JP6879133B2 - Austenitic stainless steel welded member - Google Patents

Description

The present invention relates to a laser / TIG composite welding austenitic stainless steel welding member and a welding method in which laser welding and TIG welding are combined.

Laser welding uses a concentrated heat source with high energy density, so compared to arc welding represented by TIG welding, 1) high-speed deep penetration welding is possible, 2) the effect of welding heat is very small, 3) ) It has the feature that there is little welding deformation.

However, in laser welding, the cooling rate is fast, the hardness of the welded portion is higher than that of the base metal portion, and the problem is that the toughness is lowered. Conventionally known techniques for ensuring the workability of the laser welded portion are as follows.

In Non-Patent Document 1, in martensitic stainless steel, an austenite phase is formed in a high temperature region, and a martensitic structure is formed at room temperature. Therefore, the weld metal portion is remarkably hardened, and there is a concern that cracks may occur. In order to avoid this rapid cooling after welding, preheat at 200 ° C or higher, which is higher than the temperature at which martensitic transformation starts, gradually generate martensite, and take into account the effect of self-tempering of martensite. Avoids reduced toughness. However, since austenitic stainless steel is originally a material having excellent ductility, post-heat treatment after welding is not performed.

There is no example of focusing on the limited quality improvement of stainless steel sheets by the laser / TIG composite welding that was focused on in the present invention. Patent Document 1 only mentions that stainless steel can be used to reduce spatter in welding of metal materials in general.

Kazutoshi Nishimoto, Matsugo Natsume, Kazuhiro Ogawa, Cho Matsumoto: Welding of Stainless Steel, (2001), 209-210, Sanpo Publishing

Special Table 2015-526295 (published on September 10, 2015)

By welding, austenitic stainless steel has a higher hardness than the base metal. In austenitic stainless steel represented by SUS304, δ solidification occurs in which δ ferritic is formed from the liquid phase. δ-ferrite has a higher hardness than the austenite phase. The high hardness of the welded portion means that the ductility is lowered, and it is important to reduce the hardness of the welded portion even in the austenite phase having excellent ductility.

In order to solve the above problems, the present invention focused on the cooling process immediately after welding. Even if a δ ferrite phase is generated from the liquid phase, that phase is a metastable phase to the last, and the austenite phase is the final stable phase in the phase diagram. A feature of the present invention is that the region where the austenite phase appears is slowly cooled to promote the transformation from the δ ferrite phase to the austenite phase, thereby suppressing the increase in the hardness of the welded portion.
That is, the present invention is an austenitic stainless steel welded member obtained by subjecting a base material made of austenitic stainless steel to laser / TIG composite welding.
The Vickers hardness at the center of the bead of the weld is larger than the Vickers hardness of the base metal, and the difference is 50 or less.
The chemical composition of the base material austenitic stainless steel is C: 0.20% or less, Si: 1.87 % or less, Mn: 6.0% or less, Ni: 6.0% or more, Cu: It contains 4.0% or less, Cr: 12.0 to 30.0%, N: 0.30% or less, Mo: 5.0% or less, and the balance is Fe and unavoidable impurities, according to the formula (1). It is an austenitic stainless steel welded member in which the value of δF represented by is 0 or more and less than 20.0.
δF = -36C-0.13Mn-1.3Ni-30N-0.39Cu + 1.3Cr + 1.3Mo + 0.67Si-5 ... (1)
However, the value of the content (mass%) of the component corresponding to the element symbol is substituted for the position of the element symbol such as C and Mn. However, if the component is not included, it is not included in the formula.

By using the laser / TIG composite welded austenitic stainless steel in one aspect of the present invention, the Vickers hardness at the center of the weld bead and the Vickers at the base metal are not required for preheating before laser tube making or post-heat treatment after laser welding. It is possible to provide an austenitic stainless steel welded member having a small difference in hardness and excellent weld toughness.

The figure explaining the laser-TIG composite welding method which concerns on embodiment of this invention. An example of the bead appearance of the laser / TIG composite welded member according to the embodiment of the present invention. An example of a bead cross section of a laser / TIG composite welding member according to an embodiment of the present invention. Graph showing the relationship between the distance from the center of the bead and the cross-sectional Vickers hardness of the welded member subjected to laser / TIG composite welding and the member subjected to laser single welding.

Hereinafter, the present invention will be described in more detail based on Examples, but the present invention is not limited to these Examples, and various combinations are possible within the scope of the invention described in the claims. They are also included in the scope of the present invention.

<Outline of laser / TIG combined welding method>
The laser / TIG composite welding method according to the present invention will be described with reference to the drawings. FIG. 1 is a diagram illustrating TIG advance welding for the laser / TIG composite welding method according to the present invention.
In FIG. 1, reference numeral 1 is a beam of laser light for laser welding, and reference numeral 2 is a TIG welding torch. Further, reference numeral 3 is an austenitic stainless steel material which is a material. When laser / TIG composite welding is performed by this welding method, TIG welding by the TIG welding torch 2 is performed first, and then laser welding by the beam 1 of the laser beam is performed.

