JP2024112030A - Austenitic stainless steel weld metal and stainless steel submerged arc welding method - Google Patents

Abstract

[Problem] To provide a submerged arc welding method for SUS304, which can obtain a weld metal with high strength and excellent cryogenic toughness, no defects in the welded part, good corrosion resistance, and good bending performance. [Solution] The chemical composition of the austenitic stainless steel weld metal is, by mass%, C: 0.08% or less, Si: 0.2-1.0%, Mn: 1-2%, Ni: 8-10%, Cr: 20-23%, Mo: 0.75% or less, and the balance being Fe and impurities. [Selected Figure] None

Description

The present invention relates to austenitic stainless steel weld metal with high strength and excellent cryogenic toughness in submerged arc welding materials used for welding SUS304, which is mainly used in the construction of LNG storage tanks, and a stainless steel submerged arc welding method suitable for obtaining high-strength SUS304 by using the same.

In recent years, environmental issues have led to calls for the reduction of greenhouse gas emissions, and the demand for power generation and ship propulsion using liquefied natural gas as fuel is increasing, so the use of LNG is expected to expand further in the future. In addition, as a way to further reduce greenhouse gas emissions, a switch from LNG to ammonia fuel is also being considered, and it is expected that the transition in energy sources will progress rapidly.

Although 9% Ni steel is often used for marine LNG tanks, as with land-based LNG tanks, this steel cannot be used for ammonia containers due to its poor corrosion resistance. On the other hand, SUS304 is often used for land-based LNG gas holders, and is also suitable for ammonia containers due to its good corrosion resistance. Regarding the tensile performance of SUS304, the tensile strength of JIS G 4305 cold-rolled stainless steel plate and strip is specified as 520 N/ ^mm2 or more, but SUS304 with high strength has been developed using TMCP technology, and a tensile strength of 600 N/mm2 or ^more can be guaranteed.

Regarding welding materials applied to SUS304, the performance of the deposited metal in submerged arc welding is generally 520 MPa or more in tensile strength, which is specified in S308 of JIS Z 3324, quality classification and test method for stainless steel deposited metal by submerged arc welding. The actual tensile strength is about 580 MPa, which is inferior to the tensile properties of the SUS304 base material that incorporates TMCP technology, and there was an issue of undermatching of the welded part.

For example, Patent Document 1 specifies the chemical composition of the weld metal and discloses a composition system that provides a tensile strength of 600 MPa or more. However, because it contains a large amount of Mo, there is an issue of low toughness at cryogenic temperatures.

Patent Document 2 specifies the chemical composition of the deposited metal and discloses a composition system that improves toughness at cryogenic temperatures. However, the tensile strength is low and the amount of δ-ferrite is also low, so there are issues with crack resistance.

JP 2001-276977 A Japanese Unexamined Patent Publication No. 58-97494

The present invention was devised in consideration of the above-mentioned problems, and aims to provide an austenitic stainless steel weld metal that is high-strength and has excellent cryogenic toughness as a welding material for high-strength SUS304, as well as a stainless steel submerged arc welding method that can obtain the austenitic stainless steel weld metal, has no defects in the weld, has good corrosion resistance, and ensures good bending performance.

The austenitic stainless steel weld metal of the present invention was developed based on the above findings, and its gist is as follows: It has a chemical composition, by mass%, of C: 0.08% or less, Si: 0.2-1.0%, Mn: 1-2%, Ni: 8-10%, Cr: 20-23%, Mo: 0.75% or less, and the balance being Fe and impurities.

The austenitic stainless steel weld metal according to the present invention is further characterized in that the contents of Cr, Si, Ni, C, and Mn are 10 to 20 in terms of DF calculated from the following formula (1).

DF=3×[Cr]+5×[Si]-3×[Ni]-80×[C]-[Mn]-20...(1)
In formula (1), [ ] indicates the mass percentage of each component.

Furthermore, the stainless steel submerged arc welding method according to the present invention is characterized in that it uses the above-mentioned austenitic stainless steel weld metal and adjusts the welding heat input of the submerged arc welding to 35 kJ/cm or less.

The stainless steel submerged arc welding method of the present invention produces a high-strength, high-toughness, and good weld metal at cryogenic temperatures, improving the quality of the weld.

The inventors conducted various studies on the composition of the deposited metal in stainless steel submerged arc welding suitable for SUS304 and high-strength SUS304 incorporating TMCP technology, in order to improve the tensile strength and cryogenic toughness of the deposited metal, as well as the bending performance and crack resistance.

As a result, it was found that the desired high tensile strength of the deposited metal can be achieved by optimizing the Ni content and the DF value shown in the following formula (1), that the desired ductility can be achieved in the tensile test by optimizing the Si content, and that in order to satisfy the cryogenic toughness, the desired high toughness can be achieved by optimizing the Mn, Ni, Cr and Mo content.

