Papers by Author: S.I. Kim

Abstract: A modified two dimensional (2-D) Monte Carlo (MC) technique was used to simulate primary recrystallization in automotive steels containing fine particles. In order to consider anisotropic properties of grain boundary energy and grain boundary mobility, functions of boundary misorientation were introduced. Orientation-dependent stored energy developed in 80% cold-rolled interstitial free (IF) sheet steel was evaluated by reconstructing of data measured using electron back-scattered diffraction (EBSD) analysis. A subgrain method based on subgrain structure is used for quantitative analysis of the stored energy. The simulation reveals that particles affect evolution of microstructure during recrystallization. The simulation provided a theoretical foundation for understanding effect of particles on the final microstructures and crystallographic textures.

Abstract: We have studied the fracture behavior of coating layer when low and high alloying galvannealed (GA) steels are subject to forming process. To understand better powdering features in the coating layer of the steel sheets, we carried out V-bending test and a series of finite element analysis which simulates damage characteristics in the coating layer. Results showed that the powdering behavior in the coating is significantly affected by the soundness and volume fraction of phases in the coating layer. The hardness variation of coating layers attributed to different phases leads to different deformation behavior of the coating layer itself.

Abstract: Uniaxial compression tests on hot-rolled AZ31 Mg alloy were carried out at 200°C. In order to investigate the evolution of texture during plastic deformation, cylindrical specimens were compressed to the rolling and normal directions. Experimental investigation reveals that work hardening and texture evolution are strongly dependent on the loading direction. The occurrence of deformation twinning was revealed by the observation of microtexture using electron backscatter diffraction (EBSD). A visco-plastic self-consistent (VPSC) polycrystal model was used to simulate the texture evolution during the uniaxial compression. The texture evolution induced by crystallographic slip and deformation twinning can be explained by the relative activity of each deformation modes.

Abstract: The relationships between flow stress curve and microstructure evolution in strain induced dynamic phase transformation (SIDT) of low carbon steel (0.22wt.%) were quantitatively investigated. The deformation was carried out at just above Ar3 temperature (710°C) as a function of strain rate (0.01-5/sec). The softening process of SIDT was well agreed with calculated result derived from Avrami’s and constitutive equation at higher strain rate than 0.5/sec. However, the calculated results differed from the experimental curve at strain rate of less than 0.2/sec. This is due to fact that the dynamic transformation from austenite to ferrite can not be completed owing to less stored energy during hot deformation.

Abstract: Hot torsion of a C (0.22 wt%)-Mn steel was used to investigate the influence of thermomechanical arameters on the strain induced dynamic transformation (SIDT) of ferrite. The pecimens were strained as a function of strain rate (0.05/sec - 5/sec) and strain (- 5.0) at right bove Ar3 temperature. The critical strain to initiate dynamically transformed ferrite nuclei during deformation increased as increasing the strain rate. On the other hand the completion of SIDT was hifted to larger strain by decreasing strain rate. This is due to the fact that the dynamic ransformation of ferrite was processed in the interior of austenite grain as well as at grain boundary y large stored energy and many nucleation sites for high strain rate. The dynamic transformed micro-structure of ferrite was developed to higher angle and the grain size could be refined to ~3 ㎛ at strain of 3.0 and 5/sec.

Abstract: The paper examines the effect of boron (B) on the dynamic recrystallization and continuous cooling transformation (CCT) behavior in Nb-Ti microalloyed high strength interstitial free (IF) steels. For this purpose, two Nb-Ti microalloyed IF steels containing 0.003wt.% and 0.0005wt.% B, respectively, and one IF steel without B were chosen. The dynamic recrystallization behavior was investigated using hot compression testing. The character of the austenite to ferrite transformation during continuous cooling was studied by dilatometry test and CCT diagrams for the IF steels have been constructed. It was found that the initiation of dynamic recrystallization is delayed as the amount of boron increases. Addition of B retards the austenite to ferrite transformation as well. Under cooling rates of 0.5 and 1oC s-1, which correspond to slow cooling rates in the hot strip mill, the addition of B leads to the development of acicular ferrite and bainite phases. On the other hand, at similar cooling conditions the B free IF steel was observed to have a polygonal ferrite microstructure.

Abstract: This paper examines an effect of boron (B) on dynamic softening behavior, mechanical properties and microstructures for Nb-Ti added high strength interstitial free (IF) steel. For this purpose, IF steels containing 0ppm B, 5ppm B and 30ppm B were chosen. Continuous cooling compression test was performed to investigate dynamic softening behavior. Mechanical properties and microstructures of pilot hot-rolled IF steel sheet were analyzed by uni-axial tensile test and electron back-scattered diffraction (EBSD). It was found that no-dynamic recrystallization temperature (Tndrx) which can be determined from the relationship between flow stress and temperature is a constant of 955oC for all IF steels. However, an addition of B into IF steels increases work hardening rate at the temperature below Tndrx. It was also verified that B retards phase transformation of austenite into ferrite. EBSD analysis revealed that absence of B induces fine ferrite grain size and many high angle grain boundaries.

Abstract: A constitutive relation is proposed in this study by introducing a function prescrbing the volume fraction of dynamically recrystallized grains into the Voce’s equation to predict the stress-strain relations for three microalloyed medium carbon steels. We performed hot torsion test in temperature range of 900-1100 and strain rate range of 0.05-5.0s-1 to obtain the flow stress-strain curves. The calculated flow stresses are in good agreement with the measured ones. We have then applied the equations in FE analysis to predict the distribution of dynamic recrystallization volume fraction and flow stress for hot forging.

Abstract: We have investigated the static and metadynamic behaviour of the interstitial free steels and in particular the effects of the steeling elements (phosphorous and boron) on kinetics of recrystallisation. The results showed that the strain for the initiation of strain independent softening (often referred to as metadynamic recrystallisation) varies with the Zener-Hollomon parameter and steel composition. Strain rate had a strong influence on kinetics of metadynamic recrystallisation. An increase in temperature from 930oC to 1100oC led to a decrease in time for 50% softening (about one order of magnitude) in the SRX region. However, for the same temperature range, the time for 50% MDRX did not change significantly.