Effect of Microstructure on Elongation in Cold-rolled High Carbon Steel Sheets

Abstract

The effect of void initiation due to microstructural factors, including the size and distribution of spheroidized cementite and ferrite grain size, on elongation in JIS S35C and JIS S65C high carbon cold-rolled steel sheets were investigated.
Among mechanical properties after 2nd annealing, yield strength (YS) and tensile strength (TS) both increased as cold-rolling reduction increased, and total elongation (T.EI.) also increased simultaneously. This resulted in large improvement in the TS-T.EL. Balance. As mirostructural changes in this case, with the increase in cold-rolling reduction, the ferrite grain diameter decreased and cementite on the ferrite grain boundary increase.
According to the results of a detailed study of S35C, when the microstructure was composed of fine ferrite and cementite on the ferrite grain boundary, elongation was high. In the tensile deformation process, destruction of the grain boundary cementitie was frequently observed. Voids were initiated from these points of origin, and fracture then occurred rapidly. On the other hand, when the microstructure was composed of coarse ferrite grains and fine cementite existed within the ferrite grain, elongation was low. In particular, uniform elongation was markedly decreased. In this deformation process, voids were initiated from cementite on deformation bands, then joined to form cracks, resulting in fracture.