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자동차용강의 고속변형거동에 대한 예비변형과 소부경화의 영향
최일동,김동민,Bruce, D. M.,Matlock, D. K.,Speer, J. G.,박성호 대한금속재료학회 2005 대한금속·재료학회지 Vol.43 No.4
The high speed deformation behavior of TRIP steels and HSLA steels has been investigated at strain rates ranging from 10^(-2) to 10³/sec. The effects of metallurgical factors such as prestrain (2%) and bake hardening condition (170℃, 20 min) on tensile properties at various strain rates are being evaluated. Ultimate tensile strength (UTS), total elongation, strain rate sensitivity, and absorbed energy are reported. In general, UTS increases with increasing strain rate, that is, it shows positive strain rate sensitivity. And strain rate sensitivity increases sharply over 100/sec. Strength increases with increasing the amount of prestrain. The bake hardening treatment slightly influences UTS values. The absorbed energy decreases with increasing prestrain but it is not affected by bake hardening treatment. Received February 14, 2005)
Predicting Shear Failure of Dual-Phase Steels
Ji Hoon Kim,Ji Hyun Sung,D. K. Matlock,Daeyong Kim,R. H. Wagoner 한국소성가공학회 2010 기타자료 Vol.2010 No.6
Dual-phase (DP) steels are being used increasingly to make automotive panels because of their advantageous combinations of high ductility (for forming) and high strength (for service). However, their adoption has been limited because of failures during die tryout that are unpredicted by the usual methods of finite element modeling and forming limit diagrams. The failures, often called “shear failures” occur at regions of high curvature (low R/t) where sheet of thickness t is drawn over a tool radius R. Recent work revealed that the type of failure and the formability of DP steels depend not only on R/t, but also on strain rate, an effect derived from the propensity of these steels to locally heat in areas of high strain when strain rates are sufficiently high to limit heat transfer. The formability is reduced significantly by the thermal effect for rates greater than approximately 0.1/s. This result explains at least partially why forming limit diagrams, which are measured quasi-statically (and thus isothermally) do not reflect the behavior of DP steels formed industrially (at typical strain rates of approximately 10/s). In order to apply laboratory test results of draw-bend formability to industrial forming operations, the inputs to commercial finite element codes (constitutive equations, forming limits) must be adapted to the reality of the material (DP steel) and underlying physics (thermal effects on constitutive behavior). Toward this end, two procedures have been developed and tested, one numerical and one analytical. Together they predict similar forming limits and provide a path for understanding the applied formability of DP steels.
Finite Element Investigation of Hole-Expansion Formability of Dual-Phase Steels Using RVE Approach
M.G. Lee,J.H. Kim,D. K. Matlock,R. H. Wagoner 한국소성가공학회 2010 기타자료 Vol.2010 No.6
Qualitative analysis on the formability of dual-phase (DP) steels was carried out by employing the realistic microstructure based finite element (FE) method. The microstructure-based model was generated using image processing of scanning electron micrographs and mesh generation with a boundary smoothing algorithm. The model was applied to hole-expanding formability tests for two DP steels with distinct volume fractions and morphological features: Steel A with continuous ferritic phase and Steel B with continuous martensitic phase. The micromechanical simulations showed that the high volume fraction of martensite and its morphological characteristics of Steel B prohibited the plastic deformation and damage evolution in ferritic phase, thus delayed the material’s failure, while considerable damage propagation occurred along the phase boundaries in Steel A. The observed difference in hole-expansion ratio for the two microstructures was consistent with results from the FE model.
Kim, Ji Hoon,Lee, M.G.,Kim, D.,Matlock, D.K.,Wagoner, R.H. Elsevier 2010 Materials science & engineering. properties, micro Vol.527 No.27
<P><B>Research highlights</B></P><P>&z.rtrif; Robust microstructure-based FE mesh generation technique was developed. &z.rtrif; Local deformation behavior near phase boundaries could be quantitatively understood. &z.rtrif; Macroscopic failure could be connected to microscopic deformation behavior of multi-phase steel.</P> <P><B>Abstract</B></P><P>A qualitative analysis was carried out on the formability of dual-phase (DP) steels by introducing a realistic microstructure-based finite element approach. The present microstructure-based model was constructed using a mesh generation process with a boundary-smoothing algorithm after proper image processing. The developed model was applied to hole-expansion formability tests for DP steel sheets having different volume fractions and morphological features. On the basis of the microstructural inhomogeneity observed in the scanning electron micrographs of the DP steel sheets, it was inferred that the localized plastic deformation in the ferritic phase might be closely related to the macroscopic formability of DP steel. The experimentally observed difference between the hole-expansion formability of two different microstructures was reasonably explained by using the present finite element model.</P>