Equipment line for manufacturing seamless steel tube or pipe and method of manufacturing high-strength stainless steel seamless tube or pipe for oil wells using the equipment line

A method of manufacturing a high-strength stainless seamless steel tube with an equipment line comprising:

• a heating device,
• a piercing device,
• a rolling mill,
• a cooling system, which is arranged between the heating device and the piercing device or between the piercing device and the rolling mill, and
• a thermal insulator, which is arranged on an exit side of the rolling mill,
• the method comprising;
• heating steel in the heating device,
• piercing the steel in the piercing device into a hollow steel tube,
• cooling the hollow steel tube in the cooling system,
• forming the hollow steel tube in the rolling mill into a seamless steel tube having a predetermined size, and
• passing the seamless steel tube through the thermal insulator,
• wherein:
• the steel has a chemical composition consisting of by mass %, 0.050% or less C, 0.50% or less Si, 0.20 to 1.80% Mn, 15.5 to 18.0% Cr, 1.5 to 5.0% Ni, 1.0 to 3.5% Mo, 0.02 to 0.20% V, 0.01 to 0.15% N, 0.006% or less O, and Fe and unavoidable impurities as a balance,
• the heating in the heating device is performed such that the steel is heated to a temperature within a range from 650° C. to a temperature below a melting point of the steel,
• the cooling in the cooling system is performed such that the hollow steel tube after piercing is subjected to cooling at an average cooling rate of 1.0° C./s or more on an outer surface of steel until a cooling stop temperature of 600° C. or above and in a cooling temperature range of 50° C. or more between a cooling start temperature and the cooling stop temperature, and
• the passing through the thermal insulator is performed such that a cooling rate of the seamless steel tube is slowed down to an average cooling rate of 20° C./s or less.