Materials Transactions, Vol. 52, No. 3 (2011) pp. 447 to 451 #2011 The Japan Society for Technology of Plasticity

Analysis of 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 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 . [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 or . The fabrication of plane rounding generated in the fine-blanking process. vehicle parts begins with deep and bending of , 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. Furthermore, and 0.02 mm when cut with a cutter. eliminating the elements could cause differences in the The cross section of each cutting was positioned opposite volume constancy of plastic deformation. each other, and Fig. 7 shows an image of the cross section The cut surface of the test sample punched by fine- taken with a VH-8000 KEYENCE microscope. For these cut blanking and the cross section of the blank during the surfaces, shear droop can be seen more clearly for the 0.02- punching process are shown in Fig. 6(a) and (b), respective- mm clearance than for the 0.005-mm clearance. ly. The relation between the clearance and shear droop for The cut surface in Fig. 6(a) has a glossy shear plane across each clearance on the blanking shape as measured with an the entire surface without any fracture planes from generated Accretech Surfcom 1800D surface roughness and contour crack. Furthermore, as shown in Fig. 6(b), at the cross section measuring device is shown in Fig. 8. The clearances were set of the blank during the punching process, fiber structures to 0.005 mm, 0.01 mm and 0.02 mm with the die block. These similar to the forge flow lines due to material flow in the results show that the shear droop increased as the clearance process can be seen. From this, the shearing process became larger. in this fine-blanking process is assumed to be similar to the From the graph in Fig. 8, the experimental blanking shape phenomenon in the forging process and thus can be analyzed punched with settings of Vp ¼ 400 kN and Cp ¼ 300 kN had without applying the ductile fracture criteria such as those in smaller shear droop for both clearances than that punched eqs. (1) to (3). with settings of Vp ¼ 200 kN and Cp ¼ 150 kN. A remeshing technique is used in the analysis method; this function regenerates elements and node movement for when 4.2 FEM simulation results the element deformation is large or when an element The FEM simulation results for changing the die-block penetrates a rigid body after contacting it. The friction force clearance from 0.0 mm to 0.05 mm at constant initial 450 T. Tanaka et al.

14

12 Depth of shear droop

10

8

6 FEM Analysis (V130×C45) 4 Experiment (V200×C150)

Depth of shear droop /% Experiment (V400×C300) 2

0 Fig. 7 Cross section of cut surfaces with shear droop. 0 0.02 0.04 0.06 Clearance /mm 10 Fig. 9 Relation between clearance and depth of shear droop. V200×C150

9 V400×C300 18

16 Depth of shear droop 8 14

12 7 10

Depth of shear droop /% 8 6 Clearance 6 Depth of shear droop 0.00[mm] 0.005[mm] Depth of shear droop /% 4 0.01[mm] 0.02[mm] 5 0.03[mm] 0.05[mm] 0 0.01 0.02 2 0.08[mm] 0.10[mm]

Clearance /mm 0

Fig. 8 Relation between clearance and measured depth of shear droop. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Punch position /mm compression under analysis conditions I are shown in Fig. 9 Fig. 10 Relation between punch position and depth of shear droop. together with the test results from Fig. 8. The FEM simulation results show that shear droop increases with increasing clearance, similar to the results of droop for the FEM simulation when changing the die-block the experimental blank punching results. By considering that clearance from 0.0 mm to 0.1 mm at constant initial pressure. the shear droop is caused by material deficiencies in the For each clearance examined in the analysis, the shear clearance parts during the punching process, then the flow droop tends to increase during punching, and that rate of 10) in the material caused by Vp and Cp can be thought to increase becomes larger as the clearance increases. complement this material deficiency.9) This finding can be attributed to the material deficiencies The total reaction force of the blank holder (equivalent to of the clearance parts increasing as the punch progresses; Vp) obtained from the analysis results for a constant initial these material deficiencies are thought to increase shear compression of 0.03 mm was 130 kN, and the total reaction droop. Considering the change in shear droop from the force of the counter punch (equivalent to Cp) was 45 kN. This punching process, the shear droop must be evaluated when indicates that the analysis results for the shear droop are the punch has moved to the maximum analytical limit in larger than the punching test results because the values of Vp order to estimate the shear droop in the fine-blanking process and Cp in the analysis results are smaller than those in the by FEM simulation. test, which makes the material deficiencies in the clearance Next, Fig. 11 shows the relation between the initial parts small. compression at constant clearance and shear droop for Furthermore, the Fig. 10 shows the relation between the analysis conditions II for V-ring heights of 0.0 mm, 0.35 mm punch displacement while punching the blank and shear and 0.7 mm. These analyses are all calculated for the process Analysis of Shear Droop on Cut Surface in Fine-Blanking Process 451

