EP4793382A1

EP4793382A1 - Steel material and pressure container

Info

Publication number: EP4793382A1
Application number: EP24877319.4A
Authority: EP
Inventors: Daiki NAKANISHI; Hirokazu USUKI; Tetsuya Uchiyama; Hiroyuki Shirahata
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-10-11
Filing date: 2024-10-11
Publication date: 2026-08-19
Also published as: JPWO2025079725A1; WO2025079725A1; CN122003514A; KR20260072632A

Abstract

A steel product having a specific chemical composition in which Ceq represented by the following Formula (1) is 0.350 or more, a tensile strength is from 490 MPa to 720 MPa, a microstructure of a portion that is 1/4 of a thickness from the surface, has low-temperature transformation structures whose total area ratio is 70.0% or more, and has an average crystal grain size of 25.0 µm or less.

Ceq = [C] + [Mn] / 6 + [Ni] / 15 + [Cu] / 15 + [Cr] / 5 + [Mo] / 5 + [V] / 5

Description

Technical Field

The present disclosure relates to a steel product and a pressure vessel.

Background Art

Steel products can be used in welded structures such as buildings, bridges, ships, pipelines, offshore structures, pressure vessels(tanks). Steel products having excellent strength and adaptability to low-temperature toughness are effective in low-temperature applications.
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, Al-killed steel is applied to LPG tank whose operating temperature is about -50°C.
For the purpose of low-temperature toughness and high strength, there have been proposed various steel products for which chemical compositions and microstructures (metal structures) are specified, and methods for manufacturing the steel products (see Patent Documents 1 to 9, for example).

Patent Document 1: Japanese Patent Application Laid-Open ( JP-A) No. S61-207512
Patent Document 2: JP-A No. 2013-245360
Patent Document 3: JP-A No. 2016-148105
Patent Document 4: JP-A No. 2021-088753
Patent Document 5: JP-A No. H02-254118
Patent Document 6: JP-A No. H03-264615
Patent Document 7: JP-A No. H05-263132
Patent Document 8: JP-A No. 2021-070862
Patent Document 9: JP-A No. H09-020921

SUMMARY OF INVENTION

Technical Problem

Having both high strength and low-temperature toughness is desirable for cryogenic steels 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.
On the other hand, in general in a case where a steel plate is manufactured by direct quenching (DQ) in which hot-rolling is directly followed by water cooling, or by further tempering (T) after DQ, the toughness in the width direction (also referred to as C direction) perpendicular to the rolling direction (also referred to as L direction) will remarkably deteriorate as compared with the toughness in the L direction, due to anisotropy of the rolling direction. In some cases, the steel plate for LPG tank is, however, required to have toughness not only in the L direction, but also in the C direction.
An object of the present disclosure is to provide a steel product having high tensile strength, for which good low-temperature toughness is obtainable regardless of before or after the post weld heat treatment, and, showing a small difference in the low-temperature toughness between the directions parallel to and perpendicular to the rolling direction, and is therefore suitable for low-temperature uses.

Solution to Problem

The gist of the present disclosure is as follows.

< 1 > A steel product having a chemical composition of, in mass%,
- C: from 0.03% to 0.20%,
- Si: from 0.10% to 0.50%,
- Mn: from 1.25% to 2.00%,
- P: 0.025% or less,
- S: 0.0100% or less,
- Al: from 0.005% to 0.100%,
- O: 0.0100% or less,
- N: 0.0100% or less,
- Ni: 0 to 1.30%,
- Cu: 0 to 0.90%,
- Cr: 0 to 0.90%,
- Mo: 0 to 1.00%,
- B: 0 to 0.0050%,
- Nb: 0 to 0.020%,
- Ti: 0 to 0.050%,
- V: 0 to 0.10%,
- Mg: 0 to 0.015%,
- Ca: 0 to 0.015%,
- REM: 0 to 0.015%, and
- balance: Fe and impurities,
- and in which Ceq represented by following Formula (1) is 0.350 or more,
- wherein:
  - a tensile strength is from 490 MPa to 720 MPa, and
  - a microstructure of a portion that is 1/4 of a thickness in a thickness direction from a surface of the steel product comprises low-temperature transformation structures whose total area ratio is 70.0% or more, and has an average crystal grain size of 25.0 µm or less, $Ceq = [C] + [Mn] / 6 + [Ni] / 15 + [Cu] / 15 + [Cr] / 5 + [Mo] / 5 + [V] / 5$
  - wherein a symbol of an element in square brackets in Formula (1) represents a content (mass%) of a corresponding element contained in the steel product, provided that zero is substituted when the corresponding element is not contained.
<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:
- Ni: from 0.01% to 1.30%,
- Cu: from 0.01% to 0.90%,
- Cr: from 0.01% to 0.90%,
- Mo: from 0.01% to 1.00%, 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 0.020%,
- 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 0.015%,
- Ca: from 0.0003% to 0.015%, and
- REM: from 0.0003% to 0.015%.
<5> The steel product according to any one of <1> to <4>, wherein a Charpy impact absorption energy at -90°C at the portion that is 1/4 of the thickness is 150 J or more.
<6> The steel product according to any one of <1> to <5>, wherein, when 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 the temperature is held for 2 hours at 600°C, is carried out on the steel product, a Charpy impact absorption energy at -90°C at the portion that is 1/4 of the thickness of the heat-treated portion is 150 J or more.
<7> The steel product according to any one of <1> to <6>, wherein, in a cross-section in parallel with a rolling direction and the thickness direction of the steel product, a prior austenite grain in a portion that is 1/4 of a thickness, in the thickness direction from the surface of the steel product, has an aspect ratio of 3.5 or less.
<8> The steel product according to any one of <1> to <6>, wherein, in a cross-section in parallel with a rolling direction and the thickness direction of the steel product, a prior austenite grain in a portion that is 1/4 of a thickness, in a thickness direction from the surface of the steel product, has an aspect ratio of less than 1.5.
<9> The steel product according to any one of <1> to <8>, having an yield ratio of 80.0% or less.
<10> A pressure vessel, comprising the steel product according to any one of <1> to <9>.

