Study of advanced high-strength medium Mn steels : from alloying design to deformation mechanisms

Abstract

In recent years, driven by increasingly serious environmental issues, there has been an increasing demand for low-cost, high-performance steels for the construction of lightweight and energy-efficient structures in various industries. The advanced high-strength medium Mn steel has attracted tremendous attention due to its lean alloying contents and excellent mechanical properties. The objective of this thesis is to understand the deformation mechanisms and processing-microstructure-property relationships of medium Mn steels. First, a crystal plasticity framework incorporating deformation-induced martensitic transformation was developed. The orientation-dependent compression behaviors of the [100] and [110] oriented medium Mn steel single-crystalline micropillars were simulated. It is found that the simulated morphologies of compressed micropillars and the stress-strain curves are in good consistency with the experimental results. From the simulation, it is revealed that the martensitic transformation plays significant roles on the high strength, high work hardening rate, and notable shear band in the [110] oriented micropillar. Second, a manufacture-friendly hot rolling plus warm rolling (HR-WR) thermomechanical processing route was developed to enhance the strength-ductility of medium Mn steel via tailoring the dislocation densities. The microstructures, mechanical properties, and deformation mechanisms of this novel HR-WR steel were investigated and compared with steels produced by conventional intercritical annealing (IA) treatment. It is found that this novel HR-WR steel possesses the highest dislocation densities of both ferrite and austenite phases at initial state and throughout deformation, resulting in the highest yield stress and work hardening rate yet considerable total elongation. The ferrite phase was deformed by dense dislocations, and the austenite phase was deformed by highly distorted stacking faults and deformation-induced martensitic transformation. Third, an optimized deforming and partitioning (D&P) thermomechanical processing route was developed to produce an ultrastrong yet ductile medium Mn steel. The mechanical behaviors, microstructure evolutions, and fracture behaviors of the initial hot-rolled (HR) steel, intermedium warm-rolled (WR) steel and finial D&P steel were systematically investigated. It is noted that the key to producing the ultrastrong yet ductile D&P steel is to create large amounts of mobile dislocations. It is found that the IA processing plays an insignificant role, while the cold rolling and partitioning play critical roles on producing ultrahigh-strength and ductile D&P steels. Fourth, the strain rate and temperature dependent deformation mechanisms of a room-temperature quenching and partitioning (RT-Q&P) medium Mn steel were investigated. Surprisingly, it is found that, at both room temperature (RT) and 100 °C, the RT-Q&P steel exhibits abnormal negative strain-rate sensitivity (SRS) after yielding. It is found that that the transformation induced plasticity (TRIP) effect is not responsible for this abnormal negative SRS. It is identified, for the first time, that Cottrell atmosphere induced by carbon segregation is the major mechanism responsible for the negative SRS of the RT-Q&P steel at RT and 100 °C. The lack of Cottrell atmospheres at high-strain-rate deformation results in the absence of extra carbon dragging strengthening and lower dislocation density, both of which contribute to a lower flow stress at high strain rate and therefore the negative SRS.