High tensile cold rolled steel sheet and high tensile not dip galvanized steel sheet having improved stretch flanging property and process for producing same.

摘要

There is disclosed a high tensile cold rolled steel sheet improved in ductility, particularly stretch flanging property, containing 0.03% to 0.15% by weight of C, 0.05% or less by weight of Si, 0.5% to 1.2% by weight of Mn, 0.005% to 0.045% by weight of Nb, and 0.10% or less by weight of Al, the remainder being iron and unavoidable impurities, and the steel sheet having a uniform and fine recrystallized ferrite structure having a mean grain diameter of 20 mu m or less and an area fraction of 95% or more, said high tensile cold rolled steel sheet can be obtained by preparing, as a material, steel containing 0.03% to 0.15% by weight of C, 0.05% or less by weight of Si, 0.5% to 1.2% by weight of Mn, 0.005% to 0.045% by weight of Nb, and 0.10% or less by weight of Al, the remainder being iron and unavoidable impurities, subjecting the material to hot rolling, effecting cold rolling at a reduction rate in thickness of more than 50%, and effecting annealing in which the material is heated at a heating rate of 5 DEG C/sec or more and retained in a temperature range of 720 to 780 DEG C for 20 to 60 seconds in a continuous annealing line, and then cooling the material.