Abstract
Steel bars and billets may have center porosities caused by contraction during castng. It is important to optimize not only casting conditions but conditions of subsequent forging and rolling in order to decrease the center porosity.
Forging and rolling, the two representative processes to produce steel bars, show different deformation behaviors, productivity and cost. But no study explained the porosity closure in the both processes consistently with an identical parameter. In this study, the authors discussed the integration of the hydrostatic stress, Gm, which was usually used in free forging, as a parameter to describe the closure of the porosities. The parameter can be calculated from plastic strain and stress by CAE analysis.
It was found that the cross-sectional area of the porosity was in proportion with the hydrostatic integration Gm in single-pass rolling of plasticine, when a plasticine billet was repeatedly rolled under an H–V sequence, however, the summation of Gm did not show a clear threshold for the porosity closure. The hydrostatic integration was modified as Gm+ by subtracting a coefficient C, to reproduce the result of the plasticine test. In this case, the value C was 0.024, and the threshold of the summation of Gm+ was 0.25. The new parameter Gm+ has been successfully used to develop new pass schedules with larger Gm+ for a new rolling mill.
