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Slight clearancepunchingofultra-highstrengthsteelsheetsusingpunch

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

K.Mori n ,Y.Abe,Y.Kidoma,P.Kadarno

Department of Mechanical Engineering,Toyohashi University of Technology,Toyohashi,Aichi 441-8580,Japan

a r t i c l e i n f o

Article history:

Received 7June 2012Received in revised form 13September 2012

Accepted 14September 2012Available online 5October 2012Keywords:Punching

Ultra-high strength steel sheet Small round edge punch Slight clearance High quality

Delayed fracture

a b s t r a c t

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 0.13mm 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 1000strike punching of an ultra-high strength steel sheet having a tensile strength of 1200MPa,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.

1.Introduction

The reduction in weight of automobiles is in great demand for improving the fuel consumption.For the reduction,the use of high strength steel sheets for body-in-white parts sharply increases.Since the rise in strength of the sheet is required to satisfy the safety standards for protecting passengers in automobiles,the application of ultra-high strength steel sheets having a tensile strength above 1GPa expands.Although parts made of ultra-high strength steels have superior mechanical properties,the stamping operation becomes dif?cult with the increase in strength of the sheet.The forming load and springback become very large,and the formability is considerably small.V¨ais¨anen et al.[1]have showed small bendability of ultra-high strength steel sheets due to high strength and small ductility.Mori et al.[2]have reduced the springback in bending of ultra-high strength steel sheets by bottoming using a servo press.In the bottoming process,the stress distribution becomes uniform in the thickness direction due to slight reduction in thickness.Mori et al.[3]have eliminated the springback in bending of ultra-high strength steel sheets by heating at 7001C.Mori et al.[4]have developed a warm spline forming process of ultra-high strength steel gear drums using the rapid resistance heating.In the warm stamping process,the ?ow stress of the sheet decreases and the formability increases.

Body-in-white parts are generally punched to make many holes for joining,paint removing,attachment,reduction in weight,etc.In punching of ultra-high strength steel sheets,tools tend to wear and chip due to large punching load,and thus the tool life is short.Worn and chipped tools bring about the deterioration in dimensional accuracy of the punched hole and in the quality of the sheared edge.Inoue et al.[5]and Eriksson and Olsson [6]have examined the effect of coating of tools on the occurrence of galling in a sliding test of ultra-high strength steel sheets.Sresomroeng et al.[7]have pre-vented the occurrence of galling in deep drawing of ultra-high strength steel sheets by the CrN coating of tools.

The quality of the sheared edge in punching of ultra-high strength steel sheets deteriorates.Since the onset of cracks in the shearing is early due to the small ductility,the rough fracture surface increases.The static strength of ultra-high strength steel sheets is very high,whereas the fatigue strength is not very high,particularly for punched ultra-high strength steel sheets.Although shaving is generally used to improve the quality of the sheared edge,it is not easy to shave ultra-high strength steel sheets having large strength.Mori et al.[8,9]have improved the stretch ?angeability of ultra-high strength steel sheets by smoothing the sheared edge with a conical punch.Matsuno et al.[10]have developed a tool for punching thick high strength steel sheets.Mackensen et al.[11]have examined the effect of shearing conditions on the blanking force of high strength steel sheets.Mori et al.[12,13]have improved the quality of the sheared edge in punching of ultra-high strength steel sheets by the resistance heating.In addition,the punching of ultra-high

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International Journal of Machine Tools &Manufacture

Corresponding author.Tel.:t81532446707;fax:t81532446690.E-mail address:mori@plast.me.tut.ac.jp (K.Mori).

International Journal of Machine Tools &Manufacture 65(2013)41–46

strength steel sheets has a risk of delayed fracture induced by tensile residual stress.Mori et al.[14]have prevented the occurrence of delayed fracture in punching of ultra-high strength steel sheets by partial resistance heating.

In the present study,ultra-high strength steel sheets were punched under a slight clearance with a punch having a small round edge to improve the quality of the sheared edge.The onset of cracks in the sheet during the punching was delayed by the small round edge of the punch and the slight clearance.2.Slight clearance punching of ultra-high strength steel sheets

To improve the quality of the sheared edge,ultra-high strength steel sheets were punched under a slight clearance between a punch and die.By the slight clearance,the tensile stress during the punching is reduced,and thus the onset of cracks is delayed for ultra-high strength steel sheets having small ductility.In the slight clearance punching,however,sharp edges of the punch and die tend to chip.The punch having a small round edge shown in Fig.1was developed to prevent the chipping of the tools,where R is the radius of the round edge measured from a laser displace-ment sensor,and R ?0mm is equivalent to the conventional punch having a shape edge.The small round edge has the functions of avoiding the contact between the punch and die in the slight clearance punching and of delaying the onset of cracks.In addition,the side surface of the punch has the curved relief.The radius of the round edge was changed in the present study.

