Patent ID: 6695932
Filing Date: 2004-02-24
Classification: C21D,C22C,Y02P

Abstract:
A cold-rolled steel sheet having superior strain-aging hardenability and exhibiting a tensile strength of less than 440 Mpa and a yield ratio YR of less than 70%, the steel sheet comprising, by mass:not more than 0.15% carbon; not more than 0.4% silicon'not more than 2.0% manganese; not more than 0.04% phosphorous; not more than 0.02% sulfur; greater than 0.005% and not more than 0.02% aluminum; and 0.0050% to 0.025% nitrogen, wherein the silicon content, the manganese content, and the phosphorous content satisfy relationship (1) below: Si+Mn/5+10P<0.44 â€ƒâ€ƒ(1) where Si, Mn, P represent contents, in terms of percent by mass, of corresponding elements, wherein the ratio N/Al is not less than 0.3, the content of nitrogen in the state of solid solution is not less than 0.0010%, the balance of the composition is Fe and unavoidable impurities, and wherein the steel sheet has a structure comprising the ferrite phase and the pearlite phase, the area ratio occupied by the ferrite phase being not less than 90%, the average grain size of the ferrite phase being not more than 15 &mgr;m.