• Nuclear Engineering and Technology
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• 제46권6호
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• pp.857-862
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• 2014

Mechanical alloying under various gas atmospheres such as Ar, an Ar-$H_2$ mixture, and He gases were carried out, and its effects on the powder properties, microstructure and mechanical properties of ODS ferritic steels were investigated. Hot isostatic pressing and hot rolling processes were employed to consolidate the ODS steel plates. While the mechanical alloyed powder in He had a high oxygen concentration, a milling in Ar showed fine particle diameters with comparably low oxygen concentration. The microstructural observation revealed that low oxygen concentration contributed to the formation of fine grains and homogeneous oxide particle distribution by the Y-Ti-O complex oxides. A milling in Ar was sufficient to lower the oxygen concentration, and this led a high tensile strength and fracture elongation at a high temperature. It is concluded that the mechanical alloying atmosphere affects oxygen concentration as well as powder particle properties. This leads to a homogeneous grain and oxide particle distribution with excellent creep strength at high temperature.

• 한국재료학회지
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• 제25권1호
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• pp.9-15
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• 2015

The effect of retained and reversed austenite on the damping capacity in high manganese stainless steel with two phases of martensite and austenite was studied. The two phase structure of martensite and retained austenite was obtained by deformation for various degrees of deformation, and a two phase structure of martensite and reverse austenite was obtained by reverse annealing treatment for various temperatures after 70 % cold rolling. With the increase in the degree of deformation, the retained austenite and damping capacity rapidly decreased, with an increase in the reverse annealing temperature, the reversed austenite and damping capacity rapidly increased. With the volume fraction of the retained and reverse austenite, the damping capacity increased rapidly. At same volume of retained and reversed austenite, the damping capacity of the reversed austenite was higher than the retained austenite. Thus, the damping capacity was affected greatly by the reversed austenite.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2006년도 하계학술대회 논문집 Vol.7
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• pp.87-88
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• 2006

It is needed several large reservoirs and long time in order to remove suspended solid like steel fines and iron oxide at steel making factory. If removing rate of suspended solid in rolling coolant is improved, the productivity of working process can be increased and the area of reservoir can be reduced. Pre-treatment to add extra magnetization of suspended solid was studied Iron hydroxide and electrolytic dissociation were used for pretreatment. High gradient magnetic separation (HGMS) system was used for removing of magnetized suspended solid. Removing ratio showed over than 99% in the coolant containing magnetic fines. Magnetic properties of suspended solid were investigated after mixed with Al2(SO2)3 and organic flocculant by using electrochemical treatment.

• 한국정밀공학회지
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• 제12권5호
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• pp.69-76
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• 1995

This paper describes characteristics of EDT(Electro Discharge Texturing) and that the strip surface has influence on the paintability. The pulse mode and the capacitor mode of recently developed EDT method were investigated about the peak shape and the roughness of strip surface. And EDT method compared the above-mentioned contents with the SBT(Shot Blast Texturing) method. The peak count of the pulse mode is more than that of the capacitor mode. But the lower roughness below the Ra 3.2 .mu. m of the strip surface, the texturing time of the pulse mode is longer than the capacitor mode. In SBT method, the peak shape is similar to capacitor mode. But the EDT roll has more uniformity than the SBT roll and has more peak count than that of SBT roll. Therefore, in the painted steel sheet, EDT roll gives better surface of clarity image than the SBT roll.

• 한국산학기술학회논문지
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• 제17권3호
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• pp.457-463
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• 2016

신규 개발된 초연성 듀플렉스 스테인레스강은 강도와 연성이 우수하기 때문에 철도차량 차체에 적용하여 경량화와 가공 효율화에 기여할 수 있다. 본 연구에서는 듀플렉스 스테인레스강의 장점을 인터모달 트램 차체에 활용하기 위해 점용접성 시험을 수행하였다. 다양한 두께 (0.4mm에서 5.0mm까지)의 시편을 가공하고 점용접 시험을 통해 적정 용접조건을 찾아내었다. 인장하중 시험을 통해 점용접 이음의 인장-전단 강도를 평가하였고, 품질을 확인하기 위해 너깃 지름을 측정하였다. 인장-전단 하중 시험 결과의 분석을 통해, 이음부의 인장-전단 강도는 입열 파라미터에 비례함을 알 수 있었다. 인장-전단 강도와 너깃 지름는 모두 규격에서 요구하는 기준을 만족함도 입증되었다. 전체적인 용접성 시험 결과, 듀플렉스 스테인레스강 소재는 인터모달 트램 차체에 효과적으로 활용될 수 있음을 확인하였다.

• 한국주조공학회지
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• 제30권2호
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• pp.60-65
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• 2010

In this study, we investigate the effects of austempering heat treatment on the processing window and mechanical properties in cast and hot-rolled Fe-0.7 C-2.3 Si-0.3 Mn steel. Each specimens were austenitised at $900^{\circ}C$ for 7 min, and austempered at $260^{\circ}C,\;320^{\circ}C$, and $380^{\circ}C$ for the various periods of time from 2 min to 240 min. After heat treatment, the evaluation of stage I and stage II as performed by optical metallography, XRD, hardness test. Both cast and hot rolled specimens had similar processing window. So grain size effect is not important to the austempered high carbon high silicon cast steel. When the austempering temperature was $260^{\circ}C$, the microstructure consisted of the lower ausferrite while the upper ausferrite structure was formed at $380^{\circ}C$. As the austempering temperature increases from 260 to $380^{\circ}C$, the strength and hardness decreased, elongaton and volume fraction of austenite increased. In addition, there was no change of mechanical properties between cast and hot-rolled specimens.

