Based on analyzing the effect of calcium treatment on inclusion modification and combined commercial pilot production of pipeline steel X80 (%： ≤0.08C

≤1.85Mn

≤0.060Als) in steelworks

the limits of calcium content required for modifying Al

2

O

3

to low-melting-point calcium aluminate and the limits of sulphur content to avoid single phase CaS precipitation are analyzed and calculated by using the metallurgical thermodynamics principle

and after RH treatment of liquid containing ≤0.002% S and 0.025%~0.035% Al and calcium treated by feeding Ca-Si wire according finishing [Ca

]

Tot

= (40 ~50) x 10

-6

calculation

the examination of inclusions in liquid of tundish and in casting billet has been carried

out. Results show that the average [Ca

]

Tot

for test heats of pipeline steel X80 is 41 x 10

-6

and [S

]

is 23 x 10

-6

that are within the limits calculated by theory

and the most inclusions in calcium treated steel are compound inclusion CaO- CaS-Al

2

O

3

to get better result of calcium treatment.