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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