| 摘要 |
The invention relates principally to a hot rolled steel sheet with a yield stress greater than 690 MPa and less than or equal to 840 MPa, with strength between 780 MPa and 950 MPa, elongation at failure greater than 10% and a hole expansion ratio (Ac) greater than or equal to 50%. The chemical composition of the sheet claimed by the invention contains, whereby the contents are expressed in percent by weight: 0.40%≦̸C≦̸0.065%, 1.4%≦̸Mn≦̸1.9%, 0.1%≦̸Si≦̸0.55%, 0.095%≦̸Ti≦̸0.145%, 0.025%≦̸Nb≦̸0.045%, 0.005%≦̸Al≦̸0.1%, 0.002%≦̸N≦̸0.007%, S≦̸0.004%, P<0.020. The microstructure of the sheet claimed by the invention also consists of granular bainite, ferrite, cementite in an area percentage of less than 1.5% and titanium and niobium carbonitrides, which results in a hole expansion ratio (Ac) greater than or equal to 50%, and the ratio between the grain size DL measured parallel to the rolling direction and the grain size DN measured perpendicular to the rolling direction is less than or equal to 1.4. The invention also relates to methods for the fabrication of the sheet described above, in particular depending on whether the composition includes chromium or not. |
