JP3477957B2 - Ferritic stainless steel with excellent corrosion resistance under high temperature oxidation environment of 200-400 ° C - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a ferritic stainless steel having a surface oxide film excellent in corrosion resistance, and it is particularly desirable that the corrosion resistance is less deteriorated when used in a high temperature environment of 200 to 400 ° C. It is a feature. This is effective for use in, for example, water heaters and automobile exhaust system materials.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for ferritic stainless steels having excellent corrosion resistance and heat resistance for automobile exhaust system parts, household electric appliances, kitchen appliances, building materials, etc. Low Cr ferritic stainless steel has come to be used for the automobile muffler used.

However, the exhaust gas temperature rises with the recent increase in engine output, and the maximum temperature reached by the muffler is 40.
Since the temperature reached to 0 ° C, the oxidation was promoted, the passive film on the surface was deteriorated, and the corrosion became more severe. Therefore, the low Cr ferritic stainless steel has been insufficient in corrosion resistance. Therefore, from the viewpoint of automobile safety and longer life, materials having more excellent corrosion resistance and heat resistance have been demanded.

From such a background, Japanese Patent Laid-Open No. 3190/1990
In Japanese Patent Publication No. 55, a material containing Mo to improve corrosion resistance was proposed. Although the corrosion resistance of the base material was improved by Mo, the deterioration of the corrosion resistance due to oxidation is not taken into consideration, and therefore the expensive M
No effect was obtained that was commensurate with the inclusion of o.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a ferritic stainless steel having a surface oxide film excellent in corrosion resistance and having little deterioration in corrosion resistance particularly in a high temperature environment of 200 to 400 ° C.

[0006]

Means for Solving the Problems The present inventors investigated and studied the relationship between an oxide film formed on the surface of ferritic stainless steel and corrosion resistance from various viewpoints. As a result, it was found that the surface oxide film having a specific composition / structure exhibits excellent corrosion resistance, and that the composition of the material must be suitable for forming the film. At the same time, it is indispensable to omit the conventional pickling after finish annealing and use it in a state where a film is formed.

According to the results of experiments conducted by the present inventors, the dew point is −5 to −5.
-60 ° C., component: H _{2 in an amount} of 1 to 20% by volume, and a surface oxide film formed when finish annealing is performed at 750 to 1000 ° C. for 10 minutes or less in a reducing atmosphere consisting of the balance N ₂ . It has been found that when Cr, Si and B are concentrated, extremely excellent corrosion resistance can be obtained. Those having this surface oxide film showed superior corrosion resistance to those having a passivation film formed on the surface by polishing after finish annealing. Further, those having this surface oxide film had significantly less deterioration in corrosion resistance when exposed to a high temperature environment of 200 to 400 ° C. such as an automobile muffler environment.

This surface oxide film was analyzed by X-ray photoelectron spectroscopy (X
A detailed examination by PS) and thin film X-ray diffraction revealed that the surface oxide film with the most excellent corrosion resistance was Cr (mainly Cr ₂
Concentrated layers of O ₃ ), Si and B were observed, and the ratio of the concentration of the portion with the highest concentration of Cr or Si to the concentration of the base material exceeded a certain range. The range is such that the concentration of the most concentrated portion is higher than that of the base material.
Was 2 times or more, and Si was 5 times or more. The thickening of B was based on the visual judgment of the measurement data, but the thickening was confirmed, and the corrosion resistance was improved.

It is considered that Cr forms a dense and stable oxide layer on the surface layer and has the effect of preventing the corrosion of the base material by corrosive ions. It is also considered that Si is concentrated at the interface between the Cr ₂ O ₃ coating and the base material, improves the adhesion between the two, and consequently improves the corrosion resistance. Although the reason for B is not clear, the corrosion resistance is remarkably improved as compared with the case where the surface oxide film is concentrated and the case where there is no concentration.
In the case of Cr, if the concentration ratio between the concentrated layer and the base material is less than twice, a stable Cr ₂ O ₃ film cannot be formed on the surface layer and excellent corrosion resistance cannot be obtained. When Si has a concentration ratio of the concentrated layer to the substrate of less than 5 times, Si cannot form a SiO ₂ film sufficient to improve the adhesion between the Cr ₂ O ₃ film and the substrate, and is excellent. Corrosion resistance cannot be obtained.

As described above, the surface oxide film contains Cr and Si.
Ferritic stainless steels in which B and B are concentrated with a certain composition exhibit extremely excellent corrosion resistance, and this film is formed only under certain finish annealing conditions, and the chemical composition of the steel is also an important factor. I got the knowledge. The ferritic stainless steel of the present invention, which is excellent in corrosion resistance under a high temperature oxidation environment of 200 to 400 ° C., is made to achieve the above object based on the above findings. The technical means are as follows. That is, wt%
Then, C: 0.02% or less, Si: 0.1% or more, 1.5% or less, Mn: 1.0% or less, P: 0.05% or less, S: 0.01% or less, Cr: 10% or more, 25% or less, N: 0.02% or less, Nb: 0.002% or more, 0.02% or less, B: 0.0003% or more, 0.0025% or less, Ti: 6x (C % + N%) or more and 0.5% or less, and the balance of the finish annealing of the cold-rolled steel sheet made of Fe and the unavoidable impurities is the dew point: −5 to −60 ° C. and the component: H ₂ is 1 to 1. It is carried out at 750 to 1000 ° C. for 10 minutes or less in a reducing atmosphere consisting of 20% by volume and the balance of N ₂ , and the surface oxide film has a concentrated layer of Cr, Si and B. The concentration is Cr compared to the concentration of the base material
Is 2 times or more, and Si is 5 times or more.

