Technical field [0001]The present invention relates to a bumper beam having impact resistance. BACKGROUND [0002]The bumper beam, the impact resistance is required. The bumper beam, there is one cross section and the hat member hat shape, it is formed by closing plate which is joined to the hat member. Such bumper beam is excellent in impact absorbing capacity per mass Placing closing plates vehicle length direction outward. Thus as reverse hat a material obtained by closing plates and impact input surface. What the top plate of the hat member and the impact input surface that sequentially hat. [0003] 　For example, WO WO2016 / 117335 (Patent Document 1), the bumper beam opposite hat placing the reinforcing member inside is disclosed. Impact energy absorbing efficiency of a bumper beam is improved by the internal reinforcement member. CITATION Patent Document [0004] Patent Document 1: International Publication No. WO2016 / 117335 Summary of the Invention Problems that the Invention is to Solve [0005] 　The impact test of the motor vehicle is carried out in the future North America are planned, there is one hit the pillar in the vehicle width center of the front bumper of the motor vehicle (pole frontal collision test). In this impact test, a concentrated load is generated at the center in the longitudinal direction of the bumper beam. To pass this collision test, it is required to increase the strength of the bumper beam. To improve the strength of the bumper beam, or increase the thickness of the bumper beam, it is conceivable to or size of the bumper beam. However, incompatible yield strength increased against weight and collision of the bumper beam in such measures. [0006] 　Therefore, we examined the bumper beam shape and material so as to achieve both the improvement of weight and collision strength. Specifically, as a high mass efficiency shapes, it employs a reverse-hat, with high tensile strength material. [0007] 　We, the reverse-hat shape, the bumper beam using a high tensile strength material, while reducing the weight, was found to be able to improve the strength of the bumper beam. However, in the impact test of the bumper beam of this configuration, the top plate on the opposite side of the hat member and the indenter collision portion of the closing plate has been found that in some cases broken by an impact. [0008] 　Accordingly, the present application, a reverse-hat shape, in the case of using the high tensile strength material, discloses a bumper beam that can suppress breakage due to impact. Means for Solving the Problems [0009] 　Bumper beam according to one aspect of the present invention comprises a closing plate, and a hat member curved to be convex toward the closing plate side in the longitudinal direction. The hat member includes a top plate, two first edges located at both ends of the top plate, and two flanges that are joined to the closing plate, 2 located on the inner end of each of the two flanges one of the second ridge, between said two first edges and said two second edges, and a two vertical walls located respectively. The hat member includes a high-strength portion and a low strength portion. High-strength portion, the less longitudinally comprises two longitudinal walls each longitudinal center also formed in a region spanning 250 mm. The tensile strength of the high-strength portion is greater than or equal to 1.5GPa. Low strength section is an area for more than the height of the vertical wall in the longitudinal direction in the longitudinal direction outside of the high-strength portion of the hat member curved to be convex in the longitudinal direction on the closing plate side, the from one of the two second edges, through the two longitudinal walls and the top plate is formed in a region up to the other of said two second ridgeline. Tensile strength of the low strength portion is lower than the longitudinal center of the tensile strength of the two longitudinal walls. The invention's effect [0010] 　According to the present disclosure, a reverse-hat shape, in the case of using the high tensile strength material, it is possible to provide a bumper beam that can suppress breakage due to impact. BRIEF DESCRIPTION OF THE DRAWINGS [0011] Is a side view showing the configuration of the bumper beam in FIG. 1 embodiment. It is a perspective view of a bumper beam shown in FIG. 1; FIG. 3 is a cross-sectional view of line A-A of the bumper beam shown in FIG. 4 is a diagram for explaining the arrangement of the low-strength portion of the bumper beam. 5 is a diagram illustrating a modification of the arrangement of the low-strength portion of the bumper beam. 6 is a graph showing the distribution of bending moments generated in the cross section of the bumper beam. 7 is a side view showing a configuration example of a bumper beam comprising a reinforcing member. 8 is a cross-sectional view of line A-A of the bumper beam shown in FIG. 9 is a graph showing the distribution of bending moments generated in the cross section of the bumper beam with a reinforcing member. It is a sectional view showing a modified example of FIG. 10 reinforcing member. 11 is a sectional view showing another modified example of the reinforcing member. [12] the top plate opposite the hat of the bumper beam is a graph showing a relationship between stroke and a load when broken by an impact. DESCRIPTION OF THE INVENTION [0012] 　We cross-section member of hat-shaped (hereinafter, hat that member) and were examined for the behavior to impact of the bumper beam that is closing and the plate, in formation is joined to the hat member. Hat member has a top plate, and two longitudinal walls extending from both ends of the top plate, and two flanges extending from the top plate of the two longitudinal walls opposite end in a direction away from each other, respectively. [0013] 　Inventors have studied to improve the maximum load of the bumper beam opposite hat. Using a high strength material, by forming the bumper beam opposite hat, it was found that it is possible to increase the maximum load. However, the tensile strength (TS) is in the impact test of the reverse-hat of the bumper beam with more high strength material 1.5 GPa, when indenter collision of the closing plate, the opposite side of the top plate of the indenter collision portion is broken If it has been found that there is. 12, when the top plate of the reverse-hat of the