Effects of Initial Microstructure of Boron Steel Sheets on Hardenability and Mechanical Property after Hardening

摘要

Boron steels with the composition represented by 22MnB5 are widely used for hot stamped or induction hardened components of automotive body. The steels with three different initial microstructures were investigated in order to study transformation behavior and mechanical properties after heat treatment. One is hot-rolled steel consisting of ferrite and pearlite. Another is full-hard steel produced by cold rolling of hot-rolled steel. The other is batch annealed steel consisting of ferrite and spheroidized cementite. When short time heating above A_(c3) was employed, batch annealed steel showed higher transformation temperature in dilatometory during mild cooling than full-hard steel and lower hardness after die quench. With increasing heating time, hardenability was considered to be improved. The steel, which was heated rapidly just above A_3 and water quenched, indicated maximum hardness but inferior toughness to the steel heated far above A3. Difference between A3 and the temperature, where maximum absorbed energy at room temperature by Charpy impact test was obtained, was larger for batch annealed steel than hot-rolled steel. By taking account of boron distribution observed by SIMS, it is considered that short time heating of batch annealed steel results in undissolved cementite and insufficient segregation of boron on austenite grain boundary. Comparative experiment with boron-free steel suggests that poor hardenability at short holding time was due to the insufficient segregation of boron, On the other hand, low absorbed energy by low temperature heating was due to the existence of undissolved cementite.