Design of Advanced High Strength Steels

A new advanced high strength steels (AHSS) is designed based on Fe-C-Mn-Al composition. Martensitic steel is processed in intercritical region to achieve an ultrafine-grained duplex γ–(α + α') microstructure. The focus was on tuning the degree of austenite plasticity via controlling its stability, called austenite engineering. Interest in austenite engineering stems from transformation-induced plasticity (TRIP) effect, which is known to enhance ductility. The thermodynamic and kinetic analyses were used to optimize the annealing condition. The evolution of microstructure and mechanical properties was studied using different techniques. Due to high heating rate, the austenite reversion occurred before recrystallization of the ferrite. The final microstructure was duplex steel with globular-shaped grains. High volume fraction of the austenite phase was obtained (f_γ>40%) in very short time annealing. By increasing annealing temperature and time, austenite fraction and grain size increased. However, due to dilution of the austenite from stabilizers elements, the stability of the austenite dropped and transformed into martensite during quenching. This led in variety of austenite stabilities that resulted in different combination of mechanical properties. The critical factors influencing the onset of TRIP effect is studied and it was found that both early and delayed onset of the TRIP effect will lead to worse ductility. Hence, to achieve ultrahigh strength and excellent ductility, austenite stability shall be controlled to precisely trigger out TRIP. This study find out that discontinuous yielding or Lüders bands phenomenon can be used in ultrafine duplex steels to improve ductility. The results showed that superb combination of strength (σ_YS>1.0GPa and σ_UTS>1.4GPa) and ductility (ε_t≥20%) could be achieved in short time annealing of less than 10 minutes. This work evidence that tuning the austenite to a marginal stability enables us to design strong and ductile steels