LU82481A1 - PROCESS AND APPARATUS FOR MANUFACTURING CARBON STEEL AND LOW ALLOY STEEL USING A BASIC OXYGEN OVEN WITH BOTTOM BLOWING - Google Patents

Description

The present invention relates to the manufacture of carbon steels and low-alloy steels by means of a basic blown oxygen furnace. More particularly, the present invention relates to a method of manufacturing steel in which a gas jet is injected; in a molten bath in order to activate the agitation of this i; last during the blowing of pure oxygen from the top to the i | ~ by means of a lance.

In the steel manufacturing process with s | oxygen blowing from the top, we load cast iron in i! * fusion, scrap metal and other starting materials in t; a converter, then refining the steel into | blowing pure oxygen over the molten charge by means! an oxygen lance. At the first or intermediate stage of the blowing, the oxygen reacts vigorously with the molten bath always having a high carbon content, so that the formation of carbon monoxide is sufficient. health to vigorously shake the molten bath.

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However, since the quantity of carbon contained in the molten bath decreases at the final stage of the blowing, the formation of carbon monoxide decreases rapidly while the reaction between the molten steel and the slag rapidly decreases. As a result of this decrease in the efficiency of oxygen decarburization, i.e. the reduction in the proportion of oxygen used to effect decarburization vis-à-vis the total amount of oxygen blown in the molten bath, the presence of an excess of oxygen is inevitable, which gives rise to an oxidation of iron well beyond the equilibrium level. In addition, due to insufficient agitation of between molten steel and slag, a temperature difference is established between them, giving rise to a dephosphorization reaction which takes place by exerting a harmful influence. This characteristic is due to the fact that the molten metal is less agitated. Consequently, it has been proposed to equip an oxygen converter with an electromagnetic stirrer. It has also been proposed to add iron scrap to the bath in i fusion at the last stage of blowing in order to create, in this i bath, turbulence due to a difference in temperature cross between the grapeshot and this molten bath. However, these proposals have never been adopted in practice, • as they involve high construction costs and the cost of | supposes that they do not give rise to an effect as great as that expected.

In addition, it has been proposed to rotate or oscillate the oxygen blowing lance in order to further stir the molten metal and the slag. However, by doing so, it promotes the agitation of the slag, but not that of the molten steel.

In order to avoid these drawbacks of the prior art, it has also been proposed to inject a gas jet into a molten metal from the bottom, while blowing pure oxygen onto the molten bath by means of a lance. .

Examples of gases to be injected into the molten bath are limited to an inert gas such as argon and a neutral gas such as nitrogen. However, since argon is very expensive and a relatively large amount must be injected into the molten bath during the bottom blowing in order to properly stir I. * this bath, this inevitably results in net increase in costs. The introduction of pure nitrogen or a gas mainly consisting of nitrogen, for example compressed air, has the effect of increasing the nitrogen content of the molten bath. Therefore, it is also not practical to carry out a nitrogen blowing i

In the French patent 1,151 * 053 eb in the? U.S. Patent No. 3 * 854 * 932, the bottom blowing of various types of gas, including argon, water vapor, air, oxide, is described. carbon, etc.

! ' However, U.S. Patent No. 3 * 854 * 932, i! for example, concerns the manufacture of stainless steel, so that the main goal envisaged is the suppression of chromium oxidation. Consequently, it is necessary to carry out the process of this invention under a pressure lower than atmospheric pressure. In addition, in this patent, it is considered that these gases are equivalent. In addition, since the aforementioned French patent describes the blowing from the bottom of a relatively large quantity of gas into the molten bath, the process described in this patent is less economical.

In addition, oxygen is blown into the molten steel bath in the form of a supersonic jet using the conventional lance. As a result, at the impact surface; between oxygen and molten steel, the temperature rises to 2,000 ° C or more. Consequently, the losses of iron by evaporation (which will be called hereinafter "smoke losses") are significant. In addition, the problems posed by projections of fine particles of iron after heating, J ;! 'as well as the slag and molten steel overflows,!' are still unresolved. As a result, even in this combined blowing process, you can't expect one! significant increase in casting yield. Indeed, !; I.

(l according to the conventional method of manufacturing steel using i * i1 i of oxygen, the type '.'Laval "I nozzle is used as lan.ce and the oxi-iet is initiated -she at a sunersoniaue speed i I - (above are unavoidable. In order to avoid these drawbacks, attempts have been made to adjust the decarburization speed and the conditions of the slag during the blowing. However, it is difficult to control these factors during operation and, in fact, the best expected results could not be obtained.

