Dynamic recrystallization behavior of austenite in Nb-bearing high strength low alloy steels and stainless steel.

Abstract

Dynamic recrystallization behavior of austenite in Nb-bearing HSLA steels as well as 18-8 stainless steel was investigated by hot compression testing. The hot deformation equipment, incorporating a rapid cooling system and an on-line data analysis system, made it possible to investigate the microstructural changes associated with the restoration process by fully suppressing the static recovery and recrystallization. The stress-strain behavior to a strain of 0.70 was investigated in the range of strain rate from 5×10^-4 to 10s^-1 and of temperature from 900 to 1200°C. Dynamic recrystallization was observed in all the steels deformed under the condition of the Zener-Hollomon parameter being less than 10¹⁵s^-1, but it was not observed with the highest strain rate of 10s^-1. An increase of Nb content in the HSLA steels increased the flow stress for any deformation condition and also increased the strain at onset of dynamic recrystallization. Dynamically recrystallized grain size was primarily related to the value of the Zener-Hollomon parameter, but was not influenced by the initial grain size. The microstructural changes accompanying dynamic recovery and dynamic recrystallization were investigated by thin foil TEM observation and related to the results of hardness measurements.