The steelmaking flowsheet of tested casting bloom of steel Q420C (/%: 0.18C

0.34Si

1.40Mn

0.013P

0.011S

0. 066V

0.018Als

0.0110N) is 80t BOF-LF-CC process. The hot ductility of casting bloom of steel Q420C at 600~1400°C is tested by using Gleeble-1500D thermal simulator and the morphology of fracture and structure of steel is analyzed by optical

scanning- and transmission-electron microscope to study effect of the solidification segregation and precipitates on hot ductility of casting bloom. Results show that of steel Q420C the I brittle zone is >1250~1350°C and the Ⅲ brittle zone is 700~1050 °C

and at 1050~1250℃ the steel has better ductility with reduction of area more than 60%

Commercial pilot production results show that with controlling Als content in steel 0.015%~0.020%

properly decreasing ratio of secondary cooling water to 7~8t/h at feet roller zone

5.5~6.5t/h at Ⅰ zone and 14±1t/h at support roller zone

and controlling straightening temperature of casting bloom ≥1050℃

the cracking ratio of rolled products decreasing from original 45.3% to 4.6%.