EP4793380A1

EP4793380A1 - Steel material and pressure vessel

Info

Publication number: EP4793380A1
Application number: EP24877317.8A
Authority: EP
Inventors: Tetsuya Uchiyama; Daiki NAKANISHI; Hirokazu USUKI; 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: WO2025079723A1; CN122029298A; JPWO2025079723A1; KR20260072635A

Abstract

A steel product and a pressure vessel that contains the steel product, having a specific chemical composition having α represented by the formula below of from 5.0 to 16.0, the tensile strength is from 615 MPa to 930 MPa, the microstructure of a portion that is 1/4 of the thickness from the surface includes lower bainite and martensite, the total area ratio of the lower bainite and the martensite is 15.0% or more, the total area ratio of upper bainite, the lower bainite, and the martensite is 90.0% or more, the area ratio of retained austenite is less than 1.7%, and the Mn concentration in a center segregation region in a portion that is 1/2 of the thickness is 2.00% or less.

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

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, and 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, nickel steel such as 3.5% Ni steel is used as a material for tanks that store liquefied ethane or liquefied ethylene whose usage temperatures are around -100°C.
As with this 3.5% Ni steel, steel products that require ensured low-temperature toughness-exemplified by those used in cryogenic pressure vessels-are often made to contain Ni.
For example, Patent Document 1 proposes a cryogenic nickel steel sheet having a specific chemical composition containing 3.0% or more but less than 5.0% of Ni, an alumina cluster index of 0.030 or less, an effective crystal grain size of 12.0 µm or less, an average value of tensile strength of from 540 MPa to 610 MPa, and a Charpy impact absorption energy at - 140°C of 150 J or more.
Further, Patent Document 2 proposes a nickel-containing steel product for low temperatures that has excellent toughness and has a specific chemical composition containing 2.7% or more and 5.0% or less Ni, wherein the prior austenite grain size at the time of quenching heating is 20 µm or less, and the effective crystal grain size after a heat treatment is 12 µm or less, and the tensile strength is 450 MPa or more and 690 MPa or less.
In addition, Patent Documents 3 to 6 propose a steel product or the like, in consideration of low-temperature toughness of a welded portion.

Patent Document 1: Japanese Patent Publication ( JP-B) No. 6610352
Patent Document 2: JP-B No. 6984319
Patent Document 3: International Publication No. WO 2023/276429
Patent Document 4: Japanese Patent Application Laid-Open ( JP-A) No. 2001-123245
Patent Document 5: JP-A No. 2023-57907
Patent Document 6: JP-A No. 2011-246768

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.
An object of the present disclosure is to provide a steel product and a pressure vessel suitable for low-temperature uses, having high tensile strength, and for which good low-temperature toughness is obtainable regardless of before or after post weld heat treatment.

Solution to Problem

The gist of the present disclosure is as follows.

<1> A steel product having a chemical composition comprising, in mass%,
- C: from 0.03% to 0.20%,
- Si: from 0.01% to 0.50%,
- Mn: from 0.10% to 1.50%,
- P: 0.025% or less,
- S: 0.0250% or less,
- Ni: 4.51% or more and less than 6.10%,
- Al: from 0.001% to 0.100%,
- O: 0.0100% or less,
- N: 0.0100% or less,
- Cu: 0 to 1.50%,
- Cr: 0 to 3.00%,
- Mo: 0 to 2.00%,
- B: 0 to 0.0050%,
- Nb: 0 to 0.050%,
- Ti: 0 to 0.050%,
- V: 0 to 0.10%,
- Mg: 0 to 0.0200%,
- Ca: 0 to 0.0200%,
- REM: 0 to 0.0200%, and
- balance: Fe and impurities,
- and in which α represented by following Formula (1) is from 5.0 to 16.0,
- wherein:
  - a tensile strength is from 615 MPa to 930 MPa,
  - a microstructure of a portion that is 1/4 of a thickness, in a thickness direction from a surface of the steel product, comprises lower bainite and martensite, a total area ratio of the lower bainite and the martensite is 15.0% or more, a total area ratio of upper bainite, the lower bainite, and the martensite is 90.0% or more, and an area ratio of retained austenite is less than 1.7%, and
  - a Mn concentration in a center segregation region in a portion that is 1/2 of a thickness, in the thickness direction from the surface of the steel product, is 2.00% or less, α = 0.50 × √[C] × (1 + 0.64[Si]) × (1 + 4.10[Mn]) × (1 + 0.27[Cu]) × (1 + 0.52[Ni]) × (1 + 2.33[Cr]) × (1 + 3.14[Mo])
  - wherein a symbol of an element in square brackets in Formula (1) represents a content amount (mass%) of a corresponding element contained in the steel product, provided that zero is substituted when the corresponding element is not 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:
- Cu: from 0.01% to 1.50%,
- Cr: from 0.01% to 3.00%,
- Mo: from 0.01% to 2.00%, and
- B: from 0.0003% to 0.0050%.
<3> The steel product according to <1> or <2>, wherein the chemical composition comprises the following Group B:
[Group B]:
one or two or more selected from the group consisting of:
- Nb: from 0.001% to 0.050%,
- Ti: from 0.001% to 0.050%, and
- V: from 0.01% to 0.10%.
<4> The steel product according to any one of <1> to <3>, wherein the chemical composition comprises the following Group C:
[Group C]:
one or two or more selected from the group consisting of:
- Mg: from 0.0003% to 0.0200%,
- Ca: from 0.0003% to 0.0200%, and
- REM: from 0.0003% to 0.0200%.
<5> The steel product according to any one of <1> to <4>, wherein, in the microstructure of the portion that is 1/2 of the thickness, in the thickness direction, from the surface of the steel product, an average crystal grain size is 20.0 µm or less.
<6> The steel product according to any one of <1> to <5>, wherein an aspect ratio of a prior austenite grain in the portion that is 1/2 of the thickness, in the thickness direction, from the surface of the steel product is 1.5 or more.
<7> The steel product according to any one of <1> to <6>, wherein a Charpy impact absorption energy at -110°C at the portion that is 1/2 of the thickness is 150 J or more.
<8> The steel product according to any one of <1> to <7>, 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 -110°C at the portion that is 1/2 of the thickness of the heat-treated portion is 150 J or more.
<9> A pressure vessel, comprising the steel product according to any one of <1> to <8>.

