Patent ID: 8926770
Filing Date: 2015-01-06
Classification: B21D,C21D,C23C,C25D,G01J

Abstract:
1. A method of hot stamping a galvanized steel sheet comprising: heating a galvanized steel sheet up to a temperature lower than the boiling point of zinc, and not lower than the austenite transformation temperature of the galvanized steel sheet; then, cooling the galvanized steel sheet; and then, pressing and quenching the galvanized steel sheet, wherein in the cooling the galvanized steel sheet, change in emissivity of a surface of the galvanized steel sheet is measured in a temperature range lower than the boiling point of zinc but not lower than the ferrite transformation temperature, using an emissivity sensor with an observation wavelength of 1.4 μm or longer, the pressing and quenching is started upon completion of an alloying reaction detected based on the change in emissivity, and the measuring the change in emissivity of the surface of the galvanized steel sheet comprises: smoothing the emissivity measured using the emissivity sensor by moving averaging; then, differentiating the result of the smoothing to obtain a rate of change in emissivity; and then, determining a start point of the alloying reaction by a point where the rate of change in emissivity turns from negative into positive, and determining an end point of the alloying reaction by a point where the rate of change in emissivity turns from positive into negative after the start point.