Micro-alloyed high-strength multi-phase steel containing silicon and having a minimum tensile strength of 750 mpa and improved properties and method for producing a strip from said steel

Abstract

A high-strength multi-phase steel having minimum tensile strengths of 750 MPa and preferably having a dual-phase microstructure for a cold- or hot-rolled steel strip, in particular for lightweight vehicle construction is disclosed. The high-strength multi-phase steel has improved forming properties and a ratio of yield point to tensile strength of at most 73%. The high-strength multi-phase steel includes in mass %: C≧0.075 to ≦0.105; Si≧0.600 to ≦0.800; Mn≧1.000 to ≦0.700; Cr≧0.100 to ≦0.480; Al≧0.010 to ≦0.060; N 0.0020≦0.0120; S≦0.0030; Nb≧0.005 to ≦0.050; Ti≧0.0050 to ≦0.050; B≧0.0005 to ≦0.0040; Mo≦0.200; Cu≦0.040%; Ni≦0.040% the remainder iron, including typical elements accompanying steel that are not mentioned above, which represent contamination resulting from smelting.

What is claimed is: 
     
         1 .- 20 . (canceled) 
     
     
         21 . A high-strength multiphase steel with minimal tensile strengths of 750 MPa, preferably with a dual phase microstructure, for a cold or hot rolled steel strip with improved forming properties and a yield to tensile ratio of maximally 73%, in particular for the vehicle lightweight construction, composed of the elements (in mass %)
 C≧0.075 to ≦0.105   Si≧0.600 to ≦0.800   Mn≧1.000 to ≦0.700   Cr≧0.100 to ≦0.480   Al≧0.010 to ≦0.060   N 0.0020≦0.0120   S≦0.0030   Nb≧0.005 to ≦0.050   Ti≧0.0050 to ≦0.050   B≧0.0005 to ≦0.0040   Mo≦0.200   Cu≦0.040%   Ni≦0.040%   remainder iron and steel accompanying elements constituting smelting related impurities.   
     
     
         22 . The steel of  claim 21 , wherein at a thicknesses of the steel strip of up to 1.00 mm the Mn-content is ≦1.500%. 
     
     
         23 . The steel of  claim 21 , wherein at a thicknesses of the steel strip of >1.00 to 2.00 mm the Mn-content is ≦1.750%. 
     
     
         24 . The steel of  claim 21 , wherein at a thicknesses of the steel strip of >2.00 mm the Mn-content is ≦1.500%. 
     
     
         25 . The steel of  claim 21 , wherein at a thicknesses of the steel strip of up to 1.00 mm a sum of the contents of Mn+Si+Cr is ≧2.40 and ≦2.70%. 
     
     
         26 . The steel of  claim 21 , wherein at a thicknesses of the steel strip of 1.00-2.00 mm a sum of the contents of Mn+Si+Cr is ≧2.60 and ≦2.90%. 
     
     
         27 . The steel of  claim 21 , wherein at a thicknesses of the steel strip of >2.00 mm a sum of the contents of Mn+Si+Cr is ≧2.80 and ≦3.10%. 
     
     
         28 . The steel of  claim 21 , wherein a sum of the contents of Ti+Nb is ≧0.010% and ≦0.050%, and the N content is ≧0.0020 and ≦0.0100%. 
     
     
         29 . The steel of  claim 21 , wherein a sum of the contents of Ti+Nb is >0.050%, and the N content is ≧0.0040 and ≦0.0120%. 
     
     
         30 . The steel of  claim 21 , wherein the S content is ≦0.0020%. 
     
     
         31 . The steel of  claim 21 , wherein the S content is ≦0.0010%. 
     
     
         32 . The steel of  claim 21 , wherein the contents of silicone and manganese with regard to the strength properties to be achieved are exchangeable according to the relationship:
   YS(MPa)=160.7+147.9[% Si]+161.1[% Mn]     TS(MPa)=324.8+189.4[% Si]+174.1[% Mn].   
     
     
         33 . A method for producing a cold or hot rolled steel strip made of the steel of  claim 21 , in which a dual phase microstructure is generated during a continuous annealing, comprising:
 heating a cold or hot rolled steel strip during the continuous annealing to an annealing temperature in a range of 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.   
     
     
         34 . The method of  claim 33 , further comprising after the heating step and during the cooling to the first intermediate temperature step hot dip coating the steel strip in a hot dip bath, wherein the cooling to the first intermediate temperature is interrupted prior to entry into the hot dip bath, and after the cooling to the first intermediate temperature the steel strip is treated as set forth under a), wherein the second intermediate temperature is 200 to 250° C. and the cooling from the second intermediate temperature to room temperature is conducted with a cooling rate of about 2 and 30° C./s. 
     
     
         35 . The method of  claim 33 , wherein the steel is treated as set forth under a), wherein the second intermediate temperature is 200 to 250° C., said method further comprising after the cooling to the second intermediate temperature and prior to the cooling to room temperature,
 holding the second intermediate temperature for about 1 to 20 seconds, 
 reheating the steel strip to a temperature of about 400 to 470° C., 
 hot dip coating the steel strip, and 
 cooling the steel strip to the second intermediate temperature of 200 to 250° C. with a cooling rate of between about 15 and 100° C./s, 
 wherein the cooling from the second intermediate temperature to room temperature is conducted with a cooling rate of about 2 and 30° C./s. 
 
     
     
         36 . The method of  claim 33 , wherein the heating step is performed using a plant configuration comprising a directly fired furnace and a radiant tube furnace, and wherein the method further comprises
 increasing an oxidation potential during the heating by setting a CO-content in the directly fired furnace below 4%,   setting an oxygen partial pressure of an atmosphere of the radiant tube furnace 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*(−InB)0.25, wherein Si, Mn, Cr and B are corresponding alloy proportions in the steel in mass % and pO 2  is the oxygen partial pressure in mbar, and wherein a dew point of an overall atmosphere of the plant configuration to −30° C. or below for avoiding oxidation of the strip directly prior to immersion into a hot dip bath.   
     
     
         37 . The method of  claim 33 , wherein the heating is performed with a single radiant tube furnace, and wherein the oxygen partial pressure of the atmosphere of the radiant tube furnace satisfies the following equation,
   −12>Log  p O 2 ≧5*Si−0.25−3*Mn−05−0.1*Cr−0.5−7*(−InB)0.5
   wherein Si, Mn, Cr, and B are corresponding alloy components in the steel in mass % and pO 2  is an oxygen partial pressure in mbar, and wherein a dew point of an overall atmosphere of the plant configuration to −30° C. or below for avoiding oxidation of the strip directly prior to immersion into a hot dip bath.   
     
     
         38 . The method of the  claim 33 , further comprising adjusting a plant throughput speed to different thicknesses of respective steel strips so that heat treatment of the respective steel strips results in similar microstructures and mechanical characteristic values. 
     
     
         39 . The method of  claim 33 , further comprising after after the heat treatment skin-passing the steel strip. 
     
     
         40 . The method of  claim 33 , further comprising after the heat treatment the stretch leveling the steel strip.