Method and apparatus for refining steel

Disclosed are an improved method of refining steel and an apparatus for practicing the method. The refining method comprises, basically, carrying out the refining, particularly, decarburization, while stirring molten steel in the refining furnace by injecting gas thereinto, while supplying heat with a burner installed at the top of the furnace to the molten steel. According to this method, it is possible to start at an initial carbon content of the moltend steel lower than that of known AOD process, and complete the refining in a curtailed period of time and with a decreased oxidation loss of Cr. Thus, damage of refractory materials of the furnace is reduced, and the amount of Si necessary for reducing Cr-oxides in the latter stage of the refining is also reduced. Oxygen for the decarburization is supplied usually in the form of gas, but can be supplied from a solid oxygen source. In an alternative of the present method, powdery metal oxide, which is reducable equally to or more easily than Cr-oxides, is shot into the furnace through the burner or injected into the molten steel through a tuyere or an immersed lance. Preferably, the burner is of a type of variable flame length. Use of the burner disclosed here makes it possible to lengthened the flame at the former stage of the refining so that not only heat but also oxygen may be supplied to the molten steel surface of promoting the decarburization, and to shorten the flame at the latter stage of the refining so that only heat may be supplied.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of refining steel, particularly, 
decarburizing various steels such as low carbon steel, carbon steel, low 
alloyed steel, alloyed steel and stainless steel. The invention 
encompasses an apparatus for practicing the refining method. 
2. State of the Art 
As is well known, VOD process (vacuum oxygen decarburization) and AOD 
process (argon oxygen decarburization) are often used for refining various 
steels, particularly, stainless steel containing Cr. 
AOD process is the most typical method of refining, which comprises 
injecting Ar gas with oxygen gas into molten steel to conduct 
decarbuzization while suppressing oxidation loss of Cr by lowering CO 
partial pressure, and, as the decarburization proceeds from the high 
carbon state to the low carbon state, changing the O.sub.2 /Ar ratio of 
the gas injected into the molten steel so that the decarburization 
proceeds efficiently. As the gas for diluting O.sub.2, instead of 
expensive Ar, N.sub.2 may be used, or alternatively, steam may be used 
(CLU process). 
The AOD process is conducted without additional heat to the molten steel, 
and therefore, it is usual to start the refining at a high carbon state 
such as 1.0-2.0 weight % in the molten steel so as to utilize the heat of 
oxidatin reaction of the carbon and to prevent temperature decrease during 
the refining. 
In such a process, the period for the decarburization becomes necessarily 
long. Also, it is inevitable that a portion of Cr is oxidized, and the 
oxide must be reduced afterward by using expensive metallic Si. Further, 
it is not easy to control temperature of the molten steel to be tapped. 
These are the problems inherent in AOD process. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method of refining 
steel, which makes it possible to shorten the period of decarburization, 
to lower oxidation loss of valuable metals such as Cr, and thereby to 
shorten the period for reducing the Cr-oxides in the latter stage of 
refining as well as to decrease consumption of the reducing material. 
Another object of the invention is to provide an apparatus suitable for 
practicing the above method of refining steel, particularly, the apparatus 
using a burner with which length of the flame can be easily varied. 
The method of refining steel according to the present invention can be 
carried out by using an apparatus with heating means, such as arc furnace, 
or an apparatus without heating means, such as AOD furnace, CLU furnace, 
VOD furnace, and further, even a converter.

DETAILED EXPLANATION OF PREFERRED EMBODIMENTS 
The method of refining steel of the present invention comprises carrying 
out the refining under stirring the molten steel contained in a furnace 
such as an electric furnace, a convertor, or a refining vessel (e.g., AOD 
furnace and VOD furnace) by injecting gas there into, and supplying heat 
from a burner which is installed at the top of the furnace and burns a 
fuel such as heavy oil, powdered coal or natural gas to the molten steel 
through the surface thereof enlarged due to the stirring. 
