COMPONENT MADE OF PRESS-FORM-HARDENED, ALUMINUM-BASED COATED STEEL SHEET, AND METHOD FOR PRODUCING SUCH A COMPONENT

Abstract

In a component made of press-form-hardened, aluminium-based coated steel sheet, the coating has a covering which contains aluminum and silicon applied in the hot-dip process. The press-form-hardened component in the transition region between steel sheet and covering has an inter-diffusion zone I, wherein, depending on the layer application of the covering before heating and press hardening, the thickness of the inter-diffusion zone I obeys the following formula: I [m]<( 1/35)application on both sides [g/m.sup.2]+( 19/7). Formed on the inter-diffusion zone I is a zone having various intermetallic phases having an average total thickness between 8 and 50 m, on which zone there is in turn arranged a covering layer containing aluminum oxide and/or hydroxide having an average thickness of at least 0.05 m to at most 5 m.

Claims

1.-14. (canceled)

15. A component of press-form-hardened steel sheet, said component comprising: an aluminium-based coating applied upon the press-form-hardened steel sheet; a coat containing aluminium and silicon, said coat being applied upon the coating in a hot-dipping process; an inter-diffusion zone formed in a transition region between the steel sheet and the coat at a thickness which is defined in dependence on a layer support of the coat prior to heating and press-hardening and satisfies the following formula:
I [m]< 1/35support on both sides [g/m.sup.2]+ 19/7 wherein I is the inter-diffusion zone; a zone formed on the inter-diffusion zone and having different intermetallic phases with an average overall thickness between 8 and 50 m; and a cover layer arranged on the zone and containing aluminium oxide and/or aluminium hydroxide at an average thickness of at least 0.05 m to at most 5 m.

16. The component of claim 15, wherein, depending upon a current layer support of a starting material, the thickness of the inter-diffusion zone is formed in accordance with the following formula
I [m]< 1/35support on both sides [g/m.sup.2]+ 5/7.

17. The component of claim 15, wherein, depending upon a current layer support of a starting material, the thickness of the inter-diffusion zone is formed in accordance with the following formula
I [m]< 1/35support on both sides [g/m.sup.2] 2/7

18. The component of claim 15, wherein the cover layer has an average layer thickness of at least 0.10 m and at most 3.0 m.

19. The component of claim 15, wherein the cover layer has an average layer thickness of at least at least 0.15 m and at most 1.0 m.

20. The component of claim 15, wherein the coat has an overall porosity of less than 6%.

21. The component of claim 15, wherein the coat has an overall porosity of less than 4%.

22. The component of claim 15, wherein the coat has an overall porosity of less than 2%.

23. The component of claim 15, wherein the coat is produced in a melting bath with an Si content of 8 to 12 wt. %, an Fe content of 1 to 4 wt. %, with the remainder being aluminium and unavoidable impurities.

24. A method for producing a component from a press-form-hardened steel sheet or steel strip, said method comprising: applying an aluminium-based coat as coating on the steel sheet or steel strip in a hot-dipping process; subjecting the steel sheet or steel strip with the coat, after the hot-dipping process and before a forming process, to a treatment by anodic oxidation and/or plasma oxidation and/or hot water treatment and/or treatment in an atmosphere containing at least variable proportions of oxygen and steam, with the hot water treatment or the treatment with steam being performed at a temperature of at least 90 C.; forming a cover layer containing aluminium oxide and/or aluminium hydroxide at a thickness of at least 0.05 m to at most 5 m during surface treatment of the coat by forming oxides or hydroxides; heating at least one section of the steel sheet or steel strip to a temperature above the austenitization temperature; forming the heated steel sheet or steel strip; and cooling the steel sheet or steel strip at a rate which in at least one section is above a critical cooling rate.

25. The method of claim 24, wherein the temperature for the hot water treatment or the treatment with steam is at least 95 C.

26. The method of claim 24, wherein the cover layer is applied onto the surface of the coat in a continuous process.

27. The method of claim 24, further comprising: forming an inter-diffusion zone in a transition region between the steel sheet or steel strip and the coat, wherein, depending upon a current layer support of a starting material, the inter-diffusion zone has a thickness in accordance with the following formula
I [m]< 1/35support on both sides [g/m.sup.2]+ 19/7 wherein I is the inter-diffusion zone; and forming a zone having different intermetallic phases with a thickness between 8 and 50 m.

28. The method of claim 27, wherein, depending upon a current layer support of a starting material, the thickness of the inter-diffusion zone is formed in accordance with the following formula
I [m]< 1/35support on both sides [g/m.sup.2]+ 5/7

29. The method of claim 27, wherein, depending upon a current layer support of a starting material, the thickness of the inter-diffusion zone is formed in accordance with the following formula
I [m]< 1/35support on both sides [g/m.sup.2]+ 2/7

30. The method of claim 24, wherein the atmosphere further contains proportions of basic components selected from the group consisting of ammonia (NH.sub.3), of primary, secondary and tertiary aliphatic amines (NH.sub.2R, NHR.sub.2).

31. The method of claim 24, for use of the component in the production of a motor vehicle.

32. The method of claim 24, wherein the press-form-hardened steel sheet or steel strip is suitable for being lacquered and for resistance spot welding.

Description

[0049] The invention will be described in more detail hereinafter with the aid of the illustrated figures.

