Sung-Kyu Kim

Temperature Effect on Twin Formation Kinetics and Deformation Behavior of Fe–18Mn-0.6C TWIP Steel

Temperature effect on deformation behavior has been investigated in relation to formation kinetics of twins in a Fe-18Mn-0.6C TWIP steel. Total elongation was found to reach a maximum value of 88% at 200 °C and then decreased continuously with the increase in test temperature from 300 °C up to 600 °C. This reversed temperature dependence on ductility could be attributed to the formation kinetics of deformation twins, as was prescribed by an internal variable theory of inelastic deformation. It was found that twins became more difficult to form at higher temperatures due to insufficient internal strain energy accumulated to reduce ductility progressively in this temperature range. Dislocation glide mechanism became, however, dominant at higher temperatures above 600 °C to increase total elongation following the usual temperature dependence. Finally the stacking fault energy was related with the stability parameter, β, used in the transformation kinetics relation.

Constitutive Modeling of the Tensile Behavior of Al-TWIP Steel

High Mn steels demonstrate an exceptional combination of high strength and large ductility as a result of their high strain-hardening rate during deformation. The microstructure evolution and strain-hardening behavior of Fe18Mn0.6C1.5Al TWIP steel in uniaxial tension were examined. The purpose of this study was to determine the contribution of all the relevant deformation mechanisms—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 to account for the interaction between the two dislocation populations during straining. These coupled dislocation densities were used to simulate the contribution of dynamic strain aging to the flow stress. The model was modified to include the effect of twinning. To ascertain the validity of the model, the microstructural evolution was characterized in detail by means of transmission electron microscopy and electron back-scatter diffraction.

Effects of Aluminum Addition on Tensile and Cup Forming Properties of Three Twinning Induced Plasticity Steels

In the present study, a high Mn twinning induced plasticity (TWIP) steel and two Al-added TWIP steels were fabricated, and their microstructures, tensile properties, and cup formability were analyzed to investigate the effects of Al addition on deformation mechanisms in tensile and cup forming tests. In the high Mn steel, the twin formation was activated to increase the strain hardening rate and ultimate tensile strength, which needed the high punch load during the cup forming test. In the Al-added TWIP steels, the twin formation was reduced, while the slip activation increased, thereby leading to the decrease in strain hardening rate and ultimate tensile strength. As twins and slips were homogeneously formed during the tensile or cup forming test, the punch load required for the cup forming and residual stresses were relatively low, and the tensile ductility was sufficiently high even after the cup forming test. This indicated that making use of twins and slips simultaneously in TWIP steels by the Al addition was an effective way to improve overall properties including cup formability.

Serration Phenomena Occurring During Tensile Tests of Three High-Manganese Twinning-Induced Plasticity (TWIP) Steels

In this study, the serration phenomena of two high-Mn TWIP steels and an Al-added TWIP steel were examined by tensile tests, and were explained by the microstructural evolution including formation of localized Portevin–Le Chatelier deformation bands and twins. In stress–strain curves of the high-Mn steels, serrations started in a fine and short shape, and their height and periodic interval increased with increasing strain, whereas the Al-added steel did not show any serrations. According to digital images of strain rate and strain obtained from a vision strain gage system, deformation bands were initially formed at the upper region of the gage section, and moved downward along the tensile loading direction. The time when the band formation started was matched with the time when one serration occurred in the stress–time curve. This serration behavior was generally explained by dynamic strain aging, which was closely related with the formation of deformation bands.

Effects of Inclusions on Delayed Fracture Properties of Three TWinning Induced Plasticity (TWIP) Steels

In the present study, delayed fracture properties of a high-Mn TWinning Induced Plasticity (TWIP) steel and two Al-added TWIP steels were examined by dipping tests of cup specimens in the boiled water, after which the microcrack formation behavior was analyzed. The TWIP steels contained a small amount of elongated MnS inclusions, spherical-shaped AlN particles, and submicron-sized (Fe,Mn)3C carbides. Since MnS inclusions worked as crack initiation sites, longitudinal cracks were formed along the cup forming direction mostly by MnS inclusions. These cracks were readily grown when high tensile residual stresses affected the cracking or hydrogen atoms were gathered inside cracks, which resulted in the delayed fracture. In the Al-added steels, MnS inclusions acted as crack initiation and propagation sites during cup forming or boiled-water dipping test, but residual stresses applied to MnS might be low for the crack initiation and growth. Thus, longitudinal cracks formed by MnS inclusions did not work much for delayed fracture. AlN particles present in the Al-added steels hardly acted as crack initiation or growth sites for the delayed fracture because of their spherical shape.