Recent progress in this area is the "Q-Bop" process in which, instead of blowing pure oxygen from the top, it is blown into the molten metal through nozzles provided at the bottom of the converter.

Since, in this "Q-Bop" process, pure oxygen gas is used for the bottom blowing rather than the top blowing, it is necessary to blow another gas such as propane, in order protect the aforementioned nozzles. Consequently, in this case too, a relatively large quantity of blowing gas must be injected into the molten metal. The "uniform mixing time", which will be described below in detail, is about 10 seconds.

The main object of the present invention is

Another object of the present invention is to provide an economical process for the proper agitation of a molten bath. A method of manufacturing carbon steel and low alloy steel using a basic oxygen furnace. in a converter for making steel using oxygen.

| Yet another object of the present invention is to provide a process for the production of carbon steel and low alloy steel by means of a basic oxygen furnace, a process in which a waste gas discharged from the furnace is put into circulation and used as the only source of gas to be blown from the bottom of the oven. . s>.

'? Yet another object of the present invention ί "is to provide a process for manufacturing carbon steel and low alloy steel with better casting efficiency.

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The present invention lies in a method! for manufacturing carbon steels and low alloy steels in a basic oxygen furnace, this process being characterized in that a gas jet consisting mainly of carbon dioxide is introduced into the molten metal; · And this, by means of at least one nozzle provided in the wall; 1 base or the side wall of this basic oxygen furnace w- 1 at least partially during the lapse of time t; ï: between the start of the blowing and the pouring of the molten bath, i- the flow rate of the blowing gas through the bottom being equal to 1 / 200-1 9/100 of the speed at which the oxygen strikes the ï, t molten bath via a lance.

j | The blowing gas which is the main component? carbon dioxide, may contain carbon dioxide at | more than 50% by volume, including exhaust gas from a metal refining furnace such as a conver- || steel weaver, as well as a purified or concentrated gas from a combustion gas from a heating furnace. Among other components of the blowing gas, mention may be made of nitrogen, oxygen, etc. The more nitrogen in the blowing gas, the higher the nitrogen content of the melt. In the case of the production of non-quenched steels

NOT

usual, nitrogen can be present in the blowing gas in an amount less than 50% by volume without resulting in any inconvenience. However, it is best i '! use a gas containing less than $ 20 by volume of nitrogen if it is intended for the manufacture of steel with low nitrogen content. However, it should be noted that if a relatively large amount of nitrogen is blown into the melt, the nitrogen will be removed almost completely until the carbon content is reduced to $ 0.5 · This character! is due to the fact that the denitrification reaction a | vigorously when the carbon content is greater than $ 0.5 · Therefore, nitrogen gas can be blown into the molten bath in place of carbon dioxide! . gaseous until the carbon content is reduced to | $ 0.5. After the carbon content has been reduced to! $ 0.5 or less, bottom blowing must be done! in accordance with the present invention.

In addition, during the implementation of the process of the present invention, there is sometimes a mushroom deposit with a thickness of about 5-15 cm at the end of the nozzle as a result of the temperature difference i observed between the nozzle cooled by the blown gas and the molten bath which surrounds it. It is assumed that this deposit forms at the start of the operation and that it consists mainly of slag. As a result of the formation of | this deposit at the end of a nozzle, it becomes difficult; to blow the gas in a predetermined amount. To ! To avoid this difficulty, it is advisable to increase the! j pressure or the flow rate of the blowing gas up to a value i j y at which this deposit becomes porous following the passage of the gas {through the nozzle. It is also recommended to incorporate j! a small amount of oxygen in the blowing gas so! to use its heat generation in accordance with the following equation: i! 2C0 + 0 „= 2C0„ j; .

According to the present invention, the blowing by 1 the bottom is applied at least partially during the period of time elapsing between the start of the oxygen blowing

During the process, the blowing rate * can be varied by the bottom, for example, depending on the progress of the steelmaking reaction in the converter. For example, ± 1 is preferable to increase the blowing rate at a final stage of blowing from the top in order to compensate for the reduction in agitation due to the slowing down of the decarburization reaction. As a result, an effective ripening reaction can usefully be continued to completion, thereby resulting in a substantial reduction in quantity. of gas used.

Preferably, the blowing of carbon dioxide anhy-I is carried out by means of at least one nozzle provided in! at the bottom or in the side wall of the manufacturing converter! cation of steel using oxygen.

! The advantages of using carbon dioxide I as a blowing gas lie not only in the fact that it is less expensive than an inert gas such as

Ique argon, but it also increases twice in volume when added to the molten bath according to the following equation; C + = 2C0, thus causing violent agitation of the molten bath. In other words, I compared to argon or nitrogen, it takes less than I gas to achieve the same stirring effect. This reduction i it ·! of the amount of gas used means that equipment can be simplified, including the lines required to blow gas into the molten steel in accordance with the present invention. This is a very advantageous feature from a practical point of view.

