steel

Abstract

Provided is a steel that has a specific chemical composition in which α is from 5.0 to 16.0, the tensile strength is from 615 MPa to 930 MPa, the microstructure of a location 1 / 4 of the thickness from the surface includes lower bainite and martensite, the total of the area ratios of the lower bainite and martensite is at least 15.0%, the total of the area ratios of upper bainite, lower bainite, and martensite is at least 90.0%, and the area ratio of retained austenite is less than 1.7%. 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 material

[0001] The present disclosure relates to steel products.

[0002] Steel can be used 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 6).

[0006] Patent Document 1: JP 2019-81930 A Patent Document 2: JP 6-192729 A Patent Document 3: JP 7-331328 A Patent Document 4: WO 2007 / 034576 A Patent Document 5: WO 2020 / 184162 A Patent Document 6: WO 2014 / 017057 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 that is suitable for low-temperature applications, having high tensile strength and good low-temperature toughness regardless of whether it is before or after post-weld heat treatment.

[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 2.00% or less, P: 0.025% or less, S: 0.0250% or less, Ni: 4.51% or more and 6.10% 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 5.0 or more and 16.0 or less, the steel material having a tensile strength of 615 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 and martensite, 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 retained austenite is less than 1.7%. 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 content (mass%) of the corresponding element contained in the steel material. If the corresponding element is not contained, zero is substituted. <2> The steel material according to <1>, wherein the microstructure at a position ¼ of the thickness from the surface of the steel material in the thickness direction has an average crystal grain size of 20.0 μm or less. <3> The steel material according to <1> or <2>, wherein the Charpy impact absorption energy at −110°C is 150 J or more.<4> The steel material according to any one of <1> to <3>, wherein, when the steel material is subjected to a heat treatment in which the heating rate and cooling rate in a temperature range of 425°C or higher are 55°C / h and the steel material is held at 600°C for 2 hours, the Charpy impact absorption energy at -110°C at the heat-treated location is 150 J or more. <5> 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 to 1 / 4 of the thickness 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 to 1 / 4 of the thickness in the thickness direction is less than 1.5.

[0010] According to the present disclosure, it is possible to provide a steel material that is suitable for low-temperature applications, having high tensile strength and good low-temperature toughness regardless of whether it is before or after post-weld heat treatment.

[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 paint film. However, a surface treatment layer such as a plating layer or a paint film may be formed on the surface of the steel material according to the present disclosure. Furthermore, in a welded joint, the "base material" refers to a steel material portion that is not affected by welding, in contrast to the welded portion (weld metal and weld heat-affected zone).

[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 have 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 section (t: thickness of the steel material) of steel materials after hot rolling and accelerated cooling, performed tensile tests, and observed the microstructure. As a result, it was found that the microstructure of the 1 / 4t section of steel materials having a tensile strength of 615 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 the 1 / 4t section of steel materials after hot rolling and accelerated cooling, 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 -110°C have a total area fraction of lower bainite and martensite of 15.0% or more and an area fraction of retained austenite of less than 1.7%. 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 steel materials manufactured by controlling the cooling rate and cooling stop temperature after hot rolling, and measured the circle-equivalent diameter of the area surrounded by high-angle grain boundaries by 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 a diameter 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 have found that similar results can be obtained not only with steel materials after hot rolling and accelerated cooling, but also with steel materials after reheating and quenching.

[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, 0.10% or more, or 0.12% 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 HAZ toughness, 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, 2.00% 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, resulting in a decrease in toughness after PWHT. Therefore, from the viewpoint of ensuring the toughness of the steel material after PWHT, the Mn content is 2.00% or less in the present disclosure. The Mn content is preferably 1.80% or less, or 1.50% or less.

[0023] (P: 0.025% or less) P is an impurity element. Although there is no lower limit for the P content, 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: 4.51% or more, 6.10% or less) Ni is an element effective for improving the hardenability and toughness of steel, so in this embodiment, the Ni content is 4.51% or more. The Ni content is preferably 5.00% or more, or 5.25% or more. However, since Ni is an expensive element, in the present disclosure, from the viewpoint of cost reduction, the Ni content is 6.10% or less. The Ni content is preferably 6.00% or less, or 5.75% 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 (optional elements) in place of a portion of Fe. For example, the following optional elements may be included, but the content of these elements may be 0%.

[0030] 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 selected 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] 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, and the like.

[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, or 0.030% 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] 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 optional 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 the 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: 5.0 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 the sum of [C] and [Mo]. １／２ is synonymous with.

