Steel material and pressure vessel

Abstract

A steel material having a specific chemical composition in which alpha is 5.0 to 16.0 inclusive and beta is 0.017 or less, a tensile strength of 615 to 930 MPa inclusive, a microstructure in a portion 1 / 4 of the thickness from the surface of the steel material containing lower bainite and martensite, the total area ratio of the lower bainite and martensite being 15.0% or more, and the total area ratio of the lower bainite and martensite being 15.0% or more. The total area ratio of upper bainite, lower bainite, and martensite is 90.0% or more, and the area ratio of retained austenite is less than 1.7%. Alpha = 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]), and beta = [Mn] * [P]-[Mo] / 100.

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, pipelines, 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, depending on the operating temperature, include Al-killed steel, nickel steel, high-Mn steel, and austenitic stainless steel. For example, nickel steels such as 3.5% Ni steel are used as materials for tanks housing liquefied ethane and liquefied ethylene, which operate at 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 low-temperature pressure vessels, 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 3.0% or more but less than 5.0% Ni, an alumina cluster index of 0.030 or less, an effective crystal grain size of 12.0 μm or less, an average tensile strength of 540 MPa or more but less than 610 MPa, and a Charpy impact absorption energy of 150 J or more at -140°C.

[0006] In addition, Patent Document 2 proposes a nickel-containing steel for low temperature applications with excellent toughness, which has a specific chemical composition containing more than 2.7% and less than 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 more than 450 MPa and less than 690 MPa.

[0007] Patent document 3 discloses a method for manufacturing high-strength and high-toughness steel, which includes: heating a steel billet containing 1.0 to 8.0% Ni with a specific chemical composition to between 1000 and 1250°C, then reducing it by 20 to 60% in the temperature range of austenite recrystallization during hot rolling, then reducing it by 30 to 70% in the temperature range where austenite does not recrystallize, completing rolling at or above 650°C, then performing quenching treatment starting at a temperature above the Ar3 point and stopping at a temperature below 150°C, then further reheating it from the Ac3 point to between Ac3 and Ac3+100°C and performing quenching, and then performing tempering treatment at a temperature below the Ac1 point.

[0008] Patent Document 1: Japanese Patent No. 6610352 Patent Document 2: Japanese Patent No. 6984319 Patent Document 3: Japanese Patent Application Publication No. 1-230713 Summary of the Invention

[0009] The problem that the invention aims to solve For cryogenic steel used in cryogenic pressure vessels, it is desirable to balance high strength with guaranteed low-temperature toughness. Furthermore, cryogenic pressure vessels are manufactured by welding the steel, and post-weld heat treatment (sometimes called PWHT) is sometimes performed to remove residual stresses generated during welding. Recently, the requirements for the low-temperature toughness of steel after PWHT have become even more stringent.

[0010] 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, regardless of whether they are obtained before or after post-weld heat treatment.

[0011] Methods for solving problems The main points of this disclosure are as follows.

[0012] <1> A type of steel having the following chemical composition: (in mass%) C: Above 0.03% and below 0.20% Si: ≥0.01% and ≤0.50% Mn: ≥0.10% and ≤2.00% P: below 0.025% S: Below 0.0250% Ni: 4.51% or higher and lower than 6.10% Al: Above 0.001% and below 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 5.0 or more and 16.0 or less, and β, as expressed by equation (2) below, is 0.017 or less. The tensile strength of the aforementioned steel is above 615 MPa and below 930 MPa. 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 total area ratio of the lower bainite and the martensite is 15.0% or more, and the total area ratio of the upper bainite, the lower bainite and the martensite is 90.0% or more, and the area ratio of retained austenite is less than 1.7%.

[0013] 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. If the element is not present, substitute zero.

[0014] <2> according to <1> The steel described herein, wherein the chemical composition comprises group A below, [Group A] Choose one or more elements from the following groups: Cu: ≥0.01% and ≤1.50% Cr: ≥0.01% and ≤3.00% Mo: ≥0.01% and ≤2.00%, and B: Above 0.0003% and below 0.0050%.

[0015] <3> according to <1> or <2> The steel described herein, wherein the chemical composition comprises group B below, Group B Choose one or more elements from the following groups: Nb: ≥0.001% and ≤0.050% Ti: ≥0.001% and ≤0.050%, and V: Above 0.01% and below 0.10%.

[0016] <4> according to <1> ~ <3> The steel described in any one of the above statements, wherein the chemical composition comprises group C below, [Group C] Choose one or more elements from the following groups: Mg: ≥0.0003% and ≤0.0200% Ca: ≥0.0003% and ≤0.0200%, and REM: Above 0.0003% and below 0.0200%.

[0017] <5> according to <1> ~ <4> The steel described in any one of the above statements, wherein the average grain size of the microstructure in the portion located at 1 / 4 of the thickness in the thickness direction from the surface of the steel is 20.0 μm or less.

[0018] <6> according to <1> ~ <5> In any one of the steels, the aspect ratio of the original austenite grains at a position 1 / 4 of the thickness in the thickness direction from the surface of the steel is 1.5 or more.

[0019] <7> according to <1> ~ <6> The steel described in any one of the above-mentioned methods, wherein the Charpy impact absorption energy at -110°C in the portion representing 1 / 4 of the thickness is 150 J or more.

[0020] <8> according to <1> ~ <7> The steel described in any one of the following statements, wherein, after the steel is subjected to 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 -110°C in the portion of the thickness at the location where the heat treatment was performed is 150 J or more.

