METHOD FOR 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 ZnMgAl 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-roiled strip and that are resistant to corrosive and abrasive influences.

Claims

1. A method for producing a hot strip of bainitic multi-phase steel having a ZnMgAl coating, said method comprising: smelting a steel melt consisting of (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 rolling 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.

2. The method of claim 1, wherein the precursor material is a slab or a block or a thin slab.

3. The method of claim 1, wherein the hot strip is heated to a temperature of less than Ac1+50 C.

4. The method of claim 1, 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.

5. The method of claim 1, 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.

6. The method of claim 1, 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.

7. The method of claim 1, 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.

8. The method of claim 1, wherein a sum of the Ti and Mo contents is >0.1 weight percent.

9. The method of claim 1, 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.

10. The method of claim 1, wherein in the zinc alloy melt bath a magnesium content is less than an aluminium content.

11. The method of claim 1, wherein the zinc ahoy 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.

12. The method of claim 1, wherein the hot-dip coated hot strip has a tensile strength Rm of 780 to 980 MPa.

13. The method of claim 1, wherein the hot-dip coated hot strip has a yield strength ReH of at least 680 MPa.

14. The method of claim 1, 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%.

Description

BRIEF DESCRIPTION OF THE FIGURES

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DESCRIPTION OF PREFERRED EMBODIMENTS

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