EP2411548B1 - Method for producing partially hardened steel components - Google Patents

Description

The invention relates to a method for producing partially hardened steel components according to the preamble of claim 1.

It is known to cure and manufacture steel components by heating a planar board to an austenitizing temperature, reshaping it, and then rapidly cooling it.

It is also known to heat already cold-formed components and then in a tool, which corresponds to the final shape of the component to cool and harden.

In order to achieve hardened components with different hard areas, it is known, inter alia, to form the components of laser-welded blanks, wherein the laser-welded blanks consist of steels of different quality and hardenability. A hardenable by a corresponding increase in temperature steel is thus adjacent to a steel which is not curable at these temperatures or generally.

From the DE 197 43 802 C2 For example, a method for producing a metal mold component is known, wherein the metallic mold component is to have regions with a higher ductility, wherein the mold component is formed from a hardenable steel and first bring partial areas of a board to a temperature of 600 ° C and 900 ° C in a time of less than 30 seconds, whereupon the heat-treated board is formed in a press tool to the mold component and then the mold component is cooled in the press tool and thereby partially cured ,

In a further embodiment, which is described in this document, a mold component is first heated homogeneously to a temperature which is necessary for curing and then the circuit board in the press tool to the mold component final molded. In the press tool also takes place the required hardening. The homogeneously hardened component is then placed on a conveyor and oriented by fixations. On this conveyor, the mold components undergo a heating device in which by an inductor those areas which are to have a higher ductility, in a very short time again to a temperature of 600 ° to 800 ° C and then cooled so slowly that a renewed Hardening does not take place, but these parts are in turn ductile. This method has the disadvantage that it requires several steps and is also energy-intensive.

From the DE 200 14 361 U1 a B-pillar for a motor vehicle is known, which consists of a longitudinal profile made of steel, wherein the longitudinal profile has a first length section with a predominantly martensitic material structure and a strength above 1,400 N / mm ² and a second length section of higher ductility with a predominantly ferritic-pearlitic Material structure and a strength below 850 N / mm ² should have. To adjust these different areas, it is known from this document to isolate the longitudinal profile in the areas in which the longitudinal profile is to remain softer against the heat of the furnace by insulating elements encompass and cover the profile. Consequently, these areas should not experience significant heating, so that the overall temperature increase in these sections is well below the austenitizing temperature.

In another embodiment, the moldboard is first completely and homogeneously heated to an austenitizing temperature and brought during the transfer or transport of the board in the curing tool by targeted, not too abrupt cooling to a temperature well below the Austenitisierungstemperatur, so that when hot forming no purely martensitic structure is set. In this method, it is disadvantageous that the targeted cooling of a board or of a preformed component increases the cycle times and increases and necessitates additional method steps. When insulated against the heat of the furnace is a disadvantage that both the attachment of the insulation and the removal of the insulation means additional steps that increase the cycle time and increase the cost of the process.

From the EP 0 816 520 B1 For example, a press-cured article and a method of curing the same is known. This component is intended to include hardened and uncured areas, wherein for curing the component or for curing the profile, an inductor is used, which at least partially heats the component to an austenitizing temperature and the inductor below a cooling device is tracked, for example, with water jet, which for the Hardening necessary rapid cooling makes. Experiments have shown that this method is on the one hand very complex, the cycle times extremely lengthened and on the other hand experiments have shown that this leads to an extremely strong component distortion. Consequently, this method is not found in practice.

From the DE 10 2006 018 406 A1 For example, there is known a method of partially press-hardening a sheet metal part, wherein heat is removed from a selected portion of the sheet metal part during heating, via a body having a heat-absorbing capacity such that the temperature reached in the selected section during the heating period remains below the austenitizing temperature. In another embodiment, heat is removed by convection from the body during and / or after the heating of the sheet metal part.

The object of the invention is to provide a method for producing partially hardened steel components, which is simple and inexpensive to carry out with high process reliability and well predictable hardness values in the different areas.

The object is achieved by a method having the features of claim 1.

Advantageous developments are characterized in the subclaims.

According to the invention, in the areas which are to have no or a lower hardness, an absorption mass is applied during the heating. The term "concerns" within the meaning of the invention also includes a small spacing, in particular a spacing of 0.5 to 2 mm between absorption mass and board.

