Steel sheet assembly, method of manufacturing steel sheet assembly, and spot welding process

Abstract

A steel sheet assembly includes a plurality of lapped steel sheets which have a tensile strength of 1,470 MPa or less and a thickness of 0.3 mm to 5.0 mm, the steel sheet assembly being formed by applying, in advance, an adhesive and a carbon-supplying agent to a surface of either or both of the steel sheets to be lapped and then welding the steel sheets. A weld of the assembly has a nugget diameter of 2.8√t (mm) or more, where t (mm) denotes the thickness of a thinner one of the steel sheets on both sides of a weld interface, and the amount of C is increased by 0.02 % by mass or more as compared to the steel sheets before being applied with the adhesive and the carbon-supplying agent.

1 - 9 . (canceled) 
     
     
         10 . A steel sheet assembly comprising a plurality of lapped steel sheets having a composition containing
 C: 0.4% or less,   Si: 3.0% or less,   Al: 3.0% or less,   Mn: 0.2% to 6.0%,   P: 0.1% or less, and   S: 0.07% or less   
       on a mass basis, the remainder being Fe and inevitable impurities, and having a tensile strength of 1,470 MPa or less and a thickness of 0.3 mm to 5.0 mm,
 the steel sheet assembly being formed by applying, in advance, an adhesive and a carbon-supplying agent to a surface of either or both of the steel sheets to be lapped and then welding the steel sheets, 
 wherein a weld of the assembly has a nugget diameter of 2.8√t (mm) or more, where t (mm) denotes the thickness of a thinner one of the steel sheets on both sides of a weld interface, and 
 the amount of C is increased by 0.02% by mass or more as compared to the steel sheets before being applied with the adhesive and the carbon-supplying agent. 
 
     
     
         11 . The steel sheet assembly according to  claim 10 , wherein the composition further contains one or more selected from the group consisting of
 Cr: 0.05% to 5.0%,   V: 0.005% to 1.0%,   Mo: 0.005% to 0.5%,   Ni: 0.05% to 2.0%,   Cu: 0.05% to 2.0%,   Ti: 0.01% to 0.1%,   Nb: 0.01% to 0.1%,   B: 0.0003% to 0.0050%,   Ca: 0.001% to 0.005%, and   a REM: 0.001% to 0.005%   
       on a mass basis. 
     
     
         12 . The steel sheet assembly according to  claim 10 , wherein the carbon-supplying agent is mixed in the adhesive in advance. 
     
     
         13 . The steel sheet assembly according to  claim 10 , wherein the hardness of the formed weld is higher than that of the steel sheets before being applied with the adhesive and the carbon-supplying agent by 20 or more in terms of Vickers hardness. 
     
     
         14 . A method of manufacturing a steel sheet assembly, comprising:
 applying, in advance, an adhesive and a carbon-supplying agent to a surface of either or both of steel sheets to be lapped, the steel sheets having a composition containing   C: 0.4% or less,   Si: 3.0% or less,   Al: 3.0% or less,   Mn: 0.2% to 6.0%,   P: 0.1% or less, and   S: 0.07% or less   
       on a mass basis, the remainder being Fe and inevitable impurities, and having a tensile strength of 1,470 MPa or less and a thickness of 0.3 mm to 5.0 mm;
 lapping the steel sheets; and then 
 welding the steel sheets, 
 wherein a weld of the assembly has a nugget diameter of 2.8√t (mm) or more, where t (mm) denotes the thickness of a thinner one of the steel sheets on both sides of a weld interface, and 
 the amount of C is increased by 0.02% by mass or more as compared to the steel sheets before being applied with the adhesive and the carbon-supplying agent. 
 
     
     
         15 . The method for manufacturing the steel sheet assembly according to  claim 14 , wherein the composition further contains one or more selected from the group consisting of
 Cr: 0.05% to 5.0%,   V: 0.005% to 1.0%,   Mo: 0.005% to 0.5%,   Ni: 0.05% to 2.0%,   Cu: 0.05% to 2.0%,   Ti: 0.01% to 0.1%,   Nb: 0.01% to 0.1%,   B: 0.0003% to 0.0050%,   Ca: 0.001% to 0.005%, and   a REM: 0.001% to 0.005%   
       on a mass basis. 
     
