Steel and pressure vessels

Abstract

A type of steel and a pressure vessel comprising the steel, wherein the steel contains Ni of 2.65% to 4.45% by mass, and is composed of C, Si, Mn, P, S, Al, O, N, Cu, Cr, Mo, B, Nb, Ti, V, Mg, Ca, REM, Fe and impurities within a specified composition range, and is composed of α = 0.50 × √[C] × (1 + 0.64[Si]) × (1 + 4.10[Mn]) × (1 + 0.27[Cu]) × (1 + 0.52[Ni]) × (1 + 2.3... The α value represented by 3[Cr])×(1+3.14[Mo]) is 4.0 to 16.0, and the β value represented by β=[Mn]×[P]-[Mo] / 100 is 0.017 or less. The tensile strength is 590MPa to 930MPa. The microstructure of the part located at 1 / 4 of the thickness in the thickness direction from the surface of the steel contains lower bainite and martensite. The total area ratio of lower bainite and martensite is 15.0% or more, and the total area ratio of upper bainite, lower bainite and martensite is 90.0% or more.

Description

Technical Field

[0001] This disclosure relates to steel and pressure vessels. Background Technology

[0002] Steel can be used in welded structures such as buildings, bridges, ships, conduits, marine structures, and pressure vessels (tanks). Steels with excellent strength and low-temperature toughness under stress are effective in applications at low temperatures.

[0003] Cryogenic pressure vessels, such as liquefied gas storage tanks, use cryogenic steel. Cryogenic steels include Al-killed steel, nickel steel, high-Mn steel, and austenitic stainless steel, depending on the operating temperature. For example, nickel steels such as 3.5% Ni steel are used as materials for tanks storing liquefied ethane and liquefied ethylene at operating temperatures around -100°C.

[0004] Like this 3.5% Ni steel, most steels that need to ensure low-temperature toughness, such as those used in pressure vessels for cryogenic applications, contain Ni.

[0005] For example, Patent Document 1 proposes a nickel-containing steel plate for low temperature applications, which has a specific chemical composition containing more than 3.0% and less than 5.0% Ni, an alumina cluster index of less than 0.030, an effective crystal grain size of less than 12.0 μm, an average tensile strength of 540 MPa to 610 MPa, and a Charpy impact absorption energy of more than 150 J at -140°C.

[0006] In addition, Patent Document 2 proposes a nickel-containing steel for low temperature with excellent toughness, which has a specific chemical composition containing 2.7% to 5.0% Ni, a pre-austenite grain size of less than 20 μm during quenching heating, an effective crystal grain size of less than 12 μm after heat treatment, and a tensile strength of 450 MPa to 690 MPa.

[0007] Patent document 3 discloses a high-tensile steel containing 0.5%–3.5% Ni, with a yield strength of 670–870 N / mm², calculated from the contents of C, Si, and P as α and from the contents of C, Si, Mn, Cu, Ni, Cr, and Mo as β, within a specific range. 2 The tensile strength is 780–940 N / mm². 2 The average crystal grain size in the center of the plate is less than 35 μm, and the plate thickness is 25–200 mm.

[0008] Patent document 4 proposes a method for manufacturing a high-tensile steel containing 0.5 to 4.0% Ni with excellent weldability and low-temperature toughness.

[0009] Patent Document 1: Japanese Patent No. 6610352 Patent Document 2: Japanese Patent No. 6984319 Patent Document 3: International Publication No. 2014 / 103629 Patent Document 4: Japanese Patent Application Publication No. 2-254120 Summary of the Invention

[0010] The problem that the invention aims to solve For cryogenic steel used in cryogenic pressure vessels, a balance between high strength and ensured cryogenic toughness is desired. Furthermore, cryogenic pressure vessels are manufactured by welding steel, and post-weld heat treatment (sometimes called PWHT) is sometimes performed to remove residual stresses generated during welding. Recently, the requirements for the cryogenic toughness of steel after PWHT have been further increased.

[0011] The subject of this disclosure is to provide steel and pressure vessels suitable for low-temperature applications that have high tensile strength and good low-temperature toughness both before and after post-weld heat treatment.

[0012] Methods for solving problems The main points of this disclosure are as follows.

[0013] <1> A type of steel having the following chemical composition, expressed as a percentage by mass: C: 0.03%~0.20% Si: 0.01%~0.50% Mn: 0.10%~1.65%, P: below 0.025% S: Below 0.0250% Ni: 2.65%~4.45% Al: 0.001%~0.100% O: Below 0.0100% N: below 0.0100% 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%, Remaining components: Fe and impurities. Furthermore, α, as expressed by equation (1) below, is 4.0 to 16.0, and β, as expressed by equation (2) below, is 0.017 or less. The tensile strength of the steel is 590MPa to 930MPa. The microstructure of the portion of the steel material located at 1 / 4 of its thickness along the thickness direction from the surface comprises lower bainite and martensite, wherein the combined area ratio of the lower bainite and the martensite is 15.0% or more, and the combined area ratio of the upper bainite, the lower bainite, and the martensite is 90.0% or more.

[0014] 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) β=[Mn]×[P]-[Mo] / 100 (2) In equations (1) and (2), [element symbol] represents the content (mass%) of the corresponding element contained in the steel. Zero is used if the corresponding element is not present.

[0015] <2> according to <1> The steel, wherein the chemical composition comprises group A below.

[0016] [Group A] Selected from one or more of the following elements: Cu: 0.01%~1.50%, Cr: 0.01%~3.00% Mo: 0.01%~2.00%, and B: 0.0003%~0.0050%.

[0017] <3> according to <1> or <2> The steel, wherein the chemical composition comprises group B below.

[0018] Group B Selected from one or more of the following elements: Nb: 0.001%~0.050% Ti: 0.001%~0.050%, and V: 0.01%~0.10%.

[0019] <4> according to <1> ~ <3> The steel in any one of the following methods, wherein the chemical composition comprises group C below.

[0020] [Group C] Mg: 0.0003%~0.0200% Ca: 0.0003%~0.0200%, and REM: 0.0003%~0.0200% <5> according to <1> ~ <4> The steel described in any one of the following statements, wherein the average crystal grain size of the microstructure in a portion located at 1 / 4 of the thickness in the thickness direction from the surface of the steel is 20.0 μm or less.

[0021] <6> according to <1> ~ <5> In any one of the steels, the aspect ratio of the original austenite grains at a location 1 / 4 of the thickness in the thickness direction from the surface of the steel is 1.5 or more.

[0022] <7> according to <1> ~ <6> The steel described in any one of the following statements, wherein the Charpy impact absorption energy at -100°C in the portion representing 1 / 4 of the thickness is 150 J or more.

[0023] <8> according to <1> ~ <7> The steel described in any one of the following statements, wherein, after the steel has undergone heat treatment at a temperature range of 425°C or higher with a heating rate and a cooling rate of 55°C / h and a holding time at 600°C for 2 hours, the Charpy impact energy absorbed at -100°C in a portion of 1 / 4 of the thickness of the heat-treated portion is 150 J or more.

