• 한국재료학회지
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• 제29권8호
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• pp.511-518
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• 2019

In this study, the effect of carbon equivalent and cooling rate on microstructure and hardness of A516 steels for pressure vessel is investigated. Six kinds of specimens are fabricated by varying carbon equivalent and cooling rate, and their microstructures and hardness levels are analyzed. Specimens with low carbon equivalent consist of ferrite and pearlite. As the cooling rate increases, the size of pearlite decreases slightly. The specimens with high carbon equivalent and rapid cooling rates of 10 and $20^{\circ}C/s$ consist of not only ferrite and pearlite but also bainite structure, such as granular bainite, acicular ferrite, and bainite ferrite. As the cooling rate increases, the volume fractions of bainite structure increase and the effective grain size decreases. The effective grain sizes of granular bainite, acicular ferrite, and bainitic ferrite are ~20, ~5, and ${\sim}10{{\mu}m$, respectively. In the specimens with bainite structure, the volume fractions of acicular ferrite and bainitic ferrite, with small effective grains, increase as cooling rate increases, and so the hardness increases significantly.

• 한국주조공학회지
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• 제12권5호
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• pp.403-411
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• 1992

The growth characteristics of bainite at early stage in the fast quenched spheroidal graphite cast irons containing 0.06%Mn and 0.45%Mn during austempering process, was investigated with optical and scanning electron microscope. The following results regarding the effects of Mn and isothermal heat treatment on the morphological variation of bainitic ferrite were obtained. The morphology of bainite varies from acicular below 350$^{\circ}C$ to feather shape above 350$^{\circ}C$. The period of isothermal treatment also affects the shape of bainite at the fixed temperature. At 350$^{\circ}C$, bainite is bamboo leaf-like up to 200 secs of isothermal holding time and with further increasing time up to 300 secs, changes to a mixed structure consisting of both feather and bamboo leaf and, finally becomes all feather shape at 900 secs. The morphology of bainitic ferrite formed at early stage of 300$^{\circ}C$ isothermal treatment is similar to that of bainitic ferrite formed at 250$^{\circ}C$ or 350$^{\circ}C$ with unbranched, linear ferrite. However, bainitic ferrite divides into branches with increasing isothermal treatment, which occurs more fast at 400$^{\circ}C$ than at 350$^{\circ}C$. The difference in adding amount of Mn influences the morphology of bainitic ferrite in upper bainite. The bainitic ferrite with 0.45%Mn is observed to be more stable than that with 0.06%Mn, remaining unbranched for a longer period at the same temperature.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2000년도 춘계학술대회논문집
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• pp.196-201
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• 2000

A thermodynamic model was developed to analyze the characteristics of the heat generation during transformation of austenite in 0.186wt% and 0.458 wt%. carbon steels. The heat capacity and the heat evolved during transformation were formulated as functions of temperature and chemical composition for ferrite bainite and pearlite. in addition using the transformation dilatometer the transformation heat evolved during cooling was measured and the transformation behavior was observed. It was found that the heat capacity of ferrite was similar to those of pearlite and bainite. The heat capacity of ferrite was greater than that of bainite which was greater than that of pearlite. The molar heat of transformation to pearlite was greater than that to bainite which was greater than that to ferrite. The heats were found to be increased with decreased temperature and increasing the carbon content, It was also observed that the thermodynamic model. The heat of transformation in the higher carbon steel was greater than that in the lower carbon one. This was attributed to the lower transformation temperature and the greater amount of transformed pearlite in the higher carbon steel.

• 한국주조공학회지
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• 제9권6호
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• pp.463-473
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• 1989

This study is aimed to investigate the effect of cyclic heat treatment which is special heat treatment on the strength and toughness in Multi-phase(Ferrite-Bainite-Martensite) Ductile cast Irons. Spceimens were austenitized at eutectoid transformation temperature range(${\alpha}+{\gamma}$) for 30min and austempered at $300^{\circ}C$ and $400^{\circ}C$ for different holding times, and then quenched in ice water to obtain the multi-phase(Ferrite-Bainite-Martensite) structure from various prior structures, which was obtained by various cyclic heat treatments. As the number of cycle in cyclic heat treatment increased, volume fraction of pearlite increased and the its morphology was refined. As the number of cycle in cyclic heat treatment increased, the multi-phase(Ferrite-Bainite-Martensite) was dispersed in whole matrix as refined island phase. Particularly, martensite among the multi-phase gradually became a spherical shape. Good combination in impact energy and tensile strength was detained in $840^{\circ}C-300^{\circ}C-15min$ condition after 10 cycles in cyclic heat treatment, and its multi-phase volume fraction is Ferrite(50%)-Martensite(l3%)-Bainite(37%).

