WO2014037072A1 - Coatings containing mo on tools used for direct hot forming - Google Patents

Description

 Mo-containing coatings on tools for direct press hardening

The ever-increasing demands in modern automobile construction with regard to lightweight construction and vehicle safety can be met with new production technologies, such as direct press hardening. In direct press-hardening, a high-strength boron-manganese steel (22MnB5) is hot-formed in one process step and simultaneously hardened in the tool, which enables an increase in strength up to 1500 MPa. Due to the high process temperatures and the considerable relative movement during forming, however, very high thermal and mechanical loads result for the tool. The consequences are increased tool wear, shorter tool life and thus higher costs for reworking the tool surfaces. The present invention relates to the use of a lamination system that significantly reduces tool wear and thus significantly increases tool life. State of the art

In recent years, more and more sheets of high-strength boron-manganese steel (22MnB5) are used to manufacture body components for automobiles to meet the current requirements in terms of lightweight construction, vehicle safety and CO 2 emissions (environmental issues). These ultrahigh-strength boron-manganese steels can be hot-formed by direct press hardening in a single process step at high temperatures of approx. 800 ° C and simultaneously hardened in the mold. This results in a finished component (e.g., the B-pillar of a vehicle) having the following properties: i) reduced weight over conventional steels, ii) high dimensional stability by reducing component distortion, and iii) strengthening (martensite) up to 1500 MPa.

 Direct press-hardening comprises the steps of i) tempering at about 900 ° C (austenitic region), ii) transferring the sheet to the press, and iii) molding and curing the component by the mold-held mold. At the end of pressing, the press is held closed until the entire steel plate is sufficiently quenched (Senuma, T .: ISIJ Int. 41, 520 (2001)).

Since, in general, the formability of high-strength steel sheets at room temperature is difficult, the steel sheet is, as described above, processed at temperatures of about 800 ° C. However, the scaling of the steel surface at high temperatures

CONFIRMATION COPY To prevent, nowadays different steel coatings (Zn, AlZn, AISi) are used.

 Particularly in Europe, an aluminized steel sheet was developed as a product called USIBOR® 1500 (AlSi-coated, ArcelorMittal) for this application. This has excellent properties in terms of process lubrication, oxidation protection and corrosion resistance in subsequent use in the automobile.

Despite the very promising properties of AlSi-coated steel sheets, there are significant problems with adhesive wear of the tool surfaces. The "soft" AlSi coating of the steel sheets has a very strong tendency to adhere to the tool surface of the press at temperatures around 800 ° C This adhesive wear with significant transfer of material is often referred to as "galling". After several successive press cycles, the adhered material can cause scratches and cracks on the product being molded, and the tendency for abrasive wear of the tool due to breakouts of the adhered material is greatly increased. The result is that the operation must be stopped at regular intervals in order to remove the stops on the tool surface time-consuming.

One approach to improving the current performance of industrial press hardening in combination with AlSi-coated steel sheets is to apply PVD coatings with low friction and high wear protection to the mold. In the literature (Clarysee, F. et al .: Wear 264 (2008) 400-404) basically two different types of PVD coatings are known: nitride-based coatings (eg CrN and TiAlN) and solid lubricants, such as carbon or MoS ₂ based Layers (eg diamond-like carbon (DLC) and metal MoS ₂ composites).

In addition, Clarysse et al. (Clarysse, F. et al .: Wear 264 (2008) 400-404) describe the wear behavior (galling) of various layer systems in specially designed tests. They have observed that composite carbon-based (DLC-type and WC / C) composites show outstanding galling resistance. Accordingly, they recommended the use of this type of tool coating rather than the typical hard coatings such as CrN, TiN, CrN / TiCrN. Another known concept to improve the properties of molds for press hardening and thus the surface quality of the components produced thereby, is to nitride or carbonitride the molds, as well as to perform other surface treatments on the molds, such as plasma treatments, micro structuring, etc.

However, the better properties of the molding tool achieved by applying the above concepts do not sufficiently improve the process quality of the molding process of coated high strength steel sheets. Specifically, when AlSi-coated high strength steel sheets such as USIBOR® 1500 were used, the Galling phenomenon could not be satisfactorily reduced and remains a central problem of direct press hardening.