[0024] <9> according to <1> ~ <8> The steel described in any one of the following examples, wherein, in the case of subjecting the steel to a thermal cycle simulating welding equivalent to 2 kJ / mm at 1 / 4 of its thickness, and then performing a heat treatment in a temperature range of 425°C or higher with a heating rate and a cooling rate of 55°C / h and holding at 600°C for 2 hours, the P concentration at the original austenite grain boundaries is 3.0% or less in atomic percentage terms, and the grain boundary carbide coverage is 40% or less.

[0025] <10> according to <1> ~ <9> The steel described in any one of the following examples, wherein, after subjecting the steel to a thermal cycle simulating welding equivalent to 2 kJ / mm at 1 / 4 of its thickness, and then subjecting it to heat treatment at a temperature range of 425°C or higher with a heating and cooling rate of 55°C / h and holding at 600°C for 2 hours, the Charpy impact absorption energy at -100°C of the heat-treated portion is 50 J or more.

[0026] <11> A pressure vessel comprising <1> ~ <10> The steel as described in any one of the above.

[0027] Invention Effects According to this disclosure, steel and pressure vessels suitable for low-temperature applications can be provided that exhibit high tensile strength and good low-temperature toughness both before and after post-weld heat treatment. Here, low temperature refers to a temperature range, for example, -10 to -100°C. Attached Figure Description

[0028] Figure 1 This is an example diagram showing the discrimination results of microstructures. Detailed Implementation

[0029] The following is a detailed description of this disclosure.

[0030] Unless otherwise specified, "post-weld heat treatment" in this disclosure refers to post-weld heat treatment in accordance with the provisions of JIS Z 3700:2009 "Methods for Post-weld Heat Treatment".

[0031] In this disclosure, "steel" or "base material" refers to the portion of steel that does not include surface treatment layers such as plating or coating. However, surface treatment layers such as plating or coating may also be formed on the surface of the steel disclosed herein.

[0032] In this disclosure, the numerical range indicated by “~” refers to the range that includes the values ​​recorded before and after “~” as lower and upper limits. However, when the values ​​recorded before and after “~” are marked as “exceeding” or “below”, the numerical range refers to the range that does not include these values ​​as lower or upper limits.

[0033] Regarding the content of elements in a chemical composition, "%" refers to "mass %".

[0034] The term "process" includes not only independent processes, but also processes that cannot be clearly distinguished from other processes, as long as they achieve the intended purpose of the process.

[0035] The following describes a steel material according to one embodiment of the present disclosure. First, the research results and new insights obtained by the inventors of the present disclosure up to the completion of the steel material of the present disclosure will be described in detail.

[0036] The inventors of this disclosure conducted research to improve the strength of steel. The tensile strength of steel is ensured by the composition of its microstructure. The inventors of this disclosure collected samples from a portion of hot-rolled and accelerated-cooled steel at a distance of 1 / 4 of its thickness in the thickness direction (1 / 4t portion, where t is the thickness of the steel), conducted tensile tests, and observed the microstructure. The results showed that for steel with a tensile strength of 590 MPa to 930 MPa, the ferrite area fraction in the 1 / 4t portion of the microstructure was less than 10.0%, and the combined area fraction of upper bainite, lower bainite, and martensite was more than 90.0%. It should be noted that the combined area fraction of upper bainite, lower bainite, and martensite was determined using electron backscatter diffraction (EBSD).

[0037] Furthermore, the inventors of this disclosure conducted research to improve the toughness of steel. The toughness of steel is ensured through the composition of its microstructure. The inventors of this disclosure collected samples from 1 / 4 t of hot-rolled and rapidly cooled steel, conducted Charpy impact tests, and observed the microstructure. The results showed that the combined area fraction of lower bainite and martensite in steel with a Charpy impact absorption energy of 150 J or more at -100°C was 15.0% or more. The combined area fraction of lower bainite and martensite was determined using EBSD.

[0038] Furthermore, the inventors of this disclosure conducted research to ensure the toughness of the steel. The toughness of the steel is ensured by reducing the region enclosed by large-angle grain boundaries with a crystal orientation difference of 15° or more. The inventors of this disclosure collected samples from 1 / 4 t of steel manufactured by controlling the cooling rate and cooling stop temperature after hot rolling, and used EBSD to measure the equivalent circle diameter of the region enclosed by the large-angle grain boundaries. Hereinafter, the equivalent circle diameter of the region enclosed by the large-angle grain boundaries will be referred to as the grain size. The samples were subjected to mechanical grinding and electrolytic grinding to achieve a diameter of 4 mm. 2 In the region, analysis was performed using the EBSD device attached to the FE-SEM (Field Emission Scanning Electron Microscope). The image will be viewed at a resolution of 4 mm. 2 The average grain size (sometimes referred to as the "effective grain size") is calculated by weighting the area of ​​each grain in the measured grain size within the region. It was observed that if the average grain size of 1 / 4 t of the steel is below 20.0 μm, the toughness of the steel tends to further improve, both before and after post-weld heat treatment.

[0039] Furthermore, the inventors of this disclosure also studied steel that underwent direct quenching followed by water cooling after hot rolling and then further intermediate heat treatment, and obtained the same results as those for steel that had undergone reheat quenching.

[0040] In addition, it was learned that when samples were collected from the steel at a distance of 1 / 4t of the thickness in the thickness direction from the surface of the steel, and a welding thermal cycle equivalent to 2kJ / mm was applied, the Charpy impact absorption energy at -100°C was above 50J before and after post-weld heat treatment. After the thermal cycle and post-weld heat treatment, the P concentration at the original austenite grain boundaries was below 3.0% in atomic percentage and the grain boundary carbide coverage was below 40%.

[0041] <Chemical Composition> Next, the alloying elements constituting the chemical composition of the steel disclosed herein will be described. It should be noted that in the following description of alloying elements, the percentage of content refers to "mass %".

[0042] (C: 0.03%~0.20%) Carbon (C) is an element that increases the strength of steel. From the viewpoint of ensuring the strength of the steel used in structures, the C content in this disclosure is 0.03% or more. The C content is preferably 0.05%, 0.06%, or 0.07% or more. On the other hand, C is an element that reduces toughness. From the viewpoint of ensuring the toughness of the weld heat-affected zone (HAZ), the C content in this disclosure is 0.20% or less. The C content is preferably 0.16%, 0.14%, or 0.12% or less.

[0043] (Si: 0.01%~0.50%) Si is used as a deoxidizer and is an element that increases strength by being dissolved in steel. From the viewpoint of controlling the O concentration in the molten steel, the Si content in this disclosure is 0.01% or more. The Si content is preferably 0.03% or more, 0.05% or more, or 0.10% or more. On the other hand, if the Si content is excessive, a hard phase may form in the HAZ, reducing toughness. In addition, when PWHT is performed on the heat-affected zone after welding, the grain boundary coverage of carbides precipitated at the original austenite grain boundaries may increase, reducing the toughness after PWHT. Therefore, from the viewpoint of ensuring the toughness of the HAZ before and after PWHT, the Si content in this disclosure is 0.50% or less. The Si content is preferably 0.30% or less or 0.20% or less.