Advantageous Effects of Invention

According to the present disclosure, a steel product having high tensile strength, for which good low-temperature toughness is obtainable regardless of before or after the post weld heat treatment, and, showing a small difference in the low-temperature toughness between the directions parallel to and perpendicular to the rolling direction, and is therefore suitable for low-temperature uses, can be provided. The low temperature herein means a temperature range, for example, from -10 to -90°C.

DESCRIPTION OF EMBODIMENTS

Hereinbelow, the present disclosure is described in detail.
The "post-welding heat treatment" in the present disclosure means a post-welding heat treatment in accordance with the content provided in JIS Z 3700:2009 "Method of post weld heat treatment" unless otherwise specified.
The "steel product" or "base metal" in the present disclosure means a steel portion not including 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 surfaces of the steel product according to the present disclosure.
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.
With respect to the contents of the elements in a chemical composition, "%" means "mass%".
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.
A steel product according to an embodiment of the present disclosure will now be described. First, results of studies by the inventors of the present disclosure through which the invention of the steel product according to the present disclosure has been completed, and new findings obtained are described in detail.
The inventors of the present disclosure conducted studies to improve the strength of a steel product. The tensile strength 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 (t: the thickness of the steel product), which is a part of 1/4 of the thickness in the thickness direction from a surface of a steel product after hot rolling and accelerated cooling, and performed a tensile test and observed the microstructure. As a result, it has been found that, in the microstructure of the 1/4t portion of a steel product having a tensile strength of 490 MPa or more and 720 MPa or less, the total of area ratio of low-temperature transformation structures 70.0% or more. The total of area ratio of low-temperature transformation structures was measured using an electron backscatter diffraction method (hereinafter referred to as "EBSD").
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 manufactured by direct quenching or reheat quenching after hot rolling, 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 size. The sample was subjected to mechanical polishing and electrolytic polishing, and was analyzed in a region of 4 mm² 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 sizes measured in the region of 4 mm², in which weighting was performed by the area of the crystal grain, was taken as an average crystal grain size (occasionally referred to as an "effective crystal grain size"). An obtained finding is that, by devising of the manufacturing method in which the average crystal grain size in the 1/4t portion of the steel product is controlled to 25.0 µm or less, the toughness is improved not only in the L direction but also in the C direction, whereby the difference in the low-temperature toughness between the L direction and the C direction is reduced, and the toughness of the steel product tends to further improve regardless of before or after the post weld heat treatment.

Next, alloy elements that constitute the chemical composition of the steel product according to the present disclosure are described. In the following description of alloy elements, "%" of the content means "mass%".

(C: 0.03% or more and 0.20% or less)

C is an element that enhances the strength of the steel product. In the present disclosure, the C content is 0.03% or more from the viewpoint of securing strength of a steel product used for structures. 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; in the present disclosure, the C content is 0.20% or less from the viewpoint of securing toughness of the heat-affected zone (hereinafter, occasionally referred to as "HAZ"). The C content is preferably 0.16% or less, 0.14% or less, or 0.12% or less.

(Si: 0.10% or more and 0.50% or less)

Si is used as a deoxidizer, and is also an element that forms a solid solution in the steel and increases the strength. In the present disclosure, the Si content is 0.10% or more from the viewpoint of controlling the concentration of O contained in the molten steel. The Si content is preferably 0.11% or more, 0.12% or more, or 0.15% or more. On the other hand, if the Si content is excessive, a hard phase may be formed in the HAZ, and the toughness may be reduced. Thus, in the present disclosure, the Si content is 0.50% or less from the viewpoint of securing toughness of the HAZ. The Si content is preferably 0.40% or less, 0.30% or less, or 0.25% or less.

(Mn: 1.25% or more and 2.00% or less)

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 1.25% or more. From the standpoint of ensuring the strength of the steel product and the toughness of the HAZ, the Mn content is preferably 1.30% or more, 1.40% or more, or 1.45% 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 after PWHT, in the present disclosure, the Mn content is 2.00% or less. The Mn content is preferably 1.90% or less, 1.80% or less, 1.70% or less, or 1.60% or less.

