Abstract: Aiming at the frequent break out accidents in the second cooling zone of continuous casting bloom of Q235KZ steel，the SEM，composition analysis，viscosity measurement，and simulation calculation were used for investigation. The results showed that longitudinal crack was found on the surface of bloom and the largesized slag existed near the shell of breakout. Besides，it was found that the thickness of solidified shell was unevenly distributed. In the exit of the copper mold，the thickness of the thinnest shell was 7.9 mm，while that of the thickest was 20.2 mm. The frequency of break out was significantly reduced by increasing the alkalinity of protective slag，reducing the heat transfer rate of slag film，optimizing water distribution in the secondary cooling zone.

Key words: bloom, continuous casting, break out accident