Steel sheet and method for manufacturing same

Abstract

Provided are: a steel sheet that exhibits excellent toughness in a joint after high heat input welding, while also exhibiting low temperature toughness and a strength range required for a liquefied gas storage tank; and a method for manufacturing the steel sheet. A steel sheet according to the present invention has a prescribed component composition, has a hardness at a depth position of 1 mm from the surface of 210 HV or less, and has a microstructure in which the volume ratio of ferrite is 20% or more at a depth position of 1 / 4 of the sheet thickness, the volume ratio of the total of ferrite and bainite is 80% or more, and the volume ratio of martensite islands is 5% or less. The steel sheet has a yield strength of 440 MPa or less, and a ductility-brittleness fracture surface transition temperature vTrs of -60°C or less.

Description

Steel plate and its manufacturing method

[0001] The present invention relates to a steel plate having excellent toughness and corrosion resistance, in particular to a steel plate having excellent low-temperature toughness and ammonia stress corrosion cracking resistance for use in multipurpose tanks for carrying a mixture of liquefied petroleum gas (hereinafter referred to as LPG) and liquid ammonia, and a method for manufacturing the same.

[0002] With the recent increase in energy demand, the transportation of liquefied gas by energy carriers has become popular. For efficient operation of energy carriers, not only LPG but also liquid ammonia may be transported in the tanks.

[0003] Since these liquefied gases are transported at low temperatures, the steel plates used in the storage tanks for these liquefied gases are required to have excellent low-temperature toughness.

[0004] The steel plates used in these tanks are required to have a yield strength (YS) of 235 MPa or more. Furthermore, liquefied ammonia is known to cause stress corrosion cracking, so by controlling the yield strength (YS) to 440 MPa or less, stress corrosion cracking caused by ammonia can be avoided.

[0005] The techniques for providing the low-temperature toughness and satisfying the strength range required for liquefied gas storage tanks as described above are described in Patent Documents 1 and 2, and high low-temperature toughness and predetermined strength properties are achieved by heat treating a steel plate that has been cooled after hot rolling, or by heat treating a steel plate that has been water-cooled after hot rolling.

[0006] On the other hand, when manufacturing such liquefied gas storage tanks, steel plates must usually be joined together. Therefore, from the viewpoint of work efficiency, high-heat-input welding methods such as submerged arc welding and electrogas arc welding are often adopted. However, when such high-heat-input welding is performed, the properties of the heat-affected zone (hereinafter also referred to as "HAZ") of the steel plate may be impaired due to the large amount of heat transferred to the HAZ. For example, in the HAZ exposed to high temperatures just below the melting point during high-heat-input welding, austenite grains tend to coarsen. Such coarse austenite grains transform into upper bainite structures containing island martensite, which have poor toughness, upon subsequent cooling, which may result in a decrease in the toughness of the HAZ.

[0007] Patent Document 3 discloses a technique for suppressing coarsening of the austenite grain size in the HAZ by dispersing a large amount of TiN.

[0008] Patent Document 4 discloses a technique for reducing island martensite (MA) in the HAZ by reducing the P content in addition to reducing the C and Si contents.

[0009] In the invention described in Patent Document 5, particles such as TiN are dispersed in a ferrite-based microstructure with refined crystal grain size, thereby achieving both brittle crack propagation arrestability and HAZ toughness during high heat input welding.

[0010] Japanese Patent Laid-Open No. 10-140235 Japanese Patent Laid-Open No. 10-168516 International Publication No. 2011 / 148754 Japanese Patent Laid-Open No. 2008-163446 Japanese Patent Laid-Open No. 2015-098642

[0011] The methods described in the above Patent Documents 1 and 2 require multiple heat treatments, which poses an economic problem due to the high costs of equipment and energy required for the treatment. Furthermore, when the first quenching temperature is high, the improvement in toughness at a depth of ¼ of the plate thickness from the surface of the steel plate is small, which may result in unstable toughness in the surface layer of the steel plate exposed to high temperatures for a long time.

[0012] Furthermore, the techniques described in Patent Documents 3 and 5, which utilize TiN to improve HAZ toughness during high heat input welding, heat the HAZ to the melting temperature range of TiN during high heat input welding, which causes the TiN to decompose, eliminating the dispersion effect, and embrittlement of the steel matrix due to solute Ti and solute N produced by the decomposition of TiN, resulting in a significant decrease in HAZ toughness.

[0013] In addition, the technology described in Patent Document 4, which reduces the P content in order to reduce the amount of MA, causes variations in the suppression of the amount of MA due to the distribution of P, which is likely to segregate at grain boundaries, etc., and is therefore insufficient from the viewpoint of uniformly reducing the amount of MA in the HAZ structure.

[0014] In view of the above circumstances, an object of the present invention is to provide a steel plate that satisfies the low-temperature toughness and strength ranges required for liquefied gas storage tanks, while also providing excellent toughness in joints after large heat input welding, and a method for manufacturing the same.

[0015] In this specification, "high heat input welding" refers to welding in which the heat input is 100 kJ / cm or more, and specifically includes submerged arc welding.

[0016] In order to achieve the above object, the present inventors have conducted extensive research using an online cooling device to examine various factors affecting the low-temperature toughness and strength properties of a steel plate. As a result, they have found that desired low-temperature toughness and strength properties can be obtained by adding predetermined amounts of elements such as C, Si, Mn, and Ti to a steel plate and controlling the microstructure so that the volume fraction of ferrite at a depth of ¼ of the plate thickness from the surface of the steel plate is 20% or more, the total volume fraction of ferrite and bainite is 80% or more, and the volume fraction of island martensite is 5% or less.

[0017] Furthermore, the inventors have found that stress corrosion cracking resistance in a liquid ammonia environment is improved by controlling the yield strength (YS) of the base material to 440 MPa or less and the hardness at a depth of 1 mm from the surface of the steel plate to 210 HV or less.

[0018] Furthermore, the inventors have conducted extensive research to improve the toughness in the HAZ when large heat input welding is performed (hereinafter also referred to as "HAZ toughness due to large heat input welding"), and have obtained the following new findings (1) and (2).

[0019] (A) In order to improve HAZ toughness due to large heat input welding, it is important to suppress the coarsening of austenite grain size in the HAZ. To suppress the coarsening of austenite grain size in the HAZ, it is important to disperse a large amount of TiN. However, when subjected to large heat input welding, there is a risk that TiN will decompose due to heating to the melting temperature range, resulting in the loss of the dispersion effect. It has been discovered that, in order to suppress the decomposition of TiN, the ratio of the Ti content (mass%) to the N content (mass%) (Ti / N) is 2.00 to 4.00, and the decomposition of TiN during large heat input welding is suppressed by designing the composition so that the ratio satisfies the condition of formula (1) described below.