(P: 0.025% or less)

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: 0.0100% or less)

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 0.0100% or less. The S content is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less.

(Al: 0.005% or more and 0.100% or less)

Al is an element that is effective in deoxidation, and is an element that, by forming a nitride, refines the crystal grain at the time of quenching. Therefore, in the present disclosure, the Al content is 0.005% or more. The Al content is preferably 0.007% or more, 0.010% or more, 0.015% or more, or 0.020% or more. However, if Al is excessively contained, there is the concern that the Al will generate coarse nitride particles, 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 less, 0.070% or less, 0.060% or less, 0.050% or less, or 0.040% or less.

(O: 0.0100% or less)

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 coarse oxide particles are 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, 0.050% or less, or 0.0040% or less.

(N: 0.0100% or less)

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 0.0090% or less, 0.0070% or less, 0.0050% or less, or 0.0040% or less.
The steel product according 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]

In order to improve the strength and toughness, if necessary, the steel product according to the present disclosure may be made to contain one or two or more of the optional elements Ni, Cu, Cr, Mo, and B that are described hereinafter and have the effect of improving hardenability.

(Ni: 1.30% or less)

Ni is an element that is sometimes mixed into the steel product in the manufacturing process. However, the lower limit value of the Ni content is not limited, and may be 0%. Further, Ni has little adverse effect on the weldability and on the toughness of the HAZ, and has an effect of improving the hardenability of steel, and is therefore an element that improves the strength of the steel product. Thus, in the present disclosure, the Ni content may be 0.01 % or more. The Ni content is preferably 0.10% or more. However, Ni is an expensive element, and, from the standpoint of cost reduction, the Ni content in the present disclosure is 1.30% or less. The Ni content is preferably 1.20% or less, 1.10% or less, 1.00% or less, 0.90% or less, 0.70% or less, or 0.50% or less.

(Cu: 0.90% or less)

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 0.90% or less. The Cu content is preferably 0. 80% or less, 0.70% or less, 0.60% or less, or 0.40% or less.

(Cr: 0.90% or less)

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 0.90% or less. The Cr content is preferably 0.80% or less, 0.70% or less, 0.60% or less, or 0.50% or less.

(Mo: 1.00% or less)

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 1.00% or less. The Mo content is preferably 0.90% or less, 0.080% or less, 0.070% or less, 0.60% or less, or 0.50% or less.

(B: 0.0050% or less)

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]

In order to improve the strength, as needed, the steel product according 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: 0.020% or less)

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 0.020% or less. The Nb content is preferably 0.018% or less, 0.015% or less, 0.012% or less, 0.009% or less, or 0.007% 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)

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 less, 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)

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]

In order to improve the toughness of the HAZ, as needed, the steel product according 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: 0.015% or less)

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 coarse oxide particles and decreases the toughness of the steel. Accordingly, from the standpoint of ensuring the toughness, in the present disclosure, the Mg content is 0.015% or less. The Mg content is preferably 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0040% or less.

(Ca: 0.015% or less)

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 coarse oxide particles and decreases the toughness of the steel. Accordingly, from the standpoint of ensuring the toughness, in the present disclosure, the Ca content is 0.0150% or less. The Ca content is preferably 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0040% or less.

(REM: 0.0150% or less)

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 oxide particles and decrease the toughness of the steel. Accordingly, from the standpoint of ensuring the toughness, in the present disclosure, the REM content is 0.0150% or less. The REM content is preferably 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0040% or less.

(Balance: Fe and Impurities)

The balance of the chemical composition of the steel product according 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.
In addition to the limits on the contents of the individual elements, in the present disclosure, the range of value Ceq is limited as follows.

(Value Ceq: 0.350 or more)

The value Ceq is computed by the following Formula (1). $Ceq = [C] + [Mn] / 6 + [Ni] / 15 + [Cu] / 15 + [Cr] / 5 + [Mo] / 5 + [V] / 5$ wherein [C], [Mn], [Ni], [Cu], [Cr], [Mo] and [V] are the contents (% by mass) of C, Mn, Ni, Cu, Cr, Mo and V in the steel, respectively. In a case in which a given element is not contained, zero is substituted in.
The steel product of the present disclosure has a value Ceq of 0.350 or more. This is an index indicating hardenability of the steel product. The greater the value Ceq, the more the strength is improved. The value Ceq is preferably 0.360 or more, 0.370 or more, or 0.380 or more.
When Ceq is in an appropriate range, the HAZ toughness after welding can also be ensured. When the value Ceq is 0.350 or more, the hardenability of the base metal is ensured, the ratio of low-temperature transformation structures which are advantageously balanced between strength and toughness will increase, and the toughness is suppressed from being deteriorated. Also the structure of the HAZ portion will tend to have an increased ratio of the low-temperature transformation structure, thus also improving the HAZ toughness. The upper limit of the value Ceq is not particularly limited. From the standpoint of the balance between strength and toughness, the value Ceq is preferably 0.500 or less, 0.480 or less, or 0.460 or less.
Due to the aforementioned numerical value range relating to value Ceq (0.350 or more, preferably 0.500 or less) being satisfied, there can be provided the steel product that has excellent strength and toughness, and is suitable for low-temperature uses.

