Steel sheet and method for producing same

Abstract

To provide a steel sheet which satisfies excellent toughness in a joint after high heat input welding while satisfying low temperature toughness and strength characteristics suitable for a liquefied gas storage tank, and to provide a production method therefor.SOLUTION: The steel sheet contains predetermined elements in specific amounts, with the balance being Fe and inevitable impurities, and further has a ratio of Ti content (mass%) to N content (mass%), Ti / N, of 2.00 or more and 4.00 or less, A steel sheet having a microstructure satisfying 169 ≤ 5158 * Ti + 25563 * N ≤ 360 and having a composition in which a carbon equivalent Ceq is 0.0250% or more and 0.380% or less, a hardness at a depth position of 1 mm from a surface of the steel sheet being 210 HV or more, a volume fraction of ferrite being 20% or more and a total volume fraction of ferrite and bainite being 80% or more at a depth position of 1 / 4 of a sheet thickness from the surface of the steel sheet, and having a yield strength of not less than 235 MPa and not more than 440 MPa and a ductile-brittle transition temperature vTrs after applying a pre-strain of 5% being - 60 °C. or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel sheet and a method for manufacturing the same. [Background technology]

[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, tanks may carry not only liquefied petroleum gas (LPG) but also liquid ammonia.

[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, liquid 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 technologies for providing the low-temperature toughness and 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, for work efficiency, high-heat-input welding methods such as submerged arc welding and electrogas arc welding are often used. However, when high-heat-input welding is performed, the properties of the heat-affected zone (HAZ) of the steel plate may be impaired due to the large amount of heat transferred to the HAZ. For example, austenite grains are prone to coarsening in the HAZ exposed to high temperatures just below the melting point during high-heat-input welding. These coarsened austenite grains transform into upper bainite containing island martensite (Martensite-Austenite Constituent, hereafter abbreviated as "MA"), which has poor toughness, upon subsequent cooling, resulting 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 in the HAZ by reducing the content of P in addition to reducing the content of C and Si.

[0009] The technology described in Patent Document 5 achieves both brittle crack propagation arrestability and HAZ toughness during high heat input welding by dispersing particles such as TiN in a ferrite-based microstructure with refined crystal grain size. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 10-140235 [Patent Document 2] Japanese Patent Application Publication No. 10-168516 [Patent Document 3] International Publication No. 2011 / 148754 [Patent Document 4] Japanese Patent Application Laid-Open No. 2008-163446 [Patent Document 5] Japanese Patent Application Publication No. 2015-098642 Summary of the Invention [Problem to be solved by the invention]

[0011] The methods described in Patent Documents 1 and 2 above require multiple heat treatments, which poses an economic problem due to the high costs of the 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 raises the risk of the toughness of the surface layer of the steel plate being exposed to high temperatures for a long time becoming unstable.

[0012] Furthermore, when steel plates are subjected to high heat input welding, the HAZ is heated to the melting temperature range of TiN. Therefore, the techniques described in Patent Documents 3 and 5 that utilize TiN have the problem that TiN decomposes, eliminating the dispersion effect, and the solute Ti and N produced by the decomposition of TiN embrittle the base structure of the steel, significantly reducing the toughness of the HAZ.

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

[0014] Furthermore, when manufacturing tanks, steel materials are subjected to bending, but the toughness of steel materials that have been deformed by bending can deteriorate, and this needs to be suppressed from a safety standpoint.

[0015] In view of the above circumstances, an object of the present invention is to provide a steel plate that satisfies low-temperature toughness and strength characteristics suitable for liquefied gas storage tanks, while also providing excellent toughness in joints after large heat input welding and excellent toughness even after bending, and a method for manufacturing the same.

[0016] 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. [Means for solving the problem]

[0017] In order to achieve the above object, the present inventors have conducted extensive research using an online cooling device into various factors affecting the low-temperature toughness and strength properties of steel plates. As a result, it was discovered that if the steel plate contains predetermined amounts of elements such as C, Si, Mn, and Ti, and the microstructure is controlled so that 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, and the combined volume fraction of ferrite and bainite is 80% or more, the desired low-temperature toughness and strength characteristics can be achieved.

[0018] Furthermore, the inventors have discovered that stress corrosion cracking resistance in a liquid ammonia environment can be improved by controlling the yield strength (YS) of the base material to 440 MPa or less and controlling the hardness at a depth of 1 mm from the surface of the steel plate to 210 HV or less.

[0019] In addition, the present inventors have conducted extensive research to improve the toughness of the HAZ when large heat input welding is performed, and have obtained the following new findings.

[0020] (A) In order to improve the toughness of the HAZ when high heat input welding is performed, it is important to suppress the coarsening of the austenite grain size in the HAZ. Dispersing a large amount of TiN is important to suppress the coarsening of the austenite grain size in the HAZ. However, when high heat input welding is performed, TiN decomposes when heated to its melting temperature range, which can cause the dispersion effect to be lost. It was discovered that in order to suppress the decomposition of TiN, it is necessary to have a composition in which the ratio of the Ti content (mass%) to the N content (mass%) (Ti / N) is between 2.00 and 4.00, and the chemical composition satisfies the formula (1) described below.

[0021] (B) In order to improve the toughness of the HAZ when high heat input welding is performed, it is important to reduce the amount of island martensite (MA) in the HAZ. To minimize the formation of MA in the HAZ, it is important to control the carbon equivalent (Ceq) expressed by the formula (2) described below to 0.380% or less, and to limit the C content of the steel plate to 0.100% by mass or less and the Si content to 0.50% by mass or less. In addition, to prevent a decrease in HAZ toughness, it is also necessary to limit the Mn content to 1.80% by mass or less and the Al content to 0.060% by mass or less. Here, in the present invention, "almost no MA is generated" means that the volume fraction of MA in the microstructure of the HAZ is 5% or less.

[0022] The present invention was completed based on these findings and further investigations. That is, the gist of the present invention is as follows.

