JPH01172599A - Method for electrolytically descaling annealed band stainless steel - Google Patents

Abstract

PURPOSE:To descale an annealed band stainless steel with high current efficiency by carrying out constant-current electrolysis at the current density fulfilling specified conditions. CONSTITUTION:When an annealed band stainless steel is electrolytically descaled, constant-current electrolysis is carried out (1) at the current density higher than the current density corresponding to 1.8V potential in the anode polarization curve and (2) at the current density exceeding the maximum current density of the active state only in the overpassive region, namely, on the anode polarization curve, and the stainless steel is descaled. For the purpose, the anode polarization curve is obtained in accordance with the kind, concn., composition, and temp. condition of the scale and electrolyte based on the kind of the band stainless steel or annealing condition, and the treating condition is determined. When descaling is carried out, the conditions of the various electrolytes are appropriately changed and combined with the kind of steel, the various electrolyte conditions and current density are predetermined by model tests, the operating conditions are selected from the data, and electrolytic descaling is carried out.

Description

[Detailed description of the invention] r Industrial application field] The present invention relates to a method for electrolytic descaling of stainless steel annealed steel strip, and in particular, the method for electrolytic descaling of stainless steel strip after annealing.
This invention relates to a method of electrolytic descaling that is very effective in a short period of time and is used in the field of manufacturing stainless steel strips.

[Prior Art 1] Conventionally, as a method for descaling a stainless steel strip after annealing, a method has been adopted in which the stainless steel strip is immersed in salt and then immersed in various acids. According to this method, a hot salt (generally NaO
(molten salt of H and NaNO3), followed by electrolysis in sulfuric acid, immersion in nitric-fluoric acid, and finally immersion or electrolytic treatment in nitric acid. With such a complicated method, it is extremely difficult to increase the line speed of the descaling process and increase production efficiency.

As an electrolytic descaling method, JP-A-59-59900,
Japanese Patent Publication No. 38-12162 is known, but it takes a long time to completely descale with electrolysis alone, and rather, electrolysis is used as an auxiliary means for the subsequent acid immersion, that is, as an alternative to a salt bath. It is only fulfilling this purpose.

As a descaling method using a constant current electrolysis method, for example, Japanese Patent Publication No. 55-9079 is known. In this method, the pickling rate of stainless steel is determined by the dissolution rate of the base steel, and electrolytic treatment is performed in a solution that intersects the anode polarization curve at only one point only at the potential in the active dissolution or passive region of the steel. It is something to do. This method requires more than 10 clones of electricity for complete descaling, takes more than 20 seconds at a current density of 500 mA/crn', and is not useful for increasing the production efficiency of stainless steel sheets. Furthermore, it is difficult to obtain the glossy surface inherent to stainless steel, making it impractical.

[Problems to be solved by the invention] In order to increase the line speed while maintaining descaling performance,
As mentioned above, in order to secure the immersion time in the molten alkali salt and various acids and the electrolysis time, it is necessary to increase the length of the various immersion tanks and electrolytic cells. This requires a large amount of capital investment and a large indoor space to install it. Therefore, in reality, a low threading speed has been used.

Furthermore, since many types of salts and acids are used, controlling their concentrations is a heavy burden.

As described above, descaling of stainless annealed steel strips has conventionally been incomplete and problematic by any method.

In view of these circumstances, the object of the present invention is to address the problems of the prior art described above in descaling stainless annealed steel strips, namely: (1) Low productivity.

■ Process management is complicated.

■ Easy to cause skin irritation.

■ It is difficult to obtain a good surface condition.

■ Requires a large amount of electricity.

The purpose of this paper is to propose a method for extremely effective descaling by eliminating the above problems.

[Means to solve the problem 1 The present invention aims to solve these problems.

When electrolytically descaling a stainless annealed steel strip, (1) a current density higher than the current density corresponding to a potential of 1.8 V in the anode polarization curve (saturated calomel electrode standard, hereinafter referred to as VS, SCE), (2) and Descaling is carried out by performing constant current electrolysis only in the passive state region, that is, at a current density that satisfies two conditions: a current density that exceeds the highest current density of the active state on the anode polarization curve.

[Effect 1 Figure 1 shows an example of the anode polarization characteristics of stainless steel with oxide scale on its surface. As mentioned above, conventionally,
Generally, descaling is performed in the region called the active state or passive state in FIG. 1, and in this case, the stainless steel substrate is dissolved, so the shiny surface inherent to stainless steel cannot be obtained.

Therefore, the present inventors conducted a detailed electrochemical study on the electrolytic descaling mechanism of stainless steel, and found that the mechanism shown in Fig. 1 is necessary for rapid electrolytic removal of scale without dissolving the stainless steel base. Hyperpassive region and anode polarization curve i, = g Kel potential 1.8V
(VS, 5CE) The present invention was completed based on the breakthrough finding that extremely favorable results can be obtained by using a current density higher than that corresponding to 4.1 m.

Conventionally, it has been common knowledge that electrolysis cannot be performed in the overpassive region because gas is generated and the current efficiency is lowered.

However, if descaling is carried out in the overpassive region and at a current density higher than that corresponding to a potential of 1.8V (VS, 5CE) in the anode polarization curve, stainless steel with good surface quality can be produced in a short time. can get.

Here, simply specifying the value of current density as the electrolytic descaling condition is not possible because the relationship between potential and current density in the overpassive region differs depending on the combination of steel type and electrolyte.

