Note: Descriptions are shown in the official language in which they were submitted.
54
The present invention relates to a process for
producing high-quality steel using an oxygen top-blowing
procedure.
In the present invention and for purposes of this
application, high quality steel is defined as a steel which
contains as little phosphorus, sulphur and non-metallic
inclusions as is reasonably possible.
A known method for achieving the desired carbon
content in a melt comprises discontinuing the decarburizing
process step at a pre-selected time. However, a disadvantage
or drawback of this method is that it does not permit the
production of steels having both high carbon and low
phosphorus contents. Nevertheless, if it is desired to
obtain steels having low phosphorus content, a pre-treatment
(dephosphorizing, desulphurizing, desilicizing) of the hot
metal melt is necessary. Such pre-treatments, which have
been described primarily by Japanese steelmakers in various
printed documents, are performed outside of steelmaking
converters, in stationary or transportable ladles, and
generate high costs due in part to the expensive apparatus
which must be used. It will be appreciated that the inevita-
ble temperature losses inherent to such processes have to be
either borne or compensated through heating devices, thus
leading to increased costs and further disadvantages. It
should be noted that only by using the method described
above wherein the desired carbon content is obtained through
discontinuing the decarburization step, will low inclusion
(phosphorus) contents steel also be obtained.
~Zl3~3~354
Another known method which allows low phosphorus
contents in steel to be obtained at lower costs, involves
decarburizing the steel down to low carbon-contents, e.g.
0.05% C. However, with this method, the decarburized steel
will contain relatively high amounts of oxygen, depending
upon the conditions (for examp]e, 500 to 1000 ppm, and even
more),- the removal of which through conventional means not
only requires large amounts of expensive substances, but
also generates undesired inclusions.
It is an object of the present invention to
overcome or alleviate the above discussed and other problems
and deficiencies of the prior art and to provide a process
which permits the production of high-quality steel, with a
minimum content of non-metallic inclusions.
In accordance with the present invention, there is
thus provided a process for producing high-quality steel
having a pre-selected carbon content from a melt, which
comprises a first step of decarburizing a melt in a con-
verter by top-blowing said melt with oxygen such that the
carbon content of said melt is lowered to less than about
0.1% carbon. Next, a slag is created on the melt and carbon
is delivered into the melt at a high velocity whereby the
melt and the slag mix, the carbon being delivered to the
melt until the oxygen content of the melt is less than or
equal to about 250-400 ppm oxygen and the carbon content of
the melt is greater than or equal to about 0.04-0.07~
carbon. In a third step, recarburization of the melt is
effected whereby a melt containing as little oxygen as is
reasonably possible and having a carbon content which is as
close as possible to a pre-selected carbon content is
~Z8E~9~
achieved. In a fourth optional step, the melt upon meeting
such requirements is transferred to a ladle and treated both
with a synthetic slag and with a material that is both
deoxidizing and desulphurizing.
The first step of the process according to the
present invention comprises decarburizing of the melt in a
converter, by top-blowing oxygen and adding the well-known
substances necessary to convert silicon and phosphorus into
slag. The decarburization step generates heat and the
resultant slag becomes reactive due to the oxygen input,
thereby absorbing a portion of the phosphorus. The decarbu-
rization step is contlnued to produce a carbon content lower
than about 0.1~ C, preferably lower than 0.05~ C, regardless
of the final carbon content which is desired in the finished
steel.
The second step in the present invention comprises
treating the decarburized an oxygen-enriched melt with
carbon, within the converter. This process step is performed
by introducing into the melt, at the highest velocity
possible, carbon which is preferably in the form of comminu-
ted coal such as anthracite. This second step generates an
extraordinarily vehement reaction during which the metal
bath is both deoxidized and recarburized. It will be ap-
preciated that contrary to conventional deoxidization in a
ladle with combinations of coal, ferromanganese, ferrosili-
con, aluminum and the like, the products of deoxidization
are exclusively gaseous in the instant case and will there-
fore not yield inclusions. In this step, high amounts of
carbon monoxide are generated, which result in a forceful
mixing of the metal phase with the slag phase.
~288954
In accordance with the inventlon, care is taken to
profit from the slag reactivity. The reaction of the second
step is thus performed with the purpose of obtaining a
strong stir and an intense mixing of the bath with the slag.
An advantageous method comprises top-blowing the carbon
through a special lance, wherein the carbon particles are
suspended in an inert gas and are accelerated to high
speeds. It has been found advantageous to further bottom-
blow an inert gas in the melt during the addition of carbon,
in order to even further improve mixing of the bath and
slag.
The second step is ceased at an oxygen-content of
about 250-400 ppm oxygen. At this point, the melt still has
a very low carbon content, e.g. about 0.04-0.07% C. It has
been observed that during the addition of carbon in the
second step, the carbon at first primarily deoxidizes the
melt before recarburizing the same. In fact, the combination
of the coal injection and the bottom-stirring result in
lowering the product carbon: oxygen ratio to its thermo-
dynamic equilibrium value in accordance with the actual
temperature and gas pressure conditions in the metal bath.
