Steel materials

Abstract

This steel material comprises a specific chemical composition in which α represented by the following formula is 4.0-16.0. The steel material has a tensile strength of 590-930 MPa. In the steel material: a micro structure at a segment located 1 / 4 of the thickness from the surface includes lower bainite and martensite; the total of the area percentage of lower bainite and martensite is 15.0% or more; and the total of the area percentage of upper bainite, lower bainite, and martensite is 90.0% or more. 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

Technical Field

[0001] The present disclosure relates to steel materials.

Background Art

[0002] Steel materials can be used for welded structures such as buildings, bridges, ships, line pipes, offshore structures, pressure vessels, and tanks. Steel materials with excellent strength and stress resistance to low temperature toughness are effective for low temperature applications.

[0003] Low temperature steels are used for low temperature pressure vessels such as liquefied gas storage tanks. Depending on the use temperature, there are Al-killed steels, nickel steels, high Mn steels, and austenitic stainless steels for low temperature steels. For example, nickel steels such as 3.5% Ni steel are used as materials for tanks loaded with liquefied ethane and liquefied ethylene with a use temperature of around -100°C.

[0004] For steel materials such as this 3.5% Ni steel, which require ensuring low temperature toughness typified by low temperature pressure vessels, Ni is often contained.

[0005] For example, in Patent Document 1, a nickel-containing steel material for low temperature with excellent toughness is proposed, which has a specific chemical composition containing 2.7% or more and 5.0% or less of Ni, an original austenite grain size during quenching heating of 20 μm or less, an effective crystal grain size after heat treatment of 12 μm or less, and a tensile strength of 450 MPa or more and 690 MPa or less. Also, various steel materials with defined chemical compositions and microstructures (metallic structures) have been proposed for the purpose of low temperature toughness and high strength (see, for example, Patent Documents 2 to 9).

[0006] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2019-81930 Patent Document 2: International Publication No. 2014 / 103629 Patent Document 3: Japanese Unexamined Patent Application Publication No. 52-156121 Patent Document 4: Japanese Unexamined Patent Application Publication No. 55-104427 Patent Document 5: Japanese Unexamined Patent Application Publication No. 58-73717 Patent Document 6: Japanese Patent Application Laid-Open No. 7-331328 Patent Document 7: Japanese Patent Application Laid-Open No. 2001-123222 Patent Document 8: Japanese Patent Application Laid-Open No. 2001-123245 Patent Document 9: Japanese Patent Application Laid-Open No. 2007-46096

Summary of the Invention

Problems to be Solved by the Invention

[0007] For low-temperature steel used in low-temperature pressure vessels, it is desired to achieve both high strength and good low-temperature toughness. In addition, low-temperature pressure vessels are manufactured by welding steel materials, and post-weld heat treatment (sometimes referred to as PWHT) may be performed to remove the residual stress generated by welding. Recently, the requirements for the low-temperature toughness of steel materials after PWHT have been increasing.

[0008] The present disclosure aims to provide a steel material suitable for low-temperature applications that has high tensile strength and good low-temperature toughness regardless of whether it is before or after post-weld heat treatment.

Means for Solving the Problems

[0009] The gist of the present disclosure is as follows. <1> By mass, C: 0.03% or more and 0.20% or less, Si: 0.01% or more and 0.50% or less, Mn: 0.10% or more and 1.65% or less, P: 0.025% or less, S: 0.0250% or less, Ni: 2.65% or more and 4.45% or less, Al: 0.001% or more and 0.100% or less, O: 0.0100% or less, N: 0.0100% or less, Cu: 0 to 1.50%, Cr: 0 to 3.00%, Mo: 0 to 2.00%, B: 0 to 0.0050%, Nb: 0 to 0.050%, Ti: 0 to 0.050%, V: 0 to 0.10%, Mg: 0 to 0.0200%, Ca: 0 to 0.0200%, REM: 0 to 0.0200%, Balance: Fe and impurities and having a chemical composition in which α represented by the following formula (1) is 4.0 or more and 16.0 or less, tensile strength is 590 MPa or more and 930 MPa or less, the microstructure at a position 1 / 4 of the thickness from the surface of the steel material in the thickness direction includes lower bainite and martensite, and the total area ratio of the lower bainite and the martensite is 15.0% or more, and the total area ratio of upper bainite, the lower bainite and the martensite is 90.0% or more, steel material. α = 0.50 × √[C] × (1 + 0.64[Si]) × (1 + 4.10[Mn]) × (1 + 0.27[Cu]) × (1 + 0.52[Ni]) × (1 + 2.33[Cr]) × (1 + 3.14[Mo]) ···(1) However, [element symbol] in formula (1) represents the content (mass%) of each corresponding element contained in the steel material. When the corresponding element is not contained, zero is substituted. <2> The steel material according to <1>, wherein the microstructure at a position 1 / 4 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 -100 °C is 150 J or more. <4> When a heat treatment is performed on the steel material at a temperature range of 425 °C or higher with a heating rate and a cooling rate of 55 °C / h and held at 600 °C for 2 hours, the Charpy impact absorption energy at -100 °C at the location where the heat treatment is performed is 150 J or more. The steel material according to any one of <1> to <3>. <5> The steel material according to any one of <1> to <4>, wherein the aspect ratio of the prior austenite crystal grains at a position 1 / 4 of the thickness in the thickness direction from the surface of the steel material is 1.5 or more. <6> The steel material according to any one of <1> to <4>, wherein the aspect ratio of the prior austenite crystal grains at a position 1 / 4 of the thickness in the thickness direction from the surface of the steel material is less than 1.5.

