Description Title of Invention WELDED STRUCTURE AND METHOD FOR MANUFACTURING THE SAME 5 Technical Field [OOOl] The present invention relates to a welded structure including n~ultiples teel sheets that are joined by using resistance spot welding (hereinafter simply referred to 10 as "spot welding") at multiple locatiot~si n lapped portions in which multiple steel sheets are overlapped, and also relates to a method for manufacturing the welded structure. In particular, the present invention relates to a welded structure that il~cludes an expected deformation region that is expected to be deformed by receiving a load, and also relates to a method for manufacturing the welded structure. 15 Background Art [0002] FIG. 1 is a perspective view illustrating a structure of an autonlobile showing framework ~nembers. A front side menlbcr 2, a rear side member 3, and a 20 side sill 7 are ar~anged on a side of the autonlobile. Each of these members is disposed along a vehicle front-back direction (hereinafter simply referred to as "fiont-back direction"). The front side member 2 and the rear side member 3 are disposed in the front and back portions, respectively, and the side sill 7 is disposed in tlie intermediate portion in the front-back direction. 25 [OOO3] Floor cross members :4 atid 4', which are disposed in the intertilediate t ' , portion in the front-back direction, extend in a vehicle widthwise direction along the floor surface. A center pillar 6 extending in a vertical direction is disposed on a side in tlie iutermediate portion in the fiont-back direction. A bumper reinforcement 5 30 extendit~g in a vehicle widthwise direction is disposed at the front end of the automobile. The above tllet~tioned members are fiatnework members that foml the PCT Application No.: PCT/JP2015/057G56 2/48 franlework of the auto~nobile. [0004] The fian~eworkm embers and crash-boxes la and lb are welded structures including multiple steel sheets. The welded struch~reh as lapped portions in wllich 5 multiple steel sheets are overlapped, and these steel sheets are joined by spot welding at multiple locations. W11en a spot weld portion (hereinafter simply referred to as "weld portion") fractures (shear fractures) in the case of automobile collision, the fra~nework member camiot generate a desired deformation, and thus cannot sufficie~~tlayb sorb the collision energy. Thus, the welded structure that can 10 suppress the shear fracture is desired. [OOOS] Patent Literature 1 describes a method for allowing a base steel sheet to fracture by reducing the width of a heat-influenced portion generated by welding in the case of using a high-tensile steel sheet having a tensile strength of 440 MPa. 15 Patent Literature 2 describes a method for widely softening the outer sides of a weld nugget, and for providing equiaxial martensite structure inside the nugget, to obtain a resistance spot weld joint of the high-tensile steel sheet that is excellent both in tensile shear resistance and in cross tension prope~ties. Patent Literatures 3 and 4 describe that the nugget diameter d is made to satisfy 3 x t, ID 5 d 5 6 x t,,ln (t,,, is 20 the sheet thickness (mm) of the thinnest sheet) by controlling weld current conditions in multiple steps. Patent Literature 3 insists that the method can restrain initial expulsion and expulsion at the faying surface, arid Patent Literature 4 insists that the method can achieve a high joint strength. 25 Citation List Patent Literature [0006] Patent Lite~ature1 : JP 2001-9573A Patent Literature 2: JP 2013-78782A Patent Literatu~e3 : JP 5359571B Patent Literature 4: JP5333560B PCT Application No.: PCT/JP2015/05765G 3/48 Snmmaiy of Invention Technical Problem [0007] 5 However, the methods of Patent Literatures 1 to 4 presupposes that the nugget diameters cannot be made to be larger than a certain diametel; which inlposes limitations on fracture suppressioti at weld portions. In particular, as the tensile strength of a steel sheet becomes larger, the electric resistance becomes larger accordingly, which generates expulsion due to heat generation during welding and 10 makes the nugget diameter smaller. This has led to a problem that the weld strength cannot be stably obtained. [0008] The present invention is conceived in view of the above-described problem, and is directed to provide a welded structure and a method for tnanufact~~rintghe 15 welded structure, which are new and improved and can reduce the fiacture of the weld portions. Solution to Problem [0009] 20 According to an etnbodiment of the present invention, there is provided a welded structure including two or three steel sheets and a lapped portion in which the steel sheets are overlapped and joined by spot welding at a plurality of locations, the welded structure including a spot weld portion, and, ~vliena diameter of a nugget is d,, (nun), a tip diameter of an electrode used by the spot welding is d (mtn), and an 25 average thickness per steel sheet of the steel sheets at the lapped portion is t,,,, (mmn), the spot \veld portion satisfies a condition (a) or a condition (b) below in accordance wit11 the average thickness t,,,, (rnm). (a) d,, > d(ta,,)" when 0.5 nun it,,., < 1.1 nnn (1) (b) d,,, > 1.05d when 1.1 lmn < t,,, < 2.6 mtn (2) 30 [OOlO] The xvelded structure may include an cxpected deformation region to be PCT Application No.: PCT/JP2015/057656 4/48 subjected to plastic deformation when a load is applied, and the spot welding may be cal+ied out at least within the expected deformation region. [OOll] The expected deformation region may be free from spatter adhesion. 5 [0012] The steel sheets may have a tensile strength of 980 MPa or more. [OO 1 31 The electrode may be a combined electrode having an electrode body to be pressed against the steel sheets at the lapped portion and a ring-shaped member to be 10 pr~sseda gainst the steel sheets around the electrode body. [OO 141 The movable electrode may include a first ring-shaped member and a second ring-shaped member to be pressed against the steel sheets at the lapped portion with the first ring-shaped member and the second ring-shaped member facing 15 each other, and a first electrode body and a second electrode body, each being inserted in a through hole disposed each of the first ring-shaped member and the second ring-shaped member, to be pressed against the steel sheets at the lapped portion with the first electrode body and the second electrode body facing each other, and an electric current may flow through the steel sheets behveen the first electrode 20 body and the second electrode body. [0015] The spot weld portion satisfying the condition (a) or the condition (b) may be present in a 20 to 60% extent on the welded structure. [0016] 25 The spot weld portion preferably has an equivalelit carbon content (Ceq) of 0.13 mass% oE: more, the equivalent carbon content (Ceq) being defined by an equation (3) below: Ceq = [C] + 1/90 [Si] + Ill00 ([Mn] + [Cr])(3) wvllere 30 [C]: an average C content (mass%) of the spot weld portion; [Si]: an average Si content (mass%) of the spot weld portion; PCT Application No.: PCT/JP2015/057G56 5/48 [Mn]: an average Mn content (mass%) of the spot weld portion; and [Cr]: an average Cr content (mass%) of the spot \veld portion. [0017] The welded stmcture tnay be a rnember to be used for an automobile, for 5 example, atid in this case, the expected defornlation region may be a region to be subjected to at least one of an axial conlpression load and a bending load. [OO 1 81 According to an embodiment of the present invention, there is provided a tilethod for manufacturing a welded structure including two or three steel sheets and 10 a lapped portion in which the steel sheets are overlapped and joined by spot welding at a plurality of locations, the method includitig carrying out spot welding, the carrying out spot welding including a first step in which a first rod-shaped electrode body arid a second rod-shaped electrode body are arranged facing each other with the lapped portion being sandwiched therebetween, and a first ring-shaped member and a 15 second ring-shaped member are arranged facing each other, the first ring-shaped member having a though hole through which the first electrode body is inserted and a back end to which a first elastic body is connected and the second ring-shaped member having a through hole through which the second electrode body is inserted and a back end to which a second elastic body is connected, and a second step in 20 which the lapped portioti is pressurized by pressing a tip face of each of the first electrode body and the second electrode body against the lapped portion, and by pressing a--tip face of each of the first ring-shaped member and the second ringshaped nietiiber against the lapped portion while the first elastic body exerts a pressing pressure on the first ring-shaped member and the second elastic body exerts 25 a pressing pressure on the second ring-shaped menibel; and then an electric current is applied between the first electrode body and the second electrode.body. The first step and the second step cause a spot \veld portion to satisfy a conditioti (c) or a condition (d) below in accordance with an average thickness t,,., (mtn): (c) d,, > d(ta,te)'nx vhen 0.5 mtn 5 t,,,, < 1.1 mil (4) 30 (d) d,, > 1.05d wvl~en 1.1 mm 5 t,,, 5 2.6 mm (5) whexe d,, (nun) is a nugget diameter, d (nun) is a tip diameter of an PCT Application No.: PC'UJP2015/057G6G 6/48 electrode used by the spot welding, and t,,., (mtn) is an average thickness per steel sheet of the steel sheets at the lapped portion. Advantageous Effects of Invention 5 [0019] The welded structure and the method for manufacturing the same according to the present invention can reduce the fracture of the weld portion, as compared to known welded structures, and thus the welded structure exhibits an excellent collision energy absorbing capability. In particulal; applying this invention to 10 welded structures made of high-tensile steel sheets can provide such effects appreciably, thereby eliminating the necessity of thickening the steel sheets. Consequently, this enables the steel sheets to be thinner so that the weight of the welded structures can be further reduced. 