JPH0474852A - Ferritic stainless steel excellent in thermal fatigue resistance - Google Patents

Abstract

PURPOSE:To improve thermal fatigue resistance by incorporating specific amounts of Nb and Ti into a ferritic stainless steel and also reducing the contents of O and S as impurities. CONSTITUTION:Nb and Ti are incorporated by 0.1-0.5% and 0.05-0.3% by weight, respectively, into a ferritic stainless steel having a composition consisting of <0.03% C, <0.5% Si, <0.4% Mn, 13-16% Cr, and the balance Fe so that the ratio between both (Nb%/Ti%) is regulated to 2-6, and also the total content of O and S as impurities is limited to <=50 ppm. Owing to the addition of Nb and Ti, the ferritic stainless steel excellent in thermal fatigue resistance and formability can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a ferritic stainless steel with excellent thermal fatigue resistance and suitable as a material for, for example, automobile exhaust manifolds and household heating appliances.

<Prior art and its challenges> For example, the exhaust manifold of an automobile is a member that is exposed to high-temperature combustion gas while the engine is running, so it is one of the items that requires excellent heat resistance and high-temperature corrosion resistance. However, ductile cast iron has traditionally been the mainstream material for the exhaust manifold.

However, in recent years, as demands for reducing the weight of automobile bodies and improving engine performance have become stricter, there has been a growing trend to use stainless steel instead of conventional cast iron for manifold materials, and some have even reached the stage of considering practical applications. It has become. The stainless steel used for this purpose is JIS
SUS409Lm, which is standardized in

However, the above 5US409L steel is certainly 70.0
Although it showed good oxidation resistance and thermal fatigue resistance up to around 30°F, and was also excellent in formability into manifold shapes, acid resistance deteriorated when the operating environment temperature exceeded 800°C and reached around 900°C. It has been pointed out that the thermal fatigue resistance and thermal fatigue properties deteriorate. Therefore, considering the situation in which manifolds are being used in environments where the temperature in which they are used is rising significantly in response to increasingly high-performance engines, it was predicted that 5US409L steel would no longer be able to withstand use in the future.

Therefore, when searching for stainless steel that has better heat resistance than the above-mentioned 5US409L steel among existing materials, the top steel type 5US
430LX was found, and although steel types belonging to this type have good oxidation resistance even at temperatures around 900°C, they do not have sufficient thermal fatigue resistance or formability, and are expensive, so they are not satisfactory as materials for manifolds.

Therefore, the purpose of the present invention is to improve the existing 5US40 in terms of oxidation resistance, thermal fatigue resistance, and formability.
The object of the present invention was to provide a stainless steel mesh that has characteristics that are superior to 9L steel and that can be used as a material for manifolds used in high-performance engines, for example.

Means for Solving the Problems> The present invention was completed by the inventors of the present invention through repeated research and repeated numerous experiments in order to achieve the above object. .03% or less (hereinafter, percentages representing component proportions are expressed as weight percentages).

Si: 0.5% or less, Mn: 0.4% or less.

Cr: 13-16%, Nb: 0.1-0
．． 5% Ti: Contains 0.05 to 0.3%, the remainder consists of Fe and impurities, and a) Nb (χ) / Ti (χ) ~ 2 to 6 degrees b) 0 and S in the impurities. Total amount: 5Qppm or less By having a configuration that satisfies the condition, excellent thermal fatigue resistance, oxidation resistance,
It is characterized by having good moldability.

Hereinafter, the reason why the component ratio of the steel is limited as described above in the present invention will be explained in detail along with its effect.

Effect> C is an effective component for ensuring the strength of steel, but 0.03
If the content exceeds %, the stability of ferrite will decrease.

Since C tends to deteriorate oxidation resistance, heat resistance, formability, and weldability, the C content is set at 0.03% or less in order to maintain the above characteristics.

Needle Si has the effect of improving the oxidation resistance of steel, but 0.
If the Si content exceeds 5%, the thermal fatigue resistance and formability will deteriorate significantly, so the Si content was determined to be (L5% or less).

Mn Although Mn is a necessary element as a deoxidizing component during steel manufacturing,
If the Mn content exceeds 0.4%, it will have an adverse effect on thermal fatigue properties, so the Mn content is set at 0.4% or less.

Cr Cr has the effect of improving oxidation resistance and thermal fatigue resistance, but if the content is less than 13%, the desired effect due to the above effect cannot be obtained, while on the other hand, if the content exceeds 16%, The Cr content was determined to be 13 to 16%, since this would lead to deterioration in thermal fatigue resistance and formability, and would also be disadvantageous in terms of cost.

