 0.04 to 0.16 mass%, Si: 0.10 to 0.50 mass%, Mn: 0.60 to 1.60 mass%, P: 0.005 to 0.030 mass%, Al: 0.015 to 0.050 mass%, Ti: 0.005 to 0.020 mass%, Ca: 0.0005 to 0.0025 mass%, O: 0.0008 to 0.0025 mass%, B: 0.0005 to 0.0020 mass%, N: 0.0030 to 0.0060 mass%, the balance consisting of Fe and unavoidable impurities, satisfying the following (1) and (2) formulas, and the metal structure at a portion in the thickness direction of the steel sheet at a distance of one fourth of the thickness of the steel sheet, that is, at a position of the thickness direction t / 4, contains 90% or more of ferrite structure and pearlite structure in total in terms of area ratio, the balance consisting of one or more of island-shaped martensite structure and bainite structure, and the difference between the upper yield point and the lower yield point obtained by a tensile test is 30 MPa or less.

[0016] 3[Si] + 6.3[P] + 1.4[Al] ≥ 0.63 (1)

[0017] Here, [Si], [P] and [Al] are the contents of Si, P and Al, respectively, in mass %.

[0018] [Ca] / [O] ≥ 0.2 (2)

[0019] Here, [Ca] and [O] are the contents of Ca and O, respectively, in mass %.

[0020] Mode 2 of the present application, the steel sheet according to Mode 1, wherein one or more selected from the group consisting of V: 0.003 to 0.50 mass %, Nb: 0.003 to 0.020 mass %, Cu: 0.05 to 0.25 mass %, Ni: 0.05 to 0.25 mass %, Cr: 0.05 to 0.25 mass %, Mo: 0.05 to 0.25 mass %, Zr: 0.0001 to 0.010 mass %, Mg: 0.0001 to 0.010 mass %, and REM: 0.0001 to 0.010 mass % is further contained.

[0021] Mode 3 of the present application is a method for manufacturing a steel sheet, the metal structure at a position of one fourth of the sheet thickness in the sheet thickness direction, i.e., at a position of t / 4 in the sheet thickness direction, of the steel sheet contains ferrite structure and pearlite structure in a total of 90% or more in area ratio, and the balance is constituted by one or more of island-shaped martensite structure and bainite structure, and the difference between the upper yield point and the lower yield point obtained by a tensile test is 30 MPa or less, and in this method for manufacturing, the following steps are included:

[0022] a step of preparing a steel material containing:

[0023] C: 0.04 to 0.16 mass %,

[0024] Si: 0.10 to 0.50 mass %,

[0025] Mn: 0.60 to 1.60 mass %,

[0026] P: 0.005 to 0.030 mass %,

[0027] Al: 0.015 to 0.050 mass %,

[0028] Ti: 0.005 to 0.020 mass %,

[0029] Ca: 0.0005 to 0.0025 mass %,

[0030] O: 0.0008 to 0.0025 mass %,

[0031] B: 0.0005 to 0.0020 mass%,

[0032] N: 0.0030 to 0.0060 mass%,

[0033] the balance being Fe and unavoidable impurities,

[0034] satisfy the following (1) and (2) formulas;

[0035] rolling process, after heating the steel material to 1000 to 1150°C, hot-rolling is performed such that the reduction rate in the temperature range of 820°C or higher is 30% or more, the reduction rate in the temperature range of 790°C or higher and lower than 820°C is 10% or more, and the hot-rolling end temperature is 750°C or higher, and then, cooling is performed at an average cooling rate of 0.5 to 3.0°C / sec to a temperature of the acceleration cooling start temperature, which is lower than the Ar3 point shown in the following (3) formula and 150°C or more of the Ar3 point, and from the acceleration cooling start temperature to the acceleration cooling end temperature of 500°C or higher and 650°C or lower, cooling is performed at an average cooling rate of 4 to 9°C / sec.

[0036] 3[Si] + 6.3[P] + 1.4[Al] ≥ 0.63 (1)

[0037] Here, [Si], [P], and [Al] are the contents of Si, P, and Al, respectively, in mass%.

[0038] [Ca] / [O] ≥ 0.2 (2)

[0039] Here, [Ca] and [O] are the contents of Ca and O, respectively, in mass%.

[0040] Ar3(°C) = 910 - 310[C] - 80[Mn] - 20[Cu] - 15[Cr] - 55[Ni] - 80[Mo] (3)

[0041] - 80[Mo] (3)

[0042] Here, [C], [Mn], [Cu], [Cr], [Ni], and [Mo] are the contents of C, Mn, Cu, Cr, Ni, and Mo, respectively, in mass%.

[0043] In Mode 4 of the present invention, the method of producing a steel sheet according to Claim 3, wherein the steel material further contains at least one selected from the group consisting of V: 0.003 to 0.50 mass%, Nb: 0.003 to 0.020 mass%, Cu: 0.05 to 0.25 mass%, Ni: 0.05 to 0.25 mass%, Cr: 0.05 to 0.25 mass%, Mo: 0.05 to 0.25 mass%, Zr: 0.0001 to 0.010 mass%, Mg: 0.0001 to 0.010 mass%, and REM: 0.0001 to 0.010 mass%.

[0044] Effects of the Invention

[0045] According to the embodiments of the present invention, it is possible to provide a steel sheet and a method of producing the same, which satisfy both a range of tensile stress and a range of yield strength required for a steel sheet for an ammonia-carrying storage tank, and which have excellent low-temperature toughness and high heat input welding characteristics. DETAILED DESCRIPTION

[0046] A thick steel sheet for a storage tank for a ship, if a tensile test is performed in order to evaluate tensile characteristics and a stress-strain curve is obtained, is basically an upper yield point-lower yield point type which exhibits an upper yield point and a lower yield point, and the value of the upper yield point is used as the yield strength. The inventors have focused on the difference between the upper yield point and the lower yield point and conducted research. It has then been found that, by making the difference between the upper yield point and the lower yield point be a small value of 30 MPa or less, even if the conditions of chemical composition and production conditions are appropriately adjusted to improve low-temperature toughness and high heat input welding characteristics, it is possible to satisfy both a range of tensile stress and a range of yield strength required for a steel sheet for an ammonia-carrying storage tank.

