The characterisation of deformation behavior of high and ultra-high strength steels at warm forming temperatures

Nowadays, due to the decrease of fossil fuel resources and as well as their adverse effects on the environment, light and high strength materials have been started to be preferred especially on the new generation vehicles. In that matter, high and ultra-high strength steels, thanks to their high strength values as compared to low alloyed steels, have enabled vehicle parts to be manufactured with lesser thickness and have gained an important stance in forming the future of automotive sector. Conversely, the high strength values of high and ultra-high strength steels bring up problems in terms of their formability which limits their use in more complex shaped parts. The formability of these steels can be increased with the aid of warm forming method. However, it is first needed to characterize the deformation behaviour of these steels under the effect of warm temperatures and understand how the warm temperatures affect the deformation of these steels. For this reason, in this study, tensile tests have been conducted for DP600 and DP800 high strength steels at room temperature, 75, 175, 275 and 375 °C and for MART1200 and MART1400 ultra-high strength steels at room temperature, 175, 275 and 375 and 475 °C temperatures at two different strain rates (0,05 s-1 ve 0,005 s-1). From the results of the tensile tests, the variations in the instantaneous strain hardening exponent, instantaneous strain rate sensitivity exponent, elongation and hardening capacity and the changes in the microstructure of the steels have been investigated. According to the results of the study, for both kinds of steels, manifestations of dynamic strain aging (DSA) phenomenon have been observed at 175 °C and 275 °C temperatures and it has been concluded that the deformation processes of these steels at these temperatures would not be advantageous in terms of their formability. While for the high strength steels, an improvement has been observed at 75 °C and 375 °C, for the ultra-high strength steels improvements in elongation capacity has been observed at 375 °C and 475 °C temperatures. It has been concluded that while the improvement on the elongation capacity of high strength steels might have been due to the decrease of strength gradients between ferrite and martensite grains present in the microstructure, for the ultra-high strength steels the improvement of elongation capacity might have been due to the tempering of martensite.