Shushi Ikeda

Microstructure and Mechanical Properties of Ausformed Ultra High-Strength TRIP-Aided Steels

Ultra high-strength low-alloyed TRIP-aided steels possess high impact toughness, fatigue strength and hydrogen embrittlement performance, as well as good formability. So, it is expected that the TRIP-aided steels are applied to some automotive parts such as not only center pillar but also driving parts, spring and bolts. In the present study, the effects of ausforming conditions (temperature and strain) after austenitizing on tensile properties and toughness of TRIP-aided steels with bainitic ferrite matrix were investigated to develop a new type of ultra high-strength forging steel. TRIP-aided steel with chemical composition of 0.2%C, 0.5%Si, 1.5%Mn, 1.0%A1, 0.02%Nb and 0.1 %Mo achieved large total elongation and high impact toughness by ausforming to 0-50% strain at 600-900°C, followed by austempering at 400°C for 500s. This was mainly caused by refined microstructure and stabilized retained austenite.

VERIFICATION OF THE SHB TEST RESULTS BASED ON THE ONE-DIMENSIONAL STRESS WAVE THEORY

In order to evaluate dynamic deformation behaviors under high strain rates, Kobe Steel has developed and applied a Split-Hopkinson Bar (SHB) apparatus. This paper discusses the validity of the strain measurements and strain rates measured by this SHB apparatus. The strain waves that propagated in the incident and transmitted bars and the specimen are captured using a high-resolution type high-speed photography in detail. The strain wave propagated many times in the incident and transmitted bars and the specimen when the specimen was not broken. The amount of the deformation of the specimen decreases with the propagation frequency of the incident wave. On the other hand, to improve accuracy at the strain and strain rate calculated by the one-dimensional stress wave theory, Youngs modulus, the longitudinal wave speed, and the density were accurately determined. It was understood that the calculation value showed the strain and strain rate captured with the high-speed photography are a good agreement. As a result, the validity of the measurement accuracy of this SHB could be shown.

Computational Evaluation of Micro- to Macroscopic Mechanical Characteristics of Low-Carbon TRIP Steel under Different Conditions

Low carbon TRIP steel shows quite complicated deformation behavior. The main purpose of this study is to clarify the deformation mechanism of low carbon TRIP steel by using a numerical simulation method. According to research works in the past, continuous transformation of retained austenitic phase is essential in order that TRIP steel may show favorable ductility, which implies that appropriate control of martensitic transformation is most important for improvement of ductility. Therefore, we built models for deformation-induced martensitic transformation and performed FEM analysis using homogenization method accounting for the chemical composition, temperature, and crystal orientation. As a result, the effects of chemical composition, temperature, and crystal orientation on the deformation and transformation behavior of low carbon TRIP steel were clarified quantitatively and the conditions to realize improved ductility in TRIP steel were suggested.