Innovative ultra-short route to produce high-strength microalloyed steel with excellent ductility via high-density coherent nanocarbides

The complex production process and poor ductility limit the widespread application of high-strength microalloyed martensitic steel. The formation of large-size precipitates is one of the key factors contributing to poor ductility, as these precipitates strongly interact with dislocation motion. This study proposes an ultra-short process integrating strip casting, single-pass hot rolling, and short-time reheating to produce high-strength microalloyed martensitic steel strips with enhanced ductility. Sub-rapid solidification during strip casting enables complete solid solubility of alloying elements, forming a supersaturated matrix. Subsequent hot-rolling and reheating austenization triggered the rapid precipitation of dual-scale nanocarbides, including nano-precipitates and nano-clusters. Notably, these clusters are believed to share the same crystal structure as the austenitic matrix, whereas nano-carbides exhibit semi-coherent interfaces with the austenitic matrix. The coherent clusters minimize interfacial strain localization, preserving ductility, while semi-coherent nano-carbides contribute to precipitation strengthening. During reheating, the combined effects of high-temperature processing and coherent precipitation promote rapid nucleation of these nanocarbides. Concurrently, the pinning effect of these nano-carbides effectively restricts austenite grain growth, ultimately resulting in ultra-fine martensite microstructure upon quenching, further augmented by dispersion strengthening from nano-carbides. The synergistic interplay of grain refinement and nanoscale precipitation results in exceptional mechanical performance: a tensile strength of 1610 MPa, total elongation of 13.7%. This work demonstrates that coupling sub-rapid solidification with controlled nanoscale precipitation kinetics can overcome the traditional trade-off between strength and ductility in martensitic steels. The proposed ultra-short process offers a scalable and energy-efficient strategy for manufacturing advanced high-performance steels.