JPH0586489A - High-speed pickling method of steel-base metal - Google Patents

Abstract

PURPOSE:To provide a technique which can smoothly pickle the surface of a steel-base metal which contains iron, carbon and Cr, has grain boundary Cr deficiency and is remarkable in sensitization at a high speed. CONSTITUTION:The steel-base metal is immersed into an aq. hydrochloric acid soln. contg. 0 to 300g/l No3<-> ions in hydrochloric acid of >=100g/l concn. kept at 50 to 110 deg.C or in an aq. chloronitric acid soln. and is pickled. The steel-base metal is otherwise anodically dissolved by passing a DC current at 5 to 200A/Dm<2> current density between the steel-base metal as an anode and a cathode provided to face this anode. A pair or >=2 pairs of the electrode plates of the anodes and cathodes are otherwise disposed to face the metallic surfaces to be pickled of the steel-base metal and the surfaces to be pickled of the steel- base metal are smoothly dissolved at a high speed by indirect energization while the DC current is passed at 125 to 200A/Dm<2> between these electrodes.

Description

Detailed Description of the Invention

[0001]

The present invention relates to ordinary steel, Cr, Ni, N
Low alloy steel containing a small amount of strengthening elements such as b or C
The present invention relates to a high-speed pickling method for an oxide (scale) formed on the surface of a steel metal such as a special steel containing a large amount of r, Ni and the like.

[0002]

2. Description of the Related Art Generally, in the production of steel metal strips such as ordinary steel, low alloy steel and special steel, after hot rolling or after annealing after hot rolling, If necessary, the scale produced on the surface of the steel strip is subjected to mechanical descaling treatment such as shot blasting, followed by pickling treatment, complete descaling, and cold rolling.

Various types of pickling aqueous solutions conventionally used for pickling treatment are properly used depending on the steel type in view of the characteristics of the pickling solution, particularly the smelting ability. For example, sulfuric acid solution is N
When i increases, the melting rate becomes remarkably small, and it takes a long time to obtain the required amount of ablation in austenitic stainless steel containing Ni. The pickling ability of the nitric-hydrofluoric acid mixed aqueous solution decreases as the Cr content in the steel increases, so grain boundary C produced by omitting or simplifying the annealing.
There is a problem that intergranular corrosion occurs in special steels such as r-deficient low alloy steels and ferritic stainless steels. Therefore,
For low alloy steels and ferritic stainless steels, pickling treatment using a hydrochloric acid aqueous solution or a sulfuric acid aqueous solution is generally performed. As described above, it is difficult to satisfactorily pickle steels having different component compositions with the same pickling aqueous solution and in a short working time industrially required.

Due to such a problem, there are many documents disclosed as an efficient pickling method. For example, JP-A-59-837
No. 83 discloses an acid consisting of two steps of immersing a steel strip in a sulfuric acid aqueous solution to dissolve away the scale, remove stains (smut) adhering to the steel surface in a nitric acid aqueous solution, and passivate the surface. A washing method is disclosed. This method also requires a long time for the scale cutting, and there is a problem that intergranular corrosion occurs in a nitric acid aqueous solution for the steel metal having grain boundary Cr deficiency as described above. Further, Japanese Patent Laid-Open No. 64-288 discloses a pickling method in which steel is dipped in a sulfuric acid / nitric acid aqueous solution in which sulfuric acid and nitric acid are mixed. Since this pickling method has a pickling ability of 1 to 5 times that of an aqueous sulfuric acid solution, the scale can be removed in a short time, and intergranular corrosion is caused even for a steel metal having grain boundary Cr deficiency. It also has the advantage of not being. However, when such a sulfuric nitric acid solution is used in an actual operation, Fe ions and C eluted from the steel metal into the solution.
There has been a problem that metal ions such as r ions increase and the composition and properties of the solution change, and the pickling ability is significantly deteriorated.

