Process for manufacturing iron-carbon-manganese austenitic steel sheet with excellent resistance to delayed cracking, and sheet thus produced

1. A process of fabricating a steel sheet, comprising:

(A) casting a steel with composition, comprising Fe, hydrogen, and, by weight:

0.35%≦C≦1.05%;

15%≦Mn≦26%;

Si≦3%;

Al≦0.050%;

S≦0.030%;

P≦0.080%;

N≦0.1%;

at least one metallic element X selected from the group consisting of vanadium, titanium, niobium, molybdenum, and chromium, in a percentage as follows

0.050%≦V≦0.50%,

0.040%≦Ti≦0.50%,

0.070%≦Nb≦0.50%,

0.14%≦Mo≦2%; and

0.070%≦Cr≦2%;

to obtain a semi-product and heat-treating the semi-product at a temperature between 900 and 1000° C. for a time between 5 and 20 days;

(B) additional reheating the semi-product to a temperature between 1100° C. and 1300° C.;

(C) hot rolling the semi-product up to an-end-of-rolling temperature to obtain a sheet;

(D) directly after the hot rolling (C), coiling the sheet; and

(F) after the coiling (D), performing at least one soaking treatment where the sheet is soaked at a temperature Θ comprised between 250 and 900° C. during a time t of at least 15 s so as to obtain a hydrogen content H max after soaking, H max designating a maximal hydrogen content that can be measured from a series of at least five specimens, and a quantity X p , in weight, satisfies:

1000

⁢

H

max

X

p

≤

3.3

wherein a reheating temperature, the end-of-rolling temperature, a coiling temperature, and an annealing temperature are chosen to obtain the following quantity X p of metallic element in the form of carbides, nitrides, or carbonitrides:

0.030%≦V p ≦0.40%

0.030%≦Ti p ≦0.50%

0.040%≦Nb p ≦0.40%

0.14%≦Mo p ≦0.44%

0.070%≦Cr p ≦0.6%.

2. The process of claim 1 , wherein the temperature Θ and time t are chosen such that

1000

⁢

H

max

X

p

≤

2.5

.

3. The process of claim 1 , further comprising:

soaking the steel sheet; and

wherein a quantity X p of the at least one metallic element X in the form of carbides, nitrides, or carbonitrides, is, by weight:

0.030%≦V p ≦0.40%;

0.030%≦Ti p ≦0.50%;

0.040%≦Nb p ≦0.40%;

0.14%≦Mo p ≦0.44%;

0.070%≦Cr p ≦0.6%,

under a pure nitrogen or argon atmosphere with a dew point lower than −30° C. at a temperature Θ between 250 and 900° C., with a dynamic circulation of a regenerated atmosphere.

4. The process of claim 3 , wherein the steel sheet comprises a Zn or Zn—Y alloy coating,

wherein element Y is at least one selected from the group consisting of Ni, Cr, and Mg, but not Fe or Mn,

wherein the temperature and time of the soaking satisfy Θ(° C.)Ln(t(s))≧2200.

5. A process of fabricating a hot rolled coated steel sheet, comprising:

(A) casting a steel with a composition comprising Fe, and, by weight:

0.35%≦C≦1.05%;

15%≦Mn≦26%;

Si≦3%;

Al≦0.050%;

S≦0.030%;

P≦0.080%;

N≦0.1%;

at least one metallic element X selected from the group consisting of vanadium, titanium, niobium, molybdenum, and chromium, in a percentage as follows

0.050%≦V≦0.50%,

0.040%≦Ti≦0.50%,

0.070%≦Nb≦0.50%,

0.14%≦Mo≦2%

0.070%≦Cr≦2%; and

to obtain a semi product and heat-treating the semi-product at a temperature between 900 and 1000° C. for a time between 5 and 20 days;

(B) additionally heating the semi product to a temperature between 1100 and 1300° C.;

(C) hot rolling the semi-product with an end-of-rolling temperature of 890° C. or higher to obtain a sheet;

(D) directly after the hot rolling (C), coiling the sheet at a temperature below 580° C.;

(G) coating the sheet with a Zn or Zn—Y alloy coating to obtain a coated sheet;

(H) performing at least one soaking treatment on the coated sheet under a pure nitrogen or argon atmosphere with a dew point lower than −30° C., at a temperature Θ between 250 and 900° C. during a time t, said temperature and time satisfying: Θ(° C.)Ln(t(s))≧2200.

