EP0030360B2 - Steel-making process - Google Patents

Abstract

Oxygen is introduced onto the surface of a molten ferrous metal through at least one water-cooled lance disposed above the melt surface while an oxygen free gas in which is entrained ground solids is intermittently introduced into the molten ferrous metal through tuyeres disposed below the melt surface.

Description

The invention relates to a method for producing steel in a converter with nozzles from concentric tubes below the bath surface for blowing in ground solids for slag formation and / or supply of heat, in which oxygen is directed through a water-cooled lance and / or at least one onto the bath surface Inflation nozzle is blown onto the bath surface.

Oxygen freshening for steel production by means of the inflation process and the blow-through process with nozzles made of two concentric tubes for the oxygen and a protective medium arranged in the refractory lining, for example in the converter floor, are used in steel plants worldwide. The further development today aims to increase profitability by improving the spreading rate, reducing the amount of additives (slag formers) and media (oxygen and coolant). Another development direction is to increase the scrap rate up to the exclusive use of scrap and to supply the required energy in the form of fuels with the highest possible thermal efficiency to the melt.

Proposed solutions for this have recently become known. In one of these processes, the scrap is first preheated in the converter, and then carbon-containing, powdered fuels are passed into the melt for further energy supply.

In a process known from GB-A-2011 477, 20 to 80% of the total amount of oxygen is blown onto the melt as a free jet in order to intensively afterburn the carbon monoxide leaving the melt and, above all, to optimally transfer the heat gain resulting from the afterburning back to the To achieve melt. In this process, at least 20% of the total amount of oxygen is blown in below the bath surface. This oxygen can be loaded with powdery slag formers, for example lime dust. Furthermore, in the known method, powdered carbon carriers, for example coke dust, can be blown into the melt with the aid of a carrier gas through nozzles arranged below the bath surface, in order to increase the scrap rate in this way.

This possibility does not exist in a similar method for combined freshness described in the unpublished European application EP-A1-17963, in which 2 to 17 vol.% Of the total amount of oxygen, but no solids are blown in below the bath surface. This process therefore lacks two essential prerequisites that determine both the metallurgy and the economics of freshening.

In another method for producing steel in the converter, on the other hand, the melt is also supplied with heat by means of carbon-containing fuels. The carbon-containing fuels are introduced into the melt, while the oxygen for freshening the melt and for burning the fuels is introduced into the converter simultaneously with gas jets directed onto the bath surface and below the bath surface. The particular advantage of this process is that the fuels introduced are burned to carbon dioxide with a high thermal efficiency of approx. 30%, based on the combustion. The high level of energy utilization is achieved by supplying the oxygen to the bath surface and the associated supply of heat from the CO afterburning to the melt.

The known method also allows the number of nozzles below the bath surface to be reduced; this is associated with further advantages in steel production. A disadvantage of the known method is, however, that if the blowing rate of the carbon-containing fuels is greatly increased under certain operating conditions, there are limits to the simultaneous supply of fuel and oxygen due to the limited blowing cross section of the few nozzles below the bath surface.

In the oxygen inflation process without fresh gas supply below the surface of the bath, the deterioration of the fresh effect at low carbon contents is known as a disadvantage. With a carbon content of the melt of, for example, _ 0.1%, the decarburization rate decreases significantly, since there is no longer a concentration compensation in the melt due to the decreasing formation of CO bubbles. At the same time, the iron oxide content in the melt increases. The slower decarburization rate leads to an extension of the freshening time, and the increased iron oxide content in the slag acts as a loss. Both the extension of the fresh time and the reduction in spreading have an unfavorable effect on economy.

According to the current state of the art, the oxygen blowing process, which does not have these disadvantages, requires at least one change of soil during the operating time of a converter lining. The refractory material in the area of the oxygen nozzles in the converter base wears out at approximately twice the speed of the lining of the converter side wall. In addition to the cost of the refractory material, the working time of around 20 hours for changing the floor is lost as production time.