Advantages of the Invention

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

Brief Description of the Drawings

[0011]

Figure 1

Embodiments for Carrying Out the Invention

[0012] Hereinafter, the present disclosure will be described in detail. In the present disclosure, the "post-weld heat treatment" means a post-weld heat treatment conforming to the content defined in JIS Z 3700:2009 "Post-weld Heat Treatment Method" unless otherwise specified. In the present disclosure, the "steel material" or "base material" means a steel material portion that does not include a surface treatment layer such as a plating layer or a coating film. However, a surface treatment layer such as a plating layer or a coating film may be formed on the surface of the steel material according to the present disclosure. Further, the "base material" in a welded joint means a steel material portion that is not affected by welding in contrast to the welded portion (welded joint and heat-affected zone of the welded joint).

[0013] In the present disclosure, a numerical range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value. However, a numerical range in the case where "exceed" or "less than" is attached to the numerical values described before and after "~" means a range that does not include these numerical values as the lower limit value or the upper limit value. Regarding the content of elements in the chemical composition, "%" means "mass %". The term "process" includes not only an independent process but also a process 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 research results and the newly obtained findings of the inventors of the present disclosure who have completed the steel material according to the present disclosure will be described in detail.

[0015] The inventors of the present disclosure conducted studies to improve the strength of the steel material. The tensile strength of the steel material is ensured by the structure of the microstructure. The inventors of the present disclosure collected samples from the surface of the steel material in the thickness direction after hot rolling and accelerated cooling at the 1 / 4t part (t: thickness of the steel material) which is a part of 1 / 4 of the thickness, conducted a tensile test, and observed the microstructure. As a result, it was found that in the 1 / 4t part of the steel material with a tensile strength of 590 MPa or more and 930 MPa or less, the area ratio of ferrite is less than 10.0%, and the total area ratio of upper bainite, lower bainite, and martensite is 90.0% or more. The total area ratio of upper bainite, lower bainite, and martensite was measured using the Electron Back Scatter Diffraction method (hereinafter referred to as "EBSD").

[0016] Furthermore, the inventors of the present disclosure conducted studies to improve the toughness of the steel material. The toughness of the steel material is ensured by the structure of the microstructure. The inventors of the present disclosure collected samples from the 1 / 4t part of the steel material after hot rolling and accelerated cooling, conducted a Charpy impact test, and observed the microstructure. As a result, it was found that for a steel material with a Charpy impact absorption energy of 150 J or more at -100 °C, the total area ratio of lower bainite and martensite is 15.0% or more. The total area ratio of lower bainite and martensite was measured using EBSD.

[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 large-angle grain boundaries with a difference in crystal orientation of 15° or more. The inventors of the present disclosure collected samples from the 1 / 4t part of the steel material manufactured by controlling the cooling rate and cooling stop temperature after hot rolling, and measured the equivalent circle diameter of the area surrounded by large-angle grain boundaries by EBSD. Hereinafter, the equivalent circle diameter of the area surrounded by large-angle grain boundaries is referred to as the crystal grain size. The samples were subjected to mechanical polishing and electrolytic polishing, and analysis was performed using an EBSD device attached to an FE-SEM (field emission scanning electron microscope) in a 4mm 2 area. Among the crystal grain sizes measured in the 4mm 2 area, the value calculated by the area-weighted average weighted by the area of each crystal grain was defined as the average crystal grain size (which may be referred to as the "effective crystal grain size"). It was found that if the average crystal grain size of the 1 / 4t part of the steel material is 20.0 μm or less, the toughness of the steel material tends to further improve regardless of before and after the post-weld heat treatment.

[0018] Furthermore, the inventors of the present disclosure have found that the same results can be obtained not only for the steel material after hot rolling and accelerated cooling, but also for the steel material after reheat quenching.

[0019] <Chemical composition> Next, the alloying elements constituting the chemical composition of the steel material according to the present disclosure will be described. In the following description of the alloying elements, "%" of the content means "% by mass".

[0020] (C: 0.03% or more and 0.20% or less) C is an element that increases the strength of the steel material. From the viewpoint of ensuring the strength of the steel material 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 heat-affected zone (hereinafter sometimes referred to as "HAZ") of the weld, 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: not less than 0.01% and not more than 0.50%) Si is an element used as a deoxidizer and also dissolved in steel to increase strength. From the perspective of controlling the O concentration contained in the molten steel, in the present disclosure, the Si content is not less than 0.01%. The Si content is preferably not less than 0.03%, not less than 0.05%, or not less than 0.10%. On the other hand, when the Si content is excessive, a hard phase may be formed in the HAZ, resulting in a decrease in toughness. Therefore, from the perspective of ensuring the toughness of the HAZ, in the present disclosure, the Si content is not more than 0.50%. The Si content is preferably not more than 0.30% or not more than 0.20%.

[0022] (Mn: not less than 0.10% and not more than 1.65%) Mn is an element used as a deoxidizer and also contributes to high strength by enhancing the hardenability of steel. From the perspective of controlling the O concentration contained in the molten steel, in the present disclosure, the Mn content is not less than 0.10%. Furthermore, with Mn of not less than 0.10%, solid solution S is reduced by forming MnS, preventing hot cracking. From the perspective of ensuring the strength of the steel material and the toughness of the HAZ, the Mn content is preferably not less than 0.30% or not less than 0.50%. On the other hand, when the Mn content is excessive, the toughness after PWHT may decrease due to the segregation of Mn at the grain boundaries during PWHT. Therefore, from the perspective of ensuring the toughness of the steel material after PWHT, in the present disclosure, the Mn content is not more than 1.65%. The Mn content is preferably not more than 1.50%, not more than 1.25%, or not more than 1.10%.

