Strain Rate and Temperature Dependent Plastic Deformation Behavior of Low-Carbon TRIP Steel

Abstract

Low-carbon TRIP steel has high strength and excellent formability and is one of the key materials to meet the demand for weight reduction of automobile body parts. However, in order to disseminate the materials, the clarification of a complex plastic deformation mechanism depending on the strain rate and temperature and formulation of a proper constitutive equation is essential. The present investigation experimentally clarifies the quantitative effects of the strain rate and temperature on the material characteristics by means of tensile tests under strain rates from 1.7×10-4 to 1.7 x 10-1 and temperatures ranging from -50 to 250°C. The deformation and transformation behaviors substantially change in the range of 100 to 150°C. With regard to the relatively the low temperatures up to 100°C, retained austenite transforms into martensite. The increase in strain rate causes a rise in the flow stress of the ferrite phase and an increase in temperature, which suppresses the martensitic transformation. As a balancing effect, the stress-strain relationship expresses a high degree of strain rate independency. In the later stage of uniform deformation, the effective TRIP effect declines under the high strain rate and the uniform elongation decreases. However, the tensile strength is almost unchanged due to the very small amount of work hardening. Meanwhile, for the high temperature range above 150°C, the retained austenite transforms into bainite which requires a longer transformation time, which yields the strong strain rate dependency of the deformation behavior and the tensile strength and uniform elongation increase as the strain rate decreases.