Next, the microstructure of the steel product according to the present disclosure will be described. The microstructure of a portion of 1/4 of the thickness in the thickness direction from the surface of the steel product according to the present disclosure includes a low-temperature transformation structure. The low-temperature transformation structure is a structure in which GAM described later exceeds 0.5, for which bainite and martensite are mainly given as examples.
"Bainite" 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.
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 Ratio of Low-temperature Transformation Structures: 70% or more)

Low-temperature transformation structures are hard phases and increase the toughness of the steel product. From the standpoint of ensuring the strength and the toughness of the steel product, the area ratio of low-temperature transformation structures at the 1/4t portion is 70.0% or more. The area ratio of low-temperature transformation structures at the 1/4t portion is preferably 75.0% or more, 80.0% or more, or 85.0% or more. The total of the area ratio of low-temperature transformation structures at the 1/4t portion may be 100%.
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. Measurement is performed 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 samples may be prepared, and the averages of the respective samples may be computed. Alternatively, three points within one sample may be measured in a visual field, and the average thereof may be computed.
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 ratio of the low-temperature transformation structures 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 measurement 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 low-temperature transformation structures, 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 low-temperature transformation structures. Because the low-temperature transformation structures in the present disclosure are determined by using the GAM of EBSD as the threshold value, not only bainite and martensite, but also tempered 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.

(Average Crystal Grain Size of 1/4t Portion of Steel Product)

In the present disclosure, the average crystal grain size (effective crystal grain size) of the 1/4t portion of the steel product is 25.0 µm or less. This is because it was learned that, if the average crystal grain size of the 1/4t portion of the steel product is 25.0 µm or less, the toughness improves in the C direction, and furthermore, the toughness of the steel product tends to improve even more regardless of whether before or after PWHT. The average crystal grain size (effective crystal grain size) of the 1/4t portion of the steel product is preferably 20.0 µm or less. The smaller the average crystal grain size of the steel product, the more preferable, and therefore, the lower limit value thereof is not limited. Usually, the average crystal grain size is 10 µm or more.
Measurement of the average crystal grain size 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. A sample, which was mirror-finished by mechanical polishing, and from which a strained layer produced due to the mechanical polishing was removed by electrolytic polishing, is prepared. The sample is measured at 20 points by the following method, and an average value for the 20 points is used as the area ratio of the microstructure of the steel product. Note that 20 samples may be prepared to find the average of the samples, or visual fields at 20 points within one sample may be measured to find the average.
Note that, in some cases, the average crystal grain size of the 1/4t portion of the steel product may be substantially different between the L direction and the C direction. Therefore, as for the average crystal grain size, cross sections in both the L direction and the C direction are used as observed surfaces, and the average crystal grain size is measured on the cross sections in the individual directions. The average (overall average) of an average crystal grain size in the L direction and an average crystal grain size in the C direction is defined as the average crystal grain size in the present disclosure. Note that each of the average crystal grain size in the L direction and the average crystal grain size in the C direction is preferably 25.0 µm or less, and more preferably 20.0 µm or less.
By using an electrically polished sample, measurement of the average crystal grain size is carried out by EBSD. The measurement magnification is 200×, the number of visual fields is 20, and measurement of a range of 400 µm × 500 µm per visual field is carried out at a pitch of 0.4 µm.
The average crystal grain size is determined by weighted averaging. The average crystal grain size D_area determined by weighted averaging is calculated by the following equation, with use of area S_i and grain size d_i of the i-th crystal grain detected during the measurement, among the crystal grain sizes measured in a region of 4 mm². $D_{area} = \sum S_{i} \cdot d_{i} / \sum S_{i}$

(Aspect Ratio of Prior Austenite Crystal Grain of 1/4t Portion of Steel Product)

In the steel product according to the present disclosure, the aspect ratio of the prior austenite grain (may be referred to as prior austenite grain or prior y grain.) of the 1/4t portion of the steel product is preferably 3.5 or less, from the standpoint of suppressing the difference in the low-temperature toughness between the rolling direction (L direction) of the steel product and the direction (C direction) perpendicular to the rolling direction of the steel product.
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, an intermediate heat treatment (L) and tempering (T) may be further performed.
In a steel product according to the present disclosure subjected to DQ, the form of the prior austenite grain (occasionally referred to as a prior austenite grain or a prior y 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 in the rolling direction, the toughness of the steel product can be improved even more. This is because, by flattening the prior y grain to increase the grain boundary area, the austenite grain is substantially made minute, which is effective in refining the average crystal grain size. On the other hand, when the aspect ratio of the prior y grains is too large, the toughness in the C direction may be occasionally deteriorated. Therefore, when DQ is applied, it is preferred to control the average crystal grain size to 25.0 µm or less, and from the standpoint of suppressing the difference in the toughness between the L direction and the C direction, the aspect ratio of the prior austenite grains is preferably controlled to 3.5 or less. Note that in the steel product according to the present disclosure having gone through DQ, the aspect ratio of the prior austenite grains is usually 1.5 or more.
On the other hand, in a steel product according to the present disclosure subjected to RQ, for the form of the prior austenite grains, the aspect ratio of the prior austenite grains in the rolling direction may be less than 1.5, from the viewpoint of securing homogeneity of microstructure.
The aspect ratio of the prior austenite grains 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 etching is carried out by an etchant of a saturated aqueous 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.
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.

