Dynamic Tensile Testing of Ultrahigh Strength Hot Stamped Martensitic Steels

Tensile testing over strain rates of 0.001, 1, 100, and 200 s(-1) is performed on three novel ultrahigh strength hot stamped martensitic steels, namely 38MnB5, 15MnCr5, and 25MnVB5, in addition to the conventional "boron steel" for automotive hot stamping technologies, 22MnB5. Each steel generally demonstrates positive strain rate sensitivity (increasing tensile strength) with increasing strain rate from 0.001 to 1 s(-1), but negative strain rate sensitivity (decreasing tensile strength) with increasing strain rate from 1 to 200 s(-1). The notable exception to the above is 38MnB5, which demonstrates consistently increasing ultimate tensile strength across all four strain rates. Moreover, each steel generally demonstrates maximum elongation at strain rates of 100 or 200 s(-1). The response of 38MnB5 to increasing strain rate gives rise to significantly higher modulus of toughness (energy absorption) compared to 22MnB5 at the higher strain rates. It is concluded that 38MnB5 should provide superior "anti-intrusive" crash performance under low-speed impact owing to significantly higher tensile strength, yet superior "impact energy absorptive" crash performance under high-speed impact owing to significantly higher modulus of toughness.