RELATED ART During many years the LD process (Linz-Donauwitz process) has been used to con- vert hot iron into steel. The LD process performs the refinery of iron at low cost and a high production rate. The LD process is normally performed in a refractory lined crucible, called converter, with a vertical water-cooled oxygen lance entering the converter from above. The converter is normally tiltable for charging, tapping and deslagging. The charge is normally made up of hot iron metal plus scrap and fluxes.

Cold pig iron and iron ore can also be charged.

The essential feature of the LD process is that heat produced by the reaction of oxygen with various substances in the metal is used to achieve the desired final conditions with respect of composition and temperature. No external sources of en- ergy are used. Oxygen is blown with a supersonic speed onto the hot metal, which initiates different thermochemical reactions. The main chemical reactions during the blow are exothermic oxidatiori of carbon, silicon, manganese and iron. The heat produced by these reactions are sufficient to raise the temperature of the metal and allows the use of scrap, cold pig iron, iron ore to balance the excess heat. Some reaction elements are discharged as gas, some form the slag. The agitation and circulation of the bath is to a minor extent caused by the force of the gas jet, but the

main action is caused by the intense heat production within the converter. Slag and metal intermixes, and different reactions are promoted or obstructed. The selection of chemical reactions is partly determined by the characteristics of the oxygen blow.

A high speed of the oxygen jet means a larger penetration into the bath, promoting certain reactions to take place.

When the requested conditions, concerning e. g. carbon content and temperature are reached, the blow is interrupted. The converter is tilted down for tapping into a ladle, normally through a tap opening in the upper part of the converter. The melted metal is covered by a relatively thick slag layer, which is undesirable in the ladle. In order to prevent slag from exiting the converter, different devices are known in the state of the art. Most of such slag prevention devices include sensors positioned close to the tap opening, sensing when the metal level has decreased to a certain level, and then closing the tap opening. There are also purely mechanical devices, "floating"on the molten metal, but sinking through the slag, which plug the tap opening when the level of metal is low enough.

It is known in the art, that additional stirring during and after the oxygen blow can change the performance of the converter process. An often used method is to blow inert gases, e. g. N2 or Ar, through the bottom of the converter. This so called gas stirring is believed to promote the reactions in the metal-to-slag interface region.

The gas stirring is mainly used in the end of a blow, when CO gas evolution de- creases.

When the oxygen blow is interrupted, the metal is normally oversaturated with oxy- gen. This oxygen may react with remaining carbon, and thereby lower the carbon content even further. Such reactions also seem to be promoted by introducing gas stirring through the bottom of the converter. Also the slag composition seems to be influenced by post-stirring.

A large problem with earlier LD processes was that the lining of the converter rapidly was weared down, by mechanical and chemical wear. The wear could to some ex- tent be lowered by using harder oxygen blow, but in a normal case, the lining sur- vived only 1,000-1,500 production cycles. After that, the converter had to be teared down and relined, which caused shut-down long periods and large losses of pro- ductivity as well as the very large costs for the lining itself. In later years,"slag splashing"has been introduced. Slag splashing is the use of the remaining slag, which as such is a refractory material, to cover the interior of the converter, after a completed tapping. The slag splashing is normally performed by turning the con- verter into a vertical position again after tapping, introducing a gas jet lance and with an extremely hard blow of inert gases spread the slag over the inner surfaces of the converter. The slag will then act as a part of the lining during the following produc- tion cycle. The number of production cycles can in this manner be increased to at least 10-20,000.

SUMMARY OF THE INVENTION The above described prior art has a number of disadvantages. Firstly, gas stirring normally takes place near the centre of the converter. Gas stirring close to the side walls is difficult to achieve and is normally omitted, resulting in less stirred volumes along the side watts of the converter, as compared with the centre. Furthermore, the introduced gas volumes are moving through the melt the easiest way. The introduc- tion of solid metal pieces may therefore significantly change the flow pattern.

Therefore, the gas stirring does not significantly move the solid pieces around, and the stirring in the vicinity of the pieces is normally also reduces. As a consequence, large pieces of solid metal may be present in the melt until the process is nearly fin- ished. Furthermore, the general stirring effect may be strongly different from one production cycle to the other, which negatively may influence the reproducibility.

Gas stirring is also believed to function more as a promoter for the reactions at the metal-to-slag interface, rather than providing a reliable actual stirring. In that re-

spect, the gas stirring may in some cases even enhance composition gradients within the converter than decreasing them. Furthermore, dead volumes, which are not properly stirred by the refinery process itself, are difficult to reach and stir by gas stirring.

