Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
2000-03-30
2002-04-09
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C075S652000
Reexamination Certificate
active
06368380
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method, for melt-removing impurity elements from iron, capable of economically and efficiently removing impurity elements, that are detrimental to a stable production of steel materials having a homogeneous property, from iron scraps when the iron scraps, from various industrial and public sources, are recycled.
2. Description of the Related Art
When impurity elements called generally “tramp elements”, such as Cu and Sn, are contained above predetermined concentrations in steel materials, cracks occur on a slab surface during continuous casting or during hot rolling. Therefore, an extra process step such as surface polishing becomes necessary and invites an increase in the production cost. Moreover, productivity and yield drop, and the mechanical properties of the steel products such as elongation, toughness, deep drawability, and so forth, are remarkably deteriorated, as is well known in the art. These tramp elements are contained in only limited amounts in the molten pig iron produced by a blast furnace using an iron ore as the main raw material, but are contained in large amounts in iron scraps such as the scrap of waste home electric appliances and car scrap. Therefore, when steel products are produced by using economical iron scrap as the main raw material, high grade steels having high mechanical properties cannot be produced easily, and stable production of such steels, with a high yield, cannot be expected, either. For this reason, the development of a stable production method of high quality steel materials using the economical iron scrap has been desired.
Various methods are known for removing the tramp elements such as Cu and Sn from the iron scraps. One of the methods is the one for removing copper from the iron scrap by immersing the iron scrap in aqueous ammonia solution (refer, for example, to Japanese Unexamined Patent Publication (Kokai) No. 6-2053). The second method is the one for extracting and separating Cu, Sn, and the like from the iron scrap by immersing the iron scrap in a molten zinc bath (refer to Japanese Application No. 70269000. The third is the one for pulverizing the iron scrap, and then separating copper from iron (Japanese Patent Unexamined Patent Publication (Kokai) No. 7-253400).
The aqueous ammonia solution immersion method and the molten zinc immersion method can remove Cu exposed on the iron scrap surface. However, they cannot remove Cu contained inside the iron scrap. Moreover, these methods need a specific immersion setup having a large capacity and the processing rate is low because the immersion process is carried out at a low temperature. The pulverized iron scrap separation method needs extra process steps for pulverizing and separating the scrap. Moreover, the processing rate is low and the tramp elements cannot be removed from the steel material.
As described above, these three methods involve the problem in the removing rate of the tramp elements and in the processing rate of the iron scrap, and can never be used as a practical technology.
On the other hand, “Phase Equilibria in Iron Ternary Alloys”, The Institute of Metals (1988), pp. 157-167 describes that when at least 0.1% of C is added to a Cu-containing molten iron, the molten iron can be separated into an Fe-enriched layer and a Cu-enriched layer.
It can be anticipated from the observation described in this reference that when the molten iron is cooled down to 1,184° C. and kept under equilibrium after up to 4.5% C is added to the molten iron, the Cu concentration in the molten iron might drop down to 4% (page 160, FIG.
3
.
65
). However, the molten iron can be used as a useful raw material only after Cu concentration in the molten iron is reduced below 3%. Therefore, a practical tramp element removing technology cannot be established by applying the observation of this reference.
The magazine “Shigen-to-Sozai” , Vol. 113, No. 12(1997), published by The Mining and Materials Processing Institute of Japan, pp. 1110-1114, and Japanese Unexamined Patent Publication (Kokai) No. 11-293350 describe a method of separating and recovering copper and iron after melting a copper-containing iron scrap in a non-oxidizing atmosphere, and dissolving at least 2% of C in the iron phase.
However, this method is carried out on a laboratory scale and moreover, in a non-oxidizing atmosphere while the formation of slag is prevented. Therefore, a specific melting furnace capable of continuously keeping a non-oxidizing atmosphere must be prepared and in this point, it is not suitable for practical application. Furthermore, even if at least 2% of C is dissolved in the iron phase, the Cu concentration in this phase cannot be lowered to 3% or below and in this point, too, the method is not suitable as a practical technology.
The molten iron formed by melting the iron scraps can become useful as the iron steel raw material only when the Cu concentration in the molten iron is lowered to 3% or below, as described above. However, no reference has ever disclosed a practical technology that can reduce the Cu concentration in the molten iron to 3% or below, to the knowledge of the present inventors.
SUMMARY OF THE INVENTION
To solve the problems of the prior art described above, the present invention is directed to provide a method of melt-removing impurity elements from iron, which method can remove efficiently and economically the impurity elements such as Cu, Sn, etc, under the molten state by melting the iron scrap and then adding C and other suitable alloy elements to the molten iron, and which method can economically offer the iron scraps as a useful iron or steel raw material.
As a result of various studies to solve the problems described above, the present inventors have succeeded in removing the impurity elements such as Cu and Sn by melting the iron scrap and adding C and other suitable alloy elements to the molten iron. The present invention has been thus completed.
The gist of the present invention resides in the following points.
(1) A method of melt-removing impurity elements from iron which comprises the steps of melting an iron scrap having a composition comprising iron and iron-soluble elements in the sum of 100 mass %, and containing Cu and unavoidable impurities, in an oxygen-containing atmosphere; adding C to the molten iron within the range expressed by the formula given below; separating the molten iron into an Fe-enriched layer and a Cu-enriched layer under the molten state; separating Cu contained in the iron scrap and precipitating Cu into the Cu-enriched layer by utilizing the difference of the specific gravity between the layers; and removing Cu from the Fe-enriched layer;
5.0×(1−Cu/100)≦C≦20
(with the proviso that the unit of Cu and C is mass %).
(2) A method of melt-removing impurity elements from iron which comprises the steps of melting an iron scrap having a composition comprising iron and iron-soluble elements in the sum of 100 mass %, and containing Cu and unavoidable impurity elements, in an oxygen-containing atmosphere; adding C and at least one of Cr and Mo to the molten iron within the ranges expressed by the formulas given below; separating the molten iron into an Fe-enriched layer and a Cu-enriched layer under the molten state; separating Cu contained in the iron scrap and precipitating Cu into the Cu-enriched layer by utilizing the difference of the specific gravity between the layers; and removing Cu from the Fe-enriched layer;
3.3×(1−Cu/100)≦C≦20,
4.5×(1−Cu/100)≦Cr≦30,
4.5×(1−Cu/100)≦Mo≦30,
(with the proviso that the unit of Cu, C, Cr and Mo is mass %).
(3) A method of melt-removing impurity elements from iron which comprises the steps of melting an iron scrap having a composition comprising iron and iron-soluble elements in the sum of 100 mass %, and containing Cu and unavoidable impurities, in an oxygen-containing atmosphere; adding C and at least one of Mn, V and Ti to the molten iron within the ranges
Ueshima Yoshiyuki
Ushioda Kohsaku
Andrews Melvyn
Nippon Steel Corporation
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