Creep strength property and microstructural evolution during long-term creep of 18Cr-9Ni-3Cu-Nb-MNN (KA-SUS304J1HTB) austenitic steel (Creep strength property and microstructural evolution during long-term creep of 18Cr-9Ni-3Cu-Nb-MNN (KA-SUS304J1HTB) austenitic stee)