Abstract:
Provided is a technique in which blanks in different shapes are uniformly healed using energization healing. An energization heating process (S 1 ) is a method for heating a blank ( 1 ) by connecting a pair of electrodes ( 10, 10 ) to two different end parts of the blank ( 1 ) so as to energize the electrode pair ( 10, 10 ), wherein the blank ( 1 ) is provided with void parts (cutouts ( 4, 4 ), a hole ( 5 )) provided in a direction approximately perpendicular to the equipotential line generated between the electrode pair ( 10, 10 ), and current passages (current paths ( 20, 20 )) are arranged in the direction approximately perpendicular to the equipotential line generated between the electrode pair ( 10, 10 ) within the regions spaced by the void parts ( 4, 4, 5 ) in the blank ( 1 ). The cutouts ( 4, 4 ) are formed with the end parts of the blank ( 1 ) as open parts, the hole ( 5 ) is provided to the inside of the blank ( 1 ), and the reverse side of the side on which the current paths ( 20, 20 ) arranged in the cutouts ( 4, 4 ) are connected to the blank ( 1 ) is connected to the electrodes ( 10, 10 ).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a method for heating a blank by energization heating, and particularly to a technique of electrically heating the blank for die quenching. 
       BACKGROUND ART 
       [0002]    Die quenching is well known, in which steel plate blanks are heated by energization heating and press-formed in a mold (for example, see JP 2008-87001 A). The blank to be press-formed is heated in advance so that the moldability is improved. 
         [0003]    The blanks are heated above the predetermined temperature (where the austenaitc transformations occur), and the blanks are kept in contact with the cold mold, thereby quenching is performed with the press-forming. 
         [0004]    In the respects of environment and safe, the products made of the steel plates for automotive applications have been high strength recently. However, the high strength plates need the guarantee in accuracy of connecting the multiple products. Moreover, in order to improve productivity and reduce the number of parts, the integration of multiple parts is required. 
         [0005]    There are various techniques of answering such requirements, for instance, in order to integrate the multiple parts into one member, the high-strength blanks with desired shape (H-shape. T-shape or holed shape) are prepared, whereby the blanks with different shapes are heated and press-formed. 
         [0006]    In order to heat the blanks with different shapes uniformly, heating the blanks for a long time in the heating furnace is not preferable because the facility and energy for the furnace would cost too much. 
         [0007]    When the technique of JP 2008-87001 A is used to heat the blanks having the different shapes, in which the energization is operated from one end to the opposite end of the blank, there may be a variation in electric current flow at spaces between the electrodes where the section area changes largely. Thus, there may be a variation in current density in the blank, and it is difficult to obtain the even heating. To avoid such defectives, the multiple parts tire prepared for configuring the blank with the different shape, and the heating process and press-forming process is performed to each part, after that the multiple parts are combined into the blank. 
         [0008]    Alternatively, JP 2002-248525 A discloses the technique of heating the blank with the different shape by energization heating, in which the multiple pairs of electrodes are connected to the opposite ends of the blank and used to energize the blank. Unfortunately, the technique of JP 2002-248525 A may fail to equalize the current density in the blank, because the current density largely changes at the portion where the section area perpendicular to the energization direction largely changes (e.g., if the blank has H-shape, the connection portions between the two parallel portions and the orthogonal portion). 
         [0009]    As mentioned above, it is difficult to uniformly heat the blank that has the different shape in response to the recent requirement. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         PTL 1: JP 2008-87001 A 
         PTL 2: JP 2002-248525 A 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0012]    The present invention aims to provide a technique of evenly heating a blank having a different shape using an energization heating. 
       Technical Solution 
       [0013]    The first embodiment of the present invention is a method for heating a blank by an energization using a pair of electrodes connected with two different ends of the blank, wherein the blank has a space formed in a direction perpendicular to equipotential lines generated between the electrode pair, and a current path is arranged at both ends of a periphery separated by the space in the direction perpendicular to the equipotential lines. 
         [0014]    The second embodiment of the present invention is a method for heating a blank by an energization using a pair of electrodes connected with two different ends of the blank, wherein the blank has a space formed in a direction perpendicular to equipotential lines generated between the electrode pair, and the space comprises: a first space formed at an end of the blank, opening the end of the blank; and a second space formed inside the blank, current paths are arranged at both ends of peripheries separated by the first and second spaces in the direction perpendicular to the equipotential lines, and the current path connected to the first space is connected to the electrode. 
