EP4793383A1

EP4793383A1 - Welded joint and pressure vessel

Info

Publication number: EP4793383A1
Application number: EP24877320.2A
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: KR20260072631A; JPWO2025079726A1; CN122003513A; WO2025079726A1

Abstract

A welded joint having a specific chemical composition having Ceq represented by the following Formula (1) of from 0.350 to 0.490, the tensile strength of from 490 MPa to 720 MPa, the inclusion having a circle-equivalent diameter from 0.01 µm to 0.50 µm, and containing Ti and N, has the number density of 1.0 × 10⁵/mm² or more, at a portion that is 1/4 of a thickness and at a portion that is 1/2 of a thickness, in the thickness direction from the surface, the inclusion containing Ti and N has an average grain diameter of 150 nm or less, at the 1/4 portion and the 1/2 portion, and an effective crystal grain diameter in a region between a fusion line of the welded portion and a position 1 mm away from the fusion line in a heat-affected zone is 100.0 µm or less. A pressure vessel containing the welded joint.

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

Description

Technical Field

The present disclosure relates to a welded joint 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 for cryogenic pressure vessels such as liquefied gas storage tanks. The cryogenic steel includes Al-killed steel, nickel steel, high-Mn steel, and austenitic stainless steel, suited to the operating temperature. Many cases of constructing structures employ welding for joining steel products. In a vicinity of weld metal where the steel products are welded, a heat-affected zone (referred to as "HAZ", hereinafter) is formed. In particular, the toughness of the HAZ (HAZ toughness) is reduced as compared with that of the base metal due to coarsening of crystal grains caused by welding heat input.
For example, Patent Document 1 proposes a steel plate that excels in toughness of the base metal and the welded portion after going through multi-layer welding.
Meanwhile, Patent Document 2 proposes welding steel that excels in HAZ toughness (particularly low-temperature toughness) after going through welding under a wide range of amount of welding heat input from small heat input to large heat input.
For the purpose of low-temperature toughness and high strength, there have been proposed various steel products for which chemical compositions and inclusions are specified, and methods for manufacturing the steel products (see Patent Documents 3 and 4, for example).

Patent Document 1: Japanese Patent Application Laid-Open ( JP-A) No. 2019-183205
Patent Document 2: JP-A No. 2004-218010
Patent Document 3: JP-A No. 2012-92422
Patent Document 4: JP-A No. 2016-79461

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 welded joints after PWHT has increased even more.
An object of the present disclosure is to provide a welded joint and a pressure vessel suitable for low-temperature uses, in which a steel product having high tensile strength is used as a base metal and good low-temperature toughness is obtained regardless of before or after post weld heat treatment.

Solution to Problem

The gist of the present disclosure is as follows.

<1> A welded joint comprising a base metal made of a steel product and a welded portion,
- wherein a chemical composition of the base metal is, in mass%,
- C: from 0.03% to 0.17%,
- Si: from 0.10% to 0.50%,
- Mn: from 0.10% to 1.70%,
- P: 0.025% or less,
- S: 0.0250% or less,
- Al: from 0.015% to 0.100%,
- Ti: from 0.005% to 0.050%,
- O: 0.0100% or less,
- N: from 0.0010% to 0.0064%,
- Ni: 0 to 1.20%,
- Cu: 0 to 0.95%,
- Cr: 0 to 0.95%,
- Mo: 0 to 1.00%,
- B: 0 to 0.0050%,
- Nb: 0 to 0.034%,
- V: 0 to 0.10%,
- Mg: 0 to 0.020%,
- Ca: 0 to 0.020%,
- REM: 0 to 0.020%, and
- a balance of Fe and impurities,
- and in which Ceq represented by the following Formula (1) is from 0.350 to 0.490,
- wherein:
  - a tensile strength of the steel product is from 490 MPa to 720 MPa,
  - a number density of inclusions having a circle-equivalent diameter from 0.01 µm to 0.50 µm and containing Ti and N, which are contained in the steel product, is 1.0 × 10⁵/mm² or more, at a portion that is 1/4 of a thickness and at a portion that is 1/2 of a thickness, in a thickness direction from a surface of the steel product,
  - an average grain diameter of the inclusions containing Ti and N is 150 nm or less, at the 1/4 portion and the 1/2 portion, and
  - an effective crystal grain diameter in a region between a fusion line of the welded portion and a position 1 mm away from the fusion line in a heat-affected zone is 100.0 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 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 welded joint 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 1.20%,
- Cu: from 0.01% to 0.95%,
- Cr: from 0. 1% to 0.95%,
- Mo: from 0.01% to 1.00%, and
- B: from 0.0003% to 0.0050%.
<3> The welded joint 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: 0.001% to 0.034%, and
- V: from 0.01% to 0.10%.
<4> The welded joint 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 0.020%,
- Ca: from 0.0003% to 0.020%, and
- REM: from 0.0003% to 0.020%.
<5> The welded joint according to any one of <1> to <4>, wherein a Charpy impact absorption energy at -70°C of the heat-affected zone is 150 J or more.
<6> The welded joint according to any one of <1> to <5>, wherein, when a heat treatment is carried out on the welded joint, in which a rate of temperature increase and a rate of temperature decrease in a temperature region of 425°C or more are 55°C/h and temperature is held for 2 hours at 600°C, a Charpy impact absorption energy at -70°C of the heat-affected zone at portions subjected to the heat treatment is 150 J or more.
<7> A pressure vessel, comprising the welded joint according to any one of <1> to <6>.

