JPH03107498A - Method for dissolving steel at high speed by cathodic electrolysis - Google Patents

Abstract

PURPOSE:To stabilize continuous pickling operation over a long time by supplying electric current between steel as the cathode and the anode in an aq. sulfuric acid soln. having a specified NO3<-> ion content at a specified temp. and dissolving the steel by cathodic electrolysis. CONSTITUTION:An aq. soln. contg. 100-600g/l sulfuric acid and 5-120g/l NO3<-> ions is prepd. at 50-100 deg.C. Electric current is supplied between steel such as carbon steel, low alloy steel or special steel as the cathode and the anode confronting the cathode in the sulfuric acid soln. at 5-200A/dm<2> current density and the steel is dissolved by cathodic electrolysis. The interfering action of dissolved metal ions can be minimized and continuous high-speed pickling over a long time can be attained on an industrial scale.

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to carbon steel whose main components are iron and carbon,
The present invention relates to a high-speed cathodic electrolytic melting method for the surface of steel metals such as low alloy steel or special steel containing one or more of Or, No, Mn, etc.

Conventional technology When manufacturing thin sheet products by cold rolling steel metal,
The hot coil obtained by hot rolling is either as hot rolled or subjected to intermediate annealing, and the oxide scale generated on the surface during hot rolling or annealing is removed by mechanical descaling treatment such as shot blasting, and carbon steel is then processed. Cold rolling is performed after low alloy steel and ferritic stainless steel are immersed in a hydrochloric acid or sulfuric acid aqueous solution, and austenitic stainless steel is immersed in a mixed aqueous solution of nitric acid and butic acid to dissolve and remove scale remaining on the surface.

Descaling pickling treatment not only completely removes scale, but also dissolves and removes surface flaws in hot coils that occur during slab heating or hot rolling, from the perspective of improving the surface quality of final steel sheet products. It has an important role to play.

Therefore, there is a need for the development of a high-speed metal dissolution treatment method from the viewpoint of improving the descaling pickling efficiency and smoothing the pickling surface. As such an efficient pickling method,
Electrolytic descaling method of Publication No. 7-2800, JP-A-Sho Ei4
There is a method of immersion in a mixed acid of sulfuric acid and nitric acid as disclosed in Japanese Patent No. 288.

However, the electrolytic descaling method disclosed in Japanese Patent Publication No. 57-2800 is an electrolytic pickling and electrolytic treatment method whose main purpose is to dissolve the metal in the anode part rather than the cathode part, and
The method of immersion in a mixed acid of sulfuric acid and nitric acid disclosed in Japanese Patent No. 88 had the problem that metal dissolution completely stopped when the pickling solution deteriorated due to the interfering effect of eluted metal ions.

Problems to be Solved by the Invention In order to prevent the reduction in dissolution rate due to eluted metal ions and to develop a cathodic electrolytic dissolution treatment method that can stably perform continuous pickling operations over long periods of time, the authors have developed a number of problems. As a result of repeated experiments and studies, it was discovered that the intended purpose could be achieved by cathodic electrolytic treatment in a sulfuric acid aqueous solution to which nitric acid was added.

The present invention was completed based on this knowledge.

Means and Effects for Solving the Problems The present invention provides a method for achieving a temperature of 50 to 100°C and a temperature of 5 to 120 g.
100-800 ge containing NO3- ions of el
In a sulfuric acid aqueous solution with a concentration of 1, a current density of 5 to 20
This is a high-speed cathodic electrolytic melting method for steel metals, which is characterized by cathodic melting by applying a current of 0 A/da2.

The present invention will be explained in detail below.

Dissolution of steel metal in a strict solution is caused by ionization of the metal (M→M
""+ne-) and reduction of hydrogen ions as a cathodic reaction (
The re-reaction of 2H++2e-→H2) proceeds with simultaneous active dissolution. In this case, since metal dissolution proceeds at the natural potential, the dissolved metal ions or NO3
- ions raise the corrosion potential from the active dissolution potential range to the passive potential range and significantly reduce the rate of dissolution. On the other hand, under direct current, the dissolution of metal at the anode proceeds according to Faraday's law as in conventional electrochemical theory, but in a sulfuric acid aqueous solution containing NO3- ions, the dissolution rate at the cathode is slow. It reaches several times that of the anode part.

