METHOD OF DECARBURISATION AND DEPHOSPHORISATION OF A MOLTEN METAL

Abstract: The present invention is related to a method of combined decarburisation/dephosphorisation of a molten metal, in particular for the production of ULC and/or SULC molten steels, wherein said dephosphorisation is achieved by blowing lime containing fluxes onto the steel bath during the decarburisation reaction in the degassing device. The present invention is also related to a multifunctional lance, which can be used to supply oxygen for decarburisation, as well as the powder containing lime based flux for dephosphorisation. Extra oxygen gas or solid oxides can be supplied to maintain the dephosphorisation process.

Description: METHOD OF DECARBURISATION AND DEPHOSPHORISATION OF A MOLTEN METAL Field of the invention [0001] The present invention relates to a treatment of molten metals, in particular for the production of Ultra Low Carbon (ULC) and Super Ultra Low Carbon (SULC) molten steels, including the steps of decarburisation, dephosphorisation and chemical heating. [0002] The present invention is also related to a lance required to perform said treatment in a degasser device . Background of the invention [0003] In the production of steel, the step of decarburisation, meaning reduction of carbon-content of the molten metal in a degasser device is known. This step is performed by blowing oxygen on the molten metal in the degasser device, said oxygen being in gaseous or solid oxide form. Because of the temperature drop caused by the decarburisation reaction, a reheating of the molten metal is performed. [0004] In particular for the production of ULC steels as of SULC steels, such a combined process of decarburisation and chemical heating in the degasser by oxygen supply has been described in EP-A-0347884. [0005] In US-A-4198229, a dephosphorisation technique is described whereby a flux composition and a halide of alkali earth metal such as calcium fluoride is added to the molten metal bath. [0006] The dephosphorisation, as described in the latter document, has to be performed in the secondary metallurgy device, after the steel has passed through a converter device and prior to decarburisation in a degasser device. However, this method has the following drawbacks : - a loss of time due to this additional step in the process route, - as a consequence, the higher temperature losses, - the necessity to have an amount of dissolved oxygen available in the steel at the secondary metallurgy device . [0007] O96/16190 describes a process for blowing a pulsating stream of oxygen or an oxygen containing gas onto the surface of a molten metal bath, e.g. a molten steel in a degasser device, as a means of decarburisation. A fuel gas can be added to the oxygen stream, as well as a supply of solid oxide particles. This document also describes the lance used for the supply of said substances. Called the Mesid type 1 st generation, this lance has an annular channel for oxygen, and inside this channel, a movable circular channel for addition of solid oxides, further a Laval section for accelerating the oxygen, containing nozzles for adding fuel, and an annular cooling sleeve on the outside. [0008] In this type of lance, the oxygen is used as the transport gas for blowing solid oxides on the -molten steel, which makes it difficult to regulate both oxygen and solids flows independently from each other. [0009] Another drawback of this type of lance is the fact that the refractory of the degasser device cannot be heated from ambient temperature using said lance. An additional heating system must be provided to heat up the refractory, prior to the use of this type of lance. The closed structure of the lance mouth also causes high noise levels originating from this type of lance. [0010] Finally, the necessity of having a movable channel for powder blowing complicates the design of this type of lance. [0011] EP-A-0879896 describes an apparatus and a method for decarburisation of molten metal, said apparatus being a degasser device containing a number of fixed lance nozzles m the side of the degasser device, each having an inner tube through which gaseous oxygen is blown at supersonic speeds and an outer tube through which a cooling gas is blown. This installation provides however no possibility to add oxygen m the form of solid oxides. Aims of the invention [0012] The main aim of the present invention is to improve the state of the art methods of producing ULC and SULC steels [0013] In particular, the present invention aims to provide a method to produce ULC and SULC steels, generating a time gain m the method. [0014] A further aim of the present invention is to provide an improved lance used to blow oxygen and/or solids on a molten metal m a degasser device. Summary of the present invention [0015] The present invention is related to a method of treatment of a molten metal m a degasser device, comprising the steps of decarburisation and dephosphorisation, characterised m that said decarburisation and said dephosphorisation are performed simultaneously m said degasser device. [0016] In a preferred embodiment of the method according to the present invention, said method comprises the steps of blowing an amount of oxygen on the molten metal m said degasser device, said amount depending on the need for oxygen to perform said decarburisation, simultaneously with said decarburisation, blowing a