JPH0790697A - Descaled stainless steel - Google Patents

Abstract

PURPOSE:To provide the descaled stainless steel having brightness and high smoothness by a high-speed descaling method which does not entail a high-temp. treatment. CONSTITUTION:An annealed stainless steel sheet 1 is subjected to an electrolytic treatment in an aq. neutral salt electrolytic tank 1 and is subjected to an electrolytic treatment or an immersion treatment in an aq. alkaline soln. in an aq. alkaline soln. electrolytic tank 6. The stainless steel sheet is thereafter subjected to the electrolytic treatment in an aq. nitric acid soln. electrolytic tank 10 or the immersion treatment in an aq. nitric acid/hydrofluoric acid mixed soln. immersion tank, by which the oxide scale on the surface is removed. The stainless steel from which the scale is substantially completely removed and which has the excellent brightness and smoothness is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a descaled stainless steel, and more particularly to a stainless steel from which oxide scale produced in continuous annealing after cold rolling is removed.

[0002]

2. Description of the Related Art Cold rolled stainless steel strips are annealed to remove work hardened layers. At this time, the oxide scale formed on the stainless steel surface significantly impairs the commercial value, and it is necessary to remove it. As a removing method, there is a method of electrolyzing in a strong acid such as sulfuric acid or a method of immersing in an alkali molten salt. These methods have drawbacks such as a rough surface, a long removal time, and difficulty in handling. As a method for solving the above-mentioned difficulties, Japanese Patent Publication No. 38-1
No. 2162, a method of electrolyzing in a neutral salt aqueous solution and then dipping in a nitric acid-hydrofluoric acid mixed aqueous solution or JP-B-5
As described in JP-A-3-13173, a method of electrolyzing in a neutral salt aqueous solution and then electrolyzing in a solution containing nitrate ions has been proposed.

[0003]

The above-mentioned prior art focuses only on the scale removal of the stainless steel surface, and the glossiness and smoothness of the stainless steel surface after the scale removal, the speedup of the descaling treatment, and the molten salt. Sufficient consideration has not been given to aspects such as improvement of workability at high temperature in alkali dipping treatment, and thus there is a problem that it is difficult to improve speed of scale removal and workability. An object of the present invention is to provide a descaled stainless steel having gloss and good smoothness by a high-speed descaling method that does not involve high temperature treatment.

[0004]

Means for Solving the Problems The above-mentioned object is to carry out a step of (a) electrolyzing a scale-formed stainless steel in a neutral salt aqueous solution and a step (b) of electrolyzing or dipping in an alkaline aqueous solution. After carrying out both steps (a) and (b) in the order of step (a) and step (b) or step (b) and step (a), the stainless steel treated in both steps is further treated in an aqueous nitric acid solution. Electrolysis or a treatment including a step of dipping in a mixed aqueous solution of nitric and hydrofluoric acid.

More specifically, the above method is applied to (a) a neutral salt aqueous solution electrolytic cell having a plurality of positive and negative electrodes, and (b) an alkaline aqueous solution electrolytic cell having a plurality of positive and negative electrodes or an alkali. Both (a) and (b) of the aqueous solution immersion tank are provided in the order of (a) tank (b) tank or (b) tank (a) tank, and a plurality of positive tanks are provided behind both tanks. By treating with a stainless steel continuous descaling device equipped with a nitric acid aqueous solution electrolytic bath with a negative electrode or a nitric hydrofluoric acid mixed aqueous solution dipping treatment bath, the scale is substantially removed and the surface has excellent gloss and smoothness. A stainless steel having good properties can be obtained easily at high speed. It is preferable that each electrode provided in each electrolytic cell of the continuous descaling device is an insoluble electrode arranged so as to face continuously moving stainless steel.

