Manufacture of Ferroalloys

Abstract

A ferroalloy, particularly stainless steel, is made by melting typically low-carbon steel under foamy slag conditions, and refining the molten steel at least in part by blowing molecular oxygen into the molten steel from a lance positioned above its surface. During the refining step at least one metallurgically acceptable particulate material is introduced into the molten steel. The particulate material is selected from chromium metal, chromium-containing alloys and chromium ores.

1 . A method of making stainless steel, comprising the steps of:
 a) melting a charge of steel;   b) refining the resultant molten steel at least in part by blowing molecular oxygen into the molten steel from a lance positioned above the surface of the molten steel; and   c) during the refining step introducing into the molten steel from the lance at least one first metallurgically acceptable particulate material selected from chromium metal, chromium-containing alloys and chromium ores;   wherein the steel is melted under foamy slag conditions.   
     
     
         2 . The method according to  claim 1 , wherein no chromium metal, chromium-containing alloy or chromium ore is added to the charge of steel that is melted in the said step (a). 
     
     
         3 . The method according to  claim 1 , wherein some chromium metal or chromium-containing alloy is added to the charge of steel that is melted in the said step (a). 
     
     
         4 . The method according to  claim 1 , wherein the steel that is melted in step (a) is a low-carbon steel. 
     
     
         5 . The method according to  claim 4 , wherein the low-carbon steel is mild steel. 
     
     
         6 . The method according to  claim 1 , wherein the charge of steel is melted in an electric arc furnace. 
     
     
         7 . The method according to  claim 1 , wherein the molecular oxygen is ejected from the lance at a supersonic velocity. 
     
     
         8 . The method according to  claim 1 , wherein the said first particulate material is conveyed to the lance in a carrier gas. 
     
     
         9 . The method according to  claim 1 , in which the lance comprises a first pipe for ejecting the molecular oxygen and a second pipe for ejecting the said first particulate material. 
     
     
         10 . The method according to  claim 9 , wherein the first and second pipes are coaxial, with the first pipe surrounding the second pipe. 
     
     
         11 . The method according to  claim 1 , wherein the molecular oxygen creates a localised, intensely superheated region in the molten steel into which the first particulate material is introduced. 
     
     
         12 . The method according to  claim 1 , in which the said first particulate material comprises chromium oxide. 
     
     
         13 . The method according to  claim 12 , in which there is introduced into the molten steel through the lance a second particulate material comprising a mixture of carbon and at least one deoxidising agent. 
     
     
         14 . The method according to  claim 13 , in which the deoxidising agent is ferrosilicon, ferromanganese, aluminium or ferroaluminium. 
     
     
         15 . The method according to  claim 1 , in which the said first particulate material comprises ferrochrome. 
     
     
         16 . The method according to  claim 15 , wherein the refining step comprises a first part in which all the said first particulate material is introduced into the molten steel, and a second part in which no first particulate material is introduced into the molten steel. 
     
     
         17 . The method according to  claim 1 , additionally including the step of introducing at least one third particulate material from the lance into the molten steel, wherein the said third particulate material is selected from nickel metal, nickel-containing alloys, nickel ores, molybdenum metal, molybdenum-containing alloys and molybdenum ores. 
     
     
         18 . The method according to  claim 1 , additionally including the step of blowing into the molten steel during the refining step further molecular oxygen through at least one tuyere terminating below the level of the molten steel, and at least one other gas selected from argon, nitrogen and stem, at least some of the other gas being introduced through the same or a different tuyere from the further molecular oxygen. 
     
     
         19 . The method according to  claim 1 , wherein the said first particulate material consists of fine particles having a mean particle size of 1 mm or less. 
     
     
         20 . The method according to  claim 1 , wherein in the said step (b) the rate of introduction of the first and any other metallurgically acceptable particulate material is balanced against the total rate of introduction of the molecular oxygen so as to maintain the molten steel at a temperature not in excess of 1710° C.