Steel material

Abstract

A steel material which has a specific chemical composition in which alpha is 5.0-16.0 inclusive, has a tensile strength of 615-930 MPa inclusive, and has a microstructure in a region that is 1 / 4 of the thickness from the surface containing lower bainite and martensite, the total area ratio of the lower bainite and martensite being 15.0% or more, and the total area ratio of the lower bainite and martensite being 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 less than 1.7%. And alpha = 0.50 * [square root of] [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

TECHNICAL FIELD

[0001] The present disclosure relates to a steel material. BACKGROUND

[0002] A steel material can be used for a welded structure such as a building, a bridge, a ship, a pipeline pipe, an offshore structure, a pressure vessel, a tank, and the like. A steel material excellent in a corresponding stress of strength and low-temperature toughness is effective in use at a low temperature.

[0003] A low-temperature pressure vessel such as a storage tank for liquefied gas uses a low-temperature steel. For the low-temperature steel, depending on a use temperature, there are an Al-killed steel, a nickel steel, a high-Mn steel, and an austenitic stainless steel, and the like. For example, as a material for a tank that carries liquefied ethane or liquefied ethylene having a use temperature of about -100°C, a nickel steel such as 3.5%Ni steel is used.

[0004] As this 3.5%Ni steel, in a steel material that needs to secure low-temperature toughness, represented by a low-temperature pressure vessel, Ni is contained in many cases.

[0005] For example, Patent Literature 1 proposes a low-temperature nickel-containing steel material excellent in toughness, which has a specific chemical composition containing 2.7% or more and 5.0% or less of Ni, a prior austenite grain diameter at quenching heating of 20 μm or less, an effective crystal grain diameter after heat treatment of 12 μm or less, and a tensile strength of 450 MPa or more and 690 MPa or less.

[0006] In addition, for the purpose of low-temperature toughness and high strength, various steel materials in which a chemical composition and a microstructure (metal structure) are specified are proposed (for example, refer to Patent Literatures 2 to 6).

[0007] Patent Literature 1: Japanese Patent Application Publication No. 2019-81930 Patent Literature 2: Japanese Patent Application Publication No. H6-192729 Patent Literature 3: Japanese Patent Application Publication No. H7-331328 Patent Literature 4: International Publication No. 2007 / 034576 Patent Literature 5: International Publication No. 2020 / 184162 Patent Literature 6: International Publication No. 2014 / 017057 SUMMARY

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION For a cryogenic steel used in a cryogenic pressure vessel, it is desirable to balance high strength and securing of low-temperature toughness. In addition, a cryogenic pressure vessel is manufactured by welding a steel material, and in order to remove residual stress generated due to welding, post-weld heat treatment (sometimes referred to as PWHT) is sometimes performed. Recently, the requirement for low-temperature toughness of a steel material after PWHT is further increased.

[0009] An object of the present disclosure is to provide a steel material suitable for cryogenic use, which has high tensile strength and can obtain good low-temperature toughness regardless of before and after post-weld heat treatment.

[0010] Means for solving the problem The gist of the present disclosure is as described below.

[0011] <1> A steel material having the following chemical composition: 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%, Ca: 0 to 0.0200%, REM: 0 to 0.0200%, the remainder: Fe and impurities, and α represented by the following formula (1) is 5.0 or more and 16.0 or less, the tensile strength of the above steel material is 615 MPa or more and 930 MPa or less, The microstructure at a position 1 / 4 of the thickness in the thickness direction from the surface of the steel material contains lower bainite and martensite, the total of the area ratios of the lower bainite and the martensite is 15.0% or more, and the total of the area ratios of the upper bainite, the lower bainite, and the martensite is 90.0% or more, and the area ratio of residual austenite is less than 1.7%.

[0012] α = 0.50 x V[C] x (1 + 0.64[Si]) x (1 + 4.10[Mn]) x (1 + 0.27[Cu]) x (1 + 0.52[Ni]) x (1 + 2.33[Cr]) x (1 + 3.14[Mo]) (1) wherein [element symbol] in formula (1) represents the content (mass %) of each corresponding element contained in the steel material. In the case where the element is not contained, zero is substituted.

[0013] <2> The steel material according to <1>, wherein the average crystal grain diameter of the microstructure at a position 1 / 4 of the thickness in the thickness direction from the surface of the steel material is 20.0 μm or less.

[0014] <3> The steel material according to <1> or <2>, wherein the Charpy impact absorbed energy at -110°C is 150 J or more.

[0015] <4> The steel material according to any one of <1> to <3>, wherein in the case where the steel material is subjected to a heat treatment in which the temperature increasing rate and the temperature decreasing rate in the temperature region of 425°C or higher are 55°C / h, and the holding at 600°C is 2 hours, the Charpy impact absorbed energy at -110°C of the portion subjected to the heat treatment is 150 J or more.

[0016] <5> The steel material according to any one of <1> to <4>, wherein the aspect ratio of the prior austenite grain at a position 1 / 4 of the thickness in the thickness direction from the surface of the steel material is 1.5 or more.

[0017] <6> The steel material according to any one of <1> to <4>, wherein the aspect ratio of the prior austenite grain at a position 1 / 4 of the thickness in the thickness direction from the surface of the steel material is less than 1.5.

[0018] Effects of the Invention According to the present disclosure, a steel material suitable for low-temperature use, which has high tensile strength and can obtain good low-temperature toughness regardless of before and after a heat treatment after welding, can be provided. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a drawing showing an example of the discrimination result of the microstructure. DETAILED DESCRIPTION

[0020] Hereinafter, the present disclosure will be described in detail.

[0021] In the present disclosure, "post-weld heat treatment" means post-weld heat treatment according to the content defined in JIS Z 3700:2009 "Post-weld Heat Treatment Method" unless otherwise specified.

[0022] In the present disclosure, "steel material" or "base material" means a steel material portion not including a surface treatment layer such as a plated layer or a coating film. However, a surface treatment layer such as a plated layer or a coating film can be formed on the surface of the steel material of the present disclosure. In addition, "base material" in a welded joint means a steel material portion not affected by welding in comparison with a welded portion (weld metal and a weld heat-affected portion).

[0023] In the present disclosure, a numerical range indicated using "~" means a range including the numerical values written before and after "~" as lower limit values and upper limit values. In the case where a numerical range is indicated by writing "more than" or "less than" to the numerical values written before and after "~", the numerical range means a range not including these numerical values as lower limit values or upper limit values.

[0024] In the content of elements of the chemical composition, "%" means "mass%".

[0025] The term "step" includes not only an independent step but also a case where the desired purpose of the step can be achieved even if it cannot be clearly distinguished from other steps.

[0026] Hereinafter, a steel material of one embodiment of the present disclosure will be described. First, the research results of the inventors of the present disclosure for completing the steel material of the present disclosure, and new insights obtained thereby will be described in detail.