15 Brief Description of Drawings [0020] [FIG. 11 FIG. 1 is a perspective view illustrating an automobile st~ucture showing framework members. [FIG. 2 4 FIG. 2A is a right side view schematically illustrating a configuration 20 exaniple A of a welded structure according to the second embodiment. [FIG. 2B] FIG. 2B is a right side view schematically illustrating a configuration exan~pleB of the welded structure according to the second embodiment. [FIG. 2C] FIG. 2C is a right side view schematically illustrating a configuration exalnple C of the welded structure according to the second embodiment. 25 [FIG. 2D] FIG. 2D is a sight side view schematically illustrating a configuration example D of the welded structure according to the second einbodiment. [FIG. 3A] FIG. 3A is a side view illustrating a fiont side menlbel; in which expected deformation regions are indicated. [FIG. 3B] FIG. 3B is a side view illustrating a center pillar, in which expected 30 deformation regions are indicated. [FIG. 3C] FIG. 3C is a side view illustrating a rear side member, in which expected PCT Application No.: PC'l'lJP2015/057G5G 7/48 defor~nationrc gions are indicated. [FIG. 3D] FIG. 3D is a perspective view illustrating a side sill, in which expected deformation rcgions are indicated. [FIG. 4A] FIG. 4A is a cross-sectional view illustrating a vicinity of a spot weld 5 portion in a welded structure wvhen curvature changing regions are observed near both ends of an indentation. [FIG. 4B] FIG. 4B is a cross-sectional view illustrating the vicinity of a spot weld portion it1 a welded structure when curvature changing regions are not obse~~endea r both ends of an indentation, but a depressed amount of the indentation can be 10 identified. [FIG. 4C] FIG. 4C is a photograph showing a cut surface of the vicinity of a spot weld portion of a welded structure when a depressed amount of an indentation cannot be identified. [FIG. 5A] FIG. 5A is a schematic diagram illustrating an example of a resistance spot 15 welding apparatus that can be used in the method for manufacturing welded structures according to the present invention, showing a state of the apparatus before welding starts. FIG. 5B] FIG. 5B is a schematic diagram illustrating an example of a resistance spot . .welding apparatus that can be used in the method for manufacturing welded 20 structures according to the present invention, showing a state of the apparatus during welding. -[FIG. 6A] FIG. 6A is a schematic diagram illustrating a situation in which a weld .. nugget is formed by spot welding using the resistance spot welding apparatus illustrated in FIGS. 5A and 5B when a ring-shaped r~lelilber is not electrically 25 conductive. ?%[FIG. 6B] FIG. 6B is a schematic diagram illustrati~~ag situation in which a \veld nugget is formed by spot wvelding using the resistance spot welding apparatus illustrated in FIGS. 5A and 5B ~vliena ring-shaped member is electrically conductive. [FIG. 71 FIG. 7 is a diagram showing a relation bet\veen nugget diameters and cross 30 tension loads. [FIG. 81 FIG. 8 is an illustration including a front vie\\( and a right side view showing PCT Application No.: PCT/JP2015/057G56 8/48 shapes and din~e~~soifo a~ h~ast- chantlel member and a closillg plate included in a welded structure that is used in crushing tests by bending defonnation. [FIG. 91 FIG. 9 is a diagram showing relatious between nugget diameters and absorbed energy in bending defomlation. 5 [FIG. 101 FIG. 10 is an illustration including a front view and a right side view showing shapes and dimensions of a hat-channel member and a closing plate included in a welded structure that is used in axial crushing tests. [FIG. 111 FIG. 11 is a diagram showing a relatiall between nugget diameters and absorbed energy in axial crush deformation. [FIG 121 FIG. 12 is a characteristic diagram showing a relation between nugget diameters d,, (mm) and an average thickness per material steel sheet t,, (tnm) when a welded structure is obtained by the method according to present embodiment. [FIG. 13A] FIG. 13A is a right side view schelnatically illustrating a configuration example E of a welded structure according to a first embodiment. [FIG. 13B] FIG. 13B is a right side view schematically illustrating a cotlfiguration example F of a welded structure according tothe second embodiment. [FIG. 14A] FIG. 14A is a right side view schematically illustrating a configuration example G of the welded st~ucturea ccording to the first embodiment. [FIG. 14B] FIG. 14B is a right side view schematically illustrating a configuration example H of the welded stt~~chtarcec ording to the second embodiment. [FIG. 151 FIG. 15 is a right side view schelnatically illustrating a configuration example I of the welded structure according to the first embodiment. [FIG. 161 FIG. 16 is a right side view schematically illustrating a configuration exanlple J of the welded structure according to the first embodiment. [FIG. 171 FIG. 17 is a right side view schematically illustrating a configuration example K of the welded structure according to the first embodiment. . . Description of Embodiments [0021] Hereinafter, prefe~ved embodiments of the present invention will be described in detail with refcrellce to the appended drawings. Note that, in this PCT Al~plication No.: PCT/JP2016/05765G 9/48 specification and the appended drawings, structural elements that liave substantially the same function and structure are denoted with the same reference numerals, and repeated explanation of these structural elements is omitted. roo221 5 Now, the welded stlucture and the method for niant~facturing the welded structure according to the present invention will be described in detail. As described above, there is provided a welded structure including two or thee steel sheets and a lapped portion in ~vliicht he steel sheets are overlapped and 10 joined by spot welding at a plurality of locations, the welded stn~cturei ncluding a spot weld portion, and, wlien a diameter of a nugget is d,,, (nm), a tip diameter of an electrode used by the spot welding is d (mm), and an average thickness per steel sheet of the steel sheets at the lapped portion is t, (mnl), the spot weld portion satisfies a condition (a) or a condition (b) below in accordance with the average 15 thickness t,,, (mm). (a) d., > d(t,,,,)ln when 0.5 mm 5 ta,, < 1.1 mm (1) (b) d,,, > 1.05d when 1.1 mm 5 ta,, < 2.6 mm (2) [0023] The welded structure according to the embodiment of the present invention 20 may have a configuration in which two steel sheets are included and overlapped at the lapped portion (hereinafter referred to as "first embodimetlt"). When two steel sheets are a first and a second steel sheet;- the first steel sheet can serve as a hatchannel member, and the second steel sheet can serve as a closing plate member, for exanlple, as described later with reference to FIG. 8. 25 [0024] The welded structore according to'the embodimetit of the present invention may liave a configuration including three steel sheets (hereinafter referred to as "second embodiment"). In this case, two sheets out of the thee may be overlapped at the lapped portion, or all of the t l ~ e esh eets may be overlapped at the lapped 30 portion. The welded st~ucture of the second embodinlent, for example, may be formed of a tube-shaped body and an intermediate plate that partitions tl~e inner PCT Application No.: PC'l'/JP2015/057G5G 10148 space of the body. More specifically, configuration exa~llples illustrated in FIGS. 2A to 2D may be adopted when the three steel sheets sen7e as the first, the secoad, and the third steel sheet. [0025] 5 FIGS. 2A to 2D are right side views sche~naticallyil lustrating configuratio~i exanlples of the welded structure according to the second embodiment. FIGS. 2A, 2B, 2C and 2D illustrate a configuration example A, a configuration example B, a configuration exanlple C, and a configuration example D, respectively. A \velded structure 43 illustrated in FIG. 2A has the body formed of a first steel sheet 53 and a 10 second steel sheet 63, and the intermediate plate formed of a third steel sheet 73. The first steel sheet 53 is the hat-channel member and the second steel sheet 63 is the closing plate member. The welded structure in the configuration example A has four lapped portions, and two out of the three steel sheets are overlapped at each lapped portion. 15 [0026] Welded structures (44 to 46) illustrated in FIGS. 2B to 2D each have the body formed of a first steel sheet (54 to 56) and a second steel sheet (64 to 66). The intermediate plate is formed of a third steel sheet (74 to 76). The first steel sheet (54 to 56) and the second steel sheet (64 to 66) each have two bent portions with a 20 predetermined angle (90' in these figures). The welded strncture of the configuration examples B to D has two lapped portions, at each of which all of the three steel sheets are overlapped. .. [0027] -. - FIGS. 13A and 13B illustrate a configuration example E and a configuration 25 exanlple F, FIGS. 14A and 14B illustrate a configuration example G and a configuration example 13, and FIGS. 15, 16, and 17 illustrate a configuration ...