Technical field [0001] 　The present invention relates to a method and a welding apparatus for performing it to arc spot welding the overlapping portion of the thin steel sheet made of a high strength steel. Background technique [0002] 　Recently, in the automotive field, for weight reduction and crashworthiness improvement of the vehicle body, and the use of high-strength steel sheet is increased, more and more it increases the strength of the steel sheet used. In the process such as the mounting of the vehicle body assembly and parts made of such high strength steels are mainly spot welding is used by resistance welding. [0003] 　Bonding strength of the spot weld is evaluated tensile shear strength measured by a load tensile shear direction (TSS), the cross tensile strength is measured by a load tensile peeling direction (CTS) . The TSS and CTS are known to change in the relationship shown in FIG. 16 with increasing tensile strength of the steel sheet. That, TSS whereas improved with increasing tensile strength of the steel sheet, CTS has a tensile strength of the steel sheet is saturated at about 590 MPa, the tensile strength of the steel sheet is reduced in the reverse as the more It has become a characteristic. [0004] 　If the tensile strength was spot-welded high-strength steel sheets exceeding 590 MPa, and with the improvement of the hardenability of the base material steel plate, lowering of nuggets in toughness and ductility occurs. Further, when loaded with peeling load such as CTS spot welding, the steel sheet is stress tends to concentrate on the more weld metal (nugget) end becomes high strength. Furthermore, if the particular case is pressure higher or scattering occurs, steel thickness of the spot welded portion, thinner than the surrounding, resulting in further increase in stress level for the load application. These are due to the spot welding, when the strength of the steel sheet is high, CTS of the weld will be lowered. [0005] 　In contrast, in the arc spot welding, and welding by penetrating the steel sheet, it is possible to form a weld reinforcement portion in the weld metal, it is possible to obtain a high joint strength than spot welding. [0006] 　Patent Document 1, melted to the back side of the high-strength steel sheet which is superimposed, and a bead diameter of the formed weld bead so as to protrude from the surface of the steel sheet within a proper range in relation to the steel sheet thickness Prefecture by controlling, obtained both high CTS and TSS, arc spot weld joint welded joint having excellent joint strength can be achieved is disclosed. [0007] 　However, the weld bead shape, the steel components and steel plate thickness and the like, changes, further, the rear surface of the weld bead, since the sag under the influence of gravity, it is difficult to control the stable weld bead shape. [0008] 　As means for suppressing the sag of the back bead, Patent Document 2, a method of using the backing strip is disclosed. In Patent Document 2, in order to prevent welding of the backing strip and the weld metal, a concave portion is provided on the surface portion of the backing strip, a method of suppressing the occurrence of arc discharge to the backing strip it has been proposed. [0009] 　FIG. 1 of Patent Document 2, the weld bead having the same size as the recess portion of the backing metal is described to be formed on the back side of the superimposed steel plates. However, the present inventors, using a backing strip disclosed in Patent Document 2, an attempt was made to control the rear surface bead shape during arc spot welding, Ya weld bead diameter as disclosed in Patent Document 1 it was not possible to obtain the extra prime height. [0010] 　FIG. 2 of Patent Document 3, the atmosphere forming during arc welding, the discharge of the resin gas generated during the butt welding two vibration damping steel plate, near the weld edges of the two sheets of vibration damping steel plate the way can be cooled, said apparatus gas supply groove is recessed with a width of weld near the back of the two damping can be cooled is disclosed. [0011] 　Further, in the like Figure 3 of Patent Document 4, placed on the pedestal overlapping the two steel plates, arc spot welding devices have been disclosed. [0012] 　However, none Patent Documents 3 and 4 do not disclose a specific method to control the shape of the weld bead servings height, etc. of the weld bead. Therefore, the above-described apparatus, or has a function of forming a bead shape necessary to increase the strength of the weld is not obvious. CITATION Patent Document [0013] Patent Document 1: Laid-Open Patent Publication No. 2013-010139 Patent Document 2: Laid-Open Publication No. 06-039542 Patent Document 3: Japanese Patent Publication No. 5-104250 Patent Document 4: Japanese Patent Publication No. 2-59173 