Topic: High Mn 3^rd Generation Advanced High Strength Steel for Automotive Applications

Speaker: Prof. Dr. B. C. De Cooman

　　　　　Director, Materials Design Laboratory，Pohang University of Science and Technology, South Korea

Time:（Tuesday) Jan. 11, 2011, 9:00am-10:30am

Venue: Room 403, R&D Center

Welcome to attend!

Abstract

Steel is still the material of choice for car bodies, with 99% of the passenger cars having a steel body, and 60-70% of the car weight consisting of steel or steel-based parts. Most car makers are however routinely testing multi-materials concepts, which are not limited to the obvious use of light materials for closures, e.g. the use of Al for the front lid or thermosetting resins for trunk lids. The steel research community therefore makes a sustained effort to innovate and create advanced steels and original steel-based solutions, which help achieve demanding engineering targets in terms safety, mass containment, stability, stiffness, comfort, acoustics, corrosion, and recycling.

The contribution will review the recent development of high Mn steels. These steels achieve tensile strengths >1GPa and large tensile elongations. High Mn TWinning-Induced Plasticity (TWIP) steels are highly ductile, high strength austenitic steels characterized by a high rate of work hardening resulting from the generation of deformation-nucleated twins. Their Mn content is in the range of 15-30 mass%. Alloying additions of C, Si and/or Al are needed to obtain the high strength and the large uniform elongation associated with strain-induced twinning. Depending on the alloy system, the carbon content is either low, i.e. less than 0.05 mass-%, or high, typically in the range of 0.5-1.0 mass-%. Si and Al may be added to achieve a stable fully austenitic microstructure with low stacking fault energy in the range of 15-30mJ/m2. High Mn alloys characterized by strength ductility products 40.000-60.000MPa% have reached the stage of large scale industrial testing and the industrial focus is mainly on TWIP steels with the following compositional ranges: 15-25 mass-%Mn, with 0-3%Si, 0-3% Al and 200-6000ppm C.

The dominant deformation mode in TWIP steel is dislocation glide, and the deformation-induced twins gradually reduce the effective glide distance of dislocations which results in a “Dynamical Hall-Petch effect”.

The presentation will discuss the basic mechanical properties of TWIP steels in detail, with a particular focus on the stacking fault energy and the twinning process. Methods to suppress dynamic strain aging and delayed fracture phenomenon will be reviewed. In addition, important technical aspects will be discussed, such as normal anisotropy, stretch-forming and high strain rate performance of TWIP steel.