Edge Cracking in High Strength Steels

R . G . D A V I E S

A study has been made of the sheared edge ductility of a series of cold-roll gage high strength steels. It is found that inclusion shape control is beneficial, especially for steels with tensile strengths ranging from 345 to 550 MPa (50 to 80 ksi). Edge ductility is increased for all the materials when the quality of the edge is improved; this involves removing the heavily cold-worked sheared region either by machining, trimming (repunching), or a heat treatment which results in the recrystallization of the edge. Following a heat treatment, the steels with no inclusion shape control have hole expansion values similar to those observed in materials with inclusion shape control before heat treatment. It is suggested that for the steels with the stringer type inclusions, low edge ductility will be exhibited when fracture initiates in the deformed shear edge region and propagates along the inclusion-ferrite interface. The elimination of this cold-worked region makes crack initiation more difficult and, thus, there is greater ductility. A tem- pering study of dual-phase steels suggests that the hard martensite islands play a similar role to the stringer type inclusions in reducing sheared edge elongation. It was also observed that the load to punch out a disc is proportional to both the thickness and the tensile strength of the material.

INTRODUCTION

In many forming operations, one of the steps involves imposing a tensile strain on an edge that has been cold sheared; for example, a punched hole may be flared out so as to make a bushing retention seat. The limitation on the amount of expansion is fracture (cracking) of the edge of the hole; sheared edge cracking is often the controlling parame- ter in the stamping of many components. This cracking is often associated with elongated nonmetallic inclusions, such as manganese sulfides, so that the addition of elements such as Ti, Zr, and the rare earth metals, which produce globular sulfides, leads to isotropic fracture properties ~'2 and im- proved edge cracking res i s tance) : It was also observed 3 that the higher the strength of the steel the lower was the hole expansion; the higher the strength of the steel the lower is its tensile ductility. Data for the newer higher ductility (at a

R.G. DAVIES is with the Engineering and Research Staff, Research, Ford Motor Company, Dearborn, MI 48121.

given tensile strength level) dual-phase steels are con- flicting; Bucher and Hamburg 4 found that an air-quenched dual-phase steel had a lower percent hole expansion than a conventional HSS (high strength steel) of the same tensile strength, while Waddington, et al, 5 observed that an as-

hot-rolled dual-phase steel was superior to the conven- tional HSS.

All of the previous studies of hole expansion have been on hot-rolled gage steels while the majority of automotive stampings are made from cold-rolled gage steel with thick-

ness of less than 1.8 mm (0.074 inch). Since the act of cold reducing the steels may break up the elongated manganese sulfides, inclusion shape control by alloying additions may be less effective in the lighter gage steels. Thus, one of the aims of the present study was to clarify the role of inclusion shape control on edge cracking for a series of commercially available cold-rolled gage HSS; the tensile strengths of these materials ranged from about 300 to 700 MPa (42 to 100 ksi). In addition, the influence of prior cold-work, with the atten-

ISSN 0162-9700/83/0106-00293500.75/0 VOL. 2, NO. 4, JANUARY 1983 293 J. APPLIED METALWORKING �9 1983 AMERICAN SOCIETY FOR METALS

dant reduction in tensile elongation, and quality of the edge upon hole expansion have been investigated.

EXPERIMENTAL PROCEDURE AND MATERIALS

The compositions of the steels studied are given in Table I and their tensile properties in Table II; all the materials were approximately 1.25 mm (0.050 inch) thick. Steels number 2 to 6 in Table I are conventional HSS that are strengthened by fine grain size and the precipitation of nio- bium or titanium carbonitrides; titanium also acts to give inclusion shape control. Steels 6 and 7 are dual-phase steels

which are strengthened by the presence of 15 to 20 pet martensite in the structure; number 6 was water-quenched and number 7 air-quenched from the intercritical annealing temperature. Inclusion shape control in these latter two alloys was by the addition of rare earth metals.

