Stretch Flange Forming Research Articles

Local and global forming potential of present dual-phase steel developments

Modern car bodies are constantly relying higher proportions of high and ultra-high strength steels. In the past, the increase in material strengths led to a conflict regarding the complexity of component geometries. Limits of material formability are reached and compromises had to be made in part design. By integrating application-oriented problems into the material development, these limits are expanded with respect to the given requirements. Within the present study, for several dual-phase steels with a tensile strength ≥800 MPa, practical laboratory tests are presented and their benefits in press shop application. In particular influencing factors on stretch flange formability are presented and discussed for example cutting parameters, anisotropy, time between cutting and forming or the orthogonal strain gradient.

Accuracy of stretch flange formability prediction by considering strain gradient for actual car body parts

The effect of the strain gradient on the stretch flange deformation limit of steel sheets was investigated by hole expansion tests under various forming conditions and by FEM forming simulations. The hole expansion ratio was changed by changing the initial hole diameter and punch shapes. This study clarified the fact that deformation limit strain increases, as it does not depend on the initial hole diameter or punch shape, and thus the strain gradient increases. The deformation limit thickness strain of the stretch flange portion could be changed by changing the strain gradient. These results suggest that the formability of the stretch flange deformation area should be determined from not only the maximum principal strain but also the strain gradient. Then, the accuracy of stretch flange formability prediction by considering the strain gradient for actual car body parts in mass production was investigated. The maximum principal strain and the strain gradient of the parts were calculated by FEM or measured in the press part. These results suggest that the formability of the stretch flange deformation area for actual car body parts could not be predicted by not only the maximum principal strain or the thickness reduction, but also the conventional FLD. This investigation verified the accuracy of stretch flange formability prediction by considering the strain gradient for actual car body parts.

Effects of Edge Heating and Strain Gradient on Stretch Flange Deformation Limit of Steel Sheet

To improve the stretch flange deformation limit of ultrahigh-strength steels, a partial edge heating method was investigated. The effect of the strain gradient on the stretch flange deformation limit when a partial edge heating method was applied was investigated by the hole expansion test and FEM analysis. The stretch flange formability was improved by application of a partial edge heating method because of the recovery of the shearing microstructure and the reduction of the hardness difference of microstructure. It was found that the improvement of the deformation limit strain for the strain gradient with edge heating depends on the strain gradient region. In the low strain gradient region, the improvement margin of the deformation limit strain with edge heating was small. On the other hand, in the high strain gradient region, the deformation limit strain was markedly improved compared with that without heating. It was confirmed that the strain gradient in the direction perpendicular to the maximum principal strain could be used for the prediction of the deformation limit and also the determination of the stretch flange fracture by the FEM analysis of actual parts even under edge heating conditions.

Effect of Cut-Off Side Stiffness on Stretch Flangeability in High-Strength Steel

In high-strength steel (HSS), shear edge failure due to stretch-flange formability is a serious problem. Shear edge failure is strongly affected by cutting methods. As countermeasures, cut-off punching and double punching of the entire circumference have conventionally been considered. However, it has not been shown whether these processes can be applied to the stretch-flange forming part of the outer circumference of automotive parts. In this study, the effect of the punching allowance and the method of improving the double punching process on the outer circumferences of automotive parts were investigated using high-strength hot-rolled steel sheets. The results of hole expansion tests and observations of shear edge faces and metal flow show that partial double punching reduces the rigidity of the cut-off side and increases the collapse probability. Also, the crack propagation caused by high tensile stress near the upper blade is accelerated. In comparison with double punching of the entire circumference of a 590 MPa or 780 MPa-class high-strength steel sheet, the effect of improving the stretch-flange formability is approximately 1.5 times the hole expansion test value, as indicated by the index, up to the area where the punching exceeds the size of the sheet thickness. However, it is considered that the stiffness of the cut-off side varies depending on the curvature and the thickness of the plane, and the effect of such variations will be confirmed in a wide range of the curvature and the thickness in the future.

