Abstract:
A method for hot forming, in particular for press hardening, a component is disclosed, wherein in a local region of the component a reduced martensitic hardness is produced by locally reducing a forming pressure which is exerted on a surface of the component.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to German Patent Application No. 102014109552.9 filed Jul. 8, 2014, the disclosure of which is hereby incorporated in its entirety by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a method for hot forming, in particular for press hardening, a component, in particular for a vehicle. 
         [0004]    2. Description of Related Art 
         [0005]    On account of their high characteristic strength values, hot formed or press hardened components, respectively, are inter alia employed in vehicle construction as safety-relevant components. As the contact pressure per unit area increases, the hardness in the material increases on account of higher cooling rates. Excessive hardness in the material implies a higher probability of a fracture arising in the component which has been manufactured from the material. There is a requirement for a method which is suitable in mass production for hot forming, in particular for press hardening, components which in a predefined local region have a reduced hardness and thus a lower probability of brittle fractures arising. 
         [0006]    Some press hardening methods for producing a locally differentiated material hardness, such as partial press hardening, for example, are already known in the prior art. In the case of tailored tempering, for example, the pressing tools are only partially heated during hot forming. By way of targeted temperature control, a mixed microstructure with reduced strengths is produced in a localized manner here. However, here a martensitic formation of the material is suppressed in a targeted manner by way of reduced cooling rates. In a localized manner, tensile strengths of between 500 and 950 MPa are set. The resulting Vickers hardness is in the range of 170 HV to 320 HV, which may compromise the use of the component as a lightweight element. 
         [0007]    Likewise, there are attempts at modifying the alloy composition of the tool steel such that the tool steel has a locally reduced heat conductivity, on account of which the cooling rate of the component to be pressed drops in a localized manner and thus a lower hardness is achieved in a localized manner. In such alloy concepts with locally reduced heat conductivities according to the prior art, an operating temperature has to be first set in order to obtain a reduced cooling rate of the material to be hardened. From the point of view of production, this does not represent a reliable process. Therefore, attempts including alloy compositions of the tool steel which have been especially modified for press hardening to date have not been implemented in large-scale production environments. 
       SUMMARY OF THE INVENTION 
       [0008]    It is an object of the present invention to provide a reliable and reproducible method which is suitable for large-scale mass production for hot forming, in particular for press hardening, a component, which in predefined local regions of the component to be formed enables martensitic and/or martensitic/bainitic hardening, wherein the resulting hardness in the predefined local regions is reduced in relation to the other regions of the component. 
         [0009]    The object of the present invention is achieved by a method for hot forming, in particular for press hardening, a component, wherein in a local region of the component a reduced martensitic hardness is produced by locally reducing a forming pressure which is exerted on a surface of the component. 
         [0010]    In contrast to the prior art, the drive device according to the invention here has the advantage that a reduced martensitic hardness is obtained in the predefined local regions, such that the probability of hydrogen embrittlement on the predefined local regions is significantly reduced. On account thereof, the probability of brittle fractures arising in the predefined local regions, in particular on account of stress peaks which could result during or after the assembly of the component with other components, is again significantly reduced. This is due to the stress created on the martensitic boundary surfaces being able to be dissipated by adaptive deformations. 
         [0011]    Advantageous design embodiments and refinements of the invention may be derived from the dependent claims and from the description with reference to the drawings. 
         [0012]    In one further embodiment it is provided that regulating the locally reduced forming pressure is performed by means of an element which is spring-mounted in a lower tool and/or in an upper tool of a hardening tool. 
         [0013]    On account of the use of spring-mounted tool elements, a technical implementation of the necessary cooling conditions in order to implement a desired martensitic formation with the desired reduced hardness is enabled in an advantageous manner. On account of the use of the spring-mounted elements a locally reduced contact pressure per unit area in comparison with the other regions is achieved on the material to be formed when the hardening tool is being closed, on account of which the cooling rate is reduced in a localized manner and thus the locally resulting martensitic hardness is lower than in the other regions of the material, in particular steel sheet, to be formed. Moreover, the previously used tool steels may furthermore be advantageously further utilized in a slightly modified form. 
         [0014]    If a spring-mounted element is embedded both in the lower tool as well as in the upper tool of the hardening tool, localized reduction of a martensitic hardness can be obtained in a simple and thus advantageous manner on both facing sides of the steel sheet to be formed. If the layer thickness of the steel sheet to be formed is modest, localized spatial reduction of a martensitic hardness which permeates the respective cross section may even be obtained in the steel sheet to be formed. On account thereof, a direction for the adaptive deformations in the resulting component which are created by stress can be predetermined. 
         [0015]    In one further embodiment, that side of the spring-mounted element that faces the surface of the component contacts in a form-fitting manner the surface of the component, and a forming accuracy in the local region conforms to a remaining forming accuracy in further regions of the component. 
         [0016]    On account thereof, it can be advantageously achieved that the forming accuracy of the steel sheet to be formed is retained, since cooling takes place at full contact in the forming tool. 
         [0017]    According to one further embodiment it is provided that regulating the locally reduced forming pressure is implemented by means of a gas pressure spring and/or a disc spring and/or an elastic spring as the spring-mounted element. 
         [0018]    On account of the use of a gas pressure spring a force which is almost independent of the spring travel can be advantageously generated, wherein the space requirement is low and at the same time there exists the potential for a damping mechanism to be simultaneously integrated in the spring. 
