Method for desulfurization using arc heat under vacuum

In a method of desulfurizing a molten steel bath to 0.006 and below, the steps of

• tapping molten steel from a melting unit into a tapping vessel,
• establishing, if not already established, a basic slag on the steel in the tapping vessel,
• said basic slag being established by the addition of a small but effective quantity of burnt lime to the tapping process a fluidizing agent, and a chemical deoxidizing agent,
• fluidizing the slag by subjection of the bath and slag to a heat source external to the melting unit,
• lowering the oxygen level of the bath by subjecting the steel and slag to the simultaneous effect of a vacuum and a purging agent which passes upwardly in the bath from a remote location therein in gaseous form, the degree of vacuum and quantity of purging agent being sufficient to, by their combined effect, effect a violent agitation within the bath which causes metal droplets to be impelled upwardly into the space above the bath surface, to thereafter pass downwardly through the slag carried by the bath,
• thereafter adding additional desulfurizing materials to the bath, including burnt lime,
• fluidizing the newly added desulfurizing materials by subjecting the bath and slag to a heat source external to the melting unit,
• adding chemical deoxidizing agents to the bath in a quantity sufficient, when taken with the vacuum deoxidation effect, to lower the oxygen level of the bath to a point conducive to desulfurization,
• subjecting the molten steel, while exposed to the aforementioned system conditions, to a violent agitation in the vacuum for a period, when taken with prior vacuum exposure, to a total of at least about ten minutes,
• said agitation being sufficiently violent to cause the metal droplets to be exposed to the vacuum above the slag, and to pass through the slag upon return to the bath
• whereby the sulfur level is lowered to about .006, or below.

In a method of making alloy steel having a maximum S content of 0.003 the steps of

• forming a bath in a vacuum treatment vessel having a well fluidized highly basic slag thereon, the weight of the slag being about 2% of the weight of the bath, said slag further having a high FeO content and being deoxidized to a level conducive to further desulfurization,
• subjecting said bath to an initial vacuum degassing treatment in which the molten metal and the slag are subjected to gas purging at a low absolute pressure so as to create a violent agitation within the treatment vessel and intimate mixing of the slag and the metal,
• thereafter subjecting the bath to the combined simultaneous application of a sub-atmospheric pressure, gas purging, and an AC heating arc to increase the temperature of the bath, and
• thereafter subjecting said bath to a final vacuum degassing treatment consisting of gas purging at a low absolute pressure so as to create a violet agitation within the treatment vessel and intimate mixing of the slag and the metal.

In a method of lowering the nitrogen content of steel, the steps of

• forming a bath in a vacuum treatment vessel having a well fluidized highly basic slag thereon, the weight of the slag being about 2% of the weight of the bath, said slag further having a high FeO content and being deoxidized to a level conducive to further desulfurization,
• subjecting said bath to an initial vacuum degassing treatment in which the molten metal and the slag are subjected to gas purging at a low absolute pressure so as to create a violent agitation within the treatment vessel and intimate mixing of the slag and the metal,
• thereafter subjecting the bath to the combined simultaneous application of a sub-atmospheric pressure, gas purging, and an AC heating arc to increase the temperature of the bath, and
• thereafter subjecting said bath to a final vacuum degassing treatment consisting of gas purging at a low absolute pressure so as to create a violet agitation within the treatment vessel and intimate mixing of the slag and the metal.