HOT ROLLED STEEL SHEET AND ASSOCIATED FABRICATION METHOD

摘要

The invention relates mainly to a hot-rolled steel sheet having: a yield strength greater than 690 MPa and less than or equal to 840 MPa, strength of between 780 MPa and 950 MPa, elongation at break greater than 10 % and a hole expansion ratio (Ac) greater than or equal to 50%. The chemical composition of the sheet of the invention comprises the following, expressed by weight, namely: 0.040 % = 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. Moreover, the microstructure thereof is formed by: granular bainite, ferrite, less than 1.5 % cementite by area percentage, and carbonitrides of titanium and niobium, providing a hole expansion ratio (Ac) greater than or equal to 50%. The ratio between the grain size DL measured parallel to the rolling direction and the grain size DN measured perpendicularly to the rolling direction is less than or equal to 1.4. The invention also relates to methods for producing one such sheet, in which the composition can optionally include chromium.