bumper beam is broken by an impact, is a graph showing the relationship between the stroke and the load. As shown in FIG. 12, the top plate of the bumper beam opposite hat by impact breaking load becomes zero. Thus, strength of the bumper beam is significantly reduced. [0014] 　Inventors have studied in detail for the case where the top plate is broken. If concentrated load to the bumper beam member opposite hat as the pole before impact test is loaded, tensile strain is generated in the top plate with the bending deformation. In this case, strain is largest occurring concentrated load point. In general high tensile strength material is poor in ductility, it is often cracked and large strain is generated. As a result, in the case of using the ultra-high strength, low ductility material, it has been found that there is a possibility that the top plate is broken. [0015] 　Both forward hat and reversed hat, the contribution to the load resistance is large is hat member. On the other hand, the tensile deformation is most occurs when impacted, in order hat is closing plate, a top plate of the hat member in reverse hat. If you try to avoid breakage due to tensile deformation, the closing plate in order hat, the hat member in reverse hat, it is conceivable to high ductility or low strength. In order hat, for contribution to the closing plate of the load resistance is small, even by using a low-strength material in the closing plate, the degree of reduction in the maximum load is small. That is, in order hat, by low intensity of the closing plate, can be avoided fracture by tensile deformation. On the other hand, the opposite case hat, lowering the strength of the contribution is high hat member to load resistance in order to avoid breakage, the degree of reduction in the maximum load increases. Therefore, it is difficult achieve both the fracture suppression and high load of. Furthermore, lowering the strength of the material of the hat member, it is necessary to increase the plate thickness in order to ensure impact resistance. Therefore, it becomes difficult both high load and lighter. [0016] 　The inventors have found that in reverse hat, while ensuring the strength of the central portion occurring bending moment in the cross section is increased when subjected to impact were also investigated to suppress constituting the base material fracture. Result of studies, the longitudinal center of the tensile strength in the hat member as above 1.5 GPa, on the outside of 250mm region including the center in the longitudinal direction of the hat member, tensile from the central strength low low strength portion, longitudinal to the longitudinal direction and conceived a configuration provided for over the wall height. With this configuration, in the reverse-hat, while increasing the maximum load by a high strength material, it found that it is possible to suppress the base material fracture. Based on this finding, and conceived the following embodiment. Incidentally, whether the bumper beam is applied sequentially hat, whether being applied to the reverse-hat it can be determined from the shape. Bumper beam is because curved so as to be convex in the vehicle length direction outward. That is, the bumper beam opposite hat is curved so as to be convex toward the closing plate side. In the present invention, "hat member curved to be convex toward the closing plate side in the longitudinal direction" closing plate means hat member formed bumper beam to be positioned outside of the vehicle. [0017] 　(Configuration 1) 　Bumper beam according to an embodiment of the present invention comprises a closing plate, and a hat member curved to be convex toward the closing plate side in the longitudinal direction. The hat member includes a top plate, two first edges located at both ends of the top plate, and two flanges that are joined to the closing plate, 2 located on the inner end of each of the two flanges one of the second ridge, between said two first edges and said two second edges, and a two vertical walls located respectively. The hat member includes a high-strength portion and a low strength portion. High-strength portion, the less longitudinally comprises two longitudinal walls each longitudinal center also formed in a region spanning 250 mm. The tensile strength of the high-strength portion is greater than or equal to 1.5GPa. Low strength section is an area for more than the height of the vertical wall in the longitudinal direction in the longitudinal direction outside of the high-strength portion of the hat member curved to be convex in the longitudinal direction on the closing plate side, the from one of the two second edges, through the two longitudinal walls and the top plate is formed in a region up to the other of said two second ridgeline. Tensile strength of the low strength portion is lower than the longitudinal center of the tensile strength of the two longitudinal walls. [0018] 　Bumper beam is supported at two ends corresponding to the connection portion of the other member such as the crash box and a front side member (vehicle body mounting portion). Variant of the bumper beam when the concentrated load at the center in the longitudinal direction of the bumper beam as the pole before impact test is loaded, as well as the variations of the three-point bending pushes the central portion while supporting the both end portions tests Become. In this case, the bending moment loaded on the entire cross section of the bumper beam, concentrated load point or greatest at the longitudinal center, it becomes smaller toward the end. When loaded with the same load in addition to the longitudinal center, bending moment concentrated load point is the highest. The bending moment of the concentrated load point is lower than the bending moment of the concentrated load point in the case of a load in the longitudinal center. That is, when adding the same load, most bumper beam fragile when adding a load to the longitudinal center. Therefore, load resistance is important for the load application of the longitudinal center of the bumper beam. [0019] 　In the above configuration 1, the tensile strength in the region of the length 250mm in the longitudinal center of the hat member curved to be convex toward the closing plate side in the longitudinal direction in addition to the high-strength portion of the above 1.5 GPa, high-strength portion of the longitudinal