Steel and liquid carbon dioxide transport tank

Abstract

A steel product having a specific chemical composition having a value of α represented by the following formula of 4.0 or more and 16.0 or less, in which the tensile strength is 590 MPa or more and 930 MPa or less, the microstructure of a part of 1 / 4 of the thickness from a surface includes lower bainite, martensite, and retained austenite, the total area ratio of lower bainite and martensite is 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 from 0.2% to less than 5.0%, and a liquid carbon dioxide transportation tank including the steel product. α=0.50×√C×1+0.64Si×1+4.10Mn×1+0.27Cu×1+0.52Ni×1+2.33Cr×1+3.14Mo

Description

Technical Field

[0001] The present disclosure relates to a steel product and a liquid carbon dioxide transportation tank.Background Art

[0002] Steel products can be used as constituent material for welded structures such as buildings, bridges, ships, pipelines, offshore structures, pressure vessels and tanks. Steel products having excellent strength and adaptability to low-temperature toughness are effective in low-temperature applications.

[0003] Cryogenic steel is used in cryogenic pressure vessels such as storage tanks for liquefied gases. Al-killed steel, nickel steel, high-Mn steel, austenitic stainless steel and the like exist in a cryogenic steel, in accordance with the usage temperature. For example, nickel steel such as 3.5% Ni steel is used as a material for tanks that store liquefied ethane or liquefied ethylene whose usage temperatures are around -100°C.

[0004] As with this 3.5% Ni steel, steel products that require ensured low-temperature toughness-exemplified by those used in cryogenic pressure vessels-are often made to contain Ni.

[0005] For example, Patent Document 1 proposes a nickel-containing steel product for low temperatures that has excellent toughness and has a specific chemical composition containing 2.7% or more and 5.0% or less Ni, wherein the prior austenite grain diameter at the time of quenching heating is 20 µm or less, and the effective crystal grain diameter after a heat treatment is 12 µm or less, and the tensile strength is 450 MPa or more and 690 MPa or less.

[0006] Further, various steel products of prescribed chemical compositions and microstructures (metal structures) have been proposed for the purposes of low-temperature toughness and high strength (see, for example, Patent Documents 2 through 9). Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2019-81930 Patent Document 2: International Publication No. 2014 / 103629 Patent Document 3: JP-A No. S52-156121 Patent Document 4: JP-A No. S55-104427 Patent Document 5: JP-A No. S58-73717 Patent Document 6: JP-A No. H07-331328 Patent Document 7: JP-A No. 2001-123222 Patent Document 8: JP-A No. 2001-123245 Patent Document 9: JP-A No. 2007-46096 SUMMARY OF INVENTIONTechnical Problem

[0007] Having both high strength and the ensuring of low-temperature toughness is desirable for cryogenic steels use that are used in cryogenic pressure vessels. Further, a cryogenic pressure vessel is manufactured by welding a steel product, and, in order to eliminate residual stress that arises due to the welding, there are cases in which a post weld heat treatment (called PWHT upon occasion) is carried out. Recently, the demand for low-temperature toughness of steel products after PWHT has increased even more.

[0008] A topic of the present disclosure is the provision of a steel product and a liquid carbon dioxide transportation tank which are suited to low-temperature applications and which have good low-temperature toughness regardless of whether before or after a post weld heat treatment.Solution to Problem

[0009] The gist of the present disclosure is as follows. <1> A steel product, comprising a chemical composition of, in mass%: C: from 0.03% to 0.20%, Si: from 0.01% to 0.50%, Mn: from 0.10% to 1.65%, P: 0.025% or less, S: 0.0250% or less, Ni: from 2.65% to 4.45%, Al: from 0.001% to 0.100%, O: 0.0100% or less, N: 0.0100% or less, Cu: 0 to 1.50%, Cr: 0 to 3.00%, Mo: 0 to 2.00%, B: 0 to 0.0050%, Nb: 0 to 0.050%, Ti: 0 to 0.050%, V: 0 to 0.10%, Mg: 0 to 0.0200%, Ca: 0 to 0.0200%, REM: 0 to 0.0200%, and a balance of Fe and impurities, the chemical composition having a value of α represented by the following Formula (1) of from 4.0 to 16.0, wherein a tensile strength is from 590 MPa to 930 MPa, a microstructure of a region that is 1 / 4 of a thickness, in a thickness direction, from a surface of the steel product comprises lower bainite, martensite, and retained austenite, a total area ratio of the lower bainite and the martensite is 15.0% or more, a total area ratio of upper bainite, the lower bainite, and the martensite is 90.0% or more, and an area ratio of the retained austenite is from 0.2% to less than 5.0%: wherein a symbol of an element in square brackets in Formula (1) represents a content amount (mass%) of a corresponding element contained in the steel product, provided that zero is substituted when the corresponding element is not included. <2> The steel product according to <1>, wherein the chemical composition comprises the following Group A: [Group A]: one or two or more selected from the group consisting of: Cu: from 0.01% to 1.50%, Cr: from 0.10% to 3.00%, Mo: from 0.01% to 2.00%, and B: from 0.0003% to 0.0050%. <3> The steel product according to <1> or <2>, wherein the chemical composition comprises the following Group B: [Group B]: one or two or more selected from the group consisting of: Nb: from 0.001% to 0.050%, Ti: from 0.001% to 0.050%, and V: from 0.01% to 0.10%. <4> The steel product according to any one of <1> to <3>, wherein the chemical composition comprises the following Group C: [Group C]: one or two or more selected from the group consisting of: Mg: from 0.0003% to 0.0200%, Ca: from 0.0003% to 0.0200%, and REM: from 0.0003% to 0.0200%. <5> The steel product according to any one of <1> to <4>, wherein an aspect ratio of a prior austenite grain in the region that is 1 / 4 of the thickness, in the thickness direction, from the surface of the steel product is 1.5 or more. <6> The steel product according to any one of <1> to <4>, wherein an aspect ratio of a prior austenite grain in the region that is 1 / 4 of the thickness, in the thickness direction, from the surface of the steel product is less than 1.5. <7> The steel product according to any one of <1> to <6>, wherein, in the microstructure of the region that is 1 / 4 of the thickness, in the thickness direction, from the surface of the steel product, an average crystal grain diameter is 20.0 µm or less. <8> The steel product according to any one of <1> to <7>, wherein a Charpy impact absorption energy at -100°C is 150 J or more. <9> The steel product according to any one of <1> to <8>, wherein, in a case in which a heat treatment, in which a rate of temperature increase and a rate of temperature decrease in a temperature region of 425°C or more are 55°C / h and temperature is held for 2 hours at 600°C, is carried out on the steel product, a Charpy impact absorption energy at -100°C of portions where the heat treatment has been carried out is 150 J or more. <10> A liquid carbon dioxide transportation tank, comprising the steel product according to any one of <1> to <9>. Advantageous Effects of Invention

[0010] In accordance with the present disclosure, a steel product and a liquid carbon dioxide transportation tank which are suited to low-temperature applications and which have good low-temperature toughness regardless of whether before or after a post weld heat treatment can be provided.BRIEF DESCRIPTION OF DRAWINGS

[0011] Fig. 1 is a diagram illustrating an example of results of discriminating the microstructure.DESCRIPTION OF EMBODIMENTS

[0012] The present disclosure is described in detail hereinafter.

[0013] The "post weld heat treatment" in the present disclosure means a post weld heat treatment based on the contents prescribed in JIS Z 3700:2009 "Methods of post weld heat treatment", unless otherwise specified.

[0014] The "steel product" and the "base metal" in the present disclosure mean the steel product portion that does not include a surface treated layer such as a plating layer or a coated film. However, a surface treated layer such as a plating layer or a coated film may be formed on the surface of the steel product relating to the present disclosure.

