• Archives of Metallurgy and Materials
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• 제64권3호
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• pp.863-867
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• 2019

The effects of carbon content on the austenite stability and strain-induced transformation of nanocrystalline Fe-11% Ni alloys were investigated using X-ray analysis and mechanical tests. The nanocrystalline FeNiC alloy samples were rapidly fabricated using spark plasma sintering because of the extremely short densification time, which not only helped attain the theoretical density value but also prevented grain growth. The increased austenite stability resulted from nanosized crystallites in the sintered alloys. Increasing compressive deformation increased the volume fraction of strain-induced martensite from austenite decomposition. The kinetics of the strain-induced martensite formation were evaluated using an empirical equation considering the austenite stability factor. As the carbon content increased, the austenite stability was enhanced, contributing to not only a higher volume fraction of austenite after sintering, but also to the suppression of its strain-induced martensite transformation.

• Archives of Metallurgy and Materials
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• 제65권3호
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• pp.1001-1004
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• 2020

We investigated the austenite stability and mechanical properties in FeMnNiC alloy fabricated by spark plasma sintering. The addition of Mn, Ni, and C, which are known austenite stabilizing elements, increases its stability to a stable phase existing above 910℃ in pure iron; as a result, austenitic microstructure can be observed at room temperature, depending on the amounts of Mn, Ni, and C added. Depending on austenite stability and the volume fraction of austenite at a given temperature, strain-induced martensite transformation during plastic deformation may occur. Both stability and the volume fraction of austenite can be controlled by several factors, including chemical composition, grain size, dislocation density, and so on. The present study investigated the effect of carbon addition on austenite stability in FeMnNi alloys containing different Mn and Ni contents. Microstructural features and mechanical properties were analyzed with regard to austenite stability.

• 한국분말재료학회지
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• 제29권6호
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• pp.517-522
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• 2022

We investigate the austenite stability in nanocrystalline Fe-7%Mn-X%Mo (X = 0, 1, and 2) alloys fabricated by spark plasma sintering. Mo is known as a ferrite stabilizing element, whereas Mn is an austenite stabilizing element, and many studies have focused on the effect of Mn addition on austenite stability. Herein, the volume fraction of austenite in nanocrystalline Fe-7%Mn alloys with different Mo contents is measured using X-ray diffraction. Using a disk compressive test, austenite in Fe-Mn-Mo alloys is confirmed to transform into strain-induced martensite during plastic deformation by a disk d. The variation in austenite stability in response to the addition of Mo is quantitatively evaluated by comparing the k-parameters of the kinetic equation for the strain-induced martensite transformation.

• 한국분말재료학회지
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• 제27권5호
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• pp.414-419
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• 2020

In the present study, we investigated the austenite stability of a sintered Fe-based nanocrystalline alloy. The volume fraction of austenite was measured based on the X-ray diffraction data of sintered Fe-based nanocrystalline alloys, which were prepared by high-energy ball milling and spark plasma sintering. The sintered alloy samples showed a higher volume fraction of austenite at room temperature as compared to the equilibrium volume fraction of austenite obtained using thermodynamic calculations, which resulted from the nanosized crystalline structure of the sintered alloy. It was proved that the austenite stability of the sintered Fe-based alloy increased with a rise in the amount of austenite stabilizing elements such as Mn, Ni, and C; however, it increased more effectively with a decrease in the actual grain size. Furthermore, we proposed a new equation to predict the martensite starting temperature for sintered Fe-based alloys.

• 한국분말재료학회지
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• 제26권5호
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• pp.389-394
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• 2019

In the present study, we have investigated the effect of sintering process conditions on the stability of the austenite phase in the nanocrystalline Fe-5wt.%Mn-0.2wt.%C alloy. The stability and volume fraction of the austenite phase are the key factors that determine the mechanical properties of FeMnC alloys, because strain-induced austenite-martensite transformation occurs under the application of an external stress at room temperature. Nanocrystalline Fe-5wt.%Mn-0.2wt.%C samples are fabricated using the spark plasma sintering method. The stability of the austenite phase in the sintered samples is evaluated by X-ray diffraction analysis and hardness test. The volume fraction of austenite at room temperature increases as the sample is held for 10 min at the sintering temperature, because of carbon diffusion in austenite. Moreover, water quenching effectively prevents the formation of cementite during cooling, resulting in a higher volume fraction of austenite. Furthermore, it is found that the hardness is influenced by both the austenite carbon content and volume fraction.