[0027] The inventors of the present disclosure have conducted research in order to improve the strength of a steel material. The tensile strength of a steel material is ensured by the composition of the microstructure. The inventors of the present disclosure collected a sample from the 1 / 4t portion (t: thickness of the steel material) of a steel material after hot rolling and accelerated cooling, conducted a tensile test, and conducted observation of the microstructure. As a result, it was found that in the microstructure of the 1 / 4t portion of a steel material having a tensile strength of 615 MPa or more and 930 MPa or less, the area fraction of ferrite was less than 10.0%, and the total of the area fractions of upper bainite, lower bainite, and martensite was 90.0% or more. In addition, the total of the area fractions of upper bainite, lower bainite, and martensite was measured using electron backscatter diffraction (hereinafter referred to as "EBSD").

[0028] Further, the inventors of the present disclosure have conducted research in order to improve the toughness of the steel material. The toughness of the steel material is ensured by the constitution of the microstructure. The inventors of the present disclosure collected a sample from the 1 / 4t portion of the steel material after hot rolling and accelerated cooling, conducted a Charpy impact test, and conducted observation of the microstructure. As a result, it was found that the steel material having a Charpy impact absorbed energy of 150 J or more at -110°C had a total area ratio of lower bainite and martensite of 15.0% or more and a residual austenite area ratio of less than 1.7%. The total area ratio of the lower bainite and the martensite was measured using EBSD. The residual austenite area ratio was measured by X-ray diffraction. The volume ratio of the residual austenite measured by X-ray diffraction can be regarded as the area ratio.

[0029] Further, the inventors of the present disclosure have conducted research in order to ensure the toughness of the steel material. The toughness of the steel material is ensured by reducing the area surrounded by large-angle grain boundaries having a difference in crystal orientation of 15° or more. The inventors of the present disclosure collected a sample from the 1 / 4t portion of the steel material manufactured by controlling the cooling rate and the cooling stop temperature after hot rolling, and measured the equivalent circle diameter of the area surrounded by the large-angle grain boundaries by EBSD. Hereinafter, the equivalent circle diameter of the area surrounded by the large-angle grain boundaries will be referred to as the crystal grain diameter. The sample was subjected to mechanical polishing and electrolytic polishing, and the analysis based on the EBSD device attached to the FE-SEM (field emission type scanning electron microscope) was performed in a 4 mm 2 region. The crystal grain diameter measured in a 4 mm 2 region was the value calculated as the area-weighted average with the area of each grain being weighted. It was found that if the average crystal grain diameter of the 1 / 4t portion of the steel material was 20.0 μm or less, the steel material had a tendency to further improve the toughness regardless of before and after the heat treatment after welding.

[0030] Further, the inventors of the present disclosure have found that the same results can be obtained for the steel material after reheating and quenching, not limited to the steel material after hot rolling and accelerated cooling.

[0031] <Chemical composition> Next, the alloy elements constituting the steel material of the present disclosure will be described. In addition, in the following description of the alloy elements, "%" means "mass %".

[0032] (C: 0.03% or more and 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 a structure, 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 decreases toughness, and from the viewpoint of ensuring the toughness of a 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.

[0033] (Si: 0.01% or more and 0.50% or less) Si is an element that is used as a deoxidizer and, in addition, is solid-solved in steel to increase the 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 is sometimes formed in the HAZ, and the toughness decreases. 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.

[0034] (Mn: 0.10% or more and 2.00% or less) Mn is an element that is used as a deoxidizer and, in addition, contributes to high-strengthening by improving the hardenability of steel. From the viewpoint of controlling the O concentration contained in molten steel, in the present disclosure, the Mn content is 0.10% or more. Furthermore, by 0.10% or more of Mn, MnS is formed, thereby reducing solid-solution S and preventing hot cracking. From the viewpoint of ensuring the strength of steel 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 segregates at grain boundaries at the time of PWHT, and thus the toughness after PWHT is sometimes decreased. Therefore, from the viewpoint of ensuring the toughness of steel after PWHT, in the present disclosure, the Mn content is 2.00% or less. The Mn content is preferably 1.80% or less or 1.50% or less.

[0035] (P: 0.025% or less) P is an impurity element. The lower limit of the P content is not limited, and from the viewpoint of manufacturing cost, in the present disclosure, the P content can be 0.001% or more. On the other hand, if the P content is excessive, P segregates at grain boundaries at the time of PWHT, and thus the toughness after PWHT is sometimes decreased. 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.

[0036] (S: 0.0250% or less) S is an impurity element. The lower limit of the S content is not limited, and from the viewpoint of manufacturing cost, the S content can be 0.0001% or more in the present disclosure. On the other hand, if the S content is excessive, sometimes elongated MnS is generated in the center segregation portion, and the toughness and ductility of the steel material and the HAZ are deteriorated. From the viewpoint of securing the toughness and ductility of the steel material and the HAZ, the S content is 0.0250% or less. The S content is preferably 0.0100% or less or 0.0050% or less.

[0037] (Ni: 4.51% or more and 6.10% or less) Ni is an effective element for improving the hardenability and toughness of steel, and thus, in the present embodiment, the Ni content is 4.51% or more. The Ni content is preferably set to 5.00% or more or 5.25% or more. However, Ni is an expensive element, and from the viewpoint of cost reduction, the Ni content is 6.10% or less in the present disclosure. The Ni content is preferably 6.00% or less or 5.75% or less.

[0038] (Al: 0.001% or more and 0.100% or less) Al is an element useful for deoxidization, and is an element that makes the crystal grain diameter fine at the time of quenching by forming nitride, and thus, in the present disclosure, the Al content is 0.001% or more. However, if Al is excessively contained, it is possible that Al forms coarse nitride, and the toughness of the steel material and the HAZ is reduced. Therefore, the Al content is 0.100% or less. The Al content is preferably 0.080% or 0.050% or less.

[0039] (O: 0.0100% or less) O is an impurity element. The lower limit of the O content is not limited, and from the viewpoint of manufacturing cost, the O content can be 0.0001% or more in the present disclosure. On the other hand, if the O content is excessive, it is possible that coarse oxides are generated, and the toughness and ductility of the steel material and the HAZ are deteriorated. From the viewpoint of securing the toughness and ductility of the steel material and the HAZ, the O content is 0.0100% or less. The O content is preferably 0.0060% or less or 0.0040% or less.

[0040] (N: 0.0100% or less) N is an impurity element. The lower limit of the N content is not limited, and from the viewpoint of manufacturing cost, the N content can be 0.0001% or more in the present disclosure. From the viewpoint of securing the properties of the steel material and the toughness of the HAZ, the N content is 0.0100% or less in the present disclosure. The N content is preferably 0.0050% or less or 0.0040% or less.

