DE102013011572A1 - Method for local and distortion-free heat treatment of sheet metal or sheet-like components by local resistance heating - Google Patents

Abstract

The invention relates to a method for local and substantially distortion-free heat treatment, preferably for local strength reduction and ductility increase, of sheet metal components (200) or sheet-like components, including at least one and in particular exactly one component to be treated (200) between two opposing electrodes (110A, 110B), clamped by force application between these electrodes (110A, 110B), and resistively heated with a current (I) conducted across the electrodes (110A, 110B) at that location (X). The electrical contact points (KA, KB) between the electrodes (110A, 110B) and the component (200) in this case have a diameter (D) which is greater than 10 times the square root of the sheet thickness (s) at this point (X) is.

Description

The invention relates to a method for local and substantially distortion-free heat treatment of sheet metal components or sheet-like components.

The material properties of a metallic sheet or sheet-like component can be changed by heat treatment. By the heat treatment of solid and high-strength metallic materials z. For example, the material strength is reduced and the ductility (plastic deformability which, inter alia, has an influence on the elongation at break, fracture constriction or fracture toughness) is increased. Partial or local heat treatment can be used to produce sheet-metal or sheet-like components whose material properties are distributed over the component in a way that is suitable for use.

The prior art is based on the patents DE 10 2011 101 991 B3 and DE 10 2006 054 389 A1 directed.

The invention has for its object to provide a method for local heat treatment of sheet metal or sheet-like components, which does not have at least one associated with the prior art disadvantage or at least only to a reduced extent.

This object is achieved by an inventive method with the features of claim 1. Preferred developments and refinements emerge from both the dependent claims and from the following explanations.

The inventive method for local heat treatment of sheet metal components or sheet-like components provides that at least one and in particular exactly to be treated component between (at least) two opposing electrodes or a pair of electrodes arranged by force application between these electrodes, in particular with a defined clamping force, point-like or clamped pointwise or clamped and with a guided over the electrodes current (heating current) at this point or at this point (between the clamping electrodes) resistance heated, wherein the electrodes or the pair of electrodes together with the component or workpiece form local electrical circuit for resistance heating.

According to the invention, it is provided that the electrical contact points or contact surfaces between the electrodes and the component have a diameter which is more than 10 times, preferably more than 12.5 times, particularly preferably more than 15 times and in particular more as the 20 times the square root of the sheet thickness (which means the mathematical root value of the square root of the sheet thickness) at the nip (between the electrodes) corresponds.

The electrodes thus fulfill the first condition or relationship: D> 10 · √s, preferably D> 12.5 · √s, more preferably D> 15 · √s and in particular D> 20 · √s, where D is the Diameter of the contact points (numerical value, for example, in millimeters) and s is the sheet thickness (numerical value, for example, in millimeters) at the relevant clamping point.

With regard to an optimal method design according to the invention, it may be useful to limit the size of the contact points. Preferably, their diameter, or the electrode diameter, should not be more than 80 times, preferably not more than 70 times, more preferably not more than 60 times and in particular not more than 50 times the square root of the sheet thickness at the nip exhibit. The electrodes thus preferably also satisfy the second condition or relation: D ≦ 80 × √s, preferably D ≦ 70 × √s, particularly preferably D ≦ 60 × √s and in particular D ≦ 50 × √s.

A sheet metal component or sheet-like component is understood to mean a thin-walled workpiece. It is preferably an at least partially spatially shaped sheet-metal shaped part (in particular including flat flange sections), whose sheet thickness is preferably in a range between 0.4 mm and 5 mm and in particular between 0.8 mm and 2.5 mm. The sheet material used may be a ferromagnetic or non-ferromagnetic material such as a steel sheet or an aluminum sheet. A sheet-like or sheet-like component can also be a thin-walled profile part, for example an extruded or rolled workpiece, or a cast component, for example of a cast steel material. The sheet metal component may be uncoated or at least partially coated. As a coating is, for example, a metallic corrosion protection coating into consideration.

The contact points are the contact or contact surfaces between the electrodes and the component, which are predetermined by a corresponding configuration of the electrodes. The contact points may have any suitable surface shape (circular, elliptical, square, triangular, polygonal or the like). In particular, however, there are circular contact points whose diameter meet the above conditions or relations. For non-circular contact points, these conditions can be analogous to the smallest Diameter of the contact surface apply. Thus, the contact points or contact surfaces provided according to the invention are significantly larger than, for example, the contact points provided for resistance spot welding whose diameters are in a range D = 4 to 10 · √s, although the method according to the invention is fundamentally not comparable to resistance spot welding ,

According to the invention, the contact points formed with a pair of electrodes are thus relatively large, wherein the contact points of a pair of electrodes are preferably the same size or the same area, but this need not necessarily be the case. It is preferably provided that the electrodes of a pair of electrodes are identical in geometry and / or material. The electrodes are preferably formed from a heat-resistant material, in particular from a molybdenum, tantalum, tungsten or iron-based alloy.