DF=3×[Cr]+5×[Si]-3×[Ni]-80×[C]-[Mn]-20...(1)

In formula (1), [ ] indicates the mass percentage of each component. It was also clarified that in order to obtain sound weld metal, good crack resistance can be ensured by adjusting the Mn content appropriately, porosity defects such as blowholes are prevented by adjusting the Si content appropriately, and good corrosion resistance is achieved by adjusting the C and Cr content appropriately.

Furthermore, it was found that optimizing the amount of δ-ferrite and the upper limit of the welding heat input is effective in preventing the coarsening of ferrite and austenite phase crystal grains and suppressing deterioration of the bending performance of the deposited metal.

The present invention was made possible by the individual effects of each component in the deposited metal of submerged arc welding of austenitic stainless steel, as well as the synergistic effects of their coexistence. The reasons for adding each component and limiting their content are described below. Note that the percentage of the content of each component described below refers to the mass % relative to the total mass of the deposited metal of austenitic stainless steel.

[C in deposited metal: 0.08% or less]
C combines with Cr to reduce the Cr content, which is effective for corrosion resistance, and deteriorates the corrosion resistance of the deposited metal, so the content is set to 0.08% or less. The lower the C content, the better the corrosion resistance, so no lower limit is specified, but in order to obtain a very low-carbon deposited metal, it becomes necessary to use high-purity raw materials with few impurities in the wire and flux, which reduces economic efficiency, so the lower limit is preferably set to 0.01%, and the C content is preferably 0.01 to 0.08%. C can be added from C contained in Fe-Mn as a deoxidizer in the submerged wire and flux, etc.

[Si in weld metal: 0.2 to 1.0%]
Silicon has the effect of improving the resistance of the deposited metal to porosity defects. If the silicon content is less than 0.2%, this effect is not fully obtained, and porosity defects such as blowholes are likely to occur. On the other hand, if the silicon content exceeds 1.0%, the elongation of the deposited metal in the tensile test decreases. Therefore, the silicon content is set to 0.2 to 1.0%. Silicon can be added as a submerged wire or flux deoxidizer from metal silicon, Fe-Si, Fe-Si-Mn, etc.

[Mn in weld metal: 1-2%]
Mn has the effect of improving the hot cracking resistance of the deposited metal. If the Mn content is less than 1%, cracking is more likely to occur. On the other hand, if the Mn content exceeds 2%, elongated MnS precipitates, lowering the absorbed energy in the impact test. Therefore, the Mn content is set to 1 to 2%. Mn can be added as metallic Mn, Fe-Mn, Fe-Si-Mn, etc. as a submerged wire or flux deoxidizer.

[Ni in deposited metal: 8-10%]
Ni has the effect of stabilizing the austenite phase and improving the absorbed energy in impact tests. If Ni is less than 8%, this effect is not fully obtained, and the absorbed energy is low. On the other hand, if Ni exceeds 10%, it is difficult to form δ ferrite, and tensile strength cannot be increased. Therefore, Ni is set to 8-10%. Ni can be added from metallic Ni, etc., as a submerged wire or flux alloying agent.

[Cr in deposited metal: 20-23%]
Cr has the effect of improving the corrosion resistance of the weld metal. If the Cr content is less than 20%, the corrosion resistance decreases. On the other hand, if the Cr content exceeds 23%, Cr carbides and Cr nitrides precipitate, decreasing the absorbed energy in the weld metal impact test. Therefore, the Cr content is set to 20-23%. Cr can be added from metallic Cr, Fe-Cr, etc. as a submerged wire or flux alloying agent.

[Mo in weld metal: 0.75% or less]
Mo is set to 0.75% or less because it reduces the absorbed energy in the weld metal impact test. A lower Mo content is preferable, so no lower limit is specified. However, the raw materials for stainless steel wire rods are often produced by collecting scrap from the market and recycling it, so Mo is contained as an impurity. Since the use of high-purity raw materials that do not contain Mo reduces the economic efficiency, the lower limit is preferably set to 0.05%, and the Mo content is preferably 0.05 to 0.75%. Mo is contained as an impurity in the submerged wire.

[The content of Cr, Si, Ni, C, and Mn is 10 to 20 in terms of DF calculated from formula (1)]
DF=3×[Cr]+5×[Si]-3×[Ni]-80×[C]-[Mn]-20...(1)

In formula (1), [ ] indicates the mass percentage of each component. DF indicates the ease of crystallization of delta ferrite; the higher the DF, the greater the amount of ferrite, which has the effect of increasing the tensile strength of the weld metal. If DF is less than 10, the tensile strength of the weld metal is low. On the other hand, if DF exceeds 20, the ferrite grains become coarse, and bending performance deteriorates in bending tests of the weld metal. Therefore, the contents of Cr, Si, Ni, C, and Mn should be adjusted so that DF calculated from formula (1) is between 10 and 20.

[Welding heat input: 35 kJ/cm or less]
The higher the welding heat input, the better, since it improves the efficiency of welding work. However, if it exceeds 35 kJ/cm, the crystal grains of the austenite phase become coarse, and bending performance deteriorates in bending tests of the deposited metal. Therefore, the welding heat input should be 35 kJ/cm or less. Since austenitic stainless steel is hardly affected in mechanical performance due to hardenability related to the cooling rate, unlike carbon steel, there is no lower limit for the welding heat input.

