| Many engineering materials call for increasing strength and ductility. Unfortunately, the material properties of high ductility and high strength are usually mutually contradictory. These conflicting requirements have generated interest in transformation induced plasticity or TRIP steels, which mainly consist of a mixture bainite, ferrite and retained austenite. The superior strength and ductility of these types of steels is due to the strain induced transformation of retained austenite to martensite.;Intercritical deformation (i.e. deformation in the austenite + ferrite two phase region) has been extensively studied on C-Mn and microalloyed steels, but not on TRIP steels Intercritical rolling increases the volume fraction and decreases the grain size of ferrite. These characteristics may be beneficial to the ductility of TRIP steels, since the volume fraction of retained austenite may increase with ferrite volume fraction by increasing the level of C segregation to the untransformed austenite. As well, the size of the retained austenite may decrease with decreasing ferrite grain size, thus increasing the stability of retained austenite against strain induced transformation. The aim of this study was to investigate the effect of the intercritical deformation on the characteristics of retained austenite and resulting mechanical properties of a TRIP steel.;The TRIP steel composition studied in this work was 0.22%C - 1.55%Si - 1.55%Mn - 0.035%Nb. A range of deformation strains and temperatures in the intercritical region, and holding times in the bainite phase region (400°C) were explored. As measured by shear punch testing, the UTS values fell between 850 and 950 MPa, and the tensile elongation (T-EL) values varied from 20 to 30%. The best property combination was found to be 950 MPa UTS and 30% T-EL. Mechanical properties and microstructures were not strongly affected by intercritical deformation conditions. Holding time in the bainite region significantly affected the retained austenite. The retained austenite varied between 5% and 12%, and was maximized at 10 mins holding. |
