• Korean Journal of Materials Research
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• v.1 no.3
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• pp.115-124
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• 1991

In this study, the relationship between mechanical properties and the effects of second phase in tri-phase steel which was composed of ferrite-martensite-bainite was investigated. In order to obtain different microstructure of ferrite+martensite(DP), ferrite+bainite(F+B), and ferrite+martensite+bainite(TP, different heat treatment has been accomplished. The effects of volume fraction and microstructure of each specimen were studied on tensile property, Charpy impact energy and stretch-flangeability. As the bainite content in triphase steels increased, the tensile strength, and yield strength decreased as well as the reduction of area and strength-uniform elongation increased. However, ferrite-bainite steel had high yield ratio and yield point elongation. The Charpy impact energy of TP and F+B steel was higher than that of DP steel. In addition, the characteristics of hole expanding limit($\lambda$) of TP steel and F+B steel were higher than that of DP steel. These mechanical properties of tri-phase steel have been improved, because bainite could be deformed easily within ferrite matrix. The effect of bainite on ductility in tri-phase steel has been found to be favorable. In this experiment, tri-phase steel contained within 27% bainite volume fraction had good nechanical properties and superior stretch-flangeability.

• Journal of Welding and Joining
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• v.31 no.1
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• pp.58-64
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• 2013

The following results were obtained, microstructures and tensile properties in arc brazed joints of DP(dual phase) steel using Cu-5.3wt%Sn insert metal was investigated as function of brazing current. 1) The Fusion Zone was composed of ${\alpha}Fe+{\gamma}Cu$ and Cu23Sn2. The reason for the formation of these solid solutions. Despite, Fe & Cu were impossible to solid solution at room temperature. It's melting & reaction to something of insert metal & Base Metal (DP Steel) by Arc. Brazing Process has faster cooling rate then Cast Process, Supersaturated solid solution at room temperature. 2) The increase Hardness of Fusion Zone was directly proportional to the rise of welding current. Because, ${\alpha}Fe+{\gamma}Cu$ phase (higher hardness than the Cu23Sn2.(104.1Hv < 271.9Hv)) Volume fraction was Growth, due to increasing the amount of base metal melting by High current. 3) The results of tensile shear test by Brazing, All specimens happen to fracture in Fusion Zone. On the other hand, when Brazing Current increasing tend to rise tensile load. but it was very small, about 26-30% of the base metal. 4) The result of fracture analysis, The crack initiate at Triple Point for meet to Upper B.M/Under B.M/Fusion Zone. This Crack propagated to Fusion zone. So ruptured by tensile strength. The Reason to in the fusion zone fracture, Fusion zone by Brazing of hardness (strength) was very lower then the base metal (DP steel). In addition the Fusion Zone's thickness in triple point was thin than the base metal's thickness in triple point.

• Journal of the Korean institute of surface engineering
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• v.45 no.6
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• pp.284-288
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• 2012

A study on microstructure control of multi-phase steel have been implemented to higher strength with improved formability. However, it is well known that the high strength of steel are susceptible to hydrogen embrittlement. The mechanisms of hydrogen embrittlement is caused by complex interactions. In this paper, the test specimens were fabricated to 5 type of 590DP steels at different levels of volume faction. The hydrogen charging was conducted by electrochemical hydrogen-charge method with varying charging time. The relationship between hydrogen concentration and volume fraction of 590DP steel was established by SP test and SEM-fractography. It was shown that the hydrogen amounts charged in 590DP steels increased with increasing the volume faction of austenite. The maximum loads of the 590DP steels in SP test were sharply decreased with increasing hydrogen charging time. The results of SEM-fractography investigation showed typical brittle-fracture surfaces for hydrogen-charged 590DP steels.

• Korean Journal of Metals and Materials
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• v.47 no.5
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• pp.274-282
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• 2009

To investigate the evolution of deformation texture in dual phase (DP) steels during deep-drawing deformation, deep-drawing experiments were performed. Microtexture measurements were conducted using electron backscattered diffraction (EBSD) to analyze texture evolution. A rate-sensitive polycrystal model was used to predict texture evolution during deep-drawing deformation. In order to evaluate the strain path during deep-drawing deformation, a steady state was assumed in the flange part of a deep-drawn cup. A ratesensitive polycrystal model successfully predicted the texture evolution in DP steels during deep-drawing deformation. The final stable orientations were found to be strongly dependent on the initial location in the blank. Texture analysis revealed that the deep drawability of DP steels decreases as the true strain in the radial direction of the deep-drawn cup increases during deep-drawing deformation.