Figure 1 shows 300g of dissolved 25g of 5US430.
/L of sulfuric acid aqueous solution (80°C), the anode and cathode parts per minute of 5US430 steel under natural immersion conditions and when the steel is subjected to electrolytic pickling treatment at a current density of 80A/d+a2. This figure shows the results of measuring the dependence of dissolution depth on nitric acid content. No metal dissolution is observed under natural immersion conditions.

On the other hand, the dissolution depth at the anode part does not depend on the nitric acid content at all, but at the cathode part it increases with the nitric acid content and reaches approximately 7.
At a nitric acid content of 5 g/l, the nitric acid content reaches approximately four times that of the anode part, and then rapidly decreases, and at a nitric acid content of about 120 g/l, it reaches the same level as that of the anode part. In order to significantly increase the dissolution rate in the cathode portion, the same effect can be obtained by adding not only nitric acid but also a nitrate such as sodium nitrate.

On the other hand, in order to make this cathodic electrolytic dissolution method a practical process, it is essential to maintain the initial dissolution rate for a long time. - In the process of pickling steel metals for boat fishing, the acid concentration in the acid precipitation solution decreases as the metal dissolves, reducing its dissolving ability, so the acid concentration is strictly controlled, but the eluted metal ions It is difficult to avoid a decrease in the dissolution rate due to the interfering effect of .

On the other hand, in the cathodic electrolytic dissolution method in a sulfuric acid aqueous solution containing nitrate ions according to the present invention, even when the amount of eluted metal is 85 g/l, which is the standard for updating the sulfuric acid pickling solution, new It is possible to maintain the dissolving ability of 80% or more of the liquid.

Figure 2 shows 300 g containing Bog/l nitric acid at 90°C.
The relationship between the amount of metal ions in the solution and the dissolution depth per 80 seconds of 5US430 steel was measured when electrolytic pickling was carried out at a current density of BOA/da2 in a sulfuric acid aqueous solution with a concentration of 1/2. The high dissolution rate in the cathode part is such that even if 85 g/l of metal ions are contained, the capacity decreases by only about 20%. Figure 2 also shows the dissolution depth under natural immersion conditions (dip immersion) without applying direct current, and the new solution shows slightly higher solubility than that of the cathode, but only a few gels. The dissolution of the metal is completely stopped by the incorporation of the metal ions.

Note that the metal dissolution reaction at the cathode portion in the present invention is as shown below.

Metal dissolution reaction M+M" + n e -Electron consumption reaction 2H"+2e +H2 and No3- +2H" +e- →NO2+ H2O The reason why metal dissolution is promoted by the addition of nitrate ions is that NO3- is reduced and NO2 gas is generated, but it is thought that the reduction reaction promotes the dissolution reaction in which the metal becomes metal ions by consuming electrons.In other words, as a result of the reduction reaction, No3- ions are released outside the bath as NO2 gas, It has the characteristic that it does not remain as an interfering ion.

In addition, in the present invention, when the temperature of the electrolytic solution is 50°C or lower, the cathodic electrolytic dissolution rate is at the same level as that at the anode, and the higher the solution temperature, the higher the dissolution rate.
From a practical point of view, the temperature was set at 50 to 100°C, since the tank or piping materials deteriorate significantly at temperatures above 0°C.

Furthermore, regarding the NO3- ion concentration, as is clear from Figure 1, the cathodic electrolytic dissolution rate reaches its maximum at a concentration of 40 to 90 g/i, but the addition effect is not sufficient at a concentration of 5 g/l without swirling. , Since at a concentration exceeding 120 g/41, the cathodic electrolytic dissolution rate is at the same level as that at the anode, the concentration range was set at 5 to 120 g/i.

Regarding the concentration of the sulfuric acid aqueous solution in this case, at a thin concentration of 100 g/l, the depth of dissolution in the cathode electrolytically treated part exceeds that in the anodic electrolytically treated part cannot be obtained, and at a concentration exceeding 800 g/l, excessive dissolution may occur. Non-uniform dissolution in the form of pitting progresses on the steel surface, making it difficult to obtain a uniform surface quality and also increasing costs. Therefore, in the present invention, the concentration of the sulfuric acid aqueous solution is set to 10 to prevent heterogeneous dissolution in the form of pitting corrosion and obtain a homogeneous surface texture.
It was set as 0-600g/.

