Localized heterogeneous mixed microstructure mediated high strength and large ductility in a strip casting ultra-low carbon steel

In this work, we report that a heterogeneous mixed microstructure was developed in an ultra-low carbon steel by adjusting the single-pass hot rolling reduction of strip casting process. Microstructural analysis shows that the sample with 17% rolling reduction (HR17) has coarse prior austenite grains and evolves into a mixed microstructure, consisting of coarse ferrite (CF, 27.4 vol.%) at prior-austenite grain boundaries, fine ferrite (FF, 43.6 vol.%) and bainitic ferrite (BF, 29.0 vol.%) in grain interiors, creating localized gradient microstructure from prior austenite grain boundaries to grain interior. For the sample with 44% rolling reduction (HR44), equiaxed ferrite (EF) with a grain size of 17.4 μm was observed. Tensile tests show that although these two samples possess comparable yield strength (279 vs. 275 MPa), owing to the hetero-deformation induced (HDI) strengthening effect, the HR17 sample exhibits higher ultimate strength (380 vs. 362 MPa), greater uniform elongation (19.2% vs. 13.0%) and larger total elongation (30.2% vs. 21.1%). Deformation microstructural analysis reveals sequential participation of plastic deformation in different microstructures: initial plastic deformation localizes in soft CF that generates geometrically necessary dislocations (GNDs) pile-ups at the CF/BF interfaces, followed by coordinated plastic deformation of FF and BF, producing long-range strain gradient and HDI strengthening effect, contributing to the consistently high work hardening rate, and thus the superior tensile strength and uniform elongation.