• 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.

• Journal of the Korean Society for Heat Treatment
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• v.7 no.2
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• pp.118-128
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• 1994

Five alloys were selected randomly in the composition range showing the best shape memory effect in Fe-Mn-Si system reported by Murakami. The shape memory effects of those alloys were mainly investigated through the training treatment which consisted of the repetition of 2% tensile deformation at room temperature and subsequent annealing at $600^{\circ}C$ above $A_r$ temperature. At the same deformation degress in rolling $600^{\circ}C$-annealing for 1 hr. showed the best shape memory effect, and 10%-deformation degrees represented maxima of the shpae memory effects at all annealing temperatures, $500^{\circ}C$, $600^{\circ}C$ and $700^{\circ}C$. The shape memory effects of the alloys were increased by increasing training cycle up to 5 cycles. This was because a large number of dislocations introduced by training process gave rise to increase in the austenite yield stress, and acted as nucleation sites for stress induced ${\varepsilon}$ martensite. The thermal cycling treatment, repetition of cooling in nitrogen at $-196{\circ}C$ and heating to $300^{\circ}C$ for 5 min., did not improve the shape memory effect.