Patent Publication Number: US-6210499-B1

Title: Method of bright annealing metals having a high affinity to oxygen

Description:
FIELD OF THE INVENTION 
     This invention relates to a method of bright annealing metals having a high affinity to oxygen in a hood-type furnace or the like under a protective gas. 
     DESCRIPTION OF THE PRIOR ART 
     Medium- to high-alloy steels, non-ferrous metals and metals which in general are difficult to bright-anneal and have components with a high affinity to oxygen, e.g. chromium, manganese, silicon, titanium etc., have so far been subjected to a heat treatment, in order to mold their structure or recrystallize their microstructure after the molding steps. This is usually done in hood-type furnaces with non-enclosed understructure, where hydrogen or a mixture of hydrogen and nitrogen is used as protective gas. Due to the contact with the isolation of the understructure in the furnace oxygen residues are, however, transported by the hydrogen to the batch to be treated, e.g. to strip coils. The hydrogen resulting from the reaction will then oxidize the surface of the material to be annealed, where a further deficiency results from the fact that oxygen residues in the protective gas react with the surface of the material to be annealed. 
     When a hood-type furnace with an enclosed understructure is used, annealing may be performed under lowest dew points, but visible, disturbing oxidation residues may still remain at the metal surface of the material to be annealed. 
     Finally, the pure, extremely reducing hydrogen may reducingly attack the oxides of the annealing box, i.e. for instance of the heat-resistant steel construction of the understructure, the protective hood, the understructure fan, arid the distributor as well as existing scale, and transport the resulting moisture to the material to be annealed. By packing the material to be annealed in films, for instance, or by covering it with caps of steel that is free from elements having a high affinity to oxygen, a residual discoloration of the material to be annealed can not completely be repressed either. 
     In addition, lubricant residues from the preceding molding operations may still be present on the surface of the material to be annealed, which lubricant residues chiefly consist of water and oil, i.e. an emulsion, and evaporate during heating and react with the surface of the material to be annealed. Even by supplying a large amount of protective gas, residual discolorations can therefore not be avoided. 
     SUMMARY OF THE INVENTION 
    
    
     It is therefore the object underlying the invention to provide a method as described above, where surface defects of the material to be annealed are virtually eliminated. 
     This object is solved by the invention in that a rather pure inert gas such as argon, neon or helium is used as protective gas in cooperation with an oxygen binder, preferably titanium. 
     By using such protective gas together with an oxygen binder provided in the interior of the annealing box, reactions of oxygen carriers such as CO, CO 2 , H 2 O or oxygen, which involve an oxidation or discoloration of the material to be annealed, are prevented. In addition, the transport of oxygen and oxygen carriers from the heat-resistant material of the annealing box to the material to be annealed is prevented at all. 
     The use of nitrogen as inert gas is possible only to a restricted extent, as an undesired formation of nitride at the surface of the material to be annealed might occur with various metals. 
     If metals contaminated with residual amounts of lubricant should be subjected to bright annealing, the lubricant is first of all evaporated under a protective gas which includes a noble or inert gas such as nitrogen, and/or a reducing gas such as hydrogen, and for the subsequent bright annealing the rather pure inert gas, which is mixed with not more than 50 vol-% of a reducing gas, is then used as protective gas in cooperation with an oxygen binder, preferably titanium. 
     The process of evaporating the residual lubricant adhering to the surface may be performed under a different protective gas or protective gas mixture than the process of bright annealing, as the evaporation takes place at low temperatures, so that no disturbing surface discolorations need to be expected. After the evaporation, the heat treatment is continued by exchanging the protective gas.