JPH0586440A - Stainless steel for automobile exhaust system excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To obtain an inexpensive stainless steel for automobile exhaust system excellent in corrosion resistance to the condensed and concentrated liquid of exhaust gas by specifying the composition consisting of C, Si, Mn, P, S, N, Al, Cr, Mo, Cu, and Fe. CONSTITUTION:This steel is a stainless steel for automobile exhaust system which has superior corrosion resistance and also has a composition consisting of, by weight, <=0.02% C, 0.01-0.2% Si, 0.05-1.50% Mn, <=0.025% P, <=0.010% S, <=0.015% N, 0.005-0.1% Al, 11-25% Cr, 0.5-5.0% Mo, <=0.1% Cu, and the balance Fe with inevitable impurities and further containing, if necessary, either or both of 5(C+N) to 0.5% Nb and 5(C+N) to 0.5% Ti and further either or both of 0.001-0.03% Ca and 0.001-0.03% Ce. This steel has superior resistance characteristics to internal corrosion under the environment of the condensed and concentrated liquid of exhaust gas in the engine exhaust system of automobile, etc.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stainless steel having excellent corrosion resistance in an environment where an exhaust gas condensed and concentrated liquid is produced in an automobile exhaust system.

[0002]

2. Description of the Related Art Conventionally, an aluminum-plated steel sheet obtained by subjecting ordinary steel to aluminum plating has been widely used as a material for a muffler which is a part of an automobile exhaust system. In an environment in which exhaust gas condensate concentrate is generated in an automobile exhaust system, aluminum in the outer layer of the aluminum-plated steel sheet elutes in a short time, and the aluminum-plated alloy layer formed by reacting with the iron of the steel sheet as the inner layer is Since it is noble to iron in terms of electric potential, its anticorrosion effect was not sufficient, and it was difficult to guarantee a long life.

On the other hand, in order to improve the corrosion resistance of the base material, C
A corrosion resistant steel containing 5 to 10% of r is disclosed in JP-A-63-14.
It is disclosed in Japanese Patent No. 3240 or Japanese Patent Laid-Open No. 63-143241. However, in these component systems,
At present, it has not been possible to meet the demand for extending the service life of automobile exhaust system members in recent years, and further high-grade high Cr steel was required.

[0004]

SUMMARY OF THE INVENTION The present invention has a high Rankford value (r ≧ 1.7), and has internal corrosion (local corrosion) in an exhaust gas condensed concentrated liquid environment in an exhaust system of an engine such as an automobile or a motorcycle. And general corrosion) to provide inexpensive stainless steel having excellent resistance characteristics.

[0005]

The subject matter of the present invention is as follows. (1) By weight, C ≦ 0.02%, Si: 0.01 to
0.2%, Mn: 0.05 to 1.50%, P ≦ 0.02
5%, S ≦ 0.010%, N ≦ 0.015%, Al:
0.005-0.1%, Cr: 11-25%, Mo:
0.5-5.0%, Cu ≤ 0.1%, balance Fe
And stainless steel for automobile exhaust system with excellent corrosion resistance consisting of unavoidable impurities.

(2) C ≦ 0.02% by weight, Si:
0.01-0.2%, Mn: 0.05-1.50%, P
≤0.025%, S≤0.010%, N≤0.015
%, Al: 0.005 to 0.1%, Cr: 11 to 25
%, Mo: 0.5 to 5.0%, Cu ≦ 0.1%, and Nb: 5 (C + N)% or more and 0.5% or less,
Ti: Stainless steel for an automobile exhaust system which contains 5 or more (C + N)% or more and 0.5% or less of 1 type or 2 types, and is composed of the balance Fe and inevitable impurities and has excellent corrosion resistance.

(3) C ≦ 0.02% by weight, Si:
0.01-0.2%, Mn ≦ 0.05-1.50%, P
≤0.025%, S≤0.010%, N≤0.015
%, Al: 0.005 to 0.1%, Cr: 11 to 25
%, Mo: 0.5 to 5.0%, Cu ≦ 0.1%, Ni: 0.1 to 1.0%, W: 0.05 to
0.5%, Zr: 0.05 to 0.5%, V: 0.05 to
0.5% of 1 or 2 or more, and Nb: 5
(C + N)% or more, 0.5% or less, Ti: 5 (C + N)
% Or more and 0.5% or less of 1 type or 2 types, and stainless steel for automobile exhaust system having excellent corrosion resistance, which comprises the balance Fe and unavoidable impurities.