The mechanical properties of the ultra-high strength steel sheets used for the punching are given in Table 1.The sheets are cold-rolled ones generally used for automobile parts,and are made of dual-phase steel.The mechanical properties of the sheets were measured from the tensile test.The specimens were cut in the 0,45and 901directions with respect to the rolling direction of the sheet,and the averages of the measured values are shown.The ultra-high strength steel sheets JSC980Y and JSC1180Y have the tensile strengths above 1GPa.The reductions in area for JSC1180Y are small,low ductility.

The conditions of the punching of the ultra-high strength steel sheets are given in Table 2.The sheets were punched by a 250kN screw driven type universal testing instrument.The ratio of clear-ance between the punch and die to the thickness was ?xed to 0.8%,and the clearance ratio was changed above 20%only for

examination of effect of clearance.The very slow punching speed of 0.03mm/s was chosen to avoid the chipping of the punch having a sharp edge for a clearance ratio of 0.8%between the punch and die.A realistic punching speed was employed for repeated punching operations in Chapter 5.The length and width of the punched sheet were 50and 40mm,respectively,and the centre of the sheet was punched.Each punching test was performed at least two times to prevent the scatter of results.

Finite element simulation of the punching process using the commercial software ABAQUS was performed under an assump-tion of axi-symmetric deformation.The die,sheet holder and punch were assumed to be rigid,and the cross-section of the sheet was divided into quadrilateral ring elements.To examine the concentration of deformation during the indentation of the punch,the calculation was performed until onset of crack without special treatment of occurrence of fracture.The conditions used for the simulation of ultra-high strength steel sheet JSC980Y are given in Table 3.

3.Result of single punching of ultra-high strength steel sheets The effect of the edge radius of the punch on the deformation behaviour of the sheet during the punching for JSC980Y is shown in Fig.2,where s is the punch stroke.For R ?0mm,the cracks are caused from both edges of the punch and die,whereas the crack is initiated only from the edge of the die for R ?0.13and 0.33mm.The concentration of deformation around the edge of the punch was relaxed by the small round edge,and thus the onset of the crack from the edge of the punch was prevented.

The deformation behaviour during the punching was observed from the ?nite element simulation using the commercial software ABAQUS in detail.The distribution of equivalent strain just before the onset of crack calculated by the ?nite element simulation for JSC980Y is illustrated in Fig.3.Although deformation concen-trates at both edges of the punch and die for R ?0mm,the concentration at the edge of the punch relaxes for R ?0.13and 0.33mm,and thus the crack is generated only from the edge of the die as shown in Fig.2.

The surface and cross-section of sheared edge for JSC980Y are illustrated in Fig.4.Since the linkage of the cracks initiated from both edges of the punch and die for R ?0mm is not smooth,the secondary burnished surface is caused,and the boundary with the

fracture

Fig.1.Punches having small round edge for punching of ultra-high strength steel sheets.(a)R ?;0mm,(b)R ?0.13mm and (c)R ?0.33mm.

Table 1

Mechanical properties of ultra-high strength steel sheets used for punching.Sheet

Thickness (mm)

Tensile strength (MPa)Elongation (%)

n -value

Reduction in area (%)

JSC780Y 1.4076920.00.1538.2JSC980Y 1.22105112.80.0937.4JSC1180Y 1.211215

8.20.1026.6

Table 2

Conditions of punching of ultra-high strength steel sheets.Die material SKD11Punch material

SKH51Punch diameter (mm)

10Ratio of clearance between punch and die to thickness,c (%)0.8Punching speed (mm/s)0.03Lubricant

Rust

prevention oil

Table 3

Conditions used for ?nite element simulation of ultra-high strength steel sheet JSC980Y.Flow stress (MPa)s ?1391e 0.09

Young’s modulus (MPa)210000Poisson’s ratio 0.3Yield stress (MPa)838Coef?cient of friction

0.05

K.Mori et al./International Journal of Machine Tools &Manufacture 65(2013)41–46

surface has a difference in level.For R ?0.13and 0.33mm,the smooth surface is obtained by propagating the crack only from the edge of the die,and the secondary burnished surface does not appear.

The percentages of the rollover,burnished and fracture depths and the burr height on the sheared edge for the different edge radii are given in Fig.5.For the small round edges of the punch,the burnished surfaces become considerably large,whereas the rollover and burr for R ?0.33mm increase.It was found that the very small radius of R ?0.13mm is effective for high quality of the sheared edge of the ultra-high strength steel sheets.

The punching loads for the different edge radii of the punch are illustrated in Fig.6.As the strength of the sheet increases,the punching load increases,whereas the loads for the three punches are similar due to the small edge radius.

