Hot-Rolled Flat Steel Product and Method for the Production Thereof

Abstract

A hot-rolled flat steel product having a thickness of <1.5 and consisting of, in % by mass, C: 0.04-0.23%, Si: 0.04 0.54%, Mn: 1.4-2.9%, Ti+V, wherein 0.005%<%Ti+%V<0.15%, and optionally one or more elements of Al, Cr, Mo, and B, where AI: 0.01-1.5%, 0.02<%Mo+%Cr<1.4%, and B: 0.0005-0.005%, and the remainder consisting of iron and inevitable impurities. The structure of the flat steel product consists of, in percent by area, in sum, 50-90% ferrite and bainite ferrite, 5-50% martensite, 2-15% residual austenite and <10% other structure elements. The flat steel product has a yield point Rp0.2>290 MPa, a tensile strength Rm>490 MPa and an elongation at break A80 where A80[%]=B−Rm/37 with 31<B<51. To at least one surface of the flat steel product, a Zn coating is applied by hot-dip coating. Also a method for producing a flat steel product of this kind.

1 . A hot-rolled flat steel product, which comprises
 a steel substrate at least 1.5 mm thick,   which consists of, in % by mass,   C: 0.04-0.23%,   Si: 0.04-0.54%,   Mn: 1.4-2.9%,   Ti+V, wherein the following applies for the sum %Ti+%V of the contents of Ti and V:   0.005%≤%Ti+%V≤0.15%,   and optionally one element or a plurality of elements selected from the group consisting of Al, Cr, Mo, and B, wherein:   AI: 0.01-1.5%,   the sum of the contents %Cr+%Mo of Cr and Mo: 0.02≤%Mo+%Cr≤1.4%, and   B: 0.0005-0.005%,   and a remainder consisting of iron and unavoidable impurities, wherein the unavoidable impurities include less than 0.02% P, less than 0.005% S, less than 0.01% N and less than 0.005% Nb,   wherein the hot-rolled flat steel product has a structure which consists of, in % by area, in total 50-90% ferrite and bainitic ferrite, 5-50% martensite, 2-15% residual austenite and up to 10% other structural constituents that are unavoidable due to production,   has a yield point Rp0.2 of at least 290 MPa, a tensile strength Rm of at least 490 MPa and an elongation at break A80 determined by the following formula (1):
     A 80[%]= B−Rm/ 37 with 31 ≤B≤ 51, and 
   a corrosion protection layer based on zinc is applied to at least one surface of the steel substrate by hot-dip coating.   
     
     
         2 . The flat steel product according to  claim 1 , wherein the structure of the steel substrate contains at least 5% by area residual austenite. 
     
     
         3 . The flat steel product according to  claim 1 , wherein, for the parameter B of formula (1), the following applies: 36≤B≤46. 
     
     
         4 . The flat steel product according to  claim 1 , wherein the sum X of surface proportions of the structure of the steel substrate in which there is a Mn concentration that is more than 15% above the average value of the Mn concentration in the structure is at least 10% of the total structure. 
     
     
         5 . The flat steel product according to  claim 4 , wherein the sum X is at least 12%. 
     
     
         6 . The flat steel product according to  claim 5 , wherein the sum X is at least 15%. 
     
     
         7 . The flat steel product according to  claim 1 , wherein the corrosion protection layer contains at least 75% by mass Zn. 
     
     
         8 . A method for the production of a flat steel product according to  claim 1  comprising:
 A) producing a hot-rolled steel substrate in the form of a steel strip in at least the following sub-steps: 
 A.1) melting a steel melt, which consists of, in % by mass, C: 0.04-0.23%, Si: 0.04-0.54%, Mn: 1.4-2.9%, Ti+V, wherein the following applies for the sum %Ti+%V of the contents of Ti and V: 0.005%≤%Ti+%V≤0.15%, and optionally one element or a plurality of the elements selected from the group consisting of Al, Cr, Mo, and B, wherein AI: 0.01-1.5%, the sum of the contents of Cr and Mo: 0.02≤%Mo+%Cr≤1.4%, and B: 0.0005-0.005%, and a remainder consisting of iron and unavoidable impurities, wherein the unavoidable impurities include less than 0.02% P, less than 0.005% S, less than 0.01% N and less than 0.005% Nb; 
 A.2) casting the steel melt to form a preliminary product, which is a slab or thin slab; 
 A.3) preheating the preliminary product at a preheating temperature of at least 1150° C. and at most 1350° C.; 
 A.4) hot-rolling the preliminary product to form a hot-rolled steel strip, wherein the final temperature of the hot rolling is at least 840-980° C. and the thickness of the hot-rolled steel strip is 1.5-10 mm; 
 A.5) cooling the hot-rolled steel strip to a coiling temperature that is 510-640° C.; and 
 A.6) coiling the hot-rolled steel strip cooled to the coiling temperature; and 
 B) coating the steel substrate, which is present in the form of a hot-rolled steel strip, with a corrosion protection coating based on zinc in at least the following sub-steps, which are passed through continuously: 
 B.1) optional pickling of the hot-rolled steel strip; 
 B.2) heating the hot-rolled steel strip with a heating rate of 0.5-100° C./s to an annealing temperature of 750-950° C. and holding the hot-rolled steel strip at the annealing temperature over an annealing period of 10-1000 s; 
 B.3) cooling the hot-rolled steel strip at a cooling rate of 0.5-100° C./s to a bath entry temperature BET, for which BT≤BET≤(BT+20° C.) applies, wherein BT is the temperature of the zinc melt bath and is 450-480° C.; 
 B.4) passing the hot-rolled steel strip cooled down to the bath entry temperature BET through the zinc melt bath, which consists of up to 5% by mass Mg, up to 10% by mass AI, and a remainder of Zn and unavoidable impurities to obtain a flat steel product; 
 B.5) cooling the obtained flat steel product with a cooling rate of 0.5-100° C./s; and 
 B.6) optional skin-pass rolling of the flat steel product with a degree of skin passing of 0.3-2.0%. 
 
     
     
         9 . The method according to  claim 8 , wherein the coiling temperature is at least 530° C. 
     
     
         10 . The method according to  claim 9 , wherein the coiling temperature is at least 550° C. 
     
     
         11 . The method according to  claim 8 , wherein the coiling temperature is at most 620° C.