DESCRIPTION WELD JOINT FORMED WITH STAINLESS STEEL-BASED WELD METAL FOR WELDING A ZINC-BASED ALLOY COATED STEEL SHEET [0001] This application claims priority to Japanese Application Nos. JP 2005-282712, and JP 2006-136897, filed in Japan on September 28, 2005, and May 16, 2006, respectively. The entire contents of these applications are herein incorporated by reference; Field of the Invention [0002] One aspect of the present invention relates to a weld joint for zinc-based alloy coated steel sheets. This weld joint is formed with a stainless steel-based weld metal and may be used in building materials or automobile materials. The weld joint is excellent in corrosion resistance and liquid-metal embrittlement (LME) crack resistance at the welded portion. Background of the Invention [0003] Zinc-based alloy coated steel sheets are widely used as building materials and automobile materials because of their good corrosion resistance as structural members. Conventionally, in order to improve the corrosion resistance, after non-coated members are welded, the welded members are dipped in a zinc-based alloy bath. This applies the zinc-based alloy to-the steel member and the surface of the welded portion, which secures corrosion resistance to the whole welded structure. This method, however, provides low productivity since coating has to be conducted after the welding process. This causes an increase in manufacturing costs since additional facilities, such as a coating bath, are required. In view of this, to manufacture a structural member with good corrosion resistance and with high productivity, a method where zinc-coated steel sheets are welded to form the welded structure has been employed. [0004] JP2000-64061 discloses a zinc-base alloy coated steel sheet in which a zinc-based alloy, such as a Zn-AI-Mg-Si based alloy, is coated. The Zn-AI-Mg-Si based alloy coating has improved corrosion resistance compared to conventional zinc-coated steel sheets. [0005] In the case of manufacturing a structure by welding a zinc-based alloy coated steel sheet, however, corrosion resistance is deteriorated because the coated part of the welded metal portion is evaporated. In view of this, conventionally zinc-based alloy coated steel sheets are first welded using a carbon steel welding material and then the welded portion is coating by brushing or spraying. This additional coating process lowers productivity in the manufacturing of the structural member. [0006] As for welding of stainless steel structures where good corrosion resistance is required, a stainless steel welding materia! is used to form a welded metal with good corrosion resistance at the joint between stainless steels or stainless steel and carbon steel. However, if a stainless steel welding material is used for welding zinc-base alloy coated steel sheets, cracks occur due to liquid-metal embrittlement. This is because when the zinc-base alloy coated steel is welded, liquid-metal embrittlement cracks form at the welded portion of the stainless steel components due. to the melted coating. [0007] The a main cause for liquid-metal embrittlement cracks is thought to be that zinc-based alloy coating components remain melted on the steel sheet. This can break at the crystal grain boundary when the welded metal portion is subjected to tensile stress caused by heat contraction. This is the cause of the brittleness. Therefore, it has been common sense that the zinc-based coating must has be removed in advance when the zinc-coated steel sheets are welded using a stainless steel welding material. [0008] A phenomenon similar to liquid-metal brittleness cracks occurs when different materials, such as a stainless steel sheet and a zinc coated steel sheet, are welded. Therefore, there have been few attempts at welding zinc-coated steel sheets or at welding zinc-coated and stainless steels using a stainless steel-based welding material. [0009] JP09-267I77A discloses a manufacturing method of a steel door with good corrosion resistance. In this method, a stainless steel sheet and a zinc-coated steel sheet, both of which are about 2mm thick, are butt-welded using filler wire with a relatively high Ni content. The Ni content is such to keep the Ni equivalent of the weld metal of stainless steel-based components higher than a predetermined value in order to disperse austenite and inhibit the formation of martensite of poor ductility. This leads to prevention of cracks caused by bending after welding. [0010] In the disclosure of JP09-267 I77A, there is no description of liquid-metal brittleness cracks (zinc brittleness cracks) that usually occur immediately after the