Wire rod for graphitization heat treatment, graphite steel, and manufacturing method therefor
A graphite steel available as a material for mechanical parts of industrial machines or automobiles, and more particularly, a steel wire for graphitization heat treatment and a graphite steel and methods of manufacturing the same. The graphite steel includes, in percent by weight (wt %), 0.6 to 0.9% of carbon (C), 2.0 to 2.5% of silicon (Si), 0.1 to 0.6% of manganese (Mn), 0.015% or less of phosphorus (P), 0.03% or less of sulfur (S), 0.01 to 0.05% of aluminum (Al), 0.01 to 0.02% of titanium (Ti), 0.0005 to 0.002% of boron (B), 0.003 to 0.015% of nitrogen (N), 0.005% or less of oxygen (O), and the remainder of iron (Fe) and inevitable impurities, and satisfying Equation (1) below: wherein graphite grains are distributed in a ferrite base as a microstructure and a graphitization rate is 100%, (1) −0.003<[N]−[Ti]/3.43−[B]/0.77<0.003, wherein in Equation (1), [Ti], [N], and [B] are wt % of titanium, nitrogen, and boron, respectively.
1. A steel wire for graphitization heat treatment comprising, in percent by weight (wt %), 0.6 to 0.9% of carbon (C), 2.0 to 2.5% of silicon (Si), 0.1 to 0.6% of manganese (Mn), 0.015% or less of phosphorus (P), 0.03% or less of sulfur(S), 0.01 to 0.05% of aluminum (Al), 0.01 to 0.02% of titanium (Ti), 0.0005 to 0.002% of boron (B), 0.003 to 0.015% of nitrogen (N), 0.005% or less of oxygen (O), and the remainder of iron (Fe) and inevitable impurities, and satisfying Equation (1) below:
−0.003<[N]−[Ti]/3.43−[B]/0.77<0.003
wherein in Equation (1), [Ti], [N], and [B] are wt % of titanium, nitrogen, and boron, respectively,
wherein a number of TiN having a size of 100 nm or less is 10 or more per 100 μm 2 ,
wherein a tensile strength is 1100 MPa or less.
2. The steel wire of claim 1 , wherein an area fraction of pearlite is 95% or more.
3. A method of manufacturing the steel wire of claim 1 , the method comprising:
preparing a billet comprising, in percent by weight (wt %), 0.6 to 0.9% of carbon (C), 2.0 to 2.5% of silicon (Si), 0.1 to 0.6% of manganese (Mn), 0.015% or less of phosphorus (P), 0.03% or less of sulfur(S), 0.01 to 0.05% of aluminum (Al), 0.01 to 0.02% of titanium (Ti), 0.0005 to 0.002% of boron (B), 0.003 to 0.015% of nitrogen (N), 0.005% or less of oxygen (O), and the remainder of iron (Fe) and inevitable impurities, and satisfying Equation (1) below;
reheating the billet;
hot rolling the reheated billet into a steel wire;
coiling the steel wire; and
cooling the coiled steel wire:
−0.003<[N]−[Ti]/3.43−[B]/0.77<0.003
wherein in Equation (1), [Ti], [N], and [B] are wt % of titanium, nitrogen, and boron, respectively.
4. The method of claim 3 , wherein the reheating comprises heat-treating the billet in a temperature range of 1050 to 1150° C. for 60 minutes or more.
5. The method of claim 3 , wherein the hot rolling of the reheated billet into a steel wire comprises hot rolling the reheated billet in a temperature range of 900° C. to 1000° C.
6. The method of claim 3 , wherein the coiling comprises coiling the steel wire in a temperature range over 800° C.
7. The method of claim 3 , wherein the cooling comprises cooling the coiled steel wire to 600° C. at a cooling rate of 0.2 to 5.0° C./s.