(4) By weight, C ≦ 0.02%, Si:
0.01-0.2%, Mn ≦ 0.05-1.50%, P
≤0.025%, S≤0.010%, N≤0.015
%, Al: 0.005 to 0.1%, Cr: 11 to 25
%, Mo: 0.5 to 5.0%, Cu ≦ 0.1%, Ni: 0.1 to 1.0%, W: 0.05 to
0.5%, Zr: 0.05 to 0.5%, V: 0.05 to
0.5% of 1 or 2 or more, and Nb: 5
(C + N)% or more, 0.5% or less, Ti: 5 (C + N)
% Or more and 0.5% or less of one or two kinds, and further, Ca: 0.001 to 0.03%, Ce: 0.001 to
Stainless steel for automobile exhaust systems, which contains 0.03% of one or two kinds and has the balance Fe and unavoidable impurities and has excellent corrosion resistance.

The present invention will be described in detail below. The inventors of the present invention have exhaust systems for engines of automobiles, motorcycles, etc., especially center pipes after the center pipe, muffler,
In order to improve the corrosion resistance of so-called cold ends such as tail pipes and extend the service life of members, we analyzed the factors that corrode materials in the exhaust gas condensed concentrated liquid environment of the muffler part, and based on this result As a result of creating a simulated liquid and analyzing the change in liquid properties in the muffler using this, the condensed and concentrated liquid generated in the muffler is used to start the engine of an automobile,
It was found that the liquid properties of the liquid change drastically by being affected by the temperature change accompanying the stop.

In the exhaust gas condensed and concentrated liquid, SO is mainly contained.
_{^{4 2-, SO 3 2-, NO}} 3 -, NH 4 +, CO 3 2-, Cl -
And other organic acids are contained in trace amounts. When these are subjected to a heating cycle, they initially have a high pH (pH:
It was clarified that the condensed and concentrated liquid changed to around pH: 2 in an extreme case. Therefore, in order to prolong the corrosion-resistant life of the stainless steel used in the exhaust gas condensed and concentrated liquid environment of the muffler, it is necessary to have corrosion resistance in a wide environmental range from a neutral environment to an acidic environment. Further, the present inventors have clarified that the alloy design should be made by clarifying the relationship between the exhaust gas condensate-enriched liquid component and the alloy component that affect the corrosion behavior.

The inventors of the present invention have studied alloying components for this purpose, added Cr and Mo in appropriate ranges to the raw material, and added Cr or Cr in the exhaust gas condensate-enriched liquid.
-Understanding that the corrosion resistance of Mo-based materials is greatly affected by the amount of Cu, and therefore it is necessary to strictly control the amount of Cu, C
By strictly controlling the amount of u, 1) it is possible to prevent the occurrence of localized corrosion due to chloride ions in a neutral environment. 2) SO that tends to be concentrated by long-term use of the exhaust system.
General corrosion in an environment containing ₄ ^2- ions and SO ₃ ^2- ions,
It was found that local corrosion can be significantly prevented.

The present inventors have analyzed the actual exhaust system material in detail, and have examined the influence of Cr, Mo, Ti, and Nb, which are the main components of stainless steel, on the corrosion resistance of the member in the condensed and concentrated liquid environment. As a result of studying the effect of Ni, Cu, W, V, Zr, Ca, and Ce alone or in combination on the corrosion resistance of the member, the corrosion resistance of the present invention was improved by applying the stainless steel of the present invention to an actual exhaust system. It was discovered that Also, in the stainless steel manufacturing process of the present invention, by using the ordinary steel thin plate manufacturing process,
A stainless steel having a high Rankford value (r ≧ 1.7) can be produced, and a plate material or a pipe material having excellent workability can be obtained.

When the stainless steel of the present invention is used in an engine exhaust system for automobiles, motorcycles, etc., it exhibits excellent resistance characteristics against internal corrosion (local corrosion and general corrosion) caused by the exhaust gas condensation concentrated liquid. Hereinafter, the present invention will be described in more detail. The present inventors have conducted research to develop an exhaust system material having more excellent corrosion resistance and workability than conventional exhaust system materials for automobiles and the like, and as a result, the following points have been clarified. ..

That is, a) the Cr content of the base material is at least 1
1% is required b) In order to prevent intergranular corrosion due to precipitation of carbide in the welded portion, Ti ≧ 5 (C + N)% and / or Nb ≧ 5 (C
+ N)% is required c) The addition amount of Ti and Nb should be kept to the minimum necessary to prevent intergranular corrosion so that workability and secondary workability are not deteriorated, and the recrystallization temperature is controlled. The present inventors have made it clear that it can be manufactured by the ordinary steel manufacturing process while keeping it as low as possible.

