Effect of Thermal History on Hot Workability of Fe–Cr Multiphase Steel during Hot Working

Abstract

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 immediately before hot working on 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 ¤ ferrite and austenite, and its phase balance changes with the working temperature. The phase ratio of ¤ 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 immediately 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°C to 1150°C. Hot workability was higher under the rapid cooling condition than under the slow cooling condition in the hot working temperature range of 850°C to 1150°C, but this tendency was eliminated or reversed at temperatures below 750°C. The reason for this change was clarified by investigating both the phase balance immediately before hot working, which depends on the cooling rate, and the strength of each single phase. Therefore, in order to accurately measure the hot workability of a multiphase material, it is important to consider the thermal history immediately before hot working.

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