JPH09316606A - Ferritic stainless steel for automotive exhaust system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an inexpensive ferritic stainless steel for an automotive exhaust system excellent in cold formability, high temp. strength and high temp. oxidation resistance. SOLUTION: This stainless steel has a compsn. contg., by mass, 0.01% C and <=0.02% N so as to regulate the content of C+N to <=0.02% or 0.009 to 0.02%, contg. <0.5% Si, 0.5 to 1.2% Mn, 10 to 12.5% Cr and 0.05 to 0.3% Nb, furthermore contg. Ti in the range of 8×(C+N) to 0.3, and the balance Fe with inevitable impurities.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to moldability at room temperature and
The present invention relates to a ferritic stainless steel for an automobile exhaust system, which is excellent in strength at high temperature and resistance to high-temperature oxidation, and can keep manufacturing cost low.

[0002]

2. Description of the Related Art Various material properties are required for automobile exhaust system materials, but high strength at high temperatures is an important property because they are exposed to high temperatures for a long time. High temperature oxidation resistance is also important, but especially when it is kept at a high temperature and then cooled to room temperature, the oxide scale peels off and clogs the exhaust system, and the thickness of the material itself decreases. At issue, oxidation scale delamination resistance is an important property. Further, since the plate material is processed into a complicated shape at room temperature, room temperature moldability is also an important property. Furthermore, from the recent demand for material cost reduction, it is also important that the material is cheap. As a material for automobile exhaust system, development of a material having all of these characteristics is expected.

In response to such requirements, many steels intended for use in automobile exhaust systems have been developed,
For example, Japanese Patent Laid-Open No. 3-72053 discloses C ≦ 0.03.
%, Si ≦ 1%, Mn ≦ 1%, Cr: 13-16%, A
1: 2 to 5%, Ti and / or Nb: 0.1 to 1.0
%, Steel containing Ni ≦ 0.5% in impurities is disclosed. This steel has high thermal fatigue properties, and due to the addition of Al, it is also waiting for excellent oxidation resistance, but since it contains a large amount of Al, cracks such as bald spots easily occur when rolling the steel. However, the labor required for caring for the surface of the steel sheet increases, the production yield decreases, and the production cost increases.

In Japanese Patent Laid-Open No. 3-264652, C
≦ 0.02%, S <0.50%, Mn ≦ 1.0%, P <
0.05%, S ≦ 0.005%, Cr: 11.0 to 3
0.0%, N ≦ 0.015%, Nb: 0.3 to 0.5
%, Ti: 0.05 to 0.2% of steel is disclosed. This steel reduces the amount of solid solution C and N in the steel by the combined addition of Nb and Ti, and is excellent in formability at room temperature. It is also excellent in high temperature properties such as high temperature strength and high temperature oxidation resistance.
However, since a large amount of expensive Nb is contained, the alloy cost becomes high, and the demand for a cheap material is not satisfied.

Further, Japanese Patent Laid-Open No. 6-184705 discloses S.
i: 0.01 to 0.50%, Mn: 0.01 to 0.50
%, Cr: 11.0 to 14.0%, P ≦ 0.030%,
S <0.0050%, C + N: 0.010% to 0.02
0%, Ti + Nb: 0.4% to 0.6%, Ti + Nb ≧
A 30 × (C + N) steel is disclosed. In this steel, by defining the balance of C, N content and Ti, Nb content,
In particular, it has excellent intergranular corrosion resistance in the heat-affected zone of the welding and can be manufactured at low cost, but its high Mn content results in high-temperature oxidation resistance (especially oxidation scale peeling resistance). Inferior

Further, in JP-A-6-248394, C: 0.001 to 0.015%, N: 0.001 to 0.
015%, Si: 0.50 to 2.00%, Mn: 0.0
1 to 1.00%, P: 0.010 to 0.030%, S ≦
0.003%, Cr: 10.0 to 14.0%, Nb, T
Disclosed is a steel in which one or more of i is 20 × (C + N) to 0.6%. This steel is excellent in high temperature salt damage resistance and high temperature oxidation resistance due to the addition of Si, and is also excellent in formability at room temperature. However, since it contains a large amount of Si, cracks such as a canopy can easily occur during rolling of steel, resulting in an increase in labor required for maintenance of the surface of the steel sheet, a decrease in production yield, and an increase in production cost.

Further, Japanese Patent Laid-Open No. 3-294417 discloses C
<0.013%, Si <0.75%, Mn <1%, C
r: 11-22%, N <0.03%, Nb: 0.1
Steel with 1.0% is disclosed. This steel is excellent in high-temperature strength due to the addition of Nb and the manufacturing cost is low, but a large amount of solid solution C and N remains in the steel, and solid solution strengthening reduces the elongation of the steel sheet at room temperature. There is a problem that the room-temperature processability and moldability are poor.

