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

Abstract

The invention relates to an ultrahigh strength multiphase steel having a minimum tensile strength of 980 MPa 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; and residual iron, including customary steel-accompanying smelting-related impurities, wherein the total content of Mn+Si+Cr is ≥1.750 to ≤2.250 wt. % with a view to a processing window which is as wide as possible during the annealing process, in particular during the continuous annealing process, of cold strips of said steel.

1 .- 41 . (canceled) 
     
     
         42 . 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. 
 
     
     
         43 . The method of  claim 42 , wherein the cold strip is continuously annealed. 
     
     
         44 . The method of  claim 43 , wherein, proceeding from a selected hot strip having a specific thickness or selected hot strips having different thicknesses, cold strips with degrees of thinning by cold-rolling of 10% to 70% are produced with the end thickness to be achieved. 
     
     
         45 . The method of  claim 43 , further comprising, during the continuous annealing, increasing an oxidation potential during annealing with an installation configuration comprised of a direct fired furnace region (NOF) and a radiant tube furnace (RTF) by a CO content in the NOF of less than 4 vol. %; and setting in the RTF an oxygen partial pressure of a furnace atmosphere, which is reducing for iron, in accordance with a following equation,
   −18>Log pO 2 ≥-5*Si −0.3 -2.2*Mn −0.45 -0.1*Cr −0.4 -12.5*(−ln B) 0.25  
   
       wherein 
       Si, Mn, Cr and B designate corresponding alloy proportions in steel in wt. %, 
       pO 2  designates the oxygen partial pressure in mbar, and 
       wherein in order to avoid oxidation of the steel strip directly prior to dipping in the melting bath a dew point of a gas atmosphere is set at −30° C. or below. 
     
     
         46 . The method of  claim 43 , further comprising, during the continuous annealing, increasing an oxidation potential during annealing with an installation configuration comprised of only a radiant tube furnace (RFT) by setting in the RTF an oxygen partial pressure of a furnace atmosphere, which is reducing for iron, in accordance with a following equation,
   −12>Log pO 2 ≥-5*Si −0.25 -3*Mn −0.5 -0.1*Cr −0.5 -7*(−ln B) 0.5  
   
       wherein Si, Mn, Cr and B designate corresponding alloy proportions in steel in wt. %, 
       pO 2  designates the oxygen partial pressure in mbar, and 
       wherein in order to avoid oxidation of the steel strip directly prior to dipping in the melting bath the dew point of the gas atmosphere is set at −30° C. or below. 
     
     
         47 . The method of  claim 42 , further comprising temper-rolling the steel strip after undergoing annealing or the hot-dip finishing procedure. 
     
     
         48 . The method of  claim 42 , further comprising stretch-bending-straightening the steel strip after undergoing annealing or the hot-dip finishing procedure. 
     
     
         49 . The method of  claim 42 , further comprising:
 cutting a blank from the steel strip;   heating the blank to a temperature above Ac3;   deforming the heated blank into a component; and   hardening the component in a tool or in air.   
     
     
         50 . A steel strip produced by a method as set forth in  claim 42 , said steel strip comprising a minimum hole expansion value according to ISO 16630 of at least 20%. 
     
     
         51 . The steel strip of  claim 50 , wherein the minimum hole expansion value according to ISO 16630 is 25%. 
     
     
         52 . The steel strip of  claim 50 , comprising a minimum bending angle according to VDA 238-100 of 70° in a longitudinal direction or transverse direction. 
     
     
         53 . The steel strip of  claim 50 , comprising a minimum bending angle according to VDA 238-100 of 85° in a longitudinal direction or transverse direction. 
     
     
         54 . The steel strip of  claim 50 , comprising a minimum product value Rm×α of 100000 MPa, wherein 
       Rm is a tensile strength, and 
       α is a bending angle according to VDA 238-100. 
     
     
         55 . The steel strip of  claim 50 , comprising a minimum product value Rm×α of 120000 MPa, wherein 
       Rm is a tensile strength, and 
       α is a bending angle according to VDA 238-100. 
     
     
         56 . The steel strip of  claim 50 , comprising a delayed fracture free state for at least 6 months thus meeting the requirements of SEP 1970 for hole pull and hoop test pieces. 
     
     
         57 . 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%.