Application of Mechanical Trimming to Hot Stamped 22MnB5 Parts for Energy Saving

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1 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 15, No. 6, pp JUNE 2014 / 1087 DOI: /s Application of Mechanical Trimming to Hot Stamped 22MnB5 Parts for Energy Saving Hong-Seok Choi 1, Byung-Min Kim 1, Dong-Hwan Kim 2, and Dae-Cheol Ko 3,# 1 Precision Manufacturing Systems Division, Pusan National University, Busan , South Korea 2 Dept. of Mechanical & Automotive Engineering, International University of Korea, Gyeongnam , South Korea 3 Industrial Liaison Innovation Center, Pusan National University, Busan , South Korea # Corresponding Author / dcko@pusan.ac.kr, TEL: , FAX: KEYWORDS: Energy-saving mechanical trimming, Hot half-trimming, Hot stamping, Ultra high strength steel sheet, Trimmed surface An energy-saving mechanical trimming technology for hot stamped ultra high strength steel sheets is suggested in this study, in which the sheet thickness on the desired trim line is reduced by hot half-trimming at the same time as hot stamping, and the half-trimmed part is subsequently fully trimmed by mechanical trimming. Effectiveness of the proposed process has been investigated by FEsimulation and experiment. In this process, the hot half-trimming depth in hot stamping and the clearance between the punch and die in mechanical trimming were considered to be the main process parameters determining trimmed surface quality, trimming load, and tool wear. A high-quality trimmed surface, as compared with that produced by conventional cold trimming, can be achieved with a large hot half-trimming depth in hot stamping and small clearance in mechanical trimming. It was also shown that production time and cost can be reduced considerably by mechanical trimming using hot half-trimming, with low probability of tool chipping and reduced tool wear, as compared to conventional cold trimming and laser cutting processes. Therefore, mechanical trimming using hot half-trimming as proposed in this study can replace the high energy- and production time-consuming laser cutting process in the mass manufacturing of automotive parts. Manuscript received: April 15, 2013 / Revised: April 9, 2014 / Accepted: April 18, Introduction NOMENCLATURE C = Clearance in subsequent mechanical trimming CDV = Critical damage value D h = Hot half-trimming depth ε = Effective strain F = Stripper force m = Friction factor σ = Flow stress t = Initial thickness of sheet V = Punch velocity To meet the requirements of lightweight construction and improve crash safety in automobiles, hot stamping of quenchable boron alloyed steel sheets such as 22MnB5, 27MnCrB5, and 37MnB4 is widely used for manufacturing of automotive parts, the most commonly used steel sheet being 22MnB5. By heating the sheets in a furnace followed by hot stamping, forming load and springback can be decreased and formability increased. The stamped part is subsequently hardened by quenching in the closed tool, which leads to an ultra high strength part with ultimate tensile strength of approximately 1.5 GPa. 1-4 After hot stamping, a hot stamped part requires a subsequent shearing process such as trimming, blanking, or punching, in which excess or unnecessary material is removed from the parts. However, most manufacturers in the automotive industry encounter difficulties in cutting hot stamped parts. The extremely high strength of a hot stamped part causes severe tool wear and, in some cases, early tool failure, as shown in Fig. 1(a), when conventional mechanical shearing processes are applied to an ultra high strength steel sheet such as press hardened 22MnB5. Recently, a few studies have been performed to solve this problem. Peng et al. 5 simulated the blanking process of electrically heated material KSPE and Springer 2014

2 1088 / JUNE 2014 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 15, No. 6 Fig. 1 Problems in current hot stamping process; (a) early tool failure in mechanical trimming, (b) long production time in laser trimming for different heating conditions. Simultaneous hot stamping and warm blanking of press hardened 22MnB5 for economical blanking was suggested by So et al. 6 Mori et al. 7 developed a punching process for ultra high strength steel sheets to reduce the flow stress in the shearing zone by resistance heating; the process was then applied to punching small holes in press hardened 22MnB5. Choi et al. 8 suggested a local softening hot stamping process for reducing trimming load and improving tool wear, which made it possible to reduce the strength and obtain a ductile spheroidized bainite microstructure at the trim line through reduction of the cooling rate. Until now, it has been considered most effective for trimming or blanking of hot stamped parts to be performed simultaneously with hot stamping. However, another problem in the process is that the structure of the hot stamping