[0044] (Mn: 0.10%~1.65%) Mn is used as a deoxidizer and is an element that improves the hardenability of steel, thus contributing to high strength. From the viewpoint of controlling the O concentration in the molten steel, the Mn content in this disclosure is 0.10% or more. Furthermore, with 0.10% or more Mn, MnS is formed, thereby reducing the solid solution S and preventing hot cracking. From the viewpoint of ensuring the strength and toughness of the steel in the hardened zone (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 segregation at grain boundaries during PWHT may lead to a decrease in toughness after PWHT. Therefore, from the viewpoint of ensuring the toughness of the steel after PWHT, the Mn content in this disclosure is 1.65% or less. The Mn content is preferably 1.50% or less, 1.25% or less, or 1.10% or less.

[0045] (P: below 0.025%) P is an impurity element. There is no lower limit to the P content, but from a manufacturing cost perspective, the P content in this disclosure can be 0.001% or more. On the other hand, if the P content is excessive, P segregation at grain boundaries during PWHT may lead to a decrease in toughness after PWHT. Therefore, in this 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.

[0046] (S: below 0.0250%) S is an impurity element. There is no lower limit to the S content, but from a manufacturing cost perspective, the S content in this disclosure can be 0.0001% or more. On the other hand, if the S content is excessive, tensile MnS may be generated in the central segregation region, deteriorating the toughness and ductility of the steel and the HAZ. From the viewpoint of ensuring the toughness and ductility of the steel 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.

[0047] (Ni: 2.65%~4.45%) Ni is an effective element for improving the hardenability and toughness of steel; therefore, in this disclosure, the Ni content is 2.65% or more. The Ni content is preferably set to 3.00% or more or 3.20% or more. However, Ni is an expensive element, and from a cost-reduction perspective, in this disclosure, the Ni content is 4.45% or less. The Ni content is preferably 4.10% or less or 3.80% or less.

[0048] (Al: 0.001%~0.100%) Al is a useful element for deoxidation and also contributes to grain refinement during quenching by forming nitrides. Therefore, in this disclosure, the Al content is 0.001% or more. However, if Al is present in excess, it may form coarse nitrides, reducing the toughness of the steel and the HAZ (hardening zone). Therefore, the Al content is 0.100% or less. The Al content is preferably 0.080% or less, 0.070% or less, 0.060% or less, or 0.050% or less.

[0049] (O: below 0.0100%) O is an impurity element. There is no lower limit to the O content, but from a manufacturing cost perspective, the O content in this disclosure can be 0.0001% or more. On the other hand, if the O content is excessive, coarse oxides may form, deteriorating the toughness and ductility of the steel and the HAZ. From the viewpoint of ensuring the toughness and ductility of the steel and the HAZ, the O content is 0.0100% or less. The O content is preferably 0.0060% or less, 0.0050% or less, or 0.0040% or less.

[0050] (N: below 0.0100%) Nitrogen (N) is an impurity element. There is no lower limit to the N content, but from a manufacturing cost perspective, the N content in this disclosure can be 0.0001% or more. From the viewpoint of ensuring the properties of the steel and the toughness of the HAZ (Hyperal Zone), the N content in this disclosure is 0.0100% or less. The N content is preferably 0.0050% or less or 0.0040% or less.

[0051] The steel disclosed herein may also contain other elements (optional elements) to replace a portion of the Fe. For example, optional elements from groups A to C below can be listed, but the content of these elements may also be 0%.

[0052] [Group A] For the steel disclosed herein, in order to improve strength and toughness, it may also contain one or more of the following optional elements, Cu, Cr, Mo, and B, which have the effect of improving hardenability.

[0053] (Cu: below 1.50%) Cu is an element that can potentially be incorporated into steel during the manufacturing process. However, there is no lower limit to the Cu content; it can be 0%. Furthermore, Cu has minimal adverse effects on weldability and HAZ toughness, and it improves the hardenability of steel, thus also contributing to increased steel strength. Therefore, in this 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 formation of Cu cracks during hot rolling of steel, in this 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.

[0054] (Cr: less than 3.00%) Cr is an element that can potentially be incorporated into steel during the manufacturing process. However, there is no lower limit to the Cr content; it can be 0%. Furthermore, Cr improves the hardenability of steel, and therefore is also an element that increases the strength of steel. Therefore, in this 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 and weldability of the HAZ (hardness zone), in this 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.

[0055] (Mo: 2.00% or less) Mo is an element that can potentially be incorporated into steel during the manufacturing process. However, there is no lower limit to the Mo content, and it can be 0%. Furthermore, Mo also has the effect of improving the hardenability of steel, thus increasing its strength; additionally, it causes grain boundary segregation at the grain boundaries of steel, thereby increasing grain boundary strength. Therefore, in this 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 and weldability of the HAZ (hardness zone) and suppressing the increase in alloy cost, in this disclosure, the Mo content is 2.00% or less. The Mo content is preferably 1.20% or less or 0.80% or less.

[0056] (B: below 0.0050%) Botanicals (B) are elements that can potentially be introduced into steel during the manufacturing process. However, there is no lower limit to the B content; it can be 0%. Furthermore, B is an element that significantly improves the hardenability and strength of steel. Therefore, in this disclosure, the B content can be 0.0003% or more. However, from the viewpoint of suppressing the deterioration of the surface quality of steel billets manufactured by continuous casting, in this disclosure, the B content is 0.0050% or less. The B content is preferably 0.0030% or less or 0.0020% or less.

[0057] Group B For the steel disclosed herein, in order to improve strength, it may also contain one or more of the following optional elements Nb, Ti, and V, which have the effect of improving the strength of the steel through precipitates such as carbides and nitrides.

[0058] (Nb: below 0.050%) Nitrogen (Nb) is an element that can potentially be incorporated into steel during the manufacturing process. However, there is no lower limit to the Nb content; it can be 0%. Furthermore, Nb forms carbides and nitrides, which refine the metal microstructure and contribute to increased steel strength. Therefore, in this disclosure, the Nb content can be 0.001% or more. However, from the viewpoint of suppressing the deterioration of toughness and weldability in the HAZ (hardening zone), the Nb content is 0.050% or less. The Nb content is preferably 0.040% or less or 0.030% or less. In particular, from the viewpoint of ensuring the toughness of the steel after PWHT (Polymerized Welded Steel), the Nb content can also be 0.004% or less.

[0059] (Ti: below 0.050%) Ti is an element that can potentially be incorporated into steel during the manufacturing process. However, there is no lower limit to the Ti content; it can be 0%. Furthermore, Ti forms carbides and nitrides, which refine the metal microstructure and improve the strength of the steel. Therefore, in this disclosure, the Ti content can be 0.001% or more. However, from the viewpoint of suppressing the deterioration of the toughness and weldability of the HAZ (Hyperthermal Adhesive Zone), the Ti content is 0.050% or less. The preferred Ti content is 0.040%, 0.030%, or 0.020% or less.

[0060] In particular, from the perspective of ensuring the toughness of the steel after PWHT, the Ti content can also be less than 0.004% or less than 0.002%.

[0061] (V: below 0.10%) V is an element that can potentially be incorporated into steel during the manufacturing process. However, there is no lower limit to the V content; it can be 0%. Furthermore, V is also an element that forms carbides and nitrides, increasing the strength of steel. Therefore, in this 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 (hard alloy zone) and suppressing the increase in alloy costs, the V content is 0.10% or less. The V content is preferably 0.08% or less or 0.05% or less.