[0015] In the present disclosure, numerical ranges expressed by using "-" mean ranges in which the numerical values listed before and after the "-" are included as the lower limit value and upper limit value, respectively. However, numerical value ranges in which "exceeds" or "less than" is appended to the numerical value listed before or after the "-" mean a range in which these numerical values are not included as the lower limit value or the upper limit value.

[0016] With respect to the contents of the elements in a chemical composition, "%" means "mass%".

[0017] The term "step" is not only an independent step and includes steps that, even in a case in which that step cannot be clearly distinguished from another step, achieve the intended object of that step.

[0018] A steel product relating to an embodiment of the present disclosure is described hereinafter. The new knowledge, which was obtained as the result of studies by the inventors of the present disclosure and which was arrived at in completing the steel product relating to the present disclosure, is described in detail first.

[0019] The inventors of the present disclosure carried out studies in order to improve the strength of steel products. The tensile strength of a steel product is ensured by the configuration of the microstructure. The inventors of the present disclosure took samples from a 1 / 4t portion (t: thickness of the steel product), which is the region that is 1 / 4 of the thickness in the thickness direction from the surface of a steel product after reheated quenching, and carried out tensile test thereon, and observed the microstructures. As a result, it was learned that, at the microstructure of the 1 / 4t portion of a steel product having a tensile strength of 590 MPa or more and 930 MPa or less, the area ratio of ferrite is less than 10.0%, and the total of the area ratios of upper bainite, lower bainite and martensite is 90.0% or more. Note that the total of the area ratios of upper bainite, lower bainite and martensite was measured by using electron back scatter diffraction (hereinafter called "EBSD").

[0020] Further, the inventors of the present disclosure conducted studies to improve the toughness of the steel product. The toughness of the steel product is secured by the constitution of microstructure. The inventors of the present disclosure took a sample from a 1 / 4t portion of a steel product subjected to reheated quenching and then to intermediate heat treatment, and performed a Charpy impact test and observed the microstructure. As a result, it has been found that, in a steel product in which the Charpy impact absorption energy at - 100°C is 150 J or more, the total area ratio of lower bainite and martensite is 15.0% or more and the area ratio of retained austenite is 0.2% or more and less than 5.0%. The total area ratio of lower bainite and martensite was measured using EBSD. The area ratio of retained austenite was measured by an X-ray diffraction method. The volume ratio of retained austenite measured by the X-ray diffraction method can be regarded as the corresponding area ratio.

[0021] Further, the inventors of the present disclosure conducted studies to secure toughness of the steel product. The toughness of the steel product is secured by reducing the area of regions surrounded by high angle grain boundaries at each of which the difference in crystal orientation is 15° or more. The inventors of the present disclosure took a sample from a 1 / 4t portion of a steel product after reheated quenching, and used EBSD to measure the circle-equivalent diameter of the region surrounded by high angle grain boundaries. Hereinafter, the circle-equivalent diameter of the region surrounded by high angle grain boundaries is referred to as a crystal grain diameter. The sample was subjected to mechanical polishing and electrolytic polishing, and was analyzed in a region of 4 mm 2< with an EBSD device attached to an FE-SEM (field emission scanning electron microscope). A value calculated by the area-weighted average of the crystal grain diameters measured in the region of 4 mm 2< , in which weighting was performed by the area of the crystal grain, was taken as an average crystal grain diameter (occasionally referred to as an "effective crystal grain diameter"). A finding that, when the average crystal grain diameter at the 1 / 4t portion of the steel product is 20.0 µm or less, the toughness of the steel product tends to improve further regardless of before or after post-welding heat treatment has been obtained.

[0022] Further, the inventors of the present disclosure conducted studies also on a steel product that is, after hot rolling, subjected to direct quenching of being cooled with water as it is and then subjected to intermediate heat treatment, and have obtained similar results to a steel product after reheated quenching.<Chemical Composition>

[0023] Alloy elements that constitute the chemical composition of the steel product relating to the present disclosure are described next. Note that, in the following description of the alloy elements, the "%" of the content means "mass%".(C: 0.03% or more and 0.20% or less)

[0024] C is an element that improves the strength of the steel product. From the standpoint of ensuring the strength of the steel product that is used in a structure, in the present disclosure, the C content is 0.03% or more. The C content is preferably 0.05% or more, or 0.07% or more. On the other hand, C is an element that reduces toughness, and, from the standpoint of ensuring the toughness of the heat affected zone (hereinafter called "HAZ" upon occasion), in the present disclosure, the C content is 0.20% or less. The C content is preferably 0.16% or less, 0.14% or less, or 0.12% or less.(Si: 0.01% or more and 0.50% or less)

[0025] Si is an element that is used as a deoxidizing agent, and further, that dissolves into the steel and increases the strength. From the standpoint of controlling the O concentration contained in molten steel, in the present disclosure, the Si content is 0.01% or more. The Si content is preferably 0.03% or more, 0.05% or more, or 0.10% or more. On the other hand, if the Si content is excessive, there are cases in which a hard phase forms in the HAZ, and the toughness decreases. Accordingly, from the standpoint of ensuring the toughness of the HAZ, in the present disclosure, the Si content is 0.50% or less. The Si content is preferably 0.30% or less, or 0.20% or less.(Mn: 0.10% or more and 1.65% or less)

[0026] Mn is an element that is used as a deoxidizing agent, and further, that improves the hardenability of the steel and contributes to increasing the strength. From the standpoint of controlling the O concentration contained in molten steel, in the present disclosure, the Mn content is 0.10% or more. Moreover, due to Mn in an amount of 0.10% or more, by forming MnS, the solid-solution S is reduced, and hot cracking is prevented. From the standpoint of ensuring the strength of the steel product and the toughness of the HAZ, the Mn content is preferably 0.30% or more, or 0.50% or more. On the other hand, if the Mn content is excessive, there are cases in which the toughness after PWHT decreases due to the Mn segregating at the grain boundary at the time of PWHT. Accordingly, from the standpoint of ensuring the toughness of the steel product before and after PWHT, in the present disclosure, the Mn content is 1.65% or less. The Mn content is preferably 1.50% or less, 1.25% or less, or 1.10% or less.(P: 0.025% or less)

[0027] P is an impurity element. Although the lower limit of the P content is not limited, from the standpoint of the manufacturing cost, in the present disclosure, the P content may be 0.001% or more. On the other hand, if the P content is excessive, there are cases in which the toughness after PWHT decreases due to the P segregating at the grain boundary at the time of PWHT. Accordingly, in the present disclosure, the P content is 0.025% or less. The P content is preferably 0.016% or less, 0.012% or less, or 0.008% or less.(S: 0.0250% or less)

[0028] S is an impurity element. Although the lower limit of the S content is not limited, from the standpoint of the manufacturing cost, in the present disclosure, the S content may be 0.0001% or more. On the other hand, if the S content is excessive, there are cases in which elongated MnS is generated at the centerline segregation area, and the toughness and ductility of the steel product and the HAZ deteriorate. From the standpoint of ensuring the toughness and ductility of the steel product 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.(Ni: 2.65% or more and 4.45% or less)

[0029] Ni is an element that is effective in improving the hardenability and toughness of the steel. Therefore, in the present disclosure, the Ni content is 2.65% or more. The Ni content is preferably 3.00% or more or 3.20% or more. However, Ni is an expensive element, and, from the standpoint of cost reduction, in the present disclosure, the Ni content is 4.45% or less. The Ni content is preferably 4.10% or less, or 3.80% or less.(Al: 0.001% or more and 0.100% or less)

[0030] Al is an element that is effective in deoxidation, and is an element that, by forming a nitride, refines the crystal grain diameter at the time of quenching. Therefore, in the present disclosure, the Al content is 0.001% or more. However, if Al is excessively contained, there is the concern that the Al will generate a coarse nitride, and the toughness of the steel product and the HAZ will decrease. Accordingly, the Al content is 0.100% or less. The Al content is preferably 0.080%, or 0.050% or less.(O: 0.0100% or less)

[0031] O is an impurity element. Although the lower limit of the O content is not limited, from the standpoint of the manufacturing cost, in the present disclosure, the O content may be 0.0001% or more. On the other hand, if the O content is excessive, there are cases in which a coarse oxide is generated, and the toughness and ductility of the steel product and the HAZ deteriorate. From the standpoint of ensuring the toughness and ductility of the steel product and the HAZ, the O content is 0.0100% or less. The O content is preferably 0.0060% or less, or 0.0040% or less.(N: 0.0100% or less)

[0032] N is an impurity element. Although the lower limit of the N content is not limited, from the standpoint of the manufacturing cost, in the present disclosure, the N content may be 0.0001% or more. From the standpoint of ensuring the properties of the steel product and the toughness of the HAZ, in the present disclosure, the N content is 0.0100% or less. The N content is preferably 0.0050% or less, or 0.0040% or less.

