The present invention relates to a method of production of a welded structure, more particularly relates for example to a method of production of a wel.ded 10 structure such as a tailored blank material which uses butt laser welding of steel sheets. Background Art [0002l In the field of steel sheet for automobile use, to lighten the weight for improving fuel efficiency and 15 to improve the impact resistance, application of tailored blanking has been spreading. "Tailored blanking" means the process of press-forming a sheet member comprised of a plurality of metal sheets differing in material, thickness, tensile strength, etc. joined by butt welding 2 0 (below, "tailored blank material") into a desired shape. For the butt welding when producing the tailored blank material, laser welding is generally used. [00031 PLTs 1 to 3 disclose methods of butt welding using laser welding so as to produce a tailored blank 25 material. Citation List Patent Literature [0004] PLT 1. Japanese Patent Publication No. 2005- 21968A 30 PLT 2. Japanese Patent Publication No. 2006-187811A PLT 3. Japanese Patent Publication No. 2007-237216A Summary of Invention Technical Problem [0005] In the past, as the thickness of the metal 35 sheets used for producing a tailored blank material, 0.7 mrn or so was the lower limit. In PLTs 1 to 3 as well, the thicknesses of the steel sheets disclosed in detail in the examples were as thin as 0.7 rnm. [0006] To further lighten the weight of automobiles, reducing the thickness of the metal sheets used when producing a tailored blank material has been studied. Due 5 to this, application of thinner steel sheets which have not been used for the production of tailored blank materials and for which welding techniques have not been studied has become desired. [0007] The inventors engaged in studies for 10 application of thin steel sheets to tailored blank materials. As a result, they clarified that if using steel sheets of a thickness of 0.6 mm or less on an actual production line for butting welding by laser welding, even if the gap between steel sheets is a small 15 0.1 mm or less (including case of no gap), the problem arises of a through hole being formed at the melt part. However, PLTs 1 to 3 relating to tailored blank materials using mainly thickness 0.7 mm or more steel sheets do not report the problem of a through hole. That is, means for 2 0 dealing with this have not yet been studied. 100081 The present invention was made to solve the above problems and has as its object the provision of a method of production of a welded structure enabling prevention of the formation of a through hole at a melt 2 5 part even when butt welding a plurality of steel sheets including thickness 0.6 mm or less steel sheets. Solution to Problem [0009] The inventors etc. engaged in intensive studies on the factors influencing the formation of a through 30 hole at a melt part. As a result, they obtained the following findings: [OOlO] A through hole is formed at a melt part due to a disturbance of the flow in the melt pool at the time of welding. The stability of flow of a melt pool is greatly 35 influenced by the type of the assist gas. [ 0011 1 When performing laser welding, the lower limit of laser output not resulting in insufficient heat input is made the lower limit output, the upper limit of the laser output where the energy becomes excessive and a through hole is formed is made the upper limit output, and the range between the lower limit output and upper 5 limit output is made the suitable output range. When using assist gas comprised of the generally used Ar gas, suitable butt welding cannot be performed no matter what the laser output. [0012] When not using any assist gas at all, butt 10 welding is possible. However, sometimes the flow in the melt pool is disturbed, so a through hole sometimes forms. Stable control in actual operation is difficult. [0013] When using an assist gas comprised of a mixed gas containing a suitable flow rate of O2 gas, the 15 formation of a through hole can be suppressed. The mechanism behind the suppression of formation of a through hole by 02 gas is not clear, but can be guessed to be due to the following factors. [0014] When supplying a mixed gas containing 02 gas to 2 0 a melt part, compared with when blowing Ar gas, the tension at the back surface of the molten metal falls. For this reason, a fine, sharp shape of a keyhole is stably formed. Due to this, a flow with little disturbance is obtained and as a result, it is 2 5 considered, formation of a through hole can be stably suppressed. COO151 On the other hand, when using an assist gas comprised of Ar gas, it is considered that the tension at the back surface of the molten metal is large, a thick, 30 rounded shape of keyhole is formed, and the keyhole extends backward in the welding direction, then breaks and remains as a through hole. [0016] The present invention was made based on the above findings and has as its gist the following method 35 of production of a welded structure. COO171 (1) A method of production of a welded structure by butt welding two steel sheets by laser welding while supplying assist gas to a melt pool surface characterized in that a thickness of at least one steel sheet of the two steel sheets is 0.6 mm or less, the assist gas is a mixed gas containing 10 to 50 vol% of O2 gas, and, when an 0 2 concentration in the assist gas is C (vol%), a flow rate L of assist gas (liter/min) is 30- C10. [00181 (2) The method of production of a welded structure according to (11, wherein when a welding speed is V (m/min), an average thickness of two steel sheets is "t" (rnrn), and a spot area of the laser is A (mm2), an output of the laser is 1.42xVxtxA (kW) to 1.83xVxtxA (kW). [0019] (3) The method of production of a welded structure according to (1) or (2), wherein a front surface melt width of a melt part of the two steel sheets is made 2.3 times or less a thickness of a thinner steel sheet among the two steel sheets and a back surface melt width is made 0.5 to 1.2 times the melt width of the front surface. [0020] (4) The method of production of a welded structure according to any one of (1) to (3), wherein an 0 2 concentration in the assist gas is 15 to 30 ~01%. [0021] (5) The method of production of a welded structure according to any one of (1) to (4), wherein the assist gas is air. 100221 (6) The method of production of a welded structure according to any one of (1) to (5), wherein the assist gas is supplied to the melt pool surface so as to flow in a direction opposite to a weld advancing direction. [00231 (7) The method of production of a welded structure according to (6), wherein the assist gas is supplied so as to intersect the melt pool surface. [0024] (8) The method of production of a welded structure according to (61, wherein the assist gas is supplied so as to intersect the steel sheet in front of the melt pool in the weld advancing direction. 