According to the present invention, the flow rate of the blowing gas through the bottom is limited to less than 9/100, j], preferably, to less than 5/100 of the speed at which the oxygen l 1 gene strikes the molten bath by means of a lance, *. *

Yes

Ice which means that a relatively small amount of gas is injected into the molten bath using the bottom blowing process. If the bottom blowing gas is injected into the molten bath in an amount greater than 9/100 of the speed of oxygen blown by a lance, agitation occurs in such a vigorous manner that the yield casting is significantly reduced due to the large overflow of the molten bath. On the other hand, if the amount of blowing gas from the bottom is less than 1/200 of the blowing gas from the top, the required agitation of the molten bath cannot be achieved.

Furthermore, according to a preferred characteristic, the amount of bottom blowing gas can be reduced based on the amount of molten metal to be treated, independently of the blowing speed of pure oxygen by means of a lance. . According to this embodiment, the amount of gas to be injected into the molten bath is precisely adjusted so that the uniform mixing time is 20 seconds or more.

The term "uniform mixing time" refers to the time required to uniformly mix the molten steel and the molten slag only by the bottom blowing process. The uniform mixing time is a factor introduced by K. Nakanishi et al. ("Ironmaking and; Steelmaking" (1975) 3, 193) and it is defined as follows:

Uniform mixing time ^ = 800 x £ (sec.) £ = 28.5 x T x log (1 + z / 148) (watts / ton). 'g where.

Q = gas flow rate (Nm3 / minute) W = quantity of molten steel (tonnes) g T = bath temperature (° K) I Z = bath depth (cm). -, ........-> - ^ *** ρ * Μ "* τί ^" * * In a preferred embodiment, the uniform mixing time is greater than 30 seconds. If the quantity of gas is within the limits defined above, | we then obtain perfect agitation. If the uniform mixing time is less than 20 seconds, the agitation between the molten steel and the molten slag takes place in such a vigorous manner that the reduction of the iron oxide contained in the slag in fusion takes place! l I excessively, thereby reducing the oxide content of | iron which is effective for dephosphorization of steel in | ’Fusion. In addition, if the uniform mixing time is less than 15 seconds, significant leaks of molten steel occur through the nozzles, thereby reducing the casting efficiency of the steel.

If the uniform mixing time is more than 70 seconds, i.e. if the amount of bottom blowing gas is significantly reduced, no agitation is expected and the blowing process is practically the same as the blowing from the top of the conventional steel fabrication process using oxygen. This results in a marked increase in the total quantity of iron contained in the molten slag, as well as a reduction in the casting yield. Therefore, it is desirable to set the uniform mixing time to a value between 20 and 70 [seconds.

On the basis of the experiments which have been carried out, it can be said that, for example, when the depth of the bath is 250 cm, the uniform mixing time of 20 seconds corresponds to a blown throughput rate of 0.5 Nm3 / minute per ton of molten steel, while a K uniform mixing time of 70 seconds corresponds to an I blowing rate at the bottom of 0.02 Nm3 / minute per ton *!

Considering the fact that gaseous oxygen is discharged mainly in the form of gaseous carbon monoxide! i after the decarburization operation carried out in this 1 ″ top blown oxygen refining oven, in one of its aspects, the present invention provides a method of steel refining in which this waste gas is used; as the only source of gas to be blown from below into the molten bath · in order to agitate the latter. In this way, ! the process provides its own gas for stirring molten steel.

i; * Consequently, the present invention also resides from! a process for the manufacture of steel in a basic oxygen furnace by blowing pure oxygen from the top and by blowing from the bottom a gas consisting mainly of | of carbon dioxide, this process being characterized in that a waste gas discharged and collected from this furnace is | combined with additional oxygen and / or water vapor, the mixture thus obtained is then subjected to combustion while the combustion gas thus obtained and consisting ij mainly of carbon dioxide is used at least I partially as gas blowing from the bottom.

Since the combustion gas sometimes contains a relatively large amount of nitrogen gas, it is preferable to eliminate it or collect! ; carbon dioxide from the combustion gas, to use! then the gas rich in carbon dioxide as a bottom blowing gas. The removal of nitrogen gas from the flue gas is preferably carried out when considering the manufacture of low nitrogen steel.

For the implementation of the method described above, there is provided a steel manufacturing apparatus comprising a gas circulation system and comprising, in combination, an oven .1.