Summary of the Invention Problems that the Invention is to Solve [0014] 　The present invention has such circumstances in view, in the back surface weld bead of the arc spot welding, it is possible to obtain a predetermined welding bead diameter or excess prime height, welded joint having excellent strength in both the TSS and CTS arc spot welding method can be obtained and an object thereof is to provide a welding apparatus for performing it. Means for Solving the Problems [0015] 　The present inventors have found that, when the welding by installing a backing metal in the steel plate back surface, for heat removal due to the backing metal, to obtain a knowledge that the growth of the melted region is inhibited. Therefore, in order to expand the backside weld bead of the steel sheet, the result of studying a method of suppressing heat transfer to the backing money from the steel plate back surface of the melt region near to form a clearance between the weld portion and the backing metal as it has been found to limit the contact position of the backing metal and the steel sheet back surface in the melting zone near. [0016] 　In addition to the gravity due to the mass of the molten metal itself, the pressure caused by the pressure spraying by arc plasma stream during arc welding, i.e., the arc pressure, influence the degree of sag of the back bead. Accordingly, the present inventors have found that by suppressing the ratio of the arcing time during arc welding in a certain range, can reduce the average arc pressure during the arc welding, fall out of the arc spot welding initial weld metal It found that can prevent, and have completed the present invention. [0017] 　The gist of the present invention is as follows. (1) carbon was disposed to overlap a plurality of steel plates containing at least 0.2 wt%, there by further welding torch opposite the overlapping contact arranged backing metal to the lower steel plate welding arc spot welding method Te, wherein the backing metal is a non-contact portion that is not in contact with the lower steel plate so as to face the lower steel sheet, and a contact portion in contact with the lower steel sheet, a circle of the contact portion is the back surface weld bead as the backing metal is placed in contact with the lower steel plate at a location more than 3t from the corresponding central, short energization time the welding voltage is 10V or less, less than 30% than 60% per energized one period, arc spot welding method characterized by alternating arcing and short circuits energized continuously. 　However, t is the thickness of the top plate thickness is thin steel plate among the plurality of steel plates. (2) Average welding voltage (unit: V) at the arc generation and the arc spot welding time repeating short energizing alternately continuously / average welding current (unit: A) the value of, 0.07 ~ 0.10 (V arc spot welding method according to (1) to be a / a). (3) a period as the 5 msec ~ 20 msec, arc spot welding method as claimed in and performing arcing and short circuit current during that one period (1) or (2). (4) the contact part, and maintains the clearance between the back surface and the non-contact portion of the lower steel sheet 0.2t ~ 3t (1) ~ according to any one of (3) arc spot welding method. (5) having a weld bead diameter of circle equivalent of 3t ~ 10t, and, and forming a back surface weld bead having a weld reinforcement height of 0.2t ~ 3t (1) ~ (4) of arc spot welding method as claimed in any one. (6) contact the back surface of the lower steel plate and the contact portion, punctiform contacts, linear contact, and characterized in that at least one of the planar contact (1) of the - (5) arc spot welding method as claimed in any one. (7) When the weld bead diameter of equivalent circles of the back weld bead to form a back surface weld bead 5t greater than the backing metal was formed of copper or a copper alloy, the back surface of the lower steel plate and the contact portion arc spot welding method according to any one of to, characterized in that the contact between the surface contact (1) to (6). (8) When the weld bead diameter of equivalent circles of the back weld bead to form a back surface weld bead 5t greater than the backing metal was formed of steel, contact of the back surface of the lower steel plate and the contact portion arc spot welding method according to any one of to, characterized in that the planar contact (1) to (6). (9) the contact portion and the contact area of the back surface of the lower steel plate 10 cm 2 arc spot welding method as claimed in any one of characterized by the following (1) to (8). (10) (1) to a arc spot welding apparatus for performing the arc spot welding method as claimed in any one of (9), a non-contact portion which is spaced apart from the rear surface of the lower