Provided is a galvanized steel sheet with excellent resistance to cracking in resistance welding at a welded portion, even if crystal orientations of an Fe-based electroplating layer and a cold-rolled steel sheet are integrated at a high ratio at the interface between the Fe-based electroplating layer and the cold-rolled steel sheet. The galvanized steel sheet has a Si-containing cold-rolled steel sheet containing Si in an amount of 0.1 mass % to 3.0 mass %; an Fe-based electroplating layer formed on at least one surface of the Si-containing cold-rolled steel sheet with a coating weight per surface of more than 20.0 g/m.sup.2, and a galvanized layer formed on the Fe-based electroplating layer, where crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet are integrated at a ratio of more than 50 % at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet.

Provided is a resistance spot welding method that inhibits, in accordance with the degree of axis misalignment between electrodes, the occurrence of cracking in a weld regardless of the steel grade. In resistance spot welding methods according to the present invention, H (ms) is an electrode force retaining time after completion of current passage, D (mm) is an amount of axis misalignment between the electrodes, t (mm) is a total sum of sheet thicknesses of a plurality of overlapping steel sheets, T (MPa) is a tensile strength of a steel sheet having a highest tensile strength among the plurality of steel sheets, F (N) is an electrode force, and d (mm) is a tip diameter of one electrode of the pair of electrodes that has a smaller tip diameter. The electrode force retaining time H is specified to be a predetermined value or greater.

A resistance spot welding method able to form a welded joint having delayed fracture resistance, includes, in order, applying a pressing force P1 (kN) to the plurality of steel sheets by welding electrodes while supplying a supplied current I1 (kA), while applying the pressing force P1, supplying a current Ic (kA) during a cooling time tc (s), repeating two times or more a pressure force raising and lowering cycle of supplying a supplied current I2 (kA) to the welding electrodes while applying a pressing force P2 (kN) to the plurality of steel sheets by the welding electrodes during a pressing time tf (s), then immediately applying a pressing force P3 (kN) during a pressing time ti (s), applying the pressing force P2 during the pressing time tf, and releasing the pressing force and ending the supply of current, satisfying 0Ic<I1, 0.3I2/I1<1.0, P2/P12, tf0.2, P3<P2, and ti0.2.

Provided is an automotive member, or particularly, a steel sheet having a tensile strength exceeding 900 MPa. The automotive member has a resistance weld for fixing two or more steel sheets containing a predetermined composition, in which a maximum hardness (HV.sub.BM) in a heat-affected zone of the resistance weld is at least 1.1 times hardness (HV.sub.W) of a nugget in the resistance weld formed in a softest steel sheet of a sheet set during resistance welding, and furthermore, an average grain size of a steel sheet structure of the heat-affected zone within 2 mm in a direction at a right angle to a sheet thickness from an end part of the nugget of the high-strength steel sheet is 3 m or less, and a minimum hardness (HV.sub.min) in the heat-affected zone is at least 90% of hardness (HV.sub.) of the high-strength steel sheet before the resistance welding.

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.

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 0.5D to 1.0D 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 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.

A method of resistance welding includes providing a first steel workpiece and second steel workpiece each having a zinc (Zn) coating. A first portion of the first steel workpiece and a second portion of the second steel workpiece are heated to at least about 500 C. allowing the zinc (Zn) coating to melt and dissolution of iron Fe from the first and second steel workpieces into the melted zinc (Zn) coating to form at least one of a plurality of Iron-Zinc intermetallic compounds. The first portion of the first workpiece and the second portion of the second workpiece are cooled to a first temperature. The first workpiece is disposed in contact with the second workpiece such that the first portion of the first workpiece is aligned with the second portion of the second workpiece. The first portion of the first workpiece is welded to the second portion of the second workpiece.

A cold-rolled and heat-treated steel sheet-having a microstructure consisting of, in surface fraction: between 10% and 30% of retained austenite, said retained austenite being present as films having an aspect ratio of at least 3 and as Martensite Austenite islands, less than 8% of such Martensite A islands having a size above 0.5 m, at most 1% of fresh martensite at most 50% of tempered martensite and recovered martensite containing precipitates of at least one element chosen among niobium, titanium and vanadium.

It also provides a manufacturing method thereof.

A metallic component includes a core formed of steel. A zinc alloy layer is disposed on the core. The zinc alloy layer is formed of zinc and a very small amount of aluminum, such as 0.14 weight percent or less. A method of creating a component includes providing a steel core, providing a zinc bath consisting of essentially of 0.01 to 0.14 weight percent aluminum, and hot dipping the steel core into the zinc bath to form a zinc coating on the steel core resulting in a zinc-coated steel component. The aluminum may be provided in even lower contents, such as less than 0.08 weight percent, or even less than 0.05 weight percent. The zinc-coated steel component may then be spot welded to another component without first annealing the zinc-coated component. Rather, heat treating is performed locally at the weld joint by the welding procedure alone.