CN1350594A - Decarburization and dephosphorization of molten metals - Google Patents

Abstract

The present invention relates to a dual method for decarburisation/dephosphorisation of molten metal, in particular for the production of ULC and/or SULC molten metal, wherein said dephosphorisation is achieved by blowing lime-containing fluxes onto the steel bath during the decarburisation reaction carried out in the degassing device. The present invention also relates to a multifunctional lance which can be used to provide oxygen for decarburization as well as powder containing lime-containing flux for dephosphorization. Additional oxygen or solid oxide may be supplied to maintain the dephosphorization process.

Description

Decarburization and dephosphorization of molten metals

field of invention

The present invention relates to the treatment of molten metal, especially for the production of ultra-low carbon (Ultra Low Carbon, referred to as ULC) and ultra-low carbon (Super Ultra Low Carbon, referred to as SULC) molten steel. The treatment method includes decarburization, Dephosphorization and chemical heating steps.

The invention also relates to a blowpipe for carrying out said treatment in a degasser.

Background of the invention

In iron and steel production, the decarburization (ie reduction of the carbon content of the molten metal in a degasser) step is well known. This step is accomplished by blowing oxygen, either in gaseous or solid oxide form, onto the molten metal in a degasser. Because of the temperature drop caused by the decarburization reaction, the molten metal needs to be reheated.

EP-A-0347884 describes a dual-purpose method of decarburization in a degasser by oxygen supply and chemical heating, which is used in particular for the production of ULC steels as well as SULC steels.

US-A-4198229 describes a technique for dephosphorization by adding a flux component and an alkaline earth metal halide such as calcium fluoride to a molten metal bath.

Dephosphorization as described in the latter document has to be carried out in the auxiliary smelting unit after the steel has passed through the converter unit and before decarburization in the degasser unit. However, this method has the following disadvantages:

- time wasted due to this additional step in the routing,

- Therefore, the temperature loss is higher,

- A certain amount of available dissolved oxygen is required in the steel at the auxiliary metallurgical installation.

WO 96/16190 describes a method of blowing a pulsed flow of oxygen or oxygen-containing gas onto the surface of a molten metal bath (ie molten metal in a degasser) as a means of decarburization. Fuel gas may be added to the oxygen stream as well as to the supply of solid oxide particles. This document also describes a blowpipe for supplying said substances. The so-called Mesid-type 1st generation blowpipe, this kind of blowpipe has an annular groove for oxygen, and inside the groove there is a movable annular groove for adding solid oxides, and a Laval part containing nozzles for fuel feeding to make the oxygen acceleration, and the annular cooling jacket located on the outside.

In this type of torch, oxygen is used as the transport gas for blowing the solid oxide onto the molten steel, which makes it difficult to control the independent flow of oxygen and solids.

Another disadvantage of this type of blowpipe is that it cannot be used to heat the refractory material of the degasser from room temperature with said blowpipe. An additional heating system must be used to heat the refractory prior to using this type of blowpipe. The closed construction of the mouth of the blowpipe also makes this type of blowpipe very noisy.

Finally, the design of this type of blowpipe is complicated by the need to install movable slots for blowing the powder.

EP-A-0879896 describes a device and method for the decarburization of molten metals, said device being a degasser comprising several fixed blowpipe nozzles on the side of the degasser, each with a device for blowing gaseous oxygen at supersonic velocity. The inner tube and the outer tube for blowing cooling gas. However, this device cannot incorporate oxygen in the form of solid oxides.

purpose of invention

The main purpose of the present invention is to improve the existing production methods of ULC and SULC steels.

More specifically, the object of the present invention is to provide a method for producing ULC and SULC steels with which time can be saved.

It is a further object of the present invention to provide an improved lance for blowing oxygen and/or solids onto molten metal in a degasser.

Summary of the invention

The invention relates to a method of treating molten metal in a degasser, the method comprising the steps of decarburization and dephosphorization, characterized in that said decarburization and said dephosphorization are carried out simultaneously in said degasser.

