Effects of Vertical Angle of Conical Punch on Stretch Flangeability of High Strength Steel

To clarify the effects of the vertical angle of a conical punch on stretch flangeability, hole expansion forming tests were conducted. Test results showed that the hole expansion ratio becomes larger as the vertical angle decreases.Results also showed that the fracture strain at the fracture location on the hole edge was constant and independent of the vertical angle. This is because the hole expansion ratio was controlled not only by the fracture strain, which is independent of the vertical angle, but also by deformation uniformity along the hole edge. From the result of numerical analyses, it was determined that deformation uniformity depends on the gradient of circumferential stress along the radius direction. When the vertical angle is sharp, the circumferential stress showed a steep decline and the deformation localization was suppressed. Consequently, the hole edge deformed more uniformly and the hole expansion ratio became larger. It is concluded that in order to improve stretch flangeability of high strength steel, it is important to uniformly deform the hole edge by applying a conical punch with a sharp vertical angle.     

Gradually contacting punch for improving stretch flangeability of ultra-high strength steel sheets

A gradually contacting punch for improving stretch flangeability of ultra-high strength steel sheets having small ductility was developed. In the gradually contacting punch, tensile stress around the corner edge of the sheet in stretch flanging is decreased by gradually pressing the edge of the sheet with the inclined bottom of the punch. The critical flange heights of bent 980 and 1180 MPa ultra-high strength steel sheets without fracture with the gradually contacting punch increased by 32% and 31%, respectively. In addition, the stroke limit of the press was avoided by 2-stage flanging consisting of peripheral bending and corner bending.     

Slight clearance punching of ultra-high strength steel sheets using punch having small round edge

A slight clearance punching process of ultra-high strength steel sheets using a punch having a small round edge was developed to improve the quality of the sheared edge. No crack from the edge of the punch was generated by relaxing concentration of deformation with the punch having a small round edge, and the fracture was delayed. A small edge radius of 0.13 mm was effective for improving the quality of the sheared edge of ultra-high strength steel sheets, the increase in shiny burnished surface. The delayed fracture was prevented by the increase in compressive residual stress for the punch having the small round edge. For 1000 strike punching of an ultra-high strength steel sheet having a tensile strength of 1200 MPa, a sheared edge of high quality was produced with a TiAlN-coated punch having the small round edge. In addition, the chipping of the punch edge was prevented even for a slight clearance by the small round edge. It was found that both small round edge and slight clearance are indispensable for high quality punching of ultra-high strength steel sheets having low ductility.     

Warm and hot punching of ultra high strength steel sheet

Warm and hot punching using resistance heating was developed to improve the quality of sheared edges of an ultra high strength steel sheet. As the heating temperature increased, the depth of the shiny burnished surface on the sheared edge increased and that of the rough fracture surface decreased. The rollover depth and burr height of the sheared edge became large above 800 °C. Although the roughness of the burnished surface was almost constant, the roughness of the fracture surface increased from 650 °C. The punching load was extremely reduced by the heating, i.e. 40% of the cold punching load at 650 °C and 15% at 1070 °C. The local resistance heating of the shearing region was efficient for the warm and hot shearing. It was found that the warm and hot shearing of ultra high strength steel sheets is effective for improving the quality of the sheared edge and in reducing the shearing load.     

A new approach for user-independent determination of formability of a steel sheet sheared edge

It is common to use a forming limit curve (FLC) for a feasibility study of a deep-drawn steel part based on a finite element analysis (FEA). However, in such an approach a neglected fact is that a blank edge in industrial production is often produced by shear cutting. Especially, for many high strength steel grades, this cutting process notably reduces edge formability. An overestimation of formability of the blank edge, with an FLC, is the consequence that may lead to cracks at the sheared edge of a part. The following paper describes a new approach to determine formability of a sheet-steel sheared edge by hole expansion test that uses an FLC tool set. This approach delivers a hole expansion ratio with considerably lower scattering compared to the hole expansion according to ISO 16630. Additionally, information on the planar isotropy, flow and necking behavior of the material, is supplied. Finally, a pragmatical way of transferring test results into an FEA of the forming process for a sheet blank with a sheared edge is presented.     