*. .. exanlplc I, a configuration exanlple J, and a configuration example K, respectively. , . A welded st~ucture 80 illustrated in FIG. 13A has the body fornled of a first steel sheet 83 and a second steel sheet 93. A welded structure 82 illustrated in FIG. 13B 30 has the body formed of the first steel sheet 83 and the second steel sheet 93, and the internlediate plate is formed of a third steel sheet 84. The first steel sheet 83 is the PCT Application No.: PCT/JP20151057656 11/48 hat-channel inember, and the second steel sheet 93 is the closing plate inember. The welded structure in the configuration example E has two lapped portions, and two steel sheets are overlapped at each lapped portion. The welded structure in the configuration example F has four lapped portions, and two out of tlie three steel 5 sheets are overlapped at each lapped portion. [0028] A welded structure 100 illustrated in FIG. 14A has the body formed of a first steel sheet 103 and a second steel sheet 113. A welded structure 101 illustrated in FIG. 14B has the body formed of the first steel sheet 103 and the second steel sheet 10 113, and the intermediate plate is formed of a third steel sheet 114. The first steel sheet 103 is the hat-channel member, and the second steel sheet 113 is the closing plate member. Tlie welded structure in the configuration example G has two lapped portions, and two steel sheets are overlapped at each lapped portion. The welded structure in the configuration example H has four lapped portions, and two out of the 15 three steel sheets are overlapped at three lapped portions. Three steel sheets are overlapped at the remaining one lapped portion. [0029] A welded structure 120 illustrated in FIG. 15 has the body formed of a first steel sheet 123 and a second steel sheet 133. The first steel sheet 123 is the hat- 20 channel member, and the second steel sheet 133 is the closing plate member. Tlie welded stl-ucture in the configuration example I has three lapped portions, and two steel slieets are overlapped at each lapped portion. [0030] A welded stl-ucture 140 illustrated in FIG. 16 has the body formed of a first 25 steel sheet 143 and a second steel slieet 153. The first steel sheet 143 is the hatchannel member, and the secotid steel sheet 153 is tlie clositig plate menlibel: The ;' welded structure in the configuration example J has two lapped portions, and t\vo steel sheets are overlapped at each lapped portion. [003 I] 30 . A welded stl-i~cture1 50 illustrated in FIG. 17 has the body forn~edo f a first steel shcet 153 and a second steel sheet 163. The first steel sheet 153 is the hatPCT Application No.: PCT/JP2015/057656 12/48 channel membel; and the second steel sheet 163 is the closing plate menibel: The welded stn~cturein the configrtration example K has two lapped portions, and two steel sheets are overlapped at each lapped portion. [0032] 5 With reference to FIG. 1, such welded structures of the present embodiment can be applied, for example, to any one of crash-boxes la and lb, a front side member 2,, a rear side member 3, floor cross members 4 and 4', a bumper reinforcement 5, a center pillar 6, and a side sill 7. [0033] 10 In most cases, collision against the automobile nlay occur fsotn ahead, from behind, and from side (respectively represented by "Fr. inlpact", "Rr. impact", and "Side impact" with an accompanying thick arrow in FIG. 1). In such a case, a member near a collided portion in the automobile deforms. The above-described members each have a shape extending in the longitudinal direction. When a load is 15 applied in the collision along the longitudirial direction (that is, the axial direction) to the vicinity of an end of the member, there occurs axial crush deformation (progressive plastic buckling deformation) in which the member contracts like bellows (or is folded in pleats) in the longitudinal direction. 111 addition, when the member has a bent portion and a load is applied in the collision in a direction along 20 the longitudinal direction, there occurs bending deformation followed by buckling at the bent portion. When a load is applied in the collision to a central portion of the member in a direction pe~pendicular or oblique to the longitndinal direction of the member, there occurs bending deformation. When a load is applied to the ~netnber in a cross-sectional direction of the member, there occurs the collapse of the cross 25 section acco~npanied by bending deformation, resulting in the overall bending deformation of the tnembec [0034] , . Table 1 shows examples of the regions in u~llicll plastic defornlation is expected by receiving a load (hereinafter referred to as "expected defornlation 30 region") for each of the memnbe~s. FIGS. 3A to FIG. 3D are diagrams each illustrating the.expected deformation regions of the front side member 2, the center PC1' Application No.: PCl1IJP2015/057G5G 13/48 pillar 6, the rear side nleniber 3, and the side sill 7. [Table 11 Welded structure (niember name) Front side member Expected deformation region ratio (%I Approx. 30 to 60 Expected deformation region Axial crush deformation: region between the front end and approx. 250 mm fiorn the fiont end Bending deformation: apmox. 300 mm long I the midportion in the longitudinal directiloonn -g re-ei on in Center I Approx. 20 1 Bending deformation: region between the top end and 5 corresponding members that are niounted on an automobile. [0036] pillar Rear side member Side sill In these members that are the welded stmctore, multiple weld portions created by spot welding are fornled, for example, at equal intervals along the longitudinal directioii of each ~l~etnbeeT he \veld portions present in the 'expected 10 defonnation region satisfy either the above-mentioned condition (a) or (b). . [0037] In examples in Table 1 and FIGS. 3A to 3D, the length of the expected deformation r~ionSivit11re spect to the longitudinal direction of the weldeit'structure is approximately 20 to 60% of the whole length of the ivelded structure. Thus, in Note that "front end", "rear end", "top end", and "bottom end" are with respect to to-50 Approx. 35 to 60 Approx. 30 to 60 15 the ease that the spot \veld portions are present approximately at equal intervals over approx.-150 mm from thetop end Collapse of the cross section: region between the bottonl end and approx. 300 mm froni the bottom end Axial crush deformation: region between the rear end and approx. 300 mm from the rear end Bending deformation: approx. 300 mm long region in the mid portion in the longitudinal direction Axial crush deformation: region behveen the front end and approx. 100 mm fiom the front end Bending deformation: approx. 300 mm long region in the mid portion in the longitudinal direction the whole \velded structure in the longitudinal direction thereof, approximately 20 to 60% spot weld portions of all the spot weld portions are present in the expected deforn~ationr egion. In this ease, the reniaining spot \veld portions do not have to PCT Application No.: PCT/JP2015/057G56 14/48 satisfy either of the condition (a) or (b). [0038] Regarding the crash-boxes la and lb and the bumper reinforcement 5 (sce FIG. l), the expected deformation region covers the whole region of each membel; 5 and all of the weld portions of each member needs to satisfy either of the condition (a) or (b). [0039] The diameter of the weld nugget defined in the conditions (a) and (b) is larger than tlie nugget diameter that can be fortned by known spot welding witl~out 10 generating expulsion. [0040] The expulsion is generated due to the fact that spot welding locally creates a region of high electric current density within an expected weld region of a base steel sheet, and sudden heating and melting of the region causes the melt of the base metal 15 to spatter. Consequently, when the spatters are adhered to the welded structure, the shape of the nugget in terms of size and thickness distribution, and the like does not become uniform even if the conditions of the spot welding remains the same. As a result, the welded structure having the weld poriions that satisfy the conditions (a) and (b) cannot he obtained in all of the expected deformation regions. Moreover, 20 this does not stably provide the nuggets having a high strength, and thus causes shear fracture to occur frequently even if the conditions (a) and (b) are satisfied. [0041] The welded structure of the present embodiment does .not. cause the generation of the expulsion and adhesion of the spatters, and the melt of the heated 25 and melted base metal has not been observed to spatter out of the melted portions during n~anu@iqt,tul.in. g....T ,. hus, unifornl heating is achieved within the.-expected weld region, which causes the size and shape of the nuggets to be substantially uniform wlien the conditions of spot welding remain the same. In the welded structure of the present embodiment, satisfying either of the condition (a) or (h) makes the nugget 30 strength stably high, and the shear fracture is not likely to occur. PCT Application No.: PCT/JP2015/057G5G 15/48 In the present embodiment, wvl~en the tip diameter of an electrode used in the spot welding is uuknoww~n, the diameter of a11 indentation formed on the surface of the steel sheet of the welded structure can be regarded as the tip diameter of the electrode. The indentation is formed by the tip of the electrode being pressed on the surface of .5 the steel sheet during spot welding. Now, tile structure of the welded structure and a method for determining the diameter of the indentation will be described with , reference to a case in which two steel sheets (the first and the second steel sheets) are joined. [0043] 10 FIG. 4A is a cross-sectional view illustrating the vicinity of a spot weld portion in the welded structure. This welded structure 10 includes a first steel sheet 11 atid a second steel sheet 12. The welded structure 10 has a lapped portion in which the first and tlie second steel sheets 11 and 12 are overlapped. The first steel sheet 11 and the 15 second steel sheet 12 are joined at ~nultiplel ocations in this lapped portion by spot welding. Only one spot \veld portion 13 is shown in FIG. 4A. [0044] An indentation 15, which corresponds to the tip of an electrode used for spot welding, is formed on the surface of each of the first and the second steel sheets 11 20 and 12. In FIG. 4A, a depressed portion is the indentation 15. A nugget 14 is formed inside tlie weld portion 13. In FIG. 4A, the weld portion 13 is substantially symmetrical with respect to the plane between the first steel sheet 11 and the second steel sheet 12, and t1111s reference ~lo~lleraflosr , and related to, the i~identatioti1 5 are provided only for one of the first and the second steel sheets 1 I and 12. 25 [0045] &In. the present embodimnent, the diameter of a wel&ptigget (hereinafter referred to as-"nugget diameter") d,, is defined as the maximum length of the nugget 14 in a direction parallel to the steel sheets (11 and 12) near the nugget 14. FIG. 4A is a cross sectio~pi erpendicular to the first and the second steel sheets, illustrating the 30 cross section of the illdentation 15 including. the lllaximum diameter portion (hereinafter referred t o as "maximum diameter cross section"). The ~naximutn PCT Application No.: PCT/JP2015/057G5G 16/48 lengtli of the nugget 14 in a direction parallel to the first and the second steel sheets 11 and 12 in this cross section can be regarded as tlie nugget diameter. I11 a cut face of the weld portion 13, tlie nugget 14 displays a different color (brightness), and thereby the length of the nugget 14 in a direction parallel to tlie first arid the second 5 steel sheets I1 and 12 can be determined easily. [0046] The indentation 15 corresponds to a region that is pressed by the tip of the electrode when spot welding is carried out. When the type and thickness of the steel sheets (the first steel sheet 11 and the second steel sheet 12 in the figure) that 10 are pressed by the electrodes are the same, there is a tendency it1 which the indentation 15 becomes deeper as the nugget diameter d,,g becomes larger. [0047] The diameter d of the indentation 15 (that is, the tip diameter of the electrode used for spot welding) in the present embodiment is defined as below. 