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Experimental and analytical studies on the stretch‐bending characteristics of advanced high‐strength steel sheets, primarily used in the automotive industry, are conducted. The stretch‐bending test, in which a specimen fixed at both ends is bent at its center with sharp punches until failure occurs, is conducted in this work. Assuming that the failure of the material occurred at the maximum tensile force, a relationship between applied force and forming height is derived. Nine steels, including high‐strength low‐alloy, dual‐phase, and transformation‐induced plasticity steels are tested, and the calculated results are compared with experimental limit‐forming heights. Finally, a failure criterion based on process variables and tensile properties of the material is proposed based on the deformation in the sheet thickness direction. This article is protected by copyright. All rights reserved.

A hardening model that considers the dislocation reversal‐related mechanisms is applied to predict the mechanical behavior of low carbon (LC) and twinning‐induced plasticity (TWIP) steels under forward–reverse shear test with various pre‐strain levels. The predicted results are presented in terms of stress–strain response and texture evolution. It is shown that the proposed model, embedded in the visco‐plastic self‐consistent (VPSC) framework, well captures the changes in the reloading yield stress and the hardening evolution due to the strain path reversal. The contributions of the different mechanisms are also discussed.

In the present study, viscoplastic hardening behavior of aluminum alloy 3003 sheet was measured and existing hardening models were evaluated in terms of the accuracy for predicting strain rate and temperature-dependent flow behaviors. Moreover, a modified hardening model was proposed to capture the flow stress–strain responses under various strain rates and temperatures with enhanced accuracy. Mechanical responses of the material were measured by using uniaxial tensile test at various temperature (≈25–250 °C) and strain rates (≈0.001–0.3 s−1), and the split Hopkinson pressure bar (SHPB) test was also conducted to obtain flow behavior at the strain rate over 700 s−1 at room temperature. Based on these experimental data, two well accepted viscoplastic constitutive models—Johnson–Cook and Khan–Huang–Liang—were evaluated. Finally, the Hollomon/Voce type model was further developed, which resulted in significant improvement in predicting the flow stress curves under wide range of strain rate and temperature.

The springback behavior on two sides of TWIP (twinning-induced plasticity) and mild steels tailor welded blank (TWB) is considerably different due to the large difference in strength. To reduce springback defects in a U-draw bending process of such TWB, different non-constant blank holding force (BHF) profiles on two sides of the blank are applied in this study. A systematic approach to obtain optimal BHF-stroke profiles, which helps to reduce the springback is proposed. Control of the variable BHF aims at increasing the uniformity in through-thickness of the stress component along the stretching direction, while decreasing the risk of failure due to over-stretching. Therefore, the optimal condition would require satisfying two conflicting objectives simultaneously: (i) minimize springback deformation and (ii) minimize the forming severity, leading to a Pareto-optimal problem. The optimization procedure consists of sampling design, finite element (FE) simulations, metamodeling, and finally the calculation of a Pareto-frontier. Generated outputs from FE simulations on statistically significant sampling points are used for the construction of metamodels of optimum accuracy and complexity, which, in turn, are used to evaluate the output for any set of inputs, replacing the computing intensive FE simulations. A novel genetic algorithms based multi-objective optimization technique is subsequently applied for optimization.

In this study, lower and upper bainite microstructures were produced by austempering process of a NiCrMo low alloy steel containing 0.4% C and their mechanical properties were studied by tensile testing at room temperature. Fracture mechanisms involved were investigated on special samples cut from the necked region of the tensile specimens. The specimens were metallographically prepared and examined by scanning electron microscopy. Analysis of both microstructures revealed that at lower strains, voids were nucleated by cracking or interface decohesion of Al2O3 inclusions. However, at higher strains, especially near the fracture strain, voids were mainly initiated by decohesion of cementite/ferrite matrix interfaces. Lower bainite showed an elongation of 14% in tensile testing compared with 17% for the upper bainite, but its strength was about 20% higher. In order to present the void growth kinetics behavior of these steels, applicability of the three known models; Rice–Tracey, Chae, and Huang, was examined and it was found that in both bainitic steels, void growth kinetics closely obeyed the Rice–Tracey model. It was concluded that the main fracture mechanism in both steels was creation of micro-cracks by coalescence of voids, initiated by cementite/matrix interface decohesion, and growth of these cracks by a shear mechanism.

