Method for manufacturing hot-dip galvanized steel sheet

Abstract

Provided is a method for manufacturing a hot-dip galvanized steel sheet, with which it is possible to achieve a good plated external appearance with high plating adhesion without a deterioration in the tensile strength, even when subjecting a steel sheet having a Si-content of 0.2% by mass or greater to hot-dip galvanization. On the interior of an annealing furnace, a steel sheet is conveyed through a heating area, a soaking area, and a cooling area in this order, thereby annealing the steel sheet, and the steel sheet ejected from the cooling area is subjected to hot-dip galvanization. A reducing or non-oxidizing humidifying gas and a reducing or non-oxidizing drying gas are supplied to the soaking area. The CO-gas concentration is measured by providing a CO-gas concentration meter at a discharging section for the gases in the soaking area, the thickness of a decarburized layer on the steel sheet is calculated from the measured CO concentration, and at least one of the flow volume anddew point of the humidifying gas is controlled so that the calculated thickness of the decarburized layer becomes equal to or less than a pre-set thickness.

Description

technical field [0001] The present invention relates to a method of manufacturing hot-dip galvanized steel sheets using a continuous hot-dip galvanizing apparatus having an annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged in sequence, and an annealing furnace located in the cooling zone. Downstream hot-dip galvanizing equipment. Background technique [0002] In recent years, demand for high-tensile steel sheets (high-strength steel sheets) that contribute to weight reduction of structures has increased in the fields of automobiles, home appliances, and building materials. It was found that as high-strength steel sheets, for example, steel sheets with good hole expandability can be produced by including Si in steel, and steel sheets with good ductility that easily form residual γ by containing Si and Al can be produced. [0003] However, when a high-strength steel sheet containing a large amount of Si (especially 0.2% by mass or more...

AI Technical Summary

Problems solved by technology

Therefore, as the concentration of Si in the steel increases, the wettability decreases sharply, and missing plating often occurs.

In addition, even when missing plating is not achieved, there is a problem of poor plating adhesion

Furthermore, if Si in the steel is selectively oxidized and enriched on the surface of the steel sheet, there is also a problem that a significant delay in alloying occurs in the alloying process after hot-dip galvanizing, which significantly hinders productivity.

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