Advantageous Effects of Invention

According to the present disclosure, a welded joint and a pressure vessel suitable for low-temperature uses in which a steel product having high tensile strength is used as a base metal and good low-temperature toughness is obtained regardless of before or after post weld heat treatment can be provided. The low temperature herein means a temperature range, for example, from -10 to -70°C.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1A is a schematic diagram showing an example of a portion of a double V welded joint where a test piece used for Charpy impact absorption energy of a heat-affected zone is to be taken.
Fig. 1B is a schematic diagram showing another example of a portion of a double V welded joint where a test piece used for Charpy impact absorption energy of a heat-affected zone is to be taken.
Fig. 2A is a schematic diagram showing an example of a portion of a single-bevel welded joint where a test piece used for Charpy impact absorption energy of a heat-affected zone is to be taken.
Fig. 2B is a schematic diagram showing another example of a portion of a single-bevel welded joint where a test piece used for Charpy impact absorption energy of a heat-affected zone is to be taken.
Fig. 3A is a schematic diagram showing an example of a portion of a double-bevel welded joint where a test piece used for Charpy impact absorption energy of a heat-affected zone is to be taken.
Fig. 3B is a schematic diagram showing another example of a portion of a double-bevel welded joint where a test piece used for Charpy impact absorption energy of a heat-affected zone is to be taken.
Fig. 4 is a schematic perspective view showing a notch shape of the test piece in Fig. 2A.

DESCRIPTION OF EMBODIMENTS

Hereinbelow, the present disclosure is described in detail.
The "post weld heat treatment" in the present disclosure means a post weld 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 and the welded joint in the present disclosure. The "base metal" in the welded joint means a steel portion that is not affected by welding as compared to a welded portion (a weld metal and a heat-affected zone) of a welded joint. The "heat-affected zone" means a steel portion thermally affected by welding.
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 welded joint according to an embodiment of the present disclosure will now be described. First, new findings obtained from results of studies by the inventors of the present disclosure, through which the welded joint according to the present disclosure has been completed, are described in detail.
The inventors of the present disclosure went through investigations to ensure toughness of the welded joint. In order to refine the microstructure of the heat-affected zone, extensive investigations were conducted on particles in steel to suppress coarsening of austenite grains caused by welding heat. The inventors of the present disclosure have found that, on the premise of Al-deoxidized steel, control of Ti and N contents has an effect of suppressing growth of the austenite grain in the heat-affected zone. The suppressive effect on the growth of the austenite grain was found to be obtainable, if a grain having a pinning effect on the growth of the austenite grain is a fine Ti-based nitride having a circle-equivalent diameter from 0.01 µm to 0.50 µm, and if the Ti-based nitride has a number density of 1.0 × 10⁵/mm² or more. It has also been found that the finer the grain, the greater the pinning effect, and that the low-temperature toughness is satisfied with the average grain size of the Ti-based nitride adjusted to 150 nm or less. Note that the Ti-based nitride in the present specification means inclusions that contain Ti and N, and may occasionally be referred to simply as "inclusions". The Ti-based nitride may contain, for example, Nb and C, other than Ti and N.
The toughness of the welded joint 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 heat-affected zone of a welded joint, and the sample was subjected to mechanical polishing and electrolytic polishing. The circle-equivalent diameter of the region surrounded by high angle grain boundaries was measured with an EBSD device attached to an FE-SEM (field emission scanning electron microscope). Hereinafter, the circle-equivalent diameter of the region surrounded by high angle grain boundaries is referred to as a crystal grain size. The part where the crystal grain size is measured is a region between the position of a fusion line (hereinafter, occasionally referred to as "FL") and a position 1 mm away from the fusion line (hereinafter, "FL + 1 mm"). The crystal grain size was measured at a magnification of 80× in a region of 4 mm² along the fusion line. A value calculated by the average of the largest 10 crystal grain sizes among those measured in the region of 4 mm² was taken as an effective crystal grain size (in the present disclosure, occasionally referred to as an "effective crystal grain size of the heat-affected zone"). It has been found that, when the effective crystal grain size of the heat-affected zone of the welded joint is 100.0 µm or less, the toughness of the welded joint is ensured regardless of before or after post weld heat treatment.

Next, alloy elements that constitute the chemical composition of the steel product in the welded joint according to the present disclosure are described. In the following description of alloy elements, "%" of the content means "mass%". Further, the steel product that is a base metal of the welded joint according to the present disclosure may be referred to as a "steel product in the present disclosure".

(C: 0.03% or more and 0.17% 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 0.17% or less from the viewpoint of securing toughness of the heat-affected zone (hereinafter, occasionally referred to as "HAZ"). The C content is preferably 0.15% or less, 0.13% or less, or 0.11% 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, 0.13% 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 0.20% or less.

(Mn: 0.10% or more and 1.70% 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 HAZ toughness after PWHT, in the present disclosure, the Mn content is 1.70% or less. The Mn content is preferably 1.60% or less, 1.50% or less, 1.40% or less, 1.25% or less, or 1.10% 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.

(Al: 0.015% or more and 0.100% or less)

Al is an essential element for suppressing Ti from producing coarse oxide particles, and for promoting formation of fine Ti-based nitride. Al is essentially contained also from the standpoint of Al-deoxidized steel. The Al content in the present disclosure is therefore 0.015% or more. The Al content is preferably 0.020%, 0.025% or more, or 0.030% or more. However, if Al is excessively contained, there is the concern that the Al will generate a large amount of Al-containing inclusions, 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.

(Ti: 0.005% or more and 0.050% or less)

Ti is an essential element for formation of the Ti-based nitride. In the present disclosure, the Ti content is 0.005% or more. The Ti content is preferably 0.008%, 0.010% or more, or 0.015% or more. However, if Ti is excessively contained, the Ti-based nitride would occasionally become coarse, and would lower the number density due to aggregation and coarsening. Accordingly, the Ti content is 0.050% or less. The Ti content is preferably 0.040% or less, 0.030% or less, or 0.025% 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: 0.0010% or more and 0.0064% or less)

N is an essential element for producing the Ti-based nitride. In the present disclosure, the N content is 0.0010% or more. The N content is preferably 0.0015%, 0.0020% or more, or 0.0025% or more. However, if N is excessively contained, Ti-based nitrides may become coarse, and the number density may decrease due to aggregation and coarsening. Accordingly, the N content is 0.0064% or less. The Ti content is preferably 0.0060% or less, 0.0055% or less, 0.0050% or less, or 0.0045% or less.
The steel product in 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, one or two or more of the optional elements Ni, Cu, Cr, Mo, and B, which are described hereinafter and have an effect of improving hardenability, may be contained as necessary.

