CA1340922C - Method of producing stainless molten steel by smelting reduction - Google Patents
Method of producing stainless molten steel by smelting reductionInfo
- Publication number
- CA1340922C CA1340922C CA000597362A CA597362A CA1340922C CA 1340922 C CA1340922 C CA 1340922C CA 000597362 A CA000597362 A CA 000597362A CA 597362 A CA597362 A CA 597362A CA 1340922 C CA1340922 C CA 1340922C
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- Canada
- Prior art keywords
- molten metal
- blowing
- slag
- inert gas
- outlet
- Prior art date
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21B—MANUFACTURE OF IRON OR STEEL
- C21B13/00—Making spongy iron or liquid steel, by direct processes
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
- C21C5/28—Manufacture of steel in the converter
- C21C5/30—Regulating or controlling the blowing
- C21C5/35—Blowing from above and through the bath
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Manufacturing & Machinery (AREA)
- Mechanical Engineering (AREA)
- Manufacture Of Iron (AREA)
- Carbon Steel Or Casting Steel Manufacturing (AREA)
Abstract
The invention is concerned with a method of producing stainless molten steel by smelting reduction in a smelting reduction furnace provided with a bottom tuyere, a side tuyere and a top lance having a first outlet and a second outlet. The method of the invention comprises the steps of a) reducing raw chromium ore in the furnace by use of carbonaceous material to produce chromium molten metal at the bottom of the furnace and having slag, above the molten metal; b) blowing CO and/or inert gas through the bottom tuyere into the molten metal to cause agitation thereof; c) blowing CO and/or inert gas through the side tuyere into the molten metal so as to contact the agitated molten metal; d) concurrently blowing decarburizing oxygen through the first outlet of the top lance into the molten metal and post-combustion oxygen through the second outlet of the top lance into the slag, so that the molten metal is decarburized; e) reducing chromium material by maintaining a ratio of post-combustion to more than 0.3; f) discharging the slag after completion of reduction; and g) after discharging of the slag, concurrently blowing a decarburizing oxygen diluted with an inert gas through one of the outlets of the top lance into the molten metal and blowing an inert gas through the bottom tuyere into the molten metal so as to agitate forcibly the molten metal. The invention enables one to carry out the smelting reduction at high treatment speed and the smelting reduction and decarburization in the same furnace.
Description
The present invention relates to a method of directly producing stainless molten steel from chromium ores or chromium ore pellets via a one smelting container consecutively.
High chromium steel such as stainless steel has been convent_~onally produced using ferrochromium as raw material.. In view of saving energy and securing low production costs, a so-called smelting reduction process has recently been considered as being potentially <~ttractive, which produces high chromium molten meta:L directly using chromium raw ores (hereinafter referred to as "Cr ore"). In the smelting reduction method, Cr ores, carbonaceous materials anal others are supplied in a reduction furnace for c~irect_Ly producing high Cr molten metal.
For the smelting reduction method, there have been several proposals hitherto. One blows respectively 02 from bottom tuyeres and N2 from side blowing tuye~=es at the same time as blowing 02 from a top lance. Another one blows respectively 02 from the bottom tuyeres and 02 and N2 from the side tuyeres at the same tine as blowing 02 from the top lance. The latter is disclosE:d, for example, in Japanese Patent Application :Laid-Open 61-279, 608 (1986) .
However, each of the above-mentioned methods has problems in that the Cr reduction speed is low and treatments take a very long time. With respect to the prior art, the following drawbacks are noted:
1. The reduction of Cr ore was increased by the action of C in carbonaceous materials after Cr ore had been molten in the slag, and the melting of Cr ore was assumed to determine the Cr reduction. Therefore, important technical interests for shortening the treatment. time were focussed on specifying C
High chromium steel such as stainless steel has been convent_~onally produced using ferrochromium as raw material.. In view of saving energy and securing low production costs, a so-called smelting reduction process has recently been considered as being potentially <~ttractive, which produces high chromium molten meta:L directly using chromium raw ores (hereinafter referred to as "Cr ore"). In the smelting reduction method, Cr ores, carbonaceous materials anal others are supplied in a reduction furnace for c~irect_Ly producing high Cr molten metal.
For the smelting reduction method, there have been several proposals hitherto. One blows respectively 02 from bottom tuyeres and N2 from side blowing tuye~=es at the same time as blowing 02 from a top lance. Another one blows respectively 02 from the bottom tuyeres and 02 and N2 from the side tuyeres at the same tine as blowing 02 from the top lance. The latter is disclosE:d, for example, in Japanese Patent Application :Laid-Open 61-279, 608 (1986) .
However, each of the above-mentioned methods has problems in that the Cr reduction speed is low and treatments take a very long time. With respect to the prior art, the following drawbacks are noted:
1. The reduction of Cr ore was increased by the action of C in carbonaceous materials after Cr ore had been molten in the slag, and the melting of Cr ore was assumed to determine the Cr reduction. Therefore, important technical interests for shortening the treatment. time were focussed on specifying C
the slag composition. However, the Cr ore was inhE:rentl:y less than molten, and it was lim~_ted to speed up the reduction by accelerating the melting of Cr ore.
2. For accelerating the melting speed of Cr ore in -the slag and the reduction speed, it was con~~idered to make a post-combustion of CO
gas in t:he furnace and utilize the heat produced thereby. A known method has been adopted, which blew 02 for the post-comx>ustion from an upper part of the furnace.
If the post-combustion ratio was increased, the temperature of an exhaust gas increased, but there has not been a technique which efficient:Ly transmitted a sensitive heat of the exhaust gas to the molten metal, and as a result, i~he heat transfer efficiency was lowered and the exhaust gas at high temperatu=re was inevitably removed. The heated exhaust gas considerably damaged the wal7_ refractories and those of the exhaust gas flue. Therefore, in general, it has been con:~idered that the post-combustion ratio cou7_d not be raised as much.
For producing stainless molten steel from the Cr raw ore efficiently and economically, it is of course desirable to continuously carry out a smelting reduction and a subsequent decarburization blowing in the same fur::~ace. However, the prior art has not made studies on a method that the decarburizing treatment was carried out ai=ter smelting reduction in the same furnace for i~he following reasons:
(1) A decarburization in a converter-type container caused considerable loss of Cr by its oxidation (hereinafter referred to as "Cr oxidation loss"), and therefore, although the smelting :reduction was carried out in the converter, the decarburization depended on a vacuum sy~,tem which was less in Cr oxidation loss, such. as the RH-OB system.
(2) The decarburization required much agitating gas but on the other hand, the existing smelting :reduction method did not need the agitating gas as much. Therefore; if the container of the same converter type was used, it :had been considered that a furnace for the ,melting reduction had to have a structure different from that of the decarburizing container.
(3) For carrying out the treatments from the smelting reduction to the decarburization in the same container, the slag generated by the smelting reduction had to be removed, but an ordinary electric furnace could not remove the slag.
2. For accelerating the melting speed of Cr ore in -the slag and the reduction speed, it was con~~idered to make a post-combustion of CO
gas in t:he furnace and utilize the heat produced thereby. A known method has been adopted, which blew 02 for the post-comx>ustion from an upper part of the furnace.
If the post-combustion ratio was increased, the temperature of an exhaust gas increased, but there has not been a technique which efficient:Ly transmitted a sensitive heat of the exhaust gas to the molten metal, and as a result, i~he heat transfer efficiency was lowered and the exhaust gas at high temperatu=re was inevitably removed. The heated exhaust gas considerably damaged the wal7_ refractories and those of the exhaust gas flue. Therefore, in general, it has been con:~idered that the post-combustion ratio cou7_d not be raised as much.
For producing stainless molten steel from the Cr raw ore efficiently and economically, it is of course desirable to continuously carry out a smelting reduction and a subsequent decarburization blowing in the same fur::~ace. However, the prior art has not made studies on a method that the decarburizing treatment was carried out ai=ter smelting reduction in the same furnace for i~he following reasons:
(1) A decarburization in a converter-type container caused considerable loss of Cr by its oxidation (hereinafter referred to as "Cr oxidation loss"), and therefore, although the smelting :reduction was carried out in the converter, the decarburization depended on a vacuum sy~,tem which was less in Cr oxidation loss, such. as the RH-OB system.
(2) The decarburization required much agitating gas but on the other hand, the existing smelting :reduction method did not need the agitating gas as much. Therefore; if the container of the same converter type was used, it :had been considered that a furnace for the ,melting reduction had to have a structure different from that of the decarburizing container.
(3) For carrying out the treatments from the smelting reduction to the decarburization in the same container, the slag generated by the smelting reduction had to be removed, but an ordinary electric furnace could not remove the slag.
(4) It took considerable time for the conven-tional smelting reduction method and dec~irburization method. Accordingly, if both treatment:; were performed in the same cont=ainer,, the whole treatment time was very lone and. lowered productivity, and the furnace refractory would be considerably dam<~ged so that the operation was made dif_~icult.
For dealing ~rith the above problems of the prior art, studies were made on the mechanisms of smelting reduction and decarburization, and the actual measures therefor, and subsequently the following facts were found:
(1) As stated above, it was assumed that Cr ore was reduced by the carbonaceous material remaining in the slag after the Cr ore had been molten in the slag, but instead, it was found that almost all of the actual reductions were made by the action of C as reducing material. Therefore, the reduction speed was determined by contacting the molten metal to C:r ore heated at a high temperature, not by me:Lting the Cr ore into the slag, so that the reduction speed could be effectively increased by positively contacting the molten metal with the ores.
(2) A basic concept in the prior art was that the post-combustion could not be largely increased in view of the technical limit with res~~ect to increasing the heat transfer effi.ciencv and consumption of the refractor_Les. If 02 was blown so that the post:-combustion mainly caused the slag to forcibly agitate, the heat transfer efficiency could be increased effectively.
Thu:~, by the high post-combustion and the high hE:at transfer efficiency, the temperatures of the slag and Cr ore in the slag were increased, and the reduction speed of Cr ore by C (in the molten metal) was rai:>ed effectively as shown by the formula:
Cr203 + C = Cr + C0.
(3) The foregoing technique sometimes carried out the bottom blowing of 02 for a certain period or for a full term, but such blowing was harmful to the post-combustion. For example, if 02 wa:~ blown from the bottom, CO gas was generated to a large extent in the molten X3409 2z metal and when the molten metal was agitated forcibly, and splashes of the molten metal rea<:hed t:he region of the post-combustion, and since C reacted with 02, then the post comx~ustion was hindered. Therefore, bottom blowing had to be avoided, regardless of a part: and the full term of the reduction period.
(4) From the above items (1) and (2), a certain strong agitation was necessary for performing the smelting reduction efficiently, and therefore,, it was possible to use the furnace of the same structure as the decarburizing container .
(5) If the dE~carburization was undertaken by a comx~ination of the top blowing and the bottom bloeaing agitation under predetermined conditions, it was possible to provide the dec~irburizing treatment effectively in a short period of time while checking Cr oxidation loss.