[0041] The steel material of the present disclosure can also contain other elements (selective elements) in place of a part of Fe. For example, the following selective elements can be listed, but the content of these elements can also be 0%.

[0042] In the steel material of the present disclosure, in order to improve strength and toughness, one or two or more of the following selective elements Cu, Cr, Mo, B having an effect of improving hardenability can be contained as needed.

[0043] (Cu: 1.50% or less) Cu is an element that is sometimes mixed into the steel material during manufacturing. However, the lower limit value of the Cu content is not limited, and can be 0%. In addition, Cu has a small adverse effect on the toughness of the HAZ and the weldability, has an effect of improving the hardenability of the steel, and thus is also an element that increases the strength of the steel material. Therefore, in the present disclosure, the Cu content can 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 material, in the present disclosure, the Cu content is 1.50% or less. The Cu content is preferably 1.00% or less, 0.80% or less, 0.60% or less, or 0.50% or less.

[0044] (Cr: 3.00% or less) Cr is an element that is sometimes mixed into the steel material during manufacturing. However, the lower limit value of the Cr content is not limited, and can be 0%. In addition, Cr has an effect of improving the hardenability of the steel, and thus is also an element that increases the strength of the steel material. Therefore, in the present disclosure, the Cr content can be 0.01% or more. The Cr content is preferably 0.10% or more. However, from the viewpoint of suppressing the deterioration of the toughness of the HAZ and the weldability, in the present disclosure, the Cr content is 3.00% or less. The Cr content is preferably 2.20% or less, 1.40% or less, or 0.80% or less.

[0045] (Mo: 2.00% or less) Mo is an element that is sometimes mixed into the steel material during manufacturing. However, the lower limit value of the Mo content is not limited, and can be 0%. In addition, Mo has an effect of improving the hardenability of the steel, and thus is also an element that increases the strength of the steel material. Therefore, in the present disclosure, the Mo content can 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 viewpoint of suppressing the deterioration of the toughness of the HAZ and the weldability, and suppressing the increase in the alloy cost, in the present disclosure, the Mo content is 2.00% or less. The Mo content is preferably 1.20% or less, or 0.80% or less.

[0046] (B: 0.0050% or less) B is an element that is sometimes mixed into the steel material during the manufacturing process. However, the lower limit value of the B content is not limited, and can be 0%. In addition, B is also an element that exhibits a significant effect of improving the hardenability of the steel and increasing the strength of the steel material. Therefore, in the present disclosure, the B content can be 0.0003% or more. However, from the viewpoint of suppressing deterioration of the surface quality of the steel billet 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.

[0047] In the steel material of the present disclosure, in order to increase the strength, one or two or more of the following selected elements Nb, Ti, and V, which have an effect of increasing the strength of the steel material by precipitates such as carbides and nitrides, can be contained as needed.

[0048] (Nb: 0.050% or less) Nb is an element that is sometimes mixed into the steel material during the manufacturing process. However, the lower limit value of the Nb content is not limited, and can be 0%. In addition, Nb is also an element that forms carbides and nitrides and has an effect of refining the metal structure and increasing the strength of the steel material. Therefore, in the present disclosure, the Nb content can 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 content of Nb is preferably 0.040% or less or 0.030% or less. In particular, from the viewpoint of ensuring the toughness of the steel material after PWHT, the content of Nb can be 0.004% or less.

[0049] (Ti: 0.050% or less) Ti is an element that is sometimes mixed into the steel material during the manufacturing process. However, the lower limit value of the Ti content is not limited, and can be 0%. In addition, Ti is also an element that forms carbides and nitrides and has an effect of refining the metal structure and increasing the strength of the steel material. Therefore, in the present disclosure, the Ti content can 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 content of Ti is preferably 0.040% or less or 0.030% or less. In particular, from the viewpoint of ensuring the toughness of the steel material after PWHT, the content of Ti can be 0.004% or less or 0.002% or less.

[0050] (V: 0.10% or less) V is an element that is sometimes mixed into the steel material during the manufacturing process. However, the lower limit value of the V content is not limited, and can be 0%. In addition, V is also an element that forms carbides, nitrides, and increases the strength of the steel material. Therefore, in the present disclosure, the V content can be 0.01% or more. However, from the viewpoint of suppressing the deterioration of the toughness and weldability of the HAZ, and suppressing the increase in the alloy cost, the V content is 0.10% or less. The V content is preferably 0.08% or less, or 0.05% or less.

[0051] In the steel material of the present disclosure, in order to increase the toughness of the HAZ, one or two or more of the optional elements Mg, Ca, and REM shown below can also be contained as needed.

[0052] (Mg: 0.0200% or less) Mg is an element that is sometimes mixed into the steel material during the manufacturing process. However, the lower limit value of the Mg content is not limited, and can be 0%. In addition, Mg is also an element that forms oxides and increases the toughness of the weld heat affected portion. Therefore, in the present disclosure, the Mg content can be 0.0003% or more, 0.0006% or more, or 0.0010% or more. On the other hand, if the Mg content is excessive, sometimes coarse oxides are formed, and the toughness of the steel decreases. Therefore, from the viewpoint of ensuring the 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.

[0053] (Ca: 0.0200% or less) Ca is an element that is sometimes mixed into the steel material during the manufacturing process. However, the lower limit value of the Ca content is not limited, and can be 0%. In addition, Ca is also an element that reduces the influence of MnS, which decreases the toughness of the steel material and the weld heat affected portion, by spheroidizing sulfides in the steel material. Therefore, in the present disclosure, the Ca content can be 0.0003% or more, 0.0006% or more, or 0.0010% or more. On the other hand, if the Ca content is excessive, sometimes coarse oxides are formed, and the toughness of the steel decreases. Therefore, from the viewpoint of ensuring the 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.

[0054] (REM: 0.0200% or less) The rare-earth metal (REM) refers to the total of 17 kinds of elements of Sc, Y, and 15 kinds of lanthanoid elements of La, Ce, and Nd. The REM content refers to the total content of the 17 kinds of elements. The REM is an element that is sometimes mixed into the steel during the manufacturing process. However, the lower limit value of the REM content is not limited, and can be 0%. In addition, the REM is also an element that forms an oxide to improve the toughness of the weld heat-affected portion. Therefore, in the present disclosure, the REM content can be 0.0003% or more, 0.0006% or more, or 0.0010% or more. On the other hand, if the REM content is excessive, sometimes a coarse oxide is formed, and the toughness of the steel is reduced. Therefore, from the viewpoint of securing the toughness, in the present disclosure, the REM content is 0.0200% or less. The REM content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[0055] (Remaining portion: Fe and impurities) The remaining portion of the chemical composition of the steel of the present disclosure is iron (Fe) and impurities. The impurities refer to components that are mixed in when the steel is manufactured industrially due to raw materials such as ore, scrap iron, or other causes.