(Ni: 1.20% 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 1.20% or less. The Ni content is preferably 1.00% or less, 0.80% or less, 0.60% or less, or 0.50% or less.

(Cu: 0.95% 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 0.95% or less. The Cu content is preferably 0.90% or less, 0.80% or less, 0.60% or less, or 0.50% or less.

(Cr: 0.95% 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 0.95% or less. The Cr content is preferably 0.90% or less, 0.80% 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, 0.80% 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 in the present disclosure may be made to contain one or two of the optional elements Nb 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: 0.034% 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 0.034% or less. The Nb content is preferably 0.030% or less, 0.020% or less, 0.015% or less, or 0.009% or less. In particular, from the standpoint of ensuring the HAZ toughness after PWHT, the Nb content may be 0.004% 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 in 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: 0.020% 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 0.020% or less. The Mg content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.

(Ca: 0.020% 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 in 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 Ceq is limited as follows.

(Ceq: 0.350 or more and 0.490 or less)

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.
In the steel product in the present disclosure, the range of the value Ceq is 0.350 - 0.490. This is an index indicating hardenability of the steel product. The greater the value Ceq, the more the strength can be improved.
When the value Ceq is within an appropriate range, a superior balance of strength and toughness is achieved, and the HAZ toughness can also be ensured. When the value Ceq is 0.350 or more, the hardenability of the base metal is ensured, and the strength can be ensured. On the other hand, when the value Ceq is 0.490 or less, the hardenability of the base metal is ensured, and the toughness can be ensured without the strength becoming too hard. Moreover, the HAZ toughness also is ensured before and after PWHT without the HAZ becoming too hard.
Due to the aforementioned numerical value range relating to value Ceq being satisfied, there can be provided a welded joint that has excellent strength and toughness, and is suitable for low-temperature uses. The value Ceq is preferably 0.355 or more, 0.360 or more, or 0.370 or more. The value Ceq is also preferably 0.480 or less, 0.470 or less, 0.460 or less, or 0.450 or less.

Next, the circle-equivalent diameter of the inclusions contained in the steel product of the present disclosure, and the number density of the inclusions will be described.

(The circle-equivalent diameter of the inclusions: 0.01 µm or more and 0.50 µm or less)

In the present disclosure, the inclusions having an effect of suppressing the growth of the austenite grains in the heat-affected zone are important. After investigations by the inventors of the present disclosure, the inclusions having the pinning effect of suppressing the growth of the austenite grain in the heat-affected zone were found to be a fine Ti-based nitride having a circle-equivalent diameter from 0.01 µm to 0.50 µm. The circle-equivalent diameter of the inclusions is determined from the area of the inclusions measured by TEM observation. The inclusions having a circle-equivalent diameter of less than 0.01 µm or more than 0.50 µm do not have a remarkable pinning effect. Inclusions having a circle-equivalent diameter of less than 0.01 µm or more than 0.50 µm, occasionally contained in the steel product, will be excluded from the measurement of the number density.

(Number density of inclusions: 1.0 × 10⁵/mm² or more)

In the present disclosure, the number density of the Ti-based nitride having a circle-equivalent diameter from 0.01 µm to 0.50 µm is 1.0 × 10⁵/mm² or more, from the standpoint of promoting the pinning effect. The number density of such inclusions is preferably 2.0 × 10⁵/mm² or more, or 5.0 × 10⁵/mm² or more, wherein the larger the number density of such inclusions, the more preferable.

(Average grain size of inclusions: 150 nm or less)

In the present disclosure, it was found that the finer the grain, the greater the pinning effect, and that the low-temperature toughness is satisfied with the average grain size of the Ti-based nitride adjusted to 150 nm or less.
The grain size, the composition, and the number density of the inclusions contained in the steel product in the present disclosure are measured under a transmission electron microscope (TEM) equipped with a characteristic X-ray detector (EDX), with use of extraction replicas prepared from test pieces each sampled from a 1/4t portion that lies at 1/4 of the thickness, and from a 1/2t portion that lies 1/2 of the thickness from the surface of the steel product in the thickness direction. The number of grains having a circle-equivalent diameter from 0.01 µm to 0.50 µm is measured for an area of at least 1000 µm² or more, and is converted into the number per unit area, to obtain each number density.
Meanwhile, for identification of the composition, the number of grains will reach 1000 or more in abundant cases. Since identification of all the grains one by one needs enormous work, a possible way is that at least 50 or more grains are identified to be Ti-based nitride or not, the abundance ratio of the Ti-based nitride is determined, and the ratio is then multiplied by the number of grains having been determined, to find the number of Ti-based nitride. The average grain size (average circle-equivalent diameter) of the Ti-based nitride is determined by averaging those of 50 grains among the identified Ti-based nitrides.

(Effective Crystal Grain Size of the Heat-Affected Zone: 100.0 µm or less)

The effective crystal grain size of the heat-affected zone is 100.0 µm or less from the viewpoint of securing HAZ toughness. The effective crystal grain size of the heat-affected zone is preferably 90.0 µm or less, 80.0 µmor less, or 70.0 µm or less.
The crystal grain diameter in the heat-affected zone is measured with use of an EBSD device, in a region of 4 mm² between the position of the fusion line (FL) and the position 1 mm away from the fusion line (FL + 1 mm), wherein the largest ten crystal grain diameters among those measured are averaged, and the obtained value is taken as the effective crystal grain size.

A welded joint having a base metal made of a steel product and a welded portion in the present disclosure has mechanical properties that achieve both strength of the steel product and low-temperature toughness of the heat-affected zone. In particular, the welded joint is excellent in toughness at -70°C of the heat-affected zone, 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 in the present disclosure is manufactured to have the tensile strength mentioned above.