stainless steelsheet and the zinc-coated steel sheet are butt-welded. Jn the disclosure of JP09-267177A, a hot-dip zinc-coated steel sheet, of which the melting point of the zinc coating is high, is welded on the condition that the thickness of the steel sheet is 2mm, and the binding force at the welded portion is weak. This seems to be the reason that liquid-metal embrittlement cracks (zinc brittle cracks) do not occur when the stainless steel sheet and the zinc-coated steel sheet are butt-welded. [001 I] However, if a zinc-based alloy coated steel sheet without alloying treatment is butt-welded using the method described in JP09-267I77A on the condition that the thickness of the sheet is 3mm or more and the binding force of the welded portion is high as in a fillet weld, it is supposed to have a phenomenon similar to liquid-metal embrittlement cracks. [0012] The reason why the phenomenon similar to liquid-metal embrittlement cracks occurs easier when the thickness of the sheet to be .welded is 3mm or more and the binding force is higher, is that as the thickness of the sheet increases and/or the binding force increases, the tensile stress, caused by heat contraction of the weld metal, also increases. This causes the zinc-based alloy coating components that remain melted on the surface to break easily at the crystal grain boundary of the weld metal. [0013] When stainless steel sheets are butt-welded, delayed cracks may occur after welding. ' JP200I-9589A disclosed a method for preventing delayed cracks after welding when a high Cr content stainless steel is welded. [0014] In view of above, it is understood that a method for manufacturing a welded structure, having excellent corrosion resistance in the welded portion, is very difficult when a zinc-based alloy coated steel sheet is welded using a stainless steel-based welding material. Summary of the Invention [0015] An object of the invention is to provide a weld joint formed using a stainless steel-based welding material. This stainless steel-based welding material is particularly suited for welding a zinc-based alloy coated steel sheet and results in excellent corrosion resistance and liquid-metal embrittlement crack resistance at the welded portion. This is accomplished by inhibiting liquid-metal embrittlement cracks of the stainless-steel-based weld metal When a zinc-based alloy coated steel sheet is welded. [0016] The inventors have diligently researched improvement of corrosion resistance of welded portions by using a stainless steel-based welding material for welding zinc-based alloy coated steel sheets. The inventors have also diligently researched improvement in the inhibition of liquid-metal embrittlement cracks caused by welding stainless steel metal-based components and zinc-based alloy coatings. [0017] As a result of this research, it is found that the occurrence of liquid-metal embrittlement cracks of stainless steel-based weld metals depend on solidification mechanisms and structure. Liquid-metal embrittlement cracks can be inhibited by adjusting component compositions based on Ni equivalent and Cr equivalent in order to inhibit martensite formation in the weld metal and to accelerate ferrite formation. [0018] The present invention is made based on the knowledge obtained above and the gist of invention is described below. [0019] One aspect of the present invention relates to a stainless steel-based welding material composition and a weld joint made therefrom. The weld joint is for a zinc-based alloy coated steel sheet excellent in corrosion resistance and liquid-metal embrittlement crack resistance in the welded portion. The weld joint comprises a welded portion of weld metal made of stainless steel-based components, the weld metal comprising in mass percent (%): C: 0.01-0.1; Si:0.1-I Mn: 0.5- 2.5; Ni: 5-11; and Cr: 17-25, and the balance being iron and residual impurities, wherein the following expressions (1), (2) and (3) are met; -0.81xCr equivalent +23.2 0.95xCr equivalent - 8.1 (region II in FIG.3: high Ni equivalent, low Cr equivalent), martensite formation is inhibited, and the ductility of welded metal is maintained. However, the ferrite amount in the welded metal is reduced because of the low Cr equivalent, and yet because of the high Ni equivalent, austenite grains grow up to form a lot of coarse austenite. Therefore, the melted zinc coating easily breaks at the coarse austenite grain boundary to form liquid-metal embrittiernent cracks. [0045] According to JP09-267 177A, region II in FIG.3 is supposed to be the region to avoid cracks in the welded portion normally formed when a butt-weld joint of a stainless steel and