MIT International Journal of Mechanical Engineering Vol. 1 No. 2 Aug 2011, pp 114-118 114 Forming Limit Diagram of High Strength Steel Sheet (DP-590) Raj Kumar Sahu Shrabani Majumdar B.N. Prasad Department of Mechanical Engineering, Research and Development Divison, Department of Mechanical Engineering, Indian Institute of Technology Patna, Tata Steel Limited, National Institute of Technology, Patna-13. Jamshedpur. Jamshedpur. email: rks2010@iitp.ac.in Abstract: The paper gives an overview of mechanical properties and forming behavior of dual phase steel sheet (DP 590). DP-590 has gained acceptance as automotive structural material as it combines high tensile strength with good formability. Forming limit diagram of DP-590 steel sheet has been constructed at the static loading condition. The forming limit diagrams and strain distributions were determined following the Hecker's simplified technique. In this method, the experimental procedure involves three stages: (i) grid marking the sheet samples, (ii) punch stretching the grid marked samples to failure or onset of localized necking and (iii) measurement of strains. Major strain varies from 2.5%-8% and minor strain varies from -1.25%-5%. The nature of forming limit diagram (FLD) of the selected steel sheet compares well with published data [5]. The nature of lubricant and applied die pressure are the two variables which are to be carefully controlled to prevent the occurrence of wrinkling in the steel sheet during forming. The speed of forming during actual forming operation has been considered while finalizing the test parameters as strain-rate is known to influence the formability of steel sheet significantly. Keywords: Dual Phase steel sheet, Formability, Forming Limit curve (FLC), Forming Limit diagram (FLD),High Strength Low Alloy (HSLA). I. INTRODUCTION DUAL-PHASE STEEL is a new class of high-strength low alloy (HSLA) steels. This class is characterized by a tensile strength value of approximately 550 MPa (80 ksi) and by a microstructure consisting of about 20% hard martensite particles dispersed in a soft ductile ferrite matrix. The term dual phase refers to the predominance in the microstructure of two phases, ferrite and martensite as shown in Fig.1. However, small amounts of other phases, such as bainite, pearlite, or retained austenite, may also be present. In addition to high tensile strength, other unique properties of these steels include continuous yielding behavior, a low 0.2% offset yield strength, and a higher total elongation than other HSLA steels of similar strength. Figure 1: Ferrite-Martensite Structure of Dual Phase steel. Although limited research on dual-phase steels began in the early 1970s [1], an intense interest in these steels commenced with work done in 1975 and 1976. In 1975 researcher showed that continuous annealing in the intercritical temperature range produced steels with a ferrite-martensite microstructure and a ductility superior to that of normal precipitation-hardened or solid solution hardened HSLA sheet steels.the study also showed that the increased ductility was accompanied by an increased formability of automotive parts. Prior to this time, steels with a strength of 550 MPa (80 ksi) that were produced without intercritical heat treatment were considered to have poor formability. Large-scale efforts were begun, in many steel research laboratories to develop new steel compositions as well as alternative processing procedures for producing large tonnages of dual-phase steels. It was predicted that in the 1980s automobile manufacturers would need the large tonnages to produce more fuel-efficient cars. Gasoline mileage requirements were mandated by the government because of the gasoline shortage; it was expected that using thinner sheets of high-strength dual-phase steels for automobile bodies would save weight and enable the cars to meet these requirements. Much of this work has been reported in review papers [2]. Pertinent aspects of dual-phase steels, such as heat treatment, microstructures, mechanical properties, and new advances are discussed in this article. Typical chemical compositions of present day dual-phase steels are given in literature [1]. In general, these steels have a carbon
MIT International Journal of Mechanical Engineering Vol. 1 No. 2 Aug 2011, pp 114-118 115 content of less than 0.1%, which ensures that they can be spot welded. The carbon content also produces about 20% of the martensite in the microstructure after intercritical annealing and rapid cooling. Sheet metal forming is an important manufacturing process in automotive industry. Knowledge of the formability of sheet metal is critical to the success of sheet metal stamping process. The ability to form sheet metals into desired shapes is often limited by the occurrence of material instability leading to localized necking. For a stretched sheet metal, two forms of necking, namely diffuse necking and localized necking, may appear. Diffuse necking of sheet metal involves contraction in both the lateral and width directions. In sheet specimens, localized necking occurs after necking. During local necking, the specimen thins without further width contraction. The forming limit of a sheet metal is defined to be the state at which a localized thinning of sheet initiates when it is formed into a product shape in a stamping process. Formability of sheet metals is at present characterized by the forming limit diagram (FLD) introduced in the 1960s.The forming limit is conventionally