Influence of alloying element on continuous cooling solid phase transformation and hardness of TRIP steels

In order to research the effect of alloying elements Al and P on the solid phase transformation of transformation-induced plasticity (TRIP) steels,the continuous cooling transformation (CCT) diagrams of four C-Mn-Al-P TRIP steels with Al and P contents were determined through the dilatometric test,microstructural observation and microhardness measurement. The results reveal that Al narrows strongly the austenite phase region,increases the Ac3 point and Ms point,and results in the shift of CCT diagram to the left side and upper side. P can inhibit the precipitation of the cementite. And when its mass fraction reaches 0.14%,a significant shift of both pearlite and bainite phase regions to the right side occurs. However,P has no significant effect on proeutectoid ferrite transformation and martensite transformation. The microhardness of the steels increases with increasing the cooling rate. Full martensitie microstructure can be obtained above the critical cooling rate for each steel. The addition of solid solution strengthening elements can enhance the ferrite matrix so as to increase the microhardness of the ferrite matrix. P exhibits the largest solid solution strengthening capacity,followed by Mn,and the solid solution strengthening capacity of Al is relatively weak.