Apparatus line for manufacturing seamless steel pipe and tube and method of manufacturing duplex seamless stainless steel pipe

A method of manufacturing duplex seamless stainless steel pipes and tubes using an apparatus line for manufacturing seamless steel pipes and tubes, wherein

• a steel raw material is heated by a heating apparatus,
• the hollow material is formed by piercing the steel raw material by a piercing apparatus,
• the hollow material is formed into a hollow piece by applying hot working by a rolling apparatus,
• the hollow piece is cooled by the cooling apparatus, and
• the steel raw material has the composition which contains, by mass %,
• 0.050% or less C, 2.00% or less Si,
• 5.00% or less Mn, 0.05% or less P,
• 0.03% or less S, 16.0 to 35.0% Cr,
• 3.0 to 12.0% Ni, 5.0% or less Mo,
• 0.1% or less Al, 0.5% or less N, and
• Fe and unavoidable impurities as a balance, wherein
• the steel raw material is heated to a temperature of (δA−300° C.) to (δA+100° C.) by the heating apparatus,
• hot working is applied to the rolling apparatus, and
• using a surface temperature of the hollow piece before being cooled by the cooling apparatus as a cooling start temperature, the cooling apparatus cools the hollow piece to a cooling stop temperature having a temperature difference of at least 50° C. or more from the cooling start temperature and being equal to or above 600° C. at an average cooling speed of 1.0° C./s or more in terms of an outer surface temperature.