25

20

15 V-ring height

10 0.00[mm] 0.35[mm] 0.70[mm] (a) V-ring height 0.0[mm] (b) V-ring height 0.7[mm] Depth of shear droop /% 5 Fig. 12 Distributions of hydrostatic stress. Depth of shear droop

0 (2) As V and C become large in the fine-blanking 0 0.02 0.04 0.06 0.08 p p process, the shear droop of punched products decreases. Punch displacement of initial compression /mm (3) In the FEM simulation of the fine-blanking process, Fig. 11 Relation between initial compression and depth of shear droop. shear droop close to the experimental values was calculated by implementing a remeshing function that regenerates elements. until the high-tensile-strength steel is on the verge of (4) From the FEM simulation results, it was found that the fracturing; the values shown are taken from the shear droop shear droop of the punched product is reduced as the V-ring for shear surfaces from coordinate values of the results. For height of the die or blank holder increases. each initial compression, the shear droop is suppressed as the (5) To predict the shear droop of punched products in the V-ring height increases. Furthermore, the shear droop is fine-blanking process by FEM simulation, the shear droop suppressed as the initial compression increases for each must be predicted when the punch is moved to the maximum V-ring height. theoretical range. In the case of the disk model, when the initial compression is 0.08 mm, the hydrostatic stress distribution obtained from Acknowledgement the FEM simulation when the V-ring was absent or when the V-ring height was 0.7 mm is shown in Fig. 12(a) and (b), This research was supported in part by the 2007-8 Strategic respectively. Basic Technology Advancement Support Program. Without the V-ring, tensile stress is generated in the shear droop, while equipping the blank holder and die with the V- REFERENCES ring compressive stress around the shear droop. From these results, the provision of a V-ring controls the flow in the 1) T. Nakagawa: Theory and Practice of Fine-Blanking Precision blank material when it undergoes plastic deformation; Punching, (Nikkan Kogyo Shimbun, 1998) pp. 39–46. 2) M. Murakawa, P. Kaewtatip, M. Jin and N. Koga: Proc. 1999 Japanese through the effective compressive stress, the hydrostatic Spring conference for the Technology of Plasticity, (1999) pp. 502– pressure generated by the initial compression is thought to 503. make the shear droop smaller. 3) T. Ogawa, T. Tanaka, S. Yoshimura, Y. Tadano and S. Hagihara: The 59th conference for the Technology of Plasticity, (2008) pp. 301–302. 5. Conclusions 4) K. Hayashi: Fine-Blanking Terminology, (Press Forming Journal 2007) pp. 89–90. 5) M. Goto: J. JSTP 38 (1997) 200–205. For shear droop of high-tensile-strength steel in the fine- 6) F. Klocke, K. Sweeney and H.-W. Raedt: J. Mater. Process. Technol. blanking process, the following results were obtained from 115 (2001) 70–75. test results and analysis results using FEM simulation, 7) R. Hambli and M. Reszka: Int. J. Mech. Sci. 44 (2002) 1349–1361. considering the initial compression and the clearance 8) Y. Song, X. Xiaolong, Z. Jie and Z. Zhen: J. Mater. Process. Technol. 187–188 (2007) 169–172. between the die and punch. 9) K. Kondo: J. JSTP 29 (1988) 21–25. (1) The shear droop on products punched by the fine- 10) N. Yukawa, Y. Inukai, Y. Yoshida, T. Ishikawa and T. Jinma: J. JSTP blanking process was confirmed to become greater as the 39 (1998) 1129–1133. die-punch clearance increases.