Abstract: Experiments of intercritical annealing at different temperatures (700 ℃, 750 ℃ and 800 ℃) and cooling methods (air cooling and water quenching) were carried out for a Nb-containing medium Mn steel. The results show that the product of strength and elongation and the content of retained austenite increase first and then decrease with the increase of intercritical annealing temperature. When intercritical annealed at 750 ℃, the mechanical properties of the tested steel are the best, with the yield strength of 750 MPa, the tensile strength of 1820 MPa, and the elongation after fracture of 13.9%. As the intercritical annealing temperature increases, the dissolution of cementite promotes the increase of C and Mn contents in the matrix, and the content of C and Mn partitioning into austenite increases during heat preservation process, making the austenite more stable and the retained austenite content increasing. When the intercritical annealing temperature continues to increase, the increase of austenite content during heat preservation leads to the decrease of C and Mn partitioning into austenite, which leads to the decrease of austenite stability and the formation of a large amount of martensite during cooling. The increase of martensite content and the precipitation of large-sized clusters of (Nb, Mo)C lead to high strength and low ductility of the tested steel annealed at 800 ℃. At the same intercritical annealing temperature, the phase composition of the water cooled and air cooled tested steels is the same. At the intercritical annealing temperature of 800 ℃, the differences of the two cooling methods on the retained austenite content and mechanical properties are the most obvious, which is related to the more sufficient partitioning of C and Mn into austenite during air cooling.

Key words: Nb-containing medium Mn steel, intercritical annealing, cooling method, austenite stability, mechanical properties