JP2966188B2 - Descaling method for ferritic stainless steel annealed steel strip - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to Nb, Ti and Mo.
A high-speed continuous descaling of an annealed ferritic stainless steel strip containing at least one of the following.

[0002]

2. Description of the Related Art As a descaling method after annealing a stainless steel strip, a neutral salt electrolysis method is disclosed. this is,
After performing electrolytic treatment in a neutral salt aqueous solution mainly using sodium sulfate, perform electrolytic treatment in a nitric acid aqueous solution for a ferritic stainless steel annealed steel strip, and immerse and pickle in an austenitic stainless steel annealed steel strip in a nitric hydrofluoric acid aqueous solution. The method is described in JP-A-49-123936. Compared to the conventional method, this method has the advantages of high safety because it does not use a high-temperature alkali molten salt, and has the advantage that high-speed sheet passing processing is possible. It is one of the laws.

However, when the annealed ferritic stainless steel strip containing one or more of Nb and Ti is descaled, the surface is colored yellow during neutral salt electrolysis, and it is necessary to remove the surface by nitric acid electrolysis. A large amount of time is required for descaling. Further, when the ferrite-based annealed stainless steel strip containing Mo is descaled, a brownish substance remains on the surface due to neutral salt electrolysis, and it takes time to remove this by finish pickling. If nitric hydrofluoric acid immersion is performed instead of nitric acid electrolysis, it is possible to perform descaling in a short time because the dissolving performance of nitric hydrofluoric acid is high, but nitric hydrofluoric acid erodes the mother metal and causes rough skin. Due to the loss of surface gloss, nitric hydrofluoric acid immersion has been unsuitable for ferritic stainless steels.

In the invention described in Japanese Patent Application Laid-Open No. 2-22099, NaOH is used as an intermediate step between neutral salt electrolysis and nitric acid electrolysis.
This problem is solved by performing electrolysis in an alkaline aqueous solution such as that described above. Adding this step increases the number of steps,
The total time required for the three electrolytic treatment steps of neutral salt, NaOH, and nitric acid is almost equal to the time required for descaling general ferritic stainless steel such as SUS430 in two steps of neutral salt electrolytic treatment and nitric acid electrolytic treatment. This is equivalent, and the descaling time is greatly reduced as compared with the case without the intermediate step. However, this method requires three types of electrolyzers, and increases investment costs and running costs.

In the specification of Japanese Patent Application No. 4-47363, the inventors of the present invention disclosed a method for lowering the pH of an electrolytic solution to 0 to 3 in neutral salt electrolysis and increasing the current of the electrolytic solution above the passivation current density of stainless steel. A method for solving this problem by electrolytic treatment at a high density is presented. Using this method, Nb, T
The time required for descaling of an annealed ferritic stainless steel strip containing at least one of i and Mo is drastically reduced. However, it does not reach the descaling time of general stainless steel such as SUS430.

[0006]

SUMMARY OF THE INVENTION The present invention relates to Nb, Ti
To solve the problem of the prior art that the descaling of a ferritic stainless steel annealed steel strip containing at least one of Mo and Mo requires a very short time without causing deterioration of surface gloss or roughening of the steel strip. It is possible to provide a method which can completely descaling by using the method and has a small investment cost.

[0007]

Means for Solving the Problems The present inventors have solved this problem by introducing nitric hydrofluoric acid immersion pickling, which was conventionally considered unusable with ferritic stainless steel, to finish pickling. That is, pH 0, which has higher acidity than the conventional method,
After performing electrolysis in a neutral salt solution of ~ 3, by immersing and pickling in a nitric hydrofluoric acid solution, without adding a new intermediate step, without losing the surface gloss in almost the same time as SUS430 Descaling becomes possible.

