JPH0474899A - Production of cold rolled ferritic stainless steel strip having excellent corrosion resistance - Google Patents

Abstract

PURPOSE:To obtain a surface which maintains high gloss and has high corrosion resistance by annealing a cold rolled stainless steel strip contg. Si at a specific ratios at a specific temp., then subjecting the steel strip to descaling by neutral salt electrolysis then to descaling by dipping in nitric and hydrofluoric acids. CONSTITUTION:The cold rolled stainless steel contg. >=0.2% Si annealed at >=950 deg.C is subjected to the removal of the scale of a chromic acid system by a neutral salt electrolyzing descaling method as the 1st descaling and the surface thereof is coated with silicon oxide. The above-mentioned silicon oxide layer is then gradually dissolved to finish the steel strip to the beautiful surface by the pickling using mixed acids of nitric acid and hydrofluoric acid as the 2nd descaling. In addition, the thin and dense silicon oxide layer is made to remain to imrove corrosion resistance. The concn. and temp. of an aq. neutral salt soln. are adequately 100 to 300 g/l and 70 to 90 deg.C. The electrolytic treatment with the nitric acid is adequately executed at 50 to 200 g/l and 20 to 60 deg.C concn. and temp. of the nitric acid although these conditions vary with the conditions of the dipping in the nitric and hydrofluoric acids.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for producing a cold-rolled ferritic stainless steel strip having excellent corrosion resistance.

<Prior art> In general, when cold-rolled stainless steel strip is subjected to heat treatment such as annealing or quenching in an oxidizing atmosphere, oxide scale is formed on the surface of the copper strip, so descaling is performed to remove this oxide scale. Processing is performed.

Pickling using sulfuric acid, hydrochloric acid, nitric-fluoric acid (a mixed acid of nitric acid and hydrofluoric acid), etc. is commonly used for descaling, but the oxide scale that forms on cold-rolled stainless steel strips is dense and strong. Therefore, it is difficult to completely descale.

Therefore, as a pretreatment method to facilitate pickling, immersion treatment in molten alkali salt (salt treatment) or
-12162, electrolytic treatment in a neutral salt aqueous solution, etc. have been developed.

<Problems to be Solved by the Invention> The surfaces of steel pots descaled by these various methods not only differ in surface properties such as metallic luster and whiteness, but also in terms of corrosion resistance.

Conventionally, when it comes to descaling cold-rolled stainless steel strips, the main focus has been simply on removing the oxidized scale layer and dechromium layer, so it has not been possible to develop descaling methods that take into account surface texture and corrosion resistance.

The various descaling methods mentioned above are not used alone, so the effectiveness of each descaling process is not clear, but descaling methods using strong acids such as sulfuric acid, hydrochloric acid, and nitric-fluoric acid are effective for descaling. Due to its high capacity, the stainless steel base melts considerably, and the surface after descaling generally has a low gloss level.

On the other hand, the descaling method using neutral salt electrolysis is advantageous in obtaining a highly glossy descaled surface because there is little dissolution of the stainless steel material, but on the other hand, the descaling ability tends to decrease and it However, its use is limited due to insufficient descaling.

Regarding salt treatment, the surface tends to become somewhat rough due to its high descaling ability.

Unlike austenitic stainless steel, which has a milky white surface finish, ferritic stainless steel generally has a black shiny surface with high gloss and has high utility value. Therefore, in descaling after annealing, in order to minimize corrosion of the base material, consideration is given to selecting the acid concentration, temperature, electrolytic conditions, etc., and removing only the scale layer and the dechromized layer immediately below it. There is.

However, it is difficult to achieve stable descaling with these conventional methods, and oxide scale layers may form locally due to variations in the temperature during annealing, changes in the atmosphere, or the surface condition of the cold rolled steel strip before annealing. Alternatively, in the case of complete change, after descaling, scale remains or the surface becomes rough due to overaciding, and there is a great risk that the surface gloss will change and corrosion resistance will decrease.

The present invention provides a stable descaling method that maintains high gloss on the descaled surface of ferritic stainless steel after annealing and provides a surface with high corrosion resistance.

<Means for Solving the Problems> The present inventors have focused on the neutral salt electrolysis method, which has some difficulties in terms of descaling ability, but is significantly superior to other descaling methods in terms of surface finish. In addition to clarifying its descaling behavior, we investigated its combination with other descaling methods that supplement the descaling ability and improve corrosion resistance.

