A mechanical clinching process with preforming of a lower sheet was developed to join two ultra-high strength steel sheets having low ductility. The thickness reduction of the upper sheet around the punch corner becomes small because of the decrease in the pressure from the preformed lower sheet, and then the interlock is increased by the expansion in the radial direction in the final compression. The shapes of the die and preformed lower sheet were optimized to increase the interlock. Furthermore, the strength of the sheets joined by mechanical clinching with the preformed lower sheet was compared with that of the sheets joined by resistance spot welding. The fatigue limit of the sheets joined by clinching was greater than that of the welded sheets, while the static load of the mechanically clinched joint was smaller. It was found that the mechanical clinching process with preforming of the lower sheet was effective for joining ultra-high strength steel sheets having low ductility and high flow stress.

In the hot working of steel, the thermal history before hot working affects the phase balance, particularly in multiphase steels. Knowledge of the flow stress at elevated temperatures is indispensable for predicting the forming load and strain distribution during hot working. The aim of this paper is to investigate the influence of the thermal history before hot working on flow stress by performing hot compression tests. Duplex stainless steel was chosen as the test material as this material has a typical dual-phase structure consisting of δ ferrite and austenite, in which the phase balance displays temperature dependence. To clarify the effect of the cooling rate before hot working, the cooling rate from the heating temperature at 1250 ℃ was set to two extremely different levels (0.1 ℃/s, 30.0 ℃/s). The compression test temperature was varied in the range of 1000―1200 ℃ (50 ℃ increments). In comparison with the low cooling rate, the results showed that the flow stresses at the higher cooling rate were greatly reduced at all test temperatures. Specifically, large decreases were observed in the work hardening coefficient n, maximum strain σ_max and steady stress F ₃, whereas the strain rate sensitivity index m was increased. This behavior was caused by the deviation of the actual phase balance before hot working from the equilibrium state owing to rapid cooling, resulting in the existence of a large volume fraction of the soft δ ferrite phase during hot working.