The present invention is based on the above-mentioned discoveries and provides a process for producing stainle~;s molten steel, which enables one to carry out the ~~melting reduction at high treatment speed and th.e smelting reduction and decarburization in the same :Furnace .
In accordance with the present invention, there is thus provided a method of producing stainless molten stee~_ by smelting reduction in a smelting reduction furnace provided with a bottom tuyere, a side tuyere ~~nd a top lance having a first outlet and a second outlet. The method of the invention comprises thE~ steps of:
For dealing ~rith the above problems of the prior art, studies were made on the mechanisms of smelting reduction and decarburization, and the actual measures therefor, and subsequently the following facts were found:
(1) As stated above, it was assumed that Cr ore was reduced by the carbonaceous material remaining in the slag after the Cr ore had been molten in the slag, but instead, it was found that almost all of the actual reductions were made by the action of C as reducing material. Therefore, the reduction speed was determined by contacting the molten metal to C:r ore heated at a high temperature, not by me:Lting the Cr ore into the slag, so that the reduction speed could be effectively increased by positively contacting the molten metal with the ores.
(2) A basic concept in the prior art was that the post-combustion could not be largely increased in view of the technical limit with res~~ect to increasing the heat transfer effi.ciencv and consumption of the refractor_Les. If 02 was blown so that the post:-combustion mainly caused the slag to forcibly agitate, the heat transfer efficiency could be increased effectively.
Thu:~, by the high post-combustion and the high hE:at transfer efficiency, the temperatures of the slag and Cr ore in the slag were increased, and the reduction speed of Cr ore by C (in the molten metal) was rai:>ed effectively as shown by the formula:
Cr203 + C = Cr + C0.
(3) The foregoing technique sometimes carried out the bottom blowing of 02 for a certain period or for a full term, but such blowing was harmful to the post-combustion. For example, if 02 wa:~ blown from the bottom, CO gas was generated to a large extent in the molten X3409 2z metal and when the molten metal was agitated forcibly, and splashes of the molten metal rea<:hed t:he region of the post-combustion, and since C reacted with 02, then the post comx~ustion was hindered. Therefore, bottom blowing had to be avoided, regardless of a part: and the full term of the reduction period.
(4) From the above items (1) and (2), a certain strong agitation was necessary for performing the smelting reduction efficiently, and therefore,, it was possible to use the furnace of the same structure as the decarburizing container .
(5) If the dE~carburization was undertaken by a comx~ination of the top blowing and the bottom bloeaing agitation under predetermined conditions, it was possible to provide the dec~irburizing treatment effectively in a short period of time while checking Cr oxidation loss.
The present invention is based on the above-mentioned discoveries and provides a process for producing stainle~;s molten steel, which enables one to carry out the ~~melting reduction at high treatment speed and th.e smelting reduction and decarburization in the same :Furnace .
In accordance with the present invention, there is thus provided a method of producing stainless molten stee~_ by smelting reduction in a smelting reduction furnace provided with a bottom tuyere, a side tuyere ~~nd a top lance having a first outlet and a second outlet. The method of the invention comprises thE~ steps of:
a) reducing raw chromium ore in the furnace by use of carbonaceous material to produce chromium molten metal at the bottom of the furnace and having slag above th.e molten metal;
b) blowing CO and/or inert gas through the bottom tuyere into the molten metal to cause agitation thereof;
c) blowing CO and/or inert gas through the side tuyere into the molten metal so as to contact the agitated molten met=al;
d) concurrent:Ly blowing decarburizing oxygen through the first outlet of the top lance into the molten metal and post-combustion oxygen through the second outlet. of t:he top lance into the slag, so that the molten mE:tal is decarburized;
e) reducing chromium material by maintaining a ratio of post.-combustion to more than 0.3;
f) discharging the slag after completion of reduction; and g) after discharging of the slag, concurrently blowing a decarburizing oxygen diluted with an inert gas through one of: the outlets of the top lance into the molten m~stal and blowing an inert gas through the bottom tuyere into the molten metal so as to agitate forcibly the molten metal.
In a preferred embodiment of the invention, the decarburizin~~ oxygen and post-combustion oxygen are blown respectively through the first and second outlets of the tap lance positioned close to the surface of the slag. Preferably, the decarburizing oxygen is b:Lown through the first outlet in a flow located inside the flow of post-combustion oxygen blown through the second outlet.
According to another preferred embodiment, step (f) is carried out: by tilting the furnace so that the C
134Q9 2~
side tuyere faces downward after completion of the reduction so as to remove the slag while blowing gas through the side tuyere. After the slag is discharged, inert gas is blown through the bottom tuyere in an amouni~ of more than 0.5 Nm3/molten metal Ton-min, preferably more than 1 Nm3/molten metal Ton-min.
Accordir..g to a further preferred embodiment, the ratio of decarburizing oxygen and diluting inert gas in step (g) is changed by gradually reducing the amount of decarburizing oxygen while gradually increasing the amount of inert gas during the blowing of the decarburizing oxygen and diluting inert gas.
Further features and advantages of the invention will become :more readily apparent from the following description of preferred embodiments, reference being made to the accomp<~nying drawings, in which:
FIG. 1 :is a schematic view illustrating a method according to the invention;
FIG. 2 is another schematic view showing the smelting reduction of the invention;
FIGS. 3(a) and (b) show the preferred jetting directions of the gas from the side tuyere(s) with respect to the bottom blowing tuyere(s);
FIG. 4 is a diagram showing post-combustion ratios measL.red with respect to the method of the invention, compared with a known method of 02 blown from the bottom;
FIG. 5 is a diagram showing the relationship between the heighi~ of the top blowing lance and the heat transfer efficiency;
FIG. 6 is a diagram showing the relationship between the amount of the gas blown from the side tuyere and the heat transfer efficiency;
_ g _ FIG. 7 is a diagram showing the relationship between the post combustion in the furnace, S and P
in the molten metal and unit consumption of coke;
FIG. 8 is a diagram showing the relationship of Cr increasing rate with respect to the amount of gas blown from the bottom according to the invention;
FIG. 9 is a diagram showing the relationship between the amount of bottom blowing gas and Cr oxidation lo;~s;
FIG. 1C~ is a diagram showing changes as time passes, of C con~~entration, Cr concentration, bath temperature, post-combustion ratio, amount of oxygen blown from t:he lance, amount of gas blown from side tuyere, and amount of raw materials supplied;
FIG. 1:1 is a diagram showing the reduction treatment time in comparison with conventional methods;
FIGS. 12(a) and (b) are schematic views showing the treatmeni~s of conventional methods of FIG. 11;
FIGS. 7_3 is a diagram showing the supplying speed of Cr and Cr increasing rate in comparison with a conventional method;
FIG. 14 is a diagram showing the relationship between the decarburizing level and the Cr oxidation loss according to the invention;
FIGS. 15 to .L'7 are schematic views illustrating the dischargE~ of the slag according to the invention;
FIG. 18 is a diagram showing the gas blowing conditions in the decarburizing treatment according to Example 1 hereinbelow;
FIG. 19 is a diagram showing influences of the slag amount to Cr oxidation loss in the decarburizing treatment;
C
FIG. 20 is a diagram showing influences of the amount of Ar gas blown from the bottom to denitriding rate during :~lr rinsing; and FIG. 21. is <~ diagram showing the relationship between the C content in the molten metal and the N
content in the :molten metal after finishing the blowing when changing the diluting gas blown from the top lance and the bottom blowing gas from N2 to Ar.
Fig. 1 show; schematically the method of the invention.
A smelting reduction furnace of a converter type is used for carrying out the smelting reduction and the decarburizing treatment. The furnace has a bottom blowing tuyere 1, a side blowing tuyere 2 and a top blowing lance 3.
According to the method of the invention, Cr raw materials ~>uch as Cr ores, Cr ore pellets, carbonaceous materials and fluxes are supplied to a metal bath supported in the smelting reduction furnace, anc~ the reducing treatment is performed under the fo:Llowing conditions.
Gas blowing by the bottom tuyere 1 and gas blowing by t:ze side tuyere 2 cooperate to diffuse the molten metal into the slag and greatly increase the reduction spE~ed.
As staged a~>ove, it has been found that the reduction of Cr ore in the slag progressed by C in the molten rrietal as the main reducing substance, by which the molten metal was agitated forcibly to diffuse positively into the slag (range where Cr ores float) so as t:o increase the reducing rate.
Therefore, according to the invention, the agitating gas is supplied from the bottom blowing tuyere 1 to form an uphe;~ving part A on the metal surface. At the same time, the agitating gas is supplied from the ~34D9~~2 side tuyere 2 so that at least a portion of the gas contacts the upheaving part A, whereby the part A is splashed into the slag. An apparent specific gravity of the slag is generally 0.3 to 0.5 wt.o, and the bulk specific gravity of the Cr ore is about 3.0 wt.~. Theref~~re, Cr ore in the slag gather around the lower region of the slag and float there, as seen in Fig. 2. If the upheaving part of the molten metal is splashed by the side blowing gas, the splashed molten metal is diffused around the lower region of the slag where Cr ores exist, as shown in Fig. 2, and C in the splashed molten metal reduces Cr203, and thus high reducing speed may be achieved. For providing such effects, it is of course necessary that the agitation must be forcibly caused by blowing more gas from both the bottom tuyere and the side tuyeres, and the amount of gas to be blown is determined by the amount and the depth of the molten metal. FIG. 8 shows the relationship between the amount of gas blown (Nm3/min/ton of molten metal/one tuyere) from the bottom tuyere and Cr increasing rate in the molten metal, from which it is seen that the Cr increasing rate, that is, the Cr reducing rate is increased in accordance with the increased amount of bottom blowing gas, and an effective reducing reaction takes place.
For obtaining such behavior, it is preferable that the si~~e blown gas blows exactly against the upheaving part A in both the vertical and horizontal directions. For example, it is desirable that the bottom blowing tuyere or tuyeres 1 and the side blowing tuye:re or tuyeres 2 be positioned relative to one another, as shown in Figs. 3(a) and (b).
The side tuyere blowing gas also agitates the slag where t:he post-combustion region is formed, in C
addition to the diffusion of the molten metal as mentioned above.
The side tuyere blowing gas and the bottom blowing gas i.o be employed are limited to CO and the inert gas (I~f2 or Ar) , and 02 is not used for the following reasons.
If 02 is used for side blowing, it reacts with C
in the molten metal splashed for reducing Cr ore, and the reduction by C in the molten metal would be hindered. In addition, if 02 is used, the temperature of the refractory becomes too high and the refractory may be damaged.
Further, if C>2 is used for bottom blowing, too much CO gas is caused in the molten metal and the molten metal becomes very agitated. As a result, the splash reaches the post-combustion region (see Fig.