[0023] [1] In mass%, C: 0.020% or more and 0.100% or less, Si: 0.03% or more and 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: 0.005% or more and 0.060% or less, Ti: 0.010% or more and 0.031% or less, N: 0.0038% or more and 0.0100% or less O: 0.0100% or less and the balance consisting of Fe and unavoidable impurities, and furthermore, 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, satisfies the following formula (1), and has a component composition in which the carbon equivalent Ceq represented by the following formula (2) is 0.250% or more and 0.380% or less, and the balance consisting of Fe and unavoidable impurities, The hardness at a depth of 1 mm from the surface of the steel plate is 210 HV or less, The steel plate has a microstructure in which the volume fraction of ferrite at a depth of 1 / 4 of the plate thickness from the surface thereof is 20% or more, and the total volume fraction of ferrite and bainite is 80% or more, The yield strength is 235 MPa or more and 440 MPa or less, A steel plate having a ductile-brittle fracture transition temperature (vTrs) of -60°C or less after being pre-strained by 5%. 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 component, and is set to 0 if the component is not contained.

[0024] [2] The component composition further includes, in mass%, 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: 0.0050% 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 The steel sheet according to [1], containing one or more selected from the group consisting of:

[0025] [3] The component composition is, in mass%, C: 0.045% or more and 0.100% or less Mn: 0.50% or more and 1.80% or less; The microstructure has a bainite volume fraction of 20% or more at a depth of 1 / 4 of the plate thickness from the surface, Furthermore, the tensile strength is 400 MPa or more. [1] or [2].

[0026] [4] A steel material having a chemical composition described in any one of [1] to [3], The steel material is heated to a temperature of Ar3 point +100°C or higher and 1250°C or lower at a depth of 1 / 4 of the plate thickness from the surface thereof, Next, hot rolling is performed with a rolling start temperature of Ar3 point + 100°C or higher, a cumulative rolling reduction rate in the unrecrystallized region of 60% or higher, and a rolling end temperature exceeding the Ar3 point. After that, cooling is performed so that the cooling start temperature is above Ar3 point and the cooling end temperature is below Ar3 point, Stop the cooling once and suspend the cooling for 5 seconds or more, Then cooling is carried out, Next, a heat treatment is performed at a temperature of 400°C or higher and 580°C or lower.

[0027] [5] The heating temperature of the steel material is 950 ° C or higher at a depth of 1 / 4 of the plate thickness from the surface, Cooling begins from a temperature above the Ar3 point, and the temperature is cooled at an average cooling rate of 5°C / s or more until the temperature is below the Ar3 point and reaches Ar3 point -90°C or higher. The cooling is stopped temporarily and the cooling is interrupted for 5 seconds or more and 600 seconds or less, Next, the steel plate is cooled at an average cooling rate of 5°C / s or more at a depth of 1 / 4 of the plate thickness from the surface, 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 580°C or less. [4] A method for manufacturing a steel sheet according to the present invention.

[0028] [6] The method for producing a steel sheet according to [4] or [5], wherein the average reduction rate per rolling pass in the unrecrystallized region is 2.0% or more. [Effects of the Invention]

[0029] According to the present invention, it is possible to provide a steel plate that satisfies low-temperature toughness and strength characteristics suitable for liquefied gas storage tanks, while also providing excellent toughness in joints after large heat input welding, and a method for manufacturing the same. DETAILED DESCRIPTION OF THE INVENTION

[0030] 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.

[0031] [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 HAZ of a joint in high-heat-input welding decreases. Therefore, the C content is set to 0.100% or less. Furthermore, from the viewpoint of suppressing a decrease in the toughness of the base metal, the C content is preferably set to 0.095% or less, and more preferably set to 0.090% or less. On the other hand, if the C content is less than 0.020%, the grain size of the resulting microstructure becomes coarse, and the toughness of the base metal decreases. From the viewpoint of increasing the strength (YS, TS) of the base metal, the C content is preferably set to 0.045% or more, more preferably set to 0.050% or more, and even more preferably set to 0.055% or more.

[0032] Si: 0.03% or more and 0.50% or less Si is an element that suppresses the formation of coarse carbides and is effective in improving the toughness of the base material. To achieve this effect, a content of 0.03% or more is required. Furthermore, because it can increase the strength (YS, TS) of the base material, the content of other alloying elements can be reduced, allowing for lower-cost production. From this perspective, the Si content is preferably 0.10% or more. Furthermore, from the perspective of low-cost production, the Si content is more preferably 0.15% or more, and even more preferably 0.20% or more. On the other hand, if the Si content exceeds 0.50%, the toughness of the base material and the toughness of the HAZ of a joint in high-heat-input welding will decrease. Therefore, the Si content should be 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

[0033] Mn: 0.30% or more and 1.80% or less Mn is an element that increases the hardenability of steel and is effective in improving the toughness of the base material. To achieve this effect, a content of 0.30% or more is required. Furthermore, from the viewpoint of increasing the strength (YS, TS) of the base material, the Mn content is preferably 0.50% or more. Furthermore, from the viewpoint of reducing the content of other alloying elements and enabling lower-cost production, the Mn content is more preferably 0.70% or more, and even more preferably 0.90% or more. On the other hand, if the Mn content exceeds 1.80%, the toughness of the HAZ of the joint in high-heat-input welding will decrease, and the alloy cost will become excessively high. Therefore, the Mn content is set to 1.80% or less. Furthermore, from the viewpoint of suppressing deterioration of weldability, the Mn content is preferably 1.70% or less, and more preferably 1.60% or less.

[0034] P:0.010% or less P is an element that has adverse effects, such as reducing the toughness of the base material and the HAZ of joints in high heat input welding by segregating at grain boundaries. 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 an element that is usually inevitably contained in steel as an impurity, the P content may be industrially greater than 0%. Furthermore, excessive reduction of P leads to increased refining costs, so from a cost perspective, it is preferable to set the P content to 0.0005% or more.