For example, prior to coal addition: %carbon=0.032i ppm
oxygen=900; after coal addition: ~carbon=0.057; ppm
oxygen=350. Significantly, at the same time, it has been
observed that a surprisingly high dephosphorization of the
bath has occurred.
If especially low phosphorus content is desired,
for example, 0.008% P or less, in accordance with the
present invention, a Na2O-bearing material (i.e. suitable
lZ8895~L
known compounds that yield Na2 upon thermal decomposition)
will be added. These may be added for example, by top-
blowing, together with the carbon.
The third step in the process of the invention
comprises a treatment of the deoxidized and partly decarbu-
rized bath (taking into consideration the actual conditions
relati-ve to bath temperature, oxygen content, carbon
content, etc.). It will be appreciated that the ratio of
carbon:oxygen at a given temperature will be important with
regard to the remainder of the process.
It will also be appreciated that one of the
primary objects of the present invention is to suppress the
tendency of the steel to form inclusions. As a consequence,
the melt will be transferred to a ladle only upon containing
as little oxygen as possible and having a carbon content as
close to the desired content as possible. Preferably, the
ladle should be free from any heating means. In order to
avoid producing solid deoxidation products, the deoxidation
should be carried out with carbon as far as feasible, thus
creating only carbon monoxide and carbon dioxide.
Thus, in a first embodiment of the process ac-
cording to the present invention, treatment of the melt with
carbon continues within the converter, until the desired
carbon content is reached. If this desired content is, for
example, close to 0.10~ C, the corresponding oxygen content
will be close to 200 ppm O. Thereafter, the melt is
transferred into the ladle.
A second embodiment of the present invention
comprises continuing the treatment of the melt with carbon
in the ladle, so as to limit the amounts of carbon injected
954
in the converter. In that case, the entire amount of carbon
is added in the form of a commercially available recarburi-
zer, during the transfer in the ladle and prior to adding
alloying materials.
Whether the third process step is carried out in a
converter or in a ladle, surprisingly, the rephosphorization
that $hould normally be expected does not occur. It is
believed that this unexpected result is because the second
step does not only entail conversion of carbon and oxygen to
carbon monoxide and carbon dioxide, plus a dissolution of
carbon in iron, but also there occurs some type of inter-
action between the slag and the carbon. It is further
believed that this interaction at first is due to a strong
reactivity towards phosphorus and also partially sulphur,
and at a certain time has a maximum intensity, and towards
the end of the treatment turns into a pronounced passivity.
At the term of the treatment, the slag is passive and will
not pass on phosphorus to the melt, despite a treatment with
reducing materials.
This passivity also influences the carbon: oxygen
equilibrium in the converter through addition of materials
such as ferromanganese, by injecting these materials in the
melt and permits a lowering of the oxygen-content of the
bath, while preserving its carbon content, and not suffering
from rephosphorization. While this phenomenon generates
solid deoxidization products, due to the still elevated bath
temperature and to the bath-diameter; bath-height ratio
(which is more favorable in the converter than in the
iZ8~3~5~
ladle), these deoxidization products do not turn into
inclusions in the finished steel, but precipitate entirely
and are absorbed in the slag.
Finally, in a fourth optional step of the process
according to the present invention, the melt is treated in a
ladle with a deoxidizing and desulphurizing agent, prefer-
ably with metallic calcium, and with well known synthetic
slags. As a result, the renewed oxygen-surplus twhich is due
to the inevitable temperature drop) is removed leading to
the creation of relatively large globular oxidized products.
It will be appreciated that large globular bodies will more
readily rise and precipitate from the metal phase than small
or non-globular particles.
The above-described process of the present inven-
tion can be used in order to manufacture both semi-killed
and fully killed steels.
The following non-limiting example further illus-
trates the invention.
EXAMPLE
1) Decarburizing (in the converter) end of blow
0 in steel 950 (ppm)
C in steel 0.034 (~)
P in steel 0.022 (~)
Temperature 1700 ( C)
~2~38~
2) Deoxydation/Dephosphorization (in the converter~
Anthracite 2 (kg/t of steel)
Na2O-bearer 6 (kg Na2O/t of steel)
Carrier-Gas 0.10 (Nm N2/kg Anthracite)
Bottom-stirring
gas 0.05 (Nm Ar/t of
- steel.Minute)
0 in steel 350 (ppm)
C in steel 0.06 (%)
P in steel 0.011 (%)
Temperature 1660 (C)
3) Recarburizing
Anthracite 1.1 (kg/t of steel)
Fe-Mn 1.5 (kg/t of steel)
Carrier-Gas 0.10 (Nm3N2/kg Anthracite)
Bottom-stirring
gas 0 (Nm N2/t of
steel.Minute)
0 in steel 210 (ppm)
C in steel 0.10 (~)
Temperature 1645 (C)
4) Accurate recarburizing
Anthracite 0.55 (kg/t of steel)
0 in steel 105 (ppm)
C in steel 0.14 (%)
Temperature 1605 (C)
54
5) Accurate deoxydization (in the ladle)
Ca .45 (kg/t of steel)
Synth. slag 1.8 (kg/t of steel)
O in steel 3.S (ppm.)
S in steel .06 (~)
P in steel .012 (%)
. .