steel sheet, said in contact with the rear surface of the lower steel plate 3t greater positions from the equivalent circle center of the backside weld bead, and the contact portion to maintain the clearance 0.2t ~ 3t between the back and the non-contact portion of the lower steel plate arc spot welding apparatus characterized by having a backing metal having a. (11) the arc spot welding apparatus, arc spot welding apparatus according to (10), characterized in that a welding robot provided with the backing metal and the welding torch. Effect of the Invention [0018] 　According to the present invention, even in the welding of high-strength steel sheets of the various components, without toughness and ductility of the welded portion is reduced, since the arc spot welded joint having excellent both TSS and CTS is obtained, automobile parts the by applying the present invention in the manufacture and the vehicle body assembly such steps, it is possible to produce a superior member stronger, can contribute to safety improvement of automobile. BRIEF DESCRIPTION OF THE DRAWINGS [0019] [1] is a diagram showing an example of the arc spot welding method by gas metal arc welding. Is a diagram illustrating an example of the arc spot welding method that does not use the Figure 2 the backing metal. Is a diagram illustrating an example of the arc spot welding method using the FIG. 3 dents without the backing metal. Is a diagram illustrating an example of the arc spot welding method using backing metal with [4] dent. Is a diagram illustrating an example of the arc spot welding method using backing metal of the enlarged diameter of the FIG. 5 dents. 6 is a diagram showing an example of the presence or absence of the through hole of the welds of the upper steel plate. (A) is a through-hole is a diagram illustrating a no (b) through-holes. [7] the shape as viewed from the vertical cross-sectional diagram showing an example of a backing metal with rectangular indentations. (a) is a plan view and a side cross-sectional view of the backing metal, (b) is a diagram showing a cross-sectional view after arc spot welding using backing metal of (a). 8 is a diagram showing an example of a backing metal with hemispherical depressions. (a) is a plan view and a side cross-sectional view of the backing metal, (b) is a diagram showing a cross-sectional view after arc spot welding using backing metal of (a). 9 is a diagram showing an example of a backing metal. (a) is a plan view and a side view of a backing metal having a contact portion which contacts the steel plate back surface and the point-like, (b) is a plan view and a vertical sectional view of a backing metal having a contact portion which contacts the steel plate back surface and the linear is a diagram illustrating a. Is a diagram illustrating an example of a backing metal having a [10] contact portion which contacts the steel plate back surface and the planar. (a) is a plan view and a vertical sectional view of a backing metal having a contact portion of the rectangular contact surface, (b) is a plan view and a vertical sectional view of a backing metal having a contact portion of the rectangular contact surface having a recess is there. It is a diagram illustrating an example of FIG. 11 arc spot welding apparatus. Is a diagram illustrating an example of the installation angle adjusting means [12] the backing metal. 13 is a diagram showing an example of the presser means of the upper steel plate. 14 is a diagram showing a test piece for the cross tensile test. Is a diagram showing FIG. 15 test pieces for tensile shear test. Is a graph showing a relationship [16] Tensile strength is relative to the tensile shear strength of the steel sheet (TSS) and cross tensile strength (CTS). [17] the backside weld bead is a schematic sectional view showing a state that does not fully fused to the steel plate back surface. [FIG. 18] (a) by standard arc welding process is a schematic sectional view showing a state in which molten metal contacts the surface of the backing strip to melt down the steel plate back surface. (B) is by an arc welding method of the present invention, is a schematic sectional view showing a state where the burn through the molten metal of the arc spot welding initial is prevented. [19] in arc welding using a CMT power supply is a graph showing the relationship between the voltage and current waveforms and arcing times and short energizing time. FIG. 20 is a graph showing the relationship between the welding time of the short-circuit time constant and fusion failure. An example of the arc spot welding method of fixing the backing metal in [21] in the steel is an enlarged sectional view of the welded parts. DESCRIPTION OF THE INVENTION [0020] 　Figure 1 is a diagram showing an example of the arc spot welding method by gas metal arc welding. Thus, arc spot