'' / With basic oxygen allowing both the blowing of oxygen from the top and the blowing of a gas rich in carbon dioxide by the bottom, a device for collecting '' Mt * · '"· the waste gas from the oven, a device for burning this gas with oxygen and / or water vapor, an optional device for separating the combustion gas into carbon dioxide and nitrogen, as well as a system of pipes for conveying the molten steel in the furnace with the carbon dioxide coming from the aforementioned combustion device and separation device.

The present invention also resides in a process characterized in that the blowing oxygen is injected from the top by means of a lance on the molten metal at an outlet speed of Mach 0.8-2.0, of preferably Mach 0.8-1.5.

In another aspect, the method of manufacturing top blown oxygen steel according to the present invention resides in a method characterized in that, together with a top blown oxygen jet, a powder of charge is loaded. a slag-forming agent (flux) comprising quicklime, limestone, fluorite, dolomite, iron ore or mixtures thereof while,

At the same time, carbon dioxide is blown from below into the molten steel throughout the period of the blowing of oxygen from the top or even until the start of casting. that is to say at least partially during the period of time elapsing between the start of the soufflé ^ and the pouring of the molten bath.

When using the conventional top blown oxygen converter, the oxygen jet can be sprayed with powder by a conventional lance! blowing from the top. However, the introduction of? · I ♦ 'V' "* I powder into a high pressure oxygen line inevitably leads to high equipment costs. Consequently, a separate route is provided for directing the powder to a carrier gas to the tip of the oxygen blowing lance, as well as for mixing the powder with an oxygen jet to be dispensed by the nozzle of the lance. By doing so, the disadvantages can be eliminated of the LD-AC process without causing abrasion of the "Laval" type oxygen blast nozzle. More specifically, in an example of a process of this type and according to an embodiment of the invention (see FIGS. 6 and 7 to which reference is made below), a lance with three sheaths (four coaxial tubes) is used. ). The support gas for the flux powder is not specified and an appropriate gas can be chosen according to the composition and the granularity of the particles of the flux, the internal diameter of the pipe, the type and the flow rate of the support gas or the type of oven used. However, it is desirable to load the total quantity of a given powder with fondant I in a period of time corresponding approximately to three quarters I of the blowing period, in order to dissolve the fungus! during the period of the refining operation and | also in order to quickly form a reactive slag per- | "Putting in ensuring an efficient refining operation.

* | According to the process of the present invention, a gas is blown from below the molten steel together with the distribution of a flux using a lance and, preferably, the blowing gas by the bottom is blown at a flow rate of 0.02 to 0.5 Nm3 / minute per ton of molten steel in the case of a bath with a depth of 250 cm.

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1 '··· ^ Ι

In this interval, iron and manganese undergo less oxidation as the gas flow increases. Consequently, by choosing suitable conditions for blowing the flux, as well as for blowing the gas from below the molten bath, depending on the type of steel to be produced, it is possible to obtain a steel having a desired final composition with a high precision, high efficiency and very easy.

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The features of the present invention have been described in detail above. The present invention is particularly applicable to the manufacture of carbon steel, for example, non-quenched steel, quenched steel, I etc, as well as to the manufacture of low-alloy steel.

More particularly, the present invention provides a! satisfactory process for making low carbon steel, for example, steel containing less than 0.3% carbon.

The process of the present invention offers the advantages described below compared to the conventional process.

Since the oxidation of iron, manganese, etc. is strongly inhibited, the iron yield is significantly improved and the amount of ferro-alloys used can be reduced. In addition, since the temperature difference between the molten steel and the slag decreases, the dephosphorization is favored. Another advantage of the present invention resides in the fact that the blowing gas, Î that is to say carbon dioxide, exists in an abundant quantity in a steel manufacturing installation and this, at a low price. . This is an economic aspect of the present invention. Therefore, the present invention also has practical value if we consider that actueblemei /; / we seek to save energy and prevent pollution. of the environment.

As mentioned above, the process of the present invention can be easily implemented in the traditional steel-making process by installing at least one nozzle in the conventional basic oxygen furnace. Of course, the application of the present invention is not limited to existing oxygen converters. As far as it is possible to combine top blowing and bottom blowing, the present invention j j is applicable to any type of metal refining furnace.

