Abstract: An overlapped body 10 includes at least three steel plates 1a, 1b and 1c and in which at least one steel plate interface 2a has a contact resistance different from the contact resistance of another steel plate interface 2b. A molten pool is formed at the steel plate interfaces so as to join the steel plates 1a, 1b and 1c, and an energization point 5 is formed at the steel plate interface 2b having the largest contact resistance. Initial spot welding is performed under a condition in which a branch current is generated in the energization point 5 so as to form the molten pool. By doing so, it makes it difficult to generate the expulsion and surface flash at the steel plate interface where resistive heating is large and form the molten pool having a sufficiently large size at the steel plate interface where resistive heating is small.

Abstract: A duplex stainless steel containing, by mass %: C: not more than 0.03%, Si: not more than 0.3%, Mn: not more than 3.0%, P: not more than 0.040%, S: not more than 0.008%, Cu: 0.2 to 2.0%, Ni: 5.0 to 6.5%, Cr: 23.0 to 27.0%, Mo: 2.5 to 3.5%, W: 1.5 to 4.0%, and N: 0.24 to 0.40%, the balance being Fe and impurities, wherein a ? phase susceptibility index X (=2.2Si+0.5Cu+2.0Ni+Cr+4.2Mo+0.2W) is not more than 52.0; a strength index Y (=Cr+1.5Mo+10N+3.5W) is not less than 40.5; and a pitting resistance equivalent PREW (=Cr+3.3(Mo+0.5W)+16N) is not less than 40. This duplex stainless steel is excellent in corrosion resistance and embrittlement cracking resistance.