PURPOSE: To increase the yield point of a cold rolled steel plate and to improve the denting resistance and workability by specifying the thickness ratio between the surface hard layers having a 2-phase structure and the internal single-phase soft layer and the volume ratio of the 2nd phase of the surface 2-phase structure in the outermost surface layers.
CONSTITUTION: This cold rolled steel plate is composed of surface hard layers (a), (a) having a 2-phase structure such as a ferrite-martensite structure and an internal single-phase soft layer (b). The thickness ratio between the hard layers (a), (a) and the soft layer (b) is regulated to 0.1<td/t<0.6[where (t) is the thickness of the plate and (td) is the total thickness of the hard layers], and the volume ratio Vf of the 2nd phase of the surface 2-phase structure in the outermost surface layers is regulated to 0.05<Vf<0.60. The value of td/t can be regulated by adjusting the holding time in the continuous annealing furnace of equipment for manufacturing a cold rolled steel plate, and the value of Vf can be regulated by adjusting the hardening temp.
JITSUKAWA MASAHARU
IWATOU SHIYUUICHI
