High-strength hot-rolled flat steel product having high local cold formability and a method of producing such a flat steel product

Abstract

The present disclosure provides a high-strength hot-rolled flat steel product and a method of producing such a product. This is achieved by a high-strength hot-rolled flat steel product with high local cold formability, having a tensile strength Rm of at least 760 MPa, a yield point ratio of at least 0.8 and a hole expansion ratio of at least 30%, an elongation at break of at least 10, a measure of cold formability of at least 0.12, and a ratio of local and global cold formability of at least 5 and at most 13, and a microstructure consisting of more than 50% by volume of bainite and up to 10% by volume, of carbon-rich microstructure constituents

1 . A high-strength, hot-rolled flat steel product with high local cold-formability, having a tensile strength Rm of at least 760 MPa, a yield strength ratio of at least 0.8 and a hole expansion ratio of at least 30%, an elongation at fracture of at least 10%, a measure of cold-formability of at least 0.12, and a ratio of local and global cold-formability of at least 5, and a microstructure consisting of more than 50 vol. % bainite, carbon-rich microstructure constituents, with the remainder being ferrite, said flat steel product having a chemical composition comprising (in wt. %):
 C: in a range of 0.04 to 0.081   Si: in a range of −0.1 to 0.6;   Mn: in a range of 1.0 to 2.0;   P: max. 0.06;   S: max. 0.01;   N: max. 0.012;   Al: up to 0.06;   T i : up to 0.18 and/or Nb: up to 0.08;   Mo: up to 0.35;   with T i +Nb more than 0.06, wherein a hyperstoichiometric proportion of carbon and nitrogen is present according to the following formula:
   1.0<(C/12+N/14)/(T i /48+Nb/93+Mo/96), 
   with the remainder being iron including unavoidable, steel-associated elements, with optional addition by alloying of one or more elements of Cr, Ni, V, B or Ca,   wherein the microstructure distribution over the thickness of the flat steel product in the three regions near the surface, ¼ thickness and ½ thickness of the flat steel product comprises:
 an absolute deviation of a maximum of 12 vol. %, advantageously a maximum of 7 vol. % of the proportion of ferrite in the region near the surface or in the region of ¼ thickness of the flat steel product, or in to the region of ½ thickness of the flat steel product and/or 
 the deviation in aspect ratio of the grains in the rolling direction in the three regions of the flat steel product with respect to the mean value is less than 0.3 in each of the three regions, 
 and/or 
 in the three regions the difference in hardness HV0.1 is in each case a maximum of 20 HV 0.1, advantageously a maximum of 15 HV 0.1, even more advantageously a maximum of 10 HV 0.1 in comparison with the mean value over the entire thickness of the flat steel product. 
   
     
     
         2 . The flat steel product as claimed in  claim 1 , wherein an addition by alloying of Ca of a maximum of 0.01 wt. %. 
     
     
         3 . The flat steel product as claimed in  claim 1  or  2 , wherein the steel contains (in wt. %):
 T i +Nb: max. 0.2. 
 
     
     
         4 . The flat steel product as claimed in at least one of  claims 1  to  3 , wherein the steel contains (in wt. %):
 T i  min.: 0.02, optionally 0.04, even more optionally 0.06 
 Nb min.: 0.01 
 Mo min.: 0.05. 
 
     
     
         5 . The flat steel product as claimed in at least one of  claims 1  to  4 , wherein in that the steel contains (in wt. %):
 Cr: up to 0.6 
 Ni: up to 0.6 
 V: up to 0.2 
 B: up to 0.01 
 wherein a hyperstoichiometric proportion of carbon and nitrogen is present according to the following formula:
   1.0<(C/12+N/14)/(T i /48+Nb/93+Mo/96+V/51). 
 
 
     
     
         6 . The flat steel product as claimed in  claim 5 , wherein the steel contains (in wt. %):
 Cr: more than 0.1   Ni: more than 0.1   V: more than 0.01   B: more than 0.0005   
     
     
         7 . The flat steel product as claimed in  claim 1 , wherein the bainite is a mixture of constituents which is includes by a main constituent of at least 50 vol. % and secondary constituents, wherein the main constituent consists of bainitic ferrite hardened by precipitations of (Ti, Nb, Mo)(C,N) and/or V(C;N) and the secondary constituents consist of carbon-richer constituents, such as e.g. martensite, residual austenite, lower bainite, and pearlite. 
     
     
         8 . The flat steel product as claimed in  claim 1 , wherein the microstructure consists of more than 50 vol. % bainite and the remainder consists of ferrite. 
     
     
         9 . The flat steel product as claimed in  claim 8 , wherein the microstructure consists of more than 75 vol. % bainite and the remainder consists of ferrite. 
     
