Patent Description:
Hardening of sheet metal products is carried out to alter metallurgic properties of a piece of sheet metal. Traditionally hardening has been done by heating a piece of metal in a forge and for instance cooling it quickly using water. It has recently been suggested to partially heat a piece of sheet metal using a high-power laser.

One problem with such processes is that they are rather slow as the laser beam has to sweep the entire trace that need be heated. Of course, this could be remedied to some extent by using multiple lasers, but to a much higher cost.

<CIT> describes a tool for injection moulding with means for heating an active tool surface, the heating means including a coil for generating an oscillating magnetic field in a conductive top layer adjacent to the active tool surface. <CIT> describes a method for heating of sheet metal blanks using induction, the sheet metal blanks having a temperature distribution comprising different temperature zones after heating for subsequent hot forming or press hardening.

One object of the present disclosure is therefore to provide a hardening method which is more efficient. This object is achieved by means of a method as defined in claim <NUM>. More specifically, there is provided a method for producing a hardened sheet metal product comprises placing a sheet metal piece in a heating station, and heating selected areas of the sheet metal piece in the heating station by means of induction. In the heating station, a coil induces currents that flow in a front metal layer on a front side of the coil and first and second opposing ends of the front metal layer are interconnected by a short-circuiting arrangement running on a rear side of the coil, the short-circuiting arrangement comprising a material with lower resistivity than the front metal layer. The heated sheet metal piece is moved to a pressing station, and the sheet metal piece is pressed while cooling said heated areas.

With this method, all areas of the sheet metal piece that are to be heated can be heated simultaneously which provides for a much more efficient process. Further, the heated areas may be cooled more quickly and in a much more uniform manner, which provides improved hardening properties.

In one example, suitable e.g. for hardening steel, the front metal layer may comprise the sheet metal piece, i.e. the currents developing heat are induced in the piece to be processed itself.

In an alternative example, the front metal layer may comprise a heating layer located directly beneath the sheet metal piece, the sheet metal piece being separable from the heating layer. This is useful for treating sheet metal with very low resistivity, such as aluminum.

Inlays may be located directly beneath the front metal layer, which by conveying magnetic flows or electric currents partially reduce the development of heat in the front metal layer. This allows a pattern with any desired shape to be heated on the sheet metal piece.

A corresponding production arrangement and a corresponding heating station may also be considered.

<FIG> illustrates schematically an arrangement <NUM> for processing a sheet metal material according to the present disclosure. The arrangement <NUM> includes a heating station <NUM> and a pressing station <NUM> or press as well as a transport device <NUM>, devised to rapidly transport a heated piece of sheet metal from the heating station <NUM> to the pressing station <NUM>. This may be done such that pressing can take place within five seconds from ending heating.

The arrangement <NUM> realizes a method for producing a hardened sheet metal product that includes the following steps. First, a piece of sheet metal <NUM> is placed in the heating station <NUM> as shown in <FIG>. This may be done manually or automatically by means of an additional transport device (not shown). As illustrated, the piece of sheet metal <NUM> may be flat, although this is not necessary.

The heating station <NUM> heats the sheet metal piece, either the whole surface thereof or as illustrated in <FIG>, a pattern <NUM> on that surface. The amount of heating needed depends on the metallurgic properties of the material of the piece <NUM>, and the desired outcome of the hardening process. For instance, if a steel is hardened and a trace with a different crystalline structure is desired along the pattern <NUM>, the material in the pattern should be heated to a temperature e. g above <NUM>° C and then relatively quickly cooled.

With reference again to <FIG>, once the heating has taken place, in a manner described later, the piece <NUM> is quickly moved to the press or pressing station <NUM>, where the piece is pressed into a shape deviating from the previous shape, and optionally punched to provide cut-outs in the piece, as desired. Not only does this process reshape the piece <NUM>, but also cools the previously heated parts of the piece to provide a hardening effect, if needed. The result is a piece as shown in <FIG>. The hardened pattern <NUM> may help to provide the finished piece <NUM> with desired properties, for instance it may deform in a desired pattern thanks to a stiffer but more brittle material in the generated pattern <NUM>. This may be useful for instance for automotive products where driver and passenger safety requires that a colliding car is deformed in a predetermined way.

Returning again to <FIG>, the transport device is symbolically illustrated as a roller. However, various devices can be used to transport the piece <NUM> from the heating station <NUM> to the pressing station <NUM>, such as industrial robots, etc. In some hardening processes, it may be required that the time before the heating ends and the pressing takes place is short, typically under <NUM> seconds which sets a requirement for the transport device <NUM>.

The heating station <NUM> in <FIG> is illustrated as open upwards. However, it is possible, similar to in the pressing station <NUM> as illustrated, to provide an upper half also in the heating station <NUM> that presses against the lower half.

The heating of the selected areas of the sheet metal piece <NUM> in the heating station <NUM> is carried out by means of induction as will now be described with reference to a first example illustrated in <FIG>, showing a schematic cross section through a heating station <NUM>.

Specifically, there is used a coil <NUM> which is fed by a high frequency (typically in the range <NUM>-<NUM>) alternating current pulse. The coil is made from a low resistivity material, such as aluminum of copper, and is wound around a coil carrier <NUM>.