[0047] In order to reduce the difference between the upper yield point and the lower yield point, with respect to the composition, not only the content of each element is appropriately adjusted, but also, as detailed below, the contents of Si and P and Al are controlled so as to satisfy a prescribed relationship.

[0048] Then, when the steel material having the composition thus controlled is heated to an appropriate temperature and hot-rolled, by controlling the reduction rate for each temperature zone so that the hot-rolling end temperature is in an appropriate range, and then, after slow cooling to the Ar3 point, performing accelerated cooling at a prescribed cooling rate between the accelerated cooling start temperature below the Ar3 point and a prescribed accelerated cooling end temperature, it is possible to make the difference between the upper yield point and the lower yield point be a very small value of 30 MPa or less.

[0049] The reason for making the composition and rolling conditions appropriate like this so that the difference between the upper yield point and the lower yield point can be made to be 30 MPa or less is considered to be because this increases the mobile dislocation density in the ferrite. The technical idea of increasing the mobile dislocation density in the ferrite is based on the results of the research obtained so far, and although it is considered to be appropriate, there are difficulties in measuring the mobile dislocation density in the ferrite alone, and it should be noted that this is not intended to limit the technical scope of the present application.

[0050] To further improve the high heat input welding characteristics (low temperature toughness of the weld heat affected zone formed by high heat input welding) of the steel sheet (thick steel sheet) obtained by hot rolling under the above conditions, it was found that in addition to controlling the content of each element, it is also necessary to manage the ratio of the amount of Ca to the amount of O within an appropriate range, thereby achieving the steel sheet of the embodiment of the present application.

[0051] Details of the embodiment of the present application are shown below.

[0052] <1. Chemical composition>

[0053] The steel sheet of the embodiment of the present application contains C: 0.04 to 0.16 mass%, Si: 0.10 to 0.50 mass%, Mn: 0.60 to 1.60 mass%, P: 0.005 to 0.030 mass%, Al: 0.015 to 0.050 mass%, Ti: 0.005 to 0.020 mass%, Ca: 0.0005 to 0.0025 mass%, O: 0.0008 to 0.0025 mass%, B: 0.0005 to 0.0020 mass%, and N: 0.0030 to 0.0060 mass%.

[0054] Details of each element are described below.

[0055] [1-1. Basic components]

[0056] (C: 0.04 to 0.16 mass%)

[0057] C is an element that increases the strength of steel, and in order to ensure the desired high strength, it is necessary to contain 0.04 mass% or more. On the other hand, if it contains more than 0.16 mass%, it results in a decrease in low temperature toughness. Therefore, C is specified in the range of 0.04 to 0.16 mass%. The lower limit of the C content that is preferred for easy strength assurance is 0.06 mass%. In addition, the upper limit of the C content is preferably 0.08 mass% for higher toughness.

[0058] (Si: 0.10 to 0.50 mass%)

[0059] Si suppresses the generation of cementite and is required to be 0.10 mass% or more in order to increase the mobile dislocations in ferrite. On the other hand, if it is contained in a large amount exceeding 0.50 mass%, the high heat input welding characteristics are deteriorated. Therefore, Si is regulated to be in the range of 0.10 to 0.50 mass%. In order to further increase the contribution to the introduction of mobile dislocations, the lower limit of the Si content is preferably 0.20 mass%. In addition, in order to further increase the toughness of the joint (improve the high heat input welding characteristics), the upper limit of the Si content is preferably 0.30 mass%.

[0060] (Mn: 0.60 to 1.60 mass%)

[0061] Mn contributes to the increase in strength and is required to be added in 0.60 mass% or more. On the other hand, the addition of excess Mn deteriorates the high heat input welding characteristics. Therefore, the Mn content is regulated to be in the range of 0.60 to 1.60 mass%. In order to more contribute to the increase in strength, the lower limit of the Mn content is preferably 1.35 mass%. In addition, in order to further improve the high heat input welding characteristics, the upper limit of the Mn content is preferably 1.45 mass%.

[0062] (P: 0.005 to 0.030 mass%)

[0063] P is an impurity element that is inevitably contained in steel, but is controlled in a range smaller than the range generally allowed as an impurity element for the following reasons.

[0064] If the P content is 0.005 mass% or more, the generation of cementite is suppressed and the mobile dislocations in ferrite are increased. However, since the low temperature toughness is deteriorated, it is 0.030 mass% or less. In order to further suppress the deterioration of the low temperature toughness, the upper limit of the P content is preferably 0.01 mass%.

[0065] (Al: 0.015 to 0.050 mass%)

[0066] Al suppresses the generation of cementite and contributes to the increase in mobile dislocations in ferrite, and in addition, as a deoxidizer, contributes to the improvement in cleanliness. In order to exert both effects, it is required to be added in 0.015 mass% or more, but if it is added in more than 0.050 mass%, the cleanliness is reduced. Therefore, the Al content is regulated to be in the range of 0.015 to 0.050 mass%.

[0067] (Ti: 0.005 to 0.020 mass%)

[0068] Ti precipitates as TiN at the time of solidification of the steel, suppresses the coarsening of austenite in the weld heat-affected zone at the time of large heat input welding and serves as a nucleus for the formation of ferrite phase transformation, and contributes to the increase in toughness. In order to ensure the large heat input welding characteristics, the content of Ti is 0.005 mass% or more, but if the content of Ti is higher than 0.020 mass%, the toughness is decreased due to the coarsening of TiN, and therefore it is 0.005 to 0.020 mass%. In order to more contribute to the increase in large heat input welding characteristics, the lower limit of the content of Ti is preferably 0.009 mass%. In addition, in order to more increase the toughness, the upper limit of the content of Ti is preferably 0.015 mass%.

