Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
2001-02-15
2002-08-13
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C266S210000
Reexamination Certificate
active
06432164
ABSTRACT:
TECHNICAL FIELD
This invention relates to a method for refining molten steel inexpensively and efficiently and, more specifically, to a method for decarburizing, desulfurizing or dephosphorizing molten steel inexpensively and efficiently and a refining apparatus employed for implementing said method.
BACKGROUND ART
Requirements for steel material properties are becoming more and more demanding as steel materials are used in more severe environments. Since steel materials are widely used in the society in general, they are required to be inexpensive, too. For manufacturing steel materials having desired properties, it is necessary to lower impurities such as phosphorus, sulfur, carbon, hydrogen, etc. to the least possible amounts at steel refining processes, and it is also important to refine steel inexpensively. In this situation, it is essential to clarify the physical and chemical fundamentals and principles of steel refining reactions and develop efficient refining methods and apparatuses based thereon.
Conventionally, the technical trend of steel refining has been to divide the refining process into steps so that each of impurities has been removed under a condition tailored to facilitate the removal and to complete the steel refining through several steps. Technologies based on this philosophy have come to be widely practiced. For example, widely employed is a hot metal treatment process wherein the dephosphorizing treatment and the decarburizing treatment, which were formerly carried out using only a converter, have been divided into the dephosphorizing treatment at the step of molten pig iron and the decarburizing treatment in a converter.
At the decarburizing treatment in a converter, carbon is removed through oxidation by injecting oxygen into molten steel (oxidizing refining), but the oxygen is inevitably absorbed in the molten steel.
Oxygen concentration in molten steel becomes high especially when producing low carbon steels having a carbon concentration of 0.1% or less: for example, if blowing is stopped at a carbon concentration of 0.04%, oxygen content in the molten steel will be 0.05% or so. The carbon concentration and the oxygen concentration in molten steel are roughly in inverse proportion to each other and, hence, the lower the end point carbon concentration, the higher the oxygen concentration.
In the meantime, highly formable ultra low carbon steels have come to be used in large quantities especially for exposed panels for automobiles. For producing the ultra low carbon steels, it is necessary to lower the carbon concentration to a level of 30 ppm or less and, for this purpose, decarburizing treatment is carried out by decompression refining at a secondary refining stage after the decarburization in a converter.
At the present time, when the continuous casting method has become general, in order to prevent the occurrence of pin holes and breakouts caused by CO gas generated during casting, it is necessary to remove oxygen absorbed in molten steel by adding a deoxidizing agent, typically Al, to molten steel and trapping the oxygen as oxides. When the deoxidizing agent is entrapped in steel materials, however, it will undesirably cause cracks and defects when they are plated.
Further, the deoxidizing agent remaining in steel materials tends to appear as inclusion-induced defects in the case of low carbon steels often used as materials for stamping applications undergoing intensive working. A process to produce low carbon steels with low oxygen concentration, therefore, needs to be developed.
In this respect, a method called the carbon deoxidation method is widely known, wherein the oxygen in molten steel is removed in the form of CO gas by carbon in the molten steel. In this method a vacuum degassing apparatus equipped with a large evacuator (for example, an RH vacuum degasser) is generally employed for an effective decarburizing action.
Japanese Unexamined Patent Publication No. S53-16314, for example, discloses a method to produce Al-killed molten steel for continuous casting use wherein the end point carbon concentration at a converter is controlled to 0.05% or more and a degassing treatment is applied using a vacuum degasser before deoxidation. By this method, the pressure inside a vacuum tank is controlled within the range of 10 to 300 Torr in accordance with the progress of decarburization. Further, Japanese Unexamined Patent Publication No. H6-116626 discloses a decarburization method, with a reduced occurrence of splash, wherein molten steel in a ladle with carbon concentration reduced in a converter to 0.1 to 1.0% is decarburized by immersing a single cylindrical immersion tube into the molten steel and injecting oxygen mixed with an inert gas under a pressure of 100 Torr or more.
The methods disclosed in the Japanese Unexamined Patent Publication Nos. S53-16314 and H6-116626, however, employ so-called large decompression refining apparatuses. In the method of the Japanese Unexamined Patent Publication No. S53-16314, it is necessary to reduce the pressure to 10 Torr or so, and hence a large vacuum degasser such as a vapor jet vacuum pump is indispensable. In the method of the Japanese Unexamined Patent Publication No. H6-116626 wherein oxygen gas mixed with an inert gas is used for decarburization, on the other hand, there is a problem that expensive argon gas has to be used since, when inexpensive nitrogen gas is used instead, it is absorbed in steel adversely affecting its aging properties.
At the present time, when vacuum degassers are widely used for the purposes of decarburization and dehydrogenation of ultra low carbon steels, the degassers originally designed for degassing at a high vacuum of 1 Torr or less are often used for the production of low carbon steels. However, a high decompression refining apparatus such as an RH vacuum degasser (hereinafter sometimes called “an RH refining apparatus”) has a vacuum tank very large in height and diameter and, consequently, the volume to be evacuated is huge. For this reason, there are problems of high refining costs due to high unit consumption of refractories and high costs of utilities such as steam for a vapor jet vacuum pump required for evacuation.
Another problem is that the construction of a large decompression refining apparatus intended for the carbon deoxidation of low carbon steels is expensive and uneconomical. Further, a high decompression refining apparatus is used for producing ultra low carbon steels with a carbon concentration of, for example, 30 ppm or less and, in this case, skulls of a high carbon concentration which adhered onto the inner wall of a vacuum tank when molten steel with a carbon concentration of 0.04% or so, which is a far higher carbon concentration than an ultra low carbon steel, is processed, re-melt during the processing of an ultra low carbon steel and become the source of carbon contamination. This leads to another problem of longer decarburizing treatment time or no progress in decarburization. Some RH refining apparatuses are equipped with an LPG burner for melting and removing the skulls as a countermeasure, but such a countermeasure leads to another problem of additional costs for the equipment and the removal operation.
Looking at the desulfurizing treatment of molten steel, it is classified, generally, into hot metal desulfurization applied in the state of molten pig iron and molten steel desulfurization applied in the state of molten steel. As steel materials came to be used in more severe conditions, the required level of steel purity becomes higher. As a consequence, the application of only the hot metal desulfurization can be regarded insufficient and the molten steel desulfurization is an indispensable process step. Thus, the development of a method for efficient desulfurization and an apparatus therefor, especially for producing ultra low sulfur steels having an S concentration of 10 ppm or less, has been required.
As a response, for example, Japanese Unexamined Patent Publication No. S58-37112 proposes a method to immerse an immersion tube (the
Imai Tadashi
Kunitake Okitomo
Mukawa Susumu
Omura Norio
Sakai Wataru
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