Abstract: Steel chemical composition of which contains, by weight: 0.1%.ltoreq.C.ltoreq.0.4%; 1%.ltoreq.Mn.ltoreq.1.8%; 0.15%.ltoreq.Si.ltoreq.1.7%; 0%.ltoreq.Ni.ltoreq.1%; 0%.ltoreq.Cr.ltoreq.1.2%; 0%.ltoreq.Mo.ltoreq.0.3%; 0%.ltoreq.V.ltoreq.0.3%; Cu.ltoreq.0.35%; optionally from 0.005% to 0.06% of aluminum, optionally boron in contents of between 0.0005% and 0.01%, optionally between 0.005% and 0.03% of titanium, optionally between 0.005% and 0.06% of niobium, optionally from 0.005% to 0.1% of sulfur, optionally up to 0.007% of calcium, optionally up to 0.03% of tellurium, optionally up to 0.05% of selenium, optionally up to 0.05% of bismuth, optionally up to 0.1% of lead, the balance being iron and impurities resulting from smelting. Process for the manufacture of a forging.

Abstract: The invention relates to a method for fabricating a piece of structural steel that includes thermal or thermomechanical treatment steps that could ordinarily lower the fatigue strength and shock and impact resistance of the steel piece. In the method of the invention a workpiece of microalloy steel is fabricated containing the following elements in the mounts indicated and having a bainitic structure:______________________________________ C 0.05-0.5 wt. % Mo 0-0.5 wt. % Mn 1-2 wt. % V 0-0.30 wt. % Si 0.05-1.5 wt. % B 0-0.010 wt. % Cr 0.1-1 wt. % Ti 0-0.030 wt. % Nb 0-0.1 wt. % ______________________________________The workpiece is then subjected to a treatment that includes a heating stage wherein at least a part of said piece is subjected to a temperature in the range 500.degree.-900.degree. C., followed by a cooling stage in which at least the part of the piece is subjected to cooling at a rate greater than 500.degree. C./hr.