[0056] In addition to the limitation of the content of each element, in the present disclosure, the range of the α value is also limited as follows.

[0057] (α value: 5.0 or more and 16.0 or less) The α value is calculated by the following (1).

[0058] α = 0.50 x V[C] x (1 + 0.64[Si]) x (1 + 4.10[Mn]) x (1 + 0.27[Cu]) x (1 + 0.52[Ni]) x (1 + 2.33[Cr]) x (1 + 3.14[Mo]) (1) wherein [C], [Si], [Mn], [Cu], [Ni], [Cr], and [Mo] are the contents of C, Si, Mn, Cu, Ni, Cr, and Mo in the steel (mass%). In the case where the element is not contained, zero is substituted. In addition, V[C] has the same meaning as [C] 1 / 2 .

[0059] In the present disclosure, the range of the value of a is set to 5.0 to 16.0. It is an index indicating the hardenability of the steel material, and the higher the value of a, the more the lower bainite and martensite structure having excellent balance of strength and toughness can be formed. When a is in the appropriate range, the ratio of the lower bainite and martensite structure having excellent balance of strength and toughness of the structure and strength and toughness of the HAZ becomes high, and the toughness of the HAZ can also be ensured. When a is 5.0 or more, the hardenability of the base material is ensured, the ratio of the lower bainite and martensite having favorable balance of strength and toughness increases, and deterioration of toughness is suppressed. In addition, the ratio of the lower bainite and martensite of the structure of the HAZ also easily increases, and the toughness of the HAZ also improves. On the other hand, if the value of a is 16.0 or less, the strength of the steel material will not become too high, and the toughness can be ensured. In addition, if the value of a is 16.0 or less, the toughness after PWHT can also be ensured. In addition, the HAZ will not become too hard, and the toughness of the HAZ can also be ensured.

[0060] By satisfying the above numerical range with respect to the value of a, a low-temperature nickel-containing steel material having excellent strength and toughness can be provided. The value of a is preferably 5.5 or more, 6.0 or more, or 7.0 or more. In addition, the value of a is preferably 15.5 or less or 15.0 or less.

[0061] <Microstructure> Next, the microstructure of the steel material of the present disclosure is described. The microstructure of the portion at a depth of 1 / 4 of the thickness from the surface of the steel material of the present disclosure contains lower bainite and martensite. In addition, as bainite, in addition to lower bainite, upper bainite can also be contained.

[0062] "Bainite" is a general term for upper bainite and lower bainite. "Upper bainite" is one or both of upper bainite containing residual austenite or MA phase (martensite / austenite mixed phase) between laths, and upper bainite containing carbide between laths. "Lower bainite" is lath-like lower bainite containing carbide within laths.

[0063] "Martensite" exists in four forms of lath, butterfly, lens, and thin plate, but in the composition of the present disclosure, mainly lath martensite is generated. Lath martensite is composed of lath bundles and lath blocks composed of groups of laths that have been arranged in a specific manner, and is a structure in which one austenite grain is divided into several lath bundles.

[0064] (Total of area ratio of lower bainite and martensite: 15.0% or more) The lower bainite and the martensite are hard phases, and improve the toughness of the steel material. From the viewpoint of ensuring the toughness of the steel material, the area ratio of the lower bainite and the martensite in the 1 / 4t portion is 15.0% or more. The area ratio of the lower bainite and the martensite in the 1 / 4t portion is preferably 20.0% or more or 30.0% or more. The total of the area ratio of the lower bainite and the area ratio of the martensite in the 1 / 4t portion can be 100%.

[0065] (total of area ratios of upper bainite, lower bainite, and martensite: 90.0% or more) From the viewpoint of ensuring the strength of the steel material, the total of the area ratios of the upper bainite, the lower bainite, and the martensite in the 1 / 4t portion is 90.0% or more. The total of the area ratios of the upper bainite, the lower bainite, and the martensite in the 1 / 4t portion can be 100%. In addition, the upper bainite in the 1 / 4t portion can also be 1.0% or more.

[0066] (area ratio of residual austenite: less than 1.7%) From the viewpoint of ensuring the toughness of the steel material, the area ratio of the residual austenite in the 1 / 4t portion is less than 1.7%. The area ratio of the residual austenite in the 1 / 4t portion is preferably 1.0% or less, and can also be 0%. It is considered that this is because, in the Ni-containing thick steel sheet of the present disclosure, the Ni content is less than that of the conventional 9%Ni steel, and thus, even if the residual austenite exists at -110°C, it is not stable, and when the steel structure is subjected to plastic deformation at the crack tip, the residual austenite changes to martensite due to plastic-induced martensitic transformation. Therefore, the residual austenite at room temperature is set to less than 1.7% in terms of volume ratio.

[0067] In addition, the higher the Ni content, the more likely the volume ratio of the residual austenite is to increase, but an increase in the Ni content is also not preferable in terms of cost increase.

[0068] The observation of the microstructure of the steel material is performed using a test piece with the 1 / 4t portion of the steel material as the observation surface. Two kinds of test pieces subjected to (a) electrolytic polishing and (b) nitric acid ethanol etching are prepared. For each of the test pieces of (a) and (b), three places are measured by the following methods, respectively, and the average of the three places is taken as the area ratio of the microstructure of the steel material. Note that three of each of the test pieces of (a) and (b) can be prepared and the average of the test pieces can be taken, or the measurement can be performed at three places in the field of view in one test piece and the average can be taken.

[0069] After mirror finishing by mechanical polishing, an electrolytically polished sample was used that had been electrolytically polished to remove the strain layer generated by the mechanical polishing. The total area ratio of upper bainite, lower bainite, martensite, and retained austenite was measured using EBSD. The measurement magnification was 200 times, and the measurement was performed over a range of 400μm×400μm at a pitch of 0.4μm. The measurement was performed with the electron beam diameter of 0.4μm or less. The confidence index (hereinafter referred to as the "CI value") was set to 0.1 or above. The determination of ferrite from upper bainite, lower bainite, and martensite was performed by setting the threshold value of Grain Average Misorientation (hereinafter referred to as "GAM") to 0.5. It should be noted that the GAM value is an indicator defined in OIM-Analysis (EBSD crystal orientation analysis software manufactured by TSL Corporation of the United States). Regions with a GAM of 0.5 or less represent ferrite, while regions with a GAM exceeding 0.5 represent upper bainite, lower bainite, martensite, or retained austenite. In this disclosure, upper bainite, lower bainite, martensite, and retained austenite are defined using the EBSD GAM threshold. Therefore, these terms encompass not only upper bainite, lower bainite, martensite, and retained austenite, but also tempered upper bainite, tempered lower bainite, and tempered martensite. Comparing the microstructures of direct quenching (DQ) and subsequent tempering (T) (DQT), while tempering results in decomposition of MA and coarsening of carbides, the microstructure does not significantly change in appearance.

