EP4056720A1 - Method for producing a ferrous alloy with low carbon content - Google Patents

Abstract

The invention relates to a method for producing a ferroalloy with a low carbon content, in particular stainless and high-alloy ULC steels (ultra low carbon steels), in which a melt of the ferroalloy is first treated by adding oxygen in a converter process and then the melt prepared in this way is decarburized in a subsequent vacuum process. In order to shorten the treatment time of the melt and also to reduce the consumption of the required input materials, the invention provides that oxygen is added to the melt before the end of the converter process to form slag, with no reduction in the converter process of the slag takes place, and that the slag is only reduced in the subsequent vacuum process.

Description

The invention relates to a method for producing a ferroalloy with a low carbon content, in particular stainless and high-alloy ULC steels (Ultra Low Carbon steels), in which a melt of the ferroalloy is first treated by adding oxygen in a converter process and then the melt prepared in this way is decarburized in a subsequent vacuum process.

Such a method is from EP 2 986 743 B1 known. A vacuum converter is used here, in which a main decoupling of the melt takes place in a first stage, with oxygen being blown in via a top lance; then, in a second stage, deep decarburization takes place in the same converter under vacuum.

Further solutions, which are related to the present procedure, are from EP 2 878 684 B1 , from the WO 2006/050963 A2 , from the DE 10 2014 215 669 A1 , from the EP 2 207 905 B1 , from the EP 1 431 404 B1 and from the DE 10 2014 221 397 A1 known.

The conventional converter process in combination with a subsequent vacuum process is a technology for the production of stainless and high-alloy steel grades, which are characterized by an extremely low carbon content. It includes the production of austenitic, ferritic and duplex steel grades including all derivatives with a carbon content of less than 0.015%. Process lines (such as the AOD-VOD process or the MRP-VOD process) use this technology. Also known as Triplex. In this respect, we expressly refer to the above EP 2 986 743 B1 the applicant referred.

In a conventional converter and vacuum process, such as in the mentioned EP 2 986 743 B1 described, the process ends metallurgically with a reduction of the slag formed during the carbon refining process. The technology consists of the following treatment phases: decarburization to thermodynamic carbon equilibrium, slag reduction, tapping, deslagging and further decarburization of the melt under vacuum. The continuation of the decarburization in the vacuum vessel then takes place with a slag-free metal surface and a vacuum of approx. 100 mbar and a short post-evacuation (VCD - Vacuum Carbon Degassing) at approx. 1 mbar.

$$\left(X_n{O}_m\right)+\frac{2m}{3}\left[\mathit{Al}\right]=n\left[X\right]+\frac{m}{3}\left\{{\mathit{Al}}_2{O}_3\right\}$$

In the case of the method described at the outset, this concludes with the slag reduction for the purpose of setting the desired properties (according to the target analysis). In general, the reduction proceeds according to the following equations (1) and (2): $$\left(X_n{O}_m\right)+\frac{m}{2}\left[\mathit{si}\right]=n\left[X\right]+\frac{m}{2}\left\{{\mathit{SiO}}_2\right\}$$

$$\left(X_n{O}_m\right)+\frac{2m}{3}\left[\mathit{Al}\right]=n\left[X\right]+\frac{m}{3}\left\{{\mathit{Al}}_2{O}_3\right\}$$

"X" generally stands for a metal component such as chromium or manganese.

The above reaction equations (1) and (2) describe a chemical reduction process between slag and metal, which takes place using a vacuum and strong inert gas stirring (usually with argon).

The invention is based on the object of developing a method of the type mentioned at the outset in such a way that it is possible to achieve a reduction in the treatment time of the melt, in order to thereby enable economic advantages. Furthermore, the consumption of the required input materials should be reduced additively or alternatively.

The solution to this problem provided by the invention is characterized in that oxygen is added to the melt before the end of the converter process to form slag, with no reduction of the slag taking place in the converter process, and that reduction only takes place in the subsequent vacuum process the slag takes place.

The converter process and the vacuum process can be carried out in a single metallurgical vessel. The process can also be carried out in a plant tandem converter VOD (VOD process - Vacuum Oxygen Decarburisation).

The pressure during the vacuum process is preferably kept below 100 mbar.

The temperature of the slag when carrying out the vacuum process is preferably kept between 1720°C and 1760°C.

During the converter process, oxygen is preferably blown in until the final carbon content in the melt is below the oxygen-carbon equilibrium.

A slag former, in particular in the form of fluorspar (CaF ₂ ), aluminum oxide (Al ₂ O ₃ ) and/or sand, can be added to the melt to achieve a high degree of liquefaction of the slag.