In this case, further, Ni: 0.1% or more,
1.0% or less, Mo: 0.1% or more, 3.0% or less, C
u: 0.05% or more, 1.0% or less, Ca: 0.000
It is preferable to contain one or more of 5% or more and 0.01% or less.

[0012]

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention has been made based on the above findings. Hereinafter, the alloy component and the content of the base material will be specifically described. C: When the C content increases, the generation of carbides increases and the corrosion resistance decreases. Also, because it causes intergranular corrosion of the welded part,
The upper limit of the C content was specified to be 0.02%. In order to improve the corrosion resistance, which is a feature of the present invention, the C content is 0.0
It is preferably not more than 06%.

Si: Si is originally used as a deoxidizing agent, but according to the knowledge of the present inventors, when Si, Cr and B are concentrated on the surface during finish annealing of a cold rolled sheet. , Excellent corrosion resistance was obtained. In order to form a more stable surface oxide film with excellent corrosion resistance, Si is 0.1%.
The above is necessary. However, if Si is contained too much, it becomes hard, so the upper limit of the Si content was made 1.5%.

Mn: Mn is used as a deoxidizing agent, but the corrosion resistance decreases as the content increases, so the upper limit was made 1.0%. It is desirable that the P: P content is low from the viewpoint of corrosion resistance,
Extremely reducing the P content causes an increase in manufacturing cost, so the upper limit of the P content is defined as 0.05%.

S: S is an element that reduces corrosion resistance,
The upper limit is 0.01%. Cr: Cr is a basic element that maintains the corrosion resistance, and at least 10% is necessary in consideration of maintaining the corrosion resistance of the base material when the surface oxide film has a defect. Corrosion resistance increases as the Cr content increases, but if it is too large, it becomes expensive and the hot and cold workability is impaired.
5%.

N: N is an element that deteriorates the corrosion resistance of the welded portion, and the smaller the content, the better. In order to improve the corrosion resistance, which is a feature of the present invention, the N content is 0.0
It is desirable that it is 08% or less. Nb: Nb originally fixes C and improves corrosion resistance and workability. However, according to the findings of the present inventors, C
When added in the form of a component system based on r, Si in the coexistence with B, the corrosion resistance of the surface oxide film is remarkably improved. If it exceeds 0.02%, the effect is saturated and the finish annealing temperature is raised. If it is less than 0.002%, there is no effect.

B: B is concentrated in the oxide film to form a surface oxide film having excellent corrosion resistance. If it exceeds 0.0025%, the effect is saturated, and if it is less than 0.0003%, there is no effect. Ti: Ti fixes C or N and prevents deterioration of corrosion resistance of stainless steel. In particular, it has an effect of improving the intergranular corrosion resistance, and this requires 6 × (C% + N%) or more, so this was made the lower limit. On the other hand, if it exceeds 0.5%, it becomes hard and the workability deteriorates.

Ni: Ni is electrochemically nobler than Fe and Cr and suppresses corrosion in the active region, thus improving corrosion resistance. If it is less than 0.1%, there is no effect, and if it exceeds 1.0%, the effect is saturated and it is economically expensive. Mo: Mo stabilizes the surface oxide film and improves the corrosion resistance. Even if added in a large amount exceeding 3.0%, the effect is saturated and the cost becomes high, so the upper limit is made 3.0%. Further, if less than 0.1%, there is no effect.

Cu: Since Cu improves the corrosion resistance of stainless steel, it is added in an amount of 1.0% or less as necessary.
If it exceeds 1.0%, the effect of improving the corrosion resistance is saturated and the hot workability is deteriorated. If it is less than 0.05%, there is no effect. Ca: Ca improves the adhesion of the oxide film generated by oxidation, and thus has the effect of improving the corrosion resistance of the oxide film. However, if added excessively, it precipitates at grain boundaries and deteriorates hot workability, so the upper limit is made 0.01%. 0.000
If it is less than 5%, there is no effect.

Next, a surface oxide film having excellent corrosion resistance is formed.
The finish annealing conditions for the above will be described. Annealing temperature
750 for softening the cold rolled sheet and homogenizing the texture.
~ 1000 ° C. Harder without recrystallization below 750 ° C
If the temperature exceeds 1000 ° C, the crystal grains become coarse and the steel
Since the material of becomes brittle, the finish annealing temperature is 750-10
Limited to 00 ° C.

