Cyclic deformation behavior of a transformation-induced plasticity-aided dual-phase steel
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I.
INTRODUCTION
Recently, a new type of 600 to 1000 MPa grade highstrength dual-phase sheet steel composed of a ferrite matrix and second phase (bainite plus retained austenite) was developed for the purpose of weight reduction of various automotive structural press parts.[1–7] The steel, which is named TRIP-aided dual-phase (TDP) steel, has excellent press formability associated with the transformation-induced plasticity (TRIP)[8] of the retained austenite, since the martensite-start temperature Ms is somewhat lower than room temperature, i.e., between 0 7C and 2100 7C.[2,3,4] So, many studies[2–8] on the formability have been conducted to apply the TDP steel to automotive underbody parts such as suspension arms and wheel disks. The underbody press parts are subjected to severe cyclic loading below or above yielding stress, so that high fatigue strength may be required, as well as further improvement of formability. However, there are a few basic findings[9] on TDP fatigue properties. In general, monotonic deformation of the TDP steel is controlled by the following two items: (1) long-range internal stress resulting from a difference of plastic strain between the ferrite matrix and the second phase;[10,11] and (2) strain-induced transformation (SIT) of the retained austenite resulting in both the increases in strain-induced martensite content and the relaxation of the long-range internal stress. These items are influenced by retained austenite stability, which is controlled by retained austenite parameters (carKOH-ICHI SUGIMOTO, Associate Professor, and MITSUYUKI KOBAYASHI, Professor, are with the Department of Mechanical Systems Engineering, Shinshu University, Nagano 380, Japan. SHIN-ICHI YASUKI, formerly Graduate Student, Department of Mechanical Systems Engineering, Shinshu University, is Metallurgical Engineer with Steel Wire Rod and Bar Development, Kobe Works, Kobe Steel Ltd., Kobe 657, Japan. Manuscript submitted August 19, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
bon concentration and morphology) and deformation conditions (temperature, strain rate, and state of stress).[2–6,10,11] If the retained austenite is stable against straining, then the retained austenite particle, as a hard phase, effectively enhances the internal stress during early stages.[10,11] The stable retained austenite tends to transform to martensite over a large strain range, thereby resulting in extremely large elongations by TRIP effects.[2,3,4] On the other hand, if the retained austenite is unstable, most of the retained austenite particles transform to martensite at an early stage. The resultant strain-induced martensite considerably increases the flow stress with relatively low elongations and relaxed internal stress.[10,11] The internal stress during early stages is assumed to play an important role in fatigue properties of the TDP steel, as well as the increased martensite content. In the present work, to understand the basic fatigue property of the TDP steel, cyclic hardening-softening behavior and the internal stress
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