• Transactions of Materials Processing
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• v.19 no.6
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• pp.337-343
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• 2010

Current need of weight reduction in automotive part increases the application for high strength steel (HSS). The various types of high strength steels have been used to produce chassis part, control arms and trailing arms for weight reduction and increasing of fatigue durability such as dual phase steel (DP) and ferrite bainite steel (FB). But, DP and FB steels have proven to show inferiority in durability as well as press formability. Edge cracking occurred often in flange forming and hole expansion processes is the major failure encountered. This paper discussed the behavior of edge stretchability of high strength steel of DP and FB steels. Experimental works have been conducted to study the effect of punch clearance and burr direction on hole expansion ratio (HER). Also finite element simulation (FEM) has been preformed to clarify the mechanism of flange crack and support the experimental results on HER of DP and FB steels. It was simulated the whole process of blanking process following by hole expansion process and ductile fracture criterion named the modified Cockcroft-Latham model which was used to capture the fracture initiation. From the hole expansion tests and FEM simulation studies it was concluded that ferrite bainite steel showed better stretch-flangeability than dual phase steel. It was attributed to the lower work hardening rate of ferrite bainite steel than dual phase steel at the sheared edge.

• Journal of the Korean Society for Precision Engineering
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• v.27 no.11
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• pp.46-56
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• 2010

The present research works investigated into the low velocity impact characteristics of DP 780 high strength steel sheet with 1.7 mm in thickness subjected to free boundary condition using three-dimensional finite element analysis. Finite element analysis was carried out via ABAQUS explicit code. Hyper-elastic model and the damping factor were introduced to improve an accuracy of the FE analysis. An appropriate FE model was obtained via the comparison of the results of the FE analyses and those of the impact tests. The influence of the impact energy and nose diameter of the impact head on the force-deflection curves, impact time, absorption characteristics of the impact energy, deformation behaviours, and stress-strain distributions was quantitatively examined using the results of FE analysis. The results of the FE analysis showed that the absorption rate of impact energy lies in the range of the 70.7-77.5 %. In addition, it was noted that the absorption rate of impact energy decreases when the impact energy increases and the nose diameter of the impact head decreases. The local deformation of the impacted region was rapidly increased when the impact energy was larger than 76.2 J and the nose diameter was 20 mm. A critical impact energy, which occur the instability of the DP780, was estimated using the relationship between the plastic strain and the impact energy. Finally, characteristics of the plastic energy dissipation and the strain energy density were discussed.

• Transactions of Materials Processing
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• v.33 no.1
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• pp.43-49
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• 2024

A high strength steel sheet (HSSS) has widely used to improve the specific rigidity of parts and the safety of the passenger in automotive industries. However, the HSSS is difficult to manufacture precise parts through a forming process due to the reduced elongation and the increased elastic recovery. The goal of the paper is to investigate the improved design of bending dies for DP590 HSSS. The over forming type bending dies with cam systems added to the side of the formed part is proposed to improve the quality of the part. The effects of the die design and the forming parameter on formability and elastic recovery characteristics is examined using finite element analyses (FEAs). From the results of FEAs, proper die design and forming parameters are predicted.

• Journal of the Korean Society of Industry Convergence
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• v.20 no.1
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• pp.81-87
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• 2017

In order to reduce the weight of the blow motor case and to maintain the strength of the motor joint, the mechanical joining strength is to be predicted. The true stress - true strain curves for finite element analysis were obtained through tensile tests of HGI and DP 780 steel. The mechanical joining strength was predicted through an explicit finite element analysis and the accuracy of the predicted results was verified by actual sample test. The regression equation for predicting the mechanical joining strength to the thickness of the DP 780 steel was derived. The minimum thickness of DP 780(1.2mm), which is equivalent to the joining strength of HGI(2.6mm), was derived from the equation.

• Transactions of Materials Processing
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• v.15 no.8 s.89
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• pp.568-573
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• 2006

In order to numerically evaluate automotive hydro-formed DP-steel tubes on crash performance considering welding heat effects, the finite element simulations of crash behavior were performed for hydro-formed tubes with and without heat treatment effects. This work involves the mechanical characterization of the base material and the HAG-welded zone as well as finite element simulations of the crash test of hydro-formed tubes with welded brackets and hydro-forming of tubes. The welding heat effects on the crash performance are evaluated in efforts to improve the process optimization procedure of the engine cradle in the design stage. In particular, FEM simulations on indentations have been performed and experimentally verified for material properties of weld zone and heat affected zone.

• Transactions of Materials Processing
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• v.18 no.4
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• pp.329-335
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• 2009

In order to better predict the springback for friction stir welded DP590 steel sheet, the combined isotropic-kinematic hardening was formulated with considering the permanent softening behavior during reverse loading. As for yield function, the non-quadratic anisotropic yield function, Yld2000-2d, was used under plane stress condition. For the verification purposes, comparisons of simulation and experiments were performed here for the unconstrained cylindrical bending, the 2-D draw bending tests. For two applications, simulations showed good agreements with experiments.