High strength multi-phase steel, and method for producing a strip from said steel

Abstract

A high-strength multiphase steel with minimum tensile strengths of 580 MPa, preferably having a dual-phase structure for a cold-rolled or hot-rolled steel strip with improved forming properties, particularly for lightweight vehicle construction contains the elements (contents in mass-%): C 0.075 to ≦0.105; Si 0.600 to ≦0.800; Mn 1.000 to ≦2.250; Cr 0.280 to ≦0.480; Al 0.010 to ≦0.060; P≦0.020; N≦0.0100; S≦0.0150, remainder iron, including typical steel-accompanying elements not mentioned above, which are impurities introduced by smelting, with the condition that the Mn content is preferably ≦1.500% for strip thicknesses up to 1 mm, the Mn content is preferably ≦1.750% for strip thicknesses of 1 to 2 mm, and the Mn content is preferably ≧1.500% for strip thicknesses ≧2 mm

What is claimed is: 
     
         1 .- 16 . (canceled) 
     
     
         17 . A high strength multiphase steel with minimal strengths of 580 MPa preferably with dual phase microstructure for a cold or hot rolled steel strip having improved forming properties, in particular for the vehicle lightweight construction, composed of the following elements weight %:
 C 0.075 to ≦0.105   Si 0.600 to ≦0.800   Mn 1.000 to ≦2.250   Cr 0.280 to ≦0.480   Al 0.010 to ≦0.060   P≦0.020   N≦0.0100   S≦0.0150   remainder iron including usual steel accompanying elements not mentioned above which constitute smelting related impurities, with the proviso that at strip thicknesses up to 1 mm the Mn content is preferably ≦1.500%, that at strip thicknesses from 1 mm to 2 mm the Mn content is preferably ≦1.750%, and that at strip thicknesses 2 mm the Mn content is preferably ≧1.500%.   
     
     
         18 . The steel of claim  1 , wherein at strip thicknesses up to 1 mm a sum of Mn+Si+Cr is ≧1.88≦2.60%. 
     
     
         19 . The steel of  claim 17 , wherein at strip thicknesses of 1.00-2.00 mm the sum of Mn+Si+Cr is ≧2.2≦3.00% 
     
     
         20 . The steel of  claim 17 , wherein at strip thicknesses of ≧2.00 the sum of Mn+Si+Cr is ≧2.50≦3.53%. 
     
     
         21 . The steel of  claim 17 , wherein the N content is ≦0.0090%. 
     
     
         22 . The Steel according of  claim 17 , wherein the N content is ≦0.0080%. 
     
     
         23 . The steel of  claim 17 , wherein the S content is ≦0.0050%. 
     
     
         24 . The steel of the  claim 17 , wherein the S content is ≦0.0030%. 
     
     
         25 . A method for producing a cold or hot rolled steel strip from the steel of  claim 17 , comprising:
 heating the cold or hot rolled steel strip during a continuous annealing to an annealing temperature in the range of about 700 to 950° C.;   cooling the steel strip is from the annealing temperature to a first intermediate temperature of about 300 to 500° C. at a cooling rate between about 15 and 100° C./s;   cooling the cold or hot rolled steel strip to a second intermediate temperature of about 200 to 250° C. with a cooling rate between about 15 to 100° C./s; and   cooling the steel strip at air with a cooling rate of about 2 to 30° C./s until reaching room temperature or maintaining the cooling with a cooling rate between about 15 to 100° C./s from the first intermediate temperature to room temperature, wherein a dual phase microstructure is generated during the continuous annealing.   
     
     
         26 . The method of  claim 25 , further comprising hot dip galvanizing the strip in a hot dip bath, wherein subsequent to the heating and subsequent cooling the cooling is halted prior to entering into the hot dip bath, and after the hot dip galvanizing the cooling is continued with a cooling rate between about 15 and 100° C./s until reaching an intermediate temperature of about 200 to 250° C., and subsequently the steel strip is cooled on air with a cooling rate between about 2 and 30° C./s until reaching room temperature. 
     
     
         27 . The method of  claim 25 , further comprising hot dip galvanizing the steel strip in a hot dip bath, wherein after the heating and subsequent cooling to the intermediate temperature of about 200 to 250° C. and prior to entering the hot dip bath the temperature is held for about 1 to 20 s and subsequently the steel strip is reheated to the temperature of about 420 to 470° C. and after the hot dip galvanizing the steel strip is cooled until reaching the intermediate temperature of about 200 to 250° C. with a cooling rate between about 15 and 100° C./s, and subsequently the steel strip is cooled on air with a cooling rate of about 2 and 30° C./s until reaching room temperature. 
     
     
         28 . The method of the  claim 25 , wherein the heating step is carried out in a plant configuration composed of a directly fired furnace region and a radiant-tube furnace, the method further comprising increasing an oxidation potential in the annealing by setting a CO-content below 4%, setting an atmosphere of the furnace to be reducing and setting a dew point at −30° C. or below −30° C. so as to avoid oxidation of the strip prior to immersion into the hot dip bath. 
     
     
         29 . The method of  claim 25 , wherein the annealing is carried out by solely utilizing a radiant-tube furnace, wherein a dew the dew point in the furnace atmosphere is −30° C. or above −30° C. 
     
     
         30 . The method of  claim 29 , wherein the dew point in the furnace atmosphere is −25° C. or −20° C. 
     
     
         31 . The method of  claim 25 , further comprising adjusting a throughput speed as a function of varying thicknesses of individual steel strips during the heat treatment thereby establishing comparable microstructure states and mechanical characteristic values in the individual steel strips. 
     
     
         32 . The method of  claim 25 , further comprising subsequent to the heat treatment skin passing the steel strip. 
     
     
         33 . The method of  claim 25 , further comprising aligning the steels strip stretch bending subsequent to the heat treatment.