TITLE OF THE INVENTION METHOD FOR FILLET ARC WELDING OF HIGH-STRENGTH THIN STEEL SHEET TECHNICAL FIELD 1 This invention relates to a method for fillet arc welding of high-strength thin steel sheet and more specifically relates to a method of fillet arc welding high-strength thin steel sheet capable of improving the toe shape of a fillet arc welded joint produced by gas-shielded arc welding and improving the fatigue properties of the fillet arc welded joint. 2 Structures to which the present invention can be preferably applied include automobile body structural members, particularly suspension parts and other parts critical to safety. BACKGROUND ART 3 In the automotive industry, for example, the gas-shielded arc welding is ordinarily set to a higher speed than in other industrial sectors in order to boost production line efficiency. The welding speed is generally set at 60 cm/min or higher and it is not uncommon for it to be set at 100 cm/min or higher. 4 Such high-speed arc welding is possible because the thickness of the steel sheets used in the automotive industry is usually 6 mm or less, and, for example, even in the case of suspension parts, whose thickness is relatively great, is usually 4 mm or less. In other words, this thin sheet thickness makes it possible to ensure at least the specified joint strength even if the amount of metal deposited by the arc welding is small. Moreover, when the sheet thickness is large, i.e., greater than 6 mm, for instance, if arc welding capable of depositing the amount of metal needed to attain the - 2 - specified joint strength is to be performed at a welding speed of 60 cm/min or greater, the welding current and welding voltage must be made proportionally higher, while the risk of an adverse effect on weld bead shape also increases. Thus arc welding in the automotive industry can be said to be characterized by higher welding speed than in other industries. 5 However, under such high-speed arc welding condition, the weld bead shape, especially the shape of the weld toe, deteriorates. Namely, the weld toe flank angle (see FIG. 2) enlarges, so that stress concentrates at the weld toe and degrades the fatigue strength of the welded joint. High welding speed degrades the weld bead shape because the weld pool grows increasingly slender in proportion as the welding speed is higher, so that molten metal tends to solidify before spreading out sufficiently. On the other hand, the increasing concern about the global environment has in recent years made the reduction of CO2 emission through improved fuel economy an urgent issue also in the automotive industrial sector. Reduction of vehicle weight itself is an effective way to improve fuel economy, and reducing the thickness of the steel sheet used in the vehicle is an effective means toward this end. However, reduction of steel sheet thickness equates to increasing the stress borne by the steel sheet, and stress increase poses not only a problem regarding merely static strength but also a problem regarding fatigue strength. In other words, even if static strength is sufficient, an issue arises in that sheet thickness reduction, i.e., weight reduction, cannot be pursued owing to fatigue strength considerations. 6 Welded joint fatigue strength is said generally to be almost totally independent of the material and to be governed by mechanical factors such as stress concentration, attributable to weld bead shape, and weld residual stress. Further, owing to the aforesaid tradeoff between production efficiency improvement and fatigue - 3 - strength assurance, the practice in high speed welding has been to adopt as the means for weld toe shape improvement and the means for welded joint fatigue strength improvement a method such as that of smoothing the weld toe by grinder finishing or the like or that of imparting compressive residual stress to the weld toe by shot-peening or the like. These are known as post-weld processes that undesirably increase production costs. 7 As another way to overcome the problem of welded joint fatigue, it has been proposed to design the composition of the weld material to have a low transformation temperature and thus improve fatigue strength by lowering the residual stress of the weld toe (see Patent Documents 1 and 2; such a weld material is hereinafter called a "high-fatigue-strength weld material"). Although this method regulates the weld material composition, its aspect of reducing residual stress makes it a dynamical factor control method, and as such is a highly effective method capable of achieving a high-fatigue-strength joint merely by varying the weld material. 8 Further, as taught by Patent Documents 3 and 4 and Non-patent Document 1, technologies are available for establishing a broad bead shape by restricting the compositions of the weld material and the steel sheet. For example, the technologies of Patent Document 3 and Non-patent Document 1 add S to a content of greater than 0.01% and not greater than 0.06%, thereby improving weld toe shape by lowering the surface tension of the molten pool. In addition. Patent Document 4 teaches a technique for regulating the total of the Si and Mn contents of the steel sheet. 9 Patent Document 10 teaches a technology regarding a welding method focused on fatigue properties in the gas-shielded arc fillet welding of overlapping thin steel sheets. This method defines the chemical composition of the weld metal so as to improve the bead toe shape. - 4 - Patent Documents 5 to 9 teach additional technologies related to steel sheets excellent in fatigue strength. PRIOR ART REFERENCES 0010 Patent Documents Patent Document 1 Unexamined Patent Publication (Kokai) No. 11-138290 Patent Document 2 Unexamined Patent Publication (Kokai) No. 2004-001075 Patent Document 3 Unexamined Patent Publication (Kokai) No. 2002-361480 Patent Document 4 Unexamined Patent Publication (Kokai) No. 2007-177279 Patent Document 5 Unexamined Patent Publication (Kokai) No. 2004-143518 Patent Document 6 Unexamined Patent Publication (Kokai) No. 2000-248330 Patent Document 7 Unexamined Patent Publication (Kokai) No. 11-189842 Patent Document 8 Unexamined Patent Publication (Kokai) No. 07-316649 Patent Document 9 Unexamined Patent Publication (Kokai) No. 2003-003240 Patent Document 10 Unexamined Patent Publication (Kokai) No. 2002-45963 0011 Non-patent Document Non-patent Document 1 Proceedings of the National Conference of the Japan Welding Society, 2007, Vol. 81, pp 236 - 237 SUMMARY OF THE INVENTION Problem to be Solved by the Invention 0012 Notwithstanding, even in the case of using the high- fatigue-strength weld materials set out in Patent Documents 1 and 2, degradation of the welded joint toe shape is still undesirable from the aspect of fatigue - 5 - strength. This is because, between the two major factors governing joint fatigue strength, namely residual stress and stress concentration, the technological focus of these high-fatigue-strength weld materials is on residual stress and no attention is directed to improving stress concentration. Of particular note is that, as pointed out earlier, welding is conducted at higher welding speed in the automotive and similar industries than in other sectors, and a strong need is felt for welding at higher speeds. The more welding speed is increased in response to this need, the more irregular the bead shape becomes, and the resulting increase in stress concentration is undesirable from the standpoint of fatigue strength improvement. Thus, in the automotive industry, for instance, the improvement of welded joint fatigue properties achievable by utilizing the high-fatigue-strength weld materials set out in Patent Documents 1 and 2 has been limited. 0013 Moreover, the techniques set out in Patent Documents 3 and 4 and Non-patent Document 1 are all aimed at achieving greater weld bead width than in the prior art. Weld bead width can indeed be a convenient index representative of overall welded joint shape, but fatigue strength depends greatly on the weld toe, which is the region of stress concentration. In other words, the shape of one part of the welded joint determines the properties of the whole welded joint, which is a unique fatigue strength tendency not observed in static strength. Therefore, in order to improve fatigue strength, less attention should be placed on weld bead width, a property of the whole welded joint, and more attention be focused on weld toe shape, namely the shape of only one part of the welded joint. The technologies taught by Patent Documents 3 and 4 and Non-patent Document 1 are applied to improve static strength, namely welded joint tensile fracture strength, and it is not clear whether they are effective technologies for fatigue strength enhancement. - 6 - 14 The prior art technologies set out in Patent Documents 5 to 8 all relate to base material fatigue strength. Since a steel material does not have a stress concentration region, the fatigue strength of the material is considered to be proportional to its static strength, so that these technologies are not necessarily effective for improving the fatigue strength of a welded joint. 15 Patent Document 9 teaches a technology related to the fatigue strength of a heat-affected zone (HAZ), but the welded joint dealt with is a butt welded joint and the stress concentration in this case is not as high as in a fillet arc welded joint. In fact, almost all vehicle suspension parts and the like are produced using fillet arc welding. In view of this, it is not clear whether the technique set out in Patent Document 9 can improve the fatigue strength of the structures having fillet arc welded joints with high stress concentration that are used abundantly in the automotive and other industrial sectors. 16 Moreover, the technologies taught by the prior art set out in Patent Documents 5 to 9 relate to the fatigue strength of base materials without welded joints or to the fatigue strength of butt welded joints with relatively small stress concentration. In an actual structure, fatigue cracking starts from the site with the greatest stress concentration, and this determines the fatigue strength of the whole structure. In other words, no improvement of structural fatigue properties is possible without improving the fatigue strength of the fillet lap joint, whose stress concentration is greater than that of a butt welded joint. Although the object of Patent Document 10 is to improve the fatigue strength of a thin steel sheet fillet lap joint, it is directed to embodiments with welding speeds of 80 - 110 cm/min (conventionally considered high-speed welding) and is not compatible with the still higher speed welding required - 7 - today. Patent Document 10 provides a technology that makes the joint fatigue strength at least 12% of the steel sheet fatigue strength. A steel sheet per se is flat and has no stress concentration region, so that for these and other reasons, the fatigue strength of a steel sheet is generally considered to be proportional to its tensile strength, with steel sheet tensile strength being around 60% and no more than 70% at the highest. Therefore, the teaching of Patent Document 10 amounts to a technique that in the case of a steel sheet tensile strength of 780 MPa makes the fatigue strength of the welded joint 780 x 0.7 x 0.12 = 66 MPa or greater. However, a fatigue strength on the order of 66 MPa or greater is within the range of what can be achieved just by suitably selecting the welding conditions, particularly the welding speed, or other such method. 0017 Against this backdrop, an urgent need has emerged for the development of high-strength thin steel sheet fillet lap joints that enable accelerated welding speeds while also ensuring the fatigue strength of increasingly higher strength thin steel sheets. To be specific, a steel sheet fillet arc welding method is desired that is not only capable of ensuring adequate fatigue strength at the current high-velocity welding speeds of around 100 cm/min (80 cm/min - 110 cm/min) but also capable of achieving fillet arc welded joints of high-strength thin steel sheet with excellent toe shape and ensuring high fatigue strength even at welding speeds exceeding the conventional high speed of 110 cm/min. As the fatigue strength required by a thin steel sheet welded joint is about 250 MPa, possession of a fatigue strength of at least 250 MPa is defined as a pass/fail guideline in the present invention. The reason for defining 250 MPa as the target in the present invention will be explained. First, an across-the-board value unrelated to the steel material strength was chosen because the fatigue strength of a welded joint does not depend on the type of steel - 8 - material, i.e., consideration was given to the distinctive feature of the joint fatigue strength being the same for both a 490 MPa class steel material and a 780 MPa class steel material. The value takes into account the characteristic that while the fatigue strength of a steel material depends on the tensile strength of the material, that in the case of a welded joint does not depend on the steel material. Second, the fatigue strength of a welded joint as welded, i.e., in the state with no fatigue property improvement measure being taken, is approximately 200 MPa. Realization of a fatigue strength of 250 MPa would correspond to more than a 20% strength increase and be advantageous in terms of fatigue design. This is a value that might in some cases make it possible to change the sheet thickness. So 250 MPa is used as a guideline in the present invention. 