Process for producing a high-cleanliness steel

Abstract

A process for producing a high-cleanliness steel is provided which can produce, without relying upon a high-cost remelting process, steel products having cleanliness high enough to satisfy requirements for properties of mechanical parts used under severe environmental conditions. The production process comprises the steps of: transferring a molten steel produced in an arc melting furnace or a converter to a ladle furnace to refine the molten steel; subjecting the molten steel to circulation-type degassing; and casting the molten steel into an ingot, wherein, in transferring the molten steel to the ladle furnace, a deoxidizer including aluminum and silicon is added to previously deoxidize the molten steel, that is, to perform tapping deoxidation before refining in the ladle refining furnace.

1 . A process for producing a high-cleanliness steel, comprising the steps of: 
 (a) producing a molten steel by oxidizing refining in an arc melting furnace or a converter;    (b) tapping the molten steel from the arc melting furnace or the converter to a ladle at a temperature of at least 100° C. above a melting point of the steel;    (c) circulating the molten steel in a circulation vacuum degassing device to perform a first circulation vacuum degassing step;    (d) reduction refining the molten steel in the ladle furnace;    (e) circulating the refined molten steel through a circulation vacuum degassing device to perform a second circulation vacuum degassing step to further vacuum degas the molten steel; and    (f) casting the twice vacuum degassed molten steel into an ingot.    
     
     
         2 . The process according to  claim 1 , wherein the refining in the ladle furnace of step (d) is carried out for not more than 60 minutes, and the second degassing step (e) is carried out for not less than 25 minutes.  
     
     
         3 . A high-cleanliness steel produced by the process according to  claim 1 .  
     
     
         4 . The high-cleanliness steel according to  claim 3 , wherein the content of oxygen in the steel is not more than 10 ppm.  
     
     
         5 . The high-cleanliness steel according to  claim 3 , wherein the number of oxide inclusions having a size of not less than 20 μm as detected by dissolving the steel product in an acid is not more than 40 per 100 g of the steel product.  
     
     
         6 . The high-cleanliness steel according to  claim 3 , wherein the predicted value of the maximum inclusion diameter in 30000 mm 2  as calculated according to statistics of extreme values is not more than 60 μm.