With using 120 t top and bottom combined blown converter-LF refining-VD-Φ150 ~ Φ200 mm round billet casting flow sheet

the development and production for casting round billet of J55 (37Mn5)

L80 ( TC80

0.24% ~ 0. 28%C

1.40% ~ 1. 55Mn)

N80 (36Mn2V) and Pl 10 (26CrMn4) grade oil case steel have been completed at Tiansteel by the measures including controlling residual deleterious elements content in metal； intensifying converter dephosphorizing in early phase

controlling end [C

]

≥0. 10% and after tapping slag layer thickness in ladle W50 mm； controlling ingredient of refining slag and end slag ( FeO + MnO) ≤ 1.0% ； increasing bottom argon flow rate to 200 ~ 300 L/min in VD process ； and shielding casting in whole casting process. Production results show that for J 55 grade steel the t

otal oxygen content is ≤25 x 10

-6

and P ≤0. 020% ； for L80

N80 and P110 grade steels the total oxygen content ≤20 x 10

-6

and P≤0. 015% ； the residual deleterious elements ( Pb + Sn + As + Sb + Bi) ≤ 140 x 10

-6

the rating of total inclusions in steel ≤2. 5 and the ratings of central porosity and shrinkage cavity are respectively ≤1.0.