The gas to be injected into the molten steel in the furnace may be either 
an oxidative gas (O.sub.2 or O.sub.2 -enriched air) which causes the 
decarburization reaction in the molten steel, or an inert gas (e.g., Ar 
and N.sub.2). Further, both of the oxidative gas and the inert gas may be 
used in such a way that the former is surrounded by the latter so as to 
protect the tuyere. 
The oxidative gas supplied to the burner may be air. It is, however, 
preferable to use pure oxygen gas or gas containing 30 volume % or more of 
O.sub.2 (such as mixed gas of O.sub.2 with N.sub.2 or Ar) so that the 
flame may be of a higher temperature. 
The flame from the burner is preferably so directed that it may jet to the 
surface of the molten steel. 
In the carbon content range of the molten steel where the decarburization 
reaction rate is determined by the oxygen supply rate, i.e., usually about 
0.30 weight % or more of C, it is advisable to supply O.sub.2 of the 
amount more than theoretically necessary for burning the fuel so that the 
CO generated on the molten steel surface due to the decarburization 
reaction may be burnt to CO.sub.2 and that the heat energy of the reaction 
may be utilized. The energy of burning is absorbed by the molten steel 
through the surface thereof enlarged due to the stirring and raises the 
temperature of the molten steel. 
Supplying O.sub.2 in the amount excess to the theoretically necessary 
amount for burning the fuel can be done either by increasing the amount of 
the oxidative gas supplied to the burner or by supplying additional 
O.sub.2 to the molten steel surface separately to the supply through the 
burner. 
The apparatus for practicing the present method of refining steel comprises 
a furnace body to contain the molten steel, gas supply means to inject a 
gas for stirring the molten steel in the furnace, a burner supplying heat 
to the molten steel through the surface thereof enlarged by the stirring, 
and means for supplying fuel and oxygen to the burner. A practical 
apparatus is equipped with a CO-measuring means to determine the 
CO-content in the exhaust gas form the furnace. When necessary, further 
means for supplying O.sub.2 to the molten steel surface separately to the 
burner is installed. 
FIG. 1 shows an embodiment of the present apparatus. The refining apparatus 
has a furnace lid 3 on the furnace body 2 which contains molten steel 1, 
and the furnace body is equipped with a tuyere 4 at the lower part 
thereof, which tuyere comprises an inner tube 4a for injecting O.sub.2 gas 
and an outer tube 4b for injecting Ar gas. The tuyere 4 constitutes the 
gas supplying means for stirring the molten steel. The furnace body 2 has 
an inclined tapping mouth at the top thereof. The furnace lid 3 is 
equipped with a burner 6, which supplies heat to the surface of the molten 
steel 1. The burner 6 has a conduit 6a to the fuel supplying means and a 
conduit 6b to the O.sub.2 supplying means, and the structure of the burner 
is such that can be cooled. The burner 6 is installed in the vertical 
direction so that the flame 7 from the burner 6 may be jetted to the 
molten steel surface. Thus, supply of heat to the molten steel surface can 
be done efficiently through the surface enlarged due to the stirring, and 
O.sub.2 is blown in the direction reverse to the stream of CO gas 
generated on the molten steel surface so that burning of the CO gas may 
occur near the surface. 
The fuel supply means for supplying fuel to burner 6 comprises a fuel line 
11, a flow meter 12, a valve 13, a fuel tank 14, a pressurized medium 
supply line 15, a valve 16 and a pressure gauge 17. The O.sub.2 (pure 
oxygen gas or a mixed gas containing O.sub.2) supply means comprises an 
O.sub.2 line 21, valves 22 and 23, and a flow meter 24. 
The furnace lid 3 has, in addition to the hole 3a for inserting the burner 
6, an outlet 3b for exhaust gas, so as to introduce the exhaust gas into a 
dust correcting hood 25. The exhaust gas outlet 3b is equipped with a 
CO/CO.sub.2 -meter for determining the CO/CO.sub.2 ratio in the exhaust 
gas. 
