Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Patent
1988-09-28
1990-05-22
Rosenberg, Peter D.
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
75 1051, 75 1052, C22B 400
Patent
active
049274570
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of manufacturing low carbon ferro-chromium whose nitrogen content is extremely low and whose Cr yield is sufficiently high.
PRIOR ART
Low carbon ferro-chromium, i.e., a Fe-Cr alloy comprising 60 wt. % or more of Cr and 0.10 wt. % or less of C, is manufactured by the so-called Perrin method, usually in the following procedure. First, Cr ore and calcined lime are supplied into an electric furnace, and are melted therein. The molten slag is poured into a ladle, and an amount of SiCr is added, as a reducing agent, to the molten slag in the ladle. The molten slag containing SiCr is poured from the ladle into another ladle, then from the second ladle into the first ladle. This transfer of the molten metal between two ladles, called "reladling", is repeated several times. As a result, the molten metal and the molten slag are stirred together, thereby accelerating the reduction of the molten metal.
The molten metal contacts air during the reladling and inevitably absorbs nitrogen. Consequently, the low carbon ferro-chromium will have a high nitrogen content. How much nitrogen the molten metal absorbs during the reladling depends on the temperature of the molten metal and the number of times the molten metal contacts air. There are two alternative methods of suppressing the nitrogen absorption. The first altertative is to decrease the basicity of the primary slag within the furnace (CaO/Cr.sub.2 O.sub.3), thereby to lower the temperature of the molten metal. The second alternative is to perform reladling less times, thus decreasing the number of times the molten metal contacts air.
When the basicity of the primary slag is decreased, however, the electric furnace will consume more power to melt the Cr ore, and the slag will contain more Cr. Further, the ratio of the effectively used amount of each raw material to the total supplied amount will decrease, and the productivity will also decrease. Still further, the linings of the electric furnace and the ladles will likely be eroded.
On the other hand, when the ladling is repeated less times, the reducing of the molten slag does not proceed sufficiently fast. The feeding rate of the reducing agent must therefore be decreased. When the reducing agent is fed at a lower rate, then the Cr content in the slag will rise, inevitably lowering the yield of Cr very much. In this case, too, consumption efficiency of each raw material will decrease, and the productivity will also decrease.
Hence, the manufacturing cost of the low carbon ferro-chromium is high, no matter whether the basicity of the primary slag is decreased, or the reladling is repeated is decreased less times. In either conventional method of suppressing the nitrogen absorption, it is difficult to maintain the nitrogen content of the molten metal at a desired value.
Accordingly it is an object of the present invention to provide a method of manufacturing low carbon ferro-chromium whose nitrogen content is extremely low and whose Cr yield is sufficiently high.
Another object of this invention is to provide a method of manufacturing low carbon ferro-chromium, wherein the nitrogen content of molten metal can be controlled and maintained at a low level.
DISCLOSURE OF THE INVENTION
According to a first aspect of the invention, there is provided a method of manufacturing low carbon ferro-chromium, comprising the steps of melting ores including oxides of Cr and Fe by means of a furnace, thus forming molten slag; pouring the molten slag into a ladle; adding a reducing agent to the molten slag in the ladle; and introducing an oxidizing gas or an inert gas into the melt in the ladle, thereby bubbling the molten metal and accelerating reduction of the molten slag, said gas being introduced into the melt metal at a rate of more than 7.0 l/min. but not exceeding 29.0 l/min, for each ton of the melt in the ladle.
According to a second aspect of the invention, there is provided a method of manufacturing low carbon ferro-chromium, comprising the steps of melting ore
REFERENCES:
patent: 2001015 (1935-05-01), Feild
patent: 3854932 (1974-12-01), Bishop
Doyama Hisao
Kawaguchi Sotoaki
Ohta Kazumi
Yano Yutaka
Nippon Kokan Kabushiki Kaisha
Rosenberg Peter D.
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