The metallurgical flow sheet for ultra-low temperature steel 9Ni (/% : 0. 03 ~ 0. 05C

0. 15 〜0. 30Si

0. 60 ~0. 70Mn

≤0

003P

≤0. 002S

9. 0 ~9. 5Ni

0. 02 ~6. 04Al) is 180t hot metal pretreatment-1801 top and bottom combined blown converter-LF-feeding Si-Ca wire-RH-180 ~250 mm slab casting. With controlling [P

]

≤0.001 5% by dephosphorizing in ladle during converter tapping

[S

]

≤0. 001% by desulphurizing in LF and controlling [Al

]

0. 02% ~ 0. 03% by adding Al bullets and feeding Al wire

RH vacuum ≤200 Pa in feeding silicon-calcium wire process before RH process

controlling [N

]

≤25 X 10

-6

and [ H

]

≤1. 0 x 10

-6

in ≤200 Pa vacuum for 20 min during RH refining

using electromagnetic stirring

soft reduction

and whole shielding casting process

straightening temperature of slab ≥950℃ by increasing casting speed

and slab cooling in slow cooling pit furnace at 300℃ for 40 h

the [ P

]

[ S

]

[ H

]

[ N

]

and [0

]

(/10

-6

) decrease respectively to 15

9

0. 5

26 and 10

the crack occurring ratio decreases to 0

and the qualified ratio of nondestructive testing is up to 100%.