Method for producing a steel strip with improved bonding of metallic hot-dip coatings

Abstract

A method for producing a steel strip containing, in addition to iron as the main component and unavoidable impurities, one or more of the following oxygen-affine elements in wt. %: Al: more than 0.02, Cr: more than 0.1, Mn: more than 1.3 or Si: more than 0.1, where the surface of the steel strip is cleaned, oxidation-treated and annealed. The treated and annealed steel strip is subsequently coated with a hot-dip coat. In order to be less cost-intensive and to achieve uniform, reproducible adhesion conditions for the coat, the steel strip is oxidation-treated prior to the annealing at temperatures below 200° C., where on the surface of the steel strip, with the formation of oxides with iron from the steel strip, an oxide layer is formed, which contains iron oxide and is reduction-treated during the course of the annealing under a reducing atmosphere to achieve a surface consisting substantially of metallic iron.

Claims

1. A method for producing a steel strip containing, in addition to iron as a main component and unavoidable impurities, one or more of the following oxygen-affine elements in wt. %: Al: 0.02 or more, Cr: 0.1 or more, Mn: 1.3 or more, and Si: 0.1 or more, the method comprising: cleaning a surface of the steel strip; oxidation-treating the steel strip at temperatures below 200° C., wherein on the surface of the steel strip, with the formation of oxides with iron from the steel strip, an oxide layer is formed which contains iron oxide; annealing the steel strip, wherein the oxide layer which contains iron oxide is reduction-treated during the annealing under a reducing atmosphere to achieve a surface of the steel strip consisting essentially of metallic iron; and coating the steel strip with a hot-dip coat; wherein the oxide layer formed on the surface of the steel strip has a minimum thickness of at least 5 nm and of up to 500 nm.

2. The method as claimed in claim 1, wherein the oxidation: treating takes place at temperatures below 150° C.

3. The method as claimed in claim 1, wherein the annealing takes place at temperatures of 660° C. to 880° C.

4. The method as claimed in claim 1, wherein the steel strip contains one or more of the following oxygen-affine elements in wt. %: Al: 0.02 to 15, Cr: 0.1 to 9, Mn: 1.3 to 35 and Si: 0.1 to 10.

5. The method as claimed in claim 4, wherein the steel strip contains one or more of the following oxygen-affine elements in wt. %: Al: 0.02 to 3, Cr: 0.2 to 1, Mn: 1.5 to 7, and Si: 0.15 to 3.

6. The method as claimed in claim 1, wherein the oxidation: treating is anodic oxidation.

7. The method as claimed in claim 1, wherein the oxidation treating is plasma oxidation or a wet-chemical method in media which give off oxygen.

8. The method as claimed in claim 1, wherein the oxide layer formed on the surface of the steel strip has a thickness of 10 nm to 200 nm.

9. The method as claimed in claim 8, wherein the oxide layer formed on the surface of the steel strip has a thickness of 30 nm to 150 nm.

10. The method as claimed in claim 6, wherein the anodic oxidation is performed at current densities between 50 and 400 A/dm.sup.2 and in a 20 to 60% NaOH solution or KOH solution at an electrolyte temperature of at least 45° C. to at most 3 K below a boiling temperature of the electrolyte.

11. The method as claimed in claim 1, wherein the annealing is performed in a continuous annealing furnace at an annealing temperature of 700° C. to 880° C. and a heating rate of 5 K/s to 100 K/s, with a reducing annealing atmosphere consisting of 2 to 30% H2 and 98 to 70% N2, and a dew point between +15 and −70° C., and a holding time of the steel strip at annealing temperature between 30 s and 650 s with subsequent cooling to a temperature between 400° C. and 500° C., and wherein the subsequent coating the steel strip comprises coating the steel strip with a metallic coat.

12. The method as claimed in claim 11, wherein the annealing temperature is 750 to 850° C., the heating rate is from 10 to 50 K/s, the annealing atmosphere has 2 to 10% H2, the remainder being N2, and a dew point between −10 to −50° C. and a holding time of the steel strip at annealing temperature of 60 to 180 s.

13. The method as claimed in claim 1, wherein coating the steel strip comprises coating the steel strip with a metallic coat, and wherein the metallic coat is chosen from at least one of: aluminum-silicon (AS/AlSi), zinc (Z), zinc-aluminum (ZA), zinc-aluminum-iron (ZF/galvannealed), zinc-magnesium-aluminum (ZM/ZAM), zinc-manganese-aluminum, and aluminum-zinc (AZ).

14. The method as claimed in claim 1, wherein the steel strip produced by the method is used for producing parts for motor vehicles or for producing press-form-hardened components of motor vehicles.

