Simultaneous enhancement of strength and ductility via nanoscale Cu precipitates and ultrafine filmy retained austenite in a novel quench-partitioned and tempered steel

Abstract

A novel Cu-bearing Quench-Partitioned and Tempered (Q&P-T) steel was designed to achieve an excellent strength–ductility balance. Tempering after Q&P treatment increased the tensile and yield strength from 1263 and 706 MPa to 1393 and 1003 MPa, and simultaneously improved the uniform and total elongation from 13.2 and 21.1% to 17.0 and 24.0%, respectively. Dilatometric, XRD and APT analyses as well as SEM and TEM observations revealed that the multiphase structure consisting of fresh martensite, tempered martensite and ultrafine filmy retained austenite was formed via Q&P-T process. Copious Cu precipitates with a diameter of 0.66 nm–5.89 nm, which formed during tempering at 500 °C for 30 min can account for the remarkable increase in strength. Cu particles invisible under TEM may not harm the ductility due to their coherency with the matrix. Whilst the retained austenite fraction decreased slightly by tempering, it is likely that carbon partitioning further proceeded between the austenite and the martensite matrix, and increased the stability of ultrafine filmy retained austenite, which led to the notable enhancement of ductility.