For refining steel, O.sub.2 gas is injected into the molten steel contained 
in the furnace body 2 through the inner tube 4a of the tuyere 4 and Ar gas 
through the outer tube 4b to effect stirring and decarburization of the 
molten steel, and the flame 7 from the burner is directed to the molten 
steel surface enlarged due to the stirring, thereby supplying the heat to 
the molten steel instead of conventional heat supply by heat of oxidation 
raction of C. 
According to the present method of refining steel, different from the 
conventional technology, it is not necessary to use an initial carbon 
content in the molten steel prior to refining so high as 1.5-2.0 weight % 
for the purpose of sufficient heat supply, but it is possible to reduce 
the initial carbon content as low as 0.8 weight %. This shortens the 
necessary period for the decarburization, and suppresses oxidation of 
valuable metals such as Cr, thereby reduces consumption of Si used for 
reduction of the valuable metal oxides. Decrease of the reducing agent 
consumption results in formation of less slag and shortens the period 
necessary for removing the slag. Shortened refining period also results in 
less damage in the refractory materials. 
It is preferable to use, as the oxidative gas supplied to the burner 6, a 
mixed gas containing more than 30 volume % of O.sub.2 or even pure oxygend 
gas to obtain flame 7 of a higher temperature. The decarburization 
reaction will be further promoted and oxidation loss of the valuable 
metals such as Cr can be thus reduced. 
In the process of refining, at the range of carbon content of the molten 
steel where the decarburization rate is determined by oxygen supply (as 
noted above, usually, C: 0.30 weight % or more), CO gas in the amount 
corresponding to the amount of O.sub.2 supplied through the inner tube 4a 
of the tuyere 4 is generated on the molten steel furface. It is preferable 
to determine the generated amount of CO by the CO/CO.sub.2 -meter 26 and 
supply O.sub.2 in the amount more than theoretical necessity for the fuel 
through the O.sub.2 line 21 or a separately equipped (not shown in the 
Figures) O.sub.2 -injecting nozzle to the molten steel 1 so as to burn the 
CO generated from the molten steel surface into CO.sub.2. 
The oxygen to be supplied to the molten steel as noted above may be not 
only in the form of gas, but also in the solid state. Thus, an alternative 
of the present method of refining steel comprises carrying out the 
refining while stirring the molten steel in the furnace by injecting gas, 
while supplying heat through the burner equipped at the top of the furnace 
to the molten steel through the surface thereof enlarged due to the 
stirring, and while supplying a solid oxygen source in the powder form to 
the burner by a carrier gas so as to shoot the powder to the molten steel 
surface with the flame. 
Examples for one of the groups of the solid oxygen source are: oxides of 
metals which are equally to or more easily reduced as chronium oxides, 
such as chromite sand, mill scale, iron ore, nickel oxide, molybdenum 
oxide and tangsten oxide. Another group comprises substances which 
decompose at the temperature of the molten steel to generate CO.sub.2 such 
as carbonates and bicarbonates of calcium, sodium and barium. 
Both of the above solid oxygen sources give good results when they are in 
the form of relatively fine powder, such as those of particle size 500 
micron or less. 
As the carrier gas, whole or a part of the O.sub.2 gas for burning the fuel 
may be used. In some cases, air or O.sub.2 -enriched air will do. 
As the gas to be injected into the molten steel in the furnace, inert gas 
such as Ar is the most preferable. If there is no problem, depending on 
the kind of steel, N.sub.2 may be use. From the view to promote the 
decarburization, it is necessary to inject an oxidative gas (O.sub.2 gas 
or a gas containing large quantity of O.sub.2) having decurburization 
effect in the molten steel. It is preferable, as done in the AOD process, 
to use both Ar gas and the oxidative gas. 
