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Bong H.J.,Graduate Institute of Ferrous Technology | Leem D.,Graduate Institute of Ferrous Technology | Kim J.H.,Pusan National University | Lee M.G.,Korea University
Metals and Materials International | Year: 2016

The friction coefficient for aluminum alloy 3003 was determined from a specially designed tip test and finite element (FE) simulations. Measured radial tip distance after the tip test was compared to the FE simulations by iteratively changing friction coefficient and the best fitting friction coefficient was determined. To consider strain rate effect on flow stress response during large plastic deformation, a new combined Hollomon-Voce hardening law was proposed. The friction under three different surface conditions was considered by the proposed inverse FE analysis. The results showed that there was obvious strain rate effect on the predicted punch load in the tip test. Moreover, the different friction coefficients were numerically determined for punch/workpiece and die/workpiece interfaces. Two possible causes of this difference were discussed by the analysis on contact normal pressure and slip velocity distributions of the two interfaces. © 2016 The Korean Institute of Metals and Materials and Springer Science+Business Media Dordrecht

Bong H.J.,Graduate Institute of Ferrous Technology | Barlat F.,Graduate Institute of Ferrous Technology | Lee M.-G.,Korea University | Kim J.H.,Stainless Steel Products Research Group
Metals and Materials International | Year: 2016

Two-stage forming process for manufacturing micro-channels of bipolar plate as a component of a proton exchange membrane fuel cell was optimized. The sheet materials were ultra-thin ferritic stainless steel (FSS) sheets with thicknesses of 0.1 and 0.075 mm. For the successful micro-channel forming in the two-stage forming approach, three process variables during the first stage were selected: punch radius, die radius, and forming depth. In this study, the effect of the three process variables on the formability of ultra-thin FSSs was investigated by finite element (FE) simulations, experiments, and central composite design (CCD) method. The optimum forming process designed by the CCD showed good agreement with those by experiments and FE simulations. The newly adopted optimization tool, CCD, was found to be very useful for optimization of process parameters in the multi-step sheet metal forming processes. © 2016 The Korean Institute of Metals and Materials and Springer Science+Business Media Dordrecht

Ha D.J.,Pohang University of Science and Technology | Lee J.S.,POSCO | Kim N.J.,Pohang University of Science and Technology | Kim N.J.,Graduate Institute of Ferrous Technology | Lee S.,Pohang University of Science and Technology
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2012

In the present study, mechanisms of sticking that occurs during hot rolling of modified STS430J1L ferritic stainless steels were investigated by using a pilot-plant-scale rolling machine, and the effects of alloying elements on sticking were analyzed by the high-temperature oxidation behavior. The hot-rolling test results indicated that the Cr oxide layer formed in a heating furnace was broken off and infiltrated the steel, thereby forming Cr oxides on the rolled steel surface. Because the surface region without oxides underwent a reduction in hardness rather than the surface region with oxides, the thickness of the surface oxide layer favorably affected the resistance to sticking. The addition of Zr, Cu, and Ni to the ferritic stainless steels worked in favor of the decreased sticking, but the Si addition negatively affected the resistance to sticking. In the Si-rich steel, Si oxides were continuously formed along the interfacial area between the Cr oxide layer and the base steel, and interrupted the formation and growth of the Cr oxide layer. Because the Si addition played a role in increasing sticking, the reduction in Si content was desirable for preventing sticking. © 2011 The Minerals, Metals & Materials Society and ASM International.

Kim D.,Korea Institute of Materials Science | Kim H.,Seoul National University | Kim J.H.,Pusan National University | Lee M.-G.,Korea University | And 4 more authors.
International Journal of Plasticity | Year: 2015

In order to evaluate the forming limits of planar anisotropic multilayer sheet materials, the Marciniak-Kuczynski (M-K) model was formulated based on the strain-rate potential. A common practice to predict the forming limits of sheet materials has been based on yield stress potentials defined in the stress field. As an alternative, a method based on plastic stain-rate potentials, which are especially convenient to apply for rigid-viscoplasticity, was considered in this work. The formulation based on the strain-rate potential facilitates the modeling of forming limits for multilayer sheet materials because the number of unknown variables is significantly reduced by assuming the iso-strain condition for each layer without delamination. As for the strain-rate potential, Srp2003-2d, which is the pseudo-conjugate of the yield stress potential Yld2000-2d, was applied along with Hill's 1948 strain-rate potential for comparison. In the approach proposed, rigid-viscoplasticity was formulated according to the incremental deformation theory based on the minimum plastic work path. The rotation of anisotropic symmetry axes in the groove region was also properly accounted for in the formulation. For verification purposes, the predicted forming limit criteria such as the strain-based forming limit diagram (FLD), the stress-based forming limit diagram (FLSD) and the effective strain-based forming limit diagram (x-EPS) were experimentally validated for a monolithic aluminum alloy (AA5182-O) sheet and a three-layer AA5182-O/polypropylene/AA5182-O (AA/PP/AA) sandwich sheet, which confirmed good agreement. In addition, the path-sensitivity of the strain-based FLD and the path-insensitivity of the FLSD and x-EPS were numerically proven based on the M-K model for the three-layer sandwich sheet. Finally, the M-K model was compared with a maximum force model most recently developed. © 2015 Elsevier Ltd.

