High strength thin steel sheet for the superior press formability and surface quality and galvanized steel sheet and method for manufacturing the same
A high strength thin steel sheet that is mainly used for structural members and inner and outer panels for a vehicle, a galvanized steel sheet, and methods of manufacturing the same. The high strength thin steel sheet for superior press formability includes, 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 meeting 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 relation of Ti≧0.028×Al. This thin steel sheet is galvanized or galvannealed.
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.
2. The method according to the claim 1 , wherein the steel slab further includes at least one selected from the group consisting of, by weight percent, 0.01 to 1.0% Cr, 0.005 to 0.3% Mo, and 0.001 to 0.1% Nb.
3. The method according to claim 1 further comprising galvanizing or galvannealing the steel sheet after quenching and annealing.
4. The method according to claim 2 further comprising galvanizing or galvannealing the steel sheet after quenching and annealing.