         [0015]    In the advantageous embodiment of the present invention, the electrode pair is configured as bar electrodes disposed in parallel, and connected to the two opposite ends of the blank, and the current path is arranged perpendicular to the arrangement direction of the electrode pair. 
         [0016]    Preferably, the current path is made of a material having lower electric resistance. 
         [0017]    More advantageously, the end periphery separated by the space in the blank, to which the current path is connected, is formed as an inclined line or a curved line, and the current path is connected to the inclined or curved line of the blank via an extension material made of the same material as the blank and disposed perpendicular to the arrangement direction of the electrode pair. 
         [0018]    In the embodiment of the present invention, the blank comprises: a first portion extended straightly from one end to the opposite end of the blank; a second portion extended with curved shape from the one end to the opposite end of the blank and combined to the first portion at the opposite end; and a third portion connecting the middle portions of the first and second portions, and one of the electrode pair to which the one end of the blank is connected is longer than the other one to which the opposite end of the blank is connected. 
         [0019]    The third embodiment of the present invention is an apparatus for heating a blank by an energization using a pair of electrodes connected with two different ends of the blank, wherein the blank has a space formed in a direction perpendicular to equipotential lines generated between the electrode pair, a current path is provided with at both ends of a periphery separated by the space in the direction perpendicular to the equipotential lines, the electrode pair is configured as bar electrodes disposed in parallel, and connected to the two opposite ends of the blank, and the current path is arranged perpendicular to the arrangement direction of the electrode pair. 
       Advantageous Effects of Invention 
       [0020]    According to the embodiment of the present invention, when operating the energization healing to the blank having the different shape formed with a portion where the section area changes such as spaces, the spaces are bypassed and the current density in the blank is equalized. Therefore, the blank having the different shape is heated evenly by using the energization heating. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0021]      FIG. 1  illustrates a blank. 
           [0022]      FIG. 2  illustrates an energization heating process. 
           [0023]      FIG. 3  illustrates an electrode pair and equipotential lines generated between the electrode pair. 
           [0024]      FIG. 4  shows a conventional energization heating process. 
           [0025]      FIG. 5  shows a distribution of current density by the conventional energization heating process. 
           [0026]      FIG. 6  shows a distribution of current density by the present energization heating process. 
           [0027]      FIG. 7  depicts an alternative pair of electrodes and equipotential lines generated by the electrode pair. 
           [0028]      FIG. 8  illustrates an alternative embodiment of the blank. 
           [0029]      FIG. 9  illustrates an alternative energization heating process. 
           [0030]      FIG. 10  shows an electrode pair and equipotential lines generated by the electrode pair. 
       
    
    
     EXPLANATION OF NUMERALS 
       [0031]      1 : blank,  10 : electrode,  20 : current path,  50 : blank,  60 : electrode pair,  70 : group of current paths,  80 : group of extension materials 
       DESCRIPTION OF EMBODIMENTS 
       [0032]    Referring to attached drawings, embodiments of a method for energization heating according to the present invention are described below. 
         [0033]    In the energization heating method, blanks are energized and heated. After the energization heating, the blanks are delivered to die quenching process or hot press process. 
         [0034]    During the die quenching process, the blanks, which have been heated above a predetermined temperature by the energization heating method of the present invention, are press-formed with the blanks rapidly quenched in a press mold. 
         [0035]    The die quenching process is required to improve the quality of press-forming and of quenching. In other respects, the objective is to heat the blanks evenly such that the blanks to be delivered to the die quenching process are heated above the predetermined temperature where the qualities of press-forming and quenching are guaranteed. 
         [0036]    Moreover, to reduce the number of process and the number of members, it is required to prepare the blanks ready to be used as a product through subsequent processes such as die quenching and trimming, that is, the blanks having different shapes from rectangular, and to directly transfer from the energization heating process to die quenching process. 
         [0037]    The present invention provides a new energization heating technique solving the above problems, and the embodiments of the invention are described below. 