welding method, the steel plate 1 to be welded are arranged two-ply and, perpendicular to the plate surface, or the welding wire 3 of gas metal arc welding torch 2 which is arranged substantially vertically thereby generating an arc from while feeding a welding wire 3, a method of welding two steel plates to each other. To ensure proper weld strength, a predetermined weld bead diameter W, it is necessary to form the extra prime height h. [0021] 　The present invention, in such arc spot welding method, the steel plate of the welding torch side and upper steel plate, when the welding torch side surface of the surface of the upper steel plate by arranging a backing metal on the back of the lower steel plate in welding from the surface, and a requirement to limit the contact position between the back surface of the lower steel plate and the contact portion of the backing metal. 　In addition to the requirements, the present invention is, by suppressing the ratio of the arc generation time and short energizing time during arc welding in a certain range, to reduce the average arc pressure during the arc welding It is a requirement. 　The present invention, by the two requirements are those which make it possible to be able to obtain a predetermined back surface weld bead size and extra prime height, to obtain a welded joint having excellent strength in both the TSS and CTS . [0022] 　Hereinafter, the arc spot welding method of the present invention will be described in detail with reference to the drawings. [0023] (Backing metal contact portion and conditions of the contact position between the back surface of the lower steel sheet) 　The present inventors, directed to steel sheet having a thickness of 2mm with the addition of 0.3% C which weld metal are easy to burn through in, was examined for the formation phenomenon of the back weld bead in the case of performing arc spot welding. Here, the weld bead diameter to a target was 10 mm (5t), excess prime height 1mm (t / 2). [0024] 　First, without using a backing metal was investigated the shape of the back surface weld bead in the case of performing arc spot welding. Figure 2 is a diagram showing an example of the arc spot welding method that does not use the backing metal. Thus, without using a backing metal, when performing arc spot welding, when both of the steel sheet to try to sufficiently melt, backside weld bead sag increase, no extra prime height of the target is obtained It was. [0025] 　Next, using the backing metal of various shapes were examined the shape of the back surface weld bead in the case of performing arc spot welding. [0026] 　Figure 3 is a diagram showing an example of the arc spot welding method using no dents backing metal. In this way, the backing metal 4 having no recess in close contact to the lower steel plate backside arc spot welding, it is impossible to backside weld bead sag, excess prime height of the target can not be obtained. The diameter of the back weld bead under the influence of heat removal by the backing metal is reduced. [0027] 　Figure 4 is a diagram showing an example of the arc spot welding method using backing metal with a recess. 4, in order to achieve an appropriate backside weld bead shape was 10mm diameter recessed portion of the backing metal 4, the depth 1 mm. If such a backing metal 4 dents with and in close contact with the lower steel plate backside arc spot welding, excess prime height increased to about 1 mm, excess prime height of the target is obtained. However, the backside weld bead diameter under the influence of heat removal backing metal remained as small as about 4 mm. [0028] 　Figure 5 is a diagram showing an example of the arc spot welding method using backing metal of the enlarged diameter recess. Figure 5 is to reduce the contact area between the backing metal 4 and the lower steel plate rear surface, while suppressing the heat removal backing metal 4, in order to enlarge the diameter of the back weld bead, the backing metal recess of by enlarging the diameter to 15mm. If such a backing metal 4 dents a is adhered to the lower steel plate backside arc spot welding, backside weld bead diameter of about 10mm, excess prime height was obtained weld shape was 1mm about the target . [0029] 　Thus, the back surface weld bead shape to the target is obtained, rather than by providing the recess in the backing metal, and than by enlarging the diameter of the recess, the backing metal in the melting area near This is because the heat removal is suppressed. Then, in order to suppress the heat removal backing metal, it is conceivable to form the backing metal and the lower steel plate rear surface does not contact. However, the support of the steel plate, and, because of the positioning of the backing metal, it is necessary to contact the backing metal and the lower steel plate rear surface. Therefore, at a position apart from the rear the weld bead, is brought into contact with the contact portion of the backing metal and the lower steel plate back surface, so as to suppress the heat loss of backing metal, it is effective to place the backing metal. [0030] 　Or more, and summarize the study results, are as follows. 　