Precipitation conditions of magnesium oxide and its compounded inclusions in liquid in X80 pipeline steel was calculated by thermodynamics

that were calculating according to aim acid-soluble aluminium content 0.025% in pipe-line steel at 1873 K

as 0.0008%≤[Mg

]

≤0.0059%

the compounded inclusion MgO ·Al

2

O

3

formed in steel

and as [Mg

]

≤ 0.0059%

the MgO should formed in steel.With[Ti

]

0.015%

as [Mg

]

=0.0014%

the compounded inclusion 2MgO ·TiO

2

formed

and as 0.025% ≤ [Al

]

≤0.047%

the compounded inclusion Al

2

O

3

·TiO

2

formed in steel.The analysis on pipeline steel melted with 50 kg vacuum induction furnace by SEM and EDS showed that in steel treated by magnesium the percentage of inclusion less than 2 μm was 85%

2~5 μm wa

s 14.5% and 5～10 um was only 0.5%. The deoxidation products in pipeline steel treated by magnesium were 2MgO ·TiO

2

MgO ·Al

2

O

3

etc

which could be combined with sulfide and nitride to form compounded inclusions.