Shi Hoon Choi

Mathematical model to simulate thermo-mechanical controlled processing in rod (or bar) rolling

This study presents a mathematical model for computing the thermo-mechanical parameters such as the strain and strain rate at a given pass and the temperature variation during rolling and cooling between inter-stands (pass), to assess the potential for developing “Thermo-Mechanical Controlled Process” technology in rod (or bar) rolling, which has been a well-known technical terminology in strip (or plate) rolling since the 1970s. The model has then been applied to a four-pass (oval-round or round-oval) bar rolling sequence for predicting the pass-by-pass AGS, by incorporating the equation for recrystallization behavior and AGS evolution being widely used in strip rolling. The predicted AGS was compared with those obtained from the hot torsion experiment. Results revealed the proposed model, coupled with the recrystallization behavior and AGS evolution model developed for strip (or plate) rolling, might be applied directly to rod (or bar) rolling. We also found that the recrystallization behavior during rod rolling was significantly influenced by the method for calculating the thermo-mechanical parameters, especially the strain.

Study of Texture Development and Anisotropy of Mechanical Properties of API-X80 Line Pipe Steel for Spiral-Welded Pipe

This study has been conducted to analyze the effect of texture and microstructure on the anisotropy of yield strength and Charpy fracture toughness of an X80 line pipe steel. The texture and microstructure were investigated by X-ray diffractometer and electron backscattered diffraction (EBSD). The yield strength and impact energy were measured along 0o (longitudinal), 30o and 90o (transverse) to the rolling direction. It was found that the microstructure of the developed steel consisted of fine acicular and polygonal ferrite with small pearlite and martensite or retained austenite (MA constituents). The major components of textures were {332}<113> and {113}<110> orientations. In order to investigate the effect of both morphological and crystallographic texture on yield strength anisotropy, the prediction of the plastic property was carried out by using a viscoplastic self-consistent (VPSC) polycrystal model. The predicted anisotropy of yield strength with VPSC model assuming ellipsoidal grain shape was in a good agreement with experimental observation. EBSD results showed that the density of {001} cleavage planes of Charpy specimen, 30 degree to rolling direction, was the highest compared with that of other specimens. Therefore, the highest susceptibility to the cleavage fracture, i.e. increased ductile-brittle transition temperature, can be seen in the 30 degree direction.

Influence of Boron on Mechanical Properties and Microstructures of Hot-Rolled Interstitial Free Steel

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.

Influence of Boron on Dynamic Recrystallization and Continuous Cooling Transformation of High Strength Interstitial Free Steels

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.

Prediction of Earing in IF Steels by Using Crystal Plasticity Finite Element Method

The effect of five ideal texture components ({001}<110>, {112}<110>, {111}<110>, {111}<112> and {554}<225>) typical in interstitial free (IF) steels on the development of ears was investigated using crystal plasticity finite element method (CPFEM). For the polycrystal model, the material behavior is described using crystal plasticity theory where each integration point in the element is considered to be a single grain of polycrystalline IF steel. The experimental earing profile for a IF steel was also compared to the earing profile predicted by CPFEM.

Evaluation of Stored Energy in Cold-Rolled IF Steels from EBSD Data and Its Application to Recrystallization Modeling

This study has been conducted to evaluate the orientation-dependent stored energy of deformed grains from the data measured using electron back-scattered diffraction (EBSD) analysis. The stored energy in deformed grains is an important term as a driving force for static recrystallization of cold-rolled steels. Subgrain method based on subgrain structure is used for quantitative analysis of the stored energy developed in cold-rolled interstitial free (IF) steels to a reduction of 80%. The orientation dependent stored energy term was used in Monte Carlo technique to simulate static recrystallization of IF steels.