A die set, mounted in an Instron tensile testing machine, was used to punch the holes; the die d i ame te r was 12.70 mm (0.500 inch) and the punch sizes varied from 12.19 to 12.67 mm (0.480 to 0.499 inch). For the very duc- tile materials, a die diameter of 9.52 mm (0.375 inch) with a 9.40 mm (0.370 inch) punch was used. A preliminary

experiment revealed that there was no discernible difference

Table I. Compositions, in Wt Pct, of Materials (All Aluminum Killed)

No. Material C Mn Si V Nb Ti Other

1. AKDQ 0.08 0.31 0.03 2. SAE 940X 0.07 0.22 0.04 - - 0.02 - - 3. SAE950X-1 0.08 0.35 0.06 - - 0.04 - - 4. SAE950X-2* 0.07 0.40 0.02 0.16 - - 5. SAE 980X* 0.06 0.45 0.20 - - 6. DP 90-1' 0.07 0.71 0.38 0.06 P, R.E.* 7. DP90-2* 0.11 1.54 0.53 0.02 R.E.*

*Rare Earth Metals *Inclusion Shape Controlled

Table II. Mechanical Properties of As-Received Materials

0.2 Pet Yield Tensile Uniform Total Strength Strength Elongation Elongation

Material MPa (ksi) MPa (ksi) Pet Pct

AKDQ 137 (19.8) 291 (42.2) 34.9 45.7 SAE 940X 283 (41.0) 412 (59.7) 22.6 31.0 SAE 950X-1 374 (54.2) 495 (71.8) 19.6 25.2 SAE 950X-2 354 (51.3) 465 (67.5) 19.8 25.0 SAE 980X 614 (89.1) 682 (98.9) 10.4 13.9 DP 90-1 402 (58.3) 612 (88.8) 18.8 24.9 DP 90-2 340 (49.3) 680 (98.7) 22.8 27.3

in hole expansion between the two sizes of holes. However, the larger the hole the greater is the accuracy of mea- surement. In addition, the die-set could be located on a compression load cell so that it was possible to measure the load required to punch the hole. The holes were expanded in an Olsen ball machine with a 34.92 mm (1.375 inch) diameter ball; the burr side of the hole was away from the ball and the test was terminated at the first sign of fracture. For consistency, all the tests were done by one operator.

Hole expansion is defined as (dF-dl)/dt where dF is the diameter at fracture and d/the initial diameter. The values of the hole expansion presented throughout this paper are the average of at least five determinations.

RESULTS AND DISCUSSION

Punch Load

With the die-set on the compression load cell, the maximum load required to punch out a disc was measured, as a func- tion of thickness, for most of the HSS listed in Table I as well as for a 2024 T3 aluminum alloy and 303 stainless steel. The punch diameter was 12.45 mm (0.490 inch). The different thicknesses were obtained by carefully grinding, from both sides, the original material. It can be seen in Figure 1 that, over the very limited thickness range in- vestigated, the load is proportional to the thickness, and that

0 0 5 0 0 4 ms 0 0 5 0 0 6

i i i ,

DP 9 0 - 6 0 0 0

980

24

' o ~ ' , 'o ' , '~ ' ,'~ T H I C K N E S S , mrn

Fig. 1 - -Punch load as a function of material thickness.

294 VOL. 2, NO. 4, JANUARY 1983 J. APPLIED METALWORKING

the slope of the load v s thickness line increases with in-

creasing strength of the material. The variation of punch load with the tensile strength of the

materials at a constant thickness of 1.2 mm (0.048 inch) is shown in Figure 2. The data are plotted against tensile strength instead of yield strength since the action of punch- ing out a disc involves a fracture process; the fracture stress is more closely related to the tensile strength than any other common measure of the mechanical properties of thin sheet material. It can be seen in Figure 2 that the load is a linear function of the tensile strength for the conventional HSS and the aluminum alloy. However, the loads for the dual-phase and stainless steels are above this linear line; this is possibly a reflection of the high strain hardening rates found in these types of materials. The higher the strain hardening rate the more work has to be done in the shearing portion of the hole

punching operation.

Influence of Punch Diameter

One of the first investigations was concerned with the influ- ence of punch size on the subsequent hole expansion. I t was considered possible that changes in the gap between punch and die would alter the proportion of material sheared to fractured in the punch operation, and thus lead to differences in the hole expansion behavior. All the steels were tested

ksi 4 0 60 80 I 00

28

24

Z v

6 2 O 0 J

16

12

I I I

950-1 0

Y 0

AKDQ

I I I

5 0 0 4 0 0 500

I I I

303 S S z~

/ /

DP-90-2 o

/ DP~ 90-1 .

