Influence of aluminum and phosphorus on solid transformation of TRIP steels

In order to study the effect of alloy element(Al and P) on solid phase transformation of transformation-induced plasticity(TRIP) steels,Continuous cooling transformation(CCT) diagrams and properties of four kinds of C-Mn-Al-P TRIP steels with different aluminum and phosphorus contents were examined by dilatometric test and microstructure observation using optical microscopy.Al made the formation of ferrite and pearlite shift to the left side,and the formation of bainite and martensite to higher temperatures on the CCT diagram as Al narrows the austenite phase region strongly and increases the Ac3 point and Ms point.P reduces kinetics of the cementite precipitation,and when its content reaches 0.14 wt%,it causes a shift of the forming temperature of pearlite and bainite to the right side on the CCT diagram significantly.No significant effect of P on ferrite and martensite transformation is observed.The Bs points of the investigated steels(A6,567 ℃;P1,556 ℃;P2,514 ℃;P3,548 ℃) are estimated using para-equilibrium shear growth model,which is in agreement with the experimental values.The results also indicate that Al increases the Bs point as it both reduces ΔμFγ→e,cαh(increases the driving force of phase transformation) and ΔGmsh,eFare(reduces the shear resistance),and P decreases the Bs point as it increases ΔGsmh,eFaer.