The high Mn (≥ 25wt.%) TWIP (TWinning Induced Plasticity) steels with low stacking fault energy are currently one of the most attractive materials in automotive industry due to their unique combination of high strength and good ductility during mechanical loading. However, the conventional manufacturing processes have a lot of problems such as nozzle clogging during casting, high temperature oxidation, or poor surface quality with the high Mn. Therefore, there are a lot of effort to reduce Mn content, but the steels could not have an appropriate stacking fault energy (SFE) for twinning with decreasing Mn content. In this study, it was tried to make TWIP steels with lean Mn, that is, 18Mn TWIP (Fe-18Mn-1.5Al-0.6C) steel with average grain size 5 ㎛ and 12Mn TWIP (Fe-12Mn-2Si-0.9C) steel with average grain size 13 ㎛. Tensile properties of the lean Mn TWIP steels were determined at different strain rate range of 10-4s-1≤≤3102s-1 in order to investigate the effect of strain rate on the mechanical properties. The correlation between twinning behavior and flow stress at various strain rates was studied by strain-controlled test.
X-ray diffraction analysis revealed that neither - nor -martensite was formed. These steels with low Mn content were deformed by strain-induced twinning and showed total elongation of about 60% and ultimate tensile strength of about 1,100 MPa at room temperature.
In the case of stress-strain behavior at quasi-static strain rate (10-4s-1≤≤10-1s-1), the flow stress of both steels was decreased with increasing strain rate and exhibited a negative strain rate sensitivity because the volume fraction of twinned grain was decreased with increasing strain rate at the same amount of strain. Stress is directly related to the volume fraction of twinned grain.
In the case of stress-strain behavior at high strain rate (100s-1≤≤3102s-1), the effect of strain rate on the flow stress of 18Mn TWIP steel with average grain size 5 ㎛ was not significant and strain rate sensitivity was nearly zero. The strength of 12Mn TWIP steel with average grain size 13 ㎛ was rapidly increased at the strain rate over 100s-1. The flow stress was increased with increasing strain rate at the low strain region depending on strain rate strengthening. But the flow stress was decreased with increasing strain rate at the large strain region. This meant that flow stress was dependant on strain rate at the low strain region and both deformation twin and strain rate took part in stress-strain behavior at the large strain rate.
12Mn TWIP steel with average grain size 13 ㎛ appeared positive strain rate sensitivity to flow stress compared to 18Mn TWIP steel with average grain size 5 ㎛ up to 10% strain, therefore 12Mn TWIP steel was profitable in terms of absorbed energy in car crash resistance.
The main factor in high strain hardening of TWIP steel is the role of twins. In addition to the classical mechanism of dislocation gliding, TWIP steel deforms also by twinning inside the austenite grains. While straining, the fraction of deformation twins and the density of twin boundaries increases. The several deformation twins in the same austenite grain contribute to reduce the effective grain size, and then microstructure is finer. When local necking is occurred, twin boundaries are acting as strong barriers to gliding dislocations in a similar manner as grain boundaries of austenite matrix do. The instantaneous hardening rate, strain hardening exponent(n) is maintained at a high strain level as deformation takes place in other local areas which possess lower flow stress. This phenomenon leads to the excellent combination of high strength and high ductility.