[0020] (B) In order to improve HAZ toughness in large heat input welding, it is important to reduce island martensite (MA) in the HAZ. To minimize the formation of MA in the HAZ, it is important to control the carbon equivalent (C) expressed by the formula (2) described below to 0.380 or less, and to limit the C content and Si content of the steel plate to 0.100 mass% or less and 0.50 mass% or less. In order to prevent a decrease in HAZ toughness, it is also necessary to limit the Mn content to 1.80 mass% or less and the Al content to 0.060 mass% or less. Here, in the present invention, "minimally minimizing the formation of MA" means that the volume fraction of MA in the microstructure of the HAZ is 5% or less.

[0021] The present invention was completed based on these findings and further investigations. That is, the gist of the present invention is as follows.

[0022] 1. A steel sheet comprising, by mass%, C: 0.020% or more and 0.100% or less, Si: 0.50% or less, Mn: 0.30% or more and 1.80% or less, P: 0.010% or less, S: 0.0100% or less, Al: 0.060% or less, Ti: 0.010% or more and 0.031% or less, N: 0.0038% or more and 0.0100% or less, and O: 0.0100% or less, wherein with respect to Ti and N, Ti / N, which is the ratio of the Ti content (mass%) to the N content (mass%), is 2.00 or more and 4.00 or less, and satisfies the following formula (1), and further has a component composition consisting of a carbon equivalent Ceq represented by the following formula (2) of 0.190 or more and 0.380 or less, with the balance being Fe and unavoidable impurities, A steel sheet having a hardness of 210 HV or less at a depth of 1 mm from the surface, a microstructure in which, at a depth of 1 / 4 of the sheet thickness from the surface, the volume fraction of ferrite is 20% or more, the total volume fraction of ferrite and bainite is 80% or more, and the volume fraction of island martensite is 5% or less, a yield strength of 440 MPa or less, and a ductile-brittle fracture transition temperature vTrs of -60°C or less. 169≦5158×Ti+25563×N≦360 (1) Ceq=C+Mn / 6+(Cu+Ni) / 15+(V+Mo+Cr) / 5 (2) where, in the above formulas (1) and (2), each element symbol represents the content (mass%) of each element, and if the element is not contained, it is set to 0.

[0023] 2. The steel sheet according to 1 above, wherein the chemical composition further contains, in mass%, one or more elements selected from Cu: 1.00% or less, Ni: 1.00% or less, Cr: 1.00% or less, Mo: 0.50% or less, V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, Nb: 0.050% or less, B: 0.0010% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, REM: 0.0050% or less, Sn: 0.05% or less, Zn: 0.05% or less, Pb: 0.10% or less, As: 0.05% or less, Sb: 0.10% or less, and Bi: 0.10% or less.

[0024] 3. The steel plate according to 1 above, containing, by mass%, C: 0.045% or more, and Mn: 0.50% or more, wherein the carbon equivalent Ceq is 0.250 or more, the steel plate has a microstructure in which the volume fraction of bainite at a depth position of 1 / 4 of the plate thickness from the steel plate surface is 20% or more, and the steel plate has a tensile strength of 400 MPa or more.

[0025] 4. The steel plate according to 2 above, containing, by mass%, C: 0.045% or more, and Mn: 0.50% or more, wherein the carbon equivalent Ceq is 0.250 or more, the steel plate has a microstructure in which the volume fraction of bainite at a depth position of 1 / 4 of the plate thickness from the steel plate surface is 20% or more, and the steel plate has a tensile strength of 400 MPa or more.

[0026] 5. A steel material having the chemical composition described in any one of items 1 to 4 is subjected to Ar at a temperature at a depth of 1 / 4 of the plate thickness from the surface of the steel material. ３ The temperature is then increased to a rolling start temperature of 1250°C or higher. ３ The rolling end temperature is set to Ar ３ After that, the temperature is increased to 1000 K. ３ a cooling step of cooling the steel plate to a temperature below the temperature point at which the steel plate is heated, stopping the cooling for at least 5 seconds, and then cooling the steel plate at an average cooling rate of 100°C / s or less at a depth of 1 / 4 of the plate thickness from the surface of the steel plate, and completing the cooling step when the temperature at a depth of 1 / 4 of the plate thickness from the surface of the steel plate is 400°C or higher.

[0027] 6. A steel material having the chemical composition described in any one of items 1 to 4 is heated to a temperature of 950°C or more and 1250°C or less at a depth of 1 / 4 of the plate thickness from the surface of the steel material, and then the rolling start temperature is set to Ar. ３ The rolling end temperature is set to Ar ３ After that, hot rolling is performed to a temperature of 1000°C or more. ３ Cooling begins from a temperature above the Ar point. ３ Ar below the point ３6. The method for producing a steel plate according to claim 5, wherein the steel plate is cooled at an average cooling rate of 5°C / s or more until the temperature reaches a temperature of −90°C or higher, the cooling is stopped once and the cooling is interrupted for 5 seconds or more and 600 seconds or less, and subsequently the steel plate is cooled at an average cooling rate of 5°C / s or more and 100°C / s or less at a depth of 1 / 4 of the plate thickness from the surface thereof, and the cooling is completed when the temperature at a depth of 1 / 4 of the plate thickness from the surface of the steel plate is 400°C or more and 580°C or less.

[0028] According to the present invention, it is possible to provide a steel plate that can satisfy the low-temperature toughness and strength range required for liquefied gas storage tanks while also providing excellent toughness in joints after large heat input welding, and a method for manufacturing the same.

[0029] In the present invention, it is important that the steel sheet and the steel material used for its production have the above-mentioned chemical composition. Therefore, the reason for limiting the chemical composition of the steel in the present invention to the above-mentioned will first be explained. Note that "%" regarding the chemical composition means "mass %" unless otherwise specified.

[0030] [Component Composition] C: 0.020% or More and 0.100% or Less C is an element that increases the hardenability of steel. If the C content exceeds 0.100%, the toughness of the joint HAZ in high heat input welding decreases. Therefore, the C content is set to 0.100% or less. Furthermore, from the viewpoint of suppressing the decrease in the toughness of the base metal and the joint HAZ, the C content is preferably set to 0.095% or less, and more preferably set to 0.090% or less. On the other hand, from the viewpoint of increasing the strength (YS, TS) of the base metal, the C content is set to 0.020% or more, preferably 0.045% or more, more preferably 0.050% or more, and even more preferably 0.055% or more.

