LASER WELDED JOINT, VEHICLE COMPONENT, MANUFACTURING METHOD OF LASER WELDED JOINT, AND MANUFACTURING METHOD OF VEHICLE COMPONENT [Technical Field of the Invention] [0001] The present invention relates to a laser welded joint, a vehicle component, a manufacturing method of the laser welded joint, and a manufacturing method of the vehicle component. Priority is dairi1ed on Japanese Patent Application No. 2014-234957, tiled on November 19,2014, the content of which is incorporated herein by reference. [Related Art] [0002] Recently, in an automobile field, a need of using a high-strength steel sheet as a member for a frame, a member for a chassis, and a member for a panel has been increased for reducing the weight of a vehicle body and improving collision safety. Thus, a high-strength steel sheet having formability similar to that of a low-strength steel sheet in the related art has been developed, and has been practically used for a vehicle body for an automobile (simply referred to as a vehicle body below). [0003] In the related art, a vehicle body is assembled in a manner that a plurality of components obtained by performing press forming of a steel sheet is joined to each other by spot welding or arc welding. Recently, in order to reduce the number of components for a vehicle body and to more reduce the weight of the vehicle body, tailored blank (for example, see Non-Patent Document 1) has been used when a - 1 - vehicle body is manufactured. The tailored blank refers to a technology of performing press forming of a sheet material (also referred to as "a tailored blank material" below) so as to have a desired shape. The tailored blank material is obtained ina manner that a plurality of metal sheets which are different from each other in material, sheet thickness, tensile strength, and the like is butted and welded so as to be integrated. Generally, laser welding is used for butting and welding when a tailored blank material is manufactured. [0004] However, there is a problem in that, if press forming is performed on a . ';. tailored blank material obtained by laser-welding high-strerigth steel sheets; in , . accordance with precise drawing and bending, a crack occurs in weld metal. NonPatent Document 1 discloses that, in a tailored blank material manufactured by laser welding, stress may be concentrated at a starting portion and an ending portion for welding due to shrinkage, and thus the tailored blank material may be broken. [0005] Non-Patent Document 2 discloses that, regarding arc welding, in a case where hydrogen exists in weld metal, hydrogen is collected at a stress concentrated portion generated by welding, and thus so-called delayed fracture occurs. It is known that the delayed fracture can be suppressed by preheating before welding or heat treatment after welding. [0006] Patent Docwnents 1 to 3 disclose a technology for preventing an occurrence of a crack in a weld joint. Specifically, Patent Document 1 discloses that concentration of hydrogen of weld metal in a weld steel pipe obtained by arc-welding a butting portion is set to be equal to or less than a predetermined value, and thus the - 2 - occurrence of a crack by hydrogen embrittlement when pipe expansion or pipe contraction is corrected can be suppressed. [0007] Patent Document 2 discloses that, in weld metal formed by gas shielded arc welding using a flux-cored wire, a chemical composition is controlled to be in a predetermined range, and number density and volume fraction of residual austenite particles are controlled to be equal to or more than predetermined values, and thus hydrogen embrittlement resistance is improved. [0008] l'atent Doeurheilt3·:hscloses that lhe voluihe of a porosity at the.cciltral part in a thickness direction of a slab is reduced, and thus it is possible to improve hydrogen-induced crack resistance performance of a steel sheet obtained from the slab. [Prior Art Document] [Patent Document] [0009] [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2006-263814 [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2012-176434 [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2007-136496 [Non-Patent Document] [0010] [Non-Patent Document 1] "Welding Methods and Forming Characteristics of Tailored Blanks (TBs)", Sinnittetsu giho (378), p.35-39, 2003 - 3 - '' . .- ;-' ·' ,· [Non-Patent Document 2] "Joining and welding technologies Q&A I 000", Q-04-01-01, [online], The Japan Welding Engineering Society, 1999, [searched on September 24, 2014], Internet address [Non-Patent Document 3] "Porosity Formation in C02 Laser Welding of Steel Sheets", Journal of the Japan Welding Society 19(2), p.241-251, 2001 [Disclosure of the Invention] [Problems to be Solved by the Invention] [0011] Ina tailoreli blank matenal as disclosed in Non-Patent Document l;':r'; :,· •· phenomenon as follows is considered as a cause of a crack occurring in weld metal. As described above, in a tailored blank material manufactured by laser welding, shrinkage occurs at a starting portion and an ending portion for welding. It is considered that the shrinkage causes residual stress to occur at the starting portion or the ending portion, and a crack occurs. An insufficient sheet thickness of a laserwelded portion (weld bead portion), which occurs when optimizing laser welding conditions or butting conditions of metal sheet is not possible is also considered as a cause of a crack occurring. However, a crack may occur even when beads are apparently formed well, and describing the cause of a crack occurring is not possible only based on the residual stress and the insufficient sheet thickness of a laser-welded portion. Weld metal at the laser-welded portion is rapidly cooled after welding. Thus, a hard martensite structure having low ductility is generated in the weld metal. Thus, ductility of the weld metal is degraded. The degradation of ductility may be the cause of a crack occurring at a time of forming. However, even in weld metal having the same hardness, a crack may or may not occur at a time of forming, and the cause - 4 - .,,·-, ·,· thereof is not clarified yet. [0012] As described above, delayed fracture disclosed in Non-Patent Document 2 occurs by collecting hydrogen, and can be suppressed by preheating before welding or heat treatment after welding. However, a crack occurring in weld metal when a tailored blank material is press-formed occurs during formation for a short time (within several seconds), and is considered to occur by a cause separate from the cause of delayed fracture occurring by collecting hydrogen. Thus, it is difficult that the occurrence of a crack of weld metal of a tailored blank material is prevent only by . preheating before welding or heat treatment after welding." · [0013] In the technology of Patent Document 1, in a welded steel pipe obtained by arc welding, the occurrence of a crack by hydrogen embrittlement when pipe expansion or pipe contraction is corrected can be suppressed. However, in a forming type of large deformation with drawing and bending for a short time, as in press forming of a thin steel sheet, an influence of hydrogen on a crack in weld metal is hardly clarified. Laser welding is different from arc welding from a point of a welding mechanism and a welding atmosphere. The cause of a crack occurring in weld metal is quantitatively hardly clarified in addition