[0033] The steel product relating to the present disclosure may contain other elements (optional elements) instead of some of the Fe. The following optional elements in Groups A to C are given as examples, but the contents of these elements may be 0%.[Group A]

[0034] In order to improve the strength and toughness, as needed, the steel product relating to the present disclosure may be made to contain one or two or more of the optional elements Cu, Cr, Mo, and B that are described hereinafter and have the effect of improving the hardenability.(Cu: 1.50% or less)

[0035] Cu is an element that is sometimes mixed into the steel product in the manufacturing process. However, the lower limit value of the Cu content is not limited and may be 0%. Further, Cu has little adverse effect on the weldability and on the toughness of the HAZ, and has the effect of improving the hardenability of steel, and therefore, is an element that improves the strength of the steel product. Thus, in the present disclosure, the Cu content may be 0.01% or more. The Cu content is preferably 0.10% or more. However, from the standpoint of suppressing the occurrence of Cu cracking at the time of hot rolling of the steel product, in the present disclosure, the Cu content is 1.50% or less. The Cu content is preferably 1.00% or less, 0.80% or less, 0.60% or less, or 0.50% or less.(Cr: 3.00% or less)

[0036] Cr is an element that is sometimes mixed into a steel product in the manufacturing process. However, the lower limit value of the Cr content is not limited, and may be 0%. Further, Cr is also an element that improves the strength of a steel product because it has the effect of increasing the hardenability of the steel. Therefore, in the present disclosure, the Cr content may be 0.01% or more. The Cr content is preferably 0.10% or more. However, from the standpoint of suppressing deterioration in the toughness of the HAZ and weldability, in the present disclosure, the Cr content is 3.00% or less. The Cr content is preferably 2.20% or less, 1.40% or less, or 0.80% or less.(Mo: 2.00% or less)

[0037] Mo is an element that is sometimes mixed into a steel product in the manufacturing process. However, the lower limit value of the Mo content is not limited, and may be 0%. Further, Mo is also an element that improves the strength of a steel product because it has the effect of increasing the hardenability of the steel. Therefore, in the present disclosure, the Mo content may be 0.01% or more. The Mo content is preferably 0.05% or more, 0.10% or more, 0.20% or more or 0.30% or more. However, from the standpoints of suppressing deterioration in the toughness of the HAZ and weldability, and suppressing an increase in the alloy cost, in the present disclosure, the Mo content is 2.00% or less. The Mo content is preferably 1.20% or less, or 0.80% or less.(B: 0.0050% or less)

[0038] B is an element that is sometimes mixed into a steel product in the manufacturing process. However, the lower limit value of the B content is not limited, and may be 0%. Further, B is also an element that exhibits a marked effect of increasing the hardenability of steel and improves the strength of a steel product. Therefore, in the present disclosure, the B content may be 0.0003% or more. However, from the standpoint of suppressing deterioration in the surface quality of a steel slab manufactured in continuous casting, in the present disclosure, the B content is 0.0050% or less. The B content is preferably 0.0030% or less, or 0.0020% or less.[Group B]

[0039] In order to improve the strength, as needed, the steel product relating to the present disclosure may be made to contain one or two or more of the optional elements Nb, Ti, and V that are described hereinafter and have the effect of increasing the strength of the steel product by precipitates such as carbides or nitrides.(Nb: 0.050% or less)

[0040] Nb is an element that is sometimes mixed into a steel product in the manufacturing process. However, the lower limit value of the Nb content is not limited, and may be 0%. Further, Nb is also an element that forms a carbide or a nitride, and has the effect of refining the microstructure, and improves the strength of the steel product. Therefore, in the present disclosure, the Nb content may be 0.001% or more. However, from the standpoint of suppressing deterioration in the toughness of the HAZ and weldability, 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 standpoint of ensuring the toughness of the steel product after PWHT, the Nb content may be 0.004% or less.(Ti: 0.050% or less)

[0041] Ti is an element that is sometimes mixed into a steel product in the manufacturing process. However, the lower limit value of the Ti content is not limited, and may be 0%. Further, Ti is also an element that forms a carbide or a nitride, and has the effect of refining the microstructure, and improves the strength of the steel product. Therefore, in the present disclosure, the Ti content may be 0.001% or more. However, from the standpoint of suppressing deterioration in the toughness of the HAZ and weldability, the Ti content is 0.050% or less. The Ti content is preferably 0.040% or less, 0.030%, or 0.020% or less. In particular, from the standpoint of ensuring the toughness of the steel product after PWHT, the Ti content may be 0.004% or less, or 0.002% or less.(V: 0.10% or less)

[0042] V is an element that is sometimes mixed into a steel product in the manufacturing process. However, the lower limit value of the V content is not limited, and may be 0%. Further, V is also an element that forms a carbide or a nitride, and improves the strength of the steel product. Therefore, in the present disclosure, the V content may be 0.01% or more. However, from the standpoints of suppressing deterioration in the toughness of the HAZ and weldability, and suppressing an increase in the alloy cost, the V content is 0.10% or less. The V content is preferably 0.08% or less, or 0.05% or less.[Group C]

[0043] In order to improve the toughness of the HAZ, as needed, the steel product relating to the present disclosure may be made to contain one or two or more of the optional elements Mg, Ca, and REM that are described hereinafter.(Mg: 0.0200% or less)

[0044] Mg is an element that is sometimes mixed into a steel product in the manufacturing process. However, the lower limit value of the Mg content is not limited, and may be 0%. Further, Mg is also an element that forms an oxide and improves the toughness of the heat affected zone. Therefore, in the present disclosure, the Mg content may be 0.0003% or more, 0.0006% or more, or 0.0010% or more. On the other hand, if the Mg content is excessive, there are cases in which the Mg forms a coarse oxide and decreases the toughness of the steel. Accordingly, from the standpoint of ensuring the toughness, in the present disclosure, the Mg content is 0.0200% or less. The Mg content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.(Ca: 0.0200% or less)

[0045] Ca is an element that is sometimes mixed into a steel product in the manufacturing process. However, the lower limit value of the Ca content is not limited, and may be 0%. Further, Ca is also an element that, by spheroidizing the sulfide within the steel product, mitigates the effect of the MnS that decreases the toughness of the steel product and the heat affected zone. Therefore, in the present disclosure, the Ca content may be 0.0003% or more, 0.0006% or more, or 0.0010% or more. On the other hand, if the Ca content is excessive, there are cases in which the Ca forms a coarse oxide and decreases the toughness of the steel. Accordingly, from the standpoint of ensuring the toughness, in the present disclosure, the Ca content is 0.0200% or less. The Ca content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.(REM: 0.0200% or less)

[0046] Rare earth metal (REM) is a collective term for a total of 17 elements that are the two elements of Sc and Y and fifteen lanthanoid elements such as La, Ce, Nd. The REM content means the total content of the aforementioned 17 elements. REMs are elements that are sometimes mixed into a steel product in the manufacturing process. However, the lower limit value of the REM content is not limited, and may be 0%. Further, REMs are also elements that form oxides and improve the toughness of the heat affected zone. Therefore, in the present disclosure, the REM content may be 0.0003% or more, 0.0006% or more, or 0.0010% or more. On the other hand, if the REM content is excessive, there are cases in which the REMs form coarse oxides and decrease the toughness of the steel. Accordingly, from the standpoint of ensuring the toughness, in the present disclosure, the REM content is 0.0200% or less. The REM content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.(Balance: Fe and Impurities)

[0047] The balance of the chemical composition of the steel product relating to the present disclosure is iron (Fe) and impurities. Impurities mean components that are mixed due to raw materials such as ore and scrap, and other factors, at the time of industrially manufacturing the steel product.

