| 摘要 |
PROBLEM TO BE SOLVED: To obtain a cold rolled steel sheet having tensile strength of 590b MPa or more and good in processability by using precipitation strengthening of fine carbide without solid-solution strengthening of Si, Mn or the like.SOLUTION: There is provided a cold rolled steel sheet having a composition containing, by mass%, C:0.07 to 0.12%, Si+Al:0.05 to 0.15%, Mn:0.15 to 0.45%, P:0.005 to 0.010%, S:0.004% or less, N:0.0045% or less, Ti:0.09 to 0.14% and the balance Fe with inevitable impurities and having Tidefined by Ti=Ti-(3.43 N+1.5S) of 2.4≤4C/Ti≤4.5, a ferrite phase with volume fraction of 90 to 95% and cementite with volume fraction of 5 to 10% where the ferrite phase is a ferrite phase consists of a recrystallized structure having recrystallization ratio of 50% or more and recovery structure, Ti carbide deposits in the ferrite phase, average length of the Ti carbide is less than 10 nm and a deposition percentage of Ti is 85% or more. |