coated steel is subjected to bending. However, the inventors' evaluation of a sample piece which is made by welding a zinc-based alloy coaled steel sheet using stainless-steel-based welding wire shows that liquid-metal brittleness cracks occurred in the sfainless-sfeeJ-based weJd metal. This fact suggests that the mechanism of cracking due to bending of a butt-welded joint of a stainless steel and a coated steel disclosed in JP09-267I77A is different from liquid-metal embrittlement cracking of a stainless-steel-based weld metal, which is the subject matter of the present invention. Meanwhile, in the case where the Ni equivalent meets the expression (1) below, (i.e., region I in FIG.3: high Cr equivalent, medium Ni equivalent) formation of martensite in the weld metal is inhibited, the ductility of the weld metal is maintained, and the ferrite phase is formed in 15% or more. Thus, the melted zinc coating is prevented from breaking into the welded metal; which leads to inhibition of liquid-metal embrittlement cracks. [0046] -0.81xCr equivalent+23.2 2 content ranges; preferably from 0.5 to 2.5%. As raw materials, the following can be used alone or in combination: rutile, titan slag, iluminite and titanate, such as potassium titanate, and sodium titanate. [0074] Silicon dioxide' (SiCb) is a necessary component to form an encapsulated slag. However, if the SiO2 content is less than 1.5%, the encapsulation cannot reach a sufficient level, which lowers function of anti-oxidization for weld metal and does not provide a good appearance. If the SiO2 content exceeds 3.5%, the slag tends to burn dry, peelability worsens and the weld slag increases. Therefore, the limit of the SiO2 content preferably ranges from 1.5 to 3.5%. As for raw materials for SiO2, silica sand, silica, wollastonite, zircon sand and potassium feldspar may be used. [0075] Zirconium dioxide (ZrO2) provides the slag with fluidity. However, if the ZrO2 content is less than 0.5%, the fluidity becomes insufficient. If the ZrO2 content is more than 2.5%, the slag becomes stiff, peelability worsens and the weld slag increases. Therefore, the limit of the SiO2 content preferably ranges from 0.5 to 2.5%. As for raw materials, zirconium oxide, zirconium flower, and Zilcon sand may be used. [0076] If the content of total slag components in the flux is less than 6.5%, the slag encapsulation is not sufficient. If the content of total slag components in the flux is more than 9.5%, the weld slag increases and it become easy to have slag inclusrons. Therefore, the total of the slag components in the flux preferably ranges from 6.5 to 9.5%. [0077] The present invention is applicable to coated steel sheets such as zinc coated steel sheets, Zn-Al-based alloy coated steel sheets, Zn-AI-Mg-based alloy coated steel sheets and Zn-AI-Mg-Si-based alloy coated steel sheets. The coating amount is preferably 50g/m2 or more per surface in terms of securing corrosion resistance and 1 50g/m2 or less per surface in terms of welding workability. [0078] In order to make more highly corrosion free weld joints, sheets with greater corrosion resistance, for instance, Zn-AI-Mg-Si-based alloy coated steel sheets, should preferably be used. In such sheets, the Al content is 2-19%, the Mg content is 1-10%, the Si content is 0.01-2% and the balance is Zn. . ' [0079] In the present invention, low-alloy structural steels are mainly used as a basic material for zinc-based alloy coated steel sheets. However, it is not necessary to define the components/composition of the basic material of such zinc-based alloy coated steel sheets unless the components/composition of the welding material is diluted during welding to be outside of ranges defined by the present invention. In this meaning, the present invention can be applied to a weld joint between different materials, such as welding a zinc-based coated steel sheet and a stainless-steel-based steel sheet. [0080] The present invention can be applied to any shape of weld joint, for example, a fillet joint, a lap fillet joint or a butt joint. As described above, liquid-metal embrittlement cracking occurs particularly when a binding force is applied to the welded portion. Therefore, it is more meaningful to apply the present invention to a fillet weld joint made by fillet welding of a zinc-based alloy coated steel sheet having a thickness of 3mm or more where the binding force is very strong. [0081] As for welding the method, any of MIG arc welding, MAG arc welding and carbon dioxide gas arc welding can be used. As for the welding conditions, there are no specific limited conditions. However, melting the basic material of the steel