METHOD FOR PRODUCING A PRESS-HARDENED SHEET STEEL PART HAVING AN ALUMINIUM-BASED COATING, INITIAL SHEET METAL BLANK, AND A PRESS-HARDENED SHEET STEEL PART MADE THEREFROM

Abstract

A method for producing a press-mold-hardened part includes providing a steel strip having an aluminium-based coating; applying an inorganic, iron-containing conversion layer to the aluminium-based coating with a layer weight in relation to iron of 3-30 mg/m2; cold-rolling the steel strip to form a flexibly rolled strip with strip sections of different sheet thickness; cutting an initial sheet metal blank out of the flexibly rolled strip, with the blank having different sheet thicknesses with thinnest and thickest sheet sections; press-mold-hardening the initial sheet metal blank to form a part. Alternatively, the cold-rolling can take place before the cutting, and the application of the conversion layer can take place before or after the cutting, or, instead of the cold-rolling, at least two steel strip sections having an aluminium-based coating and different sheet thicknesses can be welded together, where the application of the conversion layer can take place before or after welding.

Claims

1. A method for producing a press-form-hardened component, comprising the steps of: providing a steel strip having an aluminum-based coat; applying an inorganic iron-containing conversion layer on the aluminum-based coat having a layer weight related to iron of 3-30 mg/m.sup.2; cold-rolling the steel strip into a flexibly rolled strip having strip portions of different sheet thickness; cutting a starting blank from the flexibly rolled strip, wherein the starting blank has different sheet thicknesses with a thinnest and a thickest sheet portion; and press-form-hardening the starting blank to form a component.

2. A method for producing a press-form-hardened component, comprising the steps of: providing a steel strip having an aluminum-based coat; cold-rolling the steel strip into a flexibly rolled strip having strip portions of different sheet thickness; cutting a starting blank from the flexibly rolled strip, wherein the starting blank has different sheet thicknesses with a thinnest and a thickest sheet portion; before or after cutting the starting blank, applying an inorganic iron-containing conversion layer locally or to the entire surface of the aluminum-based coat having a layer weight related to iron of 3-30 mg/m.sup.2 at least in the region of the thickest sheet portion; and press-form-hardening the starting blank to form a component.

3. A method for producing a press-form-hardened component, comprising the steps of: providing at least two steel strip portions having an aluminum-based coat which have different sheet thicknesses; welding the steel strip portions together to form a starting blank, wherein the starting blank has different sheet thicknesses with a thinnest and a thickest sheet portion; before or after welding said steel strip portions together, applying an inorganic iron-containing conversion layer locally or to the entire surface of the aluminum-based coat having a layer weight related to iron of 3-30 mg/m.sup.2 at least in the region of the thickest sheet portion; and press-form-hardening the starting blank to form a component.

4. The method as claimed in claim 1, wherein the inorganic iron-containing conversion layer on the aluminum-based coat has a layer weight related to iron of 5-25 mg/m.sup.2.

5. The method as claimed in claim 1, wherein the inorganic iron-containing conversion layer on the aluminum-based coat is formed by applying a solution of iron compounds in an external current-free reaction with the aluminum-based metallic coat.

6. The method as claimed in claim 1, wherein the thinnest sheet portion of the starting blank has at most 80% of the thickness of the thickest sheet portion of the starting blank.

7. A starting blank for producing a press-form-hardened steel component having an aluminum-based coat, wherein the starting blank has different sheet thicknesses, and wherein an inorganic iron-containing conversion layer is formed on the aluminum-based coat with a layer weight related to iron of 3-30 mg/m.sup.2, advantageously 5-25 mg/m.sup.2, particularly advantageously 7-20 mg/m.sup.2.

8. The starting blank as claimed in claim 7, wherein the starting blank is produced from a flexibly rolled strip consisting of steel.

9. The starting blank as claimed in claim 7, wherein the starting blank is produced from strip portions consisting of steel which are welded together.

