Springback in High-Strength Steels R. G. D A V I E S A study has been made of springback after the forming of a simple flange in CRLC, SAE 950X and 980X, and a dual-phase steel," the variables investigated were material thickness, die gap, anvil radius and prior cold work. It was found that springback increased as the strength o f the steel, anvil radius and die gap increased and as the sheet thickness decreased. Dual-phase steel which has tensile strength similar to that of SAE 980X steel initially had springback characteristics similar to SA E 950X steel. However, after about 15 to 20 percent cold work, the dual-phase and the SAE 980X steels exhibit similar amounts of springback. It was also noted that for a given material springback is a unique function of the material-thickness-to-anvil-radius ratio. The results of this investigation can be used to predict the variation in springback expected from the spread in strength usually observed for a given grade of steel, and to allocate the amount of springback between material and die effects when making materials substitutions.

I. I N T R O D U C T I O N In order to increase the fuel efficiency of the automobile, while still maintaining acceptable size and cost, low carbon steels are being replaced by thinner-gage high-strength steels (HSS). One of the limitations in the application of HSS is on the ability to form these steels into the desired shapes; the higher the strength of a steel the lower is its ductility, i.e. formability. In addition to the intrinsic change in formability (draw and stretch) with strength, there is also the problem of springback of the component when it is removed from the stamping die. Springback is a complex phenomenon for, in addition to being dependent upon material properties such as strength, it is influenced both by die parameters (die gap and bend radii), and the shape and prior mechanical history of the component being formed. It is R. G. DAVIES, Ford Motor Co., Scientific Research Staff, Dearborn, MI 48121. J.

APPLIEDMETALWORKING

recognized that information developed from laboratory forming studies may not be directly applicable to components produced in stamping plants. However such information could act as a guide for component and die designers who are faced with problems such as substituting high strength for low carbon steels. Thus, the influence of material properties such as strength and thickness, and die parameters on the springback after a simple 90 degree flange operation has been investigated.

II. E X P E R I M E N T A L P R O C E D U R E The initial part of this investigation was concerned with the influence of material thickness and die variables on springback in four different types of steel. The compositions and initial thicknesses of the steels studied are given in Table I; these materials range from a cold-rolled low carbon (CRLC) steel to conventional SAE 950X and 980X steels, which are strengthened by ultrafine carbides and a dual-phase steel containing

ISSN 0162-9700/81/0105-0045500.75/0 9 1981AMERICANSOCIETYFOR METALS

VOLUME1, NUMBER4--45

/ / / / / / / / / / /

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Fig. l-- Engineering stress-strain curves for the four different steels investigated.

about 18 pct martensite. The stress:strain curves for the four steels are shown in Fig. 1; these curves were obtained using a standard sheet tensile sample with a 50 mm (2 in.) by 12.5 mm (0.5 in.) gage section. It can be seen that dual-phase steel does not show a yield plateau as do the other three steels. The dual-phase steel has a yield stress lower than the SAE 950X and a tensile strength comparable to the SAE 980X steel. In order to study the effect of specimen thickness on springback, sheets of the steels were ground in equal amounts from each side. Thus a series of samples were obtained with thicknesses ranging from 0.8 mm (0.031 in.) up to the original thickness, in steps of about 0.25 mm (0.010 in.). Grinding to produce thinner specimens has the advantage that all the different thicknesses of a given type of steel have the same strength; this was confirmed by tensile testing each thickness of all four materials. Following these initial experiments, a study was made of the influence of cold work on the springback of four steels with compositions similar to those given in Table I but with starting thicknesses of approximately 3.1 mm (0.125 in.). These steels were cold worked both by rolling and by tensile prestraining in the rolling direction of the sheet. Springback specimens were cut from the rolled sheet in both the longitudinal and transverse directions.

The apparatus for the flange simulation test is shown schematically in Fig. 2. The specimen, 25 mm (1 in.) wide by approximately 150 mm (6 in.) long and of thickness t, was fastened to the anvil A and then bent over the radius R to a full 90 deg by punch B. The forming variables studied were R, the anvil radius, which was varied from 12.5 mm (0.5 in.) to 3.1 mm (0.125 in.), and the gap between punch and anvil, d, which was varied up to about 2.5 times material thickness. This apparatus was set up in an Instron testing machine with the punch attached to the cross head and the anvil fastened to the compression base plate; gap d was changed by shimming the anvil A. A punch radius of 12.5 mm (0.50 in.) was used for all the tests; a preliminary study showed that varying the punch radius between 1.6 mm (0.063 in.) and 12.5 mm (0.50 in.) had no effect on the observed springback. The final angular position of the two arms of the specimens after upioading was measured on a shadowgraph machine with an accuracy of about _ 1/4 deg.

