Welded structure and method for manufacturing the same

Abstract

There is provided a welded structure that can reduce fractures at welded portions. The welded structure includes two or three steel sheets and a lapped portion in which the steel sheets are overlapped and joined by spot welding at a plurality of locations, the welded structure including a spot weld portion, and, when a diameter of a nugget is d ng (mm), a tip diameter of an electrode used by the spot welding is d (mm), and an average thickness per steel sheet of the steel sheets at the lapped portion is t ave (mm), the spot weld portion satisfies d ng >d(t ave ) 1/2 when 0.5 mm≦t ave <1.1 mm and d ng >1.05d when 1.1 mm≦t ave ≦2.6 mm, in accordance with the average thickness t ave (mm).

1 . A welded structure including two or three steel sheets and a lapped portion in which the steel sheets are overlapped and joined by spot welding at a plurality of locations, the welded structure comprising:
 a spot weld portion, and, when a diameter of a nugget is d ng  (mm), a tip diameter of an electrode used by the spot welding is d (mm), and an average thickness per steel sheet of the steel sheets at the lapped portion is t ave  (mm), the spot weld portion satisfies a condition (a) or a condition (b) below in accordance with the average thickness t ave  (mm),
   ( a )  d   ng   >d ( t   ave ) 1/2  when 0.5 mm≦ t   ave <1.1 mm  (1)
 
   ( b )  d   ng >1.05 d  when 1.1 mm≦ t   ave ≦2.6 mm  (2).
 
   
     
     
         2 . The welded structure according to  claim 1 ,
 wherein the welded structure includes an expected deformation region to be subjected to plastic deformation when a load is applied, and the spot welding is carried out at least within the expected deformation region.   
     
     
         3 . The welded structure according to  claim 1 ,
 wherein the expected deformation region is free from spatter adhesion.   
     
     
         4 . The welded structure according to  claim 1 ,
 wherein the steel sheets have a tensile strength of 980 MPa or more.   
     
     
         5 . The welded structure according to  claim 1 ,
 wherein the electrode is a combined electrode having an electrode body to be pressed against the steel sheets at the lapped portion and a ring-shaped member to be pressed against the steel sheets around the electrode body.   
     
     
         6 . The welded structure according to  claim 5 ,
 wherein the combined electrode includes:
 a first ring-shaped member and a second ring-shaped member to be pressed against the steel sheets at the lapped portion with the first ring-shaped member and the second ring-shaped member facing each other; and 
 a first electrode body and a second electrode body, each being inserted in a through hole disposed each of the first ring-shaped member and the second ring-shaped member, to be pressed against the steel sheets at the lapped portion with the first electrode body and the second electrode body facing each other, and 
   wherein an electric current flows through the steel sheets between the first electrode body and the second electrode body.   
     
     
         7 . The welded structure according to  claim 1 ,
 wherein the spot weld portion satisfying the condition (a) or the condition (b) is present in a 20 to 60% extent on the welded structure.   
     
     
         8 . The welded structure according to  claim 1 ,
 wherein the spot weld portion has an equivalent carbon content (Ceq) of 0.13 mass % or more, the equivalent carbon content (Ceq) being defined by an equation (3) below:
   C eq =[C]+1/90[Si]+1/100([Mn]+[Cr])  (3)
 
   
       where
 [C]: an average C content (mass %) of the spot weld portion; 
 [Si]: an average Si content (mass %) of the spot weld portion; 
 [Mn]: an average Mn content (mass %) of the spot weld portion; and 
 [Cr]: an average Cr content (mass %) of the spot weld portion. 
 
     
     
         9 . The welded structure according to  claim 2 ,
 wherein the welded structure is a member to be used for an automobile, and the expected deformation region is to be subjected to an axial compression load.   
     
     
         10 . The welded structure according to  claim 2 ,
 wherein the welded structure is a member to be used for an automobile, and the expected deformation region is to be subjected to a bending load.   
     
     
         11 . A method for manufacturing a welded structure including two or three steel sheets and a lapped portion in which the steel sheets are overlapped and joined by spot welding at a plurality of locations, the method comprising:
 carrying out spot welding, the carrying out spot welding including
 a first step in which a first rod-shaped electrode body and a second rod-shaped electrode body are arranged facing each other with the lapped portion being sandwiched therebetween, and a first ring-shaped member and a second ring-shaped member are arranged facing each other, the first ring-shaped member having a through hole through which the first electrode body is inserted and a back end to which a first elastic body is connected and the second ring-shaped member having a through hole through which the second electrode body is inserted and a back end to which a second elastic body is connected, and 
 a second step in which the lapped portion is pressurized by pressing a tip face of each of the first electrode body and the second electrode body against the lapped portion, and by pressing a tip face of each of the first ring-shaped member and the second ring-shaped member against the lapped portion while the first elastic body exerts a pressing pressure on the first ring-shaped member and the second elastic body exerts a pressing pressure on the second ring-shaped member, and then an electric current is applied between the first electrode body and the second electrode body, 
   wherein the first step and the second step cause a spot weld portion to satisfy a condition (c) or a condition (d) below in accordance with an average thickness t ave  (mm):
   ( c )  d   ng   >d ( t   ave ) 1/2  when 0.5 mm≦ t   ave <1.1 mm  (4)
 
   ( d )  d   ng >1.05 d  when 1.1 mm≦ t   ave ≦2.6 mm  (5)
 
   where d ng  (mm) is a nugget diameter, d (mm) is a tip diameter of an electrode used by the spot welding, and t ave  (mm) is an average thickness per steel sheet of the steel sheets at the lapped portion.