to the content of hydrogen. [0014] In the technology of Patent Document 2, in weld metal formed by using a flux -cored wire, number density and volume fraction of residual austenite particles are controlled to be equal to or more than predetermined values, and thus hydrogen embrittlement resistance of the weld metal is improved. However, in manufacturing a laser welded joint as a material of a vehicle body, generally, a wire is not used for - 5 - '·· suppressing an increase of manufacturing cost. In a case where a wire is not used at a time of welding, a structure and a configuration of weld metal are determined in accordance a chemical composition of a material to be welded. Thus, it is difficult that the number density and the volume fraction of residual austenite particles are controlled as in Patent Document 2~ Further, in laser welding, a material is melted, and then weld metal is rapidly cooled. Because of this, it is also _substantially difficult that the number density and the volume fraction of residual austenite particles .are controlled. [0015] In the tedmology ofl'atent Document 3, volume·tri;·anthhrui · . ·' : internal pressure is increased, and a crack occurs in this porosity as a starting point when weld metal is plastic-deformed during press forming. Thus, in order to suppress the occurrence of a crack in weld metal when press forming is performed, it is important to control distribution density of porosities having a diameter of 2 fllll to 50 fllll. Thus, in the weld joint according to the embodiment, distribution density of porosities having a diameter of 2 fllll to 50 fliTl in weld metal is controlled to be equal to or less than 5.0 pieces/mm2 , preferably equal to or less than 4.0 pieces/mm2 , and further preferably equal to or less than 3.0 pieces/mm2 . Thus, the occurrence of a crack when press forming is performed is suppressed. A lower limit value of the distribution density of porosities of 2 flm or larger in weld metal is not particularly limited. Since an effect is also saturated when the lower limit value is set to be less than 0.01 pieces/mm2 , the lower limit value may be set to 0.01 pieces/mm2 [0032] - 14 - (I ij I t , ·~ • . ! ' The distribution density of porosities is obtained in a manner as follows. Firstly, weld metal is cut in a direction vertical to an elongation direction of a welding line and a cross section obtained by the cutting is subjected to mirror polishing. The cross section subjected to mirror polishing is observed by a scanning electron microscope (SEM). The observation is performed at a magnification of 2000 or more, and at least a region of 250000 fJm2 or larger in one cross section is observed. Then, the number of porosities having a diameter of2 fJm to 50 J.Lm is counted. Similar . observation is performed on two cross sections or more which are different from the above-described cross section. The obtained number of porosities is divided by an observation area, am} a valueobtained by the division is set as distribution density or porosities. An observation position in the cross section is not particularly designated. However, it is assumed that a region as large as possible in a site which is obviously determined as weld metal is observed. In a case where a boundary between a steel sheet and a weld metal portion after mirror polishing is not clear, it is desired that etching is performed in advance, and a boundary is marked, and thus the weld metal portion is caused to be in a state where the weld metal portion can be obviously determined. The diameter of a porosity is obtained in a manner that an area of the porosity is obtained, and the obtained area is converted into an equivalent circle diameter. [0033] 3. Distribution density of oxide inclusions having a diameter of3 J.Lm or more in weld metal: 0.1 to 8.0 pieces/mm2 An oxide inclusion having a diameter of 3 J.Lm or more functions as a trap site in which diffusible hydrogen in a metal lattice of weld metal just after welding is trapped. Thus, the oxide inclusion can suppress flowing of diffusible hydrogen into a - 15 - ~-~~-~~~~~~=~~-~~-~----'"--~· ~~~~ porosity. Thus, internal pressure by non-diffusible hydrogen in a porosity can be controlled, and thus it is possible to prevent the occurrence of a crack in weld metal when press forming is performed. In order to obtain this effect, in the weld joint according to the embodiment, distribution density of oxide inclusions having a diameter of 3 f.!m or more is set to be equal to or more than 0.1 pieces/mm2 . The distribution density of oxide inclusions having a diameter of 3 f.!m or more is preferably equal to or more than 0.2 pieces/mm2 , and further preferably equal to or more than 0.3 pieces/mm2 • In a case where the distribution density of oxide inclusions having a diameter , ' uLi filll onn:ore inweld·11letal is more than 8.0 pieces/mm2;· a orackrrlay ocm.tF:'rt the oxide inclusion as a starting point. Accordingly, in the weld joint according to the embodiment, the distribution density of oxide inclusions having a diameter of 3 J.!m or more in weld metal is set to be equal to or less than 8.0 pieces/mm2 , preferably equal to or less than 6.0 pieces/mm2 , and further preferably equal to or less than 4.0 pieces/~. The oxide inclusion is not particularly limited, and includes oxide which is represented as alumina and contains AI as the main component; oxide which contains Si or Mn as the main component; oxide and oxysulfide which contain Mg as the main component; oxide which contains Ti as the main component; oxide and oxysulfide which contains Ca as the main component; oxide and oxysulfide which contain REM (La, Ce, and the like) as the main component; oxide which contains plural types of the above-described elements like (Mg, Ti, AI) oxide; and the like. [0034] The distribution density of oxide inclusions is obtained in a manner as follows. Firstly, weld metal is cut in a direction vertical to an elongation direction of a welding line and a cross section obtained by the cutting is subjected to mirror polishing. - 16 - The cross section subjected to mirror polishing is observed by a scanning electron microscope (SEM). The observation is performed at a magnification of 2000 or more, and at least a region of 250000 11m2 or larger in one cross section is observed. Then, the number of oxide inclusions having a diameter of} 11m or more is counted. Similar observation is performed on two cross sections or more which are different from the above-described cross section. The obtained number of oxide inclusions is divided by an observation area, and a value obtained by the division is set as distribution density of oxide inclusions. The diameter of an oxide inclusion is obtained in a manner that an area of the oxide inclusion is obfuined, and the obtained area i's converted into arr equivafe1'11i circle diameter. Because inclusions other than oxide inclusions are provided in weld metal, element analysis is performed by using EDS (energy dispersive X-ray spectroscopy) or WDS (wavelength dispersive X-ray spectrometry) which is mounted in a SEM. Thus, distinguishment is performed. When distinguishment between a porosity and an inclusion is performed, EDS or WDS is also preferably used. [0035] 4. Chemical composition of weld metal A chemical composition of weld metal of the weld joint according to the embodiment