[0069] (Ca: 0.0005 to 0.0025 mass%)

[0070] Ca is effective for the suppression of the coarsening of grains in the weld heat-affected zone including large heat input welding, and therefore the content of 0.0005 mass% or more is required. On the other hand, if the content of Ca is excessive, the toughness is decreased due to the decrease in cleanliness, and therefore it is 0.0025 mass% or less.

[0071] (O: 0.0008 to 0.0025 mass%)

[0072] O, in combination with Ca in the weld heat-affected zone, suppresses the crystallization of coarse TiN, and is effective for the increase in low-temperature toughness (large heat input welding characteristics) in the weld heat-affected zone at the time of large heat input welding. In order to effectively exert this effect, the content of O of 0.0008 mass% or more is required. On the other hand, if the content of O is excessive, the toughness is decreased due to the decrease in cleanliness, and therefore it is 0.0025 mass% or less.

[0073] (B: 0.0005 to 0.0020 mass%)

[0074] In order to ensure the toughness (low-temperature toughness) by suppressing the coarsening of the grain diameter in the weld heat-affected zone at the time of large heat input welding, the content of B is 0.0005 mass% or more. On the other hand, if the content of B is excessive, the toughness is decreased. Therefore, the content of B is 0.0005 to 0.0020 mass%. In order to more contribute to the increase in toughness, the lower limit of the content of B is preferably 0.0009 mass%. In addition, in order to more increase the toughness, the upper limit of the content of B is preferably 0.0015 mass%.

[0075] (N: 0.0030 to 0.0060 mass%)

[0076] N decreases the toughness in the solid solution state, and therefore the content of 0.0060 mass% or less is required. On the other hand, since N also has the effect of refining the grains by forming AlN, the content of 0.0030 mass% or more is required. Therefore, the content of N is 0.0030 to 0.0060 mass%.

[0077] In addition, in order to further improve the low temperature toughness, the upper limit of the N content is preferably 0.0050 mass%.

[0078] (Relationship to be satisfied by Si content, P content and Al content)

[0079] The more the mobile dislocations in the ferrite increase, the smaller the difference between the upper yield point and the lower yield point becomes. In the ferrite-pearlite two-phase structure, if the elements that are difficult to be solid-solved in the cementite are more, the pearlite transformation temperature moves to the low temperature side, and the mobile dislocations formed by the transformation expansion are more introduced into the ferrite. The parameter shown on the left side of the following (1) is an index corresponding to the amount of the mobile dislocations introduced via the pearlite transformation, with respect to the elements Si, P and Al that are difficult to be solid-solved in the cementite, considering the diffusion coefficients of the respective elements. In order to sufficiently reduce the difference between the upper yield point and the lower yield point to 30 MPa or less, the parameter is 0.63 or more as shown in (1).

[0080] 3[Si] + 6.3[P] + 1.4[Al] ≥ 0.63 (1)

[0081] Here, [Si], [P] and [Al] are the contents of Si, P and Al in mass%.

[0082] Further, in order to further increase the mobile dislocations, it is preferable that the parameter on the left side of (1) is 0.74 or more (i.e., 3[Si] + 6.3[P] + 1.4[Al] ≥ 0.74).

[0083] (Relationship to be satisfied by Ca content and O content)

[0084] CaO suppresses the formation of coarse TiN at the time of solidification of the steel, and thus contributes to the improvement of the low temperature toughness (high heat input welding characteristic) of the weld heat affected portion at the time of high heat input welding.

[0085] The parameter shown on the left side of (2) is an index corresponding to the amount of CaO. In order to exert the effect of improving the high heat input welding characteristic, the parameter is 0.2 or more.

[0086] [Ca] / [O] ≥ 0.2 (2)

[0087] Here, [Ca] and [O] are the contents of Ca and O in mass%.

[0088] Further, in order to obtain a greater effect of improving the high heat input welding characteristic, it is preferable that the parameter on the left side of (2) is 0.3 or more (i.e., [Ca] / [O] ≥ 0.3).

[0089] The basic components are as described above, and in one of the preferred embodiments, the balance is iron and unavoidable impurities. As the unavoidable impurities, the mixing of elements that are incorporated due to the conditions of raw materials, supplies, manufacturing equipment, and the like is permitted. As representative unavoidable impurities, S can be cited, and even if it is contained at 0.05 mass% or less, there is no problem. As examples of impurity elements other than S, As, Sn, Sb, and H can be cited.

[0090] Also, for example, P is generally treated as an unavoidable impurity element, but there are also elements for which the composition range is separately specified as described above. Therefore, in the present specification, the case where the balance is "unavoidable impurities" is a concept that excludes elements for which the composition range is separately specified.

[0091] (1-2. Selected additive elements)

[0092] Further, in other preferred embodiments of the present application, elements other than those described above can be added as necessary, without impairing the effects of the embodiments of the present application. As examples of the content of such selectively added elements, one or more selected from the group consisting of V: 0.003 to 0.50 mass%, Nb: 0.003 to 0.020 mass%, Cu: 0.05 to 0.25 mass%, Ni: 0.05 to 0.25 mass%, Cr: 0.05 to 0.25 mass%, Mo: 0.05 to 0.25 mass%, Zr: 0.0001 to 0.010 mass%, Mg: 0.0001 to 0.010 mass%, and REM: 0.0001 to 0.010 mass% can be cited.

[0093] These selective elements further improve the properties of the steel depending on the element contained. The effects of each of the selective elements are shown below.

[0094] (V: 0.003 to 0.50 mass%)

[0095] V is an element useful in improving the quenching property to ensure high strength. To allow this effect to be exhibited, the V content is made 0.003 mass% or more. The V content is preferably 0.01 mass% or more. However, if it is contained in excess, the toughness of the HAZ at the time of large heat input welding deteriorates, so the V content is 0.50 mass% or less. The V content is preferably 0.25 mass% or less.

