Patent ID: 9598753
Date: 2017-03-21
CPC Classifications: C21D,C22C,C23C

Claim:
1. A method of manufacturing a high strength thin steel sheet for superior press formability, the method comprising: hot-working a steel slab, which comprises, by weight percent, 0.06 to 0.4% C, 1.0 to 5.0% Mn, 0.05 to 2.5% Si, 0.01 to 2.0% Ni, 0.02 to 2% Cu, 0.01 to 0.04% Ti, 0.05 to 2.5% Al, 0.005 to 0.1% Sb, 0.0005 to 0.004% B, 0.007% or less N, and balance Fe and inevitable impurities, and which meets relation of Ni+0.5×Mn+0.3×Cu≧0.9, which is defined as Ni*, and Al/Ni*≦1.3 at a same time, and a relation of Ti≧0.028×Al, at a temperature above Ar3; hot-rolling the steel slab to form a hot-rolled steel sheet and coiling the hot-rolled steel sheet at a temperature between 500° C. and 700° C.; pickling and cold rolling the hot-rolled steel sheet to form a cold-rolled steel sheet; annealing the cold-rolled steel sheet at a temperature at which a fraction of austenite has at least 30%; and quenching the annealed steel sheet from a temperature directly above a martensite forming temperature to a temperature below a bainite forming temperature, maintaining the quenched steel sheet at the temperature below the bainite forming temperature for at least 30 seconds, and cooling the quenched and maintained steel sheet, wherein the cooled steel sheet comprises austenite having a volume fraction of at least 33%, the austenite having a lath shape, a short side of which is less than 350 nm.