The production flowsheet for cold-drawn tube of steel 25Mn(/%: 0.24C

0.25Si

0.87Mn

0.016P

0.011S) is hot-metal pretreatment-90t EAF-LF-VD-Φ250mm round bloom HCC-piercing-hol rolling to Φ197mmx17mm tube-cold rolling to Φ185mmx13mm tube. The mottled spots delect of internal surface of steel 25Mn cold-drawn tube is analyzed by using optical microscope

scanning electron microscope (SEM)

energy spectnam meter and electron probe analyzer. Results show that the mottled spots defect is warped skin and concave pitting；the warped skin defect is caused by aggregated CaO-Al

2

O

3

-(MgO) compound inclusions which deformation is different from base steel and the aggregated growing up of

Al

2

O

3

-MnS-CaS compound inclusions fall off in cutting process to form concave pitting. With optimizing the original aluminium deoxidizing process to silico-calcium alloy deoxidizing process

decreasing the refining slag basicity from original 6.76 to 2.26

increasing the refining slag (MgO) from 5.33% to 7.23% and decreasing the ( Al

2

O

3

) from 26.05% to 12.76% to modify the brittle inclusions in steel to silicate inclusions with better ductility

therefore the internal mottled spots defect of internal surface of steel 25Mn cold-drawn tube is eliminated.