EFFECT OF ULTRA-FAST CONTINIOUS ANNEALING ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF LOW Si GRADE Nb-Ti MICROALLOYING TRIP STEEL

Abstract

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· %.