Papers by Author: Duk Lak Lee

Abstract: In order to save natural resources and to reduce production costs, many industries have conducted studies on new developed steels and manufacturing processes. For instance, the use of high-strength bolts can decrease the number, size and weight of bolts used in a car, thereby decreasing fuel consumption. For this reason, steel makers are developing boron-added steel for high-strength bolts, which eliminates production processes. However, it has been pointed out that this boron-added steel is sensitive to austenite grain coarsening. The austenite grain coarsening does not occur uniformly, hence it can affect mechanical properties such as tensile, fatigue and so on. Therefore, the aim of this study is to investigate the abnormal behavior of grain growth and to determine the optimum amount of alloying elements in boron-added steel for use in making high-strength bolts.

Abstract: Microstructures of cold drawn pearlitic steel wires were investigated by three-dimensional atom probe (3D-AP) to understand the influence of alloying elements on the decomposition of cementite. Before cold drawing, Si is mostly located in the ferrite phase, while Cr is located in the Fe3C phase and the amount of Mn is similar in Fe3C and in ferrite. Higher Si amount leads to higher dissolution rate of cementite and Cr has a little effect on cementite decomposition during drawing.

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: The low carbon steel of 0.22wt% carbon was tested to estimate the dynamic phase transformation behavior from austenite to ferrite. The samples were deformed at just above Ar3 temperature by hot torsion at condition of strain rate (0.5/sec) and strain (5.0). The flow curve obtained at just above Ar3 significantly differed from others due to dynamic transformation. Based on the analysis of flow stress curve and observation of micro-structure evolution, the initiation and finish points of strain induced dynamic transformation (SIDT) could be determined. An inflection point observed at early deformation range (0.2–0.3) from the work-hardening rate and stress plot meant that new ferrite grains were nucleated in austenite matrix and these nuclei could be also confirmed by optical microscope. Subsequently in strain range of 0.7-1.0, the flow stress had the maximum value and new fine ferrite grains were dynamically generated inside untransformed austenite grains as well as prior austenite grains. The dynamic phase transformation induced by deformation made eventually fine ferrite grains under 3 ㎛ and decreased stress level with a fixed gradient.

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.