Abstract: The influence of the cooling rate immediately before hot working on flow stress and the microstructure of 25% Cr δ ferrite-austenite duplex stainless steel was investigated by a hot compression test at 1000-1200℃. In spite of the fact that the compression temperature was the same, the condition of rapid cooling immediately before hot working greatly reduced the maximum stress σmax and work hardening coefficient n by reason of a larger volume fraction of supercooled δ ferrite phase. Two different flow stresses were introduced in a hot rolling FE analysis model in order to compare the influence of the thermal history immediately before hot rolling on the rolling load and strain distribution. The results of the FE analysis showed that the flow stress under the condition of rapid cooling immediately before hot working resulted in an approximately 30% lower rolling load, and that rolling strain was concentrated on the plate surface portion compared with the center of plate thickness. This research indicated that it is necessary to consider the thermal history immediately before hot working in the measured flow stress when the hot-rolled material has a dual phase structure and when the change in the volume fractions of the phases with temperature change is delayed in comparison with the thermal equilibrium state.
Authors: Shunsuke Sasaki, Tatsuro Katsumura, Hiroki Ota, and Jun Yanagimoto