JPH052760B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention is applicable to steels such as carbon steel whose main components are iron and carbon, and low alloy steel or special steel containing one or more of Ni, Cr, Mo, Mn, etc. The present invention relates to a cathodic electrolytic treatment method for metal surfaces. [Conventional technology] Conventionally, steel metals of various shapes such as shaped steel and steel plates were
Molten steel produced in a melting furnace such as a converter or electric furnace is forged through an ingot making process or a continuous casting process.
It is manufactured through hot working such as rolling, cold working, and heat treatment such as normalizing and annealing. However, when high alloy steel such as stainless steel or high manganese steel containing large amounts of Mn, Ni, Cr, Mo, etc. is pickled after hot rolling, cracks and cracks are scattered locally on the surface of the plate. , there was a problem that the appearance of the product deteriorated. This is particularly problematic in thin steel sheet materials and surface-treated materials that have been plated or chemically treated. Therefore, before hot working, the steel metal piece undergoes a refining process that includes local care and overall grinding, and then mechanical cleaning such as shot blasting or wire brushing after hot working or after hot working and annealing. After pretreatment using the method, carbon steel, low alloy steel, and ferritic stainless steel are immersed in an aqueous solution of hydrochloric acid or sulfuric acid, and austenitic stainless steel is immersed in a mixed aqueous solution of nitric acid and fluoric acid to prevent surface scratches and cracks that occur at high temperatures. A method of dissolving and removing scale is being used. In particular, the gloss or beautiful surface of stainless steel largely depends on the effects of hydrochloric acid or sulfuric acid pickling, or sulfuric acid pickling. however,
When immersed in these pickling solutions and subjected to pickling treatment,
Due to the grain boundary properties of the material to be pickled, the grain boundaries on the surface are corroded, and when this is cold rolled, for example, in ferrite stainless steel, there is a problem in that minute scratches called gold dust occur. For this reason, as a pickling method that prevents or reduces the occurrence of microscopic scratches caused by pickling, the
``A method of pickling using a mixed aqueous solution of ferric chloride and hydrochloric acid'' in Publication No. 28351, and ``A method of pickling using a mixed aqueous solution of ferric chloride and hydrochloric acid,'' and ``A method of pickling using a mixed aqueous solution of ferric chloride and hydrochloric acid,'' and ``After descaling by mechanical treatment such as shot blasting, diluted hydrochloric acid. A pickling method has been developed that involves washing and then pickling with dilute nitric acid. In this pickling method, steel metal is immersed in a pickling solution heated to a temperature of about 50 to 60℃.From the perspective of descaling, hydrochloric acid pickling can be used to reduce the weight of ordinary steel to low-cost ferrite stainless steel (11 to 13 wt%). Cr), sulfuric acid pickling changes from ordinary steel to high Cr ferrite stainless steel (11~25wt%
It has the advantage that the same pickling bath can be applied to
Furthermore, it has the advantage that it does not preferentially corrode the grain boundary chromium-deficient layer due to grain boundary chromium carbide precipitation that occurs during hot working or its annealing process, and does not generate grain boundary microgrooves. It has the disadvantage of generating grain boundary microgrooves due to segregation. In addition, it is impossible to descale austenitic stainless steel containing Ni with both hydrochloric acid pickling and sulfuric acid pickling, and other pickling methods such as nitric acid pickling are required. There is no way to descale steel, ferritic stainless steel, and austenitic stainless steel using the same pickling bath, and it is necessary to install more than one pickling line or to replace the pickling bath. Furthermore, in the hydrochloric acid pickling method, sulfuric acid pickling method, and nitric acid pickling method, the steel metal is immersed in a pickling bath and descaling is performed under natural corrosion conditions, so an optimal pickled surface can be obtained. There are many problems in this respect, and in many cases two or three or more pickling treatments are required, so there is an urgent need to develop pickling technology that can control the amount of surface abrasion during descaling pickling. . As an efficient electrolytic pickling method for removing surface scale of steel metals in a short time, there is an electrolytic descaling method disclosed in Japanese Patent Publication No. 57-2800. This method involves applying a direct current voltage between an anode plate placed in a sulfuric acid or nitric acid aqueous solution and a cathode plate having a length 2 to 4 times the length of the anode plate, and passing a stainless steel strip between the two electrodes. This method discloses an electrolytic descaling method using an electric current method, and attempts to improve descaling performance by increasing the ratio of the area of the cathode plate to the area of the anode plate to increase the anodizing time. There are two types of electrolytic pickling: an anodic pickling method and a cathodic pickling method.The anodic pickling method has no fear of hydrogen embrittlement and the higher the temperature and current density, the faster the descaling speed. Generally, an anodic electrolytic pickling method is used because the underlying metal is hardly dissolved in the pickling method. In this way, in electrolytic pickling or electrolytic polishing of steel metals, when the metal is melted by passing current through the pickled or polished surface in an electrolytic solution, the metal to be pickled or polished is used as an anode and a direct current is applied. A common method is to energize. On the other hand, at the cathode, the hydrogen ions in the pickling solution are reduced and released as hydrogen gas, and it is said that the dissolution of the metal does not