During resistance heating, an exactly definable, stable and preferably also relatively low contact resistance advantageously arises with the electrodes pressed against the component at both sides against the component due to the large-area contact points. Furthermore, due to these large-area contact points in conjunction with the applied clamping force, a substantially distortion-free local heating (and cooling) is made possible. Surprisingly, therefore, the effect is achieved that the local heat treatment substantially distortion-free, which in particular a geometric distortion is meant takes place. The compressive force or clamping force on the electrodes is preferably adjusted so that a defined and stable electrical contact resistance is present and at the same time no or only a minimal thermal or geometric distortion occurs.

Another possible positive effect is a substantially locally exactly limited homogeneous heating of the component, in particular over the component thickness (uniform heat distribution over the sheet metal cross section), between the electrodes.

The method according to the invention preferably serves for locally reducing the strength and increasing the ductility of solid and high-strength steel components and in particular sheet-metal shaped parts, in particular with the aim of simplifying subsequent processing steps and / or producing a component which is suitable for use.

• – eine schnelle Erwärmungsphase mit einem Temperaturanstieg von mindestens 50 K/sec;
• – eine Temperaturhaltephase (Plateau-Phase), die mindestens eine Sekunde (1 sec) und bevorzugt mehrere Sekunden andauert, und
• – eine langsame Abkühlungsphase mit einem Temperaturabfall von höchstens 25 K/sec, bevorzugt höchstens 20 K/sec und insbesondere von nicht weniger als 5 K/sec.

It is preferably provided that the resistance heating according to the invention, in particular with a constant electrode force curve, has at least three distinguishable phases:

• A rapid heating phase with a temperature increase of at least 50 K / sec;
• - A temperature-holding phase (plateau phase), which lasts at least one second (1 sec) and preferably several seconds, and
• - A slow cooling phase with a temperature drop of at most 25 K / sec, preferably at most 20 K / sec and in particular not less than 5 K / sec.

Each phase can be executed exactly based on the current flow. In order to be able to precisely set the current flow or if necessary to be able to control or regulate it, at least one temperature measuring device, in particular integrated in a control loop, can be provided.

Interchangeable films, in particular metal foils, may be arranged between the electrodes and the component (but at least between one electrode and the component), thereby protecting the component surface and / or the electrodes and / or setting more defined and reproducible contact resistances can. In particular, it is provided that the films are thinner than the component at the relevant point (terminal point). The films at the clamping point can be changed after each heating cycle. Preferably, the films are designed as bands that are automatically moved after each heating process. Analogous effects can also be achieved by using interchangeable caps and preferably plug-in caps or plug-on caps (in particular made of a metallic material, for example as stated above) for the electrodes. The caps can be changed and / or reworked after each heating cycle or after several heating cycles.

The component can be a solid or high-strength steel component. In particular, it is a press-hardened sheet steel component made of a steel sheet material, wherein the steel sheet material (for example, a manganese-boron steel such as a 22MnB5 steel material) has a strength of up to 1600 MPa and more (e.g. also up to 2000 MPa). The high-strength material properties are generally based on a martensitic material structure.

It is preferably provided that such a solid, high-strength and / or press-hardened steel component is heated at the point clamped between the electrodes by means of local resistance heating to a temperature below the Ac1 temperature. (For example, the Ac1 temperature for a 22MnB5 material is about 720 ° C.) By heating below the Ac1 temperature (eg, in a range of 600 ° C to 650 ° C for a 22MnB5). Material), a local softening and Ductilitätssteigerung by a tempering effect (formation of tempered martensite) can be achieved.

Furthermore, it can preferably be provided that such a solid, high-strength and / or press-hardened steel component is heated at the point clamped between the electrodes by means of local resistance heating to a temperature above the Ac1 temperature and in particular above the Ac3 temperature. (The Ac3 temperature for a 22MnB5 material is, for example, about 780 ° C) This is partially or completely austenite, which then at a correspondingly slow cooling (at most 25 K / sec and preferably at most 20 K / sec to the formation of neumartensite) leads to the formation of more ductile and less solid ferrite, pearlite and / or bainite. Depending on the heating temperature and the temperature holding time, local strength reductions of several hundred MPa can be achieved.

The component can be brought to the electrodes by means of a handling device, such as, for example, an industrial robot, and moved relative to the electrodes. Likewise, the electrodes, in particular robotically guided, can be brought to the component and moved relative to the component, for which purpose the electrodes can be accommodated, for example, a C-frame or the like, while the component is held, for example, in a handling device. The component can be held during the local heat treatment with the handling device and, for example, be supported in this way. If the component is to be heat treated at multiple locations, the component and / or the electrodes may be advanced after each heating cycle. If larger areas or sections of a component are to be heat-treated, they can be subdivided into individual subregions which are successively heat treated in a suitable sequence with the method according to the invention.

