Process for manufacturing iron-carbon-maganese austenitic steel sheet with excellent resistance to delayed cracking
A process for fabricating a steel sheet is provided. The process includes soaking a steel sheet. The steel has a composition including iron, carbon, manganese, silicon, aluminum, sulfur, phosphorus and nitrogen and at least one metallic element X chosen among vanadium, titanium, niobium, molybdenum, and chromium. A quantity X p of metallic element under 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%; or 0.070%≤Cr p ≤0.6%. The soaking step occurs under a pure nitrogen or argon atmosphere with a dew point lower than −30° C. at a soaking temperature θ between 250 and 900° C. and with a dynamic circulation of a regenerated atmosphere.
1. A process for fabricating a steel sheet, comprising:
soaking a steel sheet, comprising 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 chosen among 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%;
wherein a quantity X p of metallic element under 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 soaking temperature θ comprised between 250 and 900° C., with a dynamic circulation of a regenerated atmosphere and wherein the soaking temperature θ is below a recrystallization temperature.
2. The process of claim 1 , wherein the steel sheet comprises a Zn or Zn—Y alloy coating, wherein element Y is at least one element selected from the group consisting of consisting of Ni, Cr, and Mg and wherein the temperature and time of the soaking satisfy θ(° C.)Ln(t(s))≥2200.
3. The process of claim 2 , wherein θ (° C.)Ln(t(s))≥2450.
4. The process of claim 2 , wherein θ (° C.)Ln(t(s))≥2750.
5. The process of claim 1 , wherein the soaking is performed by continuous annealing.
6. The process of claim 1 , wherein the soaking is performed by batch annealing.
7. The process of claim 6 , wherein the soaking is performed by open coil annealing.
8. The process of claim 1 , wherein the soaking is performed by induction heating.
9. The process of claim 6 , wherein the soaking is performed with transversal electromagnetic field.
10. The process of claim 1 , further comprising the step of:
cold forming the steel sheet to obtain a part,
the soaking step being performed before or after the cold forming step.
11. The process of claim 1 , wherein the steel includes 0.70%<C≤1.05%.
12. The process of claim 1 , wherein the steel includes 0.35%≤C<0.50%.
13. The process of claim 1 , wherein the steel includes boron.
14. The process of claim 1 , 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 a quantity X p , by weight, such that:
1000
H
max
X
p
≤
3.3
.
15. The process of claim 1 , 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 a quantity X p , by weight, such that:
1000
H
max
X
p
≤
2.5
.
16. The process of claim 14 , wherein
2.5
≤
1000
H
max
X
p
≤
3.3
.
17. 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%.