III. RESULTS AND DISCUSSION a) Material Thickness and Die Variations A large amount of data was obtained in this study of springback since it involved four steels with seven different thicknesses of each steel, all of which were bent over three anvil radii with four different die gaps. The data were analyzed by first plotting springback as a function of the d / t (die-gap-to-material-thickness) ratio; Figs. 3 and 4 show the springback as a function of for the three different radii, R, of the anvil at the two extremes of material thicknesses studied. A hotrolled low carbon steel (HRLC) was used for comparison at the 2.37 mm (0.095 in.) thickness since a CRLC steel for this thickness was not available; this H R L C steel was approximately 10 pct stronger than the CRLC steel. There are several general points that should be noted from this data: 1)The relative ranking of the materials is independent of anvil radius, die gap or material thickness; the springback is least in the low carbon d/t

Table I. Composition (Wt Pct, Balance Iron) and Initial Thickness of Alloys Studied Type of Steel CRLC SAE 950X SAE 980X Dual-phase

C

Mn

0.08 0.10 0.06 0.11

0.31 0.54 1.25 1.54

46--VOLUME 1, NUMBER4

Si

V

Ti

Nb

Thickness mm (in.)

0.05 -- 1.63 (0.064) --- 0.12 -- 2.36(0.93) -0.10 2.41 (0.095) 0.53 0.07 --- 2.39 (0.094)

J. APPLIED METALWORKING

steels and greatest in the SAE 980X steel. The dualphase steel shows slightly more springback than the SAE 950X steel but considerably less than the Sae 980X steel. 2) For a constant material thickness and die gap, the larger the anvil radius the greater the amount of springback. 3) For a constant die gap and anvil radius, the thinner the material the greater the springback, and 4) The smaller the die gap the smaller the springback. These general findings are in agreement with the results of more limited studies of springback in both ferrous and non-ferrous alloys? .2 The influence of material thickness and anvil radius on springback is more clearly seen in Fig. 5 where springback at an arbitrarily chosen value of d / t = 1.5 is plotted against material thickness. It can be seen that for all the steels, the thinner the material and the larger the anvil radius the greater the springback. It was found that when the springback at a given d / t ratio was plotted against t / R , the ratio of material thickness to anvil radius, a single line could be put through the data points for each material. This is illustrated in Fig. 6 where the data from Fig. 5 has been

replotted; it can be seen that the maximum scatter is about 1 deg. A comparison of the springback for all four steels as a function of t / R is shown in Fig. 7; springback is now taken at d / t -- 1.05, that is, a 5 pct clearance between die and material, which is much closer to normal die practice. One of the first points to notice is that the dual-phase and SAE 950X have very similar springback characteristics at all t / R ratios. In addition it can be seen that the ratio of the amount of springback between the different steels is a function of the t / R ratio; for example, at t / R = 0.5 SAE 980X has three times the springback of CRLC, while at t / R = O. 1 the SAE 980X exhibits just over twice the springback of the CRLC steel. b) Influence of Prior Cold Work

For the specimens cut from.the longitudinal directiom the change in yield stress with percent reduction by rolling is shown in Fig. 8; transverse cut samples showed a similar variation. The main point to note is that while initially the dual-phase steel is weaker than

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Fig. 3--Springback as a function of die-gap-to-material-thickness ratio for the four steels and three anvil radii at a material thickness of 2.37 mm (0.095 in.). J. APPLIED METALWORKING