direction outside the region, the low-strength portion is provided over the height of the vertical wall of the region in the longitudinal direction. This configuration can increase the maximum load because of the high strength of the bending moment becomes largest portion when the load caused by an impact is input to the central. Further, since the low-strength portion is provided in the region outside the length 250mm including the central longitudinal, when the load is applied at the center, towards the low-strength portion than the high strength portion is easily deformed earlier . That is, by providing the low-strength portion to be deformed origin other than the longitudinal center of the bumper beam (concentrated load point), before the high-strength portion in the concentrated load point is deformed, making it possible to locally deform the low strength portion it can. Further, the low strength portion, ductile since the tensile strength is lower than in the center. Low strength portions, one from to the other of the second ridge is formed over the two longitudinal walls and the top plate. Therefore, the base material rupture is less likely to occur due to deformation in the top plate and two vertical walls which low strength portion is formed. Since the low-strength portion has a high ductility, even low intensity portion is locally deformed, hardly broken. That is, the low strength portion, rather than break, bend extends. Thus, according to the configuration 1, a reverse-hat shape, while still ensuring a maximum load and using high-strength materials 1.5 GPa ≦ TS, it can provide a bumper beam that can inhibit matrix fracture. [0020] 　Incidentally, the vertical wall height, the length of the vertical wall in a direction perpendicular to the closing plate. That is, the distance between the first ridge and the second ridge line in a direction perpendicular to the closing plate, the vertical wall heights. Bumper beam may also be referred to as a bumper reinforcement. [0021] 　(Configuration 2) 　In the above configuration 1, the low-strength portion of the tensile strength (TS) is preferably less than 1.5 GPa. With TS <1.5 GPa, it is possible to suppress the base material rupture and ensure ductility of low strength portion. [0022] 　(Configuration 3) 　In the above structure 2, the tensile strength TS of the low-strength portion is preferably greater than 590 MPa. 590 MPa M3). For example, between the longitudinal center and one of the support portions of the hat member 1, the distance from the longitudinal center distance LP larger area, by providing the low-strength portion 10L, the seat as shown in broken line Mb2 or dashed Mb3 it is possible to obtain a bending up moment distribution. Dashed Mb3 may seat when the distance from the longitudinal center of the seat bends down moment (= M4) at the distance LP larger area, which is even lower than the longitudinal center of the seat bends down moment (= M1) (M1> M4) It shows a longitudinal distribution of bending up moment. [0064] 　The deformation behavior of the case of the seat bends down moment distribution indicated by the broken line Mb2 will be described. In this case, when the state shown in the last or the linear G3 bending moment occurring in the cross section in the longitudinal direction of the center of the hat member 1 reaches the longitudinal center of the seat bends down moment = M1 by the load, at a distance LP from the longitudinal center P1 the resulting bending moment in the cross section reaches the seat bends down moment = M3 at the position P1 (M1> M3). Therefore, than the longitudinal center of the cross-section of the hat member 1, toward the cross section at the position P1 buckles first. [0065] 　Accordingly, even when the load is input to the longitudinal center of the closing plate 2 hat member 1, the local deformation occurs at a position P1 away from the longitudinal center by a distance LP. The maximum load at the time of deformation due to the load when the seat bends down moment distribution indicated by the broken line Mb2 becomes substantially the same level as the maximum load when the seat bends down moment distribution shown in broken line Mb1. [0066] 　If the seat bends down moment distribution indicated by the broken line Mb3, the state indicated by the line G2, i.e., when the bending moment occurring in the longitudinal center of the cross section reaches M2 (M1> M2), the bending moment generated in the cross-sectional position P1 reaches the seat bends down moment = M4 (M3> M4) at the position P1. In this case, the cross section of the position P1 buckles. Thus, when the seat bends down moment distribution indicated by the broken line Mb3, as compared with the case of the seat bends down moment distribution indicated by the broken line Mb2, the maximum load during deformation due to the load becomes smaller. Therefore, in view of enhancing the maximum load, a seat bent down moment in the longitudinal center, it is preferable to appropriately set the difference ΔMb the seat bending down moment where you want to generate a local deformation. [0067] 　Difference ΔMb of the seat bends down moment, for example, can be adjusted difference in tensile strength of the low strength portion 10L and the high-strength portion 10H, the difference in thickness, the vertical wall of height H by differentially. From the viewpoint of production efficiency, the difference in the tensile strength of the low strength portion 10L and the high-strength portion 10H, it is preferable to adjust the difference ΔMb seat bent up moment. [0068] 　For example, the intensity difference between the longitudinal center of the vertical wall 1b, as the distance between the longitudinal center end closer to the low-strength portion 10L is large, the tensile strength of the tensile strength and high-strength portion 10H of the low-strength portion 10L it can be increased. As an example, it may be set the strength of the low strength portion as follows. [0069] 　(Intensity setting example of the low strength portion) 　use to FIG. 4, the intensity setting example of a low strength portion. If low-strength portion is in the interval L1, i.e., one quarter of the distance LS between greater than the distance LL is 125mm from the center 1m of the vertical wall 1b to the low strength portion 10L in the longitudinal direction, and two support portions 1s the following cases (125mm