As a result, in ferritic stainless steel strips containing 0.2% Si or more, when the annealing temperature rises above a certain temperature and the oxide scale layer thickens, descaling becomes rapidly difficult using the neutral salt electrolytic descaling method. We conducted a compositional analysis of the oxide scale layer and the surface after neutral salt electrolysis, and found that under these conditions, as shown in Figure 2, oxidation mainly composed of chromium and iron oxides is formed by annealing. It was discovered that the layer that appears to be silicon oxide that forms at the interface between the scale and the substrate is not removed at all by neutral salt electrolysis, but rather becomes concentrated on the surface.

The new silicon oxide N formed with neutral salt electrolysis has a yellow ten-bar color appearance, so it may be judged as remaining scale, or such a layer may be formed in other descaling methods. It was discovered that it is possible to finish a beautiful surface with high corrosion resistance when effectively utilized as described below, and this led to the present invention.

That is, in order to achieve the above object, the present invention 1: After annealing a cold-rolled ferritic stainless steel strip containing 0.2% or more of Si at a temperature of 950° C. or higher,
A method for manufacturing a ferritic stainless steel rolled copper strip having excellent corrosion resistance is provided, which is characterized in that descaling is performed by neutral salt electrolysis, followed by descaling by immersion in nitric-fluoric acid.

Here, it is preferable to further perform nitric acid electrolytic treatment after the nitric-fluoric acid immersion.

The present invention will be explained in more detail below.

The present invention involves removing chromium oxide scale from a ferritic stainless steel rolled steel strip annealed at 950°C or higher using a neutral salt electrolytic descaling method as a first descaling process, and coating the surface with silicon oxide. After that, the second
For descaling, pickling with a mixed acid of nitric acid and hydrofluoric acid gradually dissolves the silicon oxide layer, creating a beautiful surface and leaving a very thin and dense silicon oxide layer to improve corrosion resistance. is a feature.

Thereafter, in order to passivate or stabilize the silicon oxide, immersion in nitric acid or electrolytic treatment may be performed as necessary.

In this way, in the present invention, since the silicon oxide layer formed at the interface between the chromium oxide scale layer generated during annealing and the base material is used, the Si content in the steel is limited to 0.2% or more. Ru.

If the Si content is less than 0.2%, a sufficient silicon oxide layer cannot be obtained in the neutral salt electrolysis in the first descaling process, resulting in rough skin due to the acid in the second descaling process.

Further, the upper limit is not particularly set, or it is preferably 2% or less since the toughness of the material decreases if it exceeds 2%.

The annealing temperature also needs to be 950° C. or higher to obtain a sufficient silicon oxide layer. However, if the temperature exceeds 110℃, the crystal grains will become coarser and the toughness will decrease.
It is desirable to keep the temperature below 0°C.

The annealing atmosphere may be an oxidizing atmosphere, such as a combustion gas atmosphere.

Furthermore, the cold-rolled steel strip that is the raw material of the present invention is limited to ferritic stainless steel in order to have a beautiful surface finish.
A similar finished surface cannot be obtained with austenitic stainless steel because the oxide scale structure is different.

In the neutral salt electrolysis of the present invention, Na salts and K salts such as sulfuric acid, nitric acid, and hydrochloric acid can be used alone or in combination as neutral salts, but from the viewpoint of economy and surface finish, sodium sulfate is used. is suitable.

The concentration and temperature of the neutral salt aqueous solution are 100 to 300, respectively.
g/u, 70-90°C is appropriate.

The appropriate current density is 2 to 15 A/dm2 for both the anode reaction current density and the cathode reaction current density.

In the nitric-fluoric acid immersion in the present invention, the mixed acid consisting of nitric acid and hydrofluoric acid used has a nitric acid concentration of 3 to 15% and a hydrofluoric acid concentration of 0.3 to 3%, depending on the steel type and annealing conditions of the applied cold-rolled copper strip.
The silicon oxide layer formed by the neutral salt electrolysis increases as the silicon oxide layer increases, and the concentration and bath temperature become high.

Further, in order to further improve the corrosion resistance, nitric acid electrolytic treatment may be performed after immersion in nitric acid hydrofluoric acid.The nitric acid electrolytic treatment depends on the conditions of nitric acid immersion, but the nitric acid concentration and temperature are 50 to 200 g/ ρ, 20~
60°C is appropriate.

Further, the appropriate current density is 2 to 15 A/dm2.

<Example> The present invention will be specifically explained below based on Examples.

(Example 1) Cold-rolled plates of 0.5 mm thickness of three types of ferritic stainless steels with different amounts of Si shown in Table 1 were heated to 90 mm as shown in Table 2.
After annealing at 0 to 1000°C, descaling was performed by the method shown in Table 2.