] In the accompanying drawings: | Figure 1 is a side elevational view I schematically illustrating the basic bottom blown oxygen furnace used according to the present invention; Figure 2 is a graph showing the blowing diagrams according to the present invention; Figure 3 is a diagram illustrating the system of the steel fabrication apparatus according to this | invention; | Figures 4 and 5 are graphs showing | the effect exerted by the present invention; i | . FIG. 6 is a view from the bottom of the lance I with three sheaths to be used in an embodiment of the present invention, and | . FIG. 7 is a longitu- \ 'j = dinal cross section of this lance, this section being taken along the

-I

line a-a of figure 6.

a '* § According to the present invention, I * I cast iron, scrap iron and d.' other starting materials! in a basic bottom-blown oxygen furnace, -a- \ ri f • that is to say the converter 1 illustrated in FIG. 1. At the bottom of this oxygen converter, two to ten concentric nozzles 2 are provided During the operation of this converter, a lance 4 projects pure oxygen onto the surface or into the molten metal 3j while the bottom blowing gas, mainly consisting of carbon dioxide, is blown into this molten metal by means of the nozzles 2. In this example, the nozzles are arranged in two rows. However, it should be noted that the structure, the arrangement and the number of nozzles are not particularly limited.

In a preferred embodiment of the present invention, a gas jet consisting mainly of carbon dioxide and distributed by pipes 5 is introduced into the molten metal 3 by concentric nozzles 2 provided at the bottom of the basic oxygen furnace and this , at least partially during the period from the start of blowing to the casting of the molten bath, the flow rate of this bottom blowing gas being less than 9/100 of the flow rate of oxygen blown onto the molten bath by la lance In one embodiment of this | invention, the uniform mixing time is set to 20 | seconds or more.

! * We will now describe another form of realization! tion of the invention with reference to Figure 3 l an oven i | 31 steel fabrication using oxygen has an i! . launches 32 blowing oxygen from the top and a series

S

S of gas blowing nozzles 33 from the bottom. Oxygen i | gaseous is blown into the furnace 31 by the lance 32 of souffla ^ from the top in order to effect decarburization, this gaseous oxygen then being evacuated mainly in the form of oxide s .j.

T

carbon gas. A gas mainly consisting of carbon dioxide is sprayed, through the bottom blowing nozzles 33, into the oven 31 where it decomposes according to the reaction C + C02 -> 2C0, to then be removed from the oven.

These two supplies of gaseous carbon monoxide are captured in a hood 34 provided at the top of the oven 31 · They contain less than $ 20 by volume of nitrogen gas because they come to replace the atmosphere when they are captured in the hood.

The gas captured in the hood 34 is first of all freed of dust by a dust collector 35}

Then it is transferred to a gasometer 36, where it is kept momentarily before undergoing the subsequent treatments. The gas contained in the gasometer 36 is mixed with oxygen and / or water vapor before entering a burner 37 · The gas contained in the gasometer 36 can be burned with oxygen from a tank 4 * designed to supply oxygen to the blowing lance from the top 32, this gas possibly being dehumidified before being transferred to a carbon dioxide tank 39 or, after combustion-

Ition, the gas can be subjected to denitrification in a CO 2 / N 2 separator 38 in order to increase the CO 2 content before the transfer of this gas into the carbon dioxide tank 39.

I The carbon dioxide transferred to its tank: Ü 39 is brought by the flow regulator 40 to the nozzle 33! 'I gas blowing from the bottom. Alternatively, this anhydride; carbon dioxide can be combined with untreated gas coming i from the gasometer 36 before being conveyed to the nozzle 33 · 'The gas projected into the bath by the bottom blowing nozzle 33 is again captured in the hood 34 with l carbon monoxide from decarburization. From then on, each supply of carbon monoxide and anhydride. carbonic gas maintains recycling in the path described above.

Another embodiment of the invention will now be described with reference to Figures 6 and 7 · The | bottom of a 2 ton converter blowing pure oxygen from the top has two nozzles (inside diameter: 8 mm) through which a gas must be blown from below the molten bath. The lance used for blowing oxygen from the top is a lance with three sheaths (four coaxial tubes) 61, as shown in the bottom view and the longitudinal cross section of Figures 6 and 7 J the center of the disc 63 of the end 62 of this lance has a single nozzle opening 65 with a diameter of 10 mm acting as a passage 64 for the distribution of a flux powder, this opening being surrounded by three nozzle openings 67 each having a diameter 4 * 2 mm and serving as a passage 66 for the distribution of oxygen. This lance allows the powder to be blown against the surface of the molten bath 68 when it is mixed with the oxygen which has to be sprayed by the three peripheral points.

The present invention will be described in more detail by the examples of implementation below.

j, Example 1

For the implementation of the present invention, a conventional oxygen converter with a capacity j is used; 25Ο tonnes. At the bottom of this converter, four jets with a diameter of 10 mm are installed. In this converter, j as the main raw materials, we load 215 tonnes of pig iron, 35 tonnes of scrap iron and, as other starting material, 3 tonnes of quicklime. The composition of the melt is as follows (in% by weight): 4 * 63% of C, * 19 - i ί $ 0.48 of Si, $ 0.45 of Mn, $ 0.123 of P, $ 0.0018 of S, $ 0.0038 of N, the remainder being iron with accidental impurities. The temperature of this cast iron is 1.385 ° C.

