Abstract: A high-strength, heat-resisting ferritic steel pipe or tube for boiler use containing 0.03 to 0.15% C, 0.1 to 1.5% Mn, 8 to 13% Cr, 1.8 to 3.0% W, 0.05 to 0.30% V, 0.02 to 0.12% Nb, 0.02 to 0.05% N, 0.02 to 0.4% Mo, and up to 0.25% Si. This pipe or tube has an improved high-temperature creep rupture strength and an excellent weldability and toughness.

Abstract: A boiler tube having improved high temperature strength, improved high temperature corrosion resistance, and resistivity to embrittlement during the service thereof, comprising an outer surface layer and an inner part, the outer surface layer consisting essentially, by weight, of 0.03-0.20% carbon, 1.5-4.0% silicon, 0.1-3.0% manganese, 13-25% chromium, 13-40% nickel, 0.5-3.0% at least one kind selected from the group consisting of molybdenum and tungsten, 0.05-0.5% at least one kind selected from the group consisting of titanium, niobium and vanadium, and the balance iron and inevitable impurities, the inner part consisting essentially, by weight, of 0.03-0.20% carbon, 0.3-1.0% silicon, 0.1-3.0% manganese, 13-25% chromium, 13-40% nickel, 0.5-3.0% at least one kind selected from the group consisting of molybdenum and tungsten, 0.05-0.5% at least one kind selected from the group consisting of titanium, niobium and vanadium, and the balance iron and inevitable impurities. Boron of 0.001-0.

Abstract: A ferritic heat-resisting steel with an excellent toughness comprises0.03 to 0.10% by weight of carbon,0.1 to 1.0% by weight of silicon,1.5% by weight or less of manganese,1.5% to 2.7% by weight of molybdenum,7.0% to 10.0% by weight of chromium,0.01 to 0.1% by weight of niobium,0.02 to 0.12% by weight of vanadium,0.01 to 0.10% by weight of alloying element consisting of at least one member selected from the group consisting of rare earth elements having atomic numbers 57 through 71 and yttrium, andthe balance consisting of iron and unavoidable impurities, in which steel the ratio of the sum of the numbers of niobium and vanadium atoms to the number of carbon atoms is in the range of from 0.35 to 0.80; and the quantity (Cr%/30+Mo%/10-C%) and the content in percent of said alloying element fall on or within an irregular pentagon indicated in FIG.