tool set becomes very complex in order to carry scrap out of the die. Studies on the development of new tool materials, tool surface treatments, and tool designs have also been performed to enhance tool life, 9-11 which requires expensive tool manufacturing and tool maintenance. Despite these efforts, laser cutting technology, shown in Fig. 1(b), is still used as an alternative to mechanical shearing processes for press hardened 22MnB5 in actual industrial fields. Laser cutting has high energy consumption owing to the long production time in mass manufacturing, resulting in increased costs. Therefore, it is necessary to develop an effective trimming technology for mass production of hot stamped parts with low manufacturing cost and energy consumption. The objective of this study is to suggest a new energy-saving mechanical trimming technology for hot stamped ultra high strength steel sheets. With this technology, the sheet thickness on the desired trim line is reduced by hot half-trimming at the same time as hot stamping; the half-trimmed part is subsequently fully trimmed by press working. Effectiveness of the proposed mechanical trimming using hot half-trimming is investigated by FE-simulation and experiment. Because the reduced sheet thickness in hot half-trimming at elevated temperature and the clearance between the punch and die in mechanical trimming at room temperature are important parameters for obtaining a smooth trimmed surface in the final product, their effects on the quality of the trimmed surface, trimming load, and tool wear are also discussed in this study. 2. Energy-Saving Mechanical Trimming Using Hot Half- Trimming For effective trimming of press hardened ultra high strength steel Fig. 2 Conventional and energy-saving hot stamping process chain Fig. 3 Schematic diagram of mechanical trimming using hot halftrimming; (a) simultaneous hot stamping and hot half-trimming, (b) subsequent mechanical trimming sheets, a new mechanical trimming process using hot half-trimming is suggested in this study. In the conventional hot stamping process (direct hot stamping process) chain, as shown in Fig. 2, a sheet blank is heated up to 950 o C for approximately 5 min in a furnace for austenitic transformation, then transferred to the press and formed and quenched simultaneously by the water-cooled die, leading to martensitic transformation. Hot stamped parts for automotive panels are trimmed to remove excess material or punched to make holes for joining with other parts. However, their high hardness and strength after press hardening causes difficult problems in mechanical cutting of these parts, such as severe wear and early chipping and failure at the tool edge. Therefore, laser cutting is currently used as an alternative process to mechanical cutting, despite long production time. Mechanical trimming of hot stamped parts can be effectively achieved

3 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 15, No. 6 JUNE 2014 / 1089 Table 1 Mechanical properties of 22MnB5 and process conditions for FE-simulation Process Hot half-trimming Material 22MnB5 Mechanical trimming Press hardened 22MnB5 Flow stress J-MatPro σ = 2250σ 0.05 (MPa) Friction factor (m) Punch velocity (mm/s) 2π N S V = h h 60 Process parameter Dh = 0.8, 1.0 mm N: 25 (rpm) C = 0.1, 0.2, 0.3 mm h: punch stroke (mm) S: crank diameter (mm) Fig. 5 Determination of critical damage value, CDV; (a) hot trimming of 22MnB5, (b) cold trimming of press hardened 22MnB5 Fig. 4 FE-modeling of mechanical trimming using hot half-trimming; (a) hot half-trimming, (b) subsequent mechanical trimming when hot half-trimming is used simultaneously with hot stamping as shown in Fig. 2. A schematic diagram of mechanical trimming using hot half-trimming is shown in Fig. 3. Near the end stage of hot stamping, sheet thickness on the desired trim line is reduced by hot half-trimming (Fig. 3(a)). Subsequently the hot half-trimmed part is fully trimmed by mechanical trimming (Fig. 3(b)). Pre-deformation of the sheet on the trim line before mechanical trimming can decrease trimming load and lead to longer tool life and shorter production time. 3. FE-Modeling of Energy-Saving Mechanical Trimming 3.1 Initial FE-model In order to investigate the effectiveness of mechanical trimming using hot half-trimming, the process was simulated using the commercial FE-software DEFORM. The process is modeled and simulated under plane strain condition, as shown in Fig. 4. Al-Si pre-coated 22MnB5 is modeled as a plastic material using approximately 15,000 elements; the tool is modeled as a rigid body. Although the thickness of the pre-coated 22MnB5 is known to be 1.6 mm, the initial thickness of the sheet is modeled as 1.66 mm. taking into account the Al-Si coating layer on both sides of the sheet with a total thickness of 0.06 mm. Because hot half-trimming is performed at elevated temperature, flow stresses of 22MnB5 at different temperatures