[0005]

SUMMARY OF THE INVENTION The present invention provides an aqueous pickling solution which improves the pickling and lapping efficiency of scale formed on the surface of steely metal and smoothes the surface properties of steely metal products from the above problems. Results obtained from many experiments and studies aimed at searching for
High-speed acid of steel metal for dipping or electrolytic treatment of steel metal in hydrochloric acid aqueous solution containing 300 g / l or less NO ₃ ⁻ ions in g / l hydrochloric acid and kept at a temperature of 50 to 110 ° C. It is a washing method.

The present invention will be described in detail below. In the present invention, the pickling aqueous solution has a concentration of 100 to 450 g / l.
Use hydrochloric acid. Hydrochloric acid was selected as the basic component because it has a better base metal fusing ability than sulfuric acid and shortens the pickling time of steel metal. Such action and effect of hydrochloric acid cannot be said to be obtained at any concentration in an aqueous solution containing NO ₃ ⁻ ions. FIG. 1 shows a hydrochloric acid aqueous solution having a temperature of 80 ° C. and a hydrochloric acid aqueous solution containing NO ₃ ⁻ ions.
Mechanical descaling of hot rolled steel strip with scale of SUS430 with high sensitization having grain boundary Cr deficiency and high pressure water containing sand iron particles, followed by surface smoothness when immersed for 60 seconds, intergranular corrosion and pores The results of a survey on the occurrence of food are shown below. Further, FIG. 2 shows the amount of base metal ablation in each pickling aqueous solution corresponding to FIG. 1, and a larger value indicates an index of faster descaling speed.

That is, when the hydrochloric acid has a low concentration of less than 100 g / l, there is a problem that the base metal has insufficient smelting ability and a long time is required for pickling. If the concentration exceeds 450 g / l, there is a problem that the fusing capacity reaches the supersaturation region. Therefore, in the present invention, the concentration of hydrochloric acid is limited to 100 to 450 g / l. Further, in order to increase the fusing capacity of the ground iron, it is necessary to add NO ₃ ⁻ ions to the hydrochloric acid aqueous solution. Further, NO ₃ added to the pickling aqueous solution having a concentration of 100 to 450 g / l
^- dissolving ability of the base steel is increased to increase the ion content.
However, an excessive content of NO ₃ ⁻ ions exceeding 300 g / l causes a problem of significantly increasing intergranular corrosion, as shown in FIG. Therefore, in the present invention, the content of NO ₃ ⁻ ions is limited to 300 g / l or less from the range in which intergranular corrosion does not occur and the amount of scale cutting is increased.

As described above, the hydrochloric acid aqueous solution having the composition of the present invention having a high fusing capacity is heated and kept at a temperature of 50 to 110 ° C. and contains a large amount of ordinary steel, low alloy steel, and Cr, Ni, Mo. A steel metal such as a special steel is immersed or electrolyzed to dissolve and remove the scale and base iron formed on the surface of the steel metal. The heating of the hydrochloric acid aqueous solution at this time is performed in order to efficiently ablate the base iron, and the ablation ability at that time becomes smaller as the temperature becomes lower and increases as the temperature becomes higher. The temperature was set to 50 to 110 ° C. in consideration of maintenance problems.

Regarding the electrolytic treatment, one or two or more pairs of electrode plates, an anode and a cathode, are provided between the steel metal as an anode and a cathode provided so as to face the anode, or as opposed to the steel metal. While applying a direct current between the arranged electrodes, the scale and base metal generated on the steel metal surface are dissolved and removed. The current density at this time is not particularly limited, but a range of 5 to 200 A / Dm ² is preferable from the viewpoint of efficient and stable long-term ablation. Especially 200A /
If the current density exceeds Dm ² , the solution temperature will rapidly rise due to the solution resistance, and the hydrochloric acid aqueous solution will be severely deteriorated.

