Steel and liquid carbon dioxide transport tank

Abstract

Provided is a steel and a liquid carbon dioxide transport tank comprising said steel, the steel having the following properties: a specific chemical composition in which α, represented by the following equation, is 4.0 to 16.0; a tensile strength of 590 MPa to 930 MPa; the microstructure at a site that is 1 / 4 of the thickness from the surface comprises lower bainite, martensite and retained austenite; the total area ratio of lower bainite and martensite is 15.0% or more; the total area ratio of upper bainite, lower bainite, and martensite is 90.0% or more; and the area ratio of retained austenite is 0.2% to less than 5.0%. α=0.50×√[C] ×(1+0.64[Si]) ×(1+4.10[Mn]) ×(1+0.27[Cu]) ×(1+0.52[Ni]) ×(1+2.33[Cr]) ×(1+3.14[Mo])

Description

Steel and liquid carbon dioxide transport tanks

[0001] The present disclosure relates to steel products and tanks for transporting liquid carbon dioxide.

[0002] Steel can be used as a structural material for welded structures such as buildings, bridges, ships, line pipes, marine structures, pressure vessels, tanks, etc. Steel, which has excellent strength and low-temperature toughness, is effective for low-temperature applications.

[0003] Low-temperature steels are used for low-temperature pressure vessels such as liquefied gas storage tanks. Low-temperature steels include Al-killed steel, nickel steel, high-manganese steel, and austenitic stainless steel, depending on the operating temperature. For example, nickel steels such as 3.5% Ni steel are used as materials for tanks carrying liquefied ethane and liquefied ethylene, which have an operating temperature of around -100°C.

[0004] Like this 3.5% Ni steel, Ni is often contained in steel materials that require low-temperature toughness, such as those used in low-temperature pressure vessels.

[0005] For example, Patent Document 1 proposes a nickel-containing steel material for low-temperature use that has a specific chemical composition containing 2.7% to 5.0% Ni, a prior austenite grain size of 20 μm or less when heated for quenching, an effective crystal grain size of 12 μm or less after heat treatment, and a tensile strength of 450 MPa to 690 MPa, and has excellent toughness. Furthermore, various steel materials with defined chemical compositions and microstructures (metal structures) have been proposed with the aim of achieving low-temperature toughness and high strength (see, for example, Patent Documents 2 to 9).

[0006] Patent Document 1: JP 2019-81930 A Patent Document 2: WO 2014 / 103629 Patent Document 3: JP 52-156121 A Patent Document 4: JP 55-104427 A Patent Document 5: JP 58-73717 A Patent Document 6: JP 7-331328 A Patent Document 7: JP 2001-123222 A Patent Document 8: JP 2001-123245 A Patent Document 9: JP 2007-46096 A

[0007] Low-temperature steels used in low-temperature pressure vessels are desired to have both high strength and low-temperature toughness. Cryogenic pressure vessels are manufactured by welding steel materials, and post-weld heat treatment (PWHT) is sometimes performed to remove residual stresses caused by welding. Recently, there has been an increasing demand for low-temperature toughness after PWHT.

[0008] An object of the present disclosure is to provide a steel material and a tank for transporting liquid carbon dioxide that are suitable for low-temperature applications, having high tensile strength and good low-temperature toughness regardless of whether they are subjected to post-weld heat treatment or not.

[0009] The gist of the present disclosure is as follows. <1> In mass%, C: 0.03% or more and 0.20% or less, Si: 0.01% or more and 0.50% or less, Mn: 0.10% or more and 1.65% or less, P: 0.025% or less, S: 0.0250% or less, Ni: 2.65% or more and 4.45% or less, Al: 0.001% or more and 0.100% or less, O: 0.0100% or less, N: 0.0100% or less, Cu: 0 to 1.50%, Cr: 0 to 3.00%, Mo: 0 to 2.00%, B: 0 to 0.0050%, Nb: 0 to 0.050%, Ti: 0 to 0.050%, V: 0 to 0.10%, Mg: 0 to 0.0200%, A steel material having a chemical composition of Ca: 0 to 0.0200%, REM: 0 to 0.0200%, balance: Fe and impurities, and wherein α, represented by the following formula (1), is 4.0 or more and 16.0 or less, the steel material having a tensile strength of 590 MPa or more and 930 MPa or less, the microstructure of a region at 1 / 4 of the thickness from the surface of the steel material in the thickness direction contains lower bainite, martensite, and retained austenite, the sum of area ratios of the lower bainite and the martensite is 15.0% or more, the sum of area ratios of upper bainite, the lower bainite, and the martensite is 90.0% or more, and the area ratio of the retained austenite is 0.2% or more and less than 5.0%. α=0.50×√[C]×(1+0.64[Si])×(1+4.10[Mn])×(1+0.27[Cu])×(1+0.52[Ni])×(1+2.33[Cr])×(1+3.14[Mo]) (1) where the [element symbols] in formula (1) represent the contents (mass%) of the corresponding elements contained in the steel material. If the corresponding element is not contained, zero is substituted. <2> The steel material according to <1>, wherein the chemical composition includes the following Group A: [Group A] One or more selected from the group consisting of Cu: 0.01% or more and 1.50% or less, Cr: 0.10% or more and 3.00% or less, Mo: 0.01% or more and 2.00% or less, and B: 0.0003% or more and 0.0050% or less. <3> The steel material according to <1> or <2>, wherein the chemical composition includes the following Group B:[Group B] One or more elements selected from the group consisting of Nb: 0.001% or more and 0.050% or less, Ti: 0.001% or more and 0.050% or less, and V: 0.01% or more and 0.10% or less <4> The steel material according to any one of <1> to <3>, wherein the chemical composition includes the following Group C. [Group C] One or more elements selected from the group consisting of Mg: 0.0003% or more and 0.0200% or less, Ca: 0.0003% or more and 0.0200% or less, and REM: 0.0003% or more and 0.0200% or less <5> The steel material according to any one of <1> to <4>, wherein the aspect ratio of prior austenite grains at a position at 1 / 4 of the thickness from the surface of the steel material in the thickness direction is 1.5 or more. <6> The steel material according to any one of <1> to <4>, wherein the aspect ratio of prior austenite grains in a region from the surface of the steel material in the thickness direction to one-quarter of the thickness is less than 1.5. <7> The steel material according to any one of <1> to <6>, wherein the microstructure in a region from the surface of the steel material in the thickness direction to one-quarter of the thickness has an average crystal grain size of 20.0 μm or less. <8> The steel material according to any one of <1> to <7>, wherein the Charpy impact absorption energy at -100°C is 150 J or more. <9> The steel material according to any one of <1> to <8>, wherein, when the steel material is subjected to a heat treatment in which the heating rate and cooling rate are 55°C / h in a temperature range of 425°C or more and the steel material is held at 600°C for 2 hours, the Charpy impact absorption energy at -100°C is 150 J or more at the heat-treated region. <10> A tank for transporting liquid carbon dioxide, comprising the steel material according to any one of <1> to <9>.

