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Analysis of Shear Droop on Cut Surface of High-Tensile-Strength Steel in Fine-Blanking Process*1

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Analysis of Shear Droop on Cut Surface of High-Tensile-Strength Steel in Fine-Blanking Process*1 Materials Transactions, Vol. 52, No. 3 (2011) pp. 447 to 451 #2011 The Japan Society for Technology of Plasticity Analysis of Shear Droop on Cut Surface of High-Tensile-Strength Steel in Fine-Blanking Process*1 Toru Tanaka1, Seiya Hagihara2, Yuichi Tadano2, Shuuhei Yoshimura2;*2, Takuma Inada2;*2, Takanobu Mori3 and Kenji Fuchiwaki4 1Industrial Technology Center of Saga, 114 Yaemizo, Nabeshima, Saga 849-0932, Japan 2Department Mechanical Engineering, Saga University, 1 Honjo, Saga 840-8502, Japan 3Mori Iron Works Corporation Ltd., 2078 Ide, Kashima, Saga 849-1302, Japan 4Hatano Seimitsu Corporation Ltd., 183-7 Hirasawa, Hatano, Kanagawa 257-0015, Japan The fine-blanking process is used in the production of automobile parts and other metal components. Although the fine-blanking process can produce sheared surfaces with higher precision than the punching process, shear droops on cut surfaces are also formed, as in the punching process. It is important to determine the causes of the formation of shear droops, but the mechanism is difficult to determine experimentally. Here, the finite element method (FEM) is adopted to study the causes of the formation of shear droops. The cut surfaces in the present experiments have fine sheared surfaces but no fracture surfaces. Although a combination of the fracture criterion and element-kill method is used for many simulations of the fine-blanking process, fine sheared surfaces cannot be evaluated by the combination of these methods. In the present calculations, an adaptive remeshing technique for FEM is used to create fine sheared surfaces. The shear droop is associated with the initial compression by and the subsequent clearance of the punches and dies. Results are obtained for various clearances and initial compressions in the fine-blanking process for high-strength steel, and the experimental and calculation results are compared. In the present paper, we show that the shear droops are affected by the clearance of and initial compression by the punches and die. [doi:10.2320/matertrans.P-M2010828] (Received June 25, 2010; Accepted November 26, 2010; Published January 26, 2011) Keywords: shearing, finite element method, fine blanking, high-tensile-strength steel 1. Introduction The actual shearing is non-uniform deformation with plastic fracture, and measuring the stress and strain during the Recently, the automobile industry has been faced with deformation process is difficult, which makes assessing the various issues such as cost reduction and safety measures for propriety of shearing or the quality of the product extremely high-speed vehicles. In particular, fuel efficiency and high- difficult. Hence, effectively applying finite element method speed safety are critical issues, and each manufacturer has (FEM) simulations to the shearing phenomenon is actively plans to increase mileage by making the vehicle itself lighter being researched.2,3) through replacement of normal steel parts with high-tensile- In this study, we investigate the effectiveness of FEM strength steel parts that have the same strength but are thinner. analysis by comparing experimental data from a punching Mass production of a particular part with a press allows for test with the results of shearing analysis using FEM. In large reductions in production time and production cost in addition, we examine the effects on shear droop of shear comparison with casting or machining. The fabrication of plane rounding generated in the fine-blanking process. vehicle parts begins with deep drawing and bending of sheet metal, and a majority of parts are made using shearing 2. Punching Experiment on Irregularly Shaped Teeth processes such as punching or cutting. In general, when the entire surface has been sheared, approximately 15% of the In order to verify the effects of shear droop in the fine- sheet thickness is due to shear droop, 20% is due to the shear blanking process, an experiment was conducted on irregu- plane and 65% is due to the fracture surface combined with larly shaped teeth. the post-shear shaving process. Figure 1 shows the experimental blanking plate. The plate Here, we focus on the fine-blanking process, which is a has a central hole of 25 mm and a maximum diameter precise shearing method that does not require secondary of 100 mm; four teeth are formed with internal angles of processing.1) The fine-blanking process is a high-precision 30,60,90 and 120, and three groups of four teeth are shearing process where a blank is placed under pressure and positioned regularly around the shape. The cusp radius, R,of the malleability of the material is improved through the each group is 1.0 mm, 0.5 mm and 0.3 mm, respectively. effects of hydrostatic pressure, which protects against the Furthermore, three positions of 100 mm arc with clearance occurrence of