Kwang Geun Chin

New Trends in Advanced High Strength Steel Developments for Automotive Application

The body design with light weight and enhanced safety is a key issue in the car industry. Corresponding to this trend, POSCO is developing various automotive steel products with advanced performance. Conventional advanced high strength steels such as DP and TRIP steels are now expanding their application since the steels exhibit higher strength and ductility than those of conventional solution and precipitation strengthened high strength steels. Efforts have been made to enhance the mechanical performance of these steels such as ductility, hole expansion ratio, deep drawability, etc. Current research is focused on development of extra- and ultra-AHSS. Extra-AHSS are designed to utilize nano-scale retained austenite embedded in fine bainite and martensite. Ultra-AHSS are designed to have austenite as the major phase, and the ductility is enhanced primarily by continuous strain hardening generated during forming. These steels including extra- and ultra-AHSS are believed to be the next generation automotive steels which will replace the existing high strength steels due to their extremely high strength and ductility combinations.

Constitutive modeling of TWIP steel in uni-axial tension

High Mn steels demonstrate an exceptional combination of high strength and ductility due to their high work hardening rate during deformation. The microstructure evolution and work hardening behavior of Fe18Mn0.6C1.5Al TWIP steel in uni-axial tension were examined. The purpose of this study was to determine the contribution of all the relevant deformation mechanism : slip, twinning and dynamic strain aging. Constitutive modeling was carried out based on the Kubin-Estrin model, in which the densities of mobile and forest dislocations are coupled in order to account for the continuous immobilization of mobile dislocations during straining. These coupled dislocation densities were also used for simulating the contribution of dynamic strain aging on the flow stress. The model was modified to include the effect of twinning.

Effect of N on Microstructure and Tensile Behavior of TWIP Steel

The present study focused on the design and testing of new 3rd generation TWIP steels to find reduced Mn content alternatives to the existing high Mn FeMnC and FeMnAlC alloy systems. In order to investigate the effect of nitrogen addition, 12Mn0.6C-N was examined and 18Mn0.6C-N steel was used as a reference. Effects of nitrogen and manganese on the microstructure and mechanical properties were investigated by X-ray diffraction, optical microscopy, scanning electron microscopy and tensile testing. The deformation microstructures of 12Mn0.6C-N were mixtures of twinned austenite and 5~10% ε-martensite. Only deformation twinning was observed in 18Mn0.6C-N during the deformation due to its higher stacking fault energy. 18Mn0.6C-N steel exhibited higher strength and elongation than 12Mn0.6C-N steel. The effect of the strain rate on the tensile behavior of both materials was also examined. While 18Mn0.6C-N showed clear negative strain rate sensitivity, 12Mn0.6C-N did not show a clear relationship between flow stress and strain rate. The effect of annealing temperature on the tensile behavior and microstructure was also examined.

On the Stretch-Flangeability of High Mn TWIP Steels

Edge stretching is an important formability issue when it comes to apply sheet steels to automotive industry. Anisotropy, strain hardening and toughness are closely related to hole expansion properties. In this paper, hole expansion properties of a high Mn fully austenitic Twinning Induced Plasticity (TWIP) steel are compared with three other types of single-phase sheet steels. The effects of r-value, n-value, m-value and post-uniform elongation on the edge stretch-flangeability are discussed. It was found that the post-uniform elongation and the strain rate sensitivity have a pronounced effect on hole expansion properties.

The Evolution of Texture and Stored Energy during Recrystallization of IF High Strength Steel Investigated by Means of Orientation Imaging Microscopy

The evolution of texture and stored energy was studied for Ti bearing and Ti-Nb bearing IF high strength steel undergoing a double cold rolling and annealing treatment by using Orientation Imaging Microscopy. Ti bearing IF high strength steel showed a similar texture evolution of the α and γ fiber components to that of IF steel. On the contrary, Ti-Nb bearing IF high strength steel displayed a different texture evolution from the one observed in the former steels. The difference in texture and stored energy evolution between both high strength steels seemed to be affected by the segregation behavior of solute alloying elements such as P.

Simulation of Primary Recrystallization in Automotive Steels by Consideration of Particle Pinning

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.

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.