During certain stages of the converter process, the reaction going on in the con- verter is so intense and powerful that the whole content of the converter merely is a heterogeneous mixture of molten metal, slag and gas. In such situations there is risk for slopping, i. e. part of the reaction material boils out over the top of the converter.

Obviously, slopping causes severe damages and has to be avoided. The risk for slopping will vary with the type of oxygen blow ; hard oxygen blow reduces the risks somewhat.

However, a major disadvantage with devices according to prior art is the incompati- bility of gas stirring and slag splashing. By using the above described slag splashing method, which significantly enhance the productivity of a converter, gas stirring be- comes more or less impossible to practise. The small holes in the bottom of the con- verter used for the gas flow, will immediately be plugged by the distributed slag. Ac- tions for keeping the opening open or reopening of the openings are difficult and expensive. According to the state of the art, one has to choose either stirring or slag splashing.

Another disadvantage with converters according to the state of the art occurs at the emptying phase. During the emptying phase the molten metal exiting through the tap opening give rise to a vortex. As in other cases, the vortex tends to draw surface particles down in the centre of the vortex. This is also the case during tapping a converter. This means that even before any level sensor may determine that the slag-to-metal interface is close to the tap opening, large amounts of slag has been sucked into the vortex and down in the ladle.

An object of the present invention is therefore to present an improved device and method for a converter process, which method removes or significantly decreases the above mentioned disadvantages with prior art. It is also an object of the present invention to present a device and method which is functional together with slag splashing as well as without.

The above object is achieved by a method and a device according to the independ- ent claims. A converter process according to the present invention comprises intro- duction of iron into a converter, injection of oxygen, promoting a refinery process and inducing stirring, and emptying the converter, and is characterised by the appli- cation of an electromagnetic travelling field through the converter, inducing an addi- tional stirring flow. The converter process according to the present invention may also comprise slag splashing. A device according to the present invention similarly comprises a converter enclosure, a gas injection lance and is characterised by an electromagnetic stirrer acting through the enclosure walls.

In a preferred embodiment of the method, the electromagnetic stirring application is performed during and/or after the oxygen injection, whereby the strength and/or fre- quency can be varied with time, e. g. when the risk for slopping is high, when solid pieces are added or according to a predetermined schedule, such as a periodical application. A preferred embodiment also includes application of the electromag- netic travelling fields during the emptying step counteracting vortex creation and removing slag from the area around the tap opening.

In a preferred embodiment of the device, the electromagnetic stirrer is arranged re- movably in close connection with the converter enclosure, and most preferably at a nonmagnetic, e. g. stainless steel, window.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and features of the present invention will be apparent in the following detailed description, in connection with the enclose drawings, in which: Fig. 1 is a vertical cross-sectional view of an embodiment of a converter according to the present invention; Fig. 2 is an illustration of a converter according to an embodiment of the present invention during an emptying stage; and Fig. 3a-c are horizontal cross-sectional views of different embodiments of electro- magnetic stirrers giving different types of stirring flow patterns.

DETAILED DESCRIPTION OF EMBODIMENTS The following description presents a few explanatory embodiments of the present invention. These embodiments should be seen only as examples and should not considered as any limitations.

According to fig. 1, a converter, generally referred to as 1, comprises an enclosure, generally referred to as 8, and a water-cooled gas injection lance 2 having one end 6 thereof introduced into the enclosure 8. The enclosure 8 comprises an outer shell 18, preferably of a metallic material, and a lining 20 of refractory material covering the inner side of the outer shell 18. The lining 20 protects the outer shell 18 from the heat, wear and chemical reactivity present inside the enclosure 8. The lining 20 is very thick, often in the range of 600-900 mm. The enclosure 8 comprises a bottom part 10, side walls 12 and a top part 14 with an opening 16. The enclosure 8 has a generally rounded-off shape ; forming a gradual transition from bottom 10 to side walls 12 and from side walls 12 to top part 14, respectively. The general form of the enclosure 8 has a height-to-width ratio which normally is larger than unity.

Solid pieces containing iron, such as scrap, cold pig iron or iron ore are loaded into the converter by a charging crane (not shown). An amount of hot molten metal 5 is spout-poured from charging ladles (not shown) into the converter 1 through the opening 16. The gas injection lance 2 is removed and the converter 1 is normally slightly tilted during this loading process. The converter is brought into a vertical position and the gas injection lance 2 is subsequently lowered through the opening 16. An oxygen flow of super sonic speed is applied with the gas injection lance 2 and oxygen is flowing from the front end 6 of the gas injection lance 2 towards the surface of the hot molten metal 5.

