MOTHER PLATE FOR MANUFACTURING COLD-ROLLED STEEL SHEET, COLD-ROLLED STEEL SHEET WITH HIGH STRENGTH AND HIGH DUCTILITY, AND MANUFACTURING METHODS THEREFOR

摘要

PROBLEM TO BE SOLVED: To provide a cold-rolled steel sheet having high strength, high ductility and high formability for extension flange, and to provide a manufacturing method therefor.;SOLUTION: This method for manufacturing the cold-rolled steel sheet comprises sequentially a hot-rolling step of hot-rolling a steel raw material containing 0.05-0.40% C, of which the quantity of Si, Mn, P, and Al is controlled, then cooling it down to 500°C at a cooling rate of 30°C/s or higher, and coiling it up, a reheating step of holding it at 550-700°C to finely disperse spheroidal cementite, subsequently a cold-rolling step, and a continuous annealing step of heat treating it to hold it at an Ac₁ transformation temperature or higher but 850°C or lower, cooling it down to 350-500°C at a cooling rate of 10°C/s or higher, holding it in the temperature range for 10-600 s, and cooling it down to a room temperature. Thereby, the method forms a composite metal structure consisting of ferrite phases as a primary phase, and the phases which contain residual γ phases and further low-temperature transformation phases, as a secondary phase, wherein an average particle diameter of the secondary phase is as fine as 3.0 μm or less, the residual γ phase is such a fine and stable phase as to satisfy a predetermined expression for a relation among the residual γ quantity, dissolved C quantity in the residual γ, and the average particle diameter; and hence remarkably improves ductility and formability for extension flange.;COPYRIGHT: (C)2003,JPO