EP4663802A1

EP4663802A1 - Nickel-containing steel sheet for low-temperature applications and tank for low-temperature applications in which said steel sheet is used

Info

Publication number: EP4663802A1
Application number: EP24779067.8A
Authority: EP
Inventors: Naoki Hashimoto
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2023-03-30
Filing date: 2024-03-01
Publication date: 2025-12-17
Also published as: KR20250150043A; CN120769929A; WO2024202929A1; JP2024143355A

Abstract

A nickel-containing steel sheet for low-temperature applications, including C: 0.01 to 0.12 mass%, Si: 0.01 to 0.18 mass%, Mn: 0.2 to 1.8 mass%, P: 0.0100 mass% or less (including 0 mass%), S: 0.0100 mass% or less (including 0 mass%), Al: 0.001 to 0.100 mass%, N: 0.0080 mass% or less (including 0 mass%), Mo: 0.01 to 0.10 mass%, Ni: 8.75 to 10.0 mass%, Cu: 0.70 mass% or less (including 0 mass%), Cr: 0.20 mass% or less (including 0 mass%), and a balance: Fe and inevitable impurities, wherein a DI value is 1.03 or more and 1.65 or less, and a value calculated using a predetermined formula is 7.06 or less.

Description

TECHNICAL FIELD

The present disclosure relates to a nickel-containing steel sheet for low-temperature applications and a tank for low-temperature applications in which the steel sheet is used.

BACKGROUND ART

In recent years, restrictions in the energy industry have been loosened in Japan, and a shift from coal and oil having a large carbon dioxide (CO₂) emission coefficient to LNG, which is clean energy having a small emission coefficient, has been advanced. In addition, the demand for liquefied natural gas (LNG) is expanding as environmental protection activities have become more active around the world. Thus, construction of a tank for low-temperature applications to be used at a cryogenic temperature (-196°C) which can be used for a marine LNG fuel tank and the like is increasing. A nickel-containing steel sheet for low-temperature applications such as 9% Ni steel is excellent in toughness at a cryogenic temperature, that is, low-temperature toughness, and thus, the steel sheet is widely used as a material for tanks for transporting and storing liquefied low-temperature gases such as LNG. The LNG tank is designed and constructed in particular in consideration of safety, and thus, low-temperature toughness is regarded as important in the weld joint in addition to the steel material and welding material to be used, and the Charpy impact absorbed energy value is defined in each standard including ASTM (American Society for Testing and Materials).
When a structure such as a tank for low-temperature applications is constructed using such a nickel-containing steel plate, welding work is indispensable, and it is indispensable to secure low-temperature toughness of a weld joint portion (welded portion) generated through welding.
Patent Document 1 discloses that reduction of Si and P is effective as a method for improving the low-temperature toughness of the two-phase HAZ (intercritically reheated heat-affected zone, hereinafter it may be referred to as "IC-HAZ"). Patent Document 1 also describes that refinement of martensite locally generated in the IC-HAZ contributes to improvement of low-temperature toughness.
Patent Document 2 discloses reduction of Si and addition of Mo as a method capable of improving the CTOD characteristics of a HAZ.
Patent Document 3 describes, as a method for producing 9% Ni steel, that toughness of a weld joint portion is improved by reducing Si. It is shown that the amount of MA in a HAZ heated to the two-phase region of ferrite and austenite is reduced by reducing Si.
Patent Document 4 discloses reduction of Si, Al, and N as a method for improving the HAZ toughness of a Toe part (end part). It is also shown that the effect is obtained because of the promotion of auto-temper with Si and Al reduction and the reduction of AlN inclusions.
Patent Document 5 discloses controlling contents of C, Si, Al, and Mo as a method for improving toughness of a base material and a weld joint portion. It is also shown that the effect is obtained because of the structure refinement at the time of tempering with the addition of Mo and the suppression of the generation of a hardening phase.
Patent Document 6 discloses a method for reducing Si and performing hot rolling in two stages as a method for producing 9% Ni steel excellent in toughness of a base material and a weld joint portion.