fracture. However, even in the fine-blanking of 0.005 mm, 0.01 mm and 0.02 mm are formed regularly process, product accuracy is affected by the burr formation, around the plate. shear droop and bulging that occur in the localized shear This punching experiment was run with an irregularly planes. toothed die block; the blank material was from a steel sheet of thickness t (¼ 4 mm) of hot-rolled high-tensile-strength steel *1This Paper was Originally Published in Japanese in J. the JSTP 51-588 for vehicles (SPFH590). (2010) 50–54. The die block used in the experiment had four guide posts *2Graduate Student, Saga University (Fig. 2), and the blank punch was constructed at the centre of 448 T. Tanaka et al. Fig. 3 Analysis model of fine-blanking process. Table 1 Material properties. Fig. 1 Experimental blanking shape. Young’s modulus [GPa] 200 Poisson’s Ratio [-] 0.3 Yield Stress [MPa] 477 800 600 400 Fig. 2 Die block for fine-blanking press. stress /MPa Flow 200 the die block. Moreover, the die block’s dimensions were width of 400 mm, depth of 450 mm and height of 340 mm. 0 Normally, when a compressive force acts on the blank in 0 0.03 0.06 0.09 0.12 the fine-blanking process, the load from the hydraulic Plastic strain pressure acting on the blank holder is Vp, and that acting 4) on the counter punch is Cp. Fig. 4 Flow stress versus plastic strain. This punch experiment was run with a hydraulic 4000 kN press under conditions of Vp ¼ 200 kN, Cp ¼ 150 kN and Vp ¼ 400 kN, Cp ¼ 300 kN with a punch/counter-punch block interior consists of deformable high-tensile-strength speed of 5 mm/s during punching. steel and the punch, counter punch, blank holder and die as rigid bodies. 3. FEM Simulation The material model is an isotropic material following the plastic flow rule. To find the material properties, a tensile test 3.1 Analysis model and material properties was carried out on a JIS5 test sample of hot-rolled high- Using the commercial FEM software MSC Marc2008, the tensile-strength steel for vehicles (SPFH590). shearing process was simulated and the factors affecting The material properties for the elastic range of the high- shear droop in the fine-blanking process were investigated. tensile-strength steel were obtained (Table 1). The relation- Figure 3 shows a schematic diagram of the analysis model. ship of flow stress versus plastic strain is shown in Fig. 4. The analyzed part is a disk, and the blank diameter is 20.0 mm, the part diameter is 12.0 mm and the plate thickness 3.2 Analysis conditions t is 4.0 mm. Here, the analysis model is an axisymmetric The fine-blanking process is defined as follows by model with the axis at the center and the structure of the die- controlling the displacement of each tool through the punch, Analysis of Shear Droop on Cut Surface in Fine-Blanking Process 449 Fig. 5 Schematic diagram of displacement control of the tools in each step of the fine-blanking process. (a) Cut surface of products (b) Cross section of products counter-punch, blank holder and die; the process is analyzed as a time-independent quasi-static phenomenon. Fig. 6 Fine-blanking products. (1) The blank is pushed onto the V-ring on the blank holder. (2) The blank is pressurized by the punch and blank holder. Table 2 Analysis conditions. (3) The part is punched out using the punch and counter Displacement of Clearance punch. Combination initial compression [mm] Figure 5 shows a schematic diagram of the displacement [mm] control of the tools in each step. 0.0,0.005 Until now, in FEM simulations of shearing, the shear I 0.03 0.01,0.02,0.03 phenomenon has typically been considered under the ductile 0.05,0.08,0.1 5–7) fracture criteria described by eqs. (1) to (3). II 0.001,0.03,0.08 0.0 Z "f exp 1:5 H d" ¼ C Rice and Tracey ð1Þ 1 Z0 eq "f max acting on the contact surface between the blank material and d" ¼ C Cockcroft and Latham ð2Þ 2 each tool and between each tool was not considered in this Z0 eq "f analysis. H 1 þ d" ¼ C Oyane ð3Þ For the analysis conditions shown in Table 2, the relation- 3 0 eq ship between the clearance and shear droop in the case of Here, H is the hydrostatic pressure, eq is the Von Mises constant initial compression and the relationship between the stress, max is the maximum principal stress, " is the shear droop and initial compression with no clearance were equivalent strain, "f is the equivalent strain at fracture, is analyzed. the material constant, and C1, C2 and C3 are material specific values at the start of fracturing. 4. Experimental Results Analyses using ductile fracture conditions such as these show shear planes by element-kill method from the ductile 4.1 Punching test results for irregularly shaped teeth fracture assessment that determine a fracture has been For the experimental blanking shape punched in the reached.8) However, in this method, the roughness or fineness 4000 kN hydraulic press, shear droop of the shear plane was is affected by the elements eliminating the state of the shear observed in the 100 mm arcs with clearances of 0.005 mm planes, and is largely dependent on meshing.
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