Technical field [0001]The present invention relates to a lap fillet arc welded joint. This application claims priority based on Japanese Patent Application No. 2017-157961, filed in Japan on Aug. 18, 2017, which is incorporated herein by reference. BACKGROUND [0002] In the field of motor vehicles, for environmental protection, together with improved fuel economy by reducing vehicle weight, the improvement of collision safety has been demanded. Conventionally, in order to improve weight reduction and crashworthiness of the vehicle body, such as to optimize the body structure with using the thickness of thin high-strength steel sheet as a vehicle body structural member, various techniques developed done ing. Note that among the structural members of a motor vehicle, a plurality of high-strength steel sheet also includes a weld joint having a base material. In the automotive field, as a manufacturing method of the welded joint, lap fillet arc welding method in which the fillet arc welding in a state of superimposing two high-strength steel sheets have been widely adopted. In this specification, a welded joint produced by such lap fillet arc welding method, referred to as lap fillet arc welded joint. [0003] 　As automotive structural member, the structural member used in environments with external force load of vibration or repeated, in addition to the usual static tensile strength, to withstand the forces repeatedly act, sufficient fatigue strength It is required to be equipped with. The fatigue strength of the steel sheet (fatigue limit), rises in proportion to the tensile strength, fatigue strength of lap fillet arc welded joint, it is generally known that lower than the fatigue strength of the steel sheet which is a base material It is. [0004] 　One of the causes, the fatigue strength of lap fillet arc welded joint include to have a dependency on the weld toe shape. Figure 12 is a weld toe shape as a parameter, a graph showing a relationship between fatigue strength of lap fillet arc welded joint and (fatigue limit) and the tensile strength of the base material steel plate. In the following, referred to as fatigue characteristics of the relationship between the fatigue strength tensile strength. In Figure 12, as a reference, but also shows the fatigue characteristics of the base material steel plate alone (see straight line C0). As indicated by the straight line C0, the fatigue strength of the base steel sheet alone, increases in proportion to the tensile strength. [0005] 　12, the curve C1 is toe shape of the weld metal shows the fatigue characteristics when steep, curve C2 is toe shape of the weld metal shows fatigue characteristics when gentle. As shown by the curve C1, when toe shape of the weld metal is steep, be increased tensile strength of the base material steel plate, the fatigue strength of lap fillet arc welded joint is hardly increased, the base material steel plate only to remain at very low levels compared to the fatigue strength of the single. [0006] 　This is because, if the toe shape of the weld metal of the sharp shape, because the stress concentration of the toe portion becomes very high. As a result, as shown in FIG. 13, when the lap fillet arc welded joint continue to receive repeated tensile load, at an early stage, at the boundary between the toe portion and the base steel sheet (lower plate) 200 of the weld metal 100 fatigue crack 300 is generated in a molten boundary. Incidentally, FIG. 13 is a schematic view of the junction between the weld metal 100 and the base material steel plate 200, as seen in section perpendicular to the weld line of the weld metal 100 having a steep toe portion shape. [0007] 　On the other hand, as shown by the curve C2 in FIG. 12, when toe shape of the weld metal is gentle, the fatigue strength of lap fillet arc welded joint is lower than the fatigue strength of the base material steel plate alone, the base material steel plate It increases with an increase in the tensile strength. The reason for this (see FIG. 14) when the gentle shape toe shape of the weld metal, because it is possible to prevent the stress is concentrated on the toe. Therefore, conventionally, as a technique for improving the fatigue strength of lap fillet arc welded joint, a technique for controlling the toe shape of weld metal on a gentle shape is generally adopted. [0008] For example, the following Patent Document 1, as a technique for stably controlling the toe shape of weld metal on a gentle shape, the fillet arc welding overlapped with welding material having a specific chemical composition (welding wire) by performing a technique for improving the wettability of the molten metal to the base material steel plate is disclosed. Welding material disclosed in Patent Document 1, by mass%, C: 0.005 ~ 0.40%, Mn: 0.2 ~ 8.0%, and Si: 1.1% greater than 8.0% It contains further, Se: 0.01 to 1% and / or Te: 0.01-1%. [0009] The following Patent Document 2, in order to improve the fatigue strength of lap fillet arc welded joint, not only controls the toe shape of weld metal on a gentle shape, the weld metal to the hardness of the heat affected zone by controlling the ratio of the hardness within a specific range, a technique for reducing the local stress and strain concentration due to the difference in hardness occurring in the vicinity fusion boundary is disclosed. Here, the hardness of the weld metal, a hardness Hv of the weld metal in 0.5mm away the weld metal side from a molten boundary FL of weld toe (FL-0.5). The hardness of the heat affected zone, a hardness Hv of the heat-affected zone of 0.5mm away the HAZ side from the melt boundary FL (FL + 0.5). Specifically, in the technique of Patent Document 2, the ratio of the hardness of the weld metal to the hardness of the heat affected zone (Hv (FL-0.5) / (FL + 0.5)) is 0.3 to zero. It is controlled within the range of 9. CITATION Patent Document [0010] Patent Document 1: Japanese Patent 2002-361481 JP Patent Document 2: Japanese Patent 2008-178910 JP Summary of the Invention Problems that the Invention is to Solve [0011] 　As described above, conventionally, as a technique for improving the fatigue strength of lap fillet arc welded joint, a technique for controlling the toe shape of weld metal on a gentle shape it is generally adopted. However, as shown by the curve C2 in FIG. 12, tensile strength of the base material steel plate becomes more than 950 MPa, even if even controlling the toe shape of weld metal on a gentle shape, the lap fillet arc welded joint the fatigue strength is significantly reduced it was found by the study of the present inventors. [0012] 　Recently, among the weight and improve the collision safety of the automobile is strongly required, in order to utilize the high strength thin steel sheet having a high tensile strength of at least 950MPa as a structural member of a motor vehicle, achieving various conditions such as fatigue strength it is very important research topic to establish possible joining techniques (welding technology). Accordingly, the