described as a curve in a plot of major principal strain vs. minor principal strain. It must cover as much as possible the strain domain which occurs in industrial sheet metal forming processes. The curves are established by experiments that provide pair of the values of the limiting major and minor principal strains obtained for various loading patterns.heecker [3] developed an experimental method to determine the forming limit curves. A widely used technique is to print or etch a grid of small diameters circles on the sheet before deformation. The principal strains can be found by measuring the major and minor diameters after straining. These values of the neck or fracture give the failure condition, while the strains away from the failure indicate safe condition. As the experimental measurement of these strains is time consuming and expensive process, it would be useful if the forming limit strains can be predicted using theoretical methods. The locus of the forming limit is called the forming limit curve (FLC). Forming speed, the lubrication condition,the thickness of sheets,the strain hardening and the anisotropy properties of the steel sheets affects the forming limit curve(flc). Frictional forces is the key thing while punching. Hence some lubrication is provided between the blank and punch which reduces the friction and improves the strain distribution and delays local thinning. S.B. Kima, H. Huha, H.H. Bokb, M.B. Moonb [4] evaluated that the elongation at fracture for CQ increases at a high strain rate while the elongation at fracture for DP590 decreases slightly in relation to the corresponding value for a quasistatic strain rate. The uniform elongation and the strain hardening coefficient decrease gradually when the strain rate increases. The results confirm that the strain rate has a noticeable influence on the formability of steel sheets. Thus, the forming limit diagram of high-speed tests should be considered in the design of high-speed sheet metal forming processes. Amit Kumar Gupta, D. Ravi Kumar [5] evaluated that the limit strains of the uncoated steel are higher than those of galvanized IF steel sheets in plane strain and biaxial stretching, conditions. As the coating thickness increased, the limit strains have increased marginally and the value of coefficient of friction at the punch-sheet interface for the uncoated sheet is higher than in case of the galvanized sheets and it could be due to action of Zn coating as a solid lubricant at the interface. The present paper is organized as follows: The details on the experimental procedure, strain measurement and strain distribution are detailed in the next section. The details of results and discussion are outlined in section 3 and the applications of forming process are detailed in Section 4. Finally, conclusions of this paper are stated in section 5. II. EXPERIMENTAL PROCEDURE The forming limit diagrams and strain distributions were determined by following the Hecker's simplified technique [6]. In this method, the experimental procedure involves three stages-grid marking the sheet samples, punch stretching the grid marked samples to failure or onset of localized necking and measurement of strains. A. Grid-marking and Punch stretching Grid marking on the sheet samples was done by using a nonconducting grid of 5mm diameter circles. The grid pattern was printed on sheet samples electro-chemical grid marking technique. Punch stretching experiments were conducted (up to necking/fracture of the cup/dome) using suitably designed and fabricated tools (punch, upper and lower dies) on a 40-t capacity single action hydraulic press. The blank holder and ram capacity of machine was 700 KN and 600KN respectively. The dimensions of the specimens used for the grid marking was [(200 200), (200 175), (200 150), (200 125), (200 100), (200 75), (200 50) and (200 25), all in mm].three samples of each dimension were prepared. A hemispherical punch, fabricated on a numerically controlled machine, was used in this study. The load-displacement data during the experiments were obtained with a computerized data acquisition system consisting of a load cell and rotary encoder. The different strain states (tension-tension, plane strain and tensioncompression) during punch stretching were obtained by varying the width of the samples (between 25 and 200 in steps of 25 mm). A draw bead of 70 mm diameter was provided on dies to restrict the material flow from outside. Sufficient blank holding pressure was applied using the upper die to clamp the material in the draw bead using a spring-loaded blank holder. For each blank width three specimens were tested to get maximum number of data points. The punch velocity was maintained almost constant at 1mm/ min. Minor variations in punch velocity cause very small changes in the strain rate, which does not affect the flow behaviour of low carbon steel. The circles on the steel samples became ellipses after deformation falling into safe, necked and
