PURPOSE: To easily produce a plated steel strip having superior rust-preventing property at a low cost by applying hot-dip Zn alloy coating to the surface of a steel strip and then carrying out cooling and heat treatment under respectively specified conditions.
CONSTITUTION: A cold rolled steel strip is plated with a hot dipping bath of Zn alloy, such as Zn-Fe, Zn-Mn, Zn-Ni, Zn-Al, and Zn-Cr, of 700-1200°C and then heated up to 700-950°C to undergo recrystallization annealing. Subsequently, the steel strip is cooled from >650°C down to room temp. - 400°C at ≥30°C/sec cooling rate, or, after the recrystallization annealing, the steel strip is cooled rapidly down to 250-450°C at ≥40°C/sec cooling rate and held at this temp. for 1-5min or the steel strip is cooled down to 650-800°C at 3-14°C/sec cooling rate, cooled down to 250-450°C at ≥40°C/sec cooling rate, and held at this temp. for 1-5min, or, the plated steel strip is cooled down to 650-800°C at 3-14°C/sec cooling rate and successively cooled down to 250-350°C at ≥40°C/sec cooling rate.
OKA MASARU