15 In FIG. 4A, regions where the cuwature changes can be identified near both ends of the indentation 15. The inside surfaces of the indentation 15 include a bottom surface that is curved with a large curvature radius (or a flat surface), and inclined surfaces that are formed on the periphety of the bottom surface and are inclinedas a whole with respect the bottom surface and that have a curvature radius 20 smaller than that of the bottom surface. In the cross sectiot~ in FIG. 4A, the curvature of the contour of the indentation 15 changes between the bottom surface andthe inclined surfaces. In this case, the diameter d of the indentation 15 is a distance- between two points l6a and 16b where the curvature changes (or tlie curvature change becomes maximum) in this cross section (~iiaximum diameter cross 25 section). [OW481 * , .*. >.>-, . .. FIG. 48 illustrates the vicinity of the weld po~tion 13, showing the maximum diameter cross section of a portion it1 whicli a region where the curvature changes in the indentation 15 is not observed but a depressed aniount D of tlie 30 indentation 15 is identifiable. In FIG. 4B, the same reference numerals are used for cotnpotients similar to those in FIG. 4A, thereby omitting the description thereof. PCT Al~pLication No.: PCT/JP2015/057656 17/48 In this case (including the case in which the region \vliere the curvature changes are observable near only one end of the indentation 15), the length of the indentation 15 (depressed region) in the direction parallel to the steel sheets (I 1 and 5 12) in this cross section is regarded as the diatneter d of the indentation 15. [0050] FIG. 4C is a photograph showing a cut face of a portion near the weld portion'l3 that is cut at the maximum diameter cross section. In this portion, the depressed atiiount of the indentation 15 is not identifiable. In FIG. 4C, the same 10 reference numerals are used for components similar to those in FIG. 4A, thereby omitting the description thereof. A hardened portion 17 generated by heat influence is present around the nugget 14. The hardened portion 17 is formed by the fist and the second steel sheet 11 and 12 as base metals being heated to the austenite region and then being quenched during spot welding. 15 [0051] When the depressed amount of the indentation 15 is not identifiable, the diameter of the hardened portion 17 generated by heat influence in the maximum diameter cross section of the first or tlie second steel sheet 11 or 12 is regarded as the diatneter d of the indentation 15. In the cross. section, the brightness of the 20 hardened portion 17 generated by heat influence is different from that of another portion, and thus tlie diameter of the hardened portion 17 generated by heat influence I can be determined easily. [0052] ..~ M~liilet lie tip diameter of the electrode used for spot welding is determined 25 . from tlie diameter of the indentation by the method described with reference to FIGS. 3*,:4A to 4C, the tip diameter may instead be deter~ilined-firomth e liiaximum diatneter of .a region having a high Cu (copper) concentration on the surface of the steel sheet. The electrodes to be used for spot welding contain Cu, and CLI is transferred to the surface of the steel sheet dnring spot welding. Thus, the region having a high Cu 30 concentration on the smface of the steel sheet corresponds to the tip of the electrode. [0053] PCT Application No.: PCT/JP2015/057G56 1 8/48 In either case of determining tlie diameter of the indentation by using tlic method described with reference to FIGS. 4A to 4C or by using the region having a high Cu concentration, when the diameter d of the indentation 15 of one steel sheet 11 cannot be regarded identical to the diameter d of the indentation 15 of tlie otlier 5 steel sheet 12, an average of the diameters d of the indentations 15 of both steel sheets 11 and 12 is regarded as the diameter d of the indentation 15. [0054] In the present embodiment, whcn the lapped portion in which two steel sheets are overlapped is joined by spot welding, the average thickness t,, (mm) per 10 steel sheet at the lapped portion is (tl+t2)/2, where tl (mm) and t2 (mm) represent the individual thicknesses of the two steel sheets, respectively. The average thickness t,,, is 0.5 mm or more and 2.6 mn or less. Wfien the average thickness t,,, is in this range, it is possible to form the weld pol-ti011 that satisfies the Formula (1) or (2) without generating expulsion by using a preferred method for manufacturing the 15 welded structure of tlie present embodiment (see later description). [0055] In the present embodiment, when the lapped portion in which three steel sheets are overlapped is joined by spot welding, tlie average thickness per steel sheet _ t,,., (mm) at the lapped portion is (t,+td+ts)/3, where t3 (mm), 4 (tnm), and tS (mm) 20 represent the individual thicknesses of the three steel sheets, respectively. The average thickness t,, is 0.5 mm or more and 2.6 mm or less, which is the same as in the case that the lapped portion in which two steel sheets are overlapped is joined by spot welding. When the average thickness t,.,.is in this range, it is possible to form the weld portion that satisfies Fornlula (1) or (2) without generating expulsion by 25 using the preferred method for manufacturing tlie- welded structure of the present ?.- enlbodiment (see later description). ~?fi... . . . [0056] .: . In the present embodiment, the amount of energy that can be absorbed when the wvelded structure receives impact becomes larger as the intel-val between weld 30 portions is smaller, in other words, the number of weld portions in a given unit length of the welded structure is larger. However, if the interval between \veld portions is PCT Application No.: PCT/JP2015/057666 19/48 made too small, adjacent nuggets may be overlapped with each other. This causes a sliu~lct urrent to flow in a direction toward an adjacent nugget, thereby prevetiting the nugget from being formed to have a predetermined diameter. Thus, the interval bet\veen weld portions is preferably 17 mm or more. 5 [0057] Tlie present embodiment is preferably applied to welded structures having weld portions in which Ceq, that is, an equivalent C (carbon) content, defined in Formula (3) below is 0.13 mass% or more. Ceq = [C] + 1/90 [Si] + 11100 ([Mn] + [Cr]) . (3) 10 where [C]: average C content (mass%) of weld portion; [Si]: average Si content (mass%) of weld portion; [Mn]: average Mn content (mass%) of weld portion; and [Cr]: average Cr content (mass%) of weld portion. 15 [0058] In general, as the equivaletlt carbon content in steel sheets to be welded becomes larger, the electric resistance becomes higher and spot welding becomes more difficult to carry out without generating expulsion. When the equivalent carbon content (Ceq) of the weld portion is-0.13 mass% or more, it is extremely 20 difficult to carry out spot welding without generating expulsion by using known manufacturing methods. On the other hand, it can be achieved by using the preferred method for manufacturing the welded structure of the present embodiment (see later description). [0059] 25 The equivalent carbon content (Ceq) of the weld portion is approximately r,_ equal to an equivalent carbon content based on an average chemical composition of multiple steel sheets. When two steel sheets (the first and the second steel sheets) are joined by spot welding at the lapped portion in which the two sheets are overlapped, aud when the thicknesses of the two steel sheets are different, the 30 average chemical cornposition of the two steel sheets is calculated as a. weighted average bet\veen the average chemical composition of the first steel sheet and the PCT Application No.: PCT/JP2015/057656 20148 average chenlical composition of the second steel sheet \\lit11 respect to the thicknesses tl and t2. More specificall>: the equivalent carbon content (Ceqplateo)f the first and the second steel sheets based on their average chemical coltlpositions is given in Formula (6) belo~v. 5 Ceq,late = RI[C]I + R2[C]2 + 1/90 (Rl[Si]l + R2[Si]2) + 11100 (Rl[Mn]l + R2[M1112 + RI [Crl I + R2[Crl2) (6) where [C]I: average C content (mass%) of tile first steel sheet; [CI2: average C content (mass%) of the second steel sheet; [Sill: average Si content (mass%) of the first steel sheet; [Si]2: average Si content (mass%) of the second steel sheet; [Mnll: average Mn content (mass%) of the first steel sheet; [Mnlz: average Mn content (mass%) of the second steel sheet; [CrIl: average Cr content (mass%) of the first steel sheet; [Cr]2: average Cr content (mass%) of tlie second steel sheet; RI = tl/(tl+ t2); and R2 = t2l(tl + t2). [0060] When three steel sheets (the first, the second, and the third steel sheets) are 20 joined by spot welding at the lapped portion in which the three sheets are overlapped, and when the thicknesses of the three steel sheets are different, the average chemical composition of the three steel sheets is calculated as a weighted average among the average chemical composition of the first steel sheet, the average chemical composition of the second steel sheet, arid the average chemical composition of the 25 third steel sheet with respect to tlie tl~icknesses t3, t4 and ts. More specifically, the equivalent carbon content (Ceqplalc)tfotrh e first, the second, and the third steel sheets based on their average chemical compositions is given in Formula (7) below. Ceq,~,,, = R3[Cl3 + &LC14 + Rs[CIS + 1/90 (R3[Si13 + R4[Sil4 + Rs[Sil~) + 11100 (R3[Mn]3 + &[Mn]4 + Rs[Mn]s + R3[Cr]3 + R.I[Cr]4 + Rs[Cr]s) (7) 30 where [C]3: average C content (mass%) the first steel sheet; PCT Application No.: PCT/JP2015/067656 21/48 [C]4: average C content (mass%) of the second steel sheet; [CIS: average C content (mass%) of the third steel sheet; [Si]3: average Si content (mass%) of the first steel sheet; [%I4: average Si content (mass%) of the second steel sheet; [Sils: average Si content (mass%) of the third steel sheet; [MnI3: average Mn content (mass%) of the first steel sheet; [Mnl~a:v erage Mn content (mass%) of the second steel sheet; [Mnls: average Mn content (mass%) of the third steel sheet; [Cr]3: average Cr content (mass%) of the first steel sheet; 10 [Crld: average Cr content (mass%) of the second steel sheet; [Crls: average Cr content (mass%) of the third steel sheet; R ~ t d ( t 3+ t4 + 1s); %=t4/(t3 + 4 + tS); and Rs=ts/(t3 + 4 + ts). 