[0010] According to the present disclosure, it is possible to provide a steel material and a tank for transporting liquid carbon dioxide that are suitable for low-temperature applications, having high tensile strength and good low-temperature toughness regardless of whether the steel material is subjected to post-weld heat treatment or not.

[0011] FIG. 10 is a diagram showing an example of the discrimination results of the microstructure.

[0012] The present disclosure will be described in detail below. In this disclosure, unless otherwise specified, "post-weld heat treatment" refers to post-weld heat treatment conforming to the content specified in JIS Z 3700:2009 "Post-weld heat treatment method." In this disclosure, "steel material" or "base material" refers to a steel material portion that does not include a surface treatment layer such as a plating layer or a coating film. However, a surface treatment layer such as a plating layer or a coating film may be formed on the surface of the steel material according to the present disclosure.

[0013] In this disclosure, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. However, when the numerical values ​​before and after "to" are followed by "greater than" or "less than," the numerical range does not include these numerical values ​​as the lower or upper limits. Regarding the content of elements in a chemical composition, "%" means "mass %." The term "process" does not only refer to an independent process, but also includes processes that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

[0014] Hereinafter, a steel material according to an embodiment of the present disclosure will be described. First, the results of the studies by the inventors of the present disclosure that led to the completion of the steel material according to the present disclosure and the new findings obtained will be described in detail.

[0015] The inventors of the present disclosure conducted studies to improve the strength of steel materials. The tensile strength of steel materials is ensured by the composition of the microstructure. The inventors of the present disclosure collected samples from the 1 / 4t portion (t: thickness of the steel material), which is a portion of the steel material that is 1 / 4 of the thickness from the surface in the thickness direction after reheating and quenching, performed tensile tests, and observed the microstructure. As a result, it was found that the microstructure of the 1 / 4t portion of steel materials having a tensile strength of 590 MPa or more and 930 MPa or less had an area fraction of ferrite of less than 10.0%, and a total area fraction of upper bainite, lower bainite, and martensite of 90.0% or more. The total area fraction of upper bainite, lower bainite, and martensite was measured using electron backscatter diffraction (hereinafter referred to as "EBSD").

[0016] Furthermore, the inventors of the present disclosure conducted studies to improve the toughness of steel materials. The toughness of steel materials is ensured by the composition of the microstructure. The inventors of the present disclosure collected samples from 1 / 4t sections of steel materials that had been reheated and quenched, and then subjected to intermediate heat treatment, performed Charpy impact tests, and observed the microstructure. As a result, it was found that steel materials having a Charpy impact absorption energy of 150 J or more at -100°C have a total area fraction of lower bainite and martensite of 15.0% or more, and an area fraction of retained austenite of 0.2% or more but less than 5.0%. The total area fraction of lower bainite and martensite was measured using EBSD. The area fraction of retained austenite was measured by X-ray diffraction. The volume fraction of retained austenite measured by X-ray diffraction can be considered as an area fraction.

[0017] Furthermore, the inventors of the present disclosure have conducted studies to ensure the toughness of steel materials. The toughness of steel materials is ensured by reducing the area surrounded by high-angle grain boundaries, where the difference in crystal orientation is 15° or more. The inventors of the present disclosure collected samples from the 1 / 4t section of the steel material after reheating and quenching, and measured the circle-equivalent diameter of the area surrounded by high-angle grain boundaries using EBSD. Hereinafter, the circle-equivalent diameter of the area surrounded by high-angle grain boundaries is referred to as the grain size. The samples were subjected to mechanical polishing and electrolytic polishing, and the diameters of the areas surrounded by high-angle grain boundaries were measured to obtain a grain size of 4 mm. ２ Analysis was carried out in the region of 4 mm using an EBSD device attached to a FE-SEM (field emission scanning electron microscope). ２ The average grain size (sometimes referred to as "effective grain size") was calculated as an area-weighted average of the grain sizes measured in the region 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 40, 41, 42, 43, 44, 45, 46, 47, 48, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 112, 113, 114, 115, 120, 122, 123, 124, 130, 132, 140, 152, 162, 170, 182, 190, 192, 193, 194, 195, 10

[0018] Furthermore, the inventors of the present disclosure also investigated steel materials that had been subjected to direct quenching, i.e., water cooling after hot rolling, and then intermediate heat treatment, and obtained results similar to those of steel materials that had been reheated and quenched.

[0019] <Chemical Composition> Next, the alloying elements that make up the chemical composition of the steel material according to the present disclosure will be described. Note that in the following description of the alloying elements, "%" in the content means "mass %."

[0020] (C: 0.03% or more, 0.20% or less) C is an element that increases the strength of steel. From the viewpoint of ensuring the strength of steel used in structures, in the present disclosure, the C content is 0.03% or more. The C content is preferably 0.05% or more, or 0.07% or more. On the other hand, C is an element that reduces toughness, and from the viewpoint of ensuring the toughness of the weld heat affected zone (hereinafter sometimes referred to as "HAZ"), in the present disclosure, the C content is 0.20% or less. The C content is preferably 0.16% or less, 0.14% or less, or 0.12% or less.

[0021] (Si: 0.01% or more, 0.50% or less) Si is used as a deoxidizer and is also an element that dissolves in steel to increase strength. From the viewpoint of controlling the O concentration contained in molten steel, in the present disclosure, the Si content is 0.01% or more. The Si content is preferably 0.03% or more, 0.05% or more, or 0.10% or more. On the other hand, if the Si content is excessive, a hard phase may be formed in the HAZ, resulting in a decrease in toughness. Therefore, from the viewpoint of ensuring the toughness of the HAZ, in the present disclosure, the Si content is 0.50% or less. The Si content is preferably 0.30% or less, or 0.20% or less.

[0022] (Mn: 0.10% or more, 1.65% or less) Mn is used as a deoxidizer and is an element that improves the hardenability of steel and contributes to high strength. From the viewpoint of controlling the O concentration contained in molten steel, the Mn content is 0.10% or more in the present disclosure. Furthermore, 0.10% or more of Mn forms MnS, thereby reducing solute S and preventing hot tearing. From the viewpoint of ensuring the strength of the steel material and the toughness of the HAZ, the Mn content is preferably 0.30% or more, or 0.50% or more. On the other hand, if the Mn content is excessive, Mn may segregate to grain boundaries during PWHT, thereby reducing the toughness after PWHT. Therefore, from the viewpoint of ensuring the toughness of the steel material before and after PWHT, the Mn content is 1.65% or less in the present disclosure. The Mn content is preferably 1.50% or less, 1.25% or less, or 1.10% or less.