0018 Therefore, in the light of these problems of the prior art, the object of the present invention is to provide a method for fillet arc welding of high-strength thin steel sheet which can achieve excellent weld toe shape and improve fillet arc welded joint fatigue properties even in gas-shielded arc welding at a welding speed exceeding 80 cm/min, more notably exceeding 110 cm/min, and particularly a method for fatigue property improvement of welded joints in steel sheets of 700 MPa or greater tensile strength for which fatigue strength improvement is strongly desired. MEANS FOR SOLVING THE PROBLEM 0019 Adopting the aforesaid point of view, the inventors pursued an assiduous study of the effects of weld toe shape, with a focus on welding speed and the compositions of the steel sheet and the welding wire. They discovered that limiting especially the Si content within the steel sheet and welding wire weld enables toe shape improvement even at a welding speed higher than 80 cm/min, more notably higher than 110 cm/min up to 150 cm/min, and - 9 - further discovered a relationship between steel sheet Si content and welding wire Si content that produces a weld toe shape improving effect. The present invention was achieved through such research, and the gist thereof is as set out below. 0020 (1) A method for fillet arc welding of high-strength thin steel sheet of a tensile strength of 700 MPa or greater, which method is characterized in that: welding speed is greater than 80 cm/min, particularly from greater than 110 cm/min to not greater than 150 cm/min; the thin steel sheet contains, in mass%, C : 0.02 - 0.15%, Si: 0.2 - 1.8%, Mn: 0.5 - 2.5%, P: 0.03% or less, and S: 0.02% or less; and Si contents of the thin steel sheet and a wire for the welding are selected in a combination that makes Exp. 1 below have a value of 0.32 or greater, 0021 Si (steel sheet) + 0.1 x Si (wire) ... (Exp. 1), where Si (steel sheet) is the Si content of the thin steel sheet and Si (wire) is the total Si content of the wire for welding. 22 (2) A method for fillet arc welding of high-strength thin steel sheet as set out in (1), characterized in that the thin steel sheet and the wire for welding are selected in a combination that makes Exp. 1 have a value of 0.40 or greater. 23 (3) A method for fillet arc welding of high-strength thin steel sheet as set out in (1) or (2), characterized in that the thin steel sheet further contains, in mass%, Al: 0.005 - 0.1%. 0024 (4) A method for fillet arc welding of high-strength thin steel sheet as set out in any of (1) to (3), characterized in that the thin steel sheet further contains, in mass%, one or more of - 10 - Ti : 0.005 - 0.1%, Nb: 0.005 - 0.1%, V: 0.01 - 0.2%, Cr: 0.1 - 1.0%, and Mo: 0.05 - 0.5%. 0025 (5) A method for fillet arc welding of high-strength thin steel sheet as set out in any of (1) to (4), characterized in that as the wire for welding is used a solid wire for welding comprising, in mass%, C : 0.03 - 0.15%, Si: 0.2 - 2.0%, Mn: 0.7 - 2.5%, P: 0.05% or less, S: 0.08% or less, Cu: 0.5% or less (including 0%), and a balance of Fe and unavoidable impurities. 0026 (6) A method for fillet arc welding of high-strength thin steel sheet as set out in any of (1) to (5), characterized in that the solid wire for welding further contains, in mass%, one or more of Ti : 0.01 - 0.5%, Nb: 0.01 - 0.1%, V: 0.05 - 0.3%, Cr: 0.05 - 1.0%, Mo: 0.05 - 0.7%, and Ni: 0.3 - 12.0%. 0027 (7) A method for fillet arc welding of high-strength thin steel sheet as set out in (6), characterized in that the Ni content of the solid wire for welding is limited to, in mass%, Ni: 4.0 - 12.0%. 0028 (8) A method for fillet arc welding of high-strength thin steel sheet as set out in any of (5) to (7), characterized in that the S content of the solid wire for welding is limited to, in mass%, S: 0.02 - 0.08%. 0029 (9) A method for fillet arc welding of high-strength - 11 - thin steel sheet as set out in any of (1) to (4), characterized in that: the wire for welding is a flux-cored wire for gas-shielded arc welding comprising flux charged into a steel shell with no slit-like seam; the wire contains in the steel shell or the flux or in the two of them, a total of, in mass% of the wire as a whole, C (not including C in SiC): 0.01 - 0.20%, Si (not including Si in SiC and Si02) : 0.05 -1.2%, Mn: 0.2 - 2.5%, P: 0.03% or less, and S: 0.06% or less; and the flux charged into the steel shell further contains in mass% of the wire as a whole, SiC: 0.05 - 1.2%, and further contains one or more of Si02, AI2O3, Na20, and K2O in a total of 0.05 - 0.4%, and a balance of Fe and unavoidable impurities. 0030 (10) A method for fillet arc welding of high- strength thin steel sheet as set out in (9), characterized in that: the flux-cored wire for welding further contains in the flux charged into the steel shell, in mass% of the wire as a whole, graphite: 0.02% or greater; and total C equivalent of the flux-cored wire for welding as defined by Exp. 2 is 0.15 - 0.45%, 0031 Total C equivalent = [graphite] + 0.3 x [SiC] ... (Exp. 2), where [graphite] and [SiC] mean the mass% of graphite and SiC as a proportion of the whole wire. 0032 (11) A method for fillet arc welding of high- strength thin steel sheet as set out in (9) or (10), characterized in that: the flux-cored wire for welding further contains in - 12 - the steel shell or the flux or in the two of them, in mass% of the wire as a whole, one or more of Ni: 0.1 - 5.0%, Cr: 0.1 - 2.0%, Mo: 0.1-2.0%, and Cu: 0.1 - 0.5%, in a total of 0.1 - 6.0%. 0033 (12) A method for fillet arc welding of high- strength thin steel sheet as set out in any of (9) to (11), characterized in that the flux-cored wire for welding further contains in the steel shell or the flux or in the two of them, in mass% of the wire as a whole, B: 0.001 - 0.015%. 0034 (13) A method for fillet arc welding of high- strength thin steel sheet as set out in any of (9) to (12), characterized in that the flux-cored wire for welding further contains in the steel shell or the flux or in the two of them, in mass% of the wire as a whole, one or more of Nb, V and Ti in a total of 0.005 - 0.3%. 35 (14) A method for fillet arc welding of high-strength thin steel sheet as set out in any of (9) to (13), characterized in that the flux charged into the steel shell further contains, in mass% of the wire as a whole, 0.05 - 0.5% of a nonoxide arc stabilizer. 36 (15) A method for fillet arc welding of high-strength thin steel sheet as set out in any of (9) to (14), characterized in that the flux-cored wire for welding further contains in the steel shell or the flux or in the two of them, in mass% of the of the wire as a whole, S: 0.02 - 0.06%. 0037 (16) A method for fillet arc welding of high- strength thin steel sheet as set out in any of (1) to (15), characterized in that: the method for fillet arc welding of high-strength thin steel sheet is gas-shielded arc welding; and a shielding gas is used that comprises, in mass%. - 13 - COs: 5% - 25%, O2: 4% or less (including 0%), and a balance of Ar and unavoidable impurities. (17) A fillet arc welded joint of a high-strength thin steel sheet of a tensile strength of 700 MPa or greater, which joint is a gas-shielded arc welded joint characterized in that: welding speed is greater than 80 cm/min, particularly from greater than 110 cm/min to not greater than 150 cm/min; the thin steel sheet contains, in mass%, C : 0.02 - 0.15%, Si: 0.2 - 1.8%, Mn: 0.5 - 2.5%, P: 0.03% or less, S: 0.02% or less, and a balance of iron and unavoidable impurities; and Si contents of the thin steel sheet and a wire for the welding are selected in a combination that makes Exp. 1 of (10) have a value of 0.32 or greater. (18) A fillet arc welded joint of a high-strength thin steel sheet as set out in (17), characterized in that the thin steel sheet and the wire for welding are selected in a combination that makes Exp. 1 have a value of 0.40 or greater. EFFECT OF THE INVENTION 0038 In accordance with the present invention, even when a high-strength thin steel sheet of 700 MPa or greater is fillet lap-welded at a high welding speed exceeding 80 cm/min, more notably from greater than 110 cm/min to not greater than 150 cm/min, the weld toe shape is smooth, the stress concentration of the weld toe can be reduced in proportion, and the fatigue strength of the welded joint can be enhanced. Of particular note is that the method for fillet arc welding of high-strength thin - 14 - steel sheet provided by the present invention is of very-high industrial significance owing to the fact that it is outstandingly effective not only in the automotive industry but also in other industrial sectors that urgently require faster welding speeds and the fact that it is a technology capable of simultaneously achieving both productivity improvement and fatigue strength improvement. BRIEF DESCRIPTION OF THE DRAWINGS 0039 FIG. 1 is a schematic diagram for explaining how adding Si of smaller atomic radius than Fe to a steel material changes the Fe atom locations from those prior to the addition. FIG. 2 is a set of schematic diagrams for explaining the effect on molten pool beam when a difference occurs in the spread of the welding arc. FIG. 3 is a diagram showing how flank angle varied as a function of the Si content of a solid wire for welding and the Si content of the steel sheet when fillet lap-welding was performed at welding speeds of 100 cm/min, 112 cm/min, 120 cm/min, and 150 cm/min. FIG. 4 is a schematic diagram for explaining flank angle and undercut depth in a fillet arc welded lap joint. FIG. 5 is a schematic diagram for explaining the relationship between flank angle and fatigue strength. FIG. 6 is a set of schematic diagrams for explaining the shape and stress loading direction of a fatigue test piece used in embodiments of the present invention. MODES FOR CARRYING OUT THE INVENTION 40 The present invention is explained in detail in the following. 41 The fatigue properties of the welded joint that is the subject of the present invention are explained first. 42 Metal fatigue, unlike static strength, is a fracture - 15 - phenomenon under stress loading within the elastic range. Stress is loaded cyclically and the number of cycles to failure defines the fatigue life. Generally, if 2 million or more stress loadings are repeated without fracture occurring, the load stress at this time is called the fatigue strength. As metal fatigue is a fracture phenomenon at loaded stress within the elastic range, it differs from static strength in many aspects. For example, static strength is little affected by stress concentration or residual stress present in the welded joint. Although grinder finishing of a weld toe is highly effective for improving fatigue strength, it hardly changes the static strength. This static strength involves plastic deformation. When a stress concentration region like a weld toe is present, this region experiences only plastic strain, and from the viewpoint of static strength, regions other than the stress concentration region are merely relied on for strength, so that the strength of the welded joint as a whole is maintained. When tensile stress is already present, as in the case of residual stress, the self-regulation that is a characteristic of residual stress ensures the presence of compressive residual stress that unfailingly offsets the tensile residual stress, so that even if yield condition is immediately reached at the tensile residual stress region, this region still provides static strength because the yield condition has not been reached at the compressive residual stress region. The static strength of the welded joint as a whole is therefore unaffected by these factors. Thus, the overall shape of the weld bead, including the weld bead width, is important for static strength. 