[0035] S: 0.0100% or less S is an element that exists in steel as sulfide-based inclusions such as MnS and acts as a fracture initiation site, adversely affecting the toughness of the base material. Therefore, it is desirable to keep the S content as low as possible. S also 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%. However, since S is usually an element that is inevitably contained in steel as an impurity, the S content may be greater than 0% industrially. Furthermore, excessive reduction of S leads to increased 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.

[0036] Al: 0.005% or more, 0.060% or less Al acts as a deoxidizer, reducing oxide-based inclusions and improving the toughness of the base metal and joint HAZ, while also refining crystal grains. To achieve these effects, the Al content is set to 0.005% or more. The Al content is preferably set to 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, resulting in a decrease in 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 set to 0.050% or less, and more preferably 0.040% or less.

[0037] 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 the amount of solute N. Therefore, the inclusion of Ti can improve the toughness of the base metal and the joint HAZ. In order to obtain HAZ toughness particularly in large heat input welding, the Ti content is set to 0.010% or more. 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 will actually decrease. Therefore, the Ti content is set to 0.031% or less. The Ti content is preferably set to 0.029% or less, and more preferably 0.027% or less.

[0038] N: 0.0038% or more and 0.0100% or less N is an element that combines with Ti to precipitate as TiN, contributing to the refinement of the structure 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. The N content is preferably set to 0.0080% or less, and more preferably to 0.0060% or less.

[0039] O: 0.0100% or less O is an element that forms oxides and acts as a fracture initiation point, adversely affecting the toughness of the base material and the joint HAZ. Therefore, the O content is limited to 0.0100% or less. Furthermore, the O content is preferably 0.0050% or less, and more preferably 0.0030% or less. The lower limit of the O content is not particularly limited and may be 0%, but O is usually an element that is inevitably contained in steel as an impurity. Therefore, from an industrial standpoint, the O content may be greater than 0%. Furthermore, excessive reduction of O leads to increased refining costs, so from a cost perspective, the O content is preferably 0.0020% or more.

[0040] In addition to the above basic components, the composition may optionally contain one or more elements selected from the group consisting of up to 1.00% Cu, 1.00% Ni, 1.00% Cr, 0.50% Mo, 0.50% V, 0.50% W, 0.50% Co, 0.050% Nb, 0.0050% B, 0.0050% Ca, 0.0050% Mg, 0.0050% REM, 0.05% Sn, 0.05% Zn, 0.10% Pb, 0.05% As, 0.10% Sb, and 0.10% Bi. Cu, Ni, Cr, Mo, V, W, Co, Nb, B, Ca, Mg, or REM may be added to improve strength or toughness.

[0041] 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 added as desired. When Cu is added, 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, if the Cu content exceeds 1.00%, it will result in a deterioration in toughness and an increase in alloy costs. Therefore, when Cu is added, the Cu content is preferably 1.00% or less, and more preferably 0.50% or less.

[0042] Ni: 1.00% or less Ni, like Cu, is an element that improves the strength of steel sheets and can be added as desired. When Ni is added, in order to obtain the above effect, the Ni content is preferably 0.01% or more, more preferably 0.20% or more. On the other hand, if the Ni content exceeds 1.00%, it leads to deterioration of weldability and an increase in alloy costs. Therefore, when Ni is added, the Ni content is preferably 1.00% or less, and more preferably 0.50% or less.

[0043] 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 added as desired. When Cr is added, 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 deteriorate the weldability and increase the alloy cost. Therefore, when Cr is added, the Cr content is preferably 1.00% or less, and more preferably 0.50% or less.

[0044] Mo: 0.50% or less Mo, like Cu, is an element that improves the strength of steel sheets 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 costs. Therefore, when Mo is added, the Mo content is preferably 0.50% or less, and more preferably 0.25% or less.

[0045] 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 deteriorate the weldability and increase the alloy cost. Therefore, when V is added, the V content is preferably 0.50% or less, and more preferably 0.25% or less.

[0046] 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, if the W content exceeds 0.50%, it will deteriorate the weldability and increase the alloy cost. Therefore, when W is added, the W content is preferably 0.50% or less, and more preferably 0.25% or less.

[0047] 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, if the Co content exceeds 0.50%, it will deteriorate the weldability and increase the alloy cost. Therefore, when Co is added, the Co content is preferably 0.50% or less, and more preferably 0.25% or less.

[0048] 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, and can be contained as desired. 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 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less.

[0049] B: 0.0050% or less B is an element that has the effect of significantly improving hardenability even in trace amounts, and can be added as desired. The addition of B can improve the strength of the steel sheet. When B is added, in order to obtain the above effect, the B content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more. On the other hand, if the B content exceeds 0.0050%, the weldability decreases. Therefore, when B is added, the B content is 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less.

[0050] 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, and can be added as desired. By adding Ca, the morphology of sulfide-based inclusions can be controlled to be spherical, thereby improving the toughness of welds and other parts. When Ca is added, the Ca content is preferably 0.0005% or more, and more preferably 0.0020% or more, to achieve the above effect. 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 added, the Ca content is set to 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less.

[0051] Mg: 0.0050% or less Like Ca, Mg is an element that bonds with S and inhibits the formation of MnS and other elements that elongate in the rolling direction, and can be added as desired. The addition of Mg controls the morphology of sulfide-based inclusions so that they assume a spherical shape, thereby improving the toughness of welds and other parts. When Mg is added, in order to achieve this effect, the Mg content is preferably 0.0005% or more, 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 added, the Mg content is set to 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less.

[0052] 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, and can be optionally contained. The inclusion of REM controls the morphology of sulfide-based inclusions so that they assume a spherical shape, thereby improving the toughness of welds and other parts. When REM is contained, 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 contained, the REM content is set to 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less.

[0053] Sn: 0.05% or less Since Sn 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 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.

[0054] Zn: 0.05% or less Zn vaporizes as white smoke during molten iron treatment, damaging steelworks equipment, so it is desirable to keep the Zn content as low as possible. There is no particular lower limit for the Zn content, and it 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.

[0055] Pb: 0.10% or less Since Pb reduces joint toughness, it is desirable to keep the content as low as possible. There is no particular lower limit for the Pb content, and it 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.