2/ o

I I

600 700

6000

5 0 0 0

m

4 0 0 0

3 0 0 0

TENSILE STRENGTH, M Po

Fig. 2-- Punch load as a function of tensile strength of the materials at a constant thickness of 1.2 mm (0.048 in.).

over a range of punch diameters from 12.45 to 12.65 mm (0.490 to 0.498 inch) and, as shown in Figure 3, the hole expansion is independent of punch diameter. For some of the materials, including the dual-phase steels, the punch range was increased to 12.25 mm (0.480 inch) which is a 4 pct clearance between punch and die; even with this expanded range of punch diameters no consistent variation in hole expansion was observed. Metallographic examination of the edge of the punched hole did not show any change in the ratio of sheared to fractured surface with change in punch diameter. Because of this lack of influence of punch diameter on subsequent hole expansion, most of the other experi- ments were conducted with a 12.55 m m (0.494 inch)

punch.

Inclusion Shape Control and Edge Quality

The hole expansion as a function of tensile strength for the steels with both punched and reamed holes is shown in Figure 4; the reamed holes, which were prepared by punch- ing a 12.25 m m (0.480 inch) hole and then reaming to 12.70 m m (0.500 inch), will be of higher quality since the edge will have less damage associated with it. This figure shows that the data points fall into two groups, an upper set for the alloys with inclusion shape control and for all the steels with reamed holes, and a lower set for the steels without inclusion shape control and for the high alloy dual phase steel (DP 90-2). The line representing the data from Reference 4 is for hot-rolled HSS with inclusion shape con- trol; without the inclusion shape control the hole expansion

ins.

0 . 4 9 0 0 . 4 9 4 0 . 4 9 8

140

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• 80 U..I

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0 <> <> 0 -<> D P - 9 0 - 2

| I I I

12.4 12.5 12.6 12.7

PUNCH D I A . , m m

Fig. 3 - - Hole expansion as a function of punch diameter for some of the materials investigated.

J. APPLIED METALWORKING VOL. 2, NO. 4, JANUARY 1983 295

180

160

140

~120 (/') z

x w

w 8 0 .d 0 -r-

60

4 0

20

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950- I ~ �9

9 5 0 - 2 {~ [] [] DP- 90-1 $ �9

\ ~ DP-90-2 I~ ia o

I I I I I

5 0 0 4 0 0 5 0 0 6 0 0 700

TENSILE S T R E N G T H , MPo

Fig. 4- - Hole expansion as a function of tensile strength for both punched and reamed holes. Lower curve is for steels without inclusion shape control.

was approximately 20 pct at a tensile strength of 620 MPa (90 ksi). Thus, at comparable tensile strengths there appears to be little difference in the edge cracking resistance be- tween hot- and cold-rolled gage HSS.

It can be seen in Figure 4 that the higher quality edge, i .e. , for the reamed holes, has a large affect on the expan- sion of the noninclusion shape-controlled materials, and only a minor influence in the steels with inclusion shape control. With a better quality edge there is essentially no difference in the hole expansion behavior of the steels with and without inclusion shape control. This indicates that it is not just the presence of stringer type inclusions that leads to the low hole expansion in the nontreated steels. The low hole expansion must be a result of the interaction between the damage put in by the punch and the stringer inclusions; it is possible that the damage at the edge results in delami- nation at the inclusion-metal interface and this incipient crack then propagates at low strains when the edge is stretched.

The increase in hole expansion due to inclusion shape control or better quality edge, that is, the difference between the lines in Figure 4 is, as shown in Figure 5, a function of the tensile strength of the steel; the greatest increase is found for steels whose tensile strength is approximately 400 MPa (58 ksi). Thus, SAE 940 to 960 grade steels,

~1OO z O

80

x w 60 z

4 o

W

2 o z

ksl 40 80

I I I

120 I I

/

I /

\

i I I I

200 4 0 0 600 I000 TENSILE STRENGTH, M Po

\ I

8 0 0

Fig. 5 - - Inc r ea se in hole expansion (difference between the 2 solid lines in Fig. 4) due to inclusion shape control or reamed holes as a function of tensile strength.

whose tensile strengths vary from 345 to 550 MPa (50 to 80 ksi), will benefit the most from inclusion shape control. A possible explanation for this observation is that at the lowest strength levels, where the materials are very ductile, any cracks produced by the punch are blunted upon sub- sequent expansion. For the highest strength steels, which have such limited ductility, any flaws introduced by the punch operation are relatively unimportant.

