JPH06264208A - Production of hot-dip zn-al alloy plated steel sheet for coating - Google Patents

Abstract

PURPOSE:To stably produce a hot-dip Zn-Al alloy plated steel sheet for coating by which the generation of a hexagonal pattern is prevented on the plated face and excellent in appearance by plating a steel sheet with a hot-dip Zn-Al alloy and immediately executing quenching spraying the mist of water or a specified aq. soln. thereon. CONSTITUTION:A cold rolled steel sheet annealed in a reducing atmosphere is passed in a Zn-Al alloy plating bath contg. 3.5 to 70.0% Al to stick a hot-dip Zn Al alloy on the surface of the steel sheet, and after that, the coating weight of the hot dip Zn-Al alloy is regulated to 20 to 40g/m<2> per face of the steel sheet by a gas wiping method. Immediately after that, the mist of water or an aq. soln. of an endothermic reaction compound with a Zn-Al alloy of phosphate, multipolymer phosphate, nitrate or the like is sprayed on the face of the hot-dip Zn-Al alloy plated sheet, and quenching is executed. The plated steel sheet for coating free from the generation of a hexagonal pattern which is the defect of appearance on the plated face, having a smooth surface and excellent in corrosion resistance and workability can be obtd.

Description

Detailed Description of the Invention

[001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-dip Zn-Al alloy-plated steel sheet for coating having a smooth surface.

[002]

2. Description of the Related Art Hot-dip Zn-Al alloy-plated steel sheets are remarkably superior in corrosion resistance and workability to hot-dip Zn-plated steel sheets. Further, the molten Zn-Al alloy is
Since the viscosity is smaller than that at the same temperature, the amount of plating adhered can be reduced by gas wiping. For this reason,
The light weight coated Zn-Al alloy plated steel sheet has corrosion resistance,
It has been used as a plated steel sheet for painting with excellent workability.

As in the case of the hot dip galvanized steel sheet, this coated galvanized steel sheet is manufactured by pretreating the steel sheet and immersing it in a hot dip Zn-Al alloy plating bath, and then applying a coating amount by the gas wiping method. Is carried out in a step of naturally cooling, but as a plating bath, the Al content is 3.5 to 5.
70.0% is used, and the coating weight is adjusted to 20 to 40 g / m ² per side.

However, when the hot-dip Zn-Al alloy-plated steel sheet is naturally cooled after adjusting the coating amount, impurities may be accumulated in the crystal grain boundaries of the Zn-Al alloy during the cooling process,
Because of segregation or the like, the crystal contracts and the periphery is dented, resulting in a hexagonal pattern. This pattern becomes more remarkable as the coating amount is smaller, and when painted, it becomes noticeable and impairs the appearance of the coated steel sheet. The tortoiseshell pattern can be improved to some extent by applying a skin pass, but if the skin pass reduction ratio is increased to completely erase it, the plating metal will adhere to the work roll, causing scratches on the surface.

[0095]

Therefore, the present invention is directed to molten Zn-Al for coating which does not generate a hexagonal pattern during plating.
The present invention provides a method for producing a base alloy-plated steel sheet.

[0096]

According to the present invention, immediately after plating, water or a mist of an aqueous solution of an endothermic reaction compound with a Zn-Al alloy is sprayed onto a plated steel sheet to quench it.

[0097]

Since the Zn-Al alloy immediately after plating is in a molten state, when a mist of water is sprayed, the plated layer is abruptly solidified before the crystal grows. Therefore, local accumulation or segregation of impurities does not occur, and a hexagonal pattern does not occur. Further, when the aqueous solution of the endothermic reaction compound with the Zn-Al alloy is sprayed, the endothermic reaction compound reacts with the Zn-Al alloy and the cooling of the plating layer is further promoted, so that the crystal is further difficult to grow. . Examples of the endothermic reaction compound with the Zn-Al alloy include phosphates, polyphosphates, and nitrates. Examples of the phosphate include ammonium phosphate, sodium ammonium hydrogenphosphate, diammonium hydrogenphosphate, sodium phosphate and its hydrate, sodium dihydrogenphosphate and its hydrate, disodium hydrogenphosphate and its water. Japanese union, etc., as the polypolymeric phosphate, sodium pyrophosphate, sodium tripolyphosphate, sodium tetrapolyphosphate, sodium hexapolyphosphate and the like, further, as the nitrate, cobalt nitrate, iron nitrate, nickel nitrate, Zircozol nitrate, copper nitrate or the like may be used.

[0085]

[Example] The thickness of the plating bath was 0.5 m in a non-oxidizing furnace type pilot continuous hot dip galvanizing line of Zn-Al alloy.
m, a width of 300 mm is passed through the cold rolled steel plate at a speed of 52 m / min, and the mist of water or an aqueous solution of an endothermic reaction compound with a Zn-Al alloy is immediately adjusted with a nozzle after adjusting the coating amount by the gas wiping method. It was sprayed on the plated steel sheet and the generation state of the hexagonal pattern was investigated. The results are shown in Table 1.

[0109]

[Table 1] (Note 1) The coating weight is on one side.

[0101]

As described above, according to the present invention, it is possible to prevent the appearance of a hexagonal pattern on the light weight coated Zn-Al alloy plated steel sheet for coating and improve the appearance after coating.

Claims (1)

[Claims] 1. A steel sheet is pretreated and then dipped in a Zn-Al alloy plating bath having an Al content of 3.5 to 70.0% to obtain a coating weight of 20 to 40 g / m per side. ^{In the} method for producing a plated steel sheet adjusted to ² , immediately after plating, a mist of water or an aqueous solution of an endothermic reaction compound with a Zn-Al-based alloy is sprayed onto the plated steel sheet, and is rapidly cooled. Method for producing base alloy plated steel sheet.