Method relating to manufacturing of steel

Abstract

The invention concerns a method for the manufacturing of steel in an electric arc furnace, comprising melting of charged steel raw material, substantially iron carrier, characterised in that at least 5 weight-%, preferably at least 10 weight-%, of charged iron carrier consist of granulated pig iron, here denominated GPI.

1 . Method relating to manufacturing of steel in an electric arc furnace comprising melting charged steel raw material, substantially iron carrier, characterised in that at least 5 weight-%, preferably at least 10 weight-%, of charged iron carrier consists of granulated pig iron, herein denominated GPI.  
     
     
         2 . Method according to  claim 1 , charecterised in that said GPI satisfies the following conditions, namely: 
 a) that it has a chemical composition containing 0.2-3% Si, 2-5% C, 0.1-6% Mn, the remainder essentially only iron and impurities which can normally exist in pig iron produced in the blast furnace process or other shaft furnace process, e.g. in Capola furnace,    b) that it has a melting point<1350° C., and    c) that it consists of essentially homogenous particles with substantially round or oval shape obtainable by granulation of a melt with the above mentioned composition, comprising disintegration of a stream of said melt to drops, which are cooled in a water bath to form a granulate.    
     
     
         3 . Method according to  claim 2 , characterised in that it comprises decarburisation through oxygen gas injection into molten metal formed in the furnace.  
     
     
         4 . Method according to  claim 3 , characterised in that the silicon in said GPI is oxidised at the decarburisation to silicon dioxide, SiO2, which essentially is collected in a top slag in the furnace, for the control of the slag composition, wherein there is added any basic slag former, substantially containing Ca- and/or Mg-carriers in such an amount that the slag composition will satisfy the requirement  
       
         
           
             
               
                 
                   2 
                   , 
                   
                       
                   
                    
                   8 
                 
                 ≤ 
                 
                   
                     CaO 
                     + 
                     MgO 
                   
                   
                     SiO 
                     2 
                   
                 
                 ≤ 
                 
                   3 
                   , 
                   
                       
                   
                    
                   6 
                   , 
                 
               
                
               
                   
               
             
           
           
           
               
           
         
       
       preferably the requirement  
       
         
           
             
               
                 3 
                 , 
                 
                     
                 
                  
                 1 
               
               ≤ 
               
                 
                   CaO 
                   + 
                   MgO 
                 
                 
                   SiO 
                   2 
                 
               
               ≤ 
               
                 3 
                 , 
                 
                     
                 
                  
                 3 
               
             
           
           
           
               
           
         
       
     
     
         5 . Method according to any of claims  1 - 3 , characterised in that the steel is produced batch-wise in the electric arc furnace and that said GPI is added to the electric arc furnace at an initial stage of the charging procedure in order quickly to form a pool of molten metal in the furnace.  
     
     
         6 . Method according to  claim 5 , characterised in that said steel raw material at least partly is basket charged, at least GPI being added with the first basket in the charging procedure.  
     
     
         7 . Method according to  claim 5  or  6 , characterised in that said GPI is injected in the pool of molten metal which initially is formed or added and/or in the pool of molten metal that successively is formed in the furnace.  
     
     
         8 . Method according to any of claims  1 - 4 , characterised in that the furnace is operated semi-continuously, i.e. with batch-wise bottom-tapping of a portion, preferably 40-60% of the steel melt, and that GPI is charged continuously or semi-continuously to remaining pool of molten metal and/or to the successively growing pool of molten metal.  
     
     
         9 . Method according to any of claims  1 - 8 , characterised in that said GPI to at least 80 weight-% consists of particles having a particle size between 2 mm and 25 mm measured in the largest dimension of the particles.  
     
     
         10 . Method according to any of claims  1 - 9 , characterised in that said GPI has a bulk density of 3.5-5.5, preferably 4-5 kg/l.  
     
     
         11 . Method according to any of claims  1 - 10 , characterised in that said GPI is preheated by the flue gases from the furnace before charging, preferably that said GPI is preheated continuously by the flue gases before continuous or semi-continuous charging.  
     
