This invention relates to a method of processing ferrous metals including the heating of, and removal of sulphur from, a molten ferrous metal.

The molten ferrous metal is heated by the energy produced in an exothermic reaction between heating agents. The heating agents are usually oxygen gas blown onto the surface of the ferrous metal and aluminium (or silicon) dissolved in the molten ferrous metal. Aluminium (or silicon) is added to the molten ferrous metal at a controlled rate, whilst oxygen gas is simultaneously blown onto the surface of the ferrous metal at a controlled rate. However, in steel making the surface of the molten ferrous metal is covered with a layer of slag which inhibits the dissolving of the aluminium (or silicon) in the ferrous metal, and the transfer of oxygen gas to the surface of the ferrous metal, thereby reducing the efficiency of the heating process.

Sulphur is removed from the ferrous metal once it has been deoxidised. Desulphurisation is achieved by a chemical reaction between the sulphur dissolved within the molten ferrous metal and the slag thereon and, usually, injection of lime/spar and/or calcium containing agents such as calcium suicide. It is therefore desirable to have the maximum possible slag/molten metal contact surface for desulphurisation. As the sulphur content of the molten ferrous metal is reduced, its tendency to absorb nitrogen from the atmosphere increases. The absorption of nitrogen can have a detrimental effect on the properties of the finished product and it is therefore necessary to prevent or minimise the flow of nitrogen (or air containing nitrogen) across the surface of the molten ferrous metal during desulphurisation so as to minimise the pick-up of nitrogen by the metal .

The present invention provides a method of processing ferrous metals wherein the molten ferrous metal is stirred by bubbling a gas chrough the molten ferrous metal. A ring is then partially immersed in the molten ferrous metal within a substantially slag free portion of the surface of the molten ferrous metal to form a bounded substantially slag free area.

The molten ferrous metal is heated by the introduction of exothermically reacting heating agents to the said slag free area, the said ring is removed from the molten ferrous metal when the ferrous metal has reached a pre-determined temperature. Air is then excluded from the volume above the molten ferrous metal and sulphur removed therefrom.

Preferably the exothermically reacting agents are oxygen gas and granulated aluminium or ferro-silicon or a mixture thereof.

Preferably the method of the invention is such that air is excluded from the volume above the molten ferrous metal by placing a cover thereover and maintaining the volume at a higher pressure than that of the surrounding atmosphere.

The present invention also provides apparatus for processing ferrous metals including a receptacle for receiving the molten ferrous metal, a gas supply for bubbling gas through the molten ferrous metal, a moveable ring for defining a bounded substantially slag free area on the surface of the molten ferrous metal, and a moveable receptacle cover for excluding air from the volume above the surface of the molten ferrous metal; wherein the bubbling gas produces a substantially slag free portion on the surface of the molten ferrous metal, the said ring is moveable between a first position such that it is partially immersed in the molten ferrous metal substantially within the slag free portion produced by the bubbling gas and a second position such that it is separated from the molten ferrous metal, the receptacle cover is movable between a first position such that it excludes the surrounding atmosphere from the volume above the molten ferrous metal and a second position such that the volume is open to the surrounding atmosphere, and the receptacle cover and said ring are moveable independently of each other.

Preferably the apparatus of the present invention further includes control means for controlling the rate of supply of heating agents and monitoring means for determining the temperature of the molten ferrous metal wherein the control means form a closed loop control system.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: -

Figure 1 is a transverse section through an apparatus for steel making embodying the present invention, with the receptacle cover in the first position and the ring in its second position;

Figure 2 is a transverse section through an apparatus for steel making embodying the present invention with the receptacle cover in its first position and the ring in its first position,* and

Fiσure 3 is a transverse section through an apparatus for steel making embodying the present invention, with the receptacle cover in the second position and the ring in its first position.

Referring to Figure 1, a ladle 1 carried on a ladle transport car 3 and containing deoxidised steel 2 is enclosed within a substantially airtight chamber 4.

The ladle 1 is provided with an argon stir bottom plug 5. The argon stir bottom plug 5 is a porous plug through which argon gas may be introduced into the molten steel. The steel is stirred by the introduction of the argon gas through the bottom plug 5. The vigorous bubbling of the inert argon gas stirs the ladle contents thereby homogenising the temperature and chemical composition of the molten steel. The flow rate of the argon gas can be adjusted to suit the metallurgical requirement. For example, the argon flow rate is reduced during heating to slow down the rate of dispersion of aluminium.

The chemical composition of the steel is adjusted by the addition of weighed amounts of deoxidants and ferro-alloys. Cooling of the liquid steel is done by the controlled addition of cooling scrap, added in the same way as other ferro-alloys.

There are two discinct layers in the ladle, a layer of molten steel 2 and a layer of slag 14 on the steel's surface.

The bubbling argon gas produces a slag free zone on the surface of the molten steel. The size of this slag free zone varies with the argon gas flow rate. The boundary of the slag free zone changes with the action of the gas and the slag free zone repeatedly expands and contracts .

The chamber 4 is provided with an argon stir top lance 6, an oxygen gas lance 7, an aluminium delivery tube 9, an refractory ring 10 and a water cooled ladle hood 16.