(Charpy Impact Absorption Energy at -70°C of the Heat-Affected Zone)

In order to secure high toughness at low temperature, in the welded joint of the present disclosure, the Charpy impact absorption energy at -70°C of the heat-affected zone is preferably 150 J or more. By the fact that the welded joint of the present disclosure has a low-temperature toughness in which the Charpy impact absorption energy at -70°C of the heat-affected zone is 150 J or more, a tank manufactured using the welded joint of the present disclosure can be suitably used for, for example, transportation of liquid carbon dioxide. The Charpy impact absorption energy at -70°C of the heat-affected zone is defined as a numerical value measured by, as shown in Figs. 1A, 1B, 2A, 2B, 3A, and 3B, using a sample 14 taken from each of a region including the position of a fusion line (FL) of a welded portion 12 and a region including a position 1 mm away from the fusion line (FL + 1 mm) in a 1/4t portion of a base metal 10 of a welded joint 20. In each figure, "15" represents a weld metal, and "16" represents a heat-affected zone (HAZ). As shown in Fig. 4, a notch 18 is formed such that, when the width direction X, the thickness direction Y, and the length direction Z of the base metal 10 are defined, the length direction of the notch 18 of the test piece 14 is parallel to the thickness direction Y of the base metal 10 and the depth direction of the notch 18 is parallel to the length direction Z of the base metal 10. The length direction Z of the base metal 10 is the rolling direction, and the width direction X is a direction perpendicular to the rolling direction Z and the thickness direction Y.
In a case where the sample is collected from the welded joint with a double-V groove, as illustrated in Fig. 1A, a notch 18 for FL is formed at a position where the weld metal 15 accounts for 50%, and the total portion of the heat-affected zone 16 and the base metal 10 accounts for 50%. Meanwhile, as illustrated in Fig. 1B, the notch 18 for FL+1 mm is formed at a position 1 mm away from FL towards the base metal.

(Charpy Impact Absorption Energy at -70°C of the Heat-Affected Zone after PWHT)

For a low temperature tank, for the purpose of preventing fracture, PWHT may be performed on a welded portion after assembly into a transport tank. When the time during which the welded joint is heated to a temperature range of 425°C or higher is longer, the HAZ toughness tends to decrease. For the welded joint of the present disclosure, when a PWHT in which the holding temperature is 600°C, the holding time is 2 hours, and each of the rates of temperature increase and temperature decrease is 55°C/h in a temperature range of 425°C or higher is performed on the steel product, the toughness of the heat-affected zone subjected to the PWHT is preferably 150 J or more as a Charpy impact absorption energy at -70°C.
The HAZ toughness may be reduced by PWHT. Although the cause is not clear, it is presumed that P (phosphorus) or Mn diffuses into grain boundaries and inclusions grow or aggregate in the microstructure, and consequently the brittleness is reduced and the toughness is reduced. A reduction in toughness due to PWHT is suppressed by limiting the content amounts of P and Mn and reducing the average crystal grain size of the heat-affected zone.
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 using an impact blade having a radius of 2 mm in accordance with the provision of JIS Z2242:2018. The Charpy impact test is performed three by three at five temperatures, and the brittle fracture rates are measured to calculate vTrs. In the Charpy impact test, a V-notched test piece which is taken from the position of a 1/4t portion of the steel product and of which the longitudinal direction is a direction (L direction) parallel to the length direction of the steel product is used.
Also, the Charpy impact absorption energy of the heat-affected zone is measured by a Charpy impact test at -70°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, as shown in Figs. 1A, 1B, 2A, 2B, 3A, 3B, and 4, a V-notched test piece 14 taken from a welded joint 20 at positions corresponding to a 1/4t portion and a 1/2t portion (not shown) of the steel product is used.
The shape of the steel product in 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 welded joint according to the present disclosure is not particularly limited either; since the welded joint has mechanical properties that achieve both strength and low-temperature toughness and can exhibit excellent low-temperature toughness particularly even after PWHT, the welded joint 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)

An example of a method for manufacturing a steel product in the present disclosure will be described. In the manufacture of the steel product in the present disclosure, condition control of the steelmaking process is important to control the circle-equivalent diameter and the number density of the Ti-based nitride. First, a predetermined amount of Al is added for deoxidation, under a condition where the molten steel temperature is controlled to 1650°C or lower. Next, a predetermined amount of Ti is added under a condition where O concentration in the molten steel is controlled to 0.0050% or less, followed by casting. The time after the addition of Ti up to casting takes 120 minutes or less.
Since Ti is also a deoxidizing element, the O concentration in the molten steel before the Ti addition needs to be lowered, in order to disperse a large amount of the Ti-based nitride. In the present disclosure, the O concentration in the molten steel before the Ti addition is 0.0050% or less. The O concentration in the molten steel before the Ti addition is preferably 0.0040% or less, 0.0035% or less, or 0.0030% or less.
The time from the Ti addition up to the casting affects the circle-equivalent diameter and the number density of the Ti-based nitride. After the Ti addition, particles generated in the molten steel coalesce and grow larger. The time from the Ti addition up to the casting is 120 minutes or less in the present disclosure, from the standpoint of suppressing coarsening of the inclusions and ensuring the number density. The time from the Ti addition up to the casting is preferably 90 minutes or less, 75 minutes or less, or 60 minutes or less.
Thereafter, a steel slab at the end of solidification is subjected to soft reduction, to manufacture the steel slab. The soft reduction is a technique of applying pressure at the end of solidification of the steel slab, which effects to cause relatively uniform dispersion of the Ti-based nitride in the direction of plate thickness. Rolling reduction ratio of the soft reduction is 2.0% or more, from the standpoint of uniform dispersion of the Ti-based nitride. The rolling reduction ratio of the soft reduction is determined from the difference between the thickness of an original slab before soft reduction and the thickness of the slab after soft reduction. Rolling reduction ratio of soft reduction (%) = 100 × ([original slab thickness] - [slab thickness after soft reduction])/[original slab thickness]
The method for manufacturing a steel product in 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, is then subjected to direct quenching (DQ), in which it is directly water-cooled, or to reheated quenching (RQ) , in which it is allowed to cool, reheated, and water-cooled, and is thus made into a steel product. Further, intermediate heat treatment (L) and tempering (T) may be performed. In the case of RQ, the step preceding the reheating does not necessarily need to be allowing to cool, and may be water cooling. The manufacturing step after the hot rolling is selected from the combinations of DQ, RQ, L, and T mentioned above, and is, for example, DQT, RQT, DQLT, or RQLT.