Advantageous Effects of Invention

According to the present disclosure, there can be provided a steel product and a pressure vessel suitable for low-temperature uses, having high tensile strength, and for which good low-temperature toughness is obtainable regardless of before or after post weld heat treatment. The low temperature herein means a temperature range, for example, from -10 to -110°C.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a diagram illustrating an example of results of discriminating the microstructure.

DESCRIPTION OF EMBODIMENTS

The present disclosure is described in detail hereinafter.
The "post weld heat treatment" in the present disclosure means a post weld heat treatment based on the contents prescribed in JIS Z 3700:2009 "Method of post weld heat treatment Method", unless otherwise specified.
The "steel product" and the "base metal" in the present disclosure mean the steel product portion that does not include a surface treated layer such as a plating layer or a coated film. However, a surface treated layer such as a plating layer or a coated film may be formed on the surface of the steel product relating to the present disclosure.
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 relating to an embodiment of the present disclosure is described hereinafter. The new knowledge, which was obtained as the result of studies by the inventors of the present disclosure and which was arrived at in completing the steel product relating to the present disclosure, is described in detail first.
The inventors of the present disclosure carried out studies in order to improve the strength of steel products. The tensile strength of a steel product is ensured by the configuration of the microstructure. The inventors of the present disclosure took samples from a 1/4t portion (t: thickness of the steel product), which is the portion that is 1/4 of the thickness in the thickness direction from the surface of a steel product after hot rolling and accelerated cooling, and carried out tensile test thereon, and observed the microstructures. As a result, it was learned that, at the microstructure of the 1/4t portion of a steel product having a tensile strength of 615 MPa or more and 930 MPa or less, the area ratio of ferrite is less than 10.0%, and the total of the area ratios of upper bainite, lower bainite and martensite is 90.0% or more. Note that the total of the area ratios of upper bainite, lower bainite and martensite was measured by using electron back scatter diffraction (hereinafter called "EBSD").
Further, the inventors of the present disclosure carried out studies in order to improve the toughness of steel products. The toughness of a steel product is ensured by the configuration of the microstructure. The inventors of the present disclosure took samples from the 1/2t portion of a steel product after hot rolling and accelerated cooling, and carried out Charpy impact test, and observed the microstructures. As a result, it was found that, in a steel product exhibiting a Charpy impact absorption energy of 150 J or more at -110°C, the total of the area ratios of lower bainite and martensite is 15.0% or more, and the area ratio of retained austenite is less than 1.7%. Note that the total of the area ratios of lower bainite and martensite was measured by using EBSD. Note that the area ratio of the retained austenite was measured by X-ray diffraction. The volume ratio of the retained austenite measured by the X-ray diffraction may be considered to be the area ratio.
In addition, a sample was taken from the 1/2t portion of the thickness in the thickness direction from the surface of the steel product, and a center segregation region was observed. From the result, the steel product having a Charpy impact absorption energy of 100 J or more at -110°C was found to have a Mn concentration in the center segregation region of 2.00% or less, regardless of before or after the post weld heat treatment.
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 1/2t portion of a steel product manufactured by controlling cooling rate and cooling stoppage temperature 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"). A finding that, when the average crystal grain size at the 1/2t portion of the steel product is 20.0 µm or less, the toughness of the steel product tends to improve further regardless of before or after post-welding heat treatment has been obtained.

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

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

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

(Si: 0.01% or more and 0.50% or less)

Si is an element that is used as a deoxidizing agent, and further, that dissolves into the steel and increases the strength. From the standpoint of controlling the O concentration contained in molten steel, in the present disclosure, the Si content is 0.01% or more. The Si content is preferably 0.03% or more, 0.05% or more, 0.10% or more, or 0.12% or more. On the other hand, if the Si content is excessive, there are cases in which a hard phase forms in the HAZ, and the toughness decreases. Accordingly, from the standpoint of ensuring the HAZ toughness, in the present disclosure, the Si content is 0.50% or less. The Si content is preferably 0.30% or less, or 0.20% or less.

(Mn: 0.10% or more and 1.50% 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 0.10% or more. Moreover, due to Mn in an amount of 0.10% or more, by forming MnS, the solid-solution S is reduced, and hot cracking is prevented. From the standpoint of ensuring the strength of the steel product and the toughness of the HAZ, the Mn content is preferably 0.30% or more, or 0.50% or more. On the other hand, if the Mn content is excessive, there are cases in which the toughness after PWHT decreases due to the Mn segregating at the grain boundary at the time of PWHT. Accordingly, from the standpoint of ensuring the toughness of the steel product after PWHT, in the present disclosure, the Mn content is 1.50% or less. The Mn content is preferably 1.45% or less, 1.40% or less, or 1.35% 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: 0.0250% 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 center segregation region, and the toughness and ductility of the steel product and the HAZ deteriorate. From the standpoint of ensuring the toughness and ductility of the steel product and the HAZ, the S content is 0.0250% or less. The S content is preferably 0.0100% or less or 0.0050% or less.

(Ni: 4.51% or more and less than 6.10%)

Ni is an element that is effective in improving the hardenability and toughness of the steel. Therefore, in the present embodiment, the Ni content is 4.51% or more. The Ni content is preferably 5.00% or more, or 5.25% or more. However, Ni is an expensive element, and, from the standpoint of cost reduction, in the present disclosure, the Ni content is less than 6.10%. The Ni content is preferably 6.00% or less, or 5.75% or less.

