Mechanical properties of a microalloyed bainitic steel after hot forging and tempering

摘要

Mechanical properties of a newly developed microalloyed bainitic steel were investigated after the hot forging, air cooling and tempering process.The microstructure of the as-forged bainitic steel mainly consists of granular bainite and ~20 vol.% martensite.The fraction of retained austenite remains unchanged until tempering at 200℃, above which it decreases significantly.The in-crease of tempering tem perature leads to decreases of both ultim ate tensile strength and total elongation but decreases of both yield strength and reduction of area.The maximum and mini-mum values of impact toughness were observed after tempering at around 200 and 400℃, re-spectively.These effects are mainly attributed to the decomposition of martensite/austenite con-stituents and the tempering effects in martensite.The tempering of the forged bainitic steel at around 200℃ results in an excellent combination of strength and toughness, which is compara-ble to that of the conventional quenched-and-tempered 40Cr steel.Therefore, low-tempering treatment coupled with post-forging residual stress relieving is a feasible method to further im-prove the mechanical properties of the bainitic forging steel.