A highly corrosion-resistant spot weldment can be produced at low cost without occurrence of prominent protrusions and the like on the surface. The spot weldment is joined by a nugget formed inside stacked sheet materials through bringing a pair of electrodes arranged opposite to each other into pressure contact with the stacked sheet materials from outside and energizing the stacked sheet materials from the electrodes. The nugget has a diameter that is ≥4√t (t: thickness of sheet material) and a flattening level of 3.5 to 8, which is a ratio of diameter to thickness. Both outer surface parts of the sheet materials are free from protrusions formed due to bulging of molten metal. Even when the electrodes are made of a copper alloy, the increased amount of Cu in the outer surface parts is 0.2 mass % or less with respect to the component composition before spot welding.

A joint structure formed by resistance welding a plurality of superposed steel sheets includes a high tensile strength steel sheet having a tensile strength of at least 590 MPa and containing a chemical component having a carbon equivalent Ceq of at least 0.53%. The high tensile strength steel sheet has a decarburized layer between a base plate and a galvanized layer formed on at least one of a surface on a superposition surface side and a surface on a welding electrode side of the high tensile strength steel sheet, or has a decarburized layer on a superposition surface of the high tensile strength steel sheet adjacent to a galvanized layer of a galvanized steel sheet to be superposed. The decarburized layer has a thickness of at least 5 μm and at most 200 μm, and the carbon equivalent Ceq is a value defined by a formula: Ceq=C+Si/24+Mn/6.

Provided is a resistance spot welding method suitable for manufacturing a weld joint exhibiting excellent delayed fracture resistance. The resistance spot welding method includes sandwiching two or more overlapped steel sheets between a pair of welding electrodes, applying current to the steel sheets while pressing the steel sheets, forming a nugget on overlapping surfaces of the steel sheets to join the steel sheets, and after the joining, directly or indirectly irradiating the nugget with sound waves having a frequency of 10 Hz or more and 100000 Hz or less so that a sound pressure level on a surface of the steel sheet is 30 dB or more.

A galvanized steel sheet includes: a steel sheet having a chemical composition satisfying an equivalent carbon content Ceq of 0.35% or more and less than 0.60%, and a specified steel microstructure; and a galvanized layer on a surface of the steel sheet. The retained austenite has a solute C content of 0.6% or more by mass, and retained austenite grains with an aspect ratio of less than 2.0 constitute 50% or more of all retained austenite grains. In 90-degree bending at a curvature radius/thickness ratio of 4.2 in a rolling (L) direction with respect to an axis extending in a width (C) direction, an L cross section in a 0 to 50 μm region from a surface of the steel sheet on a compression side has a number density of voids of 1000/mm.sup.2 or less, and the galvanized steel sheet has a tensile strength of 590 MPa or more.

A method for producing a high-strength steel sheet having high ductility, formability and weldability includes providing a cold-rolled sheet, with a composition containing: 0.15% ≤C≤0.23%, 1.4% ≤Mn≤2.6%, 0.6% ≤Si≤1.3%, with C+Si/10≤0.30%, 0.4% ≤Al≤1.0%, with Al≥6(C+Mn/10)−2.5%, 0.010% ≤Nb≤0.035%, 0.1% ≤Mo≤0.5%, annealing the sheet at 860° C.-900° C. to obtain a structure consisting of at least 90% austenite and at least 2% intercritical ferrite, quenching to a temperature between Ms-10° C. and Ms-60° C. at a rate Vc higher than 30° C./s, heating to a temperature PT between 410° C. and 470° C. for 60 s to 130 s, hot-dip coating the sheet, and cooling to room temperature. The microstructure includes 45% to 68% of martensite, consisting of 85% to 95% partitioned martensite having a C content of at most 0.45%, and fresh martensite; 10% to 15% retained austenite; 2% to 10% intercritical ferrite; 20% to 30% lower bainite.

A joint structure, includes: a first member including a high tensile strength steel; and a second member including a high tensile strength steel and superposed on the first member, the first member and the second member being resistance welded to each other, in which a gap between the first member and the second member is between 0 mm and 3 mm, a nugget is not formed at a joint portion between the first member and the second member, or when the nugget is formed at the joint portion between the first member and the second member, a diameter D1 of the nugget satisfies D1<5 mm, and a decarburized layer is provided on at least one of a superposition surface of the first member, on which the second member is superposed, and a superposition surface of the second member, on which the first member is superposed.

The present disclosure provides a method of resistance spot welding that can inhibit Liquid Metal Embrittlement-induced cracking in zinc-coated steel plates irrespective of the degrees of tensile strength. One aspect of the present disclosure provides a method of resistance spot welding. The method comprises welding a workpiece with a resistance spot welding apparatus. The workpiece includes two or more steel plates in an overlapping state. The two or more steel plates include at least one steel plate coated with zinc. The welding includes welding while compressing a high-tensile steel plate among the two or more steel plates in a direction intersecting a direction of a thickness of the high-tensile steel plate. The high-tensile steel plate has a tensile strength higher than a tensile strength of another steel plate among the two or more steel plates.

The present disclose provides a method of resistance spot welding that can inhibit Liquid Metal Embrittlement-induced cracking in zinc-coated steel plates irrespective of the plate thicknesses. One aspect of the present disclosure provides a method of resistance spot welding that includes welding a workpiece with a resistance spot welding apparatus. The workpiece includes two or more steel plates in an overlapping state. The two or more steel plates include at least one steel plate coated with zinc. The welding includes causing a cooling rate of a high-tensile steel plate among the two or more steel plates to be higher than a cooling rate of an other steel plate among the two or more steel plates. The high-tensile steel plate has a tensile strength higher than a tensile strength of the other steel plate.

A steel workpiece includes an alloy substrate comprising iron, about 1.4 to about 2.0 weight percent aluminum, and about 0 to about 1.0 weight percent silicon. The steel workpiece further includes a coating comprising zinc. A method of spot welding a workpiece stack-up that includes a pair of the steel workpieces includes providing the stack-up, contacting first and second electrodes to the steel workpieces, passing an electrical current through the stack-up, forming a weld nugget from molten mixing of the alloy substrates of the pair of steel workpieces, forming a boundary layer between the coating and the alloy substrate from dispersion of the coating into the alloy substrate and reaction of the zinc with the aluminum and the silicon to prevent molten mixing of the coating within the alloy substrate, and ceasing passage of the electrical current.

A system for increasing joint strength and reducing embrittlement in a resistance spot weld of metal workpieces is disclosed. The system comprises a stackup of first and second metal workpieces, and an interface member disposed between the first and second metal workpieces. The interface member comprises a peripheral wall defining a hollow inner portion. The peripheral wall has a first open end extending to a second open end. The first open end is in contact with the first metal workpiece defining a first weld portion thereon. The second open end is in contact with the second metal workpiece defining a second weld portion thereon. The system further comprises a first electrode configured to contact the first metal workpiece to heat the peripheral wall at the first weld portion and join the first metal workpiece with the first open end of the peripheral wall. The system further comprises a second electrode configured to contact the second metal workpiece to heat the peripheral wall at the second weld portion and join the second metal workpiece with the second open end of the peripheral wall to define a weld joint. The system further comprises a power source configured to power the first and second electrodes and a controller configured to control the power to the first and second electrodes to heat the peripheral wall.