Method of producing a hot strip of a bainitic multi-phase steel having a zn-mg-al coating, and a corresponding hot strip

Abstract

The invention relates to a method for producing a hot-rolled strip composed of a bainitic multi-phase steel and having a Zn—Mg—Al coating, comprising the following steps: melting a steel melt containing (in weight percent): C: 0.04-0.11, Si: <=0.7, Mn: 1.4-2.2, Mo: 0.05-0.5, Al: 0.015-0.1, P: up to 0.02, S: up to 0.01, B: up to 0.006, and at least one element from the group Nb, V, Ti in accordance with the following condition: 0.02<=Nb+V+Ti<=0.20, the remainder being iron including unavoidable steel-accompanying elements resulting from the melting process, casting the steel melt into a preliminary material, in particular a slab or a block or a thin slab, hot rolling the preliminary material into a hot-rolled strip having a final rolling temperature in the range of 800 to 950° C., cooling the hot-rolled strip to a winding temperature less than 650° C., winding the hot-rolled strip at a winding temperature less than 650° C., cooling the wound hot-rolled strip to room temperature in still air, wherein the microstructure of the wound hot-rolled strip then has a bainite fraction greater than 50% after the hot rolling, heating the hot-rolled strip to a temperature greater than 650° C. and less than Ac3, in particular less than Ac1+50° C., cooling the hot-rolled strip to zinc bath temperature, hot-dip coating the heated hot-rolled strip in a zinc alloy molten bath containing (in weight percent): Al: 1.0-2.0, Mg: 1.0-2.0, the remainder being zinc and unavoidable impurities. The invention further relates to the hot-rolled strip produced in accordance with the method above and to shaped, dynamically highly loadable components, in particular motor vehicle parts, that are produced from said hot-rolled strip and that are resistant to corrosive and abrasive influences.

What is claimed is: 
     
         1 .- 14 . (canceled) 
     
     
         15 . A method for producing a hot strip of bainitic multi-phase steel having a Zn—Mg—Al coating, said method comprising:
 smelting a steel melt containing (in weight percent) 
 C: 0.04-0.11 
 Si: <=0.7 
 Mn: 1.4-2.2 
 Mo: 0.05-0.5 
 Al: 0.015-0.1 
 P: up to 0.02 
 S: up to 0.01 
 B up to 0.006 
 and at least one element from the group Nb, V, Ti according to the following condition: 
 0.02<=Nb+V+Ti<=0.20 
 with a remainder being iron and unavoidable elements being steel-associated or a result of smelting; 
 casting the steel melt to form a precursor material; 
 hot roiling the precursor material to form a hot strip having an end rolling temperature in a range of 800 to 950° C.; 
 cooling the hot strip to a reeling temperature of less than 650° C.; 
 reeling the hot strip at a reeling temperature of less than 650° C.; 
 cooling the reeled hot strip to room temperature in stationary air, with reeled hot strip after hot rolling having a microstructure with a bainite content of greater than 50%, 
 heating the hot strip to a temperature of greater than 650° C. and less than Ac3; 
 cooling the hot strip to zinc bath temperature, 
 hot-dip coating the heated hot strip in a zinc alloy melt bath containing (in weight percent) 
 Al: 1.0-2.0 
 Mg: 1.0-2.0 
 with the remainder being zinc and unavoidable impurities. 
 
     
     
         16 . The method of  claim 15 , wherein the precursor material is a slab or a block or a thin slab. 
     
     
         17 . The method of  claim 15 , wherein the hot strip is heated to a temperature of less than Ac1+50° C., 
     
     
         18 . The method of  claim 15 , wherein annealing and heating the hot strip at a temperature of greater than 650° C. and less than Ac3, in particular less than Ac1+50° C., take place in one working step and the hot strip is hot-dip coated immediately after the heating and cooling to zinc bath temperature. 
     
     
         19 . The method of  claim 15 , wherein the hot strip is hot-dip coated in a zinc alloy melt bath at a bath temperature of 405 to 470° C., preferably 410 to 430° C. 
     
     
         20 . The method of  claim 15 , wherein the steel melt has a C content of 0.06 to 0.10 weight percent, an Si content of 0.05 to 0.50 weight percent, and a total of the contents of Nb+V+Ti is in a range of 0.05 to 0.20 weight percent. 
     
     
         21 . The method of  claim 15 , wherein the content of each of the alloying elements from the group Nb, V, TI in the steel melt is at least 0.005 weight percent. 
     
