Abstract: Low alloy steel for the manufacture of molds for plastics or for rubber, the chemical composition of which comprises, by weight, 0.24% to 0.35% of carbon, 1% to 2.5% of manganese, 0.3% to 2.5% of chromium, 0.1% to 0.8% of molybdenum plus tungsten divided by 2, up to 2.5% of nickel, 0% to 0.3% of vanadium, less than 0.5% of silicon, 0.002% to 0.005% of boron, 0.005% to 0.1% of aluminum, 0% to 0.1% of titanium and less than 0.02% of phosphorus. The chemical composition must furthermore satisfy the relationship:U=409(% C)+19.3[% Cr+% Mo+% W/2+% V]+29.4(% Si)+10(% Mn)+7.2(% Ni)<200and the relationship:R=3.82(% C)+9.79(% Si)+3.34(% Mn)+11.94(% P)+2.39(% Ni)+1.43(% Cr)+1.43(% Mo+% W/2)<11.