By comparison and analysis on the chemical composition

production process and inspection qualification rate of 中 130-150 mm hot rolled bar of H13 steel and H11 steel

it is pointed out that average vanadium content in steel increasing from 0.5% to 1.0% is the key factor leading to inspection disqualification of steel bar. According to the results of defect analysis and the macrostructure of ①380 min CC bloom

it is concluded that the reason of low qualification rate of H13 steel flaw detection is that the center shrinkage hole of CC billets is not welded in rolling process. By improving the continuous casting process and rolling cogging-down process

including cast speed increasing from 0.58 m/min to 0.65 m/min

improving S-EMS from 650 A

5 Hz to 200 A

8 Hz

in cogging-down process increasing reduction at 6

8 and 9 pass from original -60 mm to 70 ~85 mm

change bloom size from Φ380 mm to Φ500 mm

the qualification rate of Φ130 mm H13 steel bar increases from about 50% to 99. 8%

and that of 0150 mm H13 steel bar from original 15.5% to 97.2%.