临界退火冷却方式对含铌中锰钢奥氏体稳定性和力学性能的影响

针对含铌中锰钢进行了不同退火温度(700、750和800 ℃)和不同冷却方式(空冷、水冷)下的临界退火试验。结果表明,随着临界退火温度的升高,强塑积和残留奥氏体含量呈现先升高再降低的趋势。在750 ℃临界退火水冷后,试验钢的力学性能最佳,屈服强度达到750 MPa,抗拉强度为1820 MPa,断后伸长率为13.9%。随着临界退火温度升高,试验钢中渗碳体逐渐溶解,基体中C和Mn含量增多,在保温过程中配分进入奥氏体的C和Mn含量增多,导致奥氏体更稳定,残留奥氏体含量增多。当临界退火温度进一步升高,保温时奥氏体含量的增多导致配分进入奥氏体的C和Mn浓度降低,导致奥氏体稳定性降低,在冷却过程中形成大量马氏体。马氏体的增多和大尺寸团簇状(Nb,Mo)C的析出导致800 ℃临界退火后试验钢的高强度和低塑性。在相同临界退火温度下,水冷和空冷后试验钢的相组成相同。在800 ℃临界退火时,两种冷却方式对残留奥氏体含量和力学性能引起的差异最为明显,这与空冷过程中C和Mn向奥氏体配分更充分有关。

Effect of cooling method of intercritical annealing process on austenite stability and mechanical properties of Nb-containing medium Mn steel

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.