The reasons for limiting the constituent elements of the deposited metal and welding method for submerged arc welding of low-temperature austenitic stainless steel of the present invention have been described above, but the remainder of the deposited metal is Fe and impurities. There are no particular restrictions on the impurities, but from the standpoint of the elongation of the deposited metal in a tensile test, the absorbed energy in an impact test, and the crack resistance, it is preferable that the deposited metal have P: 0.05% or less, S: 0.01% or less, N: 0.05% or less, and Cu: 0.5% or less.

The submerged wire may be a solid wire, but flux-cored wire containing flux may also be used. The submerged arc flux may be either molten or sintered. In the case of sintered flux, alloy elements and deoxidizers that are lacking in the wire can be added, and the deposited metal composition can be adjusted to have the contents of each of the components listed above.

The effects of the present invention will be specifically explained using examples, but the present invention is not limited to the following examples.

A weld metal test was conducted using a wire with a diameter of 4.0 mm shown in Table 1 and the flux shown in Table 2. The base material used was SUS304 with a plate thickness of 20 mm shown in Table 3.

Using these materials, weld metal performance was evaluated in accordance with JIS Z 3324:2010, quality classification and test methods for stainless steel weld metals by submerged arc welding.

The integrity of the weld was examined according to the radiographic testing method for stainless steel welded joints in JIS Z 3106:2001, and a 500mm weld length was checked for defects. In classifying the images of defects, Class 1 was considered to be good.

The mechanical properties of the deposited metal were evaluated by taking A0 tensile test pieces and V-notch impact test pieces according to JIS Z 3111:2005, and side bending test pieces according to JIS Z 3122:2013 from the above welded parts, and tests were performed. In the tensile test, a tensile strength of 600 MPa or more was considered good, and an elongation of 35% or more was considered good. In the impact test, a Charpy impact test was performed three times at a test temperature of -196°C, and an average absorbed energy of 34 J or more was considered good. In the bending test, a 180° bending test was performed using a push jig with a radius of 20 mm (diameter 40 mm), and a defect-free bend test surface with no cracks or openings was considered good. In addition, the pass/fail criteria of the test method and judgment criteria in the JIS Z 3841 semi-automatic welding technology certification were used as a reference, and even if small scratches occurred, a total length of cracks and openings of 3 mm or less was considered good.

The corrosion resistance was evaluated according to JIS G 0573:1999, 65% nitric acid corrosion test method for stainless steel, by taking a test piece from the weld metal and immersing it once in 65% boiling nitric acid for 24 hours to measure corrosion weight loss. A corrosion weight loss of less than 10 g/ ^m2 /h was considered good. The results are summarized in Table 4.

In Table 4, weld metals No. 1 to No. 11 are examples of the present invention, and welding rods No. 12 to No. 17 are comparative examples.

The weld metals No. 1 to 11, which are examples of the present invention, had extremely satisfactory results, with the appropriate amounts of C, Si, Mn, Ni, Cr, and Mo, no welding defects, and good mechanical performance. In addition, Nos. 1, 2, 4, 5, 6, 7, 8, and 11 had high tensile strength and better bending performance because the DF value and welding heat input were appropriate.

The weld metal No. 12 in the comparative example had poor corrosion resistance because the C content of the weld metal was high.

Weld metal No. 13 had blowholes because the Si content of the weld metal was low. Also, the Cr content of the weld metal was low, so the corrosion resistance was poor.

The weld metal No. 14 had a low elongation in the tensile test because the weld metal had a high Si content. Also, the weld metal had a high Cr content, so the absorbed energy of the weld metal was low.

In weld metal No. 15, the Mn content of the weld metal was low, so cracks occurred. In addition, the Ni content of the weld metal was low, so the absorbed energy of the weld metal was low.

The weld metal No. 16 had a high Mn content, so the absorbed energy of the weld metal was low. Also, the weld metal had a high Ni content, so the tensile strength was low.

Weld metal No. 17 had a high Mo content, so the absorbed energy of the weld metal was low.

Claims (3)

The chemical composition, in mass%, is
C: 0.08% or less;
Si: 0.2-1.0%,
Mn: 1 to 2%,
Ni: 8 to 10%,
Cr: 20 to 23%;
Mo: 0.75% or less;
The balance is austenitic stainless steel weld metal which is Fe and impurities. The austenitic stainless steel weld metal according to claim 1, characterized in that the contents of Cr, Si, Ni, C and Mn are each 10 to 20 in terms of DF calculated from the following formula (1):
DF=3×[Cr]+5×[Si]-3×[Ni]-80×[C]-[Mn]-20...(1)
In formula (1), [ ] indicates the mass percentage of each component. A stainless steel submerged arc welding method using the austenitic stainless steel weld metal of claim 1 or 2, characterized in that the welding heat input of the submerged arc welding is adjusted to 35 kJ/cm or less.