and a reason of limiting the composition will be described below. [0036] C: 0.05 to 0.30% C is an element which is solid-dissolved in weld metal during welding, and has an influence on hardness and a metallographic structure of the weld metal, and on viscosity of a weld pool when laser welding is performed. . The weld metal is rapidly - 17 - cooled after being molten by laser welding. Thus, the weld metal easily has a martensite structure, and hardness of the weld metal strongly depends on a content of C. If the content ofC is less than 0.05%, it is difficult to set hardness of weld metal to be equal to or more than 350 in I-IV. As described above, in the present invention, a weld joint in which average hardness of weld metal is equal to or more than 3 50 in I-IV is set as a target. Accordingly, the content of C is set to equal to or more than 0.05%. From such a viewpoint, the content of C may be set to be equal to or more than 0.10%, 0.15%, or 0.20%. If the content ofC in weld metal is more than 0.30%, the hardness of the weld mstal easily beco'mes more than 540 in HV and a crack ·ea~ily oecm:s'in 'the ·weld metal when press forming is performed. Accordingly, the content of C is set to equal to or less than 0.30%. From such a viewpoint, the content of C is preferably equal to or less than 0.25%, more preferably equal to or less than 0.20%, and more preferably equal to or less than 0.15 %. [0037] Si: 0.005 to 3.0% Si has an etl'ect of controlling phase transformation so as to control components of a metallographic structure of a steel sheet. In addition, Si has an influence on an amount of diffusible hydrogen in weld metal and on formation of a porosity. Thus, Si is important for controlling the occurrence of a crack in weld metal when press forming is performed. If a content of Si in weld metal is more than 3. 0%, the reason is not clear, but an amount of diffusible hydrogen inserted in metal lattice during welding is increased. Thus, a crack easily occurs in weld metal when press forming is performed. Further, as the amount of Si is increased, fluidity of a molten portion at a time of welding is increased, and an amount of fine porosities is decreased. - 18 - . , The reason is not clear, but, the amount of porosities is reversed to have a tendency of an increase, in a case which is contrary to a case where the content of Si is more than 3.0. Accordingly, the total content of Si is set to be equal to or less than 3.0%. In a case of being considered from such a viewpoint, the total content of Si is preferably equal to or less than 2.3%, more preferably equal to or less than 2.0%, and more preferably equal to or less than I. 7%. If the content of Si is less than 0.005%, oxide in weld metal is increased, and thus a crack may occur when press forming is performed. Thus, the content of Si is equal to or more than 0.005%, more preferably equal to or more than 0.01 %, and more preferably equal to or more than O.O:.i% . ,. [0038] AI: 0.005 to 1.0% Similar to Si, AI also has an effect of controlling phase transformation so as to control components of a metallographic structure of a steel sheet. In addition, AI has an influence on an amount of diffusible hydrogen in weld metal, and thus has an influence on an occurrence motion of a crack in the weld metal when press forming is performed. If a content of AI in the weld metal is more than 1.0%, the reason is not clear, but an amount of diffusible hydrogen inserted in metal lattice during welding tends to be increased. Thus, a crack easily occurs in weld metal when press forming is performed. Accordingly, the content of AI is set to be equal to or less than 1.0%. In a case of being considered from such a viewpoint, the total content of Al is preferably equal to or less than 0.8%, more preferably equal to or less than 0.6%, and more preferably equal to or less than 0.4%. The content of AI may be equal to or more than 0.005%. However, if the content of A1 is less than 0.005%, oxide in weld metal is increased, and thus a crack - 19 - may occur when press forming is performed. In a case of being considered from such a viewpoint, the content of AI is preferably equal to or more than 0.005%, more preferably equal to or more than 0.1 %, and more preferably equal to or more than 0.5%. [0039] Mn: 0.5 to 6.0% Mn is an element which is contained in a steel sheet in order to control a metallographic structure, and, as a result, is contained in weld metal. Jf a content of Mn is less than 0.5%, hardenability is largely degraded. Thus, even though a large amount of C is contained, it is difficult to stably se~ hardness of weld metal to be equal to or more than 350 in: HV. In the present mventiori; a weld joirit in which a'Ver-agb'~ · hardness of weld metal is equal to or more than 350 in HV is set as a target. Thus, the content ofMn is set to be equal to or more than 0.5%, preferably equal to or more than 1.0%, and more preferably equal to or more than 1.5%. If the content of Mn in weld metal is more than 6. 0%, weld metal is enbrittled, and thus a crack may occur in the weld metal. Accordingly, the content ofMn is set to be equal to or less than 6.0%, preferably equal to or less than 4.0%, and more preferably equal to or less than 2.0%. [0040] P: more than 0% and 0.04% or less P may be used for ensuring strength of a steel sheet constituting a joint. However, P is an element which enbrittles a welded portion. Thus, if a content of P is more than 0.04%, a crack occurs regardless of control of distribution of porosities or the amount of diffusible hydrogen. Thus, an upper limit is set to be 0.04%, and preferably 0.03%. A lower limit has any value as long as the limit is more than 0. However, - 20 - ---- - ---, -·-:-c-.-~--' ' because, if the lower limit is too low, manufacturing cost such as refining cost is increased, the lower limit may be set to 0.0001%. [0041] S:more than 0% and 0.01% or less Sis an element which allows fluidity of weld metal (melted metal) in welding to be increased, and allows an amount of porosities to be decreased. However, S is an element which enbrittles a welded portion. If a contentof S is more than 0.01 %, a crack occurs regardless of control of distribution of porosities or distribution density of oxide inclusions. Thus, an upper limit is set to 0.01 %. Alower liinit has any value as long as the limit is more thliti 0: ·'However, because, if the lower limit is too low, manufacturing cost such as refining cost is increased, the lower limit may be set to 0.0001%. [0042] N: more than 0% and 0.01% or less N is an element used for ensuring strength of a steel sheet constituting a joint, and has an effect of reducing a grain size of weld metaL However, if an amount ofN is more than 0.01 %, for example, coarse nitride in weld metal is formed, and thus a tendency of enbrittlement becomes stronger. Accordingly, an upper limit thereof is set to 0.01 %. A lower limit has any value as long as the limit is more than 0. However, because, if the lower limit is too low, manufacturing cost such as refining cost is increased, the lower limit may be set to 0.0001%. [0043] 0: more than 0% and 0.01% or less 0 is an element having an influence on distribution of oxide inclusions in weld metaL If a content is more than 0.01 %, density of oxide inclusions is increased, - 21 - and thus a crack for propagating oxide inclusions during press forming is caused to occur. Thus, an upper limit is set to 0.01%: [0044] Weld pool fluidity index a: 0.3 to 3.0 C, Si, and S are elements having an influence on fluidity of a weld pool (melted metal) in welding. Specifically, as the contents of Si and S are increased, fluidity of a weld pool is improved. As the content of C is reduced, fluidity of.a weld pool is improved. In the weld joint according to the embodiment, C, Si, and S control · co!'nponcnts ofwel7000xC-400 .. ·Expression (6) t (min)2>8000xC-400 ···Expression(7) ·, '. ,. . ·.