JPH0657392A - Production of hot dip zinc-coated steel sheet excellent in blackening resistance - Google Patents

Abstract

PURPOSE:To obtain a galvanized steel sheet by hot dip zinc-coating excellent in blackening resisting property by chromatetreating a zinc (alloy) plated steel sheet after treating it with an aqueous soln. of molybdate. CONSTITUTION:The surface of the steel sheet is galvanized by hot dip-coated with zinc or a zinc alloy. Then, water-soluble molybdate is dissolved in water to have, for example, 20 to 200g/l concentration and PH of 5 to 13. After treating the galvanized steel sheet with aqueous soln. of molybdate, what is called, a chromate-treating in which the aqueous soln. of chromic anhydride or liquid mixture of chromic anhydride and colloidal silicic acid is applied thereon is executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel sheet with a hot dip galvanized steel having excellent blackening resistance.

[0002]

2. Description of the Related Art In recent years, zinc-based hot-dip galvanized steel sheets have been required to be further improved in appearance, corrosion resistance, workability, paintability, etc., as demand has increased and applications have diversified. For the improvement of appearance, after sprinkling, before the molten zinc layer is solidified, spraying water droplets on the surface of the zinc layer to achieve a zero spangle treatment that reduces the size of zinc crystals. Skin pass rolling is performed to improve the plate shape.

To improve the corrosion resistance, phosphoric acid is added to these solutions by a chromate treatment in which a chromic anhydride aqueous solution or a mixed solution of chromic anhydride and silicic acid colloid, which has been widely practiced in the past, is applied. A method for improving the corrosion resistance (Japanese Patent Publication No. 60-18751) or a method for adding phosphoric acid, a fluoride, a cobalt salt or the like (Japanese Patent Laid-Open No. 57-1).
No. 74469) has been proposed.

However, a zinc-based hot-dip galvanized steel sheet, which has been subjected to chromate treatment for improving corrosion resistance after being subjected to skin pass rolling, has a part or whole surface of the galling surface during storage in a warehouse. The problem of discoloration to gray-black, that is, blackening, often occurs. It This blackening phenomenon is a phenomenon that appears on the plating surface of a zinc-based hot-dip galvanized steel sheet in the early stage of corrosion, and is often seen in chromate materials that are accelerated in a high temperature and high humidity atmosphere and are skin-pass rolled.

As a method for preventing such a blackening, for example, firstly, a steel sheet which has been subjected to melt plating is subjected to treatments such as skin pass rolling and leveler processing, and before being subjected to chromate treatment, To heat treatment to a temperature of 200 ° C. or higher (JP-A-55-131178), secondly,
After the melt plating is performed, an aqueous solution containing cobalt or iron ions is sprayed on the surface of the plating layer at 170 ° C. or higher, and then chromate treatment is performed (Japanese Patent Laid-Open No. 1-1299).
No. 78), as a third method, after melt-plating, treatment with a solution containing cobalt or nickel ions, and then chromate treatment (Japanese Patent Publication No.
-49982) and the like have been proposed.

However, when the first method is carried out in the plating line, the heating equipment becomes large and it is difficult to control the surface oxide film. According to the second method, after plating,
When skin pass rolling is performed, the effect of preventing blackening is significantly reduced. Furthermore, according to the third method, it is difficult to control the deposition amount of nickel or cobalt, and when the deposition amount is small, the effect of preventing blackening is small, while when it is too large, the surface has a black tint. And the corrosion resistance is deteriorated.

As described above, according to the conventional method for preventing the blackening of the galvanized steel sheet, a large heating facility is required, which is industrially disadvantageous, or the process control is complicated. Therefore, there are various problems such as difficulty in obtaining a stable effect.

[0008]

SUMMARY OF THE INVENTION The present invention has been made to solve various problems as described above in preventing the blackening of a conventional hot-dip galvanized steel sheet, and is industrially advantageous. An object of the present invention is to provide a method for producing a steel sheet with hot dip galvanizing which is excellent in blackening resistance.

