WO2024190921A1 - Steel material - Google Patents

Abstract

This steel material comprises a specific chemical composition in which α represented by the following formula is 4.0-16.0. The steel material has a tensile strength of 590-930 MPa. In the steel material: a micro structure at a segment located 1/4 of the thickness from the surface includes lower bainite and martensite; the total of the area percentage of lower bainite and martensite is 15.0% or more; and the total of the area percentage of upper bainite, lower bainite, and martensite is 90.0% or more. α=0.50×√[C]×(1+0.64[Si])×(1+4.10[Mn])×(1+0.27[Cu])×(1+0.52[Ni])×(1+2.33[Cr])×(1+3.14[Mo])

Description

Steel

This disclosure relates to steel materials.

Steel can be used for welded structures such as buildings, bridges, ships, line pipes, marine structures, pressure vessels, and tanks. Steel has excellent strength and low-temperature toughness, making it effective for low-temperature applications.

Low-temperature steels are used for low-temperature pressure vessels such as storage tanks for liquefied gas. Low-temperature steels include Al-killed steel, nickel steel, high Mn steel, and austenitic stainless steel, depending on the operating temperature. For example, nickel steels such as 3.5% Ni steel are used as materials for tanks that carry liquefied ethane and liquefied ethylene, which have operating temperatures of around -100°C.

Ni is often included in steel materials that require low-temperature toughness, such as this 3.5% Ni steel, such as low-temperature pressure vessels.

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

Patent Document 1: JP 2019-81930 A Patent Document 2: International Publication No. 2014/103629 Patent Document 3: JP 52-156121 A Patent Document 4: JP 55-104427 A Patent Document 5: JP 58-73717 A Patent Document 6: JP 7-331328 A Patent Document 7: JP 2001-123222 A Patent Document 8: JP 2001-123245 A Patent Document 9: JP 2007-46096 A

Cryogenic steels used in cryogenic pressure vessels are expected to achieve both high strength and low-temperature toughness. Cryogenic pressure vessels are also manufactured by welding steel materials, and may be subjected to post-weld heat treatment (sometimes called PWHT) to remove residual stresses caused by welding. Recently, there has been an increased demand for low-temperature toughness of steel materials after PWHT.

The objective of this disclosure is to provide a steel material that is suitable for low-temperature applications, having high tensile strength and good low-temperature toughness regardless of whether it is before or after post-weld heat treatment.

The gist of the present disclosure is as follows.
<1> In mass%,
C: 0.03% or more, 0.20% or less,
Si: 0.01% or more, 0.50% or less,
Mn: 0.10% or more, 1.65% or less,
P: 0.025% or less,
S: 0.0250% or less,
Ni: 2.65% or more, 4.45% or less,
Al: 0.001% or more, 0.100% or less,
O: 0.0100% or less,
N: 0.0100% or less,
Cu: 0 to 1.50%,
Cr: 0-3.00%,
Mo: 0-2.00%,
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: 0-0.0200%,
REM: 0-0.0200%,
The balance is Fe and impurities, and has a chemical composition in which α represented by the following formula (1) is 4.0 or more and 16.0 or less,
The tensile strength is 590 MPa or more and 930 MPa or less,
a microstructure at a portion of a steel material that is 1/4 of the thickness from the surface in the thickness direction includes lower bainite and martensite, a sum of area ratios of the lower bainite and the martensite is 15.0% or more, and a sum of area ratios of the upper bainite, the lower bainite, and the martensite is 90.0% or more;
Steel.
α=0.50×√[C]×(1+0.64[Si])×(1+4.10[Mn])×(1+0.27[Cu])×(1+0.52[Ni])×(1+2.33[Cr])×(1+3.14[Mo]) ...(1)
In the formula (1), the [element symbol] represents the content (mass%) of the corresponding element contained in the steel material. When the corresponding element is not contained, zero is substituted.
<2> The steel material according to <1>, wherein the microstructure at a portion of the steel material that is ¼ of the thickness from the surface in the thickness direction has an average crystal grain size of 20.0 μm or less.
<3> The steel material according to <1> or <2>, having a Charpy impact absorption energy of 150 J or more at −100° C.
<4> The steel material according to any one of <1> to <3>, wherein when the steel material is subjected to a heat treatment in which the heating rate and the cooling rate are 55°C/h in a temperature range of 425°C or more and the steel material is held at 600°C for 2 hours, the Charpy impact absorption energy at -100°C in the place where the heat treatment was performed is 150J or more.
<5> The steel material according to any one of <1> to <4>, wherein the aspect ratio of prior austenite grains at a portion of 1/4 of the thickness from the surface of the steel material in the thickness direction is 1.5 or more.
<6> The steel material according to any one of <1> to <4>, wherein the aspect ratio of prior austenite grains at a portion from the surface of the steel material to 1/4 of the thickness in the thickness direction is less than 1.5.

This disclosure makes it possible to provide a steel material that is suitable for low-temperature applications, having high tensile strength and good low-temperature toughness regardless of whether it is before or after post-weld heat treatment.

FIG. 13 is a diagram showing an example of a discrimination result of a microstructure.

The present disclosure will be described in detail below.
In this disclosure, unless otherwise specified, the term "post-weld heat treatment" refers to a post-weld heat treatment conforming to the contents specified in JIS Z 3700:2009 "Post-weld heat treatment method".
In the present disclosure, the term "steel material" or "base material" refers to a steel material portion that does not include a surface treatment layer such as a plating layer or a coating film. However, a surface treatment layer such as a plating layer or a coating film may be formed on the surface of the steel material according to the present disclosure. In addition, the term "base material" in a welded joint refers to a steel material portion that is not affected by welding, in contrast to the welded portion (welded joint and welded heat-affected zone).