2 and 3 show an example of the bead appearance and bead cross section of austenitic stainless steel subjected to laser / TIG composite welding. As shown in FIG. 2, the amount of sputtering is small, and as shown in FIG. 3, the undercut is as small as 0.1 mm.

Hereinafter, matters specifying the present invention will be described. In addition, "%" indicating the content of each element means "mass%" unless otherwise specified.

C is a strong austenite-forming element and an element effective for improving the strength, but excessive addition causes coarse Cr carbides to precipitate in the recrystallization treatment, which causes intergranular corrosion resistance and deterioration of weldability. Therefore, C is preferably 0.20% or less (not including 0%).

Si is usually used for the purpose of deoxidation, but the steel of the present invention has the purpose of solid solution strengthening by adding Si. However, when the amount of Si is high, there is an effect of remarkably promoting the formation of the martensite phase during cold working. Further, if Si exceeds 5.0%, high temperature cracking is likely to be induced, which causes various problems in manufacturing. Therefore, 5.0% or less is desirable.

Mn is an element for ensuring non-magnetism after cold rolling. Further, Mn is an element that increases the solid solubility of N. It is also necessary to maintain non-magnetism after cold working. However, even if a large amount of nitrogen is added under pressure, surface defects due to the occurrence of blow holes and coloring during bright quenching occur. Therefore, the upper limit is preferably 6.0% or less (excluding 0%).

Like Mn, Ni is an element for ensuring non-magnetism after cold rolling. In order to maintain non-magnetism after cold rolling, 6.0% or more is required, and it is necessary to adjust the amount of Ni according to the contents of Si and Mn.

Cu is also an element for ensuring non-magnetism after cold rolling. However, since excessive addition deteriorates hot workability and causes cracks, the component range is preferably 4.0% or less.

Cr is an essential component in terms of corrosion resistance. At least 12.0% Cr is required to provide the intended corrosion resistance. However, since Cr is also a ferrite forming element, if it is made too high, a large amount of δ ferrite phase will be generated at high temperature. Therefore, austenite-forming elements (C, N, Ni, Mn, Cu, etc.) must be added to suppress the δ ferrite phase. However, if it is contained in a large amount, δ ferrite cannot be suppressed only by adjusting by adding an austenite-forming element, and non-magnetism cannot be secured. Therefore, the upper limit is set to 30%.

N is an effective element for maintaining non-magnetism, which is a main feature of the present invention, and obtaining high strength. Since excessive addition of N causes blowholes during casting, it is necessary to devise measures such as nitrogen pressure melting, and even considering this, the upper limit is preferably 0.30% or less.

ΔF was defined as the following equation (1) as an index of the amount of δ ferrite solidified from the liquid phase after welding. The austenite-forming element represented by C and N has a positive coefficient, and the ferrite-forming element represented by Cr and Si has a negative coefficient.
δF = -36C-0.13Mn-1.3Ni-30N-0.39Cu + 1.3Cr + 1.3Mo + 0.67Si-5 ... (1)
After welding when the value of δF is 0 or more, δ ferrite is first generated from the liquid phase. If δF exceeds 20.0, cracks occur in hot workability, so the upper limit was set to 20.0.

Mo has the effect of improving corrosion resistance and finely distributing the carbonitride by aging treatment. However, if a large amount of Mo is added, a large amount of δ ferrite is formed at a high temperature, so the component range of Mo is preferably 5.0% or less.

Ti is an element that forms a carbonitride and is effective in maintaining corrosion resistance after welding, but if it is 0.50% or more, surface scratches on the steelmaking slab are likely to occur, which poses a problem in terms of production. Therefore, the upper limit is set to 0.50%.

Nb is also an element that forms carbonitride and is effective in maintaining corrosion resistance after welding, and is effective in increasing the strength during aging treatment, but it causes a decrease in hot workability due to an increase in high-temperature strength, so the upper limit is set. It was set to 0.50%.

B is an element effective in preventing the occurrence of edge cracks in the hot-rolled steel strip caused by the difference in deformation resistance between the δ ferrite phase and the austenite phase in the hot rolling temperature range, but excessive addition causes a low melting point boulder. Since it becomes easy to form and conversely deteriorates hot workability, it was set to 0.010% or less.

Al is an element effective for deoxidation and oxidation resistance, but the upper limit is set to 4.0% because excessive addition causes surface defects.

The following elements are not described in the claims, but may be contained.
P: An element harmful to hot workability. In particular, if it exceeds 0.050%, the effect becomes remarkable, so it is preferably 0.050% or less.
S: An element that easily segregates at grain boundaries and promotes deterioration of hot workability due to embrittlement at grain boundaries. If it exceeds 0.020%, the effect becomes remarkable, so it is preferably 0.020% or less.
V and Zr: V are useful elements in terms of improving workability due to the effect of precipitating solid solution C as a carbide, and Zr is a useful element in terms of improving workability and toughness by capturing oxygen in steel as an oxide. However, since the manufacturability is lowered when a large amount is added, the appropriate contents of V and Zr are 0.01 to 0.30%.
O is 0.02% or less because it forms oxide-based non-metal inclusions and lowers the cleanliness of steel, which adversely affects press formability and bendability.
In addition to these, Ca, Mg, Co, REM, etc. may be contained in the scrap that is the raw material during melting, but unless it is contained in a large amount, laser / TIG composite welding austenitic stainless steel welding It does not affect the characteristics of the parts.

