Effects of Cu and Nb on martensite hardening and transformation-induced plastic deformation behavior of maraging TRIP-aided steel

Martensite hardening and transformation-induced plastic deformation (TRIP) are crucial to the mechanical properties of steels. In this study, maraging TRIP-aided steel was developed. The addition of Cu and Nb can effectively promote transformation-induced plastic deformation, this is due to the characteristic of the alternating combination of soft and hard phases. In contrast, the addition of Nb has a more significant effect on martensite hardening, which can be attributed to the smaller effect of Nb on the martensite transformation start temperature (Ms). The multi-phase structure and precipitation strengthening, lead to the ultimate tensile strength and elongation of the maraging TRIP-aided steel is ∼1300 MPa and ∼24%, respectively. It was found that the prior austenite formed a continuous network surrounding bainite and martensite. The high-density dislocations in bainite and martensite increased the hardness. In addition, the metastable austenite phase was transformed into the martensite phase, and a heterostructure with the alternating distribution of bainite/austenite was formed during deformation. The refinement of grains effectively reduced the concentration of stress. Massive nanometer-scaled precipitates were formed in Nb-containing steel after peak aging, and the co-precipitation of Nb-rich and Mo-rich phases occurred in the steel. The large-sized Laves-Fe2(Nb, Mo) effectively enhanced the strength of steel, and the fine and dense Ni3Ti was beneficial to the simultaneous improvement of strength and plasticity. Therefore, the jointed mechanisms of martensite hardening, and the TRIP effect improved the strength while retaining good plasticity.