Plasticity-enhancement Mechanisms in Ultra-high Strength 8-12%Mn Multi-phase Steel

Abstract

High Mn austenitic steels are being considered for use in automotive car body applications as they combine ultra-high strength with large ductility. This is in part due to various strain hardening enhancing mechanisms which are affected by the thermodynamic stability, grain size, Stacking Fault Energy (SFE) and the chemical composition of the austenite phase.

Although the TWIP steel shows excellent balance of strength and ductility, high production cost due to high content of Mn (18~30 mass-%) alloys makes commercialization challenging. Many researchers have tried to reduce the Mn content and maintain the mechanical properties by alloying alternative elements, i.e. N, C, Al and Cu.

A new design concept for a ductile ultra-high strength steel was developed in the course of research for the present doctoral thesis. The plasticity-enhancement combines both of the TWIP effect and the TRIP effect. The two plasticity-enhancing mechanisms occur in succession during deformation. The TWIP+TRIP steel has a two-phase microstructures consisting of ferrite and austenite. The steel typically has a yield strength in the range of 600-1000 MPa, and the UTS is in the range of 1000-1200 MPa. The TWIP effect occurs in the low strain range. At intermediate strains, the volume fraction of deformation twins reaches saturation and the TRIP effect is activated. The martensite typically nucleates at twin intersections. The succession of the TWIP and TRIP plasticity-enhancing mechanisms results in an ultra-high strength and a large ductility.

In the present study, the microstructure-mechanical properties relationship for 8-12% Mn multi-phase steel was investigated. A pronounced intercritical annealing temperature dependence of the tensile behavior was observed. The annealing temperature dependence of the retained austenite volume fraction, composition and the grain size was analyzed experimentally and the effect of the microstructural parameters on the kinetics of mechanical twinning and strain-induced martensite formation was quantified. A dislocation-density based constitutive model was developed to predict the mechanical properties of 10% Mn multi-phase steel. The model also allows for the determination of the critical strain for dynamic strain aging effect.

In addition, the ferrite transformation mechanism and tensile behavior of 8-12%Mn multi-phase steel was studied by means in situ neutron diffraction measurements and microstructural analysis. During intercritical annealing, the ferrite transformation occurred at twin boundaries resulting in an ultra-fine-grained austenite and ferrite microstructure. The in-situ neutron measurements showed that the ferrite was much harder than austenite. In addition, the strain-hardening of 12%Mn multi-phase steels was controlled by twinning-induced plasticity and transformation-induced plasticity occurring in succession, while the yield strength of 12%Mn multi-phase steel was mainly controlled by ferrite yield strength.