first powder, consisting of a lime based flux, on said molten metal, to decrease the phosphorus content of said metal, simultaneously with said dephosphorisation and said decarburisation, blowing a further amount of oxygen on said molten metal, said further amount depending on the need for oxygen to perform sa d dephosphorisation [0017] According to a preferred embodiment of the method according to the present invention, said metal is steel and said method is performed m an RH-vessel to produce Ultra Low Carbon and Super Ultra Low Carbon steels. [0018] Said oxygen for decarburisation and/or dephosphorisation may be supplied m gaseous form or m the form of a second powder containing solid oxides, such as a powder containing substantially Fe 2 0 3 . [0019] Said lime based flux is a powder containing calcium oxide (CaO) . It may for example consist of 70% CaO and 30% calcium fluoride (CaF 2 ) . [0020] The amount of said lime based flux lies between 1 and 4 kg per ton metal . The rate at which said l me based flux is blown on said molten metal is at least 50 kg/minute, and preferably 100 kg/mmute. [0021] The present invention is equally related to a multifunctional lance used for the treatment of molten metal, m particular of steel m a degasser device, said lance comprising a central channel, coaxially surrounded by a channel which is cooled by a fluid, said lance being characterised m that : said central channel is fixed to the inside wall of said cooling channel by way of at least two ribs - a channel is placed eccentrically with respect to said central channel . [0022] According to one embodiment of the the lance according to the present invention, an axial opening is present m one of said ribs, said eccentric channel being inserted m sa d axial opening. [0023] According to another embodiment, said eccentric channel is inserted m the space between two of [0024] Said eccentric channel may be a powder-supply channel, said powder supply channel being connected to a first supply system for a powder consisting of a lime based flux and to a second supply system for a powder containing solid oxides and to a transport gas supply system. Said eccentric channel may be fixed, or it may be movable m axial direction. [0025] According to a preferred embodiment, said cooling channel is equipped with a ring at its mouth section, said ring having an inner diameter ranging from 0.8 X D to 1.6 X D, where D is the outer diameter of the central channel. Preferably, said ring has an inner diameter ranging from 0.9 X D to 1.2 X D. [0026] According to a preferred embodiment, said ring has an opening through which the stream of powder coming from the eccentric channel may flow to the molten metal. [0027] According to a preferred embodiment, the mouth section of the central channel is placed at a distance (a) from the mouth section of said lance, so that (a) is maximum 3 times D', wherein D' is the inner diameter of said mouth section of said central channel. [0028] According to a preferred embodiment, an actuator is used to move said lance in a vertical direction. [0029] The lance according to the invention may further comprise an ignition device. [0030] According to a preferred embodiment, regulating devices are present to regulate the flow of the various substances to the lance according to the present invention. Brief description of the drawings [0031] Figure 1 situates the invention within the different typical process stages at the steel works. [0032] Figure 2 describes industrial test results of the classical route and of the route following the invention. [0033] Figure 3 describes the process using a top blowing lance according to the invention. [0034] Figures 4a and 4b describe the lance according to a first embodiment of the invention. Figure 4a represents the front view, figure 4b represents the section view along A-A' . [0035] Figure 5 represents a section view of a second preferred embodiment of the lance according to the present invention. Detailed description of the invention [0036] Figure 1 places the invention amongst the stages of the metallurgical process, and compares the result to the process of the prior art, for the production of ULC and SULC steels. [0037] After the treatment of the pig iron m the converter, the steel may follow the classical routes (100, 200) or the invention route 300. [0038] The classical route is subdivided m two routes having 2 or 3 treatment stages between the converter and the continuous casting device. In the first classical route 100 (without dephosphorisation) , the steel is going directly to the degasser device. Directly after the decarburisation and chemical heating, the SULC or ULC steel undergoes a deoxidisation an and alloying treatment m the degasser or m the secondary metallurgy device. In the second classical route 200 the steel is first dephosphorised m the secondary metallurgy device and then treated m the same way as the previous case. [0039] According to the invention route 300, the steel goes to the degasser device for decarburisation, and it is simultaneously dephosphorised by means of a powder containing lime based fluxes which is blown together with or independent of oxygen on the surface of the steel into the degasser device. Directly after the decarburisation/dephosphoπsation, the SULC or ULC steel undergoes an oxidisation and alloying treatment m the degasser or m the secondary metallurgy device. The