Grain refinement technology for duplex stainless steel using rapid cooling immediately before hot working

Abstract

For the purpose of grain refinement and improvement of the hot formability of duplex stainless steel, a new concept of the thermomechanical processing method using the austenite phase transformation from the δ ferrite phase in the hot working temperature range was proposed. The supercooled δ ferrite phase is generated by rapid cooling immediately before hot working, and crystal grains are refined by the austenite transformation induced by hot working immediately after rapid cooling. Furthermore, since the new thermomechanical processing method is characterized by a large amount of soft δ ferrite phase during hot working, reduction of hot flow stress and improvement of hot ductility are possible in addition to grain refinement. The material used here was 25 % Cr duplex stainless steel, and the temperature history was controlled by combining induction heating and nitrogen gas cooling. It was confirmed that the δ ferrite phase was obtained as the parent phase by initial heating at 1320 °C, and a supercooled δ ferrite phase was generated by rapid cooling at 30.0 °C/s from the initial heating temperature to the hot working temperature of 1000 ℃. When uniaxial compression of 60 % at strain rates of 0.1, 1.0 and 10.0 s −1 was applied to the supercooled δ ferrite phase after cooling to 1000 ℃, nucleation of austenite grains was induced by the degree of supercooling and compressive strain, and as a result, the microstructure was successfully refined by strain-induced fine austenite grains. The grain diameter of the austenite grains obtained by strain-induced transformation was several μ m. The flow stress under thermomechanical processing, which succeeded in microstructure refinement, was much lower than that under the condition simulating the conventional process. The hot ductility investigated in the uniaxial tensile test was also greatly improved under thermomechanical processing. Finally, the results of a detailed crystal orientation analysis revealed that the nucleated fine austenite grains obtained by strain-induced transformation have a K–S relationship with the supercooled δ -ferrite parent phase.