The oxygen lance 7 has a drive 8 which controls the

separation between the oxygen outlet of the lance 7 and the surface of the molten steel. The position of the lance is automatically adjusted after each treatment to compensate for the wear of the lance.

The argon stir top lance 6 may be moved between a first position such that the outlet end of the lance 6 is immersed in the molten steel and the argon gas bubbles therethrough and a second position remote from the molten steel. The bubbling of the argon gas stirs the steel in a manner similar to that described above for the argon stir bottom plug 5. The argon stir top lance 6 may also be used to inject powdered additives into the liquid steel. Such additives are injected together with argon gas acting as their carrier.

A slag free zone 11 is created on the surface of the steel by the argon bubbles breaking therethrough. This applies irrespective of whether the argon gas is introduced via the bottom plug 5 or top lance 6. The refractory ring 10 is lowered into the substantially slag free zone 11 when this is at or near its greatest extent (or when it is at least as great as the diameter of the refractory ring) . The partially immersed refractory ring then encloses a bounded substantially slag free area 19. This area will then remain substantially slag free whilst the ring 10 is so immersed. The ring 10 is a refractory faced steel cylinder supported by three arms 20, raised and lowered by a three rope winch system operated by the ring winch 18. The ring may have any circumferential shape provided that it defines a central aperture for enclosing the slag free area i.e. the ring need not be circular. When the ring has been lowered into its treatment position, the three vertical hangers, 35 which support the ring are clamped by three clamps, 36 to prevent further movement of the ring.

It is important to note that the use of the refractory ring has a beneficial effect on the efficiency of heating. The volume of metal enclosed within the ring is relatively small, at any given time, although the liquid steel within the ring is changed and recirculated by the effect of the argon stirring. Adding the aluminium to a relatively small volume of liquid steel means that oxygen is blown at a locally aluminium rich liquid. This encourages the reaction between oxygen and aluminium which is the required exothermic reaction rather than

the undesired reaction between oxygen and other oxidisable elements such as iron, manganese and carbon. The use of the refractory ring therefore reduces losses of carbon and manganese quite significantly, as opposed to heating methods where the aluminium addition is rapidly diluted, as would be the case for example if the aluminium was added to the full volume of the steel in the ladle.

To increase the efficiency still further, the rate of aluminium addition is automatically controlled and monitored throughout the heating process, depending on the required rate of heating. The oxygen flow rate is also controlled and monitored throughout the heating process, in stoichiometric ratio with the addition rate of the aluminium, avoiding the oxidation of other elements. The ability of the apparatus of the invention to enclose a relatively small, slag free volume of steel is therefore highly advantageous.

A camera 26 is provided which allows the visual monitoring of the surface of the steel and/or slag within the ladle 1.

The argon top lance 6 may be lowered and immersed in the molten steel so as to promote stirring thereof as described above. The argon lance 6 may also be used for injecting suitable powdered desulphurisation agents (e.g. lime/spar and/or calcium containing agents such as calcium suicide) .

It should be noted that although the bulk of the desulphurisation is done by the injection of a suitable powdered desulphurisation agent through an immersed top lance, significant desulphurisation results from contact between the slag layer and liquid steel layer. Maximum slag/molten steel surface contact is required so as to render desulphurisation as efficient as possible. For desulphurisation, therefore, the full surface area of the molten steel should be in contact with the slag. In addition, to maintain the highly reducing conditions required for desulphurisation and so as to minimise take up of nitrogen by the molten steel, the ingress of atmospheric oxygen and nitrogen respectively into the ladle must be minimised. This means that although the full area of the surface of the molten steel in the ladle should be exposed to the top slag layer, air should be excluded from the volume 13 above the molten steel and the slag.

The ladle hood 16 is water cooled and consists of two distinct portions, a moveable ladle cover portion 27 and a fixed ladle cover portion 28 with a seal member 32 therebetween. The moveable cover portion 27 may be moved from a first position where it is resting on the ladle trunnion band reinforcing flange 33 to a second position separated from flange 33 so as to allow communication between the volume 13 and the surrounding atmosphere. A positive pressure (larger than that of the surrounding atmosphere in the chamber 4) is created within the ladle cover 16 when this is in its first position. The ladle hood is sealed on the ladle trunnion band reinforcing flange, 33 rather than the ladle lip since the mating surface is much cleaner and flatter, thereby providing a more effective seal.

The moveable ladle cover portion 27 is raised and lowered by a hood winch 17. The ladle hood winch 17 and the refractory ring winch 16 are independently operable. This allows one to adjust the conditions within the ladle so as to maximise the efficiency of the heating and desulphurisation steps. This feature of the present invention allows one to maximise the efficiency of the steel making process despite the conflicting metallurgical requirements for the heating and desulphurisation processes .