[0046] In the present disclosure, the α value is set to a range of 5.0 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 5.0 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. Furthermore, 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 set to 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 set to 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 5.5 or more, 6.0 or more, or 7.0 or more. Furthermore, 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, contains lower bainite and martensite. Furthermore, the bainite may contain upper bainite in addition to lower bainite.

[0049] "Bainite" is a general term for upper bainite and lower bainite. "Upper bainite" refers to either or both of 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 of the area ratio of lower bainite and the area ratio of martensite in the 1 / 4t portion may be 100%.

[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 / 4t portion is 90.0% or more. The total area ratio of upper bainite, lower bainite, and martensite in the 1 / 4t portion may be 100%. Furthermore, the upper bainite in the 1 / 4t portion may be 1.0% or more.

[0053] (Area Fraction of Retained Austenite: Less than 1.7%) From the viewpoint of ensuring the toughness of the steel material, the area fraction of the retained austenite in the 1 / 4t portion is less than 1.7%. The area fraction of the retained austenite in the 1 / 4t portion is preferably 1.0% or less, and may be 0%. This is thought to be because the Ni-containing thick steel plate of the present disclosure has a lower Ni content than conventional 9% Ni steel, and even if retained austenite exists at −110°C, it is unstable. When the steel structure undergoes plastic deformation at the crack tip, the retained austenite transforms into martensite through plasticity-induced martensitic transformation. For this reason, the volume fraction of retained austenite at room temperature is set to less than 1.7%. Furthermore, while a higher Ni content tends to increase the volume fraction of retained austenite, an increase in the Ni content is undesirable because it leads to increased costs.

[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 fraction of upper bainite, lower bainite, martensite, and retained austenite is measured by EBSD using electropolished samples that have been 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 made 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, and martensite 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 +M) have dense and uniformly distributed carbides. The total area fraction of lower bainite, martensite, and retained austenite can be 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 can be 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.

[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, chemically polished, and used to measure the volume fraction of retained austenite by X-ray diffraction using a Mo tube. Quantification was 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 was used. The integrated intensity of the diffraction peak was 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] (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. Effective grain size D determined by weighted averaging ａｒｅａ is 4 mm ２ Among the grain sizes measured in the region, the area S of the i-th grain detected during measurement ｉ , particle size d ｉ It is calculated using the following formula: ａｒｅａ =ΣSi·d ｉ / ΣS ｉ

[0059] (Aspect ratio of prior austenite grains at 1 / 4 thickness portion of steel material) The prior austenite grains (sometimes referred to as prior austenite grains) of the steel material of the present disclosure may have a shape flattened in the rolling direction. If the prior austenite grains at a portion 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 substantially refines the austenite grains and is effective in refining the effective crystal grain size. The aspect ratio of the prior austenite grains is usually 4.0 or less, and may be 3.5 or less.

[0060] On the other hand, from the viewpoint of ensuring homogeneity of the microstructure, the aspect ratio of the prior austenite grains in the 1 / 4t portion may be less than 1.5, or may be 1.4 or less, or 1.3 or less.

[0061] 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.

[0062] <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 -110°C, and can also exhibit excellent low-temperature toughness even after PWHT.

[0063] (Tensile strength: 615 MPa or more and 930 MPa or less) In the present disclosure, the tensile strength of the steel material is set to 615 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 in the present disclosure is also manufactured to have the above-mentioned tensile strength.

[0064] (Yield Ratio) The yield ratio (YR = [yield strength] / [tensile strength] × 100) of the steel material according to the present disclosure is not particularly limited, but is preferably 90% or less. If there is no yield point, the yield strength is determined using 0.2% proof stress.

[0065] (Charpy impact absorption energy at -110°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 -110°C. Because the steel material of the present disclosure has low-temperature toughness such that the Charpy impact absorption energy at -110°C is 150 J or more, a transport tank made of the steel material of the present disclosure can be suitably used, for example, for transporting liquid carbon dioxide. The steel material of the present disclosure may also have low-temperature toughness such that the Charpy impact absorption energy at -110°C is 100 J or more. The Charpy impact absorption energy at -110°C is a value measured using a sample taken from a 1 / 4 position of the thickness.

[0066] (Charpy Absorbed Energy at −110°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 toughness of the base material tends to decrease as the time for which the base material is heated to a temperature range of 425°C or higher increases. When the steel material of the present disclosure undergoes 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 such that the Charpy impact absorption energy at −110°C is 150 J or more. The Charpy impact absorption energy at −110°C after PWHT may be 100 J or more. The Charpy impact absorption energy at -110°C after PWHT is also a value measured using a sample taken from the 1 / 4 position of the thickness. Note that PWHT may reduce the toughness of steel. While 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 therefore 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.

