Abstract: Numerical simulation methods were adopted in order to improve ultrasonic quality rate of rolled product of medium-carbon alloy steel by analyzing flow field during the filling and solidification process of solid phase fractions of 8.4 t steel ingot mould riser, the internal defects of shrinkage and porosity of ingot was also forecasted . The calculation results indicated that the filling process was stabilized, but shrinkage and porosity were generated in the ingot solidification process at the middle and inner ingot, which further extended upward and to mould riser for lacking enough heat insulation, the rate of porosity was 1.6% to 2.0%. The internal ultrasonic quality rate of ingot improved by taking following measures coupled with other technology：altering material quality of thermal insulation layer of the 8.4 t of steel ingot mould riser，specially speaking, the density of thermal insulation layer being decreased from1.07 g/cm3 to 0.67 g/cm3, the thickness being increased from 45mm to 60mm. Others technology were also adopted such as adding heat generation agent(1.0kg/t) and thermal insulation agent, strengthening heat insulation effect from axial and radial side of riser，guaranteeing enough liquid steel during solidification process for ingot. Finally, the ultrasonic quality rate for 250 mm×250 mm and 280 mm×280 mm rolled product increased by 1.19% and 6.73% respectively.

Key words: molded, middle carbon alloyed steel, numerical simulation, quality of ingot