Effect of bainitic transformation during BQ & P process on the mechanical properties in an ultrahigh strength Mn-Si-Cr-C steel

A medium carbon Mn-Si-Cr alloyed steel was treated by a novel bainite-based quenching and partitioning (BQ & P) process: after full austenization, the steel was firstly austempered at 300 degrees C, 320 degrees C, 340 degrees C, 360 degrees C and 380 degrees C for 30 min, and then quenched to 120 degrees C, followed by partitioning at 360 degrees C for 45 min. The multiphase microstructures containing carbide-free bainite (CFB, bainitic ferrite lath plus filmy retained austenite), martensite and retained austenite were characterized by optical microscope, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and dilatometer analysis. An optimum combination of strength and ductility was achieved in the BQ & P steel when the bainitic austempering temperature is 360 degrees C (ultimate tensile strength: 1495 MPa; uniform elongation and total elongation: 26.2% and 31.8%; the reduction of area: 47.9%). Besides the transformation-induced plasticity effect of the retained austenite and the composite effect of the multiphase after BQ & P treatment, the formation of carbide free bainite also plays a significant role on the enhanced mechanical properties. The carbide-free bainite could improve the damage resistance of the multiphase due to the additional strain-hardening capacity within the local plasticity deformation zone near the tip of micro-cracks. In this case, the fraction and distribution of CFB should be controlled properly and the macrosegregation should be avoided.