FLAT STEEL PRODUCT HAVING AN IMPROVED ZINC COATING

Abstract

The present disclosure relates to a flat steel product comprising a steel substrate with, present at least on one side of the steel substrate, anticorrosion coating composed of zinc and unavoidable impurities. This anticorrosion coating has continuous microchannels which connect the steel substrate to an ambient atmosphere. Additionally, the present disclosure relates to a process for producing a flat steel product of this kind.

Claims

1-14. (canceled)

15. A flat steel product comprising: a steel substrate having, present at least on one side of the steel substrate, anticorrosion coating composed of zinc and unavoidable impurities, wherein the anticorrosion coating has continuous microchannels which connect the steel substrate to an ambient atmosphere.

16. The flat steel product as claimed in claim 15, wherein the microchannels have a density which is greater than 50 mm.sup.1.

17. The flat steel product as claimed in claim 16, wherein the microchannels have a density which is greater than 100 mm.sup.1.

18. The flat steel product as claimed in claim 3 wherein the microchannels run substantially perpendicular to the surface of the steel substrate.

19. The flat steel product as claimed in claim 19 wherein the microchannels have an angular distribution having a full width at half-maximum of more than 30.

20. The flat steel product as claimed in claim 19 wherein the anticorrosion coating has a thickness d of 5-10 m.

21. The flat steel product as claimed in claim 20 wherein the anticorrosion coating has a blocking effect for hydrogen permeation which is not more than 80%.

22. The flat steel product as claimed in claim 21 wherein the anticorrosion coating has a hydrogen permeation time which is less than 500 s.

23. The flat steel product of claim 22 wherein the anticorrosion coating has a hydrogen permeation time which is less than 150 s.

24. The flat steel product as claimed in claim 23 wherein the anticorrosion coating is applied by physical vapor deposition.

25. The flat steel product as claimed in claim 24 wherein the steel substrate has a tensile strength of more than 1200 MPa.

26. The flat steel product as claimed in claim 25 wherein the steel substrate is a multiphase steel.

27. The flat steel product as claimed in claim 26 wherein the steel substrate is one of a cold-rolled and hot-rolled multiphase steel.

28. A process for producing a flat steel product as claimed in claim 27, with steps as follows: producing or providing a steel substrate; degreasing; pickling; and applying the anticorrosion coating composed of zinc and unavoidable impurities to the steel substrate by means of physical vapor deposition; where the anticorrosion coating has a thickness d and the ratio of thickness of the anticorrosion coating d to the coating rate r on application of the anticorrosion coating is less than 800 s.

29. The process as claimed in claim 28 wherein the temperature of the steel substrate on application of the anticorrosion coating is greater than 150 C.

30. The process as claimed in claim 28 wherein the anticorrosion coating composed of zinc and unavoidable impurities is applied to the steel substrate by means of physical vapor deposition by providing the steel substrate in a coating chamber, where the pressure in the coating chamber is regulated and where zinc as coating material is caused to flow into the coating chamber at an inflow point, the zinc being conditioned to a temperature.

31. The process as claimed in claim 30 wherein pressure and temperature are adjusted such that the temperature is above the dew point of the coating material and the pressure to between 1 mbar and 100 mbar.

32. The process as claimed in claim 31 wherein pressure and temperature are adjusted such that the temperature is above the dew point of the coating material and the pressure to between 10 mbar and 100 mbar.

Description

[0107] The invention is elucidated in more detail using the figures, in which

[0108] FIG. 1 shows a schematic representation of a flat steel product having an anticorrosion coating;

[0109] FIG. 2 shows an image of a polished section of a flat steel product having an anticorrosion coating;

[0110] FIG. 3 shows a schematic detail of a microchannel;

[0111] FIG. 4 shows a schematic detail of a microchannel;

[0112] FIG. 5 shows an angular distribution of the microchannels.

[0113] FIG. 1 shows a schematic representation of a flat steel product 13. The flat steel product 13 comprises a steel substrate 15 and an anticorrosion coating 17 present on one side of the steel substrate 15. The anticorrosion coating 17 consists of zinc and unavoidable impurities. The anticorrosion coating 17 has continuous microchannels 19 which connect the steel substrate 15 to an ambient atmosphere 21. (For greater clarity, of the 18 microchannels represented, only the microchannel disposed on the far right is provided with a reference sign.) FIG. 2 shows a perpendicular polished section of the flat steel product 13. This is the exemplary embodiment with number 10 in table 1, which is elucidated below. The flat steel product 13 comprises a steel substrate 15 composed of a cold-rolled multiphase steel having the analysis A as indicated below: [0114] C: 0.11 wt % [0115] Si: 0.43 wt % [0116] Mn: 2.44 wt % [0117] Optionally one or more of the following elements: [0118] P: 0.01 wt % [0119] S: 0.002 wt % [0120] Al: 0.03 wt % [0121] Cr: 0.62 wt % [0122] Cu: 0.05 wt % [0123] Mo: 0.07 wt % [0124] N: 0.004 wt % [0125] Ni 0.05 wt % [0126] Nb: 0.038 wt % [0127] Ti: 0.022 wt % [0128] V: 0.007 wt % [0129] B: 0.0013 wt % [0130] Sn: 0.02 wt % [0131] Ca: 0.002 wt % [0132] Balance iron and unavoidable impurities.