[0021] <9> according to <1> ~ <8> In any one of the steels, wherein a thermal cycle simulating welding equivalent to 2 kJ / mm is applied to a portion of the aforementioned thickness, and the steel is further 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 at 600°C for 2 hours, the P concentration in the original austenite grain boundaries is 3.0% or less in atomic percentage terms, and the grain boundary carbide coverage is 40% or less.

[0022] <10> according to <1> ~ <9> The steel described in any one of the above-mentioned methods, wherein a heat cycle simulating a welding temperature of 2 kJ / mm is applied to a portion of the thickness of the steel, and the steel is further 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 absorption energy at -110°C at the location where the heat treatment was performed is 50 J or more.

[0023] <11> A pressure vessel comprising <1> ~ <10> The steel as described in any one of the above.

[0024] 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 of, for example, -10 to -110°C. Attached Figure Description

[0025] Figure 1 This is an example of the results of the microscopic tissue analysis. Detailed Implementation

[0026] The following is a detailed description of this disclosure.

[0027] 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".

[0028] In this disclosure, "steel," "base material of 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.

[0029] In this disclosure, the numerical range represented by "~" refers to the range that includes the values ​​recorded before and after "~" as lower and upper limits. When the values ​​before and after "~" are marked "exceeding" or "below", the numerical range refers to the range that does not include these values ​​as lower or upper limits.

[0030] Regarding the content of elements in a chemical composition, "%" refers to "mass %".

[0031] The term "process" not only includes independent processes, but also includes processes that cannot be clearly distinguished from other processes, as long as the desired purpose of the process can be achieved.

[0032] 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 in completing the steel material of the present disclosure will be described in detail.

[0033] 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 rapidly cooled steel, specifically a 1 / 4t portion (t: the thickness of the steel), located 1 / 4 of the thickness in the thickness direction from the surface. Tensile tests were performed, and the microstructure was observed. The results showed that in the microstructure of the 1 / 4t portion of steel with a tensile strength of 615 MPa or higher and 930 MPa or lower, the area fraction of ferrite was less than 10.0%, and the combined area fraction of upper bainite, lower bainite, and martensite was 90.0% or higher. Furthermore, the combined area fraction of upper bainite, lower bainite, and martensite was determined using electron backscatter diffraction (EBSD).

[0034] 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 accelerated-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 -110°C was 15.0% or more, and the area fraction of retained austenite was less than 1.7%. The combined area fraction of lower bainite and martensite was determined using EBSD. The area fraction of retained austenite was determined by X-ray diffraction. The volume fraction of retained austenite determined by X-ray diffraction can be considered as the area fraction.

[0035] Furthermore, the inventors of this disclosure conducted research to ensure the toughness of the steel. Toughness is ensured by reducing the region surrounded 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 measured the equivalent circle diameter of the region surrounded by large-angle grain boundaries using EBSD. Hereinafter, the equivalent circle diameter of the region surrounded by 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 The area was analyzed using the EBSD device attached to a FE-SEM (Field Emission Scanning Electron Microscope). At 4mm... 2 Among the grain sizes measured in the region, the average grain size (sometimes referred to as the "effective grain size") is calculated by weighting the area of ​​each grain. The following observation was obtained: if the average grain size of the steel in 1 / 4 t portion is below 20.0 μm, the steel tends to exhibit further improved toughness regardless of whether it is before or after post-weld heat treatment.

[0036] Furthermore, it is known that when samples are collected from a portion of the steel surface at a distance of 1 / 4t of the thickness in the thickness direction, and a welding thermal cycle equivalent to 2kJ / mm is applied, regardless of whether the steel has a Charpy impact absorption energy of 50J or more at -110℃, after thermal cycling and welding heat treatment, the P concentration in the original austenite grain boundaries is less than 3.0% in terms of atomic percentage concentration, and the grain boundary carbide coverage is less than 40%.

[0037] Furthermore, the inventors of this disclosure have discovered that the same results can be obtained not only for hot-rolled and accelerated-cooled steel, but also for steel that has been reheated and quenched.

[0038] <Chemical Composition> Next, the alloying elements constituting the chemical composition of the steel disclosed herein will be described. Furthermore, in the following description of the alloying elements, the percentage (%) refers to "mass %".

[0039] (C: Above 0.03% and below 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% or more 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% or less, 0.14% or less, or 0.12% or less.

[0040] (Si: ≥0.01% and ≤0.50%) Si is an element used as a deoxidizer and also dissolved in steel to increase its strength. From the viewpoint of controlling the O concentration in 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, 0.10% or more, or 0.12% or more. On the other hand, if the Si content is excessive, a hard phase may sometimes form in the HAZ, reducing toughness. Furthermore, when PWHT is applied to the heat-affected zone after welding, the grain boundary coverage of carbides precipitated at the original austenite grain boundaries increases, sometimes reducing the toughness after PWHT. Therefore, from the viewpoint of ensuring HAZ toughness 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.

[0041] (Mn: ≥0.10% and ≤2.00%) Mn is used as a deoxidizer and also contributes to high strength by improving the hardenability of steel. From the viewpoint of controlling the O concentration in 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 dissolved S and preventing hot cracking. From the viewpoint of ensuring the strength of the steel and the toughness of the HAZ (hardening zone), the Mn content is preferably 0.30% or more or 0.50% or more. On the other hand, if the Mn content is excessive, Mn segregates at grain boundaries during PWHT (purified water hardening), which may sometimes reduce the toughness after PWHT. Therefore, from the viewpoint of ensuring the toughness of the steel after PWHT, the Mn content in this disclosure is 2.00% or less. The Mn content is preferably 1.80% or less or 1.50% or less.