Edge Deformat ion

One method of obtaining a measure of the amount of dam- age put in by the punch operation is to form a series of holes and then to machine away various amounts from the edge of the hole. In the present study this was accomplished by punching a series of holes with diameters from 12.25 to 12.65 mm (0.480 to 0.498 inch) and then reaming them all to 12.70 mm (0.500 inch). The SAE 950-1 steel was chosen for this experiment since it shows a large difference in the hole expansion between the as-punched and reamed holes (Figure 4). The results of this progressive removal of mate- rial on the subsequent hole expansion are shown in Figure 6 along with the results from a repunching operation which is described later in this section; it can be seen that it is necessary to remove approximately 0.2 mm of material (0.4 mm off the diameter) before the value of the hole expansion approaches that for a fully reamed hole.

In another series of exper iments , holes 12.25 mm (0.480 inch) were punched into the SAE 950-1 sheet and then a series of larger sized punches were used to remove an annulus of material. The results of this repunching or trimming operation are also shown in Figure 6; it can be seen that the increase in hole expansion is significant but con- siderably less than that produced by reaming. Thus, the repunching must remove some of the damage produced by

296 VOL. 2, NO. 4, JANUARY 1983 J. APPLIED METALWORKING

0.10 ins. 0.20

120

I10

- I 0 0 z o z 90 x " ' 80 W ..J o -r 70

60

50

I I

SAE 950-1

o > t t f o ~ REAMED

y ~ ~ ~ 2 , ~ z ~ / ~ "~/z~ a ' ~ R E PUNCH ED

/

4C i i i i I l I 0 0.1 0.2 0.5 0.4 0.5 0.6 0.7

REMOVED FROM DIAMETER, mm Fig. 6--Change in hole expansion for the SAE 950-1 steel with the removal of material from the edge both by machining and repunching.

the initial punch, but it must also introduce some lesser amount of damage. The ring of material cut out by the trim punch had a different set of constraints upon it during its cutting than did the solid slug during the initial punching. These changes in constraints could lead to variations in edge damage which would be reflected in differences in hole

expansion behavior.

H e a t - T r e a t e d E d g e s

The expansion of an edge containing no cold work was studied by annealing the steels in a salt pot after the holes had been punched. It was found, as shown in Table III, that for the SAE 940 steel, one minute at 650 ~ was sufficient to eliminate the damage. Thus, all the conventional steels were annealed for five minutes at 650 ~ tensile specimens annealed at the same time as the sheets containing the holes showed a variation in tensile strength of less than 14 MPa (2 ksi) from the original values. The damage around the holes in the dual phase steels was removed by intercritically annealing at 760 ~ for five minutes followed by cooling at an appropriate rate. After annealing, a new hole was punch- ed into the steel coupon so as to ascertain by the subsequent expansion that there had been no gross changes in the sheet

material properties. Hardness measurements were also used to study the influ-

ence of annealing on the sheared edges. Microhardness traverses up to the sheared edge for both the SAE 940 and DP 90-2 materials in the as-punched and the annealed

Table III. Influence of Time at 650 *C on the Hole Expansion of the SAE 940 Steel

