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, Japan2Department Mechanical Engineering, Saga University, 1 Honjo, Saga 840-8502, Japan3Mori Iron Works Corporation Ltd., 2078 Ide, Kashima, Saga 849-1302, Japan4Hatano 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 processcan produce sheared surfaces with higher precision than the punching process, shear droops on cut surfaces are also formed, as in the punchingprocess. 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 presentexperiments have fine sheared surfaces but no fracture surfaces. Although a combination of the fracture criterion and element-kill method is usedfor many simulations of the fine-blanking process, fine sheared surfaces cannot be evaluated by the combination of these methods. In the presentcalculations, an adaptive remeshing technique for FEM is used to create fine sheared surfaces. The shear droop is associated with the initialcompression by and the subsequent clearance of the punches and dies. Results are obtained for various clearances and initial compressions in thefine-blanking process for high-strength steel, and the experimental and calculation results are compared. In the present paper, we show that theshear 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

Recently, the automobile industry has been faced withvarious issues such as cost reduction and safety measures forhigh-speed vehicles. In particular, fuel efficiency and high-speed safety are critical issues, and each manufacturer hasplans to increase mileage by making the vehicle itself lighterthrough replacement of normal steel parts with high-tensile-strength steel parts that have the same strength but are thinner.

Mass production of a particular part with a press allows forlarge reductions in production time and production cost incomparison with casting or machining. The fabrication ofvehicle parts begins with deep drawing and bending of sheetmetal, and a majority of parts are made using shearingprocesses such as punching or cutting. In general, when theentire surface has been sheared, approximately 15% of thesheet thickness is due to shear droop, 20% is due to the shearplane and 65% is due to the fracture surface combined withthe post-shear shaving process.

Here, we focus on the fine-blanking process, which is aprecise shearing method that does not require secondaryprocessing.1) The fine-blanking process is a high-precisionshearing process where a blank is placed under pressure andthe malleability of the material is improved through theeffects of hydrostatic pressure, which protects against theoccurrence of fracture. However, even in the fine-blankingprocess, product accuracy is affected by the burr formation,shear droop and bulging that occur in the localized shearplanes.

The actual shearing is non-uniform deformation withplastic fracture, and measuring the stress and strain during thedeformation process is difficult, which makes assessing thepropriety of shearing or the quality of the product extremelydifficult. Hence, effectively applying finite element method(FEM) simulations to the shearing phenomenon is activelybeing researched.2,3)

In this study, we investigate the effectiveness of FEManalysis by comparing experimental data from a punchingtest with the results of shearing analysis using FEM. Inaddition, we examine the effects on shear droop of shearplane rounding generated in the fine-blanking process.

2. Punching Experiment on Irregularly Shaped Teeth

In order to verify the effects of shear droop in the fine-blanking process, an experiment was conducted on irregu-larly shaped teeth.

Figure 1 shows the experimental blanking plate. The platehas a central hole of �25mm and a maximum diameterof �100mm; four teeth are formed with internal angles of30�, 60�, 90� and 120�, and three groups of four teeth arepositioned regularly around the shape. The cusp radius, R, ofeach group is 1.0mm, 0.5mm and 0.3mm, respectively.Furthermore, three positions of �100mm arc with clearanceof 0.005mm, 0.01mm and 0.02mm are formed regularlyaround the plate.

This punching experiment was run with an irregularlytoothed die block; the blank material was from a steel sheet ofthickness t (¼ 4mm) of hot-rolled high-tensile-strength steelfor vehicles (SPFH590).

The die block used in the experiment had four guide posts(Fig. 2), and the blank punch was constructed at the centre of

*1This Paper was Originally Published in Japanese in J. the JSTP 51-588

(2010) 50–54.*2Graduate Student, Saga University

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

the die block. Moreover, the die block’s dimensions werewidth of 400mm, depth of 450mm and height of 340mm.

Normally, when a compressive force acts on the blank inthe fine-blanking process, the load from the hydraulicpressure acting on the blank holder is Vp, and that actingon the counter punch is Cp.

4)

This punch experiment was run with a hydraulic 4000 kNpress under conditions of Vp ¼ 200 kN, Cp ¼ 150 kN andVp ¼ 400 kN, Cp ¼ 300 kN with a punch/counter-punchspeed of 5mm/s during punching.

3. FEM Simulation

3.1 Analysis model and material propertiesUsing the commercial FEM software MSC Marc2008, the

shearing process was simulated and the factors affectingshear droop in the fine-blanking process were investigated.

Figure 3 shows a schematic diagram of the analysis model.The analyzed part is a disk, and the blank diameter is20.0mm, the part diameter is 12.0mm and the plate thicknesst is 4.0mm. Here, the analysis model is an axisymmetricmodel with the axis at the center and the structure of the die-

block interior consists of deformable high-tensile-strengthsteel and the punch, counter punch, blank holder and die asrigid bodies.

The material model is an isotropic material following theplastic flow rule. To find the material properties, a tensile testwas carried out on a JIS5 test sample of hot-rolled high-tensile-strength steel for vehicles (SPFH590).