[0067] (Charpy Impact Absorption Energy at -110°C After Thermal Cycle) In order to ensure high toughness after a thermal cycle test simulating a weld at low temperatures, the steel material of the present disclosure preferably has a Charpy impact absorption energy of 50 J or more at -110°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 -110°C after the thermal cycle, and therefore a transport tank made of the steel material of the present disclosure can be suitably used, for example, for transporting liquid carbon dioxide. The Charpy impact absorption energy at -110°C after the thermal cycle may be 40 J or more. The Charpy impact absorption energy at -110°C after the thermal cycle is a value measured by using a sample taken from a position 1 / 4 of the thickness of the steel material as a thermal cycle test piece, giving the sample 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, and then taking a Charpy test piece from the sample.

[0068] (Thermal Cycle, Charpy Absorbed Energy at -110°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, in which the heating rate and cooling rate are 55°C / h in a temperature range of 425°C or higher, and the steel is held at 600°C for 2 hours, after which Charpy test specimens are taken and measured. In this case, the toughness of the portion subjected to the PWHT is preferably such that the Charpy impact absorption energy at -110°C is 50 J or more. The Charpy impact absorption energy at -110°C of the portion subjected to the PWHT after the thermal cycle test may be 40 J or more.

[0069] 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.

[0070] Tensile strength (TS) and yield strength (YS) are measured by a tensile test in accordance with JIS Z2241:2011. JIS 14A test specimens are used for the tensile test, taken from the quarter-thickness position and with the longitudinal direction parallel to the width direction of the steel material (C direction). TS and YS are measured using three test specimens and calculated by averaging the measured values. Based on the average values ​​of TS and YS, the yield ratio YR (%) is calculated by (YS / TS) x 100. Charpy impact absorption energy is measured by a Charpy impact test at -110°C using an impact blade with a radius of 2 mm in accordance with JIS Z2242:2018. Charpy impact absorption energy is measured using three test specimens and calculated by averaging the measured values. For the Charpy impact test, a V-notch test piece is used, which is taken from a position at 1 / 4 of the thickness of the steel material and has a longitudinal direction parallel to the width direction of the steel material (C direction).

[0071] 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 structural steel, a steel pipe, or the like. However, steel pipes and structural steel include steel materials formed by joining steel plates, such as welded steel pipes and welded structural steel, as well as structural steel joined with rivets. The thickness of the steel material (thickness of the flange in the case of structural steel) such as a steel plate, a steel strip, a structural steel, or a steel pipe 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.

[0072] 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 tank for storing and transporting liquefied gas, in particular liquid carbon dioxide.

[0073] (Method for manufacturing steel material) The method for manufacturing steel material according to the present disclosure is not particularly limited, but the steel material according to the present disclosure is, for example, produced by melting steel satisfying the above-mentioned chemical composition and then continuously casting it into a steel billet. The steel billet is heated, hot-rolled, and then directly water-cooled (direct quenching (DQ)), or hot-rolled, then naturally cooled, reheated, and water-cooled (reheat quenching (RQ)), to produce a steel material. In the case of RQ, naturally cooling before reheating is not necessarily required, and water cooling may also be used. Furthermore, tempering (T) may be performed.

[0074] (1) DQT: Direct quenching (DQ), tempering (T) (2) RQT: Natural cooling or water quenching, reheat quenching (RQ), tempering (T)

[0075] (1) DQT From the viewpoint of production costs, DQT is preferred in the production of steel materials according to the present disclosure, and an example of a preferred production process is shown below.

[0076] When the steel material according to the present disclosure is produced by DQ, the heating temperature of the steel billet to be hot rolled is set to Ac from the viewpoint that hot rolling is performed in a temperature range in which the metal structure of the rolled material is austenite. ３ The heating temperature of the slab is preferably 1000°C or higher from the viewpoint of reducing the deformation resistance. On the other hand, the heating temperature of the hot rolling is 1250°C or lower from the viewpoint of suppressing the coarsening of heated γ grains. The heating temperature of the hot rolling 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.

[0077] 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 20% or higher, more preferably 30% or higher, 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]

[0078] Non-recrystallization temperature range rolling is hot rolling performed at a temperature of the rolled material during rolling of less than 900°C. The cumulative reduction in non-recrystallization temperature range rolling is preferably 20% or more, more preferably 30% 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]

[0079] 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.

