JPH06670A - Welding method for ferritic stainless steel - Google Patents

Abstract

PURPOSE:To obtain a welded joint having high toughness with the ferritic stainless steel having high Cr content as an object. CONSTITUTION:While a filler wire such as a power filler or a foil-like filler to satisfy the inequality I is supplied so that the quantity of Ni in weld metal to exceed 7weight%, laser beam welding is performed. Ni (weight%)-Creq>=24 (weight%) ... inequality I wherein Creq=%Cr+%Mo+1.5X%Si+0.5X%Nb (weight%). Accordingly, coarsening of crystal grains is prevented and a weld zone excellent in toughness can be obtained. Consequently, the rupture of the coil joining weld zone in line passing and subsequent lowering of productivity can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for welding ferritic stainless steel having a high Cr content, and more particularly to a method for welding ferritic stainless steel capable of obtaining a welded joint excellent in toughness by laser welding.

[0002]

2. Description of the Related Art Ferritic stainless steel has excellent corrosion resistance and oxidation resistance at high temperatures, and since it does not contain Ni and is relatively inexpensive, it has recently become a member of automobile exhaust systems such as exhaust manifolds and mufflers. The use as a material for is expanding.

Problems in welding this ferritic stainless steel include brittleness at 475 ° C., embrittlement due to σ phase precipitation, coarsening of crystal grains, delayed cracking due to diffusible hydrogen,
In order to prevent such problems from occurring, various studies have been made on heat treatment after welding and welding materials. For example, as for the welding material, a common metal material is usually used, but if brittleness of the weld is a problem, a welding material for austenitic stainless steel containing Ni is also used. . As a result, an austenite phase is precipitated in the weld metal, coarsening of crystal grains of the weld metal is suppressed, the amount of hydrogen solid solution is increased, and embrittlement of the weld is prevented.

Recently, when welding a thin plate having a thickness of about 5 mm or less, a laser welding method is often applied. Since laser welding has a small heat input during welding,
The heat effect and heat strain on the material to be welded are small, and such a thin plate can be welded without deformation. Also,
Since the welded part is rapidly cooled, the embrittlement of (1) and (2) are suppressed, and the thermal effect is small, so that the coarsening region of the crystal grains is narrow. Further, gas shielding of the welded portion is easy, diffusible hydrogen dissolved in the weld metal can be suppressed to a low level, and delayed cracking due to diffusible hydrogen does not pose a problem. As described above, laser welding is essentially suitable for welding ferritic stainless steel as compared with arc welding and flash butt welding, and in recent years, a coil joint welder has been introduced into a plate making line.

Ferrite-based stainless steel is expected to be used more frequently in environments where it comes into contact with higher-temperature exhaust gas, because it is expected that demands for improving fuel efficiency of automobiles will increase in the future. Hereinafter, "%" of alloying elements and "%" of fillers represent "% by weight")
It is presumed that there will be many uses as high-temperature steels containing the above and further improved corrosion resistance and oxidation resistance containing Si and Nb.

When welding such a material containing a large amount of ferrite-forming elements and having a high Cr equivalent, it is difficult to prevent coarsening of crystal grains even by laser welding. When the laser welded portion was actually evaluated, it was found that coarsening of crystal grains occurred in both the weld metal and the weld heat affected zone (HAZ), and the bending toughness was not sufficient. In particular, coil joint welds are repeatedly bent by loopers and bridle rolls during line threading, so if the bend toughness of the welds is not sufficient, problems such as breakage in the line occur, resulting in a significant decrease in productivity. Connect

As one method of controlling the structure of this welded portion, a method of adding a filler is adopted in laser welding. Usually, when performing coil splicing welding by laser welding, if the end surface shape of the material to be welded is irregular, a filler is added. However, the amount of addition is about 10 to 30% of the total molten metal, and for example, as shown in the Schaeffler structure chart of FIG. 1, a stainless steel with a Cr equivalent of 22% (“base material” in the figure) When welding the SUS 30
The amount of austenite in the weld metal is extremely small even if 30% of the filler wire for 4 (point a in the figure) is added. In the case of CO ₂ welding, since the groove is taken, the ratio of the deposited metal can be set to about 40 to 80% and the austenite phase can be sufficiently precipitated, but in the case of laser welding, there is almost no gap in the material. Since they are butt-welded and welded together, such a large amount of filler cannot be added.