       First Embodiment 
       [0038]    Referring to  FIGS. 1 to 5 , an energization heating process S 1  as a first embodiment of the energization healing method is described below, in which a blank  1  is energized and heated. 
         [0039]    The blank  1 , as a heating object in the energization heating process S 1 , is made of a material with conductivity and quenchability such as steel. The blank  1  is a plate having a “different shape.” 
         [0040]    The “different shape” means the shape different from rectangle that is used for the object to be heated in the conventional energization heating process. For instance, the different shape is a H-shape, a T-shape, or a holed shape that is obtained by trimming a rectangular part or integrating some parts, and the blank with such shape is used as a product after the die quenching process and trimming process. 
         [0041]    Furthermore, a blank having rectangular shape, into which multiple parts with different resistances are integrated by laser welding or the like, is accounted as the different shape in the invention, because when energizing such blank, the current density varies in response to the electrical resistances of the multiple parts and it is difficult to provide the even heating distribution. 
         [0042]    For the convenience of the explanation, the upper-lower direction and left-right direction of the blank  1  are defined as the upper-lower direction and left-right direction in  FIG. 1 , respectively. 
         [0043]    As shown in  FIG. 1 , the blank  1  has two lateral portions  2  and two vertical portions  3 , and the ends of the vertical portions  3  are connected to the sides of the lateral portions  2 , thereby integrated into one part. 
         [0044]    The lateral portions  2  are disposed in parallel and extended from one end to the opposite end of the blank  1  (in the left-right direction). The vertical portions  3  are disposed in parallel and extended perpendicular to the left-right direction (in upper-lower direction). 
         [0045]    The blank  1  has two cutouts  4  at the both ends and a single hole  5  at the center. The cutouts  4  are disposed at the both opposite ends of the blank  1  and partially open the ends of the blank  1  rectangularly. The hole  5  is a rectangular opening disposed at the center of the blank  1 , surrounded by the portions of blank  1 . The blank  1  is formed in the holed shape, in which the cutouts  4  and the hole  5  are removed from the rectangular shape. 
         [0046]    The way of preparing the blank  1  is to trim the cutouts  4  and the hole  5  from the rectangular plate or to combine the lateral portions  2  and the vertical portions  3  (prepare a tailored blank). 
         [0047]    In the blank  1 , prepared in the above-described manner, the connecting portions between the lateral portions  2  and the vertical portions  3  are formed as a portion where the section area changes largely along the upper-lower direction perpendicular to the line from the left end to the right end, and as a portion where the section area changes largely along the left-right direction perpendicular to the line from the upper end to the lower end. 
         [0048]    In other words, the cutouts  4  and the hole  5  make the blank  1  defined as the object having the large variation in section area along not only the left-right direction but also the upper-lower direction. 
         [0049]    As illustrated in  FIG. 2 , in the energization heating process S 1 , a pair of electrodes  10  and multiple current paths  20  are used to heat the blank  1 . 
         [0050]    The electrode pair  10  and the current paths  20  are installed in an energization heating apparatus, to which the blank  1  is transferred and the energization heating process S 1  is operated. 
         [0051]    The electrode pair  10  energizes the blank  1 , and the one is used for a positive electrode and the other is used for a negative electrode. The electrode  10  is configured as a bar electrode having a longitudinal direction. The electrodes  10  are connected to a power source feeding the desired electric current, which applies current to the blank  1  through the electrodes  10 . In the blank  1 , the current occurs from the positive electrode  10  to the negative electrode  10 . 
         [0052]    The electrode  10  is extended along the upper-lower direction and has the substantially same length as the blank  1 . The electrode pair  10  is arranged to contact the both ends of the lateral portions  2  of the blank  1 , that is, both ends in one direction of the two perpendicular directions. The energization direction of the electrodes  10  is the left-right direction of the blank  1 . 
         [0053]    As shown in  FIG. 2 , the electrode pair  10  includes multiple connectors  11  provided with clamping structure for clamping the blank  1  from the thickness direction to secure the electrical connection with the blank  1  and the current paths  20 . The connector  11  includes clips to clamp the blank actuated by an air cylinder or a hydraulic cylinder, and the actuators switch the connecting/disconnecting between the electrode  10  and the blank  1 . 