Gas blowing is carried out from the top and from the bottom as described below.

The blowing of the oxygen is carried out from the top at a flow rate of 40,000 Nm 3 / h according to the diagram in FIG. 2. The blowing from the bottom is also carried out according to the diagram of this FIG. 2. As shown in FIG. starts blowing from the bottom at a flow rate of 50 Nm3 / h which is brought to 100 Nm3 / h when the oxygen blowing is started from the top. At the final blowing stage, the flow rate of the bottom blowing is brought to 200 Nm3 / h, then it is reduced to 50 Nm3 / h at the end of the blowing from the top.

The bottom blowing gas is an exhaust gas from an oxygen converter and includes, in percent by weight, $ 18 of C0, $ 63 of CC ^ d $ 16 of ^ and $ 3 of As For comparison, a conventional method of manufacturing oxygen steel is also carried out using même the same oxygen converter. The composition of the starting material loaded in the converter and the mode of blowing from the top are identical to those described above. However, in this case, the bottom blowing is not applied. The steel being considered for fabrication is a low carbon, non-quenched steel. Table 1 below summarizes the final composition of the molten steel.

/ v - 20 -! '' Table 1

Composition (% by weight) Tem- Content Rend- ___ per- in Fe ment I ~~ r I »Z Ö Z ture in C ^ P S N (oc) la tiered milk (%) ____ {%) _

Presents 0.063 0.16 0.017 0.012 0.0025 1,625 13.8 96.3; invention

Process 0.065 0.12 0.021 0.015 0.0011 1,618 19.5 95.8 classic! As can be seen from the figures in Table 1 above, the steel obtained by the process of the present invention has a composition which falls within the framework of that of non-quenched steel with a low car-

Ibone; there is also a remarkably effective dephosphorization, as well as an excellent casting yield. Example 2

In this example, example 1 is repeated, with the exception that different types of S gas are used as the bottom blowing gas.

As mentioned above, the bottom blowing gas according to the present invention may be î

An exhaust gas from a pig iron manufacturing facility or a steel manufacturing facility. Consequently, in this example, this type of exhaust gas is used as the bottom blowing gas. Gas # 1 comes from an exhaust gas from a -3 I oxygen converter and is enriched with carbon dioxide. Gas 2 comes from an exhaust gas from a hot oven and is enriched with carbon dioxide.

i 4 j · ί _

Table 2 below summarizes the final composition t of the molten steel during each test.

Table 2

Gas composition ($ by volume)

Gas n ° C02 CO N2 H2 02 1 95 0 5 0 0 2 52.6 0 43 3.2 1.2

Table 3 1 ^ Final composition ($ in Temp-Weight content) in Fe rature

Gas n ° -, -? Anf.J __C N __ {%) __ i 0.058 0.15 0.0019 14.2 1.632 2 0.063 0.15 0.0085 14.5 1.619

According to the present invention, an exhaust gas from an oxygen converter can be used as the bottom blowing gas. If the nitrogen content of this exhaust gas is less than $ 50 by volume, the nitrogen content of the steel obtained will be acceptable. Furthermore, according to the present invention, the] is vigorously stirred, the molten bath, thereby ensuring decarburization and dechosphorization to a degree sufficient for the process to! . the invention is practical.

Example 3 j I The following starting materials are loaded into

»K

an oxygen converter of the type shown in FIG. 1.

; The cocoa capacity of this converter is 250 tonnes and the S - 22 - bath depth is 250 cm. At the bottom of the converter, two concentric nozzles are provided (the inner nozzle has an internal diameter of 12.7 mm and an external diameter

Ide 15.4 mm, the width of the slot is 1.15 mm, while the external nozzle has an internal diameter of 17d7 mm | and an outside diameter of 19> 1 mm).

Cast iron: 220 tonnes.

[Mn] = about $ 0.40 and [P] = about $ 0.150 in the font.

Iron scrap: 30 tonnes.

Other materials: quicklime 9 tonnes iron ore 4> 5 tonnes light dolomite 3 tonnes fluorite 0.2 tonnes slag from converter 1.8 tonnes.

According to the process of the present invention, different types of blowing gas are injected from the bottom into the molten bath, while blowing pure oxygen from the top by means of a lance. Table 4 below summarizes the blowing conditions and the uniform mixing time obtained.