In the decarburization reaction of the molten steel with use of O.sub.2 
gas, as noted above, oxygen supply is rate-determining when the carbon 
content is high, and carbon diffusion in the molten steel becomes 
rate-determining as the carbon content becomes low. Under the latter 
condition, even if the amount of the injected O.sub.2 gas is increased to 
strengthen supply of oxygen, it is difficult to disperse the O.sub.2 gas 
uniformly in the molten steel, and the excess O.sub.2 is consumed by 
oxidation of expensive Cr. Thus, because of the low effect of 
decarburization and vigorous sprash of the molten steel, it is not 
possible to promote the decarburization only by increase of the amount of 
supplied O.sub.2 gas. 
When the solid oxygen source in the form of powder is supplied to the 
burner and shot to the molten steel surface with the flame according to 
the alternative of the present invention, the solid oxygen source is 
caught by the surface vigorously moving due to the stirring and disperses 
uniformly in the molten steel to move even to the zone where the gas does 
not disperse. 
The solid oxygen source reacts in the molten steel as follows and 
contributes to the decarburization: in cases of easily reducable metal 
oxides, for example, 
EQU Fe.sub.2 O.sub.3 +C.fwdarw.2FeO+CO 
EQU FeO+C.fwdarw.Fe+CO 
EQU Cr.sub.2 O.sub.3 +3C.fwdarw.2Cr+3CO 
and in cases of carbonates and bicarbonates, 
EQU CaCO.sub.3 .fwdarw.CaO+CO.sub.2 
EQU 2NaHCO.sub.3 .fwdarw.Na.sub.2 O+2CO.sub.2 +H.sub.2 O 
EQU CO.sub.2 +C.fwdarw.2CO 
thus supply of oxygen to whole the molten steel occurs quickly and the 
decarburization proceeds quickly. 
Because the interface of the reaction increases as the particles of the 
solid oxygen source is fine, it is preferable to use finely divided powder 
of 500 micron or less, as noted above. 
The solid oxygen source is added with the flame and, after being heated, 
dispersed in the molten steel. Because all the above decarburization 
reactions are endothermic, heating and temperature rise of the solid 
oxygen source itself prior to the addition effects favorable for promoting 
the reactions. In cases where the carbonates or bicarbonates are used as 
the solid oxygen source, it is considered that at least a part of the 
above decomposition reactions will occur in the flame, and that the 
CO.sub.2 thus generated is jetted to the molten steel surface and causes 
the following reaction which helps the decarburization. 
EQU C+CO.sub.2 .fwdarw.2CO 
The above described alternative of the present refining method also 
shortens the period necessary for the decarburization. As the results, the 
oxidation loss of valuable metals such as Cr is decrease. 
Supply of the solid oxygen source can be carried out not only through the 
burner as described above, but also by other means. 
Another alternative of the present method of refining comprises carrying 
out the refining while stirring the molten steel in the furnace by 
injecting gas, while supplying heat through the burner equipped at the top 
of the furnace to the molten steel through the surface enlarged due to the 
stirring, and while supplying a solid oxygen source in the form of powder 
through a tuyere or an immersed lance. 
The powdery solid oxygend source added to the molten steel in accordance 
with this alternative disperses uniformly in the molten steel to move to 
the zone where the gas does not go. The solid oxygen source reacts in the 
molten steel as explained above to contribute the decarburization. As the 
results, oxygen supply to whole the molten steel proceeds quickly, and the 
period necessary for the decarburization is shortened. 
Still other alternative of the present refining method uses a burner 
preferably, an oxygen burner with which the flame length can be varied. As 
illustrated in FIGS. 9 and 10, refining is carried out while the molten 
steel 1 is stirred by injecting a gas thereinto, while supplying heat to 
the molten steel through the surface enlarged due to the stirring. In the 
former stage of the decarburization the flame 7 of the burner is long as 
seen in FIG. 9 so as to jet the fleu gas to the molten steel 1 for 
promoting the decarburization, and in the latter stage of the 
decarburization the flame of the burner is short as seen in FIG. 10 so as 
to give only heat to the molten steel. 
