JPH08283913A - Ferritic stainless steel excellent in weldability - Google Patents

Abstract

PURPOSE: To provide a ferritic stainless steel which has a weld zone, free from welding material, having fine-grained structure and capable of providing superior toughness and ductility and has excellent weldability. CONSTITUTION: This steel is a ferritic stainless steel, having a composition consisting of, by weight 0.005-0.02% C, 0.01-1.0% Si, 0.01-1.0% Mn, 10.5-24.0% Cr, 0.61-2.0% Ni, 0.001-0.03% Al, Ti in the amount in the range of (1.5×10<-4> ×[Cr])/[N] to 0.6%) where [Cr] and [N] mean respective contents (%) of Cr and N}, 0.015-0.04% N, 0.005-0.02% O, and the balance iron with inevitable impurity elements, or a ferritic stainless steel having a composition in which either or both of 0.1-2.25% Mo and 0.01-0.5% Nb are further incorporated into the composition of the above stainless steel.

Description

Detailed Description of the Invention

[0001]

The present invention relates to Cr: 10.5 to 24.
It relates to ferritic stainless steel containing 0% and having excellent weldability.

[0002]

2. Description of the Related Art Ferritic stainless steels are less expensive than austenitic stainless steels, and are also excellent in strength and resistance to stress corrosion cracking in a chloride environment. Generally, ferritic stainless steel is used as a thin plate, and in its welding, there are many cases where welding is performed without adding a welding material. Specifically, TIG tanning welding without using a welding material, resistance seam welding, etc., but in that case, due to the coarsening of the crystal grains of the weld structure, the ductility of the weld and the decrease in toughness are major problems. is there. The workability required for thin sheets is also reduced due to the low ductility of the weld.

In order to improve the reduction of ductility and toughness of such a ferritic stainless steel welded portion, for example, Japanese Patent Publication No. 55-
As disclosed in Japanese Patent No. 47102, Japanese Patent Laid-Open No. 2-107744, etc., restrictions on the amounts of C and N, Ti, Nb,
Addition of stabilizing elements, such as JP-B-55-4710
As disclosed in Japanese Patent Laid-Open No. 2-109809 and Japanese Patent Laid-Open No. 50-109809, techniques such as addition of Al have been invented. In these inventions, since C and N are harmful to toughness and ductility in ferrite, C and N are kept low from the beginning (high purification), or C and N are changed to Ti, Nb and A in the solid phase.
It is intended to be fixed by l. On the other hand, even if these conventional techniques are used, in the case where a member including a welded part is subjected to strong working, insufficient ductility of the welded part often remains a problem. That is, the fundamental problem is that the solidification structure of the weld is
It is significantly coarser than the base metal.

[0004]

On the other hand, the ductility of the welded portion is further increased with the expansion of the application of ferritic stainless steel for automobile exhaust systems and home appliances, and the diversification of the design and design of the welded structure. A ferritic stainless steel excellent in toughness is strongly desired. The present invention has been made from such a background, and an object of the present invention is to provide a ferritic stainless steel excellent in ductility and toughness of a welded portion than before.

[0005]

Means for Solving the Problem That is, the present inventors have used ferritic stainless steels of various component systems,
TIG tanning welding without using any welding material was carried out, the weld structure, ductility and toughness of the weld were investigated, and the effect of the constituent elements of the steel on them was examined. As a result, unlike the conventional knowledge, the N content of the steel material is not suppressed to a low level, but rather, a certain appropriate level is secured and an appropriate amount of Ti corresponding to that is added, whereby equiaxed crystallization and fine grain of the weld structure are achieved. It has been newly revealed that the improvement of weldability and ductility of welds is achieved. Also, in this case, the addition of Al is rather harmful to the refinement of the structure and the improvement of ductility, while the addition of N
It was also clarified that the addition of i was effective. further,
It was also found that the effect of grain refining was not sufficiently obtained when the oxygen level of the steel material was extremely low.

