The surface transversal cracks occur on steel 65Mn (/%： 0.65C

0.24Si

1. 00Mn

0.014P

0.006S) 2.5mm X 10mm flat wire cold-drawn and rolled by φ6.5mm Coil. With analysis on defects it is obtained that due to casting billet surface carburization the abnormal structure of carbide block with strip distribution forms on coil surface

and in cold-drawing process the micro cracks form at abnormal structure and in flat-rolling process the micro cracks develop and combine to macro cracks. During casting process the liquid entraps enriched carbon layer of mold flux led to casting billet local surface carburization. With improving the 150 mm X 150 mm billet casting process including decreasing liquid surface fluctuating extend from original 7~8 mm to 3~4 mm

increasing depth of immersion n

ozzle from 70~80 mm to 90~100 mm

optimizing viscosity of mold powder from 0.35 Pa

·s

to 0.40 Pa

·s

and decreasing casting speed from 2.1~2.4 m/min to 2.1~2.2 m/min

the ratio of surface cracks of steel 65Mn flat wire decreases from original 2.33% to 0.