Continuous cooling transformation diagrams of TRIP steels with phosphorus

In order to research the effect of alloy elements on the phase transformation of transformation-induced plasticity (TRIP) steels, continuous cooling transformation (CCT) diagrams of four kinds of C-Mn-Al-P TRIP steels, with different aluminum contents and with or without microalloy element P, were studied by using optical microscopy and microhardness measurements. Al makes the ferrite forming and pearlite forming temperatures to the left side, and the bainite forming and martensite forming to the higher temperatures of the diagram, as Al narrows the austenite phase region strongly and increases the Ac₃ point and Ms point. P can reduce the kinetics of cementite precipitation, and when its mass fraction reaches 0.14%, it causes a shift of the pearlite forming and bainite forming temperatures to the right side significantly. However, P has no significant effect on ferrite transformation and martensite transformation. The results also reveal that the microhardness increases as the cooling rate increases, and the microstructure will be fully martensitic when the cooling rate reaches the critical rate of the steel.