The metallurgical flow sheet of tested pipeline steel X52 (/% : 0.070~0.0790

0.22Si

1.29~1.32Mn

0.008~0.009P

0.002S

0.028Al

0.0016~0.0017T[O

]

0.0053〜0.0065N) is hot-metal pretreatment- 220t BOF-LF-VD-230 m

m slab CC. The main steelmaking processes are adding aluminium shot in tapping of BOF； main ingredient of LF refining slag- (/%) : 54.41~58.54CaO

7.66~8.42SiO

2

22.57~24.69A1

2

O

3

4.15〜4.58MgO

0.78~2.89FeO ； and feeding calcium wire treatment after VD. The evolution of inclusions in steel during steelmaking process is studied by auto scanning electron microscope with ASPEX ( Application Specific Products employing Electron Beam and X-ray technology). Results show that before LF refining the inclusions in liquid are AI2O3 cluster and granule

after LF refining the Al

2

O

3

is modified to globular CaO-MgO-Al

2

O

3

inclusion

then modified to globular CaO-Al

2

O

3

-CaS inclusion after calcium treatment; by VD treatment the number of inclusions in liquid decreases from 30〜35/mm

2

to 7~15/ mm

2

and the T[O

]

and [N

]

markedly decrease to 12 x 10

-6

~13 x 10

-6

and 41 x 10

-6

~48 X 10

-6

. from end soft stirring to tundish the number of inclusions in liquid dramatically increases to 33/mm

2

at end VD the average size of inclusions is up to maximum i. e. 2.6~3.6 μm

but in tundish that decreases to 1.9~2.1 μm: during feeding wire and soft blowing process the T[O

]

and [N

]

increase to some extent

it shows that the secondary oxidation of liquid occurs； with LF refining

VD and calcium treatment the S content in steel decreases lo 16 x 10 ; and the high (SiO

2

) and ( FeO + MnO) content in tundish shielding flux lead to resulfurization of liquid in tundish.