[0056] 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. There is no particular lower limit for the As content, and it 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.

[0057] 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.

[0058] 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. There is no particular lower limit for the Bi content, and it 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.

[0059] 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 objectives of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap.

[0060] The component composition of the present invention further satisfies the following conditions.

[0061] Ti / N: 2.00 or more and 4.00 or less Ti and N precipitate as TiN during steel solidification, suppress austenite coarsening in the HAZ, and contribute to high toughness by serving as ferrite transformation nuclei. These elements play an important role in the present invention. In addition to the content requirements described above, Ti / N, the ratio of the Ti content (mass%) to the N content (mass%), is 2.00 to 4.00. If Ti / N is less than 2.00, the amount of TiN produced decreases, and the solute N that does not form TiN reduces HAZ toughness. Therefore, Ti / N is set to 2.00 or more. Ti / N is preferably set to 2.10 or more, and more preferably set to 2.20 or more. On the other hand, if Ti / N exceeds 4.00, TiN coarsens, reducing 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. Here, Ti / N means the content (mass%) of each element in the steel, and Ti / N means the ratio of the Ti content (mass%) to the N content (mass%).

[0062] 169≦5158×Ti+25563×N≦360 (1) In formula (1), each element symbol represents the content (mass %) of each component.

[0063] The composition of the present invention satisfies the above formula (1). Conventional techniques for improving toughness during high heat input welding using TiN have had problems with TiN decomposition in the HAZ of high heat input welding, resulting in the loss of the dispersion effect and the formation of solute Ti and N by TiN decomposition, which embrittle the steel matrix, significantly reducing HAZ toughness. Therefore, to suppress 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 175 or greater, more preferably 180 or greater, and even more preferably 190 or greater. On the other hand, if the value of 5158×Ti+25563×N exceeds 360, a large amount of TiN is formed, which actually reduces HAZ toughness. Therefore, the value of 5158×Ti+25563×N is set to 360 or less. From the viewpoint of further improving toughness, 5158×Ti+25563×N is preferably set to 330 or less, more preferably 300 or less, and even more preferably 270 or less.

[0064] Ceq: 0.250% or more and 0.380% or less In the composition of the present invention, the carbon equivalent Ceq defined by the following formula (2) is 0.250% or more and 0.380% or less, which is an essential condition for satisfying the toughness of the HAZ of a joint during large heat input welding. Ceq=C+Mn / 6+(Cu+Ni) / 15+(V+Mo+Cr) / 5 ···(2) In formula (2), each element symbol represents the content (mass%) of each component, and is set to 0 if the component is not contained.

[0065] If Ceq exceeds 0.380%, the yield strength (YS) of 440 MPa or less required to avoid ammonia-induced stress corrosion cracking cannot be achieved, and the toughness of the base metal and the joint HAZ also deteriorates. Therefore, Ceq is set to 0.380% or less. To improve the toughness of the base metal and the joint HAZ, Ceq is preferably set to 0.370% or less, and more preferably to 0.360% or less. On the other hand, if Ceq is less than 0.250%, the hardenability deteriorates, thereby reducing the toughness of the base metal. Therefore, Ceq must be set to 0.250% or more. 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 set to 0.260% or more. To achieve a higher tensile strength (TS), Ceq is more preferably set to 0.300% or more, and even more preferably to 0.350% or more.

[0066] The steel sheet of the present invention has the above-mentioned composition and also satisfies the following. -Hardness of 210HV or less at a depth of 1mm from the surface of the steel plate - A microstructure in which 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, and the total volume fraction of ferrite and bainite is 80% or more. - Yield strength is between 235 MPa and 440 MPa Ductile-brittle fracture transition temperature vTrs after applying -5% pre-strain is -60°C or less

[0067] The reasons for limiting the hardness, microstructure, yield strength, and ductile-brittle fracture transition temperature vTrs of the steel plate as described above will be explained below.

[0068] [Hardness of 210HV or less at a depth of 1mm from the surface of the steel plate] The hardness at a depth of 1 mm from the surface of the steel plate 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 (Stress Corrosion Cracking) 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 130 HV or more is preferable. The hardness at a depth of 1 mm from the surface of the steel plate can be measured by the method described in the Examples below.

[0069] Normally, in steel sheets that are subsequently cooled after hot rolling, the surface layer, which cools faster than the interior, has a higher hardness. As will be described later in the steel sheet manufacturing conditions, excessive hardening of the surface layer can be prevented by temporarily halting the cooling after hot rolling and setting a higher temperature at which the second cooling is stopped.

[0070] [Microstructure] The microstructure of the steel sheet of the present invention will be described.

[0071] (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, and the total volume fraction of ferrite and bainite is 80% or more) In the steel plate of the present invention, the microstructure at a depth of 1 / 4 of the plate thickness from the surface of the steel plate has a ferrite volume fraction of 20% or more and a combined volume fraction of ferrite and bainite of 80% or more. 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 properties of the entire steel plate. If the volume fraction of ferrite is less than 20%, the yield strength (YS) of the base material increases excessively, making it difficult to achieve a yield strength (YS) of 440 MPa or less. Furthermore, the toughness of the base material decreases. Therefore, the volume fraction of ferrite is set to 20% or more, preferably 30% or more, and more preferably 40% or more. 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, preferably 85% or more, and more preferably 90% or more. The upper limit of the total volume fraction of ferrite and bainite is not particularly limited, and may be 100%.

[0072] (Volume fraction of bainite is 20% or more (preferred mode)) The volume fraction of bainite is not particularly limited as long as the microstructure satisfies the above conditions, but from the viewpoint of achieving a tensile strength (TS) of 400 MPa or more, the volume fraction of bainite is preferably 20% or more.

[0073] (Remnant tissue) The remainder other than ferrite and bainite may include martensite in addition to pearlite and austenite. The volume fraction of each structure in the remainder structure does not need to be particularly limited, but it is preferable that the remainder structure be pearlite.

[0074] The volume fractions of various microstructures can be measured by the methods described in the examples below.