MIT International Journal of Mechanical Engineering Vol. 1 No. 2 Aug 2011, pp 114-118 116 failed zone. The punch stretched samples of different width are shown in Fig. 2.The major and minor strains?1 and?2 of deformed ellipses lying in safe, necked and fractured regions were determined by measuring the major and minor diameters of the ellipses in both longitudinal and transverse directions of the sample using Argus software. and measurement of strains as shown in Fig. 4. Grid marking on the sheet samples was done using a non-contacting grid of 3 mm diameter circles and grid center to center distance of 4 mm. The grid pattern was printed on sheet samples electrochemical grid marking technique. Punch stretching experiments were conducted (up to necking/fracture of the cup/dome) using suitably designed and fabricated tools (punch, upper and lower dies) on a 130-t capacity (60t punch and 70t blank holder) single action hydraulic press. A 50.8 mm radius polished steel hemispherical punch, fabricated on a numerically controlled machine, was used in this study. Schematics of the punch-die assembly are shown in Figs. 5(a) and (b). Figure 2: Punch stretched samples of dual phase sheet B. Strain Measurement After sheet metal is formed the marked circles will deform into ellipses of different sizes. Strain is calculated from the following formula as per Fig. 3. Major strain = (major axis length original circledia) 100 Original circledia Minor strain = (minor axis length - original circledia) 100 Original circledia Figure 4: Out of plane formability test apparatus Figure 3: Major and Minor strains of punched samples After that, pictures of formed sample were taken and analysed by grid circle analyser.grid Circle Analyser consist of digital array camera with a built-in light source, keyboard, and CRT display. The image of given deformed circle is displayed on the CRT and a least squares curve fitting program selects the most suitable ellipse, which is displayed simultaneously. The major and minor strains computed from the equation for ellipse and the diameters of the original circle are displayed on the screen. And lastly by using ARGUS Software the FLD were drawn. The forming limit diagram and strain distributions were determined by following the Hecker's simplified technique. In this method, the experimental procedure mainly involves three stages-grid marking the sheet samples, punch stretching the grid marked samples to failure or onset of localized necking Figure 5 (a): Forming press. Figure 5 (b): Punch and die set used in the out of plane formability test (Courtesy Tata Steel)
MIT International Journal of Mechanical Engineering Vol. 1 No. 2 Aug 2011, pp 114-118 117 The load-displacement data during the experiments was obtained with a computerized data acquisition system consisting of a load cell and a rotary encoder. The different strain states (tension-tension, plane strain, and tensioncompression) during punch stretching were obtained by varying the width of the samples (between 25 and 200 mm in steps of 25 mm). A draw bead of 130 mm diameter was provided on the dies to restrict the material flow from outside. Sufficient blank holding pressure was applied using the upper die to clamp the material in the draw bead using a springloaded blank holder. Two to three specimens were tested to get maximum number of data points. The punch velocity was maintained almost constant at 1 mm/min. Minor variations in punch velocity cause very small changes in the strain rate, which does not affect the flow behaviour of low carbon steel as it is known that most of the common metals including low carbon steels have very low sensitivity to changes in strain rate at room temperature. The strain rate sensitivity index (m) of low carbon steel at room temperature is less than 0.01. The circles on the sheet samples became ellipses after deformation falling into safe, necked and failed zone. The major and minor strains e1 and e2 of the deformed ellipses lying in safe, necked and fractured regions were determined by measuring the major and minor diameters of the ellipses in both longitudinal and transverse directions of the sample using a strain analyzer set-up. The forming limit curve was drawn clearly demarcating the safe limiting strains from the unsafe zone containing the necked and fractured ellipses by plotting the major strains versus the minor strains. To represent the strain distribution in the punch stretched samples with different strain states, the major and minor strains were measured for all the ellipses along the longitudinal direction of the samples and lying at the center of blank, which represent the radius and tangential strains in the samples at these locations. The ellipse, which lies exactly under the punch, usually undergoes minimum deformation and it lies at the center of the longitudinal meridian of the deformed sample. The center of this ellipse is referred to as the pole. The radial and tangential strains were plotted as a function of their distance from the pole. III. RESULTS AND DISCUSSION A. Strain Distribution Profiles The strain distribution profiles, plotted for all the blank widths by measuring the major and the minor strains along the longitudinal direction of the specimens, are shown in Fig. 6 for sheet. The general features observed from the strain distribution profiles are very similar to earlier studies on formability of low carbon steel sheets of different grades and hence are not discussed here in detail. Steep strain gradients imply non-uniform distribution of strain and reduction of stretchability. It is known that the strain hardening coefficient 'n' influences the ability of a material to distribute the strains uniformly. Figure 6: Deformation at different parts in the samples For samples of [[200 200] to [200 25], all in mm] the major strain is shown in Fig. 5 and percentage of minor and major strains are shown in Fig. 6. Strain values are very high at breaking point and it's greater at the gripped surface because of stretching. At