According to the analysis on outer fold defect of Φ85 mm x 8 mm tube of 37Mn5 steel (0. 39% C

1. 30% Mn)

it is obtained that the ~ 20 μm brittle inclusions-silicate and SiO2 existing at grain boundary in steel is main reason for formation of tube outer fold. With controlling thickness of top slag ≤50 mm in ladle after LD tapping

(FeO)≤l%

white slag time ≥15 min and soft argon blowing ≥10 min in LF refining

shield casting in whole process

the [ 0

]

is (11. 6 - 17. 1) x 10

-6

and the rating of inclusions in steel of A series is 0 ~ 2. 0，of B and C series is 0 and of D series is 0. 5 ~ 1. 0

that are available to control the formation of outer fold of steel tube.