(Al: 0.001% 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 0.001% 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, or 0.050% 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, 0.0050% 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 0.0050% 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, as needed, the steel product according to the present disclosure may be made to contain one or two or more of the optional elements Cu, Cr, Mo, and B that are described hereinafter and have the effect of improving the hardenability.

(Cu: 1.50% or less)

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

(Cr: 3.00% or less)

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

(Mo: 2.00% or less)

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

(B: 0.0050% or less)

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: 0.050% 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 0.050% or less. The Nb content is preferably 0.040% or less, or 0.030% or less. In particular, from the standpoint of ensuring the toughness of the steel product after PWHT, the Nb content may be 0.004% or less.

(Ti: 0.050% or less)

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

(V: 0.10% or less)

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: 0.0200% 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 0.0200% or less. The Mg content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.

(Ca: 0.0200% or less)

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 0.0200% 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 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 0.0200% or less. The REM content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.

(Balance: Fe and Impurities)

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 respective elements, in the present disclosure, the range of value α is limited as follows.

(Value α: 5.0 or more and 16.0 or less)

Value α is computed by following formula (1). α = 0.50 × √[C] × (1+0.64[Si]) × (1+4.10[Mn]) × (1+0.27[Cu]) × (1+0.52[Ni]) × (1+2.33[Cr]) × (1+3.14[Mo])
Wherein [C], [Si], [Mn], [Cu], [Ni], [Cr] and [Mo] are the contents (mass%) of C, Si, Mn, Cu, Ni, Cr and Mo in the steel. In a case in which a given element is not contained, zero is substituted in. Note that √[C] has the same meaning as [C]^1/2.
In the present disclosure, the range of value α is 5.0 - 16.0. This is an index expressing the hardenability of the steel product. The greater the value α, the more that lower bainite and martensite microstructures having a superior balance of strength and toughness can be formed. When α is in the appropriate range, in the microstructure of the HAZ as well, the ratio of the lower bainite and martensite microstructures having a superior balance of strength and toughness becomes high, and the HAZ toughness also can be ensured. When α is 5.0 or more, the hardenability of the base metal is ensured, and the ratio of the lower bainite and martensite having a favorable balance of strength and toughness increases, and a deterioration in toughness is suppressed. Further, in the microstructure of the HAZ as well, it is easy for the ratio of the lower bainite and martensite to increase, and the HAZ toughness also improves. On the other hand, if value α is 16.0 or less, toughness can be ensured without the strength of the steel product becoming too high. Further, when value α is 16.0 or less, the toughness after PWHT also can be ensured. Moreover, the HAZ toughness also is ensured without the HAZ becoming too hard.
Due to the aforementioned numerical value range relating to value α being satisfied, there can be provided a nickel-containing steel product for low temperatures that has excellent strength and toughness. Value α is preferably 5.5 or more, 6.0 or more, or 6.6 or more. Further, value α is preferably 15.5 or less or 15.0 or less.

(Mn Concentration in 1/2t Portion of Steel Product)

The Mn concentration in the center segregation region in the portion that is 1/2 of the thickness, in the thickness direction from the surface of at a widthwise center portion of the steel product, is 2.00% or less. Due to the Mn concentration in the center segregation region kept at 2.00% or less, excellent low-temperature toughness can be exhibited regardless of before or after PWHT. When the Mn concentration in the center segregation region is excessive, Mn would segregate in the grain boundary during PWHT, so that the toughness after PWHT would be reduced. Therefore in the present disclosure, the Mn concentration in the center segregation region is 2.00% or less, from the standpoint of ensuring toughness of the steel product after PWHT. The Mn concentration in the center segregation region is preferably 1.90% or less, or 1.80% or less.

The microstructure of the steel product according to the present disclosure is described next. The microstructure of the portion that is 1/4 of the thickness in the thickness direction from the surface of the steel product according to the present disclosure contains lower bainite and martensite. Further, in addition to lower bainite, upper bainite also may be contained as bainite.
"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 Ratios of Lower Bainite and Martensite: 15.0% or more)

Lower bainite and martensite are hard phases and increase the toughness of the steel product. From the standpoint of ensuring the toughness of the steel product, the area ratio of lower bainite and martensite at the 1/4t portion is 15.0% or more. The area ratio of lower bainite and martensite at the 1/4t portion is preferably 20.0% or more, or 30.0% or more. The total of the area ratio of the lower bainite and the area ratio of the martensite at the 1/4t portion may be 100%.

(Total of Area Ratios of Upper Bainite, Lower Bainite, and Martensite: 90.0% or more)

From the standpoint of ensuring the strength of the steel product, the total of the area ratios of upper bainite and lower bainite and martensite at the 1/4t portion is 90.0% or more. The total of the area ratios of upper bainite, lower bainite and martensite at the 1/4t portion may be 100%. Further, the upper bainite of the 1/4t portion may be 1.0% or more.

(Area Ratio of Retained Austenite: less than 1.7%)

