Three-dimensional morphology and formation mechanism of grain boundary ferrite in low carbon microalloyed steel
CAO Sheng-li1,2,3, WU Shao-wen4, ZHANG Cai-jun1,2, ZHANG Qing-jun2,3
1. College of Metallurgy and Energy, North China University of Science and Technology, Tangshan 063210, Hebei, China; 2. Hebei Province High Quality Steel Continuous Casting Engineering Technology Research Center, Tangshan 063210, Hebei, China; 3. Comprehensive Testing and Analyzing Center, North China University of Science and Technology,Tangshan 063210, Hebei, China; 4. State Key Laboratory of Advanced Metallurgy,University of Science and Technology Beijing, Beijing 100083, China
Abstract：In order to better understand the three-dimensional morphology and formation mechanism of grain boundary ferrite in low carbon microalloyed steel, the 3D morphology of the different location was observed by focused ion beam scanning electron microscope equipped with electron back-scatter diffraction(EBSD), and the growth rate under test conditions was calculated by in-situ observation technology, finally the solute diffusion model was calculated by Matlab,from two aspects of test results and the theoretical data for the demonstration and analysis. The results show that the grain boundary surface ferrite does not grow along the austenite parent phase grain, but grows in a certain angle toward the grain. At the same time, it only grows near the austenite grain boundary and cannot grow too deep. Its three-dimensional morphology is flat on the front and thin strip on the side, accompanied with a large number of pits and defects. The grain boundary ribbed ferrite and austenite at the grain boundary have fixed interface on one side, and the other side is angular and grows into the grain. The three-dimensional morphology of ridged ferrite is like a triangular pyramid with a pointed tip. The adhesion surface is smooth, tidy, and the side has a deep gully. The findings were calculated using in situ observation techniques, the grain boundary ferrite precipitated at 725-775 ℃ has a faster growth rate, Locating between 100-250 μm/s, and the grain boundary ferrite precipitated at other temperatures has a lower growth rate. The distribution of grain boundary ferrite growth rate with temperature in local equilibrium/ para-equilibrium mode is obtained, the results find that the growth rate of grain boundary ferrite is mainly distributed in the para-equilibrium mode, and the growth rate of intragranular ferrite is only controlled by carbon diffusion.
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2022, Vol. 57 