         [0054]    The clamping structure of the connectors  11  contained in the electrode pair  10  enables to maintain the contact between the blank  1  and the electrodes  10 . The clamping-type connectors reduce the influence of the deformation such as curving or roll back of the blank  1  that occurs during the energization heating and provide the uniform heating, compared with contact-type connectors. 
         [0055]    If the blank  1  is configured in rectangular, the equipotential lines generated from the positive electrode  10  to the negative electrode  10  are shown in  FIG. 3 . As shown in  FIG. 3 , the bar electrodes  10  generate the equipotential lines parallel to the arrangement direction of the electrodes  10 . 
         [0056]    Actually, the blank  1  has the cutouts  4  and hole  5  extended perpendicular to the equipotential lines between the electrodes  10 . In the embodiment, the cutouts  4  are spaces between the electrodes  10  and the blank  1 , and the hole  5  is space disposed inside of the blank  1 , whereby these spaces act as non-energized areas and bring the variation in current density. 
         [0057]      FIG. 4  shows the conventional energization heating process, in which the blank  1  is heated by the electrode pair  10 . 
         [0058]    The energization to the blank  1  is operated in one direction (from right to left in drawing) by using the electrodes  10 . There occurs current from the right side to the left side of the lateral portions  2  of the blank  1 . 
         [0059]    In the connecting area A where the lateral portions  2  and the vertical portion  3  are connected, the vertical length is sum of the lateral portions  2  and the vertical portion  3 . Therefore, in the connecting area A, the section area perpendicular to the energization direction is locally large and there is a large variation in the current density, so that the electric current hardly passes through the vertical portions  3 . 
         [0060]    In detail.  FIG. 5  depicts the variations, shown in below (1) and (2). 
         [0061]    (1) The connecting points B between the lateral portion  2  and the vertical portion  3  make right angles, and the passing direction of the electric current extremely changes at the connecting point B. The electric current gathers to the connecting points B, so that the current density is high. 
         [0062]    (2) The lateral portions  2  are directly connected to the electrodes  10 , and the current density in the lateral portions  2  is high. The resistance at the current branch from the lateral portion  2  to the vertical portion  3  is large, and therefore the current density in the vertical portions  3  is low. 
         [0063]    As described above, if the conventional energization heating process using the electrode pair  10  is performed to the blank  1  that has the different shape, it may fail to heat evenly due to the variation in current density. 
         [0064]    In the present embodiment, as shown in  FIG. 2 , the electrode pair  10  energizes the blank in one direction (from right to left in drawing), and the electric current is bypassed through the current paths  20  to the vertical portions  3 . 
         [0065]    The current paths  20  are plate electrodes made of the material having lower electrical resistances than the blank  1  (e.g. when the blank  1  is made of steel, the current path  20  is made of cupper or carbon), and are connected with the blank  1 . The current paths  20  are extended along the left-right direction and arranged parallel to the lateral portions  2 . 
         [0066]    The current paths  20  are divided into three sections to connect the right electrode  10  with the right vertical portion  3 , the right vertical portion  3  with the left vertical portion  3  and the left vertical portion  3  with the left electrode  10  (alternatively, the three sections are integrated as one member). The electrode paths bypass the non-energized areas between the electrodes  10  defined by the cutouts  4  and the hole  5  of the blank  1  to which the electrode pair  10  is connected. 
         [0067]    Via the current paths  20 , the electric current passes from the positive electrode  10  where the current density is high to the negative electrode  10  through the vertical portions  3  where the current density is low. 
         [0068]    In the embodiment, the cutouts  4  are the openings formed at the ends of the blank  1 , so that the ends of the current paths  20  disposed in the cutouts  4  are connected to the electrodes  10 . The hole  5  is the opening surrounded by the blank  1 , so that the ends of the current paths  20  disposed in the hole  5  are connected to the blank  1 . 
         [0069]    As shown in  FIG. 6 , when energizing between the electrodes  10 , the electric passage from the electrode  10  to the lateral portions  2  is bypassed via the current paths  20 , thereby passing the current to the vertical portions  3 . Hence, the current density in the vertical portions  3  is increased, and the current density in the blank  1  is equalized. 