2 ) , and C in the molten metal reacts with the above-mentioned 0~; for the post-combustion so that the post-combust=ion is hindered. However, if 02 is used, a cooling gas (such as C3Hg) must be added for the refractory whose temperature rises too much, so that the bottom blowing gas is increased and splashes of the molten metal by the compulsive agitation are accelerated. FIG. 4 shows the investigations of the post-combustion ratio actually measured to the post-combustion ratio (PC02/(DC02+02 in ores)) determined in respect of the bottom blowing N2 according to the invention and the bottom blowing 02 according to the comparative example, instead of N2. It is seen that the post-combustion is hindered by 02 blown from the bottom.
CO as the agitating gas and N2 and Ar as the inert gas may be used independently or in admixture.
The dec:arbur:izing 02 is blown into the molten metal from i=he top lance 3, and concurrently 02 for .e the post-comr~ustion is blown into the slag. The top blowing lance 3 i~> provided with a first outlet for the decarburi.zing 02 and a second outlet for the post combustion 0~>. The decarburizing 02 is blown through the first outlet in a flow located inside the flow of post-combustion 02 blown through the second outlet.
A high post-~~ombustion is provided, while the post-combustion region is formed mainly within the slag. The past-combustion region is thus formed in the slag, ~~.nd the slag is forcibly agitated by blowing the gas from the side tuyere, whereby high heat transfer efficiency is provided while the high post-combustion i,s secured. Therefore, the post-combustion O~~ must: be blown into the slag such that the post-comx~ustion is formed in the slag.
It is necest~ary that the height of the top blowing lance be determined at an appropriate level with respect to the levels of the slag or the molten metal. That is, the second outlet of the top blowing lance 3 can be positioned above the slag surface or thereunder. If it is too high, the post-combustion region would not be formed in the slag so that the heat transfer efficiency is lowered. On the other hand, if it is too low, the post-combustion region would not be properly formed.
Fig. 5 shows the relationship between the height of the free end of the lance from the slag surface and the heat transfer efficiency, wherein, if the lance is too f:ar above the slag surface, a satisfactory heal: transfer efficency cannot be obtained. Fig. 6 shows the relationship between the amount of gas blown from the side tuyere and the heat transfer efficiency, wherein the required heat transfer can be obtained by blowing a large amount of C
gas from the side tuyere so as to forcibly agitate the slag.
The post-combustion ratio is defined as:
(C02 + H~ O) / (CO + C02 + H2 + H20) which is the gas content bearing on an exhaust gas.
According to the invention, the above-mentioned reducing treatment. is performed at a post combustion ratio of more than 0.3. Since high heat transfer efficiency can be effected, and if the post-combustion ratio is increased as stated above, a high reducti~~n treatment ability (reducing speed) can be provided. In addition, if the post-combustion ratio is increased, the additional amount of the carbonaceous material (mainly coal) can be decreased.
Consequently,, an initial amount of carbonaceous material may be decreased, and because almost all of the P components in the molten metal are brought by the carbonaceous material, P in the molten metal can be lowered. If the post-combustion ratio is increased, a gaseous desulfurization vigorously takes place, and ~' in the molten metal is also decreased.
Accordingly, the post-combustion ratio should be more than 0.3. Fiq. 7 shows the relationship, in the smelting reduction, between the post-combustion ratio, the initial amount of coke, and P and S
contents in the molten metal, wherein if the post-combustion ratio i.s maintained to more than 0.3, the initial amount of coal is controlled, and P and S are decreased appropriately.
The slag is discharged, after having accomplished the smelting reduction, and the decarburizat_ion blowing is carried out consecutively in the same :Furnace under the following conditions:
(1) 02 is supplied exclusively through the top blcwing 7_ance 3, and not through the furnace bottom;
(2) 02 suppl_Led is not pure but is diluted with an inert gas; and (3) the inert gas is supplied from the bottom blowing t:uyeres 1.
The known AO:D process employs blowing from the side of the furnace and studies have found that 02 was a significant cause of the Cr oxidation loss.
That is, since the static pressure is added in the 02 bottom blowing, the CO partial pressure is increased and consequently the decarburizing reaction is hindered, and the decarburizing 02 oxidizes Cr. For this reason, in the method of the invention, 02 is not blown from the furnace bottom, but supplied from the top blowing lance 3.
It was found that, if the top blowing were merely made with 02, the Cr oxidation loss could not be avoided. This is because the decarburizing reaction takes place most vigorously around a fire point made by blowing oxygen from the top lance, but with only 0~, causing CO partial pressure to become very high, at that point, and the decarburizing reaction is obstructed and 02 oxidizes Cr. Thus, in the method of the invention 02 diluted with an inert gas (N2 or ~~r) is blown from the top lance, thereby decreasing CO partial pressure around the fire point and accelerating the decarburizing reaction. It is preferable to supply a large amount of gas from the top blowing :Lance for shortening the treatment time.
Further,, in accordance with the invention, inert gas (N2 or A.r) is blown from the bottom tuyeres 1 to forcibly agitate t:he molten metal and accelerate the mixture of the molten metal and 02 from the top f~~.-. ~a 'k~~,...~
~3~09 zz lance, and ;gin efi=ective decarburization is possible with checking the Cr oxidation loss by a combination of the compulsive agitation of the metal by the inert gas blown from the bottom and the top blowing of 02 diluted with an inert gas.
For forcibly agitating the molten metal, it is necessary to blow a considerable amount of inert gas.
Actually, for decreasing the Cr oxidation loss to not more than 1°., it is necessary to blow the inert gas in an amount of more than 0.5 Nm3/ton-min. (ton-min.:
every minute per 1 ton of molten metal), and for decreasing t:he Cr oxidation loss to not more than 0.5%, it i;s necessary to blow the inert gas in an amount of more than 1 Nm3/ton-min: If the amount of gas is too considerable, the molten metal would be splashed. Therefore, the gas is blown in an amount of not more than 0.5 to 5 Nm3/ton-min, preferably 1 to 3 Nm3/ton-min. Fig. 9 shows the relationship between the amount of bottom blowing gas and the Cr oxidation loss. 02 is used effectively in the decarburizing reaction by :blowing considerable gas from the bottom, and the Cr ~~xidation loss is appropriately checked.
As a comparison, Fig. 9 shows cases of existing methods, and in the AOD, for example, the Cr oxidation loss is quite substantial with respect to the amount o:E bottom blowing gas.
It is :referable to supply a large amount of oxygen from the top lance for shortening the treatment time.
The high Cr molten metal to be treated by the method of the invention is produced by melting ferrochromium or by a so-called direct molten reduction.
According to the invention, the entire process from the production of Cr molten metal to the production of stainless steel may be performed in the same container rationally and at high productivity by decarburizing Cr molten metal as mentioned above, which can be produced by any of the above-mentioned processes.
In the above decarburization blowing, for avoiding the Cr oxidation loss, it is effective to reduce the amount of oxygen supplied while decreasing the C level. However, in supplying oxygen from the top blowing lance, it is limited in view of lowering the blowing ;pressure to reduce the amount supplied in the same no~:zle, and the amount supplied is reduced to about one-half the maximum.
In dealing with the above-mentioned problem, it is preferable to gradually increase the amount of inert gas cLuring the blowing of decarburizing 02 while gradually reducing the amount of decarburizing 02, thereby enabling reduction of the oxygen supply without considerably lowering the blowing pressure.
Increasing of the amount of inert gas and reducing the amount of oxygen suppled may be accomplished successively or stepwise. With respect to the gas :blowing, for example, the amount of gas (02+N2 or Ar) blown from the top lance is determined to be always 3 Nm~3/ton-min, and the amount of oxygen supplied is reduced as follows, in response to C
level.
C: more than 3 wt.% .... 3 to 4 Nm3/ton-min.
C: less than 3 wt.%
to 2 wt.% .... 2 to 3 Nm3/ton-min.
C: less than 2 wt.o to 0.5 wt.o ... 1 to 2 Nm3/ton-min.
C: less than 0.5 wt.o... 1 Nm3/ton-min.
C in the molten metal during gas blowing can be determined i:n known manner by integrating the amount c 13409 2~
of oxygen or measuring the solidifying temperature of the molten metal.
When the invention is actually practised, the following processes are dealt with.
Charging - slagging . rising temperature -smelting reduction of Cr ores - discharging slag - ~~ecarburizing - tapping.
The charging process herein means to charge Fe sources such as i~he molten metal and form a metal bath in the furxlace. In the slagging and rising temperature proce~,s, the oxygen is supplied into the bath, and the coal materials and the flux are charged to the molten metal so as to form the slag for reducing Cr ores, and the temperature of the bath is raised suffi.cientT_y for the reduction to occur. In the smelting reduction, the Cr ores, the carbonaceous materials an~3 the flux are supplied in succession. At the end of this process, a finishing reduction is carried out without supplying the Cr ore; and the reducing treatment is accomplished when the Cr concentration reaches a desired value.
The following non-limiting examples illustrate the invention.
F'YZ1MDT.F' 1 3.7 t of molten metal were charged into a smelting reduction furnace of the converter type, and the smelting reduction was performed by supplying Cr ores, coke and f=lux, and 5.5 t of molten metal containing 18 wt.% Cr were produced. Subsequently, the decarburization blowing was carried out to produce stainless molten metal. Fig. 10 shows the changes of C:r and C concentrations in the metal, bath temperature and t:he post-combustion ratio OD, the amount of 02 supplied and the amount of raw material charged.
Fig. 11 shows the smelting reduction time (from start to finish) i.n the present example in comparison with the treatment time of conventional practices shown in Figs . 12 and (b) . Conventional method ( ( a) 1 ) blows fine powder coal and 02 from a top lance, and agitating ga.s from bottom tuyeres, and conventional method (2) blows 02 onto the slag and N2 from bottom tuyeres as well as N2 and 02 from side blowing tuyeres under the following conditions.
Conventional method (1) Top blowir..g 02 1700 Nm3/Hr (Finishing reduction period) Bottom blowing rd2 350 Nm3/Hr (Finishing reduction period) Molten metal 10 Tons Cr ore 4600 Kg (injected from lance) Coal materials 6700 Kg (injected from lance) Conventional method (2) Top blowing 02 1000 Nm3/Hr (Finishing reduction period) Bottom blowing N2 120 Nm3/Hr (Finishing reduction period) Lateral blowing N2 350 Nm3/Hr (Finishing reduction period) Molten metal 5 Tons Cr ore (powder) 5000 Kg (onto the slag) Coal materials 3200 Kg (onto the slag) The Cr concentration of conventional method (2) was only about 6 too 7 wt. o as shown in Fig. 11, while the Cr concentration of conventional method (1) reached the objective of 1.8 wt. o as shown in Fig. 12, but the treatment took 7.20 minutes. On the other hand, according t~o the invention, the Cr concentration reached 18 wt.o in 60 minutes, that is, in half the C
time of conventional method (1), which shows the excellent treatment function of the present invention.
Fig. 13 shows the Cr increasing speed with respect to the Cr supplying speed (supplying speed of Cr ore calculated as the Cr amount) according to the invention, revealing the high Cr increasing speed in comparison with the above methods (1) and (2).
In this example, C was decarburized from 6.7 to 0.038 wt.o in about 40 minutes. As is seen, although the decarburization went down to a low carbon range, the Cr oxidation loss was as low as about 0.5 wt. o.