CONVENTIONAL ART DOCUMENT

PATENT DOCUMENT

Patent Document 1: JP-A-S61-238911
Patent Document 2: JP-A-H04-371520
Patent Document 3: JP-A-H07-126749
Patent Document 4: JP-B-5126780
Patent Document 5: JP-A-2002-129280
Patent Document 6: JP-A-2013-142197

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

Automatic TIG welding (GTAW) and SAW are the mainstream of the welding method of a tank side plate vertical joint and a side plate horizontal joint which occupy most of the welded portions in the production of a tank for low-temperature applications such as clean energy LNG. However, GTAW is a welding method inferior in efficiency, and SAW is also inferior in efficiency because construction is performed with the maximum input heat of 50 kJ/cm or less to reduce the risk of occurrence of brittle fracture from a welding heat-affected portion. Thus, there is a strong demand for application of large heat input welding in which a welding heat input amount exceeds 50 kJ/cm for the purpose of improving construction efficiency and reducing welding cost. However, in the large heat input welding, the influence of the heat-affected portion due to welding is larger than that in the conventional welding method (low heat input welding), and there is a problem that it is difficult to secure sufficient low-temperature toughness. All of the nickel-containing steel sheets disclosed in Patent Documents 1 to 6 have been evaluated for a low heat input weld joint having a welding heat input amount of 50 kJ/cm or less, and sufficient study has not been conducted from the viewpoints of MA formation suppression in the IC-HAZ where toughness most degrades when a nickel-containing steel sheet for low-temperature applications is subjected to large heat input welding and grain boundary embrittlement caused by P. Thus, in the steel sheets described in Patent Documents 1 to 6, there is a possibility that the low-temperature toughness of the weld joint sufficient to suppress the brittle fracture of the tank when large heat input welding is applied cannot be secured.
The present disclosure has been made in view of such a circumstance, and an object of the present disclosure is to provide a nickel-containing steel sheet for low-temperature applications capable of securing sufficient low-temperature toughness of a weld joint even when large heat input welding is applied, and a tank for low-temperature applications in which the steel sheet is used.

SOLUTIONS TO THE PROBLEMS

An aspect 1 of the present invention is a nickel-containing steel sheet for low-temperature applications, the steel sheet including:

C: 0.01 to 0.12 mass%;
Si: 0.01 to 0.18 mass%;
Mn: 0.2 to 1.8 mass%;
P: 0.0100 mass% or less (including 0 mass%);
S: 0.0100 mass% or less (including 0 mass%);
Al: 0.001 to 0.100 mass%;
N: 0.0080 mass% or less (including 0 mass%);
Mo: 0.01 to 0.10 mass%;
Ni: 8.75 to 10.0 mass%;
Cu: 0.70 mass% or less (including 0 mass%);
Cr: 0.20 mass% or less (including 0 mass%); and
a balance: Fe and inevitable impurities,
wherein
a DI value represented by Formula (1) shown below is 1.03 or more and 1.65 or less, and a value calculated using Formula (2) shown below is 7.06 or less: DI = 1.16 × √([C]/10) × (0.7 [Si] +1) × (3.33 [Mn] +1) × (0.35 [Cu] +1) × (0.36 [Ni] +1) × (2.16 [Cr] +1) × (3 [Mo] +1) × (1.75 [V] +1) × (200 [B] +1) ···(1)
wherein [] represents a content expressed in mass% of the element denoted therein; $5.0 \times 10^{3} [C] \times {[Si]}^{2.3} + 1.5 \times 10^{10} \times {[P]}^{3.5} / \sqrt (133 [Mo] + 1)$
wherein [] represents a content expressed in mass% of the element denoted therein.

An aspect 2 of the present invention is a tank for low-temperature applications in which the nickel-containing steel sheet for low-temperature applications according to aspect 1 is used.

EFFECTS OF THE INVENTION

According to one embodiment of the present invention, it is possible to provide a nickel-containing steel sheet for low-temperature applications capable of securing sufficient low-temperature toughness of a weld joint even when large heat input welding is applied, and according to another embodiment of the present invention, it is possible to provide a tank for low-temperature applications (for example, a tank for storing a low-temperature liquefied gas such as clean energy LNG) in which the weld joint has sufficient low-temperature toughness.