[0023] (P: not more than 0.025%) P is an impurity element. The lower limit of the P content is not limited, but from the perspective of manufacturing cost, in the present disclosure, the P content may be not less than 0.001%. On the other hand, when the P content is excessive, the toughness after PWHT may decrease due to the segregation of P at the grain boundaries during PWHT. Therefore, in the present disclosure, the P content is not more than 0.025%. The P content is preferably not more than 0.016%, not more than 0.012%, or not more than 0.008%.

[0024] (S: less than 0.0250%) S is an impurity element. The lower limit of the S content is not limited, but from the perspective of manufacturing cost, in the present disclosure, the S content may be 0.0001% or more. On the other hand, when the S content is excessive, elongated MnS is generated in the center segregation part, and the toughness and ductility of the steel material and HAZ may deteriorate. The S content is 0.0250% or less from the perspective of ensuring the toughness and ductility of the steel material and HAZ. The S content is preferably 0.0100% or less, or 0.0050% or less.

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

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

[0027] (O: 0.0100% or less) O is an impurity element. The lower limit of the O content is not limited, but from the perspective of manufacturing cost, in the present disclosure, the O content may be 0.0001% or more. On the other hand, when the O content is excessive, coarse oxides may be generated, and the toughness and ductility of the steel material and the HAZ may deteriorate. From the perspective of ensuring 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.

[0028] (N: 0.0100% or less) N is an impurity element. The lower limit of the N content is not limited, but from the perspective of manufacturing cost, in the present disclosure, the N content may be 0.0001% or more. From the perspective 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 (selected elements) in place of a part of Fe. For example, the following selected elements may be mentioned, but the content of these elements may be 0%.

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

[0031] (Cu: 1.50% or less) Cu is an element that may be mixed into the steel material during the manufacturing process. However, the lower limit value of the Cu content is not limited and may be 0%. Also, Cu has a small adverse effect on weldability and the toughness of the HAZ, and is an element that can improve the strength of the steel because it has the effect of increasing the hardenability 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 perspective of suppressing the generation 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.

[0032] (Cr: 3.00% or less) Cr is an element that may be mixed into the steel material during the manufacturing process. However, the lower limit value of the Cr content is not limited and may be 0%. Also, since Cr has the effect of enhancing the hardenability of steel, it is an element that improves the strength of the steel material. 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, 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.

[0033] (Mo: 2.00% or less) Mo is an element that may be mixed into the steel material during the manufacturing process. However, the lower limit value of the Mo content is not limited and may be 0%. Also, since Mo has the effect of enhancing the hardenability of steel, it is an element that improves the strength of the steel material. 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 the toughness and weldability of the HAZ and suppressing an increase in 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.

[0034] (B: 0.0050% or less) B is an element that may be mixed into the steel material during the manufacturing process. However, the lower limit value of the B content is not limited and may be 0%. Also, B exhibits a remarkable effect of enhancing the hardenability of steel and is an element that improves the strength of the steel material. Therefore, in the present disclosure, the B content may be 0.0003% or more. However, from the viewpoint of suppressing deterioration of the surface quality of the steel slab 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 the steel materials according to the present disclosure, in order to improve the strength, if necessary, one or more of the following selected elements Nb, Ti, and V, which have the effect of increasing the strength of the steel materials by precipitates such as carbides and nitrides, may be contained.

[0036] (Nb: 0.050% or less) Nb is an element that may be mixed into the steel material during the manufacturing process. However, the lower limit value of the Nb content is not limited and may be 0%. Also, Nb is an element that forms carbides and nitrides and has the effect of refining the metal structure, thereby improving the strength of the steel material. Therefore, in the present disclosure, the Nb content may be 0.001% or more. However, from the viewpoint of suppressing the 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. Particularly from the viewpoint of ensuring the toughness of the steel material after PWHT, the content of Nb may be 0.004% or less.

[0037] (Ti: 0.050% or less) Ti is an element that may be mixed into the steel material during the manufacturing process. However, the lower limit value of the Ti content is not limited and may be 0%. Also, Ti is an element that forms carbides and nitrides and has the effect of refining the metal structure, thereby improving the strength of the steel material. Therefore, in the present disclosure, the Ti content may be 0.001% or more. However, from the viewpoint of suppressing the 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. Particularly from the viewpoint of ensuring the toughness of the steel material after PWHT, the content of Ti 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 the steel material during the manufacturing process. However, the lower limit value of the V content is not limited and may be 0%. Also, V is an element that forms carbides and nitrides and improves the strength of the steel material. Therefore, in the present disclosure, the V content may be 0.01% or more. However, from the viewpoints of suppressing deterioration of the toughness and weldability of the HAZ and suppressing an increase in alloy cost, 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, if necessary, one or more of the following selected elements Mg, Ca, and REM.

