• Korean Journal of Metals and Materials
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• v.46 no.12
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• pp.780-787
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• 2008

For the advanced high strength steels (AHSS), high-manganese TWIP (twinning induced plasticity) steels exhibit high tensile strength (800-1000 MPa) and high elongation (50-60%). However, the TWIP steels need to be understood of delayed fracture following the cup drawing test. Among the factors to cause delayed fracture, i.e, martensite transformation, hydrogen embrittlement and residual stress, the effects of martensite transformation (${\gamma}{\rightarrow}{\varepsilon}$ or ${\gamma}{\rightarrow}{\alpha}^{\prime}$) were investigated on the delayed fracture phenomenon. Microstructural phase analysis was conducted for cold rolled (20, 60, 80% reduction ratio) steels and tensile deformed (20, 40, 60% strain) steels. For the Al-added TWIP steels, no martensite phase was found in the cold rolled and tensile deformed specimen. But, the TWIP steels with no Al addition indicated the martensite transformation. The cup drawing specimens showed the martensite transformation irrespective of the Al-addition to the TWIP steel. However, the TWIP steel with no Al exhibited the larger amount of martensite than the case of the TWIP steel with Al addition. For the reason, it was possible to conclude that the Al addition suppressed the martensite transformation in TWIP steels, therefore preventing the delayed fracture effectively. However, it was interesting to note that the mechanism of delayed fracture should be incorporated with hydrogen embrittlement and/or residual stress as well as the martensite transformation.

• Korean Journal of Materials Research
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• v.18 no.7
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• pp.394-399
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• 2008

The hydrogen embrittlement susceptibility of high strength TRIP/TWIP steels with the tensile strength of 600Mpa to 900Mpa grade was investigated using cathodically hydrogen charged specimens. TWIP steels with full austenite structure show a lower hydrogen content than do TRIP steels. The uniform distribution of strong traps throughout the matrix in the form of austenite is considered beneficial to reduce the hydrogen embrittlement susceptibility of TWIP steels. Moreover, an austenite structure with very fine deformation twins formed during straining could also improve the ductility and reduce notch sensitivity. In Ubend and deep drawing cup tests, TWIP steels show a good resistance to hydrogen embrittlement compared with TRIP steels.

• Korean Journal of Materials Research
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• v.33 no.11
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• pp.482-490
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• 2023

The purpose of this study was to analyze microstructural changes and evaluate the mechanical properties of TWIP steel subjected to variations in heat treatment, in order to identify optimal process conditions for enhancing the performance of TWIP steel. For this purpose, a homogenization heat treatment was conducted at 1,200 ℃ for 2 h, followed by hot rolling at temperature exceeding 1,100 ℃ and cold rolling. Annealing heat treatment is achieved using a muffle furnace in the range of 600 ℃ to 1,000 ℃. The microstructure characterization was performed with an optical microscope and X-ray diffraction. Mechanical properties are evaluated using micro Vickers hardness, tensile test, and ECO index (UTS × Elongation). The specimens annealed at 900 ℃ and 1,000 ℃ experienced a significant decrease in hardness and strength due to decarburization. Consequently, the decarburization phenomenon is closely related to the heat treatment process and mechanical properties of TWIP steel, and the effect of the microstructure change during annealing heat treatment.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2009.05a
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• pp.393-396
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• 2009

The mechanical properties, press formability and texture of a TWIP steel were investigated. This steel combines both high strength and high ductility due to so called TWIP effect which are related to the microstructural changes. The formation of twins during deformation leads to an increase of its mechanical properties. In this study, the texture and mechanical properties evolutions of a TWIP steel subjected to tensile tests and press trials at room temperature were investigated in relation to the feasibility of the application to automotive body parts.