The pure oxygen gas causes an ignition, whereby thermochemical reactions be- tween elements in the metal and the oxygen gas takes place. When the ignition is obtained, fluxes are added. The fluxes are required by slag chemistry to produce sufficient basic slag at an early stage of the blow. This protect the converter lining and reduces impurities to acceptable levels. The fluxes may be added in one step or batchwise.

The main reactions taking place in the converter are exothermic oxidation of carbon, silicon, manganese and iron. The burning of these elements with oxygen and the original heat content of the hot metal is sufficient to supply enough energy to melt the solid pieces loaded into the converter. The powerful reactions also creates a stirring effect on the melt and slag. However, the movement or stirring of the molten metal in the vicinity of solid pieces is limited, at least in the beginning of a refinery process, why the melting of solid pieces often takes undesirably long time.

In order to promote such melting of solid pieces, an additional stirring action is ap- plied. In order to achieve this an electromagnetic stirrer 3 is arranged at the bottom 10 and/or lower parts of the side walls 12 of the enclosure 8. The electromagnetic stirrer 3 comprises electrical conductors arranged in connection with an iron core.

The iron core covers the area close to which the stirring is to be performed. In order to have a close relationship between the enclosure outer shell and the electromag-

netic stirrer 3, the electromagnetic stirrer 3 is given a design which is in substantial agreement with the outer form of the enclosure 8.

An electromagnetic travelling field is applied by the electromagnetic stirrer 3 through the lining 20 and enclosure shell 18 of the converter, and interacts with the molten metal 5 inside the converter 1. The molten metal 5 tries to counteract the applied electromagnetic travelling field, which give rise to a mechanical force on the metal, i. e. a stirring action takes place. This additional stirring in the metal 5 is not limited to areas reached by the oxygen jet, but is applied in volumes determined by the ac- tual shape and position of the electromagnetic stirrer 3. In this way, a stirring action can be applied in areas where solid pieces are present, thus promoting the melting of such pieces.

Throughout the refinery process, the reactions develop in different manners. Gener- ally, reactions are intensified with increasing evolution of CO gas. Towards the end of the decarburization, the CO evolution decreases, why the reaction intensity is reduced. As a result, the agitation and circulation as caused by the reactions are slowing down, which in turn increases any concentration differences between differ- ent parts of the melt. Since the reactions mainly takes place close to the interface between the melt and the slag, the melt volumes close to the slag is depleted of elements to be oxidised, and the reaction intensity decreases. In this way, concen- tration variations are introduced in the melt. In order to promote the reactions in such a situation, a stirring action is preferable. In particular, a stirring action, which evens out concentration differences is preferred, rather than a stirring action pro- viding larger reaction volumes. Gas stirring is of the second type, and does not effi- ciently influence the concentration gradients.

However, in a preferred process, an electromagnetic travelling field is applied to achieve an additional stirring effect, thus promoting the homogenisation of the chemical composition throughout the entire melt. The electromagnetic travelling field can be applied in the same manner as described above. Since this situation is be-

lieved to occur at about the same time in each production cycle, the application of the electromagnetic travelling field can advantageously be carried out according to a predetermined schedule.

In order not to build in a certain determined flow pattern, but rather achieving a more random stirring of the melt, the electromagnetic travelling field can be applied with a strength and/or frequency which varies with time, e. g. in a periodical manner. An additional stirring of varying power on the melt may in certain applications be more efficient to mix the melt than a continuous one.

As described above, there is always a risk for slopping, during an LD process. The slopping risk normally decreases with increasing oxygen flow. In cases, where the reaction situation requires a reduced oxygen flow, other ways of controlling the slopping has to be used. An efficient way to reduce such a risk is to apply an elec- tromagnetic travelling field to the melt. An additional stirring of the melt, not con- nected to chemical reaction intensity, easily controls the amount of gas trapped in the melt and/or slag. By stirring the melt without introducing extra amounts of gas, the trapped volumes of gas are released and will exit from the converter through the opening in a fairly controlled manner. It is thus preferred to increase the strength and/or frequency of the electromagnetic travelling field from the stirrer 3, when there is a risk for slopping.

When the end of the oxygen blow phase of the refinery process is reached, the oxy- gen jet is turned off. In such a situation, the intense oxygen blow has also oversatu- rated the melt with oxygen. By applying an electromagnetic travelling field to the melt a certain time period after the end of the oxygen injection, an additional stirring is achieved. This stirring will-cause some of the excess oxygen to react with the re- maining carbon, bringing down both the carbon content and the excess oxygen con- centration. The reduction of the oxygen content is thus promoted by post-blow stir- ring.