Further, in JP-A-6-145906, Cr: 11 to 30%, Ti, Nb, Zr, Ta, W,
One or more of V and P: 0.06 to 0.5%, Cu: 0.3
Steels of up to 2.0% are disclosed. This steel has excellent condensed water corrosion resistance due to the addition of P, but since it contains a large amount of expensive Cu in order to improve corrosion resistance and acid resistance, the alloy cost increases and the manufacturing cost increases. .

[0009]

The steel according to the conventional development technique as described above is very excellent for a certain specific property or a plurality of properties among various properties required for materials for automobile exhaust systems. However, it does not necessarily satisfy all the required characteristics. The present invention
It is an object of the present invention to provide a ferritic stainless steel excellent in all important properties as a material for automobile exhaust system.

[0010]

DISCLOSURE OF THE INVENTION The inventors of the present invention have improved all the properties such as workability at room temperature, high temperature strength, and high temperature oxidation resistance required for automobile exhaust system materials, and are inexpensive ferrite-based materials. The composition balance of stainless steel was investigated in detail. As a result, by reducing C and N and adding Ti, the amount of solute C and N can be lowered, the workability at room temperature and the formability can be improved, and the high temperature strength can be enhanced even with a small amount of Nb added. It is possible to secure the necessary substantially solid solution Nb amount, it is possible to increase the high temperature strength without significantly increasing the alloy cost, and by limiting the Si content, the maintenance labor caused by cracking during rolling can be reduced. Suppress increase, decrease in yield,
Moreover, by defining the Mn content, Al and Si, which cause cracking during rolling, and C, which increases the alloy cost, are added.
The present invention was accomplished based on the finding that high temperature oxidation resistance (particularly oxidation scale peeling resistance) can be improved without adding a large amount of u.

That is, the gist of the present invention is C in mass%:
0.01% or less, N: 0.92% or less and C +
N is 0.02% or less or 0.009 to 0.02%, Si: less than 0.5%, Mn: 0.5 to 1.2
%, Cr: 10 to 12.5%, Nb: 0.05 to 0.3
%, And Ti: 8 × (C + N) to 0.3%, with the balance being Fe and unavoidable impurities, which is a ferritic stainless steel for automobile exhaust systems.

The reasons for limiting the components will be described below. C: Need to be 0.01% or less. 0.01% of C
If it is contained in a large amount over the range, the cold formability of steel deteriorates. N: Need to be 0.02% or less. N is 0.02%
If it is contained in a large amount over the range, the cold formability of steel deteriorates.

It is necessary that the total amount of C and N be 0.02% or less after the C and N amounts are regulated in such a range. As will be described later, C and N are fixed by adding Ti to reduce the amount of solid solution C and N and improve the workability at room temperature, but the total amount of C and N exceeds 0.02% and is contained in large amounts. Then, it becomes necessary to add a large amount of Ti, which increases the alloy cost. However, the amount of C and N is 0.0
If it is 09% or less, the melting cost of steel becomes high. Therefore, when there is a great need to reduce the melting cost of steel, it is preferable to set the C + N amount to more than 0.009%.

Si: Need to be less than 0.5%. S
i is added as a deoxidizing agent, but if it is contained in a large amount of 0.5% or more, it deteriorates the workability of steel, induces cracks during rolling, increases maintenance labor, reduces the production yield, and reduces the production cost. Is expensive.

Mn: It is necessary to exceed 0.5% and 1.2% or less. In order to form a spinel type oxide scale and to obtain excellent oxidation scale peeling resistance, at least 0.
Addition of more than 5% is required. However, 1.2%
This is because if it is added in a large amount in excess of the above, MnS is generated and the corrosion resistance decreases.

Cr: It is necessary to be 10% or more and 12.5% or less. It is a basic element of stainless steel and must be contained in an amount of 10% or more in order to obtain excellent corrosion resistance. However, even if the content exceeds 12.5%, the required corrosion resistance is already satisfied, which merely causes an increase in alloy cost.

Nb: 0.05% or more and less than 0.3% is required. A content of at least 0.05% is necessary to obtain excellent high temperature strength. However, the content is 0.3
%, The toughness of steel deteriorates significantly and alloy cost rises. Further, as the recrystallization temperature rises, high-temperature finish annealing is required, which causes an increase in energy cost. Therefore, the upper limit needs to be less than 0.3%.