In this work, a direct-drive digital servo-press functioning with a non-conventional punch displacement profile was used to enhance the drawability of DP780 and TR780 steel sheet samples. Cup drawing tests were conducted under multiple detachments between the tools and the partially deformed part. The experiments were conducted with a special die set-up that provides a decreasing blank holding force scheme during the test. The results were compared with those obtained with the same die set-up but with a more traditional V-type punch motion, which continuously keeps the contact between tool and work-piece. The percentage of successfully drawn cups and the depth of the fractured cups were considered to evaluate the performance of this approach. In addition, two sets of friction tests were conducted with a flat die on a specially designed testing machine. The friction coefficient was measured as a function of the normal contact pressure, which varies spatially and temporally during drawing. FE simulations were conducted in order to explain the differences in experimental results obtained using conventional and unconventional punch motions. In particular, the roles of a variable friction coefficient as well as that of springback after detachment of the tool from the blank were investigated. It was demonstrated that the combination of these two effects successfully explain the higher drawing performance achieved using the attach–detach mode.

Formability of a tailor welded blank (TWB) is affected by the strength ratio of the base metals joined. In this paper, formability of TWB with very high strength ratio made by joining twinning-induced plasticity (TWIP) and low carbon steels is numerically studied using a limiting dome height test. The drawbead geometry at the weaker side is modified to increase the dome height. The design of drawbead is optimized by treating it as a multi-objective problem with maximum dome height and minimum weldline movement as objectives, which were constructed as metamodels through a genetic algorithms based approach. The necessary data for the metamodeling are generated by finite element (FE) simulation using the commercial solver, LS-DYNA®. The multi-objective optimization is carried out using a predator–prey genetic algorithm. The Pareto front estimated using this evolutionary approach is validated using FE simulations and a good correlation is obtained.

To study the anisotropic hardening behaviors of dual-phase steels under strain path changes, a meso-scale finite element analysis was considered with the representative volume elements. For the constitutive model, the Homogeneous yield function based on Anisotropic Hardening (HAH model) was used as a phenomenological constitutive equations to describe complex anisotropic material responses in an efficient way. For the martensite inclusions, three different configurations – elongated, large, and small spherical shapes – with two different volume fractions – 10 and 30% – were assumed. In order to represent strain path changes, two loading conditions were considered: tension–compression and tension–orthogonal tension. The simulation results for tension–compression test showed that the Bauschinger ratio increases as the volume fraction of martensite inclusion increases. For the tension–orthogonal tension test, the hard martensite attenuated the transient flow stress characteristics, which were observed in the single ferritic phase. The effect of hard phase inclusion was analytically explained using a simple one-dimensional analysis based on the elastic-linear plastic theory.

With the increase of using aluminum alloys and high strength steels to replace traditional steels, precise prediction of sheet springback and corresponding compensation methods become more and more important for die design because these material' s higher ratio of yield strength to elastic modulus causes more serious springback problems. In this paper, a semi-analytic springback prediction and compensation model based on in-process measurement is proposed. For springback prediction, measured strain and curvature data from the actual stamping process are incorporated in a semi-analytic model, and then springback is predicted based on the elastic unloading from the residual differential stress during sheet metal forming. In the next stage of die design, a procedure to automatically compensate the tool geometry, including the fundamental mechanics analysis and the reconstruction of the tool surface, is presented. For validation purpose, a case study was carried out for the tool optimization of a double curved panel.

Part shape error due to springback is a manufacturing defect in the sheet metal forming. This problem can be corrected by adjusting the tool shape to an appropriate shape or process optimization. In this paper, a discrete curvature adjustment (DCA) strategy is developed for tool design of channel bending products. This strategy aims at generating the right tool shape in a short time using the measured data from the trial part. A dynamic compensation factor which varies with part geometry and specific process condition will also be used to adjust the tool curvatures. Applications of this method in the tool design of an asymmetrical U-shaped part, an industrial blade, and a cylindrical part were presented. The experiment results demonstrated that this strategy was able to reduce the trial times of die modification from five or six to one or two, thus saving time and cost in the industry production. Additionally, the comparison of this method with the existing displacement adjustment (DA) method was also discussed.

The mechanical and metallurgical advantages of lubrication in hot strip rolling are well proved. However, owing to the difficulties in process control, most lubrication technologies are not commercialized, and many steel companies conducted a number of studies on the development of alternative technologies. In this study, based on the theory of rolling, a stress and strain analysis at the roll bite during hot strip rolling with lubrication is initially conducted. The ranges of suitable rolling parameters are determined. Then, a coupled thermo-mechanical 3D finite element (FE) model for hot strip rolling is developed and its reliability is verified. Based on the validated 3D FE model, a detailed analysis is performed to determine the exact lubrication mechanisms during hot strip rolling. Various rolling parameters expressed in terms of surface strain development are evaluated. Finally, a simple model predicting the variation of surface strain in a single rolling stand is employed to obtain the optimum process conditions that can be used as an alternative for lubrication.