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Example 4

In this example, we use a top blowing oxygen converter with a capacity of 2 tonnes. At the bottom of this converter, there are two pipes "with a diameter of 6 mm.

Through these nozzles, carbon dioxide gas is injected into the molten metal. Oxygen needed? * for blowing from the top is loaded by straight type nozzles and ”Laval” type nozzles.

| In a series of experiments, we vary i | · The flow of carbon dioxide used for blowing "from the bottom. In another series of experiments, the speed of exit of the oxygen from the top blowing is varied.

Other experimental conditions include:

Cast iron: 2,000 kg, 1,380 ° C,

$ 4.20 C, $ 0.52 Si, $ 0.61 Mn, $ 0.121 P, $ 0.020 S

Scrap: 360 kg I Oxygen flow: 6 Nm3 / minute.

Oxygen pressure before passing through the lance: 5 kg / mm2

Distance between the end of the lance and the surface of the bath: 300 mm.

* Carbon dioxide flow rate: 0.1-2.3 Nm3 / minu i l a tonne J Oxygen jet speed: Mach 0.3-2.3 d | Blowing time: 18.6 minutes.

I // - 25 -

Under these conditions, the yield of the casting and the effect of oxygen are determined. The results obtained are summarized in FIGS. 4 and 5. FIG. 4 shows the results obtained when the carbon dioxide flow rate is varied as indicated with an oxygen exit speed of Mach 1. FIG. 5 shows the results I 'obtained when the oxygen output speed is varied as indicated at a carbon dioxide flow rate of 1 Nm minute ton.

The results shown in the form of curves in FIGS. 4 and 5 are indicated in comparison with those obtained in the conventional method of blowing oxygen from the top and without adopting blowing from the bottom.

From the results indicated in FIG. 4} it should be noted that a better casting yield (graph A) and a better oxygen effect (graph B) are obtained compared to those of the conventional method when a carbon dioxide flow rate of 0.3-2 Nm3 / minute ton at an oxygen exit velocity of Mach 1. From I to Figure 5 * ü should also be noted that an i is obtained! better casting yield (graph A) and a better oxygen effect (graph B) compared to those obtained in the conventional method when adopting an oxygen output speed of Mach 0.8-2, preferably Mach 0.8-1 at a carbon dioxide flow rate of 1 Nm3 / minute ton.

I 5 Example 5 dd dd The steel fabrication apparatus includes a 250 ton top blast oxygen converter with four bottom blast nozzles and i. of the gas circulation system comprising the elements described above with reference to. figure 3 · The conditions I - - 26 - i ε j; refinements are as follows: the cast iron is made up of $ 4.63 C, $ 0.51 Si, $ 0.43 Mn, $ 0.115 P, $ 0.023 S, the remainder being Fe 5, the temperature of the molten bath is 1.358 ° C., the rate of hot metal is 87 $ and 3 * 5 tonnes of iron ore are loaded together with auxiliary materials consisting of 11 tonnes of quick lime and 8 tonnes of dolomite The feed rate for the oxygen blown from the top is 40,000 Nm3 / h. The waste gas recovered by the above circulation system is burned, subjected to denitrification and charged at a flow rate of 1,500 Nm3 / h as blowing gas through the bottom consisting of $ 98.5 d volume of C02 and $ 1.5 in volume of N2 · The j refining system is suitable for the production of non-quenched steel! Low carbon: oxygen is blown in the same way as in the conventional process, while the bottom blowing gas is charged at a constant rate until the start of the casting.

The results of 11 refining are as follows: from the furnace, a quantity of 108,000 Nm 3 / h can be recovered from a waste gas consisting of $ 71.1 by volume of CO, by $ 15.2 by volume of CO 2, of $ 10.5 by volume of N2 and $ 3.2 by volume of H ^ O. The steel thus obtained gives the following final analysis: $ 0.0058 for C, $ 0.01 for Si, $ 0.11 for Mn, $ 0.018 for P, $ 0.020 for S, $ 0.0011 for N, the remainder being Fe, while its temperature is 1.628 ° C, which indicates that refining operations such as decarburization and denitrification were adequate for the manufacture of the desired steel.

i - 27 - * Example 6

A converter equipped with a system of this type is operated according to four different processes, namely the process of the present invention (I), the LD-AC process (it), the process in which oxygen is blown by the top and a gas from below by charging a flux in the form of a mass (ill), as well as the conventional process of blowing oxygen from the top (IV). In each case, the following conditions are adopted and the results are obtained. indicated in table 5 · Composition of the cast iron: $ 4.3

of C, $ 0.50 of Si, $ 0.58 of Mn, $ 0.125 of P and $ 0.023 of S. Temperature: 1.380 ° C

Load: 2,000 kg of cast iron and 370 kg of scrap metal Flow rate of oxygen blown from the top:! 6 Nm3 / minute ton

Powder support gas: argon gas at a flow rate of 1 Nm3 / minute

Bottom blowing gas: carbon dioxide gas at a flow rate of 1 Nm3 / minute Distance (h) between the lance and the surface of the; . molten metal: 300 mm | Blowing time: 17.3 minutes.