         [0070]    In other words, arranging the current paths  20  parallel to the lateral portions  2  makes the change of the section area along the direction perpendicular to the energization direction between the electrodes  10  small, thereby improving the evenness of the current density in the blank  1 . 
         [0071]    As described above, due to the current paths  20 , the energization heating process S 1  provides the improvement in evenness of the current density in the blank  1  and obtains even heating. Moreover, the energization heating process S 1  improves the quality and productivity in the pressing or quenching after the heating process. 
         [0072]    The current paths  20  bypass the electric current from the high current-density area toward the low current-density area. i.e. the positive electrode  10  to the vertical portions  3  which are separated from the electrodes  10  by the non-energized areas (the cutouts  4  and the hole  5 ) and extended along the orthogonal direction with respect to the energization direction. 
         [0073]    Due to this structure, overheat at the connecting points B as the intersections of the current passage is prevented, and the sufficient differential of electric potential occurs between the left and right ends of the vertical portions  3 . The current paths  20  reduce the variation in the current density and contribute to the equation of the current density. 
         [0074]    The current paths  20  connect between the peripherals of the blank  1  defined by the cutouts  4  and the hole  5 , which are extended perpendicular to the equipotential lines generated between the electrodes  10 . 
         [0075]    Thus, the vertical portions  3 , which are separated from the electrodes  10  by the spaces and thus located as the low current-density areas, are energized by bypassing through the current paths  20 , thereby equalizing the current density in the blank. 
         [0076]    The current paths  20  are arranged orthogonal to the bar electrodes  10 , namely the paths are extended in the left-right direction and the electrodes are extended in the upper-lower direction. That is, the current paths  20  are extended perpendicular to the equipotential lines generated between the electrodes  10 . 
         [0077]    The current density in the current paths  20  is even, and the bypass though the current paths are efficiently done. 
         [0078]    Moreover, the electrodes  10  are configured as the bar electrodes extended in one direction, and therefore, if the electrodes  10  are set parallel to the opposite sides of the blank  1 , the large section areas are obtained with regard to the energization direction. Thus, the uniform equipotential lines are generated and the heating efficiency is improved. 
         [0079]    The current path  20  is made of the material that has lower resistance than the blank  1 , so that the current density in the current path  20  is higher than that in the lateral portions  2 . As a result, the electric current applied from the electrode  10  is smoothly led to the vertical portions  3  via the current paths  20 . 
         [0080]    On the contrary, if the current paths  20  have higher resistance than the blank  1 , the current paths  20  are more heated than the blank  1  by the energization, thereby degrading the heating efficiency. 
         [0081]    It should be noted that the object to be heated by the energization heating process S 1  is not limited to the blank  1 . For example, the blank may be configured not only in H-shape. T-shape or rectangular with some holes inside, but also in rectangular shape, in which multiple different materials are combined and shows the current distribution due to the difference in electric resistances during the energization. 
         [0082]    If the blank to be heated occurs the variation in current density therein when a pair of electrodes energizes from one end to the opposite end, the energization heating process S 1  provides the uniform heating, in which the electric current is bypassed from the high current-density area to the low current-density area. 
         [0083]    Moreover, the blank may be a steel pipe having varying diameter, and the energization heating process S 1  is likewise applicable. 
         [0084]    The energization direction of the energization heating process S 1  is not limited to the above embodiment, and changeable in accordance with the shape of the blank  1  or heating conditions. 
         [0085]    for example, when the upper-lower direction of the blank  1  is set as the energization direction, the current paths  20  are arranged to connect the lateral portions  2  at the outer side of the vertical portions  3 . In this case, the current density in the blank  1  is also equalized. 
         [0086]    The electrodes  10  used in the energization heating process S 1  are the bar electrodes generating the even equipotential lines, and may be substituted by an electrode pair enabled to generate the even equipotential lines between the electrode pair. 
         [0087]    For example, two pairs of hemispherical electrodes  15  may work. The hemisphere electrode pairs  15  generate the equipotential lines shown in  FIG. 7 , so that the number of the electrodes or the arrangement of the electrodes is adjusted to generate the desired equipotential lines, that is, parallel lines along the ends of the blank  1 . 
         [0088]    If the blank  1  has curved ends and the connecting portions to the electrodes  10  are not straight, preparing additional electrode members corresponding to the shape of the connecting portions to the blank  1  provides the straight connection with the electrodes  10 . 