and strain rates obtained from J-MatPro12 are used in the FE-analysis of hot half-trimming. Although the sheet is heated up to approximately 950oC within the furnace, the initial temperature of the sheet just before hot stamping is measured as 750oC; this is due to heat loss during transfer to the die. Therefore, the initial temperature of the sheet in the FE-analysis of hot half-trimming is set to 750oC. As mechanical trimming is performed after press hardening, the flow stress of press hardened 22MnB5 used in the FE-analysis of mechanical trimming was obtained from a tensile test. Mechanical properties of 22MnB5 and process conditions for the FE-analysis are summarized in Table 1. In the simulation, as shown in Fig. 4, the initial length and width of the sheet were 100 and 50 mm, respectively, whereas the trimmed length was 25 mm. The hot half-trimming depth, Dh was varied from 0.8 to 1.0 mm; clearance between the punch and the die in the process was set to zero. In subsequent mechanical trimming, various clearances, C = 0.1, 0.2, and 0.3 mm, are considered as an important variable. The friction factor, m was assumed to be 0.3 in hot half-trimming and 0.1 in subsequent mechanical trimming. The corner radius of the punch and die was set to 50 µm to avoid convergence problems in the FEsimulation. 3.2 Treatment of crack initiation and propagation To simulate crack initiation and propagation in hot half-trimming and

4 1090 / JUNE 2014 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 15, No. 6 Fig. 6 Deformed shape and effective strain distribution for various hot half-trimming depths; (a) D h = 0.8, (b) D h = 1.0 mm, where A and B indicate edges formed by hot half-trimming punch and die Fig. 7 SEM images of cross-sections of hot half-trimmed part; (a) D h = 0.8, (b) D h = 1.0 mm, where A and B indicate edges formed by hot half-trimming punch and die subsequent mechanical trimming, Cockcroft-Latham s ductile fracture criterion was employed to the FE-simulation; the critical damage value (CDV) was determined by comparison of trimmed surfaces obtained from experiment with the FE-simulation. 13,14 To determine the CDV, experiments were performed for hot trimming of 22MnB5 and for cold trimming of press hardened 22MnB5. CDVs in hot and cold trimming were determined to be 2.28 and 0.48, respectively, as shown in Fig Results of FE-Simulation and Experiment 4.1 Hot half-trimming The hot half-trimming depth, D h is one of the most important parameters because it can affect the trimmed surface quality, trimming load, and tool wear in subsequent mechanical trimming. In this study, D h values of 0.8 and 1.0 mm were considered, which correspond respectively to 50% and 62.5% of the initial sheet thickness. Fig. 6 shows the deformed shape and distribution of effective strain at the final stage of hot half-trimming for D h values of 0.8 and 1.0 mm. Roll-over and burnish features are formed as the hot half-trimming punch penetrates the sheet. With increased D h, roll-over is slightly increased, but burnish is considerably increased up to approximately 32.1%. Concentration of deformation was observed at both edges of the hot half-trimming punch and die (A and B in Fig. 6), where strain values increased with the increase in D h due to large deformation. The maximum load of hot half-trimming is calculated as 14.3 and 14.6 kn for D h of 0.8 and 1.0 mm, respectively. SEM images of the cross-sections of hot half-trimmed parts with D h = 0.8 and 1.0 mm are shown in Fig. 7. A slight difference in the D h between the FE-simulation and experiments resulted from the difficulty in the accurate control of punch stroke in the experiments using a labscale mechanical press. However, the experimental results are in good agreement with those of the FE-simulation. Similar to the results of the FE-simulation, it was observed that burnish is considerably increased to 29.8% with the increase in D h, whereas roll-over is slightly increased. As shown in Figs. 6 and 7, it was found that roll-over and burnish formed during hot-half trimming can affect the trimmed surface quality in subsequent mechanical trimming. Deeper D h in hot half-trimming without separation of the sheet metal due to crack initiation and propagation during hot stamping can reduce trimming load in subsequent mechanical trimming owing to the reduced sheet thickness on the trim line, leading to longer tool life. 4.2 Subsequent mechanical trimming Because clearance between the punch and die, C is one of the most important process parameters, subsequent mechanical trimming of the hot half-trimmed part after press hardening was investigated by FEsimulation and experiment with C = 0.1, 0.2, and 0.3 mm, and D h =0.8 and 1.0 mm. Fig. 8 shows the deformed shape and distribution of effective strain in mechanical trimming for various clearances with D h = 1.0 mm. In the cases that C = 0.1, 0.2, and 0.3 mm, cracks occurred when the trimming punch strokes were 0.064, 0.068, and again mm owing to deformation concentration around the corner edge deformed previously