Time, Minutes Hole Expansion, Pct

as-punched 66 0.5 106 1 140 5 142

conditions are shown in Figure 7. It can be seen that the hardness increase from the punching action extends ap- proximately 0.3 mm (0.072 inch) back from the sheared edge and that for both of the steels there is a large increase in hardness close to the edge. From a series of experiments with sheets of these two materials rolled various amounts, it was determined that the near-sheared edge hardness of the SAE 940 corresponded to a tensile strength of about 738 MPa (107 ksi) while for the DP 90-2 the tensile strength was about 1070 MPa (155 ksi). For both materials the uniform elongation was less than 1 pct and the total elongation less than 2 pct. Thus, the material at the edges of a sheared hole is, as a result of plastic deformation, very strong and of limited ductility. After the heat treatment the SAE 940 showed a decrease in hardness close to the edge (Figure 7(a)) due to deformation induced grain growth and over-aging of the carbide particles that provide some of the strength. For the DP 90-2 steel, which is strengthened by the presence of martensite islands, the hardness after the heat treatment is constant up to the edge of the hole

(Figure 7(b)). The hole expansion data as a function of tensile strength,

for both the heat treated and freshly punched holes, are shown in Figure 8; the solid lines, which represent the data for the noninclusion shape-controlled materials and the reamed/inclusion shape-controlled steels, are taken from Figure 4. It can be seen in Figure 8 that the expansion of the newly punched holes is in very good agreement with the previous data which confirms that the heat treatment has not changed the bulk properties of the materials. In addition, the removal, by heat treatment, of the cold work or damage has resulted in an increase in the percentage hole expansion for all the materials. The increases are the most pronounced for the noninclusion shaped controlled materials and the dual- phase steels. After the heat-treatment the dual-phase steels exhibit greater hole expansion ductility than the con- ventional SAE 980X steel; the hole expansion for the DP 90-2 alloy has increased by a factor of three.

For the SAE 940, 950-1, and DP 90-2 steels, coupons containing machined and reamed holes were heat-treated as described above. The expansion of these good quality holes was essentially the same as for the reamed holes without heat treatment. This reinforces the idea that it is the removal

J. APPLIED METALWORKING VOL. 2, NO. 4, JANUARY 1983 297

3 0 0

250

Z ft. E3

- 2 0 0 03 03 W Z t~ n~ <~ I 150

0.005 i i i J I i J ,

0

as punched

0 0

\ o o -..<

z x ~

t =

,,,,,,~,, " /heat treated

/

INS. 0.010 0.015

' ' ' t ' ' ' ' I

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0

z x 1 ' zx

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0.1 0.2 0.3 0.4 DISTANCE FROM EDGE, mm

(a)

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I

2OO

0 INS.

0.005 0.010 0015 I ' ' ' ' I ' ~ I

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0 0

\ o \ 0 0

N ~ s punched

0

z x

heat treated

t I , t , I , I 0 -0.1 0.2 0.3 0.4

DISTANCE FROM EDGE,mm. (b)

Fig. 7--Microhardness traverses up to the edge of the holes for both the as-punched and annealed conditions of the (a) SAE 940 and (b) DP 90-2 steels.

of the cold-worked rim around the hole, by heat-treatment or machining, that leads to the improved edge stretching.

Tempering of Dual-Phase Steels

Dual-phase steels consist of a mixture of approximately 18 pct high carbon martensite in a ferrite matrix. Heating

2 0 0

180

~ 1 6 0 W

~ 1 4 0

z120 o

~100

x e o U.i

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4O

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Alloy S y m b o l s a s Fig. 4

Plain Symbols - Heat Treated

b o l s l i t h T i c k Marks-As Punched

0,,<.

I I I I I

3 0 0 4 0 0 5 0 0 600 7 0 0 TENSILE STRENGTH,MPa

Fig. 8 - -Ho le expansion data as a function of tensile strength for the annealed and freshly punched holes; the lines are taken from Fig. 4.

these steels up to temperatures of about 600 ~ results in some reduction in tensile strength 6 and a diminished sus- ceptibility to hydrogen embrittlement. 7 Since such changes in properties are mainly a result of the tempering of the martensite, it can be anticipated that there will be changes in hole expansion after tempering.

