Abstract:
A method and system are disclosed for treating a press hardened part by induction heating localized areas of the part to have reduced hardness. The method and system monitor an ambient temperature, cycle time, outgoing part property requirements, and outgoing part hardness in local areas. A time value and temperature value are set by a computer system for a plurality of induction heaters. A local area of the part is induction heated to soften the part in localized areas. The hardness of the localized areas is tested after induction heating.

Description:
TECHNICAL FIELD 
       [0001]    The disclosure relates to a system and method of manufacturing hot stamped steel parts to include areas that are induction heated to soften selected areas to receive fasteners. 
       BACKGROUND 
       [0002]    Press hardened steel alloys are being used for sheet metal parts incorporated in vehicle body structures that may be assembled together with rivets. One example of a press hardened steel is cold rolled boron steel sold under the designation Docol® 22MnB5. Press hardened steel can be water cooled or oil cooled to desired level of hardness from 450 to 520 HV. Press hardened steel may be annealed to reduce the hardness to 140 HV. 
         [0003]    Press hardened steel parts may be assembled to other steel parts by welding. However, new automotive assemblies may include combinations of parts made of different materials such as aluminum and composite parts. A press hardened Ultra High Strength Steel (UHSS) beam and a composite part or an aluminum part cannot be efficiently joined together in a welding operation. The preferred technique for joining such part assemblies is to rivet or otherwise fasten the parts together. The hardness of such high strength parts poses significant challenges in high volume manufacturing operations because the rivets have difficulty penetrating the press hardened UHSS beam. 
         [0004]    Press hardened UHSS parts may also be used to manufacture controlled energy absorption applications but the high hardness and tensile strength of the parts prevents substantial energy absorption. 
         [0005]    One approach to solving the above problems is to locally soften the part while in the hot stamping die. However, locally softening of the part while disposed in the hot stamping die is unreliable because it is difficult to control all of the process variables in the hot stamping die. 
         [0006]    This disclosure is directed to solving the above problems and other problems as summarized below. 
       SUMMARY 
       [0007]    According to one aspect of this disclosure, a method is disclosed for making a press hardened part with induction heated localized areas having lower hardness. The method includes monitoring an ambient temperature, system inputs including cycle time and outgoing part property requirements, outgoing part hardness in local areas, and at least one table of material properties. Time values and temperature values are set for a plurality of induction heaters. A local area of the part is induction heated and the hardness of the local area of the part is determined after induction heating. 
         [0008]    According to other aspects of this disclosure, the method may further comprise determining a temperature distribution of the part following a hot stamping operation and prior to induction heating. The method may further comprise thermally imaging the part following induction heating. 
         [0009]    According to another aspect of the method, a material property of the outgoing part (e.g., tensile strength or yield strength) is converted to a hardness value that is provided to a computer system that sets the time value and temperature value for the induction heaters for subsequent parts. 
         [0010]    According to another aspect of this disclosure, the method may further comprise measuring a material property of a part during induction heating providing a real-time in process temperature distribution data. The real-time in process temperature distribution data is provided to a computer system that sets the time value and temperature value for the induction heaters. 
         [0011]    According to a further aspect of this disclosure, the table of material properties may include a table of tempering curves, a table of material properties, and a strength-to-hardness conversion table. 
         [0012]    In one embodiment, the plurality of induction heaters may be arranged in parallel with each other and the step of induction heating may be performed simultaneously on a plurality of parts. In an alternative embodiment, the plurality of induction heaters may be arranged in series and the step of induction heating may be performed sequentially by incrementally heating the part. 
         [0013]    According to another aspect of this disclosure, a system is disclosed for locally softening a press hardened part. The system comprises an induction heater, an ambient temperature sensor, an interface for setting a cycle time and a desired part property corresponding to a hardness value in a local area, and a controller. The controller sets a time value and a temperature value for the induction heater based upon an ambient temperature, the cycle time, the desired part property, and the hardness value in the local area. 