combined decarburisation/dephosphoπsation m the method according to the invention avoids a time loss and temperature loss. Moreover, no oxygen supply or high dissolved oxygen content before degassing is necessary m the secondary metallurgy device. [0040] According to the invention route, the molten steel enters the degassing device immediately after the converter stage. [0041] In said degasser device, a combined decarburisation/dephosphoπsation is to take place. For both reactions, a sufficient amount of dissolved oxygen must be present m the molten steel bath. Oxygen can be added by way of the lance of the invention, m the form of gaseous oxygen or m the form of solid oxides. The oxygen and/or solid oxides that have to be blown into the degasser are calculated based upon the initial oxygen content and the aimed oxygen content after decarburisation. The latter depends on the desired amount of chemical reheating. [0042] If the initial oxygen content is too low, because the initial carbon content is too high to reach the necessary ultra low carbon level, forced decarburisation can be applied by blowing gaseous oxygen or solid oxides m the early stage of the decarburisation reaction by the aid of the lance according to the invention. [0043] According to the invention, a powder containing lime based fluxes is then blown immediately after or simultaneously with said oxygen blowing, m order to remove the phosphorus . [0044] Phosphorus is removed by the following reaction: (CaO) + 2 [P] + 5 [O] <-» (CaO . P 2 0 5 ) slag Eq (1) The reaction of Eq. (1) proceeds the more to the right as the oxygen content m the molten steel and the lime content of the fluxes are higher. [0045] A necessary precondition is the presence of a sufficient quantity of dissolved oxygen m the molten steel bath when starting the dephosphorisation. According to equation 1, the higher the oxygen activity m the steel, the more efficient the dephosphorisation. In order to have sufficient dephosphorisation, a sufficient oxygen content during decarburisation is needed. [0046] If the initial oxygen activity is sufficient to fulfil said requirement, only the powder containing the lime based fluxes is added. [0047] If not enough oxygen is available, oxygen can be added as gaseous oxygen or as a powder containing solid oxides (e.g. iron oxides) by the aid of the lance of the invention before or during the addition of the lime based flux. The choice between gaseous oxygen and solid oxides depends only on the aimed temperature at the end of the treatment . [0048] If the temperature would be too high, solid oxides (e.g. iron oxides) co-mπected with the lime containing fluxes by the same said lance will provide the necessary oxygen m order to promote the dephosphorisation reaction without reheating. [0049] In case the temperature is too low for the continuous casting operation, chemical reheating is realised by blowing an excess amount of oxygen and adding aluminium through the alloying gutter 15 during the decarburisation/dephosphoπsation. However, said amount of oxygen and aluminium is added preferably after the decarburisation, during the deoxidisation of the steel. [0050] The amount of lime based fluxes, added to perform the dephosphorisation is 1 to 4 kg/to steel, preferably 2 to 3 kg/to steel, and is determined by the initial phosphorus content m the liquid steel, and the aimed phosphorus level after degassing. The lime based flux amounts must be blown at a flow rate which is high enough to avoid time loss due to the powder blowing, at a minimum rate of 50 kg/ramute, and preferably at a rate of lOOkg/mmute . [0051] The slag for dephosphorisation is mainly formed by the injected powder, so dephosphorisation increases as a function of the mass and of the fluidity of the powder blown. [0052] After the treatment, the slag must have enough phosphorus capacity and must be fluid enough to prevent P pickup m the steel. Preferably, an excess of 20% lime is added to the process slag or the process slag is skimmed and replaced by a new slag, formed by addition of lime or lime containing fluxes. [0053] By prolonging the treatment time and by blowing a higher amount of lime based flux, higher phosphorus removal will be obtained if needed. [0054] The reaction will be speeded up when a good contact between the lime containing fluxes and metal is obtained. This is realised by blowing the finely dispersed powders on the melt surface m the degasser device, said powder being mixed with the liquid steel by the turbulence of the steel due to the vacuum action. The powder blowing lance is at a height preferably lower than 5 meters above the steel level m the degasser device. [0055] Figure 2 shows industrial results of the classical route (normal represented by the cloud of black points), i.e. degassing without dephosphorisation and of the route following the invention (10 white circles around their regression line), i.e with dephosphorisation during decarburisation. The graph compares P-levels after tapping from the converter (X-axis) to P-levels taken before the continuous casting device (Y-axis) . By the method according to the invention, more than 20% phosphorus removal is realised. [0056] Figure 3 describes the process using a lance according to the invention. The degassing device 1 is placed above the ladle 2 containing the liquid steel 3. The up-leg snorkel 4 and the down-leg snorkel 5 are penetrating