Austenitic steel having excellent machinability and ultra-low temperature toughness in weld heat-affected zone, and method of manufacturing the same

An austenitic steel having excellent machinability and ultra-low temperature toughness in a weld heat-affected zone, the austenitic steel having a composition comprising:

• 15 wt % to 35 wt % of manganese (Mn);
• carbon (C) satisfying 23.6C+Mn≧28 and 33.5C−Mn≦23;
• 5 wt % or less (excluding 0 wt %) of copper (Cu);
• chromium (Cr) satisfying 28.5C+4.4Cr≦57 (excluding 0 wt %);
• and
• iron (Fe) as well as other unavoidable impurities as a remainder.

A method of manufacturing an austenitic steel having excellent machinability and ultra-low temperature toughness in a weld heat-affected zone, the method comprising:

• obtaining a weld heat-affected zone by welding a steel having a composition including 15 wt % to 35 wt % of manganese (Mn);
• carbon (C) satisfying 23.6C+Mn≧28 and 33.5C−Mn≦23;
• 5 wt % or less (excluding 0 wt %) of copper (Cu);
• chromium (Cr) satisfying 28.5C+4.4Cr≦57 (excluding 0 wt %);
• and iron (Fe) as well as other unavoidable impurities as a remainder;
• and
• cooling the weld heat-affected zone at a cooling rate of 10° C./s or more.