0043 In contrast, in the case of welded joint fatigue strength, the stress condition at only a small region of the welded joint determines the characteristics of the whole welded joint. Fatigue cracking occurs at high stress concentration regions like the weld toe. Tensile - 16 - residual stress is also present at these regions. As pointed out, residual stress exhibits self-regulation, and compressive residual stress that offsets this tensile residual stress is invariably present inside the welded joint. However, since fatigue strength is determined by the stress condition of only a small part of welded joint, any compressive residual stress that may be present will not affect the fatigue strength so long as the compressive residual stress is not present at the site of fatigue cracking. This tendency also holds for stress concentration. Specifically, even if a welded joint exhibits a smooth shape overall, the fatigue strength of the whole welded joint will nevertheless be determined by any region of high stress concentration that may be present at one part thereof. Therefore, a bigger contribution to fatigue strength improvement is realized by improving the shape locally, such as by improving the weld toe flank angle, than by improving the shape of the whole welded joint, e.g., the weld bead width. Viewed from this perspective, it is impossible to say whether the teachings of Patent Documents 3 and 4 and Non-patent Document 1 offer any weld toe shape improving effect that contributes to fatigue strength enhancement. Although Non-patent Document 1 actually teaches a technique of expanding the weld bead width, it is uncertain whether increasing the width of the weld bead also makes the flank angle smaller. 0044 As can be seen from the foregoing, expansion of weld bead width and reduction of weld toe flank angle are not the same technique. Although the present invention provides a technology aimed at improving v/eld toe shape, its ultimate object is to improve the fatigue strength of the welded joint. Insofar as the welded joint has not incurred any particular defect, adequate static strength can be achieved because static strength is independent of stress concentration or residual stress, and, moreover, no particular cause for occurrence of such welded joint - 17 - defects is present within the scope of the present invention. On this point, the present invention aims to provide a technology different from Patent Documents 3 and 4 and Non-patent Document 1. On the other hand, the object of the technologies of Patent Documents 1 and 2 is to improve welded joint fatigue strength, which is the same as the object of the present invention. However, different ways for improving welded joint fatigue strength are available, including, for example, stress concentration relaxation and residual stress relaxation; and the techniques taught by Patent Documents 1 and 2 are fatigue strength improving techniques utilizing residual stress relaxation, and as such differ from the technology utilizing stress concentration relaxation taught by the present invention. In addition, post-weld peening and grinding have also long been used to deal with the welded joint fatigue issue, but these are downstream process disadvantageous in the point of production efficiency. 45 The history of the development of the present invention is explained next. 46 In general, the material factors that determine weld toe shape, including the weld bead shape, are the molten pool surface tension and the force of gravity acting on the molten metal, and it has been considered that the weld bead shape is determined by the dynamical balance between these factors. The surface tension of the molten pool is affected by its chemical composition, such as by C, Si, S, 0 and the like. It is therefore considered that suitable control of these elements is effective for weld bead shape improvement. Based on this thinking, decreasing surface tension is sufficient for reducing weld toe flank angle and doing so also works directly to expand weld bead width. So there has been a tendency to view weld bead width expansion and weld toe contact angle enlargement as possible by the same technique. While as pointed out earlier, reduction of weld toe flank angle is preferable for fatigue strength improvement, conventional - 18 - thinking is that this is also a technique for expanding weld bead width. Particularly, from the fact that the surface tension of a molten pool is determined by its composition, it follows that the issue is not whether the components are supplied from the steel sheet or from the weld material, but that it is possible to improve weld bead shape insofar as the molten pool composition falls within a prescribed range, irrespective of which source the components are supplied from. The invention taught by Patent Document 10 was achieved based on this thinking. 0047 As pointed out, the prior art focuses on the issue of molten pool composition, and when, for example, the steel sheet is deficient in S, overcomes the problem by making up for the deficiency with S from the weld material. This means that a shortage of a component of either the steel sheet or the welding material can be made up for by supplying the component from the other. On the other hand, as discussed later, the present invention utilizes a phenomenon wherein such augmentation of a steel sheet component from the weld material is not possible. It is thought that this phenomenon occurs because a material factor other than molten pool surface tension is present as a weld bead shape determining factor, but the mechanism by which it operates is not altogether clear. However, the fact that, as found in the present invention, a material factor that determines weld toe shape exists aside from the molten pool composition amounts to the discovery of a factor not noticed heretofore and, as such, opens the way to better shape improvement than could be expected up to now. 0048 In the light of the foregoing considerations, the inventors focused on welding conditions enabling welding speeds of greater than 80 cm/min, particularly greater than the conventional high welding speed of 110 cm/min, and then moved forward with assiduous studies into the factors that determine weld toe shape. As a result, they - 19 - discovered that steel sheet Si content greatly affects weld toe shape. The effect of steel sheet Si content goes beyond simply the effect on the weld metal composition caused by dilution from the steel sheet. If this were the only effect, it should be possible to realize the same effect by regulating the welding wire Si content according to the dilution ratio, but as set out below, an effect is exhibited that cannot be realized merely by regulating the Si content of the welding wire. 49 Although the points of the function of the steel sheet Si content and of why the same effect cannot be realized by supply from the welding wire have not been completely clarified, the following is believed to be a plausible explanation. 50 The additive element Si in the steel sheet is a substitution element and is the element with the smallest atomic radius among the other substitution elements, such as Mn, Ni, Cr and the like, commonly contained in a steel sheet. The position of Si among its neighbors in the periodic table is - Na, Mg, Al, Si, P, S, CI, Ar - and since an element farther to the right has more nucleus protons, its electrons are more strongly attracted, with the result that the atomic radii of elements decrease in the order listed. Although P and S neighboring Si on the right have smaller atomic radii than Si, addition of these elements to a level in line with the object of the present invention is not feasible because heavy addition of P and S degrades the properties of the steel sheet itself and also leads to degradation of weldability. Practically, therefore. Si can be considered the element with the smallest atomic radius among the substitution elements. The inventors therefore focused on the Si in the steel sheet. 51 It is thought that when an element of small atomic radius is added, the interatomic distance naturally becomes longer in proportion. On the other hand, iron is a metal and electrons move freely inside the steel sheet. - 20 - i.e., free electrons are present. When these free electrons are present in regions where the interatomic distance is great, i.e., where Si is present, the force of attraction from the iron atoms tends to decrease in proportion, so that discharge of electrons from the steel sheet occurs that much more readily. 52 The fact that electrons discharge more readily from the steel sheet means that it is easier during arc welding for the welding arc to spread that much farther. This means that the welding bead spreads more easily. Moreover, the spreading of the welding arc also helps to maintain the surface of the molten pool at a high temperature, which has an effect of keeping the surface tension of the pool small and this in turn has an effect of enabling a smooth weld toe shape. This is an effect that cannot be realized by the method of lowering molten pool surface tension through regulation of the weld material composition. FIGs. 1 and 2 are schematic diagrams for explaining this effect. 53 FIG. 1 shows the positions of Fe atoms (broken line circles in FIG. 1) before Si was added to regularly arranged iron atoms so as to deploy small atomic radius Si atoms 3 therein and compares these positions with the positions of Fe atoms 2 (black line circles in FIG. 1) after the Si addition. It can be seen that, owing to the small radius of the Si atoms, the positions of the iron atoms changed slightly, so that the interatomic distance (the gaps regions between the atoms in FIG. 1) became larger. It is thought that the electron binding energy therefore decreased to facilitate electron discharge. 54 FIG. 2 is a schematic diagram showing the effect on weld bead shape when the arc from a welding wire spread out owing to freer electron discharge. The arc behavior when little Si was added is shown in FIG. 2(a) and that at increased Si addition is shown in FIG. 2(b). In the case of FIG. 2(b), the welding arc spread widely to melt that much more of the steel sheet and thus increase the - 21 - weld bead width, while the surface temperature of the molten pool present behind the arc increased proportionally, thereby keeping the molten pool surface tension low. As a result, the toe shape of the weld bead could be made smooth. In FIG. 2(a), the welding arc was narrower and the amount of added Si proportionally smaller than in FIG. 2(b). In this case, the zone where the steel sheet could be melted was narrower by the amount that the welding arc was narrower. And since the zone rearward of the arc where the molten pool surface could be heated was also narrower, the pool surface temperature tended to be lower than in the case of FIG. 2(b). The lower temperature also tended to increase the surface tension, while in FIG. 2(b) the surface temperature of the molten pool was maintained high thanks to the wide arc in the A2 zone, and since the surface tension was therefore held low, the pool width did not become narrow. In contrast to this, in the case of FIG. 2 (a), the narrowness of the arc made it impossible to maintain the pool surface of the Al zone in FIG. 2(a) at a high temperature, so that the pool width tended to narrow with recovery of the surface tension. Rearward of the Al zone, the pool temperature was low from the viewpoint of heat conduction, and, moreover, owing to the fact that the temperature at the outer portion of the pool was lower than that inside, a difference in surface tension occurred and the temperature was low, i.e., a phenomenon of the pool being pulled toward the outside by the larger outer surface tension was observed that made the pool width large again. This is the Bl zone in FIG. 2(a). However, since the pool was narrow in the Al zone, the difficulty of making the bead toe smooth was not resolved. In the case of FIG. 2(b), the shape of the bead remained good because the A2 zone pool width did not decrease. The prior art deals with this behavior either by establishing a component system that maintains low surface tension even if the temperature of the pool - 22 - surface becomes low or by adopting a method whereby, as shown in FIG. 2(c), the welding speed is lowered so that the Al zone of FIG. 2(a) enters the welding arc, i.e., so as to become like the A3 zone of FIG. 2(c). In the present invention, the welding arc is expanded to deal with this behavior, which differentiates the invention from the prior art. 55 From the teaching of the present invention set out in the foregoing, it will be understood that the addition of Si should be to the steel sheet and that a satisfactory effect cannot be realized by addition to the welding material. More specifically, in order to expand the welding arc, it is necessary to produce an arc between the steel sheet and the welding material before the steel material and/or welding material is melted. This requires electrons to be discharged from the steel sheet to establish a flow of electricity between the steel sheet and the welding material. The effect of Si in this phenomenon, i.e., the effect of facilitating the discharge of electrons from the steel sheet, cannot be achieved by, for instance, making up for the dilution of Si from the steel sheet by supplying the molten pool with Si from the welding material. In other words, the Si in the steel sheet is critical and the same effect cannot be achieved by supplementing Si from the welding material. 