JPH0841666A - Descaling picking method - Google Patents

Abstract

PURPOSE:To prevent the pitting of the surface of a steel at the time of removing the scale on the steel surface by pickling by conducting pickling in two stages respectively in a bath with the composition regulated. CONSTITUTION:When the scale formed on the surface of a steel is removed, the steel is dipped in about 10% aq. sulfuric acid to dissolve and remove the scale consisting of Fe3O4, Fe2O3, FeO, etc. The steel surface is then washed with water, the remaining aq. sulfuric acid is neutralized with aq. NaOH, and the steel is introduced into a surface-treating tank contg. aq. KMnO4 alkalized with NaOH to make the scale which has not been dissolved in the aq. sulfuric acid easily soluble in acid. The steel is then secondly pickled after shower washing with aq. sulfuric acid to completely dissolve off the scale on the steel surface. In this operation, when the first and second aq. sulfuric acid are electrolyzed to recover sulfuric acid, the first and second aq. sulfuric acid are separately electrolyzed, hence the Mn ion content of the first aq. sulfuric acid is reduced to >=500ppm, and the steel surface is not pitted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a method of pickling a steel material for the purpose of descaling, such as pitting.
A pickling method that prevents the occurrence of

[0002]

2. Description of the Related Art Hydrochloric acid baths have been widely used as acid baths for pickling steel coils, especially wire coils, for descaling, but since fume hydrochloric acid causes corrosion of equipment and adversely affects the environment, Currently, most pickling is carried out in a sulfuric acid bath. However, the sulfuric acid bath has a drawback that pitting occurs along with descaling.

On the other hand, the acid solution after pickling is electrolyzed in a bath provided with an ion exchange membrane to separate and recover the acid and reuse it. When performing this acid recovery and reuse, the problem of pitching becomes more serious.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a pickling method which solves the above problems associated with the descaling and pickling of steel materials and which can perform descaling while avoiding the occurrence of pitting. Especially.

[0005]

The descaling and pickling method for steel products according to the present invention is a pickling method for descaling steel products in a sulfuric acid bath, which is acid bath dipping I-neutralization-surface conditioning-acid bath dipping. II-
In a method consisting of various steps of coating treatment, the total amount of Mn ions in the acid bath Immersion I is 500 ppm.
It is characterized by using those regulated below.

When the acid solution after use is electrolytically treated to separate and collect the acid for reuse by circulation, the acid solution of the acid bath dipping I and the acid bath dipping
It should be treated separately and circulated separately without mixing with the acid solution of II.

[0007]

Function: Pickling for descaling is usually 10%
Immerse the steel material in a hot H ₂ SO ₄ bath (acid bath immersion I),
It begins with the dissolution of surface oxides (Fe ₃ O ₄ , Fe ₂ O ₃ , FeO). After washing with water in a shower and neutralizing the acid in a neutralizing tank, the steel material is sent to a surface conditioning tank called a desmut adjusting tank. (By "desmutting" is an operation to modify the scale "smut" acid sparingly soluble scale of San'eki soluble.) In this case, Fe-Cr- of undissolved steel surface in H ₂ SO ₄ bath O compound, alkaline K
Change to a form that is easily soluble in acid in MnO ₄ aqueous solution.

The steel material subjected to the surface conditioning is washed with shower water and then enters a _second hot bath of H ₂ SO ₄ (acid bath dipping II) to dissolve the Fe-Cr-O compound, which was easily dissolved in the previous step. To do. The steel material that has been pickled in this way is subjected to a coating treatment for preventing rusting and facilitating the subsequent wire drawing work after shower rinsing and becomes a product.

[0009] was pursuing the cause of pitching in the above-mentioned process, the presence of Fe ³⁺ is Fe ³⁺ + Fe → 2F
It has been found that the mechanism of e ²⁺ causes pitting as a phenomenon of melting the metal of the steel material.

Fe ³⁺ ions are not generated only by dissolving Fe in H ₂ SO ₄ , but are generated by oxidation of Fe ²⁺ . The oxidizing ^ions of Fe ²⁺ to Fe ³⁺ are Mn ⁷⁺ , Mn ⁴⁺ and Cr ⁶⁺ . When steel materials are dissolved in H ₂ SO ₄ , Mn ⁶⁺ and Cr ³⁺ are generated, but since these are stable and have poor oxidizing power, it is mainly the adjustment tank that oxidizes Fe ²⁺ to Fe ^3+. It is considered to be Mn ⁷⁺ derived from MnO ₄ ⁻ . Mn ⁷⁺ is stable in acid but unstable in alkaline, and tends to oxidize other substances to become stable Mn ⁶⁺ . It is difficult to completely remove MnO ₄ ⁻ adhered to the steel material by a shower in the adjusting tank, and MnO ₄ ⁻ enters the H ₂ SO ₄ bath of the acid bath immersion II, and SO ₄ ^{2 is} used for electrolytic recovery of the acid solution. ^- is accompanied anions enters the reproduction sulfate, go mixed acid solution of acid bath immersion I.
It is understood that Mn ⁷⁺ contained in this MnO ₄ ⁻ causes pitting under alkaline conditions in the neutralization tank.