[0031] Si: 0.50% or less Si acts as a deoxidizer, but it also reduces the toughness of the base material and the toughness of the joint HAZ during high-heat-input welding. Therefore, it is preferable to keep the Si content as low as possible, but a content of 0.50% or less is acceptable. Since steel can be sufficiently deoxidized with Al, Ti, or the like, the lower limit of the Si content is not particularly limited and may be 0%. From the viewpoint of the toughness of the base material and the toughness of the joint HAZ, the Si content is preferably 0.40% or less, and more preferably 0.30% or less. On the other hand, Si is a component that can suppress the formation of coarse carbides and increase the strength of the base material. From the viewpoint of reducing the content of other alloying elements and achieving lower manufacturing costs, the Si content is preferably 0.10% or more. Furthermore, from the viewpoint of lower manufacturing costs, the Si content is more preferably 0.15% or more, and even more preferably 0.20% or more.

[0032] Mn: 0.30% or more and 1.80% or less Mn is an element that increases the hardenability of steel. If the Mn content exceeds 1.80%, the toughness of the joint HAZ in high-heat-input welding decreases, and the alloy cost becomes excessively high. Therefore, the Mn content is set to 1.80% or less. Furthermore, from the viewpoint of suppressing the deterioration of weldability, the Mn content is preferably set to 1.70% or less, and more preferably set to 1.60% or less. On the other hand, from the viewpoint of increasing the strength (YS, TS) of the base metal, the Mn content is set to 0.30% or more, and preferably set to 0.50% or more. Furthermore, from the viewpoint of reducing the contents of other alloying elements and manufacturing at lower cost, the Mn content is preferably set to 0.70% or more, and more preferably set to 0.90% or more.

[0033] P: 0.010% or less P is an element contained as an unavoidable impurity, and its segregation at grain boundaries has adverse effects, such as reducing the toughness of the base material and the toughness of the HAZ of a joint in high heat input welding. Therefore, it is desirable to keep the P content as low as possible, but a content of 0.010% or less is acceptable. The lower limit of the P content is not particularly limited and may be 0%. However, since P is usually an element inevitably contained in steel as an impurity, the P content may be industrially greater than 0%. Furthermore, since excessive reduction of P leads to increased refining costs, it is preferable to set the P content to 0.0005% or more.

[0034] S: 0.0100% or less S is an element contained as an unavoidable impurity. It exists in steel as sulfide-based inclusions such as MnS, and has an adverse effect on the toughness of the base material, such as becoming the origin of fracture. Therefore, it is desirable to reduce the S content as much as possible. Furthermore, S has an adverse effect, such as reducing the toughness of the HAZ of a joint in high heat input welding. Therefore, the S content is set to 0.0100% or less. The lower limit of the S content is not particularly limited and may be 0%. Since S is usually an element inevitably contained in steel as an impurity, the S content may exceed 0% industrially. That is, excessive reduction of S leads to an increase in refining costs, so from a cost perspective, the S content is preferably set to 0.0005% or more, and more preferably 0.0010% or more.

[0035] Al: 0.060% or less Al acts as a deoxidizer, reducing oxide-based inclusions, improving the toughness of the joint HAZ, and refining the grains. To achieve these effects, the Al content is preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, if the Al content exceeds 0.060%, the oxide-based inclusions increase and the cleanliness decreases, thereby reducing the toughness of the base metal and the toughness of the joint HAZ in high-heat-input welding. Therefore, the Al content is set to 0.060% or less. The Al content is preferably 0.050% or less, and more preferably 0.040% or less.

[0036] Ti and N are elements that play an important role in the present invention, as they precipitate as TiN during solidification of steel, suppress coarsening of austenite grains in the HAZ, and contribute to high toughness by serving as ferrite transformation nuclei. Ti and N are contained so as to satisfy the following requirements.

[0037] Ti / N: 2.00 or more and 4.00 or less If the Ti / N ratio is less than 2.00, the amount of TiN produced decreases, and the solute N that does not become TiN reduces HAZ toughness. Therefore, Ti / N is set to 2.00 or more. Note that Ti / N is preferably set to 2.10 or more, and more preferably set to 2.20 or more. Furthermore, if the Ti / N ratio exceeds 4.00, the TiN becomes coarse and reduces HAZ toughness. Therefore, Ti / N is set to 4.00 or less. Furthermore, from the viewpoint of improving HAZ toughness, Ti / N is preferably set to 3.90 or less, and more preferably set to 3.80 or less. Note that Ti / N refers to the content (mass%) of each element in the steel, and Ti / N refers to the ratio of the Ti content (mass%) to the N content (mass%).

[0038] 169≦5158×Ti+25563×N≦360 (1) Conventional techniques for improving toughness during high heat input welding using TiN have had the problem that TiN decomposes in the HAZ of high heat input welding, eliminating the dispersion effect, or the solute Ti and N produced by the decomposition of TiN embrittle the base structure of the steel, resulting in a significant decrease in HAZ toughness. Therefore, in order to suppress this TiN decomposition, it is essential to set the value of 5158×Ti+25563×N to 169 or greater. From the perspective of further improving HAZ toughness, the value of 5158×Ti+25563×N is preferably greater than 175, more preferably greater than 180, and even more preferably greater than 185. On the other hand, if the value of 5158×Ti+25563×N exceeds 360, a large amount of TiN is produced, which actually reduces HAZ toughness. Therefore, the value of 5158 × Ti + 25563 × N is set to be equal to or less than 360. From the viewpoint of further improving toughness, 5158 × Ti + 25563 × N is preferably set to be equal to or less than 330, more preferably set to be equal to or less than 300, and even more preferably set to be equal to or less than 270.

[0039] In the present invention, the ranges of the Ti and N contents are as specified above, but in accordance with the above specifications, the preferred ranges of the Ti and N contents are specifically as follows:

[0040] Ti: 0.010% or more and 0.031% or less Ti is an element that has a strong tendency to form nitrides and has the effect of fixing N and reducing solute N. Therefore, the inclusion of Ti can improve the toughness of the base metal and the joint HAZ. In order to obtain excellent HAZ toughness, particularly in large heat input welding, the Ti content is set to 0.010% or more. Furthermore, the Ti content is preferably set to 0.012% or more. On the other hand, if the Ti content exceeds 0.031%, the toughness of the joint HAZ decreases. Therefore, the Ti content is set to 0.031% or less. Furthermore, the Ti content is preferably set to 0.029% or less, and more preferably 0.027% or less.

[0041] N: 0.0038% or more and 0.0100% or less N combines with Ti to precipitate as TiN, contributing to microstructural refinement and improving the toughness of the base metal and the joint HAZ in high heat input welding. To achieve this effect, the N content is set to 0.0038% or more. Furthermore, the N content is preferably set to 0.0044% or more. On the other hand, an N content exceeding 0.0100% actually leads to a decrease in the toughness of the base metal and the joint HAZ. Therefore, from the viewpoint of suppressing a decrease in the toughness of the base metal and the joint HAZ, the N content is set to 0.0100% or less. Furthermore, the N content is preferably set to 0.0080% or less, and more preferably to 0.0060% or less.

