Ultra high strength multi-phase steel and method for producing a cold-rolled steel strip therefrom

Abstract

The invention relates to an ultrahigh strength multiphase steel with a dual-phase microstructure or a complex-phase microstructure and with a low content of residual austenite in particular for lightweight vehicle construction, to a method for producing cold-rolled steel strips from such a steel, and to steel strips produced using said method. The aim of the invention is a novel alloy concept with which the processing window for the continuous annealing of cold strips can be extended. According to the invention, this is achieved by a multiphase steel with a minimum tensile strength of 980 MPa containing (contents 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≥0.005 to ≤0.060, N≥0.0020 to ≤0.0120, S≤0.0020, Nb≥0.005 to ≤0.060, Ti≥0.005 to ≤0.060, B>0.0005 to ≤0.0010, Mo>0.200 to ≤0.300, Ca>0.0010 to ≤0.0060, Cu≤0.050, Ni≤0.050, and residual iron, including customary steel-accompanying smelting-related impurities, wherein the total content of Mn+Si+Cr is ≥2.500 to ≤3.250 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 .- 37 . (canceled) 
     
     
         38 . An ultra high strength multi-phase steel having a minimum tensile strength of 980 MPa, said multi-phase steel 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 
                   ≥0.005 to ≤0.060 
                 
                     
                   N 
                   ≥0.0020 to ≤0.0120 
                 
                     
                   S 
                    ≤0.0020 
                 
                     
                   Nb 
                   ≥0.005 to ≤0.060 
                 
                     
                   Ti 
                   ≥0.005 to ≤0.060 
                 
                     
                   B 
                    >0.0005 to ≤0.0010 
                 
                     
                   Mo 
                    >0.200 to ≤0.300 
                 
                     
                   Ca 
                    >0.0010 to ≤0.0060 
                 
                     
                   Cu 
                   ≤0.050 
                 
                     
                   Ni 
                   ≤0.050 
                 
                     
                     
                 
             
                
               
               
                
                
                
                
                
                
                
                
                
                
                
                
                
                
                
               
            
           
         
       
       the remainder being iron, including typical steel-associated, smelting-related impurities, wherein a total content of Mn+Si+Cr is ≥2.500 to ≤3.250 wt. % with regard to a widest possible processing window during annealing, in particular continuous annealing, of cold strips of said steel. 
     
     
         39 . The ultra high strength multi-phase steel of  claim 38 , wherein the total content of Mn+Si+Cr is ≥2.500 to ≤3.100 wt. % or ≥2.650 to ≤3.150 wt. % or ≥2.700 to ≤3.250 wt. %. 
     
     
         40 . The ultra high strength multi-phase steel of  claim 38 , wherein a B content is ≤0.0009 wt. %, in particular the B content is ≥0.0006 to ≤0.0009 wt. %. 
     
     
         41 . The ultra high strength multi-phase steel of  claim 38 , wherein a C content is ≤0.100 wt. % and a carbon equivalent CEV (IIW) is ≤0.62%, or a C content is ≤0.105 wt. % and a carbon equivalent CEV(IIW) is ≤0.64%, or a C content is ≤0.115 wt. % and a carbon equivalent CEV(IIW) is ≤0.66%. 
     
     
         42 . The ultra high strength multi-phase steel of  claim 38 , wherein a Mn content is ≥1.900 to ≤2.350 wt. %. 
     
     
         43 . The ultra high strength multi-phase steel of  claim 42 , wherein the Mn content is ≥1.900 to ≤2.200 wt. %, or the Mn content is ≥2.050 to ≤2.250 wt. %, or the Mn content is ≥2.100 to ≤2.350 wt. %. 
     
     
         44 . The ultra high strength multi-phase steel of  claim 38 , wherein a N content is ≥0.0020 to ≤0.0090 wt. %, when a sum of Ti+Nb+B is ≥0.010 to ≤0.070 wt. %, or a N content is ≥0.0040 to ≤0.0120 wt. %, when a sum of Ti+Nb+B is >0.070 wt. % 
     
     
         45 . The ultra high strength multi-phase steel of  claim 38 , wherein a S content is ≤0.0015 wt. %, in particular the S content is ≤0.0010 wt. %. 
     