BRIEF DESCRIPTION OF THE DRAWINGS

[Fig. 1] Fig. 1 is a graph showing a relationship between a DI value and a base material strength (yield stress and tensile strength).
[Fig. 2] Fig. 2 is a graph showing a relationship between a value of the first term of Formula (2) and an MA area fraction.
[Fig. 3] Fig. 3 is a graph showing a relationship between a value of Formula (2) and a Charpy impact absorbed energy value vE_-196 at -196°C.

DETAILED DESCRIPTION

The inventors of the present invention have conducted intensive studies to solve the above problems. As a result, the inventors of the present invention have found that a desired tensile strength can be obtained without impairing the low-temperature toughness of a base material and a joint not only by optimizing the range of the content of individual elements, but also by setting the DI value defined by Formula (1) described later to an appropriate value, and further, the formation of MA (martensite-austenite constituent, island-like martensite) and grain boundary embrittlement in an IC-HAZ at the time of large heat input welding can be suppressed and the low-temperature toughness can be more reliably improved by setting the value defined by Formula (2) described later to be within a predetermined range.
Hereinafter, each requirement defined in the embodiments of the present invention will be described in detail.

1. Chemical composition

A nickel-containing steel sheet for low-temperature applications according to an embodiment of the present invention has the chemical compositions described below.

1-1. Content of each element

[C: 0.01 mass% or more and 0.12 mass% or less]

C is one of the elements exhibiting characteristics of the embodiments according to the present invention. When the content of C exceeds 0.12 mass%, the formation of MA (island-like martensite) in an IC-HAZ at the time of large heat input welding is promoted, and the low-temperature toughness of the joint portion degrades. Thus, the upper limit of the C amount is set to 0.12 mass%. On the other hand, C is an element that increases the strength of steel and needs to be contained in an amount of 0.01 mass% or more to secure desired strength. Thus, the C content is in the range of 0.01 to 0.12 mass%. The upper limit of the C content is preferably 0.10 mass%, more preferably 0.08 mass%.

[Si: 0.01 mass% or more and 0.18 mass% or less]

Si is one of the elements exhibiting characteristics of the embodiments according to the present invention. By setting the Si amount to 0.18 mass% or less, MA formation in a weld joint portion is suppressed, and low-temperature toughness of the joint is improved. On the other hand, Si is an element necessary as a deoxidizing agent and for securing strength, and when the content is less than 0.01 mass%, the effect is not sufficient. Thus, the Si content is in the range of 0.01 to 0.18 mass%. The formation of MA in an IC-HAZ is suppressed as the Si content is lower. Thus, the upper limit of the Si content is preferably 0.14 mass%.

[Mn: 0.2 mass% or more and 1.8 mass% or less]

Mn is an element necessary for improving hardenability of steel and securing strength, but when the content is less than 0.2 mass%, the effect is not sufficient, and when the content exceeds 1.8 mass%, toughness degrades. Thus, the Mn content is in the range of 0.2 to 1.8 mass%. A preferred range of the Mn content is 0.3 to 1.2 mass%.

[P: 0.0100 mass% or less (including 0 mass%)]

P is one of the elements exhibiting the characteristics of the embodiments according to the present invention, is inevitably present in steel as an impurity, and segregates at grain boundaries to degrade the low-temperature toughness of the base material and the weld joint portion. Thus, the upper limit of the content of P is set to 0.0100 mass%. To improve the low-temperature toughness of the weld joint portion, it is desirable that the content of P is smaller.
In the present application, "including 0 mass%" means including a case where the content is a content according to an embodiment in which the element is not intentionally added, for example, the content is a content of an inevitable impurity level (a case where the element is intentionally added is not excluded as long as the content is within a predetermined range).
On the other hand, in the present application, "not including 0 mass%" means that the element is intentionally added.

[S: 0.0100 mass% or less (including 0 mass%)]

S is an element present in steel as an inevitable impurity, and when the content of S is too large, elongated MnS becomes a starting point of brittle fracture, and the toughness of the base material and the weld joint portion degrades. Thus, the upper limit of the S amount is set to 0.0100 mass%. To improve the toughness of the weld joint portion, it is desirable that the S content is smaller.

