• 한국재료학회지
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• 제27권4호
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• pp.184-191
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• 2017

This present study deals with the effect of micro-alloying elements and transformation temperature on the correlation of microstructure and tensile properties of low-carbon steels with ferrite-pearlite microstructure. Six kinds of low-carbon steel specimens were fabricated by adding micro-alloying elements of Nb, Ti and V, and by varying isothermal transformation temperature. Ferrite grain size of the specimens containing mirco-alloying elements was smaller than that of the Base specimens because of pinning effect by the precipitates of carbonitrides at austenite grain boundaries. The pearlite interlamellar spacing and cementite thickness decreased with decreasing transformation temperature, while the pearlite volume fraction was hardly affected by micro-alloying elements and transformation temperature. The room-temperature tensile test results showed that the yield strength increased mostly with decreasing ferrite grain size and elongation was slightly improved as the ferrite grain size and pearlite interlamellar spacing decreased. All the specimens exhibited a discontinuous yielding behavior and the yield point elongation of the Nb4 and TiNbV specimens containing micro-alloying elements was larger than that of the Base specimens, presumably due to repetitive pinning and release of dislocation by the fine precipitates of carbonitrides.

• 한국재료학회지
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• 제21권6호
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• pp.334-340
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• 2011

Saw wires have been widely used in industries to slice silicon (Si) ingots into thin wafers for semiconductor fabrication. This study investigated the microstructural and mechanical properties, such as abrasive wear and tensile properties, of a saw wire sample of 0.84 wt.% carbon steel with a 120 ${\mu}M$ diameter. The samples were subjected to heat treatment at different linear velocities of the wire during the patenting process and two different wear tests were performed, 2-body abrasive wear (grinding) and 3-body abrasive wear (rolling wear) tests. With an increasing linear velocity of the wire, the tensile strength and microhardness of the samples increased, whereas the interlamellar spacing in a pearlite structure decreased. The wear properties from the grinding and rolling wear tests exhibited an opposite tendency. The weight loss resulting from grinding was mainly affected by the tensile strength and microhardness, while the diameter loss obtained from rolling wear was affected by elongation or ductility of the samples. This result demonstrates that the wear mechanism in the 3-body wear test is much different from that for the 2-body abrasive wear test. The ultra-high tensile strength of the saw wire produced by the drawing process was attributed to the pearlite microstructure with very small interlamellar spacing as well as the high density of dislocation.

• 열처리공학회지
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• 제30권1호
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• pp.6-12
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• 2017

In this study tensile and impact properties of three hypo-eutectoid steels containing different micro-alloying elements were investigated in terms of microstructural factors such as pro-eutectoid ferrite grain size, pearlite fraction, interlamellar spacing, and cementite thickness. Yield point phenomenon appeared in all the steel specimens during tensile testing, and ultimate tensile stress was mainly dependent on pearlite fraction. On the other hand, the refinement of austenite grain size caused by the addition of micro-alloying elements resulted in the increment of ferrite volume fraction and carbon contents in pearlite because of the refinement of pro-eutectoid ferrite grain size. As a result, cementite thickness in pearlite increased and had an effect on deteriorating the low temperature impact toughness.

• 소성∙가공
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• 제13권5호
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• pp.415-421
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• 2004

The effect of transformation temperature on microstructural features and their effects on ductility in 0.55%C steels were investigated, compared with in 0.82%C eutectoid steel. The samples were austenitized at 100$0^{\circ}C$ for 30min. followed by quenching in a salt bath in the temperature range of 500 ~ $620^{\circ}C$. It was found that reduction of area(RA) increased with increasing transformation temperature and then, decreased after reaching its maximum value in steels containing pro-eutectoid ferrite less than 6%. The thickness of lamellar cementite was found to be the main factor controlling RA. Additionally, the presence of cementite thickness for the maximum ductility in all the tested steels was observed as about 0.015${\mu}{\textrm}{m}$ for tested steels.