The aluminium delivery tube 9 is fed from a hopper 34 via a vibratory feeder 29 and an addition hopper system 30. This addition hopper system prevents the emission of fumes during alloying, or the drawing of air into the volume 13 above the surface of the steel in the ladle 1. The airtight chamber 4 within which the ladle and its associated equipment is enclosed is provided with at least one fume extraction duct 31. This ensures that fume extraction is carried out from the chamber 4 surrounding the ladle, ladle transport car 3 and ladle hood 16 rather than directly from the ladle hood 16. The rear of the ladle transport car 3 is provided with a plate which coincides with the walls of the chamber 4 so as to form an airtight enclosure when the ladle transport car is in position within the apparatus of the present invention.

The pressure differential between the fixed hood and surrounding enclosure is constantly monitored during treatment. The measured differential is compared with a required figure. Any variation between the required and actual value is used to

open or close an air ballast valve 37 in the exhaust duct. The air ballast: valve, which is a butterfly type valve increases or decreases the air loading to the fume extraction system, effectively increasing or decreasing the extraction capacity. Modulating the extraction capacity is required to ensure a positive pressure within the fixed hood relative to the surrounding enclosure.

The method and apparatus described above allow one to optimise the conditions for distinct metallurgical operations having sometimes conflicting requirements.

The process is now illustrated by means of the following examples of time - temperature curves.

Example 1 (Table 1)

The steel arrives at the installation in the unkilled (oxidised) state. Step 1 The steel is argon stirred using bottom mounted porous plug, or tip lance, for homogenisation purposes .

Step 2 The steel is deoxidised by aluminium addition via the double gate addition hopper and alloy system, whilst the steel is argon stirred.

Step 3 The steel is sampled, and a temperature taken.

Step 4 The steel is reheated by oxygen top blowing with the refractory ring immersed by the simultaneous addition of aluminium (or ferro-silicon) whilst argon stirring using bottom plug or top lance.

Step 5 Based on the steel analysis, ferro-alloy additions are made via the double gate addition hopper and alloy system whilst argon stirring using bottom plug or top lance.

Step 6 The steel is desulphurised by the injection of lime/spar powder through a top lance using argon as a carrier gas .

Step 7 Sulphide modification is done by the injection of calcium into the steel in the form of a cored wire via a wire feed machine.

Step 8 The steel is sampled, and a temperature taken.

Step 9 The steel is reheated by oxygen top blowing with the simultaneous addition of aluminium (or ferro-silicon) whilst argon stirring using bottom plug or top lance.

Step 10 The steel is sampled, and a temperature taken.

Example 2 (Table 2)

The steel arrives at the installation in the unkilled (oxidised) state.

Step 1 The steel is argon stirred using a bottom mounted porous plug or top lance, for homogenisation purposes .

Step 2 The steel is deoxidised by aluminium addition.

Step 3 The steel is sampled, and a temperature taken.

Step 4. Based on the steel analysis, ferro-alloy additions are made via the double gate addition hopper and alloy system whilst argon stirring using bottom plug or top lance.

Step 5 Alloying of special elements or for accurate analysis control can be carried out using cored or solid wires via the wire feed machine whilst argon stirring using bottom plug or top lance.

Step 6 The steel is reheated by oxygen top blowing with the refractory ring immersed by the simultaneous addition or aluminium (or ferro-silicon) whilst argon stirring using bottom plug or top lance.

Step 7 The steel is desulphurised by the injection of

lime/spar powder through a top lance using argon as a carrier gas .

Step 8 The steel is sampled, and a temperature taken.

Step 9 The steel is reheated by oxygen top blowing with the simultaneous addition of aluminium (or ferro-silicon) whilst argon stirring using bottom plug or top lance.

Step 10 Final fine trimming additions are made based on the steel analysis using ferro-alloys via the double gate addition hopper and alloy system whilst argon stirring using bottom plug or top lances.

Step 11 The steel is sampled, and a temperature taken.

Example 3 (Table 3)

Step 1 The steel is argon stirred using a bottom mounted porous plug or top lance, for homogenisation purposes .

Step 2 The steel is deoxidised by aluminium addition.

Step 3 The steel is sampled, and a temperature taken.

Step 4 Based on the steel analysis, cored or solid wire additions are made via the wire feed machine whilst argon stirring using the bottom plug or top lance.

Step 5 The steel is reheated by oxygen top blowing with the simultaneous addition of aluminium (or ferro-silicon) whilst argon stirring using bottom plug or top lance.

Step 6 The steel is reheated by oxygen top blowing with the refractory ring immersed by the simultaneous addition of aluminium (or ferro-silicon) whilst argon stirring using bocto plug or top lance.

Step 7 The steel is desulphurised by the injection of

lime/spar powder through a top lance using argon as a carrier gas .

Step 8 Sulphide modification is done by the injection of calcium into the steel in the form of a cored wire via a wire feed machine.

Step 9 The steel is sampled, and a temperature taken.

Step 10 Final fine trimming additions are made based on the steel analysis using ferro-alloys via the double gate addition hopper and alloy system whilst argon stirring using bottom plug or top lances.

Step 11 The steel is reheated by oxygen top blowing with the simultaneous addition of aluminium (or ferro-silicon) whilst argon stirring using bottom plug or top lance.

Step 12 The steel is sampled, and a temperature taken.

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