The amount of Ti: C + N must be 8 times or more and 0.3% or less. In order to combine with C and N, to substantially reduce the amount of solute C and N in the steel, and to improve the room-temperature formability, it is necessary to contain at least 8 × (C + N). Also 0.3
%, The fixation of C and N has been sufficiently achieved, the hot workability of steel is remarkably deteriorated, cracks such as bald defects are likely to occur, and alloy cost is increased. Because.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Strengthening and hardening of steel are mentioned as factors that hinder the workability and formability at room temperature. On the other hand, by reducing the C and N contents, it is possible to suppress solid solution strengthening due to solid solution of C and N in the steel. Just C,
There is a limit to the reduction of N, and the extreme reduction makes the melting cost high, so there is a limit to the reduction of the C and N contents only. Therefore, by adding 8 times or more of (C + N) amount of Ti, the C and N remaining in the steel are fixed in the form of carbonitride of Ti, and the amount of solid solution C and N is further reduced. The workability and moldability at room temperature are improved.

Regarding high temperature strength, Nb is added to form a solid solution N.
The increase in strength due to b is effective. However, Nb has a high alloy cost and further raises the recrystallization temperature of the steel sheet. Therefore, it is desirable to reduce the amount of Nb added as much as possible. Since Nb is an element that easily forms carbonitrides like Ti, if a large amount of solute C and N remains in the steel, Nb
b forms a carbonitride and cannot secure the amount of solid solution Nb that contributes to the increase in high temperature strength. On the other hand, the developed steel reduces the C and N contents as described above, and further contains C and N due to added Ti.
By fixing N, the amount of solid solution C and N is suppressed as much as possible, and even with a small amount of Nb added, the amount of solid solution Nb necessary for increasing the high temperature strength is secured.

Regarding the high temperature oxidation resistance, it is general to reduce the amount of oxidation by adding Si. However, adding a large amount of Si is not desirable because it causes cracks such as bald spots during rolling of the steel sheet. In the case of automobile exhaust system materials,
Due to the thermal history of holding at high temperature and cooling to normal temperature, the amount of oxide scale exfoliation becomes more problematic than the increase in oxidation amount. The element effective for the oxidation-resistant scale exfoliation is Mn. Cr ₂ O ₃ is the main oxide scale of stainless steel, and scale peeling occurs due to the difference in the coefficient of thermal expansion between this and the steel base. On the other hand, the addition of Mn produces a so-called spinel-type oxide scale having a thermal expansion coefficient intermediate between that of Cr ₂ O ₃ and the steel base material, the adhesion between the base material and the oxide scale is enhanced, and the oxide scale peeling resistance is improved. Si is effective as a deoxidizing material when steel is melted, but its oxide can be a starting point of cracking during rolling. By defining the upper limit of the amount of Si added, it is possible to prevent cracking during rolling.

[0022]

EXAMPLE 12 kinds of steels A to L having the composition as shown in Table 1 were melted in a vacuum melting furnace and cast into an ingot,
Hot rolling, cold rolling and finish annealing were performed to obtain a steel plate having a thickness of 1.5 mm.

[0023]

[Table 1]

Test pieces were cut out from these steel sheets, and the average r value was measured as an index of workability at room temperature, a high temperature tensile test was used as an index of high temperature strength, and 800 ° C. × 500 hours was maintained as an index of resistance to oxidation scale peeling. The room temperature-cooled atmospheric oxidation test was performed to measure the scale peeling amount. Table 2
As the results of these tests, the average r value, the proof stress (MPa) at 800 ° C., the presence or absence of scale delamination during the atmospheric oxidation test, and the presence or absence of cracking during rolling are also shown. The steel according to the method of the present invention exhibits excellent properties in terms of formability at normal temperature, high temperature strength, and resistance to oxide scale delamination, and cracks do not occur during rolling.

[0025]

[Table 2]

[0026]

According to the present invention, it is possible to inexpensively obtain a steel having excellent properties such as room temperature formability, high temperature strength and high temperature oxidation resistance required for automobile exhaust system materials. The contribution and significance of the present invention to industry is extremely great.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Nobuhiro Fujita 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Co., Ltd. Technology Development Division (72) Inventor Shigeru Maeda 20-1 Shintomi, Futtsu-shi, Chiba Made by Shinnihon Iron & Steel Co., Ltd.

Claims (2)

[Claims] 1. In mass%, C: 0.01% or less, N: 0.02% or less, and C and N are 0.02% or less, Si: less than 0.5%, Mn: 0. 5 to 1.2%, Cr: 10 to 12.5%, Nb: 0.05 to 0.3%, and Ti: 8 × (C + N) to 0.3%, with the balance being Fe. A ferritic stainless steel for automobile exhaust systems, characterized by comprising unavoidable impurities. 2. In mass%, C: 0.01% or less, N: 0.02% or less, and C + N is 0.009 to 0.02%, Si: less than 0.5%, Mn: 0. 0.5 to 1.2%, Cr: 10 to 12.5%, Nb: 0.05 to 0.3%, and Ti: 8 × (C + N) to 0.3%, with the balance being Ferritic stainless steel for automobile exhaust systems, characterized by comprising Fe and inevitable impurities.