         [0014]    According to other aspects of this disclosure as it relates to the system, at least one table of material properties may be used to provide hardness information relating to the part to the controller. The system including the induction heater may further comprise a plurality of induction heaters arranged in parallel with each other. The induction heaters simultaneously heat a plurality of parts. Alternatively, the system may include a plurality of induction heaters arranged in series for sequentially and incrementally heating the part. The ambient temperature sensor may be a thermal imaging camera. 
         [0015]    According to another aspect of the system for locally softening sheet metal parts, the system may include a controller that receives a temperature of the parts, production rate data, desired part property data, and feedback data regarding a physical property test. An induction heater may be used that receives control signals from the controller setting a desired temperature and a desired time for induction heating the parts. 
         [0016]    The above aspects of this disclosure and other aspects are described below with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a flowchart illustrating one embodiment of a system for locally softening hot stamped parts in a manufacturing process. 
           [0018]      FIG. 2  is a flowchart of an induction heating manufacturing process with a plurality of induction heating systems arranged for parallel processing of parts. 
           [0019]      FIG. 3  is a flowchart of an induction heating manufacturing process with a plurality of induction heating systems arranged for series processing of parts. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The illustrated embodiments are disclosed with reference to the drawings. However, it is to be understood that the disclosed embodiments are intended to be merely examples that may be embodied in various and alternative forms. The figures are not necessarily to scale and some features may be exaggerated or minimized to show details of particular components. The specific structural and functional details disclosed are not to be interpreted as limiting, but as a representative basis for teaching one skilled in the art how to practice the disclosed concepts. 
         [0021]    Referring to  FIG. 1 , a flowchart is provided to illustrate the steps of a system  10  for local softening of hot stamped parts in a high volume manufacturing operation. An incoming part  12  may be a part from storage or may be a part received from a hot stamping line. The incoming part  12 , for example, may be an ultra-high strength steel (UHSS) beam  16  that may be used as a reinforcement for a door. 
         [0022]    If the incoming part  12  is received directly from a hot stamping line with residual heat, the temperature of the part may be determined by a thermal imaging camera  18 . Alternatively, the temperature profile of the part  12  may be determined by a thermistor, or the like, instead of a thermal imaging camera  18 . In either event, temperature distribution data is compiled at  20 . The temperature distribution data provides real time incoming part temperature distribution data for the part at various locations and, in particular, in locations where the part is to be induction heated for softening. The temperature distribution data at  20  is provided to a computer control system  22 . The computer control system  22  may be a general purpose computer programmed according to the algorithm illustrated by the flowchart of  FIG. 1 , or may be a programmable controller used in manufacturing systems. 
         [0023]    A thermometer  24  may be used to provide real time ambient temperatures that are compiled at  26  and provided to the computer system  22 . Ambient temperature is monitored by the system to provide a base line for thermal treatment of the incoming part  12 . 
         [0024]    System inputs at  28  are provided to the computer system  22  including the required production volume that may be expressed in terms of cycle time and the properties required for outgoing parts, for example, the hardness or extent of softening required to be obtained by the system for locally softening parts  12 . 
         [0025]    The computer system  22  is also provided with engineering data from a knowledge base  30 . The knowledge base  30  may include lookup tables that provide material tempering curves, material properties, or hardness to strength conversion tables. 
         [0026]    Based upon the incoming temperature distribution data  20 , ambient temperature data  26 , system input parameters  28  and knowledge base  30 , the computer system  22  determines process settings such as the induction heating settings for the required part temperature the required hold time for locally softening hot stamped parts. The computer system  22  sets process settings at  32  for an induction heating controller  36 . The induction heating controller  36  controls the induction heater  38 . It should be noted that according to this disclosure, a plurality of induction heaters  38  may be provided to reduce the time necessary to locally soften the in-process part  40 . 
