Abstract:
A workpiece made of plate is subjected to a treatment which locally modifies its magnetic permeability. Subsequently, the magnetic permeability of the workpiece is examined locally resolved by a probe in order to find at least one surface region which is suitable for intended processing, and the processing is performed locally limited to the selected region.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to German Patent Application No. 102015014490.1, filed Nov. 10, 2015, which is incorporated herein by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to a method for processing a workpiece made of plate. 
       BACKGROUND 
       [0003]    From DE 196 50 258 A1 a method is known, with which the properties of a metal part are modified on the surface by laser alloying. The alloying components are supplied in the form of rods or wires. Following the laser alloying, the modified regions are usually not difficult to distinguish from unalloyed regions, since during the supply of material by the rod or wire that is locally closely limited it can hardly be avoided that irregularities remain on the surface of the plate. 
         [0004]    In the application DE 10 2014 010 660.8 a method is described, with which the properties of a plate are locally modified, in that an amorphous mass containing an alloying element is locally placed onto the plate and heated together with the plate in order to alloy an alloying element contained in the mass into the part of the surface of the plate covered by the mass, while the uncovered remainder of the surface remains unalloyed. Since the amorphous mass can be applied over a large area but in a substantially thinner layer than with a rod or wire, such a treatment does not necessarily leave behind visible traces on the surface of the plate which make possible an unambiguous distinction of alloyed and unalloyed regions from one another. This makes it difficult to take into account the position of the alloyed regions during the further processing of the plate. However, when the position of the alloyed regions is not taken into account there is a high probability that a subsequent processing step is performed on a surface region which is not suitable for this, so that the processing fails or results in a product that is defective in quality. 
         [0005]    Accordingly, there is a need for a method with which it can be ensured in a simple and reliable manner that processing steps are performed on the surface regions provided for this purpose. 
       SUMMARY 
       [0006]    The present disclosure utilizes the fact that by locally alloying elements into the surface of a plate not only characteristics such as ductility, weldability etc. which are not only relevant for further processing are decisively influenced but also the ferromagnetism—which is not relevant for applications in particular in vehicle construction—and that the magnetic permeability of a surface region of a plate—in particular compared with other surface regions of the same plate—can clearly indicate whether the surface region concerned has been subjected to alloying or not. For this reason, when during the processing of a workpiece made of plate the magnetic permeability of the workpiece is examined locally resolved, the region of the plate which is suitable for an intended processing and which is not can be securely distinguished by way of the examination results, and the processing can be performed locally limited on the selected, suitable region. 
         [0007]    In automobile construction, substantially ferritic steel plates, i.e. plates with body-centered cubic lattice structure are used, which by nature are ferromagnetic. By heating to above the austenitization temperature, the lattice structure changes to face-centered cubic, and the ferromagnetism is lost. By the addition of austenite-forming alloying elements such as for example Ni, Co, Mn, C it can be achieved that the austenite state is retained even at room temperature. When the alloying of these elements stays locally selective, a plate with magnetic properties that are different from place to place can be obtained. 
         [0008]    Conversely, a ferritizing additive can be alloyed into an originally austenitic plate, for example made from a stainless steel, in order to create a region with a deviating magnetic permeability. 
         [0009]    A region, the magnetic properties of which differ from the remainder of a workpiece can be found, even if not visible with the eye, by scanning of the workpiece with a magnetic probe such as for example a Hall or Foerster probe. Based on the knowledge of the position of this surface region, a region of the workpiece to be processed—which can be the austenitic surface region itself or an adjacent ferritic region—can subsequently be located in order to perform local processing on this region. The local processing can in particular be a cutting of the workpiece in the selected region; thus the workpiece can be divided into multiple parts each of which can be further processed separately. 
         [0010]    A workpiece to be cut in this manner can have a plurality of identically formed treated regions, wherein the locating of a cut line by way of these regions results in that the treated regions are arranged in the same manner on each of the parts obtained. The workpiece can in particular be a plate strip generally uncoiled from a coil and the local processing a cutting-out of parts for further processing from the plate strip. 
         [0011]    The local processing can also be a forming of the workpiece in the selected region or the fastening of a second workpiece to the selected region. The fastening of the second workpiece can take place by welding, screwing or riveting. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present disclosure will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements. 
           [0013]      FIG. 1  shows a schematic sequence of a method according to a first configuration of the present disclosure; 
           [0014]      FIG. 2  shows a detail of the method from  FIG. 1 ; 
           [0015]      FIG. 3  show a local processing of a workpiece according to a second configuration of the method; 
           [0016]      FIG. 4  shows a local processing according to a third configuration of the present disclosure; and 
           [0017]      FIG. 5  schematically illustrates a workpiece for a fourth configuration of the method. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description. 
         [0019]      FIG. 1  schematically shows different stages of the method according to a first configuration of the present disclosure. At a first stage, shown in the right part of  FIG. 1 , a ferromagnetic steel plate to be worked is present as a plate strip  2  coiled into a coil  1 . 
         [0020]    The plate strip  2  is gradually uncoiled from the coil  1  and passes a screen printing device or, as shown in the figure, one or more spray nozzles  3 , which apply an amorphous mass  4  in periodically repeating patterns  11 , as purely exemplarily shown in a top view in  FIG. 2 , by spraying onto a part of the surface of the strip  2 , while the same passes under the nozzles  3 . The nozzles  3  can be stationary or moveable transversely to the running direction of the plate strip  2 . 