[0023] (P: 0.025% or less) P is an impurity element. There is no lower limit for the P content, but from the viewpoint of production costs, in the present disclosure, the P content may be 0.001% or more. On the other hand, if the P content is excessive, P may segregate to grain boundaries during PWHT, which may reduce toughness after PWHT. Therefore, in the present disclosure, the P content is 0.025% or less. The P content is preferably 0.016% or less, 0.012% or less, or 0.008% or less.

[0024] (S: 0.0250% or less) S is an impurity element. There is no lower limit for the S content, but from the viewpoint of production costs, in the present disclosure, the S content may be 0.0001% or more. On the other hand, if the S content is excessive, elongated MnS is generated in the central segregation portion, which may deteriorate the toughness and ductility of the steel material and HAZ. From the viewpoint of ensuring the toughness and ductility of the steel material and HAZ, the S content is 0.0250% or less. The S content is preferably 0.0100% or less, or 0.0050% or less.

[0025] (Ni: 2.65% or more, 4.45% or less) Ni is an element effective for improving the hardenability and toughness of steel, so in the present disclosure, the Ni content is 2.65% or more. The Ni content is preferably 3.00% or more, or 3.20% or more. However, since Ni is an expensive element, in the present disclosure, from the viewpoint of cost reduction, the Ni content is 4.45% or less. The Ni content is preferably 4.10% or less, or 3.80% or less.

[0026] (Al: 0.001% or more, 0.100% or less) Al is an element useful for deoxidation and also an element that refines the grain size during quenching by forming nitrides, so in the present disclosure, the Al content is 0.001% or more. However, if Al is contained in excess, Al may form coarse nitrides, which may reduce the toughness of the steel material and the HAZ. Therefore, the Al content is 0.100% or less. The Al content is preferably 0.080% or 0.050% or less.

[0027] (O: 0.0100% or less) O ​​is an impurity element. There is no lower limit for the O content, but from the viewpoint of production costs, in the present disclosure, the O content may be 0.0001% or more. On the other hand, if the O content is excessive, coarse oxides are generated, and the toughness and ductility of the steel material and HAZ may deteriorate. From the viewpoint of ensuring the toughness and ductility of the steel material and HAZ, the O content is 0.0100% or less. The O content is preferably 0.0060% or less, or 0.0040% or less.

[0028] (N: 0.0100% or less) N is an impurity element. There is no lower limit for the N content, but from the viewpoint of production costs, in the present disclosure, the N content may be 0.0001% or more. From the viewpoint of ensuring the properties of the steel material and the toughness of the HAZ, in the present disclosure, the N content is 0.0100% or less. The N content is preferably 0.0050% or less, or 0.0040% or less.

[0029] The steel material according to the present disclosure may contain other elements (selective elements) in place of a portion of Fe. For example, the following selective elements from groups A to C may be included, but the content of these elements may be 0%.

[0030] [Group A] In order to improve strength and toughness, the steel material according to the present disclosure may contain, as necessary, one or more of the following optional elements Cu, Cr, Mo, and B, which have the effect of improving hardenability.

[0031] (Cu: 1.50% or less) Cu is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the Cu content is not limited and may be 0%. Cu also has a small adverse effect on weldability and HAZ toughness, and has the effect of increasing the hardenability of steel, thereby improving the strength of the steel. Therefore, in the present disclosure, the Cu content may be 0.01% or more. The Cu content is preferably 0.10% or more. However, from the viewpoint of suppressing the occurrence of Cu cracks during hot rolling of the steel, the Cu content is 1.50% or less in the present disclosure. The Cu content is preferably 1.00% or less, 0.80% or less, 0.60% or less, or 0.50% or less.

[0032] (Cr: 3.00% or less) Cr is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the Cr content is not limited and may be 0%. Cr is also an element that has the effect of increasing the hardenability of steel and therefore improves the strength of the steel. Therefore, in the present disclosure, the Cr content may be 0.01% or more. The Cr content is preferably 0.10% or more. However, from the viewpoint of suppressing deterioration of the toughness and weldability of the HAZ, the Cr content is 3.00% or less in the present disclosure. The Cr content is preferably 2.20% or less, 1.40% or less, or 0.80% or less.

[0033] (Mo: 2.00% or less) Mo is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the Mo content is not limited and may be 0%. Mo is also an element that has the effect of increasing the hardenability of steel and therefore improves the strength of steel. Therefore, in the present disclosure, the Mo content may be 0.01% or more. The Mo content is preferably 0.05% or more, 0.10% or more, 0.20% or more, or 0.30% or more. However, from the viewpoints of suppressing deterioration of HAZ toughness and weldability and suppressing increases in alloy costs, the Mo content is 2.00% or less in the present disclosure. The Mo content is preferably 1.20% or less, or 0.80% or less.

[0034] (B: 0.0050% or less) B is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the B content is not limited and may be 0%. B is also an element that exhibits a significant effect of increasing the hardenability of steel and improves the strength of the steel. Therefore, in the present disclosure, the B content may be 0.0003% or more. However, from the viewpoint of suppressing deterioration in the surface quality of steel slabs manufactured by continuous casting, in the present disclosure, the B content is 0.0050% or less. The B content is preferably 0.0030% or less, or 0.0020% or less.

[0035] [Group B] In order to improve strength, the steel material according to the present disclosure may contain, as necessary, one or more of the following optional elements Nb, Ti, and V, which have the effect of increasing the strength of the steel material by precipitating carbides, nitrides, etc.

[0036] (Nb: 0.050% or less) Nb is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the Nb content is not limited and may be 0%. Nb also forms carbides and nitrides, has the effect of refining the metal structure, and is an element that improves the strength of the steel. Therefore, in the present disclosure, the Nb content may be 0.001% or more. However, from the viewpoint of suppressing deterioration of the toughness and weldability of the HAZ, the Nb content is 0.050% or less. The Nb content is preferably 0.040% or less, or 0.030% or less. In particular, from the viewpoint of ensuring the toughness of the steel after PWHT, the Nb content may be 0.004% or less.

[0037] (Ti: 0.050% or less) Ti is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the Ti content is not limited and may be 0%. Ti also forms carbides and nitrides, has the effect of refining the metal structure, and is an element that improves the strength of the steel. Therefore, in the present disclosure, the Ti content may be 0.001% or more. However, from the viewpoint of suppressing deterioration of the toughness and weldability of the HAZ, the Ti content is 0.050% or less. The Ti content is preferably 0.040% or less, 0.030% or less, or 0.020% or less. In particular, from the viewpoint of ensuring the toughness of the steel after PWHT, the Ti content may be 0.004% or less, or 0.002% or less.