The invention was practised by changing the decarburizin~~ level (under almost the same conditions as in Fig. 10), and the relationship between the decarburizin~~ level and the Cr oxidation loss was studied. The results are shown in Fig. 14 in comparison to the conventional processes (AOD and ZD-OB), and it can be seen that, the Cr oxidation loss was well controlled to be low in the low carbon region.
In the method. which consecutively carries out the smelting reduction and the decarburization in the same furnace, the amount of slag generated in the smelting reduction must be removed before the decarburizing treatment.
The slag is removed from the furnace mouth by tilting the furnace, but the slag around the furnace bottom is not easily removed. If it were removed forcibly, the molten metal would flow out and a large amount of yield would be lost.
In the method of the invention, the furnace is tilted so that the side blowing tuyere faces downward, and the slag is removed while the gas is blown from the side nozzle.
Fig. 15 shows slag discharge. Fig. 15(I) shows that the smelting reduction is finished where A is C
the metal and B is the slag. The furnace body is tilted so that the side blowing tuyere 2 faces downward as seen i.n Fig. 15 ( II ) and Fig. 15 ( III ) in order to dig>charge the slag B while blowing the gas (the inert gas such as N2) from the side blowing tuyere 2. In this way, the level of the slag on the furnace bottom is increased, so that only the slag is released smo~~thly and easily from the furnace mouth.
Figs. 1.6 and. 17 show examples of the furnace more smoothly discharging the slag. Fig. 16 shows the furnace body having an opening 4 at the lower part of the furnace mouth. Fig. 17 shows that a weir 5 is provided at the i=urnace mouth to keep the metal A
from escapin~~.
According to the above embodiments, only the slag may be disch<~rged. When the furnace is inclined causing the slag t=o flow out naturally, the residual slag amount is 40 to 50 Kg/molten metal Ton and the amount of metal .removed together with the slag is about 10 Kg/molten metal Ton. On the other hand, depending on the present invention, the amount of residual slag is 7_ess than 5 Kg/molten metal Ton and the amount of molten metal removed is less than 1 Kg/molten metal Ton.
The conventional decarburization blowing was performed under conditions to form a large amount of slag, and the influences of the amount of slag to the Cr oxidation loss have not as yet been carefully and quantitative:Ly studied.
On the other hand, Applicant has studied the relationship betwe>.en the amount of slag and the Cr oxidation loss, paying close attention to the slag to be formed. As a result, it was found that a relationship exists between the amount of slag and the Cr oxidation loss during decarburizing blowing, and if the blowing was done by controlling a low amount of s7_ag, that is, not more than 50 Kg/molten metal Ton, the Cr oxidation loss could be effectively lowered.
Further, it is assumed that the Cr oxidation loss is lowered by controlling the amount of slag within the above range for the following reasons . 02 blown from the top lance causes the following reactions:
C (molten metal) +..1/2 02 ~ CO (gas) (1) 2Cr (mo.lten metal) + 3/2 02 -~ Cr203 (slag) (2) From t:he above reactions (1) and (2), the following re~~ction (3) was derived:
Cr203 (;slag + 3C (molten metal) 2C:r (molten metal) + 3C0 (gas) (3) Cr203 generated by the top lance blowing 02 is reduced by C in the molten metal.
It is important to increase the concentration of Cr203 in the slag so that the reduction according to reaction (3) progresses to the right side. For increasing the Cr,203 concentration, it is necessary to decrease the amount of the whole slag so that the reaction (3) is easily effected, and as a result, the reduction of Cr20:, is accelerated and the Cr loss is effectively decreased. In addition, the furnace refractory comprises Mg0 (magnesium chromium, magnesium carbon or magnesium dolomite), and the slag contains 10 to 30 wt.~ of Mg0 by melting it. Since Mg0 combines with Cr203 and generates less fusible Mg0.Cr203 sp:inel, and if the amount of slag is great, the Cr203 concentration in the slag is lowered and reduction is difficult. The lowering effect of the Cr oxidation lcss by lowering the amount of slag was most remarkable when the blowing was done with an amount of sl<~g not more than 50 Kg/molton metal Ton.
C
1~~09 22 To practise t:he present method, the less Si and S contents in the molten steel to be decarburized, the more advantageous is the controlling (lowering) of the amoL.nt of slag. For example, during basic decarburization of the invention, more of the reducing agent such as Fe-Si can be controlled and kept low, so that the amount of slag can be easily controlled.
DECARBURIZING TREATMENT Example 1 5.5 t of molten metal containing 18 wt.o Cr were decarburized in the above-mentioned furnace in accordance with i~he different levels of the slag amounts and gas blowing amount. The decarburization was carried out by blowing decarburizing 02 diluted with N2 gas from the top lance and blowing N2 gas from the bottom tu.yeres and C in the molten metal was decreased fr~~m 6.5 wt.o to 0.03 wt.o in about 40 min.
Fig. 18 shows the amount of the blown gas in this operation.
Fig. 19 shows the relationship between the amount of slag and the Cr oxidation loss obtained in this treatment. The Cr oxidation loss became low as the amount of slag was lowered. When the amount of slag was <_ 50 kg/molten metal ton (preferably, <_ 40 kg/molten mE~tal ton), the Cr oxidation loss was remarkably lowered.
Applicant has studied the denitrification of the molten steel when low nitrogen stainless steel was produced, on a premise that N2 was used as the agitating gas during decarburization, and found that it was very effective to the nitrification of the molten metal to add a deoxidizer such as Fe-Si or Al after the decarburization for carrying out the rinsing treatment by blowing a large amount of Ar from the boti~om tuyeres.
rv_. ~3~0,9.~~
In general, Fe-Si or A1 are added to the molten metal for deoxidation and Cr reduction in the slag after decarburization, and in this regard, the agitation is practised by blowing Ar from the bottom during addition of the deoxidizer, whereby N is removed frorl the molten steel in addition to the above-mentioned Cr reduction and deoxidation.
Therefore, N can easily escape during deoxidation of the molten steel (70 - 150 ppm - less than 50 ppm) by adding Fe-Si or A1, and when molten steel is agitated with Ar, N ins more easily removed.
The bottom blowing of Ar, as mentioned above, is ordinarily performed with 0.5 to 5 Nm3/min.-molten steel ton, ~~referably 1 to 3 Nm3/min.-molten metal ton, for 5 t« 10 minutes.
For further .Lowering N in the molten steel, in addition to the denitrification rinsing treatment mentioned above, _Lt is preferable to use Ar gas for diluting the decarburizing 02, although N2 is used as the bottom blowing gas in decarburizing blowing.
Since N-absorption is most vigorous around the fire point of the lance, and if N2 is used as the dilution gas, a large amount of N is molten into the steel. In this method, since Ar is more expensive than N, Ar is used only as the dilution gas which is sufficient in a small amount to check the increase in the nitrogen concentration. After decarburizing blowing, the above nitrificatian is carried out.
Applicant has found that, with respect to N in the molten stee7_, while the decarburization is vigorous, N is l.ow, and when the decarburization progresses and the decarburizing speed is low, N
becomes remarkably higher. This is because CO gas generated by thE: decarburization absorbs N and releases it.
~,_:, _. ~3~09 2~
The higher the C concentration is in the steel, the faster i~he de:carburizing speed. When a dilution gas is used for diluting the decarburizing 02, N2 gas is preferably used. at first, and when C is low during decarburization, the dilution gas is changed from N2 to Ar and the decarburization is continued so that the producti~~n cost can be appropriately lowered.
It is preferable to change the dilution gas of the decarburizating 02 and the bottom blowing gas from N2 to ~~r in accordance with the amount of C in the molten tsteel, as shown in Fig. 21, the changing of N2 to Ar is desirable in the range of 0.8 to 2.0 wt.% of C in the molten steel. If the change is made too soon, too much. Ar gas is required. Therefore, the change should be made when C is below 2.0 wt.o. On the other h<~nd, i.f the change is made too late (C
concentration is t=oo low), as shown in Fig. 21, the denitrificat:ion is unsatisfactory. Therefore, the change should be made when C is above 0.8 wt.°s.
DECARBURIZINc.; TREATMENT Example 2 The high Cr molten metal was decarburized in steps (A) to (E) mentioned below in a furnace having a top blow_~ng lance and bottom blowing tuyeres, followed by Ar rinsing (Fe-Si supply + Ar bottom blowing), and stainless molten steel containing 18 wt.% Cr and 0.05 wt.% C was produced.
(A) Decarburi.zing blowing "op blowing gas: 02 + N2 (Dilution) Bottom blowing gas: N2 (2 Nm3/min.-molten steel ton) Ar rinse ~3ottom blowing gas: Ar (0.1 Nm3/min.-molten metal ton) (B) Decarburizing blowing Top blowing gas: 02 + N2 (Dilution) C
W 13409 2~
l3ottom blowing gas: N2 (2 Nm3/min.-molten steel ton) Ar rinse l3ottom blowing gas: Ar (0.5 Nm3/min.-molten metal ton) (C) De~~arburizing blowing 'Cop blowing gas: 02 + N2 (Dilution) Bottom blowing gas: N2 (2 Nm3/min.-molten steel ton) Ar rinse Bottom blowing gas: Ar (1 Nm3/min.-molten metal ton) (D) Decarburi.zing blowing Cop blowing gas: 02 + N2 (Dilution) Bottom blowing gas: N2 (2 Nm3/min.-molten steel ton) Ar rinse F3ottom blowing gas: Ar (2 Nm3/min.-molten metal ton) (E) Decarburi.zing blowing ~_'op blowing gas: 02 + Ar (Dilution) ~3ottom blowing gas: N2 (2 Nm3/min.-molten steel ton) Ar rinse F3ottom blowing gas: Ar (2 Nm3/min.-molten metal ton) Fig. 20 shows the influences of the amount of gas blown from the bottom to the denitrifying speed during Ar r:insing. The molten metal is denitrified effectively by Ar rinsing. In step (D), where the amount of Ar gas is 2 Nm3/min.-molten steel ton, the denitrificat_~on reaches an objective value of N: 500 to 600 ppm for a rinsing time of 4 to 5 min. In step (E) , where Ar is used for diluting the decarburizing 02, N concentration is about 1000 ppm which is half of steps (~~) to (D) when the decarburization is completed, and the=refore the denitrification reaches its objecti~Te value when the Ar rinsing is for a shorter peri~~d of time.
DECARBURIZING TREATMENT Example 3 High Cr molt:en metal was decarburized in a furnace having a top blowing lance and bottom blowing tuyeres, in accordance with steps (a) and (b) hereinbelow, followed by Ar rinsing (Fe-Si supply +
Ar bottom blowing), and . stainless molten steel containing 18 wt.% Cr and 0.05 wt.o C was produced.
a) Decarburizing blowing Top blowing gas: N2 was used for diluting the decarburizing 02 at the beginning of the decarbur ization, and changed to Ar in accordance with values of C
in the molten steel during decarburizing blowing.