The aforementioned methods contain partial solutions for the disadvantages of acid mentioned material inflation and oxygen blowing process and show how the heat supply during steel production in the converter can be increased. When oxygen is blown into the melt below and above the bath surface, in addition to the disadvantages of the complex installation for the devices for supplying oxygen below and above the melt, undesirable high hydrogen and nitrogen contents for certain steel qualities result from the nozzle protection medium of the oxygen nozzles below the bath surface. Furthermore, during decarburization, there is a weaker dephosphorization compared to the oxygen inflation process.

It is therefore the object of the invention to combine the advantages of a special slag guide, similar to the oxygen blowing process, but without increased iron losses in the slag, and the advantages of the oxygen blowing process, in particular with regard to the low final carbon contents with a lower iron oxide content of the slag connect as well as to achieve low hydrogen and nitrogen contents in the steel. In addition, the aim is to achieve a high thermal efficiency when blowing carbon-containing fuels into the melt and to improve the durability of the refractory lining in the area of the nozzles (converter floor) below the bath surface.

Finally, even with only a few nozzles in the converter base, it should be possible to blow in relatively large amounts of carbon-containing fuels.

The above-mentioned object is now achieved in that in a method of the type mentioned at the beginning, according to the invention, through the nozzles below the bath surface at least temporarily only an oxygen-free gas, at least partially loaded with the solids, and gases or oxygen containing oxygen, but less than 20% of the total total amount of oxygen into which melt is blown, and that all of the oxygen is blown onto the bath surface to freshen the melt, to afterburn the reaction gases from the melt and to burn the carbon-containing fuels in the melt.

Surprisingly, it has been shown that when blowing in oxygen-free gases below the bath surface, to which the ground solids are temporarily charged to form slag and with which carbon-containing pulverized fuels, for example coke, are introduced into the melt, are sufficient to cheaply produce steel in the converter Perform results as are known from the oxygen blowing process. In particular, low carbon contents can be adjusted in a controllable manner without higher iron losses in the slag. For example, carbon levels of 0.03% could be achieved with iron oxide levels in the slag of approximately 12%. In the oxygen inflation process, the iron oxide content of the slag is already around 25% when the carbon in the steel is still around 0.05%.

According to the invention, less than half of the nozzles normally required in the oxygen blowing process are installed in the converter bottom and / or the lower side wall below the bath surface. Usually these are the usual nozzles consisting of two concentric tubes. In special cases, however, ring slot nozzles according to German patent 2 438142 can also be used, or nozzles made from three concentric tubes are used. These three-tube nozzles have two ring gaps of approximately the same size, approximately 0.5 to 2 mm wide. The three-tube nozzle feeds the suspension of solids and inert gas into the central tube, the annular gap surrounding the central tube introduces oxygen and the outer annular gap hydrocarbons into the melt. The amount of hydrocarbon used to protect the nozzle is low and is normally 0.1 to 5%, based on the amount of carrier gas in the central tube. The proportion of oxygen in the annular gap corresponds at least to the amount of hydrocarbon. During the last fresh phase, inert gas, e.g. B. argon, or another nitrogen and hydrogen-free gas can be introduced.

The bath is to be understood as the converter volume that the completely fresh, resting steel melt occupies when the converter is in the blowing position. The bath surface is accordingly the surface of this melt.

If scrap is preheated in the converter, e.g. B. in the production of a molten steel from solid iron carriers, the nozzles in the steel bath area serve as an oil-oxygen burner for preheating scrap. As soon as there is melt in the converter, these nozzles are used to introduce carbonaceous fuels and slag formers.

• In der Entsilizierungsphase, d. h. ungefähr in den ersten 1 bis 2 Minuten der Frischzeit, dienen die Düsen zum Zuführen der Schlackenbildner, vorzugsweise Kalk. Während des Hauptfrischens. etwa den sich anschliessenden 5 bis 10 Minuten, wird durch diese Düsen die erforderliche Menge kohlenstoffhaltiger Brennstoffe, beispielsweise pulverisierter Koks oder Kohle eingeleitet. Dazu kann parallel weiterer Kalk eingeleitet werden. Zum Beispiel können zwei Düsen der Kohlenstaubförderung und eine oder mehrere Düsen gleichzeitig zum Einleiten von Schlackenbildnern dienen.
• In der Fertigfrischphase etwa in den letzten 2 bis 5 Minuten dienen die Düsen unterhalb der Badoberfläche vorzugsweise nur noch zum Einleiten wasserstoff- oder stickstofffreier Gase mit oder ohne Beladung mit Schlackenbildnern.