[0042] O: 0.0100% or less O is an element contained as an unavoidable impurity. It forms oxides and becomes the origin of fracture, adversely affecting the toughness of the base material and the joint HAZ. Therefore, the O content is limited to 0.0100% or less. The O content is preferably 0.0050% or less, and more preferably 0.0030% or less. On the other hand, the lower limit of the O content is not particularly limited and may be 0%, but O is usually an element that is unavoidably contained in steel as an impurity. Therefore, the O content may exceed 0% industrially. That is, excessive reduction of O leads to an increase in refining costs, so from a cost perspective, the O content is preferably 0.0020% or more.

[0043] Ceq: 0.190 or more and 0.380 or less In order to satisfy the toughness of the joint HAZ during large heat input welding, it is essential that the carbon equivalent Ceq, defined by the following formula (2): Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (V + Mo + Cr) / 5 (2), be 0.380 or less. If Ceq exceeds 0.380, the yield strength (YS) of 440 MPa or less, which is necessary to avoid stress corrosion cracking due to ammonia, cannot be satisfied, and the toughness of the base metal and the toughness of the joint HAZ also decrease. On the other hand, in order to achieve a yield strength (YS) of 235 MPa or more and a tensile strength (TS) of 400 MPa or more, which are suitable for tanks, Ceq is preferably 0.190 or more and 0.250 or more. In order to achieve a higher tensile strength (TS), Ceq is more preferably 0.300 or more, and even more preferably 0.350 or more.

[0044] The basic composition of the present invention contains the above components, with the balance being Fe and unavoidable impurities. This basic composition may optionally further contain one or more elements selected from Cu: 1.00% or less, Ni: 1.00% or less, Cr: 1.00% or less, Mo: 0.50% or less, V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, Nb: 0.050% or less, B: 0.0010% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, and REM: 0.0050% or less, for the purpose of improving strength characteristics or toughness. Optionally, one or more selected from Sn: 0.05% or less, Zn: 0.05% or less, Pb: 0.10% or less, As: 0.05% or less, Sb: 0.10% or less, and Bi: 0.10% or less may be contained.

[0045] Cu: 1.00% or less Cu is an element that has the effect of increasing the hardenability of steel and improving the strength of the steel sheet, and can be contained as desired. When Cu is contained, in order to obtain the above effect, the Cu content is preferably 0.01% or more, and more preferably 0.20% or more. On the other hand, a Cu content exceeding 1.00% leads to a deterioration in toughness and an increase in alloy costs. Therefore, when Cu is contained, the Cu content is preferably 1.00% or less, and more preferably 0.50% or less.

[0046] Ni: 1.00% or less Ni, like Cu, is an element that improves the strength of the steel sheet and can be contained as desired. When Ni is contained, in order to obtain the above effect, the Ni content is preferably 0.01% or more, and more preferably 0.20% or more. On the other hand, if the Ni content exceeds 1.00%, it will lead to deterioration of weldability and an increase in alloy costs. Therefore, when Ni is contained, the Ni content is preferably 1.00% or less, and more preferably 0.50% or less.

[0047] Cr: 1.00% or less Cr is an element that, like Cu, has the effect of improving the strength of the steel sheet and can be contained arbitrarily. When Cr is contained, in order to obtain the above effect, the Cr content is preferably 0.01% or more, more preferably 0.05% or more. On the other hand, if the Cr content exceeds 1.00%, it will lead to deterioration of weldability and an increase in alloy cost. Therefore, when Cr is contained, the Cr content is preferably 1.00% or less, more preferably 0.50% or less.

[0048] Mo: 0.50% or less Mo, like Cu, is an element that improves the strength of the steel sheet and can be added as desired. When Mo is added, in order to obtain the above effect, the Mo content is preferably 0.01% or more, more preferably 0.05% or more. On the other hand, if the Mo content exceeds 0.50%, it leads to deterioration of weldability and an increase in alloy cost. Therefore, when Mo is added, the Mo content is preferably 0.50% or less, more preferably 0.25% or less.

[0049] V: 0.50% or less V, like Cu, is an element that improves the strength of the steel sheet and can be added as desired. When V is added, in order to obtain the above effect, the V content is preferably 0.01% or more, more preferably 0.05% or more. On the other hand, if the V content exceeds 0.50%, it will lead to deterioration of weldability and an increase in alloy cost. Therefore, when V is added, the V content is preferably 0.50% or less, more preferably 0.25% or less.

[0050] W: 0.50% or less W, like Cu, is an element that improves the strength of the steel sheet and can be added as desired. When W is added, in order to obtain the above effect, the W content is preferably 0.01% or more, more preferably 0.05% or more. On the other hand, a W content exceeding 0.50% leads to deterioration of weldability and an increase in alloy costs. Therefore, when W is added, the W content is preferably 0.50% or less, more preferably 0.25% or less.

[0051] Co: 0.50% or less Co, like Cu, is an element that improves the strength of the steel sheet and can be added as desired. When Co is added, in order to obtain the above effect, the Co content is preferably 0.01% or more, more preferably 0.05% or more. On the other hand, a Co content exceeding 0.50% leads to deterioration of weldability and an increase in alloy cost. Therefore, when Co is added, the Co content is preferably 0.50% or less, more preferably 0.25% or less.

[0052] Nb: 0.050% or less Nb is an element that has the effect of reducing the prior austenite grain size and improving toughness by precipitating as carbonitrides. When Nb is contained, in order to obtain the above effect, the Nb content is preferably 0.005% or more, and more preferably 0.007% or more. On the other hand, if the Nb content exceeds 0.050%, a large amount of NbC precipitates, reducing toughness. Therefore, when Nb is contained, the Nb content is preferably 0.050% or less, more preferably 0.040% or less, and even more preferably 0.030% or less.

[0053] B: 0.0010% or less B is an element that has the effect of significantly improving hardenability even in small amounts. Therefore, the inclusion of B can improve the strength of the steel sheet. When B is contained, in order to obtain the above effect, the B content is preferably 0.0001% or more, more preferably 0.0002% or more, and even more preferably 0.0004% or more. On the other hand, if the B content exceeds 0.0010%, the weldability decreases. Therefore, when B is contained, the B content is preferably 0.0010% or less, more preferably 0.0008% or less, and even more preferably 0.0006% or less.

[0054] Ca: 0.0050% or less Ca is an element that bonds with S and inhibits the formation of MnS and other elements that elongate in the rolling direction. Therefore, by including Ca, the morphology of sulfide-based inclusions can be controlled to be spherical, thereby improving the toughness of welds and other structures. When Ca is included, in order to achieve the above effect, the Ca content is preferably 0.0005% or more, and more preferably 0.0020% or more. On the other hand, if the Ca content exceeds 0.0050%, the cleanliness of the steel decreases. A decrease in cleanliness leads to deterioration of surface properties due to an increase in surface defects and a decrease in bending workability. Therefore, when Ca is included, the Ca content is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less.