5 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 15, No. 6 JUNE 2014 / 1091 Fig. 8 Deformed shape and effective strain distribution at completion of mechanical trimming for various clearances; (a) C = 0.1, (b) C = 0.2, (c) C = 0.3 mm, as D h =1.0mm Fig. 9 SEM images of cross-sections of hot half-trimming with D h = 1.0 mm and subsequent mechanically trimmed 22MnB5; (a) C = 0.1, (b) C = 0.2, (c) C = 0.3 mm by the hot half-trimming punch and die (A and B in Fig. 6). For each case, with trimming punch strokes of 0.147, 0.175, and mm, scraps are fully trimmed owing to the propagation of cracks within a narrow shear band, where the smallest punch stroke is required at the smallest clearance. It was also observed that traces of the roll-over and burnish features formed by previous hot half-trimming remained during mechanical trimming. Therefore, roll-over and burnish features on the trimmed surface of the final product are dependent on hot halftrimming depth. SEM images of the cross-sections after completion of mechanical trimming for various clearances, with D h = 1.0 mm, are shown in Fig. 9. Experimental results for trimmed surface characteristics are in good agreement with those of the FE-simulation shown in Fig. 8. Similar to the results of the FE-simulation, traces of roll-over and burnish features formed by previous hot half-trimming (Fig. 7(b)) remained during mechanical trimming. Regardless of the variation in clearances, the trimmed surface characteristics had a similar tendency, except in the case where very small burr formation was detected with the increase in clearance. Because deformation in subsequent mechanical trimming is concentrated around the corner edge formed by the hot half-trimming punch and die (A and B in Figs. 6 and 7), cracking within the shear band occurs early without further formation of burnish (Figs. 8 and 9). This result means that characteristics of the sheared surface, such as roll-over and burnish formed by hot half-trimming, can directly influence the quality of the trimmed surface in subsequent mechanical trimming, and early crack occurrence in subsequent mechanical trimming can reduce trimming load and tool wear. Relatively highquality trimmed surfaces with increasing burnish and decreasing burr height were obtained from all clearances considered in this study, as compared with that of conventional cold trimming shown in Fig. 5(b). 5. Discussion 5.1 Quality of trimmed surface Hot half-trimming depth, D h and clearance between the punch and die in subsequent mechanical trimming, C can affect the quality of the trimmed surface in the final product. As the hot half-trimming depth in hot stamping is increased, burnish is considerably increased to approximately 30%, whereas roll-over is slightly increased to approximately 8.3% (Fig. 7), which indicates that hot half-trimming depth is related to the formation of roll-over and burnish of the trimmed surface in the final product. From Fig. 8, a very small punch penetration of approximately 0.064~0.068 mm is required for crack initiation in subsequent mechanical trimming. A small percentage penetration, which

6 1092 / JUNE 2014 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 15, No. 6 Fig. 10 Measured percentage of roll-over, burnish and fracture and burr height for various hot half-trimming depths, as C = 0.1 Fig. 11 Diagram of trimming load vs. punch stroke for various clearances with D h =1.0mm describes total punch penetration at completion of fracture and is usually expressed as a percentage of sheet thickness to be cut, of approximately 22.3~26.7% is required as compared with a percentage penetration of approximately 28~38% required for steel. 15 This indicates that deformation concentration around the corner edge deformed previously by the hot half-trimming punch and die (A and B in Figs. 6 and 7) and the brittle martensite structure of the sheet by press hardening result in early crack initiation and propagation. Therefore, the punch penetration required for the completion of mechanical trimming is very small, which can reduce trimming load and tool wear. For D h = 1.0 mm, the increase in clearance does not influence the trimmed surface characteristics such as roll-over, burnish, and fracture, except for very small burr formation at large clearance (Fig. 9). It is also observed that traces of roll-over and burnish features formed by previous hot half-trimming remained during subsequent mechanical trimming, which means that the hot half-trimming depth in hot stamping is a dominant factor in the trimmed surface quality of the final product. The percentages of the roll-over, burnish, and fracture and the burr height on the trimmed surface, with D h = 0.8 and 1.0 mm in hot halftrimming and C = 1.0 mm in mechanical trimming, were measured experimentally; the results are shown in Fig. 10. Burnish is increased and fracture is decreased with increased hot half-trimming depth, whereas roll-over and burr height remained constant. The above results show that hot half-trimming depth in hot stamping strongly affects the surface quality of the final product in terms of percentages of roll-over, burnish, and fracture, whereas clearance in mechanical trimming slightly affects burr height. 