[Al: 0.001 mass% or more and 0.100 mass% or less]

Al is a deoxidizing agent and is an element effective for suppressing coarsening of crystal grains and securing toughness, but when the content of Al is less than 0.001 mass%, a sufficient effect cannot be obtained. On the other hand, when the content exceeds 0.100 mass%, brittle fracture is caused starting from alumina inclusions to degrade toughness. Thus, the Al content is in the range of 0.001 to 0.100 mass%.

[N: 0.0080 mass% or less (including 0 mass%)]

N is an impurity, and the upper limit of N is set to 0.0080 mass% because N degrades the toughness of the base material and the weld joint portion through the formation of precipitates such as AlN. To improve the toughness of the weld joint portion, it is desirable that the N content is smaller.

[Ni: 8.75 mass% or more and 10.0 mass% or less]

Ni is a basic element added to secure toughness (low-temperature toughness) at a cryogenic temperature, and in the embodiments of the present invention, the Ni amount is 8.75 mass% or more. As the Ni amount is larger, more excellent low-temperature toughness is obtained, but when the Ni is added in an amount exceeding 10.0 mass%, the effect of improving characteristics against an increase in alloy cost is reduced. Thus, the Ni content is in the range of 8.75 to 10.0 mass%. From the viewpoint of securing low-temperature toughness and suppressing alloy cost, a more preferred range of the Ni content is 8.95 to 9.85 mass%.

[Mo: 0.01 mass% or more and 0.10 mass% or less]

Mo is one of the elements exhibiting characteristics of the embodiments according to the present invention, and containing Mo in an amount of 0.01 mass% or more suppresses grain boundary embrittlement caused by P in the cooling process after welding, which contributes to improvement of toughness. On the other hand, when the Mo content exceeds 0.10 mass%, the influence of toughness degradation due to carbide formation becomes larger. Thus, the Mo content is in the range of 0.01 to 0.10 mass%. When the content is in the range of 0.10 mass% or less, the effect of suppressing grain boundary embrittlement increases with an increase in the amount of Mo added. Thus, the lower limit of the Mo content is preferably 0.02 mass% and more preferably 0.03 mass%.

[Cu: 0.70 mass% or less (including 0 mass%)]

Cu is an element contained in a trace amount in steel as an inevitable impurity. Cu is usually contained at an impurity level of about 0.03 mass% or less.
On the other hand, addition of Cu in a small amount has an effect of improving the strength without impairing the toughness. Thus, Cu may be intentionally added as necessary. On the other hand, when the Cu content is more than 0.70 mass%, toughness degrades. Thus, when Cu is intentionally added, the Cu content is 0.70 mass% or less (not including 0 mass%). The lower limit of the Cu content is preferably 0.05 mass% to reliably obtain the above-described effect of improving the strength.

[Cr: 0.20 mass% or less (including 0 mass%)]

Cr is an element contained in a trace amount in steel as an inevitable impurity. Cr is usually contained at an impurity level of about 0.08 mass% or less.
Cr may be intentionally added as necessary to improve hardenability of steel and improve strength. On the other hand, when the content is more than 0.20 mass%, toughness degrades. Thus, when Cr is intentionally added, the Cr content is 0.20 mass% or less (not including 0 mass%). The lower limit of the Cr content is preferably 0.10 mass% to reliably obtain the above-described effect.

[Balance]

In one of the preferred embodiments of the present invention, the balance is iron and unavoidable impurities. As the inevitable impurities, mixing of elements (for example, As, Sb, Nb, O, and H) brought depending on the situation of raw materials, materials, production facilities, and the like is allowed.
For example, like P and S, there are elements that are usually preferred as the content is smaller, and thus are inevitable impurities, but are elements whose composition ranges are separately specified as described above. Thus, in the present application, the term "inevitable impurities" constituting the balance is a concept excluding elements whose composition ranges are separately specified.