In a preferred embodiment of the method according to the invention, said method comprises the steps of:

- blowing an appropriate amount of oxygen onto the molten metal in said degasser, said amount depending on the amount of oxygen required to carry out said decarburization,

- simultaneously with said decarburization, blowing a first powder consisting of a lime-containing flux onto said molten metal in order to reduce the phosphorus content of said metal,

- Simultaneously with said dephosphorization and said decarburization, an additional appropriate amount of oxygen is blown onto said molten metal, said additional amount depending on the amount of oxygen required to carry out said dephosphorization.

According to a preferred embodiment of the method according to the invention, said metal being steel, said method is carried out in a RH-vessel for the production of ultra-low carbon steel and super-ultra-low carbon steel.

The oxygen for decarburization and/or _{dephosphorization} can be supplied in gaseous form or in the form of a second powder comprising solid oxides, for example a powder mainly comprising _Fe2O3 .

The lime-containing flux is powder containing calcium oxide (CaO). It may consist, for example, of 70% CaO and 30% calcium fluoride (CaF ₂ ).

The dosage of the lime-containing flux is 1-4kg per ton of metal. The rate at which the lime-containing flux is blown onto the molten metal is at least 50 kg/min, preferably 100 kg/min.

The invention also relates to a multifunctional blowpipe for the treatment of molten metal, especially steel, in a degasser, said blowpipe comprising a central tank coaxially surrounded by a fluid-cooled tank, said blowpipe being characterized in that:

- fixing said central channel to the inner wall of said cooling channel with at least two ribs,

- Placing one slot in an off-centre position with respect to said central slot.

According to one embodiment of the blowpipe according to the invention, a shaft hole is located in one of the ribs, into which shaft hole the centrifugal groove is inserted.

According to another embodiment, said centrifugal groove is inserted between the interspaces of two said ribs.

The centrifuge tank may be a powder-supply tank connected to a first supply system for powder consisting of lime-containing flux and a second supply system for solid oxide-containing powder and to convey in the gas supply system. The centrifugal tank can be fixed or axially movable.

According to a preferred embodiment, the mouth of the cooling tank is equipped with a ring, and the inner diameter of the ring is 0.8XD-1.6XD, wherein D is the outer diameter of the central tank. Preferably, the inner diameter of the ring is 0.9XD-1.2XD.

According to a preferred embodiment, said ring is provided with holes through which the flow of powder from the centrifuge tank can flow into the molten metal.

According to a preferred embodiment, the mouth of the central groove is placed at a distance (a) from the mouth of the blowpipe, (a) being at most three times D', where D' is the mouth of the central groove inner diameter of the part.

According to a preferred embodiment, transmission means are used to move the blowpipe vertically.

The lance according to the invention may also comprise combustion means.

According to a preferred embodiment, a regulating device is installed to regulate the flow rate of the various substances relative to the lance of the invention.

Brief description of the drawings

Figure 1 depicts the invention within various typical process stages in a steel plant.

Figure 2 depicts a typical route and industrial test results for the route according to the invention.

Figure 3 describes the method of using the top blowpipe according to the invention.

Figures 4a and 4b depict a blowpipe according to a first embodiment of the invention. Fig. 4a represents a front view and Fig. 4b represents a sectional view along A-A'.

Figure 5 represents a sectional view of a second preferred embodiment of the blowpipe according to the invention.

Detailed description of the invention

Figure 1 places the invention into the stages of the metallurgical process used to produce ULC and SULC steels and compares the results with prior art methods.

After processing the pig iron in the converter, the steel can be processed by the typical route (100, 200) or the inventive route 300.

Typical routes are divided into two routes with 2 or 3 process steps between the converter and the continuous casting unit. In the first typical route 100 (no dephosphorization), the steel goes directly to the degasser. After decarburization and chemical heating, SULC or ULC steel is deoxidized and alloyed in a degasser or auxiliary smelting unit. In a second typical route 200, the steel is first dephosphorized in an auxiliary smelter and then treated in the same manner as in the previous case.

According to the invention route 300, the steel enters a degassing unit for decarburization while dephosphorization is carried out by means of a powder containing a lime-containing flux which is blown together with oxygen or independently into the degassing The steel surface of the device. Immediately after decarburization/dephosphorization, the SULC or ULC steel is oxidized and alloyed in a degasser or auxiliary smelting unit.

The method according to the invention combines decarburization/dephosphorization, avoiding time loss and temperature loss. Moreover, in the auxiliary smelting unit, no oxygen supply or high dissolved oxygen content is required before degassing.