The slag that forms in the converter process is preferably kept at a basicity of between 1.4 and 1.6, for which purpose lime is added to the melt.

Thus, in the last oxygen blowing period during the converter treatment, due to said final carbon content, a conditioned liquid slag with very high oxygen reactivity and low viscosity is formed. The desired high oxygen reactivity is set in a targeted manner by excess oxygen (overblowing) and the formation of metal oxides, in particular chromium.

The supply of oxygen is targeted and based on the thermodynamic carbon balance.

The excess oxygen is bound in the slag as an oxide and also dissolved in the liquid metal. The addition of oxygen is determined and adjusted in such a way that a targeted reduction with carbon is possible under the subsequent vacuum.

The desired low viscosity of the slag is ensured by a correspondingly high temperature, in particular for metal oxidation, especially of chromium.

The deep decarburization during the vacuum process when reducing the slag with carbon takes place at the high temperature mentioned and with a set oxygen-inert gas ratio.

The converter treatment of the melt (i.e. the first phase of the process) thus ends without slag reduction.

The invention thus provides that the converter reduction is moved to the vacuum step and integrated there (see the below described figures 1 and 2 ).

A deep vacuum and a high temperature of the melt in the vacuum process replace the reduction process in the converter completely and with better quality. The relocation of the converter reduction process and a special conditioning of the slag in the converter process significantly improves the economics of the process.

• Es erfolgt eine Konverter-Behandlung ohne Schlacke-Reduktion.
• Es wird eine hoher End-Schlacke-Reaktivität sichergestellt.

Compared to the previously known method, as described at the beginning, the solution according to the invention now results in the following:

• There is a converter treatment without slag reduction.
• A high final slag reactivity is ensured.

The final converter oxygen blow period not only aims at a final carbon content, but also at a conditioned slag with very high oxygen reactivity. The oxygen reactivity results from an excess of oxygen (overblowing) and the formation of metal oxides (see the Figures 3 and 4 ).

The slag has a low viscosity. It is set by a high temperature of the metal oxide formation and the use of liquefiers (such as Fluor-Spar (CaF ₂ ), Al ₂ O ₃ or sand).

The treatment temperatures of the slag are high and preferably in the range mentioned.

Before the vacuum treatment, the slagging of the melt is abolished.

The reduction of the converter slag with carbon takes place under a deep vacuum and high temperature.

The preferred high temperature and the preferred deep vacuum with a sufficient degree of oxidation of the slag and the conditions mentioned lead to complete, targeted decarburization. An external supply of oxygen (bubbles) into the melt is only necessary if the conditions or prerequisites of the process described are not met, in particular if the degree of slag oxidation or the temperature of the melt do not correspond to the thermodynamic conditions.

Accordingly, the alloy is decarburized in such a way that in the final decarburization phase of the converter process (ie in the first phase of the process) a conditioned, highly metal-oxidized slag is produced, whereupon in the vacuum process (ie in the second, subsequent phase of the process) a strong decarburization of the melt takes place. The slag reduction in the second phase proceeds with carbon under greatly reduced pressure and high temperature.

• Die Behandlungszeit (Reduktionszeit) kann verkürzt werden. Sie liegt bevorzugt zwischen 6 und 15 Minuten.
• Der durchschnittliche Inertgas-Verbrauch (beispielsweise von Argon) in der Reduktionsperiode liegt bei 5 bis 7 Nm³/t Legierung.
• Der FF-Material-Verbrauch (Verbrauch an feuerfestem Material) kann vermindert werden, insbesondere unter 6,8 kg/ t Legierung, wodurch sich eine Verlängerung des Konverter-Einsatzes um ca. 10 bis 15 % ergibt.
• Der Energiehaushalt der Schmelze kann verbessert werden.

The following advantages can thus be achieved by the proposed procedure:

• The treatment time (reduction time) can be shortened. It is preferably between 6 and 15 minutes.
• The average inert gas consumption (e.g. argon) in the reduction period is 5 to 7 Nm ³ /t alloy.
• The FF material consumption (refractory material consumption) can be reduced, in particular below 6.8 kg/t alloy, resulting in an increase in converter use of about 10 to 15%.
• The energy balance of the melt can be improved.

By eliminating skimming before vacuum treatment, the temperature loss of the melt can be reduced in the range of 15 to 23 K.

Savings of reducing agents of approx. 11.0 kg/t alloy Si or 14.5 kg/t alloy Al can be achieved.

Likewise, a saving of slag-forming agents of approx. 45 kg/t Lime alloy and 3 to 5 kg/t Fluorspar alloy can be achieved.

