WO2024202929A1 - Nickel-containing steel sheet for low-temperature applications and tank for low-temperature applications in which said steel sheet is used - Google Patents

Abstract

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

Description

Nickel-containing steel plate for low temperature use and low temperature tank using the same

This disclosure relates to nickel-containing steel plate for low temperature use and a low temperature tank using the same.

In recent years, deregulation of the energy industry has progressed in Japan, and there has been a shift from coal and oil, which have a large carbon dioxide (CO ₂ ) emission factor, to LNG, a clean energy with a small emission factor. In addition, the demand for liquefied natural gas (LNG) is expanding due to the intensification of environmental protection activities around the world. For this reason, the construction of low-temperature tanks used at extremely low temperatures (-196°C) that can be used for marine LNG fuel tanks and the like is increasing. Low-temperature nickel-containing steel plates, such as 9% Ni steel, are widely used as materials for transporting and storing liquefied low-temperature gases such as LNG, because they have excellent toughness at extremely low temperatures, i.e., low-temperature toughness. LNG tanks are designed and constructed with particular consideration given to safety, and therefore, in addition to the steel materials and welding materials used, low-temperature toughness is also important for welded joints, and Charpy impact absorption energy values are specified in various standards, including ASTM (American Society for Testing and Materials).

Welding is essential when constructing structures such as low-temperature tanks using such nickel-containing steel plates, and it is essential to ensure the low-temperature toughness of the welded joints (welds) created by welding.

Patent Document 1 discloses that reducing Si and P is an effective method for improving the low-temperature toughness of the dual-phase HAZ (intercritically reheated heat-affected zone, hereafter sometimes referred to as "IC-HAZ"). Patent Document 1 also describes that refining martensite that forms locally in the IC-HAZ contributes to improving low-temperature toughness.

Patent Document 2 discloses the reduction of Si and the addition of Mo as a method for improving the CTOD characteristics of the HAZ.

Patent Document 3 describes a manufacturing method for 9% Ni steel in which the toughness of welded joints is improved by reducing the amount of silicon. It shows that reducing the amount of silicon reduces the amount of MA in the HAZ, which is heated to the two-phase region of ferrite and austenite.

Patent Document 4 discloses the reduction of Si, Al, and N as a method for improving HAZ toughness at the toe portion. It also shows that the reduction of Si and Al promotes auto-tempering, and that this effect is due to the reduction of AlN inclusions.

Patent Document 5 discloses that the toughness of the base material and welded joints can be improved by controlling the contents of C, Si, Al, and Mo. It also shows that the effect is due to the refinement of the structure during tempering by adding Mo, and the suppression of the formation of hardened phases.

Patent Document 6 discloses a method for producing 9% Ni steel with excellent toughness in the base material and welded joints, which involves reducing Si and performing two-stage hot rolling.

Japanese Unexamined Patent Publication No. 61-238911 Japanese Patent Application Publication No. 4-371520 Japanese Unexamined Patent Publication No. 7-126749 Patent No. 5126780 JP 2002-129280 A JP 2013-142197 A

When manufacturing low-temperature tanks for LNG and other clean energy sources, automated TIG welding (GTAW) and SAW are the mainstream welding methods for the tank side plate vertical joints and side plate horizontal joints, which account for the majority of the welds. However, GTAW is a less efficient welding method, and SAW also has the problem of being less efficient, since the maximum heat input is set to 50 kJ/cm or less to reduce the risk of brittle fracture from the weld heat-affected zone. For this reason, there is a strong demand for high-heat-input welding, with a welding heat input of more than 50 kJ/cm, in order to improve construction efficiency and reduce welding costs. However, with high-heat-input welding, the impact of welding on the heat-affected zone is greater than with conventional welding methods (low-heat-input welding), and there is a problem in that it is difficult to ensure sufficient low-temperature toughness. The nickel-containing steel plates disclosed in Patent Documents 1 to 6 were all evaluated for low heat input welded joints with a welding heat input of 50 kJ/cm or less, and sufficient consideration was not given to the suppression of MA formation in the IC-HAZ, where toughness deteriorates most when low-temperature nickel-containing steel plates are welded with high heat input, and to grain boundary embrittlement due to P. Therefore, there is a risk that the steel plates described in Patent Documents 1 to 6 will not be able to ensure sufficient low-temperature toughness of the welded joints to suppress brittle fracture of the tank when high heat input welding is applied.

