Development of Third Generation Advanced High Strength Steel

Present study intends to develop third generation Advanced High Strength Steel (AHSS) from a first generation lean alloyed TRIP assisted steel with Nb and Ti micro alloying by innovative heat treatments such as Transformation Induced Plasticity (TRIP) aided Bainitic Ferrite (TBF), Dual Stabilization (DS), Harden and Temper (H&T), Quench and Partitioning (Q&P), Austenite Reverted Transformation (ART). All the steels showed tensile toughness greater than 20 GPa. %, higher than 25 GPa% for TBF, DS, Q&P after inter critical (IC) - austenitization, whereas for ART it is found to be ≥ 30 GPa. %. A versatile range of properties was achieved in the single steel by changing the heat treatment conditions to vary the microstructure such as ferrite, bainite, martensite and retained austenite. The retained austenite with the TRIP effect further enhanced the strength-elongation product to third generation AHSS regime. Due to the higher martensite content in the Q&T and Q&P after austenitization above Ac3 temperature, the steel showed very high level of strength (1300–1450 MPa) with relatively lower elongations (14.5–17%). In the TBF, DS and Q&P after inter critical annealing give ultra high strength (994–1067 MPa) with very good ductility (25–29%) due to the presence of bainite and retained austenite. The ART showed relatively lower strength (750–767 MPa) with excellent elongation (39.5–47%) due to the presence of more ferrites compared to the other heat treatment cycles. The third generation AHSS was developed by varying heat treatment cycles from a first generation TRIP assisted AHSS.