The reason why the current density was set higher than the current density corresponding to 1.8V (VS, 5CE) is that in the region where the reaction potential is 1.8V or higher, scale (iron oxide, chromium oxide) is preferentially electrolyzed and trivalent This is because iron ions and hexavalent chromium ions are formed, and electrolysis of the stainless steel base does not proceed.

If the reaction potential is lower than 1.8V (VS, 5CE), electrolysis of the scale will not occur sufficiently, and not only will it be impossible to descale in a short time, but even if it takes a long time to descale. As electrolysis progresses on the stainless steel base, the surface becomes rough, reducing its value as a stainless steel.

This will be explained in detail below using figures.

Figure 2 under the combination conditions of steel and various electrolytes.
Each anode polarization curve as shown in FIG. 3 is obtained. For example, in Figure 2, 1.8V (VS, 5
The current density corresponding to CE) intersects the anode polarization curve only at point A;
The current density corresponding to S, 5CE) intersects the anode polarization curve at three points: point 1, point 2, and point 3. Points 1 and 2 correspond to dissolution in the active region, and are different from point 3, which is dissolution in the hyperpassive region. Therefore, in such a case, it is necessary to perform descaling at a current density higher than the current density at point B, which intersects the anode polarization curve only in the overpassive region.

As described above, in the present invention, the treatment conditions are determined by determining the anode polarization curve in accordance with the steel type of the stainless steel strip to be descaled, the type of scale and electrolyte based on the annealing conditions, the concentration composition, and the temperature conditions. In this case, since it is better to have a higher current density in order to increase efficiency, it is desirable to set the current density to 100 mA/c rri'' or more if possible.

In order to implement the above, various electrolyte conditions and current densities are determined in advance through model experiments by changing the various electrolyte conditions as appropriate and combining them with the steel type, and then selecting from these and operating in the actual process. , it is practical to perform electrolytic descaling.

Note that even if the electrolyte of the present invention is an acid solution or a conventional neutral salt solution such as sodium sulfate or sodium nitrate,
Similar electrolytic descaling can be performed according to the present invention.

[Example 1 Hereinafter, the present invention will be explained based on Examples.

The test materials were steel strips passed through continuous annealing equipment (CAL) with 5UH409 and 5US304, and 5U
A cold-rolled S430 plate (1.2 mm thick) passed through a normal AP line (however, the PIC line was bypassed) was sampled and used. Table 1 shows the annealing conditions for these test materials and the state of the scale produced.

These continuously annealed test materials were subjected to electrolytic descaling in a model electrolytic cell. That is, the electrolyte composition, electrolyte temperature, current density, and electrolysis time shown in Tables 2, 3, and 4 are 5US430.5UH40, respectively.
Electrolytic descaling of No. 9,5 US304 was performed, and the results of descaling were shown.

Figure 4 shows the anode polarization curve of 5US430 corresponding to Table 2, Figures 5 and 6 show the anode polarization curve of 5UH409 corresponding to Table 3.
The anode polarization curve of Figure 7 is 5US corresponding to Table 4.
In the anode polarization curve of No. 304, the No. in Tables 2 to 4 and the numbers written in FIGS. 5 to 7 are written in correspondence with the conditions. Here No, 7, N
The electrolyte composition, electrolyte temperature, current density, and electrolysis time shown in No. 9, No. 20 are examples corresponding to conventional electrolytic descaling conditions.

Under these conditions, descaling was carried out by an electrolytic reaction based only on active dissolution using a constant current that crossed only in the active region on the anode polarization curve.

The descaling determination results were determined by visual judgment in comparison with a representative sample that had been completely descaled according to the following criteria.

X Ni scale remaining △: Slight scale ○: Completely descaled 0: Completely descaled (good gloss) Line speed 2 based on Examples No. 5, No. 13
In the actual process of 0mpm, the plate thickness is 0, 8 using a rectifier.
When a stainless steel plate with a width of 1000 mm and a width of 1000 mm was subjected to electrolytic descaling, stainless steel with good surface quality could be obtained in a short time as in the model experiment.

In addition, when the voltage in this case was measured, it was 9■. This is due to the fact that the distance between the electrode and the insulator is large, so the liquid resistance is higher than in the model experiment, and the coil generates heat.

〔Effect of the invention〕

In the method of the present invention, descaling is carried out under conditions that are in the hyperpassive region and at a current density higher than the current density corresponding to a potential of 1.8 V (VS, 5CE) in the anode polarization curve. Highly efficient constant current electrolysis can be performed by optimizing the temperature conditions, or by slightly changing the electrolyte temperature, concentration, and composition even under the same electrolytic conditions. It became possible to descale with high current efficiency.

According to the present invention, complete descaling can be carried out by electrolysis alone, but it is clear from the gist of the invention that it can also be carried out in combination with a conventional pickling method.

The electrolytic descaling method according to the present invention can be applied to all types of stainless steel, and not only can highly efficient descaling be performed with an extremely small amount of electricity as described above, but it can also be applied to all types of stainless steel. It has an excellent effect of not causing any loss of life.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing an example of an anode polarization curve to explain the present invention in detail, FIGS. 2 and 3 are schematic diagrams showing examples of different types of anode polarization curve, and FIGS. These are anode polarization curves for various electrolytes of various stainless annealed steels.

Claims (1)

[Claims] 1 When electrolytically descaling stainless steel annealed steel strip,
Determine the anode polarization curve determined by the stainless steel type and electrolyte conditions, and find a constant current that exceeds the highest current density of the active state on the anode polarization curve and is equal to or higher than the current density corresponding to a potential of 1.8 V (saturated calomel electrode reference). A method for electrolytic descaling of stainless annealed steel strip, characterized by carrying out electrolysis in a superpassive region using .