JP2746902B2 - Method for producing bright annealed ferritic stainless steel strip with excellent rust resistance - Google Patents

Description

The present invention relates to a method for producing a bright annealed ferritic stainless steel having excellent rust resistance.

<Conventional technology> Ferritic stainless steel sheet finished with bright annealing (hereinafter abbreviated as BA) has a beautiful surface gloss with black shine, so it is widely used for various interior and exterior materials for construction, automobile molding materials, kitchen equipment, etc. Have been. Bright annealing of ferritic stainless steel is generally H ₂ + N ₂ containing 50 vol% or more of H ₂ gas.
By performing the treatment in a mixed gas at a low dew point of about -50 ° C, the sheet is annealed without impairing its luster, while its rust resistance changes depending on the bright annealing conditions. It tended to be inferior to what is called JIS 2B finishing material.

Therefore, in recent years, the effect of the BA film on rust resistance has been
-42783, JP-A-61-235540 and the like. Japanese Patent Publication No. Sho 61-2760, Japanese Patent Application Laid-Open No. 60-218498, and Japanese Patent Application Laid-Open No. 59-238
No. 82 and JP-A-59-6398 are known. However, in order to obtain a BA film having excellent rust resistance, there are problems such as restrictions on the chemical composition of the material and BA conditions. In particular, it was necessary to lower the dew point, which had the greatest effect on the BA film, to below -50 ° C and close to -55 ° C. This is extremely difficult in a BA furnace for process production, and is a major problem in producing a BA film having excellent rust resistance. Further, the method of improving the corrosion resistance by post-treatment is basically to perform anodic electrolysis in a nitric acid solution, but according to investigations by the present inventors, there is variation in the effect depending on the steel type and material lot. ,
It did not necessarily bring about a sufficient improvement in corrosion resistance.

<Problems to be Solved by the Invention> As described above, the conventional technology has problems such as limitations on the steel type and BA conditions, particularly the dew point, and variations in the effect depending on the material lot. It was difficult to achieve a remarkable improvement in rust resistance.

The present invention is to provide a method for producing a bright annealed ferritic stainless steel strip with less variation and remarkably excellent rust resistance under a milder dew point constraint.

<Means for Solving the Problems> The present invention provides a ferritic stainless steel strip cold-rolled by performing bright annealing in an atmosphere having a dew point of −50 ° C. or more and −40 ° C. or less, and then having a concentration of 10 to 30%. In a nitric acid solution, the anodic electrolysis is performed in the range of the amount of anodic electrolysis expressed by the following formula according to the dew point to reduce the Mn oxide in the film and to concentrate the Si oxide. This is a method for producing a bright annealed ferritic stainless steel strip having excellent rust resistance.

However, Q: amount of electrolytic electricity (coulomb / dm ² ) D: dew point (° C.) <Operation> First, the process leading to the present invention will be described.

As a result of various studies, the present inventors have obtained the following new knowledge on the method of producing bright annealed ferritic stainless steel, and have reached the present invention.

That is: 1. It is most important to reduce the Mn oxide in the BA film to improve the rust resistance.

2. When the amount of Si oxide is increased when the amount of Mn oxide in the BA film is small, the rust resistance is further improved. However, if the amount of Mn oxide is large, the effect is small even if the amount of Si oxide is increased.

3. In order to obtain the film, BA is performed under appropriate conditions to generate an appropriate amount of film, and then anodic electrolysis is performed in a nitric acid solution (hereinafter abbreviated as nitric acid electrolysis). Is selectively dissolved and reduced, and the Si oxide is concentrated.

4. The more Si oxide in the film after BA, the more the Si concentration after nitric acid electrolysis becomes significant. Conversely, when the amount of Si oxide in the post-BA film is small, even if the amount of electrolysis is increased, the concentration of Si in the BA film hardly occurs.

5. The amount of Mn and Si oxide in the film after BA is strongly affected by the dew point of BA atmosphere.

6. If the amount of electrolysis in the nitric acid electrolysis is too large, not only the surface is roughened and the gloss is deteriorated, but also the rust resistance is lowered.

The present invention has been constructed based on the above findings. Next, the conditions of the present invention will be described in more detail.

First, in order to improve the rust resistance of the BA material, it is necessary to reduce the Mn oxide in the film after the BA and to concentrate the Si oxide. The amounts of Mn oxide and Si oxide in the final BA film are mutually dependent on the dew point during BA and the amount of electrolytic electricity in nitric acid electrolysis. In other words, when the dew point of the BA atmosphere is high, Mn
Since the amount of the oxide increases, the amount of electrolytic electricity required for selective removal of the Mn oxide increases. The amount of Si concentrated by nitric acid electrolysis is
It depends on the amount of Si oxide present in the film after BA, and the degree of concentration does not change even if the amount of electrolysis is further increased after Mn oxide is removed. Rather, if the amount of electrolysis is increased unnecessarily, the surface becomes rough, the surface gloss tends to deteriorate, and the rust resistance tends to decrease. Therefore, in order to obtain a ferritic stainless steel BA material with excellent rust resistance, BA is performed in an appropriate range with a relatively high dew point, and after increasing the amount of Si oxide in the film, the Mn oxide generated at the same time is appropriately adjusted. It is necessary to electrolytically remove Si in a range of an appropriate amount of electrolysis and to concentrate Si. The ferritic stainless steel BA material produced by such a method achieves remarkable improvement in rust resistance due to the extremely small amount of Mn oxide in the BA film and the remarkable concentration of Si oxide.