A steel product according to the present disclosure has mechanical properties that achieve both strength and low-temperature toughness. In particular, the steel product is excellent in toughness at -90°C, and furthermore can exhibit excellent low-temperature toughness even after PWHT.

(Tensile Strength: 490 MPa or more and 720 MPa or less)

In the present disclosure, the tensile strength of the steel product is set to 490 to 720 MPa. In order to reduce the weight of a large welded structure such as a transport tank, a steel product capable of securing strength of the structure even if the thickness is thin is required. Usually, a steel product selected as one used for such a use is a steel product having the tensile strength mentioned above; thus, also the steel product according to the present disclosure is manufactured to have the tensile strength mentioned above.

(Charpy Impact Absorption Energy at -90°C)

In order to ensure high toughness at low temperatures, the Charpy impact absorption energy at -90°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 -90°C of 150 J or more, a tank manufactured using the steel product of the present disclosure can be suitably used for transporting liquid carbon dioxide. Note that the Charpy impact absorption energy at -90°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 -90°C after PWHT)

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 -90°C is preferably 150 J or more. The Charpy impact absorption energy at -90°C after PWHT may be 100 J or more. The Charpy impact absorption energy at -90°C after PWHT also is a numerical value measured by using a sample taken from a position of 1/4 of the thickness.
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 size of the steel product small.

(Yield Ratio)

The yield ratio (YR (%) = yield strength/tensile strength × 100) of the steel product according to the present disclosure is preferably 83.0% or less, and more preferably 80.0% or less, from the standpoint of further improving the structural safety of the welded structure.
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 which 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.
The toughness of the steel product is evaluated based on a brittle-ductile transition temperature (vTrs) by a Charpy impact test at -90°C using an impact blade having a radius of 2 mm in accordance with the provision of 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, there are used a V-notched test piece having the longitudinal direction laid in a direction (C direction) parallel to the width direction of the steel product, and a V-notched test piece having the longitudinal direction laid in a direction (L direction) parallel to the rolling direction of the steel product, wherein both test pieces are sampled from the 1/4 thickness position of the steel product. AV-notch for the test piece sampled in the C direction is formed so as to align the longitudinal direction of the notch to the thickness direction of the steel product, and so as to align the depth direction of the notch to the rolling direction of the steel product. Meanwhile, a V-notch for the test piece sampled in the L direction is formed so as to align the longitudinal direction of the notch to the thickness direction of the steel product, and so as to align the depth direction of the notch to the width direction of the steel product.
The shape of the steel product according to the present disclosure is not particularly limited, and is a steel plate, a steel strip, a structural steel, a steel pipe, or the like. However, the steel pipe and the structural steel include a steel product obtained by joining steel plates, for example, a structural steel obtained by joining with a rivet, and the like, in addition to a welded steel pipe and welded structural steel. The thickness of a steel product such as a steel plate, a steel strip, a structural steel, or a steel pipe (for structural steel, the thickness of the flange) is not particularly limited, and is usually 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. The thickness of the steel product may be 100 mm or less, 80 mm or less, or 60 mm or less.
The use of the steel product according to the present disclosure is not particularly limited either; since the steel product has mechanical properties that achieve both strength and low-temperature toughness and can exhibit excellent low-temperature toughness particularly even after PWHT, it can be suitably used as a constituent material for a pressure vessel, specifically a tank for storing and transporting liquefied gas, particularly liquid carbon dioxide.

(Method for manufacturing a steel product)