In this disclosure, a numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits. However, when the numerical values before and after "to" are followed by "more than" or "less than," the numerical range does not include these numerical values as the lower or upper limit.
With regard to the contents of elements in chemical compositions, "%" means "mass %".
The term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

Below, we will explain the steel material according to one embodiment of the present disclosure. First, we will provide a detailed description of the results of the research conducted by the inventors of the present disclosure that led them to complete the steel material according to the present disclosure, and the new findings they gained.

The inventors of the present disclosure have conducted research to improve the strength of steel. The tensile strength of steel is ensured by the composition of the microstructure. The inventors of the present disclosure collected samples from the 1/4t portion (t: thickness of steel), which is a portion of 1/4 of the thickness from the surface of the steel after hot rolling and accelerated cooling in the thickness direction, performed tensile tests, and observed the microstructure. As a result, it was found that the microstructure of the 1/4t portion of steel with a tensile strength of 590 MPa or more and 930 MPa or less has an area ratio of ferrite of less than 10.0%, and the total area ratio of upper bainite, lower bainite, and martensite of 90.0% or more. The total area ratio of upper bainite, lower bainite, and martensite was measured using electron backscatter diffraction (hereinafter referred to as "EBSD").

Furthermore, the inventors of the present disclosure have conducted research to improve the toughness of steel materials. The toughness of steel materials is ensured by the composition of the microstructure. The inventors of the present disclosure have taken samples from the 1/4t section of steel materials after hot rolling and accelerated cooling, performed Charpy impact tests, and observed the microstructure. As a result, it was found that steel materials with a Charpy impact absorption energy of 150 J or more at -100°C have a total area ratio of lower bainite and martensite of 15.0% or more. The total area ratio of lower bainite and martensite was measured using EBSD.

Furthermore, the inventors of the present disclosure have conducted studies to ensure the toughness of the steel material. The toughness of the steel material is ensured by reducing the area surrounded by the high-angle grain boundaries where the difference in crystal orientation is 15° or more. The inventors of the present disclosure collected samples from the 1/4t part of the steel material manufactured by controlling the cooling rate and cooling stop temperature after hot rolling, and measured the circle equivalent diameter of the area surrounded by the high-angle grain boundaries by EBSD. Hereinafter, the circle equivalent diameter of the area surrounded by the high-angle grain boundaries is referred to as the grain size. The samples were mechanically polished and electrolytically polished, and an analysis was performed by an EBSD device attached to an FE-SEM (field emission scanning electron microscope) in an area of ⁴ mm ^2. The average grain size (sometimes referred to as the "effective grain size") was calculated by the area-weighted average of the grain sizes measured in the area of 4 mm 2, weighted by the area of each grain. It has been found that if the average crystal grain size of the ¼t portion of the steel material is 20.0 μm or less, the toughness of the steel material tends to be further improved regardless of whether it is before or after post-weld heat treatment.

Furthermore, the inventors of this disclosure have discovered that similar results can be obtained not only with steel material after hot rolling and accelerated cooling, but also with steel material after reheating and quenching.

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

(C: 0.03% or more, 0.20% or less)
C is an element that increases the strength of steel materials. From the viewpoint of ensuring the strength of steel materials used in structures, the C content is 0.03% or more in the present disclosure. The C content is preferably On the other hand, C is an element that reduces the toughness of the weld heat affected zone (hereinafter sometimes referred to as "HAZ"). From the viewpoint of ensuring toughness, the C content is 0.20% or less in the present disclosure. The C content is preferably 0.16% or less, 0.14% or less, or 0.12% or less. be.

(Si: 0.01% or more, 0.50% or less)
Silicon is used as a deoxidizer and is an element that dissolves in steel to increase strength. From the viewpoint of controlling the O concentration in the molten steel, the present disclosure specifies a silicon content of 0.01 % or more. The Si content is preferably 0.03% or more, 0.05% or more, or 0.10% or more. On the other hand, if the Si content is excessive, a hard phase is formed in the HAZ. Therefore, from the viewpoint of ensuring the toughness of the HAZ, the Si content is 0.50% or less in the present disclosure. The Si content is preferably 0.30% or less, Or, it is 0.20% or less.

(Mn: 0.10% or more, 1.65% or less)
Mn is used as a deoxidizer and is an element that enhances the hardenability of steel and contributes to high strength. From the viewpoint of controlling the O concentration contained in the molten steel, in the present disclosure, the Mn content is 0. . 10% or more. Furthermore, 0.10% or more of Mn forms MnS, reducing the amount of dissolved S and preventing hot cracking. From the viewpoint of ensuring the strength of the steel material and the toughness of the HAZ, The Mn content is preferably 0.30% or more, or 0.50% or more. On the other hand, if the Mn content is excessive, Mn will segregate at grain boundaries during PWHT, resulting in poor mechanical properties after PWHT. In some cases, the toughness of the steel material may decrease. Therefore, from the viewpoint of ensuring the toughness of the steel material after the PWHT, the Mn content is 1.65% or less in the present disclosure. The Mn content is preferably 1.50% or less. , 1.25% or less, or 1.10% or less.