[0062] [Group C] For the steel disclosed herein, in order to improve the toughness of HAZ, it may also contain one or more of the optional elements Mg, Ca, REM shown below, as needed.

[0063] (Mg: below 0.0200%) Mg is an element that can potentially be incorporated into steel during the manufacturing process. However, there is no lower limit to the Mg content; it can be 0%. Furthermore, Mg is also an element that forms oxides and improves the toughness of the weld heat-affected zone. Therefore, in this 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, coarse oxides may form, reducing the toughness of the steel. Therefore, from the viewpoint of ensuring toughness, in this disclosure, the Mg content is 0.0200% or less. Preferably, the Mg content is 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[0064] (Ca: below 0.0200%) Ca is an element that can potentially be incorporated into steel during the manufacturing process. However, there is no lower limit to the Ca content; it can be 0%. Furthermore, Ca is also an element that mitigates the effects of MnS, which reduces the toughness of steel and the weld heat-affected zone, by causing spheroidization of sulfides in the steel. Therefore, in this 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, coarse oxides may form, reducing the toughness of the steel. Therefore, from the viewpoint of ensuring toughness, in this 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.

[0065] (REM: below 0.0200%) Rare earth metals (REM) refer to a total of 17 elements, including Sc and Y, and the lanthanide elements such as La, Ce, and Nd. REM content refers to the total content of these 17 elements. REM is an element that can potentially be mixed into steel during the manufacturing process. However, there is no lower limit to the REM content; it can be 0%. Furthermore, REM is also an element that forms oxides and improves the toughness of the weld heat-affected zone. Therefore, in this 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, coarse oxides may form, reducing the toughness of the steel. Therefore, from the viewpoint of ensuring toughness, in this 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.

[0066] (Remaining portion: Fe and impurities) The remaining part of the chemical composition of the steel disclosed herein is iron (Fe) and impurities. Impurities refer to components that are mixed in during the industrial manufacturing of steel due to raw materials such as ores and waste, as well as other factors.

[0067] In addition to limiting the content of each element, this disclosure also limits the ranges of α and β values ​​as follows.

[0068] (α value: 4.0~16.0) The value of α is calculated using the following equation (1).

[0069] α=0.50×√[C]×(1+0.64[Si])×(1+4.10[Mn])×(1+0.27[Cu])×(1+0.52[Ni])×(1+2.33[Cr])×(1+3.14[Mo]) (1) Where [C], [Si], [Mn], [Cu], [Ni], [Cr], and [Mo] represent the content (mass%) of C, Si, Mn, Cu, Ni, Cr, and Mo in the steel. Substitute zero if the corresponding element is not present. It should be noted that √[C] and [C] are different. 1 / 2 They have the same meaning.

[0070] In this disclosure, the α value is set in the range of 4.0 to 16.0. This is an indicator of the hardenability of steel; the higher the α value, the more easily a lower bainite and martensite structure with an excellent balance of strength and toughness can be formed. When α is within an appropriate range, the ratio of lower bainite and martensite structure with an excellent balance of strength and toughness in the HAZ structure also increases, ensuring HAZ toughness. When α is 4.0 or higher, the ratio of lower bainite and martensite, which is beneficial to the hardenability of the base material and the balance of strength and toughness, increases, suppressing the deterioration of toughness. In addition, the HAZ structure is also more likely to increase the ratio of lower bainite and martensite, thus improving HAZ toughness. On the other hand, if the α value is set below 16.0, the strength of the steel will not become excessively high, ensuring toughness, and also ensuring toughness after PWHT. In addition, the HAZ will not become excessively hardened, ensuring HAZ toughness as well.

[0071] By satisfying the above-mentioned numerical range regarding the α value, nickel-containing steel for low-temperature applications with excellent strength and toughness can be provided. The α value is preferably 4.5 or higher, 5.0 or higher, 5.2 or higher, or 5.4 or higher. Furthermore, the α value is preferably 15.5 or lower, or 15.0 or lower.

[0072] (β value: below 0.017) The value of β is calculated using the following equation (2).

[0073] β=[Mn]×[P]-[Mo] / 100 (2) Where [Mn], [P], and [Mo] represent the content (mass%) of Mn, P, and Mo in the steel. Substitute zero if the corresponding element is not present.

[0074] In this disclosure, the β value is set to be 0.017 or less. The β value is an indicator of the toughness of steel after undergoing a thermal cycle simulating a weld equivalent to 2 kJ / mm² and subsequently performing PWHT. The higher the β value, the worse the toughness after undergoing a thermal cycle simulating a weld equivalent to 2 kJ / mm² and subsequently performing PWHT. When the β value is 0.017 or less, intergranular fracture can be suppressed in the Charpy test after thermal cycling and PWHT, thus ensuring toughness.

[0075] By satisfying the above-mentioned numerical range regarding the β value, it is possible to provide low-temperature nickel-containing steel with excellent toughness after thermal cycling and PWHT. The β value is preferably 0.016 or less or 0.015 or less.

[0076] <Microstructure> Next, the microstructure of the steel disclosed herein will be described. The microstructure of the steel disclosed herein, at a distance of 1 / 4 of its thickness from the surface in the thickness direction, comprises lower bainite and martensite. In addition, as bainite, upper bainite may also be included in addition to lower bainite.

[0077] "Bainite" is a general term for upper bainite and lower bainite. "Upper bainite" is one or both of the following: upper bainite containing retained austenite or MA phase (martensite / austenite mixed phase) between laths, and upper bainite containing carbides between laths. "Lower bainite" is lath-shaped lower bainite containing carbides within the laths.

[0078] Martensite exists in four forms: lath, butterfly, lenticular, and thin plate. However, lath martensite is the main component in this disclosure. Lath martensite is composed of lath bundles and lath blocks formed by groups of laths arranged in a specific manner. It is an austenite grain divided into several lath bundles.

[0079] (The combined area ratio of lower bainite and martensite is 15.0% or more.) Lower bainite and martensite are hard phases and improve the toughness of the steel. From the viewpoint of ensuring the toughness of the steel, the area ratio of lower bainite and martensite in the 1 / 4 t section is 15.0% or more. The area ratio of lower bainite and martensite in the 1 / 4 t section is preferably 20.0% or more or 30.0% or more. The total area ratio of lower bainite and martensite in the 1 / 4 t section can also be 100%.

[0080] (The total area ratio of upper bainite, lower bainite, and martensite: ≥90.0%) From the perspective of ensuring steel strength, the total area ratio of upper bainite, lower bainite, and martensite in the 1 / 4 t section should be 90.0% or more. Alternatively, the total area ratio of upper bainite, lower bainite, and martensite in the 1 / 4 t section can also be 100%. Furthermore, the upper bainite content in the 1 / 4 t section can also be 1.0% or more.

[0081] The microstructure of the steel was observed using a sample with 1 / 4 of its surface as the observation plane. Two types of samples were prepared, one subjected to electrolytic grinding and the other to nitric acid-ethanol etching. For each sample (a) and (b), measurements were taken at three locations using the method described below, and the average of the three locations was taken as the area ratio of the steel's microstructure. It should be noted that for each sample (a) and (b), three samples can be prepared separately and the average of each sample can be taken, or measurements can be taken at three locations within a single sample and the average can be taken.

