JPH06248394A - Ferritic stainless steel for automotive exhaust system apparatus excellent in high temperature salt damage corrosion resistance - Google Patents

Abstract

PURPOSE:To provide ferritic stainless steel useful as the material for automotive exhaust system apparatus having corrosion resistance in a high temp. environment in a chlorides-adhering state. CONSTITUTION:The ferritic stainless steel for automotive exhaust system apparatus excellent in high temp. salt damage corrosion resistance is one having compsn. contg., by weight, 0.001 to 0.015% C, 0.001 to 0.015% N, 0.50 to 2.00%. Si, 0.01 to 1.00% Mn, 0.010 to 0.030% P, <=0.003% S, 10.0 to 14.0% Cr, one or two kinds of Nb and Ti: 20X(%C+%N)<=%Nb+%Ti<=0.6% and contg., at need, 0.3 to 2.0% in total of one or two kinds of Mo and Ni, <=0.1% in total of one or >=two kinds selected from the group of Ca and rare earth elements and 0.05 to 0.5% Al, and the balance iron with inevitable impurities.

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 for automobile exhaust system equipment which is excellent in high temperature salt corrosion corrosion resistance, and more specifically, as a material for automobile exhaust system equipment, especially in front and center pipe parts. The present invention relates to a ferritic stainless steel having excellent corrosion resistance in a high temperature environment of around 650 ° C. with chloride attached.

[0002]

2. Description of the Related Art As a material excellent in oxidation resistance and workability used for automobile exhaust system equipment, conventionally, C 0.010% or less, N 0.010% or less, C + N 0.015% or less, S
i 1-3%, Mn 1% or less, Cr 15-26%, if necessary a small amount of Ti, Zr, Nb, Al, 1 of rare earth element.
Ferritic stainless steels containing more than one species are known.
(Japanese Patent Laid-Open No. 53-2328)

In addition to corrosion corrosion on the outer surface by rock salt spraying, which is carried out as a measure for snow melting on the road in winter in automobile exhaust system equipment, high-temperature oxidation and high-temperature salt corrosion on exhaust manifolds, front center pipes, etc.,
There are different corrosions such as inner surface side condensed liquid corrosion due to exhaust gas cooling in the muffler, and intergranular corrosion of the welded joint, and materials having different characteristics are used for each part.

12 at the front and center pipes
Welded pipes such as SUS410L and SUH409, which are% Cr steels, were used. However, due to the increase in exhaust gas temperature and the like, in addition to high temperature oxidation resistance, high temperature salt corrosion corrosion resistance of the outer surface has been required, and it has been difficult to secure corrosion resistance with conventional steels.

A typical part exposed to a high temperature salt corrosion environment is a flexible pipe part, to which an austenitic stainless steel containing Cr and Ni is applied, and a material having an excellent high temperature salt damage resistance property has been developed.

[0006] However, in the low-cost ferritic stainless steel containing almost no Ni, there are many materials excellent in high temperature oxidation resistance as compared with the conventional ones, but few materials considering high temperature corrosion in an environment where chloride is attached. In the midst of extending the life and cost of automobiles, inexpensive and highly corrosion-resistant ferritic stainless steel has been required.

[0007]

DISCLOSURE OF THE INVENTION The present invention considers the use in a front pipe and a center pipe part in an automobile exhaust system, in particular, while suppressing the addition amount of an alloy component as much as possible, the oxidation resistance and the condensation resistance required as an automobile exhaust system material. An object of the present invention is to provide a ferritic stainless steel which has not only liquid corrosion resistance but also excellent high temperature salt corrosion resistance, and also mechanical properties that enable easy pipe forming.

[0008]

Means for Solving the Problems In view of the above-described corrosion environment of automobile exhaust system equipment, the present inventors have improved high-temperature corrosion resistance of ferritic stainless steel for automobile exhaust system equipment in a chloride adhesion environment. From the viewpoint that it is essential to improve the corrosion resistance of steel itself depending on the composition, it will adversely affect the mechanical properties of steel used for automobile exhaust system equipment and each of the important costs. We have been considering measures to improve corrosion resistance. As a result, in order to improve the high temperature salt damage corrosion resistance in the environment of the automobile exhaust system, the material contains Cr in a specific range, and C, N and Nb and Ti as stabilizing elements are limited to a specific range. By adding Si, Mo, and Ni within the weight range within a specific range, the high temperature salt corrosion resistance of the material is significantly improved compared to conventional steel, and the characteristics as a material for automobile exhaust system equipment are further improved. Knowledge was obtained.