In the method according to the invention, the nozzles below the bath surface are used approximately according to the following scheme:

• In the desilication phase, ie approximately in the first 1 to 2 minutes of the fresh time, the nozzles serve to supply the slag formers, preferably lime. During the main refresh. the subsequent 5 to 10 minutes, the required amount of carbon-containing fuels, for example pulverized coke or coal, is introduced through these nozzles. Additional lime can be introduced in parallel. For example, two coal dust extraction nozzles and one or more nozzles can be used to introduce slag formers at the same time.
• In the finished fresh phase, for example in the last 2 to 5 minutes, the nozzles below the bath surface are preferably only used to introduce hydrogen- or nitrogen-free gases with or without loading with slag formers.

As a nozzle protection medium to prevent premature To prevent the nozzles from burning back in the converter lining, hydrocarbons, such as natural gas, methane, propane or heating oil, have proven their worth during the desiliconization and main fresh phase. Argon, carbon monoxide and carbon dioxide are preferably used for finishing or post-blowing for steel grades with low hydrogen and nitrogen requirements.

In the method according to the invention, oxygen can preferably be briefly blown through the central tubes of the nozzles in the bath area until after-blowing. This measure primarily serves to clear the nozzle pipes of unwanted blockages and approaches at the nozzle mouth and to set the desired mushroom-like approaches at the nozzle mouth in the desired size (diameter approx. 100 mm). The alternate operation with slag-forming carrier gas, fuel suspensions and oxygen is possible with the corresponding changeover valves. The amounts of oxygen blown in below the bath surface are small and total less than 20% of the total amount of oxygen.

It is also within the meaning of the invention, in the case of the three-tube nozzle described, in which the central suspension medium tube is surrounded by an oxygen ring gap and a second ring gap for hydrocarbons, the supply of the small amount of oxygen until the post-blowing phase and in special cases also during the post-blowing expand. The amounts of oxygen passed through are small and amount to a total of about 10% of the total amount of oxygen.

According to the invention, the oxygen is blown onto the bath surface to freshen the melt, to afterburn the reaction gases from the melt and to burn the carbon-containing fuels in the melt. A water-cooled oxygen lance has proven itself for this if oxygen is blown as a free jet onto the bath surface via one or more nozzles in the upper converter side wall. The distribution of the amount of oxygen between the lance and inflation nozzles can vary within wide limits. However, at least 1/4 of the oxygen, based on the total amount of oxygen, is passed through the side wall nozzles, as long as the lance blows in the area of the slag bath at a distance of approximately 0.2 to 1.5 m.

The use of the oxygen lance allows active slag work practically at the beginning of the freshening, probably because the slag is hotter than the molten iron itself, in which scrap still dissolves. The slag formers, mainly lime, possibly with fluorspar and / or dolomite additive, are partly charged as lump in the converter or charged to the oxygen of the blowing lance and / or the side wall nozzle in the form of lime powder. Usually about half of the lime requirement is added to the bath surface; the rest is fed through the nozzles below the bath surface. However, the ratio can be shifted up to about 3/4 in either direction. Preferably about 10 to 20% of the total limestone. charged into the converter as lump lime. This results in viscous slags prior to tapping, which are easier to hold back in the converter, and the safe return of phosphorus and sulfur from the slag to the steel melt before tapping is avoided.

This addition technique according to the invention of the `SchIackenbildIdneτ, in particular the lime, below and above the bath surface brings about an early dephosphorization and an improved desulfurization of the iron melt. The mode of action is probably such that the overheated slag on the bath surface and the blown-in oxygen advance the dephosphorization to the actual decarburization phase and the dust lime blown through the melt at relatively high carbon contents, i.e. H. low oxygen potential of the melt, which leads to intensive desulfurization. In the last fresh minutes of the finished fresh period, the melt is supplied with lime through the floor nozzles.