[0080] 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.

[0081] After the accelerated cooling, the steel may be subjected to a tempering treatment. The heating temperature in the tempering treatment is preferably 650°C or less, 620°C or less, or 590°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 350°C or more, or 400°C or more from the viewpoint of improving toughness.

[0082] (2) RQT When the steel material according to the present disclosure is produced by RQ, the heating temperature and reduction of the billet during hot rolling have little effect on the mechanical properties of the steel material. However, if the heating temperature of the billet is too low, deformation resistance increases, so the heating temperature of the billet is preferably 1000°C or higher. Furthermore, if the reduction is insufficient, initial defects from the time of billet production may remain in the center of the thickness, degrading the quality of the steel material. Therefore, the total reduction of the hot rolling (also referred to as cumulative reduction) is preferably 35% or higher. After hot rolling, the steel may be directly water-cooled or air-cooled.

[0083] After hot rolling, the steel is reheated and quenched. The reheating temperature of the steel is set to Ac, because the steel is quenched from a single austenite phase structure.３ From the viewpoint of ensuring homogeneity of the microstructure, the reheating temperature of the steel material is preferably 750°C or higher, 850°C or higher, 880°C or higher, or 900°C or higher. On the other hand, although the upper limit of the reheating temperature is not particularly specified, heating to an excessively high temperature may cause coarsening of austenite grains, resulting in a decrease in toughness, and therefore the upper limit is preferably 1000°C or lower, 950°C or lower, or 930°C or lower.

[0084] After reheating and quenching, 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.

[0085] Hereinafter, the steel material according to the present disclosure will be specifically described using examples. However, the conditions in the following examples are merely examples of conditions adopted to confirm the feasibility and effects of the present disclosure, and the present disclosure is not limited to the following examples.

[0086] <Production by direct quenching and tempering> [Production of steel material] First, a slab having the chemical composition shown in Table 1 was cast by continuous casting. 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 content is outside the scope of the present disclosure.

[0087]

[0088] 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.

[0089]

[0090] [Measurement and Evaluation] The microstructure and mechanical properties of the obtained steel materials were measured by the methods described above. The results are shown in Table 3. The meanings of the symbols for the microstructure are as follows. The remainder of the microstructure is pearlite, MA phase, and ferrite. Bu: upper bainite BL: lower bainite M: martensite Retained γ: retained austenite For toughness, the average value of the Charpy impact absorbed energy at -110°C and the average value of the Charpy impact absorbed energy at -110°C after PWHT, which was performed 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, were measured.

[0091]

[0092] Nos. 2A to 26A and 101A to 113A are inventive examples, while Nos. 28A to 32A and 114A to 118A are comparative examples. No. 28A had an α value exceeding the upper limit of the present disclosure, resulting in excessively high hardenability and excessive strength. No. 30A had an α value exceeding the upper limit of the present disclosure, resulting in excessively high hardenability and excessive strength. Because there was also too much retained γ, sufficient low-temperature toughness was not obtained. No. 31A had low low-temperature toughness after PWHT because the Mn content was outside the upper limit. No. 32A had a low cooling rate during direct quenching, resulting in an insufficient total area fraction of lower bainite and martensite, and therefore sufficient low-temperature toughness was not obtained. Nos. 114A and 115A had α values ​​below the lower limit of the present disclosure, resulting in insufficient hardenability and strength. Sufficient low-temperature toughness was also not obtained. Nos. 116A and 117A had α values ​​exceeding the upper limit of the present disclosure, resulting in excessively high hardenability and excessive strength. No. 118A had an initiation temperature for direct quenching that was too low, resulting in an insufficient total area ratio of upper bainite, lower bainite, and martensite, resulting in insufficient strength.

[0093] In contrast to the comparative examples, in the examples of the present invention, the chemical composition and microstructure of the steel material are appropriately controlled, and the tensile strength is in an appropriate range of 615 MPa or more and 930 MPa or less. In addition, low-temperature toughness at -110°C of 100 J or more is obtained regardless of before or after PWHT.

[0094] <Production by reheating, quenching, and tempering> [Production of steel material] First, a slab having the chemical composition shown in Table 4 was cast by continuous casting. The balance other than the components shown in Table 4 is Fe and impurities. Blank cells indicate that no alloying elements were intentionally added in the steelmaking process. Underlined cells indicate that the content is outside the scope of the present disclosure.

[0095]

[0096] Next, steel materials were produced from these slabs under the production conditions shown in Table 5. "Temper heat treatment" refers to the heating temperature in the tempering treatment after quenching.