This disclosure has been made in light of these circumstances, and aims to provide a nickel-containing steel plate for low-temperature use that can ensure sufficient low-temperature toughness of welded joints even when large heat input welding is applied, and a low-temperature tank that uses said steel plate.

Aspect 1 of the present invention is
C: 0.01 to 0.12% by mass,
Si: 0.01 to 0.18% by mass,
Mn: 0.2 to 1.8% by mass,
P: 0.0100% by mass or less (including 0% by mass),
S: 0.0100% by mass or less (including 0% by mass),
Al: 0.001 to 0.100% by mass,
N: 0.0080 mass% or less (including 0 mass%),
Mo: 0.01 to 0.10% by mass,
Ni: 8.75 to 10.0% by mass,
Cu: 0.70% by mass or less (including 0% by mass), and Cr: 0.20% by mass or less (including 0% by mass)
The balance is Fe and unavoidable impurities.
The nickel-containing steel sheet for low temperature use has a DI value expressed by the following formula (1) of 1.03 or more and 1.65 or less, and a value calculated using the following formula (2) of 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)
Here, [ ] indicates the content of the element shown within, expressed as mass %.

5.0×10 ³ [C]×[Si] ^2.3 +1.5×10 ¹⁰ ×[P] ^3.5 /√(133[Mo]+1) ...(2)
Here, [ ] indicates the content of the element shown within, expressed as mass %.

Aspect 2 of the present invention is a low-temperature tank using the low-temperature nickel-containing steel plate described in aspect 1.

According to one embodiment of the present invention, it is possible to provide a nickel-containing steel plate for low-temperature use that can ensure sufficient low-temperature toughness of the welded joint even when high heat input welding is applied, and according to another embodiment of the present invention, it is possible to provide a low-temperature tank (e.g., a tank for storing low-temperature liquefied gas such as LNG, which is a clean energy source) in which the welded joint has sufficient low-temperature toughness.

1 is a graph showing the relationship between DI value and base material strength (yield stress and tensile strength). 1 is a graph showing the relationship between the value of the first term of equation (2) and the MA area fraction. 1 is a graph showing the relationship between the value of formula (2) and the Charpy impact absorption energy value vE ₋₁₉₆ at −196° C.

The present inventors have conducted extensive research to solve the above problems. As a result, they have found that a desired tensile strength can be obtained without impairing the low-temperature toughness of the base metal and joint by not only optimizing the range of the content of each element but also by setting the DI value defined by the formula (1) described below to an appropriate value, and further that by setting the value defined by the formula (2) described below within a predetermined range, the formation of MA (Martensite-Austenite constituent, island martensite) and grain boundary embrittlement in the IC-HAZ during high heat input welding can be suppressed, thereby more reliably improving the low-temperature toughness, and have arrived at the present invention.
Each requirement defined in the embodiment of the present invention will be described in detail below.

1. Chemical Composition The nickel-containing steel plate for low temperature use according to the embodiment of the present invention has the chemical composition described below.

1-1. Content of each element [C: 0.01 mass% or more, 0.12 mass% or less]
C is one of the elements that characterize the embodiment of the present invention, and if its content exceeds 0.12 mass%, it promotes the formation of MA (island martensite) in the IC-HAZ during high heat input welding, and deteriorates the low-temperature toughness of the joint. For this reason, the upper limit of the C content is set to 0.12 mass%. On the other hand, C is an element that increases the strength of steel, and it is necessary to contain 0.01 mass% or more in order to ensure the desired strength. Therefore, the C content is set to a 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% by mass or more, 0.18% by mass or less]
Si is one of the elements that characterize the embodiment of the present invention. By setting the Si content to 0.18 mass % or less, the formation of MA in the welded joint is suppressed and the low temperature toughness of the joint is improved. On the other hand, Si is an element necessary as a deoxidizer and for ensuring strength, and if the content is less than 0.01 mass%, the effect is insufficient. Therefore, the Si content is set to 0.01 The lower the Si content, the more the formation of MA in the IC-HAZ is suppressed, so the upper limit of the Si content is preferably 0.14 mass%.

[Mn: 0.2 mass% or more, 1.8 mass% or less]
Mn is an element necessary for improving the hardenability of steel and ensuring strength. If the Mn content is less than 0.2 mass%, the effect is insufficient, and if the Mn content exceeds 1.8 mass%, the toughness is deteriorated. Therefore, the Mn content is set to a range of 0.2 to 1.8 mass%, and the preferred range of the Mn content is 0.3 to 1.2 mass%.