The method for manufacturing a steel product according to the present disclosure is not particularly limited; for the steel product in the present disclosure, for example, a steel slab is manufactured by smelting steel satisfying the chemical composition described above and then performing continuous casting. The steel slab is heated and hot-rolled, and is then subjected to either direct quenching (DQ), in which it is directly water-cooled, or reheated quenching (RQ) , in which it is allowed to cool, reheated, and water-cooled, and is thus made into a steel product. In the case of RQ, the step preceding the reheating is not necessarily limited to air-cooling, and may be water cooling. Further, an intermediate heat treatment (L) and tempering (T) may be performed.
From the standpoint of carrying out hot rolling in a temperature range in which the microstructure of the steel to be rolled is austenite, the heating temperature of the steel slab (steel to be rolled) on which the hot rolling is carried out is Ac₃ or more. From the standpoint of decreasing the deformation resistance, the heating temperature of the steel to be rolled is preferably 1000°C or more. On the other hand, from the standpoint of suppressing coarsening of y grains due to heating, the heating temperature of the steel to be rolled is 1240°C or less. The heating temperature of the steel to be rolled is preferably 1200°C or less. Note that Ac₃ is a value computed by the following formula. Ac₃ = 937.2 - 436.5C + 56Si - 19.7Mn - 16.3Cu - 26.6Ni - 4.9Cr + 38.1Mo + 124.8V + 136.3Ti - 19.1Nb + 198.4Al + 3315B
The element symbols in the formula mean the content (mass%) of each element contained in the steel slab.
In a case where DQ is applied, hot rolling may occasionally be 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).
The rolling in the recrystallization temperature range is hot rolling carried out with the temperature of the steel to be rolled during rolling set to 900°C or higher. The cumulative rolling reduction ratio of the rolling in the recrystallization temperature range is preferably 40% or more, 50% or more, or 60% or more, from the standpoint of refining the austenite grain size of the steel product. When the rolling in the recrystallization temperature range is carried out at high temperatures, the prior y grain size would not be refined, so that the toughness in the C direction would not be ensured in some cases. Therefore, the cumulative rolling reduction ratio in the rolling at 900 to 930°C, which is on the lower side of the recrystallization temperature range, is preferably 20% or more, or 30% or more. In addition, if the rolling in the recrystallization temperature range takes a long time, the prior y grain size will grow and coarsen, whereby the toughness in the C direction cannot be ensured. Therefore, the time from the start to the end of the rolling in the recrystallization temperature range is within 6.0 minutes. The time from the start to the end of the rolling in the recrystallization temperature range is preferably within 5.0 minutes, within 4.0 minutes, or within 3.0 minutes. 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 steel to be rolled at 900°C. In addition, the cumulative rolling reduction ratio of the rolling at 900 to 930°C in the recrystallization temperature range is obtainable from the difference between the thickness of the steel to be rolled at 930°C, and the thickness of the steel to be rolled at 900°C. Cumulative rolling reduction ratio in rolling in recrystallization temperature range (%) = 100 × ([thickness of steel slab] - [thickness of steel to be rolled at 900°C])/[thickness of steel slab] Cumulative rolling reduction ratio in rolling at 900 to 930°C in recrystallization temperature range (%) = 100 × ([thickness of steel to be rolled at 930°C] - [thickness of steel to be rolled at 900°C])/[thickness of steel to be rolled at 930°C]
Rolling in the non-recrystallization temperature range is hot rolling carried out with the temperature of the steel to be rolled during rolling being less than 900°C. From the standpoint of refining the average crystal grain size 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 steel to be rolled at 900°C and the thickness of the steel product after rolling ends. Cumulative rolling reduction ratio (%) of rolling in non-recrystallization temperature range = 100 × ([thickness of steel to be rolled at 900°C] - [thickness of steel product after rolling ends]) / [thickness of steel to be rolled at 900°C]
From the standpoint of suppressing the generation of ferrite that decreases strength, the end temperature of the hot rolling is Ar₃ 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 Ar₃ or more. Note that Ar₃ is a value computed by the following formula. ${Ar}_{3} = 910 - 310 C - 80 Mn - 20 Cu - 15 Cr - 55 Ni - 80 Mo + 0.35$
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.
From the standpoint of promoting bainitic 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.
The stoppage temperature of the accelerated cooling is 400°C or lower from the viewpoint of improving the strength of the steel product by securing upper bainite, lower bainite, and martensite. The stoppage temperature of the accelerated cooling is preferably 350°C or lower. The accelerated cooling may be performed to room temperature. The stoppage temperature of the accelerated cooling is preferably 100°C or higher from the viewpoint of dehydrogenation of the steel product.
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 Ac₃ 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 Ac₃ is a value calculated by the following formula. Ac₃ = 937.2 - 436.5C + 56Si - 19.7Mn - 16.3Cu - 26.6Ni - 4.9Cr + 38.1Mo + 124.8V + 136.3Ti - 19.1Nb + 198.4Al + 3315B
The symbol of an element in the formula means the content amount (mass%) of the element contained in the steel slab.
The steel product may be subjected to tempering treatment. The heating temperature of the tempering treatment is preferably 660°C or lower, or 640°C or lower from the viewpoint of suppressing a reduction in strength. On the other hand, the heating temperature of the tempering treatment is preferably 400°C or higher, or 500°C or higher from the viewpoint of improving toughness.
Note that the method for manufacturing a steel product according to the present disclosure is not limited to the above-described conditions, and any method may be employed as long as the above-described chemical composition, microstructure, and tensile strength are satisfied.

EXAMPLES

The steel product according 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 according to the present disclosure is not limited to the following Examples.

[Manufacturing of Steel Product]

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.