; ! Table 5! 3 ___ i Chemical analysis ($) Tempe- overflow Content of Fe dej iure de- ij ----- (° C) in ment i. : C Si Mn P S dairy ($) ί ($)

Ji ________—-- I I 0.38 - 0.30 0.012 0.019 1,680 none 6.3 +0.5 to I II 0.39 - 0.15 0.013 0.021 1,685 important 21.8 -0.7 d as much ^ III 0.38 - 0.27 0.035 0.021 1,680 none 6.5 +0.4 i IV 0.41 - 0.14 0.044 0.025 1,690 none 7.3 0

As can be seen from the table above, the process of the present invention is very practical, since the existing oxygen converter can be used by simply installing a nozzle at the bottom or in Λ the side wall. In addition, the gas to be used for the bottom blowing may be an exhaust gas coming from the converter, with or without the additional treatment for increasing the concentration of carbon dioxide. Consequently, the process of the present invention is easily applicable to the existing oxygen converter and it offers practical advantages.

/ ^ λ i! i i i j i

Claims (12)

1. A method of manufacturing carbon steel and low-alloy steel in a basic oxygen furnace, this method consisting in preparing a molten metal suitable for the manufacture of steel in this basic oxygen furnace, performing blowing through the top and blowing through the bottom, then pour the molten steel obtained, characterized in that a jet of the blowing gas through the bottom consisting mainly of carbon dioxide is introduced into the molten metal by at least one nozzle provided in the bottom or side wall of the basic oxygen furnace and this, at least partially during the period of time elapsing between the start of blowing and the pouring of the molten bath, the flow rate of the blowing gas through the bottom being 1 / 200-9 / 100 of the speed at which the oxygen projected by a viem lance hits the molten bath. 2. Method according to claim 1, characterized in that the nitrogen content of the bottom blowing gas is limited to $ 20 in maximum volume. 3. Method according to any one of claims 1 and 2, characterized in that the bottom blowing gas contains a small amount of oxygen. 4 · A method according to any one of claims 1 to 3, characterized in that the amount of the blowing gas is adjusted by the bottom so that the uniform mixing time is greater than 20 seconds. 5- A method according to claim 4λ character = = in that the uniform mixing time is between 20 and 70 seconds. 16. Method according to any one of the res-. dications 1 to 5, characterized in that it also comprises the stages consisting in collecting a waste gas evacuated from the basic oxygen furnace, combining this waste gas with, oxygen and / or steam, burning the mixture thus obtained, then recover the combustion gas formed to blow it into the molten metal at least as part i of the bottom blowing gas. 7. Method according to claim 6, characterized in that the carbon dioxide is recovered from the gas. combustion to blow it into the molten metal at least i as part of the bottom blowing gas. " 8. Method according to any one of the res-! dications 1 to 7, characterized in that the blowing gas r! by the top strikes the molten metal at an exit speed of Mach 0.8—2. 9 »Method according to claim 8, characterized in that the blowing gas from the top strikes the molten metal at an outlet speed of Mach 0.8-1.5 · 10. A method according to any one of claims 1 to 9j characterized in that a powder of a slag-forming agent is introduced into the molten metal together with the top blowing oxygen jet 11. Method according to claim 10, characterized in that the slag-forming agent is at least | - one of the following substances: quicklime, limestone,>! J fluorite, dolomite, iron ore and mixtures thereof. 12. Steel manufacturing apparatus comprising! a gas circulation system, characterized in that it comprises, in combination, an oven allowing both the blowing of pure oxygen from the top and the blowing of a gas mainly consisting of carbon dioxide ^ ·: .- - 31 - / “\ I through the bottom, an element for collecting the resided gas! duaire evacuated from the oven, an element for mixing this waste gas with oxygen and / or steam, an element for burning the mixture thus obtained, as well as an element for passing the combustion gases; formed through a system of pipes in order to inject it into the molten metal contained in this furnace. 13 · Steel manufacturing apparatus according to claim 12, characterized in that it also comprises an element intended to separate carbon dioxide from the combustion gas, the carbon dioxide thus separated being injected into the molten metal at least as part of the bottom blowing gas.