Fig. 1

zeigt schematisch ein erfindungsgemäßes Konverter-Vakuum-Verfahren mit Zwischen-Abstich,

Fig. 2

zeigt schematisch ein erfindungsgemäßes Konverter-Vakuum-Verfahren ohne Zwischen-Abstich,

Fig. 3

zeigt schematisch das Prinzip der Entkohlung bei der Reduzierung einer Schlacke mit Kohlenstoff in einer Linie aus Konverter- und VOD-Anlage (Vakuum Oxygen Decarburisation),

Fig. 4

zeigt schematisch das Prinzip der Entkohlung bei der Reduzierung einer Schlacke mit Kohlenstoff in einem Vakuum-Konverter und

Fig. 5

zeigt schematisch das Kohlenstoff-Gleichgewicht für rostfreien Stahl (d. h. thermodynamischen Gleichgewichte zwischen Kohlenstoff und Chrom) bei unterschiedlichen Prozessbedingungen.

Exemplary embodiments of the invention are shown in the drawing.

1

shows schematically a converter vacuum process according to the invention with intermediate tapping,

2

shows schematically a converter vacuum process according to the invention without intermediate tapping,

3

shows schematically the principle of decarburization when reducing a slag with carbon in a line consisting of converter and VOD plant (vacuum oxygen decarburization),

4

shows schematically the principle of decarburization when reducing a slag with carbon in a vacuum converter and

figure 5

shows schematically the carbon balance for stainless steel (ie, thermodynamic equilibria between carbon and chromium) at different process conditions.

In the Figures 1 to 4 shows the various sub-processes that result from the combined converter-vacuum process (with and without tapping), where (according to figure 3 ) the converter process can be separated from the vacuum process or (according to figure 4 ) a combination of the two sub-processes takes place.

wobei bedeutet:

(-dC/dt)

Entkohlungsgeschwindigkeit

C

aktueller Kohlenstoffgehalt

C*

Kohlenstoffgleichgewicht

τ

Entkohlungszeitkonstante

Low carbon contents are achievable in the metal-slag system with the presence of oxygen and at greatly reduced process pressure. The vacuum causes a reduction in the carbon balance. The difference between the temporary carbon content and the carbon equilibrium, naturally understood as the reaction potential, forms the driving force of the oxygen-carbon reaction. The greater the reaction potential, the greater the decarburization rate according to equation (3) below: $$\left(-\frac{\mathit{DC}}{\mathit{German}}\right)=\frac{1}{\tau }\left(C-C*\right)$$

where means:

(-dC/dt)

decarburization rate

C

current carbon content

C*

carbon balance

τ

decarburization time constant

wird das thermodynamische Gleichgewicht der X_nO_m -Reduktion durch Kohlenstoff wie folgt gemäß Gleichung (5) beschrieben: $$K_{C,X}\left(T\right)=\frac{a_X^n{p}_{\mathit{CO}}^m}{a_{X_n{O}_m}{a}_C^m}$$

With the illustrated sequence of chemical reactions in the liquid metal according to Equation (4) $$\begin{array}{l}\left\{O_2\right\}=2\left[O\right]\\ n\left[X\right]+m\left[O\right]=\left(X_n{O}_m\right)\\ \left(X_n{O}_m\right)+m\left[C\right]=n\left[X\right]+m\left\{\mathit{CO}\right\}\end{array}$$

the thermodynamic equilibrium of X _n O _m reduction by carbon is described as follows according to equation (5): $$K_{C,X}\left(T\right)=\frac{a_X^n{p}_{\mathit{CO}}^m}{a_{X_n{O}_m}{a}_C^m}$$

wobei $$\begin{array}{l}{a}_C={ƒ}_{C}C*\\ a_X={ƒ}_{X}X\\ p_{\mathit{CO}}=\frac{R}{R+0.5}p\end{array}$$

X

Metall-Komponente wie Chrom, Mangan und Eisen

aC

Kohlenstoffaktivität

fC

Kohlenstoff-Aktivitätskoeffizient

aX

Element X-Aktivität

fX

Element X-Aktivitätskoeffizient

R

O₂/ Inertgas-Verhältnis

aXnOm

Schlacken-Komponente XnOm-Aktivität

p,pCO

Prozessdruck, Kohlenmonoxid-Partialdruck

KC,X(T)

Kohlenstoff-Element X - Reaktionskonstante in Funktion der Temperatur

$$C*=\frac{{\left({ƒ}_{X}X\right)}^{\frac{n}{m}}}{{ƒ}_C{a}_{X_n{O}_m}^{\frac{1}{m}}{K}_{C,X}{\left(T\right)}^{\frac{1}{m}}}\frac{R}{R+0.5}p$$

