The production flowsheet of test steel T91 (/% :0.10C

0.30Si

0.45Mn

0.012P

0.005S

8.90Cr

0.95Mo

0.08Nb

0.22V) is 60 t UHP EAF-AOD-LF-VD-240 mm x 240 mm bloom casting- heating and continuous rolling. The examination results of macrostructure of unqualified steel T91 Φ90 mm products by ultrasonic flaw detection show that there are center cracks and hole in bar

by metallographic analysis it is obtained that the defects have three characteristics： (1) overheating hole and cracks； (2) axial intercrystalline cracks its source at casting bloom and (3) shrinkage cavity. With the technology measures including optimizing the casting I、U and HI zone secondary cooling water amount respectively from original 28、31 and 17 L/min to 15、18 and 10 L/min

and controlling the casting bloom heating temperature

the equiaxial crystal ratio in casting bloom center increases from original 14% to more than 25%

the rating of central cracks decreases from original 2.0 to less than 0. 5

and the unqualified ratio of steel T91 products by ultrasonic flaw detection decreases from original 5% to less than 0.2%.