From the standpoint of ensuring the toughness of the steel product, the area ratio of the retained austenite of the 1/4t portion is less than 1.7%. The area ratio of the retained austenite of the 1/4t portion is preferably 1.0% or less, and may be 0.0%. This is thought to be because, in the Ni-containing steel plate of the present disclosure, the Ni content is lower than conventional 9% Ni steel, and therefore, even if retained austenite were to exist at -110°C, it would be unstable, and, if the steel structure were to incur plastic deformation at the crack tip, the retained austenite would change into martensite due to plasticity-induced martensitic transformation. Thus, the retained austenite at room temperature is made to be less than 1.7% as a volume ratio (area ratio).
Further, the higher the Ni content, the easier it is for the volume ratio of the retained austenite to increase as well, and an increase in the Ni content is also not preferable in that it leads to an increase in cost.
Observation of the microstructure of the steel product is carried out by using a sample in which a 1/4t portion of the steel product serves as the observed surface. Two types of samples on which (a) electrolytic polishing or (b) nital etching is carried out are prepared. Each sample of (a) and (b) is measured at three places by the following method, and the average value of the three places is used as the area ratio of the microstructure of that steel product. Note that three of each sample of (a) and (b) may be prepared, and the averages of the respective samples may be computed. Or, three places of one sample of each may be measured in a visual field, and the average of each computed.
By using an electrolytically polished sample that, after mirror finishing by mechanical polishing, is subjected to electrolytic polishing that removes the strained layers arising due to the mechanical polishing, measurement of the total of the area ratios of the upper bainite, lower bainite, martensite and retained austenite is carried out by EBSD. The measurement magnification is 200×, and measurement of a range of 400 µm × 400 µm is carried out at a pitch of 0.4 µm. The measuring is carried out in a state in which the beam diameter of the electron beam is 0.4 µm or less. The confidence index (hereinafter called "CI value") is set to 0.1 or more. The determination of ferrite, and upper bainite, lower bainite and martensite, is carried out by setting the threshold value of the Grain Average Misorientation (hereinafter called "GAM") to 0.5. Note that the GAM value is an index defined in OIM Analysis (EBSD crystal orientation analyzing software manufactured by TSL, United States). The region in which the GAM is 0.5 or less is ferrite. The region in which the GAM exceeds 0.5 is upper bainite, lower bainite, martensite or retained austenite. Because the upper bainite, lower bainite, martensite and retained austenite in the present disclosure are determined by using the GAM of EBSD as the threshold value, not only upper bainite, lower bainite, martensite and retained austenite, but also tempered upper bainite, tempered lower bainite and tempered martensite are included. Comparing the microstructures of direct quenching (DQ) and (DQT) that is carried out thereafter up to tempering (T), although dissolution of the MA and coarsening of carbide occur after tempering, the way of looking at the microstructure does not vary greatly.
Measurement of the area ratio of the upper bainite by SEM observation is carried out by using a nital etched sample. The measurement magnification is 500×, and measurement of a range of 360 µm × 480 µm is carried out. The portion, which has a clear lath structure and at which carbide and MA have formed along the lath boundary, is upper bainite. The microstructure of the region, at which the internal structure of the microstructure is relatively coarse, and the density of carbide is low and both sparse and dense, is upper bainite. An example of the results of discriminating the microstructures is shown in Fig. 1. (A) and (B) are SEM images of the same region of a steel product that is manufactured by DQT and whose value α is 9.9. In (B), the regions surrounded by the white lines are upper bainite (Bu), and the other regions are lower bainite + martensite (B_L+M). In the portions identified as upper bainite (Bu), carbides appearing white are sparsely distributed, and regions with varying density are present. On the other hand, at the portions identified as lower bainite + martensite (B_L+M), the carbides are densely and uniformly distributed. The total of the area ratios of the lower bainite, martensite and retained austenite is determined by subtracting the area ratio of the upper bainite from the total of the area ratios of the upper bainite, lower bainite, martensite and retained austenite, which were measured as described above.
Moreover, the area ratio of the retained austenite is determined by the measuring method described hereinafter, and the total of the area ratios of the lower bainite and martensite is determined by subtracting the area ratio of the retained austenite from the total of the area ratios of the lower bainite, martensite and retained austenite.
When the total area ratio of lower bainite and martensite is more than 0% according to the above structure discrimination, lower bainite and martensite are usually included. Lower bainite and martensite can be distinguished by a SEM or TEM (transmission electron microscope), and the presence of each structure can be found.

(Area Ratio of Retained Austenite)

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

(Average Crystal Grain Size of 1/2t Portion of Steel Product)

In the present disclosure, the average crystal grain size (effective crystal grain size) of the 1/2t portion of the steel product is preferably 20.0 µm or less. This is because it was learned that, if the average crystal grain size of the 1/2t portion of the steel product is 20.0 µm or less, the toughness of the steel product tends to improve even more regardless of whether before or after PWHT. However, the average crystal grain size of the 1/2t portion of the steel product may exceed 20.0 µm. 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.
The average crystal grain size is determined by a weighted average. Average crystal grain size D_area that is determined by a weighted average is calculated by the following formula by using, of the crystal grain sizes that are measured at a magnification of 500× in a 4 mm² region, area S_i and grain size d_i of the ith crystal grain detected at the time of measurement. $D_{area} = \sum S_{i} \cdot d_{i} / \sum S_{i}$

(Aspect Ratio of Prior Austenite Grain at 1/2t Portion of Steel Product)

The form of the prior austenite grain (occasionally referred to as a prior austenite grain) in the steel product of the present disclosure may be a shape flattened in the rolling direction. When the prior austenite grain at a portion of 1/2 of the thickness in the thickness direction from a surface of the steel product is made to be a flat grain having an aspect ratio of 1.5 or more, the toughness of the steel product can be improved even more. This is because, by flattening the prior austenite grain to increase the grain boundary area, the austenite grain is substantially made minute, which is effective in reducing the effective crystal grain size. The aspect ratio of the prior austenite grain is usually 4.0 or less, and may be 3.5 or less.
The aspect ratio of the prior austenite crystal grains (hereinafter sometimes referred to as "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 portion that is 1/2 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 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, a region of 0.5 mm² is observed at a magnification of 500×, 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.

The steel product according to the present disclosure has mechanical properties that are such that the steel product has both strength and low-temperature toughness. In addition to having excellent toughness at -110°C in particular, the steel product can exhibit excellent low-temperature toughness after PWHT as well.

(Tensile Strength: 615 MPa or more and 930 MPa or less)

In the present disclosure, the tensile strength of the steel product is 615- 930 MPa. In order to reduce the weight of large welded structures such as transport tanks, a steel product that can ensure strength of a structure even if the thickness is thin is necessary. Because steel products that are selected as steel products to be used in such applications usually are steel products having the aforementioned tensile strength, the present disclosure also is manufactured to have the aforementioned tensile strength. $The yield ratio (YR (%) = yield strength / tensile strength \times 100) of the steel product$ according to the present disclosure is not particularly limited, and is preferably 90% or less. In a case in which there is no yield point, the yield strength is determined by using 0.2% proof stress.

