JPH0735556B2 - Ferritic stainless steel with excellent high temperature strength and toughness in the heat affected zone - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless steel which is excellent in high-temperature strength and toughness of a weld heat affected zone.

[0002]

2. Description of the Related Art Conventionally, ferritic stainless steel is often used as a heat and oxidation resistant material. This is because ferritic stainless steel has (1) a lower coefficient of thermal expansion than austenitic stainless steel, that is, it has excellent characteristics (thermal fatigue characteristics) in an environment in which it is repeatedly heated. (2) Joining to other parts (steel or casting) is easy. (3) It has excellent oxidation resistance in an environment where it is repeatedly heated. (4) It is inexpensive. Because there are advantages such as.

[0003]

However, since the ferritic stainless steel is inferior to the austenitic stainless steel in high temperature strength and toughness, the range of use is naturally limited, and high temperature strength and toughness are problems. It is difficult to adapt to the intended use.

In the problem of toughness, VOD or AO has been used in recent years.
Due to the extremely low C and N technologies such as D, and the addition of stabilizing elements such as Ti and Nb, generally no problem occurs in the base metal part, but especially when welding is involved, There is a problem that toughness deteriorates. Welded parts are generally melted parts and parts that have been subjected to high temperature heat history (welding heat affected parts)
In the former, the degree of deterioration of toughness is small due to the control of the sealing method, welding heat input, selection of the optimum wire, etc.

Further, in the ERW pipe forming method which has been widely used in automobile exhaust pipes in recent years, the melted portion is discharged to the outside by the upset process, which is a kind of pressure welding.
Therefore, in ERW pipe making, the toughness of the weld zone is dominated by the toughness of the weld heat affected zone.

The problems of the ferritic stainless steel (high-temperature strength, toughness of the weld heat affected zone) have prevented the expansion of applications of the ferritic stainless steel.

[0007] Therefore, an object of the present invention is to provide a ferritic stainless steel which enhances the high temperature strength of the ferritic stainless steel and at the same time has excellent toughness in the weld heat affected zone.

[0008]

The present inventors have conducted various experiments and studies on the above-mentioned problems, namely (i) high temperature strength and (ii) toughness of a weld heat affected zone, and as a result, Nb, Mo
The inventors have found that when a small amount of Co is added to the ferritic stainless steel containing alloy, the toughness of the weld heat affected zone is significantly improved while having improved high temperature strength, and the present invention has been completed.

[0009] The present inventors have also found that by adding Ti and Co to Nb and Mo-containing ferritic stainless steel in combination, the toughness of the weld heat affected zone is further improved while having improved high temperature strength. Then, the present invention was completed.

That is, in the present invention, C: 0.02 wt% or less, Si: 1.0 wt% or less, Mn: 1.0 wt% or less, Mo: 0.05 wt% or more and 2.5 wt% or less, Cr: 14 wt% 20 wt% or more, N: 0.015 wt% or less, Nb: 0.4 wt% or more and 1.0 wt% or less, Co: 0.01 wt% or more and 0.25% or less, and the balance is Fe and inevitable impurities The present invention provides a ferritic stainless steel excellent in high temperature strength and toughness of a heat affected zone.

Further, Ti may be added in an amount of 0.5 wt% or less.

The present invention will be described in more detail below.

C: 0.02 wt% or less C is harmful to the toughness of the heat-affected zone of welding, but as will be described later, the addition of Co, and the addition of Co and Ti will significantly improve the toughness. There is no problem in practical use.
Especially when it is desired to improve the toughness, 0.01 wt% or less is desirable. Also, the lower the better, the better the high temperature strength.

Si: 1.0 wt% or less Si is effective in improving the oxidation resistance, but if it exceeds 1.0 wt%, the toughness of the weld affected zone deteriorates, so the upper limit is 1.
It was set to 0 wt%.

Mn: 1.0 wt% or less Mn is preferably as small as possible from the viewpoint of workability, but the upper limit was limited to 1.0 wt% in consideration of economical efficiency during manufacturing.

Mo: 0.05 wt% or more and 2.5 wt% or less Mo is added to improve high temperature strength. The effect is 0.
It appears at more than 05 wt% but saturates at 2.5 wt%. 0.05 ~ 2.5wt% because it is a very expensive element
Limited to

Cr: 14 wt% or more and 20 wt% or less Cr is the main element that imparts oxidation resistance, but 14 wt%
If it is less than 20% by weight, sufficient oxidation resistance is not obtained, and if it exceeds 20% by weight, the toughness and workability are significantly deteriorated.

N: 0.015 wt% or less Like C, it is harmful to the toughness of the welded portion, but 0.015 wt% or less.
If it is less than wt%, there will be no practical problem.

Nb: 0.4 wt% or more and 1.0 wt% Nb is added to improve high temperature strength. Generally, addition of Nb deteriorates toughness, but the present inventors have found that addition of Co or composite addition with Co—Ti significantly improves toughness. If it is less than 0.4 wt%, the high temperature strength is not sufficient, and if it exceeds 1.0 wt%, the toughness improving effect is insufficient even if Co addition or Co—Ti composite addition is performed, so the upper limit was limited to 1.0 wt%.