According to the route of the invention, immediately after the converter stage, the molten steel enters the degassing unit.

A combined decarburization/dephosphorization takes place in the degasser. For both reactions, a sufficient amount of dissolved oxygen must be present in the molten steel bath. Oxygen can be added via the lance according to the invention as gaseous oxygen or as solid oxide. The oxygen and/or solid oxides that must be blown into the degasser are calculated based on the initial oxygen content and the target oxygen content after decarburization. The latter depends on the required chemical reheat.

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 decarburization can be implemented by blowing gaseous oxygen or solid oxides into the torch of the present invention in the early stage of the decarburization reaction.

According to the invention, immediately after or simultaneously with the oxygen blowing, the powder containing lime-containing flux is blown in for dephosphorization.

Phosphorus is removed by the following reaction: Equation (1) When the oxygen content in the molten steel and the lime content of the flux are higher, the reaction in Equation (1) tends to proceed to the right.

A predetermined condition that must be fulfilled is that there must be a sufficient amount of dissolved oxygen in the molten steel bath when dephosphorization is started. According to Equation 1, the higher the oxygen activity in the steel, the higher the dephosphorization efficiency. For adequate dephosphorization, sufficient oxygen content is required during the decarburization process.

The powder containing the lime-containing flux is only added if the initial oxygen activity is sufficient to meet the requirements.

If oxygen is not available in sufficient quantities, the lance of the invention can be used to add oxygen as gaseous oxygen or in powder form containing solid oxides such as iron oxide before or during the addition of the lime-containing flux. The choice of gaseous oxygen or solid oxide depends only on the target temperature at the end of the process.

If the temperature is too high, in order to promote the dephosphorization reaction without reheating, the same said lance is used to inject solid oxides (such as iron oxide) together with a lime-containing flux to provide the necessary oxygen.

In cases where the temperature is too low for a continuous casting operation, chemical reheating is achieved by blowing excess oxygen and adding aluminum through the alloying tank 15 during decarburization/dephosphorization. However, the stated amounts of oxygen and aluminum are preferably added during deoxidation of the steel after decarburization.

The amount of lime-containing flux added for dephosphorization is 1-4kg/ton of steel, preferably 2-3kg/ton of steel, depending on the initial phosphorus content in the liquid steel and the target phosphorus content after degassing. The amount of lime-containing flux must be blown in at a rate high enough to avoid time loss due to powder blowing, said rate being at least 50 kg/min, preferably 100 kg/min.

The slag used for dephosphorization is mainly formed by the injected powder, so dephosphorization increases the mass function and the blowing powder fluidity function.

After treatment, the slag must have sufficient phosphorus capacity and must be fluid enough to prevent P uptake in the steel. Preferably, an excess of 20% lime is added to the process slag, or the process slag is skimmed and replaced by fresh slag formed by adding lime or a flux containing lime.

Higher phosphorus removal can be achieved, if desired, by extending the treatment time and by blowing in higher amounts of lime-containing flux.

When there is good contact between the lime-containing flux and the metal, the reaction will be accelerated. This is achieved by blowing micronized powder onto the metal surface in the degasser, which powder mixes with the liquid steel through the turbulent action of the steel due to the action of the vacuum. The height of the powder blowing lance is preferably less than 5 meters above the steel level in the degasser.

Figure 2 shows the industrial results (normal distribution represented by the black dot cloud) for a typical route (i.e. degassing route without dephosphorization), and industrial results for the route according to the invention (i.e. decarburization while dephosphorization route) Results (10 white circles around their regression lines). The graph compares the P-content after tapping from the converter (X-axis) with the P-content before the continuous casting unit (Y-axis). With the method according to the invention, more than 20% of the phosphorus is removed.

Figure 3 describes the method with the blowpipe according to the invention. The degassing unit 1 is placed above the vat 2 containing liquid steel 3 . The upper leg communication pipe 4 and the lower leg communication pipe 5 allow the liquid steel to pass through the initial slag 6 . Liquid steel is pumped into the degassing chamber 7 by vacuum.

Oxygen 8 and powder 9, such as lime-containing flux, are blown through the top-blower 10 onto the top of the steel-slag surface 11 in the degassing chamber. Under blowing pressure, the oxygen and lime-containing flux are blown across the surface of the liquid steel. They then rely on the flow of internal steel through the lower leg manifold into the tank and mix with the liquid steel.