1-2. DI value

In the nickel-containing steel sheet for low-temperature applications according to the embodiment of the present invention, the DI value represented by the following Formula (1) is 1.03 or more and 1.65 or less. DI = 1.16 × √([C]/10) × (0.7 [Si] +1) × (3.33 [Mn] +1) × (0.35 [Cu] +1) × (0.36 [Ni] +1) × (2.16 [Cr] +1) × (3 [Mo] +1) × (1.75 [V] +1) × (200 [B] +1) ···(1)
Here, [] represents a content expressed in mass% of the element denoted therein. That is, for example, [C] means the content of C expressed in mass%.
To use a nickel-containing steel sheet such as 9% Ni steel sheet as a material of an LNG tank, it is necessary to set the yield stress to 590 MPa or more and the tensile strength to 680 MPa or more. On the other hand, the toughness and strength of the material are in a trade-off relationship, and when the strength is excessively high, the toughness of the base material and the weld joint is impaired. Thus, the upper limit of the tensile strength (TS) is preferably 830 MPa or less, more preferably 800 MPa or less.
The DI value defined in Formula (1) is a general parameter representing the hardenability of the steel material, and the higher the DI value, the higher the dislocation density introduced into the material at the time of quenching. From this, by controlling the DI value, it is possible to control the strength of a nickel-containing steel sheet for low-temperature applications produced by quenching-tempering, quenching-intermediate heat-tempering, or direct quenching-tempering, which is a production method generally applied to a nickel-containing steel sheet for low-temperature applications. When the DI value is in the range of 1.03 to 1.65, the strength can be set within the above range without impairing the toughness of the base material and the weld joint.

1-3. Value of Formula (2)

In the nickel-containing steel sheet for low-temperature applications according to the embodiment of the present invention, a value calculated using Formula (2) (it may be referred to as "value of Formula (2)") is 7.06 or less. $5.0 \times 10^{3} [C] \times {[Si]}^{2.3} + 1.5 \times 10^{10} \times {[P]}^{3.5} / \sqrt (133 [Mo] + 1)$
Here, [] represents a content expressed in mass% of the element denoted therein. That is, for example, [P] means the content of P expressed in mass%.
As a result of the study conducted by the inventors, the following has become clear.
When large heat input welding is applied to a nickel-containing steel sheet for low-temperature applications, the cooling rate of the HAZ decreases as the welding heat input amount increases. Thus, the amount of upper bainite formed in a relatively high temperature range from the grain boundary of austenite formed through partial reverse transformation in the IC-HAZ heated to the two-phase state of ferrite-austenite increases. As a result, the concentration of C into untransformed austenite is promoted, and the formation of MA as a hard phase becomes remarkable. This causes brittle fracture with the MA as a starting point, and thus, the Charpy impact absorbed energy of the IC-HAZ remarkably degrades.
As described above, C promotes the formation of MA by concentrating to the untransformed austenitic phase. In addition, Si increases MA through suppression of transformation of the untransformed austenitic phase into cementite phase. Thus, reduction of C and Si is effective for MA reduction in the IC-HAZ. Further, as a result of studying the influence of the components on the MA formation amount, it has become clear that the MA amount in the IC-HAZ can be organized by C×Si^2.3. Thus, the first term "5.0 × 10³[C] × [Si]^2.3" in Formula (2) represents ease of formation of MA in the IC-HAZ.
It is also known that P causes grain boundary embrittlement in a nickel-containing steel for low-temperature applications thereby reducing toughness. Since Mo has a repulsive interaction with P on the prior austenite grain boundary, Mo has an action of excluding P from the grain boundary. As a result of studies conducted by the inventors of the present invention, it has become clear that it is possible to greatly suppress grain boundary embrittlement caused by P by adding a small amount of Mo, and the degree of grain boundary embrittlement is determined by the content ratio between P and Mo. Thus, the second term "1.5 × 10¹⁰ × [P]^3.5/√(133[Mo] + 1)" in Formula (2) indicates the degree of grain boundary embrittlement caused by P.
By setting the value of Formula (2) to 7.06 or less, the formation of MA and grain boundary embrittlement in the IC-HAZ at the time of large heat input welding are suppressed, and as shown in the experimental results of Examples described later (in particular, a quadratic approximate curve in Fig. 3 to be described later in detail), even at the time of large heat input welding, the Charpy impact absorbed energy value vE_-196 of the IC-HAZ of the weld joint at -196°C is 34 J or more, and brittle fracture is suppressed even in applications such as LNG tanks where toughness at a cryogenic temperature is required, and excellent toughness of the weld joint can be secured. The Charpy impact absorbed energy value vE_-196 being 34 J or more means that the standard value (average 34 J or more) of the L-direction Charpy impact absorbed energy value of 9% Ni steel (joint) defined in the ASTM standard (ASTM A553/A553M: 2022 and ASTM A844/A844M: 2022) is satisfied.