[P: 0.0100% by mass or less (including 0% by mass)]
P is one of the elements that characterize the embodiment of the present invention, and is inevitably present in steel as an impurity. It segregates at grain boundaries and deteriorates the low-temperature toughness of the base material and the welded joint. Therefore, the upper limit is set to 0.0100 mass%. In order to improve the low-temperature toughness of the welded joint, the lower the P content, the more desirable it is.

In this specification, "including 0 mass %" means a content in accordance with an embodiment that is not intentionally added, for example, a content at the level of unavoidable impurities (it does not exclude cases where the content is intentionally added within a specified range).
On the other hand, in this specification, "not including 0 mass %" means that the element in question is intentionally added.

[S: 0.0100% by mass or less (including 0% by mass)]
S is an element present in steel as an inevitable impurity, and if the content is too high, elongated MnS becomes the starting point of brittle fracture, degrading the toughness of the base material and the welded joint. Therefore, the upper limit of the S content is set to 0.0100 mass%. In order to improve the toughness of the joint, the lower the S content, the better.

[Al: 0.001% by mass or more, 0.100% by mass or less]
Al is a deoxidizer and is an effective element for suppressing the coarsening of crystal grains and ensuring toughness, but if its content is less than 0.001 mass %, a sufficient effect cannot be obtained. On the other hand, if the Al content exceeds 0.100 mass%, brittle fracture occurs starting from the alumina inclusions, deteriorating the toughness. The range is % by mass.

[N: 0.0080% by mass or less (including 0% by mass)]
N is an impurity that deteriorates the toughness of the base material and the welded joint through the formation of precipitates such as AlN, so the upper limit is set to 0.0080 mass%. In order to improve the toughness of the joint, the lower the N content, the better.

[Ni: 8.75% by mass or more, 10.0% by mass or less]
Ni is a basic element added to ensure toughness at cryogenic temperatures (low temperature toughness), and in the embodiment of the present invention, the Ni content is set to 8.75 mass% or more. The higher the Ni content, the better the steel. Although low temperature toughness can be obtained, the effect of improving the properties is small with the increase in alloy cost when the Ni content exceeds 10.0 mass %. Therefore, the Ni content is set to the range of 8.75 to 10.0 mass %. From the viewpoint of ensuring low-temperature toughness and reducing alloy costs, the Ni content is more preferably in the range of 8.95 to 9.85 mass %.

[Mo: 0.01% by mass or more and 0.10% by mass or less]
Mo is one of the elements that characterize the embodiment of the present invention. By including Mo in an amount of 0.01 mass % or more, grain boundary embrittlement caused by P during the cooling process after welding is suppressed, and toughness is improved. On the other hand, if the Mo content exceeds 0.10 mass%, the effect of deterioration in toughness due to the formation of carbides becomes greater. Therefore, the Mo content is set to 0.01 to 0.10 mass%. %. Note that, within 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, so the lower limit of the Mo content is preferably 0. 0.02% by mass, more preferably 0.03% by mass.

[Cu: 0.70% by mass or less (including 0% by mass)]
Cu is an element contained in steel in trace amounts as an unavoidable impurity, and is usually contained at an impurity level of about 0.03 mass % or less.
On the other hand, if added in a small amount, it has the effect of improving strength without impairing toughness, so it may be added intentionally as necessary. On the other hand, if the Cu content exceeds 0.70 mass%, it deteriorates toughness. Therefore, when Cu is intentionally added, the Cu content is 0.70 mass% or less (excluding 0 mass%). In order to reliably obtain the above-mentioned effect of improving strength, the lower limit of the Cu content is preferably 0.05 mass%.

[Cr: 0.20 mass% or less (including 0 mass%)]
Cr is an element contained in steel in trace amounts as an unavoidable impurity, and is usually contained at an impurity level of about 0.08 mass % or less.
Cr may be added intentionally as necessary to improve the hardenability and strength of steel. On the other hand, if the content exceeds 0.20 mass%, it deteriorates toughness. Therefore, when Cr is intentionally added, the Cr content is set to 0.20 mass% or less (excluding 0 mass%). In order to reliably obtain the above-mentioned effects, the lower limit of the Cr content is preferably 0.10 mass%.