That is, the present invention is a ferritic stainless steel having the following composition range, and the grain refinement of the TIG lap weld is achieved, and the toughness and ductility of the weld can be improved accordingly. (1)% by weight, C: 0.005 to 0.02%, S
i: 0.01 to 1.0%, Mn: 0.01 to 1.0%,
Cr: 10.5 to 24.0%, Ni: 0.61 to
2.0%, Al: 0.001-0.03%, T
i: (1.5 × 10 ⁻⁴ × [Cr]) / [N] or more, 0.
6% or less However, [Cr] and [N] are Cr content (%) and N content (%) N: 0.015 to 0.04%, O: 0.005 to 5%, respectively.
Ferritic stainless steel containing 0.02% and the balance iron and unavoidable impurity elements. (2) Mo: 0.1 to 2.25 in addition to the stainless steel of the preceding paragraph.
%, Nb: 0.01-0.5% of 1 type or 2 types contained, The ferritic stainless steel characterized by the above-mentioned.

[0007]

The reason for limiting each component in the present invention will be described below. Unless otherwise specified, the welded portion means a welded portion obtained by melting the steel material itself without adding a welding material.

C: C is detrimental to corrosion resistance, but must be contained to some extent from the viewpoint of strength. If the amount of ultra-low C is less than 0.005%, the manufacturing cost becomes high, and 0.02%
If it exceeds%, the workability and toughness are remarkably deteriorated, and the as-welded state and the reheating cause it to combine with Cr and the like, and remarkably deteriorate the corrosion resistance of these regions. Therefore, it is limited to 0.005 to 0.02%.

Si: Si is added as a deoxidizing agent and a strengthening element, but if it is less than 0.01%, its effect is not sufficient, while if it exceeds 1.0%, the impact toughness decreases and at the time of welding. Reduces melt penetration. Therefore, 0.
It was limited to 01 to 1.0%.

Mn: Mn is also added as a deoxidizing element,
If less than 0.01%, the effect is not sufficient, while 1.0%
If it exceeds the above level, hard martensite is generated at the grain boundaries of the weld metal as it is solidified. Therefore, it is limited to 0.01 to 1.0%.

Cr: Cr is a ferrite-forming element,
It is the main element that imparts corrosion resistance. If it is less than 10.5%, sufficient corrosion resistance cannot be obtained, while if it exceeds 24.0%, the toughness and ductility of the steel material are greatly deteriorated. Therefore, 10.5
It was limited to ~ 24.0%.

Ni: Ni is an element effective in toughness and ductility. Further, addition of 0.61% or more is indispensable for grain refinement of the welded portion, but addition of more than 2.0% increases martensite generation in the welded portion, and conversely reduces toughness and ductility. Therefore, it is limited to 0.61 to 2.0%.

Al: Al is a deoxidizing element and is also effective for hot workability. It is necessary to add 0.001% or more, but if it exceeds 0.03%, the melt penetration of the welded portion is reduced and the toughness is deteriorated. Therefore,
It was limited to 0.001 to 0.03%.

Ti: Ti is a strong carbonitride-forming element, and particularly during welding, it forms a nitride in the molten material and plays a major role in making the weld particles finer. From experiments using various alloy components, it was found that the grain refinement of the weld depends on the Ti content, N content, and Cr content. As a result of recursive calculation, the lower limit of the Ti amount is represented by (1.5 × 10 ⁻⁴ × [Cr]) / [N]. Here, [Cr] and [N] are Cr content (%) and N content (%), respectively. On the other hand, if the content of Ti exceeds 0.6%, the hot workability at the time of manufacturing the base material deteriorates.

N: N, as described above, combines with Ti during welding and contributes to grain refinement of the welded portion, thereby improving toughness and ductility. In order to secure the effect, the content of 0.015% or more is necessary. On the other hand, the content of more than 0.04% also bonds with Cr and deteriorates the corrosion resistance. Therefore, it is limited to 0.015 to 0.04%.

O: O is contained in an amount of 0.005% or more, which deepens the penetration of the weld metal as a surface active element, and serves to improve the weldability and the ductility of the welded portion due to its bead shape.
However, if the amount of O exceeds 0.02%, deoxidation will eventually be insufficient, and conversely, the ductility and toughness of the welded portion will be impaired. Therefore, 0.
It was limited to 005 to 0.02%.

