Low-carbon Steel Kid- carbon Steel High- carbon Steel Stainless Steel Alloy Steel Titanium Alloy Half soft steel: carbon(%) : 0.25~0.39 tensile(kgf/mm2) : 45~50 function : spring, boating materials, furnace, steel ribs Half hard steel: carbon(%) : 0.4~0.54 tensile(kgf/mm2) : 50~62 function : needles, pin core, steel rail, wheel, base, screw