VOLUME 1, NUMBER 4--47

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the SAE 950X steel, after 15 pct reduction in thickness it is stronger than the SAE 980X steel. Springback was measured for values of R of 12.5 mm (0.5 in.) and 6.25 mm (0.25 in.) as a function of die gap for each degree of reduction, and from curves similar to Figs. 3 and 4, the springback at d / t = 1.05 was obtained. It was recognized that these values of springback were taken at slightly different t / R ratios due to the different reductions in thickness. Thus, to better compare the influence of deformation upon springback, the values obtained experimentally were corrected, by means of Fig. 7, to t / R = 0.25 or 0.50. The maximum springback correction was less than 0.5 deg. Figures 9 and 10 show the corrected springback values as a function of the percentage rolling reduction for longitudinal specimens of all four steels at t / R = 0.25 and 0.50 respectively. It can be seen that the increase in springback with percent reduction is very similar for the CRLC, SAE 950X and SAE 980X steels. 4 8 - - V O L U M E 1, NUMBER 4

However, the springback of the dual-phase steel increases rapidly with increasing percent reductions; at 0 pct reduction the dual-phase steel is similar to the SAE 950X steel while at the highest reductions the dualphase steel exhibits more springback than the SAE 980X steel. c) Influence

of Strength

Springback is often correlated with the yield stress of steels. L: As shown in Fig. 11, which is for the steels in the as-received condition, the higher the flow stress the greater is the springback. One difficulty encountered was how to define a flow stress especially as the dual-phase steel has a smooth, rounded stress-strain curve and the other three steels have discontinuous yield plateaus (Fig. 1). If the 0.2 pct offset flow stress were used for the dual-phase steel then there would be an inconsistency in the results; the 0.2 pct flow stress for J. A P P L I E D METALWORKING

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RATI 0 DIE RADIUS Fig. 6--Springback at a die-gap-to-material-thickness ratio of 1.5 as a function of the ratio of material thickness to anyil (die) radii for the four steels investigated; the three solid points in Fig. (a) are data for the hot-rolled low carbon steel. These curves were plotted from the data of Fig. 5.

J. A P P L I E D METALWORKING

VOLUME 1, NUMBER 4--49

the dual-phase steel is about 138 MPa (20 ksi) less than the yield stress for the SAE 950X steel although the dual-phase steel exhibits slightly more springback than the SAE 950X steel. It was found empirically that there was good agreement between strength and springback for all the steels (Fig. I l) if the flow stress was taken at 2 pct strain for the as-received dual-phase steel and at 0.2 pct strain or the yield plateau for the other steels. The linear relationship between flow stress and springback is, as is shown in Fig. I l, dependent upon the t / R ratio; but the rate of change of springback with flow stress (the slope of the lines) is dependent upon the value of t / R . The slopes of these lines are plotted as a function of t/R in Fig. 12 where it can be seen that the slope is 50 pct greater at t/R = 0.1 than at t/R = 0.5: the slope appears to reach a constant value for t / R 16 If)

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ratios greater than about 0.4. Springback as a function of flow stress for the steels that were prestrained by rolling and by tensile deformation, are shown in Figs. 13 and 14 for t / R = 0.25 and 0.50 respectively: in all cases the flow stress was measured after 0.2 pct plastic strain. The initial data point for each steel, that is, the springback at the lowest flow stress value, is for the undeformed, as-received material. Thus these figures show that whether strengthening is by grain size refinement and precipitation hardening (SAE 950X and 980X) or the presence of a large amount of a strong second phase (dual-phase steel) or cold work, springback is a linear function of the flow stress. Figure 14 includes the results of prestraining both by rolling and tensile deformation. It can be seen that the springback is independent of whether the specimens were cut transversely or longitudinally with respect to the rolling direction. However, prestraining in tension has resulted in a considerable decrease in springback; the SAE 950X steel deformed 15 pct in tension to a

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50--VOLUME 1, NUMBER 4

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J. APPLIED METALWORKING

stress level of 438 MPa (63.5 ksi) exhibits less springback than the initial starting material which had a yield stress of 330 MPa (48 ksi). These differences in springback between materials deformed by rolling or in tension may be a consequence of the different strain states imposed by the cold work; rolling is very close to plane strain ( ~ 0 minor strain) while tensile deformation is a draw condition ( - v e minor strain). The deformation in many stamped components is close to the plane-strain condition. In summary it can be seen from all of the above results that the amount of springback observed is a function of the strength of the material and die configuration. For the minimum springback with a given steel, the die gap should be as small as practical and the anvil radius should be less than 2.5 times the material thickness (t/R >__0.4).

