汽车用材1

钢铁材料

IMPACT-DYNAMIC BEHAVIOUR OF AL-TRIP STEEL

P. Verleysen1, J. Van Slycken1, J. Degrieck1, B.C. De Cooman2 and L. Samek2

1

Mechanics of Materials and Structures,

Department of Mechanical Construction and Production,

Ghent University (UGent)

E-mail: Patricia.Verleysen@UGent.be 2

Laboratory of Iron and Steelmaking,

Department of Metallurgy and Materials Science,

Ghent University (UGent)

ABSTRACT

In recent years TRIP-steels (TRansformation Induced Plasticity-steels) have been developed. TRIP-steels are composite steels composed of ferrite, bainite and retained austenite. During plastic deformation, the austenite phase transforms to martensite, and this gives rise to an exceptional mechanical behaviour of the material: high strength levels (yield strength, tensile strength, …) are combined with an excellent ductility. The resulting high energy dissipation makes TRIP-steels extremely suitable for energy-absorbing devices such as the bars used in the crumpling zone of a car. To guarantee a controlled dissipation of the energy released during a crash, knowledge and understanding of the impact-dynamic material properties is essential. In this contribution results of an extensive experimental program to investigate the strain rate dependent mechanical properties of a TRIP-Al-steel are presented. A split Hopkinson tensile bar set-up was used for the experiments. Next to the TRIP-material, also the three constituent phases of the TRIP-steel were produced and subjected to high strain rate loading. From the results it is clear that TRIP steels, also in dynamic circumstances, show excellent mechanical properties.

1 INTRODUCTION

Due to increased passenger safety and comfort, the weight of passenger cars has continuously increased in recent years. Since an increased weight leads to higher fuel consumption and greenhouse gas emissions, car manufacturers try to reduce the weight of the car structure, which led to an increased use of high strength steel grades. Low alloy TRIP-aided steels (Transformation Induced Plasticity) constitute a new category of steels with high tensile strengths and a high uniform elongation. The high strain rate performance of TRIP steels makes them ideally suited for safety-related structural automotive applications. During automobile collisions, these steels offer large dynamic energy absorption. This is due to a high strain rate composite effect, i.e. the complex synergy of the high strain rate behaviour of the ferrite, bainite and meta-stable austenite within the TRIP-aided steel microstructure. During a crash, the material is strained very rapidly at strain rates which may reach 1500/s. When the strain rate is high like this, the thermal conductivity of the material is not sufficient for the heat generated within the specimen to be dissipated to the environment. The temperature rises due to the quasi-adiabatic deformation. The specimen temperature can reach 100 to 120°C during dynamic loading (Samek [1]). Therefore in dynamic conditions, the strain rate and quasi-adiabatic effects play a major role during straining. Because the austenite stability varies with the temperature, next to the chemical composition, the size of the phases, the stress state and the strain rate, the effect of the temperature has to be taken into account. An eventual rapid austenite transformation to martensite in the early stages of deformation can be detrimental to ductility. In the context of TRIP-aided alloys developed for the automotive industry for increased safety requirements during crashes, the prediction of the temperature and strain rate dependent mechanical behaviour is of considerable importance.

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