Ultra high-strength air-hardening multiphase steel having excellent processing properties, and method for manufacturing a strip of said steel

A method for producing a cold-rolled or hot-rolled steel strip from a multi-phase, air-hardenable steel comprising the following elements in % by weight:

• C≥0.075 to ≤0.115
• Si≥0.400 to ≤0.500
• Mn≥1,900 to ≤2,350
• Cr≥0.200 to ≤0.500
• Al≥0.005 to ≤0.060
• N≥0.0020 to ≤0.0120
• S≤0.0030
• Nb≥0.005 to ≤0.060
• Ti≥0.005 to ≤0.060
• B≥0.0005 to ≤0.0030
• Mo≥0.200 to ≤0.300
• Ca≥0.0005 to ≤0.0060
• Cu≤0.050
• Ni≤0.050,
• remainder iron, including usual steel accompanying smelting related impurities, wherein for a widest possible process window during continuous annealing of hot rolled or cold rolled strips made from said steel a sum content of M+Si+Cr in said steel is a function of a thickness of the steel strips according to the following relationship:
• for strip thicknesses of up to 1.00 mm the sum content of M+Si+Cr is ≥2.800 and ≤3.000%,
• for strip thicknesses of over 1.00 to 2.00 mm the sum of Mn+Si+Cr is ≥2.850 and ≤3.100%, and
• for strip thicknesses of over 2.00 mm the sum of Mn+Si+Cr is ≥2.900 and ≤3.200%, said method comprising:
• continuously annealing the multi-phase, air-hardenable steel to produce a cold-rolled or hot-rolled steel strip;
• heating the cold-rolled or hot-rolled steel strip during the continuous annealing to a temperature in the range from about 700 to 950° C.;
• cooling the annealed steel strip from the annealing temperature to a first intermediate temperature of about 300 to 500° C. with a cooling rate of between about 15 and 100° C./s;
• and after the cooling to the intermediate temperature treating the steel strip as set forth under a) or b):
• a) cooling the steel strip to a second intermediate temperature of about 160 to 250° C. with a cooling rate of between 15 and 100° C./s and after cooling to the second intermediate temperature cooling the steel strip at air to room temperature;
• b) maintaining the cooling of the steel strip with a cooling rate of between about 15 and 100′C/s from the first intermediate temperature to room temperature.