Abstract: Compression deformation was introduced into the conventional quenching and partitioning process for low carbon Si-Mn steel.Effects of the hot deformation on microstructure evolution and microstructure characteristics of specimens treated by various isothermal partitioning processes were investigated. The results showed that the microstructure is refined and the fraction of high-angle grain boundary increases from 65.7% to 72.5% due to the hot deformation. Carbon-rich areas formed during transformation and partitioning process are mostly observed at the boundaries of prior austenite grains and martensite laths. With the increase of the partitioning time，the microstructure shows a tendency to tempering transformation. When the partitioning time is up to 1500s， a great number of carbides are formed and the volume fraction of retained austenite decreases to 7.9%.

Key words: low carbon Si-Mn steel, compression deformation, direct quenching, partitioning;microstructure