• 대한금속재료학회지
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• 제46권3호
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• pp.111-117
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• 2008

The effects of the dissolved cementite on the mechanical properties have been experimentally investigated. The steel wires were fabricated depending on the carbon content of 0.82 and 1.02 wt.% and drawing strain from 4.12 to 4.32. The bending fatigue resistance and torsion ductility were measured by a hunter fatigue tester and torsion tester specially designed for thin-sized wires. The results showed that as the drawing strain and carbon content increased, the fatigue resistance and the torsional ductility of the steel wires decreased, while the tensile strength increased. In order to elucidate this behavior, the microstructure in terms of lamellar spacing (${\lambda}_p$), cementite thickness ($t_c$) and morphology of cementite was observed by advanced analysis techniques such as transmission electron microscope (TEM) and 3 dimensional atom probes (3-D AP).

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2008년도 추계학술대회 논문집
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• pp.138-141
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• 2008

The effects of annealing temperature and time on mechanical properties and microstructures were investigated in cold drawn pearlitic steel wires. During annealing, the increment of the tensile strength at low temperatures found to be due to age hardening, while the decrease in the tensile strength at high temperatures was attributed to age softening, involving the spheroidization of lamellar cementite and recovery of lamellar ferrite. Since tensile strength and the occurrence of the delamination would be closely related to the dissolution of cementite, the lower annealing temperature and the increase of drawing strain caused the higher tensile strength and the easier occurrence of the delamination in cold drawn pearlitic steel wires.

• 열처리공학회지
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• 제10권2호
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• pp.101-108
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• 1997

In high carbon steels, the cementite phase is more unstable thermally than graphite, and it can lead to a marked deterioration in the formability. Many studies have been carried out to evaluate the effect of the fundamental elements on the graphitization of cementite in high carbon steels. In this present work, the effect of Mn, Cr addition on graphitization in Fe-0.65%C-1.0%Si steel has been investigated by means of hardness testing, optical microscopy and EPMA. The nucleation of graphite may mostly depend on the dissolution rate of cementite into ferrite and the number of the nucleation sites of graphite. The graphitization was promoted by the addition of 0.1%Mn in high carbon steel, but retarded by more addition than 0.5% of Mn. By the more addition of Cr than 0.1%, the graphitization of high carbon steel was strongly retarded. Because Mn was moderate anti-graphitizer, but Cr was stroug anti-graphitizer.

• 열처리공학회지
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• 제19권5호
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• pp.270-279
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• 2006

The effects of eight types of spheroidizing annealing conditions including annealing temperature, annealing time, cooling rate, and furnace atmosphere on the microstructure and hardeness were determined in SCM440 steel which has been widely used for automotive parts. The well-spheroidized structure and minimum hardness were obtained when the steel was heat-treated at $770^{\circ}C$ for 6 hours, cooled to $720^{\circ}C$ at a cooling rate of $24^{\circ}C/h$, and then kept for 7 hours at the $720^{\circ}C$ followed by air cooling. In order to increase the productivity and to save the manufacturing cost, it is desirable to apply a faster cooling rate to the spheroidizing annealing. It was found that a cooling rate of $100^{\circ}C/hr$ was the fastest cooling rate applicable to the SCM440 steel among the four cooling rates used in this study. The microstructure consisted of ferrite and very fine spheroidized cementite when the steel was annealed for 13 hours at $720^{\circ}C$ below $A_{C1}$ temperature. This was caused by the short annealing time and the retarding effect of Cr and Mo on both the dissolution of pearlite to cementite and coarsening of spheroidized cementite. The steel heat treated in air showed the decarburized layer of about $125{\mu}m$ in thickness at the surface.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2009년도 춘계학술대회 논문집
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• pp.231-234
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• 2009

The effects of annealing temperature and time on mechanical properties and microstructures were already investigated in cold drawn pearlitic steel wires. During annealing, the increment of the tensile strength at low temperatures found to be due to age hardening, while the decrease in the tensile strength at high temperatures was attributed to age softening, involving the spheroidization of lamellar cementite and recovery of lamellar ferrite. Since Between increase of tensile strength and the occurrence of the delamination would be closely related to the dissolution of cementite, the increase of drawing strain by lower annealing temperature caused the between higher tensile strength and the easier occurrence of the delamination in cold drawn pearlitic steel wires.

