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By designing the orthogonal experiment of resistance spot welding process, the range of spot welding process parameters of Q & P980 galvanized high-strength steel was determined, meanwhile the microstructure characterization and the mechanical properties analysis of the welding joint were carried out. The results show that the microstructure of nugget zone is mainly staggered lath martensite, whereas the microstructure of HAZ is composed of lath martensite, residual austenite and ferrite. The maximum average width of martensite lath in incompletely quenched zone is 4.86 μm.The microhardness test exhibits that the hardness value of the welding joint shows a "W"-shaped symmetrical distribution. The peak hardness appears in the fine-grain zone, which is 559HV, while the minimum hardness appears in the incompletely quenched zone, which is 338HV. The incompletely quenched zone shows obvious softening phenomenon. The room temperature tensile tests of the welding joints are carried out, and the peak value of maximum tension-shear load is 27.92 kN. The fracture morphology shows typical dimples, which belongs to ductile fracture.Since the melting point of zinc is lower than that of the steel matrix, the zinc layer is prior to melting and penetrates downward along the grain boundary of the matrix at the welding joints, and then the liquid metal embrittlement cracks can be observed obviously.

The aim of the present work is to evaluate the effect of ultrafast heating on the microstructure and mechanical properties of hot forming steel. The initial microstructure utilized in this study was a cold-rolled microstructure, and the test steel was heated to full austenitization at a rate of 200 °C/s, followed by water quenching. It was observed that the ultrafast heating process significantly refines both the prior austenite grains and martensite laths while inheriting high-density dislocations from the initial cold-rolled microstructure. Consequently, the coupling mechanism between dislocation strengthening and grain refinement strengthening remarkably enhanced both the yield strength and ultimate tensile strength of the test steel. Eventually, the yield strength of the hot forming steel reached 1524 MPa, along with an ultimate tensile strength of 2221 MPa and uniform elongation of 5.2%. [ABSTRACT FROM AUTHOR]

Friction stir processing (FSP) manufacturing technology was used to fabricate medium Mn advanced high-strength steel in this study. The mechanical properties and microstructure of the steel fabricated using FSP were investigated. The steel obtained a total elongation of 35.1% and a tensile strength of 1034.6 MPa, which is about 59% higher than that of the steel without FSP. After FSP, a gradient structure occurs along the thickness direction. Specifically, across the thickness direction from the base material zone to the transition zone and finally to the stirring zone, both the grain size and austenite fraction decrease while the dislocation density increases, which results from the simultaneous effect of severe plastic deformation and recrystallization during FSP. Due to the gradient structure, an obvious difference in the strain across the thickness direction of the steel occurs during the deformation process, resulting in significant hetero-deformation-induced (HDI) strengthening. The deformation mechanism analysis reveals that HDI strengthening and dislocation strengthening are the main factors in the improvement in the strength–ductility balance. The obtained knowledge sheds light on the process of fabricating medium Mn steels with excellent properties using FSP manufacturing technology. [ABSTRACT FROM AUTHOR]