[0048] In addition to the limits on the contents of the respective elements, in the present disclosure, the range of value α is limited as follows.(Value α: 4.0 or more and 16.0 or less)

[0049] Value α is computed by following formula (1). α = 0.50 × √ C × 1 + 0.64 Si × 1 + 4.10 Mn × 1 + 0.27 Cu × 1 + 0.52 Ni × 1 + 2.33 Cr × 1 + 3.14 Mo

[0050] Wherein [C], [Si], [Mn], [Cu], [Ni], [Cr] and [Mo] are the contents (mass%) of C, Si, Mn, Cu, Ni, Cr and Mo in the steel. In a case in which a given element is not contained, zero is substituted in. Note that √[C] has the same meaning as [C] 1 / 2< .

[0051] In the present disclosure, the range of value α is 4.0 - 16.0. This is an index expressing the hardenability of the steel product. The greater the value α, the more that lower bainite and martensite microstructures having a superior balance of strength and toughness can be formed. When α is in the appropriate range, in the microstructure of the HAZ as well, the ratio of the lower bainite and martensite microstructures having a superior balance of strength and toughness becomes high, and the HAZ toughness also can be ensured. When α is 4.0 or more, the hardenability of the base metal is ensured, and the ratio of the lower bainite and martensite having a favorable balance of strength and toughness increases, and a deterioration in toughness is suppressed. Further, in the microstructure of the HAZ as well, it is easy for the ratio of the lower bainite and martensite to increase, and the HAZ toughness also improves. On the other hand, if value α is 16.0 or less, toughness can be ensured without the strength of the steel product becoming too high. Further, when value α is 16.0 or less, the toughness after PWHT also can be ensured. Moreover, the HAZ toughness also is ensured without the HAZ becoming too hard.

[0052] Due to the aforementioned numerical value range relating to value α being satisfied, there can be provided a nickel-containing steel product for low temperatures that has excellent strength and toughness. Value α is preferably 4.5 or more or 5.0 or more. Further, value α is preferably 15.5 or less or 15.0 or less.<Microstructure>

[0053] The microstructure of the steel product relating to the present disclosure is described next. The microstructure of the region that is 1 / 4 of the thickness in the thickness direction from the surface of the steel product relating to the present disclosure contains lower bainite, martensite and retained austenite. Further, in addition to lower bainite, upper bainite also may be contained as bainite.

[0054] "Bainite" is a microstructure containing bainitic ferrite (α°B) that has a substructure within the grain, and is a collective term for upper bainite and lower bainite. "Upper bainite" is one of or both of upper bainite that contains retained austenite or an MA phase (martensite-austenite constituent) between the laths, and upper bainite that contains a carbide between the laths. "Lower bainite" is lath-shaped lower bainite containing a carbide within the laths.

[0055] There are four forms of "martensite" that are lath, butterfly, lenticular and plate-shaped, but mainly lath martensite is generated in the components of the present disclosure. Lath martensite is a microstructure that is composed of packets and blocks formed from groups of laths having a specific arrangement, and in which a single austenite grain is divided into plural packets.(Total of Area Ratios of Lower Bainite and Martensite: 15.0% or more)

[0056] Lower bainite and martensite are hard phases and increase the toughness of the steel product. From the standpoint of ensuring the toughness of the steel product, the area ratio of lower bainite and martensite at the 1 / 4t portion is 15.0% or more. The area ratio of lower bainite and martensite at the 1 / 4t portion is preferably 20.0% or more, or 30.0% or more. The total of the area ratio of the lower bainite and the area ratio of the martensite at the 1 / 4t portion may be 95.0% or more.(Total of Area Ratios of Upper Bainite, Lower Bainite, and Martensite: 90.0% or more)

[0057] From the standpoint of ensuring the strength of the steel product, the total of the area ratios of upper bainite and lower bainite and martensite at the 1 / 4t portion is 90.0% or more. The total of the area ratios of upper bainite, lower bainite and martensite at the 1 / 4t portion may be 95.0% or more. Further, the upper bainite of the 1 / 4t portion may be 1.0% or more.(Area ratio of retained austenite: 0.2% or more and less than 5.0%)

[0058] Retained austenite enhances the toughness of the steel product. The area ratio of retained austenite at the 1 / 4t portion is 0.2% or more from the viewpoint of securing toughness of the steel product. The area ratio of retained austenite at the 1 / 4t portion is preferably 0.3% or more, or 0.5% or more. On the other hand, the area ratio of retained austenite at the 1 / 4t portion of the steel plate according to the present disclosure is less than 5% at most. The area ratio of retained austenite at the 1 / 4t portion is preferably 3.0% or less, or 2.0% or less.

[0059] Observation of the microstructure of the steel product is carried out by using a sample in which a 1 / 4t portion of the steel product serves as the observed surface. Two types of samples on which (a) electrolytic polishing or (b) nital etching is carried out are prepared. Each sample of (a) and (b) is measured at three places by the following method, and the average value of the three places is used as the area ratio of the microstructure of that steel product. Note that three of each sample of (a) and (b) may be prepared, and the averages of the respective samples may be computed. Or, three places of one sample of each may be measured in a visual field, and the average of each computed.

[0060] By using an electrolytically polished sample that, after mirror finishing by mechanical polishing, is subjected to electrolytic polishing that removes the strained layers arising due to the mechanical polishing, measurement of the total of the area ratios of the upper bainite, lower bainite, martensite and retained austenite is carried out by EBSD. The measurement magnification is 200×, and measurement of a range of 400 µm × 400 µm is carried out at a pitch of 0.4 µm. The measuring is carried out in a state in which the beam diameter of the electron beam is 0.4 µm or less. The confidence index (hereinafter called "CI value") is set to 0.1 or more. The determination of ferrite, and upper bainite, lower bainite, martensite and retained austenite, is carried out by setting the threshold value of the Grain Average Misorientation (hereinafter called "GAM") to 0.5. Note that the GAM value is an index defined in OIM Analysis (EBSD crystal orientation analyzing software manufactured by TSL, United States). The region in which the GAM is 0.5 or less is ferrite. The region in which the GAM exceeds 0.5 is upper bainite, lower bainite, martensite or retained austenite. Because the upper bainite, lower bainite, martensite and retained austenite in the present disclosure are determined by using the GAM of EBSD as the threshold value, not only upper bainite, lower bainite, martensite and retained austenite, but also tempered upper bainite, tempered lower bainite and tempered martensite are included. Comparing the microstructures of direct quenching (DQ) and (DQT) that is carried out thereafter up to tempering (T), although dissolution of the MA and coarsening of carbide occur after tempering, the way of looking at the microstructure does not vary greatly.

