| With the social development, the public pay more and more attention to the energy crisis and environmental pollution. As the most commonly used means of transport, new automobiles body are going to be designed to have a lighter weight but higher strength, and the most promising material for the new cars is the third generation AHSS used for auto industry which combines the high strength and toughness well. It can be used as structural parts in the car to protect the drivers and reduce the whole weight to save energy. In this paper, it studies the microstructure characteristics of low carbon C-Mn-Al steel processed with Q&P heat treatment to provide theoretical support for large-scale production of the next generation AHSS.This study focused on commercial C-Mn-Al hot-rolled steel plate as experimental materials. Q&P heat treatment process was performed by full-automatic transformation test instrument. Experimental methods such as optical microscopy(OM), scanning electron microscope(SEM), X-ray diffraction(XRD) transmission electron microscope (TEM),scanning transmission electron microscopy(STEM) and electron Back-scattering Patterns (EBSD)are used to study effect of partitioning temperature and time on the volume and microstructure characteristics of retained austenite and its location; martensite lath width and microstructure characteristics; in the steels; ε-carbide precipitation; behavior of alloy elements after Q&P heat treatment.The main results are as follows:(1) In the austenitizing process of this sample material, the austenitizing temperature is1200℃, the holding time is30seconds.(2) The best partitioning temperature for the C-Mn-Al steel is400℃and the best partitioning time is longer than20seconds,the volume fraction of retained austenite is18.7%in the sample processed with PT=400℃,T=20s.(3)If the partitioning time is between1s and10s,the microstructure of retained austenite are film-form and spot which locate between martensite laths and in martensite laths respectively; PT=400℃,after partitioning10s,the main microstructure of retained austenite is strip locating in between martensite laths and in martensite grain boundaries; some lump retained austenite is found,and its width is as wide as martensite laths.(4) The width of martensite laths don’t change with the increasing of partitioning time from1s to10s at different PT,average width is about0.3μm;the martensite laths which are processed with PT=400℃, T=15s/20s decrease to0.20μm and0.11μm because of reverse austenite;In the sample processed PT=400℃,T=20s.(5) According to the image of STEM:the C atoms do enrich in the retained austenite after Q&P heat treatment; the behavior of Al atom is familiar with C atoms enriching the retained austenite; Mn&Fe atoms distribute in the sample evenly.(6) Precipitation:PT=400℃,T=10s, a little of ε-carbide is found and its length is form0.1μm to0.2μm,these acicular ε-carbide distribute in the same direction; in the samples processed with PT=400℃,T=15s/20s,large scale of acicular and rod ε-carbide is growing,its length is form0.1μm to0.3μm and the distributing angle of them is about90°,it suggests that the volume of carbide is increasing as same as retained austenite. |
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