[0006]

The scale formed by annealing on the surface of stainless steel is spinel type oxide. An ordinary (800 ° C. or higher) annealing heat treatment produces an iron / chromium spinel oxide composed of FeCr ₂ O ₃ containing Fe ₃ O ₄ . The electrolytic or dipping treatment of the stainless steel having the scale in each of the neutral salt aqueous solution, the alkaline aqueous solution, and the nitric acid aqueous solution or the nitric hydrofluoric acid mixed aqueous solution used in the method including the treatment step for removing the scale is performed as follows. It has the following action.

The neutral salt electrolysis mainly has a function of dissolving chromium in iron-chromium spinel oxide. That Cr-H ₂ O system potential -pH view of FIG. 2 (M.Pourbaix: Atl
as of Electrochemical Equilibria in Aque-ous Solut
ions (1966) Pergamon Press) show that chromium dissolves as Cr ₂ O ₇ ²⁻ by anodic polarization to +0.2 V or higher on the saturated calomel electrode standard in the neutral to acidic pH range. In ordinary neutral salt electrolysis, Na ₂ SO ₄ is used as an electrolytic salt. Na ₂ SO ₄ has the function of increasing the conductivity of the electrolytic solution. Since the pH is usually electrolyzed in the neutral to weakly acidic region, the scale is dissolved as Cr ₂ O ₇ ²⁻ .

Electrolytic treatment in an alkaline aqueous solution such as an aqueous NaOH solution or an aqueous KOH solution has the following effects. That is, chromium in the scale is dissolved as CrO ₄ ²⁻ .
It can be seen that the electrolysis potential in this case is obtained by anodic polarization at a noble potential of about -0.35 V or higher based on the saturated calomel electrode at pH 13-14. That is,
The chromium oxide can be efficiently dissolved and removed by dissolving chromium oxide as CrO ₄ ²⁻ at a considerably lower potential than in the neutral salt electrolysis.

Iron in scale is dissolved in nitric acid aqueous solution electrolysis
Has the effect of In this case, electrolysis uses stainless steel.
Perform as a sword. That is, spinel oxide scale
The iron in the iron is mixed with divalent and trivalent iron, but the usual acid water solution
Divalent iron dissolves in liquid, but trivalent iron dissolves very quickly
The degree is small. However, by reducing trivalent iron to divalent iron
Therefore, a practical dissolution rate can be obtained. Caso in nitric acid solution
Electrode supplies electrons to stainless steel to generate 3
Valent iron is reduced to divalent, and at the same time nitric acid produces Fe. ²⁺As
Dissolve and remove. Fe³⁺(Oxide) + e^-→ Fe²⁺(Ion)

The spinel type oxide scale produced on the stainless steel by the above three kinds of electrolytic treatments has high efficiency,
High workability and can be removed at high speed. In the combination of the three kinds of electrolytic treatments of the present invention, the effect is the same regardless of which of neutral salt aqueous solution electrolysis and alkaline aqueous solution electrolysis is first. It is effective to use nitric acid aqueous solution electrolysis in the final step after removing chromium oxide, which is difficult to remove.

In the present invention, the workability is remarkably improved because it does not involve the high temperature treatment unlike the conventional alkali molten salt. Further, the problem of scale dissolution rate due to slightly lower efficiency of neutral salt aqueous solution electrolysis in neutral salt aqueous solution electrolysis → nitric acid aqueous solution electrolysis is solved by performing highly efficient alkaline electrolysis, and the scale removal rate is improved.

Further, in the method for descaling stainless steel according to the present invention, the electrolytic treatment in an alkaline aqueous solution does not necessarily require electrolytic treatment when the amount of scale is small, and it is simply immersed in the aqueous solution. It can dissolve oxides. Further, the scale removing effect does not change even if the substrate is immersed in a nitric acid-hydrofluoric acid mixed aqueous solution instead of the nitric acid aqueous solution electrolytic treatment. It is more effective if the neutral salt aqueous solution electrolysis and the alkaline aqueous solution electrolysis are mainly anodic electrolysis, and the nitric acid electrolysis is mainly cathodic electrolysis.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. Example 1 FIG. 1 shows an example of an apparatus for obtaining descaled stainless steel having gloss and good smoothness.

