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

1. 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.

2. The method according to claim 1 wherein the age hardening is performed at 490 to 525° C.

3. The method for manufacturing a steel part according to claim 1 further comprising treating the steel part with a cryogenic treatment at −50° C. or lower, transforming all the austenite into martensite, wherein the temperature is less than a measured Ms by 150° C. or more, wherein the cryogenic treatment lasts between 4 hours and 50 hours.

4. The method for manufacturing a steel part according to claim 3 wherein the cryogenic treatment is performed between −80° C. and −110° C.

5. The method for manufacturing the steel part according to claim 3 wherein the cryogenic treatment is between 4 hours and 10 hours.

6. The method for manufacturing a steel part according to claim 1 further comprising a treatment for softening a crude quenched martensite carried out at 150 to 250° C. for 4 to 16 hours, followed by quiet air cooling.

7. The method for manufacturing a steel part according to claim 1 further comprising the steel part undergoing carburization or nitridation or carbonitridation.

8. The method for manufacturing a steel part according to claim 7 wherein the nitridation or carburization or carbonitridation is carried out during an ageing cycle.

9. The method for manufacturing a steel part according to claim 8 wherein the nitridation is carried out between 475 to 600° C.

10. The method for manufacturing a steel part according to claim 7 wherein the nitridation or carburization or carbonitridation is carried out during a thermal cycle before or simultaneously with the solution heat treatment.

11. A mechanical part or part for a structural member, wherein the part is manufactured by the method according to claim 1 .

12. The mechanical part according to claim 11 wherein the mechanical part is one of an engine transmission shaft, an engine suspension device, a landing gear member, a gearbox member or a bearing axle.

13. The method for manufacturing a steel part according to claim 1 wherein the steel composition contains between 2 to 3% of Co.

14. The method for manufacturing a steel part according to claim 1 wherein the steel composition contains between 0.20 to 0.25% of C.

15. The method for manufacturing a steel part according to claim 1 wherein the steel composition contains between 2 to 4% of Cr.

16. The method for manufacturing a steel part according to claim 1 wherein the steel composition contains between 1 to 1.6% of Al.

17. The method for manufacturing a steel part according to claim 1 wherein the steel composition contains between 1.4 to 1.6% of Al.

18. The method for manufacturing a steel part according to claim 1 wherein the steel composition contains between 1 to 4% of Mo.

19. The method for manufacturing a steel part according to claim 1 wherein the steel composition contains Mo+W/2 being between 1 to 2%.

20. The method for manufacturing a steel part according to claim 1 wherein the steel composition contains between 0.2 to 0.3% of V.

21. The method for manufacturing a steel part according to claim 1 wherein the steel contains Ni=12 to 14%.

22. The method for manufacturing a steel part according to claim 1 wherein the steel composition contains between traces to 0.05% of Nb.

23. The method for manufacturing a steel part according to claim 1 wherein the steel composition contains between traces to 0.25% of Si.

24. The method for manufacturing a steel part according to claim 23 wherein the steel composition contains between traces to 0.10% of Si.

25. The method for manufacturing a steel part according to claim 1 wherein the steel composition contains traces to 10 ppm of O.

26. The method for manufacturing a steel part according to claim 1 wherein the steel composition contains between traces to 10 ppm of N.

27. The method for manufacturing a steel part according to claim 1 wherein the steel composition contains between traces to 10 ppm of S.

28. The method for manufacturing a steel part according to claim 27 wherein the steel composition contains between traces to 5 ppm of S.

29. The method for manufacturing a steel part according to claim 1 wherein the steel composition contains between traces to 100 ppm of P.

30. The method for manufacturing a steel part according to claim 1 wherein the steel composition has a measured martensitic transformation temperature Ms greater than or equal to 100° C.

31. The method for manufacturing a steel part according to claim 30 wherein the measured martensitic transformation temperature Ms is greater than or equal to 140° C.