Il-Heon Son

Effect of reduction of area on microstructure and mechanical properties of twinning-induced plasticity steel during wire drawing

The effect of reduction of area (RA), 10%, 20%, and 30%, during wire drawing on the inhomogeneities in microstructure and mechanical properties along the radial direction of Fe-Mn-Al-C twinning-induced plasticity steel has been investigated. After wire drawing, the deformation texture developed into the major <111> and minor <100> duplex fiber texture. However, the <111> texture became more pronounced in both center and surface areas as the RA per pass increased. It also shows that a larger RA per pass resulted in a higher yield strength and smaller elongation than a smaller RA per pass at all strain levels. Although inhomogeneities in microstructure and mechanical properties along the radial direction decreased with increasing RA per pass, there existed an optimum RA per pass for maximum drawing limit. Insufficient penetration of strain from surface to center at small RA per pass (e.g., 10%) and high friction and unsound metal flow at large RA per pass (e.g., 30%) all resulted in heterogeneous microstructure and mechanical properties along the radial direction of drawn wire. On the other hand, 20% RA per pass improved the drawing limit by about 30% as compared to the 10% and 30% RAs per pass.

Mechanisms of tensile improvement in caliber-rolled high-carbon steel

The microstructural evolution and tensile characteristics of caliber-rolled plain carbon steels were quantitatively investigated and compared with those of the as-received plain carbon steels. The caliber-rolled steels exhibited a similar microstructure consisting of an ultrafine-grained ferritic matrix and dispersed fine cementite particles. In contrast to the general trend of caliber-rolling processes showing significant reductions in elongation while retaining a high strength, the present caliber-rolled high-carbon steel exhibited simultaneously improved strength and elongation. This distinctive tensile enhancement of the caliber-rolled high-carbon steel was attributed to the increased strain hardening rate caused by the high fraction of submicron cementite particles and to the transition of the fracture mode from a brittle state to a ductile state.

Residual stress characteristics in a non-circular drawing sequence of pearlitic steel wire

In this paper, characteristics of residual stress in pearlitic steel wire drawn by a non-circular drawing (NCD) sequence with two processing routes, NCDA and NCDB, were experimentally and numerically investigated up to the 12th pass in comparison with conventional wire drawing (WD). For experimental investigation of the axial residual stress at the surface of the drawn wire, destructive (deflection) and non-destructive methods were employed. According to the experimental results, axial surface residual stress of the drawn wire by the NCD sequence was lower and more homogeneous compared to the conventional WD. Based on the elasto-plastic numerical simulation results from the surface to the center of the drawn wire using a commercial DEFORM-3D, an empirical relationship between residual stress and reduction of area was determined to predict the residual stress evolution in the multi-pass WD, NCDA, and NCDB, in that order. From the results of this investigation, it can be construed that the NCD sequence, especially the NCDB, might be helpful in improving the residual stress characteristics of pearlitic steel wire to improve its mechanical behavior and service life.

3D NUMERICAL AND EXPERIMENTAL STUDY ON FLOW CHARACTERISTICS OF MULTI-PASS ECAP WITH AA1050

Experiments and numerical simulations were conducted to investigate flow behaviour of the specimen made of commercially pure aluminum alloy (AA1050) during multi-pass equal channel angular pressing (ECAP) for route A up to four passes. Influence of processing conditions on friction and flow behaviour was investigated by measuring load variations, microhardness distributions and microstructure changes depending on the number of passes. It was carefully simulated by employing the finite element technique by tracing the local deformation, determining the load requirement and comparing the local strain with microhardness distributions. Change of the grain size depending on the number of passes was monitored by transmission electron microscopy. The present work clearly showed flow characteristics of the deformed specimen at the central and surface regions due to the effect of the number of passes for the multi-pass ECAP.

A New Method to Compute the Behavior of Phase Transformations and Depth of the Decarburized Ferrite Layer, Scale Thickness of Steel from Measured Temperatures

An new method is proposed to predict in sequence the phase transformation behavior and the depth of the decarburized ferrite layer with simultaneous scaling of a steel during cooling process from the measured temperature history. This proposed method successively couples a set of mathematical models describing the temperature evolution, phase transformation, decarburization and oxidation. The dilatometry experiment was performed to verify the proposed method. Results reveal that the measured depth of the decarburized ferrite layer agreed well with the predicted depth. The predicted pearlite volume fraction nearly coincided with the measured value as well.