New hot-rolled high carbon steel sheets with excellent stretch-flange formability

The effect of microstructure on the stretch-flangeability of JIS S35C hot-rolled steel sheets, was studied from the viewpoint of the initiation/growth of voids in the hole-expanding process with ferrite-pearlite and spheroidized cementite steels. In a comparison of ferrite pearlite steel and spheroidized fine-grained cementite steel, total elongation was higher with the ferrite-pearlite structure, while the hole-expanding ratio (lambda) of the spheroidized cementite structure, which is an index of stretch-flangeability, was superior to that of the ferrite-pearlite structure. The hole-expanding ratio has a strong correlation with notched-tensile elongation, and a linear relationship was obtained with a long gauge length of GL = 50 mm. A notch-shaped crack occurred at the surface of the punched hole of the ferrite-pearlite steel in the hole-expanding test and rapidly propagated along the ferrite-pearlite interface in the subsequent hole-expanding process. In contrast, the punched hole surface of the spheroidized cementite steel was smooth, and after void initiation/growth in the hole-expanding process, voids developed into cracks by coalescence, resulting in a homogeneous distribution of work strain.