Tubes, pipes and hollow profiles, seamless, of circular cross-section, cold-reduced, of alloy steel, straight and of uniform wall thickness, of alloy steel containing by weight not less than 0,9 % but not more than 1,15 % of carbon, not less than 0,5 % but not more than 2 % of chromium and, if present, not more than 0,5 % of molybdenum