[Remainder]
In one preferred embodiment of the present invention, the balance is iron and inevitable impurities. As inevitable impurities, elements (e.g., As, Sb, Nb, O, H, etc.) that are brought in due to the conditions of raw materials, materials, manufacturing facilities, etc. are allowed to be mixed in.
In addition, for example, P and S are elements whose content is usually the lower the better and therefore are unavoidable impurities, but whose composition ranges are separately defined as above. Therefore, in this specification, the "unavoidable impurities" constituting the balance are a concept excluding elements whose composition ranges are separately defined.

1-2. DI Value The nickel-containing steel sheet for low temperature use according to the embodiment of the present invention has a DI value represented by the following formula (1) of 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, the brackets [ ] indicate the content of the element shown within, expressed in mass %. That is, for example, [C] means the content of C, expressed in mass %.

In order to use nickel-containing steel plate such as 9% Ni steel plate as a material for LNG tanks, the yield stress must be 590 MPa or more and the tensile strength must be 680 MPa or more. On the other hand, there is a trade-off between the toughness and strength of a material, and if the strength is excessively high, it will impair the toughness of the base material and welded joints, so the upper limit of tensile strength (TS) should be 830 MPa or less, and more preferably 800 MPa or less.

The DI value defined in the above formula (1) is a general parameter that indicates the hardenability of steel material, and the higher the DI value, the greater the dislocation density introduced into the material during hardening. For this reason, by controlling the DI value, it is possible to control the strength of low-temperature nickel-containing steel plate manufactured by quenching-tempering, quenching-intermediate heat treatment-tempering, or direct quenching-tempering, which are manufacturing methods commonly applied to low-temperature nickel-containing steel plate. If the DI value is in the range of 1.03 to 1.65, it is possible to keep the strength within the above range without impairing the toughness of the base material and welded joint.

1-3. Value of formula (2) In the nickel-containing steel sheet for low temperature use according to the embodiment of the present invention, the value calculated using formula (2) (sometimes referred to as the "value of formula (2)") is 7.06 or less.

5.0×10 ³ [C]×[Si] ^2.3 +1.5×10 ¹⁰ ×[P] ^3.5 /√(133[Mo]+1) ...(2)
Here, the brackets [ ] indicate the content of the element shown within them, expressed in mass %. That is, for example, [P] means the content of P, expressed in mass %.

As a result of the inventors' investigations, the following became clear.
When high heat input welding is applied to low-temperature nickel-containing steel plates, the cooling rate of the HAZ decreases with an increase in the amount of welding heat input. As a result, the amount of upper bainite formed at a relatively high temperature range from the grain boundaries of austenite formed by partial reverse transformation in the IC-HAZ, which is heated to a two-phase state of ferrite and austenite, increases. As a result, the concentration of C in untransformed austenite is promoted, and the formation of MA, which is a hard phase, becomes prominent. As a result, brittle fracture occurs starting from MA, and the Charpy impact absorption energy of the IC-HAZ is significantly deteriorated.

As described above, C promotes the formation of MA by concentrating in the untransformed austenite phase. In addition, Si increases MA by suppressing the cementitization of the untransformed austenite phase, so reducing C and Si is effective for reducing MA in the IC-HAZ. Furthermore, as a result of investigating the effect of components on the amount of MA formed, it was revealed that the amount of MA in the IC-HAZ can be organized as C x Si ^2.3 . Therefore, the first term of formula (2), "5.0 x 10 ³ [C] x [Si] ^2.3 ", represents the ease of MA formation in the IC-HAZ.

It is also known that P reduces toughness by causing grain boundary embrittlement in nickel-containing steel for low temperature use. Mo has a repulsive interaction with P on the prior austenite grain boundaries, and therefore has the effect of expelling P from the grain boundaries. As a result of the inventors' investigation, it has become clear that the grain boundary embrittlement caused by P can be significantly suppressed by adding a small amount of Mo, and the degree of grain boundary embrittlement is determined by the content ratio of P and Mo. Therefore, the second term of formula (2), "1.5 x 10 ¹⁰ x [P] ^3.5 /√(133 [Mo] + 1)", 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 in the IC-HAZ portion during high heat input welding and grain boundary embrittlement are suppressed, and as shown in the experimental results of the examples described later (particularly the quadratic approximation curve in FIG. 3 described in detail later), even during high heat input welding, the Charpy impact absorption energy value vE- ₁₉₆ of the IC-HAZ portion of the welded joint at -196°C is 34J or more, and brittle fracture is suppressed even in applications requiring toughness at extremely low temperatures such as LNG tanks, and excellent toughness of the welded joint can be ensured. Note that the Charpy impact absorption energy value vE _-196 being 34J or more means that the standard value (average 34J or more) of the L-direction Charpy impact absorption energy value of 9% Ni steel (joint) specified in the ASTM standards (ASTM A553/A553M: 2022 and ASTM A844/A844M: 2022) is satisfied.