• 한국분말재료학회지
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• 제25권2호
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• pp.126-131
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• 2018

In the present study, we investigate the effects of milling time and the addition of a process control agent (PCA) on the austenite stability of a nanocrystalline Fe-7%Mn alloy by XRD analysis and micrograph observation. Nanocrystalline Fe-7%Mn alloys samples are successfully fabricated by spark plasma sintering. The crystallite size of ball-milled powder and the volume fraction of austenite in the sintered sample are calculated using XRD analysis. Changes in the shape and structure of alloyed powder according to milling conditions are observed through FE-SEM. It is found that the crystallite size is reduced with increasing milling time and amount of PCA addition due to the variation in the balance between the cold-welding and fracturing processes. As a result, the austenite stability increased, resulting in an exceptionally high volume fraction of austenite retained at room temperature.

• International Journal of Korean Welding Society
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• 제2권1호
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• pp.25-28
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• 2002

In the coarse grain HAZ adjacent to the fusion line, most of the TiN particles in conventional Ti added steel are dissolved and austenite grain growth is easily occurred during welding process. To avoid this difficulty, thermal stability of TiN particle is improved by increasing the nitrogen content in steel. In this study, the effect of hlgh nitrogen TiN particle on preventing austenite grain growth in HAZ was investigated. Increased thermal stability of TiN particle is helpful for preventing the austenite grain growth by pinning effect. High nitrogen TiN particle in simulated HAZ were not dissolved even at high temperature such as 1400'E and prevented the austenite grain growth in simulated HAZ. Owing to small austenite grain size in HAZ the width of coarse grain HAZ in high nitrogen TiN steel was decreased to 1/10 of conventional TiN steel. Even high heat input welding, the microstructure of coarse grain HAZ consisted of fine polygonal ferrite and pearlite and toughness of coarse grain HAZ was significantly improved.

• 열처리공학회지
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• 제13권4호
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• pp.253-258
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• 2000

The present study aimed to develop the TRIP(transformation induced plasticity) aided high strength low carbon steel sheets using reverse transformation process. The cold-rolled 0.14C-6.5Mn steel was reverse-transformed by slow heating to intercritical temperature region and air cooling to room temperature. An excellant combination of tensile strength and elongation of $98.3kgf/mm^2$ and 44.4% appears. This combination comes from TRIP phenomena of retained austenite during deformation. The stability of retained austenite Is very Important for the good ductility and it depends on diffusion of carbon and manganese during reverse transformation. The air cooling after holding at intercritical temperature retards the formation of pearlite and provides the carbon enrichment in retained austenite, resulting the increase of elongation in cold-roiled TRIP steel.

• 한국재료학회지
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• 제11권5호
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• pp.431-437
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• 2001

본 연구는 제조공정을 달리한 0.14C-6.5Mn강을 2상영역에서 역변태처리 하였을 때 다량의 잔류오스테나이트를 생성시키기 위한 열처리 조건을 제시하고 잔류오스테나이트의 생성과 관련하여 미세조직 관찰, C, Mn의 분배거동 및 기계적성질을 조사하였다. 잔류오스테나이트는 역변태처리시 오스테나이트내에 C, Mn의 확산으로 농축되어 안정화되며 연성향상에 크게 기여한다. 30%이상의 잔류오스테나이트를 확보하기 위해서는 6457에서 역변태처리하는 것이 효과적이지만, 잔류오스테나이트의 부피 분율과 기계적안정성을 고려하면 $620^{\circ}C$에서 열처리하는 것이 바람직하다. 냉연재의 강도.연성조합값은 3강종 모두 $620^{\circ}C$에서 1시간 역변태처리한 경우 4000kg/$\textrm{mm}^2$정도로 매우 우수하지만 고온에서는 연성감소로 인하여 그 값이 현저하게 저하하였다. 0.14C-6.5Mn계 TRIP강에서 잔류오스테나이트 생성과 기계적성질에 미치는 1.1%Si 첨가효과는 매우 미약하였다.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1999년도 제3회 압연심포지엄 논문집 압연기술의 미래개척 (Exploitation of Future Rolling Technologies)
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• pp.356-365
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• 1999

The present study is aimed at developing the TRIP(transformation induced plasticity) aided high strength low carbon steel using reversion process. An excellent combination of elongation over 40% and tensile strength abut 100kgf/$\textrm{mm}^2$ achieved in processing of 0.15C-0.5 Si-6Mn steel by slow heating to intercritial temperature region and accelerated cooling into room temperature. This good combination is caused by TRIP phenomena of retained austenite in steels during deformation. The stability of retained austenite is very important for the good ductility and it depends on the diffusion of carbon and manganeses during heat treatment. The accelerated cooling after holding at annealing temperature retards the formation of pearlite and provides the carbon enrichment in retained austenite in steel, resulting in the increase in elongation of the cold-rolled TRIP steel. On the other hand, heat treating the steel at 600$^{\circ}C$ for 5 hour before cold rolling increases elongation but reduces the amount of retained austenite after reversion processing. It is accounted that the heat treating is effective for the increase in the stability of retained austenite.