[0055] Mg: 0.0050% or less Like Ca, Mg is an element that bonds with S and suppresses the formation of MnS and other elements that elongate in the rolling direction. Therefore, by including Mg, the morphology of sulfide-based inclusions can be controlled to be spherical, thereby improving the toughness of welds and other structures. When Mg is included, in order to achieve this effect, the Mg content is preferably 0.0005% or more, and more preferably 0.0020% or more. On the other hand, if the Mg content exceeds 0.0050%, the cleanliness of the steel decreases. A decrease in cleanliness leads to deterioration of surface properties due to an increase in surface defects and a decrease in bending workability. Therefore, when Mg is included, the Mg content is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less.

[0056] REM: 0.0050% or less Like Ca and Mg, REM (rare earth metal) is an element that bonds with S and inhibits the formation of MnS and other elements that elongate in the rolling direction. Therefore, by including REM, the morphology of sulfide-based inclusions can be controlled to be spherical, thereby improving the toughness of welds and other structures. When REM is included, to achieve this effect, the REM content is preferably 0.0005% or more, more preferably 0.0020% or more. On the other hand, if the REM content exceeds 0.0050%, the cleanliness of the steel decreases. A decrease in cleanliness leads to deterioration of surface properties due to an increase in surface defects and a decrease in bending workability. Therefore, when REM is included, the REM content is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less.

[0057] Sn: 0.05% or less Sn promotes cracking of the slab surface during continuous casting and heating, so it is desirable to keep the content as low as possible. The lower limit of the Sn content is not particularly limited and may be 0%. If the Sn content is 0.05% or less, the adverse effects are tolerable. The Sn content is preferably 0.03% or less, and more preferably 0.01% or less.

[0058] Zn: 0.05% or less Zn evaporates as white smoke during molten iron treatment, causing damage to steelworks equipment, so it is desirable to keep the content as low as possible. The lower limit of the Zn content is not particularly limited and may be 0%. If the Zn content is 0.05% or less, the adverse effects are tolerable. The Zn content is preferably 0.03% or less, and more preferably 0.01% or less.

[0059] Pb: 0.10% or less Since Pb reduces joint toughness, it is desirable to keep the content as low as possible. The lower limit of the Pb content is not particularly limited and may be 0%. If the Pb content is 0.10% or less, the adverse effects are tolerable. The Pb content is preferably 0.05% or less, and more preferably 0.03% or less.

[0060] As: 0.05% or less Since As promotes cracking of the slab surface during continuous casting and heating, it is desirable to keep the content as low as possible. The lower limit of the As content is not particularly limited and may be 0%. If the As content is 0.05% or less, the adverse effects are tolerable. The As content is preferably 0.03% or less, and more preferably 0.01% or less.

[0061] Sb: 0.10% or less Like Sn, Sb promotes cracking of the slab surface during continuous casting and heating, so it is desirable to keep the content as low as possible. The lower limit of the Sb content is not particularly limited and may be 0%. If the Sb content is 0.10% or less, the adverse effects are tolerable. The Sb content is preferably 0.05% or less, and more preferably 0.03% or less.

[0062] Bi: 0.10% or less Like Sn and Sb, Bi promotes cracking of the slab surface during continuous casting and heating, so it is desirable to keep the content as low as possible. The lower limit of the Bi content is not particularly limited and may be 0%. If the Bi content is 0.10% or less, the adverse effects are tolerable. The Bi content is preferably 0.05% or less, and more preferably 0.03% or less.

[0063] The balance of the above basic components and optional components consists of Fe and inevitable impurities. These inevitable impurities are impurities that are inevitably mixed in from raw materials, the manufacturing process, or manufacturing equipment, and are allowed to be present to the extent that they do not impair the object of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap.

[0064] In addition to having the above-mentioned chemical composition, the steel sheet of the present invention has a hardness of 210 HV or less at a depth of 1 mm from the surface of the steel sheet, and a microstructure in which, at a depth of ¼ of the sheet thickness from the surface of the steel sheet, the volume fraction of ferrite is 20% or more, the total volume fraction of ferrite and bainite is 80% or more, and the volume fraction of island martensite is 5% or less. The reasons for limiting the hardness and microstructure of the steel as described above will be explained below.

[0065] Hardness of 210 HV or less at a depth of 1 mm from the surface of the steel plate The hardness of the steel plate at a depth of 1 mm from the surface is set to 210 HV or less. If the hardness of the surface layer of the steel plate, specifically at a depth of 1 mm from the surface of the steel plate, is high, stress corrosion cracking in a liquid ammonia environment is promoted. Therefore, in the present invention, by setting the hardness at a depth of 1 mm from the surface of the steel plate to 210 HV or less, excellent ammonia SCC resistance can be ensured. Note that the lower limit of the hardness at a depth of 1 mm from the surface of the steel plate is not particularly limited, but is preferably about 130 HV.

[0066] Generally, in a steel sheet that has been continuously cooled after hot rolling, the surface has a higher hardness because the cooling rate is faster than that of the interior. In the present invention, as described below in connection with the manufacturing conditions of the steel sheet, the cooling after hot rolling is temporarily interrupted and the temperature at which the second cooling is stopped is set higher, thereby preventing excessive hardening of the steel sheet surface.

[0067] [Microstructure] The microstructure of the steel plate of the present invention will be described. [The volume fraction of ferrite at a depth of 1 / 4 of the plate thickness from the surface of the steel plate is 20% or more, the total volume fraction of ferrite and bainite is 80% or more, and the volume fraction of island martensite is 5% or less] The structure at a depth of 1 / 4 of the plate thickness from the surface of the steel plate has a volume fraction of ferrite of 20% or more, a total volume fraction of ferrite and bainite of 80% or more, and the volume fraction of island martensite contained in the remainder is 5% or less. The microstructure at a depth of 1 / 4 of the plate thickness from the surface of the steel plate is specified because it has a strong influence on the strength characteristics of the entire steel plate. If the volume fraction of ferrite is less than 20%, the yield strength (YS) of the base material increases, making it impossible to achieve a yield strength (YS) of 440 MPa or less. In addition, the toughness of the base material decreases. On the other hand, if the total volume fraction of ferrite and bainite is less than 80%, the toughness of the base material decreases, so the total volume fraction of ferrite and bainite is set to 80% or more. In order to achieve a tensile strength (TS) of 400 MPa or more, it is preferable that the volume fraction of bainite is 20% or more.

[0068] The remainder other than the ferrite and bainite may contain microstructures such as pearlite and austenite, but the island martensite in the remainder has higher strength but lower toughness than normal martensite, and can become the starting point of fracture. If the island martensite is contained in an amount exceeding 5% by volume, the toughness of the base material will be significantly reduced, so the volume fraction of the island martensite is set to 5% or less, preferably 3% or less, and can of course be set to 0%.

