Abstract:
In a hot press molding method for press-molding a heated material and cooling the material while holding the material in a mold, during press molding of the material, shape changes including a portion that curves with respect to a longitudinal direction of the material are molded at longitudinal opposite end portions of the material and resistance to thermal contraction force of the material in the longitudinal direction during cooling is imparted. In one embodiment of the present invention, the shape changes molded on the material during the press molding are minute protrusions provided on surfaces of the longitudinal opposite end portions of the material. The invention makes it possible to suppress thermal contraction of the material during cooling in hot press molding to thereby prevent defects associated therewith.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a hot press molding technique. 
       BACKGROUND ART 
       [0002]    Conventionally, hot press molding for cooling material heated to a quenching temperature while press-molding the material in a mold to thereby quench the material is widely used (see Patent Document 1, for example). 
       CITATION LIST 
     Patent Literature 
       [0003]    Patent Document 1: JP 2008-284574 A 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0004]    Metal material such as steel to be hot press-molded expands when heated and contracts when cooled. Because of the contraction of the material in the mold, defects such as biting of a molded article into the mold and dimensional errors of a shape of the molded article may be caused. 
         [0005]    Therefore, the technique for avoiding the defects caused by the thermal contraction of the material when the material is cooled in a hot press machine is required. 
       Technical Solutions 
       [0006]    In accordance with an aspect of the present invention, there is provided a hot press molding method for press-molding a heated material and cooling the material while holding the material in a mold, wherein, during press molding of the material, shape changes including a portion that curves with respect to a longitudinal direction of the material are molded at longitudinal opposite end portions of the material and resistance to thermal contraction force of the material in the longitudinal direction during cooling is imparted. 
         [0007]    In accordance with another aspect of the invention, in the hot press molding method, the shape changes molded on the material during the press molding are preferably a plurality of minute protrusions provided on surfaces of the longitudinal opposite end portions of the material. 
         [0008]    In accordance with another aspect of the invention, in the hot press molding method, the shape changes molded on the material during the press molding are preferably flanges molded on the longitudinal opposite ends of the material. 
         [0009]    According to another aspect of the invention, there is provided a hot press molded article obtained by any one of the above-described hot press molding methods. 
         [0010]    According to another aspect of the invention, there is provided a mold for hot pressing, the mold including an upper mold and a lower mold, for sandwiching a heated material between the upper mold and the lower mold to press-mold the material, and for holding the material in the mold to cool the material, wherein a plurality of protruding portions protruding toward the upper mold or a plurality of recessed portions recessed from the upper mold toward the lower mold are provided at a portion of the lower mold corresponding to a longitudinal end portion of the material, a plurality of recessed portions or a plurality of protruding portions corresponding to the protruding portions or the recessed portions provided to the lower mold are provided to the upper mold, and a plurality of minute protrusions are molded on a surface of the longitudinal end portion of the material by the protruding portions or the recessed portions provided to the lower mold and the recessed portions or the protruding portions provided to the upper mold during press molding with the upper mold and the lower mold. 
         [0011]    According to another aspect of the invention, there is provided a mold for hot pressing, the mold including an upper mold and a lower mold, for sandwiching a heated material between the upper mold and the lower mold to press-mold the material, and for holding the material in the mold to cool the material, wherein a flange bending from the upper mold toward the lower mold or from the lower mold toward the upper mold is molded at a longitudinal end of the material by the upper mold and the lower mold during the press molding. 
       Advantageous Effects of Invention 
       [0012]    According to the invention, it is possible to suppress thermal contraction of a material during cooling in hot press molding to thereby avoid defects associated therewith. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is a schematic diagram of a hot press machine. 
           [0014]      FIG. 2  is a perspective view of a hot press molded article. 
           [0015]      FIG. 3  is a longitudinal sectional view of a mold for hot pressing. 
           [0016]      FIG. 4  is a longitudinal sectional view of another embodiment of the mold for hot pressing. 
           [0017]      FIGS. 5(   a ) and  5 ( b ) are diagrams showing another embodiment of the hot press molded article. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0018]    As shown in  FIG. 1 , a hot press machine includes a mold  1  for hot pressing. The mold  1  for hot pressing has an upper mold  10  and a lower mold  20  and press-molds a material  2  by sandwiching the material  2  between the upper mold  10  and the lower mold  20 . The upper mold  10  is supported on a moving device and movable in a vertical direction. The lower mold  20  is fixed immovably. 