the liquid steel through the primary slag 6. The vacuum action pumps the liquid steel into the degassed chamber 7. [0057] The oxygen 8 and the powders 9, e.g. a lime based flux, are blown through the top-lance 10 to the top of the steel-slag surface 11 within the degassing chamber. Due to the blowing pressure, the blown oxygen and the lime based fluxes are penetrating the liquid steel surface. They are then entrained into the ladle by the internal steel movement, through the down- leg snorkel and mixed with the liquid steel . [0058] The CO + C0 2 produced by the reduction of the carbon m solution and other gases are evacuated through the exhaust gas duct 12. The alloying gutter 15 is used for adding substances during reheating, or after the combined decarbuπsation/dephosphorisation process, for alloying m the degasser device (see figure 1) . [0059] Figures 4a and 4b show the lance according to a preferred embodiment of the present invention. This lance offers the opportunity to mπect powder together or without extra oxygen supply, and to regulate the flow of powder mdepently from the flow of oxygen gas . The powder may consist of solid oxides for decarburisation, or of a lime based flux for dephoshphoπsation, or of a mixture of both. This means that all the oxidic and lime based flux additions can be supplied at the same time, which leads to the possibility of using the whole decarburisation time for supplying powder independently of oxygen blowing and treatment time. [0060] The lance, as shown m figures 4a and 4b comprises a central channel 13 with a Laval type section 33 at the end. To the central channel are connected : a supply system for oxygen gas 51, a supply system 52 for fuel, e.g. natural gas, and a supply system 53 for a protection gas, e.g. argon. This protection gas is sent through the central channel, when the oxygen flow is stopped, to prevent said central channel from possible damage. The central channel 13 is surrounded by an annular cooling channel 21, which is connected to the supply system of coolant 54, for example water. Between the outer wall of the central channel 13 and the inner wall 24 of the cooling channel 21 is another annular channel 19, which is equally connected to said oxygen supply system 51. [0061] The central channel 13 is fixed to the inner wall 24 of the cooling channel 21 by way of at least two ribs 14. A channel 31 is placed eccentrically with respect to the central channel 13. In the embodiment shown in figure 4b, this eccentric channel 31 is inserted in the space between two of said ribs 14. This channel 31 is connected to a supply system 55 for powder consisting of a lime based flux and to a supply system 56 for powder containing solid oxides. Said channel 31 is also connected to a supply system for an inert transporting gas 57, which may be the same gas, e.g. argon, as said protection gas. In another embodiment of the invention shown in figure 5, one of the ribs has an axial opening 17, into which the channel 31 is inserted. [0062] The channel 31 is preferably fixed. According to another embodiment of the invention, the channel 31 may however be made movable along its axis so that it can be extended until its mouth section is flush with the mouth section 25 of the lance. [0063] There is a distance (a) between the mouth section 18 of the central channel 13 and the mouth section 25 of the lance. The distance (a) is preferably smaller than three times the inner diameter D' of the mouth of the central channel 13. At the mouth section 25 of the lance, a ring 26 is placed, having an inner diameter d ranging from 0.8 X D to l.6 X D, and preferably from 0.9 X D to 1.2 X D, where D is the outer diameter of the central channel 13. Said ring 26 has an opening 27 through which the stream of powder coming from the channel 31 may flow to the molten metal. In case said channel 31 is movable, said opening 27 is large enough to accommodate said channel 31 m ts extended position. [0064] An actuator 61 is present, by which means the lance can be moved vertically along its axis, so that its mouth section 25 is placed closer or farther away from the molten metal m the degasser device. [0065] In the preferred embodiment shown m figures 4a and 4b, an ignition device 32 is present, m the form of a channel, connected to the fuel supply system 52, to the oxygen supply system 51 and to the protection gas supply system 53. This ignition device is used for igniting the flame when the lance is used as a burner, i.e. when oxygen and fuel are sent through the central channel 13, for example to heat the refractory of the degasser device, prior to decarburisation/dephosphoπsation. [0066] Regulating devices are present to regulate the flow of the various substances which are supplied to the lance, namely for the regulation of the flow of gaseous oxygen 71, coolant 76, fuel 73, powder consisting of a lime based flux 74 and powder containing solid oxides 77, transporting gas 75 and protection gas 78. [0067] Compared to the state of the art, the lance according to the present invention allows independent regulation of the addition of oxygen gas and powders. The lance may comprise an ignition device 32 allowing the heating of the refractory of the degasser device from ambient temperature. The powder channel 31 may be fixed and the open structure at the mouth section allows a noise reduction compared to existing types of lances.

