JPH07268554A - Ferritic stainless steel for automobile exhaust system excellent in formability and heat resistance - Google Patents

Abstract

PURPOSE:To produce a ferritic stainless steel in which formability and heat resistance are secured by subjecting a stainless steel having a specified componental compsn. to hot rolling and thereafter regulating the form factor of the crystalline grains in the cross section of the sheet thickness parallel to the rolling direction. CONSTITUTION:The componental compsn. of a stainless steel is constituted of the one contg., by weight, <=0.03% C+N, 0.05 to 2.0% Si, 0.1 to 2.0% Mn and 10 to 25% Cr, furthermore contg. at least one kind of 0.02 to 0.5% Ti and 0.05 to 1.0% Nb in the ranges so as to satisfy C/12+N/14<=Ti/48+Nb/93, and the balance Fe with inevitable impurities. This steel is subjected to hot rolling, and in a state being subjected to annealing after the hot rolling or as hot-rolled, the form factor of the crystalline grains in the cross section of the sheet thickness parallel to the rolling direction is regulated to <=5. Thus, the material for an automobile exhaust system can be provided.

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 sheet which is excellent in formability and heat resistance after hot-rolling and after annealing or as hot-rolling, as a material for automobile exhaust systems. Is.

[0002]

2. Description of the Related Art The exhaust gas temperature has reached 900 ° C. along with the improvement of fuel efficiency and output of automobiles, and it is said that the temperature will further increase in the future. It is also trying to improve fuel efficiency and purify exhaust gas by reducing weight and reducing heat. Against this background, automobile exhaust system materials are also required to have improved heat resistance, compactness and lightweight components, and improved moldability at the same time.

Conventionally, SUS40 has been used as an automobile exhaust system material.
Cold rolled materials such as 9D, SUS430J1L and SUS436L are used, and from the viewpoint of improving formability, hot rolled sheet annealing is applied in the manufacturing process, or a method of increasing the rolling reduction by cold rolling is used. Has been. For example, in JP-A-3-264652, the condition is that hot-rolled sheet annealing is performed. As described above, in order to improve the workability, it is taken as an important condition to anneal the hot rolled sheet and further increase the cold rolling reduction.

However, high pressure in hot rolling annealing or cold rolling is economically disadvantageous, and automobile exhaust system materials are relatively thick among thin sheets, and the sheet thickness is 1.0 to 2. 0m
m is often used. Considering this, it is difficult to obtain a high-pressure reduction rate in cold rolling. further,
Regarding the heat resistance, especially the high temperature strength, the morphology of precipitates of Ti and Nb has a great influence. Therefore, it is not desirable from the viewpoint of precipitate control that the heat history is diverse and complicated.

On the other hand, it is not necessary to strictly control surface characteristics such as surface glossiness and roughness of automobile exhaust system materials, and it is sufficient that the surface is such that fatigue characteristics are not adversely affected. Conceivable.

[0006]

DISCLOSURE OF THE INVENTION The present invention provides a ferritic stainless steel as a material for an automobile exhaust system, which secures formability and heat resistance after hot rolling and annealing or as hot rolling. The purpose is to do.

[0007]

As described above, in the automobile exhaust system material, from the viewpoint of improving workability, hot rolled sheet annealing or cold rolling under a relatively high pressure is applied in the manufacturing process to obtain a cold rolled sheet. Are being created. However, it is economically disadvantageous to anneal hot-rolled sheets or to apply high pressure to cold-rolling, and since automobile exhaust system materials are thick among thin sheets, it is difficult to obtain a high-pressure reduction rate in cold rolling. In addition, it is desirable to simplify the thermal history as much as possible from the viewpoint of controlling deposits.

Further, since the automobile exhaust system material and the like are mounted on the lower portion of the vehicle body, it is not necessary to strictly control the surface glossiness, ridging and roping.
No. 1, Japanese Patent Publication No. 9852/1992, Japanese Patent Publication No. 4-98
Unlike the technique described in Japanese Patent No. 53, it is not necessary to consider ridging resistance. For the above reason, in the present invention, the heat history is simplified by switching the cold rolled steel sheet to the hot rolled steel sheet, and the heat resistance and the formability are secured by defining the crystal grain shape factor: A ratio (aspect ratio). Is intended.