[0038] (V: 0.10% or less) V is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the V content is not limited and may be 0%. V is also an element that forms carbides and nitrides and improves the strength of steel. Therefore, in the present disclosure, the V content may be 0.01% or more. However, from the viewpoint of suppressing deterioration of the toughness and weldability of the HAZ and suppressing an increase in alloy costs, the V content is 0.10% or less. The V content is preferably 0.08% or less, or 0.05% or less.

[0039] [Group C] In order to improve the toughness of the HAZ, the steel material according to the present disclosure may contain one or more of the following selective elements Mg, Ca, and REM, as necessary.

[0040] (Mg: 0.0200% or less) Mg is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the Mg content is not limited and may be 0%. Mg is also an element that forms oxides to improve the toughness of the weld heat affected zone. Therefore, in the present disclosure, the Mg content may be 0.0003% or more, 0.0006% or more, or 0.0010% or more. On the other hand, an excessive Mg content may form coarse oxides, which may reduce the toughness of the steel. Therefore, from the viewpoint of ensuring toughness, in the present disclosure, the Mg content is 0.0200% or less. The Mg content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[0041] (Ca: 0.0200% or less) Ca is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the Ca content is not limited and may be 0%. Ca is also an element that spheroidizes sulfides in the steel, thereby reducing the effect of MnS, which reduces the toughness of the steel and the weld heat-affected zone. Therefore, in the present disclosure, the Ca content may be 0.0003% or more, 0.0006% or more, or 0.0010% or more. On the other hand, an excessive Ca content may form coarse oxides, which may reduce the toughness of the steel. Therefore, from the viewpoint of ensuring toughness, in the present disclosure, the Ca content is 0.0200% or less. The Ca content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[0042] (REM: 0.0200% or less) Rare earth metals (REM) refer to a collective term for 17 elements, including two elements, Sc and Y, and 15 lanthanoid elements, such as La, Ce, and Nd. The REM content refers to the total content of these 17 elements. REM is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the REM content is not limited and may be 0%. REM is also an element that forms oxides to improve the toughness of the weld heat affected zone. Therefore, in the present disclosure, the REM content may be 0.0003% or more, 0.0006% or more, or 0.0010% or more. On the other hand, an excessive REM content may form coarse oxides, which may reduce the toughness of the steel. Therefore, from the viewpoint of ensuring toughness, the REM content is 0.0200% or less in the present disclosure. The REM content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[0043] (Balance: Fe and Impurities) The balance of the chemical composition of the steel material according to the present disclosure is iron (Fe) and impurities. The impurities refer to components that are mixed in due to raw materials such as ore and scrap or other factors when industrially producing steel material.

[0044] In addition to limiting the content of each element, the present disclosure limits the range of the α value as follows:

[0045] (α value: 4.0 or more and 16.0 or less) The α value is calculated by the following formula (1): α = 0.50 × √[C] × (1 + 0.64 [Si]) × (1 + 4.10 [Mn]) × (1 + 0.27 [Cu]) × (1 + 0.52 [Ni]) × (1 + 2.33 [Cr]) × (1 + 3.14 [Mo]) ... (1) where [C], [Si], [Mn], [Cu], [Ni], [Cr], and [Mo] are the contents (mass%) of C, Si, Mn, Cu, Ni, Cr, and Mo in the steel. If the corresponding element is not contained, zero is substituted. Note that √[C] is [C] １／２ is synonymous with.

[0046] In the present disclosure, the α value is set to a range of 4.0 to 16.0. This is an index indicating the hardenability of a steel material, and the higher the α value, the more likely it is that a lower bainite and martensite structure with an advantageous balance of strength and toughness can be formed. When α is within an appropriate range, the ratio of lower bainite and martensite structures with an advantageous balance of strength and toughness in the HAZ structure also increases, thereby ensuring HAZ toughness. When α is 4.0 or higher, the hardenability of the base material is ensured, the ratio of lower bainite and martensite with an advantageous balance of strength and toughness increases, and toughness deterioration is suppressed. In addition, the ratio of lower bainite and martensite in the HAZ structure also tends to increase, improving HAZ toughness. On the other hand, when the α value is 16.0 or less, the strength of the steel material does not become too high, and toughness can be ensured. Furthermore, when the α value is 16.0 or less, toughness after PWHT can be ensured. Furthermore, the HAZ does not become too hard, and HAZ toughness can be ensured.

[0047] By satisfying the above-mentioned numerical range for the α value, it is possible to provide a nickel-containing steel material for low temperature use that is excellent in strength and toughness. The α value is preferably 4.5 or more, or 5.0 or more. The α value is preferably 15.5 or less, or 15.0 or less.

[0048] <Microstructure> Next, the microstructure of the steel material according to the present disclosure will be described. The microstructure of the steel material according to the present disclosure at a position from the surface to ¼ of the thickness in the thickness direction includes lower bainite, martensite, and retained austenite. Furthermore, the bainite may include upper bainite in addition to lower bainite.

[0049] "Bainite" is a structure containing bainitic ferrite (α°B) with a substructure within the grains, and is a general term for upper bainite and lower bainite. "Upper bainite" refers to either or both upper bainite containing retained austenite or MA phase (martensite-austenite mixed phase) between laths, and upper bainite containing carbides between laths. "Lower bainite" refers to lath-shaped lower bainite containing carbides within the laths.

[0050] "Martensite" exists in four forms: lath, butterfly, lens, and thin plate, but the components disclosed herein mainly produce lath martensite. Lath martensite is composed of packets and blocks consisting of groups of laths in a specific arrangement, and is a structure in which one austenite grain is divided into several packets.

[0051] (Total area ratio of lower bainite and martensite: 15.0% or more) Lower bainite and martensite are hard phases and increase the toughness of the steel material. From the viewpoint of ensuring the toughness of the steel material, the area ratio of lower bainite and martensite in the 1 / 4t portion is 15.0% or more. The area ratio of lower bainite and martensite in the 1 / 4t portion is preferably 20.0% or more, or 30.0% or more. The total area ratio of lower bainite and martensite in the 1 / 4t portion may be 95.0% or more.

[0052] (Total area ratio of upper bainite, lower bainite, and martensite: 90.0% or more) From the viewpoint of ensuring the strength of the steel material, the total area ratio of upper bainite, lower bainite, and martensite in the 1 / 4 t portion is 90.0% or more. The total area ratio of upper bainite, lower bainite, and martensite in the 1 / 4 t portion may be 95.0% or more. Furthermore, the upper bainite in the 1 / 4 t portion may be 1.0% or more.