• 한국재료학회지
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• 제1권3호
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• pp.115-124
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• 1991

본 실험은 페라이트-베이나이트-마르텐사이트로 되는 삼상조직강의 기계적성질과 조직특성과의 관계를 연구하고저 하였다. 이를 위하여 서로 다른 열처리경로를 택하여 페라이트+마르텐사이트에(DP), 페라이트+베이나이트(F+B)의 이상조직강과 페라이트+마르텐사이트에 연질의 베이나이트를 함유시킨 삼상조직강(TP)을 제작하였다. 이들 이상조직강가 삼상조직강의 인장특성, 충격특성 및 stretch-flangeability를 측정하여 각각의 조직구성과 상호연관지어 금속조직학적으로 연구, 조사하였다. 실험결과, TP강의 경우 베이나이트 부피분율의 증가에 따라서 인장강도와 항복강도는 감소하나, 단면수축율 및 강도-연성 조합은 증가하였고, 페라이트와 베이나이트로 구성된 F+B강에서는 항복현상과 높은 항복비를 보였다. 충격특성은 DP강보다 TP, F+B강에서 향상된 충격에너지값을 얻을 수 있었고, hole expanding limit($\lambda$)시험에서도 DP강보다는 TP, F+B강이 우수한 $\lambda$값을 나타내었다. 이와같은 기계적성질의 향상은 베이나이트의 영향에 의한 결과로서, 이는 경질의 마르텐사이트보다는 연질의 베이나이트ㅏ 페라이트기지와 함께 쉽게 변형이 일어나 연성의 증가에 보다 크게 작용하였기 때문이라 생각한다. 본실험의 경우 27%범위의 베이나이트를 함유하는 삼상조직강에서 좋은 기계적 성질과 우수한 stretch-flangeability를 보였다.

• 한국산학기술학회논문지
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• 제19권4호
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• pp.69-76
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• 2018

본 연구에서는 유정용 강관의 열처리 조건 및 합금원소(B, Ti)에 따른 미세조직 및 기계적 성질을 조사하였다. 실험에는 J55, J55+B,Ti 강재를 사용하였고, 열처리 조건은 각각 오스테나이트 처리온도 ($880^{\circ}C$, $910^{\circ}C$, $940^{\circ}C$), 냉각방식 (수냉, 유냉), 템퍼링 온도 (미실시, $550^{\circ}C$, $650^{\circ}C$) 이다. 열처리 조건에 따라 얻어지는 미세조직을 예측하기 위해 J55, J55+B,Ti 강재의 화학적 성분을 기준으로 평형상태도와 CCT 곡선을 예측하였다. 시뮬레이션 결과 평형상태도는 A1, A3 온도가 약 $20^{\circ}C$ 감소하였고, CCT 곡선은 B, Ti이 첨가됨에 따라 ferrite와 bainite nose 부분이 오른쪽으로 이동하였다. J55, J55+B,Ti 강재의 CCT 곡선을 기준으로 냉각속도에 따른 martensite, bainite, ferrite등 예상되는 미세조직을 예측하였고, J55 강재의 미세조직 예측값은 실제 실험값과 유사한 양상을 나타내었지만. J55+B,Ti 강재의 예측값은 실제 실험값과는 차이가 있었다. 열처리 조건이 변화됨에 따라 martensite, bainite, ferrite 등 다양한 조직이 생성되었으며, 이는 경도, 강도 및 연신율에 밀접한 영향을 미쳤다. J55시편의 수냉의 경우 martensite 조직이 형성되었고, 유냉의 경우 bainite와 ferrite 조직이 형성되었지만, J55+B,Ti시편은 B의 첨가에 의한 경화능 향상으로 냉각방식에 관계없이 martensite 조직이 형성되었다. 전반적으로 B, Ti을 첨가하면서 기계적 성질은 향상되었고, quenching 이후의 시편보다 tempering 이후의 시편에서 크게 향상되었다. 이는 Ti의 첨가로 인해 생성된 미세한 석출물이 재결정시 결정립 성장을 억제하여 미세한 오스테나이트 결정립을 생성하였고, tempering 열처리 이후에도 결정립 미세화 효과가 큰 영향을 미친 것으로 판단된다.