IV. APPLICATIONS It is recognized that the above springback data only apply to the forming of a very simple flange without the benefit of a coining or stretching operation; in practice such operations are often used to minimize springback ksi 20

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when flanges or U-channels are formed. However, it is hoped that the results may provide useful guidelines to the springback problems that occur in stamping plants. Two illustrative applications for the above data are: the spread in springback that arises from the variation in strength within a given grade of steel, and the springback problems that arise in materials substitution.

deg from increasing the die-gap/material-thickness ratio from about 1.05 to 1.5 (Fig. 4). Thus of the 7 deg increase in springback expected from the substitution of the SAE 980X approximately 1/3 of the total (or 2.5 deg) is due to die effects (not optimum anvil radius or gap) which are easily correctable.

V. SUMMARY AND CONCLUSIONS a) Variation in Strength of a Given Grade of Steel One of the major problems facing a component or die designer is the expected spread in springback that will be encountered within a given grade of steel. For example, the yield stress of SAE 950X-grade steel can vary from 345 to 470 MPa (50 to 68 ksi) which, as can be seen from Fig. 11, will lead to a 1.5 deg spread in springback at t / R = 0.4 The variation in springback is independent of the type or grade of steel but, as shown in Fig. 15, dependent on both the spread in flow stress and the t / R ratio; for a constant die arrangement the minimum spread in springback will be given by the minimum variation in flow stress. Thus, from a component assembly point of view, where springback variations can lead to difficulty in mating parts together, it is most important to have steels with consistent, minimum-variation, mechanical properties.

b) Materials Substitution Often to obtain prototype parts for evaluation, downgaged HSS is put through existing production dies which were designed for CRLC steel sheet. Even if the HSS parts exhibit no tears or wrinkles there is usually excessive springback. This springback originates both from the use of HSS and from the increase in die gap and the decrease in t / R ratio that is a result of using thinner-gage HSS. Being able to evaluate the amount of springback that comes from the HSS and from the die configuration separately may make it easier to judge whether a prototype part can be made in the appropriate production tooling. Consider for example the forming of a simple flange in CRLC steel i mm (0.04 in.) thick over a 2.5 mm (0.10 in.) radius. With a 69 MPa (10 ksi) spread in flow stress a total springback of 2 to 3 deg would have to be allowed for. If a SAE 980X steel 0.75 mm (0.03 in.) thick were substituted in the same die setup the total springback would be in the range 9 to 11 deg. This total springback is comprised of 6.5 to 8.5 deg from material substitution at constant t / R (a flow stress spread of 165 MPa (24 ksi) is not unreasonable in this material), plus 0.5 deg from decreasing t / R to 0.3 (Fig. 7), plus about 2 52--VOLUME 1, NUMBER4

A study has been made of the influence of material properties, thickness t, die gap d and anvil radius R on springback after a simple flange operation for both as-received and cold-worked low-carbon, SAE 950X and 980X and dual-phase steels. It was observed that springback increased as the anvil radius, die gap and strength increased, and as the sheet thickness decreased. Springback was found empirically to be proportional to the initial flow stress except for the as-received dualphase steel. The dual-phase steel, whose tensile strength is similar to that of SAE 980X steel, initially had springback characteristics similar to SAE 950X steel. However, after about 15 to 20 pct cold work, the dual-phase and the SAE 980X steels had similar amounts of springback. It was also noted that for a given material, springback (at constant d/t) was a unique function of t / R and that for minimum springback, t / R should be greater than 0.4, that is, the anvil radius should be less than 2.5 times the material thickness. The results from this study of simple flange forming can be used to 1) predict the springback for a variety of material and die setups, 2)to predict the variation in springback to b e expected for the spread in strength usually observed in a given grade of steel, and 3)to indicate how the springback is to be allocated between material and die effects when making materials substitutions. These results, and conclusions drawn from them, can probably be used as a good guide for the forming of any flanges but great care should be taken in extending their use to more complex stamping operations.

ACKNOWLEDGMENTS The author would like to acknowledge the very able experimental assistance of W. S. Stewart and the helpful discussions with his colleagues W. Brazier, C. L. Magee, P. Beardmore and Y. C. Liu.

References 1. C.A.Queenerand R. J. DeAngelis:TransASM, 1968,vol.61, pp. 757-68. 2. J.C. Benedykand J. R. Newnham:J. Inst. Met., 1970,vol. 98, pp. 97-101. J. APPLIEDMETALWORKING