Ultrahigh strength multiphase steel and method for producing a steel strip from said multiphase steel

A method, comprising:

• producing a pre-strip from a multi-phase steel in a state of a slab, with the multi-phase steel having a minimum tensile strength of 980 MPa in a non-quenched state containing (in wt. %)
• C≥0.075 to ≤0.115
• Si≥0.400 to ≤0.500
• Mn≥1.900 to ≤2.350
• Cr≥0.250 to ≤0.400
• Al≥0.010 to ≤0.060
• N≥0.0020 to ≤0.0120
• P≤0.020
• S≤0.0020
• Ti≥0.005 to ≤0.060
• Nb≥0.005 to ≤0.060
• V≥0.005 to ≤0.020
• B≥0.0005 to ≤0.0010
• Mo≥0.200 to ≤0.300
• Ca≥0.0010 to ≤0.0060
• Cu≤0.050
• Ni≤0.050
• Sn≤0.040
• H≤0.0010,
• with the remainder being iron, including typical steel-associated, smelting-related impurities, wherein the total content of Mn-Si+Cr is ≥1.750 wt. % to ≤2.250 wt. % with regard to a processing window which is as wide as possible during annealing of cold strips of this steel;
• hot-rolling the pre-strip into a steel strip with a hot strip thickness to be achieved;
• proceeding from a previously fixed slab thickness and a previously selected pre-strip having a defined but variable thickness, hot-rolling hot strips with a same thickness with a degree of thinning by rolling of 72% to 87% with end thickness to be achieved;
• cold-rolling the hot strip into a cold strip with an end thickness to be achieved,
• heating the steel strip cold-rolled to the end thickness during the continuous annealing to an annealing temperature in a range of approximately 700 to 950° C. to produce a required multi-phase microstructure;
• cooling the annealed steel strip from the annealing temperature at a cooling rate between approximately 15 and 100° C./s to a first intermediate temperature of approximately 300 to 500° C. followed by a cooling rate between approximately 15 and 100° C./s to a second intermediate temperature of approximately 160 to 250° C., and cooling the steel strip in air at a cooling rate of approximately 2 to 30° C./s until room temperature is reached or at a cooling rate between approximately 15 and 100° C./s from the first intermediate temperature to room temperature, or
• cooling the annealed steel strip to a temperature of approximately 400 to 470° C. such that cooling is stopped prior to entry of the steel strip into a melting bath, then the steel strip undergoes a hot-dip finishing procedure, and after undergoing the hot-dip finishing procedure, continuing cooling at a cooling rate between approximately 15 and 100° C./s to an intermediate temperature of approximately 200 to 250° C., and cooling the steel strip in air at a cooling rate of approximately 2 to 30° C./s until room temperature is reached, or
• cooling the annealed steel strip to an intermediate temperature of approximately 200 to 250° C. such that prior to entry of the steel strip into a melting bath, maintaining the steel strip at the intermediate temperature for approximately 1 to 20 s, then the steel strip is heated to a temperature of approximately 400 to 470° C., undergoes a hot-dip finishing procedure, and after undergoing the hot-dip finishing procedure, is cooled again at a cooling rate between approximately 15 and 100° C./s to an intermediate temperature of approximately 200 to 250° C., and subsequently cooled in air at a cooling rate of approximately 2 to 30° C./s to room temperature.

A steel strip, comprising in wt. %:

• C≥0.075 to ≤0.115
• Si≥0.400 to ≤0.500
• Mn≥1.900 to ≤2.350
• Cr≥0.250 to ≤0.400
• Al≥0.010 to ≤0.060
• N≥0.0020 to ≤0.0120
• P≤0.020
• S≤0.0020
• Ti≥0.005 to ≤0.060
• Nb≥0.005 to ≤0.060
• V≥0.005 to ≤0.020
• B≥0.0005 to ≤0.0010
• Mo≥0.200 to ≤0.300
• Ca≥0.0010 to ≤0.0060
• Cu≤0.050
• Ni≤0.050
• Sn≤0.040
• H≤0.0010,
• with the remainder being iron, including typical steel-associated, smelting-related impurities, wherein a total content of Mn-Si+Cr is ≥1.750 wt. % to ≤2.250 wt. %,
• said steel strip comprising a minimum hole expansion value according to ISO 16630 of at least 20%.