[0040] (Mg: 0.0200% or less) Mg is an element that may be mixed into the steel material during the manufacturing process. However, the lower limit value of the Mg content is not limited and may be 0%. Also, Mg is an element that forms oxides and improves the toughness of the heat-affected zone of welding. 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, if the Mg content is excessive, coarse oxides may be formed and the toughness of the steel may be reduced. 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 the steel material during the manufacturing process. However, the lower limit value of the Ca content is not limited and may be 0%. Also, Ca is an element that reduces the influence of MnS, which lowers the toughness of the steel material and the heat-affected zone of the weld, by spheroidizing sulfides in the steel material. 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, if the Ca content is excessive, it may form coarse oxides and lower the toughness of the steel. Therefore, from the perspective 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) mean the general term for a total of 17 elements, namely two elements, Sc and Y, and 15 lanthanoid elements such as La, Ce, and Nd. The REM content means the total content of the above 17 elements. REM is an element that may be mixed into the steel material during the manufacturing process. However, the lower limit value of the REM content is not limited and may be 0%. Also, REM is an element that forms oxides and improves the toughness of the heat-affected zone of the weld. 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, if the REM content is excessive, it may form coarse oxides and lower the toughness of the steel. Therefore, from the perspective of ensuring 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.

[0043] (Remainder: Fe and impurities) The remainder of the chemical composition of the steel material according to the present disclosure is iron (Fe) and impurities. Impurities mean components that are mixed in due to raw materials such as ore and scrap and other factors during the industrial manufacturing of the steel material.

[0044] In addition to the limitation of the content of each element, in the present disclosure, the range of the α value is limited as follows.

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

[0046] In the present disclosure, the range of the α value is 4.0 to 16.0. This is an index indicating the hardenability of the steel material. The higher the α value, the more favorable the balance between strength and toughness, and the formation of lower bainite and martensite structures can be achieved. When α is within an appropriate range, the ratio of lower bainite and martensite structures with a favorable balance between strength and toughness in the HAZ structure also increases, and HAZ toughness can be ensured. When α is 4.0 or more, the hardenability of the base material is ensured, the ratio of lower bainite and martensite with a favorable balance between strength and toughness increases, and toughness degradation is suppressed. Also, in the HAZ part, the ratio of lower bainite and martensite easily increases, and HAZ toughness also improves. On the other hand, when the α value is 16.0 or less, the steel material strength does not become too high, and toughness can be ensured. Also, when the α value is 16.0 or less, toughness after PWHT can also be ensured. Also, the HAZ does not become too hard, and HAZ toughness can be ensured.

[0047] By satisfying the above numerical range regarding the α value, a nickel-containing steel material for low temperature with excellent strength and toughness can be provided. The α value is preferably 4.5 or more, or 5.0 or more. Also, 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 portion at a thickness of 1 / 4 in the thickness direction from the surface of the steel material according to the present disclosure includes lower bainite and martensite. Further, as bainite, upper bainite may also be included in addition to lower bainite.

[0049] "Bainite" is a structure including bainitic ferrite (α°B) having a lower structure in the grains, and is a general term for upper bainite and lower bainite. "Upper bainite" is one or both of upper bainite including retained austenite or MA phase (martensite-austenite mixed phase) between laths and upper bainite including carbides between laths. "Lower bainite" is lath-like lower bainite including carbides in the laths.

[0050] "Martensite" has four forms: lath, butterfly, lens, and thin plate. In the components of the present disclosure, mainly lath martensite is formed. Lath martensite is composed of packets and blocks formed by a group of laths arranged in a specific array, 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 perspective of ensuring the strength of the steel material, the total area ratio of upper bainite, lower bainite, and martensite in the 1 / 4t part is 90.0% or more. The total area ratio of upper bainite, lower bainite, and martensite in the 1 / 4t part may be 100%. Also, the upper bainite in the 1 / 4t part may be 1.0% or more.

[0053] Observation of the microstructure of the steel material is performed using a sample with the 1 / 4t part of the steel material as the observation surface. Prepare two types of samples that have been subjected to (a) electrolytic polishing and (b) nital etching. Measure three locations for each of the samples (a) and (b) by the following methods respectively, and take the average value of the three locations as the area ratio of the microstructure of the steel material. For each of the samples (a) and (b), three samples may be prepared and the average of each sample may be taken, or measurement may be performed in three fields of view in one sample and the average may be taken.

[0054] After mirror finishing by mechanical polishing, using an electrolytic polishing sample on which electrolytic polishing is performed to remove the strain layer generated by mechanical polishing, the total area ratio of upper bainite, lower bainite, and martensite is measured by EBSD. The measurement magnification is 200 times, and the measurement in the range of 400 μm × 400 μm is performed at a pitch of 0.4 μm. The measurement is performed in a state where the beam diameter of the electron beam is 0.4 μm or less. The reliability index (Confidence Index, hereinafter referred to as "CI value") is set to 0.1 or more. The determination between ferrite and 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, or martensite. In the present disclosure, since upper bainite, lower bainite, and martensite are determined using the GAM of EBSD as a threshold value, not only upper bainite, lower bainite, and martensite but also tempered upper bainite, tempered lower bainite, and tempered martensite are included. Comparing the microstructure of direct quenching (DQ) followed by tempering (T) (DQT), after tempering, decomposition of MA and coarsening of carbides occur, but the appearance of the microstructure does not change significantly.

[0055] Using a nitriding etching sample, the area ratio of upper bainite is measured by SEM observation. The measurement magnification is 500 times, and the measurement is carried out in the range of 360 μm × 480 μm. The part with a clear lath structure and carbide or MA generated along the lath boundary is upper bainite. The structure with a relatively coarse internal structure and a sparse and uneven density of carbides in the region is regarded as upper bainite. An example of the tissue discrimination result is shown in Fig. 1. (A) and (B) are SEM images of the same region of a steel material manufactured by DQT with an α value of 9.9. The region surrounded by the white line in (B) is upper bainite (Bu), and the other regions are lower bainite + martensite (BL + M). The part judged as upper bainite (Bu) is a region where white carbides are sparse and uneven. On the other hand, in the part judged as lower bainite + martensite (BL + M), carbides are densely and uniformly present. By subtracting the area ratio of upper bainite from the total area ratio of upper bainite, lower bainite, and martensite measured above, the total area ratio of lower bainite and martensite is obtained. In addition, in the microstructure of this case, there may be residual austenite depending on the manufacturing conditions. However, since it is an extremely small amount, it is not included in the area ratio.