Ar rinse Ar supp:Ly: 2 Nm3/min.-molten steel ton for 5 minutes.
b) Decarburizing blowing Top blowing gas: N2 was used for diluting the decarburizing 02 and the bottom blowing gas at the beginning of the decarburiza tion, and changed to Ar in accordance with values of C
in the molten steel during decarburizing blowing.
Ar rinse Ar supp:Ly: 2 Nm3/min.-molten metal steel ton for 5 minutes.
C
Fig. 21 shows the influence of N in the molten steel when changing the type of gas for the dilution gas of the decarburizing 02 and the bottom blowing gas.
Table I shows the concentrations of N in the steel after the Ar rinsing (changing of N2 - Ar during decarburizin~~ blowing was done at C in the molten steel - 1 wt. o), ,end it. can be seen that low N stainless steel containing less than 200 ppm of N is easily produced according to the invention.
TABLE I
N in the molten N in the molten steel after steel after Ar decarburizing rinsing blowing (a) 500 ppm 130 ppm -( b 15 0 ppm ~ 5 0 ppm ) y
b) blowing CO and/or inert gas through the bottom tuyere into the molten metal to cause agitation thereof;
c) blowing CO and/or inert gas through the side tuyere into the molten metal so as to contact the agitated molten met=al;
d) concurrent:Ly blowing decarburizing oxygen through the first outlet of the top lance into the molten metal and post-combustion oxygen through the second outlet. of t:he top lance into the slag, so that the molten mE:tal is decarburized;
e) reducing chromium material by maintaining a ratio of post.-combustion to more than 0.3;
f) discharging the slag after completion of reduction; and g) after discharging of the slag, concurrently blowing a decarburizing oxygen diluted with an inert gas through one of: the outlets of the top lance into the molten m~stal and blowing an inert gas through the bottom tuyere into the molten metal so as to agitate forcibly the molten metal.
In a preferred embodiment of the invention, the decarburizin~~ oxygen and post-combustion oxygen are blown respectively through the first and second outlets of the tap lance positioned close to the surface of the slag. Preferably, the decarburizing oxygen is b:Lown through the first outlet in a flow located inside the flow of post-combustion oxygen blown through the second outlet.
According to another preferred embodiment, step (f) is carried out: by tilting the furnace so that the C
134Q9 2~
side tuyere faces downward after completion of the reduction so as to remove the slag while blowing gas through the side tuyere. After the slag is discharged, inert gas is blown through the bottom tuyere in an amouni~ of more than 0.5 Nm3/molten metal Ton-min, preferably more than 1 Nm3/molten metal Ton-min.
Accordir..g to a further preferred embodiment, the ratio of decarburizing oxygen and diluting inert gas in step (g) is changed by gradually reducing the amount of decarburizing oxygen while gradually increasing the amount of inert gas during the blowing of the decarburizing oxygen and diluting inert gas.
Further features and advantages of the invention will become :more readily apparent from the following description of preferred embodiments, reference being made to the accomp<~nying drawings, in which:
FIG. 1 :is a schematic view illustrating a method according to the invention;
FIG. 2 is another schematic view showing the smelting reduction of the invention;
FIGS. 3(a) and (b) show the preferred jetting directions of the gas from the side tuyere(s) with respect to the bottom blowing tuyere(s);
FIG. 4 is a diagram showing post-combustion ratios measL.red with respect to the method of the invention, compared with a known method of 02 blown from the bottom;
FIG. 5 is a diagram showing the relationship between the heighi~ of the top blowing lance and the heat transfer efficiency;
FIG. 6 is a diagram showing the relationship between the amount of the gas blown from the side tuyere and the heat transfer efficiency;
_ g _ FIG. 7 is a diagram showing the relationship between the post combustion in the furnace, S and P
in the molten metal and unit consumption of coke;
FIG. 8 is a diagram showing the relationship of Cr increasing rate with respect to the amount of gas blown from the bottom according to the invention;
FIG. 9 is a diagram showing the relationship between the amount of bottom blowing gas and Cr oxidation lo;~s;
FIG. 1C~ is a diagram showing changes as time passes, of C con~~entration, Cr concentration, bath temperature, post-combustion ratio, amount of oxygen blown from t:he lance, amount of gas blown from side tuyere, and amount of raw materials supplied;
FIG. 1:1 is a diagram showing the reduction treatment time in comparison with conventional methods;
FIGS. 12(a) and (b) are schematic views showing the treatmeni~s of conventional methods of FIG. 11;
FIGS. 7_3 is a diagram showing the supplying speed of Cr and Cr increasing rate in comparison with a conventional method;
FIG. 14 is a diagram showing the relationship between the decarburizing level and the Cr oxidation loss according to the invention;
FIGS. 15 to .L'7 are schematic views illustrating the dischargE~ of the slag according to the invention;
FIG. 18 is a diagram showing the gas blowing conditions in the decarburizing treatment according to Example 1 hereinbelow;
FIG. 19 is a diagram showing influences of the slag amount to Cr oxidation loss in the decarburizing treatment;
C
FIG. 20 is a diagram showing influences of the amount of Ar gas blown from the bottom to denitriding rate during :~lr rinsing; and FIG. 21. is <~ diagram showing the relationship between the C content in the molten metal and the N
content in the :molten metal after finishing the blowing when changing the diluting gas blown from the top lance and the bottom blowing gas from N2 to Ar.
Fig. 1 show; schematically the method of the invention.
A smelting reduction furnace of a converter type is used for carrying out the smelting reduction and the decarburizing treatment. The furnace has a bottom blowing tuyere 1, a side blowing tuyere 2 and a top blowing lance 3.
According to the method of the invention, Cr raw materials ~>uch as Cr ores, Cr ore pellets, carbonaceous materials and fluxes are supplied to a metal bath supported in the smelting reduction furnace, anc~ the reducing treatment is performed under the fo:Llowing conditions.
Gas blowing by the bottom tuyere 1 and gas blowing by t:ze side tuyere 2 cooperate to diffuse the molten metal into the slag and greatly increase the reduction spE~ed.
As staged a~>ove, it has been found that the reduction of Cr ore in the slag progressed by C in the molten rrietal as the main reducing substance, by which the molten metal was agitated forcibly to diffuse positively into the slag (range where Cr ores float) so as t:o increase the reducing rate.
Therefore, according to the invention, the agitating gas is supplied from the bottom blowing tuyere 1 to form an uphe;~ving part A on the metal surface. At the same time, the agitating gas is supplied from the ~34D9~~2 side tuyere 2 so that at least a portion of the gas contacts the upheaving part A, whereby the part A is splashed into the slag. An apparent specific gravity of the slag is generally 0.3 to 0.5 wt.o, and the bulk specific gravity of the Cr ore is about 3.0 wt.~. Theref~~re, Cr ore in the slag gather around the lower region of the slag and float there, as seen in Fig. 2. If the upheaving part of the molten metal is splashed by the side blowing gas, the splashed molten metal is diffused around the lower region of the slag where Cr ores exist, as shown in Fig. 2, and C in the splashed molten metal reduces Cr203, and thus high reducing speed may be achieved. For providing such effects, it is of course necessary that the agitation must be forcibly caused by blowing more gas from both the bottom tuyere and the side tuyeres, and the amount of gas to be blown is determined by the amount and the depth of the molten metal. FIG. 8 shows the relationship between the amount of gas blown (Nm3/min/ton of molten metal/one tuyere) from the bottom tuyere and Cr increasing rate in the molten metal, from which it is seen that the Cr increasing rate, that is, the Cr reducing rate is increased in accordance with the increased amount of bottom blowing gas, and an effective reducing reaction takes place.
For obtaining such behavior, it is preferable that the si~~e blown gas blows exactly against the upheaving part A in both the vertical and horizontal directions. For example, it is desirable that the bottom blowing tuyere or tuyeres 1 and the side blowing tuye:re or tuyeres 2 be positioned relative to one another, as shown in Figs. 3(a) and (b).
The side tuyere blowing gas also agitates the slag where t:he post-combustion region is formed, in C
addition to the diffusion of the molten metal as mentioned above.
The side tuyere blowing gas and the bottom blowing gas i.o be employed are limited to CO and the inert gas (I~f2 or Ar) , and 02 is not used for the following reasons.
If 02 is used for side blowing, it reacts with C
in the molten metal splashed for reducing Cr ore, and the reduction by C in the molten metal would be hindered. In addition, if 02 is used, the temperature of the refractory becomes too high and the refractory may be damaged.
Further, if C>2 is used for bottom blowing, too much CO gas is caused in the molten metal and the molten metal becomes very agitated. As a result, the splash reaches the post-combustion region (see Fig.
2 ) , and C in the molten metal reacts with the above-mentioned 0~; for the post-combustion so that the post-combust=ion is hindered. However, if 02 is used, a cooling gas (such as C3Hg) must be added for the refractory whose temperature rises too much, so that the bottom blowing gas is increased and splashes of the molten metal by the compulsive agitation are accelerated. FIG. 4 shows the investigations of the post-combustion ratio actually measured to the post-combustion ratio (PC02/(DC02+02 in ores)) determined in respect of the bottom blowing N2 according to the invention and the bottom blowing 02 according to the comparative example, instead of N2. It is seen that the post-combustion is hindered by 02 blown from the bottom.
CO as the agitating gas and N2 and Ar as the inert gas may be used independently or in admixture.
The dec:arbur:izing 02 is blown into the molten metal from i=he top lance 3, and concurrently 02 for .e the post-comr~ustion is blown into the slag. The top blowing lance 3 i~> provided with a first outlet for the decarburi.zing 02 and a second outlet for the post combustion 0~>. The decarburizing 02 is blown through the first outlet in a flow located inside the flow of post-combustion 02 blown through the second outlet.
A high post-~~ombustion is provided, while the post-combustion region is formed mainly within the slag. The past-combustion region is thus formed in the slag, ~~.nd the slag is forcibly agitated by blowing the gas from the side tuyere, whereby high heat transfer efficiency is provided while the high post-combustion i,s secured. Therefore, the post-combustion O~~ must: be blown into the slag such that the post-comx~ustion is formed in the slag.
It is necest~ary that the height of the top blowing lance be determined at an appropriate level with respect to the levels of the slag or the molten metal. That is, the second outlet of the top blowing lance 3 can be positioned above the slag surface or thereunder. If it is too high, the post-combustion region would not be formed in the slag so that the heat transfer efficiency is lowered. On the other hand, if it is too low, the post-combustion region would not be properly formed.
Fig. 5 shows the relationship between the height of the free end of the lance from the slag surface and the heat transfer efficiency, wherein, if the lance is too f:ar above the slag surface, a satisfactory heal: transfer efficency cannot be obtained. Fig. 6 shows the relationship between the amount of gas blown from the side tuyere and the heat transfer efficiency, wherein the required heat transfer can be obtained by blowing a large amount of C
gas from the side tuyere so as to forcibly agitate the slag.
The post-combustion ratio is defined as:
(C02 + H~ O) / (CO + C02 + H2 + H20) which is the gas content bearing on an exhaust gas.
