KR950004239B1 - Electrolytic cleaning method of austenite cold roled annealing stainless sted sheets - Google Patents

Abstract

The cold rolled annealed austenite group stainless steel sheet is annealed under oxidizing atmosphere, and electrolytic pickling treated in electrolytic solution, prepared by adding 5˜10 wt.% Na2O3 to 5˜10 wt.% aqueous sulfuric solution, at 50˜60 deg.C with 0.1˜0.3 A/cm2 current density. The descaling process provides good surface quality.

Description

Electrolytic pickling method of austenitic stainless steel

1 is a process chart showing a process of electrolytic pickling an austenitic stainless steel cold rolled and annealed steel sheet according to the present invention.

2 is a surface texture photograph of a cold rolled annealed steel sheet pickled after electrolytic descaling according to the present invention method and a comparative method.

The present invention relates to a method for pickling austenitic stainless steel cold rolled and annealed steel sheet, and more particularly, to an austenitic stainless steel which performs electrolytic descaling in place of molten salt treatment when removing an oxide scale from an austenitic stainless steel surface. The present invention relates to an electrolytic pickling method for cold rolled and annealed steel sheets.

In general, in order to manufacture stainless cold rolled steel sheet, after annealing in an oxidizing atmosphere after cold rolling, a quenching process by forced air cooling or spray (spray) or mist (mlst) is performed, and at this time the annealing oxide scale formed on the surface of stainless steel sheet Descaling is performed to remove it.

In this descaling treatment, pickling with sulfuric acid, nitric acid, nitric acid (mixture of nitric acid and hydrofluoric acid) is generally performed, but the oxidation scale formed on the stainless steel cold rolled steel sheet is dense and robust, which makes it difficult to completely descale it. .

Therefore, a pretreatment method for facilitating pickling is developed and put into practical use by preliminary descale treatment such as immersion treatment in molten alkali salt or electrolytic treatment in an aqueous neutral salt solution (Japanese Patent Application No. 62-49197, Japanese Patent Application No. 61-313619).

However, the molten salt treatment has a problem that is extremely dangerous in operation because it is a method of immersing in a solution containing a high concentration caustic soda around 500 ℃. In addition, there is a problem of causing scratches due to a foreign material indentation groove by the reaction product in the salt bath and a roll installed in the salt bath during stripping.

In addition, since the high-viscosity fluid is a high viscosity, there is a problem that the raw unit is increased due to a large amount of salt (Salt) buried in the mailing strip. And since it solidifies below 300 degreeC, uniform temperature control is needed so that the temperature in a salt bath may be substantially constant.

On the other hand, Japanese Patent Application Laid-Open No. 62-313619 discloses a method of electrolytic descaling in an aqueous solution of sodium sulfate, which is less dangerous than molten salt treatment, but takes longer to scale. The solution temperature should be 80 ℃ or higher and the solution composition contains 20wt% or more, which causes the unit increase.

The present invention has been proposed to solve the problems of the conventional methods described above, and is safe by preliminary ralscale treatment by adding sodium nitrate to an aqueous sulfuric acid solution, and austenitic stainless steel which can improve the cold acidity and quality of the pickling process. To provide an electrolytic pickling method of cold-rolled and annealed steel sheet, its purpose is.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

In the present invention, the austenitic stainless cold rolled annealing steel sheet is subjected to annealing in an oxidizing atmosphere, followed by a descaling treatment. In the method of pickling treatment, the day scale treatment is performed in a 5-10 wt% sulfuric acid solution of 5-10 wt%. In the electrolytic pickling method of an austenitic stainless cold rolled annealing steel sheet, which is carried out by electrolytic pickling solution prepared by adding NaNO ₃ to maintain a temperature range of 50-60 ° C. and a current density of 0.1-0.3 A / cm 2. It is about.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

According to the present invention, the electrolytic pickling causes the scale to be peeled off by the volume expansion force of H ₂ gas generated on the cathode side and O ₂ gas generated on the anode side by reaction between sulfuric acid and a known metal in the aqueous sulfuric acid solution.

On the other hand, the scale and volume of the sodium nitrate are caused to undergo volume expansion of the scale and the scale is peeled off.

2FeO + NaNO ₃ → Fe ₂ O ₃ + NaNO ₂

It also reacts with the spinel layer in the scale to dissolve Cr from the scale.

Cr ₂ O ₃ + 6NaNO _{3 +} 3H ₂ O → 2Na ₂ CrO ₄ + 2NaNO ₂₊ 4HNO ₃₊ H ₂ ↑

The action of anions and cations in aqueous solution is as follows.

SO ₄ ^2- : Move to the positive electrode (stainless steel plate surface) to dissolve chromium oxide on the stainless steel plate surface.