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)	AC₃ [°C]	Rolling conditions							Ar₃ [°C]	Direct quenching conditions			Reheat quenching conditions		Temper heat treatment [°C]
No	Plate thickness (mm)	AC₃ [°C]	Heating temperature [°C]	Cumulative rolling reduction ratio at 900°C or higher [%]	Cumulative rolling reduction ratio from 900 to 930°C [%]	Time from start to end of rolling in recrystallization temperature range [min]	Cumulative rolling reduction ratio at less than 900°C [%]	End temperature [°C]	Cooling after rolling	Ar₃ [°C]	Start temperature [°C]	Stoppage temperature [°C]	Cooling rate [°C/s]	Reheating temperature [°C]	Cooling after reheating	Temper heat treatment [°C]
1	25	884	1080	62	32	3.4	73	810	air-cooling	756	-	-	-	910	water-cooling	620
2	40	870	1100	55	25	3.7	63	820	water-cooling	758	800	100	4.2	-	-	580
3	50	882	1060	65	31	2.4	41	780	water-cooling	733	770	30	2.8	-	-	600
4	40	888	1180	47	20	3.8	69	830	water-cooling	753	810	120	5.6	-	-	-
5	35	878	1120	70	34	4.5	51	790	air-cooling	741	-	-	-	940	water-cooling	540
6	30	877	1100	62	30	4.7	67	820	water-cooling	756	810	300	10.4	-	-	620
7	55	877	1090	40	21	3.6	62	740	water-cooling	689	720	170	3.1	-	-	640
8	45	866	1020	45	23	3.1	66	800	air-cooling	754	-	-	-	920	water-cooling	500
9	50	891	1100	58	32	2.9	50	780	water-cooling	730	760	60	2.5	-	-	480
10	40	866	1090	51	29	2.5	66	800	air-cooling	753	-	-	-	960	water-cooling	610
11	30	877	1110	79	35	3.9	41	790	water-cooling	746	770	220	6.2	-	-	600
12	50	898	1090	54	30	2.2	55	810	water-cooling	754	800	40	3.3	-	-	-
13	40	860	1180	64	34	4.1	54	820	water-cooling	765	800	100	5.9	-	-	600
14	55	912	1070	67	35	3.9	31	850	water-cooling	775	830	250	4.4	-	-	550
15	60	918	1120	58	30	2.9	41	750	air-cooling	712	-	-	-	930	water-cooling	640
16	35	884	1080	72	34	2.8	48	770	water-cooling	734	760	160	8.2	-	-	620
17	50	898	1100	41	22	3.9	65	830	air-cooling	791	-	-	-	950	water-cooling	-
18	55	871	1120	48	28	5.8	56	760	air-cooling	716	-	-	-	970	water-cooling	590
19	30	887	1180	57	30	4.2	71	810	water-cooling	753	800	40	9.9	-	-	-
20	50	893	1120	55	26	4.4	54	800	water-cooling	765	790	100	2.4	-	-	600
21	30	900	1090	52	21	8.4	74	820	water-cooling	749	800	150	3.9	-	-	620
22	40	865	1140	61	31	3.2	57	810	water-cooling	706	790	60	4.4	-	-	550

[Measurement and evaluation]

The microstructure and mechanical properties of the obtained steel product were measured by the methods described above. Table 3 shows the results.

For the toughness, the average value of Charpy impact absorption energies (KV2) at - 90°C, and the average value of Charpy impact absorption energies at -90°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 of steel product	Average aspect ratio of prior y grain	Average crystal grain size [µm]			YS (MPa)	TS (MPa)	Yield ratio YS/TS (%)	Base metal toughness KV2 [J]		Toughness after PWHT KV2 [J]		Notes
	Low-temperature transformation structure (area ratio) [%]		Average crystal grain size [µm]						Base metal toughness KV2 [J]		Toughness after PWHT KV2 [J]
	Low-temperature transformation structure (area ratio) [%]		L direction	C direction	Overall average				L direction	C direction	L direction	C direction
1	81.2	1.3	18.8	19.2	19.0	424	532	80	282	267	281	270	Examples of present invention
2	92.5	2.4	19.2	23.9	21.6	527	664	79	221	165	225	172
3	87.8	2.0	18.4	20.9	19.7	462	564	82	292	234	290	232
4	96.5	2.5	19.5	23.2	21.4	489	641	76	228	168	220	160
5	92.7	1.2	19.5	20.2	19.9	508	645	79	225	214	211	201
6	90.5	2.6	21.0	24.0	22.5	488	600	81	192	162	183	154
7	95.5	2.7	20.4	24.1	22.3	582	704	83	212	182	184	156
8	93.3	1.3	19.5	19.6	19.6	498	634	79	194	190	193	190
9	88.8	2.2	20.3	23.2	21.8	490	628	78	262	210	254	202
10	96.1	1.1	22.5	22.9	22.7	564	685	82	161	159	154	153
11	90.8	1.8	17.1	18.8	18.0	504	611	82	274	232	257	214
12	92.6	2.3	17.6	20.9	19.3	485	624	78	272	211	270	210
13	80.2	2.1	18.2	20.2	19.2	397	502	79	281	234	270	222
14	85.1	1.7	17.8	18.9	18.4	437	558	78	316	285	304	272
15	97.7	1.2	21.0	21.1	21.1	590	712	83	199	190	198	190
16	87.5	1.8	17.2	18.6	17.9	475	584	81	311	275	299	264
17	93.6	1.3	22.6	22.6	22.6	525	672	78	208	202	206	198
18	98.2	1.2	23.5	23.6	23.6	559	701	80	176	172	163	159
19	90.7	3.1	20.4	24.7	22.6	462	601	77	202	152	200	152
20	63.2	2.5	22.2	24.3	23.3	374	476	79	238	221	235	220	Comparative Examples
21	88.4	4.2	23.2	29.8	26.5	522	647	81	172	102	170	100
22	89.4	2.4	22.3	24.1	23.2	502	631	80	164	151	69	52

