High Manganese Advanced High Strength TWIP Steel for Automotive Applications
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High Manganese Advanced High Strength TWIP Steel for Automotive Applications Bruno C. De Cooman Materials Design Laboratory, Graduate Institute of Ferrous Technology Pohang University of Science and Technology, Pohang, South Korea ABSTRACT High Mn TWinning-Induced Plasticity (TWIP) steels have mechanical properties which make them suitable for effective vehicle mass containment and an enhanced passenger safety in automotive applications. High Mn TWIP steels with additions of C and Al are fully austenitic at room temperature and have a stacking fault energy (SFE) within the narrow range of 20-30 mJ/m2 required for mechanical twin formation. The present contribution reviews the state-of-thescience on TWIP steels, and highlights those areas where there is still a lack of fundamental understanding of their properties, such as the effect of the anti-ferromagnetic transition, the influence of interstitial C, the twinning mechanism, the effect of slip and twinning on the crystallographic texture evolution and the delayed fracture phenomenon. INTRODUCTION High manganese TWIP steels are highly ductile, high strength 15-30 mass% Mn austenitic steels characterized by a high rate of work hardening resulting from the generation of deformation-nucleated twins [1-10]. The focus of the present contribution is on TWIP steels with 15-25 mass-% Mn, with 0-3 mass-% Al, 0-3 mass-%Si and 200-6000ppm C. The dominant deformation mode in TWIP steels is dislocation glide. In addition, deformation-induced twinning gradually reduces the effective dislocation glide distance and gives rise to a “Dynamical HallPetch effect”. The mechanical properties of typical TWIP steels are reviewed in figure 1.
Dislocation source
Twin
Λ
Λ
Λ: dislocation mean free path
Figure 1. Illustration of the dynamical Hall-Petch effect (left). Typical ranges for the mechanical properties of TWIP steel (right).
THERMODYNAMIC PROPERTIES OF Fe-Mn-C ALLOYS The meta-stable binary Fe-Mn diagram is shown in figure 2. Stabilizing the austenite at room temperature requires Mn contents in excess of 27 mass-% in the binary Fe-Mn alloy system. In order to obtain a stable room austenite phase in alloys with less than 25 mass-% Mn, the formation of α’ and ε martensite must be suppressed. The addition of 0.6 mass-% C makes it possible to obtain a uniform, carbide-free, austenitic microstructures and avoid the formation of ε-martensite [5]. Larger C additions result in M3C carbide formation. Figure 2 also illustrates the stability of the austenitic microstructure of Fe-18%Mn-0.6%C TWIP steel, as there is no transformation to martensite during straining. An alternative approach to obtaining TWIP steel with uniform, carbide-free, austenitic microstructures is to use a higher Mn content and no C additions. This TWIP steel composition concept requires Si and Al additions to control the SFE. The importance of the Al additions cannot be underestimated and needs further attention as it results in much improved TWIP properties [11]. 4
Equilibrium phase boundaries
Ms(α→ε)
C, mass-%
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