Calcium-containing graphite steel having excellent machinability, and manufacturing method therefor

Abstract

The present invention relates to a graphite steel having excellent machinability, and a manufacturing method therefor, and more specifically, to a calcium-containing graphite steel having machinability superior to that of normal free-cutting steel and a manufacturing method therefor.

1 . A graphite steel comprising, in percent by weight (wt %), 0.60 to 0.90% of carbon (C), 2.0 to 2.5% of silicon (Si), 0.7 to 1.3% of manganese (Mn), 0.2 to 0.5% of sulfur (S), 0.01 to 0.05% of aluminum (Al), 0.005 to 0.020% of titanium (Ti), 0.003 to 0.015% of nitrogen (N), 0.0001 to 0.050% of calcium (Ca), and the balance of iron (Fe) and inevitable impurities,
 wherein the graphite steel includes a microstructure in which graphite grains are distributed in a ferrite matrix, has a graphitization rate of 95% or more, and includes a total of 5 wt % of MnS inclusions and pearlite.   
     
     
         2 . The graphite steel according to  claim 1 , wherein the graphite steel has a graphitization rate of 99% or more. 
     
     
         3 . The graphite steel according to  claim 1 , wherein the graphite steel does not include at least one phosphorus (P) and oxygen (O). 
     
     
         4 . A method for manufacturing a graphite steel, the method comprising:
 preparing a billet including, in percent by weight (wt %), 0.60 to 0.90% of carbon (C), 2.0 to 2.5% of silicon (Si), 0.7 to 1.3% of manganese (Mn), 0.2 to 0.5% of sulfur (S), 0.01 to 0.05% of aluminum (Al), 0.005 to 0.020% of titanium (Ti), 0.003 to 0.015% of nitrogen (N), 0.0001 to 0.05% of calcium (Ca), and the balance of iron (Fe) and inevitable impurities;   hot rolling the billet to prepare a wire rod; and   performing graphitizing heat treatment on the prepared wire rod.   
     
     
         5 . The method according to  claim 4 , wherein the hot rolling comprises hot rolling in a temperature range of 900° C. to 1150° C. 
     
     
         6 . The method according to  claim 4 , wherein the graphitizing heat treatment comprises heat treatment in a temperature range of 700° C. to 800° C. for 5 hours or more. 
     
     
         7 . The method according to  claim 6 , wherein the graphitizing heat treatment is performed for 5 hours to 20 hours.