2019 年 60 巻 707 号 p. 340-345
In the hot working of multiphase steels, the thermal history before hot working affects the phase balance. The aim of this study is to investigate the effect of the thermal history just before hot working on the hot workability by performing hot uniaxial tensile 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, and its phase balance changes with the working temperature. The phase ratio of δ ferrite is higher at high temperatures, and the ratio of the austenite phase increases as the temperature decreases. To clarify the effect of the cooling rate just before hot working on hot workability, a hot tensile test was carried out with two extremely different cooling rates (0.3 °C/s, 10.0 °C/s) from the heating temperature of 1250 °C. The hot working test temperature was in the range of 650 to 1150 °C. The hot workability under the rapid cooling condition was higher than that under the slow cooling condition in the hot working temperature range of between 850 to 1150 °C, but this tendency was eliminated or reversed under conditions below 750 °C. The reason for this change was clarified by investigating both the phase balance just before hot working, which depends on the cooling rate, and the strength of each single phase. Thus, for accurate measurement of the hot workability of multiphase materials, consideration of the thermal history just before hot working is important.