According to the invention, the above-mentioned reducing treatment. is performed at a post combustion ratio of more than 0.3. Since high heat transfer efficiency can be effected, and if the post-combustion ratio is increased as stated above, a high reducti~~n treatment ability (reducing speed) can be provided. In addition, if the post-combustion ratio is increased, the additional amount of the carbonaceous material (mainly coal) can be decreased.
Consequently,, an initial amount of carbonaceous material may be decreased, and because almost all of the P components in the molten metal are brought by the carbonaceous material, P in the molten metal can be lowered. If the post-combustion ratio is increased, a gaseous desulfurization vigorously takes place, and ~' in the molten metal is also decreased.
Accordingly, the post-combustion ratio should be more than 0.3. Fiq. 7 shows the relationship, in the smelting reduction, between the post-combustion ratio, the initial amount of coke, and P and S
contents in the molten metal, wherein if the post-combustion ratio i.s maintained to more than 0.3, the initial amount of coal is controlled, and P and S are decreased appropriately.
The slag is discharged, after having accomplished the smelting reduction, and the decarburizat_ion blowing is carried out consecutively in the same :Furnace under the following conditions:
(1) 02 is supplied exclusively through the top blcwing 7_ance 3, and not through the furnace bottom;
(2) 02 suppl_Led is not pure but is diluted with an inert gas; and (3) the inert gas is supplied from the bottom blowing t:uyeres 1.
The known AO:D process employs blowing from the side of the furnace and studies have found that 02 was a significant cause of the Cr oxidation loss.
That is, since the static pressure is added in the 02 bottom blowing, the CO partial pressure is increased and consequently the decarburizing reaction is hindered, and the decarburizing 02 oxidizes Cr. For this reason, in the method of the invention, 02 is not blown from the furnace bottom, but supplied from the top blowing lance 3.
It was found that, if the top blowing were merely made with 02, the Cr oxidation loss could not be avoided. This is because the decarburizing reaction takes place most vigorously around a fire point made by blowing oxygen from the top lance, but with only 0~, causing CO partial pressure to become very high, at that point, and the decarburizing reaction is obstructed and 02 oxidizes Cr. Thus, in the method of the invention 02 diluted with an inert gas (N2 or ~~r) is blown from the top lance, thereby decreasing CO partial pressure around the fire point and accelerating the decarburizing reaction. It is preferable to supply a large amount of gas from the top blowing :Lance for shortening the treatment time.
Further,, in accordance with the invention, inert gas (N2 or A.r) is blown from the bottom tuyeres 1 to forcibly agitate t:he molten metal and accelerate the mixture of the molten metal and 02 from the top f~~.-. ~a 'k~~,...~
~3~09 zz lance, and ;gin efi=ective decarburization is possible with checking the Cr oxidation loss by a combination of the compulsive agitation of the metal by the inert gas blown from the bottom and the top blowing of 02 diluted with an inert gas.
For forcibly agitating the molten metal, it is necessary to blow a considerable amount of inert gas.
Actually, for decreasing the Cr oxidation loss to not more than 1°., it is necessary to blow the inert gas in an amount of more than 0.5 Nm3/ton-min. (ton-min.:
every minute per 1 ton of molten metal), and for decreasing t:he Cr oxidation loss to not more than 0.5%, it i;s necessary to blow the inert gas in an amount of more than 1 Nm3/ton-min: If the amount of gas is too considerable, the molten metal would be splashed. Therefore, the gas is blown in an amount of not more than 0.5 to 5 Nm3/ton-min, preferably 1 to 3 Nm3/ton-min. Fig. 9 shows the relationship between the amount of bottom blowing gas and the Cr oxidation loss. 02 is used effectively in the decarburizing reaction by :blowing considerable gas from the bottom, and the Cr ~~xidation loss is appropriately checked.
As a comparison, Fig. 9 shows cases of existing methods, and in the AOD, for example, the Cr oxidation loss is quite substantial with respect to the amount o:E bottom blowing gas.
It is :referable to supply a large amount of oxygen from the top lance for shortening the treatment time.
The high Cr molten metal to be treated by the method of the invention is produced by melting ferrochromium or by a so-called direct molten reduction.
According to the invention, the entire process from the production of Cr molten metal to the production of stainless steel may be performed in the same container rationally and at high productivity by decarburizing Cr molten metal as mentioned above, which can be produced by any of the above-mentioned processes.
In the above decarburization blowing, for avoiding the Cr oxidation loss, it is effective to reduce the amount of oxygen supplied while decreasing the C level. However, in supplying oxygen from the top blowing lance, it is limited in view of lowering the blowing ;pressure to reduce the amount supplied in the same no~:zle, and the amount supplied is reduced to about one-half the maximum.
In dealing with the above-mentioned problem, it is preferable to gradually increase the amount of inert gas cLuring the blowing of decarburizing 02 while gradually reducing the amount of decarburizing 02, thereby enabling reduction of the oxygen supply without considerably lowering the blowing pressure.
Increasing of the amount of inert gas and reducing the amount of oxygen suppled may be accomplished successively or stepwise. With respect to the gas :blowing, for example, the amount of gas (02+N2 or Ar) blown from the top lance is determined to be always 3 Nm~3/ton-min, and the amount of oxygen supplied is reduced as follows, in response to C
level.
C: more than 3 wt.% .... 3 to 4 Nm3/ton-min.
C: less than 3 wt.%
to 2 wt.% .... 2 to 3 Nm3/ton-min.
C: less than 2 wt.o to 0.5 wt.o ... 1 to 2 Nm3/ton-min.
C: less than 0.5 wt.o... 1 Nm3/ton-min.
C in the molten metal during gas blowing can be determined i:n known manner by integrating the amount c 13409 2~
of oxygen or measuring the solidifying temperature of the molten metal.
When the invention is actually practised, the following processes are dealt with.
Charging - slagging . rising temperature -smelting reduction of Cr ores - discharging slag - ~~ecarburizing - tapping.
The charging process herein means to charge Fe sources such as i~he molten metal and form a metal bath in the furxlace. In the slagging and rising temperature proce~,s, the oxygen is supplied into the bath, and the coal materials and the flux are charged to the molten metal so as to form the slag for reducing Cr ores, and the temperature of the bath is raised suffi.cientT_y for the reduction to occur. In the smelting reduction, the Cr ores, the carbonaceous materials an~3 the flux are supplied in succession. At the end of this process, a finishing reduction is carried out without supplying the Cr ore; and the reducing treatment is accomplished when the Cr concentration reaches a desired value.
The following non-limiting examples illustrate the invention.
F'YZ1MDT.F' 1 3.7 t of molten metal were charged into a smelting reduction furnace of the converter type, and the smelting reduction was performed by supplying Cr ores, coke and f=lux, and 5.5 t of molten metal containing 18 wt.% Cr were produced. Subsequently, the decarburization blowing was carried out to produce stainless molten metal. Fig. 10 shows the changes of C:r and C concentrations in the metal, bath temperature and t:he post-combustion ratio OD, the amount of 02 supplied and the amount of raw material charged.
Fig. 11 shows the smelting reduction time (from start to finish) i.n the present example in comparison with the treatment time of conventional practices shown in Figs . 12 and (b) . Conventional method ( ( a) 1 ) blows fine powder coal and 02 from a top lance, and agitating ga.s from bottom tuyeres, and conventional method (2) blows 02 onto the slag and N2 from bottom tuyeres as well as N2 and 02 from side blowing tuyeres under the following conditions.
Conventional method (1) Top blowir..g 02 1700 Nm3/Hr (Finishing reduction period) Bottom blowing rd2 350 Nm3/Hr (Finishing reduction period) Molten metal 10 Tons Cr ore 4600 Kg (injected from lance) Coal materials 6700 Kg (injected from lance) Conventional method (2) Top blowing 02 1000 Nm3/Hr (Finishing reduction period) Bottom blowing N2 120 Nm3/Hr (Finishing reduction period) Lateral blowing N2 350 Nm3/Hr (Finishing reduction period) Molten metal 5 Tons Cr ore (powder) 5000 Kg (onto the slag) Coal materials 3200 Kg (onto the slag) The Cr concentration of conventional method (2) was only about 6 too 7 wt. o as shown in Fig. 11, while the Cr concentration of conventional method (1) reached the objective of 1.8 wt. o as shown in Fig. 12, but the treatment took 7.20 minutes. On the other hand, according t~o the invention, the Cr concentration reached 18 wt.o in 60 minutes, that is, in half the C
time of conventional method (1), which shows the excellent treatment function of the present invention.
Fig. 13 shows the Cr increasing speed with respect to the Cr supplying speed (supplying speed of Cr ore calculated as the Cr amount) according to the invention, revealing the high Cr increasing speed in comparison with the above methods (1) and (2).
In this example, C was decarburized from 6.7 to 0.038 wt.o in about 40 minutes. As is seen, although the decarburization went down to a low carbon range, the Cr oxidation loss was as low as about 0.5 wt. o.
The invention was practised by changing the decarburizin~~ level (under almost the same conditions as in Fig. 10), and the relationship between the decarburizin~~ level and the Cr oxidation loss was studied. The results are shown in Fig. 14 in comparison to the conventional processes (AOD and ZD-OB), and it can be seen that, the Cr oxidation loss was well controlled to be low in the low carbon region.
In the method. which consecutively carries out the smelting reduction and the decarburization in the same furnace, the amount of slag generated in the smelting reduction must be removed before the decarburizing treatment.
The slag is removed from the furnace mouth by tilting the furnace, but the slag around the furnace bottom is not easily removed. If it were removed forcibly, the molten metal would flow out and a large amount of yield would be lost.
In the method of the invention, the furnace is tilted so that the side blowing tuyere faces downward, and the slag is removed while the gas is blown from the side nozzle.
Fig. 15 shows slag discharge. Fig. 15(I) shows that the smelting reduction is finished where A is C
the metal and B is the slag. The furnace body is tilted so that the side blowing tuyere 2 faces downward as seen i.n Fig. 15 ( II ) and Fig. 15 ( III ) in order to dig>charge the slag B while blowing the gas (the inert gas such as N2) from the side blowing tuyere 2. In this way, the level of the slag on the furnace bottom is increased, so that only the slag is released smo~~thly and easily from the furnace mouth.
Figs. 1.6 and. 17 show examples of the furnace more smoothly discharging the slag. Fig. 16 shows the furnace body having an opening 4 at the lower part of the furnace mouth. Fig. 17 shows that a weir 5 is provided at the i=urnace mouth to keep the metal A
from escapin~~.
According to the above embodiments, only the slag may be disch<~rged. When the furnace is inclined causing the slag t=o flow out naturally, the residual slag amount is 40 to 50 Kg/molten metal Ton and the amount of metal .removed together with the slag is about 10 Kg/molten metal Ton. On the other hand, depending on the present invention, the amount of residual slag is 7_ess than 5 Kg/molten metal Ton and the amount of molten metal removed is less than 1 Kg/molten metal Ton.
