An ultrahigh strength steel with ultrafine-grained microstructure produced through intercritical deformation and partitioning process

•The proposed controlled heat treatment was shown to lead to a new kind of microstructure containing deformation-induced ferrite zones.•The microstructure helped stabilize retained austenite, and led to improvement of steel’s mechanical properties.•A novel design scheme of controlled heat treatment was explored and the controlled heat treatment improves the properties of the steel.

In this work, a novel design scheme in which deformation-induced ferrite transformation (DIFT) was applied to produce fine-grained steel and the quenching is controlled by quenching and partitioning (Q&P) process has led to the development of a new kind of steel. This steel possesses excellent mechanical properties and the ductility can be further improved without compromising strength because the refined microstructure contains martensite, retained austenite and deformation-induced ferrite. The highest elongation of 15% allied with strength of 1700 MPa is obtained through hot deformation followed by Q&P treatment at 300 °C. The microstructure evolutions are discussed in terms of the current knowledge of the Q&P process and the experimental observations. The results show that the designed multiphase steels are a promising candidate for the development of the third generation of advanced high strength steels.

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