Co: 0.01 wt% or more and 0.25 wt%
Hereinafter, Co is a very important element for the present invention. FIG. 2 shows the Charpy-absorbed energy after heat treatment that simulates the toughness of the heat affected zone. 0.01 wt% Co
There is also a significant toughness improving effect by the addition of. The Charpy absorbed energy in FIG. 2 is a value at 0 ° C. If this value is 8 kgf / cm ² or more, it can be determined that the toughness is sufficient even if the processing including the welded portion is performed in winter. . Therefore, the upper limit is 0.25 wt%.

Ti: 0.5 wt% or less Ti is also a very important element in the present invention. Ti-
The effect of improving the toughness of the weld heat affected zone becomes more remarkable by the addition of Co composite. The effect of such Ti and Co will be shown in the column of Examples, but it is preferably 0.5 wt% or less.

[0022]

EXAMPLES The present invention will be specifically described below based on examples.

(Example 1) Sample materials having various compositions shown in Table 1 were prepared in a laboratory from a 30 kg steel ingot by hot rolling and annealing cold rolling as a cold rolled sheet having a thickness of 2.0 mm. 10
After annealing at 00 ° C, pickling was performed. After that, 850 ° C. high temperature tensile test and welding heat affected zone (hereinafter referred to as H
A simulation heat treatment was performed and a 0 ° C. Charpy test was performed. Usually, the high temperature tensile test is often measured by a hot rolled sheet or a round bar after casting, but the strength is greatly affected by the precipitation state of the material, so the actual use state, that is, the cold rolled annealed sheet, It was used and measured. The Charpy absorbed energy was evaluated by the average value of n = 3 at 0 ° C.

The HAZ simulation heat treatment is
It was carried out by air cooling at 1250 ° C. The HAZ toughness of the actual TIG weld was measured using the material shown in FIG. 1, and the absorbed energy was 2.5 kgf · m / cm ² or less, so it was 1250 ° C. × 10 minutes air cooling. Was the HAZ simulation heat treatment.

As shown in FIG. 2, the toughness after such a HAZ simulation is Co 0.01 to 0.
Remarkably good in the range of 25 wt%.

As a result of the above-mentioned test using the materials having the chemical components shown in Table 1, the tensile strength and HA at 850 ° C.
Table 2 shows the toughness after the Z simulation.

In contrast to SUS430LX shown as a comparative steel, the copper No. of the present invention was used. 1-3 and No. 7, 8, 9,
The Nb, Mo, and Co-added steels of 10, 11, and 12 have markedly improved strength and toughness. In particular, like Comparative Steel C, N
Although the tensile strength is improved by b and Mo, the toughness of the HAZ simulation is remarkably low when Co is not added. Further, even if Co is added like Comparative Steels A, B, D and E, if C, Nb, Mo and N are excessive, recovery of toughness by Co is not sufficient.

Even when Co is excessive as in Comparative Steel F, FIG.
As shown in, the toughness is deteriorated. In addition, T like the comparative steel G
Even if i is added, the toughness is not improved without adding Co, but the steel of the present invention No. 3 and No. As in the comparison of 5, Ti
It is understood that the toughness is remarkably improved by the addition of —Co composite. The invention steel No. As shown in Nos. 4 and 6, the high temperature strength and the HAZ toughness are very good by the Ti-Co composite addition. Therefore, it is clear that the steel within the scope of the present invention is excellent in high temperature strength and toughness of the weld heat affected zone.

[0029]

[Table 1]

[0030]

[0031]

INDUSTRIAL APPLICABILITY According to the present invention, by adding an appropriate amount of Co or Ti, Co to Nb and Mo-added steel, high temperature strength is improved, and a ferritic stainless steel excellent in toughness of a weld heat affected zone is obtained. Can be provided. Therefore, when it is used for a part to be processed including a welded portion such as a pipe of an automobile exhaust system or various combustion appliances, problems such as cracking from a heat-affected zone of welding and creep due to insufficient high temperature strength can be eliminated. .

[Brief description of drawings]

FIG. 1 Basic steel 0.01C-0.01N-18Cr-
0.6 Nb-1.0 Mo, 1050 ° C., 115
It is a figure which shows the value of the Charpy absorbed energy (average of n = 3) after water cooling or air cooling hold | maintained at 0 degreeC, 1250 degreeC, and 10 minutes.

FIG. 2 Basic steel 0.01C-0.01N-18Cr-
It is a figure which shows the influence of the amount of Co which affects the Charpy absorbed energy (average of n = 3) after 1250 degreeC and 10-minute hold air cooling in 0.6Nb-1.0Mo.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-2-175843 (JP, A) JP-A-2-175821 (JP, A) JP-A-1-8254 (JP, A) JP-A-57- 126954 (JP, A) JP 49-59722 (JP, A)

Claims (2)

[Claims] 1. C: 0.02 wt% or less, Si: 1.0 wt% or less, Mn: 1.0 wt% or less, Mo: 0.05 wt% or more and 2.5 wt% or less, Cr: 14 wt% or more and 20 wt% or less N: 0.015 wt% or less, Nb: 0.4 wt% or more and 1.0 wt% or less, Co: 0.01 wt% or more and 0.25% or less, and the balance being Fe and inevitable impurities. Ferritic stainless steel with excellent high temperature strength and toughness in the heat affected zone. 2. The ferritic stainless steel according to claim 1, further containing Ti in an amount of 0.5 wt% or less.