[0075] [Yield strength: 235 MPa or more and 440 MPa or less] The steel plate of the present invention has a yield strength (YS) of 440 MPa or less. Here, the yield strength (YS) of the steel plate is the yield point (YP) when there is a yield point, and is the 0.2% proof stress σ0.2 when there is no yield point. The yield strength (YS) is closely related to ammonia stress corrosion cracking resistance, and the IMO Gas Code and classification rules of the International Maritime Organization (IMO) stipulate that the yield point of structural members of liquefied gas bulk carriers must be 440 MPa or less to minimize the risk of ammonia stress corrosion cracking. The yield strength is set to 235 MPa or more, preferably 265 MPa or more, in order to reduce the plate thickness and the weight of the steel material. The yield strength can be measured by the method described in the Examples below.

[0076] [Ductile-brittle fracture transition temperature vTrs after applying 5% pre-strain is -60°C or less] The steel sheet of the present invention has a ductile-to-brittle fracture transition temperature vTrs of -60°C or lower after being pre-strained by 5%. From the viewpoint of further lowering the design temperature and increasing the reliability of the steel material, the ductile-to-brittle fracture transition temperature vTrs is preferably -65°C or lower, and more preferably -70°C or lower. The ductile-to-brittle fracture transition temperature vTrs of the steel sheet after being pre-strained by 5% can be -120°C or higher. The ductile-brittle fracture transition temperature vTrs after applying a pre-strain of 5% can be measured by the method described in the Examples below.

[0077] [Tensile strength of 440 MPa or more (preferred embodiment)] Basically, the higher the tensile strength (TS) of the steel sheet, the better, and steel sheets having a tensile strength of 400 MPa or more are usually used. From this viewpoint, the tensile strength of the steel sheet is preferably 440 MPa or more, and more preferably 490 MPa or more. In addition, from the viewpoint of achieving a yield strength of 440 MPa or less to ensure resistance to ammonia stress corrosion cracking, the tensile strength (TS) of the steel plate is preferably 620 MPa or less.

[0078] Next, a method for producing a steel sheet according to the present invention will be described. A steel material having the above-described chemical composition is heated and hot-rolled to obtain a hot-rolled steel sheet, which is then cooled to a cooling start temperature above the Ar3 transformation point to obtain a steel sheet. Each manufacturing condition will be explained in detail below. In the present invention, the temperature of the steel sheet is defined as the temperature at a depth of 1 / 4 of the sheet thickness from the surface of the steel sheet, unless otherwise specified.

[0079] First, the conditions for producing the steel material are not 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.

[0080] The obtained steel material is heated once and then hot-rolled. Hot-rolling is performed at a rolling start temperature of Ar3 transformation point (hereinafter also referred to as "Ar3 point" for short) + 100°C or higher and a rolling end temperature of Ar3 point or higher to produce a hot-rolled steel sheet. Next, cooling of the hot-rolled steel sheet is started until the temperature at a depth of 1 / 4 of the sheet thickness from the surface of the steel sheet is Ar3 point or lower, and the cooling is stopped once for 5 seconds or more, followed by a second cooling.

[0081] (a) Heating temperature of steel material: Ar3 point + 100℃ or more and 1250℃ or less If the heating temperature of the steel material exceeds 1250°C, austenite will coarsen, the toughness of the base material will decrease, and oxidation will become severe, which may increase oxidation loss and reduce yield. For these reasons, the heating temperature is set to 1250°C or lower. The heating temperature of the steel material is preferably set to 1150°C or lower. On the other hand, if the heating temperature of the steel material is lower than the Ar3 point + 100°C, reverse transformation to austenite will not occur, and deformation resistance will increase, which may increase the load on the hot rolling mill and make hot rolling difficult. For these reasons, the heating temperature of the steel material is set to 100°C or higher than the Ar3 point. Furthermore, 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 set to 950°C or higher, and more preferably 1000°C or higher.

[0082] Here, the Ar3 point can be determined, for example, by the following equation (3). Ar3(℃)=910-273×C-74×Mn-57×Ni-16×Cr-9×Mo-5×Cu...(3) In formula (3), each element symbol indicates the content (mass%) of the element, and is set to 0 if the element is not contained.

[0083] (b) Hot rolling start temperature: Ar3 point + 100°C or higher After the steel material is heated to the above temperature, hot rolling is initiated at a temperature of Ar3 point + 100°C or higher. If the rolling start temperature is less than Ar3 point + 100°C, the austenite grain size will become mixed grains, which will deteriorate the toughness of the base material and may make it impossible to achieve the subsequent hot rolling finish temperature and cooling start temperature. The hot rolling start temperature is Ar3 point + 100°C or higher, and preferably Ar3 point + 150°C or higher. The upper limit of the hot rolling start temperature is the heating temperature of the steel material (the upper limit of the heating temperature is 1250°C). The hot rolling start temperature is preferably 1150°C or lower to reduce the heating temperature and prevent a decrease in yield.

[0084] (c) Cumulative reduction in the unrecrystallized region: 60% or more If the cumulative reduction rate in the non-recrystallized region (which in the present invention means the region where the steel material is at a temperature below the Ar3 point + 100°C) is less than 60%, the refinement of the austenite grain size and the amount of strain will be insufficient, resulting in insufficient microstructural refinement during subsequent cooling and reduced toughness of the base material in the product steel sheet. Therefore, the cumulative reduction rate in the non-recrystallized region is specified to be 60% or more. From the viewpoint of further improving the toughness of the base material, the cumulative reduction rate in the non-recrystallized region is preferably 65% ​​or more, and more preferably 70% or more. An excessively high cumulative reduction rate increases the load on the hot rolling mill, so it is preferably 90% or less. The cumulative reduction rate in the non-recrystallized region can be calculated using the following formula: Cumulative rolling reduction rate (%)= [Total thickness reduction in the unrecrystallized region (mm)] ÷ [Thickness of the steel material before hot rolling (mm)] × 100

[0085] (d) Average reduction rate per rolling pass in the unrecrystallized region: 2.0% or more (preferred embodiment) In the non-recrystallized region (which in the present invention means a region where the steel material is at a temperature less than the Ar3 point + 100°C), the average reduction rate per rolling pass in the non-recrystallized region is preferably 2.0% or more, from the viewpoint of further refining the austenite grain size and further increasing the amount of strain, thereby further improving the base material toughness of the product steel sheet. The average reduction rate per rolling pass in the non-recrystallized region is more preferably 2.5% or more, and even more preferably 3.0% or more. Furthermore, since an excessively high average reduction rate per rolling pass in the non-recrystallized region increases the load on the hot rolling mill, it is preferable that the average reduction rate per rolling pass in the non-recrystallized region be 10.0% or less.