(1) DQT: direct quenching (DQ) and tempering (T)
(2) RQT: allowing to cool, reheated quenching (RQ), and tempering (T)
(3) DQLT: direct quenching (DQ), intermediate heat treatment (L), and tempering (T)
(4) RQLT: allowing to cool, reheated quenching (RQ), intermediate heat treatment (L), and tempering (T)

From the standpoint of the manufacturing cost, DQT is preferable in the manufacturing of the steel product in the present disclosure. An example of a preferable manufacturing process is given hereinafter.
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 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 slab is preferably 1000°C or more. On the other hand, from the standpoint of suppressing coarsening of the heated γ grains, the heating temperature of the hot rolling is 1250°C or less. The heating temperature of the hot rolling 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.
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 30% or more, 40% or more, or 50% or more. The cumulative rolling reduction ratio of the rolling in the recrystallization temperature range is determined from the difference between the thickness of the steel slab before hot rolling and the thickness of the 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 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. ${Ar}_{3} = 910 - 310 C - 80 Mn - 20 Cu - 15 Cr - 55 Ni - 80 Mo + 0.35 (t - 8)$
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 the formation of low-temperature transformation structures (mainly bainite and martensite) that improve strength, 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 homogeneity of the cooling rate and cost, 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.
After the accelerated cooling, the steel product may be subjected to tempering treatment. The heating temperature of the tempering treatment is preferably 650°C or lower, 620°C or lower, or 590°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 350°C or higher, or 400°C or higher from the viewpoint of improving toughness.

(Method for Manufacturing Welded Joint)

Next, a method for manufacturing a welded joint (a welding method) according to the present disclosure will now be described.
First, a groove is formed in an end portion of the steel product (base metal) in the present disclosure described above. The shape of the groove is any of a single V groove, a single-bevel groove, a single-J groove, a single-U groove, a double-V groove, a double-bevel groove, a double-J groove, and a double-U groove. The groove is butted, and welding is performed using a welding material. The welding material is not particularly limited, and is determined according to the desired characteristics of the welded joint as appropriate. Examples of the welding method include shielded metal arc welding, gas-shielded metal arc welding, submerged arc welding, TIG welding, and the like. The amount of welding heat input and the number of welding passes are determined according to the welding method and the plate thickness as appropriate. The amount of welding heat input is, for example, about 1.0 kJ/mm to 7.0 kJ/mm.
Specific welding conditions of shielded metal arc welding, gas-shielded metal arc welding, and submerged arc welding are shown below.
In shielded metal arc welding, the outer surface is welded, followed by back gouging, and welding of the inner surface. For double-V groove, welding is performed under the conditions in which the angle of welding of the outer surface is 55°, the angle of welding of the inner surface is 70°, the preheating temperature is 125 to 175°C, the inter-pass temperature is 125 to 175°C, and N-16 (shielded metal arc welding rod, manufactured by Nippon Steel Welding & Engineering Co., Ltd.) having a rod diameter of 5.0 mm is used as a material for forming a weld metal. The welding is performed under the conditions in which the amount of current is 140 to 270 A, the voltage is 22 to 32 V, the amount of heat input is 1.2 to 4.5 kJ/mm, and the welding speed is 9 to 18 cm/min. The number of passes is determined depending on the plate thickness.
In the gas-shielded metal arc welding, CO₂ gas or a mixed gas of Ar and CO₂ is used, and the ratio of CO₂ gas is 20%. In the case of a double-bevel groove, the angle of the groove is set to 10R in a tip portion and 30° for each of the upper layer and the lower layer, and welding is performed under the conditions in which the preheating temperature is 100 to 150°C, the inter-pass temperature is 100 to 150°C, the gas flow rate is 15 to 20 L/min for the lower layer portion and 18 to 22 L/min for the upper layer portion, and YM-70CS (a solid wire manufactured by Nippon Steel Welding & Engineering Co., Ltd.) is used as a material for forming a weld metal. The welding is performed under the conditions in which the amount of current is 250 to 270 A, the voltage is 27 to 33 V, the amount of heat input is 1.5 to 2.4 kJ/mm, and the welding speed is 30 to 40 cm/min. The number of passes is determined depending on the plate thickness.
In the submerged arc welding, in the case of a single-bevel groove, the gap of the groove is set to 10 mm and the angle is set to 30°, and welding is performed under the conditions in which the preheating temperature is 100 to 150°C, the inter-pass temperature is 100 to 150°C, Y-204B is used as a material for forming a weld metal, and NB-250H (a submerged arc welding material manufactured by Nippon Steel Welding & Engineering Co., Ltd.) is used as flux. The welding is performed under the conditions in which the amount of current is 630 to 670 A, the voltage is 27 to 33 V, the amount of heat input is 3.5 to 4.4 kJ/mm, and the welding speed is 25 to 34 cm/min. The number of passes is determined depending on the plate thickness.

