Abstract: In order to find the causes of surface linear cracks in round bar rolled from 20 steel billet, spectrum analyzer, optical microscope, scanning
electron microscopy and energy spectrum analysis were employed to examine chemical composition, microstructure of surface crack, type
of inclusion in the cracks. The results show that cracks are caused by surface pores in 20 steel billet. Surface pores which are burned in rolling
heating process are oxidized and can not be pressed. Surface linear crack defects are eventually formed in the process rolling extension.

Key words: crack, surface pores, decarburization, oxidized round spot