Jeong Y.,Korea Advanced Institute of Science and Technology | Gnaupel-Herold T.,Center for Neutron Research | Barlat F.,Graduate Institute of Ferrous Technology | Iadicola M.,U.S. National Institute of Standards and Technology | And 2 more authors.
International Journal of Plasticity | Year: 2014

Biaxial flow behavior of an interstitial free steel sample was investigated with two experimental methods: (1) Marciniak punch test with in situ X-ray diffraction for stress analysis; (2) hydraulic bulge test. The stress analysis based on X-ray diffraction using {2 1 1} lattice planes was accompanied by the use of stress factors and intergranular (IG) strains. Stress factors and IG strains were experimentally obtained ex situ on samples after prescribed equi-biaxial deformations. An elasto-viscoplastic self-consistent (EVPSC) crystal plasticity model was used to predict the stress factors and the IG strains. The model predictions of the stress factors were in good agreement with the experiments. However, the predictions of IG strains were in poor agreement with their experimental counterparts. As a result, the flow stress solely based on the computationally predicted stress factors and IG strains was unrealistic. The input of the experimental stress factors and IG strains for stress analysis improved the agreement with a reference flow curve obtained by a hydraulic bulge tester. The resulting flow curves based on X-ray diffraction were in good agreement with that of the bulge test up to an effective strain of 0.3. However, an unrealistic softening was observed in larger deformations regardless of whether the stress factor used were experimentally measured or determined from EVPSC calculations. © 2014 Elsevier Ltd.

Sohn I.,Yonsei University | Jung S.M.,Graduate Institute of Ferrous Technology
Steel Research International | Year: 2011

The hydrogen reduction behavior of iron oxide composite pellets containing Ni, Fe, and Mn from 973 K to 1173 K was compared with iron oxide and Al 2O3 containing reference composite pellets to determine the effect of metallic species on the kinetics of iron oxide reduction. The Mn and Ni containing pellets showed slightly faster initial reduction rates compared to the Fe and Al2O3 containing pellets. The effect of the metal phases was found to be more significant at lower temperatures when chemical reaction at the interface is a slower and more controlling factor. From the SEM of partially reduced pellets, a wide intermediate region between an O rich unreacted core and an Fe rich outer shell was observed. Although an initially short topochemical receding interface controlled region exists, the mixed control between the topochemical receding interface and pore diffusion was prevalent. For Fe2O3/Mn composite pellets, the thermodynamic stability of the MnO is higher and Mn can act as a reductant for iron oxide. Thus, the overall metallization of the Fe2O3/Mn composite pellets decreased compared to the other Fe2O3/metal composite pellets. From the temperature dependence of the iron-oxide/metal composite pellets, the apparent activation energy was calculated to be approximately between 15 to 20 kJ/mol, which is typical of a mixed control reduction mechanism of gas diffusion and interface reaction. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Jung S.-M.,Graduate Institute of Ferrous Technology | Yi S.H.,POSCO
Steel Research International | Year: 2013

The reduction of hematite-graphite pellet was investigated from a kinetic viewpoint in the temperature range of 1173-1473 K (900-1200°C). The experimental procedure included thermogravimetric analysis (TGA) for measuring the weight change of the pellet and quadruple mass spectrometry (QMS) for monitoring the compositional change of the product gases. By applying a uniform internal reduction model to the current system, the activation energies for the reduction of Fe3O4 to wustite and for that of wustite to Fe were evaluated to be 91.0 and 25.9 kJ mol-1, respectively. Due to the product gas analyses by QMS, it was observed that the reduction rate of hematite-graphite pellet was accelerated up to the equilibrium concentration of CO determined by the carbon gasification curve. Changeover of reduction mechanism with increasing reduction degree was explained in terms of a schematic diagram based on the results obtained in the current study. Recently, great attention has been paid to the technology of direct reduced iron (DRI) production using low grade of ores and noncoking coals for alternative iron source. The carbothermic reduction of hematite in an Ar atmosphere has been investigated by employing thermogravimetry and quadruple mass spectrometry (QMS) gas analysis with the intention of understanding in detail the reduction phenomena of carbon composite iron ore agglomerates in the rotary kilns. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Jung G.,Graduate Institute of Ferrous Technology | Woo I.S.,POSCO | Suh D.W.,Graduate Institute of Ferrous Technology | Kim S.-J.,Graduate Institute of Ferrous Technology
Metals and Materials International | Year: 2016

In the present study, liquid metal embrittlement (LME) phenomenon during high temperature deformation was investigated for 3 grades of Zn-coated high strength automotive steel sheets consisting of different phases. Hot tensile tests were conducted for each alloy to compare their LME sensitivities at temperature ranges between 600 and 900 °C with different strain rates. The results suggest that Zn embrittles all the Fe-alloy system regardless of constituent phases of the steel. As hot tensile temperature and strain rate increase, LME sensitivity increases in every alloy. Furthermore, it is observed that the critical strain, which is experimentally thought to be 0.4% of strain at temperatures over 700 °C, is needed for LME to occur. It is observed via TEM work that Zn diffuses along grain boundaries of the substrate alloy when the specimen is strained at high temperatures. When the specimen is exposed to the strain more than 0.4% at over 700 °C, the segregation level of Zn at grain boundaries seems to become critical, leading to occurrence of LME cracks. © 2016 The Korean Institute of Metals and Materials and Springer Science+Business Media Dordrecht

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