[0056] (Average crystal grain size of 1 / 4t part of steel material) In the present disclosure, the average crystal grain size (effective crystal grain size) of the 1 / 4t part of the steel material is preferably 20.0 μm or less. This is because it has been found that when the average crystal grain size of the 1 / 4t part of the steel material is 20.0 μm or less, the toughness of the steel material tends to be further improved regardless of before and after PWHT. However, the average crystal grain size of the 1 / 4t part of the steel material may also be more than 20.0 μm. Since the smaller the average crystal grain size of the steel material, the more preferable it is, the lower limit value is not limited. Usually, the average crystal grain size is 10 μm or more. The effective crystal grain size is obtained by weighted average. The effective crystal grain size D obtained by weighted average area is 4 mm 2 among the crystal grain sizes measured in the region of, the area S of the i-th crystal grain detected during measurement i , grain size d iIt is calculated by the following formula using D area = ΣSi·d i / ΣS i

[0057] (Aspect ratio of prior austenite crystal grains in the 1 / 4t part of the steel material) The morphology of the prior austenite crystal grains (which may also be referred to as prior austenite grains or prior γ grains) of the steel material of the present disclosure may be a shape flattened in the rolling direction. If the prior austenite grains in the portion of 1 / 4 of the thickness from the surface of the steel material in the thickness direction are flattened grains with an aspect ratio of 1.5 or more, further improvement in the toughness of the steel material becomes possible. This is because by flattening the prior austenite grains, the grain boundary area is increased, resulting in substantial refinement of the austenite grains, which is effective in refining the average crystal grain size. The aspect ratio of the prior austenite grains is usually 4.0 or less, and may be 3.5 or less.

[0058] On the other hand, from the viewpoint of ensuring the homogeneity of the microstructure, the aspect ratio of the prior austenite crystal grains in the 1 / 4t part may be less than 1.5. The aspect ratio of the prior austenite crystal grains in the 1 / 4t part may be 1.4 or less, or 1.3 or less.

[0059] The aspect ratio of the prior austenite crystal grains (which may be referred to as prior austenite grains) of the steel material is determined as follows. First, the L cross-section (a cross-section parallel to the rolling direction and the thickness direction of the steel material) of the portion of 1 / 4 of the thickness from the surface of the steel material in the thickness direction is mirror-polished, and corrosion is performed with a corrosion solution based on a saturated aqueous solution of 2-4% picric acid to expose 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 as the major axis ÷ minor axis. The arithmetic mean of the aspect ratios of all the calculated prior austenite grains is determined as the "aspect ratio of the prior austenite grains". Note that the maximum length of the prior austenite grain is taken as the major axis, and the maximum distance between two lines parallel to the major axis direction in contact with the grain is taken as the minor axis.

[0060] <Mechanical properties> The steel material according to the present disclosure has mechanical properties that achieve both strength and low-temperature toughness. In particular, it has excellent toughness at -100°C and can exhibit excellent low-temperature toughness even after PWHT.

[0061] (Tensile strength: 590 MPa or more and 930 MPa or less) In the present disclosure, the tensile strength of the steel material is set to 590 to 930 MPa. In order to reduce the weight of large welded structures such as transportation tanks, a steel material that can ensure the strength of the structure even with a thin thickness is required. Usually, the steel material selected for such applications is a steel material having the above-described tensile strength. Therefore, the steel material according to the present disclosure is also manufactured to have the above-described tensile strength.

[0062] (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. When there is no yield point, the yield strength is determined using the 0.2% proof stress.

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

[0064] (Charpy impact energy absorption at -100°C after PWHT) In a cryogenic tank, for the purpose of preventing rupture, PWHT may be performed on the welded portion after being assembled into a transportation tank. At this time, not only the welded portion but also the base material portion of the steel material (also simply referred to as the base material) not affected by welding is heated. When the base material is at a temperature of 425°C or higher When the heating time in the temperature range becomes longer, the toughness of the base material tends to decrease. For the steel material of the present disclosure, when PWHT is performed on the steel material with a holding temperature of 600 °C, a holding time of 2 hours, and a heating rate and a cooling rate of 55 °C / h in the temperature range of 425 °C or higher, the toughness at the location where the PWHT is performed is preferably such that the Charpy impact absorption energy at -100 °C is 150 J or more. The Charpy impact absorption energy at -100 °C after PWHT may be 100 J or more. The Charpy impact absorption energy at -100 °C after PWHT is also a numerical value measured using a sample taken from a position 1 / 4 of the thickness.

[0065] (Charpy impact absorption energy at -100 °C after thermal cycle) In order to ensure high toughness after a thermal cycle test simulating a welded part at low temperature, for the steel material of the present disclosure, the Charpy impact absorption energy at -100 °C after the thermal cycle is preferably 50 J or more. Since the steel material of the present disclosure has a low-temperature toughness with a Charpy impact absorption energy of 50 J or more at -100 °C after the thermal cycle, 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 -100 °C after the thermal cycle may be 40 J or more. The Charpy impact absorption energy at -100 °C after the thermal cycle is a numerical value measured by taking a Charpy test piece from a sample taken from a position 1 / 4 of the thickness of the steel material as a thermal cycle test piece, applying a thermal history of heating to 1350 °C at 60 °C / s, holding at 1350 °C for 1 s, and then cooling to room temperature at 20 °C / s.