That is, the structure of the present invention as means for solving the problems is as follows. (1) C: 0.001 to 0.015% by weight,
N: 0.001 to 0.015%, Si: 0.50 to 2.00
%, Mn: 0.01 to 1.00%, P: 0.010 to 0.0
30%, S: 0.003% or less, Cr: 10.0 to 14.
0%, any one or two of Nb and Ti: 20 ×
(% C +% N) ≦% Nb +% Ti ≦ 0.6%, the balance being iron and inevitable impurities, and a ferritic stainless steel for automobile exhaust system equipment excellent in high temperature salt corrosion resistance.

(2) The ferritic stainless steel according to the above (1), characterized in that it contains one or two of Mo and Ni in a total amount of 0.3 to 2.0% by weight.

(3) The total amount of one kind or two kinds or more selected from the group consisting of Ca and rare earth elements is 0.1% by weight.
The ferritic stainless steel according to the above (1) or (2), which contains 1% or less.

(4) Al of 0.05 to 0.1% by weight.
5% is contained, The ferritic stainless steel in any one of said (1) thru | or (3) characterized by the above-mentioned.

[0013]

[Operation] In the steel of the present invention, the reason why the content ratios of the constituents are numerically limited as described above will be explained below.

(A) C, N When C and N in the steel increase in content, not only the deterioration of mechanical properties and corrosion resistance but also the corrosion resistance of the heat-affected zone formed during welding are adversely affected. To C: 0.01
By limiting the amount to 5% or less and N: 0.015% or less, the above-mentioned adverse effects can be suppressed to a practically acceptable level. In addition, since extremely decreasing the content causes an increase in manufacturing cost, C: 0.001 or more, N: 0.001%
That is all.

(B) Si In Si, Si is an effective deoxidizing element and is also effective for improving the oxidation resistance, but it is an element that exerts an excellent effect on the high temperature corrosion resistance particularly in a chloride adhesion environment. Therefore, as the content increases, the high temperature salt damage resistance property improves. The content is required to be 0.50% or more in order to improve the high temperature salt corrosion resistance, but if it exceeds 2.00%, the steel becomes harder than necessary and the cold workability is deteriorated. Content is 0.5
It was limited to 0 or more and 2.00% or less.

(C) Mn Mn in steel is an effective deoxidizing element and has the effect of improving hot workability. However, if its content is high, it causes deterioration of corrosion resistance, so it is preferably low. Therefore, the Mn content is 1.00%
Limited to:

(D) P P in steel is an impurity element. Particularly, when the content exceeds 0.030%, the weldability deteriorates, so the P content is limited to 0.030% or less.

(E) S in S steel tends to remarkably deteriorate the corrosion resistance, and it is desirable that the content thereof be low. The allowable upper limit is 0.
It is 0030% or less.

(F) Cr Steel Cr is an important element that determines the basic corrosion resistance of ferritic stainless steel. In order to secure the high temperature oxidation resistance and the exhaust gas condensate corrosion resistance in the environment where chlorides do not coexist in the front and center pipe parts, 10% or more is required. However, this element has little effect of improving the high temperature corrosion resistance in a chloride-adhering environment, and adding more than necessary will increase the cost, so the upper limit was made 14%.

(G) Ti, Nb Ti and Nb in steel are added to prevent deterioration of corrosion resistance and deterioration of toughness at the welded part due to contamination of C and N due to external factors during welding. Further, Ti and Nb have the effect of suppressing the crystal grains of the base metal and the crystal grains of the heat-affected zone of welding, and Ti and Nb are equivalent elements. In order to exert such an effect,% Ti
+% Nb ≧ 20 × (% C +% N)% is required. However, when Ti and Nb are present in a large amount, Laves
Since phase precipitation becomes remarkable and deterioration of toughness becomes noticeable, the upper limit of its content is limited to% Ti +% Nb ≦ 0.6%.