[0050] FIG. 1 schematically illustrates the layer structure of the coating on a press-form-hardened component having a coating of AS and typical long heating time to achieve a thorough alloying of the coat with iron, in accordance with the prior art. For the component, a steel sheet having a coat of AS150, i.e., with a layer support of the coat of 150 g/m.sup.2, was used. Formed on the martensitic steel base material is an inter-diffusion zone Fe(Al,Si) having a thickness of 7 to 14 m, on which a zone having different intermetallic phases (e.g. Fe.sub.2SiAl.sub.2 and FeAl.sub.2) has been formed, wherein the individual phases in this zone can occur distributed in the form of lines or also clusters. By way of the oxidation in the furnace and during transfer into the press, only a very thin aluminium oxide layer having a thickness of less than 0.05 m was formed. Pores which have formed in the different zones can also be seen.

[0051] In comparison thereto, FIG. 2 illustrates the layer structure of a coating in accordance with the invention on a press-form-hardened component having an AS coating on which a cover layer in accordance with the invention containing aluminium oxide and/or aluminium hydroxide of at least 0.05 m is formed and which was produced with reduced heating times compared with the prior art. In the transition region between the steel sheet and the coating an inter-diffusion zone is formed in which aluminium and silicon have diffused into the steel Fe(Al,Si). Owing to the only very short heating time required in the furnace to austenitization temperature, this layer has e.g for AS150 a thickness of less than 7 m on average. Formed on this layer during heating is a further layer having different intermetallic phases (e.g. Fe.sub.2SiAl.sub.2 and FeAl.sub.2), wherein the individual phases in this zone can occur distributed in the form of lines or also clusters, on which a cover layer containing aluminium oxide and/or aluminium hydroxide having an average thickness of at least 0.05 m to at most 5 m is arranged.

[0052] FIG. 3 shows a graph of the thickness I in accordance with the invention of the inter-diffusion zone for a layer support of the starting material between 50 g/m.sup.2 and 180 g/m.sup.2 in accordance with the following relationship:

I [m]< 1/35support on both sides [g/m.sup.2]+ 19/7

[0053] Table 1 summarises experiments for lacquer-bonding (phosphatising treatment, typical for automobiles, and cathodic dip coating; testing after 72 hours, constant condensation-water atmosphere as per DIN EN ISO 6270-2:2005 CH) and welding suitability (resistance spot welding) of press-hardened AS150 samples at 940 C. furnace temperature and different heating times. The sheet thickness of the samples is 1.5 mm. It can be seen that a good lacquer-bonding and welding suitability are only produced at heating times of 220 s and lower if a cover layer in accordance with the invention containing aluminium oxide and/or aluminium hydroxide is provided. At short heating times of 220 s and lower, inter-diffusion layers of less than 7 m are also produced on the press-hardened component. In contrast, at the long heating times of 360 s which are part of the prior art and not in accordance with the invention, a good lacquer-bonding and welding suitability are also produced in the samples without the cover layer in accordance with the invention containing aluminium oxide and/or aluminium hydroxide, owing to the thorough alloying of the coat with iron. The thickness of the inter-diffusion layers is clearly above 7 m after a heating time of 360 s.

TABLE-US-00001 TABLE 1 Thickness In Cathodic of the accordance Thick- Cover Furnace Furnace Welding dip diffusion with the No. Material ness Support layer temperature dwell time area coating layer invention 1 22MnB5 1.5 mm AS150 No 940 C. 150 s not okay not okay <7 m No 2 22MnB5 1.5 mm AS150 Deposition 940 C. 150 s >1 kA okay <7 m Yes time a (okay) 3 22MnB5 1.5 mm AS150 Deposition 940 C. 150 s >1 kA okay <7 m Yes time b (okay) 4 22MnB5 1.5 mm AS150 Deposition 940 C. 150 s >1 kA okay <7 m Yes time c (okay) 5 22MnB5 1.5 mm AS150 No 940 C. 180 s not okay not okay <7 m No 6 22MnB5 1.5 mm AS150 Deposition 940 C. 180 s >1 kA okay <7 m Yes time a (okay) 7 22MnB5 1.5 mm AS150 Deposition 940 C. 180 s >1 kA okay <7 m Yes time b (okay) 8 22MnB5 1.5 mm AS150 Deposition 940 C. 180 s >1 kA okay <7 m Yes time c (okay) 9 22MnB5 1.5 mm AS150 No 940 C. 220 s not okay not okay <7 m No 10 22MnB5 1.5 mm AS150 Deposition 940 C. 220 s >1 kA okay <7 m Yes time a (okay) 11 22Mn85 1.5 mm AS150 Deposition 940 C. 220 s >1 kA okay <7 m Yes time b (okay) 12 22MnB5 1.5 mm AS150 Deposition 940 C. 220 s >1 kA okay <7 m Yes time c (okay) 13 22MnB5 1.5 mm AS150 No 940 C. 360 s >1 kA okay >7 m No (okay) 14 22MnB5 1.5 mm AS150 Deposition 940 C. 360 s >1 kA okay >7 m No time a (okay) 15 22MnB5 1.5 mm AS150 Deposition 940 C. 360 s >1 kA okay >7 m No time b (okay) 16 22MnB5 1.5 mm AS150 Deposition 940 C. 360 s >1 kA okay >7 m No time c (okay)