JPS6358235B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention protects the hot-dip galvanized surface of a single-sided hot-dip galvanized steel sheet by preventing acid corrosion on the hot-dip galvanized surface by an electrolytic pickling method using an acidic electrolyte in which Zn ²⁺ is dissolved in advance. The present invention also relates to a method for removing scale from a non-plated surface, which performs single-sided descaling at high speed and at low power cost. Demand for single-sided plated steel sheets has been increasing steadily in recent years, and they are used in large quantities in automobiles and the like. In particular, recently, flat plated steel sheets have been further subjected to alloying treatment to improve the adhesion of the coating on the plated surface, and this type of development has been increasing in recent years. Generally, single-sided hot-dip galvanized steel sheets are manufactured by coating only one side with hot-dip galvanized steel in a non-oxidizing atmosphere. In the case of alloyed steel sheets, they are manufactured by further alloying them by heat treatment, but the alloying treatment is usually held at a steel sheet temperature of 500°C or higher in an oxidizing atmosphere due to cost constraints of processing equipment. It is done by Therefore, iron oxide, so-called scale, is generated on the unplated surface. Scale is an incomplete iron oxide film, which hinders painting, chemical conversion treatment, etc.
It is necessary to remove this scale that has formed on the unplated surface. Conventionally, as a method for removing scale as described above, there is (a) a grinding method such as belt grinding, but it is not an economical method. (b) There is also an electrolytic pickling method. Since the plated surface is made of a Fe-Zn alloy that has high acid activity, when both sides of the steel plate are immersed in an acid solution, it is necessary to apply a negative current to the plated surface for electrolytic protection. However, Fe− with high activity in the acid solution
Electrolytic protection of Zn alloys is not necessarily an advantageous method in terms of power costs. (c) During electrolytic pickling, it is possible to immerse only the non-plated surface in an acid solution and mechanically seal the plated surface, but with the current state of the art, there is no possibility that the acid solution or its mist will get around the plated surface. It is difficult to suppress completely. Conventional descaling treatment of single-sided plated steel sheets by electrolytic pickling requires a huge amount of current to prevent corrosion on the plated surface, and to avoid this, the concentration of the electrolytic pickling solution or bath temperature has to be lowered. . For this reason, descaling took time and continuous descaling was impossible. In view of this, the present inventors conducted research in order to improve this defect circle by dissolving an appropriate amount of Zn ²⁺ in the acid solution in advance.
The plated surface is protected against corrosion with a low current, making it possible to use highly concentrated acids and high-temperature baths, shortening descaling time, and enabling high-speed descaling without zinc precipitation on the non-plated surface. After confirming this, the present invention was completed, and the present invention is manufactured by hot-dip galvanizing only one side of a steel sheet in a non-oxidizing atmosphere (a treatment in which the galvanized surface of a single-sided hot-dip galvanized steel sheet is further alloyed). For single-sided hot-dip galvanized steel sheets (including steel sheets), iron oxide scale that has formed on the non-galvanized surface has been removed.
A method for removing scale from a single-sided galvanized steel sheet, in which descaling is performed in an acidic electrolyte containing 20 mg/~100 g/Zn ²⁺ by electrolysis using an anode placed parallel to the surface of the steel sheet and the steel sheet serving as a cathode. It provides: The present invention will be explained in detail below. According to the research of the present inventors, (i) the descaling method for the non-plated surface of a single-sided plated steel sheet is the electrolytic pickling method, that is, the descaling time when applying a negative current to the non-plated surface to descale the surface is simply immersed; This can be shorter than in the case of treatment or positive current supply, and the higher the temperature of the acid solution, the shorter the descaling time. (ii) The negative current density applied to the non-plated surface is 10A/d.
m ² or more is advantageous. For example, to descale an unplated surface, use 1NH ₂ SO ₄ at 10A/70°C.
It is possible to do this in about 20 seconds with a current of dm ² , but it takes 1NH ₂ SO ₄ to prevent the color of the plated surface, and 50 A / dm at 70℃.