Change of the flame length is done preferably at the time when the 
C-content in the molten steel decreases to such level that the rate 
determining step of the decarburization reaction changes from oxygen 
supply to carbon diffusion in the molten steel. Flame length is usually 
made binarily (long-short), and this is sufficient, but may be stepwisely 
with many more steps or even gradually. 
The refining apparatus suitable for practicing the above described still 
alternative refining method comprises, as illustrated in FIGS. 9 and 10, a 
furnace body 2 to contain the molten steel 1, a gas supply means 4 for 
injecting gas into the molten steel to stirr and a burner 6 for supplying 
heat to the molten steel surface, which is equipped with means for 
supplying fuel and means for supplying oxygen to the burner: and supplying 
oxygen to the burner is carried out, as examplified in FIGS. 11 and 12, 
through both of primary oxygen nozzle 62 which is surrounding fuel nozzle 
61 and secondary oxygen nozzle which is circumferencially to the primary 
oxygen nozzle, and by changing the ratio of the primary oxygen to the 
total oxygend so that the flame length of the burner may be varied. 
The fuel to be burnt with the burner may be any of gas (e.g., propane, 
natural gas, hydrogen), liquid kerosine, heavy oil) or solid (fine 
powdered coal). 
In FIGS. 9 and 10, reference 3 indicates a furnace lid put on the furnace 
body. The furnace body 2 has an inclined tapping mouth 5 at the top, and a 
tuyere 4 for injecting O.sub.2 gas and Ar gas at the lower part thereof. 
Using the burner of the above described structure, it is possible to 
control the flame length by changing the ratio of the amount of the 
primary oxygen to the total amount of oxygen supplied to the burner 
(hereinafter the ratio of primary oxygen/total oxygen is referred to as 
"primary oxygen ratio"). As shown in FIG. 13, the flame becames long when 
the primary oxygen ratio is small, and short when the ratio is large. 
Also, as shown in FIG. 14, at a certain primary oxygen ratio, the higher 
the percentage of excess oxygen (so-called "M-value") is, the shorter the 
flame length is. 
Accordingly, by choosing these conditions, it is possible to control the 
flame length in a certain range, for example, 55-140 cm, with a constant 
burning capacity. 
As already explained, the decarburization of the molten steel using O.sub.2 
gas is carried out by reacting O.sub.2 with C to form CO or CO.sub.2, 
EQU 2C+O.sub.2 .fwdarw.2CO 
EQU 2CO+O.sub.2 .fwdarw.2CO.sub.2 
thus removing C from the molten steel. The CO.sub.2 also react with C in 
the molten steel to cause the decarburization reaction: 
EQU C+CO.sub.2 .fwdarw.2CO 
At the former stage of the decarburization, the reaction can be promoted by 
using long flame of the burner to supply CO.sub.2 along with heat. Excess 
O.sub.2 in the flame is of course effective directly for the 
decarburization. Because the molten steel flows vigorously due to stirring 
by the injected gas, jetting CO.sub.2 and O.sub.2 which are oxidizing 
agents to the molten steel surface results in efficient absorption of both 
the heat and the oxidizing agents. 
As the decarburization proceeds and the C-content becomes lower, the rate 
determing step of the decarburization reaction changes from oxygen supply 
to carbon diffusion in the molten steel, and the oxidation loss of 
valuable metals in the molten steel, particularly, Cr, becomes 
significant. The border line is, though depending on the contents of Cr, 
Ni and Mn, and the size and shape of the furnace, approximately C:0.3%. 
Therefore, at the latter stage of the decarburization, it is advantageous 
to use a short flame so that only heat may be given to the molten steel 
and that neither CO.sub.2 nor O.sub.2 may be blown to the surface. 
EXAMPLES 
The present invention will now be illustrated with reference to the 
particular examples. 