         [0027]    After induction heating, the in-process part  40  may be thermally imaged by a thermal imaging camera  42 . The thermal imaging camera  42  may be used to provide real time in-process part temperature distribution data at  44 . The temperature distribution data compiled at  44  is provided to the computer system  22  to provide real time data upon which the computer system  22  may adjust the required part temperature or required hold time. The in-process part  40  is transferred from the induction heating system  34  after induction heating is completed and the in-process part  40  may be referred to as an outgoing part  48 . The outgoing part  48  is an UHSS part that includes locally softened areas that are softened after the hot stamped parts are formed. The outgoing part  48  is tested to determine the hardness of the outgoing part  48 . The outgoing part  48  is tested by a hardness tester at  50  for future reference and for on-line, real time adjustment of the induction heating process settings. 
         [0028]    Referring to  FIG. 2 , an induction heating system  34  is shown that includes a plurality of induction heating systems arranged in parallel on-line. Incoming parts  12  are transferred by a material handling system to a plurality of induction heating systems  34 A,  34 B, and  34 N. The induction heating process is performed in a single step in the embodiment of  FIG. 2 . The parts are heated by induction heating and held at a desired temperature for locally softening portions of the incoming part over time. For example, the system may heat the localized area of the incoming part  12  to a temperature of between 700° and 870° C. The part is held for a predetermined period of time to soften the incoming part  12 . The time required for induction heating is substantial. The time required may be 10 to 20 seconds at a high temperature (e.g., 870° C.) or at lower temperatures (e.g., 700° C.) may require 40 to 60 seconds. A plurality of induction heating systems  34 A- 34 N may be provided to match the desired cycle time. An incoming part may be directed to an induction heating system  34  while other incoming parts are being processed on other induction heating systems that are in a parallel arrangement. The number of on-line induction heating systems may be selected to decrease the cycle time. Additional induction heating system capacity may be added or may be reduced to achieve the desired cycle time. One or more of the induction heating systems  34 A- 34 N may be deactivated or reactivated, as needed to meet production requirements. In the embodiment of  FIG. 2 , after induction heating, the part is removed from the induction heating system  34  and is processed as an outgoing part  48  as previously described with reference to  FIG. 1 . 
         [0029]    Referring to  FIG. 3 , a series processing induction heating process  58  is illustrated for induction heating an incoming part  12  to locally soften the part. The incoming part  12  is processed in a series of induction heating systems  60 A,  60 B to  60 N. The induction heating systems  60  each heat the incoming part  12  to a desired temperature. For example, the induction heating system at  68  may heat an incoming part  12  to 400° C. before transferring the part to an induction heating system  60 B that heats the part to 500° C. This process of incrementally heating the part continues until induction heating system  60 N that heats the part to 700° C. Less time is required to heat the part at each stage to a desired temperature for softening localized areas of the outgoing part  48 . 
         [0030]    Softening a part using induction heating can be achieved by either high temperatures and low time, or low temperatures over a long period of time. If a part is softened using a high temperature for a limited period of time, increased variability may be encountered as to the extent of softening. Softening utilizing high temperatures also creates a risk of eventual re-austenization of the part and may result in hardening the part instead of softening. The system  10  controls the temperature and time of the induction heating process to assure that the part is softened. 
         [0031]    If the system  10  for locally softening part is provided in-line with a hot stamping tool, the output of the plurality of induction heating system  34  should be matched to the cycle time of the hot stamping tool. Due to the thermodynamics and kinetics involved with heat treating metals, in this case, high UHSS materials, different approaches may be used to achieve the target properties that may be expressed in terms of hardness or yield strength. 
         [0032]    Induction cells arranged in series each increase the temperature of the part by a set increment. On the other hand, induction cells arranged in parallel process multiple parts at the same time on separate induction heating systems so that the cumulative number of parts produced meet the cycle time requirements. 
         [0033]    The embodiments described above are specific examples that do not describe all possible forms of the disclosure. The features of the illustrated embodiments may be combined to form further embodiments of the disclosed concepts. The words used in the specification are words of description rather than limitation. The scope of the following claims is broader than the specifically disclosed embodiments and also includes modifications of the illustrated embodiments.