·• ~ ·r ·.,~·. Even though the amount of diffusible hydrogen in weld metal is reduced, a crack may occur in weld metal when press forming is performed, for a period until the retention time (min) of a weld joint in a temperature range of 1 0°C to 1 00°C reaches 60 seconds, after laser welding is ended and before press forming is performed. The reason is not clear, but it is supposed as follows. For a period until the retention time reaches 60 seconds, concentration of hydrogen in weld metal is unevenly distributed. Thus, even though an average value of the amount of diffusible hydrogen is reduced, a region in which the concentration of hydrogen is high may be locally provided. Thus, the retention timet may have a lower limit which is set to 60 seconds, preferably 100 seconds, more preferably 180 seconds, in addition to setting of the lower limit value by the above Expression ( 6) or (7). [0070] Conditions other than the above-described welding conditions are not - 34 - ' - ~ - I , ' •' • ' particularly limited. However, it is known that various conditions such as a gap between steel sheets in welding, an amount of laser beams which are out of a focal point, and a pulse width of laser have an influence on formation of a porosity. Thus, the various .conditions are appropriately set in accordance with the type, an output, and the like of laser to be used. The type of a laser oscillator is also not particularly limited. For example, an oscillator of fiber laser, YAG laser, disc laser, semiconductor laser, carbon dioxide gas laser (C02 laser), and the like can be used. A plurality of steel sheets may be joined to each other by so-called laser-arc hybrid welding using a welding wire. Combination of a sheet thickness is also not particularly limited; ·· However, if a difference irr sheet thiekness:between steel sheets to be welded is more than 2 rnrn, strain is easily concentrated in weld metal, and a crack easily occurs. Thus, a difference in sheet thickness between steel sheets to be welded is preferably equal to or less than 2 mm. Examples [0071] The present invention will be more specifically described below by using examples. However, the present invention is not limited to the examples. [0072] Firstly, each of steel sheets A to Z which had a chemical composition and tensile strength shown in the following Table 1, a width of 25 mm, and a length of 250 rnrn was prepared. A plurality of steel sheets which corresponded to each of the steel sheets A to Z was prepared. Regarding the steel sheet I, steel sheets having three types of sheet thicknesses (1.0 mm, 1.2 rnrn, and 1.6 mm) were prepared, and a sheet thickness of other steel sheets was set to 1.2 rnrn. The steel sheet F is a plated steel sheet (GA) obtained by performing alloy hot-dip galvannealing on a cold-rolled steel - 35 - sheet. The steel sheet J is a plated steel sheet (GI) obtained by performing hot-dip galvanizing on a cold-rolled steel sheet. Other steel sheets are cold-rolled steel sheets (CR). ' .. -._,- ··,, . . ~' .. , - 36 - :: i_j I i'i 1:: i:;i I! ,::,.: ll• ~! ,, ' r-: ::1 i (:1 (! :i ,-I ·i :< :-I !:: ~:· [0073] [Table 1] Steel sheet Chemical composition (mass%) Symbol Type X c Si AI Mn Ni Cr Mo p s A CR O.Dl5 0.13 0.01 0.7 O.Dl 0.01 - 0.03 0.0006 B CR 0.06 0.01 0.04 1.5 0.01 0.01 - 0.03 0.0032 c CR 0.08 0.50 0.03 2.0 0.01 O.Dl - 0.03 0.0026 D CR 0.10 0.05 0.03 1.4 0.01 0.01 - 0.03 0.0022 E CR 0.10 0.50 0.03 2.0 0.01 0.3 - 0.03 0.0028 F GA 0.10 0.32 0.01 2.0 0.01 0.3 - 0.03 0.0014 G CR 0.11 0.10 0.03 1.5 0.01 0.01 - 0.04 0,0033 H CR 0.15 0.38 0.05 1.9 0.40 0.01 0.02 0,0022 I CR 0.20 1.30 0.10 2.1 0.01 0.01 0.1 0.02 0.0015 J GI 0.20 1.30 0.10 2.1 O.Dl 0.01 0.1 0.02 0.0015 K CR 0.20 1.50 0.33 5.0 O.Dl 0.3 - 0.02 0.0008 L CR 0.20 1.50 0.33 7.0 0.01 0.01 - 0.02 0.0008 M CR 0.20 0.45 0.05 1.2 9.0 0.01 - 0.02 0.0025 N CR 0.20 0.45 0.05 1.2 11.0 0.01 - 0.02 0.0025 0 CR 0.20 3.20 0.04 2.4 0.01 0.01 - 0.02 0.0018 p CR 0.23 1.50 0.03 2.0 0.10 O.Dl - 0.02 0.0012 Q CR 0.27 1.50 0.10 2.3 0.10 0.01 - 0.02 0.0009 R CR 0.30 1.80 0.10 2.2 0.01 0.01 0.2 0.02 0.0029 s CR 0.40 1.30 0.40 2.0 0.01 0.2 - 0.02 0.0005 T CR 0.42 1.65 0.45 4.0 0.01 0.01 - 0.02 0.0015 u CR 0.40 1.30 0.40 2.0 0.01 0.2 - 0.02 0.0025 v CR 0.40 1.30 0.40 2.0 0.01 0.2 - 0.02 0,024 w CR 0.40 1.30 0.40 2.0 0.01 0.2 - 0.18 0.0005 X CR 0.40 1.30 0.40 2.0 0.01 0.2 - 0.02 0.0005 - y CR 0.40 1.30 0.40 2.0 0.01 0.2 - 0.02 0.0005 z CR 0.40 1.30 0.40 2.0 0.01 0.2 - 0.02 0.0005 (x) CR: cold-rolled steel sheet, GI: hot-dip galvanized steel sheet, GA: galvannealed steel sheet - 37 - Remainder: Fe and impurities Tensile N 0 Others Strength (MPa) 0.002 0.005 - 310 0.002 0.003 : - 600 0.003 0.002 Cu:0.03 610 0.003 - 0.003 - 450 0.003 0.004 Ti:0.04, 8:0.001 1010 0.003 0.003 Ti:0.04, B:0.001 1010 0.003 0.003 Nb:0.04 620 0.003 0.002 - 1080 0.003 0.003 Ti:0.01 1000 0.003 0.002 Ti:0.01 1000 0.003 0.003 Ca:O.OOi, La:O.OOl, Ce:O.OOI 1350 0.002 0.004 . - 1420 0.003 0.002 - 1020 0.005 0.003 - 1160 0.003 0.003 - 1340 0.003 0.003 V:O.OJ 1090 0.003. 0.004 8:0.002 1180 0.003 0.003 Nb:0.02, Ti:0.02 1040 0.004 0.004 - 1060 0.002 0.005 - 1330 0.004 0.004 V:0.2 1050 0.004 0.004 V:0.2 1040 0.004 0.004 V:0.2 1085 0.004 0.01 1V:0.2 1045 0.004 0.02 Vo.2 1045 0018 0.004 [V:0.2 1045 [0074] Then, two steel sheets were appropriately selected from the steel sheets A to Z. The two selected steel sheets were butted and welded by using YAG laser. Thus, weld joints I to 57 which had a straight welding line at the central portion, and had a width of 50 mru and a length of 250 mru were manufactured. Then, heat treatment was performed on the weld joints 1 to 57 (at 30°C for a predetermined retention time). [0075] The following items are shown in the following Table 2 and Table 3. (a) "Combination of steel sheets" (b) "Sheet thickness (mm)" (c) "Chemical composition (mass%) of weld metal" (d) "Ms of steel sheet" (e) "Index a" ···weld pool fluidity index indicated by Si+ 200S-2. 7C - 38 - I I !I' li ]! I' ii li Joint " ,, n"" 24 12. ~ ()Ms= [0076] [Table 2] ion of I Sheet thickness S!~~J~eets __ j_mm) _A-A 1_ _12 -12 B-B ____ _l 1.2-1.2 ~ri ~-E >33xMn-17>_E_<:et -~]o I o.2L ~'-- 1. _ oA9 39• J9; 35 333S 29t 23 27 ).46 1.06 1.06 1.12 0.41 Underline indicates being excluded from requirements of the manufacturing method of weld joint in the present invention. - 39 - c;;:;c.===o==~=::::-.::::=-::::-_:_-;-,=-:=-.::---- ·. -· [0077] [Table 3] Joint Combination of Sheet thickness Chemical composition (mass%) of weld metal Remainder: Fe and impurities Ms+ of steel Index a No. steel sheets (mm) c Si AI Mn Ni c, Mo p s N 0 Others sheet+ 29 N-N 1.2~1.2 0.20 0.45 0.05 1.2 11.0 O.Dl 0.02 0.0025 0.0051 0.0028 - 239 0.41 30 0-0 1.2-1.2 0.20 3.20 0.04 2.4 O.ol 0.01 0.02 0.0018 0.0034 0.0032 387 3.02 31 P-P 1.2-1.2 0.23 1.50 0.03 2.0 0.10 0.01 0.02 0.0012. 