The conventional decarburization blowing was performed under conditions to form a large amount of slag, and the influences of the amount of slag to the Cr oxidation loss have not as yet been carefully and quantitative:Ly studied.
On the other hand, Applicant has studied the relationship betwe>.en the amount of slag and the Cr oxidation loss, paying close attention to the slag to be formed. As a result, it was found that a relationship exists between the amount of slag and the Cr oxidation loss during decarburizing blowing, and if the blowing was done by controlling a low amount of s7_ag, that is, not more than 50 Kg/molten metal Ton, the Cr oxidation loss could be effectively lowered.
Further, it is assumed that the Cr oxidation loss is lowered by controlling the amount of slag within the above range for the following reasons . 02 blown from the top lance causes the following reactions:
C (molten metal) +..1/2 02 ~ CO (gas) (1) 2Cr (mo.lten metal) + 3/2 02 -~ Cr203 (slag) (2) From t:he above reactions (1) and (2), the following re~~ction (3) was derived:
Cr203 (;slag + 3C (molten metal) 2C:r (molten metal) + 3C0 (gas) (3) Cr203 generated by the top lance blowing 02 is reduced by C in the molten metal.
It is important to increase the concentration of Cr203 in the slag so that the reduction according to reaction (3) progresses to the right side. For increasing the Cr,203 concentration, it is necessary to decrease the amount of the whole slag so that the reaction (3) is easily effected, and as a result, the reduction of Cr20:, is accelerated and the Cr loss is effectively decreased. In addition, the furnace refractory comprises Mg0 (magnesium chromium, magnesium carbon or magnesium dolomite), and the slag contains 10 to 30 wt.~ of Mg0 by melting it. Since Mg0 combines with Cr203 and generates less fusible Mg0.Cr203 sp:inel, and if the amount of slag is great, the Cr203 concentration in the slag is lowered and reduction is difficult. The lowering effect of the Cr oxidation lcss by lowering the amount of slag was most remarkable when the blowing was done with an amount of sl<~g not more than 50 Kg/molton metal Ton.
C
1~~09 22 To practise t:he present method, the less Si and S contents in the molten steel to be decarburized, the more advantageous is the controlling (lowering) of the amoL.nt of slag. For example, during basic decarburization of the invention, more of the reducing agent such as Fe-Si can be controlled and kept low, so that the amount of slag can be easily controlled.
DECARBURIZING TREATMENT Example 1 5.5 t of molten metal containing 18 wt.o Cr were decarburized in the above-mentioned furnace in accordance with i~he different levels of the slag amounts and gas blowing amount. The decarburization was carried out by blowing decarburizing 02 diluted with N2 gas from the top lance and blowing N2 gas from the bottom tu.yeres and C in the molten metal was decreased fr~~m 6.5 wt.o to 0.03 wt.o in about 40 min.
Fig. 18 shows the amount of the blown gas in this operation.
Fig. 19 shows the relationship between the amount of slag and the Cr oxidation loss obtained in this treatment. The Cr oxidation loss became low as the amount of slag was lowered. When the amount of slag was <_ 50 kg/molten metal ton (preferably, <_ 40 kg/molten mE~tal ton), the Cr oxidation loss was remarkably lowered.
Applicant has studied the denitrification of the molten steel when low nitrogen stainless steel was produced, on a premise that N2 was used as the agitating gas during decarburization, and found that it was very effective to the nitrification of the molten metal to add a deoxidizer such as Fe-Si or Al after the decarburization for carrying out the rinsing treatment by blowing a large amount of Ar from the boti~om tuyeres.
rv_. ~3~0,9.~~
In general, Fe-Si or A1 are added to the molten metal for deoxidation and Cr reduction in the slag after decarburization, and in this regard, the agitation is practised by blowing Ar from the bottom during addition of the deoxidizer, whereby N is removed frorl the molten steel in addition to the above-mentioned Cr reduction and deoxidation.
Therefore, N can easily escape during deoxidation of the molten steel (70 - 150 ppm - less than 50 ppm) by adding Fe-Si or A1, and when molten steel is agitated with Ar, N ins more easily removed.
The bottom blowing of Ar, as mentioned above, is ordinarily performed with 0.5 to 5 Nm3/min.-molten steel ton, ~~referably 1 to 3 Nm3/min.-molten metal ton, for 5 t« 10 minutes.
For further .Lowering N in the molten steel, in addition to the denitrification rinsing treatment mentioned above, _Lt is preferable to use Ar gas for diluting the decarburizing 02, although N2 is used as the bottom blowing gas in decarburizing blowing.
Since N-absorption is most vigorous around the fire point of the lance, and if N2 is used as the dilution gas, a large amount of N is molten into the steel. In this method, since Ar is more expensive than N, Ar is used only as the dilution gas which is sufficient in a small amount to check the increase in the nitrogen concentration. After decarburizing blowing, the above nitrificatian is carried out.
Applicant has found that, with respect to N in the molten stee7_, while the decarburization is vigorous, N is l.ow, and when the decarburization progresses and the decarburizing speed is low, N
becomes remarkably higher. This is because CO gas generated by thE: decarburization absorbs N and releases it.
~,_:, _. ~3~09 2~
The higher the C concentration is in the steel, the faster i~he de:carburizing speed. When a dilution gas is used for diluting the decarburizing 02, N2 gas is preferably used. at first, and when C is low during decarburization, the dilution gas is changed from N2 to Ar and the decarburization is continued so that the producti~~n cost can be appropriately lowered.
It is preferable to change the dilution gas of the decarburizating 02 and the bottom blowing gas from N2 to ~~r in accordance with the amount of C in the molten tsteel, as shown in Fig. 21, the changing of N2 to Ar is desirable in the range of 0.8 to 2.0 wt.% of C in the molten steel. If the change is made too soon, too much. Ar gas is required. Therefore, the change should be made when C is below 2.0 wt.o. On the other h<~nd, i.f the change is made too late (C
concentration is t=oo low), as shown in Fig. 21, the denitrificat:ion is unsatisfactory. Therefore, the change should be made when C is above 0.8 wt.°s.
DECARBURIZINc.; TREATMENT Example 2 The high Cr molten metal was decarburized in steps (A) to (E) mentioned below in a furnace having a top blow_~ng lance and bottom blowing tuyeres, followed by Ar rinsing (Fe-Si supply + Ar bottom blowing), and stainless molten steel containing 18 wt.% Cr and 0.05 wt.% C was produced.
(A) Decarburi.zing blowing "op blowing gas: 02 + N2 (Dilution) Bottom blowing gas: N2 (2 Nm3/min.-molten steel ton) Ar rinse ~3ottom blowing gas: Ar (0.1 Nm3/min.-molten metal ton) (B) Decarburizing blowing Top blowing gas: 02 + N2 (Dilution) C
W 13409 2~
l3ottom blowing gas: N2 (2 Nm3/min.-molten steel ton) Ar rinse l3ottom blowing gas: Ar (0.5 Nm3/min.-molten metal ton) (C) De~~arburizing blowing 'Cop blowing gas: 02 + N2 (Dilution) Bottom blowing gas: N2 (2 Nm3/min.-molten steel ton) Ar rinse Bottom blowing gas: Ar (1 Nm3/min.-molten metal ton) (D) Decarburi.zing blowing Cop blowing gas: 02 + N2 (Dilution) Bottom blowing gas: N2 (2 Nm3/min.-molten steel ton) Ar rinse F3ottom blowing gas: Ar (2 Nm3/min.-molten metal ton) (E) Decarburi.zing blowing ~_'op blowing gas: 02 + Ar (Dilution) ~3ottom blowing gas: N2 (2 Nm3/min.-molten steel ton) Ar rinse F3ottom blowing gas: Ar (2 Nm3/min.-molten metal ton) Fig. 20 shows the influences of the amount of gas blown from the bottom to the denitrifying speed during Ar r:insing. The molten metal is denitrified effectively by Ar rinsing. In step (D), where the amount of Ar gas is 2 Nm3/min.-molten steel ton, the denitrificat_~on reaches an objective value of N: 500 to 600 ppm for a rinsing time of 4 to 5 min. In step (E) , where Ar is used for diluting the decarburizing 02, N concentration is about 1000 ppm which is half of steps (~~) to (D) when the decarburization is completed, and the=refore the denitrification reaches its objecti~Te value when the Ar rinsing is for a shorter peri~~d of time.
DECARBURIZING TREATMENT Example 3 High Cr molt:en metal was decarburized in a furnace having a top blowing lance and bottom blowing tuyeres, in accordance with steps (a) and (b) hereinbelow, followed by Ar rinsing (Fe-Si supply +
Ar bottom blowing), and . stainless molten steel containing 18 wt.% Cr and 0.05 wt.o C was produced.
a) Decarburizing blowing Top blowing gas: N2 was used for diluting the decarburizing 02 at the beginning of the decarbur ization, and changed to Ar in accordance with values of C
in the molten steel during decarburizing blowing.
Ar rinse Ar supp:Ly: 2 Nm3/min.-molten steel ton for 5 minutes.
b) Decarburizing blowing Top blowing gas: N2 was used for diluting the decarburizing 02 and the bottom blowing gas at the beginning of the decarburiza tion, and changed to Ar in accordance with values of C
in the molten steel during decarburizing blowing.
Ar rinse Ar supp:Ly: 2 Nm3/min.-molten metal steel ton for 5 minutes.
C
Fig. 21 shows the influence of N in the molten steel when changing the type of gas for the dilution gas of the decarburizing 02 and the bottom blowing gas.
Table I shows the concentrations of N in the steel after the Ar rinsing (changing of N2 - Ar during decarburizin~~ blowing was done at C in the molten steel - 1 wt. o), ,end it. can be seen that low N stainless steel containing less than 200 ppm of N is easily produced according to the invention.
TABLE I
N in the molten N in the molten steel after steel after Ar decarburizing rinsing blowing (a) 500 ppm 130 ppm -( b 15 0 ppm ~ 5 0 ppm ) y
Claims (20)
1. A method of producing stainless molten steel by smelting reduction in a smelting reduction furnace provided with a bottom tuyere, a side tuyere and a top lance having a first outlet and a second outlet, said method comprising the steps of:
a) reducing raw chromium ore in said furnace by use of carbonaceous material to produce chromium molten metal at the bottom of said furnace and having slag above said molten metal;
b) blowing CO and/or inert gas through said bottom tuyere into said molten metal to cause agitation thereof;
c) blowing CO and/or inert gas through said side tuyere into said molten metal so as to contact said agitated molten metal;
d) concurrently blowing decarburizing oxygen through said first outlet of said top lance into said molten metal and post-combustion oxygen through said second outlet of said top lance into said slag, so that said molten metal is decarburized;
e) reducing chromium material by maintaining a ratio of post-combustion to more than 0.3;
f) discharging said slag after completion of reduction; and g) after discharging of the slag, concurrently blowing a decarburizing oxygen diluted with an inert gas through one of said outlets of said top lance into said molten metal and blowing an inert gas through said bottom tuyere into said molten metal so as to agitate forcibly said molten metal.
a) reducing raw chromium ore in said furnace by use of carbonaceous material to produce chromium molten metal at the bottom of said furnace and having slag above said molten metal;
b) blowing CO and/or inert gas through said bottom tuyere into said molten metal to cause agitation thereof;
c) blowing CO and/or inert gas through said side tuyere into said molten metal so as to contact said agitated molten metal;
d) concurrently blowing decarburizing oxygen through said first outlet of said top lance into said molten metal and post-combustion oxygen through said second outlet of said top lance into said slag, so that said molten metal is decarburized;
e) reducing chromium material by maintaining a ratio of post-combustion to more than 0.3;
f) discharging said slag after completion of reduction; and g) after discharging of the slag, concurrently blowing a decarburizing oxygen diluted with an inert gas through one of said outlets of said top lance into said molten metal and blowing an inert gas through said bottom tuyere into said molten metal so as to agitate forcibly said molten metal.
