A 0.2C-1.5Mn-1.5Si-0.6Cr-0.05Nb （wt%） steel is treated respectively by novel quenching-partitioning-tempering （Q-P-T） process and traditional quenching and tempering （Q＆T） process for comparison. X-ray diffraction analysis indicates that Q-P-T steel has about 10% retained austenite, but Q＆T steel hardly has one. With the increase of com- pression strain rate from 7 × 10^2 to 5 × 10^3 s^-1, the flow stress of Q-P-T steel increases, which demonstrates the positive strain rate effect, but does not exist in Q＆T steel. The characterization of scanning electron microscopy indicates that a large number of long, straight martensite laths in Q-P-T steel will bend or be destroyed by large compressive strain of 35% at 5 × 10^3 s^-1. However, relative small compressive s^xain of about 5% at 7× 10^2 s^-1 almost does not have any effect on the original lath morphology. The characterization of transmission electron microscopy further reveals the origin of the positive strain rate effect and the microstructural evolution during dynamic compressive deformation.
Qingguo HaoYing WangXiaoshuai JiaXunwei ZuoNailu ChenYonghua Rong
The present investigation on a designed high strength Fe-0.25C-1.48Mn-1.20Si-1.51Ni-0.05Nb （wt%） steel treated by a novel quenching-partitioning-tempering （Q-P-T） process was focused on deformation temperature dependence of mechanical properties and microstructures. The results indicate that the Q-P-T steel deformed at various deformation temperatures from -70 to 300 ℃ exhibits superior mechanical properties due to excellent thermal stability of retained austenite. The microstructural characterization by transmission electron microscopy （TEM） reveals that the high strength of the Q-P-T steel results from dislocation-type martensite laths and fcc NbC carbides or/and hcp ε-carbides precipitated dispersively in martensite matrix, while excellent ductility is attributed to the significant transformation induced plasticity （TRIP） effect produced by considerable amount of retained austenite. The relationship between mechanical properties and microstructures at different deformation temperatures was clarified.