[0070] The sample was etched with Nital and observed under a SEM to measure the area ratio of upper bainite. Measurements were made at 500x magnification within an area of ​​360 μm x 480 μm. Upper bainite refers to areas with a clear lath structure and carbides or MA formed along the lath boundaries. Upper bainite refers to areas with a relatively coarse internal structure, a sparse concentration of carbides, and a mixture of sparse and dense areas. Figure 1 Examples of microstructure identification results are shown. (A) and (B) are SEM images of the same region of a steel material produced by DQT with an α value of 9.9. The area surrounded by the white line in (B) is upper bainite (Bu), and the rest is lower bainite + martensite (B L + M). In the portion identified as upper bainite (Bu), whitish carbides are sparse and mixed with sparse and dense areas. On the other hand, in the portion identified as lower bainite + martensite (B LIn the portion (+M), carbides are densely and uniformly present. The area ratio of the upper bainite is subtracted from the total area ratio of the upper bainite, lower bainite, martensite, and retained austenite measured above to determine the total area ratio of the lower bainite, martensite, and retained austenite. Furthermore, the area ratio of retained austenite is determined using the measurement method described below, and this area ratio is subtracted from the total area ratio of the lower bainite, martensite, and retained austenite to determine the total area ratio of the lower bainite and martensite.

[0071] (Area ratio of retained austenite) The area ratio of retained austenite was determined by X-ray diffraction. The area ratio of retained austenite was determined using a sample with a portion 1 / 4 of the thickness in the thickness direction from the surface of the steel (also referred to as "1 / 4t portion" in this specification) as the measurement surface. The sample was a 2 mm thick test piece collected from a position 1 / 4 of the width from the end of the steel in the width direction, chemically polished, and used to determine the volume ratio of retained austenite by X-ray diffraction using a Mo tube. Quantification was performed based on the ratio of the integrated intensity of the (200) and (211) diffraction peaks of the ferrite phase to the integrated intensity of the (200), (220), and (311) diffraction peaks of the austenite phase, and the average value of 6 combinations was used. The integrated intensity of the diffraction peak was calculated by fitting the background based on the signals before and after the peak and subtracting the signal portion. The volume ratio determined by X-ray diffraction was regarded as the area ratio.

[0072] (Average grain size of 1 / 4t portion of steel) In the present disclosure, the average grain size (effective grain size) of the 1 / 4t portion of the steel is preferably 20.0 μm or less. This is because the following insight has been obtained: if the average grain size of the 1 / 4t portion of the steel is 20.0 μm or less, the toughness of the steel tends to be further improved regardless of before and after PWHT. However, the average grain size of the 1 / 4t portion of the steel may also exceed 20.0 μm. The smaller the average grain size of the steel, the better, so its lower limit is not limited. Usually, the average grain size is 10 μm or more. The effective grain size is obtained by weighted average. For the effective grain size D obtained by weighted average area , used in 4mm 2 The area S of the i-th grain detected during the measurement of the grain size measured in the region i , particle size d i , and is calculated using the following formula.

[0073] D area =ΣSi·d i / ΣS i (Aspect ratio of the prior austenite grains in the 1 / 4t portion of the steel) The morphology of the prior austenite grains (sometimes referred to as prior austenite particles) of the steel material of the present disclosure can be a flattened shape in the rolling direction. If the prior austenite particles at a position 1 / 4 of the thickness from the surface of the steel material in the thickness direction are flattened grains having an aspect ratio of 1.5 or more, the toughness of the steel material can be further improved. This is because the area of the grain boundary is increased by flattening the prior austenite particles, thereby effectively refining the effective crystal grain size by substantially refining the austenite grains. The aspect ratio of the prior austenite particles is usually 4.0 or less, and can be 3.5 or less.

[0074] On the other hand, from the viewpoint of ensuring the homogeneity of the microstructure, the aspect ratio of the prior austenite grains at the 1 / 4t portion can be less than 1.5. The aspect ratio of the prior austenite grains at the 1 / 4t portion can be 1.4 or less or 1.3 or less.

[0075] The aspect ratio of the prior austenite grains (sometimes referred to as prior austenite particles) of the steel material is determined as follows. First, the L-section (a section parallel to the rolling direction and the thickness direction of the steel material) at a position 1 / 4 of the thickness from the surface of the steel material in the thickness direction is mirror polished, and etched with an etching solution of a saturated aqueous solution of 2 to 4% picric acid base to make the prior austenite grain boundaries of an arbitrary rolling direction 1.0 mm x thickness direction 0.5 mm region appear.

[0076] Next, the major axis and the minor axis of each prior austenite particle are measured, and the aspect ratio of each prior austenite particle is calculated as the major axis ÷ the minor axis. The arithmetic mean of the aspect ratios of all the calculated prior austenite particles is determined as the "aspect ratio of the prior austenite particles". In addition, the maximum length of the prior austenite particles is taken as the major axis, and the maximum interval of two lines parallel to the major axis direction in contact with the grain is taken as the minor axis.

[0077] <mechanical properties> The steel material of the present disclosure has mechanical properties that balance strength and low-temperature toughness. In particular, in addition to excellent toughness at -110°C, excellent low-temperature toughness can be exhibited after PWHT.

[0078] (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 MPa to 930 MPa. In order to reduce the weight of a large welded structure such as a transport tank, a steel material that can ensure the strength of the structure even if the thickness is thin is required. In general, a steel material having the above-mentioned tensile strength is selected as a steel material used in such applications, and therefore the present disclosure is also manufactured in a manner having the above-mentioned tensile strength.

[0079] (yield ratio) The yield ratio (YR = [yield strength] / [tensile strength] x 100) of the steel material of the present disclosure is not particularly limited, but is preferably 90% or less. When there is no yield point, the yield strength is calculated using the 0.2% yield strength.

[0080] (-110°C Charpy impact absorbed energy) For the steel material of the present disclosure, in order to ensure high toughness at low temperatures, the -110°C Charpy impact absorbed energy is preferably 150 J or more. The steel material of the present disclosure, by having a low-temperature toughness of -110°C Charpy impact absorbed energy of 150 J or more, is able to adapt the transport tank formed of the steel material of the present disclosure to, for example, a transport use of liquid carbon dioxide. The steel material of the present disclosure can have a low-temperature toughness of -110°C Charpy impact absorbed energy of 100 J or more. Note that the -110°C Charpy impact absorbed energy is set to a value measured using a test piece taken from a position 1 / 4 of the thickness.