[0069] On the other hand, the remaining structure, which accounts for 20% or less in volume fraction, may contain martensite in addition to pearlite and austenite. The volume fraction of each structure in the remaining structure does not need to be particularly limited, but it is preferable that the remaining structure is pearlite. The volume fractions of various microstructures can be measured by the method described in the Examples below.

[0070] Next, a method for manufacturing a steel sheet according to the present invention will be described. A steel material having the above-described composition is heated and hot-rolled to form a hot-rolled steel sheet. ３ The steel sheet is then cooled to a temperature equal to or higher than the transformation point to obtain a steel sheet. Each manufacturing condition will be described in detail below. In the present invention, unless otherwise specified, the temperature of the steel sheet is the temperature at a depth of ¼ of the sheet thickness from the surface of the steel sheet.

[0071] First, the conditions for producing the steel material do not need to be particularly limited, but it is preferable to produce molten steel having the above-mentioned composition by a known melting method such as a converter, and then form the molten steel into a steel material such as a slab of a predetermined size by a known casting method such as a continuous casting method. Note that there is no problem with producing a steel material such as a slab of a predetermined size by an ingot casting-decomposition rolling method.

[0072] The obtained steel material is either directly hot rolled without cooling, or is heated once and then hot rolled. ３ Transformation point (hereinafter abbreviated as "Ar ３ (also called "point") + 100 ° C or higher onset temperature, Ar ３ Then, cooling of the hot-rolled steel sheet is started, and the temperature at a depth of 1 / 4 of the sheet thickness from the surface of the steel sheet reaches Ar. ３The steel plate is cooled to a temperature below this point, the cooling is stopped once and the cooling is interrupted for 5 seconds or more, and then cooling is performed at an average cooling rate of 100°C / s or less at a temperature at a depth of 1 / 4 of the plate thickness from the surface of the steel plate (second cooling), and the cooling is completed when the temperature at a depth of 1 / 4 of the plate thickness from the surface of the steel plate is 400°C or higher.

[0073] (a) Heating temperature of steel material: Ar ３ Point or more, 1250°C or less When the heating temperature of the steel material is high and exceeds 1250°C, austenite becomes coarse, the toughness of the base material decreases, and oxidation becomes significant, which may increase oxidation loss and reduce yield. For these reasons, the heating temperature is set to 1250°C or less. It is preferable that the heating temperature of the steel material is set to 1150°C or less. On the other hand, when the heating temperature of the steel material is Ar ３ If the temperature is lower than this point, the reverse transformation to austenite does not occur, and the deformation resistance increases, which increases the load on the hot rolling mill and makes hot rolling difficult. ３ From the viewpoint of improving the yield strength (YS) and tensile strength (TS) of the base material, the heating temperature of the steel material is preferably 950°C or higher, and more preferably 1000°C or higher.

[0074] Here, Ar ３ The point can be obtained, for example, by the following equation (3): Ar ３ (°C) = 910 - 273 x C - 74 x Mn - 57 x Ni - 16 x Cr - 9 x Mo - 5 x Cu (3) where each element indicates the content (mass%) of that element.

[0075] (b) Hot rolling start temperature: Ar ３ Point +100°C or higher After heating to the above temperature, Ar ３ Hot rolling is started at a temperature of 100°C or higher than the rolling point. ３ If the temperature is less than the hot rolling point +100°C, the austenite grain size will become mixed grains, which may deteriorate the toughness of the base material and make it impossible to achieve the subsequent hot rolling finish temperature and cooling start temperature.

[0076] (c) Hot rolling finish temperature: Ar ３ Point or more Ar３ The rolling is finished at a temperature equal to or higher than the Ar point. ３ If the temperature is lower than this point, ferrite will be generated and the generated ferrite will be affected by the processing, resulting in a deterioration in toughness and a larger load on the hot rolling mill. ３ The hot rolling end temperature is set to Ar ３ It is preferable to set the temperature at the temperature of +20°C or higher.

[0077] (d) Cooling Start Temperature: Next, the hot-rolled steel sheet is cooled after hot rolling. From the viewpoint of improving the yield strength (YS) and tensile strength (TS) of the base material, ３ It is preferable to cool the hot-rolled steel sheet from this point or above.

[0078] (e) Average cooling rate in the first cooling: The average cooling rate in the first cooling at a depth of 1 / 4 of the plate thickness from the surface of the steel plate is not particularly limited. From the viewpoint of improving the yield strength (YS) and tensile strength (TS) of the base material, the average cooling rate is preferably 5°C / s or more, and more preferably 10°C / s or more. There is no particular upper limit to the cooling rate, but since excessive cooling increases cooling costs, it is preferable to set the average cooling rate to 200°C / s or less. The average cooling rate is calculated by dividing the difference between the temperature (°C) at the start of cooling and the temperature (°C) at the end of cooling by the cooling time (s).

[0079] (f) Cooling stop temperature in the first cooling: The temperature at a depth of 1 / 4 from the surface of the steel plate is Ar ３ The stop temperature of the first cooling is set to a value equal to or less than the Ar value so that the volume fraction of ferrite at a depth of 1 / 4 from the surface of the steel plate and the total volume fraction of ferrite and bainite are predetermined volume fractions. ３ The cooling stop temperature is Ar ３ If the transformation point is exceeded, ferrite is not generated and the toughness decreases. ３ In order to increase the yield strength (YS) and tensile strength (TS) of the base material and to make the volume fraction of bainite 20% or more, the cooling stop temperature is set to Ar ３The temperature is preferably −90° C. or higher.

[0080] (g) Cooling stop time: 5 seconds or more After the first cooling, cooling is temporarily stopped for 5 seconds or more. By stopping cooling, ferrite can be generated. If the cooling stop time is less than 5 seconds, ferrite is not generated, resulting in a decrease in toughness and an excessively high yield strength (YS) of the base material. On the other hand, although there is no particular upper limit on the cooling stop time, in order to increase the yield strength (YS) and tensile strength (TS) of the base material to a volume fraction of bainite of 20% or more, it is preferable to set the cooling stop time to 600 seconds or less. Furthermore, by setting the stop temperature for this cooling stop and the second cooling described below higher, the hardness of the surface layer can be reduced.

[0081] (h) Second cooling rate: An average cooling rate of 100°C / s or less at a depth of 1 / 4 from the surface of the steel plate. After the cooling is stopped, cooling is resumed. The average cooling rate here is set to 100°C / s or less so that the microstructure has a predetermined volume fraction. If the average cooling rate exceeds 100°C / s, the volume fraction of island martensite becomes too high, resulting in a decrease in toughness. On the other hand, in order to increase the volume fraction of bainite to 20% or more in order to increase the yield strength (YS) and tensile strength (TS) of the base material, it is preferable to set the average cooling rate to 5°C / s or more.