     
         16 . The method for manufacturing the steel sheet assembly according to  claim 14 , wherein the carbon-supplying agent is mixed in the adhesive in advance. 
     
     
         17 . The method according to  claim 14 , wherein the hardness of the formed weld is higher than that of the steel sheets before being applied with the adhesive and the carbon-supplying agent by 20 or more in terms of Vickers hardness. 
     
     
         18 . A welding process used in the method for manufacturing the steel sheet assembly according to  claim 14 ,
 the welding process being a spot welding process performed for a heat time of 0.08 seconds or more using a convex electrode with a tip radius of curvature of 20 mm or more or a flat electrode such that the weld force F (kN) for initial 0.03 seconds of the heat time satisfies formula:
     F< 0.00125 ×TS ×(1+0.75 ×t   all )+3
 
   where TS (MPa) denotes the average strength of the steel sheets and represents the weighted mean value of the thickness of each steel sheet, and   t all  (mm) denotes the total thickness of the steel sheets (the sum of the thicknesses of the steel sheets).   
     
     
         19 . The steel sheet assembly according to  claim 11 , wherein the carbon-supplying agent is mixed in the adhesive in advance. 
     
     
         20 . The steel sheet assembly according to  claim 11 , wherein the hardness of the formed weld is higher than that of the steel sheets before being applied with the adhesive and the carbon-supplying agent by 20 or more in terms of Vickers hardness. 
     
     
         21 . The steel sheet assembly according to  claim 12 , wherein the hardness of the formed weld is higher than that of the steel sheets before being applied with the adhesive and the carbon-supplying agent by 20 or more in terms of Vickers hardness. 
     
     
         22 . The method for manufacturing the steel sheet assembly according to  claim 15 , wherein the carbon-supplying agent is mixed in the adhesive in advance. 
     
     
         23 . The method for manufacturing the steel sheet assembly according to  claim 15 , wherein the hardness of the formed weld is higher than that of the steel sheets before application of the adhesive and the carbon-supplying agent by 20 or more in terms of Vickers hardness. 
     
     
         24 . The method for manufacturing the steel sheet assembly according to  claim 16 , wherein the hardness of the formed weld is higher than that of the steel sheets before application of the adhesive and the carbon-supplying agent by 20 or more in terms of Vickers hardness. 
     
     
         25 . A welding process used in the method according to  claim 15 ,
 the welding process being a spot welding process performed for a heat time of 0.08 seconds or more using a convex electrode with a tip radius of curvature of 20 mm or more or a flat electrode such that the weld force F (kN) for initial 0.03 seconds of the heat time satisfies the following formula:
     F< 0.00125 ×TS ×(1+0.75 ×t   all )+3
 
   where TS (MPa) denotes the average strength of the steel sheets and represents the weighted mean value of the thickness of each steel sheet, and   t all  (mm) denotes the total thickness of the steel sheets (the sum of the thicknesses of the steel sheets).   
     
     
         26 . A welding process used in the method for manufacturing the steel sheet assembly according to  claim 16 ,
 the welding process being a spot welding process performed for a heat time of 0.08 seconds or more using a convex electrode with a tip radius of curvature of 20 mm or more or a flat electrode such that the weld force F (kN) for initial 0.03 seconds of the heat time satisfies the following formula:
     F< 0.00125 ×TS ×(1+0.75 ×t   all )+3
 
   where TS (MPa) denotes the average strength of the steel sheets and represents the weighted mean value of the thickness of each steel sheet, and   t all  (mm) denotes the total thickness of the steel sheets (the sum of the thicknesses of the steel sheets).   
     
     
         27 . A welding process used in the method for manufacturing the steel sheet assembly according to  claim 17 ,
 the welding process being a spot welding process performed for a heat time of 0.08 seconds or more using a convex electrode with a tip radius of curvature of 20 mm or more or a flat electrode such that the weld force F (kN) for initial 0.03 seconds of the heat time satisfies the following formula:
     F< 0.00125 ×TS ×(1+0.75 ×t   all )+3
 
   where TS (MPa) denotes the average strength of the steel sheets and represents the weighted mean value of the thickness of each steel sheet, and   t all  (mm) denotes the total thickness of the steel sheets (the sum of the thicknesses of the steel sheets).