         [0089]    That is to say, the end peripheries of the blank are not limited to the straight shapes as the blank  1 , and the energization heating process S 1  is applicable to the blanks with any end shapes. 
         [0090]    As for the blank  1 , each current path  20  is preferably located to divide the vertical portion  3  into three in the upper-lower direction. The configuration such as arrangement or number of the current paths  20  is selectable in response to the shape of the blank  1  to achieve the even current density in the blank  1 . 
         [0091]    In the other embodiment, the current paths may be configured as conductive wires, which connect the high-potential area to the low-potential area so that the electric current is bypassed from the high current-density area to the low current-density area. 
         [0092]    Alternatively, the blank is heated without connected with the current paths, detecting the heating state by capturing the heat image or simulation, and the best mode for the current paths is selected and arranged according to the detection. 
       Second Embodiment 
       [0093]    Referring to  FIGS. 8 to 10 , an energization heating process S 2  as a second embodiment of the energization healing method is described below, in which a blank  50  is energized and heated. 
         [0094]    For the convenience of the explanation, the upper-lower direction and left-right direction of the blank  50  are defined as the upper-lower direction and left-right direction in  FIG. 8 , respectively. 
         [0095]    The blank  50 , as a heating object in the energization heating process S 2 , is made of a material with conductivity and quenchability such as steel. The blank  50  is a plate member having a “different shape.” 
         [0096]    As shown in  FIG. 8 , the blank  50  has a first portion  51 , a second portion  52  and a third portion  53 , and the sides of the first portion  51  and the second portion  52  are connected to the ends of the third portion  53 , thereby integrated into one member. 
         [0097]    These portions  51 ,  52  and  53  may be made of the same materials or different materials from each other and selectable in accordance with the characteristics of the materials such as rigidity of the blank  50 . 
         [0098]    The first portion  51  is extended from one end (right end in drawings) of the two opposite ends of the blank  50  to the other end (left end in drawings). The first portion  51  is straight portion extended along the left-right direction. 
         [0099]    The second portion  52  is extended from the one end (right end in drawings) of the blank  50  to the opposite end (left end in drawings). The second portion  52  is curved downwardly from the one end (right end in drawings) to the other end (left end in drawings). At the one end (right end in drawings), the second portion  52  is separated from the first portion  51 , and at the other end (left end in drawings), the second portion  52  is combined to the first portion  51 . 
         [0100]    The third portion  53  is extended substantially perpendicular to the direction from the one end to the other end and connected with the middle portions of the first portion  51  and the second portion  52 . The third portion  53  is inclined against the upper-lower direction. 
         [0101]    The blank  50  includes a cutout  54  provided at the right end, a cutout  55  provided at the left end and a hole  56  provided at the center. The chain-dotted line in  FIG. 9  represents the outer line if the blank  50  is rectangular. 
         [0102]    The cutout  54  is an opening disposed at the right end of the blank  50 , and has a trapezoidal shape. In the blank  50 , the end periphery (left side) of the cutout  54  is formed as an inclined straight line. 
         [0103]    The cutout  55  is an opening disposed at the left upper portion of the blank  50 . In the blank  50 , the end periphery (right side) of the cutout  55  is formed as a curved line. The cutout  55  makes the vertical length in the left side of the blank  50  shorter than that in the right side. 
         [0104]    The hole  56  is a rough square opening disposed at the center of the blank  50 . In the blank  50 , the right end line defined by the hole  56  is an inclined straight line and the upper side defined by the hole is a curved line. 
         [0105]    The way of preparing the blank  50  is to trim the cutouts  54 ,  55 , and the hole  56  from the rectangular plate or to combine the first portion  51 , the second portion  52  and the third portion  53  (prepare a tailored blank). 
         [0106]    As illustrated in  FIG. 9 , in the energization heating process S 2 , a pair of electrodes  60 , a group of current paths  70  and a group of extension materials  80  are used to heat the blank  50 . 
         [0107]    The pair of electrodes  60  and the group of current paths  70  are installed in an energization heating apparatus, to which the blank  50  is transferred and the energization heating process S 2  is operated. 
