The production flowsheet of tested 280 mm x 325 mm casting bloom of steel ML20Cr is 80t top and bottom combined blowing converter-LF-CC process. The hot ductility curves of casting bloom of tested steel at 650~1200 °C are measured by Gleeble 1500 hot simulator

and the effect of adding 0.029Ti-0.0023 B on hot ductility of steel MLCr20 (/% : 0.2i〜0.22C

0.22~0.23Si

0.74〜0.76Mn

0.015~0.017P

0.002〜0.003S

0.95〜0.96Cr

0.034~0.036 Al

0.0050~0.0051N) is analyzed. Results show that in 650~ 900℃ the plasticity of steel ML20Cr containing 0.029Ti-0.0023B ( reduction of area ＞70% at 750℃) is better that of steel ML20Cr un-adding Ti-B ( reduction of area ＜50% at 750°C ). The main reason is that the prior precipitated TiN decreases the effect of fine AIN in No Ⅲ brittle zone

and the B segregate at austenite grain boundary to inhibit the formation of proeutectoid ferrite thin film

in order to effectively improve the hot ductility of steel. The commercial production results show that with adding micro Ti-B tn steel ML20Cr the cracking j-ate of casting bloom decreases from 3.7% to 0.3% .