Composite tube

A composite tube having a rib, and having an inner tube consisting of ferritic steel and an outer tube consisting of austenitic steel, wherein

• a chemical composition of the inner tube contains, in mass %:
• C:
• 0.01 to 0.30%,
• Si:
• 0.01 to 0.80%,
• Mn:
• 0.01 to 2.00%,
• P:
• 0.030% or less,
• S:
• 0.0100% or less,
• Cr:
• 0.01 to 3.50%,
• Al:
• 0.050% or less,
• N:
• 0.0005 to 0.0500%,
• one or more elements selected from a group A below, and
• the balance:
• Fe and impurities, and
• a chemical composition of the outer tube contains, in mass %:
• C:
• 0.10% or less,
• Si:
• 0.01 to 0.80%,
• Mn:
• 0.01 to 3.50%,
• P:
• 0.040% or less,
• S:
• 0.0100% or less,
• Ni:
• 5.0 to 70.0%,
• Cr:
• 15.0 to 35.0%,
• Al:
• 1.500% or less,
• N:
• 0.001 to 0.350%,
• one or more elements selected from a group B below, and
• the balance:
• Fe and impurities:
• [group A] Ni:
• 1.5% or less, Cu:
• 1.0% or less, Mo:
• 3.0% or less, W:
• 3.0% or less, V:
• 0.50% or less, Nb:
• 0.10% or less, Ti:
• 0.30% or less, B:
• 0.0200% or less, Co:
• 0.50% or less, Ca:
• 0.050% or less, Sb:
• 0.20% or less;
• [group B] Cu:
• 5.00% or less, Nb:
• 5.00% or less, Ti:
• 1.0% or less, B:
• 0.010% or less, W:
• 10.0% or less, Co:
• 15.0% or less, Ca:
• 0.010% or less.

A composite tube having a rib, and having an inner tube consisting of ferritic steel and an outer tube consisting of austenitic steel, wherein

• a chemical composition of the inner tube contains, in mass %:
• C:
• 0.01 to 0.30%,
• Si:
• 0.01 to 0.80%,
• Mn:
• 0.01 to 2.00%,
• P:
• 0.030% or less,
• S:
• 0.0100% or less,
• Cr:
• 0.01 to 3.50%,
• Al:
• 0.050% or less,
• N:
• 0.0005 to 0.0500%,
• one or more elements selected from a group A below, and
• the balance:
• Fe and impurities, and
• a chemical composition of the outer tube contains, in mass %:
• C:
• 0.10% or less,
• Si:
• 0.01 to 0.80%,
• Mn:
• 0.01 to 3.50%,
• P:
• 0.040% or less,
• S:
• 0.0100% or less,
• Ni:
• 5.0 to 70.0%,
• Cr:
• 15.0 to 35.0%,
• Al:
• 1.500% or less,
• N:
• 0.001 to 0.350%,
• one or more elements selected from a group B below, and
• the balance:
• Fe and impurities:
• [group A] Ni:
• 1.0% or less, Cu:
• 1.0% or less, Mo:
• 3.0% or less, W:
• 3.0% or less, V:
• 0.50% or less, Nb:
• 0.10% or less, Ti:
• 0.30% or less, B:
• 0.0200% or less, Co:
• 0.50% or less, Ca:
• 0.050% or less, Sb:
• 0.20% or less;
• [group B] Cu:
• 5.00% or less, Nb:
• 5.00% or less, Ti:
• 1.0% or less, B:
• 0.010% or less, W:
• 10.0% or less, Co:
• 15.0% or less, Ca:
• 0.010% or less.