Furthermore, regarding the current density in this case, a sufficient cathode solvent effect cannot be obtained with a small current density of less than 5 A/da2, and an excessively large current density of over 200 A/dm2 may cause the solution temperature to rise rapidly due to solution resistance. As well as causing
The amount of cathode solvent also becomes saturated.

Therefore, in order to effectively perform cathodic electrolytic treatment and to control the temperature of the electrolytic solution, the current density should be set at 5 to 200 A/
It was set as d+a2.

The cathodic electrolytic melting method for steel metals of the present invention as described above is as follows:
Interfering effects of metal ions introduced into the solution can be minimized, and long-term continuous high-speed pickling can be achieved at low cost on an industrial scale.

Example Carbon steel, 5US430 containing about 17% Cr, high Cr ferritic stainless steel containing about 23% Cr, and austenitic stainless steel 5US304, respectively, were melted in depth when cathodic melting was performed at 90°C for 60 seconds. 8g of melted 5US430 steel at 50g/l
/lNO3-ion-150g/sulfuric acid, 20g/sulfuric acid
O3 ion - 150g/Table 1 and 2 for sulfuric acid
Table, 8g/sN melted from 80g/l 5US430 steel
O3-ion-300g/sulfuric acid, 35g/sodium NO3-
Ion-300g/also sulfuric acid, Bog/l N O3-
Tables 3, 4 and 5 for ion-300gel sulfuric acid and 50g/l of 5US304 steel melted 1
5g 1 sentence NO3-ion - 550g 1 sentence sulfuric acid, 85g/
fLN O3-ion-550 g/l sulfuric acid is shown in Tables 6 and 7.

With conventional simple immersion without cathode energization, when a considerable amount of metal ions coexist due to deterioration during use, the rate of metal dissolution is extremely low in stainless steel other than carbon steel, but with the method of the present invention, the rate of metal dissolution is extremely low. All steel types exhibit high dissolution rates.

(Leaving space below) 0 1 Table 1 Table 2 Table 3 Table 4 5 Table 5 Table 6 Table 7 Effects of the invention According to the present invention, all types of steel, from carbon steel to ferritic stainless steel and austenitic stainless steel Descaling treatment of steel metal can be carried out in the same pickling tank and same pickling bath, and the descaling treatment is 3 to 5
Since the solvent can be used at twice the speed, it is possible to remove various surface flaws generated in the pre-pickling process under conditions of controlling the amount of solvent.

Furthermore, the adverse effect of interfering metal ions that dissolve into the pickling solution and reduce the pickling ability during descaling pickling is reduced to 20% or less, allowing stable and efficient pickling work to be continued. In addition, #washing surface properties are also smooth and smooth.

[Brief explanation of drawings]

Figure 1 shows the characteristics of 5O3430 steel at 80°C in an aqueous sulfuric acid solution containing 25 g/l of metal ions at a concentration of 300 g/l for 60 seconds at the anode and cathode under natural immersion and current flow at a DC current density of 80 A/d112. FIG. 3 is a diagram showing the dependence of metal dissolution depth on nitric acid concentration. Figure 2 shows the amount of metal per 60 seconds at the anode and cathode of 5US430 steel under natural immersion in an aqueous sulfuric acid solution containing 60 g of nitric acid at a concentration of 300 g/i at 80 °C and under current flow with a direct current density of BOA/da2. FIG. 3 is a diagram showing the dependence of dissolution depth on eluted metal ion concentration.

Claims (1)

[Claims] The temperature is 50-100℃ and 5-120g/l NO_
In a sulfuric acid aqueous solution containing 3^-ions at a concentration of 100 to 600 g/l, a current density of 5 to 200 A/d is applied between a steel metal as a cathode and an anode provided opposite to the cathode.
^2 A high-speed cathodic electrolytic melting method for steel metals characterized by cathodic melting by applying current.