Low alloy steel for the manufacture of molds for plastics

A steel for manufacturing molds for injection molding of plastics or rubber, the steel comprising, by weight:

• ______________________________________ 0.35% .ltoreq. C .ltoreq. 0 5%, 0% .ltoreq. Si .ltoreq. 0.5%, 0.2% .ltoreq. Mn .ltoreq. 2.5%, 0% .ltoreq. Ni .ltoreq. 4%, 0% .ltoreq. Cr .ltoreq. 4%, 0% .ltoreq. Mo + W/2 .ltoreq. 2%, 0 .ltoreq. Cu .ltoreq. 1%, 0% .ltoreq. V + Nb/2 + Ta/4 .ltoreq. 0.5%, 0.005% .ltoreq. Al .ltoreq. 0.2%, 0% .ltoreq. B .ltoreq. 0.015%, ______________________________________
• the remainder of the steel being iron and impurities resulting from production of the steel,
• wherein
• ______________________________________ Cr + 3 .times. (Mo + W/2) + 10 .times. (V + Nb/2 + Ta/4) .gtoreq. 1;
• R = 3.8 .times. C + 10 .times. Si + 3.3 .times. Mn + 2.4 .times. Ni + 1.4 .times. (Cr + Mo + W/2) .ltoreq. 11;
• Qu = 3.8 .times. C + 1.1 .times. Mn + 0.7 .times. Ni + 0.6 .times. Cr + 1.6 .times. (Mo + W/2) + kB .gtoreq. 3, with kB = 0.5 if B .gtoreq. 0.0005% and kB = 0 if not;
• and R .ltoreq. Max (2.33 .times. Qu - 1, 0.9 .times. Qu + 4). ______________________________________