JP3150483B2 - Pickling method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to management of a pickling liquid applied to a pickling apparatus for steel products, mainly stainless steel.

[0002]

2. Description of the Related Art FIG. 4 is a conceptual diagram of a process control of a conventional pickling apparatus. The stainless steel strip 11 to which the oxide scale has adhered is supplied to the pickling apparatus 12. Here, multi-stage nozzle 1
From 8 the pickling liquid 17 is sprayed onto the stainless steel strip 11, the oxide scale on the surface is chemically dissolved and removed before leaving the pickling device 12. The pickling liquid 17 to be sprayed contains makeup water 13,
The nitric acid solution 14, the hydrofluoric acid solution 15, and the recirculated pickling solution 19 are adjusted and mixed and reused.

Conventionally, the pickling solution is controlled by a manual method so that the chemical composition of the pickling solution has a nitrate ion concentration of 10 to 20 wt%, a hydrofluoric acid concentration of 1 to 5 wt%, and a total iron ion concentration of 0 to 20 g / liter. Had been adjusted.

[0004]

When the bath of the pickling solution is controlled only by the bath composition, the concentration range of iron ion concentration is within a certain range despite the mixing ratio described in the "Prior art" section. , While the nitrate ion concentration and hydrofluoric acid concentration are the same,
The oxidation-reduction potential of the pickling solution enters the passivation potential range of the stainless steel, so that the pickling does not proceed easily, and poor pickling may occur.

[0005] As a countermeasure, hydrofluoric acid is added to narrow the passivation potential range, so that the oxidation-reduction potential can be shifted to the superpassive range and the pickling rate can be improved. May occur and the finished surface may not be smooth and may be defective.

[0006]

According to the present invention, in order to solve the above-mentioned problems, in chemical pickling of stainless steel, the oxidation-reduction potential of the pickling solution is maintained in the active potential range in the anodic polarization characteristics of stainless steel. A pickling method characterized by automatically adjusting the replenishing amounts of an acid and water for preparing a pickling liquid and performing pickling.

[0007] In the chemical pickling of SUS304 stainless steel, the supply amount of acid and water for preparing the pickling solution is controlled by a computer so that the oxidation-reduction potential of the pickling solution is maintained in the active potential range of 250 to 550 mVAg / AgCl. Pickling method characterized by automatically adjusting and pickling.

[0008]

In the chemical pickling of stainless steel according to the present invention, the oxidation-reduction potential of the pickling solution is measured, and the supply amount of acid and water for preparing the pickling solution is adjusted by computer control so as to reduce the oxidation-reduction of the pickling solution. By maintaining the value of the potential in the active potential range in the anodic polarization characteristics of the target stainless steel, the state of the pickling solution is maintained, and the pickling is continuously and favorably performed.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In general, dissolution of an oxide scale proceeds at the interface between the oxide scale and a substrate (base material) by the following reaction. Oxidative dissolution reaction of the substrate (anode reaction)

Embedded image Oxide scale reduction dissolution reaction (cathode reaction)

Embedded image In both of these reactions, the oxidative dissolution reaction of the substrate is the rate-determining factor. Therefore, in order to increase the pickling rate, it is necessary to increase the dissolution rate of the substrate.

If the oxidation-reduction potential is maintained in the active region in the anodic polarization characteristics, a dissolution rate several hundred times to thousands of times higher than that in the passive region can be obtained. It is important to improve the pickling speed.

FIG. 3 shows nitric acid / SUS304 oxidized scale.
FIG. 4 is a graph showing the relationship between the oxidation-reduction potential in a hydrofluoric acid solution and the elution rate of an oxidation scale obtained by electrochemical measurement.
In FIG. 3, the pickling solution has an oxidation-reduction potential of 250 to 550 m.
In the active region of VAg / AgCl, the pickling speed is very fast and scale removal is performed well. However, in the passive region, the pickling speed becomes extremely slow and scale remains.

On the other hand, in the overpassive region of 800 mVAg / AgCl or more, although the pickling speed is high, pitting corrosion and the like occur on the pickled finish surface and the skin is roughened. Is not always good, and the control of the oxidation-reduction potential of the pickling solution affects the quality of the pickling result.

[0013] In the following, the present invention that always keeps the pickling effect good by maintaining the oxidation-reduction potential in the active region,
The case of SUS304 will be described with reference to the drawings. FIG. 1 is a conceptual diagram showing an apparatus for realizing the present invention.
FIG. 2 is a flowchart illustrating control of the apparatus shown in FIG.

In FIG. 1, each measurement data indicating the properties of the pickling solution is A / D-converted by an A / D converter 2 and taken into a computer 1. The computer 1 evaluates the measured data and issues an instruction to the valve driving device 3 so that the measured data falls within a preset value range. The valve driving device 3 moves each electromagnetic valve according to the instruction. It is configured.

