Process for manufacturing cold-rolled and annealed steel sheet with a very high strength, and sheet thus produced
The present invention provides a cold-rolled and annealed steel sheet with a strength greater than 1200 MPa, the composition of which includes, the contents being expressed by weight: 0.10%≤C≤0.25%, 1%≤Mn≤3%, A≥0.010%, Si≤2.990%, S≤0.015%, P≤0.1%, N≤0.008%, it being understood that 1%≤Si+Al≤3%, it being understood that Cr+3Mo≥0.3%, Ti in an amount such that Ti/N≥4 and Ti≤0.040%. A balance of the composition includes iron and inevitable impurities resulting from the smelting. The microstructure of the steel includes 15 to 90% bainite, the remainder includes martensite and residual austenite.
1. A process for manufacturing a cold-rolled steel sheet with a strength greater than 1200 MPa and an elongation at break greater than 8%, in which:
providing a steel having a microstructure comprising 15 to 90% bainite, a remainder of the microstructure consisting of islands of martensite and residual austenite and a composition comprising, in weight percent:
0.10%≤C ≤0.25%;
1%≤Mn≤3%;
Al≥0.010%;
Si≤2.990%;
S≤0.015%;
P≤0.1%;
N≤0.008%;
Mo≤0.25%; and
Cr≤1.65%;
wherein 1%≤Si+Al≤3%, and Cr+3Mo≥0.3%,
a remainder of the composition consisting of iron and inevitable impurities resulting from smelting; then
casting a semifinished product from said steel; then
heating said semifinished product to a temperature greater than 1150° C.; then
hot-rolling said semifinished product so as to obtain a hot-rolled sheet; then
coiling said sheet; then
pickling said hot-rolled sheet; then
cold-rolling said sheet with a reduction ratio of between 30 and 80% to obtain a cold-rolled sheet; and then
reheating said cold-rolled sheet at a rate V C between 5 and 15° C./s up to a temperature T 1 between Ac3 and Ac3+20° C., and holding at said temperature T 1 for a time t 1 between 50 and 150 s, then
cooling said sheet at a rate V R1 greater than 25° C./s but below 100° C./s down to a temperature T 2 between B s and (M s -20° C.),
maintaining said sheet at said temperature T 2 for a time t 2 between 150 and 350 s, and then
cooling said sheet at a rate V R2 of less than 30° C./s down to an ambient temperature.
2. The manufacturing process of claim 1 , wherein said steel further comprises, in weight percent, 0.05%≤V≤0.15%.
3. The manufacturing process of claim 1 , wherein said steel further comprises, in weight percent, B≤0.005%.
4. The manufacturing process of claim 1 , wherein said steel further comprises Ti in an amount so that Ti/N≥4 and Ti≤0.040%.
5. The manufacturing process of claim 1 , wherein the temperature T 1 is between Ac3+10° C. and Ac3+20° C.
6. The manufacturing process of claim 1 , wherein the steel has a microstructure comprising 45 to 65% bainite.
7. The manufacturing process of claim 1 , wherein the steel has a microstructure comprising 15 to 45% bainite.