Finite element analysis of the effect of blanked edge quality upon stretch flanging of AHSS

Elimination of edge cracking is one of the major challenges in flanging of advanced high-strength steels (AHSS). Several studies show that edge cracking occurs at lower strains than those predicted by the forming limit curves (FLC) and it is influenced significantly by the sheared or blanked edge quality. This study focuses on FEM modeling and experiments on blanking and hole expansion of AHSS DP590. The FEM model of blanking was developed to characterize the edge quality for different punch/die clearances. Hole expansion was simulated to demonstrate the effect of sheared edge upon stretchability. Thus, it was possible to demonstrate how metal flow, strains and stresses in blanking affect the part quality and potential edge cracking in stretch flanging.     

Computer simulation and experimental validation of stretch flanging

The axisymmetric stretch flanging process is a common secondary operation in sheet metal stamping. The process is characterized by a uniaxial state of stress at the edge of the flange. An approximate analysis, based on the assumption that the state of stress throughout the flange is mainly uniaxial, is used to model the stretch flanging (second step) process. The approximation is derived from the total strain membrane theory of plasticity which incorporates strain hardening and normal anisotropy of the material. Under such conditions, flangeability is controlled by the tensile elongation of the metal and is limited by localized necking or fracture of the flanged edge. The analysis includes a stretching limit criterion to determine the flanging limit of the material. The influence of prestretching (first step) on flangeability is modeled using the membrane shell theory with axisymmetric deformation to solve the contact condition in stretch forming. Inputs to the model are a desired flange profile, material properties, and sheet thickness. The output includes the feasibility of the flanging operation, any requirements for prestretching and the size of the trim radius needed to successfully flange the profile. The model is verified by experimental results.     

Punching of ultra-high strength steel sheets using local resistance heating of shearing zone

A punching process using local resistance heating of a shearing zone was developed to shear ultra-high strength steel sheets. The shearing zone was heated by passing electric current between the sheet holder and the knockout in order to decrease the flow stress in the shearing, and the heating of the die and punch was prevented by no contact with the sheet during the heating. Electrode pins having an individual spring were employed to attain uniform heating of the shearing zone. The welding resistance of the heads of the electrode pins to the sheet by the heating was examined for Ag-W, Cu-W, Ag + WC and W. The Cu-W pins having the highest welding resistance were employed in a punching experiment of 980 MPa level ultra-high strength steel sheets. The punching load was considerably reduced by the heating, e.g., about 1/5 of the cold punching load at 800 °C. As the heating temperature increased, the depth of the shiny burnished surface on the sheared edge increased and that of the rough fracture surface decreased.     

Factors governing hole expansion ratio of steel sheets with smooth sheared edge

Stretch-flangeability measured using hole expansion test (HET) represents the ability of a material to form into a complex shaped component. Despite its importance in automotive applications of advanced high strength steels, stretch-flangeability is a less known sheet metal forming property. In this paper, we investigate the factors governing hole expansion ratio (HER) by means of tensile test and HET. We correlate a wide range of tensile properties with HERs of steel sheet specimens because the stress state in the hole edge region during the HET is almost the same as that of the uniaxial tensile test. In order to evaluate an intrinsic HER of steel sheet specimens, the initial hole of the HET specimen is produced using a milling process after punching, which can remove accumulated shearing damage and micro-void in the hole edge region that is present when using the standard HER evaluation method. It was found that the intrinsic HER of steel sheet specimens was proportional to the strain rate sensitivity exponent and post uniform elongation.     

Failure During Sheared Edge Stretching

Failure during sheared edge stretching of sheet steels is a serious concern, especially in advanced high-strength steel (AHSS) grades. The shearing process produces a shear face and a zone of deformation behind the shear face, which is the shear-affected zone (SAZ). A failure during sheared edge stretching depends on prior deformation in the sheet, the shearing process, and the subsequent strain path in the SAZ during stretching. Data from laboratory hole expansion tests and hole extrusion tests for multiple lots of fourteen grades of steel were analyzed. The forming limit curve (FLC), regression equations, measurement uncertainty calculations, and difference calculations were used in the analyses. From these analyses, an assessment of the primary factors that contribute to the fracture during sheared edge stretching was made. It was found that the forming limit strain with consideration of strain path in the SAZ is a major factor that contributes to the failure of a sheared edge during stretching. Although metallurgical factors are important, they appear to play a somewhat lesser role.     