56 This is why the present inventors specify the Si content of the steel sheet. 57 In addition, the inventors clarified the relationship between the appropriate Si content of the steel sheet and the Si content of the welding wire. Specifically, they found that increasing the Si content of the welding wire tends to decrease the minimum Si content of the steel sheet required to improve the weld toe shape. However, they also found that when no Si is added to the steel sheet, the weld toe shape does not improve under high-speed welding conditions even if the Si content of the welding wire is increased. For - 23 - improving weld toe shape in this case, it is necessary to implement some measure that sacrifices production efficiency, such as lowering the welding speed (to 80 cm/min or less, for example). Although it is not altogether clear why increasing the Si content of the welding wire tends to decrease the minimum Si content of the steel sheet required to improve the weld toe shape, it is thought that when the Al zone in FIG. 2(a) narrows to a certain degree, the supply of Si acting to reduce surface tension from the welding material makes it possible to realize improved weld bead toe shape. 0058 FIG. 3 is a diagram in which Si content of solid wire for welding is plotted on the horizontal axis and steel sheet Si content is plotted on the vertical axis, and which shows the weld toe shape condition when, as the type of fillet arc welding, there was conducted lap fillet arc welding, which is the method most commonly used for automobile suspension parts. As there are different definitions of "flank angle," the one used with respect to the present invention is defined in FIG. 4. By the definition according to FIG. 4, the flank angle 5 is that angle among the angles formed between tangents to the weld bead and extensions of the surfaces of the steel sheets 6 and 7 which falls on the weld metal side. Some literature references use the flank angle of FIG. 4 to define the angle as (180° - flank angle), i.e., as that angle among the angles formed between tangents to the weld bead and extensions of the surfaces of the steel sheets which falls on the side opposite the weld metal, but the angle in FIG. 4 is defined as the flank angle with respect to the present invention. FIG. 3 differentiates between the case where the flank angle is 55° or less and the case where it exceeds this value. The fillet lap-welding was done on 3.2 mm thick steel sheet at welding speeds of 100 cm/min, 112 cm/min, 120 cm/min and 150 cm/min, and cross-sectional macrostructure test pieces were taken from the welded joints formed. The - 24 - flank angles at this time were measured in accordance with FIG. 4. The results are plotted in FIG. 3 using symbols assigned by welding speed and by flank angle of 55° or less and flank angle greater than 55°. The flank angles and fatigue strengths showed good correlation, and a schematic diagram illustrating their relationship is shown in FIG. 5. Here flank angle is plotted on the horizontal axis and fatigue strength on the vertical axis, with the fatigue strength shown as becoming A' when the flank angle is A. When the flank angle goes from B to A, the fatigue strength changes from B' to A'. The relationship between flank angle and fatigue strength is, as shown in FIG. 5, expressed by a straight line (or curve) passing from the upper left to the lower right, from which it can be seen that reducing the flank angle has the effect of increasing the fatigue strength. This is because the flank angle is a parameter determining stress concentration. An increase in flank angle increases stress concentration and proportionally lowers fatigue strength, while a decrease in flank angle lowers stress concentration to increase fatigue strength. From the opposite viewpoint, once the design fatigue strength of a welded joint has been decided, the upper limit of the flank angle is automatically determined. At the flank angle of 55° in FIG. 3, the fatigue strength is around 250 MPa. 0059 As shown in FIG. 3, it is possible in the steel sheet fillet arc welding method of the present invention to define a boundary between points where the flank angle is equal to or less than 55° and points where it is greater than 55°. Above this boundary is the range within which flank angle is 55° or less and good fatigue strength is ensured. The farther above this boundary, the smaller flank angle tends to be. In FIG. 3, a tendency was seen for flank angle to decrease under the four welding speed conditions of 100 cm/min, 112 cm/min, 120 cm/min and 150 cm/min in FIG. 3, but as pointed out - 25 - in the explanation of FIG. 2, flank angle grows progressively smaller as the welding speed is lowered. In other words, the weld toe shape improves. On the other hand, the object of the present invention is to reduce flank angle, which greatly affects fatigue strength, while also ensuring good welding work efficiency. So it was verified that excellent fatigue strength can be ensured even at welding speeds of 100 cm/min, 112 cm/min, 120 cm/min and higher, at which adequately high welding work efficiency can be anticipated. 60 The thickness of the thin steel sheet in the present invention is examined next. 61 The thickness of the thin steel sheet to which the present invention is applied is not particularly limited. However, owing to the fact that the technology dealt with is restricted to gas-shielded arc welding using solid wire for welding, the practical range of applicable sheet thicknesses is subject particularly to a lower limit of about 1.6 mm. This is because spot welding or laser welding is used more often than arc welding for steel sheet thinner than 1.6 mm. The upper limit of sheet thickness is defined as 4 mm. This is because the present invention is limited to 700 MPa or higher class steel sheet for which fatigue property improvement is especially critical, and the high strength of such a steel sheet makes greater thickness unnecessary. 62 The reasons for limiting the chemical composition of the steel sheet in the high-strength thin steel sheet fillet arc welding method that is one aspect of the present invention are explained next. 63 C content of less than 0.02% makes strength hard to achieve, so this is defined as the lower limit. The upper limit is defined as 0.15% because at higher content formation of carbides increases to degrade hole expansibility. 64 Mn is an element added to strengthen the steel. But - 26 - 2.5% is defined as the upper limit because excessive addition degrades ductility, On the other hand, 0.5% or greater must be incorporated to ensure strength. 65 S is an impurity in the present invention. As the A-type inclusion (JIS G0555) formed when S combines with Mn degrades not only hole expansibility but also ductility, the upper limit of S is set at 0.02%. Reducing S content to less than 0.0005% greatly increases steelmaking cost. The lower limit of S content is therefore preferably set at 0.0005%. 66 P is also an impurity in the present invention. Its upper content limit is set at 0,03% because heavy presence of P lowers ductility and also degrades secondary workability. 67 The reasons for limiting steel sheet Si are discussed next. 68 The restriction of steel sheet Si content is a fundamental aspect of the present invention. As pointed out earlier, the inventors think that the function of Si in the steel sheet is to enlarge the spread of the welding arc but realize that much still remains to be elucidated. Be that as it may, the function of steel Si referred to in the present invention differs from that of affecting the Si content of the weld metal by dilution form the base material. For example, assuming the base material dilution ratio to be 35%, then in the case where the Si content of the welding wire is 0.7% and the Si content steel sheet is 0.4%, the Si content of the weld metal can be estimated as 0.7% x 0.65 + 0.4% x 0.35 = 0.595%. In the case where the Si content of the steel sheet is 0%, if the base material dilution ratio should be the same, it follows that the same weld metal can be obtained by making the welding wire Si content 0.595% -;-0.65 = 0.915%. In this case, the weld metal comes to have the same Si content but the weld toe shape does not become the same. The weld toe shape is better when the steel sheet Si content is 0.4%. This phenomenon was not - 27 - known heretofore. However, the phenomenon occurs only when the welding speed is greater than 80 cm/min and is not observed at 80 cm/min or less. The lower limit of steel sheet Si content, 0.2%, was decided as that for Si to fulfill its function of improving the weld bead toe shape. In view of the fact that Si content diluted from the base material increases the Si content of the weld metal and combines with oxygen to form Si02 and increase the amount of slag formed on the weld metal after welding, the upper limit of steel sheet Si content is set at 1.8%. In the automotive sector, for example, the welding process is generally followed by a painting process in which the presence of slag on the weld metal surface is undesirable. This upper limit value was decided in view of this consideration. 69 The reasons for limiting the relationship between the steel sheet Si content and the welding wire Si content are taken up next. 70 As pointed out earlier, the function of steel sheet Si is different from that of regulating the Si content of the weld metal. Si affects the viscosity and surface tension of molten iron and has generally been said to influence weld toe shape through this action. However, when Si is not added to the steel sheet, no effect of improving weld toe shape is observed. But this is when the welding speed is high, i.e., greater than 80 cm/min, and this tendency becomes more pronounced with increasing speed. In other words, weld toe shape can be controlled by such improvement of viscosity, surface tension and the like when the welding speed is not so high (80 cm/min or less) but is considered to be more difficult to control as welding speed increases. Notwithstanding, changing the welding wire Si content does change the minimum Si content of the steel sheet required to improve weld toe shape. The relationship between the Si content of the steel sheet and the Si content of the welding wire is - 28 - therefore restricted. Specifically, it was found that making the value of Exp. 1 below 0.32 or greater enables weld toe shape improvement even at welding speeds of 120 cm/min or greater and higher than 110 cm/min. 0071 Si (steel sheet) + 0.1 x Si (wire) ... (Exp. 1). The condition of the value of Exp. 1 being 0.32 or greater must be satisfied irrespective of dilution of Si from the base material. This is because the essence of the present invention is not simple regulation of the weld metal composition. The present invention fundamentally differs from the prior art on this point. A straight line representing Exp. 1 = 0.32 is shown in FIG. 3. As seen from FIG. 3, flank angle is 55° or less under conditions where the value of Exp. 1 is 0.32 or greater. In other words, the lower limit of Exp. 1 determined by the steel sheet Si content and the welding wire Si content is defined as 0.32 because at high welding speed no effect of improving weld toe shape is obtained below this value. Although an upper limit is not particularly defined, the range is automatically limited by the upper limit values of steel sheet and welding wire Si contents. The value of Exp. 1 is determined by the Si contents of both the steel sheet and the wire for welding and cannot be determined by the respective Si contents individually, but no problem exists in this regard because the value can easily be determined by a person skilled in the art. 72 In the method for fillet arc welding of high-strength thin steel sheet of the present invention, still more reliable improvement of weld toe shape is enabled by further limiting the value of Exp. 1. 