The contact portion of the backing metal is, outer position than the backside weld bead to the target, i.e., the equivalent circle center of the back surface weld bead to target, in which a circle those phases diameter (W) of about or more locations, the steel plate back surface into contact with, by arranging the backing metal, it is possible to obtain a backside weld bead shape target. Incidentally, the thickness of the arc spot diameter of the back weld bead to the target in the welding method thickness t (mm) (the thinner the front steel plate or the back steel sheet if the thickness of the two or more steel plates are different steel it is in the range of 3 ~ 10t against and t.). Therefore, in order to realize a weld bead diameter of 3t which is the lower limit, it is necessary to contact portions of the backing metal is set greater than at least 3t position from the equivalent circle center of the backside weld bead. [0031] (Arcing condition) 　as described above, by enlarging the diameter of the recess provided in the backing metal, heat removal backing metal in the molten region vicinity is suppressed, it is possible to enlarge the backside weld bead shape to become. However, depending on the welding conditions to be set, as shown in FIG. 17, incomplete fusion 30 backside weld bead boundary surface is not sufficiently fused to the steel plate back surface is formed it occurs. In particular, if the clearance between the rear surface and the backing metal of the lower steel sheet overlaid exceeds the thickness t of any steel sheet, when the arc spot welding in a standard DC arc welding method and Parusumagu welding method, the fusion failure is likely to occur, such as. [0032] 　18 (a) is in the initial welding with standard arc welding process is a sectional view schematically showing a state of the welded parts. As shown in FIG. 18 (a), in a standard arc welding process, at the beginning of arc spot welding, falling melt molten metal to the steel plate back surface in contact with the surface of the gold backing (code 40), the molten metal It will be cooled to the backing gold. Thus, even if the expanded diameter of the back weld bead can not be melted integrated backside weld bead and the steel plate back surface, not a satisfactory improvement in the bonding strength can not be obtained. [0033] 　Burn through the molten metal, in addition to the gravity of the molten metal itself, it affects the arc pressure described above. Therefore, if less the ratio of the arcing time in the arc welding process, is reduced average arc pressure, can prevent burn through arc spot welding initial molten metal as shown in FIG. 18 (b) next, it was considered to be a larger backside weld bead size while melting the steel sheet back surface. If in FIG. 18 (b), the portion of the molten metal contacts the surface of the gold backing (reference numeral 50), so is minimized, it is possible to prevent cooling of the molten metal. [0034] 　In order to reduce the ratio of arcing time in the arc welding process is actively welding wire and contacting the base material, it is effective to increase the short-circuit current time. As a means, cold metal transfer that can control the arcing time and the short-circuit current time by the number 10Hz cycle (Cold Metal Transfer) power (hereinafter, simply referred to as "CMT power".) Were studied using. The CMT power source, as shown in FIG. 19, advance feeding of the welding wire, since performed while controlling arc welding and retraction, by setting the welding conditions, continuously alternating arcing and short circuit current performed, it is possible to control the ratio of the arcing time and short energizing time. Incidentally, the short energization time refers to a time in which the welding voltage is 10V or less. [0035] 　The present inventors have found that using said CMT power, thickness 1.0 mm, subjected to various welding against 1.6mm and 2.3mm steel sheet, the relationship between the fusion failure and short circuit time constant during welding investigated. The results are shown in Figure 20. In Figure 20, "○" is the molten state can be confirmed melt integration of the backside weld bead and the steel plate back surface corresponding to a good weld, "×" is confirmed interface between the back weld bead and the steel sheet back surface melt-state corresponds to the welding defect. Incidentally, to Figure 20, the evaluation of the welding in the "short time constant = 0 (%)" is performed using Parusumagu welding, 0 (%)