         [0108]    The electrode pair  60  energizes the blank  50 . The electrode pair  60  consists of a first electrode  61  connected to the one end of the blank  50  and a second electrode  62  connected to the other end of the blank  50 , and one of the electrodes  61  and  62  is used as a positive electrode and the other is used as a negative electrode. 
         [0109]    The electrodes  61  and  62  are configured as bar electrodes having longitudinal directions. The electrodes  61  and  62  are connected to a power source feeding the desired electric current, which applies current to the blank  50  through the electrodes  61  and  62 . In the blank  50 , the current occurs from the positive electrode  61  to the negative electrode  62 . 
         [0110]    The electrode  61  is extended along the upper-lower direction and has the substantially same length as the right side of the blank  50 . The electrode  62  is extended along the upper-lower direction and has the substantially same length as the left side of the blank  50 . The length of the electrode  61  is longer than that of the electrode  62 . 
         [0111]    As shown in  FIG. 9 , the electrodes  61  and  62  include multiple connectors  63  provided with clamping structure for clamping the blank  50  from the thickness direction to secure the electrical connection with the blank  50 . The connector  63  includes clips to clamp the blank actuated by an air cylinder or a hydraulic cylinder, and the actuators switch the connecting/disconnecting between the electrodes  61 ,  62  and the blank  50 . 
         [0112]    The clamp structure of the connector  63  contained in the electrodes  61  and  62  enables to maintain the contact between the blank  50  and the electrodes  61  and  62 . The clamping-type connectors reduce the influence of the deformation such as curving or roll back of the blank  50  that occurs during the energization heating and provide the uniform heating, compared with contact-type connectors. 
         [0113]    If the blank  50  is configured as rectangular plate, the equipotential lines generated from the positive electrode  61  to the earth electrode  62  are shown in  FIG. 10 . As shown in  FIG. 10 , the bar electrodes  61  and  62  generate the equipotential lines parallel to the electrodes  61  and  62  where the bar electrodes face each other and generate the equipotential lines inclined from the upper end of the electrode  61  to the upper end of the electrode  62  above the electrode  62 , that is, where the electrodes  61  and  62  do not face. 
         [0114]    Actually, the blank  50  has the cutouts  54 ,  55  and the hole  56  arranged perpendicular to the equipotential lines between the electrodes  61  and  62 . In the embodiment, the cutouts  54  and  55  are spaces between the electrodes  61 ,  62  and the blank  50  and the hole  56  is space disposed inside of the blank  50 , whereby these spaces act as non-energized areas and bring the variation in current density. 
         [0115]    In the embodiment, as shown in  FIG. 9 , the electrode pair  60  energizes the blank in one direction (from right to left in drawing), and the electric current passes through the group of current paths  70  and the group of extension electrodes  80  to the third portion  53  bypassing the cutout  54  and the hole  56  and to the electrode  62  bypassing the cutout  55  from the curved end of the second portion  62 . 
         [0116]    All of the group of current paths  70  are plate electrodes made of the material having lower electrical resistance than the blank  50  (e.g. when the blank  50  is made of steel, the each current path  70  is made of cupper or carbon), and are connected with the blank  50 . The group of current paths  70  is extended along the left-right direction. 
         [0117]    As shown in  FIG. 9 , the group of current paths  70  includes a first path  71  connecting the electrode  61  to the right side of the third portion  53 , a second path  72  connecting the left side of the third portion  53  to the right side of the second portion  52 , and a third path  73  connecting the left side of the second portion  52  to the electrode  62 . 
         [0118]    The first current path  71  is disposed at the space formed by the cutout  54  and arranged perpendicular to the equipotential lines generated between the pair of electrodes  60 . The second current path  72  is disposed at the space formed by the hole  56  and arranged perpendicular to the equipotential lines generated between the pair of electrodes  60 . The current path  73  is disposed at the space formed by the cutout  55  and arranged perpendicular to the equipotential lines generated between the pair of electrodes  60 . 
         [0119]    In the embodiment, “perpendicular to the equipotential lines” means to cross the equipotential line at right angle and at enough angle (e.g. above 45 degrees), and the “enough angle” is defined as the angle where flow of the electric current generating the equipotential lines is influenced by the current path crossing thereto. 