The absorption mass is a "cold" mass applied to the hot board during the furnace process. This mass extracts energy from the board via the contact surface or through the narrow gap via radiation. Heat transfer in the context of the invention comprises heat conduction through the support surface in direct contact with the absorption mass with the board and heat radiation at a small spacing. The mass thus partially absorbs the energy of the board, which is introduced through the furnace. Therefore, in the following, a "cold" applied mass is also referred to as absorption mass. In the invention thus takes place a heat flow from the furnace chamber through the sheet of the component in the absorption mass. Insulation does not take place.

According to the invention, the components are not partially or only briefly brought over the austenite start temperature during the heating process. As a result, the material in these areas is not / only partially converted to austenite and can not be transformed into martensite during the pressing process (press hardening) in these areas. The areas that do not convert to martensite due to the previous heat treatment during press hardening have significantly lower strength than the areas that were brought to austenite start temperature during the heat treatment and then cured in the press.

This partial non-austenitizing is achieved by partially applying the absorption mass to the component at the beginning of the heat treatment (before the component enters the oven). The absorption mass is applied to the component and partially replicates the shape of the component. When transported through the oven, this relatively large absorption mass heats up less than the component. As a result, energy is removed from the component at the support surface by the partial contact with the mass (the energy flow is always from warm to cold). The component heats up in these areas much slower and less than in the other areas where the mass is not applied.

The soft areas can be specifically adjusted by the applied absorption mass. With the same contact surface but different thicknesses of the absorption mass (also over their extent), different strengths can be generated. It is thereby possible to set almost any strength between 500 and 1500 MPa and only by varying the thickness of the absorption mass or of the material used (even over their extent), of which the absorption mass is. The strength transition range between Hard and soft material is about 20-50mm, especially 20 to 30 mm.

In addition, air gaps, in particular in the edge region may be provided to make the hardness transition even wider.

To ensure this process, it must be ensured that the absorption mass always has a correspondingly constant low temperature before it is returned to the oven. This can be realized in the series process in different ways during the return of the furnace carrier.

A large, precisely adjustable and homogeneous transition range from hard to soft causes, for example, that the component can absorb the occurring stresses homogeneously in the transition region from hard to soft or "softly" cushioning and thus prevents the component is partially loaded too much and possibly breaks in the crash and leads to component failure.
A larger transition area also prevents, with certain component geometries, the component tearing in the area of welding points introduced in the bodyshell.
It is also possible to influence the behavior of the component in the event of a crash precisely and accurately by means of precisely defined ductile areas in the area of welding points.

In order to reduce the heating of the absorption mass by the remaining furnace chamber atmosphere, heat shields are provided on the side of the absorption mass opposite the component. These heat shields can be made of different materials, in particular of ceramic or metallic materials.

In addition, the heat absorption of the absorption mass and / or the Wärmeabschirmbleche be selectively controlled by the radiation from the furnace chamber via appropriately selected emissivities (surface condition, coating, paint). In the absorption mass, the heat absorption can be influenced by the radiation of the board also targeted.

Figur 1:

eine Platine mit einer aufgesetzten Absorptionsmasse;

Figur 2:

die Aufheizkurve der Platine und der aufgelegten Absorptionsmasse;

Figur 3:

die Platine nach dem Abnehmen der Absorptionsmasse und einer durchgeführten Abkühlung;

Figur 4:

schematisch eine Absorptionsmasse, die auf ein fertig geformtes Bauteil aufgelegt ist;

Figur 5:

die Darstellung nach Fig. 4 in einer teilgeschnittenen Ansicht;

Figur 6:

die Darstellung nach Fig. 4 in einer Draufsicht;

Figur 7:

die Darstellung nach Fig. 6 in einer teilgeschnittenen Ansicht;

Figur 8:

die Darstellung nach Fig. 4 ein einer geschnittenen Ansicht;

Figur 9:

eine weitere Ausführungsform, bei der das fertig geformte Bauteil auf einer entsprechend geformten Absorptionsmasse aufliegt;

Figur 10:

zwei Aufheizkurven eines Bauteils, wobei die Temperatur im Bereich der darunter liegenden Absorptionsmasse und in einem Bereich ohne Absorptionsmasse gemessen wurde.