[0008]

The mechanism of removing the annealed scale by the neutral salt electrolysis method is generally as follows. The annealed scale of stainless steel mainly contains oxides of Cr and relatively few Fe oxides. In the neutral salt electrolysis, the Cr oxide is dissolved and removed as hexavalent ions by an oxidation reaction by performing electrolysis (anode electrode) using the treatment material as a positive electrode. However, since brownish Fe oxide remains in this electrolysis, it is removed by finishing pickling such as nitric acid electrolysis and nitric hydrofluoric acid immersion. In the case of Nb-containing steel, the Nb oxide is concentrated near the interface between the scale and the base metal, but the Nb oxide does not dissolve at all in the oxidation reaction, and thus remains on the surface after neutral salt electrolysis. This is the cause of yellowing.
The Ti-containing steel also becomes yellowish for the same reason as the Nb-containing steel.

On the other hand, nitric acid electrolysis or nitric acid hydrofluoric acid used for finish pickling dissolves the remaining Fe oxide,
By dissolving the base metal near the scale interface, the residue can be removed and removed. Therefore, it is possible to remove even Nb and Ti oxides that do not dissolve by themselves.
In the case of the Mo-containing steel, the dissolution of the base metal is suppressed, so that it takes time to remove the residue composed of Fe oxides, Nb, and Ti oxides, and yellowing. Since the nitric hydrofluoric acid immersion method has a greater ability to dissolve the base metal compared to the nitric acid electrolysis method, these residues can be removed more easily than the nitric acid electrolysis.

The present invention utilizes the fact that the amount of dissolution of Nb, Ti, and Mo-containing ferritic stainless steel by nitric acid pickling is considerably smaller than that of general ferritic stainless steel.
Provided is a method for performing nitric acid hydrofluoric acid washing while maintaining the surface metallic luster. First, in neutral salt electrolysis, the pH of the electrolyte is set to 0.
By lowering it to ~ 3, the base metal dissolution ability is slightly given to the neutral salt electrolytic treatment, and not only does Cr oxide dissolve, but also F
e oxide and Nb and Ti oxides can be removed to some extent.

The electrolyte may be sodium sulfate, which is generally used. However, when sodium nitrate is used, the dissolution of the base metal is suppressed instead of the scale dissolving ability being somewhat reduced.
Skin roughness is less. The pH is kept in the range 0 to 3. At a pH of 3 or less, the ability to dissolve the base metal increases and the descaling time decreases. Below 0, the shortening effect saturates. Further, in order to make it less than 0, the amount of the acid to be added for pH adjustment increases dramatically, and the lining of the pickling tank must be made special, which increases the cost. Regarding the polarity of the electrolysis, the anodic electrolysis is essential because the dissolution of Cr occurs by the anodic electrolysis as described above. Although cathodic electrolysis is not required, it may be performed when indirect electrolysis requires electrolysis at both electrodes. However, in this case, in order to prevent re-precipitation of Cr ions eluted in the electrolytic solution, it is preferable that the final electrode is an anode electrode.

The concentration of sodium sulfate or sodium nitrate is 50 to 250 g per liter of solution, and the temperature is 70
It is preferable that the electrolysis current density is 0.04 to 0.1 A / cm ² . If the concentration of the electrolytic solution is less than 50 g per liter of solution, the scale dissolution rate becomes extremely low even when electrolysis is performed, so that a large amount of time is required for descaling. Further, even if it exceeds 250 g per liter, there is no time reduction effect. When the temperature is lower than 70 ° C., even if electrolysis is performed, the scale dissolution rate becomes extremely low, so that much time is required for descaling. If the temperature exceeds 90 ° C., the amount of evaporation of water in the electrolytic solution increases rapidly, and the concentration control of sodium sulfate and sodium nitrate in the electrolytic solution becomes complicated. If the electrolysis current density is less than 0.04 A / cm ² , a large amount of time is required for descaling since the scale dissolution rate becomes extremely low even when electrolysis is performed. Further, even if it exceeds 0.1 A / cm ² , there is no effect of shortening the time, and the skin tends to be rough.

