The production flow sheet of 4>60 mm bar products of non-quenching and non-tempering steel C70S6 BY is 60 t UHP EAF-LF-VD-240 mm x240 mm bloom casting-c-ontinuous rolling. The parts of defects of continuous-rolled products are sampled by ultrasonic detection

defined defect position and sampling

and analyzed by macrostructure and microstructure examination. It is obtained that the defects are center cracks ； its forming cause is casting bloom non-uniform heating and producing larger cooling stress after rolling. With using the measures including improving casting bloom heating process

in¬creasing heating temperature at second zone from 1 239 X to 1 248 °C

total heating time m2.0 h

temperature of bloom at soaking zone being 1 232 龙 for 20 ~40 min

modified the cooling process after rolling from stacking cooling for 24 h to pit cooling for 38 h

the off-qualified rate of products by ultrasonic detection decreases from original 36. 80% to 0.78% .