• Journal of Welding and Joining
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• 제34권1호
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• pp.41-46
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• 2016

With the recent increase in size of ships and offshore structures, there are more demand for thicker plates. As the thickness increases, it is known that fatigue life of the structures decrease. To improve the fatigue life, post weld treatments techniques, such as toe grinding, TIG dressing and hammer peening, are typically employed. However, these techniques require additional construction time and production cost. Therefore, it is of crucial interest steels with longer fatigue crack growth life compared to conventional steels. This study investigates fatigue crack growth rate (FCGR) behaviours of conventional EH36 steel and Ferrite-Bainite dual phase EH36 steel (F-B steel). F-B steel is known to have improved fatigue performance associated with the existence of two different phases. Ferrite-Bainite dual phase microstructures are obtained by special thermo mechanical control process (TMCP). FCGR behaviours are investigated by a series of constant stress-controlled FCGR tests. Considering all test conditions (ambient, low temperature, high stress ratio), it is shown that FCGR of F-B steel is slower than that of conventional EH36 steel. From the tensile tests and impact tests, F-B steel exhibits higher values of strength and impact energy leading to slower FCGR.

• 한국주조공학회지
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• 제12권3호
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• pp.230-237
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• 1992

The purpose of present investigation is to obtain ductile cast iron with ferrite-bainite matrix by pearlite-bainite transformation treatment. Ductile cast irons having three kinds of Mn ampunt had been manufactured. Mn increased pearlite volume fraction iin as-cast ductile cast iron. Ductile cast irons of different pearlite fraction were austenitized at $875\;^{\circ}C$ for 230-350 sec or $925\;^{\circ}C$ for 130-170 sec followed by austempering at $300\;^{\circ}C$ or $400\;^{\circ}C$ for the various periods of time from 5 to 30 min. When specimen was austenitixed for 130 sec at $925\;^{\circ}C$ and for 230 sec at $875\;^{\circ}C$, pearlite was transformed into austenite. Bainite around graphite was found at $925^{\circ}C$ for 170 sec. Bainite in grain boundary of ferrite was happened at $875^{\circ}C$ for 350 sec. During the austempering process, acicular bainite was precipitated at $300^{\circ}C$ and lath bainite was precipitated at $400^{\circ}C$. Increment in manganese content restrained bainitic transformation. Retained austenie was of little quantity.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2000년도 추계학술대회 논문집
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• pp.49-52
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• 2000

The hole flanging experiments are performed on a flat circular plates with a hole in the center and the flangeability and fracture behavior of TRIP steel and ferrite-bainite duplex steel were examined. Over the ranges of conditions investigated, the flangeability of ferrite-bainite duplex steel is better than TRIP steel and the term 'hole flanging capacity' introduced in this study. Fracture behavior of TRIP steel and ferrite-bainite duplex steel was characterized by petal formation at the edge of flange. In case of crack propagation, crack tip diversion that is supposed to be responsible for flangeability occurs more severely on HSLA Steel.

• 한국주조공학회지
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• 제11권5호
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• pp.406-413
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• 1991

Ductile cast iron(DCI) with a multi-phase(ferrite-bainite-martensite) structures was produced by various special heat treatment. Intercritical heat treatment(I. C.), intermediate heat treatment(I. M.) and step quenching(S. Q.) were used to strengthen and toughen the fracture initiation sites such as graphite-marix interfaces and eutectic cell boundaries in DCI. The purpose of this study was to investigate of DCI by the special heat treatment. (I. C., I. M., and S. Q.) At a result, bainite nucleation rate at higher temperature was higher than that of at lower temperature, and shapes of bainite and martensite became bar /spheroidal type with increase of isothermal transformation time.