A stainless steel strip 1 having a scale formed on its surface in a continuous annealing furnace is introduced into a neutral salt aqueous solution electrolytic treatment tank 2. The neutral salt aqueous solution electrolytic treatment tank 2 contains Na ₂ SO ₄
A positive voltage is applied to the stainless steel strip 1 by a pair of positive electrodes 3 provided in a non-contact manner, which is filled with an aqueous solution having a concentration of 20% and a pH of 6, and a pair provided in front and rear sides of the positive electrode in a non-contact manner. Counter electrode 3'becomes a negative electrode, and by this combination, a current flows from the stainless steel strip 1 mainly for anodic electrolysis to the stainless steel strip through the Na ₂ SO ₄ aqueous solution to the counter electrode 3 '. With this current, chromium in the scale becomes Cr ₂ O ₇ ²⁻ and dissolves. Next, the stainless steel strip 1 enters the washing tank 4 and Na ₂ S remaining on the surface
Wash O ₄ . Next, the washing water is squeezed by the ringer roll 5 and then introduced into the alkaline aqueous solution electrolytic treatment tank 6.
The alkaline aqueous solution electrolysis tank 6 is filled with an aqueous solution having a concentration of 40% NaOH, a positive voltage is applied to the stainless steel strip 1 by a positive electrode 7, and a current flows through the aqueous NaOH solution to the counter electrode 7 ′ to carry out anodic electrolysis. The main electrolysis is performed. Due to the current flowing at this time, the chromium oxide in the scale becomes CrO ₄ ²⁻ and is dissolved and removed. Chromium oxide is removed on the surface of the stainless steel strip 1, leaving iron oxide. Next, the stainless steel strip 1 enters the water washing tank 8 to wash and remove the NaOH remaining on the surface, and the washing water is squeezed by the ringer roll 9. Then stainless steel strip 1
Is introduced into the nitric acid aqueous solution electrolytic treatment tank 10. The nitric acid aqueous solution electrolysis treatment tank 10 is filled with a 10% concentrated nitric acid aqueous solution, in which a current flows through the positive electrode 11 to the stainless steel strip, and the counter electrode 11 'serves as a negative electrode. Electrolysis is performed mainly by reverse cathodic electrolysis. Positive,
For the negative electrodes 11 and 11 ', an insoluble electrode such as a titanium palladium coated plate or a titanium platinum coated plate is used in order to prevent dissolution and consumption in a nitric acid aqueous solution. Since stainless steel is subjected to cathodic electrolysis here, Fe (III) in the scale becomes Fe (II) as described above, and is eluted as Fe ^{2+ in} the solution. By the above three kinds of electrolytic treatments, the scale made of iron-chromium spinel oxide on stainless steel is removed with high efficiency and high speed. Further, the stainless steel strip 1 is washed with water to remove residual HNO ₃ on the surface in a washing tank 12, dried with a ringer roll 13, dried with a dryer 14 and sent to the next step.

As is clear from Table 1, in the examples according to the present invention, the scale was completely removed, and the stainless steel surface after the scale removal had a smooth, glossy and beautiful mirror surface. On the other hand, in the comparative example according to the conventional method shown in Table 1, the scale removal was incomplete or the stainless steel surface was clouded after the removal, resulting in surface roughness.

The scale removal condition can be evaluated by observing the scale with an optical microscope of 400 times and setting 100 when the scale residual mark is zero in the visual field, and by subtracting the number of scale residual marks from 100. . Here, the value is approximately 95 or more for ◎, 70 to less than 95 for ◯, and 50 to 70.
Less than was evaluated as Δ and less than 50 was evaluated as x. Among the evaluations of the stainless surface state, the evaluations of “gloss”, “slightly cloudy” and “cloudy” were performed by Method 5 of “JIS Z 8741”. That is, the incident angle and the light receiving angle are 20
"Gloss" when the glossiness at 150 is 150 or more, 100-1
The time of 50 was "slightly cloudy", and the time of 50 to 100 was "cloudy". The annealed material before the descaling treatment had a scale with a thickness of about 0.1 μm on its surface, exhibited a bluish purple color, and had a glossiness of 50 or less. The determination of "smooth" or "large surface roughness" was made by measuring with a surface roughness meter, and when the average roughness was approximately 0.4 μm or less, it was regarded as smooth, and when it exceeded it, the surface roughness was judged as large.