Further, from the clarification by the present inventors that the corrosion resistance in an engine exhaust system of an automobile or the like in a condensed and concentrated liquid environment significantly deteriorates corresponding to the Cu content in steel, the Cu content is 0.1% or less. However, it should be kept to 0.05% or less. Further, since the steel of the present invention can be subjected to processing such as pipe making, it must be excellent in such workability or so-called secondary workability after pipe making. From such a viewpoint, in the steel of the present invention, the amount of Si is reduced to suppress hardening by solid solution Si as much as possible, and cold rolling is performed by a rolling machine having a large diameter work roll as in the manufacturing process of ordinary steel. By rolling
As in the conventional stainless steel sheet manufacturing process,
It makes it possible to produce stainless steel sheet with a high Rankford value (r value), which cannot be obtained by cold rolling with a rolling machine having a small diameter work roll, for example, Sendzimir mill.

Based on the above findings, the steel of the present invention has Cr: 11 to 25% and Mo: 0.5 to 5.0% as basic components by weight, and a Cu content of 0.1% or less, preferably. 0.0
5% or less, Ti ≧ 5 (C + N)%, Nb ≧
A component system containing 5 (C + N)% of 1 type or 2 types and the balance Fe and unavoidable impurities was prepared. The steel of the present invention characterized by this component system exhibits excellent corrosion resistance and workability as an engine exhaust system member.

In the present invention, Ni: 0.1 to 1.0% is added to the above component system in order to impart more excellent corrosion resistance.
W: 0.05 to 0.5%, V: 0.05 to 0.5%, Z
r: 0.05 to 0.5% of 1 type or 2 types or more is added. Further, in the present invention, Ca: 0.001 to 0.
03%, Ce: 0.001-0.03% of 1 type or 2
The seeds are added to suppress the generation of Mn-based inclusions, which are the starting points of local corrosion, in the exhaust gas condensation concentrated liquid environment in the engine exhaust system, and improve the corrosion resistance of steel.

Next, the reasons for limiting the components will be described. Cr
Is a basic component for ensuring the corrosion resistance of the stainless steel under the exhaust gas condensed and concentrated liquid environment in the engine exhaust system, and requires a Cr content of at least 11% in combination with Mo. Corrosion resistance of steel according to increase of Cr addition amount,
Although the oxidation resistance is improved, when the Cr content exceeds 25%, the effect is not only saturated, but it also makes the production of steel material difficult and increases the production cost.

Like Cr, Mo is a basic component for ensuring the corrosion resistance of stainless steel under the exhaust gas condensed and concentrated liquid environment in the engine exhaust system, and contains at least 0.5% Mo in combination with Cr. Quantity is needed. Addition of Mo to Cr-containing steel significantly improves the local corrosion resistance of stainless steel in an environment containing sulfate ions and chloride ions. In addition, ammonium ions (NH ₄ ⁺ ), sulfite ions (SO ₃ ^2- ) and organic acids (HCHO, HCOO)
H, CH ₃ COOH, etc.) in order to improve the corrosion resistance (general corrosion resistance, local corrosion resistance) of steel in an environment where
Addition of Mo to Cr-containing steel is extremely effective. Although the corrosion resistance of steel improves as the amount of Mo added increases, 5.
If it exceeds 0%, the production of steel becomes difficult and the production cost is increased.

Cu significantly impairs the corrosion resistance of the Cr-containing steel in the exhaust gas condensed and concentrated liquid environment in the engine exhaust system, especially in the environment where organic acids such as NH ₄ ⁺ , SO ₃ ²⁻ , and HCHO coexist. , Its content must be strictly regulated. According to the knowledge of the present inventors, the Cu content is 0.
If it exceeds 1%, the corrosion resistance of the Cr-Mo steel of the present invention is significantly impaired. Therefore, the Cu content is at most 0.1%, preferably 0.05% or less.

C, Si, Mn, P and S may be within the range that can be obtained under ordinary steelmaking conditions. That is, C causes intergranular corrosion and cracking of ferritic stainless steel, so 0.0
It should be 2% or less, and as low as possible. Also, the lower the C content is, the better the strength, workability, and toughness of the base material. However, if the C content is made extremely low, the steelmaking time is lengthened, the productivity is lowered, and the cost is increased. Let
Therefore, from the viewpoint of corrosion resistance and workability of steel, the C content is 0.02% or less, and is made as low as possible in consideration of productivity and manufacturing cost.

Si has a deoxidizing effect on steel, but 0.01
If it is less than%, the effect cannot be expressed. On the other hand, if the Si content exceeds 0.2%, the workability of steel deteriorates. From the viewpoint of the surface treatability and workability of steel, the Si content is 0.2% or less. Although Mn does not adversely affect the corrosion resistance of steel in the exhaust gas condensed and concentrated liquid environment in the engine exhaust system, it is 0.05% or more and 1.50% or less, which is the content in ordinary stainless steel. To do.