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.

Strain Rate Sensitivity of C-alloyed, High-Mn, Twinning-induced Plasticity Steel

The mechanical properties of twinning-induced plasticity (TWIP) steels are often assumed to be solely due to the reduction of the mean free path of glide dislocations resulting from deformation twinning. Other mechanisms may also play an essential role: Mn-C cluster formation, planar glide, pseudo-twinning, short range ordering, and dynamic strain ageing. The present contribution offers a critical analysis of the mechanical properties of high-Mn TWIP steels, especially in terms of Dynamic Strain Aging (DSA) and Static Strain Aging (SSA). The presentation offers new insights into the properties of TWIP steels which were obtained by using new experimental techniques such as in-situ strain analysis and high sensitivity infrared thermo-graphic imaging.

Effects of Mn Addition on Tensile and Charpy Impact Properties in Austenitic Fe-Mn-C-Al-Based Steels for Cryogenic Applications

Effects of Mn addition (17, 19, and 22xa0wt pct) on tensile and Charpy impact properties in three austenitic Fe-Mn-C-Al-based steels were investigated at room and cryogenic temperatures in relation with deformation mechanisms. Tensile strength and elongation were not varied much with Mn content at room temperature, but abruptly decreased with decreasing Mn content at 77xa0K (−196xa0°C). Charpy impact energies at 273xa0K (0xa0°C) were higher than 200xa0J in the three steels, but rapidly dropped to 44xa0J at 77xa0K (−196xa0°C) in the 17Mn steel, while they were higher than 120xa0J in the 19Mn and 22Mn steels. Although the cryogenic-temperature stacking fault energies (SFEs) were lower by 30xa0to 50xa0pct than the room-temperature SFEs, the SFE of the 22Mn steel was situated in the TWinning-induced plasticity regime. In the 17Mn and 19Mn steels, however, α′-martensites were formed by the TRansformation-induced plasticity mechanism because of the low SFEs. EBSD analyses along with interrupted tensile tests at cryogenic temperature showed that the austenite was sufficiently deformed in the 19Mn steel even after the formation of α′-martensite, thereby leading to the high impact energy over 120xa0J.

Residual Stress Analysis in Deep Drawn Twinning Induced Plasticity (TWIP) Steels Using Neutron Diffraction Method

In Twinning Induced Plasticity (TWIP) steels, delayed fracture occurs due to residual stresses induced during deep drawing. In order to investigate the relation between residual stresses and delayed fracture, in the present study, residual stresses of deep drawn TWIP steels (22Mn-0.6C and 18Mn-2Al-0.6C steels) were investigated using the finite element method (FEM) and neutron diffraction measurements. In addition, the delayed fracture properties were examined by dipping tests of cup specimens in the boiled water. In the FEM analysis, the hoop direction residual stress was highly tensile at cup edge, and the delayed fracture was initiated by the separation of hoop direction and propagated in an axial direction. According to the neutron diffraction analysis, residual stresses in 18Mn-2Al-0.6C steel were about half the residual stresses in 22Mn-0.6C steel. From the residual strain measurement using electron back-scatter diffraction, formation of deformation twins caused a lot of grain rotation and local strain at the grain boundaries and twin boundaries. These local residual strains induce residual stress at boundaries. Al addition in TWIP steels restrained the formation of deformation twins and dynamic strain aging, resulting in more homogeneous stress and strain distributions in cup specimens. Thus, in Al-added TWIP steels, residual stress of cup specimen considerably decreased, and delayed fracture resistance was remarkably improved by the addition of Al in TWIP steels.

Correlation of Yield Ratio with Materials Constants of Constitutive Equation

The yield ratio of various HSLA steels has been correlated with the materials constants of Swift equation. It has been shown that the materials constants, b and N, of Swift equation can be related to microstructural features such as the dislocation density and volume fraction of constituent phases. In particular, the constant b can be expressed as a function of volume fraction of constituent phases. It has also been shown that the yield ratio has a linear relationship with ln(b/N2). Since the microstructural features often have opposing effects on the values of b and N, careful control of microstructure is necessary to optimize the yield ratio and other properties. The possible way of decreasing the yield ratio without sacrificing other properties of HSLA steels is suggested based on the relationship between yield ratio and the materials constants of Swift equations.