Method for manufacturing martensitic stainless steel tube

A martensitic stainless steel tube including C:

• 0.15-0.22%, Si:
• 0.1-1.0%. Mn:
• 0.30-1.00% and Cr:
• 12.00-16.00% in mass %,
• wherein the scale thickness on the outer surface of the steel tube is 5-100 μm, and
• wherein the bubble content ratio satisfies the following equation (2):
• bubble content ratio (%)≦−5.20×ln(ds)+30.20 (2)
• where ds:
• scale thickness (μm), and
• ln(x):
• natural logarithm of x.

A method for manufacturing a martensitic stainless steel tube including C:

• 0.15-0.22%, Si:
• 0.1-1.0%, Mn:
• 0.30-1.00% and Cr:
• 12.00-16.00% in mass % or a martensitic stainless steel tube further including one group or more of Al:
• 0.1% or less, Ni:
• 1.0% or less and Cu:
• 0.25% or less in mass % in addition to said components, said method comprising the following steps of:
• heating an in-process steel tube for duration between 5 min. or more and 30 min. or less at a temperature of “AC3 point+20° C.” or higher to 980° C. or lower in an atmosphere containing oxygen amount of 2.5 vol. % or less and water vapor amount of 15.0 vol. % or less;
• quenching the steel tube thus heated at a cooling rate of 1-40° C./sec. from 980° C. to the A point, at a cooling rate of less than 1° C./sec. from the A point to the B point and at a cooling rate of 5-40° C./sec. from the B point to the ambient temperature, where the A point is 680-350° C. and the B point is 300-150° C.;
• and
• spraying a high-pressure water having a pressure of 490 N/mm2 or higher onto the outer surface of the steel tube during at least a part of the cooling duration from 900° C. up to the A point of said quenching.

A method for manufacturing a martensitic stainless steel tube including C:

• 0.15-0.22%, Si:
• 0.1-1.0%, Mn:
• 0.30-1.00% and Cr:
• 12.00-16.00% in mass % or a martensitic stainless steel tube further including one group or more of Al:
• 0.1% or less, Ni:
• 1.0% or less and Cu:
• 0.25% or less in mass % in addition to said components, said method comprising the following steps of:
• heating an in-process steel tube for duration between 5 min. or more and 30 min. or less at a temperature of “Ac3 point+20° C.” or higher to 980° C. or lower in an atmosphere containing amount of oxygen 1.5 vol. % or less and amount of water vapor 3-10.0 vol. % or less;
• quenching the steel tube thus heated at a cooling rate of 1-40° C./sec. from 980° C. up to the A point, at a cooling rate of less than 1° C./sec. from the A point to the B point and at a cooling rate of 5-40° C./sec. from the B point to the ambient temperature, where the A point is 680-350° C. and the B point is 300-150° C.;
• and
• spraying a high-pressure water having a pressure of 490 N/mm2 or higher onto the outer surface of the steel tube during at least part of cooling duration from 900° C. up to the A point of said quenching.