[0082] After mirror finishing via mechanical grinding, electrolytic grinding was performed. Using this electrolytically ground sample, the total area ratio of upper bainite, lower bainite, and martensite was determined using EBSD. The measurement magnification was 200x, and measurements were taken over a 400μm × 400μm area at 0.4μm intervals. Measurements were performed with an electron beam diameter of less than 0.4μm. The Confidence Index (CI value) was set to 0.1 or higher. Ferrite was distinguished from upper bainite, lower bainite, and martensite by setting the Grain Average Misorientation (GAM) threshold to 0.5. It should be noted that the GAM value is an index defined in OIM-Analysis (EBSD crystal orientation analysis software manufactured by TSL Corporation, USA). Regions with a GAM value below 0.5 are ferrite, and regions with a GAM value above 0.5 are upper bainite, lower bainite, or martensite. The upper bainite, lower bainite, and martensite in this disclosure are determined using GAM of EBSD as the threshold. Therefore, they include not only upper bainite, lower bainite, and martensite, but also tempered upper bainite, tempered lower bainite, and tempered martensite. If the microstructure of direct quenching (DQ) is compared with that of tempering (DQT), tempering causes decomposition of MA and coarsening of carbides, but the appearance of the microstructure does not change significantly.

[0083] The samples were etched using nitric acid and ethanol, and the area ratio of upper bainite was determined by SEM observation. The measurement was performed at 500x magnification over a range of 360 μm × 480 μm. Upper bainite was defined as the region with a clear lath structure and carbides and metamerism (MA) along the lath boundaries. Regions with a coarser internal structure and a sparser, mixed-density carbide concentration were also defined as upper bainite. Figure 1 Examples of microstructure identification results are shown below. (A) and (B) are SEM images of the same area of ​​steel manufactured by DQT with an α value of 9.9. In (B), the area enclosed by the white line is upper bainite (Bu), and the other areas are lower bainite + martensite (BL+M). Regarding the portion identified as upper bainite (Bu), the whitish carbides appear sparse and mixed, with areas of varying density. On the other hand, regarding the portion identified as lower bainite + martensite (BL+M), the carbides are dense and uniform. The total area ratio of lower bainite and martensite is obtained by subtracting the area ratio of upper bainite from the total area ratio of upper bainite, lower bainite, and martensite measured above.

[0084] It should be noted that when the combined area ratio of lower bainite and martensite exceeds 0% using the above-mentioned tissue differentiation method, it usually includes both lower bainite and martensite. Lower bainite and martensite can be distinguished using SEM or TEM (transmission electron microscopy), confirming the presence of each tissue.

[0085] (Average grain size of 1 / 4 t of steel) In this disclosure, the average grain size (effective grain size) of the 1 / 4 t portion of the steel is preferably 20.0 μm or less. This is because it has been observed that if the average grain size of the 1 / 4 t portion of the steel is 20.0 μm or less, the toughness of the steel tends to be further improved both before and after PWHT treatment. However, the average grain size of the 1 / 4 t portion of the steel can also exceed 20.0 μm. A smaller average grain size is preferred, therefore, its lower limit is not limited. Typically, the average grain size is 10 μm or more. The effective grain size is obtained by weighted averaging. The effective grain size D obtained by weighted averaging is... area Using a magnification of 500x for 4mm 2 The area S of the i-th grain detected during the measurement of the crystal grain size is obtained by measuring the region. i Particle size d i The following formula can be used to calculate it.

[0086] D area =ΣSi·d i / ΣS i (The aspect ratio of the original austenite grains in 1 / 4 t of the steel) The morphology of the original austenite grains (sometimes referred to as original austenite grains or original γ grains) in the steel disclosed herein can be a flattened shape in the rolling direction. If the original austenite grains located at a position 1 / 4 of the thickness in the thickness direction from the surface of the steel are set as flattened grains with an aspect ratio of 1.5 or more, further improvement in the toughness of the steel becomes possible. This is because: by flattening the original austenite grains to increase the grain boundary area, thereby achieving substantial austenite grain refinement, it becomes effective in refining the average grain size. The aspect ratio of the original austenite grains is typically 4.0 or less, and can be 3.5 or less.

[0087] The length and width of the original austenite grains are determined as follows. First, a section L (parallel to the rolling and thickness directions of the steel) located at 1 / 4 of the thickness in the thickness direction from the surface of the steel is mirror-polished. Then, an etching solution based on a saturated aqueous solution of 2-4% picric acid is used to etch the original austenite grain boundaries in any region of 1.0 mm in the rolling direction and 0.5 mm in the thickness direction.

[0088] Next, observe the 0.5mm area at 500x magnification. 2 In the region, the major and minor axes of each original austenite grain were measured, and the aspect ratio of each original austenite grain was calculated by dividing the major axis by the minor axis. The arithmetic mean of the calculated aspect ratios of all original austenite grains was determined as the "aspect ratio of the original austenite grain". It should be noted that the maximum length of the original austenite grain was taken as the major axis, and the maximum interval between the two lines parallel to the major axis direction that contact the grain was taken as the minor axis.

[0089] <Mechanical Properties> The steel disclosed herein possesses mechanical properties that balance strength and low-temperature toughness. In particular, it exhibits excellent toughness not only at -100°C but also maintains excellent low-temperature toughness after PWHT treatment.

[0090] (Tensile strength: 590MPa~930MPa) In this disclosure, the tensile strength of the steel is set to 590–930 MPa. To reduce the weight of large welded structures such as transport tanks, steel that can ensure structural strength even with thinness is required. Typically, steel with the aforementioned tensile strength is selected for use in such applications; therefore, the steel disclosed in this disclosure is also manufactured with the aforementioned tensile strength.

[0091] (Yield ratio) The yield ratio (YR (%) = yield strength / tensile strength × 100) of the steel disclosed herein is not particularly limited, but is preferably 90% or less. In the absence of a yield point, the yield strength is determined using 0.2% of the yield strength.

[0092] (Charpy shock energy at -100℃) To ensure high toughness at low temperatures, the steel disclosed herein preferably has a Charpy impact absorption energy of 150 J or more at -100°C. By possessing low-temperature toughness with a Charpy impact absorption energy of 150 J or more at -100°C, the steel of this disclosure enables tanks manufactured using this steel to be suitable, for example, for transporting liquid carbon dioxide. It should be noted that the Charpy impact absorption energy at -100°C is a value measured using a sample taken from a location one-quarter of the thickness.

[0093] (Charpy shock absorption energy at -100℃ after PWHT) In the case of cryogenic tanks, to prevent damage, welded sections are sometimes subjected to PWHT (Pre-Waste Heat Treatment) after assembly into transport tanks. During this process, not only the welded sections are heated, but also the base material of the steel unaffected by the weld (also referred to as the base material). If the base material is heated to a temperature range above 425°C for an extended period, there is a tendency for the toughness of the base material to decrease. In the case of the steel disclosed herein, after PWHT treatment at a holding temperature of 600°C for 2 hours and a heating / cooling rate of 55°C / h in the temperature range above 425°C, the preferred toughness of the PWHT-treated section is a Charpy impact absorption energy of 150 J or more at -100°C. The Charpy impact absorption energy at -100°C after PWHT can also be 100 J or more. The Charpy impact absorption energy at -100°C after PWHT is also set to a value measured using a sample taken from a location one-quarter of the thickness.

