Hardened martensitic steel having a low cobalt content, process for manufacturing a part from steel, and part thus obtained

A method for producing a part from steel, the method comprising the following steps preceding the finishing of the part providing it with its definitive shape:

• preparing a steel part having a steel composition comprising, in weight percentages, the following:
• C=0.18 to 0.30%;
• Co=1.5 to 4%;
• Cr=2 to 5%;
• Al=1 to 2%;
• Mo+W/2=1 to 4%;
• V=traces to 0.3%;
• Nb=traces to 0.1%;
• B=traces to 30 ppm;
• Ni=11 to 16%;
• Si=traces to 1.0%;
• Mn=traces to 2.0%;
• Ca=traces to 20 ppm;
• Rare earths=traces to 100 ppm;
• if N≦10 ppm, Ti+Zr/2=traces to 100 ppm where Ti+Zr/2≦10×N;
• if 10 ppm<N≦20 ppm, Ti+Zr/2=traces to 150 ppm;
• O=traces to 50 ppm;
• N=traces to 20 ppm;
• S=traces to 20 ppm;
• Cu=traces to 1%;
• P=traces to 200 ppm;
• and
• the remainder being iron and inevitable impurities resulting from smelting, the composition of the steel simultaneously satisfying the relationship Ni≧7+(3.5×Al);
• shaping the steel part by at least one operation;
• softening tempering the steel part at 600 to 675° C. for 4 to 20 hours followed by cooling in air;
• treating the steel part with a solution heat treatment at 900 to 1,000° C. for at least 1 hour, followed by fast cooling the steel part in oil or air to avoid precipitation of intergranular carbides in an austenite matrix;
• and
• age hardening the steel part at 475 to 600° C., for 5 to 20 hours.