(Charpy Impact Absorption Energy at -110°C)

In order to ensure high toughness at low temperatures, the Charpy impact absorption energy at -110°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 -110°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. The steel product according to the present disclosure may exhibit low-temperature toughness with a Charpy impact absorption energy of 100 J or more at -110°C. Note that the Charpy impact absorption energy at -110°C is a numerical value measured by using a sample taken from a position of 1/2 of the thickness.

(Charpy Impact Absorption Energy at -110°C after PWHT)

There are cases in which PWHT is carried out on the welded portions of cryogenic 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 -110°C is preferably 150 J or more. The Charpy impact absorption energy at -110°C after PWHT may be 100 J or more. The Charpy impact absorption energy at -110°C after PWHT also is a numerical value measured by using a sample taken from a position of 1/2 of the thickness.

(Charpy Impact Absorption Energy at -110°C after Thermal Cycling)

In the steel product of the present disclosure, in order to ensure high toughness at a low temperature after a thermal cycling test that simulates the welded portions, the Charpy impact absorption energy at -110°C after thermal cycling is preferably 50 J or more. Due to the steel product of the present disclosure having low-temperature toughness that is such that the Charpy impact absorption energy at -110°C after thermal cycling is 50 J or more, a tank manufactured using the steel product of the present disclosure can be suitably used for transporting liquid carbon dioxide for example. The Charpy impact absorption energy at -110°C after thermal cycling may be 40 J or more. Note that the Charpy impact absorption energy at -110°C after thermal cycling is a numerical value measured by using a sample taken from a position of 1/2 of the thickness of the steel product as a thermal cycling test piece, and providing it with a thermal history of raising the temperature at 60°C/s to 1350°C, and, after maintenance at 1350°C for 1 s, cooling at 20°C/s to room temperature, and thereafter, taking a Charpy test piece therefrom.

(Charpy Impact Absorption Energy at -110°C after Thermal Cycling and PWHT)

There are cases in which PWHT is carried out on the welded portions of cryogenic tanks after being assembled into transport tanks, in order to prevent breakage in advance. After the above-described thermal cycling test, PWHT, in which the rate of temperature increase and the rate of temperature decrease are 55°C/h in the temperature region of 425°C or more, and the steel product is held for 2 hours at 600°C, is carried out on the steel product of the present disclosure, and thereafter, a Charpy test piece is taken, and measurement is carried out. In this case, with regard to the toughness of the portion where the PWHT has been carried out, the Charpy impact absorption energy at -110°C is preferably 50 J or more. The Charpy impact absorption energy at -110°C of the portion at which PWHT is carried out after the above-described thermal cycling test may be 40 J or more.
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, thereby promoting grain boundary fracture and reducing toughness. 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.
The tensile strength (TS) and the yield strength (YS) by a tensile test in accordance with JIS Z2241:2011. In the tensile test, a JIS 14A test piece that is taken from a 1/4 thickness position and of which the longitudinal direction is a direction (C direction) parallel to the width direction of the steel product is used. Each of TS and YS is measured using three test pieces, and is calculated by averaging them. The yield ratio (YR (%)) is computed by (YS/TS) × 100, on the basis of the respective average values of TS and YS.
The Charpy impact absorption energy is measured by a Charpy impact test at -110°C using an impact blade having a radius of 2 mm in accordance with JIS Z2242:2018. The Charpy impact absorption energy is measured using three test pieces, and is calculated by averaging them. In the Charpy impact test, a V-notched test piece that is taken from a 1/2 thickness position of the steel product and of which the longitudinal direction is a direction (C direction) parallel to the width direction of the steel product is used. The V-notch of the test piece is formed such that the longitudinal direction of the notch is the thickness direction of the steel product and the depth direction of the notch is the rolling direction of the steel product.
The form of the steel product according to the present disclosure is not particularly limited, and examples are steel plates, steel strips, structural steel and steel pipes. However, steel pipes and structural steel include steel products in which steel plates are joined, e.g., in addition to welded steel pipes and welded structural steel, structural steel joined by rivets, and the like. The thickness of the steel product such as steel plates, steel strips, structural steel and steel pipes (the thickness of the flanges in the case of structural steel) is not particularly limited, and usually is 3 mm or more and 150 mm or less. The thickness of the steel product may be 6 mm or more, 10 mm or more, 15 mm or more, or 30 mm or more. Further, the thickness of the steel product may be 100 mm or less, 80 mm or less, or 60 mm or less.
Further, although the application of the steel product according to the present disclosure also is not particularly limited, the steel product according to the present disclosure has mechanical properties such that the steel product has both strength and low-temperature toughness, and, in particular, can exhibit excellent low-temperature toughness even after PWHT. Therefore, the steel product according to the present disclosure can be suitably used as a constituent material for a pressure vessel, specifically a tank that stores and transports liquefied gases, and liquid carbon dioxide in particular.

(Method for manufacturing a steel product)

The method for manufacturing a steel product according to the present disclosure is not particularly limited. The steel product according to the present disclosure is manufactured, for example, by smelting a steel that satisfies the chemical composition described above, followed by continuous casting and soft reduction to give a steel slab. The soft reduction is a technique of applying pressure at the end of solidification of the steel slab, and has an effect of reducing center segregation. The steel slab is subjected to direct quenching (DQ), in which it is heated, hot-rolled, and then directly water-cooled, to be made into a steel product.