In the electrolytic treatment of this embodiment, it is natural that the scale can be easily removed by raising the temperature of the electrolytic solution. Table 1 shows the conventional method (electrolysis of neutral salt aqueous solution + electrolysis of nitric acid aqueous solution, electrolysis of neutral salt aqueous solution + for comparison with the descaling state of the stainless steel treated in Example 1).
Comparative Examples 1 and 2 also show the case of immersion in a nitric acid-hydrofluoric acid mixed aqueous solution. The stainless steel used is ferritic S
It is a plate with a thickness of 0.5 mm of US430. The electrolysis conditions are: neutral salt solution electrolysis: anode electrolysis, current density 6 A / d
m ² alkaline aqueous solution electrolysis: anode electrolysis, current density 3 A / d
m ² nitric acid aqueous solution electrolysis: cathode electrolysis, current density 2 A / d
m ² .

[0018]

[Table 1]

Example 2 The descaling method of Example 1 shown in the potential-pH diagram (25 ° C.) of Cr—H ₂ O element in FIG. Descaling was performed by changing the order of the aqueous solution electrolytic bath 6.

That is, the stainless steel strip was first electrolyzed in an alkaline aqueous solution electrolytic cell by applying a positive voltage to the stainless steel strip. Next, a positive voltage was applied to the stainless steel strip in a neutral salt aqueous solution electrolyzer to electrolyze the neutral salt aqueous solution. Then, a negative voltage was applied to the stainless steel strip in the nitric acid aqueous solution electrolysis cell for electrolysis. The cleaning treatment and draining during each electrolysis treatment and after the electrolysis of the nitric acid aqueous solution are the same as in Example 1. By this method, the scale was completely removed, and a stainless steel strip having a smooth and glossy surface was obtained. Table 2 shows the processing conditions and processing results. Table 2 also shows other processing conditions and the processing results thereof.
This will be shown as Example 7.

[0021]

[Table 2]

[0022]

According to the present invention, the scale of stainless steel can be rapidly removed without using a high-temperature molten salt bath which is difficult to handle, and it has a beautiful and excellent surface condition which cannot be obtained by the conventional electrolytic treatment. High quality stainless steel plate can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing the steps of a descaling process.

FIG. 2 is a diagram showing a potential-pH diagram of a Cr—H ₂ O system.

[Explanation of symbols]

1 ... Stainless steel strip, 2 ... Neutral salt aqueous solution electrolytic treatment tank, 3
... Positive electrode, 3 '... Negative electrode, 4 ... Water washing tank, 5 ... Ringer roll, 6 ... Concentrated alkaline aqueous solution electrolytic treatment tank, 7, 7' ... Electrode, 8 ... Water washing tank, 9 ... Ringer roll, 10 ... Nitric acid aqueous solution Electrolysis treatment tank, 11, 11 '... Electrode, 12 ... Water washing tank, 13
… Lingalore, 14… Dryer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Teruo Yamaguchi Inventor Teruo Yamaguchi 3-1-1, Saiwaicho, Hitachi-shi, Ibaraki Hitachi Ltd. Hitachi factory

Claims (1)

[Claims] 1. A surface oxide scale after annealing, followed by electrolytic solution treatment of a neutral salt solution and electrolytic solution treatment of an alkaline aqueous solution or immersion treatment of an alkaline aqueous solution, followed by electrolytic treatment of a nitric acid aqueous solution or immersion treatment of a nitric / hydrofluoric acid mixed aqueous solution. Descaled stainless steel obtained by removing the.