The above-mentioned results are obtained for the slight clearance between the punch and die,c ?0.8%.The relationship between the percentage of rollover,burnished and fracture depths and burr height on the sheared edge and the clearance ratio for JSC980Y and R ?0.13mm is shown in Fig.7.As the clearance ratio increases,the burnished surface decreases,and the fracture surface and rollover increase.This is due to early onset of cracks induced by the increase in tensile stress around the edge of the die with the clearance as shown in Fig.8.For R ?0.13mm,the sheared edge of high quality is limited to the slight clearance.Since the ultra-high strength steel sheets have low ductility,both small round edge and slight clearance are indispensable,i.e.the relaxation of concentration of deformation around the punch edge and reduction in tensile stress around the die edge.

4.Mechanical properties of punched sheets 4.1.Delayed fracture

Punched ultra-high strength steel sheets have the risk of delayed fracture,particularly for JSC1180Y.The effect of the small round edge of the punch on the delayed fracture was examined by keeping the punched sheets in 35%concentration hydrochloric acid at room temperature.The occurrence of delayed fracture was accelerated with the high concentration of the acid.

The effect of the edge radius of the punch on the delayed fracture time for JSC1180Y is given in Fig.9,where the delayed fracture time is the time from the soak of the sheet in the acid.The occurrence of cracks was visually observed.Although the cracks were observed after 2.5h for c ?0.8%and R ?0mm,no cracks were observed up to 24h for R ?0.13and 0.33mm.On the other hand,the cracks early occur for the large clearance of c ?10%.

s =0.22 mm 0.52 mm 0.90 mm

s =0.23 mm 0.64 mm

s =0.15 mm

0.43 mm

0.75 mm

0.5 mm

Crack

Crack Sheet holder Punch

Die

1.26 mm

Fig.2.Effect of edge radius of punch on deformation behaviour of sheet during punching for JSC980Y.(a)R ?0mm,(b)R ?0.13mm and (c)R ?0.33mm.

Fig. 3.Distribution of equivalent strain just before onset of crack calculated by ?nite element simulation for JSC980Y.(a)R ?0mm,s ?0.13mm,(b)R ?0.13mm,s ?0.21mm and (c)R ?0.33mm,s ?0.24mm.

Rollover Burnished Fracture 0.5 mm

Fracture

Burr

Secondary burnished

Fracture Burr

Rollover

Burnished Rollover

Burnished

Fracture Burr

Fig. 4.Surface and cross-section of sheared edge for JSC980Y.(a)R ?0mm,(b)R ?0.13mm and (c)R ?0.33mm.

K.Mori et al./International Journal of Machine Tools &Manufacture 65(2013)41–4643

The effect of the edge radius of the punch on the residual stress in the thickness direction of the sheared edge measured by the X-ray diffraction for JSC1180Y is shown in Fig.10.For the slight clearance,the compressive residual stress is increased by the small round edge,and the delayed fracture was prevented as shown in Fig.8.On the other hand,the tensile residual stress becomes large for the large clearance.4.2.Limiting hole expansion

An expansion test of punched holes for the different edge radii of the punch was performed to examine the formability of the

punched sheet.The punched hole of the sheet was expanded with a conical punch having an angle of 601.The expansion was stopped for just driving of cracks through the thickness,and the occurrence of cracks was visually observed.The burr of the sheet was set without touch of the punch on the opposite side.

D e p t h p e r c e n t a g e [%]

0204060

80100-20

Rollover

Burnished

Fracture Fracture

Burr

Secondary

burnished D e p t h p e r c e n t a g e [%]

0204060

80100-20

Burr

Secondary burnished Rollover

Burnished Fracture

Fracture

D e p t h p e r c e n t a g e [%]

0204060

80100-20

Rollover

Burnished Fracture

Secondary burnished Fracture Burr

Fig.5.Percentages of rollover,burnished and fracture depths and burr height on sheared edge for different edge radii.(a)JSC780Y,(b)JSC980Y and (c)JSC1180Y.

P u n c h i n g l o a d [k N ]

5101520

253035J S C 780Y J S C 1180Y J S C 980Y Fig.6.Punching load for different edge radii of punch.

Clearance ratio c [%]

D e p t h p e r c e n t a g e [%]

020*********-20Rollover

Burnished

Fracture

Fig.7.Relationship between percentage of rollover,burnished and fracture depths and burr height

on sheared edge and clearance ratio for JSC980Y and R ?0.13mm.

[MPa]-300

-600

-3000

-300

-600-9000

-900

-300-600

-900Sheet holder Punch Die

Fig.8.Distribution of mean stress calculated by ?nite element simulation for JSC980Y,R ?0.13mm and s ?0.21mm.