0.0025 0.0032 V:0.03 384 1.12 32 P-P 1.2-1.2 0.23 1.50 0.03 2.0 0.10 O.Gl 0.02 0.0012 0.0025 0.0032 V:0.03 384 1.12 33 P-P 1.2-1.2 0.23 1.50 0.03 2.0 0.10 0.01 0.02 0.0012 0.0025 0.0032 V:0.03 384 1.12 34 P-P 1.2-1.2 0.23 1.50 0.03 2.0 0.10 0.01 - 0.02 0.0012 0.0025 0.0032 V:0.03 384 1.12 35 P-P 1.2-1.2 0.23 1.50 0.03 2.0 0.10 0.01 - 0.02 0.0012 0.0025 0.0032 V:0.03 384 1.12 36 Q-Q 1.2-1.2 0.27 1.50 0.10 2.3 0.10 0.01 - 0.02 0.0009 0.0031 0.0038 :0.002 355 0.95 I 37 R-R 1.2-1.2 0.32 1.80 0.10 1.2 0.01 0.01 0.2 0.02 0.0029 0 0034 0.0032 Nb:0.02, Ti:0.02 332 1.52 38 S-S 1.2-1.2 0.40 1.30 0.40 2.0 0.01 0.2 0.02 0.0005 0.0041 0.0041 - 302 0.32 39 T-T 1.2-1.2 0.42 1.65 0.45 4.0 0.01 0.01 0.02 0.0015 0.0021 0.0049 230 0.82 40 A-I 1.2-1.2 0.11 0.70 0.08 1.4 0.01 0.01 0.05 0.03 0.001i O.'J027 0.0038 Ti:8.005 462 0.62 41 A-J 1.2-1.2 0.11 0.70 0.08 1.4 0.01 0.01 0.05 0.03 0.0011 0.0025 0.0035 Ti:0.005 462 0.62 42 C-N 1.2-1.2 0.14 0.48 0.04 1.6 5.5 0.01 - 0.03 0.0026 0.0038 0.0025 Cu:0.015 348 0.62 43 C-0 1.2-1.2 0.14 1.80 0.09 2.2 0.01 0.01 - 0.03 0.0022 0.0030 0.0027 Cu:O.Ol5 422 1.86 44 C-S 1.2-1.2 0.24 1.08 0.04 2.0 0.01 0.11 - 0.03 0.001E 0.0033 0.0031 - 379 0.75 45 C-S 1.2-1.2 0.24 1.08 0.04 2.0 0.01 0.11 - 0.03 0.0016 0.0033 0.0031 - .379 0.75 46 I-T 1.2-1.2 0.31 1.48 0.08 3.1 0.01 0.01 0.05 0.02 0.0015 0.0026 0.0039 Ti:0.005 312 0.94 47 A-E 1.2-1.2 0.06 0.32 0.02 1.35 0.01 0.16 0.03 0.0017 0.0025 0.0043 Ti:0.02, B:0.0005 485 0.50 48 A-E 1.2-1.2 0.06 0.32 0.02 1.35 0.01 0.16 - O.o3 0.0017 0.0025 0.0043 Ti:0.02, B:0.0005 485 0.50 49 A-E 1.2-1.2 0.06 0.32 O.D2 1.4 0.01 0.16 0.03 0.0017 0.0025 0.0043 Ti:0.02, B:0.0005 483 0.50 50 A-F 1.2-1.2 0.06 0.23 0.01 1.4 0.01 0.16 0.03 0.0010 0.0025 0.0037 Ti;0.02, B:0.0005 483 0.27 51 D-S 1.2~1 .2 0.25 0.68 0.22 1.7 0.01 0.11 0.03 0.0014 0.0035 0.0035 . V:O.J 384 0.29 52 D-U 1.2-1.2 0.25 0.68 0.22 1. 7 0.01 0.11 0,03 0.0024 0.0034 0.0034 V:O.l 384 0.49 53 D-V 1.2-1.2 0.25 0.68 0.22 1.7 O.ot 0.11 0.03 0.0131 0.0034 0.0034 V:0.1 384 2.63 54 D-W 1.2-1.2 0.25 0.68 0.22 1.7 0.01 0.11 0.11 0.0014 0,0034 0.0034 V:O.l 384 0.29 55 D-X 1.2-1.2 0.25 0.68 0.22 1.7 0.01 0.11 0.03 0.0015 0.0034 0.0064 ' V~O.l 384 0.31 56 D-Y 1.2-1.2 0.25 0.68 0.22 1.7 0.01 0.11 0.03 0.0014 0.0034 0.0114 V:O.l 384 0.29 57 D-Z 1.2-1.2 0.25 0.68 0.22 1.7 0.01 0.11 - 0.03 0.0014 - 0.0104 0.0034 V:O.l 384 0.29 ,-+,,L _,-~1 A.-,A .. o"' --.~ ~·- 1"l .. -,." 1.-,._,..-._.-,, -~·- Underline indicates being excluded from requirements of the manufacturing method of weld joint in the present invention. - 40 - [0078] The following items are shown in the following Table 4 and Table 5. (f) "Welding rate (m/min)" (g) "Absolute humidity (g/m3)"···absolute humidity in welding atmosphere (h) "Retention time (min)"···retention time of weld joint at 30°C, after welding is ended and before press forming is performed (i) "Defined retention time (min)"···time obtained by t=7000xC-400 (j) "Amount (mass%) of oxygen in weld metal"··· amount of oxygen after welding is ended · Because a welding wire or insert metal was not used in this example, th~· · chemical composition of weld metal was substantially the same as an average composition of steel sheets except for oxygen. - 41 - I!! ·. ',j ::j :'i :! !:I :·: H ,1.,,: [;! I,,' i:i ,:.,-, I I 1:1 [0079] [Table 4] Welding conditions Joint No. Welding rate (m/min) Absolute humidity Retention time (min) after welding Defined retention time (min) (g/m') 1 3 15 10 <0 2 3 15 10 20 3 3 15 180 20 4 3 15 1080 160 5 3 15 300 160 6 3 15 60 . 160 7 15 15 300 160 R 3 15 1080 300 9 3 15 1080 . 300 10 3 15 1080 300 11 3 15 1080 370 12 3 15 1080 650 13 3 15 1200 1000 14 7 15 1200 1000 15 10 15 1200 1000 16 15 15 1200 1000 17 3 4 1200 1000 18 3 7 1200 1000 19 3 23 1200 1000 20 3 n 1200 1000 21 3 35 1200 1000 22 3 15 900 1000 23 3 15 600 1000 24 3 15 1200 1000 25 3 15 1200 1000 26 3 15 1200 1000 27 3 15 1200 1000 28 3 _15 _L_ 1200 1000 -- ---- ·--- ---- Underline indicates being excluded from requirements of the manufacturing method ofweldjoint in the present inventiOn. - 42 - Amount (mass%) of oxygen in weld metal 0.0054 0.0038 0.0038 0.0027 0.0027 0.0027 0.0027 0.0034 0.0044 0.0031 0.0031 0.0028 0.0034 I 0.0034 0.0034 0.0034 0.0030 0.0031 0.0037 0.0039 0.0042 0.0034 0.0034 0.0034 0.0028 0.0031 0.0045 0.0028 ---------- --- ----- ------ ,:.: r: ' ' [i VJ 1-.i i ,[!, ,I·,' II I [0080] [Table 5] Welding conditions Joint No. Welding rate (m/miri) Absolute humidity Retention time (min) after welding Defined retention time (min) (g/m') 29 3 15 1200 1000 30 3 15 1200 1000 31 3 15 1080 1210 32 3 4 1440 1210 33 3 7 1440 1210 34 3 15 1440 1210 35 3 ;lJ) 1440 1210 36 3 15 2160 1490 37 3 15 2880 !840 38 3 15 2880 2400 39 3 15 2880 2540 40 3 15 1080 370 41 3 15 1080 370 42 3 15 1080 580 43 3 15 1080 580 44 3 15 1080 !280 45 3 15 1920 1280 46 3 15 2880 1770 47 3 4 180 20 48 3 7 180 20 49 3 15 180 20 50 3 15 180 20 51 3 15 1440 1350 52 3 15 1440 1350 53 3 15 !440 1350 54 3 15 1440 - 1350 55 3 15 1440 1350 56 3 15 1440 1350 57 3 15 !440 1350 Underline indicates being excluded from requirements of the manufacturing method of weld joint it1 the present itlventLCirL - 43 - I Amount (mass%) of oxygen in weld metal I 0.0034 I 0.0038 0.0038 0.0034 0.0035 0.0038 0.0044 0.0044 0.0038 0.0047 0.0055 0.0044 0.0041 0.0031 0.0033 0.0037 0.0037 0.0045 0.0045 0.0046 0.0049 0.0043 0.0041 0.0040 0.0040 0.0040 0.0070 0.0!20 0.0040 -- -', .. ,·-,: [0081] Then, a draw bending test which will be described later was performed on each of the weld joints 1 to 57, and it was examined whether or not a crack occurred in weld metal. The following items are shown in the following Table 6 and Table 7. (k) "HV WM (HV)"···average hardness of weld metal (I) "Porosity distribution density (pieces/mm2)"···distribution density of porosities having a diameter of 2 J.lm to 50 J.lm in weld metal (m) "Inclusion distribution density (pieces/mm2)"···distribution density of oxide inclusions having a diameter of 3 J.lm or more in weld metal metal (n:) "Cr~ (mass ppm)"···amount CH of diffusible hydrogen in weld.metal • (o) "Right side value in Expression (5)"· .. value of3.57-0.0066xHVwM (p) "Crystal structure of martensite"--·crystal structure of martensite in weld ( q) "State of occurrence of crack"···whether or not a crack occurs, determined by a draw bending test which is performed during a period when a retention time elapses In all of the weld joints 1 to 57, the main metallographic structure was martensite. In Table 6 and Table 7, a weld joint in which a crystal structure of martensite is indicated as bee means that the main metallographic structure of weld metal is a martensite structure of a bee structure. A weld joint indicated as bet means that the main metallographic structure of weld metal is a martensite structure of a bet structure. The crystal structure of martensite was specified by an X-ray diffraction method. Specifically, lattice constants of an a axis and a c axis of a { 1 00} plane were measured by an X -ray diffraction method, and it was determined whether the crystal - 44 - structure was cubic (bee) or a tetragonal (bet), from an axis ratio c/a. In a case where a value of c/a was equal to or less than 1.007; it was assumed that a structure of martensite was a bee structure. - 45 - ,, " -;: [0082] [Table 6] "' Right side Crystal State of Joint No. HVWM Distribution density Distribution