[0082] (i) Second cooling end temperature: Cooling end temperature at a ¼ depth position from the surface of the steel plate is 400°C or higher The second cooling end temperature is set to 400°C or higher in order to adjust the hardness of the surface layer and the strength and toughness of the base material. If the cooling end temperature is lower than 400°C, the yield strength (YS) of the base material increases, the toughness decreases, and the hardness of the surface layer also increases. In order to increase the yield strength (YS) and tensile strength (TS) of the base material, it is preferable to set the cooling end temperature to 580°C or lower.

[0083] A steel plate having the above-described structure can be obtained by manufacturing a steel material having the above-described chemical composition according to the above-described manufacturing conditions. The steel plate thus obtained has strength properties and toughness suitable for a cryogenic tank for storing ammonia. Here, "excellent strength properties" means that the yield strength (YS) of the steel plate (yield point (YP) when a yield point exists, or 0.2% proof stress σ0.2 when no yield point exists) is 235 MPa or more and 440 MPa or less. The yield strength (YS) is closely related to ammonia stress corrosion cracking susceptibility, and the IMO Gas Code and classification rules of the International Maritime Organization (IMO) stipulate that the yield point must be 440 MPa or less for structural members of liquefied gas bulk carriers to minimize the risk of ammonia stress corrosion cracking.

[0084] Basically, the higher the tensile strength TS of a steel plate, the better, and steel plates having a tensile strength of 400 MPa or more are usually used. The tensile strength TS of a steel plate is preferably 440 MPa or more, and more preferably 490 MPa or more. Furthermore, if the tensile strength (TS) of a steel plate is too high, the yield strength (YS) (yield point (YP) when there is a yield point, or 0.2% proof stress σ0.2 when there is no yield point) of 440 MPa or less, which is required to ensure ammonia stress corrosion cracking resistance, cannot be satisfied, and therefore the tensile strength (TS) of a steel plate is preferably 620 MPa or less.

[0085] Molten steel having the chemical composition shown in Table 1 was produced as steel materials (slabs). These steel materials (slabs) were subjected to hot rolling and cooling under the conditions shown in Table 2.

[0086] The hardness of the obtained steel sheets was measured at a depth of 1 mm from the surface of the steel sheets (surface layer), and the structure fraction in the microstructure at a depth of ¼ of the sheet thickness from the surface of the steel sheets was measured, and the tensile properties and toughness were evaluated.

[0087] [Hardness Measurement at a Depth of 1 mm from the Surface of the Steel Plate] Vickers hardness (HV10) was measured at five points at a depth of 1 mm from the surface of a cross section perpendicular to the rolling direction in accordance with JIS Z 2244, and the average value was calculated.

[0088] [Microstructure at a depth of 1 / 4 of the plate thickness from the surface of the steel plate] A sample was taken from each of the obtained steel plates so that the observation surface was at a depth of 1 / 4 of the plate thickness from the surface of the steel plate. The surface of the sample was mirror-polished and further subjected to nital etching, and then an image of an area of ​​1000 μm × 1000 μm was taken using a scanning electron microscope (SEM). The photographed image was analyzed using an image analyzer to determine the area fraction of the microstructure, and this value was taken as the volume fraction. When the anisotropy of the microstructure is small, the area fraction corresponds to the volume fraction, so in this specification, the area fraction was taken as the volume fraction.

[0089] In either case, when determining the area fraction of the microstructure, the distinction between each structure was made as follows. The observation surface of the sample was mirror-polished, and nital etching was performed to reveal the structure, which was then observed under an SEM at a magnification of 500 to 3000 times. Ferrite was defined as a structure that did not contain isotropically grown carbides and in which the grains appeared black, while pearlite was defined as a structure in which ferrite (black) and carbides (white) appeared in a striped pattern. Bainite was defined as a structure that had a long, thin, lath-shaped ferrite structure and contained carbides with a circle equivalent diameter of 0.05 μm or more, and further, carbides were defined as 1.0 × 10 ４ pieces / mm ２ When the carbides are contained in the bainite lath structure, the carbides are divided. For example, the elongated carbides that appear between the laths of the bainite before tempering become multiple round carbides after tempering, so they can be distinguished by looking at the carbides. Martensite has a lath-shaped ferrite structure that has grown elongated like bainite, and is a structure that contains carbides with a circle equivalent diameter of 0.05 μm or less, and further, carbides with a circle equivalent diameter of 1.0 × 10 ４ pieces / mm ２ When the carbides contained more than 0.50 μm in equivalent circle diameter, the carbides were defined as tempered martensite. The carbides appeared as white dots. Austenite was defined as a structure that was present between bainite and martensite structures and did not contain carbides with a circle equivalent diameter of 0.50 μm or more.

[0090] [Strength properties] A JIS Z 2201 No. 1B test piece was taken from the entire thickness of each steel plate so that the plate width direction was the longitudinal direction of the test piece, and a tensile test was carried out in accordance with JIS Z 2241 to measure the yield strength YS (yield point YP when there was a yield point, or 0.2% proof stress σ0.2 when there was no yield point) and tensile strength (TS). Steel plates with a yield strength of 235 MPa or more and 440 MPa or less were evaluated as having excellent tensile strength.

[0091] [Base Material Toughness] A V-notch test piece according to JIS Z 2242 was taken from a portion 1 mm removed from the surface of each steel plate, with the plate width direction being the longitudinal direction of the test piece, and a Charpy impact test was carried out in accordance with JIS Z 2242 to measure the ductile-brittle fracture transition temperature vTrs. Steel plates with a vTrs of −60° C. or less were evaluated as having excellent toughness.

[0092] [Ammonia SCC Resistance] Ammonia SCC resistance was evaluated by an accelerated test in which a four-point bending test was performed in a test solution and constant-potential anodic electrolysis was performed to accelerate corrosion. Specifically, the test was performed as follows. Test specimens measuring 5 mm thick x 15 mm x 115 mm were taken from the surface of the steel sheet, ultrasonically degreased in acetone for 5 minutes, and then subjected to four-point bending to apply a stress equal to the yield strength of each steel sheet. The test cell containing the four-point bending test specimen was filled with a solution containing 12.5 g of ammonium carbamate and 1 L of liquid ammonia. The potentiostat was then used to control the potential of +2.0 V vs. Pt to the test specimen, and the specimen was immersed at room temperature (25°C). If no cracks were observed after 168 hours of immersion, the ammonia SCC resistance was judged to be good and marked with a circle; if cracks were observed, the ammonia SCC resistance was judged to be poor and marked with an x.