(Method for Evaluating Welded Joint)

After the welded joint is produced, a notch is introduced into a fusion line (FL) or a position 1 mm away from the fusion line (FL + 1 mm) as shown in Figs. 1A, 1B, 2A, 2B, 3A, 3B, and 4, and a Charpy impact test is performed. In the case where the shape of the groove is any of a single-bevel shape, a U shape, a double-bevel shape, a double-J shape, and a double-U shape, the V notch of FL is made in a tangent on the heat-affected zone side of the fusion line, and is set to include 80% or more of the heat-affected zone. In the case where the shape of the groove is either a single V groove or a double-V groove, the V notch of FL is made such that the weld metal 15 accounts for 50%, and the total portion of the heat-affected zone 16 and the base metal 10 accounts for 50%. The V-notch of FL + 1 mm is made in a position 1 mm away from the FL notch. The grain size of the microstructure in the vicinity of FL is roughly determined depending on the amount of welding heat input. Therefore, the absorption energy of the heat-affected zone largely depends on the amount of heat input regardless of the groove shape.

EXAMPLES

The steel product in 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 in 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.

[Table 2]

No.	Plate thickness (mm)	Ac₃ [°C]	Steelmaking process			Rolling conditions					Ar₃ [°C]	Direct quenching conditions			Temper heat treatment [°C]
No.	Plate thickness (mm)	Ac₃ [°C]	O concentration in molten steel at Ti addition [%]	Time from Ti addition to casting [min]	Rolling reduction ratio in soft reduction [%]	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]	Stop temperature [°C]	Cooling rate [°C/s]	Temper heat treatment [°C]
1	40	880	0.0041	82	2.5	1120	55	63	800	water-cooling	763	790	40	7.1	630
2	55	884	0.0034	54	2.3	1140	62	40	760	water-cooling	737	750	150	3.5	500
3	25	880	0.0028	72	2.6	1090	50	79	790	water-cooling	754	770	280	14.2	600
4	40	880	0.0046	69	2.3	1200	62	56	780	water-cooling	760	770	20	6.9	560
5	45	874	0.0020	112	2.1	1160	42	68	840	water-cooling	808	820	110	5.9	640
6	35	928	0.0048	100	2.2	1090	72	48	750	water-cooling	722	740	80	8.9	590
7	50	891	0.0037	94	2.4	1110	51	57	820	water-cooling	741	810	350	5.8	540
8	45	886	0.0033	58	2.4	1150	59	54	810	water-cooling	758	790	300	5.8	610
9	50	889	0.0040	45	2.8	1120	63	43	790	water-cooling	720	770	220	4.9	480
10	60	879	0.0035	71	2.1	1100	43	56	820	water-cooling	766	810	280	3.1	590
11	50	921	0.0024	69	2.5	1040	59	49	840	water-cooling	797	820	60	7.1	520
12	55	860	0.0022	48	2.6	1080	66	33	790	water-cooling	725	780	150	5.2	610
13	40	881	0.0037	58	2.5	1200	71	42	830	water-cooling	769	820	110	6.4	590
14	45	850	0.0024	90	2.4	1160	53	60	760	water-cooling	729	750	240	5.2	610
15	35	916	0.0030	82	2.3	1110	60	64	810	water-cooling	745	800	220	7.8	600
16	50	886	0.0040	57	2.7	1100	50	59	820	water-cooling	742	800	90	5.9	580
17	65	867	0.0028	69	2.7	1090	45	51	780	water-cooling	737	760	70	2.5	600
18	35	854	0.0037	60	2.2	1140	51	70	790	water-cooling	741	780	20	9.4	620
19	50	883	0.0032	59	2.2	1120	39	73	830	water-cooling	767	810	160	8.2	550
20	35	879	0.0072	68	2.2	1100	51	70	820	water-cooling	753	800	210	8.1	620
21	40	873	0.0048	145	2.3	1080	47	69	820	water-cooling	751	800	300	7.7	610
22	45	891	0.0037	81	0.0	1110	48	64	800	water-cooling	748	780	290	4.7	550
23	50	879	0.0031	69	1.1	1160	62	45	820	water-cooling	759	810	120	3.3	600
24	40	888	0.0039	71	2.2	1140	59	59	810	water-cooling	739	790	180	7.1	500
25	50	891	0.0035	62	2.3	1120	47	61	820	water-cooling	752	810	120	3.9	550
26	55	863	0.0028	53	2.5	1080	50	54	810	water-cooling	748	800	240	3.1	590
27	40	895	0.0029	76	2.5	1100	64	54	830	water-cooling	754	810	190	5.5	620

[Manufacturing of a Welded Joint]

For each of the steel products other than Nos. 1, 16, and 25 to 27, the steel product was used to produce a double-bevel groove, and multi-layer gas-shielded metal arc welding (GMAW) was performed at an amount of heat input of 2.0 kJ/mm, to manufacture a welded joint. In the GMAW welding, a mixed gas of Ar and CO₂ is used, and the ratio of CO₂ gas is 20%. For the double-bevel groove, the angle of the groove was set to 10R in a tip portion and 30° for each of the upper layer and the lower layer, and welding was performed under the conditions where the preheating temperature was 100 to 150°C, the inter-pass temperature was 100 to 150°C, the gas flow rate was 17 L/min for the lower layer portion and 20 L/min for the upper layer portion, and YM-69F (a solid wire manufactured by Nippon Steel Welding & Engineering Co., Ltd.) was used as a weld metal. The welding was performed under the conditions in which the amount of current was 260 A, the voltage was 30 V, the amount of heat input was 2.0 kJ/mm, and the welding speed was 35 cm/min. The number of passes was changed depending on the plate thickness.
For each of Nos. 1 and 16, the steel product was used to produce a single-bevel groove, and multi-layer submerged arc (SAW) welding was performed to manufacture a welded joint. In the SAW welding, the gap of the single-bevel groove was set to 10 mm and the angle was set to 30°, and welding was performed under conditions in which the preheating temperature was 100 to 150°C, the inter-pass temperature was 100 to 150°C, Y-204B was used as a weld metal, and NB-250H (a submerged arc welding material manufactured by Nippon Steel Welding & Engineering Co., Ltd.) was used as flux. The welding was performed under the conditions in which the amount of current was 650 A, the voltage was 30 V, the amount of heat input was 4.0 J/mm, and the welding speed was 29 cm/min. The number of passes was changed depending on the plate thickness.
For each of Nos. 25 to 27, the steel product was used to produce a double-V groove, and shielded metal arc welding was performed to manufacture a welded joint. In the shielded metal arc welding, the outer surface is welded, followed by back gouging, and welding of the inner surface. The welding was performed under the conditions in which the angle of welding of the outer surface was 55°, the angle of welding of the inner surface was 70°, the preheating temperature was 125 to 175°C, the inter-pass temperature was 125 to 175°C, and N-16 (shielded metal arc welding rod, manufactured by Nippon Steel Welding & Engineering Co., Ltd.) having a rod diameter of 5.0 mm was used as a material for forming a weld metal. The welding was performed under the conditions in which the amounts of current were 190 A, 220 A and 250 A, respectively, the voltage values were 23, 27, and 29 V, respectively, the amounts of heat input were 1.6, 3.0, and 4.4 kJ/mm, and the welding speed values were 16, 12, and 10 cm/min. The number of passes was changed depending on the plate thickness.