100251 (9) The method of production of a welded structure according to any one of (1) to (8), wherein the welded structure is a tailored blank material. Advantageous Effects of Invention 100261 According to the present invention, even when butt welding a plurality of steel sheets including a thickness 0.6 mrn or less steel sheet, it becomes possible to prevent the formation of a through hole at the melt part. Therefore, the welding method according to the present invention can be suitably used for production of a welded structure, in particular a tailored blank material. Brief Description of Drawings [0027] FIG. 1 is a view showing one example of a welding method according to the present invention. FIG. 2 is a view showing another example of a welding method according to the present invention. FIG. 3 is a view showing another example of a welding method according to the present invention. FIG. 4 is a view showing another example of a welding method according to the present invention. FIG. 5 is a view showing a front surface melt width and a back surface melt width of a welded structure. Description of Embodiments [0028] The method of production of a welded structure of the present invention is one which supplies an apsist gas to a melt pool surface while butting welding two steel sheets by laser welding. Below, the requirements of the present invention will be explained in detail. [0029] (A) Supply of Assist Gas In the method of production of a welded structure of the present invention, as the assist gas, a mixed gas containing 02 gas is supplied to the melt pool surface. To stabilize the flow of the melt pool and prevent the formation of a through hole at the melt part, it is necessary to make the content of the 0 2 gas in the mixed gas 10 to 50 ~01%. [0030] If the content of O2 gas is high, the oxidation reaction becomes excessive and oxides discharged as slag become greater. As a result, the weld metal is liable to become recessed in shape. Therefore, the content of the O2 5 gas in the mixed gas is preferably 30 vol% or less. [00311 If the content of the 02 gas is too low, a through hole will easily form. Furthermore, to stably broaden the melt width at the back surface side after welding, the content of the 02 gas in the mixed gas is 10 preferably 15 vol% or more. 100321 The gas ingredients other than O2 js not particularly limited. Ar, He, or other inertgas or N2 gas, CO* gas, compressed air, etc. may be suitably used. 100331 Further, as the assist gas, air containing 02 15 gas in 21 vol% or so may be used. If using an assist gas comprised of air, the manufacturing cost can be kept low. 100341 The supplying means of the assist gas is not particularly limited. For example, a usual nozzle having an ejection opening able to eject mixed gas in a 2 0 predetermined direction may be used. The type of the nozzle is also not particularly limited. For example, a flat nozzle having a rectangular shaped ejection opening or a round pipe nozzle using a circular cross-section pipe etc. may be mentioned. 2 5 [0035] To prevent a through hole from forminy in the melt part, it is preferable to supply the assist gag so that the weld metal is pushed to flow backward in the weld advancing direction. That is, at the melt pool surface, the assist gas ejected from the supplying means 30 is preferably supplied so as to flow in a direction opposite to the weld advancing direction. COO361 To achieve the above object, the assist gas is preferably supplied by directly blowing it toward the melt pool surface. That is, the ejection position and 35 direction of the assist gas are preferably adjusted so that the centerline of the assist gas ejected from the supplying means intersects the melt pool surface. [0037] To remove foreign matter etc. present at the abutted part of the steel sheets at the front of the melt pool in the weld advancing direction, the ejection position, direction and strength of the mixed gas may be 5 adjusted so that the mixed gas blown on the steel sheets at the front of.the melt pool in the weld advancing direction and reflected at the steel sheet back surface is supplied to the melt pool surface. That is, the supplying means may be placed so that the centerline of 10 the assist gas ejected from the supplying means intersects the steel sheets in front of the melt pool in the weld advancing direction. [00381 FIGS. 1 to 4 are views for explaining one example of a welding method according to the present 15 invention. When using the laser 11 fired from the welding head 1 to weld the steel sheets 2, the supplying means 3 is used to supply the mixed gas 12. In FIG. 1, the supplying means 3 set in front of the welding head 12 in the weld advancing direction A directly blows assist gas 2 0 12 comprised of mixed gas to the melt pool 13. [0039] In FIG. 2, in the same way as FIG. 1, a supplying means 3 is set in front of the welding head 1, but it is also possible to blow assist gas 12 toward the steel sheets 2 in front of the melt pool. The position of 2 5 the supplying means 3 is adjusted so that the assist gas 12 reflected at the back surface of the steel sheet 2 is supplied to the back surface of the melt pool 13. [0040] As shown in FIG. 3, it is possible to place the supplying means 3 in front of the welding head 1, blow 3 0 the assist gas 12 toward the front, then adjust the ejection position and ejection strength of the supplying means 3 so that the assist gas 12 reflected at the back surface of the steel sheets 2 is supplied to the back surface of the melt pool 13. By strongly blowing the 3 5 mixed gas 12 toward the front of the weld advancing direction A from the melt pool 13, it becomes possible to remove the rust preventive oil, slag, spatter, etc. deposited at the abutted parts 14. [0041] As shown in FIG. 4, it is also possible to place the supplying means 3 behind the welding head 1, blow the mixed gas 12 toward the front, then adjust the 5 ejection position and ejection strength of the supplying means 3 so that the mixed gas 12 reflected at the back surface of the steel sheets 2 is supplied to the back surface of the melt pool 13. 100421 The flow rate L (liter/min) of the assist gas 10 is made to satisfy LtlO and 30-C10 and 30-C