density (pieces/mm2 ) of inclusions CH value in (HV) (pieces/mm2 structure of occurrence of Note ) of porosities (mass' ppm) expression (5) martensite crack _:_ I 328 6.6 0.54 2.300 1.405 bee None Comnarative exam.Qk 2 372 1.6 0.38 2.200 1.115 bee Occurrence Comgarative examQle 3 370 1.6 0.38 l:GlO L128 bee None Example of the inventior 4 385 1.4 0.27 0.350 1.029 bee None Example of the inventior 5 390 1.4 0.27 0.850 0.996 bee None xample ofthe inventior 6 385 1.4 0.27 1.220 1.029 bee Occurrence ComQaratjye example 7 399 5.7 0.27 0.980 0.937 bee Occurrence Comnarative exarnQle 8 401 5.3 0.34 0.230 0.923 bee Occurrence Com}2arative examnle 9 401 L1 0.44 0.310 0.923 bee None Example of the inventior 10 401 3.0 0.31 0.:60 0.923 bee None Example of the inventior 11 409 2.2 0.31 0.270 0.871 bee None Example of the inventior 12 442 2.4 0.28 0.3-JO 0.653 bee None Example of the ihventior 13 482 L1 0.34 o.:•oo 0.389 bee None Example of the inventior 14 485 4.8 0.34 0.290 ' 0.369 bee None Example of the invention 15 485 5.4 0.34 0.340 0.369 bee Occur.rence Comparative example 16 495 5.9 0.34 0.380 0.303 bee Occurrence · Comuarative examnle 17 485 0.8 0.08 0.260 0.369 bee Occurrence Comuaratjve examQle 18 485 0.8 0.14 0.300 0.369 bee None Example of the invention 19 480 L1 0.57 0.350 0,402 bee None Example of the invention ?0 480 L1 1.90 0.430 0,402 ' bee 09.currence Comparative example 21 485 L1 2.40 0.520 0.369 bee Occurrence Comparative ex·ample 22 485 L1 0.34 0.390 0.369 bee Occurrence Comparative example 23 485 Ll 0.34 0.420 0.369 bee Occurrence ComQarative examQle 24 480 L1 0.34 0.300 0.402 bee None Example of the· invention 25 490 0.9 0.28 0.170 0.336 bee None Exampie of the invention 26 479 0.9 0.31 0300 0.409 bee None Example of the Invention 27 525 1.1 ' 0.45 0.300 0.105 bet Occurrence Comgarative examQ:le 28 480 2.2 I 0.28 0.250 0.402 bee --------- None Example of the invention Underline indicates being excluded from requirements of the manufacturing method of weld joint in the present invenhol";. • 46 • Y; L: [0083] [Table 7] Right side Crystal st State of Joint No. HVWM Distribution density Distribution density (pieces/mm2 ) of inclusions CH value in ructure o occurrence of Note (HV) (pieces/mm2 ) of porosities (mass r:;pm) expression (5) martensit crack e 29 523 2.2 0.34 0.270 0.118 bet Occurrence ComQarative examnle 30 485 3.7 0.38 0.450 0.369 bee Occurrence ComQarative examQle 31 505 0.9 0.38 0.310 0.237 bee Occurrence ComQarative examQle 32 510 0.9 0.04 0.150 0.204 bee Occurrence Comoarative examole 33 505 0.9 0.16 0.17G 0.237 bee None Example of the invention 34 510 0.9 0.38 0.19C 0.204 bee None Example of the invention 35 510 0.9 1.60 0.22C 0.204 bee Occurrence Comnarative examnle 36 539 1.3 0.44 0.007 0.013 bee None Example of the invention 37 lli 11.2 0.38 O.OC2 <0 bee Occurrence ComQarative examnle. 38 644 3.3 0.47 0.00. <0 bee Occurrence Comparative examnle 39 644 1.6 0.55 0.002 <0 bee Occurrel)ce ComQarative examnle 40 407 1.4 0.44 0.35C 0.884 bee None Example of the invention 41 405 1.6 0.41 0.190 0.897 bee None Example of the invention 42 430 1.6 0.31 0.280 0.732 bee None Example of the invention 43 440 0.5 0.33 0.370 0.666 'bee None Example of the invention 44 525 1.3 0.37 0.290 0.105 bee Occurrence ComQarative exampie 45 525 0.9 0.37 0.080 0.105 bcC None Example of the invention 46 56'i 1.1 0.45 0.020 <0 bee Occurrence Comparative examole 47 380 1.7 0.03 0.69 1.062 bee Occurrence ComQarative examQle 48 384 1.5 0.20 0.82 1.036 bee None Example of the invention 49 383 1.6 0.49 0.95' 1.042 bee None Example of the invention 50 383 5.1 0.43 0.88 1.042 bee Occurrence ComQarative examQle 51 520 5.7 0.41 0.08 0.138 bee Occurrence Comparative examQle 52 525 1.8 0.40 0.08' 0.105 bee None Example of the invention 53 525 0.0 0.40 o.os· 0.105 bee Occurrence ComQarative example ' 54 520 3.3 0.40 o.o8·: 0.138 bee Occurrence Comnarative examnle 55 520 3.3 2.3 0.08 . 0.138 bee;. None Example of the invention 56 515 3.3 8.5 o.w•. 0.171 bee Occurrence Comnarative examnle 57 515 3.3 0.45 0.08 0.171 bee Occurrence ComQarative example Underline indicates being excluded from requirements of the manufacturing method ofweldjoint in the present invention .'." _' - 47 - [0084] The chemical composition of weld metal was obtained by a method described in the above-described section of"5. Obtaining method of chemical composition of weld metal". Average hardness (HV) of weld metal was obtained in a manner as follows. Firstly, a weld joint was cut in a direction vertical to an elongation direction of a welding line and a cross-sectional sample for measuring hardness was produced. Hardness at four points of weld metal in the cross-sectional sample was measured with a load of 500 gf or more, by using a Vickers hardness tester. An average value of measured hardness at four points was calculated, and was set as average hardness. A measurernent point was· setto have a point of l/4t (twas a thickness of the'weii:i·m·etaf'· '·~ '.c ·. in a sheet thickness direction of the weld joint) from a surface of the weld metal. [0085] The distribution density of porosities having a diameter of 2 J.tm to 50 J.tm in weld metal was obtained in a manner as follows. Firstly, a weld joint was cut in a direction vertical to an elongation direction of a welding line and a cross section obtained by the cutting was subjected to mirror polishing. A portion corresponding to weld metal in the cross section subjected to mirror polishing is observed by a SEM, and the number of porosities having a diameter of 2 J.tm to 50 J.tm is counted. The obtained number of porosities is divided by an observation area, and a value obtained by the division is set as distribution density of porosities. SEM observation was performed on three or more different cross sections such that an observation area was set to be equal to or more than 5 mm2 • Because a porosity has various shapes, evaluation was performed by using an equivalent circle diameter of the same area. [0086] Regarding the distribution density of oxide inclusions having a diameter of 3 - 48 - I J.!m or more in weld metal, the same sample as that in porosity observation is used. The cross section subjected to mirror polishing is observed by a SEM, and the number of oxide inclusions having an equivalent circle diameter of 3 f!m or more is counted. SEM observation was performed on three or more different cross sections such that an observation area was set to be equal to or more than 5 mm2 • In a case where it was not possible to distinguish a porosity from an inclusion based on only a SEM image, analysis of oxygen and other elements was performed by using EDS mounted in a SEM. In a case where an element constituting an inclusion was not recognized, determination to be a porosity was performed. [0087] . :-·-·, ~ .. ' ·--.,.,: •• ·l,- The amount of diffusible hydrogen contained in weld metal was measured in a manner as follows. Firstly, a sample which included weld metal and was used for measuring the amount of diffusible hydrogen was cut from each of the weld joints. The sample which had been cut out was heated at a heating rate of 1 00°C/h. Hydrogen discharged from the sample when heating was performed from room temperature to 200°C was measured by gas chromatography, and the measured value was set as an amount CH of diffusible hydrogen shown in Table 6 and Table 7. Hydrogen contained in a steel sheet before welding was an amount as small as could be ignored. Thus, in this example, it was assumed that hydrogen was not contained in a steel sheet and hydrogen was contained only in weld metal. On this assumption, the amount of diffusible hydrogen in weld metal was calculated from the mass of the weld metal and an amount of hydrogen measured in the above-described manner. The mass of the weld metal was calculated by the following Expression (8). In the following Expression (8), Aw indicates a mass of weld metal. At indicates a mass of the sample. Ww indicates a width of the weld metal. Wt indicates a width (length - 49 - I L--~-==--'-----'-"''-''-'--'""'""-=- --·-"·---o..