The mechanically-induced transformation behaviour of the metastable austenite phase in a high-strength industrial TRIP-assisted Dual Phase steel was monitored in situ using high-energy synchrotron diffraction under uniaxial loading. This allowed direct quantification of the impact of the transformation of the metastable austenite phase (16 vol %), embedded in a ferrite-bainite-martensite matrix, on the work hardening behaviour of this steel. Our results show that the mechanically induced transformation of austenite does not begin until the onset of matrix yielding. We provide experimental evidence which demonstrates for the first time that the austenite transformation increases the work-hardening contribution, σw thereby supporting a driving force approach to transformation induced plasticity. The transformation work required leads to an increase in the macroscopic work-hardening rate after matrix yielding and continues to offset the decrease in the work-hardening rate in the ferrite and martensite phases up to the UTS. Further we show conclusively that martensite yielding does not occur until the completion of the mechanically induced transformation of austenite. Plastic deformation of martensite is immediately followed by local plastic instability leading to necking and ultimate failure of this material

High Young's modulus steels can be fabricated based on the concept of metal matrix composites. In this paper, a number of reinforcing ceramic phases with high Young's modulus are assessed and selected to design compositions for high-modulus steels based on thermodynamic calculations. The steel matrix composites reinforced with boride and carbide phases are produced through ingot casting and are processed thermomechanically to strips following standard processing routes for automotive products. The results show that the Young's modulus of steels in the as-cast condition can be increased using borides and carbides. However, further down-stream processing via conventional thermomechanical processing leads to a gradual degradation of the Young's modulus due to extensive void formation. The opportunities and challenges of ceramic-reinforced high-modulus steels produced via conventional ingot casting and thermomechanical processing for the automotive market are discussed.

The current work demonstrates the microstructural evolution and mechanical property evaluation of a newly designed steel composition after hot rolling in laboratory-scale rolling mill, followed by continuous cooling. The steel thus developed has typically about 80% carbide-free bainite; about 20% retained austenite and can deliver ∼1400 MPa ultimate tensile strength along with more than 20% total elongation. The presence of ultra-fine bainite plates (∼100–130 nm thick) with high dislocation density was thought to be responsible for ultra-high strength. Excellent ductility at such strength level could be due to the presence of sufficient amount of retained austenite (∼20%) thermally stable at room temperature but starts transforming to martensite during deformation exhibiting transformation-induced plasticity effect.

Current hot stamped steels can offer ultra high strength (1500 MPa) with limited elongation (∼5%). Research is ongoing to develop hot stamped steels with similar strength and improved ductility by using various microstructural approaches. The current work describes in depth the characterisation of microstructural features of a steel that was processed via a quenching and partitioning method in a modified hot stamping thermal cycle to deliver a good combination of strength and ductility. It was found that the microstructure thus produced is very complex comprising many different phases. As it is also beyond the ability of one single technique to fully characterise it, several techniques were employed in order to properly understand the various features of the different constituents of this microstructure.

This study presents the effects of silicon (Si) and manganese (Mn) concentration and of heating rate on the ferrite recrystallization kinetics in seven model alloys with different Si and Mn concentrations, which are of relevance for the development of Advanced High Strength Steels (AHSS). The recrystallization kinetics were studied with in-situ 2D X-ray Diffraction (2D-XRD) and ex-situ microstructure observations using Scanning Electron Microscopy (SEM). The experimentally observed differences in the recrystallization start temperature (T s), dependent on the Si and Mn concentrations and the heating rate, can be described by combining the non-isothermal JMAK-model with a modified version of Cahn's solute drag model. The modified Cahn model takes into account-in an approximate manner-that the interaction energy of the solute atoms with the grain boundary depends on the Si and Mn concentrations in the boundary and the Wagner interaction parameters. The collective contribution of the Si and Mn atoms to the increase in the T s with respect to the reference alloy (without Si and with very little Mn) is higher than would be expected from the simple addition of the effects of the Si and Mn concentrations alone. This means that the interaction between Si and Mn atoms leads to an additional increase in T s , i.e. a coupled solute drag effect. For the later stages of recrystallization, we have studied the change in the number density and the growth rates of the recrystallizing grains using SEM. The observations show non-random nucleation, early impingement of the grains in the normal-direction and non-constant growth rates of recrystallizing grains.