10. The starting blank as claimed in claim 9, wherein the strip portions which are welded together in each case have different strengths with a difference in tensile strength of more than 50 MPa.

11. The starting blank as claimed in claim 7, wherein the starting blank comprises a hardenable manganese-boron steels.

12. The starting blank as claimed in claim 7, wherein the inorganic iron-containing conversion layer is applied on the aluminum-based coat with a layer weight related to iron of 3-30 mg/m.sup.2 at least in the region of the thickest sheet portion on the starting blank.

13. A press-hardened component, produced from a starting blank comprising a steel substrate and having an aluminum-based coat and different sheet thicknesses, and having a thinnest and a thickest sheet portion, wherein a diffusion zone is formed between the steel substrate and the aluminum-based coat, consisting of metals of the coat and the steel substrate, wherein the diffusion zones in the different sheet thickness regions, in relation to the starting blank, have a maximum thickness difference which corresponds to the following relationship:
DI.sub.max≤8*((D1−D2)/D1), where D1: is the thickest sheet portion of the starting blank D2: is the thinnest sheet portion of the starting blank DI.sub.max: is the maximum thickness difference of the diffusion layer thicknesses on the hardened component.

14. The press-hardened component as claimed in claim 13, wherein the maximum thickness difference of the diffusion layer thicknesses on the hardened component is DI.sub.max≤6*((D1−D2)/D1).

15. The press-hardened component as claimed in claim 14, wherein the maximum thickness difference of the diffusion layer thicknesses on the hardened component is DI.sub.max≤4*((D1−D2)/D1).

16. The press-hardened component as claimed in claim 13, wherein the thickness of the diffusion zone between steel and the aluminum-based coat in the various sheet thickness regions is 2 to 14 μm or 4 to 12 μm.

17. The press-form hardened component as claimed in claim 13, wherein the component has an aluminum oxide layer of at least 50 nm thickness on the component surface in the region of the thickest sheet portion of the starting blank.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] FIG. 1 illustrates a layer structure of aluminum oxide on a steel substrate;

[0034] FIG. 2 graphical relationship of differences in diffusion layer thicknesses relative to difference in sheet thickness;

[0035] FIG. 3 illustration of test sample formed from hardenable 22MnB5;

[0036] FIG. 4 schematic summary of samples V1 to V4 treated with an iron-containing coating before or after the cold-rolling step; and

[0037] FIGS. 5a-5d illustrate the resulting heating curves of the variants V2 to V4 of FIG. 4, each in comparison with the reference measurements on variant V1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] During the tests it was found that from a layer weight of 3 mg/m.sup.2 related to iron, advantageously 5 mg/m.sup.2 related to iron, particularly advantageously 7 mg/m.sup.2 related to iron, the heating rates can be significantly increased compared to an untreated reference. The maximum layer weight should not exceed 30 mg/m.sup.2 related to iron. In addition, the increase in the heating rate is only small and the spot welding behaviour after press-hardening begins to deteriorate, which is why higher layer weights are not practical for economic and technological reasons. Advantageously, up to 25 mg/m.sup.2 related to iron, particularly advantageously up to 20 mg/m.sup.2 related to iron, are applied in order to keep the expenditure on active substances as low as possible.

[0039] The layer weights were determined using ICP-OES (optical emission spectroscopy with inductively coupled plasma). For this purpose, the conversion layer formed on the surface was chemically detached, then analysed and referenced against commercially available element standards.

[0040] The inventive treatment of the surface of the coated steel strip can be effected advantageously in a treatment part located downstream of the process part of a continuously producing hot-dip coating installation or a separate installation e.g. via spray bars with nozzles or in a dipping process. The separate installation can be e.g. a strip coating installation. Alkaline cleaning with subsequent rinsing upstream of the treatment in accordance with the invention advantageously removes the (native) oxide layer on the aluminum-based coat formed by atmospheric oxidation and thus creates a defined initial state for the deposition of the iron and/or compounds thereof in accordance with the invention.