Stretch flange forming of steel alloys through FEM simulation

In sheet metal operation flanging is a common process in which one edge of the sheet is bent to the desired angle or the desired contoured shape and the other end keep in control either itself or by blank holder or pad force. Stretch flanging involves stretching the metal circumferentially while the bending line is concave. In the stretch flanging process, the initial flange length is a critical factor. Using commercial finite element software programmed Abaqus, a finite element simulation of the stretch flanging process was performed in this research. The impact of initial flange length on the radial strain, circumferential strain, and flange thickness distribution along the free edge of rectangular-shaped sheet metal blanks is investigated in this work. Aside from maximum radial strain, maximum circumferential strain and maximum thinning in the flange are also explored as effects of initial flange length. With a reduction in initial flange length along the free edge up to the punched centre, circumferential strain and flange thickness drop while radial strain increases. With increasing initial flange length, maximum radial strain, maximum circumferential strain, and maximum thinning are observed to rise. The formability of the stretch flanging process is shown to be considerably influenced by the initial flange length.

Hot Strip Mill Processing Simulations on a Ti-Mo Microalloyed Steel Using Hot Torsion Testing

Precipitation strengthened, fully ferritic microstructures in low-carbon, microalloyed steels are used in applications requiring enhanced stretch-flange formability. This work assesses the influence of thermomechanical processing on the evolution of austenite and the associated final ferritic microstructures. Hot strip mill processing simulations were performed on a low-carbon, titanium-molybdenum microalloyed steel using hot torsion testing to investigate the effects of extensive differences in austenite strain accumulation on austenite morphology and microstructural development after isothermal transformation. The gradient of imposed shear strain with respect to radial position inherent to torsion testing was utilized to explore the influence of strain on microstructural development for a given simulation, and a tangential cross-section technique was employed to quantify the amount of shear strain that accumulated within the austenite during testing. Greater austenite shear strain accumulation resulted in greater refinement of both the prior austenite and polygonal ferrite grain sizes. Further, polygonal ferrite grain diameter distributions were narrowed, and the presence of hard, secondary phase constituents was minimized, with greater amounts of austenite strain accumulation. The results indicate that extensive austenite strain accumulation before decomposition is required to achieve desirable, ferritic microstructures.

Binder force effect on stretch flange forming of aluminum alloy

ABSTRACTThe present research investigates the effect of binder force in flanging of aluminum alloy sheets. FEM simulation of stretch flange forming is realized for zero, constant, and fixed binder forces. The finite element simulation is carried out to evaluate strain (circumferential and radial), flange thickness, position of commencement, and extension of cracks due to variation of binder force. The investigations reveal that the binder force has a significant effect on punch load, flange thickness (percentage thinning), circumferential and radial strain distribution, and commencement and extension of edge cracks. The results of FEM simulation are validated with results obtained by experimentation, and an excellent agreement is obtained.

Microstructural evolution during continuous annealing of a 980 MPa cold rolled steel grade

The development of advanced high strength steels (AHSS) is motivated by automotive industry demands for higher strength steels with better formability and specific stretch-flange formability to avoid fractures during parts forming. In response to this, the new steel grades must have special microstructures in terms of constitution, morphology, distribution and hardness. Thus, a better understanding of formation mechanisms and the characterization of these microstructures has shown to be of fundamental importance for the development of new steels. In this study the influence of heating rate and intercritical temperature on microstructural evolution and hole expansion ratio of a cold rolled 980 MPa AHSS grade was evaluated, using a Dilatometer and a Gleeble machine. The stretch-flange formability was evaluated by hole expansion tests and the microstructures were characterized using scanning electron microscopy, electron backscatter diffraction (EBSD) and nanohardness measurements. The increase of heating rate resulted in an overlap between ferrite recrystallization and austenite formation, changing the morphology of austenite from random to fibrous distribution. On the other hand, higher intercritical annealing temperature resulted in more homogeneous microstructures and higher hole expansion ratios. This microestructural homogeneity, evaluated by nanohardness, suggested that the difference in hardness between ferrite and second-phase affects micro-void formation and crack propagation during hole expansion test.

Stretch formability of high strength steel sheets in warm forming

Warm forming at temperatures from 200°C to 600°C has several advantages compared with cold forming at room temperature: better shape accuracy, better stretch flange formability and lower press load. In this study, the stretch formability of uncoated high strength steel sheets and galvannealed high strength steel sheets in warm forming was investigated by spherical stretch forming tests at temperatures from room temperature to 600°C. The experimental results showed that the maximum formable dome height without cracks in warm forming is lower than that in cold forming despite the lower material strength at elevated temperatures. In order to clarify the factors leading to poor stretch formability when using warm forming, the surface temperature and strain in the spherical stretch forming tests were examined. Sliding tests of strips of the uncoated steel sheets and galvannealed steel sheets were also carried out under the same heating conditions as the spherical stretch forming tests. The results suggested that two factors could cause the degradation of stretch forming performance during warm forming: differences in material strength between high- and low-temperature areas, especially at high initial forming temperatures of more than 400°C, and high coefficients of friction, particularly at a low initial forming temperature of 200°C. It is necessary to take both factors into account during warm forming.

