By calculation on thermodynamic equilibrium of Fe-Al-Ca-O-S system in calcium treatment process

the digrams of equilibrium curves of [ Al

]

-[O

]

[Al

]

-[Ca

]

and [Al

]

-[S

]

in each equilibrium condition at 1873K are obtained

and the effect of each component on modification of inclusions is systematically analyzed. It is found by study that in condition of 0.02%~0. 03% [Al

]

in F-grade high strength ship plate steel

in order to insure full accompli

shing modification of inclusions

the [O

]

in steel should control less than 25X10

-6

with forming 12CaO·7Al

2

O

3

in liquid the [Ca

]

is 15.24x10-

6

~19.97x10-

6

and with forming 3CaO·Al

2

O

3

in liquid the [Ca

]

is 70.87x10

-6

~ 92.88x10

-6

and with lower temperature of liquid it is available to form CaS and inhibit MnS clustered precipitation. The results of commercial production of F-grade high strength ship plate steel DNV F40 (/%: 0.092C

0.41Si

1.56Mn

0.015P

0.002S

0.032Al

0.035Nb

0.010Ti

0.080V) by 120t BOF-LF-VD-slab casting flowsheet show that with LF refining slag ingredient/% : 24.9Al

2

O

3

55.6CaO

7.7MgO

8.0SiO

2

and 1.24TFe

aluminium content in steel 0.03% and oxygen content in steel 0.0010% before adding calcium

and per heat feeding pure calcium wire 150m(0.21kg/m)

the inclusions in steel modified to spheroidal calcium aluminate inclusions from Al

2

O

3

before adding calcium.