Patent ID: 7101445

Claim:
A high-tensile-strength cold-rolled steel sheet having a high r value and excellent strain age hardenability and natural aging resistance comprising a composition, by mass %, comprising: C: 0.025 to 0.15%; of Si: 1.0% or less; Mn: 2.0% or less; P: 0.08% or less; S: 0.02% or less; Al: 0.02% or less; and N: 0.0050 to 0.0250%; wherein N/Al is 0.30 or more, the amount of dissolved N is 0.0010% or more, the balance is composed of Fe and inevitable impurities, the structure is composed of a ferrite phase having an average crystal grain diameter of 10 μm or less at an area ratio of 80% or more and a martensite phase as a second phase at an area ratio of 2% or more, and the r value is 1.2 or more.