15 [0061] Now, a chemical eonlposition of a high-tensile steel sheet, which is a dificult material for welding and which can be used for the present embodiment, will be described below by way of example. It is assumed that a chemical composition example below~is used to form a streel structure that mainly contains 20 bainite and mal-tensite as main phases, and also contains ferrite and residual austenite partially. The steel sheet containing such a structure exhibits a high strength and also a ductility to a level in which the base metal does not fracture in a collision event. The steel sheet according to the present ernbodinlent is not limited to the above-described structure or the steel sheet containing the ehetnieal composition 25 described below. In terms of the content of each element, " % means "mass% in the example below. [0062] (i) C: 0.06% to 0.35% 30 C has an effect of pronloting the generation of bainite or martensite as a main phase and also the gelleiation of residual austenite. C also has an effect of PCT Application No.: PCTlJP2015105765G 22/48 enhancing the tensile strength of a steel sheet by improving the strength of the martensite. In addition, C strengthens steel by way of solid solution hardening and provides an effect of improving the yield strength and tensile strength of the steel sheet. 5 100631 A C content of less than 0.06% may make it difficult to obtain the abovedescribed effects. Thus, the C content is set preferably at 0.06% or more, more preferably at more than 0.12%, and still more preferably at more than 0.14%. In contrast, if the C content exceeds 0.35%, the hardness of the martensite may become 10 excessively high, or the stability of the residual austenite may become excessively increased, to cause work-induced transformation to be less likely to occur. This may result in a substantial decrease in local ductility of the steel sheet. Moreover, weldability deteriorates considerably. Therefore, it is preferable to set the C content at an amount of 0.35% or less. 15 [0064] (ii) Mn: 1.0% to 3.5% Mn has an effect of promoting the generation of bainite or martensite as a main phase, and also the generation of residual austenite. Mn also strengthens steel by way of solid solution hardening and provides an effect of improving the yield 20 strength and tensile strength of a steel sheet. Moreover, Mn enhances bainite strength by way of solid solution hardening so as to provide an effect of improving local ductility of the steel sheet by improving the bainite hardness under high-strain loading conditions. [0065] 25 A Mn content of less than 1.0% may make it difficult to obtain the abovedescribed effects. Tlms, the Mn content is set preferably at 1.0% or more;more preferably at more than 1.5%, still more preferably at more than 1.8%, and especially preferably at more than 2.0%. In contrast, a Mn content of tnore than 3.5% excessively retards bainite transfornation, which fails to stabilize the residual 30 austenite so as to make it difficult to obtain a predetermined amount of residual austenite. Therefore, the Mti content is preferably set at 3.5% or less. PCT Al)plication No.: PCT/JP2015/057656 23/48 [0066] (iii) Si+Al: 0.20% to 3.0% Si and A1 have an effect of promoting the generation of residual austenite by reducing the generation of carbides in bainite to improve the uniform ductility and 5 local ductility of the steel sheet. Si aud A1 also strengthen steel by way of solid solutio~hi ardening and provides an effect of improving the yield strength and tensile strength of a steel sheet. Moreover, Si and A1 enhance bainite strength by way of solid solution hardening so as to provide an effect of improving local ductility of the steel sheet by improwring the bainite hardness under high-strain loading conditions. 10 [0067] A total content of Si and Al (hereinafter referred to as "(Si+AI) content") of less than 0.20% makes it dificult to obtain such effects. Consequently, the (Si+Al) content is set preferably at 0.20% or more, more preferably at 0.5% or more, and still more preferably at 0.8% or more. h~ contrast, a (Si+AI) content of more than 3.0% 15 will only result in saturating the above-described effects, which leads to cost disadvantage. This also results in rising the transformation temperature and thus in hampering productivity. Therefore, the (Si+Al) content is preferably set at 3.0% or less. [0068] 20 Si has an excellent solid solution hardening capability. Thus, the Si content is set preferably at 0.20% or more, and more preferably at 0.8% or more. In contrast, Si has an.effect of deteriorating the chemical conversion treatability and the weldability of the. steel sheet, and thus the Si content is set preferably at less than 1.9%, more preferably at less than 1.7%, and still more preferably at less than 1.5%. 25 [0069] (iv) P: 0.10% or less I" In ge11era1,~P is contained as an impurity. P is segregated ~ i t i grain boundaries to make steel brittle, thereby aggravating crack generation when an impact load is applied. A P content of more than 0.10% noticeably etnbrittles steel 30 due to the above-described effect, making it difficult to suppress the crack generation when an impact load is applied. The P content is preferably less than 0.020%, and PCT 11l)plication No.: PCT/JP2015/05765G 24/48 more preferably less than 0.015%. [0070] (v) S: 0.0 10% or less In general, S is contained as an inlpuritj: aud S has an effect of deteriorating 5 formability by forniing sulfide-base inclusions. The above-described effects becomes evident at a S content of more than 0.010%. The S content is preferably , 0.005% or less, more preferably less than 0.003%, and especially preferably 0.001% or less. [0071] 10 (vi) N: 0.010% or less In general, N is contained in steel as an impurity, and N has an effect of deteriorating the ductility of the steel sheet. The deterioration of the ductility becomes evident at an N content of more than 0.010%. The N content is preferably 0.0050% or less. 15 100721 As described above, the steel co~itains C: 0.06% to 0.35%, Mn: 1.0% to 3.5%, (Si+Al): 0.20% to 3.0%, P: 0.10% or less, S: 0.010% or less, N: 0.010% or less, and the balance: Fe and impurities. Here, the impurities means what comes to be mixed in from law materials such as ore and scrap, fiom manufacturing 20 environment, and so on, in the industrial steel manufacturing. The impurities are allowed as far as they do not negatively influence the steel of concern to the present embodiment. 100731 Elements that will be described hereafter are optional, and can be contained, 25 ~vhen necessary as a substitute of a portion of Fe, in the steel having the abovedescribed cbetnical conlposition. [0074] . .. (vii) One or more elements selected fsom the group consisting of Cr: 0.5% or less, Mo: 0.5% or less, and B: 0.01% or less. 30 Cr, Mo, and B have an effect of improving hardenabilitj~a tid promoting bainite generation. In addition, these eletnents have an effect of promoting the PCT Al)plicatioii No.: PCT/JP2015/057656 25/48 generation of martensite and residual austenite, and also liavc an effect of strengthening the steel by solid solution hardening to improve the yield strength and tensile strength of the steel sheet. Therefore, one or more elements selected from the group consisting of Cr, Ma, and B may be contained. To make sure of obtaining 5 the above-described effect, it is preferable to satisfy any one of Cr: 0.1% or more, Mo: 0.1% or more, and B: 0.0010% or more. [0075] Howevel; if the Cr content exceeds 0.5%, the Mo content exceeds 0.5%, or the B content exceeds 0.01%, noticeable deterioration in the uniform elongation and 10 local ductility of the steel sheet may be resulted. Thus, when one species or more of Cr, Mo, and B are contained, it is preferable to have each content within the above-described content range. [0076] pLication No.: PCT/JP2015/05765G 30148 tlie second ring-shaped member 32 first comes into contact with and presses the surface of the first steel sheet 11, which prevents the further tnoving of the second ring-shaped tnenlber 32. [0091] 5 Subsequently, at the first combined electrode 20, the first electmde body 21 continues moving toward the second steel sheet 12. At this time, the distance between the first ring-shaped member 22 and the retainer plate 25 becomes gradually smaller, which causes the first elastic body 23 (compression coil spring 23A) to defo'onn by compression. Simultaneously, at the second combined electrode 30, the 10 second electrode body 3 1 continues moving toward the first steel sheet 11. At this time, the distance between the second ring-shaped member 32 and the retainer plate 35 becomes gradually small, which causes the second elastic body 33 (compression coil spring 33A) to deform by compression. [0092] 15 Subsequently, as illustrated in FIG. SB, at the first combined electrode 20, the tip portion of the electrode tip 21a comes into contact with and presses the suiface of the second steel sheet 12, which prevents the further inoving of the first electrode body 21. Simultaneously, at the second combined electrode 30, the tip portion of the electrode tip 31a comes.into contact with and presses the surface of the 20 first steel sheet 11, which prevents the further tiloving of the second electrode body 31. [0093] By pressing the tip portions of the electrode tips 31a and 21a against the respective surfaces of the first and the second steel sheets 11 and 12 (or by flowing 25 an electric current between the first and the second electrode bodies 21 and 31 in this state), the indentations 15 (see FIGS. 4A to 4C) are fonned. %:A [0094] In FIG. 4A, the bottoni surfaces of tlie indentations 15, which are formed as depressed poitions, correspond to the portions that have come into contact with the 30 tip faces 21aa and 31aa of the first and the second electrode bodies 21 and 31 illustrated in FIGS. 5A and 5B. The iriclincd surfaces of the indentations 15 in FIG. PCT Application No.: PCT/JP2015/067G56 3 1/48 4A correspond to the inclined surfaces formed on the peripheries of the