     
         22 . The method of  claim 15 , wherein a sum of the Ti and Mo contents is >0.1 weight percent. 
     
     
         23 . The method of  claim 15 , wherein the zinc alloy melt bath has a magnesium content in a range of 1.0 to 2.0 weight percent, preferably 1.4 to 1.8 weight percent, and an aluminium content in a range of 1.0 to 2.0 weight percent, preferably 1.4 to 1.8 weight percent, with the magnesium content and the aluminium content being identical to each other. 
     
     
         24 . The method of  claim 15 , wherein in the zinc alloy melt bath a magnesium content is less than an aluminium content. 
     
     
         25 . The method of  claim 24 , wherein the zinc alloy melt bath has a magnesium content in a range of 1.0 to 2.0 weight percent, preferably 1.0 to 1.2 weight percent, and an aluminium content in a range of 1.0 to 2.0 weight percent, preferably 1.3 to 1.7 weight percent. 
     
     
         26 . The method of  claim 15 , wherein the hot-dip coated hot strip has a tensile strength Rm of 780 to 980 MPa. 
     
     
         27 . The method of  claim 15 , wherein the hot-dip coated hot strip has a yield strength ReH of at least 680 MPa. 
     
     
         28 . The method of  claim 15 , wherein the hot-dip coated hot strip has an elongation at fracture A pursuant to DIN EN ISO 6892-1:2009 of at least 10%. 
     
     
         29 . A hot strip comprising a bainitic multi-phase steel having a Zn—Mg—Al coating, said hot strip being produced by
 smelting a steel melt containing (in weight percent) 
 C: 0.04-0.11 
 Si: <=0.7 
 Mn: 1.4-2.2 
 Mo: 0.05-0.5 
 Al: 0.015-0.1 
 P: up to 0.02 
 S: up to 0.01 
 B up to 0.006 
 and at least one element from the group Nb, V, Ti according to the following condition: 
 0.02<=Nb+V+Ti<=0.20 
 with the remainder being iron and unavoidable elements being steel-associated or a result of smelting, and 
 hot-dip coating in a zinc alloy melt bath containing (in weight percent) 
 Al: 1.0-2.0 
 Mg: 1.0-2.0 
 with the remainder being zinc and unavoidable impurities, said hot strip having a microstructure after undergoing hot rolling with a bainite content of greater than 50%. 
 
     
     
         30 . The hot strip of  claim 29  for producing a deformed component of substantial dynamic stress resistance and resistance to corrosive and abrasive effects, in particular a motor vehicle part, in particular with complex component geometry, such as e.g. spring strut or undercarriage connecting rod. 
     
     
         31 . The hot strip of  claim 29 , wherein the steel melt has a C content of 0.06 to 0.10 weight percent, an Si content of 0.05 to 0.50 weight percent, and a total of the contents of Nb+V 30 Ti is in a range of 0.05 to 0.20 weight percent. 
     
     
         32 . The hot strip of  claim 29 , wherein the content of each of the alloying elements from the group Nb, V, Tl in the steel melt is at least 0.005 weight percent. 
     
     
         33 . The hot strip of  claim 29 , wherein a sum of the Ti and Mo contents is >0.1 weight percent. 
     
     
         34 . The hot strip of  claim 29 , wherein the zinc alloy melt bath has a magnesium content in a range of 1.0 to 2.0 weight percent, preferably 1.4 to 1.8 weight, and an aluminium content in a range of 1.0 to 2.0 weight percent, preferably 1.4 to 1.8 weight, with the magnesium content and the aluminium content being identical to each other. 
     
     
         35 . The hot strip of  claim 29 , wherein in the zinc alloy melt bath a magnesium content is less than an aluminium content. 
     
     
         36 . The hot strip of  claim 35 , wherein the zinc alloy melt bath has a magnesium content in a range of 1.0 to 2.0 weight percent, preferably 1.0 to 1.2 weight percent, and an aluminium content in a range of 1.0 to 2.0 weight percent, preferably 1.3 to 1.7 weight percent. 
     
     
         37 . The hot strip of  claim 29 , wherein the hot-dip coated hot strip has a tensile strength Rm of 780 to 980 MPa. 
     
     
         38 . The hot strip of  claim 29 , wherein the hot-dip coated hot strip has a yield strength ReH of at least 680 MPa. 
     
     
         39 . The hot strip of  claim 29 , wherein the hot-dip coated hot strip has an elongation at fracture A pursuant to DIN EN ISO 6892-1:2009 of at least 10%.