열연 고강도강 내에서 석출물 거동이 hole expansion ratio에 미치는 영향

Abstract

Among the various strength steels, hot rolled low-carbon high strength steel have strong advantages comparing other steels like a dual phase steel(DP), transformation-induced plasticity (TRIP) steels and complex phase(CP) steel. Hot rolled low-carbon high strength steel have balanced properties; strength, elongation and formability. In this study, hot rolled low-carbon high strength steel which have different hole expansion ratio was investigated to reveal the reason of difference.Since it is known that high angle grain boundary is advantageous to prevent the propagation of crack, EBSD was used to observe the grain size and grain boundary orientation. And nano-sized precipitates was observed by STEM and APT. STEM is very useful equipment for observing the precipitates, however, since sample is ferromagnetic and precipitate is so tiny APT was more suitable in this study. Difference of size and composition was discovered by APT.