[0096] (Nb: 0.003 to 0.020 mass%)

[0097] Nb is an element having a ferrite grain refinement effect through an austenite grain recrystallization inhibition effect. To obtain this effect, Nb is contained at 0.003 mass% or more. The Nb content is preferably 0.008 mass% or more. On the other hand, if the Nb content is excessive, the toughness decreases, so the upper limit is set to 0.020 mass%. The Nb content is preferably 0.018 mass% or less.

[0098] (Cu: 0.05 to 0.25 mass%)

[0099] Cu is an element effective for improving strength. To exert this effect, the Cu content is set to 0.05 mass% or more. The Cu content is preferably 0.10 mass% or more. If the Cu content is excessive, cracks are likely to occur during hot working, so the Cu content is set to 0.25 mass% or less, and preferably 0.20 mass% or less.

[0100] (Ni: 0.05 to 0.25 mass%)

[0101] Ni is an element useful for ensuring good low-temperature toughness of the steel sheet and for improving both the strength and the low-temperature toughness of the steel sheet. To exert this effect, the Ni content is set to 0.05 mass% or more. The Ni content is preferably 0.10 mass% or more, and more preferably 0.15 mass% or more. On the other hand, if the Ni content is excessive, the balance of the effects of Ni on strength and toughness is broken, the strength increasing effect prevails over the effect of suppressing ductile fracture at low temperatures, and the low-temperature toughness deteriorates, so the Ni content is set to 0.25 mass% or less. The Ni content is preferably 0.20 mass% or less.

[0102] (Cr: 0.05 to 0.25 mass%)

[0103] Cr is an element effective for high-strengthening of the steel sheet, and this effect increases with an increase in the Cr content, so to effectively exert this effect, Cr is contained at 0.05 mass% or more. The Cr content is preferably 0.10 mass% or more. However, if the Cr content is excessive, the strength excessively increases, and the toughness of the HAZ including the base material and the large heat input welding portion deteriorates, so the Cr content is set to 0.25 mass% or less. The Cr content is preferably 0.20 mass% or less.

[0104] (Mo: 0.05 to 0.25 mass%)

[0105] Mo is an effective element for high strength of the steel plate, and the effect increases as the content thereof increases. The content of Mo is 0.05 mass% or more in order to effectively exert the effect. The content of Mo is preferably 0.10 mass% or more. However, if the content of Mo is excessive, the strength excessively increases, and the toughness of the HAZ including the base material and the large heat input welding portion deteriorates, so the content of Mo is 0.25 mass% or less. The content of Mo is preferably 0.20 mass% or less.

[0106] (Zr: 0.0001 to 0.010 mass%)

[0107] Zr forms a nitride like Ti, and is effective for improving the large heat input welding characteristics. In order to surely obtain the effect, the lower limit of the content of Zr is 0.0001 mass%, preferably 0.0005 mass%, and more preferably 0.0010 mass%. On the other hand, if the content of Zr is excessive, the cleanliness deteriorates. Therefore, the content of Zr is 0.010 mass% or less, preferably 0.005 mass% or less, and more preferably 0.003 mass% or less.

[0108] (Mg: 0.0001 to 0.010 mass%)

[0109] Mg forms an oxide, a sulfide, and an oxysulfide, and is effective for preventing the grain coarsening of the HAZ. In order to surely obtain the effect, the lower limit of the content of Mg is 0.0001 mass%, preferably 0.0005 mass%, and more preferably 0.0010 mass%. On the other hand, if the content of Mg is excessive, the cleanliness deteriorates. Therefore, the content of Mg is 0.010 mass% or less, preferably 0.005 mass% or less, and more preferably 0.003 mass% or less.

[0110] (REM: 0.0001 to 0.010 mass%)

[0111] REM (rare earth element) forms an oxide, a sulfide, and an oxysulfide, and is effective for preventing the grain coarsening of the HAZ. In order to surely obtain the effect, the lower limit of the content of REM is 0.0001 mass%, preferably 0.0005 mass%, and more preferably 0.0010 mass%. On the other hand, if the content of REM is excessive, the cleanliness deteriorates. Therefore, the content of REM is 0.010 mass% or less, preferably 0.005 mass% or less, and more preferably 0.003 mass% or less.

[0112] The content of REM means the total content of 17 kinds of elements of Sc, Y, and 15 kinds of elements from La to Lu, and the REM means one or more kinds of elements selected from the 17 kinds of elements.

[0113] <2. Metal structure>

[0114] The steel sheet of the embodiment of the present application contains ferrite structure and pearlite structure in total of 90% or more in area ratio in the portion at a distance of one fourth of the sheet thickness from the surface in the sheet thickness direction (hereinafter sometimes referred to as "the sheet thickness direction t / 4 position". Here, t is the sheet thickness). Also, the remaining structure is one or both of island-shaped martensite structure and bainite structure.

[0115] The reason why the position of the metal structure to be evaluated is set to the one fourth position in the sheet thickness direction from the surface is because this portion is considered to be a position that is generally representative in showing the metal structure of the steel sheet. From the viewpoint of suppressing excessive increase in strength, it is necessary to make the ferrite structure and the pearlite structure 90% or more in area ratio. Also, the ratio of the ferrite structure to the pearlite structure is not particularly limited, and, for example, it is also not a problem that only the ferrite structure is 90% or more in area ratio of the total metal structure.

[0116] <3. Difference between upper yield point and lower yield point>

[0117] As described above, the steel sheet of the embodiment of the present application, in addition to the composition of each element, also manages the ratio of Ca to O within a prescribed range and the manufacturing method described later, and thereby achieves excellent low-temperature toughness and excellent large heat input welding characteristics. Also, the difference between the upper yield point and the lower yield point is made 30 MPa or less to satisfy both of these characteristics and also satisfy the range of the tensile stress (440 MPa or more and 560 MPa or less) and the range of the yield strength (325 MPa or more and 440 MPa or less) required for a steel sheet for an ammonia storage tank.