2. Manufacturing method In manufacturing the nickel-containing steel plate for low temperature use according to the embodiment of the present invention, as long as the above-mentioned chemical components are satisfied and the DI value shown in formula (1) and the value of formula (2) are within the above-mentioned appropriate ranges, a well-known manufacturing method that is applied to the manufacture of a normal nickel-containing steel plate for low temperature use can be used.
For example, a steel slab or other such steel piece that satisfies the above requirements is hot-rolled to form a hot-rolled steel sheet of a predetermined thickness, which is then heated to a temperature range of Ac ₃ or higher and quenched, and then tempered at a temperature range of Ac ₁ or lower to obtain the above steel.
In addition, a step of quenching from a two-phase region of Ac ₁ point or more and Ac ₃ point or less may be included between quenching and tempering, or the steel sheet may be produced by directly quenching a hot-rolled steel sheet after hot rolling online and then tempering it in a temperature range of Ac ₁ point or less.

The nickel-containing steel plates for low temperature according to the embodiments of the present invention obtained in this manner can be used and welded to obtain a low temperature tank (for example, a tank for storing low temperature liquefied gas such as LNG, which is a clean energy source) according to the embodiments of the present invention. In particular, by using the nickel-containing steel plates for low temperature according to the embodiments of the present invention, the welded joints have excellent low temperature toughness even when welding is performed using a large heat input welding method, and therefore a low temperature tank with sufficient low temperature toughness can be obtained.

Note that high heat input welding refers to a welding method in which the welding heat input exceeds 50 kJ/cm, as described in Note (6) of Table M4.2 on page 16 of the "2021 Rules for Steel Ships, Part M" published by Nippon Kaiji Kyokai.

1. Sample Preparation Continuously cast slabs or vacuum melted slabs produced by converter-continuous casting and having the chemical compositions shown in Table 1 were heated to 1000°C to 1200°C, then hot-rolled to the thicknesses shown in Table 2, and cooled to room temperature by air to obtain hot-rolled samples. Table 1 also lists the DI value calculated using formula (1) for each sample, the value of the first term of formula (2) "5.0×10 ³ [C]×[Si] ^2.3 " (column "First term of formula (2)" in Table 1), the value of the second term of formula (2) "1.5×10 ¹⁰ ×[P] ^3.5 /√(133[Mo]+1)" (column "Second term of formula (2)" in Table 1), and the value of formula (2).

Next, the obtained hot-rolled sample was heated to 780° C., water-quenched, and tempered at 590° C. to obtain a steel plate sample. Details of the manufacturing conditions for the steel plate samples are shown in Table 2.
A sample for thermal cycle testing was obtained by cutting out a sample having a size of 12 mm x 33 mm x 55 mm from the t/4 position of the obtained steel plate sample (a position at a distance of 1/4 of the plate thickness t from the surface (main surface) of the steel plate toward the center) so that the plate thickness direction was 12 mm and the rolling direction was parallel to the 55 mm direction.
Also, a bar-shaped test piece was taken from the steel plate sample at the t/4 position in a direction perpendicular to the rolling direction, and this was used as a sample for a tensile test.

2. Sample evaluation (low temperature toughness evaluation)
The above-mentioned heat cycle test sample was heated to 950 ° C. by high-frequency heating at a heating rate of 50 ° C. / sec and held for 10 seconds, and then cooled 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, thereby giving a reproduced thermal history of the welded joint IC-HAZ part in one pass welding with a heat input of about 380 kJ / cm. Then, a V-notch Charpy standard test sample in accordance with Japanese Industrial Standard JIS 2242: 2018 was prepared from each heat cycle test sample. Then, using the V-notch Charpy standard test sample, a Charpy impact test was performed at -196 ° C., and the Charpy impact absorption energy value vE _-196 was measured. For one type of sample (for one sample No.), the impact test was performed three times. The measurement results of vE _-196 for each of the three tests are shown in the "each" column of Table 2, and the average value is shown in the "ave." column of Table 2.

