Ultra-high-strength reinforcing bar and manufacturing method thereof

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 reinforcing bar comprises, on a cross-section basis, 5 to 15 area % of the surface layer po tion and 85 to 95 area % of the central portion,
• wherein the surface layer portion comprises tempered martensite, and the central portion comprises 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 1]
• 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.