[0042] (P: below 0.025%) P is an impurity element. There is no lower limit to the P content; 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 segregates at grain boundaries during PWHT, which can sometimes reduce the 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.

[0043] (S: below 0.0250%) S is an impurity element. There is no lower limit to the S content; from a manufacturing cost perspective, in this disclosure, the S content can be 0.0001% or more. On the other hand, if the S content is excessive, extended MnS may sometimes form 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.

[0044] (Ni: above 4.51% and below 6.10%) Ni is an effective element for improving the hardenability and toughness of steel; therefore, in this embodiment, the Ni content is 4.51% or more. The Ni content is preferably set to 5.00% or more or 5.25% or more. However, Ni is an expensive element, and from the viewpoint of cost reduction, in this disclosure, the Ni content is less than 6.10%. The Ni content is preferably less than 6.00% or less or less than 5.75%.

[0045] (Al: above 0.001% and below 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 hardness of the HAZ (hardness 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.

[0046] (O: below 0.0100%) O is an impurity element. There is no lower limit to the O content; from a manufacturing cost perspective, in this disclosure, the O content 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 HAZ. From the viewpoint of ensuring the toughness and ductility of the steel and 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.

[0047] (N: below 0.0100%) Nitrogen (N) is an impurity element. There is no lower limit to the N content; however, 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. Preferably, the N content is 0.0050% or less, or 0.0040% or less.

[0048] The steel disclosed herein may also contain other elements (selective elements) to replace a portion of the Fe. For example, selective elements from groups A to C below can be listed, but the content of these elements may also be 0%.

[0049] [Group A] In the steel disclosed herein, in order to improve strength and toughness, one or more of the following selected elements, Cu, Cr, Mo, and B, which have the effect of improving hardenability, may be included as needed.

[0050] (Cu: below 1.50%) Cu is an element that sometimes gets mixed into steel during the manufacturing process. However, there is no lower limit to the Cu content, and it can be 0%. Furthermore, Cu has little adverse effect 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 Cu cracking 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.

[0051] (Cr: less than 3.00%) Cr is an element that sometimes gets mixed into steel during the manufacturing process. However, there is no lower limit to the Cr content, and 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.

[0052] (Mo: 2.00% or less) Mo is an element that sometimes gets mixed into steel during the manufacturing process. However, there is no lower limit to the Mo content, and it can be 0%. Furthermore, Mo improves the hardenability of steel, and therefore also increases the strength of the steel, induces grain boundary segregation at the grain boundaries, and increases 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 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.

[0053] (B: below 0.0050%) Boron (B) is an element that sometimes gets mixed into steel during the manufacturing process. However, there is no lower limit to the B content, and it can be 0%. Furthermore, B is an element that significantly improves the hardenability of steel and increases its strength. 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.

[0054] Group B In the steel disclosed herein, in order to improve strength, one or more of the following selected elements Nb, Ti, and V may be included as needed, which have the effect of improving the strength of the steel through precipitates such as carbides and nitrides.

[0055] (Nb: below 0.050%) Nitrogen (Nb) is an element that sometimes gets mixed into steel during the manufacturing process. However, there is no lower limit to the Nb content, and it can be 0%. Furthermore, Nb is also an element that forms carbides and nitrides, thus refining the metal structure and increasing the strength of the steel. Therefore, in this disclosure, the Nb 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 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 be 0.004% or less.

[0056] (Ti: below 0.050%) Ti is an element that sometimes gets mixed into steel during the manufacturing process. However, there is no lower limit to the Ti content, and it can be 0%. Furthermore, Ti is also an element that forms carbides and nitrides, thus refining the metal structure and increasing 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), the Ti content is 0.050% or less. The Ti content is preferably 0.040%, 0.030%, or 0.020% or less. In particular, from the viewpoint of ensuring the toughness of the steel after PWHT (Polymerized Welded Steel), the Ti content can be 0.004% or less or 0.002% or less.

[0057] (V: below 0.10%) V is an element that sometimes gets mixed into steel during the manufacturing process. However, there is no lower limit to the V content, and it can be 0%. In addition, 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.

[0058] [Group C] In the steel disclosed herein, in order to improve the toughness of HAZ, it may also contain one or more of the following selected elements: Mg, Ca, REM, as needed.

[0059] (Mg: below 0.0200%) Mg is an element that sometimes gets mixed into steel during the manufacturing process. However, there is no lower limit to the Mg content, and it can be 0%. Furthermore, Mg is also an element that forms oxides, thus improving 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 can sometimes 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. The Mg content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[0060] (Ca: below 0.0200%) Ca is an element that sometimes gets mixed into steel during the manufacturing process. However, there is no lower limit to the Ca content, and it can be 0%. Additionally, 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 can sometimes 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.

[0061] (REM: below 0.0200%) Rare earth metals (REM) refer to a total of 17 elements, including Sc and Y, and 15 lanthanide elements such as La, Ce, and Nd. REM content refers to the total content of these 17 elements. REM is an element that sometimes gets mixed into steel during the manufacturing process. However, there is no lower limit for REM content; it can be 0%. Furthermore, REM is also an element that forms oxides, thus increasing 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 sometimes 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.

[0062] (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 ore and scrap iron, or other factors.

