Process for manufacturing cold-rolled and annealed steel sheet with a very high strength, and sheet thus produced
A process for manufacturing a cold-rolled steel sheet with a strength of at least 1200 MPa and an elongation at break greater than 10%. A steel is provided having a microstructure comprising 65 to 90% bainite. A semifinished product is cast from the steel and heated to a temperature greater than 1150° C. The semifinished product is hot rolled to obtain a hot-rolled sheet; the coiled and pickled. Cold-rolling occurs with a reduction ratio of between 30 and 80% so as to obtain a cold-rolled sheet; and then reheating occurs at a rate V c between 5 and 15° C./s up to a temperature T 1 between Ac3 and Ac3+20° C. and held at said temperature T 1 for a time t 1 between 50 and 150 s. The sheet is cooled at a rate V R1 greater than 40° C./s but below 100° C./s down to a temperature T 2 between (M s −30° C. and M s +30° C.). The sheet is maintained at temperature T 2 for a time t 2 between 150 and 350 s, and then cooled at a rate V R2 of less than 30° C./s down to an ambient temperature.
1. A process for manufacturing a cold-rolled steel sheet with a strength of at least 1200 Mpa and an elongation at break greater than 10%, the process comprising:
providing a steel having 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<0.005%;
Cr<0.005%; and
B=0%,
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 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% so as 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 40° C./s but below 100° C./s down to a temperature T 2 between (M s −30° C. and M s +30° 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;
the steel having a microstructure comprising 65 to 90% bainite, a remainder of the microstructure consisting of islands of martensite and residual austenite.
2. The manufacturing process of claim 1 , wherein the temperature T 1 is between Ac3+10° C. and Ac3+20° C.
3. The manufacturing process of claim 1 , wherein Si is 1.5%.
4. The manufacturing process of claim 3 , wherein Al is 0.04%.
5. The manufacturing process of claim 1 , wherein the temperature T 2 is less than M s .
6. The manufacturing process of claim 1 , wherein the islands of martensite are tempered during the cooling said sheet at the rate V R2 to form tempered martensite.
7. The manufacturing process of claim 1 , wherein C is 0.19%.
8. The manufacturing process of claim 1 , wherein Mn is 2.0%.
9. The manufacturing process of claim 1 , wherein C is 0.19%, Mn is 2.0%, Si is 1.5% and Al is 0.04%.
10. The manufacturing process of claim 1 , wherein an average size of said islands of martensite and residual austenite is less than 1 micron.
11. The manufacturing process of claim 10 , wherein an average distance between said islands of martensite and residual austenite is less than 6 microns.
12. The manufacturing process of claim 1 , wherein an average distance between said islands of martensite and residual austenite is less than 6 microns.