| 摘要 |
PROBLEM TO BE SOLVED: To provide a hard cold rolled steel sheet excellent in thermal stability of hardness and its manufacturing method.SOLUTION: Cold rolling is conducted on a hot rolled steel sheet containing a composition containing, by mass%, C:0.10% to 0.25%, Si:0.3% or less, Mn:0.5% to 1.0%, P:0.03% or less, S:0.02% or less, Al:0.01% to 0.08%, Cr:0.03% or less, Ti:0.010% or less, Nb:0.010% or less, B:0.0010% or less, and the balance Fe with inevitable impurities, and ferrite and pearlite, and having a structure in which the ferrite has a percentage in the whole structure of 80% or more and an average crystal particle diameter is 10 μm to 20 μm to manufacture a cold rolled steel sheet having a composition containing ferrite as a main phase and dispersed cementite. The main phase ferrite is a coated elongating particle having an average aspect ratio in a rolling direction cross section of the steel sheet of 3 or more, the cementite has percentage in the whole structure of 5% or less, and an average major diameter in the rolling direction cross section of the steel sheet is 1.5 μm to 5.0 μm. |