EXAMPLE 1 
A refining apparatus of the structure shown in FIG. 1 was constructed. The 
apparatus comprises a furnace body 2 to contain molten steel 1 and a 
furnace lid 3 thereon. The furnace body 2 is equipped with a inclined 
tapping mouth 5 at the upper part thereof and a tuyere 4 for injecting 
O.sub.2 gas and Ar gas at the lower part thereof. A burner 6 for supplying 
heat to the molten steel surface is installed at the furnace lid 3. 
Molten steel 23 tons was charged in the furnace body 2, and refining of SUS 
304 steel was carried out in 4 stages. Using a molten steel of C-content 
0.8 weight %, the refining was carried out while stirring it by injecting 
O.sub.2 gas through the inner tube 4a of the tuyere 4, and simultaneously, 
Ar gas through the outer tube 4b, and while heating it by blowing flame 7 
from the burner 6 (burning capacity: heavy oil 22 liters/min.). 
For comparison, according to the known AOD process, a molten steel of 
C-content 1.5 weight % was subjected to the refining while it was stirred 
in the same manner as above, but without heating by the burner. 
The refinging patterns are shown in Table 1 and FIG. 2A and FIG. 2B. Also, 
the heat efficiencies in the first and second steps of the refining are 
shown in Table 2. In Table 2, the sum of the effective heat is the total 
heat necessary in molten steel temperature rise+melting the alloy+melting 
the slag. 
In this example, oxidation of Cr was suppressed to about a half of the 
control example. The heating efficiency by the burner 6 was assumed to be 
40%. 
TABLE 1 
______________________________________ 
Control Invention 
(without burner) 
(with burner) 
Period C-con- Period 
C-con- 
of tent in of tent in 
Refin- the mol- Refin- 
the mol- 
Refining ing ten steel 
ing ten steel 
Stages (min.) (wt %) (min.) 
(wt %) 
______________________________________ 
Before starting 
-- 1.5 -- 0.8 
1st and 2nd stages 
16 0.3 9(.DELTA.7) 
(0.3) 
3rd and 4th stages 
25 0.005 22(.DELTA.3) 
0.005 
Reduction of Cr-oxides, 
and adjustment of alloy 
composition 
Sampling and Slag-off 
22 -- 19(.DELTA.3) 
-- 
Charge 7 -- 7 -- -- 
Total 70 -- 58(.DELTA.12) 
-- 
______________________________________ 
TABLE 2 
__________________________________________________________________________ 
Control Invention 
(without burner) (with burner) 
(Refining period: 16 min.) 
(refining period: 9 min.) 
Heat Effective 
Heat Effective 
Generation 
Efficiency 
Heat Generation 
Efficiency 
Heat 
Heat Source 
(Mcal) 
(%) (Mcal) 
(Mcal) 
(%) (Mcal) 
__________________________________________________________________________ 
Oxidation of Cr 
1070 66.5 710 540 (66.5) 
360 
Oxidation of Si 
480 66.5 320 480 (66.5) 
320 
C CO 540 66.5 360 230 (66.5) 
150 
CO CO2 420 (66.5) 
280 180 (66.5) 
120 
burner -- -- -- 1800 (40.0) 
720 
Total 2510 66.5 1670 3230 51.7 1670 
__________________________________________________________________________ 
As shown in Table 1, it was possible, according to the invention, to lower 
the carbon content of the molten steel prior to the refining from 1.5 
weight % in the convertional method to 0.8 weight % in the example. Thus, 
it was realized to curtail the decarburization period of 7 minutes for 
decreasing the carbon content in the molten steel to 0.3 weight % after 
completion of the first and the second stages. 
Also, as seen from the comparison of FIG. 2A and FIG. 2B, loss of Cr due to 
the oxidation in the example of the invention (FIG. 2A) was less than in 
the conventional refining (FIG. 2B), and therefore, the period necessary 
for reduction of the Cr-oxides in the latter stages was about 3 minutes 
shortened as seen in Table 1. 