[0063] In addition to limiting the content of each element, this disclosure also limits the range of α and β values ​​as follows.

[0064] (α value: 5.0 or higher and 16.0 or lower) The value of α is calculated using the following equation (1).

[0065] α=0.50×√[C]×(1+0.64[Si])×(1+4.10[Mn])×(1+0.27[Cu])×(1+0.52[Ni])×(1+2.33[Cr])×(1+3.14[Mo]) (1) Where [C], [Si], [Mn], [Cu], [Ni], [Cr], and [Mo] are the contents (mass%) of C, Si, Mn, Cu, Ni, Cr, and Mo in the steel. Substitute zero if the element is not present. Also, √[C] and [C]... 1 / 2 The meanings are the same.

[0066] In this disclosure, the α value is set in the range of 5.0 to 16.0. This α value is an indicator of the hardenability of steel; a higher α value allows for the formation of a lower bainite and martensite structure with an excellent balance of strength and toughness. When α is within an appropriate range, the proportion of lower bainite and martensite in the HAZ structure, which provides an excellent balance of strength and toughness, increases, thus ensuring HAZ toughness. When α is 5.0 or higher, the proportion of lower bainite and martensite, which is beneficial for the hardenability of the base material and the balance of strength and toughness, increases, and toughness degradation is suppressed. Furthermore, the proportion of lower bainite and martensite in the HAZ structure also tends to increase, thus improving HAZ toughness. On the other hand, if the α value is below 16.0, the steel strength will not become excessively high, ensuring toughness. Additionally, if the α value is below 16.0, toughness after PWHT (Power Wheat Hardening) can also be ensured. Furthermore, the HAZ will not become excessively hard, thus ensuring HAZ toughness.

[0067] 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 5.5 or higher, 6.0 or higher, or 7.0 or higher. Furthermore, the α value is preferably 15.5 or lower, or 15.0 or lower.

[0068] (β value: below 0.017) The value of β is calculated by the following equation (2).

[0069] 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 element is not present.

[0070] In this disclosure, the β value is set to be below 0.017. The β value is an indicator of the toughness of steel after a thermal cycle simulating a weld equivalent to 2 kJ / mm², followed by PWHT. The higher the β value, the more deteriorated the toughness after a thermal cycle simulating a weld equivalent to 2 kJ / mm², followed by PWHT. When the β value is below 0.017, grain boundary fracture can be suppressed in Charpy impact tests after thermal cycling and PWHT, thus ensuring toughness.

[0071] 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.

[0072] It should be noted that there is no specific lower limit for the β value. From a cost perspective, it can be above -0.010, above -0.005, or above 0.000.

[0073] <Microstructure> Next, the microstructure of the steel disclosed herein will be described. The microstructure of the portion of the steel at a distance of 1 / 4 of its thickness from the surface in the thickness direction comprises lower bainite and martensite. Furthermore, in addition to lower bainite, upper bainite may also be included as bainite.

[0074] "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.

[0075] Martensite exists in four forms: lath, butterfly, lenticular, and thin plate. However, lath martensite is the main component in this disclosure. Lath martensite consists of lath bundles and lath blocks composed of groups of laths arranged in a specific manner. It is a microstructure in which one austenite grain is divided into several lath bundles.

[0076] (The combined area ratio of lower bainite and martensite: ≥15.0%) The 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 be 100%.

[0077] (The total area ratio of upper bainite, lower bainite, and martensite is 90.0% or more.) 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 be 100%. Furthermore, the upper bainite content in the 1 / 4 t section can also be 1.0% or more.

[0078] (Area ratio of retained austenite: less than 1.7%) From the perspective of ensuring the toughness of the steel, the area ratio of retained austenite in the 1 / 4 t portion is less than 1.7%. The area ratio of retained austenite in the 1 / 4 t portion is preferably less than 1.0%, but can also be 0%. This is because, in the Ni-containing steel of this disclosure, the Ni content is less than that of conventional 9% Ni steel. Therefore, even if retained austenite exists at -110°C, it is unstable. When the steel microstructure is subjected to plastic deformation at the crack tip, the retained austenite transforms into martensite due to plastic-induced martensitic transformation. Therefore, the retained austenite at room temperature is set to be less than 1.7% in terms of volume ratio (area ratio).

[0079] It should be noted that the higher the Ni content, the easier it is for the volume fraction of retained austenite to increase, but increasing the Ni content is not preferable in terms of increasing costs.

[0080] The microstructure of the steel was observed using a sample with 1 / 4 t of the steel as the observation surface. Two types of samples were prepared, which underwent (a) electrolytic grinding and (b) nitric acid-ethanol etching. For each sample of (a) and (b), measurements were taken at three locations using the method described below, and the average of the three measurements was taken as the area ratio of the microstructure of the steel. It should be noted that for each sample of (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 on a single sample and the average can be taken.

[0081] After mirror finishing via mechanical grinding, electropolished samples, having undergone electropolishing to remove the strain layer caused by mechanical grinding, were used. The total area ratio of upper bainite, lower bainite, martensite, and retained austenite was measured using EBSD. The measurement magnification was 200x, and measurements were performed at 0.4μm intervals across a 400μm × 400μm area. Measurements were conducted with an electron beam diameter of less than 0.4μm. The Confidence Index (CI value) was set to 0.1 or higher. The distinction between ferrite and upper bainite, lower bainite, and martensite was made 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, while regions with a GAM value above 0.5 are upper bainite, lower bainite, martensite, or retained austenite. In this disclosure, upper bainite, lower bainite, martensite, and retained austenite are determined using the GAM value of the EBSD as a threshold. Therefore, it includes not only upper bainite, lower bainite, martensite, and retained austenite, but also tempered upper bainite, tempered lower bainite, and tempered martensite. Comparing the microstructure of direct quenching (DQ) and subsequent tempering (T) (DQT), tempering results in the decomposition of MA or coarsening of carbides, but the appearance of the microstructure does not change significantly.