[0081] (-110°C Charpy impact absorbed energy after PWHT) In a low-temperature tank, in order to prevent breakage, sometimes the welded portion is subjected to PWHT after being assembled in a transport tank. At this time, not only the welded portion is heated, but also the base material portion (also simply referred to as base material) of the steel material that is not affected by welding is heated. If the base material is heated for a long time in a temperature region of 425°C or more, there is a tendency for the toughness of the base material to decrease. For the steel material of the present disclosure, in the case where the steel material is subjected to PWHT with a holding temperature of 600°C, a holding time of 2 hours, and a temperature increase rate and a temperature decrease rate of 55°C / h in a temperature region of 425°C or more, the toughness of the portion subjected to the PWHT is preferably -110°C Charpy impact absorbed energy of 150 J or more. The -110°C Charpy impact absorbed energy after PWHT can be 100 J or more. The -110°C Charpy impact absorbed energy after PWHT is also set to a value measured using a test piece taken from a position 1 / 4 of the thickness.

[0082] Furthermore, there is a possibility that the toughness of the steel material decreases due to PWHT. The reason is not clear, but it is presumed that this is because P (phosphorus) and Mn diffuse at the grain boundaries, and in addition, growth or aggregation of inclusions occurs in the structure, and the brittleness decreases and the toughness decreases. By limiting the contents of P and Mn, and reducing the average crystal grain diameter of the steel material, the decrease in toughness due to PWHT is suppressed.

[0083] (-110°C Charpy impact absorbed energy after thermal cycle) For the steel material of the present disclosure, in order to ensure high toughness after a heat cycle test simulating a weld portion at low temperature, the Charpy impact absorbed energy at -110°C after the heat cycle is preferably 50 J or more. The steel material of the present disclosure, by having a low-temperature toughness with a Charpy impact absorbed energy at -110°C after a heat cycle of 50 J or more, is able to use a transport tank formed of the steel material of the present disclosure, for example, as a transport use for liquid carbon dioxide. The Charpy impact absorbed energy at -110°C after a heat cycle can be 40 J or more. Further, for the Charpy impact absorbed energy at -110°C after a heat cycle, a test piece is taken from a position of 1 / 4 of the thickness of the steel material, and used as a heat cycle test piece, and the following heat process is imparted: a temperature rise to 1350°C at 60°C / s, a 1 s hold at 1350°C, and a cooling to room temperature at 20°C / s, after which a Charpy impact test piece is taken therefrom, and used as a value for measurement.

[0084] (Charpy impact absorbed energy at -110°C after heat cycle, PWHT) In a low-temperature tank, in order to prevent breakage, sometimes a PWHT is performed on a weld portion after assembly in a transport tank. The steel material of the present disclosure, after the above-mentioned heat cycle test, a PWHT is performed in which the temperature rise and temperature drop rates in the temperature region of 425°C or more are 55°C / h, and a 2 h hold at 600°C, after which a Charpy impact test piece is taken and measured. At this time, the toughness of the portion on which the above-mentioned PWHT was performed is preferably a Charpy impact absorbed energy at -110°C of 50 J or more. The Charpy impact absorbed energy at -110°C of the portion on which the PWHT was performed after the heat cycle test can be 40 J or more.

[0085] Further, it is possible that the toughness of the steel material decreases due to the PWHT. The reason is not clear, but it is presumed that this is due to the diffusion of P (phosphorus) or Mn at the grain boundaries, and in addition, the growth or agglomeration of inclusions occurs in the structure, and the toughness decreases due to the decrease in brittleness. By limiting the contents of P and Mn, and reducing the average crystal grain diameter of the steel material, the decrease in toughness due to the PWHT is suppressed.

[0086] The tensile strength (TS) and the yield strength (YS) are measured by a tensile test in accordance with JIS Z2241:2011. In the tensile test, a JIS No. 14A test piece taken from a position of 1 / 4 of the thickness, with the direction parallel to the width direction of the steel material (C direction) as the length direction, is used. The TS and YS are measured using 3 test pieces, and are calculated by averaging them. Based on each of the average values of the TS and YS, the yield ratio YR (%) is calculated by (YS / TS) x 100.

[0087] The Charpy impact absorbed energy is measured by the Charpy impact test at -110°C using an impact knife with a radius of 2 mm. The Charpy impact absorbed energy is measured using three test pieces, and is calculated by averaging them. In the Charpy impact test, a V-notch test piece is used, which is taken from a 1 / 4-thickness position of the steel material, with the direction parallel to the width direction of the steel material (C direction) as the length direction.

[0088] The shape of the steel material of the present disclosure is not particularly limited, and is a steel sheet, a steel strip, a steel shape, a steel pipe, or the like. However, the steel pipe and the steel shape include not only a steel material made by joining a steel sheet, such as a welded steel pipe and a welded steel shape, but also a steel shape made by joining with a rivet, or the like. The thickness of the steel material (the thickness of the flange in the case of a steel shape) is not particularly limited, and is usually 3 mm or more and 150 mm or less. The thickness of the steel material can be 6 mm or more, 10 mm or more, 15 mm or more, or 30 mm or more. In addition, the thickness of the steel material can be 100 mm or less, 80 mm or less, or 60 mm or less.

[0089] In addition, the use of the steel material of the present disclosure is not particularly limited, and has mechanical properties that balance strength and low-temperature toughness, and particularly, excellent low-temperature toughness after PWHT, and thus can be suitably used as a tank for storing and transporting a liquefied gas, particularly liquid carbon dioxide.

[0090] (Method for manufacturing steel material) The method for manufacturing the steel material of the present disclosure is not particularly limited, and the steel material of the present disclosure is manufactured by continuous casting after melting a steel that satisfies the above-described chemical composition, for example. The steel billet is subjected to direct quenching (DQ) by heating, hot rolling, and direct water cooling, or the steel billet is subjected to reheating quenching (RQ) by heating and water cooling after being cooled and then reheated after hot rolling, thereby manufacturing the steel material. In the case of RQ, the steel billet can be cooled or water-cooled before reheating. Furthermore, tempering (T) can also be performed.

[0091] (1) DQT: direct quenching (DQ), tempering (T) (2) RQT: cooling or water cooling, reheating quenching (RQ), tempering (T) (1) DQT From the viewpoint of manufacturing cost, DQT is preferred in the manufacturing of the steel material of the present disclosure, and examples of a preferred manufacturing process are shown below.

[0092] In the case where the steel material of the present disclosure is manufactured using DQ, the heating temperature of the billet subjected to hot rolling is Ac3 or higher from the viewpoint of performing hot rolling from a temperature region in which the metal structure of the rolled material is austenite. The heating temperature of the billet is preferably 1000°C or higher from the viewpoint of reducing the deformation resistance. On the other hand, the heating temperature of hot rolling is 1250°C or lower from the viewpoint of suppressing the coarsening of the γ grains. The heating temperature of hot rolling is preferably 1200°C or lower. Further, Ac3 is set to a value calculated from the following formula.