tip faces 21aa and 3 1 aa and inclined with respect to the tip faces 2 1 aa and 3 1 aa in the electrode tips 21a and 31a illustrated in FIGS. 5Aand 5B. [0095] 5 The portions corresponding to the inclined surfaces of the electrode tips 21a and 31a are not clearly exhibited in the indentations 15 illustrated in FIGS. 4B and 4C. [0096] The diameters d of the indentations, which have been described with 10 reference to FIGS. 4A to 4C, correspond to the diameters of the tip faces 21aa and 3 laa of the electrode tips 21 a and 3 1 a. [0097] As described above, the lapped portion of the steel sheet set is sandwiched by the first electrode body 21 and the second electrode body 31 that are facing each 15 other. At the same time, the lapped portion is also sandwiched by the first ringshaped member 22 and the second ring-shaped member 32 that are facing each other. At this time, the pressure from the first electrode body 21 and the second electrode body 31 is applied onto the lapped portion, and the pressure from the first ringshaped member 22 and the second ring-shaped member 32 is also applied onto the 20 lapped portion. [0098] Here, the repulsive force of the first elastic body 23 that is in compressive defornlation is acting on the first ring-shaped nlember 22, and the repulsive force of the second elastic body 33 that is also in compressive deformation is acting on the 25 second ring-shaped member 32. Thereby, the first and the second steel sheets 11 . and 12 are in close contactbwith each other over a wide area due to the fact that ~iot only the region in contact with the first electrode body 21 and the second electrode body 3 1 is pressed, but also the peripheral region therearound (the region in contact with the first ring-shaped member 22 and the second ring-shaped member 32) is also 30 pressed, Thereby, the generation of sheet separation (the first steel sheet 11 is separated from the second steel sheet 12 near the portion pressed by the electrode PCT Application No.: PCT/JP2015/057G56 32/48 tips 21a and 22a) is restrained. [0099] With this state being achieved, an electric cmrent is applied between the first electrode body 11 and the second electrode body 21 by operating the power 5 supply to weld the first steel sheet 11 and the second steel sheet 12. [O 1001 , FIGS. 6A and 6B are schematic diagrams for describing the situatioa in which a \veld nugget is formed by spot welding using the resistance spot welding apparatus illustrated in FIGS. 5A and 5B. FIG. 6A illustrates the case in which the 10 first ring-shaped member 22 and the second ring-shaped member 32 are not electrically conductive, while FIG. 6B illustrates the case in which the first ringshaped member 22 and the second ring-shaped member 32 are electrically conductive. In FIGS. 6A and 6B, dotted arrows represent the flow of welding current. 15 [OlOl] As illustrated in FIG. 6A, the contact region between the first and the second steel sheets 11 and 12 extends in a wide area, encompassing not only the contact region with the first electrode body 21 and the second electrode body 3 1, but also the surrounding contact .region with the first ring-shaped member 22 and the second 20 ring-shaped member 32. Thereby noticeable sheet separation does not occur. Thus, when an electric cu~~eins ta pplied between the first electrode body 21 and the second electrode body 31, the electric current flows it1 a wider area as compared to known spot welding. . . . [O 1021 25 Especially \~11e1l the first ring-shaped member 22 and the second ringshaped nlember 32 are electrically conductive, the flow of the electric currenb'spreads to an even wider area in the first and the second steel sheets 11 and 12 as illustrated in FIG. 6B. More specifically, the electric current not only spreads in a central area and flows from the first electrode body 21 to the second electrode body 31, but also . 30 is drawn from first electrode body 21 toward the first ring-shaped member 22, and then drawn toward the second ring-shaped nletllber 32, and finally towvard the second PCT Apl~lication No.: PCTIJP2015/057656 33/48 clectrode body 31. This is because both of the first ring-shaped nle~nber2 2 aud the second ring-shaped member 32 have a high electric conductivity while the first and the second steel sheets 11 and 12 are in close contact with each other in the region ~vl~etrhee first ring-shaped member 22 and the second ring-shaped member 32 are 5 facing each other due to a strong pressure exerted by the first ring-shaped member 22 and the second ring-shaped tnetnber 32. [0.103] In general, expulsion (expulsion at the faying sorface) is generated between metal sheets. However, a large amount of electric current applying to electrodes 10 may cause the contact portion between the metal sheet and the electrode to be overheated to generate expulsion (expulsion at the outside surface) from the outer surface of the metal sheet. The embodiment illustrated in FIG. 6B, in which the first and the second ring-shaped members 22 and 32 are made electrically conductive, provides another advantage that the electric current makes a detour from the first and 15 the second electrode bodies 21 and 31 to the first and the second ring-shaped nlembers 22 and 32 which are electrically-conductive, which can suppress the heat generation at the contact portion between the electrodes and the metal sheet, and thereby can suppress the expulsion generation from the outer surface of the metal sheet. 20 [0104] Thus, the first and the second steel sheets 11 and 12 are pressed firmly by the first ring-shaped member 22 and the second ring-shaped member 32 so that the contact region between the first and the second steel sheets 11 and 12 melts over a wide area to form the nugget 14 having a large nugget diameter. In addition, 25 according to this spot welding, the appropriate welding current range can be expanded with the nugget diameter being larger. .I [0 1051 As described above, the nlethod for manufacturing the welded structure according to the present etnbodiment provides effects of suppressing the sheet 30. separation, suppressing the expulsion generation, expanding the nugget diameter, and cxpanding the appropriate welding current range. These effects have been PCT Application No.: PCT/JP2015/05765G 34/48 described by using the exen~plaryc asc with the steel sheet set illcludillg the first and the second steel sheets. These effects are not limited to the case of the steel sheet set including two steel sheets, but can be produced in the case includillg thee steel sheets. Consequently, not only the welded st~ucture according to the first 5 enlbodiment but also the welded structure according to the second embodiment can be obtained by the method for lnanufacturing the.welded structure of the present embodiment. [0106] Note that, when the steel sheet set includes thee steel sheets, the exerted 10 pressure, the current value, and the welding current pattern will be properly adjusted as the need arises. Thereby, nuggets having a large diameter can be formed as is the case of the steel sheet set having two steel sheets, which will be described in Example later. [0107] 15 In order to suppress the sheet separation sufficiently, what is impostant are the distance between the peripheral edge of the tip face 21aa of the first electrode body 21 and the inner peripheral edge of the tip face 22a of the first ring-shaped member 22, and the distance between the peripheral edge of the tip face 3laa of the second. electrode body 31 and the inner peripheral edge of the tip face 32a of the 20 second ring-shaped me~ilber 32, wvhen the electrode tips 21a and 31a are in contact with the steel sheet set. These distances are preferably as small as possible as far as no interference occurs during welding. If these distances are too large, the suppressing effect of the sheet separation becomes small, and also the electric current does not easily spread in the case that the first ring-shaped member 22 and the 25 seco~td ring-shaped member 32 are electrically conductive. These distances are preferablj+.7 tnm or less, Inore preferably 5 nlln or less, and stilk Illore preferably 3 mm or less. [0108] One elnbodinlent of the present invention has been described so far, but the 30 present invention can also be i~ilplelnented in other modes. For example, the welded structure may be a housing of an electric appliance (for example, a copying PC1' Application No.: PCT/JP2015/057G5G 35/48 machine) that is manufactured by using a thin steel sheet. In this case, tlic welded structure according to tlie present invention, for esaniple, may sufficiently absorb an impact shock so as to protect the inside of tlie appliance when the appliance falls. 5 [Esaniples] [OI 091 The tests as described below were conducted to coilfir111 tlie effects of the present invention. One of the steel sheets A to C (Mat. A to Mat. C) in Table 2 was used in the 10 tests. Table 2 shows the chemical coniposition and the properties of the steel sheet including thickness, yield strength (YS), tensile strength (TS), and breaking elongation (El) for each of the steel sheets A to C. All of the steel sheets A to C are those generally used for the framework members of autotnobiles. As shown in Table 2, the equivalent carbon content (Ceq) of each of the steel sheets A to C 15 exceeds 0.15 mass%, which has been difficult to be welded without generating expulsion when known welding methods are used. 20 A*. J [Table 21 By using tlie resistance spot welding apparatus illustrated in FIGS. 5A and 5B and a known resistance spot welding apparatus, two steel sheets were spot welded to manufacture corresponding welded structures, and then cross tension tests were Steel sheet conducted on these welded structures. 