Ultra-high-strength reinforcing bar and manufacturing method thereof

Abstract

Provided is an ultra-high-strength reinforcing bar and a method for manufacturing the same are disclosed. In an exemplary embodiment, the ultra-high-strength reinforcing bar includes an amount of 0.10 to 0.45 wt % carbon (C), an amount of 0.5 to 1.0 wt % silicon (Si), an amount of 0.40 to 1.80 wt % manganese (Mn), an amount of 0.10 to 1.0 wt % chromium (Cr), an amount greater than 0 and less than or equal to 0.2 wt % vanadium (V), an amount greater than 0 and less than or equal to 0.4 wt % copper (Cu), an amount greater than 0 and less than or equal to 0.5 wt % molybdenum (Mo), an amount of 0.015 to 0.070 wt % aluminum (Al), an amount greater than 0 and less than or equal to 0.25 wt % nickel (Ni), an amount greater than 0 and less than or equal to 0.1 wt % tin (Sn), an amount greater than 0 and less than or equal to 0.05 wt % phosphorus (P), an amount greater than 0 and less than or equal to 0.03 wt % sulfur (S), an amount of 0.005 to 0.02 wt % nitrogen (N), and the remainder being iron (Fe) and other inevitable impurities.

1 .- 8 . (canceled) 
     
     
         9 . A method for manufacturing an ultra-high-strength reinforcing bar, the method comprising steps of:
 reheating a semi-finished product comprising an amount of 0.10 to 0.45 wt % carbon (C), an amount of 0.5 to 1.0 wt % silicon (Si), an amount of 0.40 to 1.80 wt % manganese (Mn), an amount of 0.10 to 1.0 wt % chromium (Cr), an amount greater than 0 and less than or equal to 0.2 wt % vanadium (V), an amount greater than 0 and less than or equal to 0.4 wt % copper (Cu), an amount greater than 0 and less than or equal to 0.5 wt % molybdenum (Mo), an amount of 0.015 to 0.070 wt % aluminum (Al), an amount greater than 0 and less than or equal to 0.25 wt % nickel (Ni), an amount greater than 0 and less than or equal to 0.1 wt % tin (Sn), an amount greater than 0 and less than or equal to 0.05 wt % phosphorus (P), an amount greater than 0 and less than or equal to 0.03 wt % sulfur (S), an amount of 0.005 to 0.02 wt % nitrogen (N), and the remainder being iron (Fe) and other inevitable impurities, the semi-finished product having a carbon equivalent (Ceq) of 0.7 or greater as calculated according to the following Equation 1;   manufacturing a rolled material by hot-rolling the reheated semi-finished product at a finish delivery temperature of 850 to 1,000° C.; and   cooling the rolled material to a Ms temperature or below, the cooling comprising a step of subjecting the rolled material to heat recuperation to reach a temperature of 500 to 700° C.,   wherein the ultra-high-strength reinforcing bar comprises: a central portion; and a surface layer portion formed on an outer circumference of the central portion,   wherein the surface layer portion comprises tempered martensite, and the central portion is composed of a microstructure comprising ferrite, pearlite and bainite,   wherein the ferrite includes any one or more of polygonal ferrite and acicular ferrite, and   the ultra-high-strength reinforcing bar has a yield strength (YS) of 700 MPa or greater, and a tensile strength/yield strength (TS/YS) ratio of 1.25 or greater:   
       
         
           
             
               
                 
                   
                     Ceq 
                     = 
                     
                       
                         [ 
                         C 
                         ] 
                       
                       + 
                       
                         
                           [ 
                           Mn 
                           ] 
                         
                         / 
                         6 
                       
                       + 
                       
                         
                           ( 
                           
                             
                               [ 
                               Cr 
                               ] 
                             
                             + 
                             
                               [ 
                               V 
                               ] 
                             
                             + 
                             
                               [ 
                               Mo 
                               ] 
                             
                           
                           ) 
                         
                         / 
                         5 
                       
                       + 
                       
                         
                           ( 
                           
                             
                               [ 
                               Cu 
                               ] 
                             
                             + 
                             
                               [ 
                               Ni 
                               ] 
                             
                           
                           ) 
                         
                         / 
                         15 
                       
                     
                   
                 
                 
                   
                     [ 
                     
                       Equation 
                       ⁢ 
                           
                       l 
                     
                     ] 
                   
                 
               
             
           
         
         wherein [C], [Mn], [Cr], [V], [Mo], [Cu] and [Ni] are contents (wt %) of carbon (C), manganese (Mn), chromium (Cr), vanadium (V), molybdenum (Mo), copper (Cu) and nickel (Ni), respectively, in the semi-finished product. 
       
     
     
         10 . The method of  claim 9 , wherein the cooling is performed under conditions of a rolled material line speed of 6.7 to 7.2 m/s and a specific water amount of 3.7 to 3.9 l/kg. 
     
     
         11 . The method of  claim 9 , wherein the reheating is performed at a temperature of 1,050 to 1,250° C. 
     
     
         12 . The method of  claim 9 , wherein the central portion comprises a hardened core layer having a hardness of 350 Hv or greater.