Since P adversely affects the corrosion resistance of steel in the exhaust gas condensed and concentrated liquid environment in the engine exhaust system,
The content is preferably as low as 0.025% or less. Since S also affects the corrosion resistance in the exhaust gas condensed and concentrated liquid environment, the smaller the content, the better. The upper limit was set to 0.010% as a normal steelmaking condition for stainless steel. As with N, N also forms carbonitrides in the weld heat affected zone and causes intergranular corrosion, so the smaller the amount, the better. The upper limit was 0.015%.

Ti fixes C and N and prevents deterioration of corrosion resistance of stainless steel. Coexists with Ca and fixes O, S
It suppresses the formation of i and Mn-based oxides and improves hot workability and corrosion resistance. If it exceeds 0.5%, the hot workability is significantly deteriorated. Further, the Ti amount needs to be 5 (C + N)% or more from the relationship between the Ti amount and the (C + N) amount that affect the intergranular corrosion resistance, so this is the lower limit.

Nb fixes C and N similarly to Ti and prevents deterioration of corrosion resistance of stainless steel. If it exceeds 0.5%, the hot workability is deteriorated. Further, from the evaluation result of the intergranular corrosion property, the amount of Nb is required to be 5 (C + N)% or more, so this was made the lower limit. Al is added as a deoxidizing agent in the range of 0.1% or less. If it exceeds 0.1%, the corrosion resistance and hot workability deteriorate. If it is less than 0.005%, there is no effect.

Since the addition of W improves the corrosion resistance in the exhaust gas condensed and concentrated environment, it is added in an amount of 0.5% or less as necessary. If it exceeds 0.5%, the effect is saturated, and 0.05%.
Less than is ineffective. Addition of V improves the corrosion resistance in the exhaust gas condensed and concentrated liquid environment, so 0.5% is added as necessary.
Add below. If it exceeds 0.5%, the effect will be saturated,
If it is less than 0.05%, there is no effect.

Since the addition of Zr improves the corrosion resistance in the waste gas condensed and concentrated liquid environment, it is added in an amount of 0.5% or less as necessary. If it exceeds 0.5%, the effect will be saturated and 0.05
If it is less than%, there is no effect. Ca and Ce fix O in the low-sulfur content steel similarly to Al, and suppress the formation of MnS-based inclusions that are the starting points of local corrosion generation in the exhaust gas condensate-concentrated liquid,
Improves corrosion resistance. Ca and Ce are 0.001-
One type or two types are added as needed in the range of 0.03%.

Ni undergoes a heating cycle like an exhaust gas condensed and concentrated liquid to change its liquid property, and particularly improves the corrosion resistance in an environment where the pH drops to around 2 to 3, so 0.1% or more of 1 Add less than 0.0%. If it is less than 0.1%, there is no effect, and if it exceeds 1.0%, the toughness of the welded portion deteriorates.

[0029]

EXAMPLES Tables 1 to 4 show the results of comparison of the corrosion resistance between the conventional steel and the steel of the present invention. Each material is melted on a trial basis, hot-rolled, cold-rolled, annealed, and then the surface is polished with # 320, 50 ×
It processed into 70 mm and used for the test. The prepared samples were evaluated by the following tests.

(1) The evaluation of the corrosion resistance by the condensed and concentrated liquid on the inner surface of the exhaust system is as follows. (A) Since the test incorporating the environmental change due to the heating cycle most simulates the muffler environment, Local corrosion depth after repeated 50 cycles (dry and wet cycle of placing test piece in semi-immersed state for 4 hours in boiling state and then standing for 2 hours as 1 cycle), and (a) exhaust gas condensed concentrated solution heating cycle The liquid property changes drastically by receiving the water, and the pH is expected to drop to around 2 to 3 when used for a long period of time. The general corrosion behavior of was evaluated by the corrosion weight loss after 50 hours.

The condensed and concentrated liquid used in the tests (a) and (b) was analyzed by actually condensing and concentrated liquid collected from the exhaust system of the automobile, and based on the result, it was used as a laboratory accelerating liquid. The solution set as above was used. Test condensate concentrated solution; Sulfate ion 5000ppm, Sulfite ion 1000pp
m, carbonate ion 3000 ppm, nitrate ion 100 pp
m, acetate ion 10 ppm, chloride ion 1000 pp
m, all added with ammonium salts. The pH was around 8-9 but was not prepared.