[Measurement and Evaluation]

The microstructures and mechanical properties of the obtained steel products and welded joints (heat-affected zones) were measured by the above-described methods. The results are shown in Table 3. Note that "inclusions" in Table 3 mean, "inclusions having a circle-equivalent diameter from 0.01 µm to 0.50 µm, and containing Ti and N".

For the toughness of the steel product, vTrs was measured.

For the toughness of the heat-affected zone, a notch was made at FL or FL + 1 mm on the I side of the double-bevel groove or the single-bevel groove, and a Charpy impact test was performed at -70°C. For the HAZ toughness, the average value of Charpy impact absorption energies (KV2) at -70°C when welded joints were as-manufactured (as-weld toughness), and the average value of Charpy impact absorption energies at -70°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.
The toughness was measured for each of (1) a case where the steel product (base metal) was not subjected to heat treatment, (2) a case where heat treatment was performed after welding, and (3) a case where heat treatment was not performed after welding as described below, and was measured using a sample taken from a part of 1/4 of the thickness in each case.

(1) Base metal toughness
vTrs obtained by Charpy impact tests at various temperatures
(2) Welded joint toughness

The average value of Charpy impact absorption energies at -70°C after manufacturing welded joints, each obtained by a process in which a double-bevel groove, a single-bevel groove, or a double V-groove was produced, and multi-layer gas-shielded metal arc welding (GMAW) with the amount of heat input set to 2.0 kJ/mm, multi-layer submerged arc (SAW) welding with the amount of heat input set to 4.0 kJ/mm, or multi-layer shielded metal arc welding with the amount of heat input set to 1.6, 3.0 or 4.4 kJ/mm was performed

(3) Welded joint toughness after PWHT

The average value of Charpy impact absorption energy at -70°C after the following steps: manufacturing welded joints, each obtained by a process in which a double-bevel groove, a single-bevel groove, or a double-V groove was produced and multi-layer gas-shielded metal arc welding (GMAW) with the amount of heat input set to 2.0 kJ/mm, or multi-layer submerged arc (SAW) welding with the amount of heat input set to 4.0 kJ/mm, or multi-layer shielded metal arc welding with the amount of heat input set to 1.6, 3.0 or 4.4 kJ/mm was performed; and then performing 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

[Table 3]

No.	Average grain diameter of inclusions [nm]		Number density of inclusions [× 10⁵/mm²]		Base metal			Welding method		Effective crystal grain diameter of heat-affected zone [µm]		FL toughness of welded joint				Notes
	Average grain diameter of inclusions [nm]		Number density of inclusions [× 10⁵/mm²]		YS [MPa]	TS [MPa]	Toughness vTrs [°C]	Welding technique	Amount of heat input [kJ/mm]	Effective crystal grain diameter of heat-affected zone [µm]		As-weld toughness KV2 [J]		Toughness after PWHT KV2 [J]
	1/4t	1/2t	1/4t	1/2t	YS [MPa]	TS [MPa]	Toughness vTrs [°C]	Welding technique	Amount of heat input [kJ/mm]	1/4t	1/2t	1/4t	1/2t	1/4t	1/2t
1	102	114	3.7	3.1	524	640	-90	SAW	4.0	93.2	95.6	191	182	198	180
2	94	104	3.4	2.8	406	522	-95	GMAW	2.0	82.5	87.2	209	198	194	186
3	105	118	4.2	3.7	542	675	-105	GMAW	2.0	79.9	83.5	232	211	224	205
4	115	133	2.3	1.9	489	622	-95	GMAW	2.0	84.5	92.3	207	194	200	180
5	119	145	1.5	1.2	592	710	-90	GMAW	2.0	90.6	98.7	192	182	178	168
6	126	138	2.0	1.5	551	682	-90	GMAW	2.0	92.1	94.2	182	177	169	157
7	110	120	2.6	2.2	442	565	-95	GMAW	2.0	84.6	90.5	234	200	232	200
8	84	101	2.1	1.9	512	635	-90	GMAW	2.0	85.9	90.9	209	194	207	188
9	73	90	4.0	3.4	454	578	-100	GMAW	2.0	70.9	72.5	245	238	240	230	Examples of present invention
10	102	123	1.8	1.4	492	602	-95	GMAW	2.0	85.9	92.6	194	176	186	161	Examples of present invention
11	93	102	2.9	2.4	388	502	-90	GMAW	2.0	84.2	88.2	199	190	192	189
12	89	92	3.8	3.2	478	602	-100	GMAW	2.0	72.8	75.2	244	244	244	241
13	100	110	3.6	3.0	565	682	-85	GMAW	2.0	80.1	86.9	226	209	214	194
14	78	94	2.8	2.4	578	692	-90	GMAW	2.0	77.7	82.3	229	224	228	219
15	92	124	2.4	2.1	461	565	-90	GMAW	2.0	86.3	90.5	200	195	199	195
16	76	95	3.4	2.9	440	552	-105	SAW	4.0	88.2	90.9	218	216	207	201
17	84	98	3.5	3.0	537	658	-90	GMAW	2.0	77.1	81.1	244	210	236	204
18	98	109	2.2	1.7	598	704	-90	GMAW	2.0	84.1	91.1	221	202	194	181
19	114	125	2.4	1.9	369	468	-85	GMAW	2.0	83.2	92.2	154	150	151	150	Comparative Examples
20	162	172	0.7	0.5	548	660	-80	GMAW	2.0	130.4	135.2	40	38	29	26
21	158	169	0.6	0.4	552	682	-85	GMAW	2.0	126.6	132.2	39	35	37	34
22	147	158	0.8	0.5	470	594	-90	GMAW	2.0	120.2	128.4	40	37	39	36
23	139	151	0.9	0.7	534	654	-85	GMAW	2.0	108.5	119.4	55	43	53	41
24	115	122	3.8	3.2	407	524	-80	GMAW	2.0	93.7	95.2	139	132	22	14
25	108	115	1.9	1.6	410	522	-100	shielded arc	4.4	88.2	95.4	207	190	204	185	Present invention
26	99	106	2.4	2.1	492	605	-95	shielded arc	3.0	80.4	88.4	201	188	200	184
27	85	100	3.8	3.2	456	556	-95	shielded arc	1.6	77.8	79.2	243	222	240	218