• – im Wesentlichen kein Verzug (unerwünschte Geometrieänderung) des Bauteils an der lokal wärmebehandelten Stelle (deutlicher Vorteil gegenüber der Wärmebehandlung mit Laserstrahl, Flamme oder dergleichen);
• – exakte bzw. präzise lokale Wärmebehandlung wird ermöglicht, vorzugsweise zur Entfestigung und Duktilitätssteigerung;
• – sowohl bei ferromagnetischen als auch bei nicht-ferromagnetischen Metallmaterialien einsetzbar (deutlicher Vorteil gegenüber induktiver Wärmebehandlung);
• – geringe Prozessdauer (kurze Zykluszeit);
• – guter energetischer Wirkungsgrad;
• – hohe Flexibilität;
• – hervorragende Grossserientauglichkeit, wobei in besonders bevorzugter Weise vorhandene Einrichtungen verwendet werden können (Möglichkeit der einfachen Integration in bestehende Anlagen), und
• – hohe Wirtschaftlichkeit.

In summary, the invention may have the following advantages (no exhaustive list):

• Substantially no distortion (unwanted geometry change) of the component at the locally heat treated location (distinct advantage over heat treatment with laser beam, flame or the like);
• - Accurate or precise local heat treatment is possible, preferably for softening and ductility increase;
• - Can be used in both ferromagnetic and non-ferromagnetic metal materials (significant advantage over inductive heat treatment);
• - short process time (short cycle time);
• - good energy efficiency;
• - high flexibility;
• - Excellent mass production capability, with existing facilities can be used in a particularly preferred manner (possibility of easy integration into existing systems), and
• - High profitability.

The invention is explained in more detail below by way of example and not by way of limitation with reference to the schematic and not to scale figures. The features shown in the figures and / or explained below may, regardless of specific feature combinations, be general features of the invention.

1 shows in a side view the local heat treatment to a sheet metal component according to a method of the invention.

2 shows in a diagram qualitatively the force-time curve and the temperature-time course in a local heat treatment according to the invention.

3 also shows in a side view another embodiment of the invention.

4 also shows a side view of a further embodiment of the invention.

1 shows a partially illustrated sheet metal component 200 which is locally heat treated at the point indicated by X by resistance heating to reduce the material strength and increase the material ductility at this point. At the point X, the sheet metal component 200 For example, in a subsequent processing step by pre-hole riveting or screwing (direct screwing) are joined. In the sheet metal part 200 is, for example, to press-hardened sheet metal part from a high-strength steel sheet material.

The local heat treatment is carried out according to the invention by local resistance heating with the aid of two electrodes 110A and 110B between which the sheet metal component 200 is positioned and the between them the sheet metal component 200 clamp at the point X with defined clamping force F, which requires the two-sided accessibility.

The opposing electrodes 110A and 110B The electrode pair shown may be arranged in a holding frame, not shown, for example. A C-frame or the like. Such a frame can be stationary or stationary or movable, for example by connection to a robot arm. To carry out a clamping movement and for applying the clamping force F can be provided hydraulically, pneumatically and / or electric motor actuated adjusting mechanism. The electrodes 110A and 110B are identical and arranged symmetrically (without offset) with respect to the common force axis. The electrodes 110A and 110B can be formed with cooling devices, for example. With cooling fluid flow-through cooling channels.

To perform the local heat treatment, the sheet metal component 200 between the electrodes 110A and 110B positioned (or vice versa), which then perform a clamping movement and the sheet metal component 200 punctiform at the point X between them with the clamping force F clamp. At the same time, the sheet metal component 200 be pinched at other points pointwise between other electrodes. The circular electrical contact points between the electrodes 110A and 110B and the component 200 , which means the two-sided point-like and circular contact surfaces KA and KB, have a diameter D which corresponds to more than ten times the root value (or greater, as stated above) of the sheet thickness s at the point X, to which the electrodes 110A and 110B are formed accordingly. The electrodes 110A and 110B satisfy the condition or relation: D> 10 · √s (or D> 12.5 · √s; D> 15 · √s; D> 20 · √s), where D is the diameter of the contact points or Contact surfaces KA and KB and s the sheet thickness of the sheet metal component 200 at the terminal point X is. At least one of the contact points KA or KB fulfills this condition.

After a short delay (pressure build-up time), the resistance heating starts at the point X with the help of one through the electrodes 110A and 110B supplied and discharged current I, the electrodes 110A and 110B at the point X together with the sheet metal component or workpiece 200 form an electrical circuit for resistance heating.