(P: 0.025% or less)
P is an impurity element. Although there is no lower limit for the P content, from the viewpoint of production costs, in the present disclosure, the P content may be 0.001% or more. On the other hand, if the P content is excessive, If the P content is less than 0.025%, P may segregate at grain boundaries during PWHT, which may reduce the toughness after PWHT. Therefore, in the present disclosure, the P content is 0.025% or less. Preferably, it is 0.016% or less, 0.012% or less, or 0.008% or less.

(S: 0.0250% or less)
S is an impurity element. Although there is no lower limit for the S content, from the viewpoint of production costs, in the present disclosure, the S content may be 0.0001% or more. On the other hand, if the S content is excessive, If the S content is less than 0.0250, elongated MnS is generated in the central segregation portion, and the toughness and ductility of the steel material and the HAZ may deteriorate. The S content is preferably 0.0100% or less, or 0.0050% or less.

(Ni: 2.65% or more, 4.45% or less)
Since Ni is an element effective for improving the hardenability and toughness of steel, in the present disclosure, the Ni content is 2.65% or more. The Ni content is preferably 3.00% or more. Alternatively, the Ni content is 3.20% or more. However, since Ni is an expensive element, from the viewpoint 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, 0.100% or less)
Al is an element useful for deoxidization and also forms nitrides to refine the grain size during quenching. Therefore, in this disclosure, the Al content is set to 0.001%. However, if Al is contained in excess, Al may form coarse nitrides, which may reduce the toughness of the steel material and the HAZ. Therefore, the Al content is 0.100% or less. The Al content is preferably 0.080% or less, or 0.050% or less.

(O: 0.0100% or less)
O is an impurity element. Although there is no lower limit for the O content, from the viewpoint of production costs, in the present disclosure, the O content may be 0.0001% or more. On the other hand, if the O content is excessive, If the O content is too high, coarse oxides are generated, and the toughness and ductility of the steel material and the HAZ may deteriorate. From the viewpoint of ensuring the toughness and ductility of the steel material and the HAZ, the O content is 0.0100% or less. The O content is preferably 0.0060% or less, or 0.0040% or less.

(N: 0.0100% or less)
N is an impurity element. Although there is no lower limit for the N content, in terms of manufacturing costs, in the present disclosure, the N content may be 0.0001% or more. Steel properties and toughness of HAZ From the viewpoint of ensuring this, 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.

The steel material according to the present disclosure may contain other elements (selected elements) in place of a portion of the Fe. For example, the following selected elements may be included, but the content of these elements may be 0%.

The steel material disclosed herein may contain one or more of the following optional elements, Cu, Cr, Mo, and B, which have the effect of improving hardenability, as necessary to improve strength and toughness.

(Cu: 1.50% or less)
Cu is an element that may be mixed into steel materials during the manufacturing process. However, the lower limit of the Cu content is not limited and may be 0%. Cu also has an adverse effect on weldability and HAZ toughness. Since Cu has little adverse effect and has the effect of increasing the hardenability of steel, it is also an element that improves the strength of the steel material. Therefore, in the present disclosure, the Cu content may be 0.01% or more. The Cu content is The Cu content is preferably 0.10% or more. However, from the viewpoint of suppressing the occurrence of Cu cracks during hot rolling of the steel material, the Cu content is 1.50% or less in the present disclosure. It is preferably 1.00% or less, 0.80% or less, 0.60% or less, or 0.50% or less.

(Cr: 3.00% or less)
Cr is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the Cr content is not limited and may be 0%. Cr also has the effect of improving the hardenability of 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 viewpoint of suppressing deterioration of the toughness and weldability of the HAZ, the Cr content is 3.00% or less in the present disclosure. The Cr content is preferably 2.20% or less, 1.40% or less, Or, it is 0.80% or less.

(Mo: 2.00% or less)
Mo is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the Mo content is not limited and may be 0%. Mo also has the effect of improving the hardenability of steel. Therefore, in the present disclosure, the Mo content may be 0.01% or more. The Mo content is preferably 0.05% or more, and more preferably 0.10% or more. % or more, 0.20% or more, or 0.30% or more. However, from the viewpoint of suppressing deterioration of the toughness and weldability of the HAZ and suppressing an increase in the alloy cost, in the present disclosure, the Mo content is 2. The Mo content is preferably 1.20% or less, or 0.80% or less.

(B: 0.0050% or less)
B is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the B content is not limited and may be 0%. B also has a significant effect of improving the hardenability of steel. Therefore, in the present disclosure, the B content may be 0.0003% or more. However, in order to improve the surface quality of the steel slab produced by continuous casting, From the viewpoint of suppressing the deterioration of, 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.

In order to improve the strength of the steel material according to the present disclosure, one or more of the optional elements Nb, Ti, and V shown below may be contained as necessary, which have the effect of increasing the strength of the steel material by forming precipitates such as carbides and nitrides.

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

(Ti: 0.050% or less)
Ti is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the Ti content is not limited and may be 0%. Ti also forms carbides and nitrides. Ti has the effect of refining the metal structure and is also an element that improves the strength of the steel material. Therefore, in the present disclosure, the Ti content may be 0.001% or more. However, the toughness of the HAZ and the weldability may be affected. From the viewpoint of suppressing deterioration of the mechanical properties, the Ti content is 0.050% or less. The Ti content is preferably 0.040% or less, or 0.030% or less. From the viewpoint of ensuring toughness, the Ti content may be 0.004% or less, or 0.002% or less.

(V: 0.10% or less)
V is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the V content is not limited and may be 0%. V also forms carbides and nitrides. V is also an element that improves the strength of steel materials. Therefore, in the present disclosure, the V content may be 0.01% or more. However, in order to suppress the deterioration of the toughness and weldability of the HAZ and to suppress the increase in alloy costs, From a viewpoint, the V content is 0.10% or less, preferably 0.08% or less, or 0.05% or less.