( ⁱⁱⁱ ) To descale the non-plated surface of single-sided plated steel sheets, plating can be achieved by adding the optimum amount of Zn ²⁺ to the electrolytic acid solution. The anti-corrosion current on the surface can be significantly reduced. This relationship is shown in Figure 1. Figure 1 shows the influence of Zn ²⁺ concentration on the lower limit corrosion protection current of the alloyed plated surface at various acid concentrations and temperatures.
℃, 1NH ₂ SO ₄ Relationship curve between lower limit corrosion protection current density (A/dm ² ) and Zn ²⁺ concentration (mg/) in acidic conditions, 2 is similarly 75℃, 1.8NH ₂ SO ₄ , 3 is 80℃,
The respective relationship curves for 3NH ₂ SO ₄ are shown. This shows that the addition of Zn ²⁺ significantly reduces the corrosion protection current density, although it varies depending on the H ₂ SO ₄ concentration and electrolysis temperature. Furthermore, (iv) a region was also found in which the precipitation of Zn on the plated surface was sufficiently reduced and no precipitation of Zn occurred on the non-plated surface. If the concentration of Zn ²⁺ is less than 20 mg/2, the effect of reducing the anticorrosive current will be poor, and if it is more than 100 g/2, zinc will be significantly deposited on the plated surface, which has the disadvantage of impairing the properties of the plated surface as an alloyed material. The concentration of Zn ²⁺ in the electrolyte needs to be appropriately adjusted within these ranges depending on the acid concentration and temperature of the electrolyte. In addition, as the acid to be added to the electrolyte, H ₂ SO ₄ or HCl, which is commonly used for pickling steel plates, is suitable.
The acid concentration is preferably about 0.5N to 5N. 0.2N
If it is less than 5N, the descaling effect will be poor, and if it is more than 5N, cathodic protection of the plated surface will be extremely difficult. Note that if an organic inhibitor is mixed into the acid solution and used, cathodic protection of the plated surface can generally be achieved at a lower current density, but in this case, the descaling efficiency decreases. The present invention has the advantage that protection of the plated surface during electrolytic pickling can be carried out at a low current density without reducing the descaling property unlike organic inhibitors. Next, the present invention will be explained with reference to Examples. Example 1 Single-sided hot-dip galvanized steel sheet manufactured in a normal process and subjected to single-sided alloying treatment with scale still attached (base material SPC, 0.6 mmt, scale film thickness 6μ, composition
Table 1 shows the results of electrolyzing Fe ₂ O ₃ +FeO) using two lead electrodes left on a steel plate and measuring the degree of corrosion of the plated surface and the descaling time. Adjustment of Zn ²⁺ was performed with ZnSO ₄ .7H ₂ O. From the results in Table 1, if the solution has a low Zn ²⁺ content, the plated surface cannot be protected even if the corrosion protection current for the plated surface is 40A/ ^dm2 , so it is necessary to lower the bath temperature or increase the acid concentration. It is necessary to lower the descaling time and increase the descaling time. Although the use of organic inhibitors contributes to corrosion protection of the plated surface, it has the drawback of hindering descaling. As shown in Comparative Examples 16 and 17,
If the Zn ²⁺ concentration becomes excessive, more zinc will be deposited than necessary, and the plated surface will lose its properties as an alloyed material. In Example 1, a single-sided alloyed hot-dip galvanized steel sheet was used, but a normal single-sided hot-dip galvanized steel sheet may be used.

[Table] The present invention not only protects the plated surface with a relatively low anti-corrosion current, but also makes it possible to use concentrated acid and high temperature baths, and improves the chemical conversion treatment property of the plated surface, especially for alloying treatment. There are a number of benefits, including a reduction in the non-uniformity of zinc phosphate coatings commonly found in steel sheets.

[Brief explanation of the drawing]

Figure 1 shows the effect of Zn ²⁺ concentration on the lower limit corrosion protection current of the alloyed plated surface at various acid concentrations and temperatures. 1...70℃ as electrolyte, when using 1NH ₂ SO ₄ solution, 2... 75℃, 1.8NH ₂ SO ₄ as electrolyte
When using a solution, 3... 80℃ as an electrolyte,
When using _3NH2SO4 solution _,

Claims (1)

[Claims] 1 Single-sided hot-dip galvanized steel plate with 20 mg of Zn ²⁺ /
~100g/contain H ₂ SO ₄ or HCl 0.5N
A single-sided hot-dip galvanized steel sheet or single-sided alloyed steel sheet characterized by descaling by electrolysis in an acidic electrolyte containing ~5N, with an electrode placed opposite to the surface of the steel sheet serving as an anode and the steel sheet serving as a cathode. Method for removing scale from treated steel plates.