プレス成形における伸びフランジ割れ予測技術の開発

プレス成形における伸びフランジ割れ予測技術の開発

Void Generation in Cold-rolled Dual-Phase Steel Sheet Having Excellent Stretch Flange Formability

The aim of this research is to investigate generation of micro voids affecting stretch flange formability in cold-rolled Dual-Phase (DP) steel sheets having a low volume fraction of martensite. The volume fraction of martensite was the dominant factor of the hole expanding ratio instead of the difference in hardness between ferrite and martensite when the difference in hardness between ferrite and martensite was large. On the other hand, the difference in hardness became the dominant factor of the hole expanding ratio in DP steel sheets having a low difference in hardness. Micro voids around the punched hole and the fracture edge of tensile deformation were observed in order to understand these different results. The void density is able to be associated with the hole expanding ratio under the same strain condition and the void density depends on the martensite spacing.

高強度鋼板の温間張出し性に関する検討

Warm forming at temperatures from 200 to 600℃ has several advantages compared with cold forming at room temperature: better shape accuracy, better stretch flange formability and lower press load. In this study, the stretch formability in warm forming of uncoated high-strength-steel sheets and galvannealed high-strength-steel sheets were investigated by spherical stretch forming tests from room temperature to 600℃. The experimental results showed that the maximum formable dome height without cracks in warm forming is lower than that in cold forming despite the lower material strength at elevated temperatures. In order to clarify the factors leading to poor stretch formability when using warm forming, surface temperatures and strains were examined in the spherical stretch forming tests. Also, sliding tests of strips of the uncoated steel sheets and galvannealed steel sheets were carried out under the same heating conditions as the spherical stretch forming tests. The results suggested that two factors could cause the degradation of stretch forming performance during warm forming: differences in material strength between high- and low-temperature areas, especially at high initial forming temperatures of more than 400°C, and high coefficients of friction, particularly at a low initial forming temperature of 200℃. It would be necessary to take both factors into account during warm forming.

Effect of shearing clearance and angle on stretch-flange formability evaluated by saddle-type forming test

In this study, the effects of shearing clearance and shearing angle on stretch-flange formability are investigated. The saddle-type forming test is adopted for this investigation. A notch-type shearing line, which comprises straight and circular arcs, is used for this test. The results show that stretch-flange formability is improved in narrow clearances. This tendency is different from that obtained by the hole expansion test, wherein stretch-flange formability is improved in large clearances. This improvement might be caused by the increase in the area of the fractured surface in narrow clearances. Generally, the larger the fractured surface, the smaller the work hardening on a sheared edge. Furthermore, the investigation reveals the existence of an optimum shearing angle for excellent stretch-flange formability. FE simulation shows that the optimum shearing angle increases hydrostatic stress in materials during shearing. Compressive stress across the material thickness is mainly affected by shearing angle. The result of the analysis suggests that the optimum shearing angle increases the area of the fractured surface to improve stretch-flange formability.

Influence of Blank Holding Force on Stretch Flange Forming of Aluminum Alloy

The effect of blank holding force on the stretch flanging process was investigated using finite element simulation and experimental analysis. Finite element model was used to study the effect of blank holding forces on punch load and forming kinematics. An aluminum alloy 5052 sheet of 0.5mm thickness was employed for this work. Blank holding force has a great effect on punch load and significant influence on forming kinematics. Validation of FEM results were obtained by conducting experiments. A very good agreement was obtained between FEM simulation results in terms of punch load and final shape which shows the accuracy of present finite element model.