[0066] (Charpy absorption energy at -100 °C after thermal cycle and PWHT) In the case of a cryogenic tank, for the purpose of preventing rupture, PWHT may be performed on the welded joints after being assembled into a transportation tank. For the steel material of the present disclosure, after the above heat cycle test, the heating rate and the cooling rate are 55 °C / h in the temperature range of 425 °C or higher, and PWHT is performed by holding at 600 °C for 2 hours, and then a Charpy test piece is taken and measured. In that case, the toughness at the location where the PWHT is performed is preferably such that the Charpy impact absorption energy at -100 °C is 50 J or more. The Charpy impact absorption energy at -100 °C at the location where the PWHT is performed after the heat cycle test may be 40 J or more.

[0067] Note that the toughness of the steel material may decrease due to PWHT. Although the cause is not clear, it is presumed that P (phosphorus) and Mn diffuse to the grain boundaries, and the growth or aggregation of inclusions occurs in the structure, resulting in a decrease in brittleness and a decrease in toughness. The decrease in toughness due to PWHT is suppressed by limiting the contents of P and Mn and reducing the average crystal grain size of the steel material.

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

[0069] The shape of the steel material according to the present disclosure is not particularly limited and may be a steel plate, a steel strip, a section steel, a steel pipe, etc. However, steel pipes and section steels include steel materials joined by steel plates, for example, welded steel pipes and welded section steels, as well as section steels joined by rivets. The thickness of the steel material such as a steel plate, a steel strip, a section steel, or a steel pipe (the thickness of the flange in the case of a section steel) is not particularly limited and is usually 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. Also, the thickness of the steel material may be 100 mm or less, 80 mm or less, or 60 mm or less.

[0070] In addition, the use of the steel material according to the present disclosure is not particularly limited, but since it has mechanical properties that achieve both strength and low-temperature toughness and can exhibit excellent low-temperature toughness particularly after PWHT, it can be suitably used as a tank for storing and transporting liquefied gas, particularly liquid carbon dioxide.

[0071] (Manufacturing method of steel material) The manufacturing method of the steel material according to the present disclosure is not particularly limited. However, for the steel material according to the present disclosure, for example, after melting steel that satisfies the above-described chemical composition, a steel slab is manufactured by continuous casting. The steel slab is heated, and after hot rolling, direct quenching (DQ) in which it is directly water-cooled as it is, or reheat quenching (RQ) in which it is air-cooled after hot rolling and then reheated and water-cooled is performed to obtain a steel material. In the case of RQ, it is not necessarily air-cooled before reheating, and it may be water-cooled. Further, intermediate heat treatment (L) and tempering (T) may be performed. The manufacturing process after hot rolling is selected from the combinations of the above DQ, RQ, L, and T, and for example, DQT, RQT, DQLT, and RQLT.

[0072] (1) DQT: Direct quenching (DQ), tempering (T) (2) RQT: Air-cooling or water-cooling, reheat quenching (RQ), tempering (T) (3) DQLT: Direct quenching (DQ), intermediate heat treatment (L), tempering (T) (4) RQLT: Air-cooling or water-cooling, reheat quenching (RQ), intermediate heat treatment (L), tempering (T)

[0073] (1) DQT From the perspective of manufacturing cost, DQT is preferred in the manufacture of steel materials according to the present disclosure. Examples of preferred manufacturing processes are shown below.

[0074] When manufacturing the steel material according to the present disclosure by DQ, the heating temperature of the steel slab to be hot-rolled is above Ac3 from the viewpoint of performing hot rolling in a temperature range where the metal structure of the material to be rolled is austenite. The heating temperature of the steel 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 the heated γ grains. The heating temperature of the hot rolling is preferably 1200°C or lower. Note that Ac3 is the value calculated by the following formula. 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 mean the content (mass%) of each element contained in the steel slab.

[0075] Hot rolling may be composed of rolling in a temperature range where recrystallization occurs (recrystallization temperature range rolling) and rolling in a temperature range where recrystallization is suppressed (unrecrystallized temperature range rolling). Recrystallization temperature range rolling is hot rolling performed at a temperature of 900°C or higher of the material to be rolled during rolling. The cumulative reduction ratio of the recrystallization temperature range rolling is preferably 20% or more, more preferably 30% or more, from the viewpoint of refining the austenite grain size of the steel material. The cumulative reduction ratio of the recrystallization temperature range rolling is obtained from the difference between the thickness of the steel slab before hot rolling and the thickness of the material to be rolled at 900°C. Cumulative reduction ratio (%) of recrystallization temperature range rolling = 100 × ([thickness of steel slab] - [thickness of material to be rolled at 900°C]) / [thickness of steel slab]

[0076] The rolling in the non-recrystallization temperature range is hot rolling carried out at a temperature of less than 900°C for the material being rolled during rolling. From the perspective of refining the average crystal grain size of the steel material, the cumulative reduction ratio in the non-recrystallization temperature range rolling is preferably 20% or more, more preferably 30% or more. The cumulative reduction ratio in the non-recrystallization temperature range rolling is determined from the difference between the thickness of the material being rolled at 900°C and the thickness of the steel material after rolling. Cumulative reduction ratio (%) in non-recrystallization temperature range rolling = 100×([Thickness of the material being rolled at 900°C] - [Thickness of the steel material after rolling]) / [Thickness of the material being rolled at 900°C]