     
         10 . The flat steel product as claimed in  claim 1 , wherein the grain extension of all microstructure constituents in a position ½ thickness of the flat steel product and the aspect ratio of all microstructure constituents in the rolling direction of at most 2.0 and/or the mean value over the three regions near the surface, ¼ thickness and ½ thickness of the flat steel product is at most 2.0, and optionally 1.6. 
     
     
         11 . The flat steel product as claimed in  claim 1 , wherein half of the precipitations of (Ti, Nb, Mo)(C,N) and/or V(C,N) which harden the ferrite and the main constituent of bainitic ferrite have a diameter of less than 10 nm and/or the precipitations have an average spacing of less than 750 nm. 
     
     
         12 . The flat steel product as claimed in  claim 1 , wherein the ratio of shear texture components to rolling texture components increases towards the surface and has the following values:
 near the surface: min. 0.9; and   ½ thickness: max. 0.1.   
     
     
         13 . A method for producing a hot-rolled flat steel product with high local cold-formability, having a tensile strength Rm of at least 760 MPa, a yield strength ratio of at least 0.8 and a hole expansion ratio of above 30%, a measure of cold-formability of at least 0.12, and a ratio of local and global cold-formability of at least 5 and at most 13, said method comprising the steps of:
 melting a steel melt containing (in wt. %):   
       C: in a range of 0.04 to 0.08: 
       Si: in a range of 0.1 to 0.6; 
       Mn: in a range of 1.0 to 2.0; 
       P: max. 0.06; 
       S: max. 0.01; 
       N: max. 0.012; 
       Al: up to 0.06; 
       T i : up to 0.18 and/or Nb: up to 0.08; 
       Mo: up to 0.35; 
       with T i +Nb more than 0.06, and wherein a hyperstoichiometric ratio of carbon and nitrogen is present according to the following formula: 1.0<(C/12+N/14)/(T i /48+Nb/93+Mo/96) is set, with optional addition by alloying of one or more elements of Cr, Ni, V, B or Ca, with the remainder being iron including unavoidable steel-associated elements;
 casting the steel melt to form a slab or thin slab by means of a horizontal or vertical slab or thin slab casting process; 
 reheating the slab or thin slab to 1100° C. to 1270° C. and then hot-rolling the slab or thin slab with the following directly consecutive steps of: 
 rolling in the last rolling pass to a hot strip to the required final thickness at a final rolling temperature EWT, wherein the following applies:
   EWT≥EWTmin=682° C.+464 C+6445 Nb−644×Nb 0.5 +732 V−230 V 0.5 +890 T i +363 Al
 
 
 36 Si 
 cooling at an average cooling rate of 30 K/s to 150 K/s; 
 reeling the hot strip into a coil at a reeling temperature HT which is low enough to set the advantageous microstructure constituents with
   HT≤HTmax=761° C.−217×C−77×Mn+97×Si−47×Mo−53×Cr−34×Ni−21×V
 
 
 
       and on the other hand is suitable to provide sufficient precipitation-hardening in the subsequent cooling process T(t), defined by 
       17000≤HP<18800 with HP(T, t)=T(t)×(ln(t)+20), where T is indicated in K and t is indicated in h; and
 cooling in a cooling process T(t) at an average cooling rate of 5 K/h to 50 K/h between reeling temperature and 100° C., with subsequent cooling in still air to room temperature. 
 
     
     
         14 . The method as claimed in  claim 13 , wherein said hot-rolling the slab includes hot-rolling a flat steel product with a thickness of 1.6 mm to 6.0 mm. 
     
     
         15 . The method as claimed in  claim 13 , further comprising providing the flat steel product with a metallic coat electrolytically or by means of hot-dipping. 
     
     
         16 . The method as claimed in  claim 15 , wherein said providing the flat steel product comprises providing the flat steel product with a zinc-based metallic coat. 
     
     
         17 . The method as claimed in  claim 13  for producing a flat steel product as claimed in  claim 1 . 
     
     
         18 . A method of using a flat steel product as claimed in  claim 1  for producing a component in the automotive industry. 
     
     
         19 . The method as claimed in  claim 18  for producing a chassis component. 
     
     
         20 . The flat steel product according to  claim 1 , wherein the microstructure distribution over the thickness of the flat steel product in the three regions comprises:
 an absolute deviation of a maximum of 7 vol. % of the proportion of ferrite in the three regions; and/or   the deviation in aspect ratio of the grains in the rolling direction in the three regions of the flat steel product with respect to the mean value is less than 0.3 in each of the three regions; and/or   in the three regions the difference in hardness HV0.1 is in each case a maximum of 15 HV 0.1, and optionally a maximum of 10 HV 0.1 in comparison with the mean value over the entire thickness of the flat steel product.