15. The method as claimed in claim 1, wherein the oxidation treatment takes place at temperatures below 135° C.

16. The method as claimed in claim 2, wherein the annealing takes place at temperatures of 660° C. to 880° C.

17. The method as claimed in claim 3, wherein the oxidation treating is anodic oxidation, and wherein the anodic oxidation is performed at current densities between 50 and 400 A/dm.sup.2 and in a 20 to 60% NaOH solution or KOH solution at an electrolyte temperature of at least 45° C. to at most 3 K below a boiling temperature of the electrolyte.

18. The method as claimed in claim 17, wherein the annealing is performed in a continuous annealing furnace at an annealing temperature of 700° C. to 880° C. and a heating rate of 5 K/s to 100 K/s, with a reducing annealing atmosphere consisting of 2 to 30% H2 and 98 to 70% N2, and a dew point between +15 and −70° C., and a holding time of the steel strip at annealing temperature between 30 s and 650 s with subsequent cooling to a temperature between 400° C. and 500° C., and wherein the subsequent coating the steel strip comprises coating the steel strip with a metallic coat.

19. The method as claimed in claim 18, wherein the metallic coat is chosen from at least one of: aluminum-silicon (AS/AlSi), zinc (Z), zinc-aluminum (ZA), zinc-aluminum-iron (ZF/galvannealed), zinc-magnesium-aluminum (ZM/ZAM), zinc-manganese-aluminum and aluminum-zinc (AZ).

20. The method as claimed in claim 5, wherein the steel strip contains one or more of the following oxygen-affine elements in wt. %: Al: 0.02 to 1, Cr: 0.3 to 1, Mn: 1.7 to 3, and Si: 0.15 to 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates a comparison of an Fe-GDOES spectrum of an anodized and subsequently reducingly annealed, non-galvanized steel sample of an HCT980XD against a spectrum of an untreated steel sample of the same grade;

(2) FIG. 2 is a schematic illustration of the formation of the internal and external oxides;

(3) FIG. 3 is a schematic illustration of an annealing procedure prior to the hot-dip finishing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) FIG. 1 illustrates an Fe-GDOES spectrum of an anodized and subsequently reducingly annealed, non-galvanized steel sample of an HCT980XD (annealing conditions: 830° C., 165 s, TP −30° C.) in comparison with an untreated steel sample of the same grade. On the steel sample which is anodized, in accordance with the invention, the near-surface iron proportion in the selected conditions is significantly higher in comparison with the untreated reference sample. On the sample anodized, in accordance with the invention, the previously formed iron oxide could be completely reduced in the given conditions, even the porous structure of the freshly anodized surface is no longer observed after the annealing process. In comparison with the reference, the adhesion of the coat is improved by the previous anodizing of the sample.

(5) The inventive formation of the internal and external oxides is illustrated schematically in FIG. 2. By means of the inventive anodizing with subsequent annealing in an HNx atmosphere, the formation of only a few globular external oxides is achieved. By virtue of the high proportion of metallic surface, a hot-dip finishing procedure can be performed without adversely affecting the adhesion and the surface look-and-feel. The reference process is shown in FIG. 3, which illustrates the schematic of a typical annealing procedure prior to the hot-dip finishing procedure with the formation of an almost covering external oxide layer. This disrupts the subsequent wetting to a considerable extent and results in non-galvanized locations and adhesion problems of the hot-dip coat.

(6) By reason of the increased porosity, which can be advantageously achieved during anodizing, in comparison with thermally produced oxide layers, layers produced by anodizing can then still be reduced in the annealing furnace even in the case of higher oxide layer applications.

(7) The hot-dip coated steel strips produced according to the method in accordance with the invention can be used preferably, but not restrictively, for producing parts for motor vehicles, such as for producing cold-formed, hot-formed or press-form-hardened components. Basically, the following are considered as coatings for the steel strips: aluminium-silicon (AS/AlSi), zinc (Z), zinc-aluminium (ZA), zinc-aluminium-iron (ZF/galvannealed), zinc-magnesium-aluminium (ZM/ZAM) or zinc-manganese-aluminium and aluminium-zinc (AZ).

(8) In summary, when the method in accordance with the invention is applied, the following advantages are to be noted: improvement in galvanizing capability in particular in the case of an increased alloy content improvement in surface quality visually and in terms of surface defects. the development of new alloying concepts is accompanied by the mechanical-technological properties of the material and also by requirements of a subsequent coating. If the steel strip is to be hot-dip finished e.g. in a continuous method after annealing, then even in alloy development it is necessary to take into consideration that wettability must be present. The method in accordance with the invention allows a higher degree of freedom to be achieved in alloy development. As a result, costs can be saved in alloying or improved mechanical-technological properties can be achieved. possibility of measuring the oxide layer thickness prior to the annealing treatment homogeneous deposition of the oxide layer over the length and width of the strip possibility of rapid and automatic adaptation of the anodizing parameters in the event of drops in speed and a change in quality the emission ratio of the steel strip can be increased by the anodizing prior to the annealing process. Higher heating rates in the furnace result from this. It then possible to increase the strip speed for the same furnace length.