[0118] The upper yield point and the lower yield point can be obtained by performing a tensile test in accordance with JIS Z 2241 as shown in detail in the examples.

[0119] Also, it is known that the stress-strain curve obtained from the tensile test of a steel material is roughly classified into two types. One is the upper yield-lower yield type, that is, if a strain is applied, the stress increases in proportion to the strain, and when the upper yield point is reached, the stress decreases, and a region in which the stress does not increase even if the strain increases, that is, a yield shelf appears, and the stress at this time is called the lower yield point. The other is the Around Curve type, that is, a clear upper yield point and a lower yield point and a yield shelf are not observed, and the strength continuously increases together with the increase in strain.

[0120] In the encircling curve type, since a clear yield point does not appear, the stress at a strain of 0.2% is generally used as the yield strength, that is, the 0.2% yield strength. As such, since the 0.2% yield strength is used as the yield strength, and a clear upper yield point and lower yield point do not appear, in the encircling curve type, both the upper yield point and the lower yield point are equal to the 0.2% yield strength, and thus there is also the idea of regarding the difference between the upper yield point and the lower yield point as 0. However, in the strain range after the 0.2% yield strength (that is, in the region in which the strain is slightly higher than 0.2%), there is a problem in that stress continuously increases, resulting in deterioration in stress corrosion cracking resistance. Therefore, the steel sheet of the embodiment of the present application is a steel sheet in which the stress-strain curve obtained from the tensile test is of the upper yield-lower yield type, and in which the difference between the upper yield point and the lower yield point is 30 MPa or less. Furthermore, such a steel sheet can be obtained by controlling the chemical components in the above-described manner, and by applying the following production method.

[0121] <4. Production method>

[0122] A cold-worked mechanical structural steel, as described in detail below, can be produced by preparing a steel material having a prescribed composition, heating the steel material to an appropriate temperature, controlling the reduction rate for each temperature zone during hot rolling so that the finish temperature of the hot rolling is in an appropriate range, and then performing accelerated cooling at a prescribed cooling rate from a prescribed start temperature of the accelerated cooling, which is lower than the Ar3 point, to a prescribed finish temperature of the accelerated cooling, after slowly cooling to the Ar3 point.

[0123] [4-1. Preparation of a steel material having a prescribed chemical composition]

[0124] A steel material having the composition described in "1. Chemical composition" above is prepared for hot rolling in the following rolling process. The steel material can be a steel material generally used in thick plate hot rolling. As such a steel material, a cast slab can be cited. As examples of the cast slab, a slab obtained using a continuous casting method and a ingot obtained by a ingot casting method using a mold can be cited. These slabs and ingots can be subjected to surface treatment, heat treatment, machining treatment, and the like as necessary, as a steel material for rolling. In addition, the tundish used in continuous casting is preferably a hot-state recycled tundish. This is because it is possible to easily reduce the O concentration in the molten steel.

[0125] [4-2. Rolling]

[0126] The steel material described above is hot rolled.

[0127] (Heating)

[0128] First, the steel material is heated to 1000°C to 1150°C. This is because, if the heating temperature is higher than 1150°C, the austenite grain size becomes coarse, the toughness decreases, on the other hand, if the heating temperature is lower than 1000°C, it is difficult to secure the rolling temperature described later, and desired characteristics are not obtained.

[0129] (heating for hot rolling)

[0130] The steel material is heated for hot rolling. In order to refine the austenite grain size for the purpose of improving the low temperature toughness and the high heat input welding characteristics, the reduction rate is controlled for each temperature region, that is, the reduction rate in the recrystallization region is 30% or more, and the reduction rate in the unrecrystallization region is 10% or more. In the composition of the steel sheet of the embodiment of the present application described in the above "1. Chemical composition", the recrystallization region is 820°C or more, and the unrecrystallization region is 790°C or more and less than 820°C (790°C as the lower limit is actually the lower limit temperature as the austenite region). Therefore, the reduction rate in the temperature region of 820°C or more is made 30% or more, and the reduction rate in the temperature region of 790°C or more and less than 820°C is made 10% or more.

[0131] These reduction rates can be achieved by one pass of rolling, or can be achieved as the total reduction amount of multi-pass rolling. In addition, the temperature of the temperature region referred to here corresponds to the temperature of the steel material on the entry side of the roll.

[0132] Further, the recrystallization region temperature is determined by the following method. A test piece of φ12 x 12L is extracted with the t / 2 position (the position at one-half of the thickness from the surface in the thickness direction) of the rolled steel sheet as the center axis, heated to 1070°C using a thermal processing reproduction testing machine, and reduced at various reduction temperatures, and in the stress-strain curve thus obtained, the temperature region in which the strength increase due to work hardening does not monotonously increase is taken as the recrystallization temperature region.

[0133] In addition, the hot rolling end temperature (the temperature of the steel material on the exit side of the roll at the final pass of rolling) is made 750°C or more. This is in order to avoid excessive increase in strength, and to avoid deterioration of low temperature toughness.

[0134] Further, as long as the hot rolling end temperature satisfies 750°C or more, rolling in a temperature region lower than 790°C can be performed in addition to the rolling in the temperature region of 790°C or more and less than 820°C described above, or can not be performed.

[0135] (cooling after rolling)

[0136] The steel material (rolled steel sheet) is cooled immediately after the completion of hot rolling under the following conditions.

[0137] The cooling is performed at an average cooling rate of 0.5 to 3.0°C / sec from the hot rolling end temperature to the accelerated cooling start temperature, for example, by slow cooling means such as air cooling. This is to suppress the generation of bainite in the surface layer of the steel sheet. The accelerated cooling start temperature is a temperature lower than the Ar3 point and 150°C or more below the Ar3 point (a temperature lower than the Ar3 point and a temperature within 150°C of the Ar3 point).