proceed. [Problems to be Solved by the Invention] The present inventors have developed a method that dissolves the surface of steel metal in a short time and creates a beautiful appearance, contrary to the common wisdom that the underlying metal is hardly dissolved by conventional cathodic electrolytic treatment methods. As a result of many experiments and studies aimed at developing a cathodic electrolytic treatment method with specific properties, the objective was achieved by cathodic electrolysis in a pickling solution containing Cr ⁺⁶ ions in an aqueous sulfuric acid solution. I found out that this happens. The present invention was constructed based on this knowledge, and its gist is that a steel metal is used as a cathode in an aqueous sulfuric acid solution containing 5 to 200 g of Cr ⁺⁶ ions and a concentration of 100 to 600 g/. This is a cathodic electrolytic treatment method for steel metal in which a current density of 5 to 200 A/dm ² is passed between anodes placed opposite to each other. [Means and effects for solving the problems] The present invention will be described in detail below. Carbon steel that has undergone heat treatment such as high-temperature working or annealing, as well as low alloy steel or stainless steel that contains one or more of strengthening elements such as Ni, Cr, and Mo, plasticity improving elements, and corrosion resistance improving elements. In order to dissolve and remove scale formed on the surface of steel metals such as special steel or adhered contamination, and to create a good pickled surface by melting an appropriate amount of the underlying metal, the process is carried out in a sulfuric acid aqueous solution containing Cr ions. Electrolytic treatment is performed using the steel metal as a cathode. 25wt% without carbon steel and Ni in sulfuric acid aqueous solution
The following ferrite stainless steels containing Cr are surface-cut by active melting in a natural immersion state. On the other hand, when austenitic stainless steel containing Ni is simply immersed in an aqueous sulfuric acid solution, active dissolution hardly progresses. However, according to the cathodic electrolytic treatment method of the present invention, the dissolution of all steel metals including austenitic stainless steel progresses under hydrogen gas generation conditions at the cathode in an aqueous sulfuric acid solution. The present inventors investigated the effect of cathodic electrolytic treatment in various acids such as hydrochloric acid and phosphoric acid aqueous solutions in addition to sulfuric acid aqueous solution, and confirmed that cathodic dissolution proceeds in any acid, but the amount of dissolution in sulfuric acid aqueous solution was It was confirmed that it was significantly smaller than that in the previous model and was therefore impractical. The details of the mechanism of significant cathodic dissolution in an aqueous sulfuric acid solution are not currently clear, but the present inventors believe that it is due to the progress of a chemically active dissolution reaction under cathodic current application. Compared to the solution generally used as a catholyte, which is made by adding sodium chloride, hydrochloric acid, etc. to sodium hydroxide, an aqueous sulfuric acid solution exhibits an excellent feature of dissolving metals at an extremely high rate. The effect in aqueous solution is not obtained at any concentration. Figure 1 shows SUS430 in sulfuric acid aqueous solution of various concentrations (temperature 80℃).
The figure shows a comparison of the dissolution depth of steel in an anodic electrolytic treatment section and a cathodic electrolysis treatment section when the steel is electrolytically pickled. At a thin concentration of less than 100g/, the depth of dissolution in the cathodic electrolytically treated area will not exceed that of the anodic electrolytically treated area, and at a concentration exceeding 600g/, excessive dissolution will occur, causing uneven dissolution in the form of pitting corrosion on the steel surface. death,
This makes it difficult to obtain a uniform surface quality and also becomes a factor in increasing costs. Therefore, in the present invention, the concentration of the sulfuric acid aqueous solution was set to 100 to 600 g/ml in order to obtain a homogeneous surface texture by melting the steel metal surface and preventing uneven dissolution in the form of pitting corrosion. Furthermore, the present invention includes sodium dichromate (Na ₂ Cr ₂ O ₇ ), potassium dichromate (K ₂ Cr ₂ O ₇ ),
In an aqueous solution of sulfuric acid such as chromic anhydride (CrO ₃ )
Add a compound that generates Cr ions. In a sulfuric acid ^electrolyte solution containing Cr ions, Cr ⁶⁺ ions are reduced to Cr ⁺³ ions in the cathode electrolytic treatment section, and electrons ( ^{e -} ) is consumed (Cr ⁶⁺ +3e ^- →Cr ³⁺ )
By doing so, it is possible to increase the dissolution rate of the metal, reduce the hydrogen generation reaction due to phosphorus segregated at the grain boundaries, prevent the generation of grain boundary microgrooves, and make it possible to produce a good surface quality. Figure 2 is 300g/
This shows the dissolution depth of the metal when SUS304 steel is cathodic electrolytically treated by adding various concentrations of Cr ⁶⁺ ions to a sulfuric acid aqueous solution (temperature 80°C) with a concentration of
When simply immersed in the electrolyte solution, no change is observed, but when subjected to cathodic electrolytic treatment, a remarkable depth of dissolution is observed. That is, the content of Cr ⁶⁺ ions limited in the present invention is within a range that can provide a remarkable cutting action on the surface of steel metal in a short time, and is a thin concentration of less than 5 g/ or a high concentration of more than 200 g/. That kind of effect cannot be expected. Furthermore, the present invention uses a sulfuric acid aqueous solution containing Cr ⁶⁺ ions to use a steel metal as a cathode, and a current density of 5 to
200A/dm ² is applied. The current density accelerates the dissolution rate of steel metal, and as shown in Figure 3, the current density is 50 g/
Cathodic electrolysis treatment was performed for 60 seconds in a sulfuric acid aqueous solution containing Cr ⁶⁺ ions at a concentration of 300 g/80°C.