The invention will be explained with reference to a drawing. It shows:

FIG. 1:

a board with an attached absorption mass;

FIG. 2:

the heating curve of the board and the applied absorption mass;

FIG. 3:

the board after removing the absorption mass and a cooling carried out;

FIG. 4:

schematically an absorption mass, which is placed on a finished molded component;

FIG. 5:

the representation after Fig. 4 in a partially sectioned view;

FIG. 6:

the representation after Fig. 4 in a plan view;

FIG. 7:

the representation after Fig. 6 in a partially sectioned view;

FIG. 8:

the representation after Fig. 4 a cut view;

FIG. 9:

a further embodiment in which the finished molded component rests on a correspondingly shaped absorption mass;

FIG. 10:

Two heating curves of a component, wherein the temperature was measured in the area of the underlying absorption mass and in an area without absorption mass.

According to the invention, in a first embodiment of the invention, an absorption mass is placed on a sheet to be austenitized, for example in the form of a steel square.

As absorption material is any form of heat-resistant metals such as Ampco alloys and steels, especially heat-resistant steels, but also ceramic bodies in question. Crucial criteria for usability are the thermal conductivity and the heat capacity. The absorption mass in this case has an outer shape or contour, which, if appropriate also matched to the formed part, corresponds to the areas which are to remain softer. In particular, the absorption mass can of course also have a deviating from the simple cuboid shape, complex irregular shape with recesses.

In Fig. 2 a heating curve for the board and a heating curve for absorption mass is shown.

It can be seen that the absorption mass is heated with a considerable delay and while the board in the uncovered area at 720 ° from the oven is taken to press-harden, the absorption mass and thus the underlying sheet has a temperature of less than 600 ° C, in which a rapid subsequent cooling does not lead to a cure.

The board after removing the absorption mass and cooling shows the appearance after Fig. 3 It can be seen that in the region in which the absorption mass was applied, the sheet has a substantially unaltered bright metallic appearance. The hardness transition range from the hard region to the soft region below the absorption mass is 20 mm to 50 mm, particularly 20 mm to 30 mm.

In a further advantageous embodiment, the absorption mass has a shape which is matched to the shape of a finished formed workpiece. This finished formed workpiece is then heated for the purpose of curing and cooled after heating in a mold without substantial transformation. During heating up, as in Fig. 4 shown, either the absorption mass placed on the component lying in the furnace to leak the underlying sheet with a lower temperature from the oven or, as in Fig. 9 shown, the component placed so that it rests partially on the absorption mass. The effect for warming up is the same.

In Fig. 10 a diagram is shown in which were measured at a component during the heating temperatures, namely once in the range of an underlying absorption mass and once in a region in which no absorption mass was present. It can be seen from the diagram that the temperature of the component is above the absorption mass in a non-critical range, which means that due to the significantly lower heating no hardness will be achieved here.

As already stated, the absorption mass can be designed so that either a flat board or an already preformed component in the areas that are to remain softer, rests on this absorption mass, optionally in some areas with a slightly larger air gap, in particular an air gap of 4 mm to 10 mm thickness to realize hardness transitions.

A preferred application of the absorption mass is, for example, the production of round or circular softer regions on a component or a circuit board, in particular in the flange region at locations where a joint is to be performed. This is particularly advantageous for welded joints, because it has been shown that by the heat treatment of galvanized high-hardenable steel sheets hardening by the surface of the zinc layer partially changed by oxide coatings so that the weldability is reduced. If these areas are left soft with absorption masses, in particular by an absorption mass which is elongated, for example, in the area of the flange and has rounded columnar projections on which the component rests, areas can be achieved in which the zinc surface is not adversely affected, then that here a very good weldability is maintained. Also for mechanical reasons, this is advantageous because the welds remain ductile even in these softer areas and allow so-called Ausknöpfbrüche, so that a preferred fracture pattern in the industry is achieved.

The absorption mass can be actively cooled by a cooling section after the furnace process on the return path of the furnace support. Before the absorption mass returns to the furnace, this cooling distance ensures that the temperature of the mass is always a constant low temperature having. Different cooling media can be used to cool the absorption mass, such as compressed air or nitrogen.