2. The method of claim 1, wherein said decarburizing oxygen and post-combustion oxygen are blown respectively through said first and second outlets of said top lance positioned close to the surface of said slag.
3. The method of claim 2, wherein said decarburizing oxygen is blown through said first outlet in a flow located inside the flow of post-combustion oxygen blown through said second outlet.
4. The method of claim 1, wherein after the slag is discharged, inert gas is blown through the bottom tuyere in an amount of more than 0.5 Nm3/molten metal Ton-min.
5. The method of claim 4, wherein the amount is more than 1 Nm3/molten metal Ton-min.
6. A method of producing stainless molten steel by smelting reduction in a smelting reduction furnace provided with a bottom tuyere, a side tuyere and a top lance having a first outlet and a second outlet, said method comprising the steps of:
a) reducing raw chromium ore in said furnace by use of carbonaceous material to produce chromium molten metal at the bottom of said furnace and having slag above said molten metal;
b) blowing CO and/or inert gas through said bottom tuyere into said molten metal to cause agitation thereof;
c) blowing CO and/or inert gas through said side tuyere into said molten metal so as to contact said agitated molten metal;
d) concurrently blowing decarburizing oxygen through said first outlet of said top lance into said molten metal and post-combustion oxygen through said second outlet of said top lance into said slag, so that said molten metal is decarburized;
e) reducing chromium material by maintaining a ratio of post-combustion to more than 0.3;
f) discharging said slag after completion of reduction; and g) after discharging of the slag, concurrently blowing a decarburizing oxygen diluted with an inert gas through one of said outlets of said top lance into said molten metal and blowing an inert gas through said bottom tuyere into said molten metal so as to agitate forcibly said molten metal, wherein the ratio of decarburizing oxygen and diluting inert gas is changed by gradually reducing the amount of decarburizing oxygen while gradually increasing the amount of inert gas during the blowing of said decarburizing oxygen and diluting inert gas.
a) reducing raw chromium ore in said furnace by use of carbonaceous material to produce chromium molten metal at the bottom of said furnace and having slag above said molten metal;
b) blowing CO and/or inert gas through said bottom tuyere into said molten metal to cause agitation thereof;
c) blowing CO and/or inert gas through said side tuyere into said molten metal so as to contact said agitated molten metal;
d) concurrently blowing decarburizing oxygen through said first outlet of said top lance into said molten metal and post-combustion oxygen through said second outlet of said top lance into said slag, so that said molten metal is decarburized;
e) reducing chromium material by maintaining a ratio of post-combustion to more than 0.3;
f) discharging said slag after completion of reduction; and g) after discharging of the slag, concurrently blowing a decarburizing oxygen diluted with an inert gas through one of said outlets of said top lance into said molten metal and blowing an inert gas through said bottom tuyere into said molten metal so as to agitate forcibly said molten metal, wherein the ratio of decarburizing oxygen and diluting inert gas is changed by gradually reducing the amount of decarburizing oxygen while gradually increasing the amount of inert gas during the blowing of said decarburizing oxygen and diluting inert gas.
7. The method of claim 6, wherein said decarburizing oxygen and post-combustion oxygen are blown respectively through said first and second outlets of said top lance positioned close to the surface of said slag.
8. The method of claim 7, wherein the decarburizing oxygen is blown through said first outlet in a flow located inside the flow of post-combustion oxygen blown through said second outlet.
9. The method of claim 6, wherein after the slag is discharged, inert gas is blown through the bottom tuyere in an amount of more than 0.5 Nm3/molten metal Ton-min.
10. The method of claim 9, wherein the amount is more than 1 Nm3/molten metal Ton-min.
11. A method of producing stainless molten steel by smelting reduction in a smelting reduction furnace provided with a bottom tuyere, a side tuyere and a top lance having a first outlet and a second outlet, said method comprising the steps of:
a) reducing raw chromium ore in said furnace by use of carbonaceous material to produce chromium molten metal at the bottom of said furnace and having slag above said molten metal;
b) blowing CO and/or inert gas through said bottom tuyere into said molten metal to cause agitation thereof;
c) blowing CO and/or inert gas through said side tuyere into said molten metal so as to contact said agitated molten metal;
d) concurrently blowing decarburizing oxygen through said first outlet of said top lance into said molten metal and post-combustion oxygen through said second outlet of said top lance into said slag, so that said molten metal is decarburized;
e) reducing chromium material by maintaining a ratio of post-combustion to more than 0.3;
f) tilting said furnace so that the side tuyere faces downward after completion of the reduction so as to remove said slag while blowing gas through said side tuyere; and g) after removing said slag, concurrently blowing a decarburizing oxygen diluted with an inert gas through one of said outlets of said top lance into said molten metal and blowing an inert gas through said bottom tuyere into said molten metal so as to agitate forcibly said molten metal.
a) reducing raw chromium ore in said furnace by use of carbonaceous material to produce chromium molten metal at the bottom of said furnace and having slag above said molten metal;
b) blowing CO and/or inert gas through said bottom tuyere into said molten metal to cause agitation thereof;
c) blowing CO and/or inert gas through said side tuyere into said molten metal so as to contact said agitated molten metal;
d) concurrently blowing decarburizing oxygen through said first outlet of said top lance into said molten metal and post-combustion oxygen through said second outlet of said top lance into said slag, so that said molten metal is decarburized;
e) reducing chromium material by maintaining a ratio of post-combustion to more than 0.3;
f) tilting said furnace so that the side tuyere faces downward after completion of the reduction so as to remove said slag while blowing gas through said side tuyere; and g) after removing said slag, concurrently blowing a decarburizing oxygen diluted with an inert gas through one of said outlets of said top lance into said molten metal and blowing an inert gas through said bottom tuyere into said molten metal so as to agitate forcibly said molten metal.
12. The method of claim 11, wherein said decarburizing oxygen and post-combustion oxygen are blown respectively through said first and second outlets of said top lance position close to the surface of said slag.
13. The method of claim 12, wherein said decarburizing oxygen is blown through said first outlet in a flow located inside the flow of post-combustion oxygen blown through said second outlet.
14. The method of claim 11, wherein after the slag is removed, inert gas is blown through the bottom tuyere in an amount of more thanØ5 Nm3/molten metal Ton-min.
15. The method of claim 14, wherein the amount is more than 1 Nm3/molten metal Ton-min.
16. A method of producing stainless molten steel by smelting reduction in a smelting reduction furnace provided with a bottom tuyere, a side tuyere and a top lance having a first outlet and a second outlet, said method comprising the steps of:
a) reducing raw chromium ore in said furnace by use of carbonaceous material to produce chromium molten metal at the bottom of said furnace and having slag above said molten metal;
b) blowing CO and/or inert gas through said bottom tuyere into said molten metal to cause agitation thereof;
c) blowing CO and/or inert gas through said side tuyere into said molten metal so as to contact said agitated molten metal;
d) concurrently blowing decarburizing oxygen through said first outlet of said top lance into said molten metal and post-combustion oxygen through said second outlet of said top lance into said slag, so that said molten metal is decarburized;
e) reducing chromium material by maintaining a ratio of post-combustion to more than 0.3;
f) tilting said furnace so that the side tuyere faces downward after completion of the reduction so as to remove said slag while blowing gas through said side tuyere; and g) after removing of the slag, concurrently blowing a decarburizing oxygen diluted with an inert gas through one of said outlets of said top lance into said molten metal and blowing an inert gas through said bottom tuyere into said molten metal so as to agitate forcibly said molten metal, wherein the ratio of decarburizing oxygen and diluting inert gas is changed by gradually reducing the amount of decarburizing oxygen while gradually increasing the amount of inert gas during the blowing of said decarburizing oxygen and diluting inert gas.
a) reducing raw chromium ore in said furnace by use of carbonaceous material to produce chromium molten metal at the bottom of said furnace and having slag above said molten metal;
b) blowing CO and/or inert gas through said bottom tuyere into said molten metal to cause agitation thereof;
c) blowing CO and/or inert gas through said side tuyere into said molten metal so as to contact said agitated molten metal;
d) concurrently blowing decarburizing oxygen through said first outlet of said top lance into said molten metal and post-combustion oxygen through said second outlet of said top lance into said slag, so that said molten metal is decarburized;
e) reducing chromium material by maintaining a ratio of post-combustion to more than 0.3;
f) tilting said furnace so that the side tuyere faces downward after completion of the reduction so as to remove said slag while blowing gas through said side tuyere; and g) after removing of the slag, concurrently blowing a decarburizing oxygen diluted with an inert gas through one of said outlets of said top lance into said molten metal and blowing an inert gas through said bottom tuyere into said molten metal so as to agitate forcibly said molten metal, wherein the ratio of decarburizing oxygen and diluting inert gas is changed by gradually reducing the amount of decarburizing oxygen while gradually increasing the amount of inert gas during the blowing of said decarburizing oxygen and diluting inert gas.
17. The method of claim 16, wherein said decarburizing oxygen and post-combustion oxygen are blown respectively through said first and second outlets of said top lance positioned close to the surface of said slag.
18. The method of claim 17, wherein said decarburizing oxygen is blown through said first outlet in a flow located inside the flow of post-combustion oxygen blown through said second outlet.
19. The method of claim 16, wherein after the slag is discharge, inert gas is blown through the bottom tuyere in an amount of more than 0.5 Nm3/molten metal Ton-min.
20. The method of claim 19, wherein the amount is more than 1 Nm3/molten metal Ton-min.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000597362A CA1340922C (en) | 1989-04-21 | 1989-04-21 | Method of producing stainless molten steel by smelting reduction |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000597362A CA1340922C (en) | 1989-04-21 | 1989-04-21 | Method of producing stainless molten steel by smelting reduction |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1340922C true CA1340922C (en) | 2000-03-07 |
Family
ID=33315132
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000597362A Expired - Fee Related CA1340922C (en) | 1989-04-21 | 1989-04-21 | Method of producing stainless molten steel by smelting reduction |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1340922C (en) |
-
1989
- 1989-04-21 CA CA000597362A patent/CA1340922C/en not_active Expired - Fee Related
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