Development of novel low density ultra-high strength manganese-steel with significant ductility through thermo-mechanical processing route

The development of ultra-high strength steels for automotive applications is often associated with insufficient ductility or formability properties. To overcome this problem, a high Mn alloy steel (Fe-15Mn-0.17C-2Al-3Si0.6Ti) was prepared by induction melting and was subjected to various thermomechanical treatments (TMT) to achieve high strength with high ductility. The TMT comprised of simply hot forging (at 1373K) and hot rolling at three different temperatures (1073K, 1173K, and 1273K) followed by water quenching. The forged steel displayed a tensile strength of 510 MPa with 34 % elongation, while the hot-rolled steel (at 1173K) demonstrated a remarkable improvement in both strength (approximately 2 GPa) and ductility (around 51 % elongation). The microstructure of the rolled steel showed austenite, alpha-ferrite, and precipitates of TiAl; the austenite was found to possess finer grain size (-7.9 mu m) with a significant amount of twins (volume fraction = 32 %). The high strength was achieved due to finer grain size, higher amount of twins, transformation-induced martensite, and precipitation hardening, while the substantial ductility was attributed to significant TWIP and TRIP effect.