sheet can change the components/composition of the weld metal. Therefore, it is preferable to avoid unnecessary increase in dilution by the melted basic material according to the following conditions: input heat is preferably lOKJ/cm or less, and dilution ratio of the basic material is preferably 10-40%. The dilution ratio of the basic material is defined as follows: dilution ratio of basic material = (each melted component of the basic material)/(each melted component of the basic material + each melted component of the welding material) x 100% (1) The input heat for welding can be controlled to preferably be lOKJ/cm or less, by adjusting the welding current, the welding voltage and the welding speed. [0082] The present invention can be applied to tailor blank welding, other than arc welding, by using stainless steel-based welding wire asfiller wire. Examples [0083] The welding material used is solid wire containing alloy components is designated in Table I below as (SI), (S2), S(l I), and S(12) and flux-cored wire is designated as (F3) - (FI0). [0084] The flux-cored wire contains the following slag components: Ti02: 1.7%, SiGv 2.5% and ZrO2: 1.6% per total wire weight. (SI2) is welding wire for ordinary steel. [0085] Table 2 shows the components/composition of zinc-based alloy steel sheets to be used and the coating weights. Coated steel sheet A is a zinc alloy coated steel sheet, class 590MPa, 6mm in thickness. Coated steel sheet B is a Zn-Al-Mg-Si alloy coated steel sheet, class 400MPa, 3mm in thickness. [0086] The fillet weld joint shown in FIG.4 is prepared using the welding material and zinc-based alloy coated steel sheet described above. The joint is evaluated for liquid-metal embrittlement cracks and corrosion resistance of the stainless-steel-based weld metal. [0087] The test sample shown in FIG.4 is prepared as follows. A zinc-based alloy coated steel sheet 4 is vertically placed on a horizontally set zinc-based alloy coated steel sheet 1. Then a fillet welding 5 is carried on using a stainless-steel based wire. After the welded portion is cooled off, a fillet welding 6 is performed to complete the test sample. Fillet welds 5 and 6 have similar penetration shape, which indicates that dilution ratios of basic material at Fillet welds 5 and 6 are substantially similar. [0088] The welding conditions of fillet weld 5 is welding current: 200-220A, arc voltage: 25-28 V, welding speed: 40-50cm/min., shield gas: in the case of using solid wire, a mixed gas of (argon + 2% oxygen) is used, in the case of using llux-cored wire, a mixed gas of (argon + " 20-50% carbon dioxide) is used, or in the case of using flux-cored wire, carbon dioxide gas is used. Under these welding conditions, the dilution ratio of the basic material during welding is roughly determined by the kind of shield gas. When using (argon + 2% oxygen) as a shielding gas, the dilution ratio of basic material is about 15%. When using (argon + 20-50% carbon dioxide), the dilution ratio ranges from 20 to-35% as the carbon dioxide gas ratio in the mixed gas changes from 20 to 50%. [0089] Evaluation of liquid-metal embrittlement cracks of the weld metals is performed with a color check (liquid penetrant inspection method). If a crack is not observed with the naked eye, the evaluation is "good". [0090] Evaluation of corrosion resistance is performed with a JASO-defined mixed cycle corrosion test. One cycle of the test is as follows. (I) salt-water (5%NaCI) spray, 35°C, 2hours; (2) drying (moisture 30%), 60°C, 4hours, and (3) wet condition (moisture 95%), 50°C, 2hours. A total of 120 cycles are performed and red rust is checked every 20 cycles. If red rust is not observed in the first 20 cycles, the evaluation of corrosion is "good". [0091] Table 3 shows the components/composition, the Cr equivalent and Ni equivalent, the relation to expression (I), .and evaluations of liquid-metal embrittlement cracks and corrosion resistance of each weld metal test sample. [0092] With respect to sample Nos. J-8, these meet the upper and lower limits of Ni equivalent. No liquid-metal embrittlement cracks are observed and the corrosion resistance is good. Sample Nos. 1-2 using solid wire have weld spatters, but sample Nos.3-8 using flux-cored wire have little weld spatter and good vveldability. [0093] Sample No. 7 is the sample to which Mo and N added. Red rust is observed in the area around the welded metal at 40 cycles but no red rust is observed on the welded metal itself until more than 120 cycles. Samples No. 3 and No. 8 are the samples to which the high corrosion resistant coating Zn-AI-Mg-Si alloy is applied. These samples show excellent