[0093] Ac3 = 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 refer to the content (mass %) of each element contained in the billet.

[0094] Hot rolling is sometimes composed of rolling at a temperature region in which recrystallization occurs (recrystallization temperature region rolling) and rolling at a temperature region in which recrystallization is suppressed (non-recrystallization temperature region rolling).

[0095] The recrystallization temperature region rolling is hot rolling performed at a temperature of 900°C or higher in rolling of the rolled material. The cumulative reduction ratio of the recrystallization temperature region rolling is preferably 20% or higher, and more preferably 30% or higher, from the viewpoint of the refinement of the austenite grain size of the steel material. The cumulative reduction ratio of the recrystallization temperature region rolling is calculated from the difference between the thickness of the billet before hot rolling and the thickness of the rolled material at 900°C.

[0096] Cumulative reduction ratio (%) of recrystallization temperature region rolling = 100 x ([thickness of billet] - [thickness of rolled material at 900°C]) / [thickness of billet] The non-recrystallization temperature region rolling is hot rolling performed at a temperature lower than 900°C in rolling of the rolled material. The cumulative reduction ratio of the non-recrystallization temperature region rolling is preferably 20% or higher, and more preferably 30% or higher, from the viewpoint of the refinement of the average crystal grain size of the steel material. The cumulative reduction ratio of the non-recrystallization temperature region rolling is calculated from the difference between the thickness of the rolled material at 900°C and the thickness of the steel material after rolling is completed.

[0097] Cumulative reduction ratio (%) of non-recrystallization temperature region rolling = 100 x ([thickness of rolled material at 900°C] - [thickness of steel material after rolling is completed]) / [thickness of rolled material at 900°C] From the viewpoint of suppressing generation of ferrite which reduces strength, the finish temperature of hot rolling is Ar3 or higher. After the finish of hot rolling, accelerated cooling such as water cooling is performed on the steel material. From the viewpoint of suppressing generation of ferrite which reduces strength, the start temperature of accelerated cooling is Ar3 or higher. Note that Ar3 is set to a value calculated by the following formula.

[0098] Ar3 = 910 - 310C - 80Mn - 20Cu - 15Cr - 55Ni - 80Mo + 0.35 (t - 8) The element symbols in the formula refer to the content (mass %) of each element contained in the steel material, and t refers to the thickness (mm) of the steel material.

[0099] From the viewpoint of promoting bainite transformation and martensite transformation, the cooling rate is 1.0°C / sec or higher. The cooling rate of accelerated cooling is preferably 5.0°C / s or higher or 10.0°C / s or higher. The faster the cooling rate of accelerated cooling is, the more preferable it is, but from the viewpoint of homogenization of the cooling rate, cost, and the like, it is preferably 50.0°C / s or lower or 30.0°C / s or lower. The cooling rate is a value obtained by calculating the cooling rate at the position of 1 / 4 of the thickness through simulation based on heat transfer calculation.

[0100] From the viewpoint of improving the strength of the steel material by ensuring upper bainite, lower bainite, and martensite, the stop temperature of accelerated cooling is 400°C or lower. The stop temperature of accelerated cooling is preferably 350°C or lower. Accelerated cooling can be performed to room temperature. From the viewpoint of dehydrogenation of the steel material, the stop temperature of accelerated cooling is preferably 100°C or higher.

[0101] After accelerated cooling, the steel material can be subjected to a tempering treatment. From the viewpoint of suppressing reduction in strength, the heating temperature of the tempering treatment is preferably 650°C or lower, 620°C or lower, or 590°C or lower. On the other hand, from the viewpoint of improving toughness, the heating temperature of the tempering treatment is preferably 350°C or higher or 400°C or higher.

[0102] (2) RQT When the steel material of the present disclosure is manufactured by RQ, the effects of the heating temperature and the reduction of the billet at the time of hot rolling on the mechanical properties of the steel material are small. However, when the heating temperature of the billet is too low, the deformation resistance increases, and thus the heating temperature of the billet is preferably 1000°C or higher. In addition, when the reduction is insufficient, initial defects at the time of billet manufacturing sometimes remain in the center portion of the thickness, and the material quality of the steel material decreases, and thus the total of the reductions of hot rolling (also referred to as cumulative reduction) is preferably 35% or higher. After hot rolling, water cooling can be performed directly, or air cooling can be performed.

[0103] The steel is reheated after hot rolling and then quenched. In order to quench from a single-phase austenite structure, the steel is reheated at a temperature of Ac3 or higher. From the viewpoint of ensuring homogeneity of the microstructure, the steel is preferably reheated at a temperature of 750°C or higher, 850°C or higher, 880°C or higher, or 900°C or higher. On the other hand, the upper limit of the reheating temperature is not particularly specified, but when excessively heated to a high temperature, the austenite grains sometimes coarsen, leading to a decrease in toughness, and thus is preferably 1000°C or lower, 950°C or lower, or 930°C or lower.

[0104] After the reheating and quenching, the steel can be subjected to a tempering treatment. From the viewpoint of suppressing a decrease in strength, the heating temperature of the tempering treatment is preferably 660°C or lower or 640°C or lower. On the other hand, from the viewpoint of improving toughness, the heating temperature of the tempering treatment is preferably 400°C or higher, 450°C or higher, or 500°C or higher.

[0105] Examples Hereinafter, examples are listed to specifically describe the steel of the present disclosure. However, the conditions in the following examples are one example of conditions adopted in order to confirm the workability and effects of the present disclosure, and the present disclosure is not limited to the following examples.

[0106] <Manufacture using direct quenching and tempering> [Manufacture of steel] First, a slab having the chemical composition shown in Table 1 was cast by a continuous casting method. The remainder other than the components shown in Table 1 was Fe and impurities. In addition, the blank indicates that an alloying element was not intentionally added in the steelmaking step. The underlined portion indicates that it is outside the scope of the present disclosure.

[0107] Next, the steels were manufactured from these slabs according to the manufacturing conditions shown in Table 2. The "Temper heat treatment" is the heating temperature in the tempering treatment after quenching.

[0108] [Measurement and evaluation] The microstructure and mechanical properties of the steels obtained by the above-described method were measured. The results are shown in Table 3. The meanings of the symbols of the microstructure are as follows. In addition, the remainder of the microstructure is pearlite, MA phase, and ferrite.

[0109] Bu: upper bainite BL: lower bainite M: martensite Residual γ: residual austenite Regarding toughness, the average value of Charpy impact absorbed energy at -110°C, the temperature rise rate and the temperature drop rate: 55°C / h in the temperature range of 425°C or higher in the temperature region of 600°C, the holding time: 2 hours, and the average value of Charpy impact absorbed energy at -110°C after PWHT were measured.

