Abstract: Ferritic-austenitic duplex stainless steel comprising the following components in the following proportions in % by weight:0<C.ltoreq.0.08,______________________________________ Si 0.2-2.0, Mn 0.2-2.0, Cr 19.0-30.0, Ni 3.0-9.0, Mo 1.0-5.0, Cu 0.5-3.0, Co 0.2-4.0, N 0.05-0.35, ______________________________________the balance being substantially Fe and inevitable impurities, the proportions of Cr and Ni having the correlation of19.0.ltoreq.Cr<24.0 and 3.0.ltoreq.Ni.ltoreq.8.0, or 24.0.ltoreq.Cr.ltoreq.30.0 and 4.0.ltoreq.Ni.ltoreq.9.0the micro structure of the steel containing delta-ferrite phase in an amount of 30 to 70% in area ratio.

Abstract: Composite pipe adapted for use, for example, in line pipes for crude oil or natural gas producing oil wells and having excellent corrosion resistance and mechanical properties to withstand high temperatures and high pressures, the composite pipe having an outer layer made of a material comprising in % by weight 0.05 to 0.20% of C, up to 1.0% of Si, up to 2.0% of Mn, up to 1.0% of Cr, 0.2 to 2.0% of Ni, up to 1.0% of Mo, 0.05 to 0.15% of V, up to 0.1% of Al, and the balance inevitable impurities and Fe, the material being up to 0.45 in carbon equivalent expressed by: ##EQU1## an inner layer made of stainless steel, high alloy steel or like corrosion resistant alloy, and an intermediate layer substantially intermediate between the outer layer and the inner layer in composition and having a thickness of 10 to 100 .mu.m, the intermediate layer providing a boundary between the outer and inner layers and being metallurgically joint to the outer layer and the inner layer integrally therewith.