Improvement of mechanical properties for low carbon ultra-high strength steel strengthened by Cu-rich multistructured precipitation via modification to bainite

Abstract

This work focused on ultra-high strength steel strengthened by Cu-rich multistructured precipitates and toughened by bainite. A Cr–Mo bearing bainitic steel was designed and contrasted with a ferritic steel to examine the effects of Cr and Mo on microstructural transformation and precipitation evolution as well as the corresponding mechanical properties. By adding Cr and Mo elements, bainitic steel was obtained by an air cooling process after hot rolling, thus omitting controlled cooling and off-line quenching. Cr and Mo decreased the γ/α transformation temperature, such that the ferritic matrix was modified to fine bainite. Cu-rich multistructured particles and (Nb,Ti)C particles were found to be the two main precipitation phases in this bainitic steel. Cu-rich multistructured particles contained B2-ordered structure and transition-state structure between B2-ordered and 9R structure. Notably, this Cr–Mo bearing bainitic steel had better strength and toughness compared with ferritic steel. Its yield strength reached 1155 MPa, owing to precipitation and grain boundary strengthening, which were estimated to be 596 and 311 MPa, respectively. The impact absorbed energy of this steel at −40 °C was ~55.3 J and its fracture mode a brittle-ductile mixed mode, compared with the cleavage fracture mode of ferritic steel. The small grain size of this steel compensated for toughness deterioration caused by precipitation and contributed to high impact toughness.