[0061] The area ratio of upper bainite is measured by SEM observation using a nital-etched sample. The measurement magnification is 500 times, and the measurement is performed in a range of 360 µm × 480 µm. A portion having a clear lath structure and having carbides or MA generated along the lath boundary is upper bainite. The structure of a region where the internal structure of the microstructure is relatively coarse, the density of carbides is sparse, and sparseness and denseness are mixed is defined as upper bainite. Fig. 1 shows an example of a result of structure discrimination. (A) and (B) are SEM images of the same region of a steel product manufactured by DQT and having an α value of 9.9. In (B), the region surrounded by the white line is upper bainite (Bu), and the other region is lower bainite + martensite (B L + M). In the portion determined as upper bainite (Bu), carbides, which appear white, are sparse, and sparse and dense regions are mixed. On the other hand, in the portion discriminated as lower bainite + martensite (B L + M), carbides are densely and uniformly present. The total area ratio of lower bainite, martensite, and retained austenite is determined by subtracting the area ratio of upper bainite from the total area ratio of upper bainite, lower bainite, martensite, and retained austenite measured above. Further, the total area ratio of lower bainite and martensite is determined by using a measurement method described later to determine the area ratio of retained austenite and subtracting the area ratio of retained austenite from the total area ratio of lower bainite, martensite, and retained austenite.

[0062] When the total area ratio of lower bainite and martensite is more than 0% according to the above structure discrimination, lower bainite and martensite are usually included. Lower bainite and martensite can be distinguished by a SEM or TEM (transmission electron microscope), and the presence of each structure can be found.(Area ratio of retained austenite)

[0063] The area ratio of retained austenite is measured by an X-ray diffraction method. The area ratio of retained austenite is measured using a sample in which a part of 1 / 4 of the thickness in the thickness direction from a surface of the steel product (in the present specification, also referred to as a "1 / 4t portion") is taken as a measured area. The sample is a test piece having a thickness of 2 mm; the sample is taken from a position of 1 / 4 of the width from an end portion in the width direction (a direction perpendicular to the rolling direction and the thickness direction) of the steel product, is subjected to chemical polishing, and is used for measurement of the volume ratio of retained austenite by an X-ray diffraction method using a Mo tube. Quantification was performed based on the ratio of the integrated intensities of the (200) and (211) diffraction peaks of the ferrite phase to the integrated intensities of the (200), (220), and (311) diffraction peaks of the austenite phase, and the average value of six combinations was taken. The integrated intensity of the diffraction peak is obtained by fitting the background on the basis of the signals before and after the peak and subtracting the signals. The volume ratio measured by the X-ray diffraction method is regarded as the area ratio.(Aspect ratio of the prior austenite grain at the 1 / 4t portion of the steel product)

[0064] When manufacturing a steel product according to the present disclosure, for example, after hot rolling, either direct quenching (DQ), in which the steel product is cooled with water as it is, or reheated quenching (RQ), in which the steel product is allowed to cool, then reheated, and subsequently cooled with water, is performed. After DQ or RQ is performed, intermediate heat treatment (L) is further performed.

[0065] In a steel product according to the present disclosure that is subjected to DQ, the form of the prior austenite grain (occasionally referred to as a prior austenite grain or a prior γ grain) may be a shape flattened in the rolling direction. When the prior austenite grain at a portion of 1 / 4 of the thickness in the thickness direction from a surface of the steel product is made to be a flat grain having an aspect ratio of 1.5 or more, the toughness of the steel product can be improved even more. This is because, by flattening the prior austenite grain to increase the grain boundary area, the austenite grain is substantially made minute, which is effective in reducing the average crystal grain diameter. The aspect ratio of the prior austenite grain is usually 4.0 or less, and may be 3.5 or less.

[0066] In a steel product according to the present disclosure that is subjected to RQ, for the form of the prior austenite grain, the aspect ratio of the prior austenite grain at the 1 / 4t portion may be less than 1.5 from the viewpoint of securing homogeneity of microstructure. The aspect ratio of the prior austenite grain at the 1 / 4t portion may be preferably 1.4 or less, or 1.3 or less.

[0067] The aspect ratio of the prior austenite grains (hereinafter called prior austenite grains upon occasion) of the steel product is determined as follows. An L cross-section (a cross-section parallel to the rolling direction and the thickness direction of the steel product) of the region that is 1 / 4 of the thickness in the thickness direction from the surface of the steel product is mirror polished, and corrosion is carried out by a corrosive liquid of a saturated solution base of 2 - 4% picric acid, and the prior austenite grain boundaries of an arbitrary region of 1.0 mm in the rolling direction × 0.5 mm in the thickness direction are made to appear.

[0068] Next, the long diameters and short diameters of the individual prior austenite grains are measured, and the aspect ratio of each prior austenite grain is calculated by long diameter ÷ short diameter. The arithmetic mean of the calculated aspect ratios of all of the prior austenite grains is determined as the "aspect ratio of the prior austenite grains". Note that the maximum length of the prior austenite grain is used as the long diameter, and the maximum interval between two lines, which contact the grain and are parallel to the long diameter direction, is used as the short diameter.(Average Crystal Grain Diameter at 1 / 4t Portion of Steel Product)

[0069] In the present disclosure, the average crystal grain diameter (effective crystal grain diameter) of the 1 / 4t portion at the steel product is preferably 20.0 µm or less. This is because it was learned that, if the average crystal grain diameter at the 1 / 4t portion of the steel product is 20.0 µm or less, the toughness of the steel product tends to improve even more regardless of whether before or after PWHT. However, the average crystal grain diameter at the 1 / 4t portion of the steel product may exceed 20.0 µm. The smaller the average crystal grain diameter of the steel product, the more preferable, and therefore, the lower limit value thereof is not limited. Usually, the average crystal grain diameter is 10 µm or more. The effective crystal grain diameter is determined by a weighted average. Effective crystal grain diameter D area that is determined by a weighted average is calculated by the following formula by using, of the crystal grain diameters that are measured in a 4 mm 2< region, area S i and grain diameter d i of the ith crystal grain detected at the time of measurement. D area = ΣS i • d i / ΣS i <Mechanical Properties>

[0070] The steel product relating to the present disclosure has mechanical properties that are such that the steel product has both strength and low-temperature toughness. In addition to having excellent toughness at -100°C in particular, the steel product can exhibit excellent low-temperature toughness after PWHT as well.(Tensile Strength: 590 MPa or more and 930 MPa or less)

[0071] In the present disclosure, the tensile strength of the steel product is 590 - 930 MPa. In order to reduce the weight of large welded structures such as transport tanks, a steel product that can ensure strength of a structure even if the thickness is thin is necessary. Because steel products that are selected as steel products to be used in such applications usually are steel products having the aforementioned tensile strength, the steel product relating to the present disclosure also is manufactured to have the aforementioned tensile strength.(Charpy Impact Absorption Energy at -100°C)

[0072] In order to ensure high toughness at low temperatures, the Charpy impact absorption energy at -100°C of the steel product of the present disclosure is preferably 150 J or more. Due to the steel product of the present disclosure having low-temperature toughness of a Charpy impact absorption energy at -100°C of 150 J or more, a transport tank formed from the steel product of the present disclosure can be suitably used for transporting liquid carbon dioxide. Note that the Charpy impact absorption energy at -100°C is a numerical value measured by using a sample taken from a position of 1 / 4 of the thickness.(Charpy Impact Absorption Energy at -100°C after PWHT)

[0073] There are cases in which PWHT is carried out on the welded portions of low temperature tanks after being assembled into transport tanks, in order to prevent breakage in advance. At this time, not only the welded portions, but also the base metal portion (also simply called base metal) of the steel product that is not affected by welding are heated. If the time over which the base metal is heated in the temperature range of 425°C or more is long, the toughness of the base metal tends to decrease. In the steel product of the present disclosure, in a case in which PWHT, in which the holding temperature is 600°C, and the holding time is 2 hours, and the rate of temperature increase and the rate of temperature decrease are 55°C / h in the temperature region of 425°C or more, is carried out on the steel product, with regard to the toughness of the portion where the PWHT has been carried out, the Charpy impact absorption energy at -100°C is preferably 150 J or more. The Charpy impact absorption energy at -100°C after PWHT may be 100 J or more. The Charpy impact absorption energy at -100°C after PWHT also is a numerical value measured by using a sample taken from a position of 1 / 4 of the thickness.(Charpy Impact Absorption Energy at -100°C after Thermal Cycling)