Delayed fracture in cold blanking of ultra-high strength steel sheets

Hydrogen-induced delayed fracture at cold-blanked edges of 1–1.5 GPa ultra-high strength steel sheets was investigated. The blanked edges undergo large shear deformation and tensile residual stress, and thus the risk of delayed fracture is high, especially for the 1.5 GPa sheet. The effects of residual stress, surface quality and hardness of the sheared edge on the occurrence of delayed cracking were examined. Delayed cracking was caused by press blanking, whereas no cracking occurred for laser blanking because of compressive residual stress. For the 1.5 GPa sheet, delayed cracking was prevented by heating above 250 °C and a stain above 0.005.     

Analysis of sheared edge formability of aluminum

Edge quality produced by shearing processes often leads to reduced material formability which was observed in multiple studies and summarized in the reference literature. The intention to make the sheared edge performance more predictable has motivated development of several experimental techniques such as the hole expansion test and the half dogbone tensile test. The paper presents a detailed review of published results for both of these techniques and illustrates very limited research dedicated to sheared edge performance of aluminum alloys. The experimental study, performed on a broadly used aluminum alloy, 6111-T4, illustrated the effects of cutting clearance on longitudinal, transverse and diagonal orientations of the trim line relative to the rolling direction. For all sheet orientations, increasing the cutting clearance resulted in a substantial reduction in material stretchability along the sheared surface. However, for all investigated conditions a cutting clearance of 5% of material thickness resulted in stretching performance similar to the standard tensile test. In this case the sheared edge does not affect the stretching behavior of tested material. The analysis of material prestrain on sheared surface stretchability for a variety of combinations of minor and major strains indicated that for the widely accepted industry standard gap of 10% of the material thickness, the prestrain has significant effects on stretchability which only gets stronger with increased thinning of the sheet in the prestraining process. For an extended clearance of 40%, the effect of prestrain was less visible indicating that the sheared edge has a stronger effect on these cutting conditions than prestrain.Analysis of the effect of the cutting angle on stretchability indicated that higher elongations were observed with cutting angles of 10° and 20° for broadly used 10% clearance compared to orthogonal cutting with an identical clearance.The results of half dogbone tensile tests were compared with the results of hole expansion tests performed on the same sheet material. This comparison indicated that a substantial amount of localization occurs in the hole expansion test and leads to a much higher hole expansion ratio for small cutting clearances compared to the total elongations observed in tensile tests. However, the local strains measured in the area adjacent to fracture in the tensile test were above the hole expansion ratio.     

Shear cutting of press hardened steel: influence of punch chamfer on process forces, tool stresses and sheared edge qualities

In order to cope with the difficulties of shearing operations of press hardened steels, this work attempted to optimize these processes. Herefore, it is necessary to reduce the press forces and stresses in tools, while still obtaining adequate sheared edge quality levels. To reach this goal, different punch chamfer angles (0°, 2°, 7° and 20°) and relative cutting clearances (5, 10 and 15?% of the sheet thickness) were tested in a cutting tool, which was adapted in a way that different active elements could be mounted. The tool was equipped with a measurement system, which allowed the determination of the process forces in three dimensions at each punch stroke. Basis was an AlSi coated 22MnB5 sheet with a thickness of 1.5?mm. In addition a finite element model was developed to predict the stress distribution in tools and the sheared edge qualities. According to the experimental results the application of a 20° chamfer angle succeeded to reduce the forces and stresses of tools, but the sheared parts had a poor quality. In contrast, the 7° chamfer angle gave lowest tool stresses and sufficient part qualities, but the forces were very high. The simulation results agreed with the experimental data, except for the prediction of the rollover zone. These deviations were attributed among others to the presence of the AlSi coating, which was not considered.     

超高强度钢与镀锌双相钢电阻点焊接头强度试验

试验研究了超高强度硼钢板/镀锌双相钢板的电阻点焊接头质量缺陷及其产生原因,通过正交试验设计,重点讨论了焊接电流、通电时间和电极压力对点焊接头强度的影响.结果表明:超高强度硼钢板/镀锌双相钢点焊中超高强度钢板侧更易出现飞溅和烧穿问题,通电时间和焊接电流强度时点焊接头拉剪强度影响显著,这类钢板组合的焊接应优先采用大电流、短...     