The coil carrier <NUM> may comprise a material with high resistivity, and that has a high relative magnetic permeability. Soft magnetic composites, such as for instance SOMALOY, comprising ferromagnetic granules that are sintered to a desired shape with an insulating plastic material is one example of materials suitable for this purpose.

The coil <NUM> and the coil carrier <NUM> will induce strong electric currents that flow in conductive neighboring elements. A main current loop is formed by the sheet metal piece <NUM> and a short-circuit arrangement <NUM>. The short-circuit arrangement <NUM> interconnects opposing edges <NUM>, <NUM> of the sheet metal piece <NUM>, which in this example forms a front metal layer. 'Front' here relates to the surface on a front side of the coil <NUM> and the surface of the heating station <NUM> where heating is intended to take place. Opposing edges <NUM>, <NUM> of the front metal layer, where the coil turns, are interconnected by a short-circuiting arrangement <NUM> running on a rear side of the coil <NUM>. Thus, strong alternating currents will run in the direction indicated by arrows in the drawing, while likewise alternating magnetic fields run perpendicular to the currents. The currents will develop heat in this loop. However, if the sheet metal piece <NUM> to be heated and making up the front metal layer is a steel that allows hardening, and, the short-circuiting arrangement <NUM> is made of e.g. copper or aluminum, most of that heat will be developed in the higher resistivity sheet metal piece <NUM> that becomes heated in a very efficient way. Generally, the short-circuiting arrangement <NUM> may comprise a material with lower resistivity than the front metal layer.

An intermediate conductive layer <NUM> may be placed in between the coil carrier <NUM> and the front metal layer <NUM>. This intermediate layer <NUM> may be electrically/galvanically insulated from neighboring layers but may itself be highly conductive, for instance made of copper or aluminum and may up to be a few centimeters thick. The coil <NUM> induces currents in the lower face of the intermediate layer <NUM>, and those currents run, due to the skin effect, close to the surface of the layer <NUM>, along the lower face, a first end face, the upper face and a second end face back to the lower face to form a closed loop close to the outer boundaries of the intermediate layer <NUM>. Therefore, strong currents will be present in the top surface of the intermediate layer <NUM> that assist in driving currents through the front metal layer <NUM> by induction.

A thermally insulating layer <NUM> may be placed beneath, typically directly beneath the front metal layer <NUM>. This layer serves to reduce the conduction of heat from the front metal layer <NUM> such that the latter can reach higher temperatures. Materials such as glass, ceramic compositions as for instance including yttrium stabilized zirconium, YSZ, or different plastic materials such as KAPTON, may be considered for this purpose.

<FIG> illustrates schematically a stack of layers in a second example of a heating station <NUM>'. This stack is intended for the heating of sheet metal pieces <NUM> which themselves have a very low resistivity, such as aluminum. In this example a stationary front metal layer <NUM> is provided that is connected to the short-circuiting arrangement <NUM>. That connection may be more or less permanent, and the stationary layer <NUM> may have similar properties as the sheet metal piece <NUM> making up the front metal layer in the example in <FIG>, i.e. high a conductivity, but still lower than the one of the short-circuiting arrangement.

In this example, therefore, it is the stationary layer <NUM> that is heated, and this heat in turn is conveyed to the sheet metal piece <NUM> stacked on the stationary layer <NUM>. Then, the sheet metal piece <NUM> is separated from the stationary layer and moved to the pressing station. This arrangement for instance allows hardening of aluminum. While the heating will not be as effective as in the previous example, temperature requirements are not as high.

<FIG> illustrates varying of the heating over a sheet metal piece's <NUM> surface. This may be accomplished by means of inlays that are located directly beneath the front metal layer, whether this is the sheet metal piece <NUM> itself as in <FIG> or a stationary metal layer as in <FIG>. The inlays may therefore be located in milled recesses in the underlying, thermally insulating layer <NUM>. The inlays may comprise electrically highly conductive pieces <NUM>, such as copper or aluminum. Alternatively, highly magnetically conductive pieces <NUM> with high relative magnetic permeability such as used in the coil carrier can be considered. The electrically conductive pieces <NUM> will locally divert the electric current from the front metal layer. Similarly, the highly magnetic pieces will divert the magnetic field from the front metal layer. In either case areas <NUM> with reduced heat development will result in the sheet metal piece, which allows selective hardening of the piece <NUM>. Additional cooling may optionally be provided, that cools areas not intended to be heated to some extent, e,g, by means of a fluid flow, for instance in the intermediate layer <NUM>.

Claim 1:
Method for producing a hardened sheet metal product comprising the steps of:
- placing a sheet metal piece (<NUM>) in a heating station (<NUM>),
- heating selected areas (<NUM>) of the sheet metal piece (<NUM>) in the heating station (<NUM>) by means of induction, wherein a coil (<NUM>) induces alternating currents that flow in a front metal layer (<NUM>; <NUM>) on a front side of the coil (<NUM>), where heating is intended to take place, and first (<NUM>) and second (<NUM>) opposing ends of the front metal layer (<NUM>; <NUM>) are interconnected by a short-circuiting arrangement (<NUM>) running on a rear side of the coil (<NUM>), the short-circuiting arrangement (<NUM>) comprising a material with lower resistivity than the front metal layer (<NUM>; <NUM>),
- moving the heated sheet metal (<NUM>) piece to a pressing station (<NUM>), and
- pressing the sheet metal piece (<NUM>) while cooling said heated areas (<NUM>).