The CO+CO ₂ and other gases generated by the reduction of carbon in the solution are discharged through the exhaust gas conduit 12 . The alloying tank 15 is used to feed substances for alloying in the degasser during the reheating process, or after the combined decarburization/dephosphorization process (see Figure 1).

Figures 4a and 4b show a blowpipe according to a preferred embodiment of the present invention. This lance provides the opportunity to inject powder with or without an additional supply of oxygen, as well as to regulate the powder flow independently of the oxygen flow. The powder may consist of a solid oxide for decarburization, or a lime-containing flux for dephosphorization, or a mixture of both. This means that all oxidation feeds and lime-containing flux feeds can be supplied simultaneously, which makes it possible to supply powders throughout the decarburization time independent of oxygen blowing and its treatment time.

The blowpipe as shown in Figures 4a and 4b comprises a central groove 13 with a Laval-shaped portion 33 at its end. The central tank is connected with: an oxygen supply system 51 , a fuel (such as natural gas) supply system 52 and a protective gas (such as argon) supply system 53 . In order to avoid possible damage to the central tank when the flow of oxygen is stopped, the protective gas is conveyed through the central tank. The central tank 13 is surrounded by an annular cooling tank 12 which is connected to a cooling liquid (eg water) supply system 54 . Between the outer wall of the central tank 13 and the inner wall 24 of the cooling tank 21 is another annular tank 19 which is likewise connected to said oxygen supply system 51 .

The central tank 13 is fixed to the inner wall 24 of the cooling tank 21 by at least two ribs 14 . The slot 31 is placed in an off-center position with respect to the central slot 13 . In the embodiment shown in FIG. 4 b , this eccentric groove 31 is inserted into the space between said ribs 14 . The tank 31 is connected to a supply system 55 of powder consisting of lime-containing flux and a supply system 56 of powder comprising solid oxide. The tank 31 is also connected to a supply system 57 for an inert transport gas (which may be the same gas as the protective gas, such as argon), in another embodiment of the invention shown in FIG. 5 , One of the ribs has a shaft hole 17 into which a slot 31 is inserted.

The groove 31 is preferably fixed. However, according to another embodiment of the invention, the slot 31 can be displaced in its axial direction so that it can extend until its mouth abuts the mouth 25 of the blowpipe 25 .

There is a distance (a) between the mouth 18 of the central tank 13 and the mouth of the blowpipe 25 . The distance (a) is preferably less than three times the inner diameter D' of the mouth of the central groove 13. At the mouth 25 of the blowpipe is placed a ring 26 with an inner diameter d of 0.8XD to 1.6XD, preferably 0.9XD to 1.2XD, where D is the outer diameter of the central groove 13 . The ring 26 has holes 27 through which the flow of powder from the groove 31 can flow into the molten metal. In case the slot 31 is movable, the hole 27 is sufficiently large to accommodate the slot 31 in the extended position.

In the case of a transmission 61, by means of which the torch can be moved vertically along its axis so that its mouth 25 can be placed closer or further away from the molten metal in the degasser.

In the preferred embodiment shown in Figures 4a and 4b, there is a combustion device 32 connected in the form of a tank to a fuel supply system 52, an oxygen supply system 51 and a protective gas supply system 53. When the lance is used as a burner, ie when oxygen and fuel are fed through the central slot 13, the burner is used to burn a flame to heat (eg prior to decarburization/dephosphorization) the refractory material of the degasser.

Regulators are provided to regulate the flow rate of various substances sent to the blowpipe, namely regulating gas oxygen 71, cooling liquid 76, fuel 73, powder consisting of flux 74 containing lime and powder 77 containing solid oxide, transport gas 75 and the flow of protective gas 78.

Compared with the prior art, the torch according to the invention can independently regulate the addition of oxygen and powder. The lance may contain a burner 32 which can heat the refractory material of the degasser from room temperature. The powder tank 31 can be fixed, and the hole structure at the mouth can reduce the noise compared with the existing type of blowpipe.

Claims (23)

1. the treatment process of molten metal in the de-gassing vessel, this method comprises decarburization and dephosphorization step, it is characterized in that described decarburization and described dephosphorization carry out in described de-gassing vessel simultaneously.