First, as a component system, it was basically based on lowering the solid solution C and the solid solution N when ductility and r value were particularly required. Further, Ti and Nb were added alone or in combination, and C and N were fixed to further improve the workability. Here, the addition amount of Ti and Nb is C +
The lower limit was limited so that it was excessive with respect to the amount of N. Also,
Ti and Nb also contribute to high temperature strength and oxidation resistance improvement.

Further, by defining the A ratio (aspect ratio) as a shape factor of crystal grains, workability and heat resistance are improved. That is, the workability is ensured by equiaxing the crystal grain shape in a certain range, and at the same time, the solid solution amount of the high temperature strengthening element in the product plate is promoted by remelting the precipitate. For high temperature strengthening, it is common to add Nb or Mo. When improving the high temperature strength and the thermal fatigue resistance, the existing state of Nb and Mo becomes a particular problem.
That is, it is effective for high temperature strengthening to have Nb and Mo dissolved in solid solution in "Materials and Processes" VOL. 4 (1
991) p. 1796 etc. Therefore,
In order to improve the high temperature strength and heat fatigue resistance, a step of suppressing precipitation of these elements in each heat treatment step as much as possible in order to secure the amount of solid solution Nb and the amount of solid solution Mo in the state of the final product plate. It is necessary to set the conditions. For this reason, it is desirable to simplify the heat history, and it is important to secure the amount of solid solution in the product plate.

For the above reasons, in the present invention, the crystal grain shape factor A ratio (aspect ratio =
By defining the number of grains per unit length in the plate thickness direction / the number of grains per unit length in the rolling direction to be 5 or less, the forming process can be performed in the state after hot rolling annealing or in the hot rolling as it is. It is intended to obtain a ferritic stainless steel for automobile exhaust systems, which has excellent properties and thermal fatigue resistance.

[0012]

FUNCTION C + N: C and N deteriorate the moldability and have a strong affinity with Ti and Nb to lower the high temperature strength. Further, since it is an austenite former, it must be kept low from the viewpoint of suppressing martensite formation, and the upper limit of the amount of C + N was set to 0.03%. Also,
When the effect of further improving the workability and improving the high temperature strength by the solid solution Nb is utilized, the amount of C + N is preferably 0.015% or less.

Si: Si is a deoxidizing element, and is added in an amount of 0.05% or more in order to improve oxidation resistance. on the other hand,
Addition of Si in excess of 2.0% decreases ductility and hardens the steel (increases yield stress), so 2.0%
Was set as the upper limit. Mn: Mn is a deoxidizing element, so 0.1% or more was added. On the other hand, Mn deteriorates the oxidation resistance and is also an austenite former, so the upper limit was made 2.0% from the viewpoint of suppressing martensite formation.

Cr: Cr secures corrosion resistance, which is the basic performance of stainless steel, and at the same time, assures oxidation resistance capable of coping with a high exhaust gas temperature of an automobile, that is, at least 600 ° C. To achieve this, 10% or more is added. On the other hand, when the amount of Cr exceeds 25%, the effect of further improving the corrosion resistance and the oxidation resistance becomes small and the workability also deteriorates, so the upper limit was made 25%. Further, if considering both the oxidation resistance up to 1000 ° C. and the processability, 10 to 20% is sufficient, and this range is economically desirable.

Ti: Ti is used to fix C and N, and to improve the moldability, the content is 0.02% or more, and C / 12.
The range was + N / 14 ≦ Ti / 48 + Nb / 93. Further, Ti contributes to high temperature strengthening by producing fine precipitation during hot rolling. However, addition of more than 0.5% causes coarsening of the precipitate and reduces high temperature strength. And Further, in order to improve the workability, T of 0.1% or more is used.
i addition is desirable. Nb: Nb fixes C and N similarly to Ti, and is 0.05% or more and C / 12 + N / 14 ≦ Ti / 48 + Nb / 9 in order to fix C and N and improve moldability.
The range was set to 3. Further, Nb contributes to high temperature strengthening by solid solution strengthening, but addition of more than 1.0% deteriorates toughness, so 1.0% was made the upper limit. Further, Nb is preferably added in an amount of 0.2% or more from the viewpoint of improving workability and high temperature strength, and is preferably 0.6% or less from the viewpoint of improving toughness.