corrosion resistance including both the welded portion and the area in the vicinity of the weld. [0094] Sample Nos. 9-15 are comparison examples that are out of the range defined by the present invention. None of sample Nos. 9-12. meets the right part of expression (I) (upper limit of Ni equivalent), and all show brittleness cracks. Sample No. 1 1 has an excess amount of Mo andN. [0095] Sample Nos. 9-12 meet the condition: -0.7xCr equivalent + 20 < Niequivalent (lower limit of Ni) disclosed in JP09-267I77A, but do not meet upper limit of Ni defined by the present invention. Therefore, these samples do not form sufficient amounts of ferrite phase in the welded metal, and thus form brittleness cracks. (Table Removed) [0096] Sample No. 13 does not meet left part of expression (I) (Ni lower limit), and brittleness crack is observed. Sample Nos. 4, 5 and 13 use the same welding wire. However, sample No. 13, where carbon dioxide is used as a shielding gas, has an increased penetration at the welded portion, which causes ah increase of dilution of the basic material, and thus, the final components/composition of the welded metal fall out of the range defined by the present invention. [0097] Sample Nos. 14 and 15 use a welding wire for ordinary steel. Needless to say, the weld metal is lacking in Cr and Ni. Therefore, the corrosion resistance is poor and red rust forms at an early stage. This weld metal has ferrite rich welding components of soft steel. Thus, although the conditions of the present invention are not met, brittleness cracks do not occur. [0098] Sample Nos. 3-1 1 and Sample No. 13 use flux-cored wire as a welding material. In these samples, solidified slag from welding covers the surface of the welded metal to inhibit oxidization, which enables maintenance of a metallic luster. Table I (Table Removed) Table 2 (Table Removed) Table 3 (Table Removed) [0102] All cited patents, publications, copending applications, and provisional applications referred to in this application are herein incorporated by reference. [0103] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. CLAIMS 1. A weld joint for a zinc-based alloy coated steel sheet, comprising a welded portion of weld metal made of stainless steel-based components, the weld metal 1 in mass percent . C: 0.01-0.1; Si: 0.1-1; Mn: 0.5- 2.5; Ni: 5-11; and Cr: 17-25, and the balance being iron and residual impurities, wherein the following expressions (1), (2) and (3) are met; -0.8IxCr equivalent +23.2 < Ni equivalent < 0.95.\Cr equivalent - 8.1 ...(1) Ni equivalent = Ni + 30xC+0.5xMn + 30xN ...(2) . Cr equivalent = Cr +Mo + 1.5xSi .-.(3) wherein each of each of Ni, C, Mn, N, Cr, Mo and Si represents content (mass %) of each component element contained in the stainless steel-based weld metal. 2. The weld joint according to claim 1, wherein the stainless steel-based weld metal further comprises in mass (%): Mo: 0.5-2; and N: 0.05-0.15, . wherein each of Mo and N represents content (mass %) of each component element contained in the stainless steel-based weld metal. 3. The weld joint according to claim 1 or 2, wherein the composition of the stainless steel-based weld metal is adjusted by a solid wire or a flux-cored wire, wherein the solid wire or the flux-cored wire comprises the following metal components in mass (%) per total wire mass: C: 0.01-0.05; Si: 0.1-1; Mn: 0.5- 3; Ni: 7-12; Cr: 24-30; and at least one of Mo: not greater 2, or N: 0.17, and the balance being iron and residual impurities wherein each of Ni, C, Mn, Cr, Si, Mo and N represents content (mass %) of each component element contained in the solid wire or the flux-cored wire. 4. The weld joint according to claim 3, wherein the flux-cored wire comprises the following as slag component in mass (%) per total wire mass: Ti02: 0.5-2.5; SiCb: 1.5-3.5; and ZrCb: 0.5-2.5, wherein total amount of the slag component ranges from 6.5 to 9.5. 5. The weld joint according to claim 1, wherein the zinc-based alloy coating of the zinc-based alloy coated steel sheet comprises the following in mass (%) with the balance being zinc and residual impurities: AI: 2-19; Mg: 1-10; and Si: 0.01-2. 6. The weld joint according to claim 1, wherein the weld joint is suitable for welding zinc-based alloy coated steel sheets having a thickness of 3mm or more, wherein a shape of the weld joint is a fillet weld joint. 7. A stainless steel-based weld metal composition comprising, in mass percent (%), the following: C: 0.01-0.1; Si: 0.1-1; Mn: 0.5-2.5; Ni: 5-11: and Cr: 17-25, and the balance being iron and residual impurities, wherein the following expressions (1), (2) and (3) are met; -0.81xCr equivalent +23.2