Process for producing hot-rolled steel strip

A process for producing a hot-rolled steel strip with a tensile strength of between 760 and 940 MPa and a hole expansion ratio of at least 50%, the steel having a final microstructure comprising ferrite, bainite and at least 3% of martensite, wherein the total volume of a ferrite phase and a bainite phase is not smaller than 80%, optionally also tempered martensite, retained austenite and/or fine carbides having an average diameter of not larger than 30 nm, and wherein the final microstructure does not contain pearlite and/or coarse Fe3C and has a steel composition comprising, in weight %:

• 0.07 to 0.13% C,
• 0.75 to 1.30% Mn,
• 0.6 to 1.4% Cr,
• 0.1 to 0.49% Si,
• 0.03% P or less,
• up to 0.05% S,
• up to 0.001% B,
• 0.07 to 0.15% Ti,
• 0.003 to 0.6% Al,
• up to 0.01% N,
• optionally calcium in an amount consistent with a calcium treatment for MnS inclusion control or REM in an amount consistent with a treatment for MnS inclusion control, and
• remainder iron and inevitable impurities associated with the steelmaking process;
• the process comprising:
• providing a steel slab or thick strip of the steel composition, optionally calcium treated, by:
• continuous casting,
• thin slab casting,
• belt casting, or
• strip casting;
• optionally followed by reheating the steel slab or thick strip at a reheating temperature of at most 1300° C.;
• hot-rolling the steel slab or thick strip at a hot-rolling finishing temperature above Ar3 so the steel composition is still austenitic during a last hot-rolling pass;
• cooling the hot-rolled steel strip by means of continuous cooling or interrupted cooling at a cooling rate of at least 20° C./s to a coiling temperature between Ms and Bs which is between 440° C. and 600° C.;
• and
• coiling the hot-rolled steel strip at said coiling temperature.