[0094] (P concentration at the original austenite grain boundaries after thermal cycling of 1 / 4 t of the steel and after PWHT) The steel disclosed herein preferably exhibits a P concentration of less than 3.0% (atomic percentage concentration) at the proto-austenite grain boundaries after thermal cycling at a location 1 / 4 of the steel's thickness along the thickness direction, following PWHT. For the determination of the P concentration at the proto-austenite grain boundaries, a focused ion beam (FIB) is used to cut around the region containing the proto-austenite grain boundaries. Then, at the tip of a nanoneedle, the region containing the proto-austenite grain boundaries is fixed and collected using Pt deposition at a size of approximately 100 μm × 100 μm, and further thinned to below 100 nm. Subsequently, an aberration-corrected transmission electron microscope (TEM) is used. Under scanning transmission electron microscopy (STEM), with electron beams entering parallel to the proto-austenite grain boundaries, a 20 nm × 20 nm region is scanned at 1.8 million times magnification and 0.35 nm / pixel. Elemental analysis is performed by detecting the characteristic X-rays using energy-dispersive X-ray fluorescence (EDS). The signal is accumulated in a direction parallel to the grain boundary with a width of 2 pixels. The signals of P and Fe are quantified to obtain the line profile of P near the grain boundary. The P concentration obtained by accumulating the signal and quantifying the element directly above the grain boundary is taken as the P grain boundary segregation. The unit is expressed as atomic percentage concentration. Data is obtained for a total of 3 grain boundaries, the average value is calculated and evaluated. The lower the P grain boundary segregation, the better the intergranular fracture is suppressed, and the better the HAZ toughness after thermal cycling and PWHT is ensured. Preferably, it is 2.5% or less in atomic percentage concentration, more preferably 2.0% or less.

[0095] (The portion of the steel surface that is 1 / 4 t away from the steel surface in the thickness direction after thermal cycling and PWHT, which is 1 / 4 of the thickness of the original austenite grain boundary, is the coverage rate of the grain boundary carbide.) The steel disclosed herein preferably exhibits a grain boundary carbide coverage of 40% or less at a portion of the steel's thickness along a thickness of 1 / 4 of its surface after thermal cycling and PWHT. Regarding the grain boundary carbide coverage of the original austenite grain boundaries, the sample after thermal cycling and PWHT at 1 / 4 of its thickness is electrolytically ground, and the images are observed along the original austenite grain boundaries using a scanning electron microscope equipped with a field emission electron gun in double electron images. The magnification is appropriately varied depending on the original austenite grain size, but images are obtained, for example, at 10,000x magnification. By obtaining double electron images of one grain boundary quantity, the overall length of the grain boundary and the length of the region containing carbides are measured. The grain boundary carbide coverage is calculated by dividing the length of the region containing carbides by the overall length of the grain boundary and multiplying by 100. Data from five grain boundary quantities are obtained, the average value is calculated, and an evaluation is performed. The lower the coverage of grain boundary carbides, the better the intergranular fracture can be suppressed, and the better the HAZ toughness after thermal cycling and PWHT can be ensured. Preferably, it is 37% or less, and more preferably 35% or less.

[0096] (Charpy shock energy absorbed at -100°C after thermal cycling) To ensure high toughness after simulating thermal cycling tests of welded parts at low temperatures, the steel disclosed herein preferably has a Charpy impact absorption energy of 50 J or more at -100°C after thermal cycling. By exhibiting low-temperature toughness with a Charpy impact absorption energy of 50 J or more at -100°C after thermal cycling, the steel of this disclosure enables tanks manufactured using this steel to be suitable for, for example, the transport of liquid carbon dioxide. The Charpy impact absorption energy at -100°C after thermal cycling can also be 40 J or more. It should be noted that the Charpy impact absorption energy at -100°C after thermal cycling is measured using a sample taken from one-quarter of the thickness of the steel as a thermal cycling test piece. This sample is subjected to a thermal process of heating at 60°C / s to 1350°C, holding at 1350°C for 1 s, and then cooling to room temperature at 20°C / s. The Charpy test piece is then collected from this sample as the measured value.

[0097] (Charpy shock absorption energy at -100°C after thermal cycling and PWHT) In the case of cryogenic tanks, to prevent damage, welded sections are sometimes subjected to thermal cycling (PWHT) after assembly into transport tanks. The steel disclosed herein, after the aforementioned thermal cycling test, undergoes PWHT at a temperature range of 425°C or higher, with a heating and cooling rate of 55°C / h, and is held at 600°C for 2 hours. Charpy test specimens are then collected for measurement. In this case, the toughness of the PWHT-treated section preferably exhibits a Charpy impact absorption energy of 50 J or more at -100°C. The Charpy impact absorption energy at -100°C of the PWHT-treated section after the aforementioned thermal cycling test can also be 40 J or more.

[0098] It should be noted that PWHT may reduce the toughness of steel. The reason is unclear, but it is speculated that the diffusion of phosphorus (P) and manganese (Mn) to grain boundaries, and the resulting growth or aggregation of inclusions in the microstructure, leads to decreased brittleness and thus reduced toughness. The reduction in toughness caused by PWHT can be suppressed by limiting the content of P and Mn, thereby reducing the average grain size of the steel.

[0099] Tensile strength (TS) and yield strength (YS) in the examples were determined by tensile testing according to JIS Z2241:2011. In the tensile test, JIS 14A test pieces were used, taken from a position at 1 / 4 thickness, with the length direction parallel to the width direction of the steel (the direction perpendicular to both the rolling and thickness directions) (direction C). TS and YS were measured using three test pieces, and calculated by averaging them. Based on the average values ​​of TS and YS, the yield ratio (YR (%)) was calculated by (YS / TS) × 100.

[0100] Charpy impact absorption energy is determined according to JIS Z2242:2018 using an impact blade with a radius of 2 mm at -100°C via a Charpy impact test. The Charpy impact absorption energy is calculated by averaging three test pieces. In the Charpy impact test, a V-notch test piece is used, taken from a point 1 / 4 the thickness of the steel, with its length direction parallel to the width direction of the steel (C direction). The V-notch of the test piece is formed such that the length direction of the notch corresponds to the thickness direction of the steel, and the depth direction of the notch corresponds to the rolling direction of the steel.

[0101] The shape of the steel disclosed herein is not particularly limited, and includes steel plates, steel strips, structural steel, steel pipes, etc. However, steel pipes and structural steel include steel products formed by joining steel plates, such as structural steel products joined by rivets in addition to welded steel pipes and welded structural steel. The thickness of steel products such as steel plates, steel strips, structural steel, and steel pipes (the thickness of the flange for structural steel) is not particularly limited, and is generally 3mm to 150mm. The thickness of the steel products can be 6mm or more, 10mm or more, 15mm or more, or 30mm or more. In addition, the thickness of the steel products can be less than 100mm, less than 80mm, or less than 60mm.

