베이나이트 변태를 활용한 초고강도강의 미세조직학적 설계

Abstract

Phase transformation behavior and mechanical properties of bainitic microstructure are important topics to investigate before any further studies for designing advanced high strength steels (AHSS), since most of AHSS grades include bainitic ferrite in the final phase constitution. Despite much research on bainite transformation, the occurrence of transformation stasis still remains a highly controversial topic. The present study deals with different perspectives on the phenomenon and analyzes it with a reasonable accuracy by suggesting a new modification of the Gibbs energy balance (GEB) model. Mechanical properties of bainitic microstructure were investigated in medium-carbon spring steels, and bainitic ferrite is found to be a good alternative to conventional tempered martensite since it shows good ductility with comparable strength. Mechanical properties from quenching-and-partitioning (Q&P) were examined in medium-carbon TRIP steels, which forms bainitic ferrite during partitioning. Q&P microstructure, as a mixture of martensite, bainitic ferrite and retained austenite, shows a finer morphology than austempered one, leading to better performance in every examined aspect. Meanwhile, a new prediction model based on the modified GEB model is suggested for Q&P phase fraction and has succeeded in describing phase constitution according to heat treatment condition and chemical composition.