-======·,_~.::--~" ~~ in a direction vertical to an elongation direction of a welding line) of the sample. Aw=AtxWw/Wt···Expression (8) [0088] The width Ww of weld metal was obtained in a manner as follows. Firstly, the sample was cut in a direction vertical to an elongation direction of a welding line, and a width at each of positions of liSt (t indicates a thickness of weld metal), 1/4t, l/2t, 3/4t, and 7/8t from a surface of weld metal was measured. An average value of measured width of five points was calculated, and the calculated average value was set as the width Ww of weld metal. [0089] Ametallographic structure of weld metal was observed by a SEM. Specifically, a sample which included weld metal and was used for observing a metallographic structure was cut from each of the weld joints. Across section of weld metal in the sample which had been cut out was observed by a SEM. The crystal structure of martensite in weld metal was samely determined by an X-ray diffraction method. [0090] Next, a draw bending test will be described. In the draw bending test, firstly, lubricant oil was applied onto both sides of each of the above-described weld joints I to 57. Press forming (hat forming) was performed on each of the weld joints 1 to 57 at room temperature and a punch speed of 60mm/min by using a press test device I 00. The press test device 100 includes a punch 101, a dice 102, and a blank holder 103 which are illustrated in FIG. 3. As illustrated in FIG. 3, a diameter d of the punch 101 in the press test device 100 was set to 100 mm. A radius rp of a shoulder oftbe punch was set to I 0 mm. A radius rct of a shoulder ofthe dice was set to 5 mm. A - 50 - clearance c between the punch I 0 I and the dice I 02 was set to 3 mm. A blank holding force (BHF) by the blank holder I 03 when press forming was performed was adjusted such that tensile strength applied to a vertical wall of the weld joint during press forming was 0.5 times tensile strength of one having low strength among two steel sheets which were joined to each other. A forming height was set to 60 mm. The weld joint was installed in the press test device so as to cause a welding line to pass through a substantially center of an upper surface of the punch. [0091] Weld meta! of each of the weld joints I to 57 after press forming. was visually obs~tved•bya magnifier, and it was examined whether or nof ti'crack occurred. As a result, as shown in Table 6 and Table 7, in the weld joint according to the example of the present invention, a crack in weld metal after press forming was not confirmed. [0092] In a weld joint 1 according to a comparative example, porosity distribution density was increased by a small weld pool fluidity index o.. Thus, it was supposed that an influence of internal pressure by non-diffusible hydrogen in a porosity was largely received. However, in the weld joint I, since the average hardness of weld metal was small, a crack in the weld metal was not confirmed. [0093] In weld joints 2, 6, 22, 23, 31, and 44 according to the comparative example, it is supposed that the retention time is short, and thus it is not possible to sufficiently reduce internal pressure by non-diffusible hydrogen in a porosity, and a crack occurs from the porosity as a starting point in a draw bending test. [0094] ln weld joints 7, 15, and 16 according to the comparative example, it is - 51 - -----------.------ ---. - __ :::-=::,-- 1- .-. •-' .,- r ._-·v,~' '. supposed that the welding rate is fast, and thus it is not possible to ~up press an amount of porosities, and an influence of internal pressure by non-diffusible hydrogen in a porosity causes a crack to occur from the porosity as a starting point in a draw bending test. In a weld joint 15, it is supposed that a value ofCH indicating an amount of hydrogen in metal lattice can be reduced to an appropriate value, but, diffusion of the amount of non-diffusible hydrogen in a porosity is insufficient, and an influence of internal pressure causes a crack to occur from the porosity as a starting point in a draw bending test. [0095] · In a weld joihf8 a(;cording to the comparative example; .porosity-distributil'ln density was increased by a small weld pool fluidity index a. Thus, it is supposed that it is not possible to sufficiently reduce internal pressure by non-diffusible hydrogen in a porosity, and a crack occurs from the porosity as a starting point iu a draw bending test. [0096] In a weld joint 17 according to the comparative example, absolute humidity in welding was low, and thus it was not possible to sufficiently increase distribution density of oxide inclusions. It is supposed as follows. The value of CH indicating an amount of hydrogen in metal lattice can be reduced to an appropriate value, but it is not possible to sufficiently obtain an effect of trapping diffusible hydrogen by oxide inclusions. Thus, it is not possible to sufficiently reduce internal pressure by nondiffusible hydrogen in a porosity and a crack occurs from the porosity as a starting point in a draw bending test. [0097] In weld joints 20, 21, and 35 according to the comparative example, it is - 52 - i,, supposed that absolute humidity in welding is high; and thus it is not possible to I I 1: sufficiently reduce internal pressure by non-diffusible hydrogen in a porosity and a crack occurs from the porosity as a starting point in a draw bending test. ' [0098] In a weld joint 27 according to the comparative example, it is supposed that the content of Mn at a melted metal portion is excessive, and thus weld metal is enbrittled, and a crack occurs in the weld metal. [0099] In a weld joint 29 according to the comparative example, it is supposed that · the content ofNi at a melted metal portion is excessive-; arid thus weld metahs·, .;· enbrittled, and a crack occurs in the weld metal. [0100] In a weld joint 30 according to the comparative example, it is supposed as follows. The content of Si at a melted metal portion is excessive, and thus, the amount CH of hydrogen in metal lattice is excessive. Thus, it is not possible to sufficiently reduce internal pressure by non-diffusible hydrogen in a porosity and a crack occurs from the porosity as a starting point in a draw bending test. [0101] In weld joints 32 and 47 according to the comparative example, absolute humidity in welding was low, and thus it was not possible to sufficiently increase distribution density of oxide inclusions. It is supposed as follows. The value of C11 indicating an amount of hydrogen in metal lattice can be reduced to an appropriate value, but it is not possible to sufficiently obtain an effect of trapping