[0010]

【Example】

(Example 1) Low C containing 7% Cr with iron and carbon
r steel, SUS430, which is a chromium-based stainless steel containing about 17% Cr, and SUS304, a high-carbon austenitic stainless steel containing 18% Cr and 8% Ni (C: 1.2%, Cr: 0.4). %) And Cr-Mo steel (C
r: 17%, Mo: 1%) A mechanically descaled 10t coil with mechanical descaling is used, in which high-temperature water is blown with sand iron particles mixed with a hot coil that is not subjected to annealing or is annealed for a short time. Table 1 shows the results of the tests conducted together with the comparative method. According to the method of the present invention, all steel types exhibit a high fusing rate, the descale pickling efficiency can be improved, and the pickling surface can be smoothed.

The pickled steel strip thus obtained together with the comparative material is subjected to tandem cold rolling by a rolling mill train having a large diameter roll (work roll having a diameter of 200 to 600 mm) and a small diameter work roll (work roll having a diameter of 100 mm or less). After the cold rolling step by the combination of the cold rolling by the Sendzimir mill having the above and the cold rolling (3t → 0.4t) only by the Sendzimir mill, bright annealing was performed. As a result of testing the gold dust of the product plate thus obtained, no gold dust was generated and the surface gloss was good. On the other hand, the comparative material with intergranular corrosion generated gold dust.

[0012]

[Table 1]

(Example 2) 7% Cr with iron and carbon
Low Cr steel containing, SUS430 of chromium type stainless steel containing about 16.5% Cr and 18% Cr, N
SU of austenitic stainless steel containing 8% i
Regarding S304, a hot coil with remarkable sensitization, which has been annealed for a short time or annealed for a short time, is sprayed by mixing sand iron particles in high-pressure water and sprayed with mechanical descaling. When the anodic dissolution pickling is performed by passing an electric current with a current density of 80 A / Dm ² between the cathode and the cathode provided in the A total of 8 electrode plates (width 1200 mm, 2 pairs of cathode and anode on one side of the steel strip)
Length of 400 mm), the polarity is vertically symmetrical, the distance between the electrodes is 30 to 100 mm, and the depth of welding and the presence of intergranular corrosion in the case of the indirect energization method in which a direct current is applied between the electrodes Table 2 shows the results of the experiment for evaluating According to the method of the present invention, all steel types exhibit a high fusing rate, the descale pickling efficiency can be improved, and the pickling surface can be smoothed.

The pickled steel strip thus obtained is subjected to tandem cold rolling by a rolling mill train having a large diameter roll (work roll having a diameter of 200 to 600 mm) and a small diameter work roll (1
Cold rolling process by a combination of cold rolling with a Sendzimir mill having a work roll having a diameter of 00 mm or less) and cold rolling with only a Sendzimir mill (4t → 0.
After 3t), bright annealing was performed. As a result of testing the gold dust of the product plate thus obtained, no gold dust was generated and the surface gloss was good.

[0015]

[Table 2]

[0016]

EFFECTS OF THE INVENTION According to the present invention, a steel material having a grain boundary Cr deficiency in which annealing of a hot-rolled steel strip such as a low alloy steel or a special steel containing iron, carbon and Cr is omitted or the annealing is simplified. Since we have established a technology that enables smooth pickling and high-speed pickling of metal surfaces, pickling speed can be increased and ferritic stainless steel and austenitic stainless steel can be pickled with the same pickling solution. The effect is enormous.

[Brief description of drawings]

FIG. 1 is a drawing of a mechanically descaled material in which a hot-rolled steel strip of SUS430 having a grain boundary Cr deficiency and a remarkable sensitization without annealing is mixed with sand iron grains in high-pressure water and sprayed at 80 ° C. The surface properties after 60 seconds of salt nitric acid pickling are shown.

FIG. 2 shows the ablation ability of hydrochloric acid concentration and nitric acid concentration of the materials tested in FIG.

Claims (1)

[Claims] 1. A hydrochloric acid solution having a concentration of 100 to 450 g / l.
It contains NO ₃ ⁻ ions of not more than 00 g / l and has a temperature of 50
A high-speed pickling method for a steel metal, comprising immersing or electrolytically treating the steel metal in an aqueous hydrochloric acid solution which is heated and kept at 110 ° C.