     
         12 . Method according to an of claims  1 - 11 , characterised in that as a steel raw material there is added to the furnace, besides said GPI, also scrap which contains impurities in form of one or more of the residual metals belonging to the group of metals consisting of Cu, Ni, Mo, Zn and Sn, wherein the addition of said GPI dilutes the content of said residual metals in the steel melt being formed.  
     
     
         13 . Method according to any of claims  1 - 11 , characterised in that as a steel raw material there is added to the electric arc furnace, besides said GPI, also directly reduced iron, here denominated DRI, which contains in weight-% 75-90% metallic iron, 0.2-3% C, 2-7% gangue material, substantially SiO 2 +Al 2 O 3 , the balance being substantially iron oxide, FeO (iron bound as oxides), wherein GPI is added at least in such extent that its content of silicon and carbon in combination with carbon in added DRI will reduce the iron oxide of said DRI to metallic iron, at the same time as the oxidation of Si and C in said GPI generates heat at least in a sufficient amount to compensate for the cooling action caused by the gangue material and the iron oxide in added DRI.  
     
     
         14 . Method according to claims  12  and  13 , characterised in that as a steel raw material there is added to the furnace, besides GPI, also scrap containing impurities in the form of one or more of the residual metals belonging to the group of metals consisting of Cu, Ni, Mo, Zn and Sn, wherein the addition of said GPI will dilute the content of said residual metals in the steel melt that is being formed, wherein also directly reduced iron being added, herein denominated DRI, containing in weight-% 75-90% metallic iron, 0.2-3% C, 2-7% gangue material, substantially SiO 2 +Al 2 O 3 , the balance being substantially iron oxide, FeO (iron bound as oxides), wherein GPI is added at least in such extent that its content of silicon and carbon in combination with carbon in added DRI will reduce the iron oxide of said DRI to metallic iron, at the same time as the oxidation of Si and C in said GPI generates heat at least in a sufficient amount to compensate for the cooling action caused by the gangue material and the iron oxide in added DRI.  
     
     
         15 . Method according to any of the previous claims, characterised in that 10-20%, preferably 30-50% of the steel raw material consists of said GPI.  
     
     
         16 . Method according to  claim 1 , characterised in that steel raw material consists of said GPI to 100%.  
     
     
         17 . Method according to  claim 1 , comprising the formation of a foaming slag with a temperature of 1500-1750° C. in the furnace on top of the surface of the bath of molten metal, and the supply of oxygen in the form of oxygen gas to the melt to oxidise at least part of carbon existing in the melt for heat generation and to generate gas in the form of Co and/or Co 2  as a contribution to the slag foaming, wherein the supply of oxygen to the melt also brings about oxidation of other metal elements than silicon in the melt, herein referred to as valuable metal elements, which enter the top slag from where they at least to an essential degree are recovered to the melt through addition of reduction agents to the top slag, characterised in that during at least one phase of the one phase of the production process, a doping agent in the form of a particle-formed, granulated product is added to the top slag with the aim of creating improved conditions for the reduction of the oxidised, valuable metal elements in the top slag, participating in the reduction process itself, contributing to and/or maintaining the slag foaming as well as adding metal to the melt, said doping agents fulfilling the following requirements, namely: 
 a) that it has a chemical composition containing 0-5% Si, 2-7% C, 0-3% Mn, the remainder essentially only iron and impurities which can normally exist in pig iron produced in the blast furnace process or other shaft furnace process,  
 b) that it has melting point<1350° C., and  
 c) that it consists of essentially homogeneous particles with substantially round or oval shape obtainable by granulation of a melt with above-mentioned composition, comprising disintegration of a stream of said melt to drops, which are cooled in a water bath to form a granulate.  
 
     
     
         18 . Method according to  claim 17 , characterised in that the particles which are added to the slag consist of particles which to at least 80 weight-% consist of particles having a particle size varying between 0.5 and 5.5 mm measured in the largest dimension of the particles.