A multilayer steel includes a core formed of transformation-induced plasticity (TRIP) steel. A decarburized layer is exterior to the core on at least one side thereof. The decarburized layer has reduced carbon content relative to the core. A zinc-based layer is exterior to the decarburized layer. The decarburized layer may have a composition of at least 80 percent ferrite, such that LME is reduced or mitigated. In some configurations, the decarburized layer is between 10-50 microns thick. A method of creating a coated advanced high-strength steel component is also provided. An apparatus for forming a coated advanced high-strength steel is also provided. The core of the multilayer steel may have a carbon weight-percent of less than or equal to 0.4. The decarburized layer of the multilayer steel may have a carbon weight-percent of less than or equal to 50 percent of the carbon weight-percent of the core.

A resistance spot welding method for joining two or more steel sheets including at least one zinc-coated steel sheet. The method includes a first current applying step and a second current applying step. The first current applying step involves forming a nugget having a nugget diameter of 3?t or more and 4.5?t or less by setting a current value I1 (kA) and a weld time, where t is a thickness of the thinnest steel sheet among the overlapping steel sheets. The second current applying step involves growing the nugget by repeating a cooling step for maintaining a zero-current state for 10 ms or more and less than 160 ms and a current applying step for applying a current for 20 ms or more and less than 200 ms at a current value I2 (kA) greater than or equal to the current value I1 (kA).

A welding electrode and a method of using the welding electrode for resistance spot welding are disclosed. The welding electrode includes a body and a weld face. The weld face includes a central dome portion and a shoulder portion that surrounds the central dome portion and extends from an outer circumference of the weld face upwardly and radially inwardly to the central dome portion. The central dome portion has a series of radially-spaced ringed ridges that project outwardly from a base dome face surface. The series of radially-spaced ringed ridges on the central dome portion includes an innermost ringed ridge and an outermost ringed ridge. The outermost ringed ridge on the central dome portion has a radial inner side surface and a radial outer side surface. The radial outer side surface extends below the base dome face surface down to the shoulder portion of the weld face.

A method for welding multiple workpieces together includes applying a force to the multiple workpieces, generating ultrasonic vibration, transferring the ultrasonic vibration to the multiple workpieces to breakdown an oxide layer, generating an electric current, transmitting the electric current to heat up the workpieces, and synchronizing the ultrasonic and resistance heating operations. A welding system includes an ultrasonic vibration unit that generates an ultrasonic vibration and transfers the ultrasonic vibration to multiple workpieces to breakdown an oxide layer, a resistance heating unit that generates an electric current and transmits the electric current to heat up the workpieces, a workpiece mount that includes electrodes configured to receive the generated current and/or clamp the multiple workpieces during a welding process, and a controller configured to synchronize an operation of the ultrasonic vibration unit and an operation of a resistance heating unit.

This disclosure relates to weldability of steel alloys that provide weld joints which retain hardness values in a heat affected zone adjacent to a fusion zone and which also have improved resistance to liquid metal embrittlement due to the presence of zinc coatings.

A resistance spot welding method in which a high strength zinc-based coated steel sheet used in a sheet set has an equivalent carbon content of 0.20% or more and a tensile strength of 780 MPa or more. The method includes performing a first energization step such that a current value at the end of the energization, a current value Is at the start of the energization, and a thickness of the thinnest steel sheet among steel sheets overlapping each other satisfy specified relationships. The method includes performing a second energization step in which a cooling step of holding a non-energization state for 10 ms or longer and shorter than 160 ms and performing energization at a current value equal to or more than the current value at the end of the first energization step for 20 ms or longer and shorter than 200 ms are repeated.

A spot welded member includes a spot weld formed by holding a sheet stack of multiple steel sheets between a pair of electrodes and spot-welding the sheet stack, in which at least one of the multiple steel sheets is a high-strength zinc-based coated steel sheet having a tensile strength of 780 MPa or more, the high-strength zinc-based coated steel sheet having a coating with an Al content of 0.5% or more by mass, and in which the heat shock region of the spot weld outwardly extending from an edge of a corona bond area includes a coated layer including an FeAl alloy layer having an average thickness of 0.3 ?m or more and a zinc-based coated layer having an average thickness of 2.0 ?m or more on the FeAl alloy layer at the interface between the base steel sheet of the high-strength zinc-based coated steel sheet and the coating.

A steel welded member comprising a plurality of Zn-based plated steel materials, each having a Zn-based plating layer on a surface of a steel material, joined together through at least one spot weld, wherein the steel material contains, by mass %, C: 0.05 to 0.40%, Si: 0.2 to 3.0%, and Mn: 0.1 to 5.0%, the steel welded member includes, by area ratio, 20 to 80% of a ? phase (Fe.sub.3 Zn.sub.1 0) and at least one oxide with a long axis of 0.5 ?m or more in a region up to 0.5 mm from an end part of a pressure weld of the spot weld.

This spot welded joint includes a first steel sheet having tensile strength of 1100 MPa or higher and hard martensitic structure as main structure, a second steel sheet stacked on the first steel sheet, a nugget having diameter D at an interface between the first and second steel sheets and formed between the first and second steel sheets, and a hardness control region occupying, in a cross section of the first steel sheet in sheet thickness direction that passes a nugget center, a region that is the first steel sheet in sheet thickness direction and is from 0.5?D to 1.0?D from the nugget center in sheet surface direction and in which difference between maximum value and minimum value in hardness in the region is 80 HV or less, and the maximum hardness value in the region is lower than the maximum hardness value of the first steel sheet.

A multilayer steel includes a core formed of transformation-induced plasticity (TRIP) steel. A decarburized layer is exterior to the core on at least one side thereof. The decarburized layer has reduced carbon content relative to the core. A zinc-based layer is exterior to the decarburized layer. The decarburized layer may have a composition of at least 80 percent ferrite, such that LME is reduced or mitigated. In some configurations, the decarburized layer is between 10-50 microns thick. A method of creating a coated advanced high-strength steel component is also provided. An apparatus for forming a coated advanced high-strength steel is also provided. The core of the multilayer steel may have a carbon weight-percent of less than or equal to 0.4. The decarburized layer of the multilayer steel may have a carbon weight-percent of less than or equal to 50 percent of the carbon weight-percent of the core.