The processes of steel-making, rolling and heat treatment have being constantly improving in Xingcheng Special Steel Co., Ltd, and the complete sets of advance production technology and experiences have been formed. A new -50 degrees C high-strength steel plate for large sphere tank has been successfully developed. This steel was micro-alloyed with right amount of Nb, and it has excellent metallurgical quality, lower residual elements and higher performance stability. The relationships between the heat treatment process and the microstructure and mechanical property have been investigated. The results showed that the steel has good strength and toughness, especially, the central portion of the steel has excellent low-temperature toughness, and it can fully meet the technical requirements, namely the yield strength ReL >= 500 MPa, the tensile strength 610MPa <= Rm <= 730MPa, the elongation A >= 17%, the -50 degrees C impact energy KV2 >= 80J (in the center of plate thickness).