From the evaluation results of Tables 2 and 4, the steel of the present invention containing Cr-Mo coexistent has excellent characteristics in both general corrosion and local corrosion as compared with the conventional Cr base steel. However, when the Cu content exceeds 0.1%, the general corrosion rate and the local corrosion depth are increased. The steel of the present invention whose Cu content is specially controlled in this manner sufficiently exhibits its corrosion resistance function in a range where the environment changes in a wide range from neutral to weakly acidic depending on the usage conditions such as automobile exhaust gas condensation concentrated liquid. To be done.

From the general corrosion characteristics and the local corrosion characteristics shown in Table 1, the steels of the present invention showed excellent corrosion resistance in all steel types as compared with the comparative steels. This indicates that the steel of the present invention exhibits excellent corrosion resistance for a long period of time in a condensate-concentrated liquid environment with severe corrosiveness such as engine exhaust gas, and is extremely effective in practice. The local corrosiveness evaluations in Tables 2 and 4 were evaluated according to the above (A) and (A).

Among the workability, the Rankford value is used as a workability evaluation standard for pipes and plate materials. Since the r value can be significantly improved by using the ordinary steel manufacturing process, the r value of 1.70 or more was used as the standard. Further, as a secondary workability evaluation, after cold working from a plate thickness of 0.6 mm to 0.4 mm, tight bending was performed, and 1 depending on the degree of cracking.
A ranking of (no cracking) to 6 (severe cracking) was performed. Rank 1 was targeted from the standpoint of workability of plates and tubes.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

[Table 3]

[0038]

[Table 4]

[0039]

EFFECTS OF THE INVENTION According to the present invention, it is possible to provide a stainless steel which exhibits excellent corrosion resistance in an exhaust gas condensed and concentrated liquid environment in an engine exhaust system and is excellent in workability, which is industrially beneficial. There are some big ones.

Claims (4)

[Claims] 1. C ≦ 0.02% by weight, Si: 0.0
1 to 0.2%, Mn: 0.05 to 1.50%, P ≦ 0.
025%, S ≦ 0.010%, N ≦ 0.015%, A
1: 0.005-0.1%, Cr: 11-25%, M
o: Stainless steel for an automobile exhaust system, which contains 0.5 to 5.0% and Cu ≦ 0.1%, and which is excellent in corrosion resistance and is composed of the balance Fe and unavoidable impurities. 2. C ≦ 0.02% by weight, Si: 0.0
1 to 0.2%, Mn: 0.05 to 1.50%, P ≦ 0.
025%, S ≦ 0.010%, N ≦ 0.015%, A
1: 0.005-0.1%, Cr: 11-25%, M
o: 0.5 to 5.0%, Cu ≤ 0.1%, Nb: 5 (C + N)% or more, 0.5% or less, Ti: 5
(C + N)% or more and 0.5% or less 1 type or 2 types, and stainless steel for automobile exhaust systems excellent in corrosion resistance which consists of the balance Fe and unavoidable impurities. 3. By weight, C ≦ 0.02%, Si: 0.0
1 to 0.2%, Mn ≦ 0.05 to 1.50%, P ≦ 0.
025%, S ≦ 0.010%, N ≦ 0.015%, A
1: 0.005-0.1%, Cr: 11-25%, M
o: 0.5 to 5.0%, Cu ≤ 0.1%, Ni: 0.1 to 1.0%, W: 0.05 to 0.5%,
Zr: 0.05 to 0.5%, V: 0.05 to 0.5%, containing one or more kinds, and further Nb: 5 (C + N).
% Or more, 0.5% or less, Ti: 5 (C + N)% or more,
Stainless steel for automobile exhaust systems, which contains 0.5% or less of one or two kinds and has the balance Fe and unavoidable impurities and is excellent in corrosion resistance. 4. By weight, C ≦ 0.02%, Si: 0.0
1 to 0.2%, Mn ≦ 0.05 to 1.50%, P ≦ 0.
025%, S ≦ 0.010%, N ≦ 0.015%, A
1: 0.005-0.1%, Cr: 11-25%, M
o: 0.5 to 5.0%, Cu ≤ 0.1%, Ni: 0.1 to 1.0%, W: 0.05 to 0.5%,
Zr: 0.05 to 0.5%, V: 0.05 to 0.5%, containing one or more kinds, and further Nb: 5 (C + N).
% Or more, 0.5% or less, Ti: 5 (C + N)% or more,
It contains 0.5% or less of one or two, and further Ca:
0.001-0.03%, Ce: 0.001-0.03
% Stainless steel for automobile exhaust systems, which contains 1% or 2% and has the balance Fe and unavoidable impurities and has excellent corrosion resistance.