The oven supports can be modified in such a way that you can attach the absorption mass by means of robots or suitable device on the furnace support and remove. This can be realized in the series process as follows. The furnace supports are returned above the furnace. The oven holders stay for about 20 seconds always in the same place. There, a robot or a suitable device can be positioned, which removes the hot absorption mass from its holder and then attaches a cold absorption mass. The hot absorption mass may be fed to a cooling circuit (active or passive) which cools the hot absorption mass until reuse. This ensures that the absorption mass always extracts the same energy from the component in the oven during the oven process.

Partial austenitizing may be followed by partial press hardening.

• Die Bauteilgeometrie wird prozesssicher gewährleistet, da das Bauteil beim Presshärten während des Abkühlens im Presswerkzeug gehalten wird.
• keine Erhöhung von Taktzeiten beim Presshärten
• kein extra Anlassen notwendig
• Es sind beliebige Festigkeiten zwischen 500 MPa und 1.500 MPa, je nach eingesetzter Absorptionsmasse auf den Ofenträgern gezielt erreichbar.
• überschaubare Investitionskosten
• Die Größe des jeweils duktilen Bereiches kann je nach Anwendungsfall frei variiert werden.
• relativ schmaler Härteübergangsbereich zwischen hart und weich

The advantages of the invention are:

• The component geometry is guaranteed to be reliable, since the component is held during press hardening during cooling in the pressing tool.
• no increase in cycle times during press hardening
• no extra starting necessary
• There are arbitrary strengths between 500 MPa and 1500 MPa, depending on the used absorption mass on the furnace carriers targeted.
• manageable investment costs
• The size of each ductile area can be varied freely depending on the application.
• relatively narrow hardness transition range between hard and soft

In order not to produce surface contamination or efflorescence of the component surface by the applied absorption mass must be ensured that the support surface of the absorption mass is not dirty and not scaled by the constant heating and Abkühlvorgangvorgang. It is either a suitable material to use as an absorption mass or a corresponding surface coating of advantage.

Claims (5)

1. A method for producing partially hardened steel components in which a blank composed of a hardenable sheet steel is subjected to a temperature increase that is sufficient for a quench-hardening and after reaching a desired temperature and optionally after a desired holding time, the blank is transferred to a forming tool in which the blank is formed into a component and at the same time quench-hardened or the blank is cold-formed and the component obtained by the cold-forming is then subjected to a temperature increase, the temperature increase being carried out so that a component temperature required for a quench-hardening is reached and the component is then transferred into a tool in which the heated component is cooled and thus quench-hardened; during the heating of the blank or component for the purpose of increasing the temperature to a temperature required for the hardening, absorption masses rest against and/or are spaced with a small gap apart from regions that are intended to have a lower hardness and/or higher ductility; and with regard to its expansion and thickness, its thermal conductivity, and its thermal capacity and/or with regard to its emissivity, the absorption mass is dimensioned so that the thermal energy acting on the component in the region to remain ductile flows through the component into the absorption mass, characterized in that on one or more surfaces of the absorption mass that are oriented toward the furnace chamber, shielding plates are provided, which shield the absorption mass from radiation emanating from the furnace chamber.
2. The method as recited in claim 1, characterized in that an absorption mass is used, which is composed of a heat-resistant metal such as an Ampco alloy, a steel, or the like, and at least one surface of the absorption mass is embodied as contoured so that rests against the blank or component and/or is spaced apart from it by a small gap, in particular a gap of 0.5 mm to 2 mm, or, in order to adjust hardness transition zones, is spaced apart from the blank or component in some areas by slightly larger air gaps, in particular gaps of 4 to 10 mm.
3. The method as recited in one of the preceding claims, characterized in that the absorption mass or masses is/are situated on a support with which the blank or component is conveyed through a heating device such as a furnace and as it travels through the heating device, the blank or component rests on the absorption mass or masses.
4. The method as recited in one or more of the preceding claims, characterized in that the heat absorption of the absorption mass from the furnace chamber and/or from the component is controlled by adjusting the emissivities of the surface of the absorption mass.
5. The method as recited in one or more of the preceding claims, characterized in that the thermal absorption of the shielding plates from the furnace chamber is controlled by adjusting the emissivities of the surface of the shielding plates.

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