Microstructure and deformation mechanism of Si-strengthened intercritically annealed quenching and partitioning steels

Abstract

To address the growing demand and stringent requirements for lightweight steel, quenching and partitioning (QP) steel has attracted significant attention due to its excellent strength–ductility balance. However, to date, reports on the mechanism of intercritical annealing QP have been limited. Thus, this study investigated the effect of the addition of 1.3 wt% to 2.5 wt% Si on the microstructure and mechanical properties of intercritically annealed QP steel. Neutron diffraction and quasi-in situ electron backscatter diffraction were used to analyze the deformation mechanisms of commercial-grade QP1180 steel and Si-strengthened QP steel. The microstructure of the QP steel consisted of ferrite, martensite, and retained austenite (RA). Si increased the volume fractions of ferrite and RA. The Si-strengthened QP steel with a multiphase structure, including 43% ferrite, 13% RA, and 43% martensite, exhibited better tensile strength (1330 MPa), higher elongation (21.5%), and lower yield ratio (0.615) than commercial-grade QP1180 steel. The mechanical stability of larger RA grains is lower than that of finer grains. RA experienced additional stress with the ferrite yield and Si promoted interphase deformation accommodation behavior. The interphase deformation accommodation mechanism rather than the orientation-dependent mechanism plays a key role in controlling the onset of the deformation-induced martensite (DIM) transformation. Thus, the DIM transformation was triggered before the yield of RA, and the residual RA after the DIM transformation exhibited a non-negligible stress distribution. • QP2.5Si has higher strength, better plasticity, and a lower yield ratio. • The additional lattice strain in the FCC phase promotes DIM transformation. • Residual RA after DIM transformation has a non-negligible stress distribution.