High-strength electric resistance welded steel tube and production method therefor

Abstract

A high-strength electric resistance welded steel tube has a composition including, in terms of percent by mass, C: 0.05 to 0.20%, Si: 0.5 to 2.0%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01 to 0.1%, N: 0.005% or less, and the balance being Fe and unavoidable impurities; and a structure which is a dual phase structure including a ferrite phase and a martensite phase, with a volume ratio of the martensite phase being 20 to 60%, wherein a tensile strength TS is 1180 MPa or more, an elongation El in a tube axis direction is 10% or more, and a yield ratio is less than 90%; and after application of a 2% prestrain and baking finishing that includes a heat treatment of 170° C.×10 min, a strength increase (BH value) is 100 MPa or more and a yield ratio is 90% or more.

1 . A high-strength electric resistance welded steel tube having a composition including, in terms of percent by mass, 
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                   C: 0.05 to 0.20% 
                   Si: 0.5 to 2.0% 
                 
                     
                   Mn: 1.0 to 3.0% 
                   P: 0.1% or less 
                 
                     
                   S: 0.01% or less 
                   Al: 0.01 to 0.1% 
                 
                     
                   N: 0.005% or less, 
                 
                     
                     
                 
             
                
               
               
                
                
                
                
                
               
            
           
         
       
       and the balance being Fe and unavoidable impurities,
 and a structure which is a dual phase structure including a ferrite phase and a martensite phase, with a volume ratio of the martensite phase being 20 to 60%, wherein a tensile strength TS is 1180 MPa or more, an elongation El in a tube axis direction is 10% or more, and a yield ratio is less than 90%; and after application of a 2% prestrain and baking finishing that includes a heat treatment of 170° C.×10 min, a strength increase (BH value) is 100 MPa or more and a yield ratio is 90% or more. 
 
     
     
         2 . The high-strength electric resistance welded steel tube according to  claim 1 , wherein the composition further includes, in terms of percent by mass, at least one selected from Cu: 1.0% or less, Ni: 1.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.05% or less, Ti: 0.05% or less, W: 0.05% or less, and B: 0.0050% or less. 
     
     
         3 . The high-strength electric resistance welded steel tube according to  claim 1 , wherein the composition further includes, in terms of percent by mass, Ca: 0.0050% or less and/or REM: 0.0050% or less. 
     
     
         4 . A method for producing a high-strength electric resistance welded steel tube, the method comprising a hot rolling process of hot-rolling a steel into a hot-rolled sheet; a cold-rolling process of pickling the hot-rolled sheet and cold-rolling the pickled hot-rolled sheet to prepare a cold-rolled sheet; an annealing process of annealing the cold-rolled sheet into a cold-rolled annealed sheet so as to prepare a material for a steel tube; and a tube production process of continuously forming the material for a steel tube into a substantially cylindrical open tube and electric-resistance-welding the open tube to prepare an electric resistance welded tube,
 wherein the steel has a composition including, in terms of percent by mass,   
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                   C: 0.05 to 0.20% 
                   Si: 0.5 to 2.0% 
                 
                     
                   Mn: 1.0 to 3.0% 
                   P: 0.1% or less 
                 
                     
                   S: 0.01% or less 
                   Al: 0.01 to 0.1% 
                 
                     
                   N: 0.005% or less, 
                 
                     
                     
                 
             
                
               
               
                
                
                
                
                
               
            
           
         
       
       and the balance being Fe and unavoidable impurities,
 in the hot-rolling process, the hot rolling is conducted at a finishing temperature equal to or higher than an Ar a  transformation point and at a coiling temperature of 500 to 700° C. to prepare the hot-rolled sheet, 
 in the annealing process, after soaking is performed at a temperature in a two-phase temperature region ranging from an Ac 1  transformation point to an Ac 3  transformation point, the sheet is cooled at an average cooling rate of 10° C./s or more to a temperature in the range of 600 to 750° C. and then rapidly cooled at a cooling rate of 500° C./s or more from the temperature in the range of 600 to 750° C. to room temperature, and then soaking is performed in the temperature range of 150 to 300° C., 
 the forming is performed by a roll forming method involving a cage roll method, and 
 the electric resistance welded tube has a tensile strength TS of 1180 MPa or more, an elongation El in a tube axis direction of 10% or more, and a yield ratio less than 90%, and exhibits, after application of a 2% prestrain and baking finishing that includes a heat treatment of 170° C.×10 min, a strength increase (BH value) of 100 MPa or more and a yield ratio of 90% or more. 
 
     
     
         5 . The method for producing a high-strength electric resistance welded steel tube according to  claim 4 , wherein the composition further includes, in terms of percent by mass, at least one selected from Cu: 1.0% or less, Ni: 1.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.05% or less, Ti: 0.05% or less, W: 0.05% or less, and B: 0.0050% or less. 
     
     
         6 . The method for producing a high-strength electric resistance welded steel tube according to  claim 4 , wherein the composition further includes, in terms of percent by mass, Ca: 0.0050% or less and/or REM: 0.0050% or less.