[0102] Furthermore, the steel disclosed herein is not particularly limited in its application, but due to its mechanical properties that balance strength and low-temperature toughness, especially its excellent low-temperature toughness after PWHT, it can be suitable as a constituent material for pressure vessels, specifically for tanks used to store and transport liquefied gases, particularly liquid carbon dioxide.

[0103] (Methods for manufacturing steel) The method for manufacturing the steel disclosed herein is not particularly limited, but the steel disclosed herein is manufactured into billets by continuous casting, for example, after smelting steel that meets the above-described chemical composition. The billets are heated, and after hot rolling, they are directly water-cooled and quenched (DQ), or they are cooled, reheated, and water-cooled and quenched (RQ) to produce steel. The reheating process for RQ does not necessarily require cooling; water cooling may also be performed. Furthermore, intermediate heat treatment (L) and tempering (T) may also be performed. The manufacturing process after hot rolling is selected from combinations of DQ, RQ, L, and T, such as DQT, RQT, DQLT, and RQLT.

[0104] (1) DQT: Direct Quenching (DQ) and Tempering (T) (2) RQT: Cooling, reheating and quenching (RQ), tempering (T) (3) DQLT: Direct Quenching (DQ), Intermediate Heat Treatment (L), Tempering (T) (4) RQLT: Cooling, reheating and quenching (RQ), intermediate heat treatment (L), tempering (T) From the perspective of manufacturing cost, DQT is preferred in the manufacture of the steel disclosed herein, and examples of preferred manufacturing processes are shown below.

[0105] From the viewpoint of hot rolling in a temperature range where the microstructure of the rolled material is austenitic, the heating temperature of the hot-rolled steel billet is Ac3 or higher. From the viewpoint of reducing deformation resistance, the heating temperature of the steel billet is preferably 1000°C or higher. On the other hand, from the viewpoint of suppressing the coarsening of heated γ-grains, the hot-rolling heating temperature is 1250°C or lower. The hot-rolling heating temperature is preferably 1200°C or lower. It should be noted that Ac3 is a value calculated using the following formula.

[0106] 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 steel billet.

[0107] Hot rolling is sometimes composed of rolling in the temperature zone where recrystallization occurs (recrystallization temperature zone rolling) and rolling in the temperature zone where recrystallization is inhibited (non-recrystallization temperature zone rolling).

[0108] Recrystallization temperature zone rolling is hot rolling performed at a temperature of 900°C or higher. From the viewpoint of refining the austenite grain size of the steel, the cumulative reduction rate in recrystallization temperature zone rolling is preferably 20% or more, and more preferably 30% or more. The cumulative reduction rate in recrystallization temperature zone 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.

[0109] Cumulative reduction rate (%) during rolling in the recrystallization temperature range = 100 × ([bill thickness] - [thickness of rolled material at 900℃]) / [bill thickness] Non-recrystallization temperature zone rolling is hot rolling performed at a temperature below 900°C. From the viewpoint of refining the average grain size of the steel, the cumulative reduction rate in non-recrystallization temperature zone rolling is preferably 20% or more, more preferably 30% or more. The cumulative reduction rate in non-recrystallization temperature zone rolling is calculated from the difference between the thickness of the rolled material at 900°C and the thickness of the steel after rolling.

[0110] Cumulative reduction rate (%) during rolling in the non-recrystallization temperature zone = 100 × ([thickness of the rolled material at 900℃] - [thickness of the steel after rolling]) / [thickness of the rolled material at 900℃] From the viewpoint of suppressing the formation of ferrite, which reduces strength, the finishing temperature of hot rolling is Ar3 or higher. After hot rolling, the steel is subjected to accelerated cooling such as water cooling. From the viewpoint of suppressing the formation of ferrite, which reduces strength, the starting temperature of accelerated cooling is Ar3 or higher. It should be noted that Ar3 is a value calculated using the following formula.

[0111] 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, and t refers to the thickness of the steel (mm).

[0112] From the viewpoint of promoting bainitic and martensitic phase transformations, the cooling rate is 1.0 °C / s or higher. For accelerated cooling, a cooling rate of 5.0 °C / s or higher or 10.0 °C / s or higher is preferred. A faster accelerated cooling rate is preferable, but from the viewpoints of homogenization of cooling rate and cost, a rate of 50.0 °C / s or lower or 30.0 °C / s or lower is preferred. The cooling rate is obtained by calculating the cooling rate at 1 / 4 of the thickness using simulations based on heat transfer calculations.

[0113] From the viewpoint of improving steel strength by ensuring the presence of upper bainite, lower bainite, and martensite, the stop temperature for accelerated cooling is 400°C or below. The stop temperature for accelerated cooling is preferably 350°C or below. Accelerated cooling to room temperature is also possible. From the viewpoint of steel dehydrogenation, the stop temperature for accelerated cooling is preferably 100°C or above.

[0114] After accelerated cooling, the steel can also be tempered. From the viewpoint of suppressing strength reduction, the tempering temperature is preferably 650°C or below, 620°C or below, or 590°C or below. On the other hand, from the viewpoint of improving toughness, the tempering temperature is preferably 350°C or above or 400°C or above.

[0115] Example The following examples illustrate the steel materials disclosed herein. However, the conditions in the following examples are merely examples used to confirm the feasibility and effectiveness of this disclosure, and the steel materials disclosed are not limited to the following examples.

[0116] [Steel Manufacturing] First, slabs with the chemical compositions shown in Table 1 are cast using a continuous casting method. The remainder besides the components shown in Table 1 consists of Fe and impurities. Additionally, blank columns indicate that no alloying elements were intentionally added during the steelmaking process.

[0117] The underlined part indicates that it is outside the scope of this disclosure.

[0118] [Table 1] Next, these slabs are used to manufacture steel under the manufacturing conditions shown in Table 2. "Tempering heat treatment" refers to the heating temperature during the tempering process after quenching.

[0119] [Table 2] [Measurement and Evaluation] The microstructure and mechanical properties of the obtained steel were determined using the methods described above. The results are shown in Table 3. The meanings of the symbols for the microstructure are as follows. It should be noted that for microstructures identified as lower bainite + martensite (B... L In the region (+M), the presence of lower bainite and martensite was confirmed by SEM observation. Additionally, the remaining microstructure consisted of pearlite, MA phase, retained γ (austenite), and ferrite.

[0120] Bu: Upper Bainite BL: Lower Bainite M: Martensite Regarding toughness, the test was conducted using a sample taken from 1 / 4 of the thickness, for both cases where the steel (base material) was not heat-treated and cases where the following heat treatments (2) to (4) were performed.

