Abstract: Steel for the manufacture of molds for injection molding of plastics, wherein its chemical composition contains, 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%, 0%.ltoreq.Ti+Zr/2.ltoreq.0.3%, 0%.ltoreq.S+Se+Te .ltoreq.0.2%, 0%.ltoreq.Pb+Bi.ltoreq.0.2%, 0%.ltoreq.Ca.ltoreq.0.1%, the remainder being iron and impurities resulting from the production, the analysis simultaneously satisfying the following relationships: 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)+k B.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).

Abstract: Steel for the manufacture of molds, and especially of molds for injection molding of plastics, whose chemical composition comprises, by weight: 0.17%.ltoreq.C.ltoreq.0.27%, 0%.ltoreq.Si.ltoreq.0.5%, 0%.ltoreq.Mn.ltoreq.2%, 0%.ltoreq.Ni.ltoreq.2%, 0%.ltoreq.Cr.ltoreq.3%, 0%.ltoreq.Mo+W/2.ltoreq.1.5%, 0%.ltoreq.V+Nb/2+Ta/4.ltoreq.0.5%, 0.002%.ltoreq.B.ltoreq.0.015%, 0.005%.ltoreq.Al.ltoreq.0.2%, optionally at least one element taken from sulfur, selenium and tellurium, the sum of the contents of these elements being smaller than or equal to 0.2% optionally at least one element taken from titanium and zirconium, in contents such that the sum of the titanium content and of half the zirconium content is smaller than or equal to 0.3%, optionally at least one element taken from lead and bismuth, the sum of the contents of these elements being smaller than or equal to 0.2%, optionally calcium in a content lower than or equal to 0.

Abstract: Steel useful for the manufacture of molds for the injection molding of plastics, the chemical composition of which includes, by weight: 0.03%.ltoreq.C.ltoreq.0.25%, 0%.ltoreq.Si.ltoreq.0.2%, 0%.ltoreq.Mn.ltoreq.0.9%, 1.5%.ltoreq.Ni.ltoreq.5%, 0%.ltoreq.Cr.ltoreq.18%, 0.05%.ltoreq.Mo+W/2.ltoreq.1%, 0%.ltoreq.S.ltoreq.0.3%, at least one element chosen from Al and Cu in contents of between 0.5% and 3%, optionally 0.0005%.ltoreq.B.ltoreq.0.015%, optionally at least one element taken from V, Nb, Zr, Ta and Ti, in contents of between 0% and 0.3%, optionally at least one element taken from Pb, Se, Te and Bi, in contents of between 0% and 0.3%, the remainder being iron and impurities resulting from the processing, especially nitrogen; the chemical composition additionally satisfying the relations: Kth=3.8.times.C+9.8.times.Si+3.3.times.Mn+2.4.times.Ni+.alpha..times.Cr+1. 4.times.(Mo+W/2).ltoreq.15, with .alpha.=1.4 if Cr<8% and .alpha.=0 if Cr.gtoreq.8%; Tr=3.8.times.C+1.07.times.Mn+0.7.times.Ni+0.57.times.Cr+1.

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.

Abstract: The agent in question is cobalt added in contents of between 0.05% and 2% to steels containing from 0.05% to 0.6% of carbon and less than 10% of alloying elements taken from silicon, manganese, nickel, chromium and molybdenum, to produce optionally normalized blocks, plates, bars or pieces of large size, with improved hydrogen cracking resistance and with improved weld-ability and suitability for thermal cutting. The invention also relates to a process for employing the agent and to the pieces thus obtained.