FIG. 1 shows the condition range in which the present inventors have found as a result of various studies and dramatically improved rust resistance. Region A is the proper range indicated by the present invention. If the dew point in the B region is lower than −50 ° C., the dew point is too low.
Even if the nitric acid electrolysis is performed, the rust resistance is improved by the selective dissolution of the Mn oxide, but is not drastically improved because the degree of concentration of the Si oxide is small. In the C region, the amount of electrolysis is small and the removal of Mn oxide is insufficient. In the D region, the surface gloss is deteriorated and the rust resistance is also reduced due to excessive electrolysis. If the dew point in the E region exceeds -40 ° C, a large amount of electrolysis is required to remove Mn oxides, and as a result, the gloss deteriorates and the rust resistance tends to decrease before seeing a dramatic improvement. Become.

Here, the boundary between area A and area C is The boundary between area A and area D , And the boundary is within the scope of the present invention. Here, Q represents the amount of electrolytic electricity (coulomb / dm ² ), and D represents the dew point (° C.).

Typical examples of the change of the BA film as described above are shown in FIGS. Each figure shows the results of analyzing the BA film in the depth direction by glow discharge spectroscopy. The horizontal axis indicates the time of sputtering in the depth direction, and the vertical axis indicates the degree of concentration of each element at that time. Mn and Si oxides were both formed thicker when BA was performed at a dew point of -48 ° C. 1 coulomb / dm in Fig. 3
Although the ^second nitric acid electrolysis can not be taken is Mn oxide, when the fourth Figure 5 electrolytic coulomb / dm ² and Si remarkably concentrated with Mn oxides are hardly removed, rusting resistance dramatically improves. On the other hand, as shown in FIG. 5, when BA was performed at a dew point of −55 ° C., the oxides of Mn and Si were thin.
Although the n-oxide is removed, the degree of concentration of the Si oxide is small, and therefore, the rust resistance is little improved.

The nitric acid concentration during the nitric acid electrolysis is suitably 5 to 30%, below which the Si oxide is not sufficiently concentrated, and above which the Si oxide is altered and rust resistance deteriorates. Will come.
The temperature of the nitric acid solution is suitably from room temperature to 60 ° C. If it is higher than that, the surface becomes rough and the gloss is greatly deteriorated. In addition, it was confirmed that the current density during nitric acid electrolysis had almost no effect, and depended only on the quantity of electricity.

Further, the anodic electrolysis treatment of the present invention may be anodic electrolysis treatment alone, or may be carried out alternately depending on the case by dividing cathodic electrolysis and anodic electrolysis into several times. The energization method may be direct energization or indirect energization. In short, it is important to secure the total amount of electricity for anodic electrolysis.

<Examples> Table 1 shows the compositions. Using a cold rolled sheet of SUS 430 steel, a typical ferritic stainless steel, and 19Cr-CuNb-added steel, a ferritic stainless steel for exterior,
BA treatment was performed in laboratory under various dew point conditions. The used atmosphere gas is a mixed gas of H ₂ 75% + N ₂ 25%. Thereafter, electrolysis was performed in a 10% nitric acid solution (30 ° C.). As for electrolysis, cathodic electrolysis was performed first, and then anodic electrolysis was performed, assuming equipment in the field line. Although cathodic electrolysis is not always necessary, it often comes inevitably in on-site facilities. The energization method was direct energization.

Table 2 shows the dew point at BA, nitric acid electrolysis conditions, and the results of the rust resistance test. The rust resistance test used was a CASS test specified in JIS D0201. After spraying CASS solution (5% NaCl + acetic acid 0.1-0.3% + cupric chloride 0.26g / l: pH 3.0-3.1) for 16 hours at 49 ° C, stop spraying and wet and dry at 49 ° C for 8 hours. The rust condition after the test was evaluated by the rust area ratio.

As shown in Table 2, those according to the method of the present invention have remarkably improved rust resistance.

<Effects of the Invention> According to the present invention, it has become possible to stably and dramatically improve the rust resistance of bright annealed ferritic stainless steel.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the appropriate dew point and the amount of electrolysis of the present invention, and FIGS. 2 to 6 are graphs showing the results of analysis of the BA film by glow discharge spectroscopy.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-1-123098 (JP, A) JP-A-61-253323 (JP, A) JP-A-59-6398 (JP, A) JP-A-59-6398 23882 (JP, A)

Claims (1)

(57) [Claims] 1. A cold-rolled ferritic stainless steel strip is subjected to bright annealing in an atmosphere having a dew point of −50 ° C. or more and −40 ° C. or less, and then in a nitric acid solution having a concentration of 10 to 30%. Excellent rust resistance characterized by anodic electrolysis treatment in the range of anodic electrolysis electricity amount represented by the following formula according to the following, reducing Mn oxide in the film and concentrating Si oxide Method for producing bright annealed ferritic stainless steel strip. However, Q: amount of electrolytic electricity (coulomb / dm ² ) D: dew point (℃)