Hot rolled steel having low compressive strength loss after being processed into steel pipe, and manufacturing method therefor

Abstract

Provided are a hot rolled steel having small compressive strength loss after being processed into a steel pipe, and a manufacturing method therefor. The hot rolled steel of the present invention comprises, by wt %, 0.15% or less of carbon (C), 2.5% or less of silicon (Si), 2.0% or less of manganese (Mn), 0.05% or less of aluminum (Al), nitrogen (N) and boron (B) in a sum of 0.002 to 0.008%, and a balance of Fe and inevitable impurities, wherein 250<450C+95Si+70Mn is satisfied, a microstructure is a mixed structure of ferrite and pearlite, the average grain size of the ferrite is 8 to 25 μm, and a value defined by relation 1 is less than 20%.

1 . A hot rolled steel having small compressive strength loss after being processed into a steel pipe, the hot rolled steel comprising:
 by wt %, 0.15% or less of carbon (C), 2.5% or less of silicon (Si), 2.0% or less of manganese (Mn), 0.05% or less of aluminum (Al), nitrogen (N) and boron (B) in a sum of 0.002 to 0.008%, and a balance of Fe and inevitable impurities,   wherein 250<450C+95Si+70Mn is satisfied,   a microstructure is a mixed structure of ferrite and pearlite,   an average grain size of the ferrite is 8 to 25 μm, and   when tensile strength under a 0.3% stretching condition is referred to as σ Tensile  and compressive strength when the stretched and deformed material is compressed by 0.2% again is referred to as σ compressive , a value is less than 20% defined in the following Relational Expression 1.   
       
         
           
             
               
                 
                   
                     
                       
                         ( 
                         
                           
                             σ 
                             
                               Tensile 
                                 
                             
                           
                           - 
                           
                             σ 
                             compressive 
                           
                         
                         ) 
                       
                       
                         ( 
                         
                           σ 
                           Tensile 
                         
                         ) 
                       
                     
                     × 
                     1 
                     ⁢ 
                     0 
                     ⁢ 
                     0 
                   
                 
                 
                   
                     [ 
                     
                       Relational 
                       ⁢ 
                            
                       Expression 
                       ⁢ 
                           
                       1 
                     
                     ] 
                   
                 
               
             
           
         
       
     
     
         2 . The hot rolled steel having small compressive strength loss after being processed into a steel pipe of  claim 1 , wherein the tensile strength σ Tensile  of the hot rolled steel is 385 MPa or more, and impact toughness at 0° C. is 50 J or more. 
     
     
         3 . A method for manufacturing a hot rolled steel having small compressive strength loss after being processed into a steel pipe, the method comprising:
 finish hot-rolling a steel slab including, by wt %, 0.15% or less of carbon (C), 2.5% or less of silicon (Si), 2.0% or less of manganese (Mn), 0.05% or less of aluminum (Al), nitrogen (N) and boron (B) in a sum of 0.002 to 0.008%, and a balance of Fe and inevitable impurities at a finish temperature of 950 to 1030° C.; and   cooling the finish hot-rolled steel sheet to a temperature within a range of 580 to 730° C. at a cooling rate of 5 to 50° C. and coiling the finish hot-rolled steel sheet,   wherein a temperature change of the cooled hot-rolled steel sheet is controlled to be within 20° C. for 3 seconds immediately before coiling it.   
     
     
         4 . The method for manufacturing a hot rolled steel having a small compressive strength loss after being processed into a steel pipe of  claim 3 , wherein in the coiled hot rolled steel, a microstructure is a mixed structure of ferrite and pearlite, and an average grain size of the ferrite is 8 to 25 μm, and
 when tensile strength under a 0.3% stretching condition is referred to as σ Tensile  and compressive strength when the stretched and deformed material is compressed by 0.2% again is referred to as σ compressive , a value is less than 20% defined in the following Relational Expression 1. 
 
       
         
           
             
               
                 
                   
                     
                       
                         ( 
                         
                           
                             σ 
                             
                               Tensile 
                                 
                             
                           
                           - 
                           
                             σ 
                             compressive 
                           
                         
                         ) 
                       
                       
                         ( 
                         
                           σ 
                           Tensile 
                         
                         ) 
                       
                     
                     × 
                     1 
                     ⁢ 
                     0 
                     ⁢ 
                     0 
                   
                 
                 
                   
                     [ 
                     
                       Relational 
                       ⁢ 
                            
                       Expression 
                       ⁢ 
                           
                       1 
                     
                     ] 
                   
                 
               
             
           
         
       
     
     
         5 . The method for manufacturing a hot rolled steel having a small compressive strength loss after being processed into a steel pipe of  claim 4 , wherein the tensile strength (Tensile of the hot rolled steel is 385 MPa or more, and impact toughness at 0° C. is 50 J or more.