EP1190104B1

EP1190104B1 - Method of decarburisation and dephosphorisation of a molten metal

Info

Publication number: EP1190104B1
Application number: EP00926586A
Authority: EP
Inventors: Niek Van Poucke; Leo Peeters; Frank Haers; Danny Dobbelaere
Original assignee: SMS Mevac GmbH
Current assignee: SMS Mevac GmbH
Priority date: 1999-05-07
Filing date: 2000-05-05
Publication date: 2003-03-05
Anticipated expiration: 2020-05-05
Also published as: CN1350594A; ATE233828T1; DE60001576D1; ES2193954T3; TR200103025T2; BR0011233A; WO2000068442A1; ZA200108634B; KR20020005741A; CA2371652A1; EP1190104A1; TR200201466T2; PL353443A1; DE60001576T2; JP2002544376A; BR0011233B1; AU4528900A

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

Field of the invention

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.

Background of the invention

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.
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.
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.
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.

WO96/1619 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.
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.
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.
Finally, the necessity of having a movable channel for powder blowing complicates the design of this type of lance.
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 in 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 in the form of solid oxides.

Aims of the invention

The main aim of the present invention is to improve the state of the art methods of producing ULC and SULC steels
In particular, the present invention aims to provide a method to produce ULC and SULC steels, generating a time gain in the method.

Summary of the present invention

The present invention is related to a method of treatment of a molten metal in a degasser device, comprising the steps of decarburisation and dephosphorisation, characterised in that said decarburisation and said dephosphorisation are performed simultaneously in said degasser device.
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 in 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 said dephosphorisation.

According to a preferred embodiment of the method according to the present invention, said metal is steel and said method is performed in an RH-vessel to produce Ultra Low Carbon and Super Ultra Low Carbon steels.
Said oxygen for decarburisation and/or dephosphorisation may be supplied in gaseous form or in the form of a second powder containing solid oxides, such as a powder containing substantially Fe₂O₃.
Said lime based flux is a powder containing calcium oxide (CaO). It may for example consist of 70% CaO and 30% calcium fluoride (CaF₂).
The amount of said lime based flux lies between 1 and 4 kg per ton metal. The rate at which said lime based flux is blown on said molten metal is at least 50 kg/minute, and preferably 100 kg/minute.

Brief description of the drawings

Figure 1 situates the invention within the different typical process stages at the steel works.
Figure 2 describes industrial test results of the classical route and of the route following the invention.
Figure 3 describes the process using a top blowing lance.

Detailed description of the invention

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.
After the treatment of the pig iron in the converter, the steel may follow the classical routes (100, 200) or the invention route 300.
The classical route is subdivided in 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 in the degasser or in the secondary metallurgy device. In the second classical route 200 the steel is first dephosphorised in the secondary metallurgy device and then treated in the same way as the previous case.
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/dephosphorisation, the SULC or ULC steel undergoes an oxidisation and alloying treatment in the degasser or in the secondary metallurgy device.
The combined decarburisation/dephosphorisation in 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 in the secondary metallurgy device.
According to the invention route, the molten steel enters the degassing device immediately after the converter stage.
In said degasser device, a combined decarburisation/dephosphorisation is to take place. For both reactions, a sufficient amount of dissolved oxygen must be present in the molten steel bath. Oxygen can be added by way of the lance of the invention, in the form of gaseous oxygen or in 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.
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 in the early stage of the decarburisation reaction by the aid of the lance according to the invention.
According to the invention, a powder containing lime based fluxes is then blown immediately after or simultaneously with said oxygen blowing, in order to remove the phosphorus.
Phosphorus is removed by the following reaction: (CaO) + 2 [P] + 5 [O] ↔ (CaO.P₂O₅)slag The reaction of Eq. (1) proceeds the more to the right as the oxygen content in the molten steel and the lime content of the fluxes are higher.
A necessary precondition is the presence of a sufficient quantity of dissolved oxygen in the molten steel bath when starting the dephosphorisation. According to equation 1, the higher the oxygen activity in the steel, the more efficient the dephosphorisation. In order to have sufficient dephosphorisation, a sufficient oxygen content during decarburisation is needed.
If the initial oxygen activity is sufficient to fulfil said requirement, only the powder containing the lime based fluxes is added.
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.
If the temperature would be too high, solid oxides (e.g. iron oxides) co-injected with the lime containing fluxes by the same said lance will provide the necessary oxygen in order to promote the dephosphorisation reaction without reheating.
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/dephosphorisation. However, said amount of oxygen and aluminium is added preferably after the decarburisation, during the deoxidisation of the steel.
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 in 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/minute, and preferably at a rate of 100kg/minute.
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.
After the treatment, the slag must have enough phosphorus capacity and must be fluid enough to prevent P pickup in 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.
By prolonging the treatment time and by blowing a higher amount of lime based flux, higher phosphorus removal will be obtained if needed.
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 in 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 in the degasser device.
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.
Figure 3 describes the process using a lance. 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.
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.
The CO + CO₂ produced by the reduction of the carbon in 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 decarburisation/dephosphorisation process, for alloying in the degasser device (see figure 1).

Claims

A method of treatment of a molten steel in a degasser device, for the production of ULC and/or SULC steel, comprising the steps of decarburisation and dephosphorisation, characterised in that said dephosphorisation is performed by blowing lime containing fluxes onto the steel bath during the decarburisation reaction.
The method according to claim 1, wherein said decarburisation comprises the addition of oxygen to said molten steel, in 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 steel and the further addition of oxygen to said molten steel, in an amount depending on the need for oxygen to perform said dephosphorisation.
The method according to claim 1 or 2, wherein said oxygen for decarburisation and/or dephosphorisation is supplied in gaseous form.
The method according to claim 1 or 2, wherein said oxygen for decarburisation and/or dephosphorisation is supplied in the form of a powder containing solid oxides.
The method according to claim 4, wherein said powder contains substantially Fe₂O₃.
The method according to claim 1 or 2, wherein said lime based flux is a powder containing calcium oxide (CaO).
The method according to claim 6, wherein said powder consists of 70% CaO and 30% calcium fluoride (CaF₂).
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 steel.
The method according to claim 1 or 2, wherein the rate at which said lime based flux is blown on said molten steel is at least 50 kg/minute, and preferably 100 kg/minute.