At the end of a refinery process, the gas injection lance 2 is removed from the con- verter. Any external stirrer is preferably removed from its position relative the con- verter. The stirrer 3 can be fixed at a bearer, which easily, preferably remote con- trolle, can be removed from the converter 1. The converter is tilted in order to let the molten metal flow out through the tap opening. In order to prevent vortex crea- tion in the area around the tap opening, an electromagnetic travelling field is applied to the molten metal through the walls of the converter. This is achieved by arranging an electromagnetic stirrer as close as possible to the tap opening. The electromag- netic travelling field gives rise to a stirring of the molten metal. Preferably, the elec- tromagnetic travelling field is applied in such a way that the creation of the vortex is counteracted by the flow induced by the electromagnetic travelling field. Since the creation of vortices depends on small initial disturbances, which subsequently are enhanced, a relatively smalt external stirring, disturbing this initial vortex creation is enough to prevent a vortex creation. By applying an electromagnetic travelling field in such a direction, that a flow in the molten metal is created, which flow is directed substantially away from the tap opening at the interface between slag and metal, an additional effect can be reached. Additional to the vortex killing, the movement of the metal surface away from the tap opening will transport the floating slag away from the area around the tap opening, further decreasing the risk for tapping slag into the tapping ladle. Such a situation is schematically shown in fig. 2.

Stirring by electromagnetic travelling fields is known as such in other metallurgical applications. However, such use normally originates from a wish to obtain a slight mixing of the metallic melts, and does not promote the outcome of the metallurgical process in a similar fashion. It is thus not obvious for anyone skilled in the art to se- lect such methods and employ them in several aspects in a converter process. The applications which normally uses electromagnetic stirrers are smaller facilities, re- quiring moderate power of the electromagnetic travelling field. In a converter proc- ess, where the amount of molten metal often may be 10 to times higher, and the converter and lining are heavy and difficult to penetrate, electromagnetic stirring is not an obvious choice due to constructional and design difficulties. However, the

present invention has surprisingly shown that electromagnetic stirring indeed is fa- vourable in many respects during a converter process, and that the advantages us- ing electromagnetic stirring are so prominent, that design difficulties are more or less neglectable in a comparison.

The fundamental design of the electromagnetic stirrer is similar to what is found in the state of the art. However, the heavy lining 12 of the enclosure 8 in a converter 1 and the amount of metal 5 put additional requirements on the electromagnetic stirrer 3. In smaller metal ovens with electromagnetic stirrers according to the state of the art, a normal power transferred to the metal 5 is in the range of 20 W per ton of metal. In a converter 1, the requirements to obtain the process advantages are in- stead in the order of 100 W-800 W per ton of metal, and since a normal amount of metal 5 in a converter 1 is in the order of 100-150 tons, the total requested power is huge. Because of this, a heating power of about 4 W/m2 is transferred to the con- verter. Efficient air or water cooling has to be performed in order to reduce this heat transfer.

The converter 1 has an outer shell and a thick lining, which the electromagnetic travelling field has to penetrate. During this penetration, the strength of the electro- magnetic travelling field decreases significantly, and is converted into heat. There are a few favourable techniques to avoid a too large loss of power in the wall. One way is to create a window in the converter, through which the electromagnetic trav- elling field may penetrate. The window is positioned in the converter wall at the po- sition for the electromagnetic stirrer. In order to reduce the losses in the window, it is favourably made of a non-magnetic material, such as e. g. austenitic stainless steel.

Other aspects of the electromagnetic stirrers may be found in the state of the art.

The stirrers basically consist of phase windings arranged around an iron core. The pole division, the geometrical shape and size of the iron core etc. are similar to what is found at smaller designs. In fig. 3a to 3c, different geometrical designs are sketched in horizontal cross section. In fig. 3a, an electromagnetic stirrer 3a con-

sists of two parts, arranged at opposite sides of the enclosure 8. The direction in which the electromagnetic travelling field of the stirrers 3a are acting on the molten metal 5 in the enclosure 8, are indicated by arrows 32a. The resulting stirring flow in the enclosure 8, is indicated by the arrows 30a. Fig. 3b illustrates a single part elec- tromagnetic stirrer 3b, positioned below the bottom 10 of the enclosure 8. The stirrer 3b acts on the molten metal 5 in the direction indicated by the arrow 32 b, and the resultant flow indicated by the arrows 30b is not rotationally symmetric. Fig. 3c shows an embodiment with two stirrer parts 3c, one stronger and one weaker, acting at the molten metal 5 according to the arrows 32c. The resultant flow 30c, has a sig- nificantly broken symmetry, which may be advantageously in certain applications.