The steel material disclosed herein may contain one or more of the optional elements Mg, Ca, and REM shown below as necessary to improve the toughness of the HAZ.

(Mg: 0.0200% or less)
Mg is an element that may be mixed into steel materials during the manufacturing process. However, the lower limit of the Mg content is not limited and may be 0%. In addition, Mg forms an oxide, It is also an element that improves the toughness of the weld 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. If the content is excessive, coarse oxides are formed, which may reduce the toughness of the steel. Therefore, from the viewpoint of ensuring toughness, the Mg content is set to 0.0200% or less in the present disclosure. The Mg content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.

(Ca: 0.0200% or less)
Ca is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the Ca content is not limited and may be 0%. Ca also dissolves sulfides in the steel into spherical particles. It is also an element that reduces the effect of MnS, which reduces the toughness of steel and weld heat affected zone, by increasing the Ca content. Therefore, in the present disclosure, the Ca content is set to 0.0003% or more, 0.0006% or more, or 0. On the other hand, if the Ca content is excessive, coarse oxides are formed, which may reduce the toughness of the steel. Therefore, from the viewpoint of ensuring toughness, In the above, 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)
Rare earth metals (REM) are a collective term for 17 elements, including two elements, Sc and Y, and 15 lanthanoid elements, such as La, Ce, and Nd. The REM content is the total content of the 17 elements. REM is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the REM content is not limited and may be 0%. REM also forms oxides. As a result, REM is also an element that improves the toughness of the weld 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, coarse oxides are formed, which may reduce the toughness of the steel. Therefore, from the viewpoint of ensuring toughness, the REM content in the present disclosure is set to 0.0200%. The REM content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.

(balance: Fe and impurities)
The balance of the chemical composition of the steel material according to the present disclosure is iron (Fe) and impurities. The impurities refer to components that are mixed in due to raw materials such as ores and scraps or other factors during industrial production of the steel material.

In addition to limiting the content of each individual element, this disclosure limits the range of the α value as follows:

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

In this disclosure, the range of the α value is set to 4.0 to 16.0. This is an index showing the hardenability of the steel material, and the higher the α value, the more the lower bainite and martensite structures with a favorable balance of strength and toughness can be formed. When α is in the appropriate range, the ratio of the lower bainite and martensite structures with a favorable balance of strength and toughness in the HAZ structure is high, and HAZ toughness can be ensured. When α is 4.0 or more, the hardenability of the base material is ensured, the ratio of the lower bainite and martensite structures with a favorable balance of strength and toughness increases, and toughness deterioration is suppressed. In addition, the ratio of the lower bainite and martensite in the structure of the HAZ portion is likely to increase, and the HAZ toughness is also improved. On the other hand, if the α value is 16.0 or less, the strength of the steel material does not become too high, and toughness can be ensured. In addition, if the α value is 16.0 or less, toughness after PWHT can be ensured. In addition, the HAZ does not become too hard, and HAZ toughness can be ensured.

By satisfying the above numerical range for the α value, it is possible to provide a nickel-containing steel material for low temperature use that has excellent strength and toughness. The α value is preferably 4.5 or more, or 5.0 or more. In addition, the α value is preferably 15.5 or less, or 15.0 or less.

<Microstructure>
Next, the microstructure of the steel material according to the present disclosure will be described. The microstructure at a portion of the steel material according to the present disclosure that is ¼ of the thickness from the surface in the thickness direction contains lower bainite and martensite. In addition, the bainite may contain upper bainite in addition to lower bainite.

"Bainite" is a structure containing bainitic ferrite (α°B) with a substructure within the grains, and is a general term for upper bainite and lower bainite. "Upper bainite" is either or both of upper bainite containing retained austenite or MA phase (mixed martensite-austenite phase) between the laths, and upper bainite containing carbides between the laths. "Lower bainite" is lath-shaped lower bainite containing carbides within the laths.

"Martensite" exists in four forms: lath, butterfly, lens, and thin plate, but the components disclosed in this disclosure mainly form lath martensite. Lath martensite is composed of packets and blocks consisting of groups of laths in a specific arrangement, and is a structure in which one austenite grain is divided into several packets.

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

(Total area ratio of upper bainite, lower bainite and martensite: 90.0% or more)
From the viewpoint of ensuring the strength of the steel material, the total area ratio of the upper bainite, lower bainite and martensite in the 1/4t portion is 90.0% or more. The total area ratio of the upper bainite, lower bainite and martensite in the 1/4t portion may be 100%. Furthermore, the upper bainite in the 1/4t portion may be 1.0% or more.

Observation of the microstructure of steel is carried out using a sample with the 1/4t part of the steel as the observation surface. Two types of samples are prepared: (a) electrolytic polishing, and (b) nital etching. Measurements are taken at three locations for each of samples (a) and (b) using the method described below, and the average of the three locations is taken as the area ratio of the microstructure of the steel. Three samples each of (a) and (b) may be prepared and the average taken for each sample, or measurements may be taken at three locations within each sample and the average taken.