If the annealing time exceeds 10 minutes, the oxide film
Thickening, formation of a Cr-free layer on the base metal and a large amount of oxide film
Corrosion resistance deteriorates with porogenization. Further consider productivity
Then, holding for 10 minutes or less is desirable. Next, the corrosion resistance
Dew point that greatly affects the formation of surface oxide film
Define the atmosphere. If the dew point is higher than -5 ℃, surface acid
The chemical film becomes thick and becomes very porous, resulting in poor corrosion resistance.
It On the other hand, if the temperature is lower than -60 ° C, the surface oxide film
The atmosphere becomes very thin and the surface becomes active.
Corrosion resistance deteriorates due to nitrification underneath. Therefore, dew
Points are limited to -5 to -60 ° C.

The atmosphere also has to determine the denseness of the surface oxide film.
The reason is that a protective surface oxide film with denseness is formed.
The order, H ₂ is from 1 to 20% by volume, the balance N ₂ mixed
Annealing in a reducing atmosphere with a mixed gas is essential. H
_{When 2} is less than 1% by volume, the surface oxide film becomes thick and the denseness is
descend. Also, when H ₂ exceeds 20% by volume, surface oxidized skin
The film becomes thin and nitrification occurs, resulting in reduced corrosion resistance. According to
H ₂ of the mixed gas is limited to 1 to 20% by volume, and the rest is
The reducing atmosphere is N ₂ gas.

[0024]

【Example】

Example 1: Ferritic stainless steel having the chemical composition shown in Table 1 was cast into a steel ingot of 50 kg by high-frequency vacuum melting, hot-rolled and cold-rolled to form a cold-rolled sheet having a thickness of 1 mm and having a volume of 5 mm. % of H _2, balance N _2, in an atmosphere of a dew point of -40 ° C., after performing the final annealing of 900 ° C. × 1 min, was processed into a test piece. For these test materials, surface analysis of the surface layer portion was performed by X-ray photoelectron spectroscopy (XPS), and changes in the concentrations of Cr, Si and B in the depth direction were examined.
For Cr and Si, the results are shown in Table 2 as the ratio of the concentration of the portion having the highest concentration to the concentration of the substrate (hereinafter referred to as the concentration rate). In addition, in Table 2, the pitting corrosion potential is measured for the as-finished as-annealed product, the as-finished after-annealing at 400 ° C. for 5 hours in the air, and the as-finished-annealed # 600 polished product. The result was shown. The pitting corrosion potential was measured at a liquid temperature of 30 ° C. by using an Ar degassed 3.5% NaCl solution by a potentiodynamic method with an insertion speed of 20 mV / min, and an anode current density of 10 μm / cm.
^The potential reaching ² was defined as the pitting potential.

As shown in Table 2, the steels of the present invention show superior corrosion resistance to the abrasives having the passive film formed thereon. Further, the corrosion resistance is less deteriorated even after high temperature oxidation in the air, and the corrosion resistance is still superior to that of the abrasive .

[0026]

[0027]

INDUSTRIAL APPLICABILITY As described above, the present invention provides a ferritic stainless steel having a surface oxide film excellent in corrosion resistance and having significantly less deterioration in corrosion resistance particularly in a high temperature environment of 200 to 400 ° C. To be The ferritic stainless steel on which this surface oxide film is formed is
It is used in a wide range of fields such as hot water supply containers and automobile exhaust system materials by taking advantage of its excellent corrosion resistance in high temperature oxidizing environments.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Susumu Sato 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Inside the Technical Research Laboratory, Kawasaki Steel Co., Ltd. (56) Reference JP-A-7-268485 (JP, A) 63-25052 (JP, B1) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 9/46-9/48

Claims (2)

(57) [Claims] 1. By weight%, C: 0.02% or less, Si: 0.1% or more and 1.5% or less, Mn: 1.0% or less, P: 0.05% or less, S: 0. 0.01% or less, Cr: 10% or more, 25% or less, N: 0.02% or less, Nb: 0.002% or more, 0.02% or less, B: 0.0003% or more, 0.0025% or less , Ti: 6 × (C% + N%) or more and 0.5% or less, and the finish annealing of the cold-rolled sheet of steel containing the balance of Fe and inevitable impurities, dew point: −5 to −60 ° C. Ingredients: H _{2 in an amount} of 1 to 20% by volume and the balance N _{2 in a} reducing atmosphere at 750 to 1000 ° C. for 10 minutes or less, and the surface oxide film has a concentrated layer of Cr, Si, and B. Then
The high temperature oxidizing environment of 200 to 400 ° C. characterized in that the concentration of the most concentrated portion is 2 times or more for Cr and 5 times or more for Si as compared with the concentration of the base material.
Ferritic stainless steel with excellent corrosion resistance under . 2. Further, by weight%, Ni: 0.1% or more and 1.0% or less, Mo: 0.1% or more, 3.0% or less, Cu: 0.05% or more, 1.0% Below, Ca: 0.0005% or more, 0.01% or less, and one or more of them are contained, and the corrosion resistance in a high temperature oxidation environment of 200 to 400 ° C. according to claim 1. Excellent ferritic stainless steel.