Method of making a multiphase hot-rolled steel strip

The present invention relates to a method for making a multiphase hot-rolled steel strip having improved mechanical properties, in particular high strength and good ductility. Currently, such strips have a thickness of between 0.7 mm and 10 mm and more often between 2 mm and 6 mm. The invention concerns a method for making a multiphase hot-rolled steel strip comprising an ultra-fast cooling operations, which consists in carrying out said ultra-fast cooling operation after controlled slow cooling of the strip on a conventional slow cooling table of the rolling mill. The controlled cooling constitutes a first slow cooling, at the output of the finishing mill, from an end-of-roll temperature to an intermediate temperature of about 750° C. to 500° C.; said first cooling determines the fraction of the first phase (ferrite) in the steel. The ultra-fast cooling (>150° C./s), which solidifies the resulting structure, lowers the temperature of the strip down to a coiling temperature, ranging between about 600° C. and room temperature, at which a second slow cooling is performed which results if the formation of the second phase (bainite or martensite).