[0138] Next, the cooling is performed at an average cooling rate of 4 to 9°C / sec from the accelerated cooling start temperature to the accelerated cooling end temperature set to a temperature of 500°C or more and 650°C or less (accelerated cooling).

[0139] If the accelerated cooling start temperature is lower than 150°C below the Ar3 point, coarse proeutectoid ferrite is precipitated, resulting in low strength. Therefore, the accelerated cooling temperature is a temperature of 150°C or more below the Ar3 point ((150°C or more below the Ar3 point).

[0140] If the average cooling rate is lower than 4°C / sec or the accelerated cooling end temperature is higher than 650°C, the density of mobile dislocations in ferrite is not sufficient, and the difference between the upper yield point and the lower yield point cannot be small enough. In addition, if the average cooling rate is higher than 9°C / sec or the accelerated cooling end temperature is lower than 500°C, bainite is excessively generated, and the strength excessively increases.

[0141] It is preferable that the accelerated cooling start temperature be higher than the accelerated cooling end temperature by 50°C or more. This is because, if the difference between the accelerated cooling temperature and the accelerated cooling end temperature is 50°C or more, the high strength effect by the accelerated cooling becomes large.

[0142] The Ar3 point can be obtained according to the following (3). In this case, when Cu, Cr, Ni, and Mo are contained only at the impurity level, the amounts thereof can be taken as 0.

[0143] Ar3 (°C) = 910 - 310[C] - 80[Mn] - 20[Cu] - 15[Cr] - 55[Ni] - 80[Mo] (3)

[0144] - 80[Mo] (3)

[0145] Here, [C], [Mn], [Cu], [Cr], [Ni], and [Mo] are the contents of C, Mn, Cu, Cr, Ni, and Mo, respectively, in mass %.

[0146] In order to further increase the density of mobile dislocations in ferrite and further reduce the difference between the upper yield point and the lower yield point, there are two means, a first means and a second means, as shown below. One or both of these two means can be adopted.

[0147] First means:

[0148] After the accelerated cooling described above, cooling is performed at an average cooling rate of 0.5 to 4.0°C / sec between 500°C and 200°C. Also, since the average cooling rate when air cooling is performed in this temperature range is 0.1 to 0.4°C / sec, it means that cooling is performed at a faster rate than usual air cooling.

[0149] The second means:

[0150] After the accelerated cooling described above, plastic working is performed so that the strain of the portion at a distance of one fourth of the plate thickness in the plate thickness direction from the surface (i.e., at the plate thickness direction t / 4 position. Here t is the plate thickness) is 0.2% or more. As such plastic working, light press-down using a pair of rolls such as upper and lower rolls, press working, and introduction of strain by stretching can be exemplified.

[0151] This plastic working, if performed after the accelerated cooling, can be performed at any temperature, for example, at a warm state or at a cold state.

[0152] The first means is preferred to be implemented since it is relatively easy to implement during cooling after hot rolling.

[0153] On the other hand, the second means is preferred not to be implemented when high productivity is desired since plastic working is additionally performed. However, it is preferred to be implemented when higher mobile dislocation density in ferrite is desired.

[0154] Also, the temperature of the steel material described in the rolling process described above can be measured by a non-contact thermometer such as a radiation thermometer, or can be measured by a contact thermometer such as a thermocouple. In addition, it can be confirmed by simulation or the like.

[0155] Examples

[0156] Hereinafter, the present application will be described more specifically by citing examples. The present application is not limited by the following examples, and can be appropriately changed and implemented within a range capable of achieving the aforementioned and hereinafter described objects, and these are included in the technical scope of the present application.

[0157] 1. Production of Test Specimens

[0158] Steel having the composition shown in Table 1 was melted by a converter. Using this molten steel, a cast slab was obtained by continuous casting.

[0159] In Table 1, the contents of C, Si, Mn, P, Al, Ti, Ca, O, B, and N as essential components, the contents of V, Nb, Cu, Ni, Cr, and Mo as optional components, and the content of S as an unavoidable impurity are shown. The balance is Fe and unavoidable impurities other than S. The expression "-" in the table means that only the impurity level is contained. In Table 1, (1) the value on the left side of the formula, (2) the value on the left side of the formula, and Ar3point calculated using (3) the formula are shown. In addition, values deviating from the embodiments of the present application are underlined.

[0160] Table 1

[0161]

[0162] After the cast sheet obtained was heated to 1000 to 1150°C, it was subjected to rolling to obtain a steel sheet sample (hot-rolled sheet) having a sheet thickness of 12 to 16 mm. In Table 2, the rolling conditions, more specifically, the reduction rate in the temperature range of 820°C or higher, the reduction rate in the temperature range of 790°C or higher and lower than 820°C (no hot-rolling is performed in the temperature range lower than 790°C), the finish rolling temperature (FRT), the average cooling rate from the finish rolling temperature to the start temperature of accelerated cooling, the start temperature of accelerated cooling, the end temperature of accelerated cooling, the average cooling rate during accelerated cooling (the average cooling rate from the start temperature of accelerated cooling to the end temperature of accelerated cooling), the cooling rate after accelerated cooling, and the presence or absence of strain introduction (when strain introduction is performed, the amount of strain is also described) are shown. In Table 2, for reference, Ar3point shown in Table 1 is also described.

[0163] Further, in Table 2, conditions deviating from the embodiments of the present application are underlined.

[0164] Air cooling was performed from the finish rolling temperature to the start temperature of accelerated cooling.

[0165] Steel sheet samples No. 6 and 7 were not subjected to accelerated cooling, and air cooling was performed from the finish rolling temperature to the temperature described in the column of "end temperature of accelerated cooling". Therefore, in steel sheet samples No. 6 and 7, the so-called "temperature at which accelerated cooling is completed" means the temperature at which the "cooling rate after accelerated cooling" is started. In addition, since air cooling was thus performed, it means that the average cooling rate in the temperature range of accelerated cooling of steel sheet samples No. 6 and 7 is much lower than 4.0°C / sec.