　The total area ratio of upper bainite, lower bainite and martensite is measured by EBSD using electrolytically polished samples that have been mechanically polished to a mirror finish and then electrolytically polished to remove the distorted layer caused by mechanical polishing. The measurement magnification is 200 times, and measurements are made over an area of 400 μm x 400 μm at a pitch of 0.4 μm. The measurement is performed with an electron beam diameter of 0.4 μm or less. The confidence index (hereinafter referred to as the "CI value") is set to 0.1 or more. The judgment between ferrite and upper bainite, lower bainite and martensite is performed by setting the threshold value of Grain Average Misorientation (hereinafter referred to as the "GAM") to 0.5. The GAM value is an index defined in OIM-Analysis (EBSD crystal orientation analysis software manufactured by TSL, USA). The region where GAM is 0.5 or less is ferrite, and the region where GAM is more than 0.5 is upper bainite, lower bainite, or martensite. In this disclosure, upper bainite, lower bainite, and martensite are determined using the EBSD GAM as a threshold value, and therefore include not only upper bainite, lower bainite, and martensite, but also tempered upper bainite, tempered lower bainite, and tempered martensite. Comparing the structure of direct quenching (DQ) and that followed by tempering (DQT), after tempering, decomposition of MA and coarsening of carbides occur, but the appearance of the structure does not change significantly.

Using a nital etched sample, the area ratio of upper bainite is measured by SEM observation. The measurement magnification is 500 times, and the measurement is performed in a range of 360 μm × 480 μm. The part that has a clear lath structure and where carbides and MA are generated along the lath boundaries is upper bainite. The structure of the region where the internal structure of the structure is relatively coarse, the density of carbides is sparse, and coarse and dense are mixed is upper bainite. Figure 1 shows an example of the structure discrimination result. (A) and (B) are SEM images of the same region of a steel material manufactured by DQT and has an α value of 9.9. In (B), the region surrounded by a white line is upper bainite (Bu), and the other regions are lower bainite + martensite (BL + M). The part determined to be upper bainite (Bu) has sparse carbides that look white and coarse and dense regions are mixed. On the other hand, in the portion determined as lower bainite + martensite (BL+M), carbides are densely and uniformly present. The total area fraction of lower bainite and martensite is obtained by subtracting the area fraction of upper bainite from the total area fraction of upper bainite, lower bainite, and martensite measured above.
Note that the microstructure in this case may contain retained austenite depending on the manufacturing conditions, but since the amount is extremely small, it is not included in the area ratio.

(Average grain size of 1/4t part of steel material)
In the present disclosure, the average grain size (effective grain size) of the 1/4t part of the steel material is preferably 20.0 μm or less. This is because it has been found that if the average grain size of the 1/4t part of the steel material is 20.0 μm or less, the toughness of the steel material tends to be further improved regardless of whether it is before or after PWHT. However, the average grain size of the 1/4t part of the steel material may be more than 20.0 μm. The smaller the average grain size of the steel material, the more preferable it is, so the lower limit is not limited. Usually, the average grain size is 10 μm or more. The effective grain size is calculated by weighted averaging. The effective grain size D _area calculated by weighted averaging is calculated by the following formula using the area S _i and grain size d _i of the i-th grain detected during measurement among the grain sizes measured in ^{a 4} mm 2 area.
D _area =ΣSi・d _i /ΣS _i

(Aspect ratio of prior austenite grains at 1/4t part of steel material)
The prior austenite grains (may be referred to as prior austenite grains or prior γ grains) of the steel material of the present disclosure may have a shape flattened in the rolling direction. If the prior austenite grains at a position 1/4 of the thickness from the surface of the steel material in the thickness direction are flat grains with an aspect ratio of 1.5 or more, the toughness of the steel material can be further improved. This is because flattening the prior austenite grains increases the grain boundary area, which substantially refines the austenite grains and is effective in refining the average crystal grain size. The aspect ratio of the prior austenite grains is usually 4.0 or less, and may be 3.5 or less.

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

The aspect ratio of prior austenite crystal grains (sometimes referred to as prior austenite grains) of a steel material is determined as follows: First, an L-section (a section parallel to the rolling direction and thickness direction of the steel material) at a location 1/4 of the thickness from the surface of the steel material in the thickness direction is mirror-polished, and then etched with an etchant based on a saturated aqueous solution of 2 to 4% picric acid to reveal prior austenite grain boundaries in an arbitrary region of 1.0 mm in the rolling direction × 0.5 mm in the thickness direction.
Next, the long and short diameters of each prior austenite grain are measured, and the aspect ratio of each prior austenite grain is calculated by dividing the long diameter by the short diameter. The arithmetic average of all the aspect ratios of the prior austenite grains thus calculated is determined as the "aspect ratio of the prior austenite grains." The long diameter is the maximum length of the prior austenite grain, and the short diameter is the maximum distance between two lines parallel to the long diameter direction that contact the grain.

<Mechanical properties>
The steel material according to the present disclosure has mechanical properties that balance strength and low-temperature toughness, and is particularly excellent in toughness at -100°C, and can also exhibit excellent low-temperature toughness even after PWHT.

(Tensile strength: 590 MPa or more and 930 MPa or less)
In the present disclosure, the tensile strength of the steel material is set to 590 to 930 MPa. In order to reduce the weight of a large welded structure such as a transportation tank, a steel material that can ensure the strength of the structure even if it is thin is required. Usually, the steel material selected for such applications is a steel material having the above-mentioned tensile strength, so the steel material according to the present disclosure is also manufactured to have the above-mentioned tensile strength.

(Yield ratio)
The yield ratio (YR = yield strength/tensile strength x 100) of the steel material according to the present disclosure is not particularly limited, but is preferably 90% or less. When there is no yield point, the yield strength is determined using the 0.2% proof stress.