[0086] The number of rolling passes in the non-recrystallized region is not particularly limited, and for example, from the viewpoint of the load on the hot rolling mill, it can be 10 or more, preferably 20 or more, and from the viewpoint of rolling efficiency, it can be 40 or less, preferably 30 or less. The average reduction rate per rolling pass in the unrecrystallized region can be calculated by the following formula. Average reduction rate per rolling pass (%) = Σ {[Reduction in thickness of steel material per rolling pass (mm)] ÷ [Thickness of steel material before each rolling pass (mm)] × 100} ÷ [Number of rolling passes]

[0087] (e) Hot rolling finish temperature: above Ar3 point Rolling is terminated at a temperature above the Ar3 point. That is, if the hot rolling end temperature is below the Ar3 point, ferrite is generated, and the generated ferrite is affected by processing, resulting in a deterioration in toughness and a greater load on the hot rolling mill. Therefore, the hot rolling end temperature is set to be above the Ar3 point. Furthermore, it is preferable that the hot rolling end temperature be set to the Ar3 point + 20°C or higher. The upper limit of the hot rolling end temperature is the Ar3 point + 100°C.

[0088] (f) Cooling start temperature: Above Ar3 point 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, the hot-rolled steel sheet is cooled from a cooling start temperature above the Ar3 point. Here, "cooling" refers to cooling by active means and does not include the temperature drop that occurs during transportation after hot rolling.

[0089] (g) Average cooling rate in the first cooling: 5°C / s or more (preferred embodiment) The average cooling rate in the first cooling 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).

[0090] The cooling method is not particularly limited, but water cooling is preferred.

[0091] (h) Cooling stop temperature in the first cooling: The temperature at a depth of 1 / 4 from the surface of the steel plate is below Ar3 point The cooling stop temperature of the first cooling is set to the Ar3 point or below 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. If the cooling stop temperature exceeds the Ar3 point, ferrite is not generated and toughness decreases. Therefore, the cooling stop temperature is set to the Ar3 point or below. The cooling stop temperature of the first cooling is preferably the Ar3 point -20°C or below. 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 metal, it is preferable to set the cooling stop temperature to Ar3 point - 90°C or more. By setting the cooling stop temperature higher, it is possible to reduce the hardness of the surface layer portion, and from this point of view, the cooling stop temperature is preferably, for example, Ar3 point - 70°C or more.

[0092] (i) Cooling stop time: 5 seconds or more After the first cooling, the cooling is temporarily stopped for 5 seconds or more. By stopping the cooling, ferrite can be generated. If the cooling stop time is less than 5 seconds, ferrite is not sufficiently 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 to the cooling stop time, it is preferable to set the cooling stop time to 600 seconds or less 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.

[0093] (j) Average cooling rate in the second cooling: 5°C / s or more (preferred embodiment) After the first cooling is stopped, cooling is resumed. 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 that the average cooling rate in the second cooling is 5°C / s or more. The average cooling rate in the second cooling can be 60°C / s or less.

[0094] (k) Cooling end temperature in the second cooling: less than 580°C (preferred embodiment) The cooling end temperature of the second cooling is not particularly limited, but in order to adjust the hardness of the surface layer and the strength and toughness of the base material and to increase the yield strength (YS) and tensile strength (TS) of the base material, the cooling end temperature is preferably less than 580°C, and more preferably 530°C or less. By setting the cooling end temperature to a higher value, the hardness of the surface layer portion can be reduced, and from this point of view, the cooling end temperature is preferably, for example, 450°C or higher.

[0095] (l) Heat treatment temperature: 400°C or higher and 580°C or lower The heat treatment temperature is set to 400°C or higher and 580°C or lower to adjust the hardness of the surface layer and the strength and toughness of the base material. If the heat treatment temperature exceeds 580°C, the yield strength (YS) and tensile strength (TS) of the base material will decrease. On the other hand, if the heat treatment temperature is lower than 400°C, the relaxation of transformation strain and the reduction of solid solution elements, which affect the strain aging characteristics, will be insufficient, and the toughness after applying 5% pre-strain will not satisfy the target characteristics. The heat treatment temperature is preferably set to 420°C or higher, more preferably 440°C or higher, and preferably 540°C or lower, more preferably 520°C or lower.

[0096] (m) Average temperature rise rate: 0.1°C / s or more, 20.0°C / s (preferred embodiment) The average heating rate during heat treatment is not particularly limited, and can be 0.1°C / s or higher. In consideration of production efficiency, it is preferably 0.5°C / s or higher, and more preferably 1.0°C / s or higher. There is no particular upper limit to the average heating rate, but excessively rapid heating increases heating costs, so the heating rate is preferably 20.0°C / s or lower. From the perspective of reducing heating costs, the heating rate is more preferably 10.0°C / s or lower, and even more preferably 5.0°C / s or lower.

[0097] (n) Heat treatment time: 60 seconds or more, 10.8 seconds or less (preferred embodiment) Since the heat treatment has an effect during the temperature rise until the target temperature is reached, there is no particular limit to the lower limit of the holding time after the target temperature is reached, but in order to ensure a uniform temperature within the steel sheet, the heat treatment time is preferably held for 60 seconds or more, and more preferably for 300 seconds or more. Since a heat treatment time exceeding 10.8 ks reduces production efficiency, it is preferable to set the heat treatment time to 10.8 ks or less. From the viewpoint of improving production efficiency, the heat treatment time is more preferably 1.8 ks or less, and even more preferably 900 s or less.