ルーフシャーパンチによる穴抜きと鋼板の伸びフランジ性の異方性を組み合わせた穴広げ率の向上

In this study, the effect of a wedge on a piercing punch on hole-expansion ratio has been investigated. As a result, the holes pierced using a wedged piercing punch showed significantly high hole-expansion ratios under certain conditions. Regarding the wedge vertical angles, the hole-expansion ratio becomes high at 80 degrees and low at 45 degrees. The sheet bending effect caused by the wedge shape contributes to these results. The small amount of bending in the case of 80 degrees induces tensile stress around the punch edge without plastic deformation, which accelerates material fracture. This acceleration decreases plastic strain on the pierced surface and leads to the high hole-expansion ratio. On the other hand, a large amount of bending in the case of 45 degrees causes plastic deformation around the punch edge, which deteriorates the hole-expansion ratio. Regarding the material direction when set on a die face, the hole-expansion ratio becomes high when the material direction at which the stretch-flange formability is the highest is at 90 degrees to the wedge tip line. In this condition, the stretch-flange formability at the weakest position resulting from piercing using the wedged punch becomes the highest in the other material directions.

高張力鋼板せん断加工部の疲労強度と伸びフランジ性の改善

高張力鋼板せん断加工部の疲労強度と伸びフランジ性の改善

自動車用鋼板におけるせん断加工工具条件が及ぼす伸びフランジ性への影響

自動車用鋼板におけるせん断加工工具条件が及ぼす伸びフランジ性への影響

Application of an Extended Stress-Based Forming Limit Curve to Predict Necking in Stretch Flange Forming

An extension of the stress-based forming limit curve (FLC) advanced by Stoughton (2000, “A General Forming Limit Criterion for Sheet Metal Forming,” Int. J. Mech. Sci., 42, pp. 1–27) is presented in this work. With the as-received strain-based FLCs and stress-strain curves for 1.6-mm-thick AA5754 and 1-mm-thick AA5182 aluminum alloy, stress-based FLCs are obtained. These curves are then transformed into extended stress-based forming limit curves (XSFLCs), which consist of the invariants, effective stress, and mean stress. By way of application, stretch flange forming of these aluminum alloy sheets is considered. The AA5754 stretch flange displays a circumferential crack during failure, whereas the AA5182 stretch flange fails through a radial crack at the edge of the cutout. It is shown that the necking predictions obtained using the strain- and stress-based FLCs in conjunction with shell element computations are inconsistent when compared with the experimental results. By comparing the results of the shell element computations with those in which the mesh comprises eight-noded solid elements, it is demonstrated that the plane stress approximation is not valid. The XSFLC is then used with results from the solid-element computations to predict the punch depths at the onset of necking. Furthermore, it is shown that the predictions of failure location and failure mode obtained using the XSFLC are in accord with the differences observed between the two alloys/gauges.

Strain Loci in Stretch Flange Forming of Aluminum Alloys

Stretch flange forming experiments of AA5182 and AA5754 sheet alloys were performed to characterize the forming properties. Identical blanks with the same cutout radius were deformed to 25, 50, 75, and 90% of the measured minimum punch depth to fracture. Strain development during stretch flange forming was tracked through strain measurement of the recovered deformation-interrupted samples. Finite element modeling of stretch flange forming was performed, and the resulted strain distribution along the stretch flange profile was compared to the experimental results. Good agreement was observed between the numerical and experimental results. Two failure modes observed in the stretch flange experiments, inner edge necking and circumferential cracking in sidewall, were related to the measured strain locus and sheet thickness.

Effect of Niobium on Zn-Coated Dual Phase Steel Sheet

Dual Phase (DP) steel sheets, mainly 590 MPa TS grade steel, have been applied to structural parts of automobile because of their good formability, large bake hardenability and high crash worthiness. Although the concept of DP steel was established as early as the mid-1970th, the literature contains little discussion of Zn-coated DP steel, which has been the main application in recent years. In manufacturing Zn-coated DP steel, chemical composition which secures adequate Zn coatability and appropriate heat cycles, including galvannealing, must be considered. In this paper, the effect of Nb on the tensile properties, stretch flange formability and bake hardenability of 590 MPa TS grade DP steel is discussed. The base chemical composition was 0.05%C-2%Mn-0.5%Cr steel. The effect of Nb on the above-mentioned properties was studied using 0.05%Nb added steel and found that the addition of 0.05%Nb results in improved elongation, stretch flangeability and bake hardenability with higher tensile strength under both simulated GA and GI heat cycles. These improved properties by the addition of Nb are brought about by the grain refinement of ferrite matrix and finer dispersion of martensite.