73 Methods for establishing the value of Exp. 1 include that of establishing it with the Si content of the steel sheet and that of establishing it with the Si content of the wire for welding. As the coefficient of steel sheet Si content is the greater in Exp. 1, the method of - 29 - establishing the value with the steel sheet might seem superior, but the weight of steel sheet in a welded structure is generally around 100 times that of weld metal. Therefore, even though the coefficient of steel sheet Si content is greater in Exp. 1, it is usually economically preferable to establish the value of Exp. 1 with the wire for welding. However, in a case where Si is added to the steel sheet to realize an effect other than that of achieving the weld toe shape improving effect that is the object of the present invention, it may not be necessary to add Si abundantly to the wire for welding. Therefore, the value of Exp. 1 can be established by limiting the Si content of the wire for welding in accordance with the Si content of the steel sheet. Moreover, additional flank angle reduction is achieved if the value of Exp. 1 is made 0.4 0 or greater. A line for the case where the value of Exp. 1 is 0.40 is also shown in FIG. 3 and can be seen to be shifted to a region upward from the case where the value of Exp. 1 is 0.32. In this case, flank angle can be further decreased to realize a still greater fatigue strength enhancing effect. The larger flank angle reducing effect realized when the value of Exp. 1 is raised above 0.40 makes it possible to further increase welding speed. For example, welding speed can be increased to 120 cm/min. 74 The reasons for limiting the essential components of the steel sheet in the present invention are as set out in the foregoing. Although it is possible in the present invention to selectively incorporate the additional elements set out below as required, these are all for ensuring the strength and workability of the steel sheet and not for improving weld toe shape. 75 The upper limit of welding speed is specified as 150 cm/min. This is because, as pointed out earlier, welding speed is one factor determining the production efficiency of a welded structure and this efficiency - 30 - improves in proportion as the speed is increased. On the other hand, excessively high speed is undesirable from the viewpoint of weld bead shape because, inter alia, it intensifies molten pool movement. Increased likelihood of the undercut 8 in FIG. 4 occurring is a particular problem. The object of the present invention is to improve welded joint fatigue strength, and flank angle reduction and other weld toe shape improving measures are means for achieving this object. Occurrence of an undercut degrades fatigue strength. So the upper limit of welding speed is specified as 150 cm/min from the viewpoint of fatigue strength improvement. It is of course not a matter of joint fatigue strength decreasing as soon as welding speed goes above 150 cm/min. Depending on the welding conditions, welding at a higher speed is possible without any problem. But, as shown in FIG. 3, it was ascertained that the present invention can achieve excellent fatigue strength even at a high welding speed of 150 cm/min. 76 Selective elements of the steel sheet are discussed next. 77 In the present invention, Al is added to the steel sheet for the purpose of deoxidation, not for the purpose of realizing the weld toe shape improvement that is an object of the present invention, and such addition falls within the prior art also disclosed in Patent Document 5 and the like. The lower limit of Al content is defined as 0.005%, which is the minimum content capable of exhibiting a deoxidation effect. When Al is added excessively, it remains in the steel as oxide. In this case, a hole expansibility problem arises. When gas-shielded arc welding is conducted in the automotive sector, it is generally applied to suspension parts, so that hole expansibility is one important property required by the steel sheet. Although not an object of the present invention, the establishment of hole expansibility was recognized as industrially significant. - 31 - The upper limit of 0.1% for Al addition was set as a value enabling achievement of hole expansibility. 78 Ti, Nb, V, Cr and Mo are added to the steel sheet for the purpose of enhancing steel sheet strength. These elements increase the strength of the steel sheet by combining with C to form carbides. However, as these elements differ in strength enhancing effect, a different content range was defined for each. 79 The lower limits of Ti and Nb content are set at 0.005% because this is the minimum value at which strength increase can be anticipated. The upper limits of Ti and Nb content are set at 0.1% because steel sheet ductility is degraded by excessive addition above this value. 80 V is an element that works similarly to Ti and Nb. However, the lower and upper limits of V content are set different from those for Ti and Nb because V is inferior to these elements in precipitation hardening. The lower limit of V is set at 0.01% because this is the minimum value at which strength increase can be anticipated. The upper limit is set at 0.2% because steel sheet ductility is degraded by excessive addition above this value. 81 Cr is an element that, like Ti, increases strength by forming carbides, but Cr has not only a precipitation hardening effect but also a solution hardening effect. However, since its precipitation hardening activity is not so great as those of Ti, Nb and V, the content range of Cr can be defined more broadly than for these elements. The lower limit of Cr is set at 0.1% because this is the minimum value at which strength increase can be anticipated. The upper limit is set at 1.0% because steel sheet ductility is degraded by excessive addition above this value. 82 Mo is an element with an effect similar to Cr. The lower limit of Mo is set at 0.05% because this is the minimum value at which strength increase can be anticipated. The upper limit is set at 0.5% because steel - 32 - sheet ductility is degraded by excessive addition above this value. 83 Reasons for limiting steel sheet components in the present invention are set out in the foregoing. 84 The reasons for limiting the components of the wires for welding are discussed next. 85 The present invention envisions use of two types of wires for welding: solid wire and flux-cored wire. 86 The reasons for limiting the components of the solid wire for welding will be taken up first. 87 C is added to ensure the strength of the weld metal. The lower limit of solid wire C content must be set higher than that of the base material because, unlike the steel sheet, the weld metal must secure strength in its condition as welded. The lower limit of C content is defined as 0.03% because strength is hard to ensure at a lower content than this. The upper limit is set at 0.15% because a risk of hot cracking of the weld metal arises above this level of addition. 88 Mn is also an element added to increase the strength of the weld metal. The upper limit of Mn content is defined as 2.5% because heavier addition causes excessive hardening. As the steel sheet that is the subject of the present invention is of the ordinarily required 700 MPa class or above, the welded joint is required to have commensurate strength. As Mn content of 0.7% or greater is required to ensure this strength, this value is defined as the lower limit value. 89 Si is an element that acts to deoxidize the weld metal. The lower limit of Si content is defined as 0. 2% because when the content is lower than this, insufficient deoxidation leads to ready formation of blowholes and the like in the weld metal. The improvement of weld toe shape that is an object of the present invention can be achieved even if Si is added to greater than the value specified as the limit by the present invention. However, the present invention deals with steel sheet in the - 33 - thickness range prevalent in the automotive and similar industrial sectors. When the Si content of the solid wire for welding is too high, the Si content of the weld metal increases and combines with oxygen to form Si02, so that the amount of slag occurring on the weld metal surface increases. In the automotive sector, for example, the welding process is followed by a painting process in which the presence of slag on the weld metal surface is undesirable. Moreover, an Ar-based shielding gas is sometimes used to reduce sputtering during welding, and in this case it is preferable to set the content of the deoxidizing element Si low. So in the present invention, 0.7% is set as the upper limit of Si content within which slag formation can be suppressed and sputter occurrence diminished. It should noted that the upper limit value is more preferably 0.6% and still more preferably 0.5%. 90 S is generally an impurity. The upper limit of S is defined as 0.08% because higher S content degrades weld metal toughness and increase the risk of weld metal hot cracking. 91 P is also an impurity in the present invention. As weld metal toughness degradation and weld metal hot cracking probability rise with increasing P content, 0.05% is defined as the upper limit of P content. 92 Cu is plated on the solid wire for welding for the dual purposes of increasing electrical conductivity and preventing wire corrosion. Therefore, addition of Cu is not absolutely necessary for achieving the weld toe shape improving effect that is one object of the present invention. However, owing to the risk of wire corrosion causing blowholes and other problems, the present invention defines the range of Cu content. That said, it must be noted that Cu addition is sometimes considered undesirable from the viewpoint of environmental preservation, and the view that Cu addition should be avoided even at some sacrifice of electrical conductivity and the like is receiving increasing acceptance. The - 34 - present invention is therefore constituted to include the case of no Cu plating, i.e., no particular lower limit of Cu content is defined, thereby including 0% in the content range. In order to realize the effect of Cu plating, the lower limit of Cu addition is desirably set at 0.05%. The upper limit of Cu content is defined as 0.5% because the effect of enhancing electrical conductivity and the like saturates above this level, while copper cracking and other harmful aspects are more likely to emerge. 93 Selective elements of the solid wire for arc welding are discussed next. 94 Although the primary purpose of the elements Ti, Nb, V, Cr, Mo and Ni that can be selectively added to the solid wire for welding is to ensure the strength of the weld metal, one of them, Ti, is also an element that stabilizes the welding arc and can therefore also be added for a purpose other than weld metal strengthening. 95 Ti is assigned a lower content limit of 0.01% because this is the minimum value at which strength increase and welding arc stabilization effects can be anticipated. The upper limit is defined as 0.5% because addition above this value excessively hardens the weld metal to cause a joint property issue. The upper and lower limits of Ti content defined here are set higher than the upper and lower limits of Ti addition that the present invention specifies for steel sheet in consideration of the fact that the Ti of the solid wire for welding is oxidized by the welding arc. 96 The lower limit of Nb content is set at 0.01% because this is the minimum value at which strength increase can be anticipated. The upper limit of Nb content is set at 0.1% because addition above this value excessively hardens the weld metal to cause a joint property issue. 97 V is an element that works to ensure strength similarly to Ti and Nb. However, the lower and upper - 35 - limits of V content are set different from those for Ti and Nb because V is inferior to these elements in precipitation hardening. The lower limit of V is set at 0.05% because this is the minimum value at which strength increase can be anticipated. The upper limit is set at 0.3% because steel sheet ductility is degraded by excessive addition above this value. 98 Cr is an element that, like Ti, increases strength by forming carbides, but Cr has not only a precipitation hardening effect but also a solution hardening effect. However, since its precipitation hardening activity is not so great as those of Ti, Nb and V, the content range of Cr can be defined more broadly than for these elements. While the lower limit of Cr is set at 0.05% because this is the minimum value at which strength increase can be anticipated, it is preferably defined as 0,1%. The upper limit is set at 1.0% because excessive addition above this value hardens the weld metal to cause a toughness issue and other problems 99 Mo is an element with an effect similar to Cr. The lower limit of Mo is set at 0.05% because this is the minimum value at which strength increase can be anticipated. Although the upper limit is set at 0.7% because excessive addition above this value degrades the toughness of the weld metal, it is preferably defined as 0.5%. 100 Ni is added mainly for two purposes. The first is to ensure the strength of the weld metal and the second to ensure the fatigue strength of the weld zone. From the viewpoint of the second purpose of ensuring fatigue strength, there is a need to define the Ni range more narrowly, so the Ni range regarding this point will be taken up later. From the viewpoint of ensuring the strength of the weld metal, the lower limit of Ni content is defined as 0.3%. This lower limit is set as the minimum value at which strength increase can be anticipated. The upper limit is defined as 12.0% because - 36 - addition above this value gives the weld metal an austenite microstructure, which poses a risk of instead lowering strength, as well as a danger of hot cracking. 