Therefore, it is concluded that the absence of Mn ⁷⁺ (and Mn ⁴⁺ , if present) in the step of acid bath dipping I is effective in preventing pitting.

As a rough guide to the allowable amount of Mn ions, the inventors have empirically established an upper limit of 500 ppm. Preferably, when the amount of Mn ions in the acid solution of Acid Bath Immersion I exceeds 300 to 400 ppm, the acid solution is renewed. As can be easily considered from the above mechanism, it is desirable that the acid obtained by electrolytically recovering the acid solution in the acid bath immersion II is not returned to the acid bath immersion I. Therefore, when electrolytically recovering and reusing the acids, the acids I and II should not be combined but treated separately and circulated in the respective acid baths. However, since it is not preferable to increase the amount of Mn ⁷⁺ even in the acid solution of the acid bath immersion II, it is necessary to newly rebuild the acid bath II when the Mn ion amount exceeds 1000 ppm.

[0013]

EXAMPLES In a descaling and pickling apparatus in which each of the following tanks was continuously connected, a bearing steel, a structural steel, and a spring steel wire rod (φ7 to 44)
mm) coil was continuously pickled.

1) Acid bath immersion I: 10% H ₂ SO ₄ , 60 m ³ , 60 ° C. 2) Shower: Water spray 3) Neutralization: NaOH aqueous solution, room temperature 4) Surface preparation: 4% KMnO ₄ + 12% NaOH, 8
5 ° C 5) Shower: Water sprinkling 6) Acid bath immersion II: 10% H ₂ SO ₄ , 12m ³ , 60 ° C 7) Shower: Water sprinkling 8) Coating treatment: Phosphate “Bonda Leube” Acid solution Analysis was carried out, and when the amount of H ₂ SO ₄ fell below a predetermined value, a part was extracted, filtered, regenerated by electrolysis, and returned to the tank. It goes without saying that new acid is replenished according to the loss. H ₂ SO ₄
The electrolytic recovery of was carried out using a three-chamber electrolysis cell equipped with two cation exchange membranes, defoaming the anolyte and catholyte (containing O ₂ gas and H ₂ gas, respectively) The liquid was further subjected to sedimentation separation of solids (Fe oxides and hydroxides) in a separation tank and circulated in the electrolytic tank. The average amount of acid solution withdrawn was 3 m ³ / hour in both cases.

When the acid solutions extracted from the acid bath immersions I and II are not separated but integrally electrolyzed and circulated, the amount of Mn ions in the acid bath immersion I reaches 500 ppm after 7 days of operation, The need for a bath arose. When both were separated and electrolyzed separately and circulated, the Mn ion concentration of the acid bath immersion I was 500 ppm or less after 30 days of operation,
The Mn ion concentration of II reached 1000 ppm and required a bath.

[0016]

By controlling the acid bath components of the descaling pickling according to the present invention, the descaling pickling of steel can be carried out while avoiding the occurrence of pitting, which is a drawback of sulfuric acid pickling. Particularly in the case of the preferred embodiment in which the acid solutions of the acid bath dips I and II are separately electrolyzed to recover the acid, the frequency of the building bath for avoiding the occurrence of pitting may be reduced to sufficiently utilize the acid. In addition to being economical, the amount of waste generated can be reduced.

Claims (2)

[Claims] 1. A pickling method for descaling a steel material in a sulfuric acid bath, which comprises acid bath dipping I-neutralization-surface conditioning-acid bath dipping II-
In a method consisting of various steps of coating treatment, the total amount of Mn ions in the acid bath Immersion I is 500 ppm.
A descaling pickling method characterized by using the following regulated ones. 2. In a descaling pickling method in which an acid solution after use is electrolytically treated to separate and collect the acid, and is reused by circulation, the acid solution of acid bath immersion I and the acid solution of acid bath immersion II are mixed. The descaling pickling method according to claim 1, wherein the descaling pickling is performed separately and separately circulated.