Method of producing tube of duplex stainless steel

Abstract

Method of producing a tube of duplex stainless steel is disclosed. The steel comprises the following composition, in weight %: C max 0.03, Si max 1.0, Mn max 1.5, P max 0.05, S max 0.03, Cr 24-26, Ni 6-8, Mo 3.0-4.0, N 0.24-0.32. The method comprises steps of: forming a tube of the duplex stainless steel, cold working the tube obtained from the step of forming a tube, and soft annealing the tube after the step of cold working by subjecting the tube to a temperature, T, within a range of 500-750° C. for a time period, t, of 0.5-5 minutes.

1 . A method of producing a tube of duplex stainless steel comprising the following composition, in weight %: 
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                   C 
                   max 0.03, 
                 
                     
                   Si 
                   max 1.0, 
                 
                     
                   Mn 
                   max 1.5, 
                 
                     
                   P 
                   max 0.05, 
                 
                     
                   S 
                   max 0.03, 
                 
                     
                   Cr 
                     24-26, 
                 
                     
                   Ni 
                     6-8, 
                 
                     
                   Mo 
                    3.0-4.0, 
                 
                     
                   N 
                   0.24-0.32, 
                 
                     
                     
                 
             
                
               
               
                
                
                
                
                
                
                
                
                
                
               
            
           
         
       
       and
 a balance of Fe and unavoidable impurities, 
 wherein the method comprises steps of:
 forming a tube of the duplex stainless steel, 
 cold working the tube obtained from the step of forming a tube, and 
 soft annealing the tube after the step of cold working by subjecting the tube to a temperature, T, within a range of 500-750° C. for a time period, t, of 0.5-5 minutes, and 
 
 wherein, in the step of soft annealing the tube, a target minimum yield strength, Rp0.2, of the duplex stainless steel is achieved following the formula:
     Rp 0.2=1642−0.96× T− 10.75× t  
 
 
 
       with a standard deviation in the target minimum yield strength Rp0.2 of 10 MPa, wherein T is temperature in degree Celsius, t is time in minutes. 
     
     
         2 . (canceled) 
     
     
         3 . The method according to  claim 1 , wherein the step of soft annealing the tube comprises subjecting the tube to a temperature within a range of 600-750° C. for a time period of 1-3 minutes. 
     
     
         4 . The method according to  claim 1 , wherein the step of soft annealing the tube is followed by a step of:
 quenching the tube.   
     
     
         5 . The method according to  claim 1 , wherein the step of cold working the tube, includes an area reduction of the tube within a range of 25-70%. 
     
     
         6 . The method according to  claim 1 , wherein the step of forming a tube comprises steps of:
 producing an ingot or a continuous casted billet of the duplex stainless steel, and   hot extruding the ingot or the billet obtained from the step of producing an ingot or a continuous casted billet into the tube.   
     
     
         7 . The method according to  claim 1 , wherein the step of forming a tube comprises steps of:
 providing a melt of the duplex stainless steel,   atomizing the melt of the duplex stainless steel to produce a powder of the duplex stainless steel,   forming the tube of the powder of the duplex stainless steel in a process that includes hot isostatic pressing the powder of duplex stainless steel.   
     
     
         8 . The method according to  claim 1 , wherein the target minimum yield strength, Rp0.2, of the duplex stainless steel is 868.25-1156.25 MPa with a standard deviation of 10 MPa. 
     
     
         9 . The method according to  claim 1 , wherein the amount of Mn in the composition is 0 to 1.2 wt %. 
     
     
         10 . The method according to  claim 1 , wherein the amount of Mo in the composition is 3.5 to 4.0 wt %. 
     
     
         11 . The method according to  claim 1 , wherein the amount of Ni in the composition is 6.5 to 7.5 wt %. 
     
     
         12 . The method according to  claim 1 , wherein the amount of N in the composition is 0.26 to 0.30 wt %. 
     
     
         13 . The method according to  claim 1 , wherein the amount of Si in the composition is 0 to 0.8 wt %. 
     
     
         14 . The method according to  claim 1 , wherein the composition further includes:
 one or more of rare earth metals, Ca, Co, Cu, Ti, Nb, W, Sn, Ta, Mg, B, Pb and Ce in a total amount of max 0.5 wt %.   
     
     
         15 . The method according to  claim 1 , wherein the composition further includes:
 Ca<0.01 wt %, Mg<0.01 wt %, Cu max 0.5 wt %, and rare earth metals <0.2 wt %.