[0077] From the perspective of suppressing the formation of ferrite that reduces strength, the finishing temperature of hot rolling is Ar3 or higher. After hot rolling, accelerated cooling such as water cooling is applied to the steel material. From the perspective of suppressing the formation of ferrite that reduces strength, the starting temperature of accelerated cooling is Ar3 or higher. Here, Ar3 is taken as the value calculated by the following formula. Ar3 = 910 - 310C - 80Mn - 20Cu - 15Cr - 55Ni - 80Mo + 0.35(t - 8) The element symbols in the formula mean the content (mass%) of each element contained in the steel material, and t means the thickness (mm) of the steel material. From the perspective of promoting bainite transformation and martensite transformation, the cooling rate is 1.0°C / s or higher. The cooling rate of accelerated cooling is preferably 5.0°C / s or higher, or 10.0°C / s or higher. Although the faster the cooling rate of accelerated cooling is, the better, from the perspectives of homogenization of the cooling rate, cost, etc., it is preferably 50.0°C / s or lower, or 30.0°C / s or lower. The cooling rate is the value calculated by simulation using heat transfer calculation for the cooling rate at the 1 / 4 position of the thickness.

[0078] From the perspective of improving the strength of the steel material by ensuring upper bainite, lower bainite, and martensite, the stopping temperature of accelerated cooling is 400°C or lower. The stopping temperature of accelerated cooling is preferably 350°C or lower. Accelerated cooling may be carried out until room temperature. From the perspective of dehydrogenation of the steel material, the stopping temperature of accelerated cooling is preferably 100°C or higher.

[0079] After accelerated cooling, the steel material may be subjected to tempering treatment. From the viewpoint of suppressing a decrease 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.

[0080] (2)RQT When the steel material according to the present disclosure is manufactured by RQ, the influence of the heating temperature and the reduction ratio of the material to be hot-rolled during hot rolling on the mechanical properties of the steel material is small. However, when the heating temperature of the material to be hot-rolled is too low, the deformation resistance increases, so the heating temperature of the material to be hot-rolled is preferably 1000°C or higher. In addition, when the reduction ratio is insufficient, initial defects during steel slab production may remain in the center of the thickness, and the quality of the steel material may deteriorate. Therefore, the total reduction ratio of hot rolling (also referred to as the cumulative reduction ratio) is preferably 35% or higher. After hot rolling, it may be directly water-cooled or air-cooled.

[0081] After hot rolling, the steel material is subjected to reheat quenching. The reheat temperature of the steel material is Ac3 or higher in order to perform quenching from an austenite single-phase structure. From the viewpoint of ensuring the homogeneity of the microstructure, the reheat 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 temperature of the reheat temperature is not particularly defined, heating to an excessively high temperature may cause coarsening of austenite grains and lead to a decrease in toughness. Therefore, it is preferably 1000°C or lower, 950°C or lower, or 930°C or lower.

[0082] After reheat quenching, the steel material may be subjected to 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.

Examples

[0083] Hereinafter, examples will be given to specifically describe the steel materials according to the present disclosure. However, the conditions in the following examples are one set of conditions adopted to confirm the feasibility and effects of the present disclosure, and the steel materials according to the present disclosure are not limited to the following examples.

[0084] <Manufacture by direct quenching and tempering> [Manufacture of Steel Material] First, slabs having the chemical compositions shown in Table 1 were cast by the continuous casting method. The balance other than the components shown in Table 1 is Fe and impurities. Also, the blank indicates that no alloying elements were intentionally added in the steelmaking process. The underlines mean that they are outside the scope of the present disclosure.

[0085] [Table 1]

[0086] Next, steel materials were manufactured from these slabs under the manufacturing conditions shown in Table 2. "Temper heat treatment" is the heating temperature in the tempering treatment after quenching.

[0087] [Table 2]

[0088] [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 of the microstructure are as follows. The balance of the microstructure is pearlite, MA phase, and ferrite. Bu: Upper bainite BL: Lower bainite M: Martensite The toughness was measured as the average value (KV2) of the Charpy impact absorption energy at -100°C, holding temperature: 600°C, holding time: 2 hours, and heating and cooling rates in the temperature range of 425°C or higher: 55°C / h, and the average value of the Charpy impact absorption energy at -100°C after PWHT.

[0089]

Table 3

[0090] No.1A to 22A, 101A to 109A are examples of the present invention, and No.23A, 26A, 110A to 114A are comparative examples. In No.23A, since the finish rolling temperature was low and the direct quenching start temperature was lower than Ar3, sufficient strength could not be obtained. In No.26A, since the Mn content was too high, sufficient low-temperature toughness could not be obtained after PWHT. In No.110A and 111A, the α value was less than the lower limit value of the present disclosure, the hardenability was insufficient, and the strength was insufficient. Sufficient low-temperature toughness could not be obtained either. In No.112A and 113A, the α value exceeded the upper limit value of the present disclosure, the hardenability was too high, and the strength was excessive. In No.114A, since the cooling rate of direct quenching was low, the total area ratio of lower bainite and martensite was insufficient, and sufficient low-temperature toughness could not be obtained.

