The X70 pipeline steel (% ： 0. 02 - 0. 05C

0. 145 〜0. 25Si

≤0. 015P

Wt). 002S

1. 70 ~ 1. 80Mn

0. 085 ~ 0. 15Nb

0. 2 - 0. 3Ni

0. 015 ~ 0. 040Al

0. 01 ~ 0. 03Ti) is produced by hot metal pretreatment - 120 t converter - RH - LF - (200 ~ 250) mm x (900 ~ 1 800) mm slab concasting process. With metal manganese to replace low- carbon ferro-manganese and partial silico-manganese to decrease carbon pickup amount in liquid after RH treatment

controlling LF liquid carbon pickup amount ≤0. 01 %

controlling pretreated hot metal [ S] ≤0. 002% and basicity of LF refining slag being about 1.5

the LF initial [ C] and [ S] are respectively 0. 03% and 0. 001 5%

and the LF end [ C] and [S] are respectively 0. 04% and 0. 000 8%

to meet the requirement of service.