     
         46 . The ultra high strength multi-phase steel of  claim 38 , wherein a Mo content is ≤0.250 wt. %. 
     
     
         47 . The ultra high strength multi-phase steel of  claim 38 , wherein a Ti content is ≥0.025 to ≤0.045 wt. %. 
     
     
         48 . The ultra high strength multi-phase steel of  claim 38 , wherein a Nb content is ≥0.025 to ≤0.045 wt. %. 
     
     
         49 . The ultra high strength multi-phase steel of  claim 38 , wherein a sum of Nb+Ti is ≤0.100 wt. %, in particular the sum of Nb+Ti is ≤0.090 wt. %. 
     
     
         50 . The ultra high strength multi-phase steel of  claim 38 , wherein a sum of Cr+Mo is ≤0.650 wt. %. 
     
     
         51 . The ultra high strength multi-phase steel of  claim 38 , wherein a sum Ti+Nb+B is ≤0.102 wt. %, in particular the sum of Ti+Nb+B is ≤0.092 wt. %. 
     
     
         52 . The ultra high strength multi-phase steel of  claim 38 , wherein a sum of Ti+Nb+B+Mo+V is ≤0.365 wt. %. 
     
     
         53 . The ultra high strength multi-phase steel of  claim 38 , wherein a Ca content is ≤0.0030 wt. %. 
     
     
         54 . The ultra high strength multi-phase steel of  claim 38 , wherein additions of Si and Mn with respect to a strength property to be achieved are exchangeable according to the following relationships:
   YS (MPa)=185+147.9 [% Si]+161.1 [% Mn]     TS (MPa)=574+189.4 [% Si]+174.1 [% Mn].   
     
     
         55 . A method, comprising:
 producing a hot strip from a multi-phase steel as set forth in  claim 38 ;   cold-rolling the hot strip to produce a steel strip with a desired end thickness; and   subsequently annealing, in particular continuously annealing, the steel strip.   
     
     
         56 . The method of  claim 55 , wherein steel strips are cold-rolled in a wide range of degrees of thinning by rolling of 10% to 70% with the desired end thickness by proceeding from a selected master hot strip having a specific thickness or selected hot strips having different thicknesses. 
     
     
         57 . The method of  claim 55 , wherein a chemical composition of the multi-phase steel is selected in dependence upon the desired end thickness of the steel strip. 
     
     
         58 . The method of  claim 57 , wherein the steel strip is cold-rolled to an end thickness of 0.50 mm to 3.00 mm and the chemical composition of the multi-phase steel is selected in dependence upon the end thickness as follows:
 end thickness 0.50 mm to 1.00 mm inclusive: sum of Mn+Si+Cr≥2.500 to ≤3.100 wt. %,   end thickness over 1.00 mm to 2.00 mm inclusive: sum of Mn+Si+Cr≥2.650 to ≤3.150 wt. %,   end thickness over 2.00 mm to 3.00 mm inclusive: sum of Mn+Si+Cr≥2.700 to ≤3.250 wt. %.   
     
     
         59 . The method of  claim 57 , wherein the chemical composition of the multi-phase steel is selected in dependence upon the end thickness as follows:
 end thickness 0.50 mm to 1.00 mm inclusive: C content ≤0.100% and carbon equivalent CEV (IIW)≤0.62%,   end thickness over 1.00 mm to 2.00 mm inclusive: C content ≤0.105% and the carbon equivalent CEV (IIW)≤0.64%,   end thickness over 2.00 mm to 3.00 mm inclusive: C content ≤0.115% and the carbon equivalent CEV (IIW)≤0.66%.   
     
     
         60 . The method of  claim 57 , wherein the chemical composition of the multi-phase steel is selected in dependence upon the end thickness as follows:
 end thickness 0.50 mm to 1.00 mm inclusive: Mn content ≥1.900 to ≤2.200%,   end thickness over 1.00 mm to 2.00 mm inclusive: Mn content ≥2.050 to ≤2.250%,   end thickness over 2.00 mm to 3.00 mm inclusive: Mn content ≥2.100 to ≤2.350%.   
     
