The invention concerns mold or mold frame parts for plastic material injection molding made of quenched and tempered martensitic stainless steel consisting, in wt. %, of: 0.02%=C=0.09%, 0.025%=N=0.12% with 0.05%=C+N=0.17%; Si=0.34% Al=0.080% with Si+23 Al=0.20% and Si+0.6 Al=0.25%; and 0.00025=Al×N=0.0020 0; 0.55%=Mn=1.8%; 11.5%=Cr=16% optionally up to 0.48% of copper, up to 0.90% of the sum Mo+W/2, up to 0.90% of nickel, up to 0.090% of vanadium, up to 0.090% of niobium, up to 0.025% of titanium, optionally up to 0.25 of sulphur, the rest being iron and impurities derived from preparation, the composition further satisfying the condition: 6.5=F═(Cr+Mo)+2(Si+V+Nb)−27(C+N)−(Ni+Mn/2+Cu/3)=13.