Mo: Mo is an element that enhances high temperature strength and high temperature salt damage resistance, but since it deteriorates ductility and weldability,
The range was 0.1 to 2.0%. Further, from the viewpoint of high temperature strength and high temperature salt damage resistance, it is desirable to add 0.4% or more of Mo. Crystal grain shape factor A of the plate thickness cross section parallel to the rolling direction: The crystal grain shape factor (aspect ratio: A) of the plate thickness cross section parallel to the rolling direction is a parameter for ensuring the formability. In the as-hot-rolled material, the band structure remains, and the ductility becomes low especially in the room temperature tensile test.
In order to ensure ductility, the crystal grain shape factor A of the plate thickness cross section parallel to the rolling direction is defined to be 5 or less. This is a parameter indicating the degree of equiaxing of crystal grains, and takes a large value when the ratio of remaining band structure is high, and takes a value of 1 when completely equiaxing.

[0017]

[Examples] Table 1 shows the chemical composition of the test steel. The sample steel is
50 kg of each was melted by vacuum melting and hot-rolled, or hot-rolled and then annealed to produce a 2 mmt hot-rolled sheet and hot-rolled annealed sheet. Each test piece was sampled from the obtained hot rolled sheet and hot rolled annealed sheet and subjected to various tests. The test results are shown in Table 2. The workability was evaluated by adhesion bendability. The steels of the present invention show good adhesion bending properties with an A ratio of 5 or less regardless of the presence or absence of hot-rolled sheet annealing. Further, a thermal fatigue test was conducted on a bendable material. As a result, the thermal fatigue life of the steel of the present invention was
9 and SUS430LX cold rolled annealed sheets have almost the same life, and it is considered that the steel of the present invention has a surface roughness inferior to that of the conventional cold rolled annealed sheets. It can be said that the effect of roughness is small.

[0018]

[Table 1]

[0019]

[Table 2]

Note) Bending property judgment criteria ◎: good, ○: small amount of wrinkles, Δ: wrinkles occurred, ×: many wrinkles, with micro-cracks In the thermal fatigue test, a 2 mm thick plate-shaped test piece was used, and 200 ° C.
→ 800 ° C. × 30 seconds → 200 ° C. The temperature was raised and lowered in a cycle of 60 seconds and ruptured when the load dropped below 10% of the maximum load (in many cases, buckling occurred).

[0021]

EFFECTS OF THE INVENTION According to the present invention, it is possible to provide a ferritic stainless steel as a material for an automobile exhaust system, in which the formability and heat resistance are ensured after hot rolling and annealing or as hot rolling. .

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akio Yamamoto 20-1 Shintomi, Futtsu City, Chiba Shin Nippon Steel Co., Ltd. Technology Development Headquarters (72) Inventor Keiichi Omura 1-1 Hibahata-cho, Tobata-ku, Kitakyushu, Fukuoka No. New Nippon Steel Co., Ltd., Yawata Works

Claims (5)

[Claims] 1. By weight%, C + N: 0.03% or less Si: 0.05 to 2.0% Mn: 0.1 to 2.0% Cr: 10 to 25%, and Ti: 0. 0.02 to 0.5% Nb: 0.05 to 1.0% of at least one kind is contained in a range satisfying C / 12 + N / 14 ≦ Ti / 48 + Nb / 93, and the balance Fe and unavoidable impurities are contained in the steel. Excellent in formability and heat resistance, characterized in that the crystal grain shape factor A of the plate thickness cross section parallel to the rolling direction is 5 or less in the state of being annealed after hot rolling or as hot rolled. Ferritic stainless steel plate for automobile exhaust system. 2. A ferritic stainless steel sheet for an automobile exhaust system which is excellent in moldability and heat resistance according to claim 1, which contains Mo: 0.1 to 2.0%. 3. A Cr content of 10 to 20%.
Alternatively, the ferritic stainless steel sheet for automobile exhaust system, which is excellent in formability and heat resistance as described in 2. 4. C + N: 0.0 according to any one of claims 1 to 3.
Ferritic stainless steel sheet for automobile exhaust system, which is excellent in forming workability and heat resistance, characterized by being 15% or less. 5. Ti: 0.1 to 0.3%, Nb: 0.2
5. The automobile exhaust system excellent in molding processability and heat resistance according to claim 4, characterized by containing at least one of 0.1 to 0.6% in a range satisfying C / 12 + N / 14≤Ti / 48 + Nb / 93. Ferritic stainless steel plate for use.