[0082] 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 a magnification of 500x within a range of 360 μm × 480 μm. Upper bainite was defined as the portion with a clear lath structure and carbides or metamerism (MA) along the lath boundaries. Upper bainite was further defined as a microstructure with a relatively coarse internal structure, sparse carbide density, and a mixture of dense and sparse areas. Figure 1 Examples of microstructure discrimination results. (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 (B). L +M). In the portion identified as upper bainite (Bu), the whitish carbides appear sparse and mixed, with areas of varying density. On the other hand, the portion identified as lower bainite + martensite (B) LIn the +M portion, carbides are dense and uniform. The total area ratios of the lower bainite, martensite, and retained austenite are obtained by subtracting the area ratio of the upper bainite from the total area ratios of the upper bainite, lower bainite, martensite, and retained austenite measured above. Then, the area ratio of the retained austenite is determined using the method described later, and subtracted from the total area ratios of the lower bainite, martensite, and retained austenite to obtain the total area ratios of the lower bainite and martensite.

[0083] It should be noted that, based on the above tissue analysis, when the combined area ratio of lower bainite and martensite exceeds 0%, it usually contains 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.

[0084] (Area ratio of retained austenite) The area ratio of retained austenite was determined by X-ray diffraction. The determination of the area ratio of retained austenite was performed using a specimen with a measurement plane at a point 1 / 4 of the steel's thickness in the thickness direction (referred to in this specification as the "1 / 4t portion"). The specimen was a 2 mm thick test piece, collected from a position 1 / 4 of the width from the end of the steel in the width direction (perpendicular to the rolling and thickness directions), chemically ground, and used to determine the volume ratio of retained austenite by X-ray diffraction using a Mo vacuum tube. Quantification was based on the ratio of the integrated intensities of the (200) and (211) diffraction peaks of the ferrite phase to the integrated intensities of the (200), (220), and (311) diffraction peaks of the austenite phase, using the average of six combinations. The integrated intensities of the diffraction peaks were obtained by subtracting the signal portion from the background based on the signal before and after the peak. The volume ratio determined by X-ray diffraction was considered as the area ratio.

[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 regardless of whether PWHT is applied. 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... area Used in 4mm 2 The area S of the i-th grain detected when measuring the crystal grain size at 500 times the measured size. i Particle size di And it is calculated using the following formula.

[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) in the steel disclosed herein can be a flattened shape in the rolling direction. If the original austenite grains at a position 1 / 4 of the thickness in the thickness direction from the surface of the steel are flattened grains with an aspect ratio of 1.5 or more, the toughness of the steel can be further improved. This is because increasing the grain boundary area by flattening the original austenite grains effectively refines the austenite grains and is effective in refining the effective 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 (sometimes called original austenite grains) in the steel are determined as follows. First, the L-section (parallel to the rolling direction and thickness direction of the steel) located at 1 / 4 of the thickness in the thickness direction from the surface of the steel is mirror-polished, and then etched with an etchant based on a saturated aqueous solution of 2-4% picric acid to expose 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". Furthermore, 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, in addition to its excellent toughness at -110°C, it also exhibits excellent low-temperature toughness after PWHT treatment.

[0090] (Tensile strength: above 615MPa and below 930MPa) In this disclosure, the tensile strength of the steel is set to 615 MPa to 930 MPa. To reduce the weight of large welded structures such as conveyor tanks, steel that can ensure the strength of the structure even with a thin layer is required. Typically, steel with the aforementioned tensile strength is selected for use in such applications; therefore, this disclosure also manufactures steel 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. When there is no yield point, the yield strength is determined using 0.2% of the elastic limit stress.

[0092] (Charpy shock energy at -110℃) For the steel disclosed herein, to ensure high toughness at low temperatures, a Charpy impact absorption energy of 150 J or more at -110°C is preferred. By possessing low-temperature toughness with a Charpy impact absorption energy of 150 J or more at -110°C, the steel disclosed herein enables tanks manufactured using this steel to be suitable for applications such as the transport of liquid carbon dioxide. The steel disclosed herein can also possess low-temperature toughness with a Charpy impact absorption energy of 100 J or more at -110°C. It should be noted that the Charpy impact absorption energy at -110°C is a value measured using a sample taken from a location one-quarter of the thickness.