25 [0112] Chemical composition (mass%) Properties (TS is measuredafter buenchinrr Retnark PCT Application No.: PCTIJP20151057G5G 36/48 As described above, the resistance spot welding apparatus illustrated in FIGS. 5A and 5B, wliicli had the first and the second combined electrodes 20 and 30 (hereinafter referred to as "rnovable electrodes"), was configured to sandwich the material with tlie first and tlie second electrode bodies 21 and 31 and with the first 5 and the second ring-shaped members 22 and 32, to apply an electric current. The known spot welding apparatus was the one that corresponded to the resistance spot welding apparatus illustrated in FIGS. 5A and 5B, but excluded the first and the second ring-shaped members 12 and 22 and the first and the second elastic bodies 23 and 33, and the electrodes corresponding to the first and the second electrode bodies 10 21 and 31 (hereinafter referred to as "normal electrodes") were configured to sandwich the material to apply an electric cul'rent. [0113] In either case, the steel sheets C (2.0 mm in thicktiess) in Table 2 were used as the two steel sheets. The average thickness per steel sheet t,,,, was 2.0 mn at the 15 lapped portion, and thus the thickness fell in a range from 1.1025 to 2.6 rnm. Spot welding was carried out with the electric currelit value being varied. The cross tension tests were condncled in accordance with the method stipulated in Japanese Industrial Standards (JIS) Z 3137. [0114] 20 FIG. 12 is a cha~acteristic diagram showing a relation between obtained nugget diameters d,,, (mm) and average thicknesses la,-,( mm) per material steel sheet .-when the welded structures are obtained by the method according to present embodiment. In the welded structure in which two-steel sheets are joined by spot welding at a plurality of locations, the average thickness on the transverse axis of FIG. 25 . 12-is t,, = (tl + 12) 12, where the thicknesses of the steel sheets at the lapped portion !;+.in \vliich two steel slieets are overlapped are represent6d by tl and tz. [01 151 According to the tnatiufacturing method of the present e~nbodiment, the relation between the nugget diameter d,, (mnl) and the average thickness t a , (mrn) 30 were obtained in the range indicated by hatching in FIG. 12. More specificallj: it was found that different characteristics in the relation among the nugget diameter d,, PCT Application No.: PCl'/JP2015/057656 37/48 (nnn), the average thickless t,,, (~rnn)a, nd the tip diameter d (111111) of the electrode used for spot welding could be obtained 011 the two sides of the boundary of the average thickness t,,., = 1.1 (mm). [0116] 5 More spccifically, when 0.5 mm i t,,, < 1.1 mm, the condition (a) below was obtained. (a) dng > d(tave)ln (1) [0117] In addition, when 1.1 mm i t,, 5 2.6 tntn, the condition (b) below was 10 obtained. (b) d,, > 1.05d (2) [0118] A thin joint in which the average thickness t,., per ~nateriasl teel sheet is less than 1.1 mm is susceptible to the generation of peel stress at a weld portion because 15 the flexural rigidity is low. To suppress the interfacial peeling caused by the peel stress at the weld portion, the nugget diameter d,, (mm) needs to be controlled to satisfy the condition (a). In contrast, when the average thickness t,,, per material steel sheet is 1.1 mtn or more, the flexural rigidity is high, and thus the interfacial peeling at a weld portiot~c an be suppressed by providing a nugget diameter d,,, (mm) 20 of more than 1.05d as indicated in the condition (b). Therefore, by satisfying the conditiot~ (a) or (b), the manufactured welded structure can provide desired deformation behavior .. . - [0119] The welded structures manufactured using the movable electrodes did not 25 - have spatters adhering thereto, but the welded structures manufactured using the ,.{ I . nornlal electrodes had spatters adhering theretb'. ..~FI G. 7 shows a relation between , the nugget diameter d,,, and the cross tension load P. In FIG. 7, the dotted line indicates that the nugget diameter equals to 1.05d (d is the tip diameter of the electrodes used in spot welding). In FIG. 7, the \velded structures according to the 30 present it~ventioat~re those using the tnovable electrodes that satisfy d,, > 1.05d. [O 1201 PCT Application No.: PCT/JP2015/057G56 38/48 Either case of using the movable electrodes or using the normal electrodes exhibits a tendency in which the cross teusion load P becomes larger as the nugget diameter d,,, becomes larger. II-Iowevel; the case of using the tiormal electrodes exhibits a large dispersion in tlie cross tension load P with the same nugget diameter d,,,, as conlpared to the case of using the ~novablee lectrodes, and has not necessarily provided a level of the cross tension load P equivalent to the case of using the movable electrodes even when d,, > 1.05d is satisfied (for welded structures having no spatter adhesion). Welded structures without spatter adhesion have stably exhibited high cross tension loads P as compared to those with spatter adhesion. In the case of using the movable electrodes, both the spot weld portions that satisfy d,,, > 1.05d and the spot weld portions that satisfy d,,, i 1.05d have been obtained. However, the welded structure that satisfies d,, > 1.05d can be stably obtained by properly setting welding conditions. (01211 When the nugget diameter d,, cannot be made larger than a certain size, there arises a limitation in suppressing fiacture at the weld portion. Especially as the tensile strength of the steel sheet becomes larger, the stress expected at the weld portion becomes larger. Thus, it is necessary to form strong welds by obtaining a large nugget diameter d,,. Moreover, as the steel sheet becomes thicker, the stress expected at the weld potlion becomes larger. Thus, it is also necessary to form strong welds by obtaining a large nugget diameter dng [O 1221 In contrast, as tlie tensile strength of the steel sheet becomes larger, the electric resistance becomes larger accordingly, causing expulsio~tlo occur due to heat generation during welding. The generation of expulsion causes the nugget diameter to be smaller and the steel sheet thickness to be smaller, and also makes it difficult to carry out stable welding. As a result, the welded steel sheets are vulnerable to peeling. Thus, it is desirable to obtain a larger nugget diameter d,,, while suppressing the expulsion generation. According to the present embodiment, the 30 steel sheets to be welded are in contact with each other firnlly and securely by pressing, against the steel sheet, tlie first ring-shaped member 22 included in the first PCT Al~plication No.: PCTIJP20151057G5G 39/48 combined electrode 20 and the second ring-shaped nlenlber 32 included in the second combined electrode 30. Thereby, the nugget diameter dng can be reliably made larger, which causes the strength of the weld portion to be substantially higher. In addition, when the first ring-shaped nlcnlber 22 and the second ring-shaped 5 member are made electrically conductive, tlie electric current flows in a wide area as illustrated in FIG. 6B.. This lowers the electric current density, which enables the generation of expulsion to be securely prevented. Consequently, even if the steel sheets having a tensile strength of 980 MPa or more are used, the nugget diameter d,,, that satisfies the condition (a) or (b) can be obtained, and the generation of expulsion 10 can be suppressed, according to the present embodiment. Thereby, the joining strength of the weld portion can be made considerably larger and tlie peeling at the weld poaion can be reliably prevented. In other words, when known manufacturing methods are used, it is difficult to obtain the nugget diameter d,, that satisfies the condition (a) or (b) especially for the steel sleet having a tensile strength of 980 MPa 15 or more. [0123] Moreover, if the movable electrodes according to the present embodiment are not used, there are not many solutions to enlarge the nugget diameter except for increasing tlie pressure exerted by the electrodes so as to flow a large amount of the 20 electric current. On the other hand, when the movable electrodes according to the present embodiment are used, the first ring-shaped member 22 and the second ringshaped member 32 are pressed against the steel sheet in the peripheries of the first electrode body 21 and the second electrode body 31, which enables tlie nugget diameter to be larger while suppressing sheet separation. Thus, while known 25 electrodes exerts a large pressure to cause the gap between the steel sheets to be wider, the rliovable electrodes according to the present embodiment enables the sheet 2'. . gap to be sufficiently small by suppressing the sheet separation. Consequently, in the deformation process in a collision event, the joint forrned of the \veld portion xvith the larger sheet gap by the knowvn method tends to enter a stress state of peeling 30 tlie weld portion, thns-causing the weld portion to be peeled.off. On the other hand, the joints having large diameter nuggets formed by tlie ~novablee lectrodes according PCll Al~plication No.: PCT/JP2015/057G56 40148 to the present etnbodiment have a sheet gap smaller than that fornied by the known clcctrodes, thereby securely suppressing the peeling off during deformation in a collision event. [0 1241 5 Crushing tests (what is called "dynamic defornlation impact .testsn) were conducted on the welded structures with the procedure as described below. The crushing tests were conducted in two different conditions, in other words, bending deformation conditions and axial crush deformation conditions, which are basic 10 testing modes related to actual automobile collisions. [0125] 1. Bending Deformation Hat-channel members and closing plate members formed of the steel sheets A and B were prepared. FIG. 8 illustrates the shapes and dimensions of the hat- 15 channel member and the closing plate member. As illustrated in FIG. 8, the hatchannel member, which was 120 tnm in width, 60 lntn in height, and 600 mtn in length, had 20 mm wide protruding portions (bent portions and flanges) at both sides in the widthwise direction. The flanges were overlapped and were spot welded with the closing plate member having the same length and width with the hat-channel . 20 member. [0 1261 Spot welding was conducted by using the movable electrodes, and also - using the normal electrodes. The intel-val between weld portions were set at 17.5 mm and 3 5 nun. 25 [0127] This welded structure was placed on two se~nicylindricajli gs (the curvature-' radius of the cylindrical portion: 30 mm) with the closing plate menlber facing upward. At this time, two jigs were positioned in parallel with each other with a 440 nnn spacing therebetween and with each cylindrical portion facing upward. 30 .