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 [μm]<(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 μm, 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 μm to at most 5 μm.

Claims

1. A component of press-form-hardened steel sheet, said component comprising: an aluminium-based coating upon the press-form-hardened steel sheet; the coating comprising a coat containing aluminium and silicon, said coat being applied upon the steel sheet 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 presshardening and satisfies the following formula:
I [μm]<1/35×layer support on both sides [g/m.sup.2]+19/7, wherein I is the inter-diffusion zone; wherein the layer support is between 50 g/m and 180 g/m.sup.2; a zone formed on the inter-diffusion zone and having different intermetallic phases with an average overall thickness between 8 and 50 μm; 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 μm to at most 5 μm.

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

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

4. The component of claim 1, wherein the cover layer has an average layer thickness of at least 0.10 μm and at most 3.0 μm.

5. The component of claim 1, wherein the cover layer has an average layer thickness of at least 0.15 μm and at most 1.0 μm.

6. The component of claim 1, wherein the coat has an overall porosity of less than 6%.

7. The component of claim 1, wherein the coat has an overall porosity of less than 4%.

8. The component of claim 1, wherein the coat has an overall porosity of less than 2%.

9. The component of claim 1, 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.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The invention will be described in more detail hereinafter with the aid of the illustrated figures. It is shown in:

(2) FIG. 1 illustrates a layer structure of a coating on a press-form-hardened component having a coating of AS and typical long heating time;

(3) FIG. 2 illustrates a layer structure of a coating in accordance with the invention; and

(4) FIG. 3 shows a graph of the thickness I in accordance with the invention of an inter-diffusion zone for a layer support of a starting material.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(5) 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 μm, 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 μm was formed. Pores which have formed in the different zones can also be seen.

(6) 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 μm 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 μm 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 μm to at most 5 μm is arranged.

(7) 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 [μm]<1/35×support on both sides [g/m.sup.2]+19/7

(8) 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 μm 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 μm after a heating time of 360 s.

(9) 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 μm No 2 22MnB5 1.5 mm AS150 Deposition 940° C. 150 s >1 kA okay <7 μm Yes time a (okay) 3 22MnB5 1.5 mm AS150 Deposition 940° C. 150 s >1 kA okay <7 μm Yes time b (okay) 4 22MnB5 1.5 mm AS150 Deposition 940° C. 150 s >1 kA okay <7 μm Yes time c (okay) 5 22MnB5 1.5 mm AS150 No 940° C. 180 s not okay not okay <7 μm No 6 22MnB5 1.5 mm AS150 Deposition 940° C. 180 s >1 kA okay <7 μm Yes time a (okay) 7 22MnB5 1.5 mm AS150 Deposition 940° C. 180 s >1 kA okay <7 μm Yes time b (okay) 8 22MnB5 1.5 mm AS150 Deposition 940° C. 180 s >1 kA okay <7 μm Yes time c (okay) 9 22MnB5 1.5 mm AS150 No 940° C. 220 s not okay not okay <7 μm No 10 22MnB5 1.5 mm AS150 Deposition 940° C. 220 s >1 kA okay <7 μm Yes time a (okay) 11 22Mn85 1.5 mm AS150 Deposition 940° C. 220 s >1 kA okay <7 μm Yes time b (okay) 12 22MnB5 1.5 mm AS150 Deposition 940° C. 220 s >1 kA okay <7 μm Yes time c (okay) 13 22MnB5 1.5 mm AS150 No 940° C. 360 s >1 kA okay >7 μm No (okay) 14 22MnB5 1.5 mm AS150 Deposition 940° C. 360 s >1 kA okay >7 μm No time a (okay) 15 22MnB5 1.5 mm AS150 Deposition 940° C. 360 s >1 kA okay >7 μm No time b (okay) 16 22MnB5 1.5 mm AS150 Deposition 940° C. 360 s >1 kA okay >7 μm No time c (okay)