The medium carbon micro-alloyed steel containing niobium is crack-sensitive steel,which is easy to crack in the continuous casting process and seriously deteriorates the surface quality of the bloom.The causes of corner cracks in 46MnVNbS5 steel were analyzed using an optical microscope,scanning electron microscope with energy dispersive spectrometer,three-dimensional etching technology,high temperature confocal laser scanning microscope and Thermo-Calc software.The surface crack distribution of 320 mm×425 mm casting bloom was observed,and no obvious cracks were found in the range of 0-20 mm of the inner arc,and only pitted defects were observed on the samples 20 mm away from the inner arc.There are obvious cracks in the range of 20-40 mm,and the crack size is generally greater than 10 mm.At the distance of 25 mm from the inner arc surface,cracks were observed only at the low multiplier surface.At 30 mm,35 mm,and 40 mm from the inner arc surface,there were cracks on multiple observation surfaces of the sample.The crack grows along the dendrite growth direction.A large number of cou-pling precipitates of manganese sulfide and carbonitride in and near cracks were determined by SEM-EDS.The sur-face of the crack specimen was corroded and it was found that the crack spread along the ferrite film.The thermody-namic results show that manganese sulfide is the main inclusion in the solid-liquid zone of steel.The niobium-rich carbonitride and vanadium-rich carbonitride will be precipitated successively,which is consistent with the types of inclusions obtained by experiments.The cracking zone(620.1-794.3℃)of steel containing niobium was deter-mined by high-temperature confocal laser microscopy,and numerical simulation was applied to simulate the corner temperature field of continuous casting bloom,and the cooling system was optimized to avoid the cracking zone of medium carbon micro-alloyed steel containing niobium.This study provides the methods to solve the crack of cast-ing bloom and has important practical significance.
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