Steel sheet having high tensile strength and ductility
A hot-rolled steel sheet having a tensile strength greater than 800 MPa and an elongation at break greater than 10% is provided. A composition of the steel includes, the contents being expressed by weight: 0.050%≤C≤0.090%, 1%≤Mn≤2%, 0.015%≤Al≤0.050%, 0.1%≤Si≤0.3%, 0.10%≤Mo≤0.40%, S≤0.010%, P≤0.025%, 0.003%≤N≤0.009%, 0.12%≤V≤0.22%, Ti≤0.005%, Nb≤0.020% and optionally, Cr≤0.45%. A balance of the composition includes iron and inevitable impurities resulting from the smelting. A microstructure of the sheet or part includes, as a surface fraction, at least 80% upper bainite, and a remainder includes lower bainite, martensite and residual austenite. A sum of the martensite and residual austenite, as a surface fraction, is less than 5%.
1. A hot-rolled steel sheet or part comprising:
a tensile strength greater than 800 MPa;
an elongation at break greater than 10%;
a composition of the steel comprising, the contents being expressed by weight:
0.050%≤C≤0.090%;
1.4%≤Mn≤1.8%;
0.015%≤Al≤0.050%;
0.1%≤Si≤0.3%;
0.10%≤Mo≤0.40%;
S≤0.010%;
P≤0.025%;
0.003%≤N≤0.009%;
0.12%<V≤0.22%;
Ti<0.005%;
Nb≤0.020%; and
a balance of the composition comprising iron and inevitable impurities resulting from the smelting; and
a microstructure of the sheet or part comprising:
at least 80% upper bainite, as a surface fraction;
a remainder consisting of lower bainite, martensite and residual austenite; and
a sum of the martensite and residual austenite, as a surface fraction, being less than 5%.
2. The steel sheet or part according to claim 1 , wherein the composition of the steel comprises, the content being expressed by weight:
0.050%≤C≤0.070%.
3. The steel sheet or part according to claim 1 , wherein the composition of the steel comprises, the content being expressed by weight:
0.070%≤C≤0.090%.
4. The steel sheet or part according to claim 1 , wherein the composition of the steel comprises, the content being expressed by weight:
0.020%≤Al≤0.040%.
5. The steel sheet or part according to claim 1 , wherein the composition of the steel comprises, the content being expressed by weight:
0.12%<V≤0.16%.
6. The steel sheet or part according to claim 1 , wherein the composition of the steel comprises, the content being expressed by weight:
0.18%≤Mo≤0.30%.
7. The steel sheet or part according to claim 1 , wherein the composition of the steel comprises, the content being expressed by weight:
Nb≤0.005%.
8. The steel sheet or part according to claim 1 , wherein the composition of the steel comprises, the content being expressed by weight:
Cr≤0.45%.
9. The steel sheet or part according to claim 1 , wherein the composition of the steel comprises, the content being expressed by weight:
0.20%≤Cr≤0.45%.
10. The steel sheet or part according to claim 1 , wherein the sheet or said part is coated with a zinc-based or aluminum-based coating.
11. The steel part with a composition and a microstructure according to claim 1 , manufactured by a process comprising the steps of:
heating at a temperature T of between 400 and 690° C.;
warm-drawing a temperature range of between 350° C. and (T-20° C.); and then cooling down to ambient temperature.
12. A welded assembly comprising:
at least one steel sheet or part according to claim 1 welded by a high-energy-density beam.
13. A hot-rolled steel sheet or part comprising:
a tensile strength greater than 800 MPa;
an elongation at break greater than 10%;
a composition of the steel comprising, the contents being expressed by weight:
0.050%≤C≤0.090%;
1%≤Mn≤2%;
0.015%≤Al≤0.050%;
0.1%≤Si≤0.3%;
0.10%≤Mo≤0.40%;
S≤0.010%;
P≤0.025%;
0.003%≤N≤0.009%;
0.12%<V≤0.22%;
Ti<0.005%;
Nb≤0.020%;
Cr≤0.45%; and
a balance of the composition consisting of iron and inevitable impurities resulting from the smelting; and
a microstructure of the sheet or part comprising:
at least 80% upper bainite, as a surface fraction;
a remainder consisting of lower bainite, martensite and residual austenite; and
a sum of the martensite and residual austenite, as a surface fraction, being less than 5%.
14. The steel sheet or part according to claim 13 , wherein the composition of the steel comprises, the content being expressed by weight: 1.4%≤Mn≤1.8%.