[0053] (Area ratio of retained austenite: 0.2% or more, less than 5.0%) Retained austenite increases the toughness of steel material. From the viewpoint of ensuring the toughness of steel material, the area ratio of retained austenite in the 1 / 4t portion is 0.2% or more. The area ratio of retained austenite in the 1 / 4t portion is preferably 0.3% or more, or 0.5% or more. On the other hand, the area ratio of retained austenite in the 1 / 4t portion of the steel plate according to the present disclosure is at most less than 5%. The area ratio of retained austenite in the 1 / 4t portion is preferably 3.0% or less, or 2.0% or less.

[0054] Observation of the microstructure of steel is carried out using a sample with the 1 / 4 t portion of the steel as the observation surface. Two types of samples are prepared: (a) electrolytic polishing and (b) nital etching. Measurements are taken at three locations for each of (a) and (b) samples using the method described below, and the average value of the three locations is taken as the area ratio of the microstructure of the steel. Three samples of each of (a) and (b) may be prepared and the average taken for each sample, or measurements may be taken at three visual fields within each sample and the average taken.

[0055] The total area ratios of upper bainite, lower bainite, martensite, and retained austenite are measured by EBSD using electropolished samples that are mechanically polished to a mirror finish and then electropolished to remove the strain layer caused by mechanical polishing. The measurement magnification is 200x, and measurements are performed over a 400 μm x 400 μm area at a 0.4 μm pitch. The measurement is performed with an electron beam diameter of 0.4 μm or less. The confidence index (hereinafter referred to as the "CI value") is set to 0.1 or greater. The determination of ferrite, upper bainite, lower bainite, martensite, and retained austenite is performed by setting the threshold value of grain average misorientation (hereinafter referred to as "GAM") to 0.5. The GAM value is an index defined in OIM-Analysis (EBSD crystal orientation analysis software manufactured by TSL, USA). The region where GAM is 0.5 or less is ferrite, and the region where GAM exceeds 0.5 is upper bainite, lower bainite, martensite, or retained austenite. In the present disclosure, upper bainite, lower bainite, martensite, and retained austenite are determined using the EBSD GAM as a threshold, and therefore include not only upper bainite, lower bainite, martensite, and retained austenite, but also tempered upper bainite, tempered lower bainite, and tempered martensite. Comparing the structures of direct quenching (DQ) and direct quenching followed by tempering (DQT), decomposition of MA and coarsening of carbides occur after tempering, but the appearance of the structure does not change significantly.

[0056] Using a nital-etched sample, the area ratio of upper bainite is measured by SEM observation. The measurement magnification is 500x, and the measurement is performed over an area of ​​360 μm x 480 μm. The area with a clear lath structure and where carbides and MA are formed along the lath boundaries is upper bainite. The structure in the region where the internal structure is relatively coarse and the carbide density is sparse and a mixture of coarse and dense is considered to be upper bainite. Figure 1 shows an example of the results of structure determination. (A) and (B) are SEM images of the same region of a steel material manufactured by DQT and with an α value of 9.9. In (B), the region surrounded by the white line is upper bainite (Bu), and the other region is lower bainite + martensite (B). Ｌ The part judged to be upper bainite (Bu) has sparse white carbides and coarse and dense regions mixed together. On the other hand, the part judged to be lower bainite + martensite (B Ｌ In the portion determined as "+M", carbides are densely and uniformly present. The total area fraction of lower bainite, martensite, and retained austenite is determined by subtracting the area fraction of upper bainite from the total area fraction of upper bainite, lower bainite, martensite, and retained austenite measured above. Furthermore, the total area fraction of lower bainite and martensite is determined by determining the area fraction of retained austenite using the measurement method described below and subtracting it from the total area fraction of lower bainite, martensite, and retained austenite. Note that when the sum of the area fractions of lower bainite and martensite is greater than 0% according to the above structure determination, lower bainite and martensite are usually present. Lower bainite and martensite can be distinguished using SEM or TEM (transmission electron microscope), and the presence of each structure can be confirmed.

[0057] (Area Fraction of Retained Austenite) The area fraction of retained austenite is measured by X-ray diffraction. The area fraction of retained austenite is measured using a sample with a measurement surface at a location ¼ of the thickness from the surface of the steel material in the thickness direction (also referred to as the "¼t portion" in this specification). The sample is a 2 mm thick test piece, collected from a position ¼ of the width from the end of the steel material in the width direction (direction perpendicular to the rolling direction and thickness direction). Chemical polishing is performed, and the volume fraction of retained austenite is measured by X-ray diffraction using a Mo tube. Quantification is performed based on the ratio of the integrated intensities of the (200) and (211) diffraction peaks of the ferrite phase to the integrated intensities of the (200), (220), and (311) diffraction peaks of the austenite phase, and the average value of six combinations is used. The integrated intensity of the diffraction peak is determined by fitting the background based on the signals before and after the peak and subtracting the signal. The volume fraction measured by X-ray diffraction is considered to be the area fraction.

[0058] (Aspect Ratio of Prior Austenite Grains at 1 / 4t Portion of Steel Material) When manufacturing a steel material according to the present disclosure, for example, after hot rolling, direct quenching (DQ), in which the material is directly water-cooled, or reheat quenching (RQ), in which the material is allowed to cool, reheated, and water-cooled, is performed. After DQ or RQ, an intermediate heat treatment (L) is further performed. In a steel material according to the present disclosure that has been subjected to DQ, the prior austenite grains (sometimes referred to as prior austenite grains or prior γ grains) may have a shape flattened in the rolling direction. If the prior austenite grains in the region from the surface of the steel material to 1 / 4 of the thickness in the thickness direction are flat grains with an aspect ratio of 1.5 or more, the toughness of the steel material can be further improved. This is because flattening the prior austenite grains increases the grain boundary area, which essentially refines the austenite grains and is effective in refinement of the average grain size. The aspect ratio of the prior austenite grains is usually 4.0 or less, and may be 3.5 or less. In the steel material according to the present disclosure when RQ is performed, the aspect ratio of the prior austenite grains in the 1 / 4t portion may be less than 1.5 from the viewpoint of ensuring homogeneity of the microstructure. The aspect ratio of the prior austenite grains in the 1 / 4t portion may preferably be 1.4 or less, or 1.3 or less.

[0059] The aspect ratio of prior austenite crystal grains (sometimes referred to as prior austenite grains) in a steel material is determined as follows. First, an L-section (a cross section parallel to the rolling direction and thickness direction of the steel material) at a location 1 / 4 of the thickness from the surface of the steel material in the thickness direction is mirror-polished, and then etched with an etchant based on a saturated aqueous solution of 2 to 4% picric acid to reveal the prior austenite grain boundaries in an arbitrary region of 1.0 mm in the rolling direction × 0.5 mm in the thickness direction. Next, the major axis and minor axis of each prior austenite grain are measured, and the aspect ratio of each prior austenite grain is calculated by dividing the major axis by the minor axis. The arithmetic mean of all the calculated aspect ratios of the prior austenite grains is determined as the "aspect ratio of the prior austenite grain." The major axis is the maximum length of a prior austenite grain, and the minor axis is the maximum distance between two lines parallel to the major axis direction that contact the grain.