Further, less oxidation loss of Cr in the present invention enabled saving 
of the metallic Si, which was necessary for reducing the Cr-oxides, of 
about 3.5 Kg/ton-molten steel and curtailment in about 2 minutes of the 
period for removing the slag. The curtailment of the operation period of 
12 minutes in total assured a longer life of the refractory materials. 
EXAMPLE 2 
A refining apparatus as shown in FIG. 3 was constructed. The apparatus has, 
in addition to the structure as used in Example 1, a means for supplying 
the solid oxygen source to the molten steel surface at the burner 6. 
Molten steel 3000 kg prepared in an arc furnace was charged in the above 
furnace for refining SUS304 steel. The operation conditions were as 
follows: 
______________________________________ 
(Tuyere operation conditions) 
Former stage: Ar supply 900 Nl/min. 
O.sub.2 supply 
1800 Nl/min. 
Period 10 min. 
Latter stage: Ar supply 2000 Nl/min. 
O.sub.2 supply 
1000 Nl/min. 
Period 19 min. 
(Burner operation conditions) 
Fuel: Kerosin 1.0 1/min. 
O.sub.2 gas: 2.0 Nm.sup.3 /min. 
Solid oxygen source: 
Mill scale (350 mesh pass, 
4.0 kg/min.) 
______________________________________ 
Relations between the intermediate C-contents (averages of the carbon 
contents at the beginning and the end of the decarburization refining) and 
the rate of decarburization were plotted to give FIG. 4, in which the data 
of the known AOD process are also shown for comparison. 
The graph shows that use of the solid oxygen source according to the 
present invention promotes the decarburization reaction. 
The valuable metals such as Cr in the molten steel were oxidized in an 
amount as small as 75-65% of the amount in the conventional method, and 
the reducing agent (ferro-silicon) was saved at 3.5-4.5 kg/ton-steel. 
EXAMPLE 3 
A refining apparatus as shown in FIG. 5 was constructed. The apparatus has, 
in addition to the structure used in Example 1 and shown in FIG. 1, a 
means for injecting solid oxygen source 7 in the powdery form at the 
tuyere 4. 
Molten steel 3000 kg prepared in an arc furnace was charged in this furnace 
and subjected to refining for obtaining SUS 304 steel. 
In the relatively high carbon state (where the intermediate C-conten 
decrease from 0.6 to 0.2%) and in the relatively low carbon state (where 
the intermediate C-content decreases from 0.2 to 0.02%), chromite sand 
(350 mesh pass) or mill scale (the same) was used as the solid oxygen 
source and injected through the tuyere. The relations between the 
intermediate C-contents and the decarburization rates were plotted in 
comparison with the data according to the known AOD process to give FIG. 6 
(high carbon state) and FIG. 7 (low carbon state). 
For comparison, operations without burner were also carried out. 
The operation conditions are as follows: 
______________________________________ 
O2 Ar Solid Oxygen 
Reference 
(Nm.sup.3 /min) 
Source (kg/min) 
Burner Heating 
______________________________________ 
(Former stage of decarburization . . . FIG. 6) 
.DELTA. 1.8 0.9 chromite sand 4.0 
no 
1.8 0.9 chromite sand 4.5 
yes 
.quadrature. 
1.8 0.9 mill scale 3.2 
no 
1.8 0.9 mill scale 3.0 
yes 
O 1.8 0.9 -- no 
(Latter stage of decarburization . . . FIG. 7) 
.DELTA. 1.0 2.0 chromite sand 3.2 
no 
1.0 2.0 chromite sand 3.3 
yes 
.quadrature. 
1.0 2.0 mill scale 2.3 
no 
1.0 2.0 mill scale 2.3 
no 
O 1.0 2.0 -- yes 
______________________________________ 
The graphs of FIG. 6 and 7 show that show use of the solid oxygen source 
enhanced the decarburization. 