The influence of tempering for five minutes at various temperatures up to 550 ~ on both of the dual-phase steels has been investigated; the initial starting condition was quenched into brine from 760 ~ for DP 90-1 and forced air cooled at approximately 30 ~ per second from 760 ~ for DP 90-2. The holes were punched into the coupons following the tempering treatment. Figure 9 shows that the tempering leads to a two-fold increase in the hole expansion for both the alloys, although there is a considerable differ- ence in the tempering temperature for maximum hole ex- pansion between the steels. This difference in temperature could be due to a combination of effects such as quenching stresses in the DP 90-1 and higher carbon content in the martensite and auto tempering 6 during the air cooling of the DP 90-2. However, the maximum hole expansion after tem- pering is considerably less than if the damage to the punched edge has been removed by heat treatment. The differences in the hole expansion data for the as-received materials (Figure 4) is a consequence of the heat-treatments the steels

298 VOL. 2, NO. 4, JANUARY 1983 J. APPLIED METALWORKING

o~ z" 0 0,) Z

x h i

LLI --I 0 "1-

60

50

40

30

~ A

-I / ~ f ~

,, / DP-90-2

/ o

| I I I I I

0 200 TEMPER!NG

400 600 o TEMPERATURE, C

Fig. 9 - -Hole expansion as a function of tempering temperature for the 2 dual-phase steels; DP 90-1 was initially water quenched from 760 ~ while DP 90-2 was forced air cooled from the same temperature.

receive at the mill; the DP 90-2 is shipped in the as-air- cooled condition while the DP 90-1 is tempered at around 250 ~ following the water-quench.

The dilemma noted in the introduction, where as-rolled dual-phase steels exhibit greater hole expansion ductility than air-cooled dual-phase steels, can now be understood in terms of tempering response. The as-rolled steels will, dur- ing their manufacture, be held at an elevated temperature for a considerable time and, therefore, the martensite in these steels will be tempered. As noted above, the more temper- ing of the martensite the greater is the hole expansion for dual-phase steels.

SUMMARY AND CONCLUSIONS

The edge stretching ability of cold-roll gage high strength steels is dependent upon both the quality of the steel (inclu- sion shape control) and the quality, or amount of damage, in the edge. Steels with inclusion shape control, i .e. , globular

inclusions, have significantly better edge ductility than the materials whose inclusions are in the form of stringers. The SAE 940 to 960 steels will benefit the most from inclusion shape control.

A correlation of hardness and the mechanical properties of heavily cold-rolled steels indicates that the as-sheared edges have very high yield and tensile strengths and very low ductilities. The removal of the cold-worked region around the sheared edge either by machining or heat treat- ment results in dramatic increases in the pct hole expansion especially for the noninclusion shape controlled and the dual-phase steels. This suggests that it is not the presence of the stringer inclusions p e r se that is extremely detrimental to the edge ductility, but it is the interaction of these inclu- sions with the deformed region that leads to the low hole expansion results. It is probable that the fracture initiates in the low ductility cold-worked region and then propagates easily along the interface of the stringer inclusions. If there are no easy crack paths, then the small initial cracks will be blunted and there will be enhanced ductility. Similarly, if there is no low ductility deformed edge to initiate a crack, there will be increased ductility. The response of the hole expansion of the dual-phase steels to both edge quality and tempering indicates that the martensite islands in these steels act in an analogous manner to the stringer type inclusions.

ACKNOWLEDGMENTS

The author is grateful to W. S. Stewart for technical assis- tance in all phases of this study, to R.S. Terlecki for the design of the punch set-up, and to his colleague, P.H. Thornton, for critically reviewing the manuscript.

REFERENCES

1. L. Luyckx, J. R. Bell, A. McLean, and M. Korchynsky: Metall. Trans., 1970, vol. 1, pp. 3341-50.

2. E. J. Lichy, G.C. Duderstadt, and N.L. Samways: J. Metals, 1965, pp. 769-800.

3. B.N. Ferry and E.J. Paliwoda: SAE Preprint 74018, February 1974. 4. J. H. Bucher and E. G. Hamburg: SAE Preprint 770164, February 1977. 5. E. Waddington, R. M. Hobbs, and J. L. Duncan: J. Appl. Metalwork.,

1980, vol. 1, no. 2, pp. 35-47. 6. R. G. Davies: Proceedings of Symposium "Fundamentals of Dual-Phase

Steels," eds. R. A. Kot and B. L. Bramfitt, published by Met. Soc. of AIME, 1982, pp. 265-77.

7. R.G. Davies: Metall. Trans. A, 1981, vol. 12A, pp. 1667-72.

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