[0060] (Average grain size of 1 / 4t portion of steel material) In the present disclosure, the average grain size (effective grain size) of the 1 / 4t portion of the steel material is preferably 20.0 μm or less. This is because it has been found that if the average grain size of the 1 / 4t portion of the steel material is 20.0 μm or less, the toughness of the steel material tends to be further improved regardless of whether it is before or after PWHT. However, the average grain size of the 1 / 4t portion of the steel material may be more than 20.0 μm. The smaller the average grain size of the steel material, the more preferable it is, so there is no lower limit. Usually, the average grain size is 10 μm or more. The effective grain size is determined by weighted averaging. The effective grain size Darea determined by weighted averaging is 4 mm ２ Among the crystal grain sizes measured in the region, Darea is calculated by the following formula using the area Si and the grain size di of the i-th crystal grain detected during measurement: Darea=ΣSi·di / ΣSi

[0061] <Mechanical Properties> The steel material according to the present disclosure has mechanical properties that combine strength and low-temperature toughness. In particular, it has excellent toughness at -100°C, and can also exhibit excellent low-temperature toughness even after PWHT.

[0062] (Tensile strength: 590 MPa or more and 930 MPa or less) In the present disclosure, the tensile strength of the steel material is set to 590 to 930 MPa. In order to reduce the weight of large welded structures such as transport tanks, steel material that can ensure the strength of the structure even if it is thin is required. Typically, steel material selected for such applications has the above-mentioned tensile strength, and therefore the steel material according to the present disclosure is also manufactured to have the above-mentioned tensile strength.

[0063] (Charpy impact absorption energy at -100°C) In order to ensure high toughness at low temperatures, the steel material of the present disclosure preferably has a Charpy impact absorption energy of 150 J or more at -100°C. Because the steel material of the present disclosure has low-temperature toughness with a Charpy impact absorption energy of 150 J or more at -100°C, a transport tank made of the steel material of the present disclosure can be suitably used for transporting, for example, liquid carbon dioxide. The Charpy impact absorption energy at -100°C is a value measured using a sample taken from a 1 / 4 position of the thickness.

[0064] (Charpy Impact Absorption Energy at −100°C After PWHT) In cryogenic tanks, PWHT may be performed on welds after assembly into transport tanks in order to prevent fracture. During this process, not only the welds but also the base material portion of the steel (also simply referred to as base material) unaffected by the welds are heated. The longer the time the base material is heated to a temperature range of 425°C or higher, the lower the toughness of the base material tends to decrease. When the steel material of the present disclosure is subjected to PWHT at a holding temperature of 600°C, a holding time of 2 hours, and a heating rate and cooling rate of 55°C / h in a temperature range of 425°C or higher, the toughness of the portion subjected to the PWHT is preferably 150 J or more in terms of Charpy impact absorption energy at −100°C. The Charpy impact absorption energy at −100°C after PWHT may be 100 J or more. The Charpy impact absorption energy at −100° C. after PWHT is also a value measured using a sample taken from the ¼ position of the thickness.

[0065] (Charpy Impact Absorption Energy at -100°C After Thermal Cycle) In order to ensure high toughness at low temperatures after a thermal cycle test simulating a weld, the steel material of the present disclosure preferably has a Charpy impact absorption energy of 50 J or more at -100°C after the thermal cycle. The steel material of the present disclosure has low-temperature toughness with a Charpy impact absorption energy of 50 J or more at -100°C after the thermal cycle, making it suitable for use in transport tanks for transporting liquid carbon dioxide, for example. The Charpy impact absorption energy at -100°C after the thermal cycle may be 40 J or more. The Charpy impact absorption energy at -100°C after the thermal cycle is measured using a sample taken from a position 1 / 4 of the thickness of the steel material as a thermal cycle test piece, and the sample is subjected to a thermal history of heating to 1350°C at 60°C / s, holding at 1350°C for 1 second, and then cooling to room temperature at 20°C / s, followed by taking a Charpy test piece from the sample.

[0066] (Thermal Cycle, Charpy Impact Absorption Energy at -100°C After PWHT) In low-temperature tanks, PWHT may be performed on welds after assembly into transport tanks in order to prevent fracture. After the thermal cycle test, the steel material of the present disclosure undergoes PWHT, which involves heating and cooling rates of 55°C / h in a temperature range of 425°C or higher and holding at 600°C for 2 hours, after which Charpy test specimens are taken and measured. In this case, the toughness of the area where the PWHT was performed is preferably such that the Charpy impact absorption energy at -100°C is 50 J or more. The Charpy impact absorption energy at -100°C of the area where the PWHT was performed after the thermal cycle test may also be 40 J or more.

[0067] PWHT may reduce the toughness of steel. Although the cause is unclear, it is presumed that the diffusion of P (phosphorus) and Mn to grain boundaries and the growth or aggregation of inclusions in the structure reduce brittleness and thus toughness. The reduction in toughness due to PWHT can be suppressed by limiting the P and Mn contents and reducing the average crystal grain size of the steel.

[0068] The tensile strength (TS) and yield strength (YS) in the examples are measured by a tensile test in accordance with JIS Z2241:2011. For the tensile test, a JIS 14A test piece is used, taken from the 1 / 4 thickness position, with the longitudinal direction parallel to the width direction of the steel (C direction). TS and YS are measured using three test pieces and calculated by averaging the measurements. The Charpy impact absorption energy is measured by a Charpy impact test at -100°C using an impact blade with a radius of 2 mm in accordance with the provisions of JIS Z2242:2018. The Charpy impact absorption energy is measured using three test pieces and calculated by averaging the measurements. For the Charpy impact test, a V-notch test piece is used, taken from the 1 / 4 thickness position of the steel, with the longitudinal direction parallel to the width direction of the steel (C direction). The V-notch of the test piece is formed so that the longitudinal direction of the notch is the thickness direction of the steel material and the depth direction of the notch is the rolling direction of the steel material.

[0069] The shape of the steel material according to the present disclosure is not particularly limited, and may be a steel plate, a steel strip, a steel section, a steel pipe, or the like. However, steel pipes and steel sections include steel materials formed by joining steel plates, such as welded steel pipes and welded steel sections, as well as steel sections joined with rivets. The thickness of the steel material (thickness of the flange in the case of steel sections) such as steel plates, steel strips, steel sections, and steel pipes is not particularly limited, and is typically 3 mm or more and 150 mm or less. The thickness of the steel material may be 6 mm or more, 10 mm or more, 15 mm or more, or 30 mm or more. The thickness of the steel material may also be 100 mm or less, 80 mm or less, or 60 mm or less.