[0110] No. 2A to 26A, 101A to 113A are examples of the present disclosure, and No. 28A to 32A, 114A to 118A are comparative examples.

[0111] The α value of No. 28A exceeds the upper limit value of the present disclosure, and the hardenability is too high, and the strength is too high.

[0112] The α value of No. 30A exceeds the upper limit value of the present disclosure, and the hardenability is too high, and the strength is too high. Because the residual γ is also too much, sufficient low-temperature toughness is not obtained.

[0113] No. 31A has low low-temperature toughness after PWHT because Mn deviates from the upper limit.

[0114] No. 32A has insufficient low-temperature toughness because the cooling rate of direct quenching is low, and the total area ratio of lower bainite and martensite is insufficient.

[0115] No. 114A, 115A has insufficient hardenability and strength because the α value is lower than the lower limit value of the present disclosure, and sufficient low-temperature toughness is not obtained.

[0116] No. 116A, 117A has too high hardenability and strength because the α value exceeds the upper limit value of the present disclosure.

[0117] No. 118A has insufficient strength because the starting temperature of direct quenching is too low, and the total area ratio of upper bainite, lower bainite, and martensite is insufficient.

[0118] In the examples of the present disclosure, the chemical composition and the microstructure of the steel material are appropriately controlled, the tensile strength is in an appropriate range of 615 MPa or more and 930 MPa or less, and in addition thereto, low-temperature toughness of 100 J or more at -110°C is obtained before and after PWHT, in comparison with the comparative examples.

[0119] Manufacture by reheating quenching and tempering [Manufacture of steel material] First, a slab having the chemical composition shown in Table 4 was cast by a continuous casting method. The remainder other than the components shown in Table 4 is Fe and impurities. In addition, the blank indicates that an alloying element is not intentionally added in the steelmaking step. The underlined indicates outside the range of the present disclosure.

[0120] Next, steel materials were produced from these slabs according to the production conditions shown in Table 5. "Temper heat treatment" is the heating temperature in the tempering treatment after quenching.

[0121] [Measurement and Evaluation] The microstructure and mechanical properties of the steel materials obtained by the above method were measured. The results are shown in Table 6. The meanings of the symbols of the microstructure are as follows. In addition, the remaining part of the microstructure is MA phase, ferrite.

[0122] Bu: upper bainite BL: lower bainite M: martensite Residual γ: residual austenite Regarding toughness, the average value of Charpy impact absorbed energy at -110°C, the average value of Charpy impact absorbed energy at -110°C after PWHT with a temperature maintaining temperature: 600°C, a temperature maintaining time: 2 hours, a temperature rise rate and a temperature drop rate in a temperature region of 425°C or higher: 55°C / h were respectively measured.

[0123] No. 2B to 24B, No. 101B to 113B are examples of the present disclosure, and No. 25B to 29B, No. 114B to 119B are comparative examples.

[0124] The α value of No. 25B is lower than the lower limit value of the present disclosure, the hardenability is insufficient, the strength is insufficient. Also, sufficient low temperature toughness cannot be obtained.

[0125] The α value of No. 26B exceeds the upper limit value of the present disclosure, the hardenability is too high, the strength is too high.

[0126] No. 29B has a Mn deviating from the upper limit, so the low temperature toughness after PWHT becomes low.

[0127] The α value of No. 114B, 115B is lower than the lower limit value of the present disclosure, the hardenability is insufficient, the strength is insufficient. Also, sufficient low temperature toughness cannot be obtained.

[0128] The α value of No. 116B, 117B exceeds the upper limit value of the present disclosure, the hardenability is too high, the strength is too high.

[0129] No. 118B has a reheat quenching temperature that is too low, the total area ratio of upper bainite, lower bainite and martensite is insufficient, the strength is insufficient.

[0130] The α value of No. 119B exceeds the upper limit value of the present disclosure, the hardenability is too high, the strength is too high. In addition, because the residual γ is also too much, sufficient low temperature toughness cannot be obtained.

[0131] In the present application example, in contrast to the comparative example, the chemical composition and the microstructure of the steel material are appropriately controlled, the tensile strength is in an appropriate range of 615 MPa or more and 930 MPa or less, and in addition thereto, the low-temperature toughness at -110°C of 100 J or more is obtained before and after PWHT. In a particularly preferred constitution of the present disclosure, the low-temperature toughness at -110°C of 150 J or more is obtained before and after PWHT.

[0132] Industrial applicability The steel material of the present disclosure can be mainly used for a transport tank suitable for liquefied carbon dioxide. In addition, the steel material of the present disclosure can also be used for other welded structures such as buildings, bridges, ships, line pipes, marine structures, pressure vessels, and tanks.

[0133] The disclosure of Japanese Patent Application No. 2023-042399 and Japanese Patent Application No. 2023-042402 filed on March 16, 2023 is incorporated herein by reference in its entirety. All documents, patent applications and technical standards cited in the present specification are incorporated herein by reference to the same extent as the particular document, patent application or technical standard is specifically and individually indicated to be incorporated by reference.

Claims

1. A steel material having the following chemical composition: 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: less than 0.025%, 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: 0~1.50%, Cr：0~3.00%、 Mo: 0~2.00%, B：0~0.0050%、 Nb: 0~0.050%, Ti: 0~0.050%, V：0~0.10%、 Mg: 0~0.0200%, Ca: 0~0.0200%, REM: 0~0.0200%, The rest: Fe and impurities, Furthermore, α represented by the following formula (1) is 5.0 or more and 16.0 or less, The tensile strength of the steel is 615 MPa or more and 930 MPa or less. The microstructure of a portion located 1 / 4 of the thickness in the thickness direction from the surface of the steel material comprises lower bainite and martensite, the total area ratio of the lower bainite and the martensite being 15.0% or more, the total area ratio of the upper bainite, the lower bainite, and the martensite being 90.0% or more, and the area ratio of retained austenite being 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 [element symbol] in formula (1) represents the content (mass %) of each corresponding element contained in the steel material. When the element is not contained, zero is substituted.

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

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

4. The steel material according to any one of claims 1 to 3, wherein When the steel is heat treated at a temperature range of 425°C or higher with a heating rate and a cooling rate of 55°C / h and maintained at 600°C for 2 hours, the Charpy impact absorbed energy of the heat-treated portion at -110°C is 150 J or more.

5. The steel material according to any one of claims 1 to 4, wherein The aspect ratio of prior austenite grains at a location that is 1 / 4 of the thickness in the thickness direction from the surface of the steel material is 1.5 or greater.

6. The steel material according to any one of claims 1 to 4, wherein The aspect ratio of prior austenite grains at a location that is 1 / 4 of the thickness in the thickness direction from the surface of the steel material is less than 1.5.

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