The influence of the heat generation during deformation on the mechanical properties and microstructure of the selected TWIP steels

Abstract

The TWIP (Twinning Induced Plasticity) steels are one of the most promising materials in reducing the weight of vehicles. Despite a lot of research on TWIP steel, there are some issues that are not explained enough. Due to the future use of TWIP steel and the manufacturing of the final part by metal forming, three issues still need to be clarified. The first one, which is the most important, is the increase of the temperature due to the conversion of the deformation work into heat. TWIP steel has a high limit strain, strength and lower conductivity than conventional steel, therefore the heat generation of TWIP steel is greater than for other materials. The second and third issues are combined. They concern the influence of V microadditions on the stress–strain curves, the strain hardening coefficient n and the strain rate sensitivity coefficient m under cold deformation conditions. These properties determine the cold formability of TWIP steels. In the research, two TWIP steels were used with and without V microadditions (MnAl and MnAl-V steel). The special methodology using strain and temperature measurement systems as well as light and scanning electron microscopy (SEM) were applied. Research shows a significant increase of the temperature in the material due to high plastic deformations as well as a high level of yield stress. In the neck area, for the highest strain rate of 0,1 s -1, at the moment of rupture, the temperature reaches more than 200 °C. The difference between the average temperature in the rupture area and the maximum temperature is equal to 100° C. Its high increase can lead e.g. to changes in the deformation mechanism from twinning to dislocation gliding, which is also connected with a worsened workability, and thus also energy consumption of the bodywork elements. MnAl-V steel has better or similar ductility for the deep drawing in comparison to MnAl steel at low strain rates for almost isothermal conditions (constant temperature during deformation). However the MnAl steel has better ductility for the larger strain rates over 0.1 s−1 then there is large heat concentration in a very narrow area for MnAl-V steel. The obtained results are very important from an application point of view. The strain rate sensitivity coefficient m of the steel MnAl has very low, and even negative, values, which can make the production of complicated drawpieces difficult. Higher values of the strain rate sensitivity coefficient are exhibited by steel MnAl-V, i.e. at the level of 0,05, which is almost constant in the whole range of the obtained deformations.