Effect of matrix structures on TRIP effect and mechanical properties of low-C low-Si Al-added hot-rolled TRIP steels

We applied different hot-rolling direct quenching and partitioning (HDQ&P) processes to a low-C low-Si Al-added steel and obtained eight TRIP-assisted steels with different matrix structures, viz, martensite, ferrite/bainite, ferrite/martensite and ferrite/bainite/martensite. The microstructures were characterized using SEM, TEM and XRD. The mechanical properties were investigated by means of uniaxial tensile tests. Quasi in-situ tensile tests in combination with EBSD and microscopic digital image correlation (μ-DIC) analyses were performed on microstructure regions containing ferrite, martensite and retained austenite to investigate the TRIP effect and composite effect. Considering both the chemical and strain partitioning among the structure components, we analyzed the specific influence of the matrix structures on the austenite stabilization, martensitic transformation of retained austenite and ductility of the material. The results show that the TRIP steel with a martensitic matrix exhibits high ultimate tensile strength (UTS) and yield ratio reaching up to 1200 MPa and 0.87, with the product of strength and elongation (PSE) of about 18000 MPa%. The introduction of 8%–25% (in area fraction) of ferrite leads to a decrease of the yield strength in 100–200 MPa, but without significant reduction of the UTS. The increase of the ferrite fraction to 30%–35% results in an obvious decrease of the UTS and yield ratio to about 950 MPa and 0.6. Retained austenite, with the amount of 14 vol %, was stabilized in the TRIP steel with a martensitic matrix. The introduction of ferrite (8%–35% in area fraction) and granular bainite can promote the carbon partitioning, thus enhancing the stabilization of retained austenite. The TRIP steel with a martensitic matrix exhibits a slight martensitic transformation of retained austenite because of the low deformability of the martensite. For TRIP steel with a matrix composed of ferrite and martensite, the low deformation compatibility of the soft and hard structure components also leads to a week martensitic transformation of retained austenite. The introduction of granular bainite can effectively improve the deformation uniformity and enhance the martensitic transformation during deformation. The TRIP effect and the composite effect of matrix structures jointly control the ductility of the TRIP steels. To optimize the ductility, we not only need to enhance the TRIP effect but also to improve the deformation compatibility of the