(H) Mo, Ni In steel, Mo and Ni are effective elements for exerting high temperature corrosion resistance in a chloride adhering environment, and are positively added if necessary. Mo and Ni are equivalent elements for exerting such effects, and it is necessary to add one or two kinds of them in a total amount of 0.3% or more in order to obtain a predetermined addition effect. However, in order to prevent cost increase, the upper limit of the total amount of one or two kinds is set to 2.0% or less.

(I) Ca and rare earth elements Ca and rare earth elements have the effect of enhancing the high temperature oxidation resistance because they improve the adhesion of the oxide film formed by high temperature oxidation. Is an even element. However, if added excessively, it precipitates at the grain boundaries and adversely affects the hot workability, and the allowable upper limit value is 0.1%.

(J) Al In the Al steel, it is a known technique to add Al for the purpose of improving the oxidation resistance, but in the steel of the present invention, it is added for the purpose of increasing the high temperature strength. However, when Al is added in an amount of 0.05% or more, the high temperature strength increases, but when it exceeds 0.5%, the high temperature strength decreases, so the lower limit is made 0.05% or more and the upper limit is 0.5% or less. And

[0024]

【Example】

[Example 1] First, ferritic stainless steel having the composition shown in Table 1 was cast, each ingot was forged after the casting, and then hot rolling was performed at 1200 ° C. Further, the hot-rolled sheet thus obtained was annealed and air-cooled, then cold-rolled to 1.5 mm, and subjected to 940 ° C. finish annealing, and then processed into a test piece.

[0025]

[Table 1]

The test piece thus prepared was subjected to a high temperature salt corrosion test under the conditions shown in FIG. 4, and the test results are also shown in Table 1. The test results are shown by the amount of reduction in plate thickness. From the results shown in Table 1, it is understood that the steel of the present invention has excellent high temperature salt damage corrosion resistance.

The effects of addition of Si, Mo and Ni in steel are shown in FIGS. 1 and 2. From these results, it can be confirmed that Si, Mo and Ni are effective elements for high temperature salt corrosion resistance.

Example 2 Of the ferritic stainless steels having the composition shown in Table 1, steels having different Si contents were subjected to the normal temperature tensile test, and the test results are shown in Table 2 together with the corresponding compositions. It was FIG. 4 shows the relationship between the elongation of the plate and the Si content in the room temperature tensile test. It can be seen from FIG. 4 that the steel of the present invention having a Si content of 2.0% or less has excellent high temperature salt corrosion resistance and good elongation characteristics, and can be easily pipe-formed.

[0029]

[Table 2]

[0030]

As described above, the steel of the present invention exhibits excellent corrosion resistance in the front and center pipes of an automobile exhaust system and, at the same time, sufficiently satisfies other properties required for an automobile exhaust system steel. Therefore, it is possible to contribute to the further improvement of the durability of the automobile without causing a significant increase in cost, which brings industrially useful effects.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the Si content in a ferritic stainless steel having the composition shown in Table 1 and the reduction in plate thickness by a high temperature salt corrosion test.

FIG. 2 is a graph showing the relationship between the Mo + Ni content in ferritic stainless steels having the composition shown in Table 1 and the reduction in plate thickness by a high temperature salt corrosion test.

FIG. 3 shows the elongation and S of the plate in the normal temperature tensile test shown in Table 2.
It is a graph which shows the relationship with i content.

FIG. 4 is a flow chart showing high temperature salt damage test conditions for ferritic stainless steel according to the present invention.

Claims (4)

[Claims] 1. C: 0.001 to 0.015 in% by weight.
%, N: 0.001 to 0.015%, Si: 0.50 to 2.
00%, Mn: 0.01 to 1.00%, P: 0.010
0.030%, S: 0.003% or less, Cr: 10.0
14.0%, any one or two of Nb and Ti: 2
Ferrite stainless steel for automobile exhaust system excellent in high temperature salt corrosion corrosion resistance, characterized by 0 × (% C +% N) ≦% Nb +% Ti ≦ 0.6% and the balance being iron and unavoidable impurities. 2. One or two of Mo and Ni in weight%.
The ferritic stainless steel according to claim 1, wherein the total content of the seeds is 0.3 to 2.0%. 3. The ferrite according to claim 1 or 2, characterized in that the total amount of one or more selected from the group consisting of Ca and rare earth elements is 0.1% or less by weight. Stainless steel. 4. The ferritic stainless steel according to claim 1, wherein the ferritic stainless steel contains 0.05 to 0.5% by weight of Al.