(MA area fraction)
The amount of MA in the thermal cycle test sample after the above-mentioned simulated thermal history was measured. After wet polishing, the sample was etched with Repellant after excluding the end 3 mm of the thermal cycle test sample to which the simulated thermal history had been given. The structure of a 155 μm×202 μm area was observed with an optical microscope at a magnification of 400 times to identify MA from the contrast, and the area fraction of MA was calculated. The results are shown in Table 2.

(Tensile test)
The above-mentioned bar-shaped test pieces were used to measure the yield stress and tensile strength by a tensile test in accordance with JIS Z2241: 2022. The measurement results are shown in Table 2 as "base material strength" (strength of the material before the simulated thermal history of the welded joint IC-HAZ portion was applied).

Figure 1 shows the relationship between the DI value and the base material strength (yield stress and tensile strength), Figure 2 shows the relationship between the value of the first term of formula (2) and the MA area fraction, and Figure 3 shows the relationship between the value of formula (2) and the Charpy impact absorption energy value vE _-196 (average value of three samples) at -196°C. The dotted line in Figure 3 is a quadratic approximation curve.

(Good or bad judgement)
Regarding the base material strength, samples with a yield stress of 590 MPa or more and a tensile strength of 680 MPa or more and 830 MPa or less, and with an average Charpy impact absorption energy value vE _-196 at -196°C of 34 J or more and all three samples with a vE _-196 of 27 J or more were judged to be "good", and samples not satisfying any one of these criteria were judged to be "poor". The pass/fail judgment results are shown in Table 2.

As can be seen from Tables 1 and 2, Samples No. 1 to 8, which have the chemical composition specified in the embodiment of the present invention and have DI values and values of formula (2) within the specified range, have sufficient strength and low-temperature toughness. In other words, they have the strength required as a structural material for application to low-temperature tanks such as LNG, and can ensure sufficient low-temperature toughness for low-temperature tanks even when subjected to thermal history in the IC-HAZ, which is prone to toughness degradation during high-heat-input welding, and therefore can be said to contribute to improving welding construction efficiency by applying high-heat-input welding.

On the other hand, sample No. 9 has an excessive amount of Si and an insufficient amount of Mo, and the value of formula (2) is outside the predetermined range. For this reason, when a thermal history of high heat input welding is applied, the formation of MA is remarkable, and further, the influence of grain boundary embrittlement due to P is large, and sufficient low-temperature toughness cannot be secured when high heat input welding is applied.
In addition, in sample No. 10, the value of formula (2) is outside the predetermined range because the content ratio of Mo to P is small, so that the effect of grain boundary embrittlement due to P becomes significant, and sufficient low-temperature toughness cannot be ensured when large heat input welding is applied.

This application claims priority from Japanese Patent Application No. 2023-055986, filed on March 30, 2023. Japanese Patent Application No. 2023-055986 is incorporated herein by reference.

Claims (2)

C: 0.01 to 0.12% by mass,
Si: 0.01 to 0.18% by mass,
Mn: 0.2 to 1.8% by mass,
P: 0.0100% by mass or less (including 0% by mass),
S: 0.0100% by mass or less (including 0% by mass),
Al: 0.001 to 0.100% by mass,
N: 0.0080 mass% or less (including 0 mass%),
Mo: 0.01 to 0.10% by mass,
Ni: 8.75 to 10.0% by mass,
Cu: 0.70% by mass or less (including 0% by mass), and Cr: 0.20% by mass or less (including 0% by mass)
The balance is Fe and unavoidable impurities.
A nickel-containing steel sheet for low temperature use, having a DI value expressed by the following formula (1) of 1.03 or more and 1.65 or less, and a value calculated by the following formula (2) of 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)
Here, [ ] indicates the content of the element shown within, expressed as mass %.

5.0×10 ³ [C]×[Si] ^2.3 +1.5×10 ¹⁰ ×[P] ^3.5 /√(133[Mo]+1) ...(2)
Here, [ ] indicates the content of the element shown within, expressed as mass %. A low-temperature tank using the nickel-containing steel plate for low temperatures described in claim 1.

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