• 소성∙가공
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• 제31권4호
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• pp.207-213
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• 2022

In this study, variations in the microstructure and hardness of a low-carbon SCM415 steel with austenitizing temperature and cooling rate are investigated. When the austenitizing temperature is lower than the A₁ temperature (738.8 ℃) of the SCM415 steel, the microstructures of both the air-cooled and water-cooled specimens consist of ferrite and pearlite, which are similar to the microstructure of the initial specimen. When heat treatment is conducted at temperatures ranging from the A₁ temperature to the A₃ temperature (822.4 ℃), the microstructure of the specimen changes depending on the temperature and cooling rate. The specimens air- and water-cooled from 750 ℃ consist of ferrite and pearlite, whereas the specimen water-cooled from 800 ℃ consists of ferrite and martensite. At a temperature higher than the A₃ temperature, the air-cooled specimens consist of ferrite and pearlite, whereas the water-cooled specimens consist of martensite. At 650 ℃ and 700 ℃, which are lower than the A₁ temperature, the hardness decreases irrespective of the cooling rate due to the ferrite coarsening and pearlite spheroidization. At 750 ℃ or higher, the air-cooled specimens have smaller grain sizes than the initial specimen, but they have lower hardness than the initial specimen owing to the increased interlamellar spacing of pearlite. At 800 ℃ or higher, martensitic transformation occurs during water cooling, which results in a significant increase in hardness. The specimens water-cooled from 850 ℃ and 950 ℃ have a complete martensite structure, and the specimen water-cooled from 850 ℃ has a higher hardness than that water-cooled from 950 ℃ because of the smaller size of prior austenite grains.

• 열처리공학회지
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• 제8권3호
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• pp.205-212
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• 1995

Effect of Mn addition on rolling contact fatigue of C-base induction hardened bearing steels has been investigated to develop inexpensive surface-hardened bearing steels with improved resistance to rolling contact fatigue. Fatigue tests were conducted in elasto-hydrodynamic lubricating conditions at a shaft speed of 5,000rpm, under max. Hertzian stress of $492kg/mm^2$. It was found in the C-Mn steels that effective depth of induction hardened layer and amount of retained austenite were slightly increased in comparison with those of C-base steels. finer interlamellar spacing of pearlite in the C-Mn steels was also observed using TEM. Decomposition of retained austenite during rolling contact fatigue was smaller in quantity in the C-Mn steels than C-base steels. This might be associated with enhanced mechanical stability of retained austenite with addition of Mn. Statistical analysis of fatigue life for C-Mn steels using Weibull distribution indicated that improved resistance to rolling contact fatigue was mainly attributed to transformation induced plasticity and mechanical stability of retained austenite.

• 대한기계학회논문집
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• 제12권3호
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• pp.505-512
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• 1988

본 연구에서는 상술한 $\boxDr$한계정유길이$\boxUl$를 찾아내는 것을 주요수단으로 하여, 그 전파 하한계응력으로서의 피로한도와의 관계를 정량적으로 검토하고, 미소크랙전파 거동이 미시조직 또는 피로부하이력등에 민감함을 감안하여, 이들 인자가 미소크랙 전 파의 하한계조건에 미치는 영향에 관하여 검토하기로 하였다.

• 한국재료학회지
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• 제2권6호
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• pp.452-460
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• 1992

공냉에 의한 중탄소 비조질강과 직접 소입 방법에 의한 저탄소 비조질강의 강도와 인성에 대하여 미량 첨가 원소와 온도 및 냉각 속도의 영향을 조사하였다. 공냉에 의한 중탄소 비조질강은 페라이트-펄라이트 조직으로 V+Nb 복합첨가로 강도와 인성의 조합을 이룰 수 있었으며 연구 결과 최적 조합은 0.40C+0.12V+0.07Nb에 의한 인장 강도 831MPa, 충격 52.1J이었다. 직접 소입 방법에 의한 저탄소 비조질강은 마르텐사이트 조직으로 Mo에 의한 강도와 인성의 조합을 이를 수 있었으며 가장 양호한 조합은 0.12C+0.10V+0.03Nb+1.13Mo에 의한 인장강도 855MPa, 충격인성 108J로써 중탄소 비조질강에 비해 충격 인성이 2배 정도 향상되었다. 가열온도 110$0^{\circ}C$가 120$0^{\circ}C$에 비해 더 양호한 강도 및 충격 인성을 나타내었고, 냉각속도는 1.$2^{\circ}C$/s가 가장 양호한 결과를 나타내었다. 또한 합금원소와 온도 및 냉각속도가 강도, 충격인성, 오스테나이트 결정립 크기 그리고 펄라이트 층간거리에 미치는 영향을 중회귀분석하여 계량화하였다.