2. Production method

In the production of the nickel-containing steel sheet for low-temperature applications according to the embodiment of the present invention, a known production method applied to the production of a normal nickel-containing steel sheet for low-temperature applications can be used as long as the above-described chemical components are satisfied, and the DI value shown in Formula (1) and the value of Formula (2) are within the above-described appropriate ranges.
For example, the nickel-containing steel sheet for low-temperature applications can be obtained by subjecting a steel piece such as a slab that satisfies the above-described requirements to hot rolling to form a hot-rolled steel sheet having a predetermined sheet thickness, then heating and quenching the hot-rolled steel sheet to a temperature range of the Ac₃ temperature or higher, and then tempering the hot-rolled steel sheet in a temperature range of the Ac₁ temperature or lower.
A step of quenching from a two-phase region of the Ac₁ temperature or higher and the Ac₃ temperature or lower may be included between quenching and tempering, or the hot-rolled steel sheet after hot rolling may be directly quenched online and then tempered in a temperature region of the Ac₁ temperature or lower.
By using the nickel-containing steel sheet for low-temperature applications according to the embodiment of the present invention thus obtained and welding the steel sheets, a tank for low-temperature applications (for example, a tank for storing a low-temperature liquefied gas such as clean energy LNG) according to the embodiment of the present invention can be obtained. In particular, by using the nickel-containing steel sheet for low-temperature applications according to the embodiment of the present invention, the weld joint has excellent low-temperature toughness even when welding is performed by a large heat input welding method, and thus a tank for low-temperature applications having sufficient low-temperature toughness can be obtained.

The large heat input welding means, for example, a welding method in which a welding heat input amount exceeds 50 kJ/cm as described in Table M4.2, Remarks (6) on page 16 of "2021 Steel Ship Regulation, edition M" issued by NIPPON KAIJI KYOKAI (ClassNK).

EXAMPLES

1. Sample preparation

A continuous casting slab produced through converter-continuous casting or a slab produced through vacuum melting having the chemical compositions shown in Table 1 was heated to 1000°C or higher and 1200°C or lower, then rolled to a sheet thickness shown in Table 2 through hot rolling, and cooled to room temperature through air cooling, whereby a hot rolled sample was obtained. In Table 1, the DI value calculated using Formula (1) of each sample, the value of the first term "5.0 × 10³[C] × [Si]^2.3" of Formula (2) (column "First term of Formula (2)" in Table 1), the second term "1.5 × 10¹⁰ × [P]^3.5/√(133[Mo] +1)" of Formula (2) (column "Second term of Formula (2)" in Table 1), and the value of Formula (2) are also described.
Next, the obtained hot rolled sample was heated to 780°C, then subjected to water quenching, and tempered at 590°C, whereby a steel sheet sample was obtained. The details of the production conditions of the steel sheet sample are shown in Table 2.
A sample for thermal cycle test was obtained by cutting out the obtained steel sheet sample in a size of 12 mm × 33 mm × 55 mm from a t/4 position (a position at a distance of 1/4 of the sheet thickness t from a surface (main surface) of the steel sheet toward the center) such that the sheet was cut in 12 mm in a sheet thickness direction and cut in 55 mm parallel to the rolling direction.

In addition, a rod-shaped test piece was taken from a direction perpendicular to the rolling direction at the t/4 position of the steel sheet sample and used as a sample for tensile test.

[Table 1]