[0093] (Charpy shock absorption energy at -110℃ after PWHT) In cryogenic tanks, to prevent breakage, welded sections are sometimes subjected to PWHT (Pre-Waste Heat Treatment) after assembly into the transport tank. In this case, not only the welded section is heated, but also the base material of the steel (also simply referred to as the base material), which is not affected by the weld. If the base material is heated to a temperature range above 425°C for an extended period, the toughness of the base material tends to decrease. For the steel of this disclosure, when PWHT is performed on the steel at a holding temperature of 600°C for 2 hours, and with a heating and cooling rate of 55°C / h in the temperature range above 425°C, the preferred toughness at the PWHT location is a Charpy impact absorption energy of 150 J or more at -110°C. The Charpy impact absorption energy at -110°C after PWHT can be 100 J or more. The Charpy impact absorption energy at -110°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% atomically at the original austenite grain boundaries after thermal cycling at a location 1 / 4 of the steel's thickness along the thickness direction from the surface, following PWHT. In determining the P concentration of the original austenite grain boundaries, a focused ion beam (FIB) is used to cut around the region containing the original austenite grain boundaries. Then, Pt is deposited and fixed at the tip of a nanoneedle in a size of approximately 100 μm × 100 μm and collected, further thinned to less than 100 nm. Then, using aberration-corrected transmission electron microscopy, scanning transmission electron microscopy is employed, with the electron beam parallel to the original austenite grain boundaries, at a magnification of 1.8 million times and a resolution of 0.35 nm / pixel for a 20 nm × 20 nm region. Elemental analysis is performed by detecting the generated characteristic X-rays using energy-dispersive X-ray fluorescence (EDS). The signals of P and Fe are accumulated at a width of 2 pixels in a direction parallel to the grain boundary to obtain the line profile of P near the grain boundary. The P concentration obtained directly above the grain boundary through signal accumulation and elemental quantification is taken as the P grain boundary segregation. The unit is expressed as atomic percentage concentration. Data are obtained at a total of 3 grain boundaries, the average value is calculated and evaluated. The lower the P grain boundary segregation, the better it can suppress grain boundary fracture and ensure HAZ toughness after thermal cycling and PWHT. Preferably, it is 2.5% or less, more preferably 2.0% or less in atomic percentage concentration.

[0095] (The coverage of grain boundary carbides at the original austenite grain boundaries after thermal cycling at a point 1 / 4 of the thickness from the surface of the steel and after PWHT) 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 the steel's thickness after thermal cycling and PWHT (partially heat-treated). Regarding the grain boundary carbide coverage of the original austenite grain boundaries, a sample of the steel after thermal cycling and PWHT at 1 / 4 of its thickness is subjected to electrolytic grinding, and secondary electron microscopy is performed along the original austenite grain boundaries using a field emission electron microscope. The magnification is appropriately varied depending on the original austenite grain size, for example, images are acquired at 10,000x magnification. Secondary electron images of one grain boundary portion are acquired, and the overall length of the grain boundary and the length of the carbide-containing region are measured. The length of the carbide-containing region is divided by the overall length of the grain boundary, multiplied by 100, and the grain boundary carbide coverage is calculated. Data from five grain boundary portions are acquired, and the average value is used as the grain boundary carbide coverage for evaluation. The lower the grain boundary carbide coverage, the better it can suppress grain boundary fracture and ensure HAZ toughness after thermal cycling and PWHT. Preferably, it is 37% or less, more preferably 35% or less.

[0096] (Charpy shock energy absorbed at -110°C after thermal cycling) For the steel disclosed herein, in order to ensure high toughness after simulating thermal cycling tests of the welded parts at low temperatures, it is preferable that the Charpy impact absorption energy at -110°C after thermal cycling is 50 J or more. By possessing low-temperature toughness with a Charpy impact absorption energy of 50 J or more at -110°C after thermal cycling, the steel of this disclosure enables tanks manufactured using the steel of this disclosure to be suitable for applications such as the transport of liquid carbon dioxide. The Charpy impact absorption energy at -110°C after thermal cycling can be 40 J or more. Furthermore, for the Charpy impact absorption energy at -110°C after thermal cycling, a sample taken from 1 / 4 of the steel's thickness is used as a thermal cycling test piece. This sample is subjected to a thermal process 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. The Charpy impact test piece is then collected from this process to obtain the measured value.

[0097] (Charpy shock absorption energy at -110°C after thermal cycling and PWHT) In cryogenic tanks, to prevent breakage, welded parts are sometimes subjected to PWHT (Potential Welding Temperature) after assembly into the transport tank. 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 impact test specimens are then collected and measured. At this point, the preferred toughness of the location where the PWHT was performed is a Charpy impact absorption energy of 50 J or more at -110°C. The Charpy impact absorption energy at -110°C at the location where the PWHT was performed after the aforementioned thermal cycling test can be 40 J or more.

[0098] It should be noted that PWHT can sometimes reduce the toughness of steel. The reason for this is not yet clear, but it is speculated that it is due to the diffusion of phosphorus (P) and manganese (Mn) to the grain boundaries, and the formation of carbides at the grain boundaries, which reduces toughness. The reduction in toughness caused by PWHT can be suppressed by controlling the content of P and Mn to reduce the average grain size of the steel. Furthermore, by limiting the content of P and Mn to satisfy the relevant equation β, the P concentration at the original austenite grain boundaries (atomic percentage) should be below 3.0%, and the grain boundary carbide coverage should be below 40%, to suppress the reduction in toughness after thermal cycling and PWHT.

[0099] Tensile strength (TS) and yield strength (YS) were determined according to the tensile test method of JIS Z2241:2011. In the tensile test, JIS 14A test pieces were used, taken from the 1 / 4 thickness position, with the length direction parallel to the width direction of the steel (the direction perpendicular to the rolling direction and thickness direction) (C direction). TS and YS were determined 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 energy is determined according to JIS Z2242:2018 using an impact blade with a radius of 2 mm at -110°C via a Charpy impact test. The Charpy impact energy is calculated by averaging three test pieces. In the Charpy impact test, a V-cut test piece is used, taken from a position 1 / 4 of the steel thickness, with its length direction parallel to the width direction of the steel (C direction). The V-cut of the test piece is formed such that the length direction of the cut corresponds to the thickness direction of the steel, and the depth direction of the cut corresponds to the rolling direction of the steel.