diffusible hydrogen by oxide inclusions. Thus, it is not possible to sufficiently reduce internal pressure by non-diffusible hydrogen in a porosity and a crack occurs from the porosity - 53 - as a starting point in a draw bending test. [0102] In weld joints 37, 38, 39, and 46 according to the comparative example, it is supposed that the content of C at a melted metal portion is excessive, and thus average hardness of weld metal deviates a suitable range, and a crack occurs in the weld metal. [0103] In weld joints 50 and 51 according to the comparative example, porosity distribution density was increased by a small weld pool fluidity index a. Thus, it is supposed that an influence of internal pressure by non-diffusible hydrogen in a ··porosity causes a crack to occur by from the porosity· as ·a starting point in a· draW' · · bending test. [0104] In a weld joint 53 according to the comparative example, it is supposed that the content of S at a melted metal portion is excessive, and thus a crack occurs in the weld metal. [0105] In a weld joint 54 according to the comparative example, it is supposed that the content of P at a melted metal portion is excessive, and thus a crack occurs in the weld metal. [0106] In a weld joint 56 according to the comparative example, it is supposed that the content of 0 at a melted metal portion is excessive, and thus oxide inclusions are excessively generated and a crack occurs in the weld metal. [0107] In a weld joint 57 according to the comparative example, it is supposed that - 54 - the content ofN at a melted. metal portion is excessive, and thus a crack occurs in the weld metal. [01 08] As described above, in the weld joint according to the present invention, even in a laser welded joint using a high-strength steel sheet in which tensile strength is higher than 350 MPa, it is possible to suppress the occurrence of a crack in weld metal when press formingis performed. That is, according to the weld joint of the present invention, it is understood that excellent formability is obtained. [Industrial Applicability] [0 109] According to the present invention, even in a case where a laser welded joint including a high-strength steel sheet is press-formed, it is possible to prevent the occurrence of a crack in weld metal. Thus, for example, even in a case using a highstrength steel sheet which has a high carbon equivalent, it is possible to integrally manufacture components by tailored blank and to realize reduction in weight of a vehicle body and improvement of safety. The laser welded joint according to the present invention can be also used as a panel component and a chassis component, in addition to a frame component of a vehicle body. Claims 1. A laser welded joint comprising: weld metal provided between a plurality of steel sheets, wherein, the weld metal contains, as a chemical composition, by mass%, 0.05% to 0.30% of C, 0.005% to 3.0% of Si, 0.005% to 1.0% of AI, 0.5% to 6.0% ofMn, more than 0% and 0.04% or less ofP, more than 0% and 0.01% or less ofS, more than 0% and 0.01% or less ofN, more than 0% and 0.01% or less ofO, 0% to 1.0% ofCu, O%to 0.2% ofNb+Ti+V, 0% to 0.01% ofCa+REM, 0% to 0.01% ofB, 0% to 5.0% ofCr, 0% to 10.0% ofNi, 0% to 1.0% ofMo, and a remainder .consisting ofF e and impurities, 0.3<:Si+200xS-2.7xC<:3.0 is satisfied, ·\'I average hardness of the weld metal is 350 to 540 in Vickers hardness, distribution density of porosities having a diameter of 2 }.!ill to 50 rtm in the weld metal is equal to or less than 5.0 pieces/mm2 , and - 56 - ' ·.-. .-.. :_-.·------- -.-.,-,--.-- distribution density of oxide inclusions having a diameter of 3 11m or more in the weld metal is 0.1 to 8.0 pieces/mm2 • 2. The laser welded joint according to claim I, wherein at least one selected from the group consisting of 0.0001%to l.Oo/oofCu, 0.0001% to 0.2% ofNb+ Ti+V, 0.0001% to 0.01% ofCa+REM, 0.0001% to 0.01% ofB, 0.0001 'Yo to 5.0% of Cr, 0.0001% to 10.0% ofNi, and 0.0001%to l.Oo/oofMo is contained as the chemical composition of the weld metal, by mass%. 3. The laser welded joint according to claim 1 or 2, wherein an amount CH of diffusible hydrogen in the weld metal satisfies the following Expression ( 1) in a unit of mass ppm. CH"S3.570-0.0066xHVwM Expression (1) Here, HV WM in Expression (1) indicates average hardness of the weld metal in Vickers hardness. 4. The laser welded joint according to any one of claims 1 to 3, wherein 80% or more of a metallographic structure in the weld metal is martensite, and a structure of the martensite is a bee structure. - 57 - 5. The laser welded joint according to any one of claims 1 to 4, wherein a value ofMs represented by the following Expression (2) is equal to or more than 250. Ms=561-474xC-33xMn-17xNi-17xCr-21 xMo Expression (2) 6. The laser welded joint according to any one of claims 1 to 5, wherein at least one of the plurality of steel sheets is a plated steel sheet. 7. A vehicle component comprising: · ·' · '·· " ' ·..-. ' '·, the laser welded joint according to any one of claims 1 to 6. 8. A manufacturing method of a laser welded joint which is the laser welded joint according to any one of claims 1 to 6, the method comprising: performing laser welding of a plurality of steel sheets at a welding rate of 8 rnlmin or slower in an atmosphere in which absolute humidity is equal to or less than 5 to 25 g/m3 , so as to form weld metal for joining the plurality of steel sheets; and retaining the plurality of steel sheets after welding, in a temperature range of 10°C to 1 oooc for a time defined by the following Expression (3), wherein the weld metal contains, as a chemical composition, by mass%, at least one selected from the group consisting of 0.05% to 0.30% ofC, 0.005% to 3.0% ofSi, 0.005% to 1.0% of AI, 0.5%to 6.0% ofMn, - 58 - I more than 0% and 0.04% or less ofP, more than 0% and 0.01% or less ofS, more than 0% and 0.01% or less ofN, more than 0% and 0.01% or less ofO, 0% to 1.0% ofCu, 0% to 0.2% ofNb+Ti+V, 0% to 0.01% ofCa+REM, 0% to O.ol% ofB, 0% to 5.0% ofCr, 0% to 10.0% ofNi, 0% to 1.0% ofMo, and a remainder consisting of Fe and impurities, and 2.7xC:<:3.0 is satisfied. 127000xC-400 Expression (3) ., ..... - ... 0.3:<:Si+200xSHere, in Expression (3), t indicates a time in a unit of minute. 9. The manufacturing method of a laser welded joint according to claim 8, wherein the weld metal contains, as the chemical composition, by mass%, at least one selected from the group consisting of 0.0001% to 1.0% ofCu, 0.0001% to 0.2% ofNb+ Ti+V, 0.0001% to 0.01% ofCa+REM, 0.0001% to O.ol% ofB, 0.0001% to 5.0% ofCr, 0.0001% to 10.0% ofNi, and - 59 - 0.0001% to 1.0% ofMo. 10. The manufacturing method of a laser welded joint according to claim 8 or 9, wherein a value ofMs represented by the following Expression (4) is equal to or more than 250. Ms=561-474xC-33xMn-17xNi-17xCr-21 xMo Expression (4) 11. The manufacturing method of a laser welded joint according to any one of claims 8 to 10, · wherein at least one of the plurality of steel sheets is a plated steel sheet. 12. A manufacturing method of a vehicle component; which performs press forming on the laser welded joint according to any one of claims 1 to 6. 13. A manufacturing method of a vehicle component, which performs press forming on the laser welded joint manufactured by the manufacturing method according to any one of claims 8 to 11 ..