As to the temperature of the molten steel, the temperature changes 
accompanying decrease of C-content in the molten steel in the case where 
the mill scale was used as the solid oxygen source are shown in FIG. 8. 
The graph in FIG. 5 shows that, because the decarburization reactions by 
the solid oxygen sources are endothermic reactions, the temperature drop 
in the present method is much more than in the AOD process. On the other 
hand, the temperature drop substantiates that the decarburization by the 
solid oxygen source actually proceeds. Further, the graph of FIG. 8 proved 
that the same final refining temperature as that of the AOD process can be 
attained by using the burner heating according to the present invention. 
In the examples of this invention the valuable metals such as Cr in the 
molten steel were oxidized only to the extent of 60-70% of the 
conventional technology, and saving of the reducing agent (ferrosilicon) 
was 4-5 kg/ton-steel. In the cases where no burner heating was used, 
oxidation of the valuable metals was in the level of 80-70% of the 
conventional technology, and the reducing agent of 2-3 kg/ton-steel was 
saved. 
EXAMPLE 4 
A refining apparatus of the structure shown in FIGS. 9 and 10 as 
constructed. The burner has, as illustrated in FIGS. 11 and 12, a fuel 
nozzle 61 at the center and a slit-shaped primary oxygen nozzle 62 
surrounding the fuel nozzle, and a secondary oxygen nozzle 63 consisting 
of multiple holes distributed on a circle around the primary oxygen 
nozzle. The slits of the primary oxygen nozzle has a somewhat concentrical 
taper. 
Kerosin was fed to the burner. Oxygen in various amounts just necessary for 
burning the kerosin was also fed with different primary oxygen ratios. The 
flame length varied as shown in FIG. 13, which shows that, the higher the 
primary oxygen ratio, the longer the flame length is. Variation of the 
"M-value" under a constant primary oxygen ratio resulted in the tendency 
that, as shown in FIG. 14, at a higher M-value the flame length become 
somewhat shorter. 
The above refining apparatus received each 3 tons of molten steel prepared 
in an arc furnace, which was refined to produce SUS 304 steel in 
accordance with the present method or the known AOD process. 
The operation conditions are as follows: 
______________________________________ 
Burner operation 
Former Latter 
conditions Stage State 
______________________________________ 
Fuel supply (kerosin) 
2.01 l/min 1.0 l/min 
M-value 1.0 1.0 
Primary oxygen ratio 
0.2 1.0 
Flame length 140 cm 55 cm 
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Tuyere operation 
Present AOD 
conditions Invention Process 
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(Former Stage) 
Ar injection 900 Nl/min 900 Nl/min 
O2 injection 1800 Nl/min 
1800 Nl/min 
Period 10 min 22 min 
(Latter Stage) 
Ar injection 2000 Nl/min 
2000 Nl/min 
O2 injection 1000 Nl/min 
1000 Nl/min 
Period 19 min 19 min 
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Rise of the molten steel temperature and decrease of C-content in the 
molten steel during the former stage of the decarburization is shown in 
FIG. 15, and rise of the molten steel temperature and decrease of 
Cr-content in the molten steel during the latter stage of the 
decarburization is shown in FIG. 16. 
The graph in FIG. 15 shows that the decarburization according to the 
present method proceeds more quickly than in the AOD process. Because the 
molten steel temperature rises due to the burner heating, the present 
method can start at a carbon content lower than that used in the AOD 
process. Also, even in the latter stage of the decarburization, the burner 
heating helps the temperature rise, and this enables starting at a lower 
initial C-content and curtails the period of the former stage of the 
decarburization. 
The graph of FIG. 16 shows that the temperature rise is rapid also at the 
latter stage of the decarburization according to the present invention, 
and hence, even if the molten steel temperature at the beginning of the 
latter stage is low, it will reach a necessary level in the same period of 
time as that of the AOD process, and that the oxidation loss of Cr during 
the latter stage is approximately equal to that of the AOD process.