[0098] (o) Average cooling rate: 1°C / s or more and 100°C / s or less (preferred embodiment) The average cooling rate after heat treatment is not particularly limited, but from the viewpoint of production efficiency, it is preferably 1°C / s or more, more preferably 3°C / s or more, and even more preferably 5°C / s or more. The upper limit of the average cooling rate after heat treatment is not particularly limited, but excessively rapid cooling increases cooling costs, so the average cooling rate after heat treatment is preferably 100°C / s or less. From the viewpoint of reducing cooling costs, it is more preferable that the average cooling rate after heat treatment be 10°C / s or less.

[0099] The cooling method is not particularly limited, and examples thereof include air cooling and water cooling.

[0100] The steel plate of the present invention can be obtained by manufacturing a steel material having the above-mentioned chemical composition according to the above-mentioned manufacturing conditions. The steel plate thus obtained satisfies the low-temperature toughness and strength properties suitable for liquefied gas storage tanks, and also satisfies excellent toughness in joints after large heat input welding, and is particularly suitable for use in multipurpose tanks that carry a mixture of liquefied petroleum gas (LPG) and liquid ammonia. [Example]

[0101] Molten steel having the chemical composition shown in Table 1 was melted to prepare steel materials (slabs). These steel materials (slabs) were heated, hot rolled, cooled and heat treated under the conditions shown in Table 2.

[0102] The hardness of the obtained steel sheets was measured at a depth of 1 mm from the surface (surface layer), and the microstructure fraction was measured at a depth of 1 / 4 of the sheet thickness from the surface, and the tensile properties and toughness were evaluated.

[0103] [Hardness measurement at a depth of 1 mm from the surface of the steel plate] For a cross section perpendicular to the rolling direction, Vickers hardness (HV10) was measured at five points at a depth of 1 mm from the surface of the steel plate in accordance with JIS Z 2244, and the average value was calculated.

[0104] [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 located 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. When the anisotropy of the microstructure is small, the area fraction corresponds to the volume fraction, so in this specification, the area fraction is referred to as the volume fraction.

[0105] In either case, when determining the area fraction of the microstructure, the distinction between each structure was made as follows. The steel material was mirror-polished and etched with nital to reveal the structure, which was then observed under an SEM at magnifications of 500 to 3000 times. Ferrite (abbreviated as F in Table 3) was a structure that did not contain isotropically grown carbides and the grain interiors appeared black, while pearlite (abbreviated as P in Table 3) was a structure in which ferrite (black) and carbides (white) appeared in a striped pattern. Bainite (abbreviated as B in Table 3) was 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 contained carbides with a diameter of 1.0 × 10 4 pieces / mm 2 When it contains more than 1.0 x 10 carbides, it is defined as tempered bainite. In a tempered structure, the carbides are divided, and for example, the long and thin carbides that appear between the laths of bainite have become multiple round carbides, so it is easy to distinguish by looking at the carbides. Martensite has a lath-shaped ferrite structure that has grown long and thin like bainite, and is a structure that contains carbides of less than 0.05 μm in equivalent circle diameter, and further, carbides of 1.0 x 10 carbides are defined as 1.0 x 10 carbides. 4 pieces / mm 2 If the structure contains more than 0.50 μm in diameter, it is defined as tempered martensite. Carbides appear as white dots. Austenite is defined as a structure that exists between bainite and martensite structures, has a circle equivalent diameter of 0.50 μm or more, and is not made up of carbides.

[0106] [Strength characteristics of steel plates] A JIS Z 2201 No. 1B test piece was taken from the full thickness of each steel plate so that the plate width direction coincided with 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 if there was a yield point, or 0.2% proof stress σ0.2 if 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 strength properties.

[0107] [Toughness after 5% pre-strain] Tensile test specimens with a parallel section width of 60 mm, a parallel section length of 230 mm, and a total length of 500 mm were taken from the full thickness of each steel plate in a direction parallel to the rolling direction and subjected to 5% plastic strain using a tensile testing machine. Next, aging heat treatment was performed at 250°C for 1 hour. Subsequently, V-notch test specimens according to JIS Z 2242 were taken from a location 1 mm removed from the surface of each steel plate, with the rolling direction aligned with the longitudinal direction of the test specimen. Charpy impact tests were performed according to JIS Z 2242 to measure the ductile-brittle fracture transition temperature (vTrs). Steel plates with a vTrs value of -60°C or less were evaluated as having excellent toughness after bending.

[0108] [Ammonia SCC resistance] Ammonia SCC resistance was evaluated by an accelerated test in which a four-point bending test was carried out in a test solution and constant-potential anodic electrolysis was carried out to accelerate corrosion. Specifically, the following procedure was carried out. Test specimens measuring 5 mm thick x 15 mm x 115 mm were taken from the surface of the steel sheets, ultrasonically degreased in acetone for 5 minutes, and then subjected to four-point bending to a stress equal to the 100% YS of each steel sheet. The test cell containing the four-point bending test specimens was filled with a solution containing 12.5 g of ammonium carbamate and 1 L of liquid ammonia. The potentiostat was then used to apply a potential of +2.0 V vs. Pt to the test specimens, and the specimens were immersed at room temperature (25°C). After 168 hours of immersion, the test specimens were removed from the test solution and visually inspected. Ammonia SCC resistance was judged good if no cracks were observed on the surface of the test specimen, and poor if cracks were observed.

[0109] [Joint HAZ toughness] Test plates for joints taken from each steel plate were given a Y-groove and then submerged arc welding was performed with a welding heat input of 100 kJ / cm using commercially available low-temperature steel welding wire to produce joints using high heat input welding. The yield strength (YS) and toughness of the resulting joints were evaluated. The test methods were as follows: NK U4 impact test specimens were taken from the joint surface to a depth of 1 mm, with the HAZ as the notch position, so that the weld metal and base metal were 50% each above the notch. 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) was taken as the toughness of the HAZ. -60℃ A HAZ toughness of 100 J or more was evaluated as excellent.

