JPH0657500A - Method for pickling high-cr ferritic stainless steel sheet - Google Patents

Abstract

PURPOSE:To rapidly remove an oxide film at a low cost by AC-electrolyzing a Cr-contg. steel sheet coated with an oxide film in an aq. soln. having specified contents of sulfuric acid, BF4 and NO3. CONSTITUTION:A steel sheet contg. >=14wt.% Cr and coated with an oxide film is AC-electrolyzed in an aq. soln. and pickled. The aq. soln. contains 10-300g/l of sulfuric acid, 1.5-8g/l of BF<->4 and 7-80g/l of NO<->3 as a sulfate. However, when the content of sulfuric acid is 10-60g/l, sulfuric acid (g/L)/nitrate (g/L)>=1. Consequently, a pickling method of high industrial value excellent in economical efficiency and pollution resistance is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing an oxide film from ferritic and martensitic stainless steel sheets containing 14% by weight or more of Cr.

[0002]

2. Description of the Related Art Cr-containing steel sheets that have undergone heat history such as hot rolling and hot-rolled sheet annealing, or Cr that has been annealed after cold rolling.
The presence of the oxide film formed on the surface of the steel sheet contains many problems such as decreasing the smoothness of the final product surface and commercial value, shortening the life of the die during drawing, and making welding work difficult. Therefore, it is necessary to completely remove it. Therefore, for example, "Handbook of Steel, April 5, 1937, Maruzen K.
K, page 1258, Table 15.3, "nitric acid and hydrofluoric acid", "sulfuric acid, nitric acid and chromic acid", "sulfuric acid, nitric acid and hydrofluoric acid".
Method of immersing in various pickling solutions such as etc. at high temperature, method of electrolyzing in neutral salt such as sodium sulfate called Rusner method, method of cleaning in high temperature molten salt called molten salt immersion method Are known.

[0003]

Among these oxide film removal methods, the pickling method is one in which the Cr content of the target stainless steel sheet is more than a certain amount, or the steel sheet is used in order to improve the use performance of the steel sheet. Elements to be added, such as Mo, Nb, T
When i, Ni, and Cu are present, the removal efficiency of the oxide film is extremely reduced, which causes various problems in manufacturing. In addition, the Rusner method and the molten salt dipping method require a treatment time of several tens of seconds, so that it is necessary to lengthen the treatment line in order to ensure productivity.

It has been conventionally said that it is extremely difficult to chemically dissolve an oxide film of a stainless steel sheet with a solution containing no hydrofluoric acid. To remove the oxide film on the surface, the acid penetrating from the crack portion of the oxide film is the base material. It is mentioned that H ₂ is dissolved and H ₂ generated at that time mechanically peels off the oxide film.
Therefore, it takes a long time to completely remove the oxide film on the surface by this method. On the other hand, the dissolution of the oxide film is relatively easy when using a nitric acid-hydrofluoric acid aqueous solution,
Generation of NO _x, waste treatment, not favored in terms of operational safety.

[0005]

According to the present invention, in alternating electrolytic pickling, when a fluoroborate is added to a pickling solution based on a sulfuric acid aqueous solution, an oxide film is dissolved by an anodic reaction, and when a nitrate is added, a cathodic reaction is performed. By utilizing the fact that the dissolution of the base material is accelerated, the surface finish of the stainless steel sheet is achieved in a short time and in a low pollution environment.

That is, according to the present invention, a steel sheet containing 14% by weight or more of Cr with an oxide film attached is used in an amount of 10 to 300 g / L.
Sulfuric acid, 1.5-8 g / L BF ₄ ^- and 7-80 g /
L of NO ₃ ^- (provided that added in the form of sulfate) consists of (except sulfuric acid when sulfuric acid is 10 to 60 g / L (g / L)
/ Nitrate (g / L) ≧ 1. ) The gist is a method of pickling a Cr-containing steel sheet by alternating electrolysis in an aqueous solution.

