| 摘要 |
PROBLEM TO BE SOLVED: To provide a high-tensile-strength hot-dip galvanized steel sheet having an adequate surface appearance free from an unplated part. SOLUTION: This manufacturing method includes reducing the surface of a steel sheet containing 0.3 mass% or more Si in a temperature region of 400 to 800°C by using a direct flame burner before the steel sheet is hot-dip galvanized. The method also includes, in the above step, mixing a substance having a flux action (for instance, boron compound, fluorine compound, and compounds of rosin, amine and amide) into a combustion gas to burn the mixture. The manufacturing method specifically comprises the step of: heating the steel sheet from room temperature to 200°C by using an atmospheric gas; heating it from 200 to 600°C by using the direct flame burner; heat-treating it from 600 to 850°C through radiant heating; subsequently galvanizing it at 480°C which is a temperature of the sheet entering a galvanizing bath by using the galvanizing bath which contains 0.135% Al (and saturation amount of Fe) and is kept into the temperature of 460°C. Thus, the manufacturing method reduces the surface of the steel sheet by using the direct flame burner, thereby prevents the external part from being oxidized, and prevents oxides from forming on the surface due to Si, Mn, Cr and the like concentrated in the surface from the inner part. COPYRIGHT: (C)2008,JPO&INPIT
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