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

(Charpy impact absorption energy at -100°C after PWHT)
In low-temperature tanks, in order to prevent destruction, PWHT may be performed on the welded parts after assembly into a transport tank. At this time, not only the welded parts but also the base material part of the steel material that is not affected by welding (also simply referred to as the base material) are heated. If the time for which the base material is heated to a temperature range of 425°C or more is long, the toughness of the base material tends to decrease. In the steel material of the present disclosure, when PWHT is performed on the steel material with a holding temperature of 600°C, a holding time of 2 hours, and a heating rate and a cooling rate of 55°C/h in a temperature range of 425°C or more, the toughness of the part where the PWHT was performed is preferably 150J or more in Charpy impact absorption energy at -100°C. The Charpy impact absorption energy at -100°C after PWHT may be 100J or more. The Charpy impact absorption energy at -100°C after PWHT is also a value measured using a sample taken from the 1/4 position of the thickness.

(Charpy impact absorption energy at -100°C after thermal cycling)
In order to ensure high toughness after a thermal cycle test simulating a welded portion at low temperature, the steel material of the present disclosure preferably has a Charpy impact absorption energy of 50 J or more at -100 ° C after the thermal cycle. Since the steel material of the present disclosure has a low-temperature toughness of 50 J or more at -100 ° C after the thermal cycle, a transport tank made of the steel material of the present disclosure can be suitably used, for example, for transporting liquid carbon dioxide. The Charpy impact absorption energy at -100 ° C after the thermal cycle may be 40 J or more. The Charpy impact absorption energy at -100 ° C after the thermal cycle is a value measured by using a sample taken from a 1/4 position of the thickness of the steel material as a thermal cycle test piece, heating it to 1350 ° C at 60 ° C / s, holding it at 1350 ° C for 1 s, and then cooling it to room temperature at 20 ° C / s, and then taking a Charpy test piece from there.

(Charpy absorbed energy at -100°C after thermal cycle, PWHT)
In low-temperature tanks, in order to prevent destruction, PWHT may be performed on the welded portion after assembly into a transport tank. After the above-mentioned thermal cycle test, the steel material of the present disclosure is subjected to PWHT in which the heating rate and cooling rate are 55°C/h in a temperature range of 425°C or higher and the steel is held at 600°C for 2 hours, and then Charpy test pieces are taken and measured. In this case, the toughness of the portion where the PWHT was performed is preferably such that the Charpy impact absorption energy at -100°C is 50 J or more. The Charpy impact absorption energy at -100°C of the portion where the PWHT was performed after the thermal cycle test may be 40 J or more.

In addition, PWHT can sometimes reduce the toughness of steel. The reason for this is not clear, but it is presumed that the diffusion of P (phosphorus) and Mn to grain boundaries and the growth or aggregation of inclusions in the structure reduce brittleness and therefore toughness. The reduction in toughness due to PWHT can be suppressed by limiting the P and Mn content and reducing the average crystal grain size of the steel.

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

The shape of the steel material according to the present disclosure is not particularly limited, and may be a steel plate, a steel strip, a steel section, a steel pipe, etc. However, steel pipes and steel sections include steel materials formed by joining steel plates, such as welded steel pipes and welded steel sections, as well as steel sections joined with rivets. The thickness of steel materials such as steel plates, steel strips, steel sections, and steel pipes (the thickness of the flange for steel sections) is not particularly limited, and is usually 3 mm or more and 150 mm or less. The thickness of the steel material may be 6 mm or more, 10 mm or more, 15 mm or more, or 30 mm or more. The thickness of the steel material may also be 100 mm or less, 80 mm or less, or 60 mm or less.

Furthermore, the uses of the steel material disclosed herein are not particularly limited, but since it has mechanical properties that combine strength and low-temperature toughness, and can exhibit excellent low-temperature toughness even after PWHT, it can be suitably used as a tank for storing and transporting liquefied gas, especially liquid carbon dioxide.

(Method of manufacturing steel products)
The method for producing the steel material according to the present disclosure is not particularly limited, but the steel material according to the present disclosure is, for example, produced by melting steel satisfying the above-mentioned chemical composition and then continuously casting a steel slab. The steel slab is heated, hot-rolled, and then directly water-cooled (direct quenching (DQ)), or hot-rolled, cooled, reheated, and water-cooled (reheat quenching (RQ)), to produce a steel material. In the case of RQ, it is not necessarily required to cool naturally before reheating, and water cooling may be used. Furthermore, intermediate heat treatment (L) and tempering (T) may be performed. The production process after hot rolling is selected from the above combinations of DQ, RQ, L, and T, and is, for example, DQT, RQT, DQLT, and RQLT.

(1) DQT: Direct Quenching (DQ), Tempering (T)
(2) RQT: Natural cooling or water cooling, reheat quenching (RQ), tempering (T)
(3) DQLT: Direct quenching (DQ), intermediate heat treatment (L), tempering (T)
(4) RQLT: Natural cooling or water cooling, reheat quenching (RQ), intermediate heat treatment (L), tempering (T)

(1) DQT
From the viewpoint of manufacturing costs, DQT is preferred for manufacturing the steel material according to the present disclosure, and an example of a preferred manufacturing process is shown below.

When the steel material according to the present disclosure is manufactured by DQ, the heating temperature of the steel slab to be hot rolled is Ac ₃ or more from the viewpoint of performing hot rolling in a temperature range where the metal structure of the rolled material is austenite. The heating temperature of the steel slab is preferably 1000°C or more from the viewpoint of reducing the deformation resistance. On the other hand, the heating temperature of the hot rolling is 1250°C or less from the viewpoint of suppressing the coarsening of heated γ grains. The heating temperature of the hot rolling is preferably 1200°C or less. Incidentally, Ac ₃ is a value calculated by the following formula.
Ac ₃ =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 indicate the content (mass %) of each element contained in the steel slab.