According to the invention, the lance distance in the main blowing phase can be increased approximately after half the fresh time. It is in the spirit of the invention to increase the lance distance so far, i. H. about 1.50 m above the surface of the bath so that the oxygen jet has a similar effect to the free jet of the side wall nozzle and contributes to the CO afterburning and return of the heat generated to the melt.

According to the invention, it is possible without principle disadvantages to remove the lance from the converter after approximately half the fresh time and to only blow the oxygen onto the bath via one or more side wall nozzles.

In special cases, mainly if water-cooled lances can no longer be installed when converting existing oxygen blow-through converters to the method according to the invention, it proves to be possible to work without an oxygen lance and to install oxygen inflation nozzles in the converter lining at two different levels above the bath surface. The lower installation level of the side wall nozzles is then between approx. 0.5 and 2 m above the bath surface. The nozzles are also directed towards the bath surface. In this lower installation level, one or more side wall nozzles can preferably be arranged above the converter pivot, based on the converter blowing position. The nozzles take over the described function of the water-cooled lance in the first half of the fresh season. The position of one or more nozzles in a second, higher level of the converter side wall corresponds in function to the side wall nozzles described when using a water-cooled inflation lance.

Another variant of it ^f indungsgemässen method makes it possible to work without side wall jets with a water-cooled lance above the bath surface. The lance is then only at the beginning of the fresh water during the desilication phase at the aforementioned short distance from the bath surface. Then, about 2 minutes after the start of freshness, as soon as the decarburization phase begins or carbon-containing fuels are added to the melt, the lance distance is increased to over 1.50 m, preferably over 2 m, above the bath surface. In this mode of operation, it has been shown that the oxygen jet emerging from the lance opening has a sufficient running distance in the converter space above the melt in order to ensure optimal afterburning of the reaction gas leaving the melt and return of the heat obtained to the melt. Although this procedure somewhat limits the flexibility of the lance guidance in relation to the fresh flow compared to the combination of lance / side nozzles, the advantages of the method according to the invention could also be achieved with this mode of operation. There have been no disadvantages with regard to iron slagging and the high thermal efficiency of the carbon-containing fuels passed into the melt.

In order to be able to introduce large amounts of fuel into the melt per unit of time, even if the number of nozzles below the bath surface is only small, the oxygen below the bath surface can only be introduced into the melt temporarily according to the invention. The high level of efficiency in the supply of energy by blowing in carbon-containing fuels is also achieved if oxygen is only temporarily led into the melt below the bath surface. Obviously, the temporary induction is sufficient to create conditions that favor the retransfer of the energy obtained from the afterburning of the exhaust gases in the upper converter room to the bathroom. It has been shown that during certain fresh phases it is possible to use all nozzles below the bath surface to introduce the carbon-containing fuels as a suspension with an oxygen-free carrier gas.

Another feature of the invention is to introduce the slag formers, preferably lime (CaO) in powder form through the nozzles below the bath surface. The preferred method of addition is to charge the powdered lime with oxygen.

The invention is explained in more detail below with the aid of non-restrictive examples and an illustration which shows a section through a converter.

A converter for the method according to the invention consists of a sheet steel jacket 1 with a refractory lining 2 and an exchangeable base 3, in the refractory lining nozzles 4 are arranged. The nozzles 4 are usually the known OBM nozzles made of two concentric tubes. Some or all of these floor nozzles can also be designed as three-tube nozzles.

In the converter shown, for example, two floor nozzles 4 are arranged for introducing the dried and pulverized carbon-containing fuels. The suspension of fuel, e.g. B. brown coal coke flour, with an oxygen-free carrier gas, for. B. nitrogen or argon, flows through a manifold 5 via a T-shaped distribution piece 6 to the switching valves 7 and from there to the central tubes of the nozzles 4. The switching valves 7 allow the central tubes of the nozzles 4 alternately with a fuel inert gas -Suspension or only with an oxygen-free gas, in special cases also oxygen, to supply, which flows via a line 8 to the changeover valves 7. The annular gaps of the nozzles 4 are supplied with either a liquid or a gaseous protective medium. The change from liquid to gaseous media and vice versa takes place with the aid of pressure-controlled switching valves 9, which are usually integrated in a nozzle connection flange 10. The liquids and gases are supplied to the changeover valve 9 via feed lines 11, 12.

