Effects of subzero treatment and B doping on austenite reversion are investigated in quenched and tempered Fe–9Mn–5Al-0.3C and Fe–9Mn–5Al-0.3C-0.005B (wt.%) lightweight steels. In the as-quenched condition, the steel microstructure consist of a triplex structure of austenite, ferrite, and martensite. B doping leads to a reduction in the prior austenite grain size by grain boundary segregation and precipitation of boro-carbides, which increases the stability of austenite against the athermal martensitic transformation. After tempering at 200 °C for 2 h, nano-lath reverted austenite is formed by the C-enrichment instead of the carbide precipitation owing to the high Al content. This reversion effect is promoted further by the subzero treatment at −196 °C for 0.5 h prior to tempering, which enables the remaining austenite in the as-quenched state to transform and, thus, provides additional sites for austenite reversion. In addition, the subzero treatment and B doping result in the synergistic effect of the delay of crack initiation through the transformation of retained austenite in contact with ferrite and the improvement of bonding strength. Thus, the B-doped steel subjected to quenching, subzero treatment, and tempering exhibits a very high yield strength of approximately 1 GPa, the tensile strength of over 1.3 GPa, and an excellent elongation of 29.4%, which outperform the tensile properties of conventional austenitic or (austenite + ferrite) duplex lightweight steels.
Bibliographical noteFunding Information:
This work was supported by the Korea University Grant for Dr. Seok Su Sohn, by the Fundamental Research Project of the National Research Foundation of Korea ( NRF-2019R1F1A1057687 ), by Korea Institute for Advancement of Technology ( KIAT ) grant funded by the Korea Government ( MOTIE ) ( P0002019 , The Competency Development Program for Industry Specialist), and by the BK21 Plus Center for Creative Industrial Materials.
© 2020 Elsevier B.V.
- (austenite+ferrite+martensite) triplex microstructure
- Boron doping
- Medium-Mn lightweight Steel
- Subzero treatment
- Transformation-induced plasticity (TRIP)
ASJC Scopus subject areas
- General Materials Science
- Condensed Matter Physics
- Mechanics of Materials
- Mechanical Engineering