DQT: direct quenching (DQ) and tempering (T)

From the standpoint of the manufacturing cost, DQT is preferable in the manufacturing of the steel product according to the present disclosure. An example of a preferable manufacturing process is given hereinafter.
In order to reduce center segregation during continuous casting, the steel slab is subjected to soft reduction, in which pressure is applied at the end of solidification. Depending on the casting conditions and the thickness of the steel slab, the steel slab is usually subjected to reduction with a rolling reduction of about 5 to 20 mm.
The heating temperature of the steel slab to be hot rolled (steel to be rolled) is Ac₃ or higher from the viewpoint of performing hot rolling in a temperature range in which the microstructure of the steel to be rolled is austenite. The heating temperature of the steel to be rolled is preferably 1000°C or higher from the viewpoint of reducing deformation resistance. On the other hand, the heating temperature of hot rolling is 1250°C or lower from the viewpoint of suppressing coarsening of γ grains due to heating. The heating temperature of the steel to be rolled is preferably 1200°C or lower. 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.
There are cases in which the hot rolling is structured by rolling in a temperature range in which recrystallization occurs (rolling in the recrystallization temperature range) and rolling in a temperature range in which recrystallization is suppressed (rolling in the non-recrystallization temperature range).
Rolling in the recrystallization temperature range is hot rolling carried out with the temperature of the steel to be rolled during rolling being 900°C or more. From the standpoint of refining the austenite grain size of the steel product, the cumulative rolling reduction ratio of the rolling in the recrystallization temperature range is preferably 20% or more, more preferably 30% or more. The cumulative rolling reduction ratio of the rolling in the recrystallization temperature range is determined from the difference between the thickness of the steel slab before hot rolling and the thickness of the steel to be rolled at 900°C. Cumulative rolling reduction ratio (%) of rolling in recrystallization temperature range = 100 × ([thickness of steel slab] - [thickness of steel to be rolled at 900°C]) / [thickness of steel slab]
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 20% or more, more preferably 30% 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. ${Ar}_{3} = 910 - 310 C - 80 Mn - 20 Cu - 15 Cr - 55 Ni - 80 Mo + 0.35 (t - 8)$
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 and martensitic transformation, the cooling rate is 1.0°C/s or more. The cooling rate of the accelerated cooling is preferably 5.0°C/s or more, or 10.0°C/s or more. The faster the cooling rate of the accelerated cooling, the more preferable, but from standpoints such as cost and homogeneity of the accelerated cooling, the cooling rate is preferably 50.0°C/s or less, or 30.0°C/s or less. The cooling rate is a value obtained by calculating the cooling rate at a position of 1/4 of the thickness by simulation in accordance with thermal transfer calculation.
From the standpoint of improving the strength of the steel product by ensuring the upper bainite, lower bainite and martensite, the stoppage temperature of the accelerated cooling is 400°C or less. The stoppage temperature of the accelerated cooling is preferably 350°C or less. Accelerated cooling may be carried out to room temperature. From the standpoint of dehydrogenation of the steel product, the stoppage temperature of the accelerated cooling is preferably 100°C or more.
After the accelerated cooling, a tempering treatment may be carried out on the steel product. From the standpoint of suppressing a decrease in strength, the heating temperature of the tempering treatment is preferably 650°C or less, 620°C or less, or 590°C or less. On the other hand, from the standpoint of improving the toughness, the heating temperature of the tempering treatment is preferably 350°C or more or 400°C or more.

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 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. The soft reduction is a technique of applying pressure at the end of solidification of a steel slab, in order to reduce the center segregation, and indicates the rolling reduction. The steel slab is subjected to reduction, usually at a rolling reduction of about 5 to 20 mm. "Temper heat treatment" is the heating temperature in the tempering treatment after the quenching.

[Table 2]

No.	Rolling reduction in soft reduction (mm)	Plate thickness (mm)	Mn concentration in 1/2t portion of steel sheet (mass%)	Ac₃ [°C]	Hot rolling conditions					Ar₃ [°C]	Direct quenching (accelerated cooling) conditions			Temper heat treatment [°C]
No.	Rolling reduction in soft reduction (mm)	Plate thickness (mm)	Mn concentration in 1/2t portion of steel sheet (mass%)	Ac₃ [°C]	Heating temperature [°C]	Cumulative rolling reduction ratio at 900°C or higher [%]	Cumulative rolling reduction ratio at less than 900°C [%]	End temperature [°C]	Cooling after rolling	Ar₃ [°C]	Start temperature [°C]	Stoppage temperature [°C]	Cooling rate [°C/s]	Temper heat treatment [°C]
1	10	35	1.64	697	1080	35	73	820	water-cooling	448	810	320	10.3	510
2	8	40	1.89	711	1210	23	80	840	water-cooling	442	820	180	8.9	550
3	7	35	1.90	728	1100	45	55	790	water-cooling	491	780	176	5.1	610
4	5	15	1.54	703	1150	66	68	770	water-cooling	449	750	144	6.3	620
5	13	50	1.70	703	1070	45	55	830	water-cooling	447	804	320	5.8	580
6	14	30	1.04	742	1100	53	65	790	water-cooling	479	778	280	13.1	455
7	12	60	1.35	764	1200	38	42	800	water-cooling	505	760	300	2.5	610
8	17	45	1.56	721	1150	21	70	830	water-cooling	450	815	150	4.2	490
9	11	20	1.77	690	1150	26	80	760	water-cooling	413	740	243	5.4	580
10	9	40	0.81	781	1230	34	56	800	water-cooling	517	774	123	3.3	600
11	8	60	1.05	739	1150	38	45	780	water-cooling	525	760	318	4.2	390
12	15	40	1.14	748	1000	45	50	820	water-cooling	495	803	345	20.7	550
13	20	55	0.92	792	1080	69	31	880	water-cooling	504	850	228	10.7	630
14	18	60	1.36	762	1100	37	38	830	water-cooling	507	800	210	3.7	530
15	7	30	1.07	786	1200	25	66	800	water-cooling	501	785	106	5.7	615
16	17	55	1.70	760	1150	62	31	810	water-cooling	479	778	189	6.9	420
17	19	80	1.64	705	1200	55	40	770	water-cooling	456	764	230	4.1	545
18	6	45	1.10	741	1120	40	39	810	water-cooling	473	778	379	7.0	480
19	16	60	1.65	806	1080	38	56	830	water-cooling	451	821	370	5.1	583
20	15	50	1.06	763	1150	42	56	820	water-cooling	503	810	110	2.9	473
21	7	35	1.68	737	1230	38	50	830	water-cooling	475	804	220	8.3	507
22	8	50	1.18	757	1150	35	45	790	water-cooling	502	779	234	4.2	579
23	6	60	1.81	700	1100	14	41	780	water-cooling	472	756	360	5.5	510
24	9	70	1.85	702	1250	38	13	800	water-cooling	455	780	300	4.1	470
25	10	55	0.72	756	1100	12	39	820	water-cooling	531	790	376	7.8	446
26	8	60	1.67	760	1150	33	11	830	water-cooling	444	803	310	3.9	489
27	none	30	2.27	730	1100	43	55	790	water-cooling	456	778	259	6.7	510
28	12	40	2.74	731	1150	39	47	800	water-cooling	406	786	187	5.4	530
29	14	40	1.32	769	1080	38	48	810	water-cooling	521	795	365	8.2	555
30	13	55	1.78	737	1100	39	47	820	water-cooling	452	800	230	4.8	523
31	9	45	1.09	761	1150	44	53	810	water-cooling	548	504	287	5.2	465
32	8	40	1.94	730	1100	41	33	800	water-cooling	396	785	255	8.1	530
33	13	50	1.26	745	1200	36	46	820	water-cooling	506	800	140	0.4	480

