changes, which leads to a broader band for the yield strength and tensile strength curves than for the other steel grades. Compared to room temperature a decrease of 100 MPa in yield and tensile strength can generally be observed at 80°C for strain rates up to 10s-1. However, Figure 2 indicates that the high strain rate sensitivity is similar for each temperature, i.e. the curves related to the lower yield strength at different strength levels have the same slope. This high strain rate sensitivity allows 301LN to catch up with DP500G in yield strength at low strain rates after ε& = 10s-1. At lower temperature the γ→α’ transformation increases the strain hardening rate leading to a typical discontinuous tensile curve and very high values for the tensile strength accompanied by a decrease of the uniform elongation as shown on Figures 3 and 4. The influence of strain rate is related to adiabatic heating during the deformation, which restricts the γ→α’ transformation. Hecker et al. have also observed the latter on austenitic stainless steel 304 [6]. Currently running volume fraction measurements, performed on the specimens tested at ε& = 10-2s-1 and ε& = 10s-1 at increasing deformation levels, are confirming lower martensite volume fraction at high strain levels for high strain rate conditions. However, martensite is formed more readily at low strain levels during high strain rate testing than during quasi-static loading. Energy absorption The strain rate dependency at room temperature of the energy absorption of the different steels is indicated in Figure 5. The energy absorption is calculated at 10% true strain taking into account that in a crash situation automotive parts will hardly deform until fracture. On this figure the ratio of the dynamic over static absorbed energy is also plotted showing the higher ratio for the austenitic stainless steel and the ferritic steel IFHSS260 but naturally at lower energy levels. The evolution of energy absorption over the whole range of strain rate illustrates the need to characterise materials dynamically. Indeed ZStE420 and DP500G show the higher energy absorption under quasi-static testing conditions but the absorbed energy of 301LN increases stronger with the strain rate and finally, at ε& = 1100s-1, reaches a value 6% higher than DP500G. 0 10 20 30 40 50 60 70 80 IFHSS260 ZStE420 DP500 G 301LN Energy absorption at =10% (MJ/m³) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Dynamic Eabs / Static Eabs 5.10-³ s-1 10 s-1 1100 s-1 Eabs 1100s-1 Eabs 5.10-³s-1 Room temperature Figure 5: Energy absorption at 10% true strain (columns) and dynamic over static absorbed energy ratio (line) for the investigated steel grades at room temperature The same behaviour is expected at 80°C with a more pronounced difference between dynamic and static conditions due to the lower yield strength of 301LN under quasi-static testing conditions. This great potential of dynamic energy absorption of 301LN is due to its higher strain hardening well described in Figure 6 showing the evolution of the dynamic strain hardening coefficient with strain at 80°C. Consequently, the difference in dynamic energy absorption between 301LN and the other steel grades like DP500G becomes larger when considering a higher true strain value than 10%.
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