A description will be given below according to a flowchart. First, in step S1, an optimal control range of a solution for pickling is set. In the case of stainless steel SUS304, the oxidation-reduction potential is 250 to 550 mVAg / AgCl. Next, the fluorine ion concentration is set. The fluorine ion concentration does not significantly affect the oxidation-reduction potential. The pickling rate increases as the fluorine ion concentration increases, but the surface finish deteriorates,
In addition, it is desirable to set as low a value as possible because it may cause deterioration of the work environment of humans.

Next, in step S2, the oxidation-reduction potential of the pickling solution 17 in the supply tank 16 is measured by the oxidation-reduction electrode 4, and if the potential is higher than the set value in step S3, the process proceeds to step S4, and the electromagnetic valve 7 is turned on. Open to supply the supply water 13 to the supply tank 16. If the measured potential is low, step S
Proceeding to 5, the electromagnetic valve 8 is opened to supply the acid solution 14, and the oxidation-reduction potential is measured again. If the measured potential falls within the set potential range, the process proceeds to step S6, and the previously opened electromagnetic valve 7 or 8 is closed.

Next, the fluorine ion concentration of the pickling solution 17 in the supply tank 16 is measured using the fluorine ion electrode 5 in step S7. If the fluorine ion concentration is higher than the set range in step S8, the process proceeds to step S4, in which the electromagnetic valve 7 is opened to supply the makeup water 13 to the supply tank 16. If the fluorine ion concentration is lower than the set range, step S9
Then, the electromagnetic valve 9 is opened to supply the hydrofluoric acid solution 15. If the fluorine ion concentration falls within the set range, the process proceeds to step S10, and the previously opened electromagnetic valve 7 or 9 is closed.

To control the pickling liquid, a supply tank 16 is supplied with make-up water 13, an acid solution 14, and a hydrofluoric acid solution 15.
Therefore, since the amount of the pickling liquid increases, the liquid amount in the supply tank 16 is measured by the supply tank level meter 6 in step S11.
If the amount of liquid has increased, the process proceeds to step S12, the electromagnetic valve 10 is closed, and the pickling liquid 20 is blown down. On the other hand, if the liquid amount is insufficient, the process returns to step S4 and the electromagnetic valve 7
Is opened to supply makeup water 13. If the liquid amount is within the specified range, the electromagnetic valve 10 is opened to recirculate the pickling liquid in step S13, and normal operation is performed.

Although SUS304 has been described above as an example, the same effect can be obtained by setting the optimum oxidation-reduction potential and fluorine ion concentration for each of different steel types. In this case, the same effect can be obtained even if a person constantly monitors and operates the control valve, but in the present invention, automation by online measurement is realized by computer control.

[0020]

According to the present invention, since the control is not based on the conventional liquid composition but based on the oxidation-reduction potential, there is no danger of the finished surface being roughened or the pickling rate being reduced due to an improper mixture amount.
The pickling condition can always be maintained optimally. In addition, since the acid solution is controlled based on the oxidation-reduction potential, automatic control can be easily performed by on-line computer control, and labor costs can be reduced.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of process control of an acid pickling apparatus according to an embodiment of the present invention.

FIG. 2 is a control flowchart of the pickling apparatus according to the embodiment of the present invention.

FIG. 3 is a diagram showing the relationship between the oxidation-reduction potential of a SUS304 oxidation scale in a nitric acid / hydrofluoric acid solution and the elution rate of the oxidation scale obtained by electrochemical measurement.

FIG. 4 is a conceptual diagram of process control of a conventional pickling apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Computer 2 A / D converter 3 Valve drive device 4 Redox electrode 5 Fluoride ion electrode 6 Supply tank level meter 7 Electromagnetic valve (for makeup water control) 8 Electromagnetic valve (for acid solution control) 9 Electromagnetic valve (hydrofluoric acid solution) 10) Solenoid valve (for recirculating pickling liquid control)

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Yukio Kanda 4-6-22 Kannon Shinmachi, Nishi-ku, Hiroshima-shi, Hiroshima Works, Mitsubishi Heavy Industries, Ltd. (56) References JP-A-5-263279 (JP, A) ( 58) Field surveyed (Int.Cl. ⁷ , DB name) C23G 1/00

Claims (2)

(57) [Claims] In the chemical pickling of stainless steel, the supply amount of acid and water for preparing a pickling solution is adjusted so as to maintain the oxidation-reduction potential of the pickling solution in an active potential range in the anodic polarization characteristics of stainless steel, A pickling method characterized by pickling. 2. In the chemical pickling of SUS304 stainless steel, the oxidation-reduction potential of the pickling solution is set to 250 to 550 mVAg.
A pickling method characterized by automatically adjusting the replenishing amounts of acid and water for preparing a pickling solution so as to maintain the active potential range of / AgCl 2 and pickling.