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

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

The end temperature of the hot rolling is Ar ₃ or more from the viewpoint of suppressing the formation of ferrite that reduces strength. After the end of the hot rolling, the steel material is subjected to accelerated cooling such as water cooling. The start temperature of the accelerated cooling is Ar ₃ or more from the viewpoint of suppressing the formation of ferrite that reduces strength. Here, Ar ₃ is a value calculated by the following formula.
Ar ₃ =910-310C-80Mn-20Cu-15Cr-55Ni-80Mo+0.35(t-8)
The element symbols in the formula represent the content (mass%) of each element contained in the steel material, and t represents the thickness (mm) of the steel material.
From the viewpoint of promoting bainite 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 it is, but from the viewpoint of homogenizing the cooling rate, cost, etc., the cooling rate is preferably 50.0°C/s or less, or 30.0°C/s or less. The cooling rate is a value calculated by simulating the cooling rate at a 1/4 position of the thickness using heat transfer calculation.

The accelerated cooling stop temperature is 400°C or lower from the viewpoint of improving the strength of the steel by ensuring upper bainite, lower bainite, and martensite. The accelerated cooling stop temperature is preferably 350°C or lower. Accelerated cooling may be performed down to room temperature. The accelerated cooling stop temperature is preferably 100°C or higher from the viewpoint of dehydrogenating the steel.

After accelerated cooling, the steel may be subjected to a tempering treatment. From the viewpoint of preventing a decrease in strength, the heating temperature of the tempering treatment is preferably 650°C or less, 620°C or less, or 590°C or less. On the other hand, from the viewpoint of improving toughness, the heating temperature of the tempering treatment is preferably 350°C or more, or 400°C or more.

(2) RQT
When the steel material according to the present disclosure is manufactured by RQ, the heating temperature and reduction of the material to be rolled during hot rolling have little effect on the mechanical properties of the steel material. However, if the heating temperature of the material to be rolled is too low, the deformation resistance increases, so the heating temperature of the material to be rolled is preferably 1000°C or higher. In addition, if the reduction is insufficient, initial defects at the time of manufacturing the steel billet may remain in the thickness center, and the quality of the steel material may deteriorate, so the total reduction of the hot rolling (also called the cumulative reduction) is preferably 35% or higher. After hot rolling, the steel may be water-cooled or air-cooled as it is.

The steel material is reheated and quenched after hot rolling. The reheating temperature of the steel material is Ac ₃ or more because quenching is performed from an austenite single phase structure. From the viewpoint of ensuring homogeneity of the microstructure, the reheating temperature of the steel material is preferably 750°C or more, 850°C or more, 880°C or more, or 900°C or more. On the other hand, the upper limit of the reheating temperature is not particularly specified, but since heating to an excessively high temperature may cause austenite grains to coarsen and lead to a decrease in toughness, the upper limit is preferably 1000°C or less, 950°C or less, or 930°C or less.

After reheating and quenching, the steel may be subjected to a tempering treatment. The heating temperature in the tempering treatment is preferably 660°C or less, or 640°C or less, from the viewpoint of suppressing a decrease in strength. On the other hand, from the viewpoint of improving toughness, the heating temperature in the tempering treatment is preferably 400°C or more, 450°C or more, or 500°C or more.

Below, the steel material according to the present disclosure will be specifically explained using examples. However, the conditions in the following examples are merely examples of conditions adopted to confirm the feasibility and effects of the present disclosure, and the steel material according to the present disclosure is not limited to the following examples.

<Manufacturing by direct quenching and tempering>
[Steel manufacturing]
First, a slab having the chemical composition shown in Table 1 was cast by a continuous casting method. The balance other than the components shown in Table 1 is Fe and impurities. Also, blanks indicate that no alloying elements were intentionally added in the steelmaking process.
The underlines mean that they are outside the scope of this disclosure.

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

[Measurement and Evaluation]
The microstructure and mechanical properties of the obtained steel material were measured by the above-mentioned methods. The results are shown in Table 3. The meanings of the symbols of the microstructure are as follows. The remainder of the microstructure is pearlite, MA phase, and ferrite.
Bu: upper bainite BL: lower bainite M: martensite Toughness was measured by measuring the average value of the Charpy impact absorption energy at -100°C (KV2) and the average value of the Charpy impact absorption energy at -100°C after PWHT with a holding temperature of 600°C, a holding time of 2 hours, and a heating rate and a cooling rate of 55°C/h in a temperature range of 425°C or higher.

 

Nos. 1A to 22A and 101A to 109A are examples of the present invention, and Nos. 23A, 26A, and 110A to 114A are comparative examples.
In No. 23A, the rolling end temperature was low and the direct quenching start temperature was lower than that of Ar ₃ , so sufficient strength was not obtained.
No. 26A had too much Mn content and therefore could not obtain sufficient low-temperature toughness after PWHT.
Nos. 110A and 111A had an α value less than the lower limit of the present disclosure, and thus had insufficient hardenability and strength. Sufficient low-temperature toughness was also not obtained.
In Nos. 112A and 113A, the α value exceeded the upper limit of the present disclosure, the hardenability was too high, and the strength was excessively high.
In No. 114A, the cooling rate of the direct quenching was low, so the total area ratio of lower bainite and martensite was insufficient, and sufficient low-temperature toughness was not obtained.

