A resistance spot welding method includes the steps of: removing at least part of oil on a surface of a welding target material by energization between a pair of electrodes; and after the oil removal step, forming a nugget in an overlapped portion of the welding target material by energization between the pair of electrodes.

A method for producing a zinc or zinc-alloy coated steel sheet with a tensile strength higher than 900 MPa, for the fabrication of resistance spot welds containing in average not more than two Liquid Metal Embrittlement cracks per weld having a depth of 100 μm or more, with steps of providing a cold-rolled steel sheet, heating cold-rolled steel sheet up to a temperature T1 between 550° C. and Ac1+50° C. in a furnace zone with an atmosphere (A1) containing from 2 to 15% hydrogen by volume, so that the iron is not oxidized, then adding in the furnace atmosphere, water steam or oxygen with an injection flow rate Q higher than (0.07%/h×α), α being equal to 1 if said element is water steam or equal to 0.52 if said element is oxygen, at a temperature T≥T1, so to obtain an atmosphere (A2) with a dew point DP2 between −15° C. and the temperature Te of the iron/iron oxide equilibrium dew point, then heating the sheet from temperature T.sub.1 up to a temperature T.sub.2 between 720° C. and 1000° C. in a furnace zone under an atmosphere (A2) of nitrogen containing from 2 to 15% hydrogen and more than 0.1% CO by volume, with an oxygen partial pressure higher than 10.sup.−21 atm., wherein the duration t.sub.D of heating of the sheet from temperature T.sub.1 up to the end of soaking at temperature T.sub.2 is between 100 and 500 s., soaking the sheet at T.sub.2, then cooling the sheet at a rate between 10 and 400° C./s, then coating the sheet with zinc or zinc-alloy coating.

A steel sheet for the manufacture of a press hardened part is provided, having a composition of: 0.15%≤C≤0.22%, 3.5%≤Mn<4.2%, 0.001%≤Si≤1.5%, 0.020%≤Al≤0.9%, 0.001%≤Cr≤1%, 0.001%≤Mo≤0.3%, 0.001%≤Ti≤0.040%, 0.0003%≤B≤0.004%, 0.001%≤Nb≤0.060%, 0.001%≤N≤0.009%, 0.0005%≤S≤0.003%, 0.001%≤P≤0.020%. A microstructure has less than 50% ferrite, 1% to 20% retained austenite, cementite, such that the surface density of cementite particles larger than 60 nm is lower than 10{circumflex over ( )}7/mm.sup.2, and a complement of bainite and/or martensite, the retained austenite having an average Mn content of at least 1.1*Mn %. Press-hardened steel part obtained by hot forming the steel sheet, and manufacturing methods thereof.

A method for producing a resistance-welded member made of three or more sheets including a plated steel sheet includes: a main energizing by performing energization with a first current value while compressing the steel sheet with a first compressive force to form a nugget; a subsequent energizing by performing, after the main energizing, energization such that the current value gradually decreases from the first current value while compressing with a second compressive force greater than the first compressive force; and holding an electrode while maintaining the second compressive force after the subsequent energizing. The second compressive force and a total sheet thickness, compression rise delay time, and a downslope time and an electrode holding time satisfy respective predetermined conditions.

A method for producing a resistance-welded member made of three or more sheets including a plated steel sheet that includes: a first energizing with a first current value while compressing the steel sheets with a first compressive force to form a nugget; a subsequent energizing of, after the first energizing, energizing with a second current value smaller than the first current value while compressing the steel sheets with a second compressive force greater than the first compressive force; and holding an electrode by maintaining the second compressive force after the subsequent energization. The second compressive force and a total sheet thickness, the first current value and the second current value, and a subsequent energization time and an electrode holding time satisfy predetermined conditions respectively.

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.

A method of joining an aluminum workpiece and an adjacent overlapping steel workpiece by reaction metallurgical joining, and the resultant metallurgical joint formed between the two workpieces, are disclosed. The method involves compressing a reaction material located between the aluminum and steel workpieces and heating the reaction material momentarily to form a metallurgical joint that comprises bonding interface between the reaction material and the steel workpiece and a bonding interface between the reaction material and the aluminum workpiece. The reaction material is formulated to be able to interact with both aluminum and steel in order to establish the bonding interfaces of the metallurgical joint. Moreover, the practice of oscillating wire arc welding may be employed to deposit the reaction material in the form of a reaction material deposit onto the steel workpiece prior to assembling the steel and aluminum workpieces in a workpiece stack-up.

A method of resistance spot welding a workpiece stack-up that includes an aluminum workpiece and an overlapping adjacent steel workpiece so as to minimize the thickness of an intermetallic layer comprising Fe—Al intermetallic compounds involves providing reaction-slowing elements at the faying interface of the aluminum and steel workpieces. The reaction-slowing elements may include at least one of carbon, copper, silicon, nickel, manganese, cobalt, or chromium. Various ways are available for making the one or more reaction-slowing elements available at the faying interface of the aluminum and steel workpieces including being dissolved in a high strength steel or being present in an interlayer that may take on a variety of forms including a rigid shim, a flexible foil, a deposited layer adhered to and metallurgically bonded with a faying surface of the steel workpiece, or an interadjacent organic material layer that includes particles containing the reaction-slowing elements.

Aluminum alloy workpieces and/or magnesium alloy workpieces are joined in a solid state weld by use of a reactive material placed, in a suitable form, at the joining surfaces. Joining surfaces of the workpieces are pressed against the interposed reactive material and heated. The reactive material alloys or reacts with the workpiece surfaces consuming some of the surface material in forming a reaction product comprising a low melting liquid that removes oxide films and other surface impediments to a welded bond across the interface. Further pressure is applied to expel the reaction product and to join the workpiece surfaces in a solid state weld bond.

A system for spot welding two metal sheets together includes a plurality of conductive particles placed in an interface between the two metal sheets, a first electrode, and a second electrode. The first electrode and the second electrode are arranged to clamp the two metal sheets together such that the conductive particles become embedded into the metal sheets at the interface between the metal sheets. A conductive path is formed from the first electrode to the second electrode through the metal sheets and the plurality of metallic particles.