Papers by Author: Young Seog Lee

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: A maximum spread model dependent on rolling speed is proposed for rod rolling process following the analysis of deformation behavior of steel under compression and four-pass continuous high speed rod rolling test. Results reveal that amount of spread depends on a region which restricts plastic flow of workpiece during deformation. Increasing rolling speed (strain rate) leads to decrement of friction coefficient and, in turn, enlarge deformation-restricted region in the workpiece and consequently reduce amount of spread during deformation. Hence, the rolling speed dependent spread model could be expressed in terms of flow stress, which is a function of strain, strain rate and temperature and finally the model is proposed in a concise form.

Abstract: In this study, we introduce an approach which simulates crack propagation under mixedmode loading condition. In comparison with the conventional element removing method which eliminates any element that satisfies a prescribed failure criterion near the crack tip, the present approach selects a set of elements ahead of the crack tip on the crack growth direction and removes them one by one when the element meets a prescribed failure criterion. Compact tension shear (CTS) specimens of type 304 stainless steel were used for failure testing. Finite element simulation has been carried out to simulate crack profiles and compared with observed ones. Results showed the proposed element removing algorithm is useful for crack growth simulation under mixed mode loading condition. The experimentally measured crack growth profile is in an agreement with the predicted ones.

Abstract: In this paper, we present an analytic approach for the prediction of roll force to be applicable to the billet rolling process which consists of a series of horizontal stands and vertical stands. The approach is based on the deformation shape and employs the assumption that the deformation homogeneously occurs in three directions. Strain and strain rate are calculated by the geometric relationships between those components and the prescribed deformation functions. By integrating the stress components along the rolling direction, roll force is finally obtained. The prediction accuracy of the proposed model is examined through comparison experimentally mesured roll force with computed ones.

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: 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.