Fracture toughness of high-alloy austenitic-martensitic TRIP steels after Q&P processing

The recently developed austenitic-martensitic TRIP cast steel Fe-14Cr-3Ni-3Mn-0.4Si-0.11N-0.15C was subjected to different Quenching & Partitioning (Q&P) treatments in order to achieve a variation of the microstructural and mechanical properties. Subsequently, the fracture properties of three material variants were studied by means of tensile tests and fracture mechanical 3-point-bending tests to determine J-a fracture resistance curves. Due to Q&P treatment, the steel achieved considerable strength and ductility values (UTS of about 1500 MPa with a total elongation of almost 30%) which qualify it for the 3rd generation of AHSS. The fracture toughness behavior was significantly influenced by the initial -martensite content as well as by the austenite stability, which could be adjusted by varying the Q&P parameters. If the austenite stability was low, the formation of deformation-induced -martensite became possible. This TRIP effect is known to be beneficial for fracture toughness of austenitic steels. However, the experimental results suggest that there was a contrary effect of embrittlement due to metastable austenite which undergoes martensitic transformation already in the early stages of deformation. Therefore, the Q&P parameters have to be carefully chosen in order to achieve a remarkable combination of strength, ductility and fracture toughness of the investigated high-alloy austenitic-martensitic TRIP steel.