A stainless steel plate (annealed material) having a thickness of 3.0 mm having the components and compositions shown in Table 1 was used as a material, and laser / TIG composite welding or laser single welding was performed. Steel No. in Table 1 A to F are examples of the present invention in which the chemical composition values are within the range of the present invention, Steel No. GI are other steels (comparative examples). No filler metal was used.

Welding was butt welding and the end face was machined. Welding conditions are as follows. When laser / TIG composite welding is performed, the distance between the torch for TIG welding and the torch for laser welding is set to 3 mm. Further, the assist gas for laser welding was used only when performing laser single welding, and was not used when performing laser / TIG composite welding.
Arrangement: TIG precedent or laser precedent Laser welding: Output 4 kW,
Spot diameter φ0.6 mm,
Inclination 0 °,
Assist gas Ar100%, 40L / min

TIG welding: receding angle 30 °,
Current 300A,
Arc length 1.5 mm,
Shield gas Ar100%, 15L / min

Welding speed: Laser / TIG composite welding 8.0 m / min,
Laser single welding 4.0 m / min

Table 2 summarizes the Vickers hardness at the center of the bead, the Vickers hardness at the base metal, and the difference between the two, between the welded member subjected to laser / TIG composite welding and the welded member subjected to laser single welding. The Vickers hardness measurement was obtained from the average of a total of three points, the plate thickness center t / 2 and the plate thickness t / 4 (front and back). The hardness of the base metal portion is defined as a three-point average value of 1.5 mm, 1.75 mm, and 2.0 mm from the center of the bead before welding.

As shown in Table 2, in the welded member of the example of the present invention, the Vickers hardness of the bead center portion of the welded portion is higher than the Vickers hardness of the base metal portion, but the difference satisfies the difference of 50 or less. .. In particular, when laser / TIG composite welding is performed, the difference in Vickers hardness is smaller in the TIG welding precedent than in the laser welding precedent.

A specific example is shown in FIG. In FIG. 4, the austenitic stainless steel of the A steel shown in Table 1 was used as the material, and the No. 1 was subjected to laser / TIG composite welding (TIG precedent). No. 1 and No. 1 with laser single welding. It is a graph which shows the relationship between the distance from the bead center part of the welding member, and the cross-sectional Vickers hardness about 16 welding members. The Vickers hardness is highest in the central part of the bead part, but when comparing the Vickers hardness difference between the central part of the bead part and the base metal part, No. No. 1 was clearly laser-only welded No. 1. Hardness increase is suppressed more than 16.

No. No. 13 has a Vickers hardness difference of 50 or less, but the value of δF in G steel is −1.6 and δ ferrite is not formed. No. No. 14 is H steel, the amount of C exceeds the claimed range, carbides are generated during welding cooling, and the corrosion resistance is significantly reduced. No. In No. 15, the δF value of the I steel was 25.3, and the amount of δ ferrite produced was too large, and even with the hardness lowering effect of laser / TIG composite welding, the Vickers hardness difference of 50 or less could not be satisfied.

1 Laser beam for laser welding 2 Torch for TIG welding 3 Material

Claims (3)

An austenitic stainless steel welded member in which a base metal made of austenitic stainless steel is subjected to laser / TIG composite welding.
The Vickers hardness at the center of the bead of the weld is larger than the Vickers hardness of the base metal, and the difference is 50 or less.
The chemical composition of the base material austenitic stainless steel is C: 0.20% or less, Si: 1.87 % or less, Mn: 6.0% or less, Ni: 6.0% or more, Cu: It contains 4.0% or less, Cr: 12.0 to 30.0%, N: 0.30% or less, Mo: 5.0% or less, and the balance is Fe and unavoidable impurities, according to the formula (1). An austenitic stainless steel welded member in which the value of δF represented by is 0 or more and less than 20.0.
δF = -36C-0.13Mn-1.3Ni-30N-0.39Cu + 1.3Cr + 1.3Mo + 0.67Si-5 ... (1)
However, the value of the content (mass%) of the component corresponding to the element symbol is substituted for the position of the element symbol such as C and Mn. However, if the component is not included, it is not included in the formula. According to claim 1, the chemical composition of the base material further contains at least one of Ti: 0.50% or less, Nb: 0.50% or less, and B: 0.010% or less in mass%. The austenitic stainless steel welded member described. The chemical composition of the base material further contains Al: 0.78 to 4.0 % in mass%.
The austenitic stainless steel welding member according to claim 1 or 2.

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