D e l a y e d f r a c t u r e t i m e [h ]

Cracks

c =0.8%

Fig.9.Effect of edge radius of punch on delayed fracture time for JSC1180Y.

K.Mori et al./International Journal of Machine Tools &Manufacture 65(2013)41–46

The relationship between the limiting expansion ratio and the average Vickers hardness of the sheared edge for JSC980Y is given in Fig.11,where the limiting expansion ratio is the change in diameter of the hole divided by the diameter before the expan-sion and the hardness was measured in the cross-section at 0.05mm from the sheared edge.As the hardness increases,the limiting expansion ratio decreases,and the ratio is the highest for R ?0mm.The delay of the onset of the crack for the small round edges of the punch brings about increase in plastic deformation in the sheared edge as shown in Fig.3,and thus the limiting expansion ratios for the small round edges are reduced by the increase.The hardnesses for R ?0.13and 0.33mm are almost the same,and thus the limiting expansion ratios are similar.The punch having the small round edge is inappropriate to burring processes of punched holes for forming a ?ange.

5.Repeated punching of ultra-high strength steel sheets Since the punch having an edge radius of 0.13mm exhibits high performance in the single punching operation,this punch

was employed for a repeated punching operation of the ultra-high strength steel sheet JSC1180Y.The tungsten carbide punch (1150HV)was treated by the TiN (2200HV)and TiAlN (3500HV)coatings having 2m m in thickness.The clearance ratio was 1.0%and the number of strikes was n ?1000.A CNC servo press having a capacity of 800kN was employed,and the punching speed was 10spm.

The surface and cross-section of the sheared edge for the repeated punching of JSC1180Y are shown in Fig.12.The surface of the sheared edge for the punch without the coating becomes rough after n ?100,whereas the quality of the surface for the TiN and TiAlN-coated punches is high even after n ?1000.

The surface of the punch for the repeated punching of JSC1180Y is given in Fig.13.The edge of the punch for R ?0mm chipped from the early stage,and the chipping progressed.The chipping of the edge was prevented by the small round edge,and the wear of the round edges was not observed without change of the coating layer.Although the galling occurs around the punch edge after n ?100for the punch without the coating,slight and no pick-ups appear after n ?1000for the TiN and TiAlN-coated punches,respectively.It was found that the TiAlN-coated punch having a small round edge is useful for actual punching opera-tions of ultra-high strength steel sheets.

6.Conclusions

A slight clearance punching process using a punch having a small round edge was developed to improve the quality of the sheared edge for ultra-high strength steel sheets.The onset of

-1000

-800-600

-400-2000200

400R e s i d u a l s t r e s s [M P a ]

Fig.10.Effect of edge radius of punch on residual stress in sheared edge for JSC1180Y.

L i m i t i n g e x p a n s i o n r a t i o [%]

51015

2530Average Vickers hardness of sheared edge [HV0.05]

Fig.11.Relationship between limiting expansion ratio and average Vickers hardness of sheared edge for JSC980Y.

n =1000

n =1n =1000

n =1n =100

n =1

0.5 mm

Rollover

Burnished

Fracture

Burr

Rollover

Burnished

Burr

Rollover

Burnished

Burr

Fig.12.Surface and cross-section of sheared edge for repeated punching of JSC1180Y.(a)No coating,(b)TiN coating and (b)TiAlN coating.

K.Mori et al./International Journal of Machine Tools &Manufacture 65(2013)41–4645

cracks in the punching process was delayed by the small round edge of the punch.The following results were obtained:

1.The concentration of deformation around the edge of the punch was relaxed by the small round edge,the onset of a crack from the edge of the punch was prevented,and thus the burnished surfaces became considerably large.

2.A very small edge radius of 0.13mm was effective for high quality of the sheared edge of ultra-high strength steel sheets.

3.The high quality of the sheared edge for the edge radius of 0.13mm was limited to the slight clearance between the punch and die.

4.For the punched sheet with the punch having the small round edge,the delayed fracture was prevented and the fatigue strength was improved due to large compressive stress around the sheared edge.

5.The TiAlN-coated tungsten carbide punch having the small round edge had high galling resistance for the 1000strike punching.Acknowledgements

The authors acknowledge Dijet Industrial Co.,Ltd.and Bellso-nica Co.,Ltd.for providing the punches and sheets,respectively.

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1 mm

n =0

Pick-up

n =0

n =1000

n =0n =1000

0.1 mm

Chipping

Fig.13.Surface of punch for repeated punching of JSC1180Y.(a)R ?0.13mm,no coating,(b)R ?0.13mm,TiN coating,(c)R ?0.13mm,TiAlN coating and (d)R ?0mm,TiAlN coating,n ?600.

K.Mori et al./International Journal of Machine Tools &Manufacture 65(2013)41–46