According to API Spec 5CT1988, there are 10 types of casing steel grades, including H-40, J-55, K-55, N-80, C-75, L-80, C-90, C-95, P-110, and Q-125.Casing shall be supplied with threads and couplings, or in one of the following pipe end forms: flat end, round thread with or without couplings, partial trapezoid thread with or without couplings, straight thread, special end work, sealing ring construction.According to SY/T6194-96, domestic casing should be tied with steel wire or steel belt.The exposed part of each sleeve and coupling thread shall be screwed with a guard ring to protect the thread.