Martensitic stainless steel composition, method for making a mechanical part from said steel and resulting part

1. Martensitic stainless steel, characterised in that its composition is, in percentages by weight:

9%≦Cr≦13%

1.5%≦Mo≦3%

8%≦Ni≦14%

1%≦Al≦2%

0.5%≦Ti≦1.5% with Al+Ti≧2.25%

traces≦Co≦2%

traces≦W≦1% with Mo+(W/2)≦3%

traces≦P≦0.02%

traces≦S≦0.0050%

traces≦N≦0.0060%

traces≦C≦0.025%

traces≦Cu≦0.5%

traces≦Mn≦3%

traces≦Si≦0.25%

traces≦O≦0.0050%

and is such that:

M S (° C.)=1302−42Cr−63Ni−30Mo+20Al−15W−33Mn−28Si−30Cu−13Co+10Ti≧50

Cr eq/Ni eq≦1.05

with Cr eq (%)=Cr+2Si+Mo+1.5Ti+5.5Al+0.6W

Ni eq (%)=2Ni+0.5Mn+30C+25N+Co+0.3Cu.

2. Steel according to claim 1 , characterised in that 10%≦Cr≦11.75%.

3. Steel according to claim 1 , characterised in that 2%≦Mo≦3%.

4. Steel according to claim 1 , characterised in that 10.5%≦Ni≦12.5%.

5. Steel according to claim 1 , characterised in that 1.2%≦Al≦1.6%.

6. Steel according to claim 1 , characterised in that 0.75%≦Ti≦1.25%.

7. Steel according to claim 1 , characterised in that traces≦Co≦0.5%.

8. Steel according to claim 1 , characterised in that traces≦P≦0.01%.

9. Steel according to claim 1 , characterised in that traces≦S≦0.0010%.

10. Steel according to claim 1 , characterised in that traces≦S≦0.0005%.

11. Steel according to claim 1 , characterised in that traces≦N≦0.0030%.

12. Steel according to claim 1 , characterised in that traces≦C≦0.0120%.

13. Steel according to claim 1 , characterised in that traces≦Cu≦0.25%.

14. Steel according to claim 1 , characterised in that traces≦Si≦0.25%.

15. Steel according to claim 1 , characterised in that traces≦Si≦0.10%.

16. Steel according to claim 1 , characterised in that traces≦Mn≦0.25%.

17. Steel according to claim 16 , characterised in that traces≦Mn≦0.10%.

18. Steel according to claim 1 , characterised in that traces≦O≦0.0020%.

19. Method of making a mechanical part from steel with high mechanical strength

and resistance to corrosion, characterised in that:

producing a semi-finished product by preparing then transforming a hot ingot having a composition according to claim 1 ;

carrying out a heat treatment of melting said semi-finished product at a temperature between 850 and 950° C., immediately followed by a cryogenic treatment of rapid cooling to a temperature below or equal to −75° C. without interruption below the point of transformation Ms and during a sufficient time in order to ensure complete cooling throughout the thickness of the part;

aging annealing is carried out between 450 and 600° C. for an isothermal maintenance period of 4 to 32 hours.

20. Method according to claim 19 , characterised in that the said cryogenic treatment is quenching with dry ice.

21. Method according to claim 19 , characterised in that the said cryogenic treatment is carried out at a temperature of −80° C. for at least 4 hours.

22. Method according to claim 19 , characterised in that, between the said melting treatment and the said cryogenic treatment, isothermal quenching is carried out at a temperature higher than the point of transformation Ms.

23. Method according to claim 19 , characterised in that, after the cryogenic treatment and before the ageing annealing, cold shaping and a thermal melting treatment are carried out.

24. Method according to claim 20 , characterised in that at least one thermal homogenisation treatment between 1200 and 1300° C. is carried out for at least 24 hours on the ingot or during the transformation of the ingot into the semi-finished product, but before the last of the hot transformations.

25. Mechanical part made from steel with high resistance to corrosion and high mechanical strength, characterised in that it has been obtained by the method according to claim 19 .

26. Mechanical part according to claim 25 , characterised in that it is a casing of an aircraft landing gear.