The material properties for the elastic range of the high-tensile-strength steel were obtained (Table 1). The relation-ship of flow stress versus plastic strain is shown in Fig. 4.

3.2 Analysis conditionsThe fine-blanking process is defined as follows by

controlling the displacement of each tool through the punch,

Fig. 1 Experimental blanking shape.

Fig. 2 Die block for fine-blanking press.

Fig. 3 Analysis model of fine-blanking process.

Table 1 Material properties.

Young’s modulus [GPa] 200

Poisson’s Ratio [-] 0.3

Yield Stress [MPa] 477

0

200

400

600

800

0 0.03 0.06 0.09 0.12

Flow

str

ess

/MPa

Plastic strain

Fig. 4 Flow stress versus plastic strain.

448 T. Tanaka et al.

counter-punch, blank holder and die; the process is analyzedas a time-independent quasi-static phenomenon.(1) The blank is pushed onto the V-ring on the blank

holder.(2) The blank is pressurized by the punch and blank holder.(3) The part is punched out using the punch and counter

punch.Figure 5 shows a schematic diagram of the displacement

control of the tools in each step.Until now, in FEM simulations of shearing, the shear

phenomenon has typically been considered under the ductilefracture criteria described by eqs. (1) to (3).5–7)Z �""f

0

exp 1:5�H

�eq

� �d �"" ¼ C1 Rice and Tracey ð1Þ

Z �""f

0

�max

�eqd �"" ¼ C2 Cockcroft and Latham ð2Þ

Z �""f

0

1þ ��H

�eq

� �d �"" ¼ C3 Oyane ð3Þ

Here, �H is the hydrostatic pressure, �eq is the Von Misesstress, �max is the maximum principal stress, �"" is theequivalent strain, �""f is the equivalent strain at fracture, � isthe material constant, and C1, C2 and C3 are material specificvalues at the start of fracturing.

Analyses using ductile fracture conditions such as theseshow shear planes by element-kill method from the ductilefracture assessment that determine a fracture has beenreached.8) However, in this method, the roughness or finenessis affected by the elements eliminating the state of the shearplanes, and is largely dependent on meshing. Furthermore,eliminating the elements could cause differences in thevolume constancy of plastic deformation.

The cut surface of the test sample punched by fine-blanking and the cross section of the blank during thepunching process are shown in Fig. 6(a) and (b), respective-ly.

The cut surface in Fig. 6(a) has a glossy shear plane acrossthe entire surface without any fracture planes from generatedcrack. Furthermore, as shown in Fig. 6(b), at the cross sectionof the blank during the punching process, fiber structuressimilar to the forge flow lines due to material flow in theforging process can be seen. From this, the shearing processin this fine-blanking process is assumed to be similar to thephenomenon in the forging process and thus can be analyzedwithout applying the ductile fracture criteria such as those ineqs. (1) to (3).

A remeshing technique is used in the analysis method; thisfunction regenerates elements and node movement for whenthe element deformation is large or when an elementpenetrates a rigid body after contacting it. The friction force

acting on the contact surface between the blank material andeach tool and between each tool was not considered in thisanalysis.

For the analysis conditions shown in Table 2, the relation-ship between the clearance and shear droop in the case ofconstant initial compression and the relationship between theshear droop and initial compression with no clearance wereanalyzed.

4. Experimental Results

4.1 Punching test results for irregularly shaped teethFor the experimental blanking shape punched in the

4000 kN hydraulic press, shear droop of the shear plane wasobserved in the �100mm arcs with clearances of 0.005mmand 0.02mm when cut with a cutter.

The cross section of each cutting was positioned oppositeeach other, and Fig. 7 shows an image of the cross sectiontaken with a VH-8000 KEYENCE microscope. For these cutsurfaces, shear droop can be seen more clearly for the 0.02-mm clearance than for the 0.005-mm clearance.

The relation between the clearance and shear droop foreach clearance on the blanking shape as measured with anAccretech Surfcom 1800D surface roughness and contourmeasuring device is shown in Fig. 8. The clearances were setto 0.005mm, 0.01mm and 0.02mm with the die block. Theseresults show that the shear droop increased as the clearancebecame larger.

From the graph in Fig. 8, the experimental blanking shapepunched with settings of Vp ¼ 400 kN and Cp ¼ 300 kN hadsmaller shear droop for both clearances than that punchedwith settings of Vp ¼ 200 kN and Cp ¼ 150 kN.

4.2 FEM simulation resultsThe FEM simulation results for changing the die-block

clearance from 0.0mm to 0.05mm at constant initial

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

Fig. 6 Fine-blanking products.

Table 2 Analysis conditions.

Combination

Displacement of

initial compression

[mm]

Clearance

[mm]

0.0,0.005

I 0.03 0.01,0.02,0.03

0.05,0.08,0.1

II 0.001,0.03,0.08 0.0

Analysis of Shear Droop on Cut Surface in Fine-Blanking Process 449

compression under analysis conditions I are shown in Fig. 9together with the test results from Fig. 8.