6. The process of claim 5 , further comprising, between the coiling (D) and the coating (G):

(E) performing at least one cold rolling on the sheet; and

(F) performing at least one annealing treatment on the sheet, wherein the annealing treatment comprises a heating rate V h of between 2 and 10° C./s, at a temperature T s of between 700 and 870° C. for a time between 30 and 180 s and a cooling rate of between 10 and 50° C./s.

7. The process of claim 4 or 5 , wherein Θ(° C.)Ln(t(s))≧2450.

8. The process of claim 4 or 5 , wherein Θ(° C.)Ln(t(s))≧2750.

9. The process of claim 3 or 5 , wherein soaking temperature Θ is below recrystallization temperature.

10. The process of claim 1 , 3 , or 5 , wherein the soaking is performed by continuous annealing.

11. The process of claim 1 , 3 , or 5 , wherein the soaking is performed by batch annealing.

12. The process of claim 11 , wherein the soaking is performed by open coil annealing.

13. The process of claim 1 , 3 , or 5 , wherein the soaking is performed by induction heating.

14. The process of claim 13 , wherein the soaking is performed with a transversal electromagnetic field.

15. The process of claim 1 , 3 , or 5 , wherein the steel sheet is cold formed to obtain a part, and the soaking is performed before or after cold forming of the part.

16. The process of claim 1 , 3 , or 5 , wherein the steel comprises:

0.70%≦C≦1.05%.

17. The process of claim 1 , 3 , or 5 , wherein the steel comprises:

0.35%≦C≦0.50%.

18. The process of claim 1 , further comprising:

(E) after the coiling, cold rolling and annealing the sheet, wherein the soaking treatment (F) is performed after the cold rolling and annealing (E).

19. The process of claim 1 , wherein the steel comprises boron.

20. The process of claim 3 , wherein the steel comprises boron.

21. The process of claim 5 , wherein the steel comprises boron.

22. The process of claim 3 , wherein the steel comprises hydrogen and the sheet has a hydrogen content, H max , designating the maximal hydrogen content that can be measured from a series of at least five specimens, and the quantity, X p , by weight, such that:

1000

⁢

H

max

X

p

<

3.3

.

23. The process of claim 3 , wherein the steel comprises hydrogen and the sheet has a hydrogen content, H max , designating the maximal hydrogen content that can be measured from a series of at least five specimens, and the quantity, X p , by weight, such that

1000

⁢

H

max

X

p

≤

2.5

.

24. The process of claim 5 , wherein the soaking treatment (H) is performed after the coating (G).

25. The process of claim 1 , wherein

1000

⁢

H

max

X

p

<

3.3

.

26. The process of claim 5 , wherein the steel comprises hydrogen and the sheet has a hydrogen content, H max , designating the maximal hydrogen content that can be measured from a series of at least five specimens, and the quantity, X p , by weight, such that:

1000

⁢

H

max

X

p

<

3.3

.

27. The process of claim 5 , wherein the steel comprises hydrogen and the sheet has a hydrogen content, H max , designating the maximal hydrogen content that can be measured from a series of at least five specimens, and the quantity, X p , by weight, such that

1000

⁢

H

max

X

p

≤

2.5

.

28. The process of claim 22 , wherein

2.5

≤

1000

⁢

H

max

X

p

≤

3.3

.

29. The process of claim 26 , wherein

2.5

≤

1000

⁢

H

max

X

p

≤

3.3

.

30. The process of claim 1 , further comprising, at least one element selected from the group consisting of boron, nickel, and copper, in a percentage as follows

0.0005%≦B≦0.010%,

Ni≦2%, and

Cu≦5%.

31. The process of claim 3 , further comprising, at least one element selected from the group consisting of boron, nickel, and copper, in a percentage as follows

0.0005%≦B≦0.010%,

Ni≦2%, and

Cu≦5%.

32. The process of claim 5 , further comprising, at least one element selected from the group consisting of boron, nickel, and copper, in a percentage as follows

0.0005%≦B≦0.010%,

Ni≦2%, and

Cu≦5%.