Abstract: Based on the conditions of defects of HSLA steel during solidification，the process parameters of continuous casting were analyzed. The surface cracks of  bloom
 were investigated by optical microscopy, SEM and EDS, the precipitates were analyzed by TEM. Furthermore, the hot ductility was studied by Gleeble-1500 thermal simulator. The results showed that ferrite film, formed in straightening area, is in the range from 17-25 μm, would lead to stress concentration at grain boundaries. Cracks would be induced by the higher content of copper. In addition, the surface temperature of  bloom  was improved 40 ℃ with the casting speed increasing from 0.58 m/minto 0.63 m/min. In order to decrease the cracks during continuous casting, the mass fraction of nitrogen in steel  should be controlled  in 45×10^-6, the  basicity of mold powder and viscosity should be controlled  at 1.11, 0.85Pa·s, and the  molten slag layer should  greater than 8 mm.

Key words: microalloyed steel, bloom, surface defects, brittle zone, precipitates