• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2002년도 춘계학술대회 논문집
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• pp.159-162
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• 2002

In this paper the stress-strain curves of bearing steels at hot working conditions are obtained by compression test with a computer controlled servo-hydraulic Gleeble 3800 testing machine and elongations and reductions of area of the bearing steels are obtained by tensile test with a computer controlled servo-hydraulic Gleeble 1500 testing machine. These tests have been focused to obtain the flow stress data and optimal hot forging conditions under various conditions of strain rates and temperatures. The strain rate sensitivity exponent and reduction of area of the materials are evaluated. Experimental results are resented for various conditions of temperatures and strain rates.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2009년도 춘계학술대회 논문집
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• pp.417-420
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• 2009

The hot forming process of a CAM for vessel engine was designed by finite element (FE) simulation and experimental analysis. An aim of process design was to achieve the near-net shaped CAM forgings by hot forging process. Based on the compression test results of the low alloy steel, deformation processing map was generated using the superposition approach between the dynamic materials model (DMM) and flow stability and/or instability criteria. From the processing map, the initial heating temperature was determined as $1200^{\circ}C$. FE analysis was simulated to predict the formation of rolling defects and deformed shape with different forging designs. Optimum process design suggested in this work was made by comparing with the CAM for vessel engine manufactured by actual forging process.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2009년도 춘계학술대회 논문집
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• pp.102-106
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• 2009

Thermal energy control is a important factor in a large size casting and forging. Good control of thermal energy makes characteristics and defect of large cast-forged part, such as large sized forged shell. We have studied about not only large size ring forging process and after heat treatment by FEM simulation. Also, changes of temperature and microstructure for forged shell were predicted. Therefore, we can choose the proper heat treatment condition by FEA. The sectional properties confirmed by practical experiment and evaluation have presented possibilities of process design by computational analysis.

• 한국분말야금학회:학술대회논문집
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• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part 1
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• pp.559-560
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• 2006

Powder forging is used for heavy-loaded parts (rings of rolling-contact bearings, gears etc.) production. Rolling contact fatigue is material property values of which characterize possibility of practical utilization of such parts. Rolling contact fatigue of some steels obtained out of prealloyed powders Astaloy CrM, Atomet 4601, Atomet 4901 and powder blends iron-carbon-nickel by hot forging is studied in the present paper. Effect of various kinds of heat and thermomechanical treatment on rolling contact fatigue is determined. Thermomechanical treatment provides optimal values of rolling contact fatigue. In this case steel structure contains up to 40% of retained metastable austenite which is transformed to martensite on trials. Thus typically crack is generated on residual pores and non-metallic inclusions instead of martensite zones in wrought steels.

• 한국정밀공학회지
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• 제15권7호
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• pp.129-138
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• 1998

The objective of this study is to demonstrate the ability of a computer simulation of microstructural evolution in hot forging of C-Mn steels. The development of microstructure is strongly dependent on process variables and metallurgical factors that affect time history of thermodynamical variables such as temperature, strain. and strain rate during deformation. Then finite element method is applied for the prediction of microstructural evolution, and it should be coupled with heat transfer analysis to consider the change of thermodynamical properties during forming process. In this study, Yada's recrystallization model and rigid-thermoviscoplastic finite element method are employed in order to analyze microstructural evolution during hot forging process. To show the validity and effectiveness of the proposed method, experiments are accomplished and the results of experiments are compared with those of simulations.

• 한국재료학회지
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• 제2권5호
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• pp.343-352
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• 1992

고온 압축 시험을 이용하여 열간 단조용 비조질강의 고온 변형 거동을 온도, 변형률속도, 합금원소에 따라 조사하였다. 고온 압축 시험에서 얻은 유동 응력 곡선의 형태와 조직관찰로부터 고온 변형 기구는 동적 재결정임을 알 수 있었다. 최대응력에 이르는 변형률은 온도가 증가할수록 작아지고 변형률속도가 빠를수록 크게 나타났다. Nb-V-Mo강은 Nb-V강에 비하여 최대응력은 증가하였으나 동적 재결정은 빨라졌다. 1.2Mn-0.09Nb강은 1.0Mn-0.05Nb강에 비하여 최대응력은 증가하였으나 동적재결정은 지연되었다. C-Nb-V강은 C강에 비하여 최대응력이 증가하였으며 동적 재결정은 지연되었다. 열간변형에 대한 구성방정식은 멱수법칙의 형태를 가졌다. Zener-Hollomon 파라미터가 증가할수록 동적 재결정립은 미세해졌고, 동적 재결정립과 Zener-Hollomon 파라미터와의 관계는 멱수법칙으로 정량화할 수 있었다.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2004년도 춘계학술대회 논문집
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• pp.395-399
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• 2004

Four microalloyed steels containing B were investigated in terms of hardenability, mechanical properties and microstructure depending upon the cooling rates in order to develop the steel grade for the cold forged fasners. The alloy with the largest DI value among 4 alloys, which contains $0.12\%\;C,\;1.54\%\;Mn,\;0.65\%\;Cr,\;0.11\%V,\;0.040\%Ti\;and\;0.0033\%B$, showed the larest shift to the right hand side in the TTT diagram, implying the wide allowable cooling rate range subsequent to hot rolling in long bar processing, Mechanical tests indicated that yield strength are dependent upon the DI value in water quenched specimens but other properties showed almost the same values. In the same grade of steel, the increase in cooling rates causes the decrease in elongation but the increase in strength, reduction of area and Charpy impact values. Microstructural examination in steel grade with the larest DI values revealed martensitic structure In the water quenched state, a mixture of martensite and bainite in the oil quenched, and ferrite + pearlite in the air cooled and the forced air cooled but the latter showed finer microstructure.

• 한국정밀공학회지
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• 제31권12호
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• pp.1093-1100
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• 2014

The goal of this paper is to investigate preliminary the applicability of 3D printing technologies for the development of the hot bulk forming process and die. 3D printing technology based on the plastic material was applied to the preform design of the hot forging process. Plastic hot forging dies were fabricated by Polyjet process for the physical simulation of the workpiece deformation. The feasibility of application of Laser-aided Direct Metal Rapid Tooling (DMT) process to the fabrication of the hot bulk metal forming die was investigated. The SKD61 hot-working tool steel was deposited on the heat treated SKD61 using the DMT process. Fundamental characteristics of SKD 61 hot-working tool steel deposited specimen were examined via hardness and wear experiments as well as the observation of the morphology. Using the results of the examination of fundamental characteristics, the applicability of the DMT process to manufacture hot bulk forming die was discussed.

• 한국정밀공학회지
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• 제13권11호
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• pp.106-113
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• 1996

Using stainless steel as the cold forged parts especially the outer parts of automobile is gradually increasing because it can bear up against the erosion and the wear. During cold forging of the stainless steel the working pressure acting on die surface are very high therefore the wear on die surface can be greatly increased. In cold forging processes, die failure must be considered before die design. One of the main reasons of die failure in industrial application of metal forming technologies is wear. The die wear affects the tolerances of forged parts, metal flow and costs of processes etc. The only way to to control these failures is to develop methods which allow prediction of the die wear and which are suited to be used in the desing stage in order to optimize the process. In this paper, the rigid-plastic finite element method was combined with the wear prediction routine and then the forward extrusion process using stainless steel was analysed simultaneously. To minimize the die wear the FPS algorithm was applied and the optimal conditions of die configuration are suggested.

• 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.