[0041] The concentration of the deployment solution and the temperature thereof, the treatment time, the spray pressure, the shear of the sprayed-on solution relative to the surface of the metal strip to be treated, and the volume brought into contact with the surface can influence the amount of iron deposited on the surface.

[0042] The teaching of the invention further comprises a press-hardened component produced from a starting blank having an aluminum-based coat, having different sheet thicknesses with a thinnest and a thickest sheet portion, which is characterised in that a diffusion zone is formed between the steel substrate and the aluminum-based coat, consisting of metals of the coat and the steel substrate, wherein the diffusion zones in the different sheet thickness regions, in relation to the starting blank, have a maximum thickness difference which corresponds to the following relationship:

DI.sub.max≤8*((D1−D2)/D1),

[0043] where

[0044] D1: is the thickest sheet portion of the starting blank

[0045] D2: is the thinnest sheet portion of the starting blank

[0046] DI.sub.max: is the maximum thickness difference of the diffusion layer thicknesses on the hardened component.

[0047] In an advantageous embodiment of the invention, the maximum thickness difference DI.sub.max corresponds to the following relationship:

DI.sub.max≤6*((D1−D2)/D1)

[0048] In a particularly advantageous embodiment of the invention, the maximum thickness difference DI.sub.max corresponds to the following relationship:

DI.sub.max≤4*((D1−D2)/D1)

[0049] These relationships are graphically illustrated in FIG. 2. The three straight lines represent the aforementioned relationships DI.sub.max≤8*((D1−D2)/D1), DI.sub.max≤6*((D1−D2)/D1) and DI.sub.max≤4*((D1−D2)/D1). The region above the solid line which represents DI.sub.max≤8*((D1−D2)/D1) indicates the region which could hitherto be achieved by the prior art. The region below the solid straight line forms the region in accordance with the invention.

[0050] In accordance with the invention, the thickness of the diffusion zone between steel and the aluminum-based coat in the different sheet thickness regions should advantageously be between 2 and 14 μm, particularly advantageously between 4 and 12 μm, in order to ensure a sufficiently high but not excessive degree of alloying.

[0051] To explain these relationships, results from laboratory tests are described hereinafter.

[0052] Sheet metal strips consisting of the hardenable steel 22MnB5 having a sheet thickness of 1.5 mm and an aluminum-silicon coat with a nominal layer weight of 150 g/m.sup.2 on both sides were half-rolled by 50% in a laboratory cold-rolling stand and cut to a sample size of 200×600 mm.sup.2, wherein the thickness transition lay in the middle (FIG. 3).

[0053] Thermocouples were applied at the edges of the samples and the heating rate was recorded in both sample regions in a furnace preheated to 920° C. Subsequently, the thickness of the diffusion layer was determined metallographically in a plurality of regions of the sample.

[0054] This procedure was likewise performed with samples which were treated with an iron-containing coating before or after the cold-rolling step. The tested variants V1 to V4 are as follows: V1—rolling (reference); V2—pre-coating, subsequent rolling; V3—rolling, subsequent pre-coating; V4—rolling, subsequent partial pre-coating. FIG. 4 shows a schematic summary of these different variants V1 to V4 (not to scale).

[0055] FIGS. 5a to 5d illustrate the resulting heating curves of the variants V2 to V4, each in comparison with the reference measurements on variant V1. In addition, the temperature difference between the thick and thin sample part is also illustrated as a function of the heating time. It can be clearly seen how, by means of the iron-containing pre-coating, the heating rates are aligned, in particular by reason of the very large increase in the heating rate in the thick sample part. This leads to a significant enlargement of the process window during heating in the course of the press-form hardening procedure.

[0056] Table 1 summarises the resulting diffusion layer thicknesses which were determined metallographically on several specimens from the respective sample regions (thick/thin) and averaged. The exploded view of this table 1 is provided for reasons of clarity. The diffusion layer thickness was determined on the basis of the current version of the VW works standard TL 4225.