101 With respect to the S content of the solid wire for welding, the present invention defines a range within which S can be positively utilized to a degree that does not adversely affect the joint properties. S can be expected to produce a weld toe shape improving effect by lowering weld metal viscosity. The S content of the weld metal can be established either by the method of adding S to the steel sheet or by the method of adding it to the welding wire, but between these two methods, that of addition to the solid wire is preferable because the method of addition to the steel sheet causes steel sheet property issues. However, the method of addition to the solid wire experiences the problem of hot cracking mentioned earlier when too much S is added, so the upper limit is set at 0.08%. When it is desired to improve weld toe shape by positive utilization of S, it suffices to make the amount of S addition 0.02% or greater. Generally, addition of S to 0.02% or greater may lead to a weld metal toughness problem. However, this depends on the properties reguired by the welded joint and can be dealt with by suitably deciding S content based on the balance betv/een weld toe shape improvement and required toughness. 102 In the present invention, aside from the use of Ni in the solid welding wire to ensure weld metal strength, a method can also be implemented of using it to lower the transformation start temperature of the weld metal and thus positively reduce the residual stress of the toe of the weld, thereby improving the fatigue strength of the welded joint by dint of this effect. This method amounts to positive adoption in the present invention of the high-fatigue-strength weld material technology, which high-fatigue-strength weld material technology has already been published in, for example. Patent Documents - 37 - 1 and 2. This technology and the technology provided by the present invention can be used in combination as required with no tradeoff between the two. The lower limit of Ni added to the solid wire for welding is set at 4.0% because this is the minimum value at which enhancement of fatigue strength by Ni addition can be anticipated. The upper limit was defined as 12.0% because addition above this value increases austenite in the microstructure of the weld metal, which decreases the amount of transformation expansion and may sometimes lead to no transformation occurring, in which cases no improvement of fatigue strength can be expected. 103 As incorporation on Ni within the content range of 4.0 to 12.0% increases the hot cracking susceptibility of the weld metal, in this Ni range the upper limit of S content is preferably specified as 0.01% and more preferably as 0.006%. 104 The basic composition of the solid wire for welding of the present invention is as set out in the foregoing. 105 The flux-cored wire is discussed next. 106 The wire used for thin sheet welding in the automotive and other sectors is generally a solid wire. Among the reasons for this are that solid wire is cheaper than flux-cored wire and that that solid wire leaves less slag on the weld and is therefore advantageous for post-weld painting. But the point of solid wire being more economical holds only when the amount of wire produced is relatively great, and when production is low, flux-cored wire can be produced at lower cost than solid wire. This is because in the case of a solid wire a change in wire composition requires remaking of the wire material itself but in the case of a flux-cored wire can be dealt with simply by adjusting the composition of the charged flux, without changing the composition of the whole wire. In light of this, the inventors concluded that it would be beneficial to provide a technology enabling fatigue strength improvement with a flux-cored wire. - 38 - 107 Now follows an explanation as to why the steel shell of the flux-cored wire is limited to a shell with no slit-like seam that might allow entry of ambient air. 108 As compared with a solid wire, a flux-cored wire not only has the aforesaid problem of excessive slag generation but also has a problem of a large amount of hydrogen. Therefore, in the manufacture of the flux-cored wire, the flux to be charged into the wire is dried beforehand to reduce the amount of hydrogen. However, even in the case of a flux-cored wire charged with dried flux, if the steel shell of the flux-cored wire has a slit-like seam through which ambient air can enter, the amount of hydrogen increases if the flux takes up moisture through the seam. As a technique for improving fatigue properties, the present invention includes that of reducing residual stress by controlling the composition of the flux-cored wire, but this composition is unavoidably relatively high in alloying elements, namely, a composition high in cold cracking susceptibility. Since cold cracking can be prevented by reducing the amount of hydrogen, the present invention prevents the flux from absorbing moisture by limiting the shell of the flux-cored wire to one having no slit-like seam vulnerable to air invasion. 109 The reasons for limiting the composition of the flux-cored wire are explained next. 110 In the flux-cored wire, C other than that in SiC is included mainly in the steel shell so as to prevent breakage in the drawing process during wire production. Although C other than that in SiC also acts to lower the transformation temperature of the weld metal, the present invention enables thorough reduction of weld metal transformation temperature by controlling the SiC content of the flux charged into the steel shell in accordance with the makeup of the composition. In order to utilize C in the steel shell to prevent breaks in the wire drawing process, the lower limit of C content, not including C in - 39 - Sic, must be made 0.01%. On the other hand, since excessive addition of C to the steel shell leads to hardening that may cause breakage during drawing, the upper limit of C content, not including C in SiC, is defined as 0.20%. 111 It should be noted that when iron powder is charged into the steel shell as flux, the C of the steel powder is counted as C other than that in SiC. Therefore, in order to minimize hardening by C in the steel shell during wire drawing, it is preferable to set the C content of the steel shell at 0.15% and supply the remaining amount of C from the C content of the iron powder added as flux. 112 The lower limit of Si content, not including Si in Sic and Si02, is defined as 0.05% so as to realize a weld metal deoxidation effect during arc welding. On the other hand, since excessive addition of Si other than Si in SiC and Si02 hardens the weld metal, which is undesirable from the viewpoint of joint properties, the upper content limit is specified as 1.2%. 113 Mn is a required element for ensuring the strength of the weld metal and is assigned a lower content limit of 0.2% because weld metal strength is hard to achieve at a content lower than 0.2%. The lower limit of Mn content can be set below that of the solid wire for welding in the present invention because some degree of strength is ensured by addition of C. The upper limit of Mn content is set at 2.5% because too much Mn degrades the toughness of the weld metal. 114 P is an unavoidable impurity element of the weld metal and is assigned an upper content limit of 0.03% because heavy presence of such an impurity element degrades toughness. 115 Like P, S is also generally an unavoidable impurity element of the weld metal, but S is said to contribute to bead shape improvement by lowering the surface tension of the molten pool. It is therefore put to effective use in - 40 - the present invention. However, the flux-cored wire of the present invention is limited to a composition range for achieving residual stress reduction, and since it is therefore high in C, it is necessary from the viewpoint of hot cracking to set the S content lower than in the case of the solid wire of the present invention. So the upper limit of S content is defined as 0.06%. 116 Si02, AI2O3, Na20 and K2O contained in the flux are usually referred to as slag material. Prior to flux-cored wire production, these compounds serve as binders during granulation of the flux components, and after the flux components have been charged into the steel shell, they function as lubricants that reduce friction between the inner surface of the steel shell and the flux in the course of drawing down to a prescribed wire diameter. In the present invention, by utilizing the lubricant activity of added SiC, good workability can be established in the wire drawing process even if the slag material constituted by these oxides is reduced below the conventional level. Nevertheless, the lower limit of the total content of one or more of Si02, AI2O3, Na20 and K2O is defined as 0.05% because at a total content below 0.05% the aforesaid workability is hard to realize and wire quality and production efficiency issues emerge. On the other hand, the upper limit of the total content of one or more of Si02, AI2O3, Na20 and K2O is defined as 0.4% because at a total content above 0.4% a problem of degraded paintability arises due to profuse generation of slag in the weld zone. 117 Sic is an essential component in the present invention because it ensures the presence of an appropriate amount of Si in the flux-cored wire, functions as a source of the major element C for lowering the transformation start temperature of the weld metal, and exhibits the lubrication and deoxidizing properties of Sic. 118 The lower limit of SiC content is defined as 0.05% - 41 - because below this content the improvement in wire workability by the lubrication and deoxidization activities of SiC and the effect of slag reduction are insufficient. However, increasing the SiC content of the flux poses an issue of weld metal hardening and the possibility of no transformation of the weld metal occurring due to an abundance of austenite structure, and in such a case, the trouble taken to include SiC in flux-cored wire comes to no avail. The upper limit of SiC contained in the flux-cored wire is therefore defined as 1.2%. The Si content of a flux-cored wire tends to be higher than the upper limit of a solid wire. This is because the C in SiC combines with oxygen to form CO that escapes outside the welding arc, so that even if the amount of Si included in the flux-cored wire is greater than the upper Si content limit of a solid wire, the amount of slag formed on the weld metal surface is little affected. 119 The reasons for limiting the essential components of the flux-cored wire in the present invention are as set out in the foregoing. 120 The reasons for limiting the selective elements of the flux-cored wire are discussed next. 121 The function of graphite in the present invention is to serve as an alternative to SiC. While graphite is cheaper than SiC as a C source, it has a production issue in that its light weight makes it apt to become airborne during flux-cored wire manufacture. The present inventors nevertheless treat graphite as a selective element, not just because of its low cost, but also because it is more effective than SiC in the point of lubricating action in wire drawing. But both SiC and graphite are C sources, so to take this fact into account, Exp. 2 was formulated to express total C equivalent and limit total C content. 122 Total C equivalent = [graphite] + 0.3 x [SiC] ... (Exp. 2) . The lower limit of graphite, 0.02%, was defined - 42 - because the effects of graphite addition are not exhibited below this value. Although the present invention does not specify an upper limit for graphite, the upper limit of graphite is automatically determined because the range of Exp. 2 is restricted. Moreover, the lower limit of Exp. 2, 0.15%, is defined because when the lower limit is set lower than this, the graphite content must be made less than 0.02%. On the other hand, the upper limit of 0.45% was defined because addition above this value makes the C level of weld metal so high that problems arise with respect to the hardenability, toughness and cracking susceptibility of the weld metal. 