[0008]

DISCLOSURE OF THE INVENTION The present invention solves the above problems and, when welding ferritic stainless steel, prevents coarsening of crystal grains and can provide a welded portion excellent in toughness. It is intended to provide a welding method.

[0009]

[Means for Solving the Problems] In order to achieve the above object, the present inventors have proposed SUS 409, SUS 430, and SUS 430-based high-temperature steel as typical examples of ferritic stainless steels. A method for obtaining a welded joint with high toughness by laser welding was examined for steel containing 0.7% Nb (Nb-added steel) and steel containing 2.8% Si (Si-added steel). As the filler wire, a wire for mild steel (YCW2) and a wire for SUS304 (Y308L) specified by JIS were used.

The examination results are shown in FIG. The toughness was evaluated by a repeated bending test. The welding method and repeated bending test method are the same as those in the examples described later. As shown in this figure, SUS 4 with a low Cr content
With 09 (symbol: S409), high toughness is secured by using any of the above wires, and SUS 430 (S430) is slightly inferior to SUS 409, but toughness with practical use with any wire Indicates. However, Si-added steel (S430S)
With Nb-added steel (S430N of the same), the toughness is remarkably low regardless of which wire is used, because the weld metal part breaks brittlely due to repeated bending.

In view of the fact that the amount of filler metal that can be added by laser welding is as small as 10 to 30%, the present inventors have proposed a filler having a composition capable of controlling the structure of the welded portion even if the amount added is small. We examined what kind of thing is best. As a result, pure Ni or
When Ni-based alloy is used, the Ni equivalent of the weld metal becomes 10 to 30%, austenite and ferrite, or a mixed structure containing martensite, or austenite single phase, SUS 430, Si-added steel, Nb-added It was confirmed that sufficient toughness was secured for any of the steels. For example, as shown in FIG. 1, when laser welding the above-mentioned stainless steel having a Cr equivalent of 22%, Ni wire was
The composition of the weld metal is shown when it is added so as to be up to 30%, but it has an austenite single phase or a structure mainly composed of austenite.

The present invention has been made on the basis of such knowledge, and its gist is the following method for welding ferritic stainless steel.

A filler wire, a powder filler or a foil filler satisfying the following formula (1) is mixed with Ni in the weld metal to 7
% Welding method of ferritic stainless steel, characterized in that laser welding is performed while supplying an amount exceeding 100%.

Ni (%) − Cr _eq ≧ 24 (%) (1) where Cr _eq =% Cr +% Mo + 1.5 ×% Si + 0.5 ×% Nb The ferritic stainless steel is the Cr equivalent ( Cr _eq )
Is approximately 12% or more of stainless steel that is difficult to weld without impairing toughness, and specifically, the above-mentioned SUS409, SUS
430, Si-added steel and Nb-added steel.

The filler wire, powder filler or foil filler satisfying the formula (1) means pure Ni or Ni-based alloy filler wire, powder filler or foil filler, as described later.

[0016]

In the method of the present invention, the laser welding is performed while supplying the filler wire, the powder filler and the like satisfying the above formula (1) in order to make the weld metal a tough structure in which the austenite phase is partially precipitated. is there.

FIG. 3 is a diagram showing the results of repeated bending tests in which SUS 430, Si-added steel and Nb-added steel were subjected to butt welding by laser welding using a filler wire of pure Ni and subjected to repeated bending tests. The chemical composition of the test material, the plate thickness,
The welding method and the repeated bending test method are the same as those in the examples described later.