• Korean Journal of Materials Research
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• v.20 no.7
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• pp.385-391
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• 2010

Twinning-induced plasticity (TWIP) steels have attracted great attention due to their excellent mechanical properties of high tensile strength (over 800MPa) and high ductility (over 50%), which result from the high strain hardening due to the mechanical twin formation during plastic deformation. The purpose of this study is to investigate the effect of annealing temperature and alloying elements on the mechanical properties of Fe-18Mn-0.6C TWIP steel. In 1.5%Al TWIP steel with 0.123%Ti content, the average recrystallized grain size was reduced to 2.5 ${\mu}m$ by cold rolling and annealing at $800^{\circ}C$ for 5 min, because of the pinning effect of the fine TiC carbides on grain coarsening. The tensile strength was decreased and the ductility was improved with the increase of the annealing temperature. However, a reversion of hardness and yield strength happened between $750^{\circ}C$ and $800^{\circ}C$ due to TiC and $M_3C$ type precipitation. 0.56% Ni added TWIP steel exhibited relatively lower yield strength, because Ni precipitates were not formed during the annealing process. When this specimen was annealed at $800^{\circ}C$ for 5min, the tensile strength and elongation were revealed at 1096MPa and 61.8%, respectively.

• Journal of Welding and Joining
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• v.32 no.2
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• pp.9-13
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• 2014

Friction stir welding (FSW) was successfully performed about 900MPa grade Twinning Induced Plastisity (TWIP) steel. A PCBN tool with convex-type shoulder was applied. Optimal process conditions were deduced. Microstructure and mechanical properties such as hardness, tensile strength and impact absorbed energy were observed and evaluated, respectively. An optical microscope (OM) and a field emission scanning electron microscope (FE-SEM) was selected for observing the grain structure.

• Corrosion Science and Technology
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• v.22 no.4
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• pp.297-303
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• 2023

This study aimed to evaluate hydrogen permeation behaviors of pre-strained twinning-induced plasticity steel with or without Zn coating using electrochemical permeation technique. In contrast to un-strained and 30% strained samples, permeation current density was measured in the 60% strained sample. Tensile pre-straining at 60% involved microstructural modifications, including a high level of dislocation density and stacking fault with a semi-coherent twin boundary, which might provide a high diffusion path for hydrogen atoms. However, reproducibility of measurements of hydrogen permeation current was low due to non-uniform deformation and localized stress concentration. On the other hand, the permeation current was not measured in pre-strained TWIP steel with Zn coating. Instead, numerous blisters with some cracks were observed on the surface of the coating layer. In locally damaged Zn coating under tensile straining, hydrogen atoms could relatively easily permeate through the coating layer. However, they were trapped at the interface between the coating layer and the substrate, which might delay hydrogen penetration into the steel substrate.

• Journal of the Korean Society for Heat Treatment
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• v.33 no.2
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• pp.57-64
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• 2020

In the present study, we investigated whether the surface oxidation of C-bearing TWIP steel ℃curs in the air during specimen delivery from an annealing furnace to a water bath and how the microstructure and tensile properties are influenced by surface oxidation. A cold-rolled Fe-18Mn-0.6 (wt%) steel was exposed in the air for 5 s after annealing at various temperatures (750℃, 850℃ and 1000℃) for 10 min in a vacuum, and then water-quenched. For comparison, another specimen, which had been quartz-sealed in a vacuum, was annealed at 1000℃ for 10 min and immediately water-quenched without exposure to air. The 750℃ and 850℃-annealed specimens and the quartz-sealed specimen showed a γ-austenite single phase in the entire specimen due to negligible surface oxidation. However, the 1000℃-annealed specimen exhibited a dual-phase microstructure consisting of ε-martensite and γ-austenite at the sub-surface due to decarburization. Whereas the specimens without decarburization revealed high elongations of 70-80%, the decarburized specimen exhibited a low elongation of ~40%, indicating premature failure due to cracking inside the decarburized layer with ε-martensite and γ-austenite.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2009.10a
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• pp.259-262
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• 2009

The influence of pre-strain in low-cycle fatigue behavior of Fe-18Mn-0.05Al-0.6C TWIP steel was studied by conducting axial strain-controlled tests. As-received plates were deformed by rolling with reduction ratios of 10 and 30%, respectively. A triangular waveform with a constant frequency of 1 Hz was employed for low cycle fatigue test at the strain amplitudes in the range of ${\pm}0.4{\sim}{\pm}0.6$ pct. The results showed that low-cycle fatigue life was strongly dependent on the amount of pre-strain as well as the strain amplitude. Increasing the amount of prestrain, the number of reversals to failure was significantly decreased at high strain amplitudes, but the effect was negilgible at low strain amplitudes. A new model for predicting fatigue life of pre-strained body has been devised adding a correction term of ${\Delta}E_{pre-strain}$ to the energy-based fatigue damage parameter.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2007.05a
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• pp.122-122
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• 2007