Object of the invention

The object of the present invention is to provide a coating for molding tools which satisfactorily improves the service life and the wear behavior of the tool. The coating should provide sufficient abrasive wear protection, sufficient adhesive wear protection, sufficient adhesion and sufficient temperature stability (phase stability and oxidation). Basically, the coating should improve galling protection, as typically observed with AlSi-coated steel sheets during press-hardening, compared to coatings that are currently used (TiAlN and AlCrN).

Description of the invention

According to the invention, the object is achieved by applying to the tool a layer system which contains one or more layer packages in which, as the distance from the substrate increases, a high-temperature-active smear layer follows a high-temperature-stabilized layer (HT layer). In the context of the present description, the term "lubricating layer" is to be understood as a high-temperature-active lubricating layer Preferably, a plurality of such layer packages are implemented successively as a alternating layer system The number of layer packages and the respective layer thickness can correspond to the prevailing high-temperature tribo contact and the resulting mechanical and chemical stress of the tool be set. The bandwidth can thus extend from bi-layer to multilayer to nano-laminated / structured multilayer.

The lubricating layers of the layer packages based on the HT layers are preferably realized by partially replacing the metallic constituents occurring in the HT layer by one or more element (s) which promote the lubricating properties of the layer.

Specifically, the inventive solution of the problem can be achieved for example by a alternating layer system of molybdenum-rich and low-molybdenum layers. The molybdenum-poor layers in the alternating layer system form the HT layers and have, for example, a composition corresponding to (Mel, Me 2, Mo _a ) N. The molybdenum-rich layers form the lubricating layers and have a composition corresponding to (Me3, Me4, Mo _b ) N. Here, a and b indicate the metallic content in at.%, And 0 <a <b <1 and Mel, Me2, Me3 and Me4 are elements of the group formed by Al, Cr and Ti and preferably: Mel = Me3 and / or Me2 = Me4

Preferably, the maximum molybdenum concentration in the molybdenum rich layers is at least 5 at.%, More preferably at least over 10 at.%, Above the minimum molybdenum concentration of the adjacent low molybdenum layers.

The molybdenum-rich layers of the alternating layer system can be deposited, for example, both by means of PVD methods using individual components of material sources (targets) and by means of PVD methods using multicomponent material sources.

The molybdenum-rich layers of the alternating layer system, to further improve lubrication, may contain one or more other elements selected from the group consisting of C, O, Si, V, W, Zr, Cu, and Ag.

The molybdenum-poor layers of the alternating layer system may, in order to further improve the high-temperature stability, for example, by improving the mechanical and chemical properties, one or more further elements and mixtures thereof from the group formed by Si, W, Zr and B. Preferably, the total layer thickness between 4 and 10 μηι, more preferably between 6 and 8 μηι. The inventors have reason to believe that the layer oxidation taking place at the high temperatures (about 800 ° C.) releases molybdenum, which subsequently partially reacts to what are known as Magneli phases. It is known that such Magneli phases have excellent lubricating properties (solid state lubrication). An indication of this is the advantageous influence of copper, which, as a catalyst, facilitates oxidation and thus optimizes HT lubrication.

Accordingly, in addition to the molybdenum-containing lubricating layers described here, such layers are generally suitable as lubricating layers which form Magneli phases. These so-called "shear structures" are formed, for example, by oxides of vanadium, tungsten, titanium or even molybdenum and are characterized by crystallographic shear planes / slip planes with low shear strength / shear stress.

The invention will now be described by way of example in detail and by way of example.

According to a first example of the present invention, a 2μηι thick (Tio _.5 Alo.5) N layer is applied to a press-hardening mold. Then follow 4 layer packages, each layer package 0.5μηι a thick (Tio.3Alo.3Moo. ₄₎ on which a 0.5 μπι thick (Tio _.5 Al _0.5) N layer follows. The layer system is completed by a 0.5 μm thick (Ti ₀ .3 Al 3 .Moo. 4).

