Abstract: High-strength high-ductility steel whose chemical composition, by weight, comprises from 0.15% to 0.35% of carbon, from 0% to 3% of silicon, from 0% to 3% of aluminium, from 0.1% to 4.5% of manganese, from 0% to 9% of nickel, from 0% to 6% of chromium, from 0% to 3% of the sum of tungsten divided by two plus molybdenum, from 0% to 0.5% of vanadium, from 0% to 0.5% of niobium, from 0% to 0.5% of zirconium, at most 0.3% of nitrogen and, optionally, from 0.0005% to 0.005% of boron, optionally from 0.005% to 0.1% of titanium, optionally at least one element taken from Ca, Se, Te, Bi and Pb in contents less than 0.2%, the balance being iron and impurities resulting from smelting; the chemical composition furthermore satisfying the relationships: 1%.ltoreq.Si+Al.ltoreq.3% and 4.6.times.(%C)+1.05.times.(%Mn)+0.54.times.(%Ni)+0.66.times.(%Mo+%W/2)+0.5 .times.(%Cr)+K.gtoreq.3.8 where K=0.5 when the steel contains boron, K=0 when the steel does not contain boron.