After solving Equation (5) and using Equation (6) below, Equation (7) can be obtained as given below, from which the dependency of the carbon balance on the external process pressure according to Equation (8) can be seen: $$a_C=\frac{a_X^{\frac{n}{m}}{p}_{\mathit{CO}}}{a_{X_n{O}_m}^{\frac{1}{m}}{K}_{C,X}{\left(T\right)}^{\frac{1}{m}}}$$

whereby $$\begin{array}{l}{a}_C={ƒ}_{C}C*\\ a_X={ƒ}_{X}X\\ p_{\mathit{CO}}=\frac{R}{R+0.5}p\end{array}$$

X

Metal component such as chromium, manganese and iron

aC

carbon activity

fC

carbon activity coefficient

aX

Element X activity

fX

Element X activity coefficient

R

O ₂ / inert gas ratio

aXnOm

Slag component XnOm activity

p,pCO

Process pressure, carbon monoxide partial pressure

KC,X(T)

Carbon element X - reaction constant as a function of temperature

$$C*=\frac{{\left({ƒ}_{X}X\right)}^{\frac{n}{m}}}{{ƒ}_C{a}_{X_n{O}_m}^{\frac{1}{m}}{K}_{C,X}{\left(T\right)}^{\frac{1}{m}}}\frac{R}{R+0.5}p$$

As simulations according to Equation (8) have shown, the carbon-chromium reaction equilibrium is established under atmospheric pressure, a temperature of 1,700 °C, a chromium content of 18 % and an O ₂ /inert gas ratio of 2 (Blas -Level 2:1, R=1.33) at 0.25% on. The result says that a further supply of oxygen after reaching equilibrium, in the case of 0.25%, forces a strong oxidation of metallic elements, especially chromium, and promotes the reactivity of the melt.

In the further vacuum process at a pressure of 0.1 bar and at an R ratio of 0.22, there is a sharp shift in the equilibrium to the level of 0.0078% (78 ppm) and limits further decarburization. In this regard, on figure 5 Referenced where shown.

How out figure 5 with the depiction of the equilibria, both the process management through the vacuum and the temperature control as well as the controlled blowing of oxygen and inert gas or stirring gas (ie the O ₂ /inert gas ratio) are very flexible. The relevant parameters have a significant impact on the efficiency of decarburization. Depending on the alloy type being produced, different variations between the temperature-pressure ratio and the O ₂ /inert gas ratio can be selected within the framework of the preconditions. The temperature is set in the converter process and the pressure in the vacuum process; the setting of the ratio applies to both sub-processes. It should be noted that the endothermic course of the slag reduction must be taken into account in the energy balance of the process. It causes the metal temperature to decrease.

A process control calculates, optimizes and implements the relevant parameters and the conditions of the process. The determination of the decarburization between the end of the converter process and the target analysis after the vacuum step is decisive in order to achieve the carbon target analysis. This value further determines the necessary oxygen demand for metal oxidation. Corrections to the amount of oxygen are not excluded and may result from further need for temperature adjustment and slag former additions related to basicity.

Claims (8)

Process for producing a ferroalloy with a low carbon content, in particular stainless and high-alloy ULC steels (Ultra Low Carbon steels), in which a melt of the ferroalloy is first treated in a converter process by adding oxygen and then in a subsequent vacuum process the melt prepared in this way is decarburized,
characterized,
that oxygen is added to the melt before the end of the converter process to form slag, with no reduction of the slag taking place in the converter process, and that the slag is only reduced in the subsequent vacuum process. Method according to Claim 1, characterized in that the converter process and the vacuum process are carried out in a single metallurgical vessel. Method according to Claim 1, characterized in that the converter process and the vacuum process are carried out in a plant tandem converter VOD (vacuum oxygen decarburization process). Method according to one of Claims 1 to 3, characterized in that the pressure during the vacuum process is kept below 100 mbar. Method according to one of Claims 1 to 4, characterized in that the temperature of the slag is kept between 1,720°C and 1,760°C when the vacuum process is carried out. Method according to one of Claims 1 to 5, characterized in that oxygen is blown in during the converter process until the final carbon content in the melt is below the oxygen-carbon equilibrium. Method according to one of Claims 1 to 6, characterized in that a slag former, in particular in the form of fluorspar (CaF ₂ ), aluminum oxide (Al ₂ O ₃ ) and/or sand, is added to the melt to achieve a high degree of liquefaction of the slag becomes. Method according to one of Claims 1 to 7, characterized in that the slag forming in the converter process is kept at a basicity of between 1.4 and 1.6, for which purpose lime is added to the melt.

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