[0121] (1) Base material toughness Average Charpy impact energy at -100℃ (KV2) (2) Toughness of the base material after PWHT The average Charpy shock absorption energy at -100℃ after PWHT, set with a holding temperature of 600℃, a holding time of 2 hours, and a heating and cooling rate of 55℃ / h in the temperature range above 425℃. (3) Thermal cycling toughness The average Charpy impact absorption energy at -100°C after a thermal cycle (heating at 60°C / s to 1350°C, holding at 1350°C for 1s, and then cooling to room temperature at 20°C / s) is calculated. (4) Toughness after thermal cycling PWHT The average Charpy shock absorption energy at -100°C after PWHT was obtained by heating to 1350°C at 60°C / s, holding at 1350°C for 1s, cooling to room temperature at 20°C / s, and setting the holding temperature to 600°C, holding time to 2 hours, and heating and cooling rates of 55°C / h in the temperature range above 425°C. [Table 3] In Table 3, "P grain boundary segregation at the original γ grain boundary after thermal cycling PWHT [at.%]" refers to the P concentration (atomic %) at the original austenite grain boundary after thermal cycling and post-weld heat treatment. "ND" indicates below the detection limit. "Carbide coverage at the original γ grain boundary after thermal cycling PWHT [%]" refers to the carbide coverage at the original austenite grain boundary after thermal cycling and post-weld heat treatment.

[0122] Examples No. 1 to 26 are examples of the present invention, and examples No. 27 to 35 are comparative examples.

[0123] No. 27, due to the Si content exceeding the upper limit, has a grain boundary carbide coverage exceeding 40% after thermal cycling and PWHT, thus failing to achieve sufficient low-temperature toughness.

[0124] No. 28, due to the Mn content exceeding the upper limit, also has an β value exceeding the upper limit, and therefore cannot achieve sufficient low-temperature toughness after thermal cycling and PWHT.

[0125] No. 29 Because the amount of P deviates from the upper limit, the value of β also deviates from the upper limit. After thermal cycling and PWHT, the amount of P grain boundary segregation in the original austenite grain boundaries exceeds 3%, which makes it impossible to obtain sufficient low-temperature toughness.

[0126] Because the β value of No. 30 is outside the upper limit, it cannot achieve sufficient low-temperature toughness after thermal cycling and PWHT.

[0127] The α value of No. 31 is lower than the lower limit of this disclosure, resulting in insufficient hardenability and strength. It also cannot achieve sufficient low-temperature toughness.

[0128] The α value of No. 32 exceeds the upper limit of this disclosure, resulting in excessive hardenability and strength.

[0129] The starting temperature of direct quenching of No. 33 is too low, the total area ratio of upper bainite, lower bainite and martensite is insufficient, and the strength is insufficient.

[0130] The α value of No. 34 exceeds the upper limit of this disclosure, resulting in excessive hardenability and strength. It also fails to achieve sufficient low-temperature toughness.

[0131] No. 35 suffers from insufficient total area ratio of lower bainite and martensite due to the low cooling rate of direct quenching, thus failing to achieve adequate low-temperature toughness.

[0132] In contrast to the comparative examples, all of the present invention examples (No. 1 to 26) not only had their chemical composition and microstructure of the steel appropriately controlled, with tensile strengths ranging from 590 MPa to 930 MPa within an appropriate range, but also exhibited high Charpy impact absorption energy at -100°C, both before and after PWHT. In particular, those with excellent properties achieved low-temperature toughness of 150 J or more. Specifically, for No. 1 to 22, where the average grain size of the 1 / 4 t portion was 20.0 μm or less, low-temperature toughness of 150 J or more was achieved at -100°C, both before and after PWHT. Furthermore, both thermal cycling toughness and post-PWHT thermal cycling toughness achieved low-temperature toughness of 50 J or more at -100°C.

[0133] Industrial availability The steel disclosed herein can be primarily used as transport tanks for liquefied carbon dioxide. Additionally, the steel disclosed herein can also be used in other welded structures such as buildings, bridges, ships, conduits, marine structures, and pressure vessels (tanks).

[0134] The entire disclosure of Japanese Patent Application No. 2023-176324, filed on October 11, 2023, is incorporated herein by reference. All documents, patent applications, and technical standards described in this specification are incorporated herein by reference to the same extent as those specifically described therein.

Claims

1. A type of steel having the following chemical composition, expressed as a percentage by mass: C：0.03%～0.20%、 Si: 0.01%~0.50% Mn: 0.10%~1.65%, P: below 0.025% S: Below 0.0250% Ni: 2.65%~4.45% Al：0.001%～0.100%、 O: Below 0.0100% N: below 0.0100% 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%, Remaining components: Fe and impurities. Furthermore, α, as expressed by equation (1) below, is 4.0 to 16.0, and β, as expressed by equation (2) below, is 0.017 or less. The tensile strength of the steel is 590MPa to 930MPa. The microstructure of the portion of the steel material located at 1 / 4 of its thickness along the thickness direction from the surface comprises lower bainite and martensite, wherein the combined area ratio of the lower bainite and the martensite is 15.0% or more, and the combined area ratio of the upper bainite, the lower bainite, and the 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]) (1) β=[Mn]×[P]-[Mo] / 100 (2) in, In equations (1) and (2), the [element symbol] represents the content of the corresponding element in the steel by mass % and is substituted into zero if the corresponding element is not included.

2. The steel according to claim 1, wherein, The chemical composition includes the following group A. [Group A] Selected from one or more of the following elements: Cu: 0.01%~1.50%, Cr：0.01%～3.00%、 Mo: 0.01%~2.00%, and B：0.0003%～0.0050%。 3. The steel according to claim 1 or claim 2, wherein, The chemical composition includes the following group B. Group B Selected from one or more of the following elements: Nb: 0.001%~0.050% Ti: 0.001%~0.050%, and V：0.01%～0.10%。 4. The steel according to any one of claims 1 to 3, wherein, The chemical composition comprises the following group C. [Group C] Selected from one or more of the following elements: Mg: 0.0003%~0.0200% Ca: 0.0003%~0.0200%, and REM: 0.0003%~0.0200%.

5. The steel according to any one of claims 1 to 4, wherein, The average crystal grain size of the microstructure in a portion of the steel material located at 1 / 4 of its thickness in the thickness direction is less than 20.0 μm.

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

7. The steel according to any one of claims 1 to 6, wherein, The Charpy impact absorption energy at -100°C is above 150 J at 1 / 4 of the thickness.

8. The steel according to any one of claims 1 to 7, wherein, When the steel is subjected to heat treatment at a temperature range of 425°C or higher, with a heating and cooling rate of 55°C / h and a holding time of 600°C for 2 hours, the Charpy impact energy absorbed at -100°C in a portion of 1 / 4 of the thickness of the heat-treated part is 150 J or more.

9. The steel according to any one of claims 1 to 8, wherein, When the steel is subjected to a thermal cycle simulating welding at 2 kJ / mm at 1 / 4 of its thickness, and then subjected to heat treatment at a temperature range above 425°C with a heating and cooling rate of 55°C / h and held at 600°C for 2 hours, the P concentration at the original austenite grain boundaries is less than 3.0% by atomic percentage and the grain boundary carbide coverage is less than 40%.

10. The steel according to any one of claims 1 to 9, wherein, A heat cycle simulating welding equivalent to 2 kJ / mm was applied to a portion of the steel at 1 / 4 of its thickness. Then, heat treatment was performed at a temperature range above 425°C with a heating and cooling rate of 55°C / h and a holding time of 600°C for 2 hours. The Charpy impact absorption energy at -100°C in the heat-treated portion was above 50 J.

11. A pressure vessel comprising the steel according to any one of claims 1 to 10.

Images