     
         61 . The method of  claim 55 , wherein in order to produce a required microstructure, the steel strip cold-rolled to the end thickness is heated during the continuous annealing to a temperature in a range of approximately 700 to 950° C., and the annealed steel strip is subsequently cooled 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., then at a cooling rate between approximately 15 and 100° C./s to a second intermediate temperature of approximately 160 to 250° C., then the steel strip is cooled 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. 
     
     
         62 . The method of  claim 55 , wherein in order to produce a required microstructure, the steel strip cold-rolled to the end thickness is heated during the continuous annealing to a temperature in a range of approximately 700 to 950° C. and, during a hot-dip finishing procedure after the heating and subsequent cooling to a temperature of approximately 400 and 470° C., cooling is stopped prior to entry into a melting bath and, after undergoing the hot-dip finishing procedure, cooling is continued at a cooling rate between approximately 15 and 100° C./s to an intermediate temperature of approximately 200 to 250° C., and then the steel strip is cooled in air at a cooling rate of approximately 2 to 30° C./s until room temperature is reached. 
     
     
         63 . The method of  claim 55 , wherein in order to produce a required microstructure, the steel strip cold-rolled to the end thickness is heated during the continuous annealing to a temperature in a range of approximately 700 to 950° C. and, during a hot-dip finishing procedure after the heating and subsequent cooling to an intermediate temperature of approximately 200 to 250° C. prior to entry into a melting bath, the temperature is maintained for approximately 1 to 20 s and then the steel strip is heated to a temperature of approximately 400 to 470° C. and, after undergoing the hot-dip finishing procedure, cooling at a cooling rate between approximately 15 and 100° C./s to an intermediate temperature of approximately 200 to 250° C. and subsequently cooling in air at a cooling rate of approximately 2 to 30° C./s to room temperature. 
     
     
         64 . The method of  claim 55 , wherein, during the continuous annealing, an oxidation potential during annealing with an installation configuration comprised of a direct fired furnace region (NOF) and a radiant tube furnace (RTF) is increased by a CO content in the NOF of less than 4 vol. %, wherein in the RTF an oxygen partial pressure of a furnace atmosphere which is reducing for iron is set according to the 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 the steel in wt. % and pO 2  designates the oxygen partial pressure in mbar, and in order to avoid oxidation of the strip directly prior to dipping in a melting bath a dew point of the gas atmosphere is set at −30° C. or below. 
     
     
         65 . The method of  claim 55 , wherein during annealing only with a radiant tube furnace an oxygen partial pressure of a furnace atmosphere satisfies the 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 the steel in wt. % and pO 2  designates the oxygen partial pressure in mbar, and in order to avoid oxidation of the strip directly prior to dipping in a melting bath a dew point of the gas atmosphere is set at −30° C. or below. 
     
     
         66 . The method of  claim 55 , wherein when strips of different thickness are involved during continuous annealing comparable microstructure states and mechanical characteristic values of the strips are set by adapting a rate of installation throughput rate during heat treatment. 
     
     
         67 . The method of  claim 55 , wherein the steel strip is temper-rolled subsequent to the annealing or hot-dip finishing procedure. 
     
     
         68 . The method of  claim 55 , wherein the steel strip is stretch-bend-straightened subsequent to the annealing or hot-dip finishing procedure. 
     
     
         69 . The method of  claim 55 , further comprising:
 cutting a blank from the steel strip;   heating the blank to a temperature above Ac3;   deforming the heated blank to produce a component; and   hardening the component in a tool or in air.   
     
     
         70 . The method of  claim 55 , wherein the steel strip is part of a sub-peritectic grade chain in the absence of vanadium as a microalloy element. 
     
     
         71 . A steel strip produced by a method as set forth in  claim 55 , said steel strip comprising a minimum hole expansion according to ISO 16630 of 20%, in particular of 25%. 
     
     
         72 . The steel strip of  claim 71 , further comprising a minimum bending angle according to VDA 238-100 of 70° in a longitudinal direction or transverse direction, in particular of 85°. 
     
     
         73 . The steel strip of  claim 71 , further comprising a minimum product value Rm×α (tensile strength×bending angle according to VDA 238-100) of 100000 MPa°, in particular of 120000 MPa°. 
     
     
         74 . The steel strip of  claim 71 , further comprising a delayed fracture free state for at least 6 months thus meeting a requirement of SEP 1970 for hole pull and hoop test pieces.