The FEM simulation results show that shear droopincreases with increasing clearance, similar to the results ofthe experimental blank punching results. By considering thatthe shear droop is caused by material deficiencies in theclearance parts during the punching process, then the flowin the material caused by Vp and Cp can be thought tocomplement this material deficiency.9)

The total reaction force of the blank holder (equivalent toVp) obtained from the analysis results for a constant initialcompression of 0.03mm was 130 kN, and the total reactionforce of the counter punch (equivalent to Cp) was 45 kN. Thisindicates that the analysis results for the shear droop arelarger than the punching test results because the values of Vp

and Cp in the analysis results are smaller than those in thetest, which makes the material deficiencies in the clearanceparts small.

Furthermore, the Fig. 10 shows the relation between thepunch displacement while punching the blank and shear

droop for the FEM simulation when changing the die-blockclearance from 0.0mm to 0.1mm at constant initial pressure.

For each clearance examined in the analysis, the sheardroop tends to increase during punching, and that rate ofincrease becomes larger as the clearance increases.10)

This finding can be attributed to the material deficienciesof the clearance parts increasing as the punch progresses;these material deficiencies are thought to increase sheardroop. Considering the change in shear droop from thepunching process, the shear droop must be evaluated whenthe punch has moved to the maximum analytical limit inorder to estimate the shear droop in the fine-blanking processby FEM simulation.

Next, Fig. 11 shows the relation between the initialcompression at constant clearance and shear droop foranalysis conditions II for V-ring heights of 0.0mm, 0.35mmand 0.7mm. These analyses are all calculated for the process

Fig. 7 Cross section of cut surfaces with shear droop.

5

6

7

8

9

10

0 0.01 0.02

Clearance /mm

Dep

th o

f sh

ear

droo

p /%

V200×C150

V400×C300

Depth of shear droop

Fig. 8 Relation between clearance and measured depth of shear droop.

0

2

4

6

8

10

12

14

0 0.02 0.04 0.06

Clearance /mm

Dep

th o

f sh

ear

droo

p /%

FEM Analysis (V130×C45)

Experiment (V200×C150)

Experiment (V400×C300)

Depth of shear droop

Fig. 9 Relation between clearance and depth of shear droop.

0

2

4

6

8

10

12

14

16

18

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Punch position /mm

Dep

th o

f sh

ear

droo

p /%

0.00[mm] 0.005[mm]0.01[mm] 0.02[mm]0.03[mm] 0.05[mm]0.08[mm] 0.10[mm]

Clearance

Depth of shear droop

Fig. 10 Relation between punch position and depth of shear droop.

450 T. Tanaka et al.

until the high-tensile-strength steel is on the verge offracturing; the values shown are taken from the shear droopfor shear surfaces from coordinate values of the results. Foreach initial compression, the shear droop is suppressed as theV-ring height increases. Furthermore, the shear droop issuppressed as the initial compression increases for eachV-ring height.

In the case of the disk model, when the initial compressionis 0.08mm, the hydrostatic stress distribution obtained fromthe FEM simulation when the V-ring was absent or when theV-ring height was 0.7mm is shown in Fig. 12(a) and (b),respectively.

Without the V-ring, tensile stress is generated in the sheardroop, while equipping the blank holder and die with the V-ring compressive stress around the shear droop. From theseresults, the provision of a V-ring controls the flow in theblank material when it undergoes plastic deformation;through the effective compressive stress, the hydrostaticpressure generated by the initial compression is thought tomake the shear droop smaller.

5. Conclusions

For shear droop of high-tensile-strength steel in the fine-blanking process, the following results were obtained fromtest results and analysis results using FEM simulation,considering the initial compression and the clearancebetween the die and punch.

(1) The shear droop on products punched by the fine-blanking process was confirmed to become greater as thedie-punch clearance increases.

(2) As Vp and Cp become large in the fine-blankingprocess, the shear droop of punched products decreases.

(3) In the FEM simulation of the fine-blanking process,shear droop close to the experimental values was calculatedby implementing a remeshing function that regenerateselements.

(4) From the FEM simulation results, it was found that theshear droop of the punched product is reduced as the V-ringheight of the die or blank holder increases.

(5) To predict the shear droop of punched products in thefine-blanking process by FEM simulation, the shear droopmust be predicted when the punch is moved to the maximumtheoretical range.

Acknowledgement

This research was supported in part by the 2007-8 StrategicBasic Technology Advancement Support Program.

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0

5

10

15

20

25

0 0.02 0.04 0.06 0.08

Punch displacement of initial compression /mm

Dep

th o

f sh

ear

droo

p /%

0.00[mm]0.35[mm]0.70[mm]

V-ring height

Depth of shear droop

Fig. 11 Relation between initial compression and depth of shear droop.

(a) V-ring height 0.0[mm] (b) V-ring height 0.7[mm]

Fig. 12 Distributions of hydrostatic stress.

Analysis of Shear Droop on Cut Surface in Fine-Blanking Process 451