[0128] Using a hydraulic compression tester, the se~nicylindrical impactor (the PCT Al~plication No.: PCTlJP20151057656 41/48 curvature radius of the cylindrical portion: 150 mm) was collided f~omab ove against the mid postion in the longitudinal direction of the welded structure in this state, at a velocity of 8.6 km/h with the cylindrical portion facing downward. [0129] 5 On this occasion, the displacement of the inlpactor was measured, and also the defornlation resistance following the bending defor~nation -was measured by using piezoelectric load cells built in the impactor. The test displacement, which is an amount of the impactor movement, was set at 60 nun. Based on the curve of the deformation resistance against the test displacement, the deformation resistance was 10 integrated with respect to the test displacement (from the point the impactor collided with the welded stn~cture to the point the impactor moved 45 t m ) to obtain an absorbed energy U when the welded stri~cturew as subjected to bending deformation. [0130] FIG. 9 shows a relation between the nugget diameter d,, and the absorbed 15 energy U. As shown in FIG. 9, in either case of using the steel sheet A (Mat. A) or of using the steel sheet B (Mat. B), the absorbed energy U generally becomes larger as the nugget diameter d,, becomes larger. When the steel sheet A is used and the nugget diameter dng exceeds 1.05d, the absorbed energy U stably exhibits high values. [0131] 20 When the steel sheet A is used, the difference in the interval between the weld portions does not cause the absorbed energy U to become largely different. In the case of using the steel sheet B, when the welded structures using the movable electrodes are compared with each other, the welded structures having an interval of 17.5 nnn tend to exhibit higher absorbed energies as compared to the nrelded 25 structures having an interval.of 35n1m. As shown in Table 2, the steel sheet B has an equivalent carbon co~ltent higher than that of the steel sheet A. When a\,.steel sheet ivith a high equiva1ent:carbon content (for exatnple, a steel sheet with Ceq > 0.3) is used, it is preferable to make the interval smaller to obtain a suficiently high absorbed energy. 30 [0132] 2. Axial Crush Deformation PCT Application No.: PCT/JP2015/057G5G 42/48 Hat-channel members aud closing plate members formed of the steel sheets C (having a thicluiess of 1.6 mm) were prepared. FIG. 10 illustrates the shapes and dimensions of the hat-channel member and the closing plate member. As illustrated in FIG. 10, the hat-channel member, which was 120 mnl or 126 in width, 60 mm iu 5 height, and 300 nlm in length, had 20 mm or 23 nnn wide protruding portions @ent portions and flanges) at both sides in the widtl~wise direction. The length of each bent portion in the widthwise direction of the hat-channel member is 5 nnn, and thus the width of the flauge is 15 nlm or 18 mm. The flanges were overlapped and were spot welded wit11 the closing plate member having the same letlgth and width with 10 the fiat-channel member. [0133] Spot welding was conducted by using the tnovable electrodes, and also using the normal electrodes. The interval between weld portions were set at 35 mnl. [0134] 15 The axial direction of the welded structure was aligned to a vertical direction, and then the falling weight having a mass of 850 kg was fallen freely from a height of 4.83 111 and was collided from above against the welded structure in the axial direction thereof at a velocity of 35 kmlh. On this occasion, the displacement of the falling weight was measured, and the deformation resistance of the welded 20 sttucture was also measured by using piezoelectric load cells. Based on the curve of the deformation resistance against the displacement of the falling weight, the deformation resistance was integrated with respect to the displacement of the falling weight (from the point the falling weight collided with the welded structure to the ' point the falling weight moved 200 mnl) to obtain an absorbed energy U when the 25 welded structure was subjected to axial crush deformation. [0135] I , _ *'- FLG. 11 sho\vs a relation between the nugget diameter d,, and the absolbed energy U. In the legend in FIG. 11, "If = 20 mm" represents the case where a flange width is 20 tnm, and "lf= 23 n1111' represents the case where a flange width is 23 mm. 30 .As illustrated in FIG. 11, the absorbed energy U generally becomes larger as the nugget diameter d,, becomes largel; and the absorbed energy U stably exhibits high PCT Al~plication No.: PCT/JP2015/067G56 43/48 values xvllen the ilugget diameter dng exceeds 1.05d. [0136] Incidentally, while the welding using the movable electrodes described above is used as a main joining method, laser welding or adhesion may be used 5 together as an auxiliaty joining method. In this case, an adhesive such as an electrically conductive weldbond, etc., can be used. [0137] It should be understood by those skilled in the art that various tnodificatiotls, conlbinations, sub-combinations and alterations may occur depending on design 10 requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof. Reference Signs List welded structure first steel sheet second steel sheet third steel sheet .spot weld portion nugget indentation PCT Application No.: PCTIJP20151057G6G 44/48 CLAIMS Claim 1 A welded structure including two or three steel sheets and a lapped portion in wllich the steel sheets are overlapped and joined by spot welding at a plurality of 5 locations, the welded structure connprising: a spot weld portion, and, when a diameter of a nugget is d,,, (mm), a tip diameter of an electrode used by the spot welding is d (mm), and an average thickness per steel sheet of the steel sheets at the lapped portion is t,., (mm), the spot weld portion satisfies a condition (a) or a condition (b) below in accordance with the 10 average thickness t,, (mnl). (a) d,, > d(t,,,)ln when 0.5 lmn 5 ta,-, < 1.1 nlm (1) (b) d,, > 1.05d when 1.1 lnnl 5 t,,, 5 2.6 mm (2) Claim 2 15 The welded structure according to claim 1, wherein the welded structure includes an expected deformation region to be subjected to plastic deformation wl~ehen a load is applied, and the spot welding is carried out at least within the expected deformation region. 20 Clairn 3 T11e welded structure according to claim 1 or 2, wherein the expected deformation region is free from spatter adhesion. Claim 4 25 The welded structure according to any one of clai~ns1 to 3, .:, \vherein the steel sheets ha\7e a tensile strength of 980 MPa or more , . Claim 5 The welded structure according to any one of claims 1 to 4, 30 \\herein the electrode is a combined electrode having an electrode body to be pressed against the steel sheets at the lapped portion and a ring-shaped lne~nbetro PCT Application No.: PCTlJP20151057G5G 45/48 be pressed against the steel sheets around the electrode body. Claitn 6 The welded structure according to claim 5, wherein the movable electrode includes: a first ring-shaped nlember and a second ring-shaped member to be pressed against the steel sheets at the lapped pol-tion with the first ring-shaped member and the second ring-shaped member facing each other; and a first electrode body and a second electrode body, each being 10 inserted in a though hole disposed each of the first ring-shaped member and the seco~ld ring-shaped member, to be pressed against the steel sheets at the lapped portion with the first electrode body and the second electrode body facing each other, and wherein an electric current flows through the steel sheets between the first 15 electrode body and the second electrode body. Claim 7 The welded structure according to any one of claitns 1 to 6, wherein the spot weld portion satisfying the condition (a) or the condition 20 (b) is present in a 20 to 60% extent on the welded structure. Claim 8 The welded structure according to any one of claims 1 to 7, wherein the spot weld portion has an equivalent carbon content (Ceq) of 25 0.13 mass% or more, the equivalent carbon content (Ceq) being defined by an equation (3) below: . . Ceq = [C] + 1/90 [Si] + 1/100 ([Mn] + [Cr])(3) wl~ere [C]: an average C content (mass%) of the spot weld portion; [Si]: an average Si content (mass%) of the spot weld portion; MI^]: an average Mn conte~l(tm ass%) of the spot weld portion; and PCT Application No.: PCT/JP2015/06765G 46/48 [Cr]: an average Cr content (mass%) of tllc spot \veld portion. Claim 9 The welded structure according to any one of clai~lls2 to 8, wherein the welded structure is a member to be used for an automobile, and the expected deformation region is to be subjected to an axial compression load.. Claim 10 The welded structure according to any one of claims 2 to 9, 10 wherein the welded structure is a member to be used for an automobile, and the expected deformation region is to be subjected to a bending load. Claim 11 A method for manufacturing a welded structure including two or three steel 15 sheets and a lapped portion in which tlie steel sheets are overlapped and joined by spot welding at a plurality of locations, the method comprising: carrying out spot welding, the carrying out spot welding including a first step in which a first rod-shaped electrode body and a second rod-shaped electrode body are arranged facing each other with the lapped portion 20 being sandwiched therebetween, and a first ring-shaped member and a second ringshaped member are arranged facing each other, the first ring-shaped member having . . a tlnough hole through ~vllicht he first electrode body is inserted and a back end to which a first elastic body is co~ulecteda nd the second ring-shaped member having a through hole through \vhich the second electrode body is inserted and a back end to 25 which a second elastic body is conllected, and .,. a second step in which the lapped portion is pressurized by pressing +. a tip face of each of the first electrode body and the second electrode body against the lapped portion, and by pressing a tip face of each of the first ring-shaped member and the second ring-shaped member against the lapped po~tionw hile the first elastic 30 body exerts a pressing.pressure on the first ring-shaped member and .the second elastic body exerts a pressing pressure on the second ring-shaped member, and then PCT Applicatio~i No.: PCT/JP2015/057656 47/48 . an electric current is applied between the first electrode body and the second .. . electrode body, -2 ' wherein the first step and the second step cause a spot weld portion to satisfy a condition (c) or a condition (d) below in accordance with an average 5 thickness t,,, (mm): (c) dng > d(tnve)'" when 0.5 mrn 5 t,,, < 1.1 - mm (4); (d)dn,>1.O5dwhen 1.1 mm_