         [0021]    The amorphous mass contains at least one alloying element such as for example chromium or manganese, which during the alloying has an austenitizing effect on the regions of the plate strip  2  coated with the mass  2 . In addition, the mass  2  can also contain other elements which when alloyed-in impart the coated region of the plate strip  2  desired properties for the further processing, which can vary depending on the type of the intended further processing. 
         [0022]    Following the application of the amorphous mass  4  on the plate strip  2 , the same passes through an oven  5 , in which it is subjected to a heat treatment, during which the alloying elements out of the amorphous mass  4  diffuse into the surface regions of the plate  2  coated with the mass  4 . The heat treatment takes place under an inert gas atmosphere, for example nitrogen, in order to exclude undesirable reactions of the plate strip  2  or of the mass  4  with oxygen of the air. 
         [0023]    Following the heat treatment in the oven  5 , the plate strip  2 , as indicated by a dashed line  6 , can be directly separated by a punching tool or knife  7  into parts  8 ; however, the present disclosure is applicable particularly advantageously in the case where the plate strip  2  following the heat treatment is coiled into a coil  9  and the coil  9  is only subsequently uncoiled again for separating the workpieces  8 . Accordingly, in particular the coating with the amorphous mass  4  and the heat treatment in the oven  5  as specified by a customer can be carried out in the steel plant, possibly even immediately following the rolling and prior to the initial coiling, and the coil  9  locally alloyed in such a manner supplied to the customer. Following the delivery of the coil to the customer, separation and subsequent further processing of the parts  8  then takes place in the production facilities of the customer. 
         [0024]    The alloyed regions which are austenitized at least on the surface and the unalloyed regions of the plate strip  2  are distinguishable from one another by way of their different magnetic permeability. When the coil  9  is uncoiled at the customer and in the process the plate strip  2  is guided past a magnetic probe such as for example a Foerster probe  10 , the passage of the repeating patterns  11  in front of the probe  10  result in that the same supplies a periodic signal μ(t) that is representative for the magnetic permeability of the passing material, corresponding to the small diagram in  FIG. 2 , by way of which the position of the patterns  11  on the plate strip  2  is inferred and the movement of the knife  7  synchronized with the plate strip  2  in such a manner that the plate strip is severed in each case in a region  12  between two patterns  11  and a complete, well-centered pattern  11  is allocated to each part  8  severed by the knife  7 . 
         [0025]    A part  8  obtained in this manner forms the workpiece  8  of a following processing step shown in  FIG. 3 . The pattern  11  in this case includes two alloyed or austenitic regions, a ring  13  and a star  14 . The workpiece  8  can be placed for further processing by deep-drawing on a drawing die  15 . Here, three magnetic probes  10  are arranged on a circle the diameter of which corresponds to the ring  13 ; these can be contained in a hold-down which is not shown. When all three probes  10  measure a low magnetic permeability μ corresponding to an austenitic surface, the ring  13  then lies on the circle of the probes  10 , and the workpiece  8  is correctly centered with respect to the drawing die  15 . Accordingly, when a drawing mandrel  16  is lowered onto the workpiece  8 , the resulting deformation and stretching of the plate is limited to a non-alloyed ferritic region  17  of favorable ductility between the ring  13  fixed between drawing die  15  and hold-down and the star  14 . 
         [0026]      FIG. 4  shows a cup-shaped workpiece  18 , which could be obtained out of the workpiece  8  by the deep-drawing step of  FIG. 3 . The star-shaped alloyed region  14  is located on the bottom of the cup and in its middle a further workpiece may be fastened by welding, riveting, screwing or the like. In order to find the center of the star  14 , two magnetic probes  10  are guided over the bottom of the cup in approximately parallel paths  19  crossing the rays of the star  14 . In the process, two points  20 ,  21  are determined on each path  19 , at which the path  19  of a probe  10  crosses the rays of the star  14 . A point of intersection  22  of a straight line running through the points  20  and a straight line running through the points  21  indicates the center point of the star  14 , so that a tool  23  such as for example a drill, a welding rod or the like can be exactly aligned with the point  22  and with its help the second workpiece which is not shown be fastened exactly in the middle of the star  14 . 
         [0027]    The method according to the present disclosure is not necessarily limited to the differentiation of alloyed and non-alloyed regions and the utilization of their various properties. Accordingly it is by all means possible for example to apply patterns onto the plate strip which consists of two or more differently composed amorphous masses in order to obtain from this a workpiece  8  as shown in  FIG. 5 . Among the amorphous masses used there can be one which when alloyed into the plate supplies a region  24  with properties that are desirable for a certain type of further processing, but the magnetic properties of which do not differ to a sufficient degree from those of the unalloyed plate in order to allow a direct recognition of the region  24  by the magnetic probe  10  or its shape—for example since composed of many identical elements—is poorly suited for unambiguous position measurement. In order to unambiguously define in this region  24  a point  25 —for example for fastening a further workpiece thereto—a second mass can be applied which in the subsequent heat treatment forms austenitic regions  26  which are securely detectable on the treated plate and from the position of which that of the point  25  can be easily derived. 
         [0028]    While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.