Claims: CLAIMS 1. A method of treatment of a molten metal m a degasser device, comprising the steps of decarburisation and dephosphorisation, characterised m that said decarburisation and said dephosphorisation are performed simultaneously m said degasser device. 2. The method according to claim 1, wherein said decarburisation comprises the addition of oxygen to said molten metal, m an amount which is depending on the need for oxygen to perform said decarburisation, and wherein said dephosphorisation comprises the addition of a lime based flux to said molten metal and the further addition of oxygen to said molten metal, m an amount depending on the need for oxygen to perform said dephosphorisation . 3. The method according to claim 1 or 2, wherein said oxygen for decarburisation and/or dephosphorisation is supplied m gaseous form. 4. The method according to claim 1 or 2 , wherein said oxygen for decarburisation and/or dephosphorisation is supplied m the form of a powder containing solid oxides. 5. The method according to claim 4, wherein said powder contains substantially Fe 2 0 3 . 6. The method according to claim 1 or 2 , wherein said lime based flux is a powder containing calcium oxide (CaO) . 7. The method according to claim 6, wherein said powder consists of 70% CaO and 30% calcium fluoride (CaF 2 ) . 8. The method according to claim 1 or 2 , wherein the amount of said lime based flux lies between 1 and 4 kg per ton molten metal . 9. The method according to claim 1 or 2, wherein the rate at which said lime based flux is blown on said molten metal is at least 50 kg/mmute, and preferably 100 kg/mmute. 10. A multifunctional lance used for the treatment of molten metal, m particular of steel m a degasser device, said lance comprising a central channel (13), coaxially surrounded by a channel (21) which is cooled by a fluid, said lance being characterised m that : said central channel (13) is fixed to the inside wall (23) of said cooling channel (21) by way of at least two ribs (14) , a channel (31) is placed eccentrically with respect to said central channel (13) . 11. The lance according to claim 10, wherein an axial opening (17) is present m one of said ribs (14), said channel (31) being inserted m said axial opening (17) . 12. The lance according to claim 10, wherein said channel (31) is inserted m the space between two of 13. The lance according to any one of the claims 10 to 12, wherein said channel (31) is a powder- supply channel, said powder supply channel being connected to a first supply system (55) for a powder consisting of a lime based flux and to a second supply system (56) for a powder containing solid oxides and to a transport gas supply system (57) . 14. The lance according to any one of the claims 10 to 13, wherein said channel (31) is fixed. 15. The lance according to any one of the claims 10 to 13, wherein said channel (31) is movable in axial direction. 16. The lance according to any one of the claims 10 to 15, wherein said cooling channel (21) is equipped with a ring (26) at its mouth section, said ring having an inner diameter ranging from 0.8 X D to 1.6 X D, where D is the outer diameter of the central channel (13) . 17. The lance according to claim 16, wherein said ring has an inner diameter ranging from 0.9 X D to 1.2 X D. 18. The lance according to claim 16 or 17, characterised by the fact that the ring (26) has an opening (27) . 19. The lance according to any one of the claims 10 to 18, wherein the mouth section of the central channel (13) is placed at a distance (a) from the mouth section (25) of said lance, so that (a) is maximum 3 times D' , wherein D' is the inner diameter of said mouth section of said central channel (13) . 20. The lance according to any one of the claims 10 to 19, wherein an actuator (61) is used to move said lance in a vertical direction. 21. The lance according to any one of the claims 10 to 20, further comprising an ignition device (32) . 22. The lance according to claim 21, comprising a device (76) , regulating the flow of coolant towards the cooling channel (21) , three devices (71) , regulating the flow of gaseous oxygen towards the central channel (13), the surrounding channel (19) and the ignition device (32), two devices (73), regulating the flow of fuel to the central channel (13) and to the ignition device (32), a device (74) , regulating the flow of a powder, consisting of a lime based flux to the channel (31) , - a device (77) , regulating the flow of a powder, containing solid oxides to the channel (31) , a device (75), regulating the flow of inert gas, e.g. Argon to the channel (31) , two devices (78) , regulating the flow of a protection gas, e.g. argon to the oxygen supply channel (13) and to the ignition device (32) . 23. Use of the method according to any one of the claims 1 to 9 or of the lance according to any one of the claims 10 to 22 in order to produce ultra low carbon or super ultra low carbon steels in a RH-vessel .

Inventors: DOBBELAERE DANNY, HAERS FRANK, PEETERS LEO, VAN POUCKE NIEK, DOBBELAERE, DANNY, HAERS, FRANK, PEETERS, LEO, VAN POUCKE, NIEK
Applicants: DOBBELAERE DANNY, HAERS FRANK, PEETERS LEO, POUCKE NIEK VAN, SIDMAR NV, DOBBELAERE, DANNY, HAERS, FRANK, PEETERS, LEO, SIDMAR N.V., VAN POUCKE, NIEK
Application year: 2000
Main classification: C21C7
IPC codes: C21C5, C21C7