[0091] In contrast to the comparative examples, in all examples of the present invention (No.1A to 22A, 101A to 109A), the chemical composition and microstructure of the steel material are appropriately controlled, the tensile strength is in an appropriate range of 590 MPa or more and 930 MPa or less, and in addition, regardless of before and after PWHT, the Charpy impact absorption energy at -100 °C is high, and particularly good ones have a low-temperature toughness of 150 J or more.

[0092] <Manufacturing by Reheating Quenching and Tempering> [Manufacture of Steel Material] First, a slab having the chemical composition shown in Table 4 was cast by the continuous casting method. The balance other than the components shown in Table 4 is Fe and impurities. Also, the blank indicates that no alloying elements were intentionally added in the steelmaking process. The underlines mean that it is outside the scope of the present disclosure.

[0093]

Table 4

[0094] Next, steel materials were produced from these slabs under the manufacturing conditions shown in Table 5. "Temper heat treatment" is the heating temperature in the tempering treatment after quenching.

[0095]

Table 5

[0096] [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 pearlite, MA phase, and ferrite. Bu: Upper bainite BL: Lower bainite M: Martensite Toughness was measured as the average value of the Charpy impact energy (KV2) at -100°C, holding temperature: 600°C, holding time: 2 hours, heating rate and cooling rate in the temperature range of 425°C or higher: 55 °C / h, and the average value of the Charpy impact energy at -100°C after PWHT was measured respectively.

[0097]

Table 6

[0098] No. 1B to 21B, 101B to 109B are examples of the present invention, and No. 22B to 25B, 110B to 113B are comparative examples. In No. 22B, since α was too small, sufficient hardenability could not be obtained, sufficient strength could not be obtained, and sufficient low-temperature toughness could not be obtained either. In No. 23B, since the reheat temperature was low and lower than Ac3, sufficient strength could not be obtained, and sufficient low-temperature toughness could not be obtained either. No. 24B had too little Ni, so sufficient low-temperature toughness could not be obtained. No. 25B had too much Mn content, so sufficient low-temperature toughness could not be obtained after PWHT. For No. 110B and 111B, the α value was less than the lower limit of the present disclosure, the hardenability was insufficient, and the strength was insufficient. Sufficient low-temperature toughness could not be obtained either. For No. 112B and 113B, the α value exceeded the upper limit of the present disclosure, the hardenability was too high, and the strength was excessive.

[0099] In contrast to the comparative examples, in all the inventive examples (No. 1B to 21B, 101B to 109B), the chemical composition and microstructure of the steel material are appropriately controlled. The tensile strength is in an appropriate range of 590 MPa or more and 930 MPa or less. Moreover, regardless of before and after PWHT, the Charpy impact absorption energy at -100 °C is as high as 125 J or more, and particularly good ones have a low-temperature toughness of 150 J or more.

Industrial Applicability

[0100] The steel material according to the present disclosure can be mainly used for transportation tanks of liquefied carbon dioxide. Moreover, the steel material according to the present disclosure can also be used for other welded structures such as buildings, bridges, ships, line pipes, offshore structures, pressure vessels, and tanks.

[0101] The disclosures of Japanese Patent Application No. 2023-042400 and Japanese Patent Application No. 2023-042401 filed on March 16, 2023 are incorporated herein by reference in their entirety. All documents, patent applications, and technical standards described in this specification are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually stated.

Claims

1. By mass percentage, C: 0.03% or more and 0.20% or less, Si: 0.01% or more and 0.50% or less, Mn: 0.10% or more and 1.65% or less, P: 0.025% or less, S: 0.0250% or less, Ni: 2.65% or more and 4.45% or less, Al: 0.001% or more and 0.100% or less, O: 0.0100% or less, N: 0.0100% or less, Cu: 0 to 1.50%, Cr: 0 to 3.00%, Mo: 0 to 2.00%, B: 0 to 0.0050%, Nb: 0 to 0.050%, Ti: 0 to 0.050%, V: 0 to 0.10%, Mg: 0 to 0.0200%, Ca: 0 to 0.0200%, REM: 0 to 0.0200%, The balance: Fe and impurities and having a chemical composition in which α represented by the following formula (1) is 4.0 or more and 16.0 or less, the tensile strength is 590 MPa or more and 930 MPa or less, the microstructure of the part at a thickness of 1 / 4 in the thickness direction from the surface of the steel material includes lower bainite and martensite, and the total area ratio of the lower bainite and the martensite is 15.0% or more, and the total area ratio of upper bainite, the lower bainite and the martensite is 90.0% or more, a steel material. α = 0.50 × √[C] × (1 + 0.64[Si]) × (1 + 4.10[Mn]) × (1 + 0.27[Cu]) × (1 + 0.52[Ni]) × (1 + 2.33[Cr]) × (1 + 3.14[Mo])... (1) However, [element symbol] in formula (1) represents the content (mass%) of each corresponding element contained in the steel material. When the corresponding element is not included, zero is substituted.

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

3. The Charpy impact energy absorption at -100°C is 150 J or more. The steel material according to Claim 1.

4. When the heating rate and the cooling rate are 55°C / h in the temperature range of 425°C or higher, and a heat treatment of holding at 600°C for 2 hours is performed on the steel material, the Charpy impact energy absorption at -100°C at the location where the heat treatment is performed is 150 J or more. The steel material according to Claim 1.

5. The aspect ratio of the prior austenite crystal grains of the part at a thickness of 1 / 4 in the thickness direction from the surface of the steel material is 1.5 or more. The steel material according to any one of Claims 1 to 4.

6. The steel material according to any one of claims 1 to 4, wherein the aspect ratio of the prior austenite crystal grains at a position 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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