A new strengthening mechanism driven by disruptive shear and solute segregation during warm rolling in 1.4 GPa class 12.5 wt% Al added-FeMnC ultra-lightweight steel

Abstract

Novel strengthening of Fe‒29.1Mn‒12.5Al‒1.35C‒4.95Cr steel achieved by warm rolling was investigated. The solution-treated steel consisted of a γ-matrix containing nano-sized κ-carbide ((Fe,Mn)3AlC) and elongated prior ferrite, which was transformed into FeAl-type B2 and Fe3Al-type D03 phases. The solution-treated steel exhibited poor strain hardening owing to glide softening associated with κ-carbide shearing by dislocations. However, after warm rolling with a reduction ratio of 30% at 300 °C, the yield and tensile strengths significantly increased from 917 to 1300 MPa and 1025 to 1419 MPa, respectively. The tensile test conducted at 300 °C to simulate warm rolling exhibited serrated flows, indicating dynamic strain aging (DSA). Atom probe tomography exhibited that the C atoms in κ-carbide were swept away along the slip direction by disruptive shear during rolling at 300 °C. The swept C atoms along the slip direction interacted strongly with dislocations at 300 °C, with repeated pinning and breakaway of dislocations from the C atoms. This contributed to significant strengthening owing to the formation of a solute-rich atmosphere after warm rolling. The results of the tensile tests at 300 °C indicated that the degree of strengthening was proportional to the pre-strain level. Tensile strength of 1.4 GPa can be achieved with good ductility (17% elongation) by warm rolling. This novel warm-rolling strengthening method expedites the potential application of Fe‒29.1Mn‒12.5Al‒1.35C‒4.95Cr as a 1.4 GPa class ultra-lightweight steel.