Flat Steel Product, Method for the Production Thereof, and Use of Such a Flat Steel Product

Abstract

A cold-rolled flat steel product has a tensile strength of 750-940 MPa, a high strength, an improved weldability and optimized shaping properties, and can be produced at low cost. The cold-rolled flat steel product consists of a steel composed, in percent by mass, of C: 0.040-0.100%; Mn: 2.10-2.50%; Si: 0.10-0.40%; Cr: 0.30-0.90%; Ti: 0.020-0.080%, B: 0.0005-0.0020%; N: 0.003-0.010%; Al: up to 0.10%; Ca: up to 0.005%; P: up to 0.025%; S: up to 0.010%; optionally one or more of the following elements: Mo: up to 0.20%; Nb: up to 0.050%; Cu: up to 0.10%; V: up to 0.020%; Ni: up to 0.10%, the remainder being iron and unavoidable impurities, and total content of impurities is limited to at most 0.5% by mass and contents of phosphorus (“P”) and sulfur (“S”) belong to the impurities.

1 . A cold-rolled flat steel product having a tensile strength of 750-940 MPa, the steel substrate of which:
 consists of a steel which, in % by mass, consists of   C: 0.040-0.100%,   Mn: 2.10-2.50%,   Si: 0.10-0.40%,   Cr: 0.30-0.90%,   Ti: 0.020-0.080%,   B: 0.0005-0.0020%,   N: 0.003-0.010%,   Al: up to 0.10%,   Ca: up to 0.005%,   P: up to 0.025%,   S: up to 0.010%,   optionally one or more of the following elements:   Mo: up to 0.20%,   Nb: up to 0.050%,   Cu: up to 0.10%,   V: up to 0.020%,   Ni: up to 0.10%   and as the remainder iron and unavoidable impurities, wherein the total fraction of impurities is limited to at most 0.5% by mass, and the fractions of phosphorus (“P”) and sulfur (“S”) belong to the impurities, and   has a dual phase structure that consists of 10-40% by volume of martensite, 30-90% by volume of ferrite including bainitic ferrite, no more than 5% of residual austenite, and the remainder of other structural constituents unavoidable due to the production process.   
     
     
         2 . The flat steel product according to  claim 1 , wherein its strain hardening exponent n, measured in the expansion interval between 0.2-2.2%, is at least 0.22%. 
     
     
         3 . The flat steel product according to  claim 1 , wherein the following applies for the Ti fraction, in % Ti:
   %  Ti< 11×(%  N +%  B )
   
       where % N=the given N fraction and % B=the given B fraction. 
     
     
         4 . The flat steel product according to  claim 1 , wherein its tensile strength Rm is 780-900 MPa, its elastic limit Rp0.2 is 440-650 MPa, and its elongation at break A80 is more than 13%. 
     
     
         5 . The flat steel product according to  claim 1 , wherein it has a hole expansion ratio HER of greater than 20% determined in accordance with DIN ISO 16630. 
     
     
         6 . The flat steel product according to  claim 5 , wherein the hole expansion ratio HER with a conical punch of 180° is at least 15%, and with a conical punch of 50° is at least 25%. 
     
     
         7 . The flat steel product according to  claim 1 , wherein it has a drawing depth of greater than 33 mm, as determined in an LDH test. 
     
     
         8 . The flat steel product according to  claim 1 , wherein it is coated with a corrosion-inhibiting layer applied by dip coating or electrolytic coating. 
     
     
         9 . A method for producing a cold-rolled flat steel product formed according to  claim 1 , comprising the following steps:
 a) melting a steel melt comprising, in % by mass, C: 0.040-0.100%, Mn: 2.10-2.50%, Si: 0.10-0.40%, Al: up to 0.10%, Cr: 0.30-0.90%, Ti: 0.020-0.080%, B: 0.0005-0.0020%, Ca: up to 0.005%, P: up to 0.025%, S: up to 0.010%, N: 0.003-0.010%, up to 0.20% Mo, up to 0.050% Nb, up to 0.10% Cu, up to 0.020% V, and up to 0.10% Ni, and as the remainder iron and unavoidable impurities;   b) casting the melt to make a precursor, such as a slab or thin slab,   c) hot rolling the precursor at a hot rolling end temperature of 850-980° C., to make a hot-rolled strip;   d) coiling the hot-rolled strip at a coiling temperature of 480-650° C.;   e) pickling the hot-rolled strip;   f) cold rolling the hot-rolled strip to form a cold-rolled flat steel product having a total degree of cold rolling of 25-70%;   g) annealing the cold-rolled flat steel product in a continuous furnace at an annealing temperature GT of 780-920° C.;   h) cooling the cold-rolled flat steel product heated to the annealing temperature GT to a cooling end temperature KET of 380-500° C.,   wherein the cold-rolled flat steel product heated to the annealing temperature GT is cooled to a cooling end temperature KET in two steps, wherein the cold-rolled flat steel product in the first step of its cooling is cooled from the given annealing temperature GT to an intermediate temperature ZT lying in the range of 750-620° C., with a cooling rate AR1 which is greater than 1.5 K/s, and in the second step from the intermediate temperature ZT to the given cooling end temperature KET, with a cooling rate AR2 for which the following applies: AR2>4×AR1   or   wherein the cold-rolled flat steel product heated to the annealing temperature GT is cooled to a cooling end temperature KET in two steps, wherein the cold-rolled flat steel product in the first step of its cooling is cooled from the given annealing temperature GT to an intermediate temperature ZT lying in the range of 700-450° C., with a cooling rate AR1 which is greater than 5 K/s, and in the second step from the intermediate temperature ZT to the given cooling end temperature KET, with a cooling rate AR2 for which the following applies: AR2<(AR1)/3;   i) optionally: cooling or heating the cold-rolled flat steel product from the cooling end temperature KET to a bath entry temperature BT of 450-490° C., and conveying through a melt bath consisting of zinc or a zinc alloy with a Zn fraction of at least 75 wt. %;   j) cooling the emerging cold-rolled flat steel product to room temperature, and/or cooling the cold-rolled flat steel product from the cooling end temperature KET to room temperature; and   k) optionally: skin-pass rolling the cold-rolled flat steel product with a skin-pass degree of max. 2%, preferably 0.2-0.7%.   
     
     
         10 . The method according to  claim 9 , wherein the coiling temperature is 500-600° C. 
     
     
         11 . The method according to  claim 9 , wherein the annealing temperature GT is 810-890° C. 
     
     
         12 . (canceled) 
     
     
         13 . An axially stressed component comprising longitudinal members and cross members, wherein a material of the component is the cold-rolled flat steel product of  claim 1 . 
     
     
         14 . A bending-stressed component comprising a B-pillar, a B-pillar reinforcement, or a sill of an automotive body, wherein a material of the component is the cold-rolled flat steel product of  claim 1 .