Production process to get super-low phosphorus content in high grade pipeline steel has been calculated by thermodynamics to study the conditions for LD dephosphorization and controlling resulfurization and the conditions for LF desulfurization and controlling rephosphorization. As 100 t LD end [O

]

is 0. 055%

the converter end [S

]

and [P

]

well be less than 0. 004% in the conditions of slag basicity being more than 4.5

( FeO) less than 20%

sulfur and phosphorus content in slag being respectively less than 0.036 5% and 1. 18%

During ladle furnace refining

0. 004% or 0. 006% phosphorus content in steel are got in conditions of refining slag basicity 5. 5 ~6

(Al

2

O

3

) respectively less than 18. 8% and 21. 0%

( FeO) respectively more than 1. 72% and 1. 28%

and ( P

2

O

5

) less than 0. 012%.