| 摘要 |
The invention relates mainly to a sheet of hot-rolled steel with an elastic limit of more than 680 MPa at least in the direction across the rolling direction, and no higher than 840 MPa, with strength of 780 MPa and 950 MPa, elongation at break higher than 10% and hole expansion ratio (Ac) no lower than 45%, in which the chemical composition consists of, the contents being expressed as weight percentages: 0.04% ≤ C ≤ 0.08%, 1.2% ≤ Mn ≤ 1.9%, 0.1 % ≤ Si ≤ 0.3%, 0.07% ≤ Ti ≤ 0.125%, 0.05% ≤ Mo ≤ 0.35%, 0.15% ≤ Cr ≤ 0.6% when 0.05% ≤ Mo ≤ 0.11% or 0.10% ≤ Cr ≤ 0.6% when 0.11 % ≤ Mo ≤ 0.35%, Nb ≤ 0.045%, 0.005% ≤ Al ≤ 0.1 %, 0.002% ≤ N ≤ 0.01 %, S ≤ 0.004%, P ≤ 0.020%, and optionally 0.001% ≤ V ≤ 0.2%, the remainder consisting of iron and inevitable impurities from the production process, in which the microstructure consists of granular bainite having a surface percentage higher than 70%, and ferrite having a surface percentage lower than 20%, the possible complement consisting of lower bainite, martensite and residual austenite, the sum of the contents of martensite and residual austenite being lower than 5%. The invention also relates to the method for manufacturing such a sheet. |