Note that neutral salt electrolysis was performed using a 200 g/fl aqueous sodium sulfate solution at 80°C for 10 seconds at 5 A/dm2, and nitric-fluoric acid immersion was performed using a bath consisting of 5% nitric acid and 1% hydrofluoric acid at a temperature of 50°C.
The sample was immersed in the mixed acid for 40 seconds. Nitric acid electrolysis is performed using 10% nitric acid at 50℃ for 5 A/dm2 x 5 seconds.
A, sulfuric acid electrolysis is 5% sulfuric acid, 5 A/dm2x at 50℃
The electrolytic treatment was performed for 5 seconds. Salt treatment was carried out in a salt bath consisting of sodium hydroxide and sodium nitrate at 420℃ for 10 minutes.
It was immersed for seconds.

In Table 2, A is neutral salt electrolysis - nitric acid immersion, B is neutral salt electrolysis - nitric acid dipping, C is neutral salt electrolysis - nitric acid electrolysis,
D is sulfuric acid electrolysis - nitric acid immersion, E is salt treatment - sulfuric acid dipping -
Nitric acid electrolysis, F indicates salt treatment, neutral salt electrolysis, and nitric acid immersion, respectively.

After descaling, the surface was measured for gloss and whiteness, and a cycle corrosion test was conducted to compare corrosion resistance and oxidation resistance.

Corrosion resistance was evaluated by carrying out a 15-cycle corrosion test as described below with descaling.

35℃ salt water spray x 2 hours - Dry 60℃ x 2 hours - Wet 5
5°C x 2 hours (evaluation) ○... Rust area rate: less than 10% △... Rust area rate, 10-20% ×... Rust area rate: 20% Super oxidation resistance is After heat treatment at 200° C. for 2 hours in the atmosphere, a 15-cycle corrosion test was conducted and evaluated as described below.

(Evaluation) ○... Rust area ratio: less than 10% △... Rust area ratio: 10-20% X... Rust area ratio: 20% Super high surface gloss and excellent corrosion resistance What can be changed is
This is only the case where steel grades S2 and S3 with high Si were descaled by descaling method A or B at an annealing temperature of 950° C. or higher (invention examples 1 to 5). When the annealing temperature is low, the surface becomes rough, the gloss decreases, and the corrosion resistance also decreases (Comparative Example 1.2).

In descaling method C, which involves neutral salt electrolysis followed by nitric acid electrolysis, it is difficult to descale during high-temperature annealing as in Comparative Examples 3 to 7.
Moreover, descaling tends to be uneven, and gloss and corrosion resistance become low.

In the case of steel type S1 with a low Si content, when immersed in nitric hydrofluoric acid after neutral salt electrolysis as in Comparative Examples 13 to 17, the surface roughness is noticeable and the corrosion resistance is low.

Further, although descaling is almost possible using the descaling method F, corrosion resistance is not sufficient (Comparative Examples 8 to 12).

Looking at the effects of the descaling method on steel type S3 with high Si content at the same annealing temperature of 975°C, corrosion resistance was particularly low with the descaling C method, which has a low descaling ability (Comparative Example 18), and with the descaling method F, which has a low descaling ability. Even in (Comparative Examples 19 to 21), the corrosion resistance after atmospheric oxidation is significantly lower than that of methods A and B (inventive example 4.5).

Table <Effects of the Invention> Since the present invention is constructed as described above, descaling after annealing is performed by neutral salt electrolysis followed by nitric-fluoric acid immersion, thereby improving the gloss of the descaling surface. It is possible to maintain high corrosion resistance and obtain a surface with high corrosion resistance.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the surface composition of a cold-rolled ferritic stainless steel strip before descaling. FIG. 2 is a graph showing the surface composition of the steel strip shown in FIG. 1 after neutral salt electrolysis. Figure 3 is shown in Figure 2! It is a graph showing the surface composition after rough immersion of f in nitric hydrofluoric acid. F I G, 3

Claims (2)

[Claims] (1) After annealing a cold-rolled ferritic stainless steel strip containing 0.2% or more of Si at a temperature of 950°C or higher, descaling is performed by neutral salt electrolysis, followed by descaling by nitric-fluoric acid immersion. A method for producing a cold-rolled ferritic stainless steel strip having excellent corrosion resistance. (2) The method for producing a cold-rolled ferritic stainless steel strip with excellent corrosion resistance according to claim 1, wherein a nitric acid electrolytic treatment is further performed after the nitric-fluoric acid immersion.