采用EBSD和TEM对不同超快速连续退火条件下的低Si系Nb-Ti微合金化TRIP钢进行了显微组织观察, 并探讨了拉伸性能. 结果表明, 100 ℃/s的加热速率和短时保温制度, 使铁素体晶粒细化, 并保留了热轧过程中的弥散细小的微合金元素碳氮化物析出, 因此提高了钢的强度和塑性. 缓冷制度对消除钢的屈服平台有显著作用, 而强度有所下降; 钢的强度随着退火温度的升高而升高. 退火温度在830 ℃时, 残余奥氏体形貌多呈膜状结构与贝氏体铁素体板条相伴出现, 使钢的强度和塑性达到了最佳的配合: 抗拉强度748 MPa, 屈服强度408 MPa, 均匀延伸率21.3%, 加工硬化指数0.27, 强塑积15932.4 MPa·%.

Si-containing transformation induced plasticity (TRIP) steel is noted for good balance of excellent formability and high strength as the advanced high strength steel (AHSS). The advantage of this steel can be attributed to the TRIP effect, which is the transformation of the retained austenite. Furthermore, the local increase in specific volume caused by the TRIP effect can help to close propagating cracks. It is favorable for the automotive structural components based on the high work hardening rate and energy absorption behavior. Low Si-containing can optimize the galvanized performance of the cold rolling TRIP steel, and the ferrite stabilization can be compensated by adding Al. Microalloying with Nb and Ti may provide effective means for further strengthening via grain refinement and precipitation strengthening. The ultra-fast continuous annealing comprised of rapid heating and short austempering is a new-style process for grain refinement and precipitation solidifying. However, the influences of the process on the cold rolling low Si TRIP steel, especially the austenite transformation characteristics and their effects on microstructure and mechanical properties, were rarely reported. Therefore, in this work the microstructures of low Si grade Nb-Ti microalloying TRIP steel under different ultra-fast continuous annealing conditions were observed via EBSD and TEM, and the tensile properties were discussed. The results show that the polygonal ferrite is refined by heating rate of 100 ℃/s and short asutempering procedure. The dispersive and fine microalloyed carbonitrides formed during the hot-rolling stage are reserved. Therefore, the strength and ductility are enhanced simultaneously. The slow cooling procedure can effectively contribute to eliminate the yield point, while the strength is slightly decreased. As the annealing temperature increasing, the strength is enhanced. When the annealing temperature is 830 ℃, the morphology of retained austenite consists of alternated film and bainite-ferrite plates, resulting in optimal combination of strength and ductility: tensile strength 748 MPa, yield strength 408 MPa, uniform elongation 21.3%, work hardening exponent 0.27, balance of strength and ductility is 15932.4 MPa·%.