[0009]

A method for producing a hot-dip galvanized steel sheet having excellent blackening resistance according to the present invention is a method for producing a galvanized steel sheet having a zinc or zinc alloy-based hot-dipped galvanized steel surface. It is characterized in that it is treated with an aqueous solution containing a water-soluble molybdate, followed by chromate treatment. In the method of the present invention, sodium molybdate, potassium molybdate, ammonium molybdate, etc. are preferably used as the water-soluble molybdate. These may be used alone or as a mixture of two or more kinds. In the method of the present invention, the concentration of the molybdate aqueous solution is preferably in the range of 20 to 20 g / l, and the liquidity is pH.
Is preferably in the range of 5 to 13, and the treatment temperature is 20.
The range of -50 ° C and the treatment time of 1-25 seconds are preferable.

When the concentration of the molybdate aqueous solution is less than 20%, the effect of preventing the blackening of the molten zinc plated steel sheet is 200 g / l even by the treatment according to the present invention.
If the concentration exceeds the range, the surface of the steel sheet with the hot-dip galvanized steel may be bluish. When the treatment temperature is lower than 20 ° C., the treatment requires a long time. On the other hand, when it exceeds 50 ° C., the plated surface is etched and the surface gloss is reduced, which is not preferable. Further, when the pH of the treatment liquid is lower than 5 or higher than 13, the plating surface exhibits a black tint, which is not preferable.

In the method of the present invention, the zinc-plated steel sheet may be treated with an aqueous solution containing a water-soluble molybdate by any method such as a dipping method and a spray method. However, it is essential to wash the zinc-plated steel sheet with an aqueous solution containing a water-soluble molybdate after washing with water. After this washing with water, the steel sheet may or may not be dried.

In the method of the present invention, as described above,
The method of chromating the zinc-based plated steel sheet after treatment with the aqueous solution containing a water-soluble molybdate is not particularly limited in terms of the treatment liquid used, the treatment conditions, and the like.

[0013]

As described above, according to the method of the present invention,
Molten zinc with excellent blackening resistance is obtained by treating a zinc-plated steel sheet with zinc or zinc alloy-based fusion plating on the surface of the steel sheet with an aqueous solution containing a water-soluble molybdate, followed by chromate treatment. A plated steel plate can be obtained.
According to the method of the present invention, stable effects can be obtained in such a wide range of conditions such as the concentration of the treatment liquid, the treatment temperature and the time.

[0014]

The present invention will be described below with reference to examples.
The present invention is not limited to these examples.

Example 1 As a material-plated steel sheet, a fine spangle (zero spangle) molten zinc-plated steel sheet having a zinc adhesion amount of 60 g / m ² on one side was subjected to skin pass rolling with an elongation of 1%, and then the present invention was used. Then, chromate treatment was carried out to evaluate the blackening resistance. Table 1 shows the pretreatment conditions of chromate and the results of evaluation of the amount of chromium adhesion and blackening resistance.

Chromate Treatment Conditions The components of the chromate treatment liquid are as follows. Chromic anhydride 20 g / l Phosphoric acid 8 g / l Silica colloid 20 g / l Chromate Treatment temperature is 30 ° C. Treatment time is 2 seconds (immersion)
After that, the roll was squeezed and dried with warm air at 90 ° C.

Blackening acceleration test A 70 mm × 150 mm test piece was held at an angle of 60 ° and left for 10 days in an atmosphere of temperature 50 ° C. and humidity 98%, after which the blackening of the plated surface was visually judged. did. ⊚ indicates no black discoloration, ∘ indicates a very slight discoloration, Δ indicates a noticeable discoloration, and x indicates a marked discoloration. The appearance color tone and gloss after chromate treatment were visually evaluated. ○: white surface with glossy surface, △: white surface with low gloss, ×
Means a low-gloss surface with a blue tint.

[0018]

[Table 1]

Claims (1)

[Claims] 1. A blackening resistance, which comprises subjecting a zinc-plated steel sheet having a zinc or zinc alloy-based melt-plated surface to a zinc-plated steel sheet with an aqueous solution containing a water-soluble molybdate, followed by chromate treatment. A method for manufacturing a steel sheet with a hot-dip galvanized steel, which is excellent.