The present invention also includes one or two of Mo and Nb.
Can contain seeds. Mo: Mo is an element that improves corrosion resistance particularly in a chloride environment, and addition of 0.1% or more is effective, but 2.2
If it exceeds 5%, the ductility and toughness of the base material and the welded part deteriorate. Nb: Nb is combined with C to suppress carbide precipitation particularly in the welded portion and improve corrosion resistance. The addition of 0.01% or more is effective, but the addition of more than 0.5% reduces the ductility and toughness of the base material and the welded portion.

[0018]

EXAMPLES The effects of the present invention will be specifically described below based on examples. Steel with the chemical composition shown in Table 1 was melted by vacuum melting, and hot rolling, hot rolled sheet annealing, pickling, cold rolling, finish annealing, and pickling were performed under known conditions to obtain a sheet thickness of 1.0 mm. The steel plate of After performing melt run TIG welding on the steel sheets under the conditions shown below, the average crystal grain size corresponding to a circle was calculated from a cross-sectional microscope of the welded portion, and the ductility of the welded portion was measured by the Erichsen test. Table 2 shows the results. Welding condition: Welding current 60A Welding voltage 10V Welding speed 70cm / min Tungsten electrode diameter 2.4mm Electrode tip angle 40 ° Shield gas Torch Ar 20 l / min After Ar 35 l / min Back Ar 3 l / min

[0019]

[Table 1]

[0020]

[Table 2]

Regarding the microstructure of the weld metal, in the comparative steel, ferrite crystal grains coarsely extend from the boundary of the fusion zone and are so-called butt-solidified at the center of the weld metal, whereas in the steel of the present invention, from the boundary of the fusion zone. Grain growth is suppressed, and most of the weld metal consists of equiaxed ferrite crystal grains. Therefore, in the former case, the ferrite sometimes has a considerably large crystal grain size corresponding to an elongation circle of 1 mm or more, but in the latter case, it is equiaxed and is about 100 μm. It is considered that the difference in elongation in the Erichsen test reflects this, and the Erichsen values of the welds of the present invention steels are all 9 to 10 mm, which is clearly improved from that of the comparative steel, and the value of the base metal A value equal to or close to (10 to 11 mm) is shown. Further, the fracture position in the Erichsen test is relatively random in the weld metal in the steel of the present invention, whereas in the comparative steel, it sharply fractures along the line of butt solidification at the center of the weld. The effect of grain refining in the steel of the present invention and the adverse effect on the ductility of coarse grain and butt solidification in the comparative steel can be understood from this.

The present invention relates to a ferritic stainless steel excellent in the characteristics of a welded portion not using a filler metal. Therefore, applicable welding methods include TIG welding without a filler, plasma welding, Laser welding and electron beam welding are possible. In that case, TIG welding is backside welding, plasma, laser, and electron beam welding are full penetration welding, and the range of weldable conditions is determined by the plate thickness, target welding speed, beam power, etc. in any case.

[0023]

As described above, the present invention makes it possible to provide a ferritic stainless steel in which coarse grains in the welded portion, which has been a problem in the past, are suppressed and ductility of the welded portion is improved. However, the industrial effect can be said to be extremely large.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Nobutaka Yurioka 20-1 Shintomi, Futtsu City, Chiba Nippon Steel Corporation Corporate Technology Development Division

Claims (2)

[Claims] 1. By weight%, C: 0.005-0.02%, Si: 0.01-1.0%, Mn: 0.01-1.0%, Cr: 10.5-24. 0%, Ni: 0.61 to 2.0%, Al: 0.001 to 0.03%, Ti: (1.5 × 10 ⁻⁴ × [Cr]) / [N] or more,
0.6% or less However, [Cr] and [N] are Cr content (%) and N content (%), respectively. Ferritic stainless steel containing N: 0.015 to 0.04%, O: 0.005 to 0.02%, and the balance being iron and inevitable impurity elements. 2. The stainless steel according to claim 1, further containing one or two of Mo: 0.1 to 2.25% and Nb: 0.01 to 0.5% by weight. Ferritic stainless steel characterized by.