• 비파괴검사학회지
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• 제17권4호
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• pp.270-277
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• 1997

소성이방성은 판재의 가공성을 결정하는 중요한 인자로써 주로 기계적인 인장시험에 의해 측정되고 평가되어 왔다. 압연 방향에 대하여 상대적인 방향으로 진행하는 초음파의 속도로부터 집합조직의 결정분포함수를 구할 수 있으며, 결정분포함수의 계수인 결정방위계수는 소성이방성의 평균값들과 상관 관계를 갖는다. 본 실험에서는 초음파 속도로 구한 결정방위계수와 압연강판의 소성이방성을 비교하여 서로의 상관 관계를 구하였으며, 측정결과 average normal anisotropy, $\bar{\gamma}$는 ${\pm}0.082$, average planar anisotropy, ${\Delta}r$는 ${\pm}0.096$의 편차 내에서 예측 가능하였다. 초음파 속도 측정에는 공진주파수측정법이 적용되었고, 초음파의 송수신에는 EMAT가 이용되었다.

• 한국재료학회지
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• 제5권7호
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• pp.858-868
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• 1995

다른 탄소함량을 갖는 두 Hadfield 강에서 압연에 따른 결정방위조직(Texture)과 마르텐사이트 상변태를 연구하였다(0.65wt.%, 1.35wt.%) 두 Hadfield 강의 적층결함에너지 차이가 매우 적음에도 불구하고 (약 2mJm$^{-2}$ ) 결정방위조직은 차이를 보였다. 0.65wt.% 탄소 강의 경우, 낮은 변형구간에서는 낮은 적층에너지 금속과 비슷한 결정방위조직을 보였으나 높은 변형에서는 {111}, {110}<001>과 같은 이상 결정방위조직이 나타났고, 이것은 입계 또는 일단의 전단띠에(shear bands) 형성된 $\alpha$ 마르텐사이트의 방해에 의한 것으로 생각된다. 이와는 대조적으로 1.35wt.% Hadfield강은 전변형구간에서 낮은 적층결함에너지를 갖는 금속과 비슷한 결정방위조직을 보이고 있다. 이것은 변형중에 유도된 마르텐사이트의 양이 적게 형성되는 사실에 기인하며, 변형에 따른 $\alpha$ 마르텐사이트 변태는 교류자화율과 금속입자 시험으로 알 수 있다.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2005년도 춘계학술대회 논문집
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• pp.106-109
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• 2005

Springback is a common phenomenon in sheet metal forming, caused by the elastic recovery of the internal stresses after removal of the tooling. Recently, advanced high strength steels (AHSS) such as TRIP and DP are finding acceptance in the automotive industry because their superior strength to weight ratio can lead to improved fuel efficiency and assessed crashworthiness of vehicles. The major troubles of the automotive structural members stamped with high strength steel sheets are the tendency of the large amount of springback due to the high yield strength and the tensile strength. The amount of springback is mainly influenced by the type of the yield function and anisotropic model induced by rolling. The discrepancy of the deep drawn product comparing the data of from the product design induced by springback must be compensated at the tool design stage in order to guarantee its function and assembly with other parts. The methodology of compensation of the low shape accuracy induced by large amount of springback is developed by the expert engineer in the industry. Recently, the numerical analysis is introduced in order to predict the amount of springback and to improve the shape accuracy prior to tryout stage of press working. In this paper, the tendency of springback is evaluated with respect to the blank material. The stamping process is analyzed fur the front side member formed with AHSS sheets such as TRIP60 and DP60. The analysis procedure fully covers the binderwrap, stamping, trimming and springback process with the commercial elasto-plastic finite element code LS-DYNA3D.

• 한국분말재료학회지
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• 제28권2호
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• pp.110-119
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• 2021

In this study, the high-temperature oxidation properties of austenitic 316L stainless steel manufactured by laser powder bed fusion (LPBF) is investigated and compared with conventional 316L manufactured by hot rolling (HR). The initial microstructure of LPBF-SS316L exhibits a molten pool ~100 ㎛ in size and grains grown along the building direction. Isotropic grains (~35 ㎛) are detected in the HR-SS316L. In high-temperature oxidation tests performed at 700℃ and 900℃, LPBF-SS316L demonstrates slightly superior high-temperature oxidation resistance compared to HR-SS316L. After the initial oxidation at 700℃, shown as an increase in weight, almost no further oxidation is observed for both materials. At 900℃, the oxidation weight displays a parabolic trend and both materials exhibit similar behavior. However, at 1100℃, LPBF-SS316L oxidizes in a parabolic manner, but HR-SS316L shows a breakaway oxidation behavior. The oxide layers of LPBF-SS316L and HR-SS316L are mainly composed of Cr₂O₃, Fe-based oxides, and spinel phases. In LPBF-SS316L, a uniform Cr depletion region is observed, whereas a Cr depletion region appears at the grain boundary in HR-SS316L. It is evident from the results that the microstructure and the high-temperature oxidation characteristics and behavior are related.