Nos. 1 to 19 are Examples of the present invention, and Nos. 20 to 22 are Comparative Examples.
In No. 20, since Ceq was too small, sufficient hardenability was not obtained, and sufficient strength was not obtained.
In No. 21, since the time from the start up to the end of the rolling in the recrystallization temperature range rolling was too long, the prior y grain size was not refined, also the aspect ratio of the prior γ grain size was large, the toughness of the base metal in the C direction and the toughness after PWHT were inferior, and both values of the toughness were 70 J lower than the toughness in the L direction.
In No. 22, since the Mn content was excessive, sufficient low-temperature toughness was not obtained after PWHT.
In contrast to the Comparative Examples, in all the Examples of present invention (Nos. 1 to 19), the chemical composition and microstructure of the steel product were appropriately controlled, the tensile strength was in an appropriate range of 490 MPa or more and 720 MPa or less, and furthermore the Charpy impact absorption energy at -90°C was high in both the C direction and the L direction, regardless of before or after PWHT. In Examples with particularly excellent properties, a low-temperature toughness of 150 J or more was obtained.

Industrial Applicability

The steel products according to the present disclosure can be used mainly as material for transport tanks of liquefied carbon dioxide, for example. Further, the steel products according to the present disclosure can also be used in manufacturing other welded structures such as buildings, bridges, ships, pipelines, offshore structures, and pressure vessels (tanks).
The disclosures of Japanese Patent Application No. 2023-176327 filed on October 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

A steel product having a chemical composition of, in mass%,
C: from 0.03% to 0.20%,

Si: from 0.10% to 0.50%,

Mn: from 1.25% to 2.00%,

P: 0.025% or less,

S: 0.0100% or less,

Al: from 0.005% to 0.100%,

O: 0.0100% or less,

N: 0.0100% or less,

Ni: 0 to 1.30%,

Cu: 0 to 0.90%,

Cr: 0 to 0.90%,

Mo: 0 to 1.00%,

B: 0 to 0.0050%,

Nb: 0 to 0.020%,

Ti: 0 to 0.050%,

V: 0 to 0.10%,

Mg: 0 to 0.015%,

Ca: 0 to 0.015%,

REM: 0 to 0.015%, and

balance: Fe and impurities,

and in which Ceq represented by following Formula (1) is 0.350 or more,

wherein:
a tensile strength is from 490 MPa to 720 MPa, and

a microstructure of a portion that is 1/4 of a thickness in a thickness direction from a surface of the steel product comprises low-temperature transformation structures whose total area ratio is 70.0% or more, and has an average crystal grain size of 25.0 µm or less, $Ceq = [C] + [Mn] / 6 + [Ni] / 15 + [Cu] / 15 + [Cr] / 5 + [Mo] / 5 + [V] / 5$

wherein a symbol of an element in square brackets in Formula (1) represents a content (mass%) of a corresponding element contained in the steel product, provided that zero is substituted when the corresponding element is not contained.
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:
Ni: from 0.01% to 1.30%,

Cu: from 0.01% to 0.90%,

Cr: from 0.01% to 0.90%,

Mo: from 0.01% to 1.00%, and

B: from 0.0003% to 0.0050%.
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 0.020%,

Ti: from 0.001% to 0.050%, and

V: from 0.01% to 0.10%.
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 0.015%,

Ca: from 0.0003% to 0.015%, and

REM: from 0.0003% to 0.015%.
The steel product according to any one of claims 1 to 4, wherein a Charpy impact absorption energy at -90°C at the portion that is 1/4 of the thickness is 150 J or more.
The steel product according to any one of claims 1 to 5, wherein, when 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 the temperature is held for 2 hours at 600°C, is carried out on the steel product, a Charpy impact absorption energy at -90°C at the portion that is 1/4 of the thickness of the heat-treated portion is 150 J or more.
The steel product according to any one of claims 1 to 6, wherein, in a cross-section in parallel with a rolling direction and the thickness direction of the steel product, a prior austenite grain in a portion that is 1/4 of a thickness, in the thickness direction from the surface of the steel product, has an aspect ratio of 3.5 or less.
The steel product according to any one of claims 1 to 6, wherein, in a cross-section in parallel with a rolling direction and the thickness direction of the steel product, a prior austenite grain in a portion that is 1/4 of a thickness, in a thickness direction from the surface of the steel product, has an aspect ratio of less than 1.5.
The steel product according to any one of claims 1 to 8, having an yield ratio of 80.0% or less.
A pressure vessel, comprising the steel product according to any one of claims 1 to 9.