As shown in this figure, when the Ni addition amount, that is, the Ni content of the weld metal exceeds 7%, high toughness in which the number of repeated bending cycles exceeds 20 is exhibited. In the case of steel (S430S) and Nb-added steel (S430N), precipitation of austenite phase is insufficient and the effect of improving toughness is small. Although this result is obtained by using a pure Ni filler wire, similar results can be obtained by using a Ni-based alloy filler wire having a high Ni content.

The ratio of the filler metal that can be added by laser welding is usually 30% or less as described above. Therefore, for example, a Fe-Ni binary alloy filler is used and the addition amount is the maximum value. If it is 30%, the Ni content of the weld metal is 7
In order to exceed 0.1%, an Fe-Ni alloy containing about 24% or more Ni should be used. On the other hand, when a ferrite-forming element such as Cr is contained in the filler, it is naturally necessary to increase the Ni content. As a result of examining the increase rate, increasing the amount of Ni equivalent to the Cr equivalent contained in the filler, that is, the Ni content after subtracting the Cr equivalent contained in the filler, as in the above formula (1). Use a filler whose amount is 24% or more,
It was confirmed that a weld metal rich in toughness can be obtained by welding so that Ni in the weld metal exceeds 7%.

The upper limit of the Ni content is not specified, but the reason is that the amount of filler that can be added in laser welding is limited as described above, and the upper limit is naturally determined. If the usual addition amount is in the range of 10 to 30%, good toughness can be obtained by setting the Ni content of the weld metal within the above range. The same result can be obtained by using a powder filler or a foil filler instead of the filler wire.

When ferritic stainless steel is subjected to CO ₂ welding or the like using the above-mentioned pure Ni or Ni-based alloy filler wire, the cooling rate at the welded portion is low, and thus hot cracking is likely to occur. That is, the use of a filler containing Ni as a main component is effective only in laser welding in which the amount of filler added is limited and the weld is rapidly cooled, and has the effect of improving the toughness of the weld of the ferritic stainless steel. I can say.

When carrying out the method of the present invention, when a protrusion of the bead excess occurs, it is removed by means of grinding or other similar means in order to prevent breakage in the heat affected zone.

FIG. 4 is a schematic view of a welding cross section when a pure Ni filler wire is used for ferritic stainless steel and laser welding is performed while changing the gap at the time of butting. (a) The figure shows a gap of 0 mm as it is usually done.
When the amount of filler added was 10 to 30%, there was no excess and no cracking was observed in the repeated bending test.
Shows good toughness. (c) The figure shows the case where the gap is enlarged to 0.3 mm and the filler is added up to 60% so that there is no overhang of protrusions, the weld metal has an austenite-based structure, but cracks do not occur and toughness Is no problem at all. That is, the high Ni content is not harmful per se. on the other hand,
(b) In the figure, the gap is 0 mm, the amount of filler added is 60%, there is extra protrusion, and cracks occur in the heat-affected zone in Si-added steel and Nb-added steel. This is because the heat-affected zone is also somewhat brittle due to the coarsening of the structure, and due to the protrusion of the overfill, stress concentrates on the interface between the overfill and the heat-affected zone, and that portion started as a crack. By removing the surplus by grinding or the like, concentration of stress can be relieved and cracking can be prevented.

[0024]

[Example] As a test material, a hot-rolled steel sheet having a thickness of 3 mm and a width of 100 mm and a chemical composition shown in Table 1 was shirred,
The end faces were abutted as they were, welded by laser welding, and a repeated bending test was performed to evaluate the toughness of the welded portion.

A carbon dioxide laser with an output of 5 kw was used as a laser oscillator, the beam was focused by a lens having a focal length of 254 mm, and 30 liters of Ar was sprayed as a processing gas at a rate of 2 to 2.5 m / min at a welding speed. Welded. As the filler, three kinds of wires (0.9 mmφ) for SUS 304, pure Ni, and Ni alloy shown in Table 2 were used, and these were added to the molten pool by a wire feeder at a feed rate of 0 to 6 m / min. When Ni wire is used, its addition ratio becomes the Ni content of the weld metal as it is.

