The production flow sheet for oil casing tube steel (/% ： 0.26-0.29C

0.25-0.35Si

0.40-0.50Mn

≤0.009P

≤0.004S

0.95-1.05Cr

0.09-0.11V

0.02-0.04Al

0.015-0.020Ti

≤0.0060N) is hot metal pretreatment-120 t BOF-argon blowing-LF-CaSi wire feeding-RH-Alloying-CaSi wire feeding-soft argon blowing-Φ200 mm round bloom casting process. By commercial test as the N content in produced round casting bloom is 67 x 10

-6

and by thermodynamic analysis as the N content in steel is more than 50 x 10

-6

the more than 2 μm TiN inclusions easily form in casting bloom. With controllin

g BOF end [N

]

≤ 30 X 10

-6

LF end [S

]

≤ 25 X 10

-6

[O

]

≤ 25 X 10

-6

and [N

]

≤ 35 X 10

-6

after alloying RH end [N

]

≤ 35 x 10

-6

and [H

]

≤ 1.5 x 10

-6

stably feeding CaSi wire speed 300-400 m/min and controlling tundish [N

]

≤ 40 x 10

-6

and strict casting-shielding process

the N content in casting bloom shall be ≤ 50 X 10

-6

the amount of TiN inclusions in steel decreases markedly and there are not larger size TiN inclusions to be found in steel.