Method for refining molten metal bath to control nitrogen

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Broadly in the practice of the present invention, the decarburizing method thereof is performed in a conventional decarburizing vessel and the vessel may, in addition, be provided with tuyeres beneath the surface of the bath.
In the conventional manner oxygen or oxygen and nitrogen in combination are introduced to the bath from the lance, tuyeres or the use of a lance and tuyeres in combination.
A portion of the oxygen reacts with the carbon in the molten bath to evolve carbon oxides, which are removed in gaseous form.
In accordance with the invention, at the end of the refining period wherein carbon in the bath has been reduced to a selected level, an inert gas, such as argon, is introduced to the bath from a lance that directs the gas onto or beneath the surface of the bath.
Typically, a slag reductant, such as aluminum and/or silicon, is introduced with the inert gas for purposes of recovering chromium and other metallics to the bath which have been oxidized to the slag during decarburization.
The oxygen level of the bath is also reduced to a low or minimum value when the reductant is added.
For purposes as used herein, the term inert gas is used to refer to any gas which is substantially nonreactive with the molten metal and includes argon, xenon, neon, helium, and mixtures thereof.
In combination with the inert gas introduced from the lance, inert gas may simultaneously be introduced through tuyeres.
With the invention, since the argon is supplied from a lance, the argon supply rate is more rapid than with prior art practices wherein the inert gas is supplied solely from the tuyeres.
Consequently, the nitrogen removal achieved by the introduction of the inert gas is more rapid and efficient, thereby reducing the time required to complete the overall refining operation.
In addition, by introducing the inert gas from a lance as opposed to solely from tuyeres, the stirring action is such as to be less severe with respect to refractory wear