Technical field [0001] 　The present invention relates to a door impact beam disposed in the interior of a car door. Background technique [0002] 　Car door is constructed by assembling the inner panel, for example, by hemming (180 ° bending) which mainly maintains the strength form the skeleton of the outer panel and the door to create the appearance. Door locking mechanism is provided between the outer panel and the inner panel, door components such door glass and window regulator is accommodated. [0003] 　The recent automobiles, in order to improve safety during side impact, the door impact beam is a reinforcing member elongated disposed by fixing the both ends toward the car front-rear direction, the outer panel and the inner panel it is often provided between. Door impact beam, for example, a closed section type to a round pipe or the like as a material, for example, is divided into an open cross-section form having a cross section of the groove-shaped, any type of door impact beam also load the impact load due to a side collision Once, to absorb impact energy by generating a three-point bending to an impact load input position and the working point deformation. [0004] 　The door impact beam as is, since it is necessary to be arranged so as to avoid interference with various door components described above in a narrow space between the outer panel and the inner panel, to efficiently absorb impact energy in a small cross-section it is required. For such door impact beams, it has been made various proposals in the past. [0005] 　A groove bottom portion in Patent Document 1, and two ridge portions continuous to the groove bottom, and two vertical wall portions continuous respectively to the two ridge portions, two curves each successive thereto two vertical wall portions parts and, these two in a thin sheet steel door impact beam having an open cross-sectional shape of the hat-shaped having two flanges respectively continuous to the curved portion, a part-one ridge-groove of one of the vertical wall portion bottom-place the reinforcing plate in a part of the other ridge portion - the other vertical wall portion through improved between flat-ridge portion of the groove bottom part, the hat-shaped collision open cross-section by suppressing cross-sectional collapse, the invention is disclosed for improving the absorption performance of the collision energy. [0006] 　Patent Document 2, in a substantially thin sheet steel door impact beam having an open cross-sectional shape of the U-shape, by forming a bead that protrudes toward the inside top of the U-shaped impact energy absorbing performance of the the invention reduces the risk of fracture while increasing is disclosed. [0007] 　Incidentally, Patent Document 3 relates to a center pillar of an automobile, the invention in which a deformation promoting bead and the auxiliary bead bend top plan of reinforcement arranged as a reinforcing material for the resin outer panel is disclosed . CITATION Patent Document [0008] Patent Document 1: Laid-Open Patent Publication No. 2009-196488 Patent Document 2: Laid-Open Patent Publication No. 2008-284934 Patent Document 3: Laid-Open Patent Publication No. 2005-1615 Summary of the Invention Problems that the Invention is to Solve [0009] 　In the invention disclosed in Patent Document 1, in the open cross-sectional shape of the hat-shaped at the time of collision, the groove bottom center portion is discontinuous for cross-sectional collapse occurred, would be a load reduction occurs due to the cross-sectional collapse, collision energy easy absorption performance is reduced. [0010] 　Further, in the invention disclosed in Patent Document 2 is considered to distortions generated by the load of the impact load is dispersed deflection is suppressed in order to form a bead that protrudes toward the inside top of the U-shaped although, a discontinuous shape wavy top U-shaped to form a bead that protrudes toward the inside top of the U-shape. Accordingly easily bending the top of the U-shape when the impact energy load, tends distortion is concentrated on the corner portion of the top of the U-shape (ridge equivalent portion). This especially because elongation is less material (e.g., tensile strength or more high-tensile steel sheet 780 MPa) constituting the door impact beam by, there is a possibility that breaking of the impact energy by the load occurs, not obtained the desired impact energy absorbing performance there is a problem and the like. [0011] 　Furthermore, in the invention disclosed in Patent Document 3, the auxiliary bead is only with the load transfer capabilities to bending deformation promoting bead, that are not directly involved in the collision energy absorption performance. Means for Solving the Problems [0012] 　The present inventors have found that, when the deformation by bending to the door impact beam comprising a body having an open cross-sectional shape of the hat-shaped impact load loaded progresses, with decreasing collision load caused by the collapse of the cross section of the open cross-sectional shape, deformation have increased the risk of fracture by localized, a result of intensive studies to obtain a cross-sectional shape can disperse the deformation with high resistance to collapse of the cross-section, typically have a hat-shaped section in the door impact beam having a body made of a thin steel plate of the molded body, a portion of the groove bottom portion of the body, forming with its bead shape protruding with a curved surface to the outside of the cross-sectional shape in the longitudinal direction Accordingly, by finding that can solve the above problems, and completed the present invention by further repeated studies. [0013] 　The present invention is as listed below. 　(1) is configured to include a body of elongate a molded body of a metal plate, wherein the body and the door mounting portion are respectively formed on both ends of the elongated longitudinal, between these door mounting portion bending is arranged and a deformation generating section, the bending deformation generating section, the groove bottom and groove bottom respectively continuous to the two ridge portions and the two ridge portion continuous to the two vertical wall portions and the two one of a door impact beam with two curved portions respectively continuing the vertical wall portion and two flanges respectively continuous to the two curved portions, a portion of the groove bottom, which acts on the door impact beam as opposed to the impact load, and protrudes in a shape having a curved surface to the outside of the cross-sectional shape of the bending deformation generating unit, a door impact beam, characterized by having a bead formed toward the longitudinal direction of the body . [0014] 　(2) the bending deformation generating unit, which has an open cross-sectional shape of the hat-shaped or less hat height 50 mm, cross-sectional circumferential length of the groove bottom is 10 mm ≦ L ≦ 35 mm, 　the bead has the following (1) formula and (2) door impact beam according relationship to satisfy (1) of the formula. 　1 mm