[0093] [Joint HAZ Toughness] Test plates for joints taken from each of the steel plates were subjected to Y-groove processing, and submerged arc welding was performed with a welding heat input of 100 kJ / cm using a commercially available welding wire for low-temperature steel to produce joints by high-heat input welding. The yield strength (YS) and toughness were evaluated using the obtained joints. The test methods are as follows. NK U4 impact test specimens were taken so that the test specimen surface was located 1 mm deep from the joint surface and the HAZ was the notch position. Charpy impact tests were performed on the taken test specimens at a test temperature of -60°C, and the average absorbed energy vE of three test specimens taken under the same conditions was calculated. －６０℃ (unit: J) is the toughness of the HAZ, and vE －６０℃ The results of the evaluation are shown in Table 2.

[0094]

[0095]

[0096] As can be seen from Tables 1 and 2, all of the invention examples have a surface hardness of 210 HV or less, a yield strength (YS) of 440 MPa or less, a ductile-brittle fracture transition temperature vTrs of -60°C or less, and steel plates excellent in ammonia stress corrosion cracking resistance and high heat input joint HAZ toughness.

[0097] Steel plates Nos. 1, 16 to 28, which are examples of the invention and were manufactured under suitable manufacturing conditions, also achieved a high tensile strength (TS) of 400 MPa or more.

[0098] On the other hand, steel plates Nos. 2, 4, 5, 8, 10, 12, and 15, which correspond to comparative examples and whose manufacturing conditions are outside the scope of the present invention, are inferior to the invention examples in at least one of surface hardness, low-temperature toughness, and ammonia SCC resistance, or have excessive yield strength YS. Furthermore, steel plate No. 30, which corresponds to a comparative example, has a high carbon content and therefore has poorer low-temperature toughness than the invention examples. Steel plates Nos. 31, 34, 35, 36, 38, 40, and 41 have higher contents of various elements than the invention examples, and therefore have poorer low-temperature toughness and HAZ toughness than the invention examples. Steel plate No. 33 has a high Mn content and therefore has poorer HAZ toughness than the invention examples. Steel plates Nos. 37 and 39 have low Mn or Ti contents and therefore have poorer low-temperature toughness than the invention examples. Steel plate Nos. 44, 45, 46, and 47 have Ti / N or formula (1) outside the specified range, and therefore have inferior HAZ toughness compared to the invention examples.

Claims

1. A steel sheet having a composition comprising, by mass%, C: 0.020% or more and 0.100% or less, Si: 0.50% or less, Mn: 0.30% or more and 1.80% or less, P: 0.010% or less, S: 0.0100% or less, Al: 0.060% or less, Ti: 0.010% or more and 0.031% or less, N: 0.0038% or more and 0.0100% or less, and O: 0.0100% or less, wherein, with regard to Ti and N, a ratio of the Ti content (mass%) to the N content (mass%), Ti / N, is 2.00 or more and 4.00 or less, and the following formula (1) is satisfied, and further, a carbon equivalent Ceq represented by the following formula (2) is 0.190 or more and 0.380 or less, with the remainder being Fe and unavoidable impurities; A steel sheet having a hardness of 210 HV or less at a depth of 1 mm from the steel sheet surface, a microstructure in which the volume fraction of ferrite at a depth of 1 / 4 of the sheet thickness from the steel sheet surface is 20% or more, the total volume fraction of ferrite and bainite is 80% or more, and the volume fraction of island martensite is 5% or less, a yield strength of 440 MPa or less, and a ductile-brittle fracture transition temperature vTrs of -60°C or less. 169≦5158×Ti+25563×N≦360... (1) Ceq=C+Mn / 6+(Cu+Ni) / 15+(V+Mo+Cr) / 5... (2) In the above formulas (1) and (2), each element symbol represents the content (mass%) of each element, and 0 is used when the element is not contained.

2. The steel plate according to claim 1, wherein the chemical composition further contains, in mass%, one or more selected from the following: Cu: 1.00% or less, Ni: 1.00% or less, Cr: 1.00% or less, Mo: 0.50% or less, V: 0.50% or less, W: 0.50% or less, Co: 0.50% or less, Nb: 0.050% or less, B: 0.0010% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, REM: 0.0050% or less, Sn: 0.05% or less, Zn: 0.05% or less, Pb: 0.10% or less, As: 0.05% or less, Sb: 0.10% or less and Bi: 0.10% or less.

3. The steel plate according to claim 1, containing, by mass%, C: 0.045% or more, Mn: 0.50% or more, further having a component composition in which the carbon equivalent Ceq is 0.250 or more, with the balance consisting of Fe and unavoidable impurities, having a microstructure in which the volume fraction of bainite at a depth position of 1 / 4 of the plate thickness from the steel plate surface is 20% or more, and having a tensile strength of 400 MPa or more.

4. The steel plate according to claim 2, containing, by mass%, C: 0.045% or more, Mn: 0.50% or more, further having a component composition in which the carbon equivalent Ceq is 0.250 or more, with the balance consisting of Fe and unavoidable impurities, having a microstructure in which the volume fraction of bainite at a depth position of 1 / 4 of the plate thickness from the steel plate surface is 20% or more, and having a tensile strength of 400 MPa or more.

5. A steel material having the composition described in any one of claims 1 to 4 is subjected to Ar at a temperature at a depth position of 1 / 4 of the plate thickness from the surface of the steel material. ３ The temperature is then increased to 1,250° C. or higher. ３ The rolling end temperature is set to Ar + 100 ° C or higher. ３ After that, the temperature is increased to Ar ３ cooling the steel plate until the temperature reaches or below the melting point, stopping the cooling once and suspending the cooling for 5 seconds or more, subsequently cooling the steel plate at an average cooling rate of 100°C / s or less at a depth position of 1 / 4 of the plate thickness from the surface of the steel plate, and finishing the cooling when the temperature at a depth position of 1 / 4 of the plate thickness from the surface of the steel plate is 400°C or higher.

6. A steel material having the composition described in any one of claims 1 to 4 is heated to a temperature of 950°C or more and 1250°C or less at a depth position of 1 / 4 of the plate thickness from the surface of the steel material, and then the rolling start temperature is set to Ar. ３ The rolling end temperature is set to Ar + 100 ° C or higher. ３ Then, the steel is subjected to hot rolling to a temperature of 1000° C. or higher. ３ Cooling is started from a temperature above the Ar point. ３ Ar below point ３ 6. The method for producing a steel plate according to claim 5, further comprising the steps of: cooling the steel plate at an average cooling rate of 5°C / s or more until the temperature reaches a temperature point −90°C or more; once stopping the cooling and interrupting the cooling for a period of 5 seconds to 600 seconds; subsequently, cooling the steel plate at an average cooling rate of 5°C / s or more and 100°C / s or less at a depth position of 1 / 4 of the plate thickness from the surface of the steel plate; and finishing the cooling when the temperature at a depth position of 1 / 4 of the plate thickness from the surface of the steel plate is 400°C or more and 580°C or less.

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