切角板坯对纯铜薄板矩形盒拉深影响的试验及数值分析

在大量纯铜薄板矩形盒拉深试验的基础上,结合有限元计算,分析了矩形盒的拉深特性。指出,矩形切角板坯使法兰曲边变形分布得以改善,非拉深变形抵抗弱化,提高了拉深成形性。有限元模拟结果显示矩形板坯拉深断裂点产生在凸模肩部转角附近,且始终承受两向不等拉伸,应变计算值与实际拉深测定值相符;切角板坯的拉深断裂点则转移至接近凹模口的侧壁处,且始终处于压剪组合变形状态,断裂时板厚应变相当小,认为倾向于剪切断裂。切角板坯断裂点的拉、压应力组合效应使该点板厚几乎不变,是提高拉深极限的重要因素之一。     

Joining of high strength steel and aluminium alloy sheets by mechanical clinching with dies for control of metal flow

High strength steel and aluminium alloy sheets were joined by mechanical clinching with dies for control of metal flow. Since the sheets undergo plastic deformation for the joining during the mechanical clinching, the high strength steel sheets tend to fracture due to the small ductility. For the upper high strength steel sheet, fracture was caused by the concentration of deformation around the corner of the punch, and cracks were caused by the tensile stress generated in the bulged bottom into the groove of the die for the lower high strength steel sheet. To prevent these defects, metal flow of the sheets was controlled by optimising a shape of the die. For the upper high strength steel sheets, the depth of the die was decreased to prevent the concentration of deformation around the corner of the punch. On the other hand, the groove of the die was eliminated to reduce the tensile stress for the lower high strength steel sheets. The sheets below SPFC780 and SPFC980 were successively joined with the aluminium alloy sheet for the upper and lower high strength steel sheets, respectively.     

Piercing of Steel Sheet by using Hydrostatic Pressure

In order to improve the dimensional accuracy of the hole made by piercing a sheet/tube in hydroforming, some piercing methods using hydrostatic pressure are examined. In piercing by pushing a steel sheet into the pressurised liquid with a punch, the sheet is warped and the edge of the hole sinks (edge drop). In piercing by pushing the sheet outwards with hydrostatic pressure while the punch recedes, warping is suppressed but burr is formed. When the sheet is first pushed with the punch into the liquid and then piercing is completed by receding of the punch, burr is not formed and edge drop decreases by about 50%.     

Plastic Joining of Ultra High Strength Steel and Aluminium Alloy Sheets by Self Piercing Rivet

Ultra high strength steel and aluminium alloy sheets were plastically joined by a self piercing rivet driven through the upper sheet and spread in the lower sheet with a die. The self piercing rivet directly pierces into the sheets without drilling the sheets beforehand unlike the conventional rivets. Insufficient driving though the upper sheet and fracture of the lower sheet occur due to the high hardness and low ductility of the ultra sheet, respectively. An ultra high strength steel sheet having a tensile strength of 980MPa and an aluminium alloy sheet were successfully joined by optimising shapes of the die.     

The effect of die materials on the strain distribution in electro-galvanized sheet steel forming

The punch load and strain distribution of two deformed sheet steels, aluminum killed drawing quality steel (AKDQ Bare) and electro-galvanized drawing quality steel (AKDQ E.G.), are examined under the various process conditions including, die materials, punch speed, blank holding force, drawbead height and lubricant. The punch load and strain distribution ot Bare sheet steel forming is higher than that of E.G.sheet steel on the Kirkesite die set and are reversed on the GM 241 die set. The punch load and strain distribution on the Kirkesite die set is lower than those of the GM 241 die set. The changes of punch load and strain distribution ot the deformed cup for two sheet steels are affected by the frictional behavior of each sheet steel. It shows that the changes of frictional behavior having to be considered in the die design.     

Warm and Hot Stamping of Ultra High Tensile Strength Steel Sheets Using Resistance Heating

A warm and hot stamping process of ultra high tensile strength steel sheets using resistance heating was developed to improve springback and formability. In this process, the decrease in temperature of the sheet before the forming is prevented by directly heating the sheets set into the dies by means of the electrical resistance, the so-called Joule heat. Since the heating time up to 800°C is only 2 seconds, the resistance heating is rapid enough to synchronise with a press. The effects of the heating temperature on the springback and formability of ultra high tensile strength steel sheets were examined. The springback in hat-shaped bending of the high tensile strength steel sheets was eliminated by heating the sheet. In addition, the ultra sheet having a tensile strength of 980MPa was successfully drawn by the heating. The heating temperature is optimum around 600°C due to the small springback and oxidation and the increase in hardness.