Time dependence of transformation process of Si-Mn TRIP steel during high-speed tensile impact testing

摘要

In the present paper, the time dependence of the transformation of retained austenite to martensite in high strength low alloy TRIP steel during high-speed tensile impact testing is analysed in detail. The dynamic tensile testing was in the strain rate range from 300 to 1600 s~(-1). The high strain rate stress-strain curves were characterised by undulations with a period of 50 - 80 μs during plastic deformation. This phenomenon, characteristic of TRIP-aided steel, is believed to reveal the time-dependent of transformation of retained austenite to martensite. Due to the presence of retained austenite islands of different stability, the deformation-induced transformation of retained austenite to martensite was found to be time-dependent in the strain rate range of 400 to 900 s~(-1). As expected, steel of the same chemical composition but with a dual-phase (DP) microstructure did not reveal this time-dependent transformation process in the stress-strain curves in the same strain rate range. Both TRIP and DP steel exhibited no time-dependent transformation for strain rates larger than 1000 s~(-1). By comparing the volume fraction of retained austenite on tested specimens at same position, it was found that the amount of transformed austenite in the specimens exhibiting a clear time-dependent transformation was larger than in the specimen with no clear time-dependent transformation.