The thermodynamics and kinetics theoretical basis of modification of inclusions by calcium treatment is expounded. Through the test by LF-VD flowsheet

at calcium-free treatment conditions

except low carbon steel

the castability of medium and high carbon steel is good

the Ca content in the steel can be controlled by ≤0.001%

and the hit rate of non-metallic inclusions of rating D and Ds W1

0 is obviously improved. By optimizing ingredient of refining slag and using a method of later sulfur-alloying for sulfur-containing aluminum killed steel

it can improve the morphology of inclusions

resulting in fine

diffuse distribution of ( Ca / Mn) • S inclusions to improve the cutting performance of steel.