The floor nozzles 4 work, for example, for preheating solid iron supports as burners. Then liquid hydrocarbons, e.g. B. light heating oil, through line 11, via the changeover valve 9 in the nozzle ring gap and through line 8 via the changeover valve 7 oxygen in a stoichiometric amount for the oil combustion through the central tube of the nozzle 4. As soon as there is melt in the converter and covers the nozzle orifices , is switched to the powdered fuel supply, and at the same time the annular gaps of the nozzles 4 are supplied with gaseous protective media, for example hydrocarbons such as natural gas or propane. The melt can consist of molten steel or post-charged pig iron.

The other floor nozzles are constructed in principle in the same way and serve to supply oxygen-free gases, to which powdered slag formers, in particular CaO and / or carbon-containing fuels, are charged as required. However, all of the floor nozzles can only be loaded temporarily with a suspension of carbon-containing fuel and an oxygen-free gas.

The bottom nozzles for the introduction of the slag formers, only one of which is shown, are evenly charged with the gas-CaO suspension via a collecting line and a lime distributor (not shown). Gaseous hydrocarbons have proven to be reliable as a protective medium in the annular gap, in particular when oxygen or oxygen-containing gases flow briefly through the central tubes of the nozzles. During the The nozzles are operated as burners to preheat the solid feedstocks in the converter.

An oxygen nozzle 14 d. · H is located in the lining 2 of the converter 1 above one of the converter pivot pins 13. an inflation nozzle or side wall nozzle. This inflation nozzle 14 preferably consists of two concentric tubes, the oxygen also flowing through the central tube and a nozzle protection medium through the annular gap. The outlet opening of the nozzle 14 on the inside of the converter lining 2 is at least 2 m above the bath surface 15. In the case shown, this installation height is approximately 3 m. At least 1/4 of the total amount of oxygen flows through the side wall nozzle. The oxygen jet emerges from the nozzle opening at approximately the speed of sound and acts as a free jet in the gas space of the converter. It sucks a multiple of its volume of the reaction gases escaping from the melt in the converter space above the melt. A significant proportion of the carbon monoxide of these reaction gases, experience has shown that at least 20% is burned to e0 ₂ . The heat generated is almost completely transferred to the melt in the operating mode described, and there is no overheating of the upper converter lining. The heat radiation of the free jet, which is at a high temperature (estimated at approx. 2800 ° C), is apparently absorbed by the gases in the converter room contaminated with dust, slag and steel droplets.

Further oxygen is blown onto the bath surface by means of the water-cooled oxygen lance 16. In this case, it is a lance with four outlet openings. In the illustrated mode of operation with lance and side nozzle, the lance is controlled in such a way that it moves close to the bath surface 15 at the start of freshness and the lance distance is increased with increasing freshness. When the amounts of oxygen are divided between the side nozzle and the lance, at least 25% of the total amount flows through the side nozzle, but preferably 30 to 50%.

If all of the oxygen is inflated only through the water-cooled lance, the lance distance from the bath surface 15 should be at least 1.50 m after the start of blowing, but no later than after the desilication phase.

When an oxygen-free gas is supplied through the nozzles 4 below the bath surface with at least temporary loading of pulverized solids, it is possible to maintain a sufficient bath movement, even against fresheners, at very low carbon contents, in order to produce foam slag as in the case of the oxygen inflation process, and to avoid a sharp increase in the iron oxide content of the slag. Roughly 10 to less than 20% of the amount of oxygen as an oxygen-free gas below the bath surface is sufficient as a rough orientation value.

A 200 t converter, which worked according to the method according to the invention, had a water-cooled oxygen lance and two side wall nozzles in the converter hat. During the freshness of approx. 12 minutes, approx. 7000 Nm ³ oxygen was blown through the oxygen lance as when inflating oxygen and approx. 3,000 Nm ³ oxygen through the two side wall nozzles onto the bath surface. There were eight nozzles for oxygen-free gas below the surface of the bath. During the first approx. 8 blowing minutes, a total of approx. 1,000 Nm ³ nitrogen flowed through the nozzles below the bath surface, loaded with a total of 10 t of dust lime for slag formation and 5 t of coke flour to increase scrap by 10 percentage points.