[0101] The shape of the steel materials disclosed herein is not particularly limited, and includes steel plates, steel strips, structural steel, steel pipes, etc. However, steel pipes and structural steel include not only steel materials formed by joining steel plates, such as welded steel pipes and welded structural steel, but also structural steel materials joined by riveting. The thickness of steel materials 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 3 mm or more and 150 mm or less. The thickness of the steel materials can be 6 mm or more, 10 mm or more, 15 mm or more, or 30 mm or more. In addition, the thickness of the steel materials can be less than 100 mm, less than 80 mm, or less than 60 mm.

[0102] Furthermore, the steel disclosed herein is not particularly limited in its application, possessing mechanical properties that balance strength and low-temperature toughness. In particular, it exhibits excellent low-temperature toughness even after PWHT treatment, making it suitable as a structural material for pressure vessels, specifically tanks for storing and transporting liquefied gases, especially liquid carbon dioxide.

[0103] (Methods for manufacturing steel) The method for manufacturing the steel disclosed herein is not particularly limited. For example, after smelting steel that meets the above chemical composition, a steel billet is manufactured by continuous casting. The steel billet is subjected to direct quenching (DQ), which involves heating, hot rolling, and then directly water-cooling, or reheat quenching (RQ), which involves cooling, reheating, and then water-cooling, thereby producing the steel. For RQ, cooling is not necessary before reheating, and water cooling is also acceptable. Furthermore, tempering (T) may also be performed.

[0104] (1) DQT: Direct Quenching (DQ) and Tempering (T) (2) RQT: Cooling, reheating and quenching (RQ), 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] When manufacturing the steel disclosed herein using DQ (Digital-to-Qitar) technology, from the viewpoint of hot rolling in the temperature range where the microstructure of the rolled material is austenitic, the heating temperature of the hot-rolled billet is Ac3 or higher. From the viewpoint of reducing deformation resistance, the heating temperature of the 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. Furthermore, Ac3 is set to a value calculated by 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 sometimes consists of rolling in the temperature range where recrystallization occurs (recrystallization temperature range rolling) and rolling in the temperature range where recrystallization is inhibited (non-recrystallization temperature range 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 of recrystallization temperature zone rolling is preferably 20% or more, and more preferably 30% or more. The cumulative reduction rate of recrystallization temperature zone rolling is calculated from the difference between the thickness of the billet before hot rolling and the thickness of the material being rolled at 900°C.

[0109] Cumulative reduction rate (%) during rolling in the recrystallization temperature range = 100 × ([bill thickness] - [thickness of the rolled material at 900℃]) / [bill thickness] Non-recrystallization temperature zone rolling is hot rolling performed at a temperature below 900°C during rolling. From the viewpoint of refining the average grain size of the steel, the cumulative reduction rate of non-recrystallization temperature zone rolling is preferably 20% or more, more preferably 30% or more. The cumulative reduction rate of 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 set as a value calculated by 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 a value calculated by simulation based on heat transfer calculations at the cooling rate at one-quarter of the thickness.

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

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

[0115] Example The following examples illustrate the steel of this disclosure. However, the conditions in the following examples are examples used to confirm the feasibility and effectiveness of this disclosure, and this disclosure is 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 compositions shown in Table 1 consists of Fe and impurities. Additionally, blank columns indicate that alloying elements were not intentionally added during the steelmaking process. Underlined elements indicate elements outside the scope of this disclosure.

[0117] Next, steel is manufactured from these slabs according to the manufacturing conditions shown in Table 2. “Temper heat treatment” refers to the heating temperature during the tempering process after quenching.

[0118] [Measurement and Evaluation] The microstructure and mechanical properties of the 1 / 4 t portion of the steel obtained by the above method were determined. The results are shown in Table 3. The symbols for the microstructure are as follows. It should be noted that for the microstructure determined to be lower bainite + martensite (B... L In the region (+M), SEM observation confirmed the presence of lower bainite and martensite. The remaining microstructure consisted of pearlite, MA phase, and ferrite.

[0119] Bu: upper bainite BL: lower bainite M: Martensite Residual γ: Residual austenite Regarding toughness, the test was conducted on both the case where the steel (base material) was not heat-treated and the case where it was heat-treated as described in (2) to (4), using samples taken from a portion of 1 / 4 of the thickness.

[0120] (1) Base material toughness Average Charpy shock absorption energy at -110℃ (2) Toughness of the base material after PWHT The average Charpy shock absorption energy at -110°C after PWHT was measured at a holding temperature of 600°C, a holding time of 2 hours, and a heating and cooling rate of 55°C / h in the temperature range above 425°C. (3) Thermal cycling toughness The average Charpy impact absorption energy at -110°C was obtained after a thermal process (thermal cycling) involving heating to 1350°C at 60°C / s, holding at 1350°C for 1 second, and then cooling to room temperature at 20°C / s. (4) Toughness after thermal cycling PWHT The average Charpy shock absorbed energy at -110°C was obtained after a thermal process (thermal cycling) involving heating to 1350°C at 60°C / s, holding at 1350°C for 1 second, and cooling to room temperature at 20°C / s, followed by holding at 600°C for 2 hours, with heating and cooling rates of 55°C / h in the temperature range above 425°C. In Table 3, "P-grain boundary segregation of the original γ grain boundary after thermal cycling PWHT [at.%]" refers to the P concentration (atomic %) in the original austenite grain boundary after thermal cycling and post-weld heat treatment (PWHT), where "ND" is below the detection limit. "Carbide coverage of the original γ grain boundary after thermal cycling PWHT [%]" refers to the carbide coverage in the original austenite grain boundary after thermal cycling and post-weld heat treatment (PWHT).

