Papers by Author: Li Wang

Abstract: Surface oxides of medium manganese steel treated under different hot-dip galvanizing processes were analyzed by X-ray photoelectron spectroscopy, scanning electron microscope microscopy and energy dispersive spectrometer techniques. Combined with thermodynamic calculation, the effects of dew point and alloying elements on the formation of oxides were investigated. It was shown that many oxides such as Cr₂O₃ and Al₂O₃, which deteriorate galvanizing performance largely, were formed at lower dew point, and that the formation of Cr and Al oxides could be effectively inhibited at higher dew point. It was also shown that the formation of MnO could be controlled by regulating the rate of Al/Mn in the composition of experimental steel, thus reducing the surface defect.

Abstract: Press hardening steel has become a much used material in car body manufacturing due to its excellent safety and lightweight potential. In some recent car models press hardening steel has reached already a weight share of more than 20% in the body structure while it is estimated that it could reach even around 40% in the future. However conventional press hardening steel based on the alloying concept 22MnB5 was designed originally not for automotive application. In spite of the high strength level, press hardening steel has generally low toughness due to the relatively high carbon content and its martensitic microstructure. Particularly important is the ductile-to-brittle transition temperature at low temperature, which might lead to unexpected failure in cold climate regions. Furthermore, hydrogen embrittlement is a major concern in press hardening steel as previous results indicated that only a few ppm of hydrogen could induce delayed cracking. Generally it is important that impact energy should be absorbed by the material to avoid unexpected disintegration of the structure. This material capability is usually characterized by the toughness. All these characteristics have not yet been systematically investigated in press hardening steel. Consequently, no attempts have been made to optimize conventional press hardening steel for obtaining better toughness, lower ductile-to-brittle transition temperature and reduced sensitivity to hydrogen induced cracking.A generally proven approach of improving the resistance against brittle failure in high strength steel is the refinement of microstructure, which can be efficiently achieved by Nb microalloying. This paper will introduce modified alloy designs for press hardening steel and explain fundamentally the metallurgical effects of Nb microalloying on the improvement of crack propagation resistance, bendability and delayed cracking behavior induced by hydrogen penetration in press hardening steel. The results reveal better toughness, improved bending behavior and increased resistance against hydrogen embrittlement.

Abstract: Transformation-induced-plasticity (TRIP) –aided steels are a promising solution for producing lighter, crash-resistant car bodies, due to their high-strength and large uniform elongation. The influences of the austempering temperature and time on the microstructure mechanical properties and the transformation behavior of a Si-Mn TRIP steel are investigated in this paper.