In a slightly modified variant of the first example, the (Ti _0.3 Alo _.3 Moo _.4 ) N layers may in turn be formed as nanolayers. This can be achieved, for example, by separating the materials of the layers from separate material sources (targets) in a PVD process. For example, a TiAl target can be arranged next to a Mo target in the vacuum chamber of a coating installation, during which the substrates to be coated in the vacuum chamber are mounted on a so-called carousel past the targets. If an HT layer is to be deposited, the Mo target will not start up taken and only from the TiAl target is coated. If a smear layer is to be deposited, the Mo target is also put into operation.

According to a second example of the present invention, a 2μηι thick (Ti _0.3 Al ₀ .7) N layer is applied to a press-hardening mold. Then follow 4 layer packages, each layer package 0.5μηι a thick (Ti _{0. 8} i _.42 Alö Moo. ₄₎ contains the N a (Ti _0.3 Al _0.7) N layer follows 0.5μιη thick. The layer system is completed by a 0.5 μm thick (Tio.18Al _{0 42} Moo. ₄ ) N. This layer system can also be realized as a variant in the form of a nanolayer structure.

According to a third example of the present invention, a 2μπι thick (Alo _.65 Cro _.25 Sio _.05 ) N layer is applied to a press-hardening mold and Si can optionally also be omitted. Subsequently, 4 layer packages follow, whereby each layer package contains a 0,5μιτι thick (Al _{0, 42} Croj ₈ Moo. ₃₅ Cuo _. O ₅ ) N on which a 0.5μιτι thick (Alo.7Cro.3) N layer follows. The layer system is completed by a 0.5 μm thick layer (Alo. ₄₂ Cro.i8Moo.35Cuo.o ₅ ). The Mo-containing layers are again realized in the example as nanolayers.

Tests with the layers according to the examples have shown that the corresponding coated molds have significantly improved properties over the uncoated tools or over conventionally coated tools.

In the context of the present description, a press-hardening molding tool with a layer system applied to the substrate has been disclosed, which contains one or more layer packages in which a smearing layer on a high-temperature-stabilized layer (HT layer) follows with increasing distance from the substrate.

One or more of the smear layers may contain a Magneli phase. One or more of the smear layers may contain a molybdenum concentration that is above the molybdenum concentration in the HT layers.

The base material from which the lubricating layers are constructed may substantially correspond to the base material constituting the HT layer. One or more of the lubricating layers may include one or more of the group consisting of C, O, Si, V, W, Zr, Cu, and Ag to further improve the lubricating properties.

One or more of the HT layers may contain one or more further elements and mixtures thereof from the group formed by Si, W, Zr, and B to improve their high temperature properties.

Claims

claims 1. Press-hardening mold with a coating system applied to the substrate, which contains one or more layer packages, in which with increasing distance from the substrate, a smear layer on a high temperature-stabilized layer (HT layer) follows. 2. Press hardening mold according to claim 1, characterized in that the lubricating layer contains a Magneli phase. 3. Press-hardening mold according to one of claims 1 or 2, characterized in that the lubricating layer contains a molybdenum concentration which is above the molybdenum concentration in the HT layer. 4. Press-hardening mold according to claim 3, characterized in that the HT-layer has a composition corresponding to (Mel, Me2, Mo _a ) N and the lubricating layer has a composition corresponding to (Me3, Me4, Mo _b ) N, where a and b indicates the metallic content in at% and 0 <a <b <l and where Mel, Me2, Me3 and Me4 are elements from the group formed by Al, Cr and Ti and preferably: Mel = Me3 and / or Me2 = me4 5. Press hardening mold according to claim 4, characterized in that the maximum molybdenum concentration in the lubricating layer is at least 5 at.%, Preferably at least 10 at.% Above the minimum molybdenum concentration of an adjacent HT layer. 6. Press hardening mold according to one of the preceding claims, characterized in that, the base material from which the lubricating layer is constructed essentially corresponds to the base material from which the HT layer is constructed. A press-hardening mold according to any one of the preceding claims, characterized in that the lubricating layer for further improving the lubricating properties contains one or more of the elements selected from the group consisting of C, O, Si, V, W, Zr, Cu, and Ag. 8. Press-hardening mold according to one of the preceding claims, characterized in that the HT-layer to improve the high temperature properties, one or more further elements and mixtures thereof from the group formed by Si, W, Zr and B.