Many techniques have been developed on the vacuum degassing of molten steel. The RH and DH reactors, which are suitable to the high productivity of converter, have occupied the mainstream. The DH process had constantly been installed until the latter half in 1970's, but has not been set up since 1980's. On the other hand, the RH process has continually been installed up to the present. The reason why RH has occupied the mainstream and has been developed further is examined from the viewpoint of the rate enhancement of the degassing reaction by the enlargement of RH and DH reactors. With the enlargement of the RH and DH reactors, the ratio of the upper surface area of steel in vacuum vessel to total reaction area decreases. On the other hand, the surface area of bubbles by Ar injection increases. Accordingly, the contribution of bubbles to the
A/
V (reaction area/metal volume) value becomes larger than that of the upper surface of steel in vacuum vessel. Although the argon gas can be also injected in the DH process, it is rather difficult to conduct the Ar injection in comparison with the RH process. It is not possible to sufficiently demonstrate the rate enhancement effect of the bubbles on the degassing reaction in DH reactor. Accordingly, from the viewpoint of the
A/
V value, the enlargement of the reactor is more advantageous to RH reactor. On the other hand, the circulation mass flow rate of steel similarly increases with increasing the heat size in both RH and DH reactors. It can be concluded that, with the enlargement of the RH and DH reactors, the Ar injection plays more important role on the rate enhancement of degassing reaction. In RH reactor, the molten steel is continuously circulated by the Ar injection. On the other hand, in DH reactor, the vacuum vessel repeats the periodic (uncontinuous) up and down. In this respect, the RH process is superior to the DH process. It can be considered that this is one of the reasons why the RH process has been more developed.
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