the breaking surface strain value is around 95% to 113% and at the same periphery strain is highly greater and its value is around 45% to 75%.In general, major strain varies from -8.0% to 40%. All percentage of strains (Major, Minor) are shown in Fig. 7. Colorless area [refer Fig. 5] is the area where etching wasn't proper and that's why grid points were not clear. Initially sheet has some strain values because of some tension and compression in sheet during cutting and its placement which produces some error in strain values in it of around 0.0-2.0%. Figure 7: Strain percentage for a formed sheet of dual phase steel sheet B. Construction of Forming limit diagram The forming limit diagram of the sheet was obtained by conducting punch-stretching experiments on specimens of different widths as explained earlier. Because of the large scatter in the measured strains with varying blank width and also due to the overlap of some points (the maximum safe strains and the strain in the portions where necking has just started), it is difficult to draw a very precise curve that indicates the onset of failure. Therefore, it is more appropriate to show the forming limit diagram by a band rather than as a line. Fig. 8 shows the forming limit diagram of the selected steel sheet. The area below the lower line of the curve is the safe working zone for the sheet for all possible combinations of
MIT International Journal of Mechanical Engineering Vol. 1 No. 2 Aug 2011, pp 114-118 118 strains. Above the upper line of the curve, the sheet metal is certain to fail by necking/fracture. The area within the curve represents the critical region where the sheet is likely to develop the necking/onset of failure. V. CONCLUSIONS Forming limit diagram of DP-590 steel sheet has been constructed at the static loading condition. Major strain varies from 2.5%-8% and minor strain varies from -1.25%-5%. The nature of forming limit diagram (FLD) of the selected steel sheet compares well with published data [5]. The nature of lubricant and applied die pressure are the two variables which are to be carefully controlled to prevent the occurrence of wrinkling in the steel sheet during forming. The strain rate is the key parameter which influences the formability of steel sheet. Therefore, for determining the forming speed (as the formability varies in relation to strain rate) strain rate is considered. Figure 8: Forming limit curve The specimens with dimensions (25 200, 50 200, 75 200, 100 200, 125 200 all in mm) represent the strain paths varying from uniaxial to plane strain conditions, whereas the specimens with dimensions (150 200, 175 200, 200 200, all in mm), represent the strain paths varying from biaxial to plane strain conditions. The specimen dimensions covering the entire range of FLD are shown in Fig. 8. The following points have been observed: 1. Major strain varies from 2.5% to 8%. 2. Minor strain varies from -1.25% to 5%. In many cases while deforming the sheet metal, the component fractures at certain point. The causes of failure are parameters related to forming process. IV. APPLICATIONS OF FORMING PROCESS Forming processes are widely used in Automotive, Aerospace, Appliance, Cookware etc. The products made by sheet-forming processes include a large variety of shapes and sizes, ranging from simple bends to double curvatures with shallow or deep recesses. Typical examples are metal desks, appliance bodies, aircraft panels, beverage cans, auto bodies, and kitchen utensils. Stamping is an efficient and low cost process for high volume manufacturing in automotive industry. About 70% of parts are formed by stamping process which saves time as well as money. Figure 9: Application in Automotive industry ACKNOWLEDGEMENT One of the authors (RKS) is thankful to all the faculty members of Mechanical Engineering Department, N.I.T. Jamshedpur for their kind co-operation and useful suggestion from time to time during the work. Help and support provided by the researchers and technicians of R&D Division of TATA STEEL during experimental work is gratefully acknowledged. REFERENCES [1] G.R. Speich, Department of Metallurgical Engineering, Illinois Institute of Technology ASM Handbook, Volume 1, Properties and Selection: Irons, Steels, and High-Performance Alloys (ASM International) (1990) 424-429. [2] N., Venkata Reddy, Jian Cao, "Incremental Sheet Metal Forming: A Review" Dept. of Mechanical Engg. IIT, Kanpur and Northwestern, University, Evanston. [3] S.S. Heeker, Simple Technique for Determing Forming Limit Curves, Sheet Metal Ind. 52(1975) 671-676. [4] S.B. Kima, H. Huha, H.H. Bokb, M.B. Moonb, Journal of Materials Processing Technology, 2010. [5] Amit Kumar Gupta and D. Ravi Kumar, Formability of Galvanised Interstitial Free Steel Sheets, Journal of Materials Processing Technology, v 172, 2006, pp. 225-237. [6] Taylor B., Formability Testing of Sheet Metals, Metals Handbook, ASM. [7] Rajkumar Sahu, "Fatigue and Formability Behaviour of Dual Phase Steel Sheets" Mtech Thesis NIT Jamshedpur, 2010. [8] Lang, L., Danckert, J., Nielsen, K.B., 2005. Multilayer Sheet Hydroforming: Experimental and Numerical Investigation into the very thin layer in the Middle, J. Mater. Proc. Technol. 170, 524-535. [9] Lee, M.G., Kim, D., Kim, C., Wenner, M.L., Wagoner, R.H., Chung, K., 2005. Springback Evaluation of Aautomotive Sheets based on Isotropic-kinematic Hardening Laws and Non-quadratic Anisotropic Yield Functions. Part II. Characterization of Material Properties. Int. J. Plast. 21, 883-914.