Hot deformation mechanisms, mechanical properties and microstructural evolution of a HP-Nb steel

In this study hot deformation mechanisms, mechanical properties and microstructural evolution of a HP-Nb steel used as reformers tube in oil and gas industries was examined. Hot tensile tests were performed on the as-cast samples in temperature range from 700 °C to 1000 °C with 100 °C interval and under various strain rates. Microstructural analyses of the hot tensile samples were performed using optical microscope and Field Emission Scanning Microscope (FE-SEM). Constitutive equations were constructed to define deformation regimes. In that regard, and based on apparent activation energy, strain rate sensitivity and Zener-Hollomon parameters were calculated. Results showed that increasing temperature after 800 °C substantially reduced the flow stress. Such an effect was a little offset by increasing the strain rate. Strain rate sensitivity was remarkably increased at 800 °C and again at 1000 °C which indicated an increase in ductility due to the occurrence of dynamic recrystallization (DRX) at these temperatures. Strain rate sensitivity was relatively low at 700 °C and 900 °C with the latter being higher than 700 °C. It was seen that increasing the deformation temperature to 900 °C led to a decrease in the possibility of DRX due mainly to strain induced precipitation (SIP) and solute drag effect of Nb element. It was also observed that increasing the engineering strain rate from 0.002 s⁻¹ significantly increased the flow stress due probably to the domination of work hardening over restoration phenomena, particularly at 700 °C. As well, it was found that when deformation temperature increased, particularly at lower strain rate regimes, more dynamically recrystallized grains were developed. A contour map of Zener-Hollomon parameter together with the strain rate and temperature is constructed which defined the deformation regime under various combinations of strain rates and temperatures.