構造部材

摘要

PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet excellent both in deep drawability and fatigue characteristic, and its manufacturing method.SOLUTION: A steel containing, by mass%, C:0.015 to 0.15%; Si:2.0% or less; Mn:0.1 to 3.0%; P:0.05% or less; S:0.05% or less; sol.Al:0.001 to 0.1%; N:0.001 to 0.01%; and O:0.01% or less is rolled with multistage path hot rolling under conditions that time between rolling paths of a prior final rolling path and a final rolling path is 0.3 to 4.0 sec., a completion temperature of the final rolling path is the Arpoint to 780°C, and cooling time after completion of rolling to 720°C is within 0.4 sec., then applied to the cold rolling with a rolling reduction of 40 to 90% and then annealed. The obtained cold rolled steel sheet has TS[=(TS+2×TS+TS)/4] of 300 MPa or more, YR[=(YR+2×YR+YR)/4] of 0.67 or more, |Δr| of 0.20 or less, r/|Δr| of 4.7 or less [Δr=(r-2×r+r)/2, r=(r+2×r+r)/4]. Suffixes 0, 45, and 90 mean a rolling direction, 45° direction to the rolling direction, and 90° direction to the rolling direction respectively.