Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Reexamination Certificate
1998-07-31
2001-05-29
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
C075S010500, C075S010510, C075S010480, C075S551000, C075S500000, C075S531000, C075S543000
Reexamination Certificate
active
06238453
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a process for producing stainless steels, and particularly for producing special steels containing chromium and chromium-nickel, in a smelting arrangement having at least two vessels for supplying a steel foundry.
2. Description of the Related Art
Usually, an electric furnace of conventional construction is used in the production of chromium-containing, and chromium-nickel-containing stainless steels. The electric furnace is constructed as a D.C. or A.C. furnace in which scrap and other iron-containing metallic raw material, e.g., pig iron or DRI (Direct Reduced Iron), are melted together with an adequate amount of alloying elements or alloy carriers. The raw or base material which is melted for this purpose is tapped off into a ladle at a temperature of 1670° C. to 1700° C. The ladle is subsequently emptied into a converter wherein the melt, which contains approximately 2.5% carbon and approximately 1% silicon, is first oxidized or refined by oxygen. The carbon content is next reduced by mixtures of oxygen and nitrogen, and later by mixtures of oxygen and argon.
Depending on the application of different process techniques, decarburization is carried out to produce a final carbon content of less than 0.1%. Resulting chromium losses in the slag must then be recovered by reduction with ferrosilicon or secondary aluminum.
Further, it is known in a three-step process technique to tap off the metal from the converter at carbon contents of approximately 0.2% to 0.3% and subsequently to bring the metal to the final carbon content in a separate vacuum oxidation process.
A disadvantage that the previously known methods have in common is that decanting or reladling the melt one or more times results in high temperature losses. These losses must be compensated. For by using a high tapping temperature resulting in a high amount of energy consumption in the primary melting vessel, such as the electric arc furnace. In addition to the high amount of energy consumption, the known methods disadvantageously cause increased electrode and refractory wear in the electric furnace. Furthermore installation of the converter required for the second process step requires substantially high construction heights for a surrounding building in order to accommodate a blowing lance and exhaust gas system.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for the production of stainless steels having fewer process steps and lower energy consumption in the individual process steps than in the known art. A further object is to provide operating equipment that can be constructed at a lower height.
The process of the present invention begins by melting, in a first vessel, a first charge having mostly iron-containing raw scrap materials and partially carbon-containing alloy carriers. At a temperature of 1460° C., the melt is decarburized by the injection of oxygen, such as by blowing, so as to reduce the carbon content to less than 0.3%. Next the melt is heated to a tapping temperature of between 1620° C. to 1720° C. The carbon content is then reduced to 0.1%. The process of melting a second charge in a second vessel is accomplished simultaneously with the decarburizing of the first charge in the first vessel.
The process according to the present invention is carried out in a smelting device having at least two vessels being operated in parallel. Either electrodes for melting the charge, or blowing lances for top-blowing and/or blowing in oxygen and oxygen mixtures can be used. The vessels thus serve initially as smelting units and then as refining units. This has the advantage that the melt can be processed and brought to a desired temperature without experiencing temperature losses caused by decanting. Scrap, ferronickel, nickel, ferrochromium and other metallic iron-containing raw materials are melted in each of the vessels at different times, preferably by electrical energy. This results in a base metal containing mostly iron and having a chromium and nickel content close to the final analysis of the steel quality to be produced, particularly as austenites, ferrites and martensites.
In an advantageous embodiment of the present invention, when using high carbon-containing and/or high silicon-containing ferrochromium, oxygen is blown in by a lance so that the silicon content is reduced. After a melt temperature of 1500° C. to 1600° C. is reached, in the same vessel, the electrode arm is swiveled out. An oxygen lance is swiveled in, which together with nozzles located in a bottom in a side wall of the vessel, oxidizes the melt with oxygen. Of course, mixtures of oxygen and nitrogen, argon, hydrocarbon, and steam can also be used to oxidize the melt. For an average blowing period of 20 to 40 minutes, and at an oxygen injection rate of 0.1 to 2.0 Nm
3
/t×min., for the oxygen lance and the injection nozzles, the melt is decarburized to a final carbon content of 0.10% to 0.015%.
The amount of heat liberated by the blowing process can be utilized to add coolant, as for example, Ni, FeNi, ferrochromium, scrap and other iron-containing metallic raw materials such as pig iron masses, DRI or alloying agents, in order to adjust the target analysis and target temperature.
After blowing, the slag is reduced by reducing agents such as ferrosilicon, aluminum or secondary aluminum with the addition of slag developers such as lime and fluorspar for recovering oxidized chromium. The produced steel and/or the slag are/is tapped off. The vessel is again filled with scrap and alloy carriers, the electrodes are swiveled in, and the scrap and alloy carriers are melted by the electrodes.
The smelting process can and the subsequent blowing process run successively in each of the respective vessels of synchronously between the vessels. After 80 to 120 minutes, a melt can be prepared from one vessel, or in the case of synchronous production of both vessels, a melt is prepared every 40 to 60 minutes for further-processing, in a continuous casting plant.
The simultaneous use of two vessels not only has the advantage, of continuously supplying a continuously casting plant, but is also advantageous with respect to energy. After the smelting in the first vessel, for example, the-still hot-electrodes drawn out of the first vessel can then be introduced into the second vessel to begin the smelting therein. This process reduces energy consumption and electrode loss.
The blowing process is carried out to the lowest carbon content which naturally leads to high stress of the refractory material in the vessel hearth or bottom. Therefore, the blowing process, in an advantageous embodiment of the present invention, is terminated when a carbon content of 0.2% to 0.4% is reached. In this embodiment, metal and slag are emptied together into a ladle. The slag is removed by decanting and by scraping. The ladle with the metal melt is then transferred into a vacuum vessel, as is known, so that by blowing oxygen the metal melt is refined to a final carbon content.
Utilizing this process, the typically elaborate installation of a converter for the blowing process is not necessary particularly in the preferred embodiments of the present invention, so that investment costs for the process can be decisively reduced. Furthermore, there is no energy-consuming decanting of the base metal which has been melted by electrical energy, especially from the transporting ladle into the converter, in order to refine the carbon content by using oxygen.
For a particularly high degree of oxidation of silicon, another preferred embodiment of the present invention adds that oxygen during the smelting of the charge. A door lance is used in this embodiment so as to avoid special construction steps.
An example of the invention is shown in the accompanying drawing.
REFERENCES:
patent: 3379815 (1968-04-01), Parker
patent: 3507642 (1970-04-01), Shaw
patent: 3575696 (1971-04-01), Rehmus
patent: 3746325 (1973-07-01), Freeberg et al.
patent: 4599107 (1986
Kappes Horst
Rose Lutz
Ulrich Klaus
Vorwerk Hartmut
Cohen & Pontani, Lieberman & Pavane
King Roy
Mannesmann AG
McGuthry-Banks Tima
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