Cold rolled steel sheet and hot rolled steel sheet excellent in bake hardenability and resistance to ordinary temperature aging and method for their production

摘要

To provide a steel sheet excellent in painting bake hardenability and anti aging property at room temperature: containing, in mass, 0.0001 to 0.20% of C, 2.0% or less of Si, 3.0% or less of Mn, 0.15% or less of P, 0.015% or less of S, and, in addition, 010% or less of Al and 0.001 to 0.10% of N so as to satisfy the expression 0.52Al/N5 and, further, one or more of 2.5% or less of Cr, 1.0% or less of Mo and 0.1% or less of V so as to satisfy the expression (Cr3.5MO39V) 0.1, with the balance consisting of Fe and unavoidable impurities; having the value of BE170, evaluated after applying a 2% tensile deformation and then a heat treatment at 170 C. for 20 min., being 45 MPa or more, and any of the value of BH160, evaluated after applying a 2% tensile deformation and then a heat treatment at 160 C. for 10 min., and the value of BH150, evaluated after applying a 2% tensile deformation and then a heat treatment at 150 C. for 10 min., being 35 MPa or more; and having the yield point elongation at a tensile test after applying a heat treatment at 100 C. for 1 h. being 0.6% or less.