[0110] The evaluation results thus obtained are shown in Table 3.

[0111] [Table 1]

[0112] [Table 2] TIFF2026001684000003.tif255112

[0113] [Table 3]

[0114] As can be seen from Table 3, all of the inventive examples have a hardness of 210 HV or less at a depth of 1 mm from the surface of the steel plate, a yield strength (YS) of 235 MPa or more and 440 MPa or less, and a ductile-brittle fracture transition temperature vTrs of -60°C or less, which are within the range specified by the present invention, and the obtained steel plates have excellent ammonia stress corrosion cracking resistance and HAZ toughness of high heat input welded joints.

[0115] Steel plates Nos. 1 and 17 to 29, 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.

[0116] On the other hand, steel plates Nos. 2, 4, 5, 6, 9, 11, 15, and 16, which correspond to comparative examples and were manufactured under conditions different from those of the present invention, are inferior to the invention examples in at least one of surface hardness, yield strength (YS), low-temperature toughness, and ammonia SCC resistance. Steel plates Nos. 31, 32, 34, 35, 36, 38, 40, and 41 contain various elements in amounts greater than the invention examples, and are therefore inferior to the invention examples in low-temperature toughness and HAZ toughness. Steel plate No. 33 has a high Mn content and is therefore inferior to the invention examples in HAZ toughness. Steel plates Nos. 37 and 39 have low N or Ti contents and are therefore inferior to the invention examples in low-temperature toughness. Steel plate No. 43 has a high Ceq and is inferior to the invention examples in surface hardness, yield strength (YS), low-temperature toughness, and ammonia SCC resistance. Steel plates Nos. 44, 45, 46, 47, and 48 have Ti / N or formula (1) outside the specified range, and therefore have inferior HAZ toughness compared to the invention examples. Steel plates Nos. 49, 50, 51, and 52 have a lower content of either C, Si, Mn, or Al compared to the invention examples, and therefore have inferior toughness at low temperatures compared to the invention examples. [Industrial Applicability]

[0117] According to the present invention, a steel plate is provided that satisfies low-temperature toughness and strength characteristics suitable for liquefied gas storage tanks, while also exhibiting excellent toughness in joints after large heat input welding and excellent toughness even after bending, along with a manufacturing method thereof.

Claims

1. In mass%, C: 0.020% or more and 0.100% or less, Si: 0.03% or more and 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: 0.005% or more and 0.060% or less, Ti: 0.010% or more and 0.031% or less, N: 0.0038% or more and 0.0100% or less; O: 0.0100% or less and the balance being Fe and unavoidable impurities, and further having a Ti / N ratio of 2.00 to 4.00, which is the ratio of the Ti content (mass%) to the N content (mass%), satisfying the following formula (1), and having a carbon equivalent Ceq represented by the following formula (2) of 0.250% to 0.380%, The hardness at a depth of 1 mm from the surface of the steel plate is 210 HV or less, The steel plate has a microstructure in which the volume fraction of ferrite at a depth of ¼ of the plate thickness from the surface thereof is 20% or more, and the total volume fraction of ferrite and bainite is 80% or more, The yield strength is 235 MPa or more and 440 MPa or less, A steel plate having a ductile-brittle fracture transition temperature vTrs of -60°C or less after being pre-strained by 5%. 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 component, and is set to 0 when no component is contained.

2. The component composition further includes, in mass %, 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: 0.0050% 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 The steel sheet according to claim 1, comprising one or more selected from the group consisting of:

3. The component composition is, in mass%, C: 0.045% or more and 0.100% or less; Mn: 0.50% or more and 1.80% or less; The microstructure satisfies a bainite volume fraction of 20% or more at a depth position of 1 / 4 of the plate thickness from the surface, Furthermore, the tensile strength is 400 MPa or more. The steel sheet according to claim 1.

4. The component composition is, in mass%, C: 0.045% or more and 0.100% or less; Mn: 0.50% or more and 1.80% or less; The microstructure satisfies a bainite volume fraction of 20% or more at a depth position of 1 / 4 of the plate thickness from the surface, Furthermore, the tensile strength is 400 MPa or more. The steel sheet according to claim 2.

5. A steel material having a component composition according to any one of claims 1 to 4, The temperature at a depth of 1 / 4 of the plate thickness from the surface of the steel material, Ar ３ Heating to a temperature of 100°C or higher and 1250°C or lower, Next, the rolling start temperature was set to Ar ３ The rolling end temperature is set to 100°C or higher than the rolling point, the cumulative reduction rate in the unrecrystallized region is 60% or higher, and the rolling end temperature is Ar ３ Hot rolling is carried out to exceed the rolling point. After that, the cooling start temperature is Ar ３ The cooling stop temperature exceeds Ar ３ Cooling is performed below 100°C, Stop the cooling once and suspend the cooling for 5 seconds or more, Then cooling is carried out, Then, heat treatment is performed at a temperature of 400°C or higher and 580°C or lower. Steel plate manufacturing method.

6. The heating temperature of the steel material is 950°C or higher at a depth of 1 / 4 of the plate thickness from the surface, Ar ３ Cooling begins from a temperature above the Ar point. ３ Ar below point ３ Cool at an average cooling rate of 5°C / s or more until the temperature reaches -90°C or higher. The cooling is stopped once and the cooling is interrupted for 5 seconds or more and 600 seconds or less, Subsequently, the steel plate is cooled at an average cooling rate of 5°C / s or more at a depth of 1 / 4 of the plate thickness from the surface thereof, 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 580°C or less. The method for manufacturing a steel sheet according to claim 5.

7. The method for producing a steel sheet according to claim 5, wherein the average reduction rate per rolling pass in the unrecrystallized region is 2.0% or more.

8. The method for producing a steel sheet according to claim 6, wherein the average reduction rate per rolling pass in the unrecrystallized region is 2.0% or more.

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