EFFECT OF ULTRA-FAST CONTINIOUS ANNEALING ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF LOW Si GRADE Nb-Ti MICROALLOYING TRIP STEEL
LUO Zongan, LIU Jiyuan, FENG Yingying, PENG Wen
State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819
Cite this article:
LUO Zongan, LIU Jiyuan, FENG Yingying, PENG Wen. EFFECT OF ULTRA-FAST CONTINIOUS ANNEALING ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF LOW Si GRADE Nb-Ti MICROALLOYING TRIP STEEL. Acta Metall Sin, 2014, 50(5): 515-523.
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·%.
Fund: Supported by National High Technology Research and Development Program of China (No.2013AA031302) and Fundamental Research Funds for the Central Universities (No.090307004)
Table 1 Mechanical properties and volume fraction of retained austenite of the steel after different continuous annealing processes
Fig.2
不同连续退火工艺下钢的SEM像
Fig.3
不同连续退火试样的拉伸工程应力-应变曲线
Fig.4
不同连续退火工艺下钢的晶界分布图
Fig.5
不同工艺条件下的EBSD定量分析
Fig.6
不同工艺条件下残余奥氏体分布EBSD分析
Fig.7
工艺III试样中残余奥氏体的TEM明场像、暗场像和衍射斑标定
Fig.8
工艺III试样中贝氏体板条间残余奥氏体的TEM明场像及其与贝氏体铁素体取向关系分析
Fig.9
工艺III试样中各相组织和位错的TEM形貌及残余奥氏体的选区衍射
Fig.10
工艺III试样中析出相的TEM像及其EDS分析
Fig.11 Microstructures of the steel after hot rolling (a) and cold rolling (b)
Fig.12 Thermal expansion curve of the steel with different heating rates(Ac10.5 and Ac30.5 represent the critical temperatures under 0.5 ℃/s heating rate, Ac1100 and Ac3100 represent the critical temperatures under 100 ℃/s heating rate)
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