About 40 Nm ^{3 of} natural gas were passed through the annular gaps of the nozzles during the time mentioned. In the last four blowing minutes, 500 Nm ³ argon was introduced into the melt via the nozzles below the bath surface. Without taking into account the additional melted scrap from the fuel supply (coke flour), the scrap rate in the described procedure could be increased by 6 t, corresponding to 3 percentage points, compared to the oxygen inflation process. Output was also improved by 1.5%. This is mainly due to the low iron oxide content of the slag of approx. 15% compared to 25% in the oxygen inflation process and a lower iron loss in the exhaust gas of approx. 0.5% compared to 1.2% in the inflation process.

Similarly advantageous values could be set in the same 200 t converter if all the oxygen was passed through the water-cooled lance and the nozzles below the bath surface were only operated with a suspension of an oxygen-free carrier gas and slag formers or carbon-containing fuels. However, compared to the usual oxygen inflation method, the lance distance (distance of the lance opening from the bath surface) was increased shortly after the start of blowing, about 1 minute later, to about 1.50 m and after another minute to about 2 m.

A significant advantage of the method according to the invention has been the improvement in soil durability compared to the oxygen blowing method. With the usual floor lining of approx. 1 m thickness, there was no need to change the floor for each converter lining. The improvement in soil durability is most likely due to the lower number of nozzles compared to the oxygen blowing process and the use of oxygen-free gases.

The essential feature of using oxygen-free gas below the bath surface with and without loading with solids (slag formers and / or carbon-containing fuels), for example, up to approx. 20% of the total oxygen or discontinuous small amounts of oxygen, but not more than 10% of the total amount of oxygen, brings a number of advantages.

Claims (8)

1. Process for the production of steel in a converter comprising tuyeres of concentric tubes beneath the barth surface for the blowing in of crushed solids for slag formation and/or heat supply, wherein oxygen is blown onto the bath surface through a water-cooled lance and/or at least one blowing on tuyere directed onto the bath surface, characterized in that, through the tuyeres beneath the bath surface, at least from time to time only an oxygen-free gas charged at least partially with the solids and also oxygen- containing gases or oxygen, but in any case in total less than 20 % of the entire oxygen quantity, are blown into the melt, and that the entire oxygen for refining the melt, for afterburning the reaction gases from the melt and for burning the carbon-containing fuels in the melt is blown onto the bath surface.

2. Process according to claim 1 characterized in that CaO, dolomite, fluorspar, calcium carbide or mixtures of these are introduced through the tuyeres beneath the bath surface.

3. Process according to claim 1 or 2, characterized in that carbon-containing, pulverized fuels, such as coal coke dust, lignite coke, graphite and mixtures of these in suspension with an oxygen-free carrier gas are introduced into the melt through the tuyeres beneath the bath surface.

4. Process according to one or more of claims 1 to 3, characterized in that nitrogen, carbon dioxide, carbon monoxide, natural gas, methane, propane inert gases, for example argon and mixtures of these, serve as oxygen-free carrier gases for the solids.

5. Process according to one or more of claims 1 to 4, characterized in that slag forming materials are charged as lump lime into the converter or are blown in the form of lime powder onto the bath surface.

6. Process according to one or more of claims 1 to 5, characterized in that the oxygen is blown onto the bath surface only with a water-cooled lance and the distance between the lance opening and the bath surface after the desiliconization phase is at least 1,5 m.

7. Process according to one or more of claims 1 to 6, characterized in that the oxygen feed onto the bath surface is effected through one or more tuyeres which are built into the converter lining and are protected by a protective medium against premature burning-back and that the gas jet emerging from the tuyere opening acts for an appreciable distance as a free jet and sucks in reaction gases from the converter space before it strikes the bath surface in the converter.

8. Process according to one or more of claims 1 to 7 characterized in that, with simultaneous oxygen feed through a water-cooled lance and one or more blowing on tuyeres directed onto the bath surface, at least one quarter of the total quantity of oxygen is conducted through the blowing-on tuyeres.

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