To provide a manufacturing method of a seamless steel tube by a plug mill tube-making line by which the generation of a carburized layer on the inside surface is suppressible and seamless alloy steel tube excellent in corrosion resistance which is manufactured by this method.
At the time of manufacturing the seamless steel tube of alloy steel containing, by weight, ≤0.10% C and 4-30% Cr by the plug mill tube- making line, either of graphite, graphitic lubricant and non-graphitic lubricant is used for a lubricant for inside surfaces and, meanwhile, elongation rolling is executed at a thickness rolling reduction of ≤2.5 mm. In this way, the product tube in which the amount ΔC of carburization in the position 100 μm apart in the thickness direction from the inside surface of the tube is not more than 3 times the carbon content of a base material and which is excellent in corrosion resistance can be obtained.
UEDA MASAKATSU