         [0019]    The material  2  is metal material such as steel which can be quenched. The material  2  is charged into the mold  1  for hot pressing after heated to a quenching or higher temperature by electric heating or the like. The material  2  is molded into a predetermined product shape (see  FIG. 2 ) by press molding with the mold  1  for hot pressing and held for a predetermined time in the mold to thereby be cooled and quenched (hot press-molded). 
         [0020]    As shown in  FIG. 2 , the material  2  is molded from a rectangular shape having one direction in a longitudinal direction into a molded article  3  having a hat-shaped section continuous along the longitudinal direction by the hot press molding. In other words, by bending molding of the material  2  along a short-side direction with the mold  1  for hot pressing, the material  2  is molded into the molded article  3  having the hat-shaped section. 
         [0021]    As the molded article  3  formed by using the mold  1  for hot pressing according to the embodiment, there are a rocker outer and the like besides a pillar outer shown in  FIG. 2 . 
         [0022]    As shown in  FIG. 2 , protrusions  30 ,  30 , . . . are molded on a top face at longitudinal opposite end portions of the molded article  3 . The protrusions  30  are molded by curving molding of portions of the material  2 . In other words, the protrusions  30  are shape changes including the curved portion inclined with respect to the longitudinal direction of the material  2 . The plurality of protrusions  30  are molded as minute protrusions protruding upward (or downward) into hemispherical shapes from a flat face of the material  2 . 
         [0023]    The protrusions  30 ,  30 , . . . are molded on the top face of the molded article  3  and sprinkled at predetermined intervals in the longitudinal direction and the short-side direction. Size, shape, and the number of protrusions  30 ,  30 , . . . are set so as not to hinder mounting operation, assembly operation, and the like in later steps, for example, i.e., not to impair the function of the molded article  3  as a product. 
         [0024]    As described above, the minute protrusions  30  are protrusions having three-dimensional shapes such as hemispheres, oval hemispheres, cubes, rectangular parallelepiped, and frustums formed in such size as not to impair the function of the molded article  3  as the product and protruding to a front face side or a back face side from the flat face portion (especially, the flat face extending in the longitudinal direction) of the molded article  3 . The number of protrusions  30  to be provided is set so as to be able to impart sufficient resistance to thermal contraction force in the longitudinal direction of the material  2  in view of the shape and the size. 
         [0025]    For example, protruding lengths of the protrusions  30  from the surface are preferably equal to or shorter than thickness of the material  2 . If the thickness of the material  2  is 1.4 mm, diameters of the protrusions  30  formed into the hemispherical shapes are preferably 3 mm or smaller. 
         [0026]    As shown in  FIG. 3 , recessed portions  11 ,  11 , . . . recessed upward are formed on the upper mold  10  and protruding portions  21 ,  21 , . . . protruding upward are formed on the lower mold  20 . These recessed portions  11 ,  11 , . . . and the protruding portions  21 ,  21 , . . . are provided to mold the protrusions  30 ,  30 , . . . on the material  2  and have shapes corresponding to the protrusions  30 ,  30 , . . . . The respective protrusions  30  are molded by sandwiching portions of longitudinal opposite end portions of the material  2  between the respective recessed portions  11  and the respective protruding portions  21  in the press molding. 
         [0027]    By sandwiching the material  2  between the upper mold  10  and the lower mold  20  to mold the protrusions  30 ,  30 , . . . in this manner and then holding the material  2  in the mold for a predetermined time to thereby cool the material  2 , the hot press molded article  3  is obtained. 
         [0028]    As described above, in the embodiment, the protrusions  30 ,  30 , . . . are formed by sandwiching the material  2  between the recessed portions  11 ,  11 , . . . of the upper mold  10  and the protruding portions  21 ,  21 , . . . of the lower mold  20  during the press molding with the mold  1  for hot pressing, i.e., before cooling in the mold proceeds and the material  2  is hot press-molded into the molded article  3 . 
         [0029]    Here, when the heated metal material is cooled, the material contracts from the longitudinal opposite end portions toward a central portion and an amount of contraction in the longitudinal direction is greater than that in the short-side direction, in general. To cope with this, if the protrusions  30 ,  30 , . . . are formed on the longitudinal opposite end portions and shapes of the opposite end portions are changed to be inclined with respect to a direction of the flat face of the material  2 , it is possible to impart resistance to contraction force to contraction starting points. Therefore, it is possible to satisfactorily prevent the contraction in the longitudinal direction during cooling to thereby prevent deformation and biting into the mold. 
