Arcelormittal

Javier Gil Otin

Antoine Moulin

A process for manufacturing a cold-rolled steel sheet with a strength of at least 1200 MPa and an elongation at break greater than 10%. A steel is provided having a microstructure comprising 65 to 90% bainite. A semifinished product is cast from the steel and heated to a temperature greater than 1150&#xb0; C. The semifinished product is hot rolled to obtain a hot-rolled sheet; the coiled and pickled. Cold-rolling occurs with a reduction ratio of between 30 and 80% so as to obtain a cold-rolled sheet; and then reheating occurs at a rate Vbetween 5 and 15&#xb0; C./s up to a temperature Tbetween Ac3 and Ac3+20&#xb0; C. and held at said temperature Tfor a time tbetween 50 and 150 s. The sheet is cooled at a rate Vgreater than 40&#xb0; C./s but below 100&#xb0; C./s down to a temperature Tbetween (M&#x2212;30&#xb0; C. and M+30&#xb0; C.). The sheet is maintained at temperature Tfor a time tbetween 150 and 350 s, and then cooled at a rate Vof less than 30&#xb0; C./s down to an ambient temperature.

Process for manufacturing cold-rolled and annealed steel sheet with a very high strength, and sheet thus produced

The present invention provides a cold-rolled and annealed steel sheet with a strength greater than 1200 MPa, the composition of which includes, the contents being expressed by weight: 0.10%&#x2264;C&#x2264;0.25%, 1%&#x2264;Mn&#x2264;3%, A&#x2265;0.010%, Si&#x2264;2.990%, S&#x2264;0.015%, P&#x2264;0.1%, N&#x2264;0.008%, it being understood that 1%&#x2264;Si+Al&#x2264;3%, it being understood that Cr+3Mo&#x2265;0.3%, Ti in an amount such that Ti/N&#x2265;4 and Ti&#x2264;0.040%. A balance of the composition includes iron and inevitable impurities resulting from the smelting. The microstructure of the steel includes 15 to 90% bainite, the remainder includes martensite and residual austenite.

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Madrid, Spain

Javier Gil Otin