[0074] In the steel product of the present disclosure, in order to ensure high toughness at a low temperature after a thermal cycling test that simulates the welded portions, the Charpy impact absorption energy at -100°C after thermal cycling is preferably 50 J or more. Due to the steel product of the present disclosure having low-temperature toughness that is such that the Charpy impact absorption energy at -100°C after thermal cycling is 50 J or more, a transport tank formed from the steel product of the present disclosure can be suitably used for transporting liquid carbon dioxide for example. The Charpy impact absorption energy at -100°C after thermal cycling may be 40 J or more. Note that the Charpy impact absorption energy at -100°C after thermal cycling is a numerical value measured by using a sample taken from a position of 1 / 4 of the thickness of the steel product as a thermal cycling test piece, and providing it with a thermal history of raising the temperature at 60°C / s to 1350°C, and, after maintenance at 1350°C for 1 s, cooling at 20°C / s to room temperature, and thereafter, taking a Charpy test piece therefrom.(Charpy Impact Absorption Energy at -100°C after Thermal Cycling and PWHT)

[0075] There are cases in which PWHT is carried out on the welded portions of low temperature tanks after being assembled into transport tanks, in order to prevent breakage in advance. After the above-described thermal cycling test, PWHT, in which the rate of temperature increase and the rate of temperature decrease are 55°C / h in the temperature region of 425°C or more, and the steel product is held for 2 hours at 600°C, is carried out on the steel product of the present disclosure, and thereafter, a Charpy test piece is taken, and measurement is carried out. In this case, with regard to the toughness of the portion where the PWHT has been carried out, the Charpy impact absorption energy at -100°C is preferably 50 J or more. The Charpy impact absorption energy at -100°C of the portion at which PWHT is carried out after the above-described thermal cycling test may be 40 J or more.

[0076] Note that there are cases in which the toughness of the steel product decreases due to PWHT. Although the reason for this is not clear, it is assumed that P (phosphorus) and Mn diffuse at the grain boundaries, and growth or aggregation of inclusions arises within the microstructure, and due thereto, the brittleness decreases and the toughness decreases. A decrease in the toughness due to PWHT is suppressed by limiting the contents of P and Mn and making the average crystal grain diameter of the steel product small.

[0077] The tensile strength (TS) and the yield strength (YS) in Examples are measured by a tensile test in accordance with JIS Z2241:2011. In the tensile test, a JIS 14A test piece that is taken from a 1 / 4 thickness position and of which the longitudinal direction is a direction (C direction) parallel to the width direction of the steel product is used. Each of TS and YS is measured using three test pieces, and is calculated by averaging them.

[0078] The Charpy impact absorption energy is measured by a Charpy impact test at -100°C using an impact blade having a radius of 2 mm in accordance with JIS Z2242:2018. The Charpy impact absorption energy is measured using three test pieces, and is calculated by averaging them. In the Charpy impact test, a V-notched test piece that is taken from a 1 / 4 thickness position of the steel product and of which the longitudinal direction is a direction (C direction) parallel to the width direction of the steel product is used. The V-notch of the test piece is formed such that the longitudinal direction of the notch is the thickness direction of the steel product and the depth direction of the notch is the rolling direction of the steel product.

[0079] The form of the steel product relating to the present disclosure is not particularly limited, and examples are steel plates, steel strips, structural steel and steel pipes. However, steel pipes and structural steel include steel products in which steel plates are joined, e.g., in addition to welded steel pipes and welded structural steel, structural steel joined by rivets, and the like. The thickness of the steel product such as steel plates, steel strips, structural steel and steel pipes (the thickness of the flanges in the case of structural steel) is not particularly limited, and usually is 3 mm or more and 150 mm or less. The thickness of the steel product may be 6 mm or more, 10 mm or more, 15 mm or more, or 30 mm or more. Further, the thickness of the steel product may be 100 mm or less, 80 mm or less, or 60 mm or less.

[0080] Further, although the application of the steel product relating to the present disclosure also is not particularly limited, the steel product relating to the present disclosure has mechanical properties such that the steel product has both strength and low-temperature toughness, and, in particular, can exhibit excellent low-temperature toughness even after PWHT. Therefore, the steel product relating to the present disclosure can be suitably used as a constituent material for a tank that stores and transports liquefied gasses, and liquid carbon dioxide in particular.(Method for manufacturing a steel product)

[0081] For the steel product according to the present disclosure, a steel slab is manufactured by smelting steel satisfying the chemical composition described above and then performing continuous casting. The steel slab is subjected to either direct quenching (DQ), in which it is heated, hot rolled, and then directly water-cooled, or reheat quenching (RQ), in which it is hot rolled, air-cooled, reheated, and then water-cooled. After that, intermediate heat treatment (L) is further performed to obtain a steel product. Note that, in the case of RQ, the hot rolled steel does not necessarily have to be air-cooled before reheating, but water cooling may be used. Moreover, tempering (T) may be carried out.

[0082] The heating temperature of the steel to be rolled is Ac 3 or higher from the viewpoint of performing hot rolling in a temperature range in which the microstructure of the steel to be rolled is austenite. The heating temperature of the steel to be rolled is preferably 1000°C or higher from the viewpoint of reducing deformation resistance. On the other hand, the heating temperature of the steel to be rolled is 1250°C or lower from the viewpoint of suppressing coarsening of heated γ grains. The heating temperature of the steel to be rolled is preferably 1200°C or lower. Note that Ac 3 is a value calculated by the following formula. Ac 3 = 937.2 − 436.5 C + 56 Si − 19.7 Mn − 16.3 Cu − 26.6 Ni − 4.9 Cr + 38.1 Mo + 124.8 V + 136.3 Ti − 19.1 Nb + 198.4 Al + 3315 B

[0083] The symbol of an element in the formula means the content amount (mass%) of the element contained in the steel slab.

[0084] There are cases in which the hot rolling is structured by rolling in a temperature range in which recrystallization occurs (rolling in the recrystallization temperature range) and rolling in a temperature range in which recrystallization is suppressed (rolling in the non-recrystallization temperature range).

[0085] Rolling in the recrystallization temperature range is hot rolling carried out with the temperature of the rolled steel during rolling being 900°C or more. From the standpoint of refining the austenite grain diameter of the steel product, the cumulative rolling reduction ratio of the rolling in the recrystallization temperature range is preferably 30% or more, 40% or more, or 50% or more. The cumulative rolling reduction ratio of the rolling in the recrystallization temperature range is determined from the difference between the thickness of the steel slab before hot rolling and the thickness of the rolled steel at 900°C.

[0086] Rolling in the non-recrystallization temperature range is hot rolling carried out with the temperature of the rolled steel during rolling being less than 900°C. From the standpoint of refining the average crystal grain diameter of the steel product, the cumulative rolling reduction ratio of the rolling in the non-recrystallization temperature range is preferably 30% or more, 40% or more, or 50% or more. The cumulative rolling reduction ratio of the rolling in the non-recrystallization temperature range is determined from the difference between the thickness of the rolled steel at 900°C and the thickness of the steel product after rolling ends.

[0087] From the standpoint of suppressing the generation of ferrite that decreases strength, the end temperature of the hot rolling is Ar 3 or more. After hot rolling ends, accelerated cooling such as water cooling is carried out on the steel product. From the standpoint of suppressing the generation of ferrite that decreases strength, the start temperature of the accelerated cooling is Ar 3 or more. Note that Ar 3 is a value computed by the following formula. Ar 3 = 910 − 310 C − 80 Mn − 20 Cu − 15 Cr − 55 Ni − 80 Mo + 0.35 t − 8

[0088] The element symbols in the formula mean the content (mass%) of each element contained in the steel product, and t means the thickness (mm) of the steel product.

