Simple ladle refining method

Details Simple ladle refining method Simple ladle refining method

2000-04-25

2001-10-30

Andrews, Melvyn (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Processes

Producing or treating free metal

C266S225000

Reexamination Certificate

active

06309443

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a simplified process for refining a molten steel previously refined in a refining furnace, and particularly to a process for efficiently heating a molten steel.
BACKGROUND ART
It has been widely known that, in a steelmaking process, at the time of supplying a molten steel primary-refined by a converter or an electric furnace to a casting process represented by continuous casting, the molten steel is treated in advance by a simplified secondary refining apparatus for the purpose of adjusting the chemical composition and temperature.
Japanese Unexamined Patent Publication (Kokai) No. 53-149826 discloses a process for heating a molten steel comprising immersing an immersion snorkel in a molten steel to provide a protective wall while the molten steel is being stirred by blowing a gas through a gas-blowing nozzle in the bottom of a ladle, and blowing oxygen gas onto the molten steel through an oxygen-blowing pipe while an oxidation reaction agent is being added to a region surrounded by the protective wall through a supply snorkel. Moreover, Japanese Unexamined Patent Publication (Kokai) No. 61-235506 discloses a process for refining a molten steel in a ladle comprising blowing an oxidizing gas through a lance onto the surface of a molten steel within an immersion snorkel inserted into a ladle, while the molten steel is stirred by blowing an inert gas through the bottom of the ladle, wherein prior to blowing the oxidizing gas through a top-blowing lance, an oxidation reaction agent is added within the immersion snorkel, and then blowing the oxidizing gas and adding the oxidation reaction agent are continuously conducted through the top-blowing lance.
However, although the structure of the top-blowing lance and the conditions for blowing oxygen significantly influence the efficiency for heating a molten steel, neither the influences nor the conditions for efficiently heating a molten steel are disclosed in the patent publication. Japanese Unexamined Patent Publication (Kokai) No. 61-129264 discloses a process for refining a molten steel in a ladle comprising inserting an immersion snorkel into a ladle while a molten steel is being stirred by blowing an inert gas through the bottom of the ladle, and blowing an oxidizing gas onto the molten steel surface within the immersion snorkel through a lance, wherein an annular lance comprising an inner pipe and an outer pipe is used as the lance mentioned above, oxygen and an inert gas are supplied to the inner pipe and the outer pipe, respectively, while the blowing pressure P
1
of the oxidizing gas from the inner pipe is being made lower than the blowing pressure P
2
of the inert gas from the outer pipe (P
1
<P
2
), and an oxidation reaction agent is added within the immersion snorkel in accordance with the conditions for blowing oxygen.
However, the process has the problem of a high gas cost because a large amount of a costly inert gas is used therein. On the other hand, Sumitomo Kinzoku, vol. 45-3, page 66 and the following pages (1993) discloses an embodiment of using a castable annular pipe as an oxygen lance. However, since a lance having a castable structure lacks durability, the lance has the disadvantage of being costly.
DISCLOSURE OF INVENTION
An object of the present invention is to provide a simplified refining process capable of efficiently heating a molten steel in a short period of time while the scattering and adhesion of splashes and the erosion of refractories are suppressed.
In order to achieve the object mentioned above, the present invention provides a process as described below.
(1) A simplified ladle refining process for refining a molten steel in a ladle comprising inserting an immersion snorkel into a ladle and blowing an oxidizing gas onto the surface of a molten steel within the immersion snorkel through a lance while the molten steel is being stirred by blowing an inert gas through the bottom of the ladle, wherein the lance has a ratio (d
0
/d
t
) of a nozzle outlet diameter d
0
(mm) to a nozzle throat diameter d
t
(mm) of from 1.2&agr; to 2.5&agr; wherein &agr; is calculated by the formula (1):
&agr;=[(1
/M·{
(1+0.2×
M
2
)/1.2}
3
]
½
  (1)
wherein M={5×(P
{fraction (2/7)}
−1)}
½
wherein P is a back pressure (kgf/cm
2
, absolute pressure).
The oxygen-blowing lance desirably has a water-cooled structure. Moreover, an alloy containing Al and Si is added to the molten steel during blowing oxygen so that the molten steel has an Al content of 1.6×S−1.9×S kg/t or a Si content of 1.25×S−1.5×S kg/t wherein S (Nm
3
/t) is a unit requirement of oxygen, and the heat produced by such oxidation reactions is desirably utilized.
(2) The simplified ladle refining process according to (1), wherein the ratio (L/a) of a cavity depth L (mm) of the molten steel surface calculated by the formulas (2) to (8) to a fire spot diameter a (mm) calculated by the formulas (3) and (9) is defined to be from 0.5 to 0.005:
0.016×
L
½
=H
c
/(
LH+L
)  (2)
wherein LH is a distance (lance gap, mm) between the lance and the molten steel surface, and H
c
is a jet core length (mm) calculated by the formula (3):
H
c
=f×M
op
×(4.2+1.1×
M
op
2
)×
dt
  (3)
wherein M
op
depends on the lance shape and is obtained by solving the formula (4):
d
0
/d
t
=[(1/
M
op
)×{(1+0.2×
M
op
2
)/1.2}
3
]
½
  (4)
and f is calculated by the formula (5) or (6):
f=
0.8
X−
0.06 (
X<
0.7)  (5)
f=−
2.7
X
4
+17.7
X
3
−41
X
2
+40
X−
13(
X>
0.7)  (6)
wherein X=P
o
/P
op
wherein P
op
is calculated by the formula (7) using M
op
, and P
o
is calculated by the formula (8):
P
op
={(
M
op
2
/5)+1}
{fraction (7/2)}
  (7)
P
o
=F/
(0.456×
n×d
t
2
)  (8)
wherein F is an oxygen supply rate (Nm
3
/hr), and n is a number of nozzles, and
a=
0.425×(
LH−Hc
)+
d
t
  (9)
(3) The simplified ladle refining process according to (1) or (2), wherein the ratio (D/a) of an immersion snorkel diameter D (mm) to the fire spot diameter a (mm) is defined to be from 1.5 to 8.
(4) The simplified ladle refining process according to any one of (1) to (3), wherein the ratio (LH/a) of the lance gap LH (mm) to the fire spot diameter a (mm) is defined to be from 2 to 3.5.
In the process of the present invention, an oxidation reaction agent is added to a molten steel within the immersion snorkel while the interior of the immersion snorkel is being maintained at the atmospheric pressure, and the molten steel can be heated by oxidizing the oxidation reaction agent.
Alternatively, the molten steel can be heated by oxidizing an oxidation reaction agent having been added to the molten steel in advance while the interior of the immersion snorkel is being maintained at the atmospheric pressure.
Furthermore, prior to or subsequent to heating the molten steel by oxidizing the oxidation reaction agent under the atmospheric pressure as described above, the molten steel can be decarburized by blowing an oxidizing gas onto the molten steel surface within the immersion snorkel through the lance while the interior of the immersion snorkel is evacuated.


REFERENCES:
patent: 5540753 (1996-07-01), Takano et al.
patent: 6017380 (2000-01-01), Kitamura et al.
“Sumitomo Metals”, vol. 45-3 (1993), pp. 66-72.

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