• Corrosion Science and Technology
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• 제5권1호
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• pp.23-26
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• 2006

Recent development of ultrafine grained (UFG) low carbon steels by using equal channel angular pressing (ECAP) and their room temperature tensile properties are reviewed, focusing on the strategies overcoming their inherent mechanical drawbacks. In addition to ferrite grain refinement, when proper post heat treatments are imposed, carbon atom dissolution from pearlitic cementite during ECAP can be utilized for microstructural modification such as uniform distribution of nano-sized cementite particles or microalloying element carbides inside UFG ferrite grains and fabrication of UFG ferrite/martensite dual phase steel. The utilization of nano-sized particles is effective on improving thermal stability of UFG low carbon ferrite/pearlite steel but less effective on improving its tensile properties. By contrast, UFG ferrite/martensite dual phase steel exhibits an excellent combination of ultrahigh strength, large uniform elongation and extensive strain hardenability.

• 열처리공학회지
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• 제12권1호
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• pp.55-65
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• 1999

Graphitization of cementite in high carbon steels at subcritical temperature is reported. We have studied about the effects of Ni in these steels on graphitization. The chemical compositions of the specimens were Fe-(0.54, 0.7)%-C-1.0%Si-0.1%Mn-(0.2~1.0)%Ni. After annealing at $650^{\circ}C$, $680^{\circ}C$ and $700^{\circ}C$ during various time the microstructures and hardness change were observed. In order to recover the initial hardness of high carbon steel, dissolution treatment of graphite was performed at $870^{\circ}C$. In case of 0.7%C steel, graphitization was accelerated rather 1.0%Ni addition steel than 0.2% and 0.54%Ni addition steels but the graphite is coarser. In case of 0.54%C-0.2%Ni steel, graphite particles were distributed relatively homogeniously and finely. Nickel addition promotes graphitization of these steels but makes graphite blocky.

• 열처리공학회지
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• 제26권6호
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• pp.300-305
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• 2013

The effects of solute carbon atoms on the thermal expansion coefficients of ferrite and austenite as well as austenitization behavior were investigated by comparing carbon-free ferrite and carbon-containing ferrite. The thermal expansion coefficients and austenitization start and finish temperatures were measured using a dilatometer. Solute carbon atoms at elevated temperatures above the cementite dissolution temperature (650 K) decreased the thermal expansion coefficients of both ferrite and austenite. In addition, minute amount of carbon atoms dissolved in ferrite stimulated austenite nucleation during continuous heating, resulting in the lower starting temperature of austenitization.

• Advances in materials Research
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• 제6권3호
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• pp.245-255
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• 2017

High energy ball milling is employed to produce iron matrix- multiwall carbon nanotube (MWCNT) reinforced composite. The damage caused to MWCNT due to harsh ball milling condition and its influence on interfacial bonding is studied. Different amount of MWCNT is used to find the optimal percentage of MWCNT for avoidance of the formation of chemical reaction product at the matrix - reinforcement interface. Effect of process control agent is assessed by the use of different materials for the purpose. It is observed that ethanol as a process control agent (PCA) causes degradation of MWCNT reinforcements after milling for two hours whereas solid stearic acid used as process control agent, allows satisfactory conservation of MWCNT structure. It is further noted that at a high MWCNT content (~ 2wt.%), high energy ball milling leads to reaction of iron and carbon and forms iron carbide (cementite) at the iron-MWCNT interface. At low percentage of MWCNT, dissolution of carbon in iron takes place and the amount of reinforcement in iron matrix composite becomes negligibly small. However, under the present ball milling condition (ball to metal ratio~ 6:1 and 200 rpm vial speed) iron-1wt.% MWCNT composite of good interfacial bonding can retain the tubular structure of reinforcing MWCNT.

• 열처리공학회지
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• 제11권2호
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• pp.140-149
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• 1998

The graphitization is affected by the addition of small amount of the elements, such as Si, Al, Ni, B, Cr and Mn etc. Boron is well known as the most effective element for the graphitization of cementite in high carbon steels. But a study on quantitative analysis of B effect on the graphitization is few reported. Therefore the effect of boron addition in Fe-0.65%C-1.0%Si-0.5%Mn steels on the graphitization is investigated quantitatively using hardness tester, optical microscope and scanning electron microscope, neutron induced microscopic radiography. The graphitization in high carbon steels is promoted with 0.003~0.005%B addition. But the graphitization in steels which has no boron takes long holding time at $680{\sim}720^{\circ}C$. The hardness of quenched steel containing 0.003%B is higher than that of 0.005%B added steel due to complete dissolution of fine graphites into the austenite. The 0.003%B added high carbon steel graphitized at $680^{\circ}C$ for 25hr is useful steel for the agricultural implements and automobile parts which needed a good formability and high hardness.