Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
1998-07-24
2001-05-22
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C266S208000
Reexamination Certificate
active
06235084
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for decarburizing steel melts in a closed metallurgical vessel that is attached to a vacuum unit and into which oxygen is fed via a lance and combustible material is fed via a feed device. The invention also relates to a hollow device for implementing this process.
2. Description of the Prior Art
In what is known as “forced decarburization,” oxygen is added during the decarburization phase. The addition of oxygen is always necessary when the oxygen present in the steel is insufficient for decarburization or is so low that the required C removal is not completed in the available time. In processes of this type, for example, immersion tubes of an RH vessel are submerged into the melt. When pressure reduction begins in the RH vessel, the decarburization process begins simultaneously as a function of the pressure reduction. When a low pressure of p<100 mbar is reached, through a hollow oxygen lance O
2
is blown for approximately 1 to 3 minutes. During the deep vacuum phase, self-decarburization takes place; after deoxidation, decarburization is ended.
During decarburization, up to 70% CO is formed. Part of this gas automatically reacts with part of the added oxygen to form CO
2
. The degree of post-combustion in this method is less than 30%.
Moreover, it is also a metallurgical practice to add aluminum for the purpose of chemically heating steel melts in atmospheric units. During such chemical heating, the energy gained from the combustion of the aluminum with the added oxygen is used to heat the melt.
In addition to its use in purely thermal heating, aluminum may also be used with other substances to treat the melt. For example, EP 0 110 809 discloses a process for treating steel in a ladle with reactive slags. This process calls for a metal-thermal reaction, whereby oxygen is blown through a lance into a bell submerged in the melt. Combustible metal substances react and, as reactive slags form, a neutral or reductive flush gas is blown in below the tube in which the steel treatment occurs.
The disadvantage of this process, which is used for the desulphurization, deoxidation and purification reaction of steel melts, is the formation of reactive slags, that are created in the bell submerged into the molten metal.
Further, EP 0 347 884 B1 discloses a process for the degasification and desulphurization of molten steel, wherein steel is fed through a container into a vacuum chamber. Arranged in the vacuum chamber at a given distance is an oxygen lance, from which oxygen or a gas containing oxygen is blown in for the purpose of combusting the CO in the surface region of the molten steel located in the vacuum chamber. An amount of oxygen fed through the lance is in accordance with a predetermined ratio of (CO+CO
2
)/waste gas quantity or CO/(CO+CO
2
),.
From this process, it is not possible to derive the chemical heating of the melt under particular pressure conditions and the blowing in of a defined quantity of surplus oxygen.
SUMMARY OF THE INVENTION
The object of the invention is to create a process and a device for decarburizing a steel melt that, while realizing a high degree of oxidic purity, shorten the decarburizing time and/or reduce the final carbon content.
According to the invention, a process for decarburizing a metal melt in a closed mettalurgical vessel that is connected to a vacuum unit includes reducing pressure in the vessel to below 100 mbar, introducing replenishment oxygen to implement the removal of carbon, introducing a predetermined additional amount of oxygen, and introducing a combustible metallic substance with the additional amount of oxygen.
According to the invention, in addition to the replenishment oxygen used for carbon removal during the decarburization phase, additional oxygen is blown in simultaneously with a metallic combustion substance that is added in a distributed fashion during the first 10 minutes following completion of the step of adjusting the pressure to below 100 mbar.
In known vacuum units, until now, only killed cast (Al, Si or Al—Si deoxidation) melts and non-killed cast melts (decarburization melts) have been chemically heated after decarburization and subsequent deoxidation. The reason is the decrease in the oxygen needed for decarburization upon addition of the heating aluminum. The energy gain that results, during the reaction, from the combustion of the aluminum with the added oxygen is utilized. However, the decarburization reaction is sharply slowed in this process and the decarburization oxygen to be expected is not achieved.
According to the invention, this advantage is avoided, and the temperature loss occurring during decarburization is compensated for, by means of the heating process using aluminum or similar products. With the proposed addition of oxygen, a partial oxygen surplus of limited duration occurs in the melt during the first 10 minutes of blowing time after the adjustment of the pressure to below 100 mbar. The partial oxygen surplus is the extra oxygen needed during decarburization or non-killed melts in vacuum units to combust metallic combustion substances or combustible mixtures without disadvantageously influencing the decarburization process. This surplus has positive thermodynamic and kinetic effects and promotes the decarburization process in a surprising manner. The decarburization reaction [C]+[O]=(CO), which is highly pressure-dependent and, in particular, temperature-dependent, is accelarated. This is because the strong overheating that occurs briefly during the chemical heating of a partial melt, especially in the RH vessel, has a catalytic effect on the decarburization reaction.
Furthermore, the chemical heating means, e.g., granular aluminum, can be used in a special manner to accelerate decarburization. Along with the thermodynamic effect, the reaction kinetics are influenced by the A
1
2
O
3
particles formed during heating. These deoxidation products act as foreign germinative bodies and thus act in a forcing manner on the speed of decarburization, especially by forming CO bubbles.
In an advantageous embodiment, a combination lance is used to convey both the oxygen and the metallic combustion substances. However, when especially coarse-grained materials are used, it is proposed that they be fed to the vessel via a separate tube.
This process permits the realization of every partial temperature increase during decarburization in a vacuum. This has the advantage of compensating for typical temperature losses due, for example, to inadequately preheated treatment vessels or steel ladles or to delays resulting from transport or extended treatment times.
The targeted chemical heating of decarburization melts during the decarburization phase makes it possible to reduce the converter or ultra high power (UHP) furnace tap temperatures.
In converter furnaces, this reduction in tap temperatures facilitates higher durability, high variability in solid scrap use, and shorter tap-to-tap times, and in electric arc furnaces, the reduction in tap temperatures facilitates shorter tap-to-tap times, lower specific electrolode use and lower specific energy use.
REFERENCES:
patent: 5902374 (1999-05-01), Kitamura et al.
patent: 5931985 (1999-08-01), Schoeler et al.
patent: 53-81418 (1978-07-01), None
patent: 789 591 (1980-12-01), None
Dittrich Rainer
Haers Frank
Peeters Leo
Scholer Horst-Dieter
Wiegmann Volker
Andrews Melvyn
Cohen & Pontani, Lieberman & Pavane
Mannesmann AG
LandOfFree
Method for decarburizing steels melts does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for decarburizing steels melts, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for decarburizing steels melts will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2440186