[0166] The average cooling rate after accelerated cooling means that, in the case of a sample in which the average cooling rate from 500°C to 200°C, that is, the average cooling rate after accelerated cooling, is "air cooling", the average cooling rate after accelerated cooling is in the range of 0.1 to 0.4°C / sec.

[0167] In addition, the sample to which strain was introduced after accelerated cooling (sample 3) was subjected to plastic working of 1.5% in strain at the t / 4 position in the thickness direction by light press-down with a set of reduction in the thickness direction of 85% in the first pass, 75% in the second pass, and 65% in the third pass at ordinary temperature after cooling to ordinary temperature using upper and lower rolls. Also, the value of the introduced strain was derived by comparing the stress-strain curves obtained by tensile test before and after the introduction.

[0168] Table 2

[0169]

[0170] 2. Sample Evaluation

[0171] For each steel sheet, the following detailed methods were used to perform observation of the microstructure, tensile test, Charpy impact test, and HAZ toughness evaluation.

[0172] <Observation of Metal Structure>

[0173] For each steel sheet sample, the area ratio of ferrite and pearlite was measured using an optical microscope at a magnification of 100 times and observing a field of view of 600 μm x 800 μm at the t / 4 position in the thickness of the same position as the impact test piece extraction position described later, using image analysis software. In addition, for samples in which the total area ratio of ferrite + pearlite was not 100%, it was also confirmed what the portion other than ferrite and pearlite, i.e., the remaining structure, was.

[0174] <Tensile Test>

[0175] From the t / 4 position in the thickness of each steel sheet sample, a No. 4 test piece (round bar shape) of JIS Z2241 was extracted perpendicular to the rolling direction and the thickness direction (extracted with the center axis of the round bar being the t / 4 position in the thickness), or a No. IB test piece of JIS Z 2241 (flat plate tensile, total thickness extraction) was used to perform a tensile test in accordance with JIS Z 2241, obtaining a stress-strain curve. Then, based on this stress-strain curve, the upper yield point and the lower yield point determined by JIS Z 2241 were obtained. The difference between the upper yield point and the lower yield point obtained for each sample is shown in Table 3.

[0176] Based on the obtained stress-strain curve, the tensile stress (tensile strength) was also obtained. In addition, the upper yield point was taken as the yield strength. The tensile stress and the yield strength are also shown in Table 3. The tensile stress was acceptable if it was 440 MPa or more and 560 MPa or less, and the yield strength was acceptable if it was 325 MPa or more and 440 MPa or less.

[0177] <Charpy Impact Test>

[0178] The center axis of the test piece was set to a position 6 mm in depth from the surface of the steel sheet test piece, and the longitudinal direction of the test piece was made parallel to the rolling direction, and thus three full-size Charpy impact test pieces (V-notch test pieces according to JIS Z 2202) were extracted from each of the steel sheet test pieces. For the obtained Charpy impact test pieces, Charpy impact tests were performed at -40°C, and the absorbed energy vE －40℃ was measured. The average value of the results of these three Charpy impact tests (vE －40℃ as (ave.)) is shown in the column of "vE －40℃ " in Table 3. The value of vE －40℃ of 200 J or more was determined to be sufficient in terms of low-temperature toughness.

[0179] HAZ Toughness Evaluation

[0180] A test piece of 55 mm (in the rolling direction) x 323 mm (in a direction perpendicular to the rolling direction and the plate thickness direction) x 12.5 mm (in the plate thickness direction) was extracted from the t / 2 position of the steel sheet test piece (the extraction was performed so that the center of the test piece in the plate thickness direction was at the t / 2 position). The obtained test piece was cooled at a rate controlled so that the cooling time from 800°C to 500°C was 340 seconds after being held at 1460°C for 60 seconds. This simulates the thermal cycle when a large heat input (about 9 kJ / mm) welding of single-sided SAW is performed. From these test pieces, three full-size Charpy impact test pieces (V-notch test pieces according to JIS Z 2202) were extracted, and Charpy impact tests were performed at -20°C, and the absorbed energy vE －20 was measured. The average value of the results of these three Charpy impact tests (vE －20℃ as (ave.)) is shown in the column of "HAZ toughness vE －20℃ " in Table 3. The value of HAZ toughness vE －20℃ of 30 J or more was determined to be sufficient in terms of HAZ toughness (large heat input welding characteristics).

[0181] Further, the HAZ toughness evaluation was performed only for steel sheet test pieces No. 1, 2, 4, and 8. Steel sheet test piece No. 2 and steel sheet test piece No. 3 differed only in the presence or absence of the introduction of strain in terms of chemical composition and manufacturing conditions. That is, since the chemical composition and the reduction rate during rolling that affect the HAZ toughness (large heat input welding characteristics) were the same, it is considered that the HAZ toughness of steel sheet test piece No. 3 is the same as that of steel sheet test piece No. 2.

[0182] Among the characteristics shown in Table 3, the underlined ones are outside the scope of the embodiments of the present application.

[0183] Table 3

[0184]

[0185] From Tables 1 to 3, the following can be investigated.

[0186] Steel sheet samples No. 2, 3, and 4 all satisfy all the requirements of the chemical composition and the manufacturing conditions prescribed by the embodiment of the present application. As a result, as shown in Table 3, the requirements of the metal structure and the difference between the upper yield point and the lower yield point are satisfied, and the tensile stress, the yield strength, the low-temperature toughness, and the high heat input welding characteristics are all excellent characteristics.

[0187] Steel sheet sample No. 1, since the prescribed chemical composition is not satisfied, the difference between the upper yield point and the lower yield point is large, although the tensile stress and the yield strength satisfy the prescribed requirements, the high heat input welding characteristics are poor. In addition, not satisfying the formula (2) is also a reason for the low high heat input welding characteristics.