Nb Nb has the effect of significantly improving the thermal fatigue resistance and formability of steel when added in combination with Ti at a predetermined ratio described below. However, if the content is less than 0.1%, the desired effect due to the above action cannot be obtained, and on the other hand, if the content exceeds 0.5%, thermal fatigue resistance and formability tend to deteriorate. , the Nb content was determined to be 0.1 to 0.5%.

Ti Ti also exhibits a similar effect when coexisting with Nb, so it is an essential component in the steel of the present invention.
, less than 0.05%, sufficient effect cannot be obtained; on the other hand, 0.3%
If the Ti content exceeds 0.05 to 0.0%, thermal fatigue resistance and formability tend to deteriorate.
It was set at 3%.

In the steel of the present invention, the Nb/Ti ratio is an extremely important requirement, and even if the Nb/Ti ratio is less than 2, on the contrary
Even if it exceeds 100%, the effect of improving thermal fatigue resistance cannot be ensured. Therefore, the Nb/Ti ratio was limited to 2-6.

Figure 1 shows C: 0.01%, Si: 0.2%, Mn: 0
．． 1%.

Cr: 14.0%, O: 15ppm, S
: A thermal fatigue test in which a ferritic stainless steel mesh containing 5 ppm and further containing Nb and Ti at various [Nb/Ti ratios] is completely restrained, and heating and cooling are repeated between 200°C and 900°C in this state. The results are shown below. In addition, "thermal fatigue life" in FIG. 1 is the number of times of heating and cooling until cracking occurs. The results shown in FIG. 1 also confirm that an excellent effect of improving thermal fatigue resistance can be ensured only when the Nb/Ti ratio is within the range of 2 to 6.

○, SO Both O and S are impurity elements that form nonmetallic inclusions, but if their total amount exceeds 50 ppm, both thermal fatigue resistance and formability tend to deteriorate.
0. The total S content was set at 50 ppm or less.

In addition, in the ferritic stainless steel mesh according to the present invention, Ni can be contained in an amount comparable to that of ordinary ferritic steel.

Next, the effects of the present invention will be explained in more detail with reference to Examples.

<Example> First, steel having each component composition shown in Table 1 was melted, and was made into a plate material through hot rolling and cold rolling.

Next, a test piece was taken from the obtained copper plate, and the thermal fatigue resistance and
Oxidation resistance, formability and weldability were investigated.

The thermal fatigue resistance was evaluated by fully restraining the test piece at 20
A thermal fatigue test was conducted by repeatedly heating and cooling between 0° C. and 900° C., and the number of times until cracking occurred (thermal fatigue life) was measured.

The oxidation resistance was evaluated by heating the test piece to 900° C. in the atmosphere and examining the oxidation state when the test piece was held for 200 hours.

The moldability was evaluated by subjecting the test piece to an Erichsen test and based on the obtained Erichsen test.

Weldability was evaluated by subjecting a TIG welded test piece to a bending test and observing the occurrence of cracks in the welded portion.

These results are also shown in Table 1.

As is clear from the results shown in Table 1, the steels of the present invention have excellent formability and weldability equivalent to 5US409L, and also have 5U in terms of thermal fatigue resistance and oxidation resistance.
It has been rated better than S409L, and it is 16
Expensive steel types with a Cr content exceeding % (SUS430LX, etc.)
It shows good formability and weldability that cannot be expected with 900
It can be seen that this is a structural material that exhibits sufficient performance even in the high temperature range of around ℃.

On the other hand, it is clear that comparative materials whose compositional conditions deviate from those defined by the present invention are inferior in terms of thermal fatigue resistance, oxidation resistance, formability, and weldability.

<Summary of Effects> As explained above, according to the present invention, in addition to providing a relatively low-cost material for automobile exhaust manifolds that can be used at higher temperatures than conventional ductile cast iron, it is also possible to provide a material for automobile exhaust manifolds that can be used at higher temperatures than conventional ductile cast iron. By applying the steel of the present invention to heat-resistant parts, etc., it is possible to significantly improve their performance and service life.
Industrially extremely useful effects are brought about.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the Nb/Ti ratio and the thermal fatigue life of ferritic stainless steel. Applicant Nippon Stainless Co., Ltd.

Claims (1)

[Claims] C: 0.03% or less, Si: 0.5% or less, Mn: 0.
4% or less, Cr: 13-16%, Nb: 0.1-0.5
%, Ti: 0.05 to 0.3%, and the remainder is F
A heat-resistant material consisting of e and impurities, and satisfying the following conditions: a) Nb (%) / Ti (%) = 2 to 6, b) total amount of O and S in impurities: 50 ppm or less Ferritic stainless steel with excellent fatigue resistance.