[Measurement and Evaluation]

The microstructure and the mechanical properties of the obtained steel product were measured by the methods described above. The results are shown in Table 3. The meanings of the symbols of microstructure are as follows. For the region determined as lower bainite + martensite (BL + M), the presence of lower bainite and martensite was found by SEM observation. The balance of the microstructure is pearlite, an MA phase, or ferrite.

Bu: upper bainite
BL: lower bainite
M: martensite
Retained γ: retained austenite

The toughness was measured for (1) a case where the steel product (base metal) was not subjected to heat treatment, and for cases where heat treatment processes of (2) to (4) below were carried out, with use of a sample taken from the portion that is 1/2 of the thickness for each case. For the toughness of (3) and (4) below, only a part of the steel products were measured, and the steel products not measured were left blank in Table 3.

(1) Base metal toughness

Average value of Charpy impact absorption energy at -110°C

(2) Base metal toughness after PWHT

Average value of Charpy impact absorption energy at -110°C after PWHT in which the holding temperature was 600°C, the holding time was 2 hours, and each of the rate of temperature increase and the rate of temperature decrease in a temperature range of 425°C or higher was 55°C/h

(3) Toughness after Thermal Cycle

Average value of Charpy impact absorption energy at -110°C, after application of thermal history (thermal cycle) in which temperature is raised to 1350°C at 60°C/s, held at 1350°C for 1 s, and then cooled down to room temperature at 20°C/s

(4) Toughness after Thermal Cycle and PWHT

Average value of Charpy impact absorption energy at -110°C, after application of thermal history (thermal cycle) in which temperature is raised to 1350°C at 60°C/s, held at 1350°C for 1 s, and then cooled down to room temperature at 20°C/s; and after PWHT in which the holding temperature was 600°C, the holding time was 2 hours, and each of the rate of temperature increase and the rate of temperature decrease in a temperature range of 425°C or higher was 55°C/h

[Table 3]

No.	Microstructure (area ratio)			Average aspect ratio of prior γ grain	Average crystal grain size [µm]	YS [MPa]	TS [MPa]	YR [%]	Base metal toughness KV2 [J]	Base metal toughness after PWHT KV2 [J]	Toughess after thermal cycle KV2 [J]	Toughness after thermal cycle and PWHT KV2 [J]	Notes
No.	BL+M [%]	Bu+BL+M [%]	Retained γ [%]	Average aspect ratio of prior γ grain	Average crystal grain size [µm]	YS [MPa]	TS [MPa]	YR [%]	Base metal toughness KV2 [J]	Base metal toughness after PWHT KV2 [J]	Toughess after thermal cycle KV2 [J]	Toughness after thermal cycle and PWHT KV2 [J]	Notes
1	19.2	90.2	0.0	2.7	17.3	544	622	87	185	180	134	129	Examples of present invention
2	26.1	92.3	0.3	2.9	17.6	553	651	85	178	165
3	24.9	92.7	0.2	2.6	17.4	538	632	85	185	171
4	27.2	94.6	0.1	2.7	16.3	515	619	83	177	165
5	31.3	94.3	0.2	2.8	16.5	533	641	83	176	174	132	130
6	33.2	93.3	0.4	3.0	17.1	560	657	85	188	170
7	38.7	94.4	0.3	3.2	15.6	568	663	86	190	188
8	33.4	92.1	0.3	2.9	18.2	578	669	86	186	189
9	37.6	92.7	0.4	3.1	17.5	569	671	85	177	180
10	53.2	93.9	0.6	2.7	14.1	622	722	86	285	280	182	183
11	51.4	94.8	0.5	2.6	13.7	635	759	84	286	287
12	64.3	96.2	0.6	3.3	12.8	654	781	84	280	261
13	67.2	95.4	0.7	3.4	11.9	663	748	89	315	296
14	63.1	94.3	0.6	3.1	12.5	671	756	89	233	228
15	68.4	96.1	0.5	3.0	15.3	660	766	86	253	234	180	165
16	67.2	96.3	0.4	2.9	16.3	654	742	88	213	201
17	89.3	96.8	0.7	3.2	15.2	723	829	87	244	236
18	91.6	97.5	1.1	2.7	14.8	736	914	81	278	235
19	97.2	98.1	0.0	2.9	15.3	744	843	88	256	237
20	94.4	97.4	0.8	3.1	16.0	751	912	82	215	207
21	93.2	98.3	1.2	3.0	14.8	746	856	87	251	232	167	146
22	97.1	97.4	1.4	2.8	15.3	751	871	86	245	228
23	35.8	92.2	0.4	3.3	26.7	529	635	83	121	118
24	36.9	92.8	0.3	1.5	27.9	538	652	83	115	101
25	39.2	93.2	0.5	2.9	28.2	546	681	80	132	129	140	138
26	94.1	97.1	1.2	1.5	27.9	664	781	85	138	110
27	44.4	95.4	0.4	3.1	17.2	561	654	86	131	54	110	25	Comparative Examples
28	64.8	96.3	0.7	2.9	15.8	664	791	84	125	49	114	25
29	5.2	67.2	0.2	3.1	38.9	510	588	87	72	81	41	45
30	96.4	98.4	1.6	2.8	16.8	885	998	89	146	127
31	22.9	67.3	0.0	2.7	15.5	413	507	81	155	143
32	95.3	97.1	1.9	2.8	17.1	924	1053	88	110	97
33	8.2	84.4	0.1	2.9	17.3	525	649	81	71	67