• – einer schnellen Erwärmungsphase mit einem Temperaturanstieg von mindestens 50 K/sec, wie mit der steilen Rampe a veranschaulicht;
• – einer Temperaturhaltephase, insbesondere in einem der oben beschriebenen Temperaturbereiche, die mindestens eine Sekunde (1 sec) und bevorzugt mehrere Sekunden andauert, wie mit dem Plateau b veranschaulicht; und
• – einer langsamen Abkühlungsphase mit einem Temperaturabfall von höchstens 25 K/sec und insbesondere von nicht weniger als 5 K/sec, wie mit der flachen Rampe c veranschaulicht.

The resistance heating and the associated temperature profile T (t) at the point X, wherein the current waveform I (t) of the electric heating current I is controlled or regulated in accordance with the heating effects, takes place at a constant electrode force curve F (t) (which also non-constant electrode power curve F (t) can be provided) in three defined phases, as in 2 illustrates:

• A rapid heating phase with a temperature increase of at least 50 K / sec, as illustrated by the steep ramp a;
• A temperature maintenance phase, in particular in one of the temperature ranges described above, which lasts at least one second (1 second) and preferably several seconds, as illustrated by the plateau b; and
• - A slow cooling phase with a temperature drop of at most 25 K / sec and in particular of not less than 5 K / sec, as illustrated by the flat ramp c.

These phases can be controlled or regulated.

The resistance heating is followed by a retention time, within which the sheet metal component 200 at the point X between the electrodes 110A and 110B remains trapped (holding pressure).

The local resistance heating at the point X, depending on the heating temperature and temperature hold time, leads to a locally exactly limited and distortion-free strength reduction and ductility increase in the between the electrodes 110A and 110B clamped area M of the sheet material. In the sheet material forms evenly around the heat-treated area or around the heat-treated zone M around a small or narrow transition zone U from.

3 shows a variant in which between the electrodes 110A and 110B and the sheet metal component 200 in each case an electrically conductive and in particular metallic foil 120A / 120B is arranged, which makes a direct contact between the electrodes 110A and 110B and the sheet metal component 200 prevented. The previous explanations apply analogously to this variant. The film thickness is not included in the determination of the minimum contact areas KA and / or KB.

4 shows a variant in which the electrodes 110A and 110B exchangeable metallic plug caps 130A and 130B are attached, over which the contact surfaces KA and KB are formed (the plug caps 130A and 130B are shown in a sectional view). The previous explanations apply analogously to this variant. The cap thickness is not included in the determination of the minimum contact surfaces KA and / or KB.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102011101991 B3 [0003]
• DE 102006054389 A1 [0003]

Claims (10)

Method for local and substantially distortion-free heat treatment, preferably for local strength reduction and ductility increase, of sheet metal components ( 200 ) or sheet-like components, including at least one component to be treated ( 200 ) between two opposing electrodes ( 110A . 110B ) by force application between these electrodes ( 110A . 110B ) clamped point-like and with a via the electrodes ( 110A . 110B ) is resistively heated at this point (X), the electrical contact points (KA, KB) between the electrodes ( 110A . 110B ) and the component ( 200 ) has a diameter (D) greater than 10 times the square root of the sheet thickness (s) at that point (X). Method according to claim 1, characterized in that exactly one component ( 200 ) between the electrodes ( 110A . 110B ) is arranged. A method according to claim 1 or 2, characterized in that the contact points (KA, KB) are substantially circular. Method according to one of the preceding claims, characterized in that the electrodes ( 110A . 110B ) are formed identically. Method according to one of the preceding claims, characterized in that the local resistance heating, in particular at a constant electrode force profile (F (t)), comprises at least three phases: - a rapid heating phase (a) with a temperature rise of at least 50 K / sec; A temperature maintenance phase (b) lasting at least 1 second; and - a slow cooling phase (c) with a temperature drop of at least 5 K / sec and at most 25 K / sec. Method according to one of the preceding claims, characterized in that the electrodes ( 110A . 110B ) are formed of a heat-resistant material, in particular of a molybdenum, tantalum, tungsten or iron-based alloy. Method according to one of the preceding claims, characterized in that between the electrodes ( 110A . 110B ) and the component ( 200 ) exchangeable films ( 120A . 120B ) or that for the electrodes ( 110A . 110B ) exchangeable plug caps ( 130A . 130B ) be used. Method according to one of claims 1 to 7, characterized in that it is in the component ( 200 ) is a solid or high-strength steel component and that this is heated by means of local resistance heating to a temperature below the Ac1 temperature. Method according to one of claims 1 to 7, characterized in that it is in the component ( 200 ) is a solid or high-strength steel component and that this is heated by means of local resistance heating to a temperature above the Ac1 temperature and in particular above the Ac3 temperature. Method according to one of the preceding claims and in particular according to claim 8 or 9, characterized in that it is in the component ( 200 ) is a press-hardened sheet metal part made of a steel sheet material.

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