2. add oxygen according to the process of claim 1 wherein that described decarburization comprises in described molten metal, the add-on of oxygen depends on carries out the needed oxygen amount of described decarburization; Wherein said dephosphorization comprises and adds limy fusing assistant and further add oxygen in described molten metal in described molten metal, and the oxygen add-on depends on carries out the required oxygen amount of described dephosphorization.

3. according to the method for claim 1 or 2, the described oxygen that wherein said decarburization and/or dephosphorization are used is with the gas form supply.

4. according to the method for claim 1 or 2, the described oxygen that wherein said decarburization and/or dephosphorization are used is to contain the powder type supply of soild oxide.

5. according to the method for claim 4, wherein said powder mainly contains Fe ₂O ₃

6. according to the method for claim 1 or 2, wherein said limy fusing assistant is for containing the powder of calcium oxide (CaO).

7. according to the method for claim 6, wherein said powder is by 70%CaO and 30% Calcium Fluoride (Fluorspan) (CaF ₂) form.

8. according to the method for claim 1 or 2, the consumption of wherein said limy fusing assistant is a 1-4kg/ ton molten metal.

9. according to the method for claim 1 or 2, the speed that wherein said limy fusing assistant blows on the described molten metal was at least 50kg/ minute, preferred 100kg/ minute.

10. be used for handling the especially multi-functional blowpipe of de-gassing vessel steel of molten metal, described blowpipe comprises by the central channel (13) by fluid-cooled groove (21) coaxial surrounding, and described blowpipe is characterised in that:

-described central channel (13) is fixed on the inwall (23) of described cooling tank (21) with at least two ribs (14),

-groove (31) is placed on eccentric position with respect to described central channel (13).

11. according to the blowpipe of claim 10, its central axis hole (17) is positioned at one of them of described rib (14), described groove (31) is inserted in the described axis hole (17).

12. according to the blowpipe of claim 10, wherein said groove (31) is inserted between the space of two described ribs (14).

13. according to each blowpipe among the claim 10-12, wherein said groove (31) is powder-filling tank, and described powder supplies groove is connected to first supply system (55) that is used for the powder be made up of limy fusing assistant and second supply system (56) and the transport gas supply system (57) that is used to contain the soild oxide powder.

14. according to each blowpipe among the claim 10-13, wherein said groove (31) is a pickup groove.

15. according to each blowpipe among the claim 10-13, wherein said groove (31) can move vertically.

16. according to each blowpipe among the claim 10-15, wherein said cooling tank (21) is equipped with ring (26), the internal diameter of described ring is 0.8XD-1.6XD, and wherein D is the external diameter of central channel (13).

17. according to the blowpipe of claim 16, the internal diameter of wherein said ring is 0.9XD-1.2XD.

18., it is characterized in that ring (26) has a hole (27) according to the blowpipe of claim 16 or 17.

19. according to each blowpipe among the claim 10-18, wherein the mouth of central channel is placed on apart from the distance (a) of described blowpipe mouth and locates, and (a) is three times of D ' to the maximum, wherein D ' is the internal diameter of the described mouth of described central channel (13).

20., wherein adopt transmission mechanism (61) to make described blowpipe vertical shifting according to each blowpipe among the claim 10-19.

21., also comprise combustion unit (32) according to each blowpipe among the claim 10-20.

22. the blowpipe according to claim 21 comprises

-device (76) is regulated the coolant rate to cooling tank (21),

-three devices (71) are regulated the gas oxygen flow to central channel (13), circumferential groove (19) and combustion unit (32),

-two devices (73) are regulated the fuel flow to central channel (13) and combustion unit (32),

-device (74) is regulated the powder be made up of the limy fusing assistant flow to groove (31),

-device (77), adjusting contains the flow of the powder of soild oxide to groove (31),

-device (75) is regulated rare gas element such as the argon gas flow to groove (31),

-two devices (78) are regulated protective gas such as argon gas to oxygen supply channel (13) with to the flow of combustion unit (32).

23. according among the claim 1-9 each method or according to the production Ultra-low carbon or surpass application in the ultra low-carbon steel in the RH-container of each blowpipe among the claim 10-22.

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