Nos. 1 to 26 are Examples of the present invention, and Nos. 27 to 33 are Comparative Examples.
In No. 27, soft reduction was not carried out, so that the Mn concentration in the center segregation region exceeded 2.00%, and the sufficient low-temperature toughness was not obtained.
In No. 28, the amount of Mn was excessive, so that the Mn concentration in the center segregation region exceeded 2.00%, and the sufficient low-temperature toughness was not obtained.
In No. 29, the value α was below the lower limit value of the present disclosure, so that the hardenability was insufficient, and the strength was insufficient. The low-temperature toughness was found to be lower.
In No. 30, the value α exceeded the upper limit value of the present disclosure, so that the hardenability was too high, and the strength was excessive.
In No. 31, the start temperature of direct quenching was too low, so that the total area ratio of the upper bainite, the lower bainite, and the martensite was insufficient, and the strength was insufficient.
In No. 32, the value α exceeded the upper limit value of the present disclosure, so that the hardenability was too high, and the strength was excessive. Also the retained γ was excessive, so that sufficient low-temperature toughness was not obtained.
In No. 33, the cooling rate in direct quenching was low, so that the total area ratio of the lower bainite and the martensite was insufficient, and sufficient low-temperature toughness was not obtained.
In contrast to Comparative Examples, in all Examples of the present invention (Nos. 1 to 26), the chemical composition and the microstructure of the steel product were appropriately controlled, the tensile strength was in an appropriate range of from 615 MPa to 930 MPa, and furthermore the Charpy impact absorption energy at -110°C was high regardless of before or after PWHT. In particular, in Nos. 1 to 22 having an average crystal grain size of 20.0 µm or less at the 1/2t portion, a low-temperature toughness of 150 J or more was obtained at -110°C regardless of before or after PWHT, and also the toughness after thermal cycle and the toughness after thermal cycle and PWHT were found to be given by a low-temperature toughness of 100 J or more at -110°C.

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. 2023-176325 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 comprising, in mass%,
C: from 0.03% to 0.20%,

Si: from 0.01% to 0.50%,

Mn: from 0.10% to 1.50%,

P: 0.025% or less,

S: 0.0250% or less,

Ni: 4.51% or more and less than 6.10%,

Al: from 0.001% to 0.100%,

O: 0.0100% or less,

N: 0.0100% or less,

Cu: 0 to 1.50%,

Cr: 0 to 3.00%,

Mo: 0 to 2.00%,

B: 0 to 0.0050%,

Nb: 0 to 0.050%,

Ti: 0 to 0.050%,

V: 0 to 0.10%,

Mg: 0 to 0.0200%,

Ca: 0 to 0.0200%,

REM: 0 to 0.0200%, and

balance: Fe and impurities,

and in which α represented by following Formula (1) is from 5.0 to 16.0,

wherein:
a tensile strength is from 615 MPa to 930 MPa,

a microstructure of a portion that is 1/4 of a thickness, in a thickness direction from a surface of the steel product, comprises lower bainite and martensite, a total area ratio of the lower bainite and the martensite is 15.0% or more, a total area ratio of upper bainite, the lower bainite, and the martensite is 90.0% or more, and an area ratio of retained austenite is less than 1.7%, and

a Mn concentration in a center segregation region in a portion that is 1/2 of a thickness, in the thickness direction from the surface of the steel product, is 2.00% or less, α = 0.50 × √[C] × (1 + 0.64[Si]) × (1 + 4.10[Mn]) × (1 + 0.27[Cu]) × (1 + 0.52[Ni]) × (1 + 2.33[Cr]) × (1 + 3.14[Mo])

wherein a symbol of an element in square brackets in Formula (1) represents a content amount (mass%) of a corresponding element contained in the steel product, provided that zero is substituted when the corresponding element is not 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:
Cu: from 0.01% to 1.50%,

Cr: from 0.01% to 3.00%,

Mo: from 0.01% to 2.00%, 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 0.050%,

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 0.0200%,

Ca: from 0.0003% to 0.0200%, and

REM: from 0.0003% to 0.0200%.
The steel product according to any one of claims 1 to 4, wherein, in the microstructure of the portion that is 1/2 of the thickness, in the thickness direction, from the surface of the steel product, an average crystal grain size is 20.0 µm or less.
The steel product according to any one of claims 1 to 5, wherein an aspect ratio of a prior austenite grain in the portion that is 1/2 of the thickness, in the thickness direction, from the surface of the steel product is 1.5 or more.
The steel product according to any one of claims 1 to 6, wherein a Charpy impact absorption energy at -110°C at the portion that is 1/2 of the thickness is 150 J or more.
The steel product according to any one of claims 1 to 7, 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 -110°C at the portion that is 1/2 of the thickness of the heat-treated portion is 150 J or more.
A pressure vessel, comprising the steel product according to any one of claims 1 to 8.