Processes for producing high-strength hot-dip galvanized steel sheet and high-strength galvannealed steel sheet

A process for producing a high-strength hot-dip galvanized steel sheet which comprises: heating a steel sheet containing, in terms of mass%, 0.005-0.12% carbon, 0.7-1.8% silicon, 0.5-2.8% manganese, up to 0.1% phosphorus, up to 0.07% sulfur, up to 1.0% aluminum, and up to 0.008% nitrogen, with the remainder being iron and incidental impurities, with a CGL having a heating zone of the DFF type or NOF type so that the heating zone has an atmospheric-gas composition with a CO/H2O volume ratio of 0.001-0.8, the steel sheet on the heating-zone outlet side has a temperature of 700 DEG C or higher and a width-direction temperature deviation less than 20 DEG C, and the average rate of heating in the heating zone from 400 DEG C to the heating-zone outlet-side temperature is 10 DEG C/s or higher; subsequently annealing the steel sheet in the temperature range of 700-940 DEG C for 15-600 seconds; successively cooling the sheet at a cooling rate of 3 DEG C/sec or higher to a temperature in the range of 440-550 DEG C; and within 200 seconds of termination of the cooling, dipping the 440-550 DEG C steel sheet in a 440-500 DEG C molten-zinc plating bath to plate the steel sheet with zinc by hot dipping. By this process, a high-strength hot-dip galvanized steel sheet is obtained which has a satisfactory deposit appearance, excellent formability, and a tensile strength of the 590 MPa class.