700MPa CLASS STEEL BAR HAVING EXCELLENT YIELD RATIO AND UNIFORM ELONGATION PROPERTY, AND METHOD FOR MANUFACTURING THE SAME

Abstract

A steel bar having yield strength of 700 MPa class, excellent yield ratio, and excellent uniform elongation property is provided. The steel bar includes bainite and ferrite included in a center of the steel bar and bainite accounts for 20 to 80% by volume.

What is claimed is: 
     
         1 . A steel bar having yield strength of 700 MPa class, excellent yield ratio, and excellent uniform elongation property, the steel bar comprising:
 bainite and ferrite included in a center of the steel bar,   wherein the bainite accounts for 20 to 80% by volume.   
     
     
         2 . The steel bar of  claim 1 , wherein the steel bar has the yield strength of 700 to 900 MPa and the yield ratio of 1.25 to 1.50. 
     
     
         3 . The steel bar of  claim 1 , wherein, when manufacturing the steel bar, a bainite structure is formed in the center of the steel bar by a rapid cooling through a Tempcore cooling process. 
     
     
         4 . The steel bar of  claim 3 , wherein the Tempcore cooling process is performed at a speed of the steel bar of 5 to 20 m/sec, a pressure of cool water of 10 to 30 bar, and an amount of water of 100 to 300 m 3 /hr. 
     
     
         5 . A method of manufacturing a steel bar having yield strength of 700 MPa class, excellent yield ratio, and excellent uniform elongation property, the method comprising:
 heating a billet for the steel bar;   hot-rolling the heated billet to be formed in a steel bar shape;   cooling a rolled body having the steel bar shape at a Tempcore; and   performing reheating and additional cooling,   wherein the cooling at the Tempcore is performed at a speed of the steel bar of 5 to 20 m/sec, a pressure of cool water of 10 to 30 bar, and an amount of water of 100 to 300 m 3 /hr.   
     
     
         6 . The method of  claim 5 , wherein, in the heating of the billet for the steel bar, the billet for the steel bar includes carbon (C) of 0.18 to 0.30 wt %, manganese (Mn) of 0.65 to 2.00 wt %, silicon (Si) of 0.13 to 0.40 wt %, vanadium (V) of 0 to 0.10 wt %, the remaining Fe, and other unavoidable impurity components. 
     
     
         7 . The method of  claim 5 , wherein, in the reheating, a temperature of the center of the steel bar is maintained at 500 to 600° C. 
     
     
         8 . The method of  claim 5 , wherein, after the reheating, a structure of the center of the steel bar includes bainite and ferrite. 
     
     
         9 . The method of  claim 5 , wherein, after the reheating and the additional cooling, the steel bar has the yield strength of 700 to 900 MPa and the yield ratio of 1.25 to 1.50.