present inventors, first, the fatigue strength of the fillet arc welded joint tensile strength of the base material steel plate overlapping in the case of more than 950MPa was investigated the cause of significantly reduced. [0013] 　As a result, as shown in FIG. 14, when the tensile strength of the base material steel plate is not less than 950 MPa, rather than melting the boundary between the weld metal 110 and the base material steel plate (lower plate) 200 having a smooth toe section shape, welding fatigue in the surface of the toe portion of the metal crack 310 were found to be generated. Incidentally, FIG. 14 is a schematic view of the junction between the weld metal 110 and the base material steel plate 200, as seen in section perpendicular to the weld line of the weld metal 110 having a smooth toe section shape. [0014] Furthermore, when observing the portion fatigue crack 310 has occurred microscopically, fatigue crack 310 from coarse ferrite grains present on the surface of the toe portion of the weld metal was found to have occurred. From the above results, the present inventors have considered why the fatigue strength of the fillet arc welded joint tensile strength of the base material steel plate overlapping in the case of more than 950MPa is significantly reduced as follows. [0015] If the tensile strength of the base material steel plate is not less than 950 MPa, the surface of the weld metal, the composite structure of ferrite and bainite (or martensite) are generated. As is well known, ferrite is a relatively soft tissue, bainite (or martensite) is relatively hard tissue. Therefore, due to the intensity difference between different tissues of hardness is believed that stress and strain tends to concentrate in the soft ferrite. Then, in an environment in which the lap fillet arc welded joint continue to receive repeated tensile load, results stress and strain is concentrated in the ferrite, at an early stage, the fatigue crack 310 has occurred ferrite grains present on the surface of the weld metal it is conceivable that. [0016] 　For such technical problems, even if the tensile strength of the base material steel plate is not less than 950 MPa, simply by performing the fillet arc welding overlapped using welding wire of higher strength, has high fatigue strength can be obtained lap fillet arc welded joint. 12, the curve C3 is a high strength using a welding wire of the weld metal toe shape of lap fillet arc welded joint obtained by performing the fillet arc welding overlapped so as to smooth more shows the fatigue characteristics. As indicated by the curve C3, simply, using the welding wire of higher strength, even if the tensile strength of the base material steel plate becomes more 950 MPa, the fatigue strength of lap fillet arc welded joint continues to rise it can be seen. [0017] 　However, as a result of further studies by the present inventors, when the fillet arc welding overlapped using welding wire of higher strength, due to an increase of the strength of the weld metal (high hardness), the hydrogen in the weld metal embrittlement cracking has been found to be more likely to occur. That is, even when the tensile strength of the base material steel plate is not less than 950 MPa, by using a welding wire of higher strength, although it is possible to increase the fatigue strength of lap fillet arc welded joint, the generation of hydrogen embrittlement cracks When you take into account, the overall strength of the lap fillet arc welded joints was found to be reduced rather. Moreover, the use of welding wire having a higher strength, causes an increase in manufacturing cost of lap fillet arc welded joint. [0018] 　On the other hand, without the use of high-strength welding wire, a method of increasing the fatigue strength of the fillet arc welding lap joint by applying the technique disclosed in Patent Document 2 is also conceivable. However, as described above, when the tensile strength of the base material steel plate is not less than 950 MPa, due to the intensity difference between the hardness of different tissues present on the surface of the weld metal, concentrated stress and strain in the soft ferrite easy it is believed that a cause of deteriorating the fatigue strength of lap fillet arc welded joint. Therefore, as in the technique disclosed in Patent Document 2, local stress and strain caused by difference in hardness occurring in the vicinity of the melting boundary (i.e. hardness difference between the heat affected zone and the weld metal) for the purpose of reducing the concentration, as the ratio of the hardness of the weld metal to the hardness of the heat affected zone were applied a method of controlling within a specific range, consisting of the base material steel plate having a tensile strength of at least 950MPa it is difficult to increase the fatigue strength of lap fillet arc welded joint. [0019] 　The present invention has been made in view of the above circumstances, without using a high-strength welding wire causing hydrogen embrittlement cracks, lap fillet arc welded joint consisting of the base steel sheet having a tensile strength of at least 950MPa aims to increase the fatigue strength of the. Means for Solving the Problems [0020] The present invention employs the following means in order to achieve the object according to the above-mentioned problems are eliminated. (1) lap fillet arc welded joint according to one aspect of the present invention, the first steel plate and a second steel plate surface and the second steel plate of the first steel sheet having a tensile strength of at least 950MPa, respectively and mutually superimposed and a weld metal which extends along the corner, which is the end surface of the at formation. In this lap fillet arc welded joint, Vickers hardness of the weld metal is less than 400 HV. Also, the lap fillet arc welded joint, when viewed in cross-section perpendicular to the weld line of the weld metal, the position of the fusion boundary on the surface of the first steel sheet and the point A; surface of the first steel plate C point position spaced 0.4mm toward the weld metal from the point a in the X-direction; a position spaced 0.5mm toward the weld metal from the point a to the point D in X direction parallel to the wherein a straight line connecting the point B and the point a, and the point a; and to; the intersection between the point D the street and the first steel plate thickness extending direction line and the weld metal surface and the point B the angle between the straight line connecting the point D, the a toe angle β of the weld metal; the total number of recesses present on the surface of the weld metal in the range between the point a and the point C NA and; of the recess, Blow having a maximum particle size of at least 10μm The number of recesses site grains contact the NB. As described above, beta, when defining the NA and NB, the weld metal satisfies the following conditional expression (1) and (2) simultaneously. ° 0