Enhancement of strength-ductility synergy in 1300 MPa strip casting steels via silicon regulated bainite transformation

A new 1300 MPa ultrahigh-strength strip casting steel with high ductility was developed in this work. The effects of silicon (Si) content on the microstructure, mechanical properties, and deformation mechanisms of the steels were investigated and discussed in detail. Our results suggest that Si can effectively retard bainitic transformation kinetics, thereby inhibiting the coarsening of the bainite laths and contributing to the increase in retained austenite (RA) volume fraction. Meanwhile, the precipitation of cementite is markedly suppressed with increasing Si content, which renders more carbon partitioned into the untransformed austenite during the bainitic transformation, thereby further increasing the volume fraction and stability of RA. Mechanical tests indicate that increasing the Si content from 0.5 wt.% to 1.5 wt.% significantly improves yield strength and tensile strength from 919 MPa and 1104 MPa to 1055 MPa and 1331 MPa, while the ductility rises from 12.2% to 17.8%. This can be attributed to the refined bainitic microstructure, which markedly enhances the strength and provides a greater capacity for dislocation multiplication and storage, thereby contributing to the enhanced work-hardening capability and ductility. Meanwhile, the higher volume fraction and enhanced stability of RA in the high Si steels also favors sustained high work-hardening capacity via sequential activations of stacking faults, nanotwinning, and ε-martensite transformation with increasing strain. These findings provide valuable insights into the design of high-performance strip casting steels.