What is claimed is:
1. A galvannealed steel sheet made of a steel having a chemical composition including by weight:
0.34%≤C≤0.45%,
1.50%≤Mn≤2.30%,
1.50%≤Si≤2.40%,
0%<Cr≤0.7%,
0%≤Mo≤0.3%,
0.10%≤Al≤0.7%,
optionally 0%≤Nb≤0.05%, and
a remainder, the remainder including Fe and unavoidable impurities,
a structure of the steel consisting of between 50% and 70% of martensite, retained austenite, and bainite.
2. The galvannealed steel sheet according to claim 1 , wherein the composition of the steel includes Al≤0.30%.
3. The galvannealed steel sheet according to claim 1 , wherein the composition of the steel includes 0.15%≤Al.
4. The galvannealed steel sheet according to claim 1 , wherein the composition of the steel includes 0.03%≤Nb≤0.05%.
5. The galvannealed steel sheet according to claim 1 , wherein the retained austenite has a C content between 0.9% and 1.2%.
6. The galvannealed steel sheet according to claim 1 , wherein the galvannealed steel sheet has a tensile strength TS of at least 1450 MPa and a total elongation TE of at least 17% as measured according to ISO standard ISO 6892-1, published in October 2009.