Indicates the cutting depth of SUS430 steel. i.e. 5A/ ^dm2
At a small current density below 200A/dm2, sufficient cathode cutting effect cannot be obtained, and at an excessively large current density exceeding 200A/ ^dm2 , the liquid temperature will rise rapidly due to solution resistance and the amount of cathode cutting will become saturated. It's coming. Therefore, in order to effectively perform cathodic electrolytic treatment and to control the temperature of the electrolytic solution, the current density should be set to 5~5.
It was set to 200A/ ^dm2 . The cathodic electrolytic treatment method for steel metal of the present invention as described above is carried out by any method such as a dipping method or a continuous method for steel metal. [Example] Carbon steel, SUS430 containing about 17% Cr,
When high Cr ferrite stainless steel and austenitic stainless steel SUS304 containing 23% are cathodicly melted at 80°C for 60 seconds, the melting depth is 10 g/Cr ⁶⁺ ion - 150 g/
Tables 1 and 2 for sulfuric acid, 100 g/of Cr ⁺⁶ ions - 150 g/of sulfuric acid, 10 g/of Cr ⁶⁺ ions - 300 g/of sulfuric acid, 75 g/of Cr ⁶⁺ ions -
300g/sulfuric acid, 180g/Cr ⁶⁺ ion −300
Tables 3, 4 and 5 respectively for g/g of sulfuric acid and 15 g/Cr ⁶⁺ ions - 550 g/
of sulfuric acid, 175 g/g of Cr ⁶⁺ ions - 550 g/sulfuric acid are shown in Tables 6 and 7. Conventional simple immersion without cathode energization is effective against non-carbon steel.

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〔Effect of the invention〕

According to the present invention, descaling treatment of all steel metals from carbon steel to ferritic stainless steel and austenitic stainless steel can be carried out in the same pickling tank.
Since it can be carried out in the same pickling bath and can perform 2 to 3 times faster machining compared to simple immersion treatment, it is possible to remove various surface flaws generated in the pre-pickling process under the conditions of controlling the amount of machining. It can be done with Furthermore, it is possible to prevent the formation of grain boundary microgrooves caused by the grain boundary segregation of phosphorus, an impurity in steel, which occurs during descaling pickling by natural immersion in a normal sulfuric acid aqueous solution, and It is also possible to prevent the generation of grain boundary microgroups due to the grain boundary chromium-deficient layer that occurs in the anode portion, resulting in a glossy surface with excellent appearance.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the dependence of the depth of metal dissolution per 60 seconds at the anode and cathode of SUS430 steel in a pure sulfuric acid solution at 80° C. with a concentration of 100 to 600 g/600 g per 60 seconds on the sulfuric acid concentration. Figure 2 is
FIG. 2 is a diagram showing the dependence of the metal dissolution depth per 60 seconds on the Cr ⁺⁶ ion concentration during natural immersion and cathode current application in a sulfuric acid solution of 300 g/concentration at 80° C. for SUS304 steel. Figure 3 shows SUS430 steel of 50g/kg at 80℃.
FIG. 3 shows the current density dependence of the metal dissolution depth at the anode and cathode per 60 seconds in a sulfuric acid solution containing Cr ⁶⁺ ions at a concentration of 300 g/min.

Claims (1)

[Claims] 100~ containing 5~200g/Cr ⁶⁺ ions
A steel metal is used as a cathode in an aqueous solution of sulfuric acid with a concentration of 600 g/dm, and a current density of 5 to 200 A/dm ² is passed between the cathode and an anode provided opposite to the cathode for cathodic dissolution. Cathodic electrolytic treatment method for steel metals.