123 In the present invention, Ni, Cr, Mo and Cu are added for the purposes of enhancing the Charpy and other mechanical properties of the weld metal and of improving the fatigue strength of the weld metal by lowering its transformation start temperature. The purpose of Cu goes beyond the foregoing to also include improvement of electrical conductivity by plating the wire with Cu. 124 Ni is an element effective for lowering the transformation start temperature of the weld metal and thus improves the fatigue strength of the welded joint, and is also an element that improves the strength, toughness and other properties of the joint. When Ni is incorporated, the lower limit of Ni content must be made 0.1%, which is the minimum amount at which an adequate joint fatigue strength improving effect can be expected in a low SiC composition, but is preferably made 0.5%. An adequate effect of lowering the weld metal transformation start temperature can be achieved at up to the upper limit of Ni content. The upper limit of Ni content is defined as 5.0% because at an Ni content higher than 5.0% no fatigue strength improvement can be expected owing to the fact that interaction with C contained in the weld metal leads to no transformation of the weld metal to bainite and martensite at low temperature, so that austenite may remain until the completion of cooling. - 43 - 125 Cr and Mo are elements that act to lower the transformation start temperature and enhance the strength and hardenability of the weld metal. The effect of Cr and Mo toward improving the strength and ensuring the hardenability of the weld metal is particularly high, exceeding that of Ni, and to utilize this effect for transforming the weld metal to martensite or other structure of low transformation temperature, thereby improving the fatigue strength of the welded joint, the content of each of Cr and Mo must be made 0.1% or greater. However, the weld metal toughness enhancing effects of Cr and Mo are inferior to that of Ni, so the upper content of each of Cr and Mo is defined as 2.0% owing to the risk of weld metal toughness degradation when included excessively. 126 Cu, like Cr and Mo, is also an element effective for lowering the transformation start temperature, and enhancing the strength and ensuring the hardenability of the weld metal. Further, Cu is sometimes plated on the surface of the wire to establish electrical conductivity. In order to realize these Cu effects of improving the strength and hardenability of the weld metal and the effect of establishing conductivity, the lower limit of Cu content must be defined as 0.1%. The upper limit of Cu content is specified as 0.5% because the weld metal is liable to sustain copper cracking when too much Cu is added to the weld metal. 127 In the present invention, the upper limit of the total content of one or more of Ni, Cr, Mo and Cu is set at 6.0%. This is because at an excessive total content higher than 6.0%, joint fatigue strength is difficult to achieve owing to the fact that low-temperature transformation to bainite and martensite does not take place in the weld metal during post-weld cooling, with the result that austenite structure remains as is. The upper limit of the total content is therefore preferably specified as 6.0%. Although the present invention does - 44 - not particularly establish a lower content limit for when one or more of Ni, Cr, Mo and Cu are added, the lower limit of the total content automatically becomes 0.1% owing to the lower content limits set for the respective additive elements. When a need to increase fatigue strength exists in addition to that of achieving the toe shape improving effect of the present invention, the lower limit of the total content is desirably set at 1.5%. Less than 1.5% may be added to improve Charpy and other mechanical properties, but whether Charpy properties should be improved or fatigue properties should be improved depends on the intention of the invention user, and determining the amount of addition is not particularly difficult for a person skilled in the art. 128 B is a hardening element that acts to ensure weld metal hardenability, make the microstructure of the weld metal a higher strength structure, and, by restraining generation of structures that commence transformation at high temperature, establish a microstructure that transforms at low temperature. As the weld metal has a higher oxygen content than the steel sheet, B may be deprived of its effects by combining with oxygen, so that in order to enable B in the weld metal to improve tensile strength and fatigue strength by the aforesaid hardenability and microstructure control, the lower limit of B content is preferably made 0.001%. The upper limit of B content is set at 0.015% because weld metal cracking is liable to occur at higher amount of addition. 129 Nb, V and Ti are all elements that increase strength by forming carbides in the weld metal, and inclusion of a small amount of one or more of Nb, V and Ti in the weld metal improves joint strength. The lower limit of the total content of one or more of Nb, V and Ti is preferably 0.005% because not much joint strength improvement can be anticipated at a content of less than 0.005%. The upper limit of the total content is - 45 - preferably defined as 0.3% because when the total content is higher than 0.3%, the strength of the weld metal rises excessively to cause joint property problems. In addition to strengthening the weld metal, Ti also functions to stabilize the welding arc, so that the content of Ti, when included, is preferably 0.003% or greater. 130 With respect to S in the flux-cored wire, the present invention defines a range within which S can be positively utilized to a degree that does not adversely affect the joint properties. S can be expected to produce a weld toe shape improving effect by lowering weld metal viscosity. The S content of the weld metal can be established either by the method of adding S to the steel sheet or by the method of adding it to the welding wire, but between these two methods, that of addition to the flux-cored wire is preferable because the method of addition to the steel sheet causes steel sheet property issues. However, the method of addition to the flux-cored wire experiences the problem of hot cracking mentioned earlier when too much S is added, so the upper limit is set at 0.06%. When it is desired to improve weld toe shape by positive utilization of S, it suffices to make the amount of S addition 0.02% or greater. Generally, addition of S to 0.02% or greater may lead to a weld metal toughness problem. However, this depends on the properties required by the welded joint and can be dealt with by suitably deciding S content based on the balance between weld toe shape improvement and required toughness. Still, when one or more of Ni, Cr, Mo and Cu are added within the range of the present invention, it is preferable from the viewpoint of cracking susceptibility to set the upper limit of S at 0.03%. 131 An arc stabilizer is an element that functions to stabilize the welding arc when included in the flux charged in the steel shell. As the Na20, K2O and the like included in the aforesaid flux function as arc stabilizers, these components are preferably included at - 46 - a level that does not interfere with the reduction of weld zone slag generation that is an object of the present invention. Moreover, since the action of an arc stabilizer need not rely on oxides like Na20 and K2O and the aforesaid arc stabilization effect can be realized with Na, Al and F compounds like cryolite (NasAlFe) , compounds other than oxides are preferably included from the viewpoint of reducing slag generation. 0132 In order to reduce weld zone slag generation and achieve an arc stabilization effect, the lower limit of non-oxide type arc stabilizer content is preferably 0.05%. The upper content limit is preferably defined as 0.5% because the arc stabilization effect remains unchanged when content of non-oxide type arc stabilizer rises above 0.5%. The reasons for limiting the shielding gas in the present invention are taken up next. 0133 CO2 and Ar are the gases that can be used as the shielding gas, but use of 100% Ar as shielding gas is impossible from the aspect of arc stability with current technology. The method of using 100% CO2 is totally feasible within the scope of conventional technology if combined with effective use of the deoxidizing element Si, and, moreover, CO2 gas has the merits of being usable as 100% of the shielding gas within the Si range taught by the present invention and of being cheaper than Ar gas. The reason for nevertheless using a shielding gas consisting primarily of Ar gas is the advantage that it can minimize sputtering. However, Ar is an inert gas and this makes a minimal amount of CO2 gas necessary. The lower limit of CO2 gas expressed in mass% of the shielding gas consisting primarily of Ar gas is defined as 5% because the welding arc is unstable below this value. The upper limit is defined as 25% because above this value sputtering increases and performance becomes not much different from the case of using 100% of CO2 gas as the shielding gas. - 47 - 134 In the present invention, it is also possible to add O2 to the shielding gas. However, the addition of O2 gas is for the purpose of lowering the cost of the shielding gas and has no direct bearing on the effect of weld toe shape improvement that is an object of the present invention. Generally, use of a shielding gas that is 100% Ar gas requires removal of O2 gas (to 0%), which increases the cost of the shielding gas. On the other hand, Ar gas containing a certain amount of O2 can be produced at relatively low cost. The presence of a certain amount of O2 gas does not impair the weld toe shape improving effect. The preferable value of the lower limit of the O2 gas composition limit range is specified as 2% because lowering the content rate below this amount has an effect on the cost of the Ar gas. The upper limit of 4% is established because an increase in the weld metal oxygen content gives rise to a toughness issue in the case higher addition than this. 135 The reasons for the limitations with respect to the method for fillet arc welding of high-strength thin steel sheet according to the present invention are as set out in the foregoing. First Set of Examples 136 Examples of the present invention are explained in the following. 137 Table 1 shows the steel sheet compositions used in a first set of examples. The first set of examples are for investigating steel sheet hole expansibility. - 48 - A< 2 > PI; t-i u 2 (TJ> >>> >>>> g -Hciacaccccc-H-H-HCCCccc-iH-H-H-Hcccca Oj 4-)0000000004-i4-i4-iOOOOOO-P-P-P-POOOOO ptl (D -H -H -H -H -H -H -H -rH -H fO (0 (0 -H -H -H -H -H -H (0 (0 0) (0 -H -rH -H -H -H M-P-P-P-P4-I-P4-1-P+J ^1 M M-P-P-M+J-P-P M U U S-l-P-P-P-P+J aJdCCCCCCCCnJronJCCCCCCiTimaJnJCGCCc; !ia)a>a)(D(Da)(Da)a)Q4aa(i)a)(Da)(ua)aaia4D4(DQ)a)>>>>>>>>ggg>>>>>i>gggg>>>>> occcccccccoooccccccooooccccc CJ M J-H_j--I_ H J--I_J-I_ H MHUUUHHMMHHUUUCJMMHHH +J c; "hJiU oooinOLr)OLoj£?oooo2SoLnooo5ir)Lr)ooLnoo "'D-i c\icrivDcri^ix>inor;oorHLno3r;Jl:oomroc\i~,rsir~ro^'3'iHO -jS ^-[^(Xla^[~(^ooa^'_2ja^r~roc\l^^r^[~^-■^~[~-^|r--oor--r^^~r--CD -H CO ^ to fO s >, ^-^ -p -H ^ -H -H M-g-^LnoinioggLnooLnOogoLoooaiL/iOLOLno^gggS -H ^ C -H ^ CO S C J5 IT) iHrOrHC\irO>H ., ■^iHrH rr)C\lC\l DJ H O, .OOOOOO ^OOO ,,000 V ej^ ,1 I ',,. ''... Q) o OOOOOO '-'ooo ooo (U O ^ , o MS • ' o o ^ ^ -C ;, en LO CO ro CM 13 ^ O rH O O O C CM ro _i CM ro fO > ^ "? , -^ , o o (-; ^ ^ _^ 0._0._ "IJ g ° - , , §§ ■^ ° ° ° ° O CD _^ r~- CD kO IT) n_i o "' oooo S"'^ o "^11 oooo O o ° oooo o cocsi'!3'00cx)r~CDOiH[^c»r~-ocriTHo«=j'ooi~'-Do„Lr)CMCMr^cx)co . 0iH0rH000rHi-l0O0rH0i-HrHiH000rHpJiHrH.H000 -^ cn ooooooooooooooooooooooooooo (U • 0) ooooooooooooooooooooo°oooooo r* ~ uoiocoiOLnLnunLnLnin-ir^cMorocMLriocriLnLOLrj.oLnLnLnLnro "J rHi-IOiHrHiHrHiHrHiH.HrHiHrHiHiHiHiHOiHrHrHpr:>-l>HrHiHiH PJ OOOOOOOOOOOOOOOOOOOOOOOOOOO M ■ Q) OOOOOOOOOOOOOOOOOOOOOOOOOOO Q^ cDc»_, ,_„„^_cMLn_, ,p^„„or^OLnro_„,inc»f^cnc^ " ^ cr^c^'-^^^^'-^^^'o'-^QP^'^ooc^cn^o^-^^ oo_^ o c^ '^CM™o'^^°°"^'^'"f"°°^"^CM^'"^'^°OOCDCMfM'^°CD -H o.ro_rocMLnr^Ln'3'rHCMmLn«ci<_cMCMOir)r^~_^^^^CMn_ O'~'o'~'o0O00O0OOOrH'~',-lrHrHOo'-'*-''~''~'oo'~' ~ ■!3'LncM^3''!i'^3''=r-!5'ro-!3x)r~-cncrioc\)m-=i'LncDt~-cocno>—icNjn'=3'LncD[^cx>cn PO ■^ OOOOOOOOOiHi-HrHrHiHiHrHiHrHCMCMCMCMCMCSlCsJCMCMCM r_| ^ mmcQpqmoQPQPQcQcQPQmpQDacQDQcQpQcflpQpQpQDQpQffipQmm o w - 49 - 139 The steel sheets having the compositions in Table 1 were heated to a heating temperature of 1150 to 1250 °C, hot rolled to a finishing temperature of 820 to 900 °C, thereafter cooled at a cooling rate of 35 to 75 °C/sec, and coiled at a coiling temperature of 400 to 550 °C to obtain hot-rolled steel sheets of 2.6 mm thickness. Table 1 also shows the steel sheet tensile strengths, which were regulated to various values by controlling the cooling rate and the like. 140 A square test piece measuring 250 mm x 250 mm was taken from each steel sheet, a circular hole of 30 mm diameter was punched in the center region of each, and a hole expansion test was then performed with a conical punch of 60° apex angle. The hole was expanded with the conical punch, the punched surface were observed for crack occurrence, diameter d was measured at the time a crack propagated to the rear surface of the steel sheet, and hole expansibility was calculated from the diameter d increase ratio {(d - 30) x 100/30}. A doubling of the diameter to 60 mm amounted to hole expansibility of 100%. 