Simultaneous enhancement of impact strength and toughness of a low carbon steel with co-precipitation of nanoparticles

This study investigates the influence of molybdenum on the strength and ductility/toughness of low-carbon ultra-high-strength steels strengthened by multiple nanoparticles. The tempered steel consists of tempered martensite, granular bainite, and nanoparticles. Increasing the tempering temperature from 500 °C to 600 °C alters the precipitation behavior, that is, the precipitation gradually evolves from dispersed Cu or NiAl nanoparticles to core-shell structured (Cu-rich core with NiAl shell) multiphase precipitates. This evolution reduces the yield strength from 1360 ± 15 MPa to 1075 ± 15 MPa and the ultimate tensile strength from 1435 ± 30 MPa to 1110 ± 30 MPa. However, tempering at 600 °C enhances ductility from 14 % to 20 % and low-temperature impact toughness from 5 J to 77 J (at −40 °C) by increasing the fraction of high angle grain boundaries and reducing the nanohardness disparities between the martensite and bainite packets. Mo effectively inhibits the multiphase nanoparticles coarsening caused by the growth and coalescence of Cu-rich particles and NiAl precipitates during high-temperature tempering by reducing the diffusion rates of Cu, Ni, and Al. Such effect substantially enhances the contribution of precipitates to mechanical properties. These findings provide a viable strategy for developing advanced steels with ultrahigh strength while retaining good ductility and toughness.