PURPOSE: To make the production of a high-strength zinc hot dipped steel plate having a low yield ratio, and excellent strength-elongation balance and stretch- flangability in the production of a zinc hot dipped steel plate by forming the structure of the steel plate into specific 3-phase composite structure.
CONSTITUTION: An ingot contg. 0.005W0.15% C, <1% Si, 0.7W2.5% Mn, <0.1% P or further <0.5% Cr is hot and cold rolled to a cold-rolled steel plate. After the steel plate is annealed in a direct firing type heating furnace of a non- oxidative atmosphere at 900W1,100°C and a radiation heating furnace of a reducing atmopshere, the steel plate is passed through a slow cooling zone, a quick cooling zone and a cooling control furnace then through the inside of a zinc hot dipping bath of about 500°C, whereby zinc hot dipping is applied thereon. The ferrite-bainite-martensite 3-phase structure contg. 5W50% area rate of bainite and 1W20% area rate of martensite is formed, and the high strength zinc hot dipped steel plate of the composite structure having a low yield ratio and excellent strength-elongation balance and stretch-flangability is obtained.
TSUKATANI ICHIROU
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