[0007]

The details of the present invention will be described below. Sulfuric acid used as the main ingredient of the present invention has a very slight oxide film dissolution reaction,
It is involved in the dissolution reaction of the base material in the part where the oxide film is dissolved.
Further, the fluoroborate salt is involved in the reaction of directly dissolving the oxide film. The mechanism is that, for example, in the case of sodium fluoroborate, when a steel sheet exists as an anode, peroxosulfuric acid generated as a result of coexisting with sulfuric acid on the surface thereof transforms an oxide film into an oxide that is easily dissolved in water by the following chemical reaction. Seem.

[0008] [Formula 1] _{Cr 2 O 3 + H 2 S} 2 O 8 + H 2 O + 20 2 + 4e - → 2CrO 4 - + 2H 2 S
_{O 4 Cr 2 O 3 + H} 2 S 2 O 8 + H 2 O + 3/20 2 + 2e - → Cr 2 O 7 - +
2H ₂ SO ₄

Nitrate increases the amount of green body dissolved in the cathodic reaction. It is considered that this is because the supply of reaction ions due to the disturbing action of H ₂ generated in large quantities at the cathode, the activation of the surface, and the surface heat generation due to energization promote the Fe dissolution of sulfate ion-nitrate ion.

Various combination experiments have established a correlation between the amounts of "sulfuric acid", "nitrate" and "fluoroborate". First, sulfuric acid needs to be 10 g / L or more in order to efficiently perform pickling, and preferably 60 g
/ L or more. Fluoroborate, such as sodium fluoroborate, was an essential component for the dissolution reaction of the oxide film. The effect amount is BF ₄ ^- as a 1.5g
/ L or more becomes remarkable, and it becomes saturated at 8 g / L. Fluorosilicates were investigated as a substitute for fluoroborate, but the effect was significantly less than that of fluoroborate. Also, this effect of fluoroborate was affected by the amounts of sulfuric acid and nitrate.

When the amount of sulfuric acid is 60 g / L or less, the nitrate is
If the amount is too large, the dissolved amount of the base material is reduced. This is NO₃ ^-of
It is thought that their existence contributed to the deactivation of the steel sheet surface.
The amount relationship between the two to prevent this is sulfuric acid / nitrate ≧ 1
It was However, this phenomenon disappears when the amount of sulfuric acid exceeds 60 g / L.
Lost If both satisfy the above quantitative relationship, fluorophore
The acid film dissolution reaction of the acid salt continued. Fluoroborate
Oxide film dissolution rate of BF _Four ^-When the amount exceeds 8 g / L
Saturated Also, the amount of sulfuric acid is 300 g / L, and nitrate
NO supplied by₃ ^-If the amount exceeds 80 g / L,
The dissolved amount became saturated. Incidentally, NO₃ ^-Is a solution of pickling solution
Add 7 g / L or more to improve the dissolving ability. Also,
NO₃ ^-NO when added with nitric acid_xA large amount of
It was convenient.

[0012]

EXAMPLES Examples of the present invention will be described below. A cold rolled stainless steel plate of SUS430 was annealed in a continuous annealing line and pickled under the conditions shown in Table 1. The results are shown in Table 1.
From this table, it is clear that the steel sheet pickled under the conditions of the present invention shows performances comparable to those of the conventional method requiring a long time in appearance, contact resistance, and corrosion resistance in all cases.

[0013]

[Table 1]

[0014]

INDUSTRIAL APPLICABILITY As described above, the present invention provides a rapid oxide film removal method suitable for cost reduction by high-speed mass production of Cr-containing steel sheet, and has an economical and industrial value excellent in pollution resistance. Is extremely high.

Claims (1)

[Claims] 1. 14% by weight or more of Cr with an oxide film attached
Steel plate containing 10 to 300 g / L of sulfuric acid, 1.5
To 8 g / L of BF ₄ ^- and 7~80g / L of NO
₃ ^- (provided that added in the form of sulfate) composed of (. Although sulfuric acid when the 10 to 60 g / L and sulfuric acid (g / L) / nitrate (g / L) ≧ 1) alternating in an aqueous solution A method for pickling a Cr-containing steel sheet, which comprises electrolyzing.