         [0019]    On account of the use of disc springs, the spring travel may be varied depending on a type of configuration; if the spring disc is assembled as a spring pack, the spring force of the individual disc spring can be multiplied by the number of disc springs, meaning a high spring force at short spring travel; if the spring disc is assembled as a spring stack, the spring force of the individual disc spring is equal to the spring force of the entire spring stack, meaning low spring force at long spring travel. On account thereof, the spring-mounted tool elements can be readily adapted in terms of their dimensions to the tool steels which are used for press hardening. 
         [0020]    In one further embodiment it is provided that the spring-mounted element is mounted in a die and/or mould region of the lower tool and/or of the upper tool of the hardening tool. 
         [0021]    On account thereof, installation space which is already present is advantageously utilized, such that no additional installation space is required. 
         [0022]    In one further embodiment it is provided that a hydraulic installation is embedded in the lower tool and/or in the upper tool of the hardening tool. 
         [0023]    On account thereof, fine tuning of the forming pressure which is exerted in a localized manner on the steel sheet to be formed is advantageously implemented. 
         [0024]    The present invention furthermore comprises a device for hot forming, in particular for press hardening, a component, characterized in that the device is configured for producing in a targeted manner a local region with a reduced martensitic hardness in the component, by locally reducing a forming pressure which is exerted on a surface of the component. 
         [0025]    The device advantageously enables to counteract a brittle martensitic formation in predefined localized regions of the component by way of a reduced martensitic hardness. 
         [0026]    In one further embodiment, the device comprises the following means: 
         [0027]    a lower tool which is configured for receiving on its upper side a component in a form-fitting manner; 
         [0028]    an upper tool which is configured so as to be pushed by way of its lower side in the direction towards the upper side of the lower tool when the device is being closed, 
         [0000]    wherein the lower tool and/or the upper tool have/has a clearance which is configured for receiving a spring-mounted element. 
         [0029]    In one further embodiment it is provided that the spring-mounted element, by way of its side that faces the surface of the component, is configured so as to surround the component in a form-fitting manner. 
         [0030]    In one further embodiment it is provided that the clearance comprises a die and/or mould region. 
         [0031]    In one further embodiment it is provided that the spring-mounted element comprises a gas pressure spring and/or a disc spring and/or an elastic spring. 
         [0032]    In one further embodiment it is provided that a hydraulic installation is incorporated in the lower tool and/or in the upper tool of the device. 
         [0033]    Further details, features, and advantages of the invention are derived from the drawing and from the following description of preferred embodiments by means of the drawing. The drawing here merely illustrates exemplary embodiments of the invention, which do not limit the substantial concept of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0034]      FIG. 1  shows a device for hot forming, in particular press hardening, a component. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0035]    A device  100  for hot forming, in particular for press hardening, a component  106  is schematically illustrated in  FIG. 1 . The device is composed of an upper tool  102  and a lower tool  104  between which the component  106  to be formed is disposed in the hot forming process, in particular when press hardening. 
         [0036]    An element  110 , which in comparison with the remaining surface of the lower tool  104  exerts a reduced forming pressure on a local region  108  of the component  106  during the press hardening process, is embedded in the lower tool  104 . On account of a reduced forming pressure being exerted in the local region  108  of the component  106 , the cooling conditions are modified such that the cooling rate in comparison with the remaining region of the surface of the lower tool  104  is reduced. On account of the reduced cooling rate in the local region  108  of the component  106  it is achieved that the resulting martensitic hardness in the local region  108  of the component  106  in comparison with the remaining regions of the component  106  is reduced. 
         [0037]    Exerting a reduced contact pressure per unit area in the local region  108  of the component  106  is achieved in that the element  110  which is embedded in the lower tool  104  is spring mounted. In one embodiment of the invention, the spring-mounted element  110  may be configured as a gas pressure spring or a disc spring or an elastic spring. Installations based on the hydraulic principle may also be incorporated in the forming table. In the case of all embodiments, the effective contact pressure per unit area acting on the local region  108  of the component  106  may be regulated by way of the spring force. 
         [0038]    In one further embodiment, the spring-mounted element  110  may be embedded in the upper tool  102 . In one further embodiment, spring-mounted elements  110  may be embedded both in the lower tool  104  as well as in the upper tool  102 . In all embodiments, the contact face which is directed towards the component  106  adjoins thereon in a form-fitting manner, such that on the one hand cooling takes place in the forming tool  100  at full contact, and on the other hand a forming accuracy is retained during press hardening. In one further embodiment, the spring-mounted element  110  is disposed in the die and/or mould region of the tool. 
         [0039]    A substantial constructive modification of previously utilized press tools is thus not required. The embodiments described of the device  100  according to the invention for press hardening meet the criteria of reproducibility, controllability, and safety, such that the resulting method according to the invention is suitable for press hardening in large-scale mass production. By way of regulating in a targeted manner the forming pressure which is exerted in the local region  108  of the component  106  by the spring-mounted element  110 , the degree of hardness of the localized martensitic formation can be set in a targeted manner, such that undesirable hydrogen embrittlement can be counteracted in a targeted manner according to the local material requirements which are predefined by stress peaks during or after the assembly of the component  106  with other components. 
       LIST OF REFERENCE SIGNS 
       [0000]    
       
           100  Device for hot forming, in particular for press hardening 
           102  Upper tool 
           104  Lower tool 
           106  Component 
           108  Local region with reduced hardness 
           110  Spring-mounted element