Composite tube and welded joint

A composite tube comprising a first tube and a second tube, wherein:

• a chemical composition of the first tube consists of, in mass %,
• C:
• 0.110 to 0.400%,
• Si:
• 0.01 to 1.00%,
• Mn:
• 0.01 to 1.20%,
• P:
• 0.0350% or less,
• S:
• 0.0150% or less,
• Sn:
• 0.0005 to 0.0400%,
• Al:
• 0.040% or less,
• N:
• 0.050% or less,
• O:
• 0.030% or less,
• the balance:
• Fe and impurities, and
• wherein, in lieu of a part of Fe, one or more elements selected from a group of, in mass %,
• a total of one or more elements selected from Cu, Ni and Co:
• 1.00% or less,
• a total of Mo and/or W:
• 4.00% or less,
• a total of one or more elements selected from V, Nb, Ti and Ta:
• 1.00% or less,
• B:
• 0.0200% or less,
• a total of Ca and/or Mg:
• 0.0100% or less, and
• REM:
• 0.0500% or less;
• a chemical composition of the second tube comprises, in mass %,
• C:
• 0.003 to 0.100%,
• Si:
• 0.01 to 1.50%,
• Mn:
• 0.01 to 2.20%,
• P:
• 0.0400% or less,
• S:
• 0.0100% or less,
• Sn:
• 0.0005 to 0.0300%,
• Ni:
• 7.0 to 52.0%,
• Cr:
• 15.0 to 27.0%,
• Al:
• 0.001 to 0.600%,
• N:
• 0.001 to 0.150%,
• O:
• 0.030% or less, and
• the balance:
• Fe and impurities;
• and
• the composite tube satisfies Formulas (i) and (ii) below:
• S
• ⁢
• i
• ave
• +
• 6
• ×
• P
• ave
• +
• 2
• ⁢
• 0
• ×
• S
• ave
• +
• 2
• ×
• S
• ⁢
• n
• ave
• ≤
• 1.1
• (
• i
• )
• 0.0015
• ≤
• 4
• ×
• S
• ave
• +
• S
• ⁢
• n
• ave
• (
• ii
• )
• where, the meaning of each symbol in the above formulas is as follows:
• Siave:
• (mass % of Si in the first tube+mass % of Si in the second tube)/2,
• Pave:
• (mass % of P in the first tube+mass % of P in the second tube)/2,
• Save:
• (mass % of S in the first tube+mass % of S in the second tube)/2,
• Snave:
• (mass % of Sn in the first tube+mass % of Sn in the second tube)/2.