[0121] Examples No. 1 to 26 are examples of the present invention, and examples No. 27 to 35 are comparative examples.

[0122] The Si content of No. 27 deviates from the upper limit, so after thermal cycling and PWHT, the grain boundary carbide coverage of the original austenite grain boundaries exceeds 40%, resulting in insufficient low-temperature toughness.

[0123] The Mn content of No.28 deviates from the upper limit, so the β value also deviates from the upper limit. After thermal cycling and PWHT, it cannot obtain sufficient low-temperature toughness.

[0124] The amount of P in No. 29 deviates from the upper limit, so after thermal cycling and PWHT, the amount of P grain boundary segregation in the original austenite grain boundaries exceeds 3%, and sufficient low-temperature toughness cannot be obtained.

[0125] The β value of No.30 deviates from the upper limit, so it cannot achieve sufficient low-temperature toughness after thermal cycling and PWHT.

[0126] The α value of No. 31 is lower than the lower limit of this disclosure, resulting in insufficient hardenability and strength. It also fails to achieve sufficient low-temperature toughness.

[0127] The α value of No. 32 exceeds the upper limit of this disclosure, resulting in excessive hardenability and excessive strength.

[0128] 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.

[0129] The α value of No. 34 exceeds the upper limit of this disclosure, resulting in excessive hardenability and strength. It also has excessive residual γ, thus failing to achieve sufficient low-temperature toughness.

[0130] The cooling rate of the direct quenching of No. 35 is low, so the total area ratio of the lower bainite and martensite is insufficient, and sufficient low-temperature toughness cannot be obtained.

[0131] In contrast to the comparative examples, in all the present invention examples (No. 1 to 26), the chemical composition and microstructure of the steel were appropriately controlled, the tensile strength was within a suitable range of 615 MPa or more and 930 MPa or less, and, regardless of whether PWHT was applied before or after, low-temperature toughness of 100 J or more at -110°C was obtained. In particular, in 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 at -110°C was obtained regardless of whether PWHT was applied before or after, and the thermal cycling toughness and the toughness after thermal cycling PWHT were also obtained at low-temperature toughness of 50 J or more at -110°C.

[0132] Industrial availability The steel disclosed herein can be primarily used, for example, in tanks for the transport of liquefied carbon dioxide. Additionally, the steel disclosed herein can also be used in other welded structures such as buildings, bridges, ships, pipelines, marine structures, and pressure vessels (tanks).

[0133] The entire disclosure of Japanese Patent Application No. 2023-176326, filed on October 11, 2023, is incorporated herein by reference. All documents, patent applications, and technical standards described herein, and the specific details of each document, patent application, and technical standard, are incorporated herein by reference to the same extent.

Claims

1. A type of steel having the following chemical composition: (in mass%), C: Above 0.03% and below 0.20% Si: ≥0.01% and ≤0.50% Mn: ≥0.10% and ≤2.00% P: below 0.025% S: Below 0.0250% Ni: 4.51% or higher and lower than 6.10% Al: Above 0.001% and below 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 5.0 or more and 16.0 or less, and β, as expressed by equation (2) below, is 0.017 or less. The tensile strength of the steel is above 615 MPa and below 930 MPa. The microstructure of the portion of the steel material extending 1 / 4 of its thickness 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, while the area ratio of retained austenite is less than 1.7%. α=0.50×√[C]×(1+0.64[Si])×(1+4.10[Mn])×(1+0.27[Cu])×(1+0.52[Ni])×(1+2.33[Cr])×(1+3.14[Mo]) (1) β=[Mn]×[P]-[Mo] / 100 (2) in, In equations (1) and (2), the [element symbol] represents the content (mass%) of the corresponding element contained in the steel. If the element is not contained, the value is zero.

2. The steel according to claim 1, wherein, The chemical composition includes the following group A. [Group A] Choose one or more elements from the following groups: Cu: ≥0.01% and ≤1.50% Cr: ≥0.01% and ≤3.00% Mo: ≥0.01% and ≤2.00%, and B: Above 0.0003% and below 0.0050%.

3. The steel according to claim 1 or claim 2, wherein, The chemical composition includes the following group B. Group B Choose one or more elements from the following groups: Nb: ≥0.001% and ≤0.050% Ti: ≥0.001% and ≤0.050%, and V: Above 0.01% and below 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] Choose one or more elements from the following groups: Mg: ≥0.0003% and ≤0.0200% Ca: ≥0.0003% and ≤0.0200%, and REM: Above 0.0003% and below 0.0200%.

5. The steel according to any one of claims 1 to 4, wherein, The average 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 -110°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 -110°C at a portion of 1 / 4 of the thickness at the location where the heat treatment was performed is 150 J or more.

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

10. The steel according to any one of claims 1 to 9, wherein, A thermal cycle simulating welding equivalent to 2 kJ / mm was applied to a portion of the steel at 1 / 4 of its thickness. The steel was further subjected to heat treatment 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 -110°C at the location where the heat treatment was performed was above 50 J.

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

Images