TABLE-US-00001 TABLE 1 Measured average diffusion thicknesses and maximum thickness difference Sheet DImax [μm] thickness DImax [mm] Region Region (Region Sample Variant D1 D2 D1 D2 D2 − D1) V1_1 V1-REF 1.5 0.75 4 9 5 V1_2 V1-REF 1.5 0.75 4 9 5 V2_1 V2-coating +  1.5 0.75 7 9 2 rolling V2_2 V2-coating +  1.5 0.75 7 10 3 rolling V3_1 V3-rolling + 1.5 0.75 7 10 3 coating V3_2 V3-rolling + 1.5 0.75 8 11 3 coating V4_1 V4-rolling + 1.5 0.75 8 8 0 partial coating V4_2 V4-rolling + 1.5 0.75 7 9 2 partial coating In accordance with the invention: DImax ≤ 8*((D1 − 6*((D1 − 4*((D1 − Sample D2)/D1) D2)/D1) D2)/D1) V1_1 NO NO NO V1_2 NO NO NO V2_1 YES YES YES V2_2 YES YES NO V3_1 YES YES NO V3_2 YES YES NO V4_1 YES YES YES V4_2 YES YES YES

[0057] These results were combined with supplementary tests, in which the influence of the iron-containing coating on the heating rate and diffusion layer thickness in the case of different sheet thicknesses, heating times and heating temperatures was examined. An almost linear increase in the thickness of the diffusion layer with the heating time could also be observed here. As a result of these tests, the previously presented formulaic relationships between the maximum thickness difference of the diffusion layer thickness and the sheet thickness difference of the starting blank were empirically determined.

[0058] As described above, an approximation of the heating rates leads to a small difference in the diffusion layer thicknesses and results in homogeneous component properties with regard to lacquering capability and spot-welding capability. What is particularly advantageous is an alloy grade related to the entire component having a diffusion layer thickness between 2 and 14 μm, particularly advantageously between 4 and 12 μm.

[0059] During production of components by press-hardening, the iron-containing pre-coating on the blank is not retained. Rather, in the course of heating, e.g. in a roller hearth furnace, an aluminum-rich oxide layer which is doped with iron cations is formed as a result of the inventive pre-coating of the starting blank with the inorganic iron-containing conversion layer. The iron cations suppress the otherwise usual self-limitation of aluminum oxide layer growth and lead to the formation of substantially thicker aluminum oxide layers during the heat treatment, wherein aluminum oxide layer thicknesses of over 50 nm are achieved.

[0060] In contrast, typical aluminum oxide layer thicknesses on press-hardened components with an aluminum-based coat without an iron-containing pre-coating are significantly lower, as described with respect to FIG. 1. Thus, at least in the region having the high sheet thickness of the starting blank, components in accordance with the invention have a thickened aluminum oxide layer of more than 50 nm which results from the iron-containing pre-coating in combination with the heating before press-hardening.

[0061] An example of an advantageous method sequence is described hereinafter: [0062] hot-rolling, acid-cleaning and optional cold-rolling of a suitable steel strip; [0063] annealing the steel strip in a hot-dip coating installation in a reducing atmosphere at temperatures between 500 and 950° C. and subsequent hot-dipping in an aluminum-based melt and applying an aluminum-based coat on the steel strip with a layer weight between 60 and 200 g/m.sup.2 on both sides; [0064] subsequently applying an inorganic iron-containing conversion layer on the aluminum-based coat having a layer weight related to iron of 3-30 mg/m.sup.2; [0065] flexibly rolling the steel strip having the aluminum-based coat so that the thin region of the resulting strip is 70% of the thickness of the thick region of the strip or less; [0066] producing blanks from the flexibly rolled strip so that thick and thin sheet portions lie within each cut blank; [0067] producing components by heating the blanks in a roller hearth furnace to temperatures between 750 and 1000° C. in order to adjust an austenitic microstructure at least in parts of the blank and subsequently forming in a tool to form a component with simultaneous rapid cooling so that a martensitic hardness microstructure is produced at least in parts of the component.