Nos. 1 to 18 and 25 to 27 are Examples of the present invention, and Nos. 19 to 24 are Comparative Examples.
In No. 19, since Ceq was too small, sufficient hardenability was not obtained, and sufficient strength was not obtained.
In No. 20, the O concentration in the molten steel before the Ti addition was too high, so that the Ti-based nitride (TiN) was coarsened, the number density was small, and sufficient joint toughness was not obtained.
In No. 21, the time from the Ti addition up to the casting was too long, so that the TiN was coarsened, the number density was small, and sufficient joint toughness was not obtained.
In No. 22, the slab at the end of solidification was not subjected to soft reduction, so that TiN was not uniformly dispersed, and sufficient joint toughness was not obtained.
In No. 23, the rolling reduction ratio in the soft reduction of the steel slab at the end of solidification was small, so that TiN was not uniformly dispersed, and sufficient joint toughness was not obtained.
In No. 24, the amount of Mn was large, so that Mn presumably segregated at the grain boundary during manufacture of the welded joint, particularly during PWHT, and the toughness after PWHT decreased.
In contrast to the Comparative Examples, in all the Examples of present invention (Nos. 1 to 18 and 25 to 27), the chemical composition of the steel product and the inclusions 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 -70°C of the heat-affected zone was high regardless of before or after PWHT and low-temperature HAZ toughness of 150 J or more was obtained.

Industrial Applicability

The welded joints according to the present disclosure can be preferably used as a constituent material for transport tanks of liquefied carbon dioxide, for example. Further, the welded joints 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).

Description of Reference Signs

10: base metal
12: welded portion
14: test piece
15: weld metal
16: heat-affected zone

The disclosures of Japanese Patent Application No. 2023-176328 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 welded joint comprising a base metal made of a steel product and a welded portion,
wherein a chemical composition of the base metal is, in mass%,

C: from 0.03% to 0.17%,

Si: from 0.10% to 0.50%,

Mn: from 0.10% to 1.70%,

P: 0.025% or less,

S: 0.0250% or less,

Al: from 0.015% to 0.100%,

Ti: from 0.005% to 0.050%,

O: 0.0100% or less,

N: from 0.0010% to 0.0064%,

Ni: 0 to 1.20%,

Cu: 0 to 0.95%,

Cr: 0 to 0.95%,

Mo: 0 to 1.00%,

B: 0 to 0.0050%,

Nb: 0 to 0.034%,

V: 0 to 0.10%,

Mg: 0 to 0.020%,

Ca: 0 to 0.020%,

REM: 0 to 0.020%, and

a balance of Fe and impurities,

and in which Ceq represented by the following Formula (1) is from 0.350 to 0.490,

wherein:
a tensile strength of the steel product is from 490 MPa to 720 MPa,

a number density of inclusions having a circle-equivalent diameter from 0.01 µm to 0.50 µm and containing Ti and N, which are contained in the steel product, is 1.0 × 10⁵/mm² or more, at a portion that is 1/4 of a thickness and at a portion that is 1/2 of a thickness, in a thickness direction from a surface of the steel product,

an average grain diameter of the inclusions containing Ti and N is 150 nm or less, at the 1/4 portion and the 1/2 portion, and

an effective crystal grain diameter in a region between a fusion line of the welded portion and a position 1 mm away from the fusion line in a heat-affected zone is 100.0 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 amount (mass%) of a corresponding element contained in the steel product, provided that zero is substituted when the corresponding element is not contained.
The welded joint 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 1.20%,

Cu: from 0.01% to 0.95%,

Cr: from 0.01% to 0.95%,

Mo: from 0.01% to 1.00%, and

B: from 0.0003% to 0.0050%.
The welded joint 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: 0.001% to 0.034%, and

V: from 0.01% to 0.10%.
The welded joint 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 0.020%,

Ca: from 0.0003% to 0.020%, and

REM: from 0.0003% to 0.020%.
The welded joint according to any one of claims 1 to 4, wherein a Charpy impact absorption energy at -70°C of the heat-affected zone is 150 J or more.
The welded joint according to any one of claims 1 to 5, wherein, when a heat treatment is carried out on the welded joint, in which a rate of temperature increase and a rate of temperature decrease in a temperature region of 425°C or more are 55°C/h and temperature is held for 2 hours at 600°C, a Charpy impact absorption energy at -70°C of the heat-affected zone at portions subjected to the heat treatment is 150 J or more.
A pressure vessel, comprising the welded joint according to any one of claims 1 to 6.