Sample No.		Chemical composition (mass%): balance is Fe and inevitable impurities										DI value	First term of Formula (2)	Second term of Formula (2)	Value of Formula (2)
Sample No.		C	Si	Mn	P	S	Al	Cu	Ni	Cr	Mo	DI value	First term of Formula (2)	Second term of Formula (2)	Value of Formula (2)
Examples	1	0.047	0.14	0.66	0.0005	0.0008	0.025	0.00	9.19	0.00	0.09	1.53	2.55	0.01	2.57
	2	0.047	0.08	0.66	0.0004	0.0014	0.027	0.00	9.27	0.00	0.03	1.27	0.70	0.01	0.71
	3	0.049	0.07	0.67	0.0002	0.0018	0.028	0.00	9.28	0.00	0.03	1.30	0.54	0.00	0.54
	4	0.054	0.08	0.66	0.0017	0.0008	0.027	0.00	9.35	0.00	0.08	1.56	0.81	0.89	1.70
	5	0.055	0.08	0.56	0.0018	0.0006	0.027	0.00	9.13	0.00	0.03	1.22	0.82	1.66	2.49
	6	0.042	0.07	0.55	0.0015	0.0006	0.025	0.00	9.09	0.00	0.03	1.04	0.46	0.88	1.34
	7	0.052	0.06	0.65	0.0007	0.0007	0.025	0.00	9.24	0.00	0.06	1.41	0.40	0.05	0.45
	8	0.051	0.06	0.64	0.0009	0.0016	0.026	0.00	9.21	0.00	0.06	1.38	0.39	0.11	0.50
Comparative Examples	9	0.047	*0.23	0.65	0.0006	0.0008	0.027	0.00	9.19	0.00	*0.00	1.26	8.00	0.08	*8.08
Comparative Examples	10	0.051	0.06	0.65	0.0030	0.0010	0.025	0.00	9.26	0.00	0.06	1.40	0.39	7.40	*7.80

* means being outside the scope defined by the embodiments of the present invention.

2. Sample evaluation

(Evaluation of low-temperature toughness)

The sample for thermal cycle test described above was heated up to 950°C at a temperature rising rate of 50°C/sec by high frequency heating and held for 10 seconds, and then cooled down from 950°C to 900°C for 6 seconds, from 900°C to 800°C for 80 seconds, from 800°C to 500°C for 240 seconds, and from 500°C to 50°C for 765 seconds, whereby a simulated thermal history of the welded joint IC-HAZ in 1-pass welding at a heat input of about 380 kJ/cm was given. Thereafter, a V-notch Charpy standard test sample conforming to Japanese Industrial Standards JIS2242:2018 was prepared from each thermal cycle test sample. Then, a Charpy impact test at -196°C was performed using the sample for V-Notch Charpy standard test, and the Charpy impact absorbed energy value vE_-196 was measured. For one sample (for one sample No.), the impact test was performed three times. The measurement results of vE_-196 of each of the three tests are shown in the column of "each." in Table 2, and the average value is shown in the column of "ave." in Table 2.

(MA area fraction)

The MA amount of the thermal cycle test sample after the simulated thermal history was given was measured. LePera corrosion was performed after wet polishing at a position excluding an end portion of 3 mm of the sample for thermal cycle test to which the simulated thermal history was given, the structure of a region of 155 µm × 202 µm was observed with an optical microscope at a magnification of 400 times, MA was identified from contrast, and an area fraction of MA was obtained. The results are shown in Table 2.

(Tensile test)

The yield stress and the tensile strength were measured through a tensile test according to JIS Z2241:2022 using the rod-shaped test piece. The measurement results are shown in Table 2 as "Base material strength" (strength of the material of the weld joint IC-HAZ before the simulated thermal history is given).
Fig. 1 shows the relationship between the DI value and the base material strength (yield stress and tensile strength), Fig. 2 shows the relationship between the value of the first term of Formula (2) and the MA area fraction, and Fig. 3 shows the relationship between the value of Formula (2) and the Charpy impact absorbed energy value vE_-196 (average value of three samples) at -196°C. The dotted line in Fig. 3 represents a quadratic approximate curve.

(Quality determination)

With respect to the base material strength, samples having a yield stress of 590 MPa or more, a tensile strength of 680 MPa or more and 830 MPa or less, and a Charpy impact absorbed energy value vE_-196 of 34 J or more as an average value and vE_-196 of 27 J or more in all the three samples at -196°C were determined as "Good", and samples not satisfying all of them were determined as "Poor". The quality determination results are shown in Table 2.

