Effect of Nb and Mo additions on the microstructure and mechanical properties of thermomechanically processed and bake hardened CMnSi TRIP steel

Bake hardening of steels during paint baking of an automotive body could result in an increase of their strength. In this work, the effect of pre-straining and bake-hardening on the microstructure and mechanical properties has been investigated in thermomechanically processed CMnSi Transformation-Induced Plasticity (TRIP) steels with and without additions of Nb and Mo. The steels were characterised before and after bake hardening at 180°C for 30 min. by transmission electron microscopy (TEM), X-ray diffraction (XRD), atom probe tomography (APT), and tensile tests. The microstructure of the thermomechanically processed (TMP) TRIP steels consisted of polygonal ferrite, bainite, retained austenite and martensite. Fine Nb and Mo carbides were detected by TEM in the microstructure of the alloyed steel. In addition, APT revealed the presence of Nb-, Mo-, Fe-, C- containing clusters. Cottrell atmospheres were also detected in the ferrite phase of the CMnSi steel. After pre-straining and bake hardening (PS/BH) both steels exhibit continuous yielding behaviour. The observed higher yield and tensile strengths of the PS/BH samples in comparison with the TMP samples were due to an increase in the number density of dislocations and their interaction with iron carbides formed in bainite and martensite. In addition, the number density of fine Nb and Mo carbides was significantly increased in the alloyed TRIP steel after PS/BH, which also contributed to significant precipitation strengthening in the NbMo steel.