NO ₃ ⁻ : SO ₄ ^2- , Na +, H ⁺ , promotes, inhibits and regulates the respective actions, and dissolves chromium oxide to Cr ⁶⁺ .

H ⁺ : The mobility to the cathode is six times that of Na +, which reduces the migration of SO ₄ ^2- , NO ₃ ^- ions to the anode. If the H ⁺ concentration is high, the base metal dissolves preferentially in reaction with the stainless base, causing pitting.

Na +: Mobility is slow to the cathode compared to H ⁺ to promote the migration of SO ₄ ^2- , NO ₃ ^- ions to the anode.

Sulfuric acid in the present invention serves to peel off the scale by volume expansion occurs on the scale by the generation of hydrogen gas (H ₂ ) from the cathode of the electrode.

At this time, the amount of aqueous sulfuric acid solution is preferably 5-10wt% in the total pickling solution, because if less than 5wt% hydrogen gas is generated less descale effect, if more than 10wt% descale exfoliation deepening excess This is because mixed acid penetrates and may cause over-acid tax.

In the present invention, the NaNO ₃ concentration added to the aqueous sulfuric acid solution is preferably set to 5-10wt%. The reason is that when the NaNO ₃ concentration is less than 5wt%, descaling or the like decreases, and when the NaNO ₃ concentration exceeds 10wt%, This is because a pit is formed on the surface of the steel sheet.

Therefore, in the present invention, the electrolytic acid wash is formed by adding 5-10 wt% of NaNO ₃ to a 5-l0 wt% sulfuric acid solution.

It is preferable to set the temperature of the aqueous sulfuric acid solution containing NaNO ₃ to 50-60 ° C. during electrolysis, because if the sulfuric acid solution temperature is less than 50 ° C., the reactivity with the scale is low, and descaling cannot be expected. If it exceeds, the reaction with the body is promoted and taxation occurs.

On the other hand, it is preferable to set the density of the externally applied current to 0.1-0.3 A / cm 2, because the current density is too small when the current density is less than 0.1 A / cm 2, so that descaling reaction cannot be expected. It is because a pit is formed due to overcorrosion when it exceeds / cm2.

As for the electrolysis time in this invention, 30-60 second is preferable like a normal operation.

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example)

After STS 304 stainless steel cold rolled steel sheet was annealed for 3 minutes in an oxidizing atmosphere at 1090 ° C. to form a dense scale on the surface, 5-10 wt /% NaNO ₃ was added to a 5% sulfuric acid solution as shown in Table 1, and the current density was 0.1-%. Electrolytic descale treatment was performed for 30-60 seconds under the condition of 0.3 A / cm 2, solution temperature: 50-60 ° C.

Next, after pickling was carried out by immersion for 30 seconds in a mixed acid solution composed of 60%, 7% HNO ₃ + 2% HF composition, the degree of descale of the surface of the steel sheet was observed, and the results are shown in Table 1 below.

On the other hand, in the following Table 1, the invention example (b), the comparative example (1), and the comparative example (4) were observed the steel plate surface structure photograph, (a) of FIG. 2, the invention example (b), (b) In Comparative Example (1), (C) shows Comparative Example (4).

○: No scale residue after pickling

△: part of scale remains after pickling

×: scale remains after pickling

◎: taxation

As shown in Table 1, in the case of the invention (ah) subjected to descaling by electrolytic treatment under conditions consistent with the present invention, it can be seen that no scale remains after pickling. It can also be confirmed in FIG. 2 (a) which is a surface tissue photograph of b).

On the contrary, when 5% sulfuric acid solution without NaNO ₃ was added, the scale was immersed in the mixed acid solution after descaling [Comparative Example (1-3)]. In case of pickling in mixed acid after being immersed in the solution (Comparative Example (4)), it can be seen that a considerable amount of scale remains, and this fact indicates that the surface tissue photographs of Comparative Examples (1) and (4) are respectively shown. It can also be seen in 2 degrees (b) and (c).

On the other hand, when NaNO ₃ 3 is added more than the range of the present invention and / or the current work is higher than the range of the present invention (Comparative Examples 95-13), it can be seen that peracid tax occurs.

As described above, the present invention performs electrolytic descaling in place of molten salt treatment when removing the oxidized scale of the surface of the austenitic stainless steel sheet, thereby improving the surface quality of the final cold rolled product, thereby reducing productivity. You can expect the improvement of the effect.

Claims (1)

In a method in which an austenitic stainless cold rolled annealing steel sheet is annealed in an oxidizing atmosphere, followed by descaling, and then pickling treatment, 5-10 wt% of NaNO ₃ is added to a 5-10 wt% sulfuric acid solution. And electrolytic pickling solution formed at a temperature range of 50-60 ° C., and electrolytic at a current density of 0.1-0.3 A / cm 2, for electrolytic pickling of an austenitic stainless cold rolled annealing steel sheet.