Moon Hi Hong

Influence of Initial Microstructure on Mechanical Properties of Cu Bearing Extra Low Carbon Steel Sheets

This paper examines the effect of initial microstructure after hot rolling on the final microstructure and mechanical properties for Cu bearing Extra Low Carbon(ELC) Steel Sheets. For this purpose, two ELC steels having different initial microstructures due to different onset time of cooling after pilot hot rolling (0.4 and 1.2 second) were selected. Mechanical properties and microstructures were analyzed as well using uni-axial tensile test, electron back-scattered diffraction (EBSD) technique following pilot rolling and continuous annealing. It has been found that the reduction of onset time of cooling gives rise to the grain refinement in hot rolled sheets. The average grain sizes of hot rolled sheetss at the onset time of 0.4 and 1.2 second are 16.7μm and 20.8 μm, respectively. In addition, the planar anisotropy of the Cu bearing ELC steel sheets has improved with reducing onset time of cooling after hot rolling. However, other mechanical properties such as strength and elongation of annealed steel are similar to both cooling condition.

Effect of Copper Sulfide on the Fatigue Properties of Bake Hardening Steels

Bake hardening steels have to resist strain aging to prevent the yield strength increment and stretcher strain during press process and to enhance the bake hardenability during baking process after painting. The bake hardening steels need to control the solute carbon and the solute nitrogen to improve the bake hardenability. Ti and/or Nb alloying for nitride and carbide precipitation and low carbon content below 0.003% are used to solve strain aging and formability problem for automotive materials. However, in the present study, the effect of micro-precipitation of copper sulfide on the bake hardenability and fatigue properties of extremely low carbon steel has been investigated. The bake hardenability of Cu-alloyed bake hardening (Cu-BH) steel was slightly higher (5MPa) than that of Nb-alloyed bake hardening (Nb-BH) steel, but the fatigue limit of Cu-BH steel was far higher (45MPa) than that of Nb-BH steel. All samples showed the ductile fracture behavior and some samples revealed distinct fatigue stages, such as crack initiation, stable crack growth and unstable crack growth.