[0133] The flat steel product 13 additionally comprises an anticorrosion coating 17 present on one side of the steel substrate 15. The anticorrosion coating 17 has a thickness d of 9 m and consists of zinc and unavoidable impurities. The anticorrosion coating 17 has continuous microchannels 19 which connect the steel substrate 15 to an ambient atmosphere 21. (For greater clarity, here again, only one of the microchannels is provided with a reference sign.) In the image detail shown, there are about 27 microchannels, corresponding to a density of 29 channels per 100 m or 290 mm{circumflex over ()}1.

[0134] The table below shows a number of exemplary embodiments and the process parameters associated with their production. For all of the samples, moreover, the coat adhesion was determined by means of the SEP1931 ball impact test. If flaking of the coat occurred during the ball impact test, the coat adhesion was classified as unsatisfactory (not OK). In the cases without flaking, the coat adhesion was classified as satisfactory (OK).

[0135] In all of the exemplary embodiments, the substrate used was a steel blank having a thickness of 1.8 mm. This steel blank consisted of a steel having the analysis indicated with reference to FIG. 2.

[0136] Example 1 is the reference sample used for determining the blocking effect S. Samples 1-10 were coated by means of vapor deposition (PVD). In the case of examples 2-8, an electron beam evaporator was used in order to melt and evaporate the zinc coating material. In the case of exemplary embodiments 9 and 10, the zinc coating material was melted and evaporated by means of an electric arc. Examples 2-5 were coated at a substrate temperature of room temperature (i.e., less than 50 C.). In these cases, anticorrosion coatings having a different thickness between 0.5 m and 12 m were generated. In all four cases, the coat adhesion was inadequate. In the case of examples 6 to 8, the substrate was preconditioned to a temperature of 200 C. With a coating rate of 8 nm/s, coat thicknesses of between 1 and 8 m were generated. Samples 6 and 7 exhibit not only good coat adhesion but also good hydrogen permeability. In the case of samples 9 and 10, the substrate was preconditioned to a temperature of 240 C. With a significantly higher coating rate of 7000 nm/s and 10 000 nm/s, respectively, coat thicknesses of 6.5 m and 9 m were generated. Samples 9 and 10 exhibit not only good coat adhesion but also good hydrogen permeability. As a comparison, samples 11 and 12 were galvanized electrolytically. Sample 12, moreover, was thermally aftertreated by being held at a temperature of 2000 for 60 minutes in a protective gas atmosphere. In both cases, the blocking effect S found is extremely high, and so hydrogen introduced remains in the substrate. These samples are therefore susceptible to hydrogen embrittlement.

TABLE-US-00002 TABLE 1 Coating transient time Coating relative to Substrate Thickness Rate r uncoated Blocking Ratio No. Type temperature/ C. d [m] [nm/s] substrate/s effect S [%] Adhesion d/r 1 uncoated 0 0 2 PVD <50 0.5 8 0 30 not OK 62.50 3 PVD <50 2.5 8 0 47 not OK 312.50 4 PVD <50 7 8 90 60 not OK 875.00 5 PVD <50 12 8 130 74 not OK 1500.00 6 PVD 200 1 8 0 18 OK 125.00 7 PVD 200 5 8 100 80 OK 625.00 8 PVD 200 8 8 >80 000 >95 OK 1000.00 9 PVD 240 6.5 7000 40 57 OK 0.93 10 PVD 240 9 10 000 40 60 OK 0.90 11 ZE 55 8 200 8000 >95 OK 40.00 as deposited 12 ZE-thermally 55 8 200 13 000 >95 OK 40.00 aftertreated

[0137] FIG. 3 shows a schematic detail of a microchannel 19 within an anticorrosion coating 17. The microchannel 19 connects the steel substrate 15 to the ambient atmosphere. The microchannel 19 runs substantially perpendicular to the surface 23 of the steel substrate 15. The bottom third of the microchannel 19 represented runs at an angle of 110 to the surface 23 of the steel substrate 15. This is shown by the angle 25 between the surface 23 of the steel substrate 15 and the tangent 27. The tangent 27 is fitted to the profile of the microchannel 19 in the bottom third. In the further profile, the microchannel makes a right curve and runs initially almost perpendicularly and then at an angle of about 70 to the surface 23 of the steel substrate 15, before the microchannel 19 in the manner of a funnel expands in the manner of a funnel to the surface of the anticorrosion coating. The profile in the region of the funnellike expansion is again almost perpendicular to the surface 23 of the steel substrate 15. In analogy to the bottom third, the respective angle is determined by fitting a tangent and determining the angle of the tangent to the surface 23. For greater clarity, only the tangent 27 fitted to the profile in the bottom third is represented.

[0138] FIG. 4 shows a schematic detail of a microchannel 19 within an anticorrosion coating 17. The microchannel 19 connects the steel substrate 15 to the ambient atmosphere. The microchannel 19 has an angle 31 of inclination. The angle 31 of inclination is determined by ascertaining the centerpoint of the near-substrate end of the microchannel 19 and connecting it to the centerpoint of the substrate-remote end of the microchannel 19. The angle of this connecting line 19 to the substrate surface 23 is referred to as the angle 31 of inclination of the microchannel 19.

[0139] FIG. 5 shows an angular distribution of the microchannels in the form of a histogram of the angles of inclination. The number of angles of inclination determined within the respective range is plotted. The step width chosen has been 5. A total of 930 microchannels have been evaluated. The full width at half-maximum is 42.