Method of decarburizing high chromium molten metal

Details Method of decarburizing high chromium molten metal Method of decarburizing high chromium molten metal

1989-04-05

1991-09-10

Rosenberg, Peter D.

Specialized metallurgical processes, compositions for use therei

Processes

Producing or treating free metal

75512, C21C 710

Patent

active

050470817

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to a method of decarburizing high Cr molten metal.


BACKGROUND OF THE INVENTION

There have been conventional decarburizing methods of high Cr molten metal known as AOD, VOD, VODC or RH-OB processes. In AOD, a mixed gas of oxygen and an inert gas from a furnace bottom is blown under an atmospheric pressure. In VOD and VODC, O.sub.2 is blown under a vacuum, and in RH-OB, a rough decarburization is performed in a converter, and subsequently a vacuum O.sub.2 blowing is carried out in a RH degasfying chamber.
However, since in the AOD process, the decarburization is carried out in the air and O.sub.2 is blown directly into the molten metal, loss of Cr by its oxidation (called as "Cr oxidation loss" hereinafter) is large, and it is disadvantageous that much Fe--Si or Al must be thrown into a slag as a reducing agent.
In the VOD, VODC and RH-OB processes, the decarburization is undertaken in the vacuum with less Cr oxidation loss, but since much oxygen is necessary for securing productivity, a vacuum facility of a large capacity therefor is required at high cost.
Also when the high Cr molten metal is decarburized by blowing in the air, the inert gas is much required to agitate the molten metal. Ordinarily available gases therefor are N.sub.2 and Ar. If a low nitrogen stainless steel is produced, N.sub.2 cannot be used because it increases N concentration in steel, and an expensive Ar must be used. If Ar were much used, a problem would arise about the production cost. From such a viewpoint, the production of the low nitrogen steel usually depends upon a vacuum blowing, by which the denitrification takes place easily.
For these foregoing problems, the present invention has been devised. It is a basic object of the invention to provide a method which could perform the decarburization in a short period of time without practising the vacuum decarburization and with checking Cr oxidation loss.
It is the other object of the invention to provide a method which could decrease N in steel, not using much Ar, thereby to enable production of low nitrogen stainless steel at low production cost.


OUTLINE OF THE INVENTION

For accomplishing these objects, to high Cr molten metal supported in a container having bottom blowing tuyeres and a top blowing lance, the invention blows decarburizing O.sub.2 diluted with the inert gas from the top lance, as well as blows the inert gas from the bottom tuyeres so as to agitate the molten metal complusively.
According to this method, the decarburization can be finished in a short time under the atmospheric pressure with checking the Cr oxidation loss
In the AOD process, O.sub.2 is blown from the bottom tuyeres. It has been found through the inventors' studies that O.sub.2 blown from the bottom largely increased the Cr oxidation loss. That is, since static pressure was added in the O.sub.2 bottom blowing practice CO partial pressure was increased, and as a result the decarburizing reaction was hindered and O.sub.2 oxidizes Cr. Therefore, the invention blew O.sub.2 not from the bottom tuyeres but from the top lance.
However, it was found that, if the top blowing were merely made with pure O.sub.2, the Cr oxidation loss could not be avoided exactly. This is why the decarburizing reaction takes place most vigorously around a fire point made by blowing oxygen from the lance, but with O.sub.2 only, CO partial pressure becomes very high there, and the decarburizing reaction is obstructed and O.sub.2 oxidizes Cr. Thus, the invention blows O.sub.2 diluted with the inert gas (N.sub.2 or Ar) from the top lance, thereby to decrease CO partial pressure around the fire point and accelerate the decarburizing reaction.
Further, in the invention, the inert gas (N.sub.2 or Ar) is blown from the bottom tuyeres to forcibly agitate the molten metal and accelerate the mixture of the molten metal and O.sub.2 sent from the top lance, and the effective decarburization is possible with checking the Cr oxidation loss by combination of the compulsive agitation of the metal by

REFERENCES:
patent: 3854932 (1974-12-01), Bishop
patent: 4514220 (1985-04-01), Tommaney
patent: 4592778 (1986-06-01), Fujii

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