[0089] From the standpoint of promoting bainitic transformation and martensitic transformation, the cooling rate is 1.0°C / s or more. The cooling rate of the accelerated cooling is preferably 5.0°C / s or more, or 10.0°C / s or more. The faster the cooling rate of the accelerated cooling, the more preferable, but from standpoints such as cost and homogeneity of the accelerated cooling, the cooling rate is preferably 50.0°C / s or less, or 30.0°C / s or less. The cooling rate is a value obtained by calculating the cooling rate at a position of 1 / 4 of the thickness by simulation in accordance with thermal transfer calculation.

[0090] From the standpoint of improving the strength of the steel product by ensuring the upper bainite, lower bainite and martensite, the stoppage temperature of the accelerated cooling is 400°C or less. The stoppage temperature of the accelerated cooling is preferably 350°C or less. Accelerated cooling may be carried out to room temperature. From the standpoint of dehydrogenation of the steel product, the stoppage temperature of the accelerated cooling is preferably 100°C or more.

[0091] In the case where RQ is performed, the steel product is, after hot rolling, subjected to reheated quenching. The reheating temperature of the steel product is Ac 3 or higher because quenching is to be performed from the structure of the austenite single phase. The reheating temperature of the steel product is preferably 850°C or higher, 880°C or higher, or 900°C or higher from the viewpoint of securing homogeneity of microstructure. On the other hand, the upper limit temperature of reheating temperature is not particularly specified, but is preferably 1000°C or lower, 950°C or lower, or 930°C or lower because heating to an excessively high temperature may cause coarsening of austenite grains and a reduction in toughness. Note that Ac 3 is a value calculated by the formula described above.

[0092] The steel product is, after being subjected to DQ or RQ, subjected to intermediate heat treatment (L). The intermediate heat treatment is held in a temperature range of Ac 1 or higher and Ac 3 or lower for the purpose of securing stable retained austenite even at low temperature. The holding time is preferably 20 minutes or more from the viewpoint of concentrating the elements into precipitated austenite. On the other hand, the holding time is preferably 120 minutes or less because heat treatment for a long time causes a reduction in productivity. The cooling is preferably water cooling (quenching). Note that Ac 1 is a value calculated by the following formula. Ac 1 = 750.8 − 26.6 C + 17.6 Si − 11.6 Mn − 22.9 Cu − 23 Ni + 24.1 Cr + 22.5 Mo − 39.7 V − 5.7 Ti + 232.4 Nb − 169.4 Al − 894.7 B

[0093] The symbol of an element in the tormula means the content amount (mass%) of the element contained in the steel product.

[0094] After the intermediate heat treatment, a tempering treatment may be carried out on the steel product. From the standpoint of suppressing a decrease in strength, the heating temperature of the tempering treatment is preferably 660°C or less, or 640°C or less. On the other hand, from the standpoint of improving the toughness, the heating temperature of the tempering treatment is preferably 400°C or more, 450°C or more, or 500°C or more.EXAMPLES

[0095] The steel product relating to the present disclosure is described concretely hereinafter by way of Examples. However, the conditions in the following Examples are examples of conditions that are employed in order to confirm the feasibility and the effects of the present disclosure, and the steel product relating to the present disclosure is not limited to the following Examples.[Manufacturing of Steel Product]

[0096] First, slabs having the chemical compositions shown in Table 1 were cast by continuous casting. The balance, which is other than the components listed in Table 1, is Fe and impurities. Further, blank cells mean that the alloy elements were not intentionally added in the steelmaking process. The underlines mean that the value is outside of the scope of the present disclosure.

[0097] Next, steel products were manufactured from these slabs under the manufacturing conditions listed in Table 2. "Temper heat treatment" is the heating temperature in the tempering treatment after the quenching. [Table 2]No.Plate thickness (mm)Ac 3 [°C]Ac 1 [°C]Rolling conditionsAr 3 [°C]Direct quenching conditionsReheated quenching conditionsIntermediate heat treatment conditionsTemper heat treatment [°C]Heating temperature [°C]Cumulative rolling reduction ratio at 900°C or higher [%]Cumulative rolling reduction ratio at less than 900°C [%]End temperature [°C]Cooling after rollingStart temperature [°C]Stopage temperature [°C]Cooling rate [°C / s]Reheating temperature [°C]Cooling after reheatingIntermediate heat treatment temperature [°C]Cooling after reheating12577964311207559870air-cooling545---910water-cooling680water-cooling52022577964311207657880air-cooling545---1050water-cooling680water-cooling55034581772011005269870air-cooling615---950water-cooling750air-cooling62043574763411606360790water-cooling5277801509.2--670water-cooling48054084166610904570860air-cooling549---970water-cooling690water-cooling62063073963611305672790water-cooling5097703307.1--650water-cooling50074081366511206651840air-cooling582---920water-cooling690water-cooling65085082869010403966760water-cooling615750206.2--710water-cooling64094582867310906260770water-cooling586760505.4--700water-cooling620104580865511004268860air-cooling604---930water-cooling680water-cooling580116075664311505841840air-cooling551---900water-cooling690water-cooling600125578063911205652800water-cooling5187901102.6--650water-cooling610133580166711004872760water-cooling616750206.2--690water-cooling560145084368610607437850air-cooling614---940water-cooling710water-cooling600154583267511205559830water-cooling629810309.3--690water-cooling610164084168611104868780air-cooling623---920water-cooling700water-cooling500175075464211306942860air-cooling545---1020water-cooling660water-cooling550184581066911404762870air-cooling598---980water-cooling720water-cooling580193080966911905969810water-cooling59980021010.1--690water-cooling570205080465011406442830air-cooling592---920water-cooling670water-cooling630214577365111204268790water-cooling537770305.1--680water-cooling590224077665810805960770air-cooling539---940water-cooling670water-cooling5501014576063711205558770air-cooling520---940water-cooling690water-cooling6001025584968810806535840air-cooling645---910water-cooling730water-cooling6401032576063611407853790air-cooling532---930water-cooling710water-cooling5701044580367211303969820air-cooling604---980water-cooling720water-cooling6001053073963311605870790water-cooling4987801007.6--680water-cooling5101065080066211005554760air-cooling593---950water-cooling710water-cooling6201076083069010906248810air-cooling596---910water-cooling760water-cooling6301083580264911105369770water-cooling592760805.8--720water-cooling6101094081864911206859830air-cooling546---920water-cooling720water-cooling6201105579763611005252820air-cooling589---910water-cooling730water-cooling6201115080766510904959800air-cooling645---930water-cooling750water-cooling6301123578767011206854810water-cooling5388001208.1--700water-cooling5801133578965011205369830air-cooling511---950water-cooling700water-cooling5601146080265011401917850water-cooling5828401101.9--750water-cooling6201154084568011203176640water-cooling6476202505.4--730water-cooling4901162580067210708813860air-cooling608---880water-cooling710water-cooling470 [Measurement and evaluation]

[0098] The microstructure and mechanical properties of the obtained steel product were measured by the methods described above. Table 3 shows the results. The meanings of the symbols of microstructure are as follows. For the region determined as lower bainite + martensite (B L + M), the presence of lower bainite and martensite was found by SEM observation. The balance of the microstructure is pearlite, an MA phase, or ferrite. Bu: upper bainite BL: lower bainite M: martensite Retained γ: retained austenite