141 Table 1 includes steel sheet composition, tensile strength, and hole expansibility. As hole expansibility generally tends to decrease with increasing steel material strength, it is inappropriate to evaluate the hole expansibility of the 700 MPa or higher class steel material that is the subject of the present invention by comparison with lower strength steel material of, for example, 400 MPa class. Relative superiority should be judged by comparison with other materials of 700 MPa or greater strength. Therefore, all steel materials shown in Table 1 other than B13 and B14 were produced under conditions that imparted strength of 700 MPa or greater. 0142 0143 Comparative examples BOl and B12 exhibited good properties, with hole expansibilities of higher than 70%. Although these steel sheets are classified as comparative - 50 - examples because their Si contents were outside the range of the present invention, it can be seen that they had good hole expansibility despite containing Si at below the lower limit of the present invention. The reason for this result is that the lower limit of Si content is not defined with a view to achieving good hole expansibility but to improve the weld toe shape, which is compared in the second and later sets of examples, so the validity of the lower Si limit specified by the present invention is not demonstrated by only this first set of examples. 014 4 Although comparative examples B13 and B14 contained Si within the range of the present invention, they respectively had Mn and C contents outside the invention range and their strengths were not of 700 MPa class. The present invention is directed to steel materials of 700 MPa class and higher for which fatigue is a serious problem., so B13 and B14 are comparative examples in the present invention. Second Set of Examples 145 A second set of examples dealing with weld toe shape improvement and fatigue testing is set out below. 146 Steel sheets among the first set of examples that exhibited hole expansibility of greater than 70% were used to fabricate fillet arc welded lap joints that were tested for weld toe shape and fatigue. The fillet arc welded lap joint is one of the most commonly used welded joint shapes in the sheet thickness range of the present invention, particularly in vehicle suspension parts. The compositions of the solid wires for welding used to fabricate the welded joints are shown in Table 2. Those outside the range of the present invention by wire composition only are indicated as comparative examples in the Remark column. Those indicated in the Remark column as invention examples are ones within the range of the present invention in terms of wire composition, but their indication in the Remark column of Table 2 is for - 51 - reference because the present invention prescribes by combination with the steel sheet. - 52 - ^51 I I I I I I I I I [ I I 1 1 I u m ^ ^ ^ ^ £ -rH -H C -H Di v-^ --- -- — U o o ° o 00 X! O o -H '^ O O T3 .H -^ . Q) S ^ ;^ m CO cN S-l r 1 * . ■ ■ ' O C> Q O O O ?J cx)CX)t^ocrirHLr)p^r^[^r-r~r-[^r-r~- j OOO>-IOiH00p-;OOOOOOOO ■^ coooooooo,oooooooo ^ ooooooo'—'oooooooo -H O RrHiHi-Hi-HiHiHi—li-l>—liHrHiHrHvHi—I ^ DJOOOOOOOOOOOOOOOO ^ oooooooooooooooo o +J ^QQi-ICNJrHOOOiHiHrHiHLn>x>Ln^oo r>OOOOtHi-liHOOOOOOOOO Q) '^ <-\ OOOOOOOOOOCDOOOOO Xi m 2iHCMoO'^Ln>X>r-~-aDcriOiHC\ioo^Ln'x> f^ -yOOOOOOOOOrHi—liHi-HiHrHiH O CO - 53 - 0148 Tables 3 to 5 show welding conditions and compositions of utilized shielding gases. The results of all examples in Tables 3 to 5 are for steel sheets of 2.6 mm thickness. The effect of varying welding speed was checked, with current conditions established as shown below to enable joint formation with a single welding pass: 60 cm/min : 120 A, 85 cm/min : 170 A 100 cm/min : 200 A, 120 cm/min : 240 A 130 cm/min : 260 A, 140 cm/min : 280 A 170 cm/min : 320 A. - 54 - TiXiXiTi "OXi^a (D (V d) 0) (UtUtUX 0)00)0) o)o);3w mcoojOTtn OTcofO^ — tji D)MCntn-H cn tn w w CCnJCCm CCrH J^ -M -rH > -H -rH -rH -H MM U Ti X} XS T> 0) T)Ti -(DO) rC H HiHMr-i U r-^ H ttO) 0) 6a)a)o)a)-H a)0)X-p-p o) pc;— —Q, — —— —--„„-_- f, X 0) 0) >>>> >>>>> u ^^ -H -H V -H -H •— -H C d -H C -H C -iH -H -rH 3 (J, ;-) 4J-H4-14-)-rH+JOO-PO-PO-P-P -P O (-H (0 n3WcOnJCOra-H-H(0-Ha)-Hn)(fl m O ^ M— M MM M+J+JM-PM+JMM—V-i o ■■^ n3M(Di-itomMfOc;cl(OC'OCrtifoa)a3 X) a4a)ao)Q4aiO)Q4Q)fl)ao)aia)fta.3ai o) r-{ ga)ga)e6>e>6>6e'-i6 M Qj O-PO-POO-POCdOCOCOOfOO O ^ c>OTC)OTucjcooHHUHUHOU>u x: 3 ^ (1) -^ O OcnS'roo ooo_;ooooooooo -Q 4-l-rHjr, D-iO-i oocTicri'^crirHrHMcriVDcMr-co M ^ m 0) •H 1 ^ S 4-1 Xi « s^^ s D) «^ e -H g d s; o 05 En^'r^oo in [-^ iH ■n<*ooxi CM m! 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T—' P*^ 'Q,'^ inomir) mm .'^{jm O Qj futl o'~'o'~'o"^o'~*ooooo'-^o^o J-aoc >-l 5'' oooo °°a)SS^-^ o\o n3 i " -p-p o p CO^ u^>. ooo oooo "^>iS'^'^ mC) ^Xi-p ooo'~''^"~'oo'~''~'"^.ooooo9'P''d^?^ <~^ -a*m.H ... .... roMx^co^ frt -^tnto-i OOO oooo s^rop-ao S T5 I . u M X) f-H rjlLnoommmLnommmo mooom^s O^ m frt -^ rouip u " cTOOOOOOOOOOOOOOOOO -POT3-H -.^^ 'J_mom o o ooo-HC-Ps-j ^-'^ ^ H 9,C*'^C>oooo^<-.r-.o'^o'~''-^'-'o'^J C:(L)C •'^ r^ooo o o oooc -Hpx: 0) -p S ^ 5—^—H —-^ I g g * r—j -H 2 _^ooooooooooo ""J o'^'~''^oc I -.H co = X! H ^ S 2 ii^a_5'"§-oS frt -rH fli o in iT) ir> in mmmommmmm-M'na' o r j^ Jj rj~* Wire No. oooooooooo-H m ininininin^^-^-^.— \J |C']|rsi|(N|cs]|{N|cN|csi|rs]lcN]l(M|(N[ oj |cN|rsi|csi|cs]lc']l.H fn m uj 03 - 65 - 161 Table 7 and Table 8 include more test items than Table 2, which is the table showing examples of the solid wire for welding. The reasons for this are, inter alia, that flux components peculiar to the flux-cored wire were present, that Charpy property data were included in Table 7 and Table 8 because the fact that the present invention defines the amount of added C higher for the flux-cored wire than for the solid wire for welding gave rise to a possible weld metal Charpy property issue, and that the number of items requiring evaluation, which included wire drawability, amount of generated slag, and dustiness when graphite was used, was greater than in the case of the solid wire. 162 The wires of Table 7 are examined first. 163 The wires numbered 100 to 110 are flux-cored wires within the range of the present invention, and those numbered 150 to 165 are outside the range of the present invention. 164 The wires of Table 7 were measured for flux dustiness, wire drawability, Charpy absorbed energy, and slag generation. Flux dustiness compares the ratio of the amount of graphite made available for producing flux to the amount of graphite in the flux just before charging into the wire. When no graphite scatters as dust, these amounts match and the dustiness is 0%, but when scattering occurs, the amount of graphite just before wire charging decreases proportionally. Dustiness is a measure of this decrease. Wire drawability is an evaluation of whether or not breakage occurred during wire production. Charpy absorbed energy was evaluated as the value obtained by butt-welding 3.2 mm steel sheets using the wire concerned, taking therefrom a H size Charpy test piece having a 2-mm V notch machined at the weld metal center region, and subjecting it to a 0°C Charpy test. Slag generation was evaluated as the weight of slag generated on the weld metal surface when bead-on-plate welding was performed to a weld bead length of - 66 - 250 mm. 165 Wires 150, 151 and 159, whose slag material contents were outside the range of the present invention, generated more than 0.1 g of slag and thus posed a paintability problem. In contrast, wires 100 to 105, which were within the range of the present invention, are shown in Table 2 all to have generated less than 0.1 g of slag, meaning that slag material must be restricted to within the range of the present invention to ensure good paintability. However, wire 150 produced a good weld bead. So a Charpy test piece was taken using this wire, and when Charpy tested, was found to have a 7 J value. This is attributable to the fact that the Mn content of wire 150 was above the range of the present invention, and Mn must be brought within the invention range to obtain good mechanical properties. 166 On the other hand, wire 151 was also below the range of the present invention in SiC content. In such a case, a problem of wire drawing difficulty and breakage during production might be expected, like in wire 154, but the wire 151 did not experience a breakage problem thanks to heavy addition of slag material. So slag amount was measured using wire 151, and slag generation was found to be higher than 0.1 g, namely, 0.34 g. This means that preventing breakage while also minimizing slag generation requires use of SiC, not slag material. 0167 Wire 152 is an example in which weld zone cracking occurred because SiC content exceeding the range of the present invention caused the value of Exp. 2 also to exceed the range of the present invention. Moreover, wire 165, whose SiC was within the range of the present invention, nevertheless sustained similar cracking because its Exp. 2 value was above the range of the present invention. Wire 153 contained Si above the range of the present invention and Charpy tested at less than 10 J owing to excessive Si. Wire 155 is an example in which the Charpy value was less than 10 J due to C - 67 - content above the range of the present invention. Wire 156 sustained weld zone blowholes and other defects because Si content was below the range of the present invention. 168 On the other hand, wire 157 could not be manufactured because a wire breakage problem arose during wire production owing to deficient steel shell strength caused by C content below the range of the present invention. Wire 158 was below the range of the present invention in Mn content and experienced wire breakage for the same reason as wire 157. 169 Wires 160 to 162 and 164, whose total of Nb, V and Ti was above the range of the present invention, had Charpy values of less than lOJ. Wire 163 contained B at higher than the range of the present invention and sustained cracking in the weld zone. 170 Wires 100 to 110, which were within the range of the present invention, all were free of breakage problems, had slag generation amounts of less than 0.1 g, and exhibited Charpy values above 10 J. 171 The wires of Table 8 are discussed next. 172 Among the wires of Table 8, wires 200 to 210 are the ones within the range of the present invention. These wires had relatively high Cu, Ni, Cr and Mo contents compared with the wires of Table 7. Wires 250 to 255 are comparative examples. 173 Wire 250 contained slag material above the range of the present invention and its slag amount was above 0.1 g, namely, 0.3 g. This tendency observed in the examples of Table 2 was thus found also in compositions comprising Cu, Ni, Cr and Mo. 0174 Although wires 251 and 252 had total contents of these four elements exceeding the range of the present invention, no particular drawbacks were evident in light of Table 3. This point is examined in the fifth set of examples set out below. 0175 Wire 253 had a total Nb, V and Ti content above the range of the present invention. - 68 - The Charpy value was therefore below 10 J, namely, 6 J. 176 Wire 254 is one without added SiC and with slag material restricted to within the range of the present invention, and in which graphite was used to prevent wire breakage. As a result, graphite dustiness amounted to 40%. Such high dustiness made wire production process control extremely difficult and posed a risk of even slight modification of the production process causing major change in wire composition. Such a situation made high-quality wire production difficult. 177 Wire 255 contained SiC at below the range of the present invention and suffered a wire breakage problem. 178 In contrast to these comparative examples, the wires 200 to 210 had slag generation rates below 0.1 g, had no wire drawability or dustiness problems, and exhibited Charpy values of 20 J and higher. Fifth Set of Examples 179 The fifth set of examples carried out fatigue testing on fillet lap-welds fabricated using those among the steel materials and wires in the first and fourth sets of examples that were found to be free of problems, namely those designated "Invention" in the Remarks columns of Tables 1, 8 and 9, and also using some designated "Comparative" in order to corroborate the effects of the present invention. 180 The tables show the shielding gases used in combination. The results of all examples in Tables 10 to 12 are for steel sheets of 2.6 mm thickness. The effect of varying welding speed was checked, with current conditions established as shown below to enable joint formation with a single welding pass: 60 cm/min : 120 A, 85 cm/min : 170 A 100 cm/min : 200 A, 120 cm/min : 240 A 130 cm/min : 260 A, 140 cm/min : 280 A 170 cm/min : 320 A. - 69 -w 0) a) rH H H 0) m m (U > > TO iH iH d) X X Q) v> CO ^ ^ ra ^~ i: Di m m Cn co S -H M rH -H M ^ TS > > > > -H-HTHc-HGCCdccc-Hcccaccccccccccac; 4J4J+JO+JOOOOOOO+JOOOOOOOOOOOOOOOO ^^ (C CD fO -H m -rH -H -H -r^ -H -H -H tC -H -rH -H -H -H -H -H -H -H -H -H ■^^ -H -H -H -H 0) MMt-l+JM-P-P4-l-P-P4-l+JS-l4-)-P-P-P-P-P-P+J+J-P+-l+J4-l-P4-l-P .^ aiQj(ig>>>>>>>g>>>>>>>>>>>>>>>> •* ooococacccccocccccccccccacccc . 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