In the repeated bending test, the test piece is sandwiched between bending jigs having arcuate curved surfaces with a radius of 60 mm facing each other on the left and right sides, and 90 ° repeated bending is performed left and right along the curved surfaces to make 20 reciprocations. Those that did not break after repeated bending were rated as good (◯), and those that did break were rated as bad (x).

The test results are shown in Table 3. In the same table, the protrusion height of the bead means that the height from the surface of the steel plate to the highest part of the bead was compared between the front side and the back side of the steel sheet, and the higher value was taken as the height. When the bead grinding was performed, it is indicated by a circle.

In Table 3, No. 1 to No. 5 are SUS 430
(Symbol: S430) for Ni wire (Ni (%) −Cr _eq ＝ 100
) Is used for laser welding. In No. 1, the addition amount of the filler was as small as 5%, the effect of improving the structure of the welded portion was not obtained, and the toughness was not sufficient. On the other hand, No.2-No.
No. 5 is an example of the present invention, and high toughness was obtained because the amount of filler added was appropriate (more than 7%). In all of these, the protrusion of the bead was small, and therefore the grinding was not performed.

No. 6 to No. 13 are the same laser welded steel (S430S) in which Si is added to SUS 430.
No. 7 has a small amount of filler added and has poor toughness,
In No. 8 to No. 11 and No. 13, the amount of filler added was appropriate and the toughness was good.

No. 12 is the case where the toughness was lowered due to the bead protruding, and No. 14 was the case where the bead produced in No. 12 was ground, and the toughness was improved by grinding.

No. 15 to No. 23 were obtained by similarly laser-welding SUS 430-added Nb-added steel (S430N). No. 15 and No. 16 were poor in toughness due to a small amount of filler added. However, No. 17 to No. 20 and No. 22 are examples of the present invention.
No. 21 is a comparative example in which the toughness was reduced due to bead protrusion, No. 2
3 is an example of the present invention when it is ground.

Nos. 24 to 26 were obtained by similarly laser-welding SUS 409 (S409), and all showed high toughness.

No. 27 to No. 30 deviate from the conditions of the present invention S
When US 304 wire (Ni (%) −Cr _eq = −31) is used and Cr content is relatively low, and SUS 409 (No.29)
Except that the toughness was not sufficient.

No. 31 to No. 34 are Ni satisfying the conditions of the present invention.
When the alloy wire (Ni (%)-Cr _eq = 34.5) was used, good toughness was obtained in all cases.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

[Table 3 (1)]

[0038]

[Table 3 (2)]

[0039]

By applying the method of the present invention, when ferritic stainless steel having a high Cr content is welded, coarsening of crystal grains can be prevented, and a welded portion having excellent toughness can be obtained. As a result, it is possible to prevent breakage of the coil splice welded portion during the line threading and decrease in productivity accompanying it.

[Brief description of drawings]

FIG. 1 is a diagram showing the composition of weld metal when laser welding ferritic stainless steel using SUS 304 wire or pure Ni wire as a filler on the Schaeffler structure diagram.

FIG. 2 is a diagram showing the toughness when laser welding various ferritic stainless steels with a mild steel filler wire or a SUS 304 filler wire.

FIG. 3 is a diagram showing the relationship between the Ni content of weld metal and the toughness of various ferritic stainless steels.

FIG. 4 is a diagram for explaining a change in bead shape due to a gap size and a filler addition amount during butt welding.

Claims (1)

[Claims] 1. A filler wire, a powder filler or a foil filler satisfying the following formula (1) is contained in the weld metal with Ni of 7
A method for welding ferritic stainless steel, characterized in that laser welding is performed while supplying an amount exceeding 5% by weight. Ni (wt%)-Cr _eq ≧ 24 (wt%) ・ ・ ・ (1) where Cr _eq =% Cr +% Mo + 1.5 x% Si + 0.5 x% Nb (wt%)