[0097]

[0098] [Measurement and Evaluation] The microstructure and mechanical properties of the obtained steel materials were measured by the methods described above. The results are shown in Table 6. The meanings of the symbols for the microstructure are as follows. The remainder of the microstructure is the MA phase and ferrite. Bu: upper bainite BL: lower bainite M: martensite Retained γ: retained austenite For toughness, the average value of the Charpy impact absorbed energy at -110°C and the average value of the Charpy impact absorbed energy at -110°C after PWHT, which was performed 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, were measured.

[0099]

[0100] Nos. 2B to 24B and Nos. 101B to 113B are invention examples, while Nos. 25B to 29B and 114B to 119B are comparative examples. No. 25B had an α value below the lower limit specified in the present disclosure, resulting in insufficient hardenability and insufficient strength. Sufficient low-temperature toughness was also not obtained. No. 26B had an α value above the upper limit specified in the present disclosure, resulting in excessively high hardenability and excessive strength. No. 29B had a Mn content outside the upper limit, resulting in low low-temperature toughness after PWHT. Nos. 114B and 115B had an α value below the lower limit specified in the present disclosure, resulting in insufficient hardenability and insufficient strength. Sufficient low-temperature toughness was also not obtained. Nos. 116B and 117B had an α value above the upper limit specified in the present disclosure, resulting in excessively high hardenability and excessive strength. In No. 118B, the reheating and quenching temperature was too low, resulting in an insufficient total area ratio of upper bainite, lower bainite, and martensite, and thus insufficient strength. In No. 119B, the α value exceeded the upper limit specified in the present disclosure, resulting in excessive hardenability and excessive strength. Furthermore, the amount of retained γ was too high, so sufficient low-temperature toughness could not be obtained.

[0101] In contrast to the comparative examples, in the examples of the present invention, the chemical composition and microstructure of the steel are appropriately controlled, and the tensile strength is in an appropriate range of 615 MPa or more and 930 MPa or less, and a low-temperature toughness at -110°C of 100 J or more is obtained, regardless of whether it is before or after PWHT. In a particularly preferred configuration of the present disclosure, a low-temperature toughness at -110°C of 150 J or more is obtained, regardless of whether it is before or after PWHT.

[0102] The steel material according to the present disclosure can be used primarily for transport tanks for liquefied carbon dioxide, but can also be used for other welded structures such as buildings, bridges, ships, line pipes, marine structures, pressure vessels and tanks.

[0103] The disclosures of Japanese Patent Application Nos. 2023-042399 and 2023-042402, filed on March 16, 2023, are incorporated herein by reference in their 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%, C: 0.03% or more, 0.20% or less, Si: 0.01% or more, 0.50% or less, Mn: 0.10% or more, 2.00% or less, P: 0.025% or less, S: 0.0250% or less, Ni: 4.51% or more, 6.10% 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 and has a chemical composition in which α represented by the following formula (1) is 5.0 or more and 16.0 or less, The tensile strength is 615 MPa or more and 930 MPa or less, A steel material in which the microstructure at a region extending from the surface of the steel material to 1 / 4 of the thickness in the thickness direction contains lower bainite and martensite, the sum of the area ratios of the lower bainite and the martensite is 15.0% or more, the sum of the area ratios of upper bainite, the lower bainite and the martensite is 90.0% or more, and the area ratio of retained austenite is less than 1.7%. 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) In the formula (1), the [element symbol] represents the content (mass%) of the corresponding element contained in the steel material. If the corresponding element is not contained, zero is substituted.

2. 2. The steel material according to claim 1, wherein the microstructure at a position from the surface of the steel material to 1 / 4 of the thickness in the thickness direction has an average crystal grain size of 20.0 μm or less.

3. The steel material according to claim 1 or 2, wherein the Charpy impact absorption energy at −110 ° C. is 150 J or more.

4. 3. The steel material according to claim 1, wherein the heating rate and cooling rate in a temperature range of 425°C or higher are 55°C / h, and when the steel material is subjected to a heat treatment in which the steel material is held at 600°C for 2 hours, the Charpy impact absorption energy at -110°C at the heat-treated portion is 150J or more.

5. 3. The steel material according to claim 1, wherein the aspect ratio of prior austenite grains at a position from the surface of the steel material to 1 / 4 of the thickness in the thickness direction is 1.5 or more.

6. 3. The steel material according to claim 1, wherein the aspect ratio of prior austenite grains at a position from the surface of the steel material to 1 / 4 of the thickness in the thickness direction is less than 1.5.