The relationship of microstructure, mechanical properties and mechanism with multi-scale analysis in 4Mn steel under different intercritical annealing temperatures

Abstract

In view of the poor applicability of medium Mn steel, this study designed an application-friendly medium Mn steel with 4 wt% Mn, Fe–4Mn-1.5Al-0.5Si-0.2C-0.05Nb. The tested steel achieves high mechanical properties with the ultimate tensile strength >1000 MPa and the total elongation >30%. Through mechanical properties testing and microstructure characterization and theoretical calculation, the influence of intercritical annealing (IA) temperature on the microstructure evolution and mechanical properties of cold-rolled steel was studied. The highest austenite volume fraction (19.8%) and the best comprehensive mechanical properties (yield strength: 653 MPa; ultimate tensile strength: 1084 MPa; total elongation: 30.2%) were achieved in the sample annealed at 720 °C through reasonable microstructure construction and coupling of various strengthening mechanisms. The response relationship between strengthening and toughening mechanism at the microscale, microstructure characteristics at the mesoscale, and mechanical properties at the macroscale were obtained. Among them, solid solution strengthening, grain boundary strengthening and dislocation strengthening play a significant role in the multi-mechanism coupling effect of yield strength. In the α+γ+α’ triplex phase system, the increase of IA temperature leads to the increase of austenite and martensite fraction, resulting in a linear negative correlation between tensile strength and ferrite fraction as the other phase. In the plastic deformation, the TRIP effect of partial austenite and dislocation multiplication of ferrite contribute to the dominant work hardening effect, in which the former mainly plays a role in the first half of the deformation stage, while the latter plays a uniform role in the whole deformation stage.