The concentration of nitric hydrofluoric acid is preferably 5 to 40 g of hydrofluoric acid and 10 to 100 g of nitric acid per liter of the mixed solution. If the amount of hydrofluoric acid is less than 5 g or the amount of nitric acid is less than 10 g per liter of solution, the metal dissolution rate becomes extremely low, so that a large amount of time is required for descaling. If it exceeds 40 g per liter of hydrofluoric acid or 100 g per liter of nitric acid, the surface roughness of the treated material due to dissolution of the base metal becomes remarkable, and the gloss is lost.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on embodiments.

As test materials, five types of ferritic stainless steel cold-rolled annealed sheets (0.5 mm thick) of A, B, C, D and E shown in Table 1 were used. A is a steel containing Nb, B is a steel containing Ti, C is a steel containing Mo, and D is a steel containing Ti and Mo.
E is SUS430 of a comparative material. Table 2 for cold rolled sheets
The samples passed through a normal cold rolling line shown in (1) (but not through the pickling line) were sampled and annealed. This annealing was performed in an experimental furnace.

The descaling process was performed using a test piece of 40 × 40 mm. In neutral salt electrolysis, only anodic electrolysis is performed by direct electrolysis. The descalability was visually determined by the following criteria in comparison with a completely descaled sample. ◎: Completely descaled and no rough skin △: Completely descaled but rough skin ×: Remaining scale or yellowing remaining In Table 3, neutral salt electrolysis was 5 under each descaling condition.
2 shows the result of descaling when immersion in nitric hydrofluoric acid for 10 seconds. From the results of Examples and Comparative Examples, when the method of the present invention is performed, SUS430 causes severe skin roughness,
It can be seen that the descaling of the steel containing b, Ti, and Mo is completed in a very short time without causing skin roughness.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

[Table 3]

[0020]

[Table 4]

[0021]

According to the present invention, a neutralized ferritic stainless steel strip containing at least one of Nb, Ti, and Mo, which requires a long time for descaling in a conventional neutral salt electrolysis method, is used. Continuous descaling can be performed at high speed without causing skin roughness only by the steps of electrolysis and nitric acid dipping and pickling, and investment and running costs are low.

Continuing on the front page (72) Inventor Shoji Nagashima 3434 Shimada, Hikari-shi, Yamaguchi Prefecture Inside Nippon Steel Corporation Hikari Works (72) Inventor Tsuyoshi Tanaga 3434 Shimada, Hikari-shi, Hikari-shi, Yamaguchi Prefecture Nippon Steel Corporation Inside the Hikari Works (72) Inventor Shunji Shoda 3434 Shimada, Hikari-shi, Yamaguchi Prefecture Nippon Steel Inside the Hikari Works (72) Inventor Shinya Kunioka 3434 Shimada, Hikari-shi, Hikari-shi, Yamaguchi Shin-Nippon Steel (58) Field surveyed (Int. Cl. ⁶ , DB name) C25F 1/06

Claims (3)

(57) [Claims] When removing the scale of an annealed ferritic stainless steel strip containing at least one of Nb, Ti, and Mo, an aqueous solution containing sodium sulfate or sodium nitrate and having a pH of 0 to 3 is used as an electrolyte. A method of descaling an annealed ferritic stainless steel strip, comprising subjecting the sheet to electrolytic treatment and subsequently immersing in a mixed solution of nitric acid and hydrofluoric acid. 2. The concentration of sodium sulfate or sodium nitrate in the electrolytic treatment is 50 to 2 per liter of the solution.
50 g, temperature 70-90 ° C., electrolysis current density 0.04
Characterized in that it is a ~0.1A / cm ^2, descaling method of ferritic stainless annealing steel strip according to claim 1. 3. The concentration of nitric acid and hydrofluoric acid in the immersion treatment is 5 to 40 g of hydrofluoric acid and 10 to 1 nitric acid per liter of the mixed solution.
The descaling method for a ferritic stainless steel annealed steel strip according to claim 1 or 2, wherein the weight is 00 g.