Annealing method for transformation Induced Plasticity of bainite

摘要

PURPOSE: A bainitic transformation heat treatment method is provided to improve forming property in a plane strain mode of the multiaxial stress state for a cold rolled strip for 800 MPa class car members using transformation induced plasticity steel satisfying superior forming property as well as high strength characteristics. CONSTITUTION: The method for bainitic transformation heat treating transformation induced plasticity steel to improve forming property for plane strain comprises a first step of heating the slab to a temperature of 1200 to 1300 deg.C by charging into a heating furnace a slab comprising 0.2 to 0.25 wt.% of C, 1.3 to 1.6 wt.% of Si, 1.4 to 1.6 wt.% of Mn, 0.01 to 0.02 wt.% of S, and a balance of Fe; a second step of furnace cooling the strip to a temperature of 500 to 600 deg.C by charging the hot rolled strip into a cooling furnace after hot rolling the slab by taking out the slab heated in the first step from the heating furnace; a third step of cold rolling the strip by taking out the strip cooled in the second step from the cooling furnace, and annealing the strip to a temperature of 750 to 850 deg.C by charging the cold rolled strip into an annealing furnace; a fourth step of charging the strip into a salt bath heat treatment furnace by taking out the strip annealed in the third step from the annealing furnace, and bainite isothermal heat treating the strip charged into the salt bath heat treatment furnace to a temperature of 300 to 450 deg.C; and a fifth step of air cooling the strip that is bainite isothermal heat treated in the fourth step using air of an ordinary temperature.