The production flowsheet of Φ108 mm x 15 mm tube shell of tested steel TP347H (/% : 0. 08C

0. 36Si

1. 27Mn

0. 020P

0.0005S

18.l0Cr

10.80Ni

0.78Nb

0.007Cu) is hot-metal pretreatment- 90 t K-OBM-S melting- VOD-LF-220 mm x220 mm bloom casting- hot-rolled to Φ110 mm tube blank- piercing to tube shell. The batches intermittent and irregular cracks at inner wall of shell occur after pickling. By examination and analysis the inner defects of shell it is defined that inner defects is double layers formed by large-external composite inclusions in base

during conical-extrusion-piercing process

the base metal produces breaking caused by uneven stress led to form inner wall cracks. With prolonging LF soft stirring time and killing time

maintaining liquid in tundish more than 20 t

increasing the quality of submerged nozzle and increasing the baking temperature of nozzle form ≥800℃ to ≥1 000℃

the rating of each type of inclusions is 0 ~ 1.0 and the rate of shell inner defects decreases form 3.41% to less than 0.01%.