[0099] For the toughness, the average value of Charpy impact absorption energies (KV2) at -100°C, and the average value of Charpy impact absorption energies at -100°C after a PWHT in which the holding temperature was 600°C, the holding time was 2 hours, and each of the rates of temperature increase and temperature decrease in a temperature range of 425°C or higher was 55°C / h were measured. [Table 3]No.Microstructure (area ratio) of steel productAspect ratio of prior γ grain Average valueAverage crystal grain diameter [µm]YR [%]YS [Mpa]TS [Mpa]Base metal toughness KV2 [J]Toughness after PWHT KV2 [J]Thermal cycle toughness KV2 [J]Toughness after thermal cycling and PWHT KV2 [J]NotesRetained γ [%]BL+M [%]Bu+BL+M [%]10.519.291.61.318.279498632185172--20.821.392.01.223.2 79502638158150--30.296.797.21.218.289748842162152--41.178.195.42.619.483602726172152--51.290.397.11.216.289737831177169--62.666.296.43.218.484632749202162--71.464.795.51.313.883567685296297--80.941.394.52.313.682543664302305--90.777.595.32.213.786648755287285--Examples of present invention101.670.896.11.314.084614732282281--111.390.098.21.317.488763872182155--120.888.198.41.818.188782892161152--131.473.295.53.113.783606734289286--141.780.197.01.214.284658784280280--151.138.494.62.015.081522644284295--161.858.395.61.413.779515652310311--170.959.796.81.121.5 84601716160152--180.435.396.51.214.380502625261265--191.285.097.32.814.687713822229202--200.76.9 74.8 1.225.683468 564 10295--Comparative examples210.874.894.92.516.38567279510923--220.496.896.91.321.289864 968 9311--1011.720.491.11.319.27851365522215410277Examples of present invention1020.821.990.81.214.1795416812422111721511031.531.590.51.218.783518621181162110961040.950.492.21.113.6815727082822801821801052.264.893.53.015.4896617422141971311221061.160.493.11.114.3866557612772681981941070.978.295.81.218.3876988021821771151091081.085.598.12.813.9897778722322271921881090.787.297.71.215.8907618411911901521501100.84.5 70.5 1.135.1 79442 562 36 24 27 19 Comparative examples1110.95.2 68.2 1.236.278432 555 42 35 21 18 1121.197.298.82.419.593898 962 57 49 39 31 1130.898.599.01.319.492902 981 62 35 36 16 1140.812.2 95.81.4 28.5 8560871890 87 - - 1150.75.9 84.2 3.034.4 78456 581 44 40 - - 1160.710.5 91.11.113.482498608136 126 - -

[0100] Nos. 1 to 19 and 101 to 109 are Examples of present invention, and Nos. 20, 21, 22, and 110 to 116 are Comparative Examples.

[0101] In No. 20, since α was too small, sufficient hardenability was not obtained, sufficient strength was not obtained, and sufficient low-temperature toughness was not obtained either.

[0102] In No. 21, since the Mn content was too large, sufficient low-temperature toughness was not obtained either before or after PWHT.

[0103] In No. 22, since the α value was too high, the strength was excessive, and sufficient low-temperature toughness was not obtained either before or after PWHT.

[0104] In Nos. 110 and 111, the α value was less than the lower limit value of the present disclosure, and the hardenability was insufficient and the strength was insufficient. Sufficient low-temperature toughness was not obtained either.

[0105] In Nos. 112 and 113, the α value exceeded the upper limit value of the present disclosure, and the hardenability was too high and the strength was excessive.

[0106] In Nos. 114 to 116, the total area ratio of lower bainite and martensite was insufficient, and sufficient low-temperature toughness was not obtained either before or after PWHT.

[0107] In contrast to the Comparative Examples, in all the Examples of present invention (Nos. 1 to 19 and 101 to 109), the chemical composition and microstructure of the steel product were appropriately controlled, the tensile strength was in an appropriate range of 590 MPa or more and 930 MPa or less, and furthermore the Charpy impact absorption energy at - 100°C was high regardless of before or after PWHT and low-temperature toughness of 150 J or more was obtained.Industrial Applicability

[0108] The steel products relating to the present disclosure can be used mainly as material for transport tanks of liquefied carbon dioxide. Further, the steel products relating to the present disclosure can also be used in manufacturing other welded structures such as buildings, bridges, ships, pipelines, offshore structures, pressure vessels and tanks.

[0109] The disclosures of Japanese Patent Application No. 2023-114105 filed on July 11, 2023 are, in their entireties, incorporated by reference into the present specification. All publications, patent applications, and technical standards mentioned in the present specification are incorporated by reference into the present specification to the same extent as if such individual publication, patent application, or technical standard was specifically and individually put forth herein.

Claims

1. A steel product, comprising a chemical composition of, in mass%: C: from 0.03% to 0.20%, Si: from 0.01% to 0.50%, Mn: from 0.10% to 1.65%, P: 0.025% or less, S: 0.0250% or less, Ni: from 2.65% to 4.45%, Al: from 0.001% to 0.100%, O: 0.0100% or less, N: 0.0100% or less, Cu: 0 to 1.50%, Cr: 0 to 3.00%, Mo: 0 to 2.00%, B: 0 to 0.0050%, Nb: 0 to 0.050%, Ti: 0 to 0.050%, V: 0 to 0.10%, Mg: 0 to 0.0200%, Ca: 0 to 0.0200%, REM: 0 to 0.0200%, and a balance of Fe and impurities, the chemical composition having a value of α represented by the following Formula (1) of from 4.0 to 16.0, wherein a tensile strength is from 590 MPa to 930 MPa, a microstructure of a region that is 1 / 4 of a thickness, in a thickness direction, from a surface of the steel product comprises lower bainite, martensite, and retained austenite, a total area ratio of the lower bainite and the martensite is 15.0% or more, a total area ratio of upper bainite, the lower bainite, and the martensite is 90.0% or more, and an area ratio of the retained austenite is from 0.2% to less than 5.0%: 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 wherein a symbol of an element in square brackets in Formula (1) represents a content amount (mass%) of a corresponding element contained in the steel product, provided that zero is substituted when the corresponding element is not included.

2. The steel product according to claim 1, wherein the chemical composition comprises the following Group A: [Group A]: one or two or more selected from the group consisting of: Cu: from 0.01% to 1.50%, Cr: from 0.10% to 3.00%, Mo: from 0.01% to 2.00%, and B: from 0.0003% to 0.0050%.

3. The steel product according to claim 1 or 2, wherein the chemical composition comprises the following Group B: [Group B]: one or two or more selected from the group consisting of: Nb: from 0.001% to 0.050%, Ti: from 0.001% to 0.050%, and V: from 0.01% to 0.10%.

4. The steel product according to any one of claims 1 to 3, wherein the chemical composition comprises the following Group C: [Group C]: one or two or more selected from the group consisting of: Mg: from 0.0003% to 0.0200%, Ca: from 0.0003% to 0.0200%, and REM: from 0.0003% to 0.0200%.

5. The steel product according to any one of claims 1 to 4, wherein an aspect ratio of a prior austenite grain in the region that is 1 / 4 of the thickness, in the thickness direction, from the surface of the steel product is 1.5 or more.

6. The steel product according to any one of claims 1 to 4, wherein an aspect ratio of a prior austenite grain in the region that is 1 / 4 of the thickness, in the thickness direction, from the surface of the steel product is less than 1.5.

7. The steel product according to any one of claims 1 to 6, wherein, in the microstructure of the region that is 1 / 4 of the thickness, in the thickness direction, from the surface of the steel product, an average crystal grain diameter is 20.0 µm or less.

8. The steel product according to any one of claims 1 to 7, wherein a Charpy impact absorption energy at -100°C is 150 J or more.

9. The steel product according to any one of claims 1 to 8, wherein, in a case in which a heat treatment, in which a rate of temperature increase and a rate of temperature decrease in a temperature region of 425°C or more are 55°C / h and temperature is held for 2 hours at 600°C, is carried out on the steel product, a Charpy impact absorption energy at -100°C of portions where the heat treatment has been carried out is 150 J or more.

10. A liquid carbon dioxide transportation tank, comprising the steel product according to any one of claims 1 to 9.

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