In contrast to the comparative examples, in all of the inventive examples (Nos. 1A-22A, 101A-109A), the chemical composition and microstructure of the steel are appropriately controlled, and the tensile strength is within an appropriate range of 590 MPa or more and 930 MPa or less. In addition, the Charpy impact absorption energy at -100°C is high regardless of before or after PWHT, and those with particularly good properties have a low-temperature toughness of 150 J or more.

<Manufacturing by reheating, quenching and tempering>
[Steel manufacturing]
First, a slab having the chemical composition shown in Table 4 was cast by a continuous casting method. The balance other than the components shown in Table 4 is Fe and impurities. Also, blanks indicate that no alloy elements were intentionally added in the steelmaking process. Underlines indicate that the contents are outside the scope of the present disclosure.

　Then, steel materials were manufactured from these slabs under the manufacturing conditions shown in Table 5. "Temper heat treatment" is the heating temperature in the tempering process after quenching.

[Measurement and Evaluation]
The microstructure and mechanical properties of the obtained steel material were measured by the above-mentioned methods. The results are shown in Table 6. The meanings of the symbols of the microstructure are as follows. The remainder of the microstructure is pearlite, MA phase, and ferrite.
Bu: upper bainite BL: lower bainite M: martensite Toughness is the average value of Charpy impact absorption energy at -100°C (KV2), holding temperature: 600°C, holding time: 2 hours, heating rate and cooling rate in the temperature range of 425°C or higher: 55
The average Charpy impact absorption energy at -100°C after PWHT at 100°C/h was measured.

 

Nos. 1B to 21B and 101B to 109B are examples of the present invention, and Nos. 22B to 25B and 110B to 113B are comparative examples.
In No. 22B, since α was too small, sufficient hardenability, strength, and low-temperature toughness were not obtained.
In No. 23B, the reheating temperature was low, lower than that of Ac ₃ , so sufficient strength and low-temperature toughness were not obtained.
No. 24B had too little Ni, and therefore could not obtain sufficient low-temperature toughness.
No. 25B had too much Mn content and therefore could not obtain sufficient low-temperature toughness after PWHT.
Nos. 110B and 111B had an α value less than the lower limit of the present disclosure, and thus had insufficient hardenability and strength. Sufficient low-temperature toughness was also not obtained.
In Nos. 112B and 113B, the α value exceeded the upper limit of the present disclosure, the hardenability was too high, and the strength was excessively high.

In contrast to the comparative examples, in all of the inventive examples (Nos. 1B to 21B, 101B to 109B), the chemical composition and microstructure of the steel are appropriately controlled, and the tensile strength is within an appropriate range of 590 MPa or more and 930 MPa or less. In addition, regardless of before or after PWHT, the Charpy impact absorption energy at -100°C is high at 125 J or more, and those with particularly good properties have a low-temperature toughness of 150 J or more.

The steel material according to the present disclosure can be mainly used for transport tanks for liquefied carbon dioxide. The steel material according to the present disclosure can also be used for other welded structures such as buildings, bridges, ships, line pipes, marine structures, pressure vessels and tanks.

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

Claims (6)

In mass percent,
C: 0.03% or more, 0.20% or less,
Si: 0.01% or more, 0.50% or less,
Mn: 0.10% or more, 1.65% or less,
P: 0.025% or less,
S: 0.0250% or less,
Ni: 2.65% or more, 4.45% or less,
Al: 0.001% or more, 0.100% or less,
O: 0.0100% or less,
N: 0.0100% or less,
Cu: 0 to 1.50%,
Cr: 0-3.00%,
Mo: 0-2.00%,
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: 0-0.0200%,
REM: 0-0.0200%,
The balance is Fe and impurities, and has a chemical composition in which α represented by the following formula (1) is 4.0 or more and 16.0 or less,
The tensile strength is 590 MPa or more and 930 MPa or less,
a microstructure at a portion of a steel material that is 1/4 of the thickness from the surface in the thickness direction includes lower bainite and martensite, a sum of area ratios of the lower bainite and the martensite is 15.0% or more, and a sum of area ratios of the upper bainite, the lower bainite, and the martensite is 90.0% or more;
Steel.
α=0.50×√[C]×(1+0.64[Si])×(1+4.10[Mn])×(1+0.27[Cu])×(1+0.52[Ni])×(1+2.33[Cr])×(1+3.14[Mo]) ...(1)
In the formula (1), the [element symbol] represents the content (mass%) of the corresponding element contained in the steel material. When the corresponding element is not contained, zero is substituted. The steel material according to claim 1, wherein the microstructure at a portion of the steel material extending from the surface to 1/4 of the thickness in the thickness direction has an average crystal grain size of 20.0 μm or less. The steel material according to claim 1 or 2, which has a Charpy impact absorption energy of 150 J or more at -100°C. A steel material according to any one of claims 1 to 3, in which, when the steel material is subjected to a heat treatment in which the heating rate and cooling rate are 55°C/h in a temperature range of 425°C or higher and the steel material is held at 600°C for 2 hours, the Charpy impact absorption energy at -100°C in the area where the heat treatment was performed is 150J or more. The steel material according to any one of claims 1 to 4, in which the aspect ratio of the prior austenite grains at a location 1/4 of the thickness from the surface of the steel material in the thickness direction is 1.5 or more. The steel material according to any one of claims 1 to 4, in which the aspect ratio of the prior austenite grains at a location 1/4 of the thickness from the surface of the steel material in the thickness direction is less than 1.5.