[0188] Steel sheet sample No. 5, the reduction rate at a temperature of 780°C or higher and lower than 820°C is insufficient, as a result, the low-temperature toughness is poor.

[0189] Steel sheet sample No. 6, the hot rolling end temperature is too low, and the cooling rate at the time of accelerated cooling is slow. As a result, the difference between the upper yield point and the lower yield point becomes large, the yield strength is too large, and the low-temperature toughness is poor.

[0190] Steel sheet sample No. 7, the cooling rate at the time of accelerated cooling is too small. As a result, the difference between the upper yield point and the lower yield point becomes large, and the yield strength is too large.

[0191] Steel sheet sample No. 8 does not contain B. As a result, the high heat input welding characteristics are poor.

[0192] Steel sheet sample No. 9, the cooling rate at the time of accelerated cooling is too large. As a result, the bainite structure is excessively generated, and the tensile stress and the yield strength are too large. In addition, steel sheet sample No. 9 does not contain B.

[0193] This application claims priority based on Japanese Patent Application, No. 2021-120011, filed on July 20, 2021. Japanese Patent Application, No. 2021-120011 is incorporated by reference in the present specification.

Claims

1. A steel sheet comprising C: 0.04 to 0.16 mass%, Si: 0.10 to 0.50 mass%, Mn: 0.60 to 1.60 mass%, P: 0.005 to 0.030 mass%, Al: 0.015 to 0.050 mass%, Ti: 0.005 to 0.020 mass%, Ca: 0.0005 to 0.0025 mass%, O: 0.0008 to 0.0025 mass%, B: 0.0005 to 0.0020 mass%, N: 0.0030 to 0.0060 mass%, a balance including Fe and unavoidable impurities, satisfying the following (1) and (2) formulas, a metal structure in a portion at a distance of a quarter of a sheet thickness from a surface, that is, at a sheet thickness direction t / 4 position, includes ferrite structure and pearlite structure in a total of 90% or more in an area ratio, and a balance includes one or more of island-shaped martensite structure and bainite structure, a difference between an upper yield point and a lower yield point obtained from a tensile test is 30 MPa or less, 3 [Si] + 6.3 [P] + 1.4 [Al] ≥ 0.63 (1) where [Si], [P], and [Al] are contents of Si, P, and Al in mass%, [Ca] / [O] ≥ 0.2 (2) where [Ca] and [O] are contents of Ca and O in mass%. one or more selected from a group consisting of V: 0.003 to 0.50 mass%, Nb: 0.003 to 0.020 mass%, Cu: 0.05 to 0.25 mass%, Ni: 0.05 to 0.25 mass%, Cr: 0.05 to 0.25 mass%, Mo: 0.05 to 0.25 mass%, Zr: 0.0001 to 0.010 mass%, Mg: 0.0001 to 0.010 mass%, and REM: 0.0001 to 0.010 mass%.

3. A method of manufacturing a steel sheet having a metal structure in a portion at a distance of a quarter of a sheet thickness from a surface, that is, at a sheet thickness direction t / 4 position, including ferrite structure and pearlite structure in a total of 90% or more in an area ratio, and a balance including one or more of island-shaped martensite structure and bainite structure, and a difference between an upper yield point and a lower yield point obtained from a tensile test is 30 MPa or less, 2. The steel sheet according to claim 1, wherein, In the method of manufacturing the steel sheet, comprising: a step of preparing a steel material comprising: C: 0.04 to 0.16 mass% Si: 0.10 to 0.50 mass%, Mn: 0.60 to 1.60 mass%, P: 0.005 to 0.030 mass%, Al: 0.015 to 0.050 mass%, Ti: 0.005 to 0.020 mass%, Ca: 0.0005 to 0.0025 mass%, O: 0.0008 to 0.0025 mass%, B: 0.0005 to 0.0020 mass%, N: 0.0030 to 0.0060 mass%, a balance including Fe and unavoidable impurities, satisfying the following (1) and (2) formulas, ​ ​ 3[Si] + 6.3[P] + 1.4[Al] ≥ 0.63 (1) Here, [Si], [P], and [Al] are the contents of Si, P, and Al, respectively, in mass %, [Ca] / [O] ≥ 0.2 (2) Here, [Ca] and [O] are the contents of Ca and O, respectively, in mass %; In the rolling process, the steel material is heated to 1000 to 1150°C, then hot-rolled with a reduction ratio of 30% or more in the temperature range of 820°C or higher, a reduction ratio of 10% or more in the temperature range of 790°C or higher and lower than 820°C, and a hot-rolling end temperature of 750°C or higher, followed by cooling at an average cooling rate of 0.5 to 3.0°C / sec to a temperature lower than the Ar3 point shown by the following (3) and higher than (Ar3 point - 150°C), and then cooling at an average cooling rate of 4 to 9°C / sec from the start temperature of the accelerated cooling to an end temperature of 500°C or higher and 650°C or lower, Ar3 (°C) = 910 - 310[C] - 80[Mn] - 20[Cu] - 15[Cr] - 55[Ni] - 80[Mo] (3) Here, [C], [Mn], [Cu], [Cr], [Ni], and [Mo] are the contents of C, Mn, Cu, Cr, Ni, and Mo, respectively, in mass %.

4. The method of producing a steel sheet according to claim 3, wherein The steel material also contains at least one selected from the group consisting of V: 0.003 to 0.50 mass %, Nb: 0.003 to 0.020 mass %, Cu: 0.05 to 0.25 mass %, Ni: 0.05 to 0.25 mass %, Cr: 0.05 to 0.25 mass %, Mo: 0.05 to 0.25 mass %, Zr: 0.0001 to 0.010 mass %, Mg: 0.0001 to 0.010 mass %, and REM: 0.0001 to 0.010 mass %.

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