Patent ID: 8518191

Claim:
A high strength cold-rolled steel sheet with superior bake hardenability, comprising, by weight %: C: 0.0016˜0.0025%; Si: 0.02% or less; Mn: 0.2˜1.2%; P: 0.05˜0.11%; S: 0.01% or less; Sol. Al: 0.08˜0.12%; N: 0.0025% or less; Ti: 0.008˜0.018%; Mo: 0.1˜0.2%; B: 0.0005˜0.0015%; and the balance of Fe and other unavoidable impurities, wherein the steel sheet is Nb-free and satisfies Equations 1 and 2: Ti*[Effective Ti]=Total Ti−(48/14)N−(48/32)S≦0 (1) C*[amount of solute carbon in grain boundaries (GB-C)+amount of solute carbon in crystal grains (G-C)]=Total C (ppm)−C in TiC=8˜15 ppm - - - (2)[in Equation 2, GB-C (that is, the amount of solute carbon in the grain boundaries) is 5˜10 ppm, and G-C (that is, the amount of solute carbon in the crystal grains) is 3˜7 ppm], and wherein the steel sheet has ASTM No. of 9 or more, a bake hardenability value (BH) of 30 MPa or more, an aging index (AI) of 30 MPa or less, and a tensile strength of 340˜390 MPa.