         [0120]    The third current path  73  contains first portions  73   a  extended in the left-right direction and a second portion  73   b  connecting the first portions  73   a  to the electrode  62  and extended in the upper-lower direction. The first portions  73   a  and the second portion  73   b  are perpendicular to the equipotential lines generated between the pair of electrodes  60 . In other words, the second portion  73   b  of the third path  73  extends the electrode  62  in the upper direction, whereby the electrode  62  and the second portion  73   b  make the vertical electrode with the same length as the electrode  61 . 
         [0121]    As described above, the group of current paths  70  bypasses the non-energized area formed by the cutouts  54 ,  55  and the hole  56  along the direction perpendicular to the equipotential lines between the electrode pair  60 . 
         [0122]    All of the extension materials  80  are made of the same materials as the blank  50  (steel or the like), and connected with the blank  50 . The group of extension materials  80  is extended along the left-right direction. The group of extension materials  80  connects the blank  50  and the group of current paths  70  at the inclined sides and curved side of the blank. 
         [0123]    As depicted in  FIG. 9 , the group of extension materials  80  is formed such that the blank  50  is straightly connected to the group of current paths  70 . That is, the ends of the group of extension materials  80  are formed as straight lines at the connections to the group of current paths  70 . 
         [0124]    The clamping structures are used to electrically connect the group of extension materials  80  to the group of current paths  70 , and as described above, the straight connections between the group of extension materials  80  and the group of current paths  70  make the clamping resistances reduced and improve the heating efficiency by means of the electric current passing through the group of current paths  70 . 
         [0125]    The clamping structures may be the same as the connectors  11  installed in the electrodes  10  as in the first embodiment. 
         [0126]    As shown in  FIG. 9 , the group of extension materials  80  includes first materials  81  connecting the first current path  71  to the right side of the third portion  53 , second materials  82  connecting the left side of the third portion  53  to the second current path  72 , a third material  83  connecting the second current path  72  to the right side of the second portion  52 , and fourth materials  84  connecting the left curved side of the second portion  52  to the third current path  73 . 
         [0127]    The way to connect the group of extension materials  80  with the blank  50  is to prepare the blank  50  including such materials or to fix the materials to the blank  50  after preparing the blank  50 . Regardless of the way to connect, the extension materials  80  are not used in the product and removed in the trimming process or the like after the energization heating process S 2 . 
         [0128]    The number or arrangement of the extension materials ( 81 ,  82 ,  83  and  84 ) of the group of extension materials  80  is not limited to the present embodiment. 
         [0129]    In the energization heating process S 2 , the energization is operated with the group of current paths  70 , and therefore the current density in the blank  50  is equalized and the uniform heating is provided. Moreover, the energization heating process S 2  improves the quality and productivity in the pressing or quenching after the process. 
         [0130]    It should be noted that the second embodiment brings the same effects as the first embodiment. 
         [0131]    Furthermore, in the present embodiment using the group of extension materials  80  to connect the group of current paths  70  to the blank  50 , the following effects are obtained. 
         [0132]    The peripherals of the cutouts  54 ,  55  and the hole  56  formed as the spaces in the blank  50  contain the curved shape (the left side of the second portion  52 ) and the inclined shape to the energization direction by the electrode pair  60  (the both sides of the third portion  53 ). Therefore, if the group of current paths  70  is directly connected to the blank  50 , there may be defects in the heating condition or the clamping condition. In the embodiment, the group of extension materials  80  is formed with the blank  50  and the group of current paths  70  is connected to the blank  50  via the group of extension materials  80 , which improves the heating property, thereby providing the even heating. 
         [0133]    In the present embodiment, the electrode pair  60  includes the electrode  61  and  62  having the different lengths from each other to correspond to the lengths of the ends of the blank  50 . However, the electrode  62  may have the same length as the maximum upper-lower length of the blank  50  (i.e., the electrode  61 ). In this ease, the equipotential lines generate by the electrode pair  60  is parallel to the arrangement direction of the electrode pair  60 . 
       INDUSTRIAL APPLICABILITY 
       [0134]    The present invention is applicable to a technique of heating by energizing a blank, and particularly to the technique of evenly heating the blank, which causes a distribution of current density while energizing by using a single pair of electrodes.