[Table 2]

Sample No.		Production conditions				Base material strength		Thermal cycle test			Quality determination
		Sheet thickness (mm)	Slab heating temperature (°C)	Quenching temperature (°C)	Tempering temperature (°C)	Yield stress (MPa)	Tensile strength (MPa)	vE_-196(J)		MA area fraction (%)
		Sheet thickness (mm)	Slab heating temperature (°C)	Quenching temperature (°C)	Tempering temperature (°C)	Yield stress (MPa)	Tensile strength (MPa)	each.	ave.	MA area fraction (%)
Examples	1	30	1100	780	590	738	772	51, 52, 53	52	0.072	Good
	2	30	1100	780	590	686	725	56, 46, 60	54	0.010	Good
	3	30	1100	780	590	674	718	60, 69, 73	67	0.012	Good
	4	50	1100	780	590	716	750	50, 55, 55	53	0.046	Good
	5	50	1100	780	590	650	699	42, 55, 52	50	0.042	Good
	6	50	1100	780	590	631	682	69, 61, 75	68	0.032	Good
	7	50	1100	780	590	693	724	54, 56, 51	54	0.021	Good
	8	50	1100	780	590	684	715	60, 54, 46	53	0.019	Good
Comparative Examples	9	30	1100	780	590	655	704	30, *22, 31	*28	0.389	Poor
Comparative Examples	10	50	1100	780	590	685	722	28, 36, 29	*31	0.017	Poor

* means being out of the good range of the quality determination.

As can be seen from Tables 1 and 2, the sample Nos. 1 to 8 having the chemical compositions defined by the embodiment of the present invention and having the DI value and the value of Formula (2) within predetermined ranges have sufficient strength and low-temperature toughness. That is, the samples have strength as a structural material necessary for application to a tank for low-temperature applications such as LNG, and the samples can secure sufficient low-temperature toughness as a tank for low-temperature applications even when a thermal history of the IC-HAZ in which toughness degradation is likely to occur is given at the time of large heat input welding. Thus, it can be said that the samples contribute to improvement of welding construction efficiency through application of large heat input welding.
On the other hand, sample No. 9 contains excessive Si and excessively small Mo, and the value of Formula (2) is also out of the predetermined range. Thus, the formation of MA is remarkable when a thermal history in large heat input welding is given, the influence of grain boundary embrittlement caused by P is large, and sufficient low-temperature toughness cannot be secured when large heat input welding is applied.
In sample No. 10, the content ratio of Mo to P is small, and thus the value of Formula (2) is out of the predetermined range. Thus, the influence of grain boundary embrittlement caused by P is remarkable, and sufficient low-temperature toughness cannot be secured when large heat input welding is applied.
This application claims priority based on Japanese Patent Application No. 2023-055986 filed on March 30, 2023 . Japanese Patent Application No. 2023-055986 is incorporated herein by reference.

Claims

A nickel-containing steel sheet for low-temperature applications, the steel sheet comprising:
C: 0.01 to 0.12 mass%;

Si: 0.01 to 0.18 mass%;

Mn: 0.2 to 1.8 mass%;

P: 0.0100 mass% or less (including 0 mass%);

S: 0.0100 mass% or less (including 0 mass%);

Al: 0.001 to 0.100 mass%;

N: 0.0080 mass% or less (including 0 mass%);

Mo: 0.01 to 0.10 mass%;

Ni: 8.75 to 10.0 mass%;

Cu: 0.70 mass% or less (including 0 mass%);

Cr: 0.20 mass% or less (including 0 mass%); and

a balance: Fe and inevitable impurities,

wherein

a DI value represented by Formula (1) shown below is 1.03 or more and 1.65 or less, and a value calculated using Formula (2) shown below is 7.06 or less: DI = 1.16 × √([C]/10) × (0.7 [Si] +1) × (3.33 [Mn] +1) × (0.35 [Cu] +1) × (0.36 [Ni] +1) × (2.16 [Cr] +1) × (3 [Mo] +1) × (1.75 [V] +1) × (200 [B] +1) ···(1)

wherein [] represents a content expressed in mass% of the element denoted therein; $5.0 \times 10^{3} [C] \times {[Si]}^{2.3} + 1.5 \times 10^{10} \times {[P]}^{3.5} / \sqrt (133 [Mo] + 1)$

wherein [] represents a content expressed in mass% of the element denoted therein.
A tank for low-temperature applications in which the nickel-containing steel sheet for low-temperature applications according to claim 1 is used.