COMPOSITE TUBE AND WELDED JOINT

A composite tube is provided that includes a first tube and a second tube. A chemical composition of the first tube includes, in mass %, C: more than 0.060% to 0.400% or less, 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, and the balance: Fe and impurities. A chemical composition of the second tube includes, 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. The composite tube satisfies formulas [Siave+6×Pave+20×Save+2×Snave≤1.1000] and [0.0015≤4×Save+Snave].