         [0030]    Since the protrusions  30 ,  30 , . . . are formed as the minute protrusions, they do not affect the product strength, deformation mode, and the like. In other words, the protrusions  30 ,  30 , . . . do not greatly increase cross-sectional moment of the shape of the product and do not serve as points where bending starts. In this manner, the protrusions  30 ,  30 , . . . do not impose a large limitation on the shape of the product and it is easy to design the molded article  3  including the protrusions  30 ,  30 , . . . . 
         [0031]    From a viewpoint of prevention of the contraction, the protrusions  30 ,  30 , . . . are preferably close to the longitudinal ends as far as the protrusions  30 ,  30 , . . . are formed in positions as not to impair functions of the molded article  3 . Here, “the positions as not to impair functions of the molded article  3 ” are positions where the molded article  3  does not affect the product strength and the deformation mode of the molded article  3 . 
         [0032]    If the product shape of the molded article  3  includes a hole, a notch, or the like on the top face or a side wall portion, the protrusions  30 ,  30 , . . . are formed in positions closer to the end portions than positions where the hole or the notch is provided. 
         [0033]    In general, when the above-mentioned hole or notch is provided to the molded article  3 , it is often formed simultaneously in the hot press molding and a processing blade such as a trim punch for this purpose is provided in the mold in many cases. By providing the protrusions  30 ,  30 , . . . on the outer sides (on the end portion sides when seen from the central portion) of the shape change as in the embodiment, it is possible to prevent thermal contraction in an area on an inner inside of the protrusions  30 ,  30  . . . . Therefore, the protrusions  30 ,  30 , . . . do not affect positions or dimensions of the hole and the notch and it is possible to improve processing quality. 
         [0034]    In the case where the product shape of the molded article  3  is the rocker outer, if protrusions  30 ,  30 , . . . are molded based on the same technical idea, it is possible to satisfactorily prevent the contraction in the longitudinal direction during cooling to thereby prevent deformation and biting into the mold. 
         [0035]    Although the protrusions  30 ,  30 , . . . have been explained as being protruding from the lower mold  20  toward the upper mold  10  (upward) in the above embodiment, they may protrude from the upper mold  10  toward the lower mold  20  (downward) and their protruding directions are not restricted. 
         [0036]    The protruding directions of the respective protrusions  30  may be selected according to the function of the molded article  3 . In other words, the upward and downward protruding directions of the protrusions  30  may be mixed. 
         [0037]    As shown in  FIG. 4 , in place of the protrusions  30 ,  30 , . . . , flanges  40 ,  40  may be provided to longitudinal opposite ends of the molded article  3 . In the embodiment, in the same manner as in the embodiment in which the protrusions  30 ,  30 , . . . are molded, portions bent with respect to a longitudinal direction (flat face direction) of a material  2  are molded at longitudinal end portions of the material  2  in press molding and resistance to contraction force in the longitudinal direction is imparted. 
         [0038]    The flanges  40 ,  40  are molded by bending the longitudinal opposite ends of the material  2  downward (or upward). The respective flanges  40  are provided to opposite ends of the molded article  3  as inclined faces having angles which are larger than 45° and which are such angles that the flanges  40  can be molded by press molding with respect to the flat face direction of the material  2 . The flanges  40  are preferably molded as the inclined faces having the angles from 60° to 80°. 
         [0039]    The flanges  40 ,  40  may be bent from the upper mold  10  toward the lower mold  20  as shown in  FIG. 4  or in an opposite direction from the lower mold  20  toward the upper mold  10 . 
         [0040]    By bending and molding the longitudinal opposite ends of the material  2  in this manner, a longitudinal length of the molded article  3  is kept as a length between the flanges  40 ,  40  and resistance to contraction force in the longitudinal direction can be imparted. Therefore, it is possible to prevent the contraction in the longitudinal direction during cooling to thereby prevent deformation and biting into the mold. 
         [0041]    The longitudinal opposite ends on which the flanges  40 ,  40  are molded are trimmed in a later step to be used as an end product or parts of the product shape. 
         [0042]    As a product shape of the molded article  3 , shapes with open longitudinal end faces may be employed as well. For example, as shown in  FIGS. 5(   a ) and  5 ( b ), a shape having a simple hat-shaped section continuous in a longitudinal direction ( FIG. 5(   a )) and a shape having a substantially hat-shaped section, with a wall face molded as an inclined face, continuous in a longitudinal direction ( FIG. 5(   b )) may be employed. 
       DESCRIPTION OF NUMERALS 
       [0043]      1 : mold for hot pressing,  2 : material,  3 : molded article,  10 : upper mold,  11 : recessed portion,  20 : lower mold,  21 : protruding portion,  30 : protrusion,  40 : flange