Jun-Yun Kang

Effect of external stress on the orientation distribution of ferrite

Abstract The effect of external stress on the orientation distribution of ferrite during the transformation was examined. Two types of carbon steel were used to evaluate the deviation angle of ferrite from the Kurdjumov–Sachs (K–S) relationship using electron backscattered diffraction. The K–S relationship was weakened when an external stress was applied during the transformation.

Phase Analysis on Dual-Phase Steel Using Band Slope of Electron Backscatter Diffraction Pattern

A quantitative and automated phase analysis of dual-phase (DP) steel using electron backscatter diffraction (EBSD) was attempted. A ferrite-martensite DP microstructure was produced by intercritical annealing and quenching. An EBSD map of the microstructure was obtained and post-processed for phase discrimination. Band slope (BS), which was a measure of pattern quality, exhibited much stronger phase contrast than another conventional one, band contrast. Owing to high sensitivity to lattice defect and little orientation dependence, BS provided handiness in finding a threshold for phase discrimination. Its grain average gave a superior result on the discrimination and volume fraction measurement of the constituent phases in the DP steel.

Texture Development in Low Carbon Sheet Steels for Automotive Application

The development of recrystallization texture in three low carbon sheet steels, conventional IF steel, fine grained IF steel, and DP steel, for automotive application was investigated using EBSD technique. The recrystallization texture at the early stage of recrystallization was very similar to that at the final stage in all three steels. A strong α-fiber texture with {112} peak and weaker γ-fiber texture with {111} peak were developed during the deformation of all three low carbon steels. After recrystallization, they changed to a strong γ-fiber texture with strong {111} component. However, the intensity of the texture components varied in the different steels. The grains of γ-fiber orientations developed from deformed grains of similar orientations and grew rapidly, consuming the neighboring grains to form a strong γ-fiber texture. In conventional IF steels, some recrystallized grains formed at grain boundaries showed weak correlation with {111} orientation, however, the growth rates of these random grains were too slow to affect the final recrystallization texture. In the fine grained IF steel, the intensity of the {001} component after recrystallization was higher than that in the conventional IF steel. Strain induced boundary migration (SIBM) was observed in the fine grained IF steel. The intensity of the γ-fiber texture was much weaker in the DP steel than in the IF steels. Nucleation of the {110} and {001} recrystallized grains from the shear bands in {111} or {110} grains was expected. The austenite transformation during the intercritical annealing of the DP steel was found to have little effect on the formation of its final recrystallization texture.

Formation of Ultrafine Cellular Microstructure Around Alumina Particles in a Low-Carbon Steel

Ultrafine cellular microstructures around alumina particles in a low-carbon steel were observed, which survived even after cyclic austenitization. This indicates that their formation is closely related to internal stress because of a structural heterogeneity during phase transformation rather than to externally applied forces or deformation. Thermo-elasto-plastic finite element analysis confirmed the evolution of a large hydrostatic pressure around an alumina particle due to thermal mismatch during cooling. Therefore, the fine cellular microstructure might be generated as a result of the hydrostatic pressure, which retards the phase transformation around the particle during cooling. In addition, we observed microstructural similarity with the same steel processed under an ultra-high pressure, which was the evidence for the role of the delay in the transformation caused by the hydrostatic pressure.