[0070] Furthermore, the uses of the steel material according to the present disclosure are not particularly limited, but since it has mechanical properties that combine strength and low-temperature toughness, and can exhibit excellent low-temperature toughness even after PWHT in particular, it can be suitably used as a constituent material for tanks that store and transport liquefied gases, in particular liquid carbon dioxide.

[0071] (Method for manufacturing steel material) The steel material according to the present disclosure is produced by melting steel satisfying the above-mentioned chemical composition and then continuously casting it into a steel billet. The steel billet is heated and hot-rolled, followed by direct water-cooling quenching (DQ), or by hot-rolling, allowing it to cool, then reheating and water-cooling (RQ). Thereafter, an intermediate heat treatment (L) is further performed to produce a steel material. In the case of RQ, it is not necessary to allow it to cool naturally before reheating, and water cooling may also be performed. Tempering (T) may also be performed.

[0072] The heating temperature of the material to be rolled is set to Ac from the viewpoint that hot rolling is performed in a temperature range in which the metal structure of the material to be rolled is austenite. ３ The heating temperature of the material to be rolled is preferably 1000°C or higher from the viewpoint of reducing the deformation resistance. On the other hand, the heating temperature of the material to be rolled is 1250°C or lower from the viewpoint of suppressing the coarsening of heated γ grains. The heating temperature of the material to be rolled is preferably 1200°C or lower. ３ is a value calculated by the following formula: Ac ３= 937.2 - 436.5C + 56Si - 19.7Mn - 16.3Cu - 26.6Ni - 4.9Cr + 38.1Mo + 124.8V + 136.3Ti - 19.1Nb + 198.4Al + 3315B The element symbols in the formula represent the content (mass%) of each element contained in the steel slab.

[0073] Hot rolling may consist of rolling in a temperature range where recrystallization occurs (recrystallization temperature range rolling) and rolling in a temperature range where recrystallization is suppressed (non-recrystallization temperature range rolling). Recrystallization temperature range rolling is hot rolling performed at a temperature of 900°C or higher during rolling. The cumulative reduction in recrystallization temperature range rolling is preferably 30% or more, 40% or more, or 50% or more from the viewpoint of refining the austenite grain size of the steel. The cumulative reduction in recrystallization temperature range rolling is determined from the difference between the thickness of the slab before hot rolling and the thickness of the rolled material at 900°C. Cumulative reduction in recrystallization temperature range rolling (%) = 100 x ([thickness of slab] - [thickness of rolled material at 900°C]) / [thickness of slab]

[0074] Non-recrystallization temperature range rolling is hot rolling performed at a temperature of less than 900°C during rolling. The cumulative reduction in non-recrystallization temperature range rolling is preferably 30% or more, 40% or more, or 50% or more from the viewpoint of refining the average crystal grain size of the steel material. The cumulative reduction in non-recrystallization temperature range rolling is determined from the difference between the thickness of the rolled material at 900°C and the thickness of the steel material after rolling is completed. Cumulative reduction in non-recrystallization temperature range rolling (%) = 100 x ([Thickness of rolled material at 900°C] - [Thickness of steel material after rolling]) / [Thickness of rolled material at 900°C]

[0075] The finishing temperature of the hot rolling is set to Ar from the viewpoint of suppressing the formation of ferrite, which reduces the strength. ３ After the hot rolling is completed, the steel material is subjected to accelerated cooling such as water cooling. The starting temperature of the accelerated cooling is set to 1000°C / 2000°F from the viewpoint of suppressing the formation of ferrite, which reduces the strength. ３ That's all. ３ is a value calculated by the following formula: ３=910-310C-80Mn-20Cu-15Cr-55Ni-80Mo+0.35(t-8) The element symbols in the formula represent the content (mass%) of each element contained in the steel, and t represents the thickness (mm) of the steel. From the viewpoint of promoting bainite transformation and martensitic transformation, the cooling rate is 1.0°C / s or more. The cooling rate of accelerated cooling is preferably 5.0°C / s or more, or 10.0°C / s or more. The faster the cooling rate of accelerated cooling, the more preferable it is, but from the viewpoints of homogenizing the cooling rate, cost, etc., it is preferably 50.0°C / s or less, or 30.0°C / s or less. The cooling rate is a value calculated by simulating the cooling rate at a 1 / 4 position of the thickness using heat transfer calculation.

[0076] The stop temperature of the accelerated cooling is 400°C or lower from the viewpoint of improving the strength of the steel material by ensuring upper bainite, lower bainite, and martensite. The stop temperature of the accelerated cooling is preferably 350°C or lower. Accelerated cooling may be performed down to room temperature. The stop temperature of the accelerated cooling is preferably 100°C or higher from the viewpoint of dehydrogenating the steel material.

[0077] When RQ is applied, the steel is reheated and quenched after hot rolling. The reheating temperature of the steel is set to Ac, because quenching is performed from an austenite single phase structure. ３ From the viewpoint of ensuring homogeneity of the microstructure, the reheating temperature of the steel material is preferably 850°C or higher, 880°C or higher, or 900°C or higher. On the other hand, although there is no particular upper limit for the reheating temperature, heating to an excessively high temperature may cause coarsening of austenite grains, resulting in a decrease in toughness, so the reheating temperature is preferably 1000°C or lower, 950°C or lower, or 930°C or lower. ３ is the value calculated by the above formula.

[0078] After DQ or RQ, the steel is subjected to intermediate heat treatment (L). The intermediate heat treatment is performed by adding Ac to the steel in order to ensure stable retained austenite even at low temperatures. １ Above, Ac ３The temperature is maintained within the following range. From the viewpoint of concentration of elements in precipitated austenite, the maintenance time is preferably 20 minutes or more. On the other hand, since a long heat treatment time leads to a decrease in productivity, the maintenance time is preferably 120 minutes or less. Cooling is preferably performed by water cooling (quenching). １ is a value calculated by the following formula: Ac １ = 750.8 - 26.6C + 17.6Si - 11.6Mn - 22.9Cu - 23Ni + 24.1Cr + 22.5Mo - 39.7V - 5.7Ti + 232.4Nb - 169.4Al - 894.7B The element symbols in the formula represent the content (mass%) of each element contained in the steel.

[0079] After the intermediate heat treatment, the steel may be subjected to a tempering treatment. The heating temperature in the tempering treatment is preferably 660°C or less, or 640°C or less, from the viewpoint of suppressing a decrease in strength. On the other hand, the heating temperature in the tempering treatment is preferably 400°C or more, 450°C or more, or 500°C or more, from the viewpoint of improving toughness.

[0080] The steel material according to the present disclosure will be specifically described below using examples, but the conditions in the following examples are merely examples of conditions adopted to confirm the feasibility and effects of the present disclosure, and the steel material according to the present disclosure is not limited to the following examples.

[0081] [Manufacturing of Steel Materials] First, a slab having the chemical composition shown in Table 1 was cast by a continuous casting method. The balance other than the components shown in Table 1 is Fe and impurities. Blank cells indicate that no alloying elements were intentionally added in the steelmaking process. Underlined cells indicate that the contents are outside the scope of the present disclosure.

[0082]

[0083] Next, steel materials were produced from these slabs under the production conditions shown in Table 2. "Temper heat treatment" refers to the heating temperature in the tempering treatment after quenching.

[0084]

[0085] [Measurement and Evaluation] The microstructure and mechanical properties of the obtained steel material were measured by the methods described above. The results are shown in Table 3. The meanings of the symbols in the microstructure are as follows. Ｌ In the regions judged to be upper bainite (+M), the presence of lower bainite and martensite was confirmed by SEM observation. The remainder of the microstructure is pearlite, MA phase, or ferrite. Bu: upper bainite BL: lower bainite M: martensite Retained γ: retained austenite Toughness was measured by measuring the average value of the Charpy impact absorption energy at -100°C (KV2) and the average value of the Charpy impact absorption energy at -100°C after PWHT, with a holding temperature of 600°C, a holding time of 2 hours, and a heating rate and cooling rate of 55°C / h in the temperature range of 425°C or higher.

[0086]

[0087] Nos. 1 to 19 and 101 to 109 are inventive examples, while Nos. 20, 21, 22, and 110 to 116 are comparative examples. No. 20 had an α value that was too small, resulting in insufficient hardenability, insufficient strength, and insufficient low-temperature toughness. No. 21 had an excessively high Mn content, resulting in insufficient low-temperature toughness both before and after PWHT. No. 22 had an excessively high α value, resulting in excessive strength, resulting in insufficient low-temperature toughness both before and after PWHT. Nos. 110 and 111 had an α value below the lower limit specified in the present disclosure, resulting in insufficient hardenability and insufficient strength. Insufficient low-temperature toughness was also not obtained. Nos. 112 and 113 had an α value that exceeded the upper limit specified in the present disclosure, resulting in excessively high hardenability and excessive strength. In Nos. 114 to 116, the total area ratio of lower bainite and martensite was insufficient, and sufficient low-temperature toughness was not obtained either before or after PWHT.

[0088] In contrast to the comparative examples, in all of the inventive examples (Nos. 1 to 19, 101 to 109), the chemical composition and microstructure of the steel were appropriately controlled, and the tensile strength was within an appropriate range of 590 MPa or more and 930 MPa or less. In addition, regardless of before or after PWHT, the Charpy impact absorption energy at -100°C was high, and low-temperature toughness of 150 J or more was obtained.

[0089] The steel material according to the present disclosure can be used primarily as a material for transport tanks for liquefied carbon dioxide, and can also be used to manufacture other welded structures such as buildings, bridges, ships, line pipes, marine structures, pressure vessels and tanks, etc.

[0090] The disclosure of Japanese Patent Application No. 2023-114105, filed on July 11, 2023, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards mentioned herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated herein by reference.

Claims

1. In mass percent, C: 0.03% or more, 0.20% or less, Si: 0.01% or more, 0.50% or less, Mn: 0.10% or more, 1.65% or less, P: 0.025% or less, S: 0.0250% or less, Ni: 2.65% or more, 4.45% or less, Al: 0.001% or more, 0.100% or less, O: 0.0100% or less, N: 0.0100% or less, Cu: 0 to 1.50%, Cr: 0-3.00%, Mo: 0-2.00%, B: 0 to 0.0050%, Nb: 0 to 0.050%, Ti: 0 to 0.050%, V: 0-0.10%, Mg: 0 to 0.0200%, Ca: 0-0.0200%, REM: 0-0.0200%, Remainder: Fe and impurities Furthermore, it has a chemical composition in which α, represented by the following formula (1), is 4.0 or more and 16.0 or less. The tensile strength is between 590 MPa and 930 MPa. A steel material in which the microstructure of the portion extending 1 / 4 of the thickness from the surface of the steel material in the thickness direction includes lower bainite, martensite, and retained austenite, the sum of the area ratios of the lower bainite and martensite is 15.0% or more, the sum of the area ratios of the upper bainite, lower bainite and martensite is 90.0% or more, and the area ratio of the retained austenite is 0.2% or more and less than 5.0%. α=0.50×√[C]×(1+0.64[Si])×(1+4.10[Mn])×(1+0.27[Cu])×(1+0.52[Ni])×(1+2.33[Cr])×(1+3.14[Mo]) ...(1) However, the [element symbol] in formula (1) represents the content (mass%) of the corresponding element contained in the steel material. If the element is not present, substitute zero.

2. The steel material according to claim 1, wherein the chemical composition includes the following group A. [Group A] Cu: 0.01% or more, 1.50% or less, Cr: 0.10% or more, 3.00% or less, Mo: 0.01% or more, 2.00% or less, B: 0.0003% or more, 0.0050% or less One or more selected from the group consisting of

3. The steel material according to claim 1, wherein the chemical composition includes the following group B. [Group B] Nb: 0.001% or more, 0.050% or less, Ti: 0.001% or more, 0.050% or less, V: 0.01% or more, 0.10% or less One or more selected from the group consisting of

4. The steel material according to claim 1, wherein the chemical composition includes the following group C. [Group C] Mg: 0.0003% or more, 0.0200% or less Ca: 0.0003% or more, 0.0200% or less, REM: 0.0003% or more, 0.0200% or less One or more selected from the group consisting of

5. The steel material according to any one of claims 1 to 4, wherein the aspect ratio of prior austenite crystal grains in the thickness direction from the surface of the steel material to a point where 1 / 4 of the thickness is present is 1.5 or more.

6. The steel material according to any one of claims 1 to 4, wherein the aspect ratio of the prior austenite crystal grains in the thickness direction from the surface of the steel material to a portion of the thickness of 1 / 4 is less than 1.

5.

7. The steel material according to any one of claims 1 to 4, wherein the microstructure of the steel material in the thickness direction from the surface of the steel material at a point where 1 / 4 of the thickness is present has an average crystal grain size of 20.0 μm or less.

8. A steel material according to any one of claims 1 to 4, wherein the Charpy impact absorption energy at -100°C is 150 J or more.

9. The steel material according to any one of claims 1 to 4, wherein, in a temperature range of 425°C or higher, the heating rate and cooling rate are 55°C / h, and when the steel material is subjected to a heat treatment in which it is held at 600°C for 2 hours, the Charpy impact absorption energy at -100°C at the heat-treated portion is 150 J or more.

10. A tank for transporting liquid carbon dioxide, comprising the steel material described in any one of claims 1 to 4.