5.2 Trimming load and its effect on tool failure A diagram of the predicted trimming load vs. punch stroke for various clearances, with D h =1.0mm, is shown in Fig. 11, as compared to conventional trimming. For all cases, maximum loads in mechanical trimming using hot half-trimming are approximately 48.3% lower than those in conventional mechanical trimming. The reason for the decrease in maximum load is that deformation in mechanical trimming is concentrated on the corner edge formed by the punch and the die in hot half-trimming (A and B in Figs. 6 and 7), and eventually cracking occurs and propagates early at a very small punch penetration depth without further formation of burnish (Figs. 8 and 9). The reduced thickness on the desired trim line during hot half-trimming can also contribute to the reduction of trimming load in subsequent mechanical trimming. The low trimming load and non-formation of burnish in mechanical trimming using hot half-trimming can prevent tool chipping and tool wear, leading to prolonged tool life. A slightly lower trimming load is required for large clearance owing to the bending effect of the sheet, which often occurs in the initial stage of mechanical trimming. Based on the results shown in Figs. 6~9, a high-quality trimmed surface can be achieved with a large hot half-trimming depth in hot stamping and a small clearance in mechanical trimming. It is also found that the quality (Fig. 9) is much better than that of conventional cold trimming (Fig. 5(b)) due to the increase of burnish and the decrease of burr height. Mechanical trimming using hot half-trimming can reduce trimming load, which leads to reduced tool chipping and tool wear as compared with conventional cold trimming. It is well known that laser cutting of center pillar takes approximately 1 min per part. But mechanical trimming proposed in this study can achieve it within a few seconds. Although quantitative comparison in production time between the proposed trimming and laser cutting was not performed in this study, it can be easily predicted that production time in mechanical trimming is much shorter than that of laser cutting. The shorter production time can contribute to the less energy consumption. Therefore, it is expected that mechanical trimming using hot halftrimming as proposed in this study can replace the current high energyand production time-consuming laser cutting process for mass manufacturing of automotive parts. 6. Conclusions An energy-saving mechanical trimming technology for hot stamped ultra high strength steel sheets has been suggested in this study, in which the sheet thickness on the desired trim line is reduced by hot half-trimming at the same time as hot stamping, and the half-trimmed part is subsequently fully trimmed by mechanical trimming. The effectiveness of the proposed process has been investigated by FEsimulation and experiment. The following conclusions can be drawn: (1) As hot half-trimming depth in hot stamping was increased, burnish was considerably increased up to approximately 30%, whereas roll-over was slightly increased up to approximately 8.3%. (2) A very small punch penetration of approximately 0.064~0.068

7 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 15, No. 6 JUNE 2014 / 1093 mm was required for crack initiation in subsequent mechanical trimming. A small percentage penetration of approximately 22.3~26.7% was required owing to deformation concentration around the corner edge deformed previously by the hot half-trimming punch and die, and brittle martensite transformation of the sheet by press hardening. (3) It was shown that roll-over and burnish features on the trimmed surface of the final product were affected by the hot half-trimming depth in hot stamping, in which burnish was increased and fracture was decreased with the increase in hot half-trimming depth, whereas rollover and burr height remained constant. (4) As the same hot half-trimming depth was applied in this process, the increase in clearance had no influence on the trimmed surface characteristics, except for very small burr formation at large clearance. (5) Maximum loads in mechanical trimming using hot half-trimming were approximately 48.3% lower than in conventional mechanical trimming, because the deformation was concentrated on the corner edge formed by the punch and the die of hot half-trimming, and eventually cracking occurred and propagated early without further formation of burnish, which can prevent tool chipping and reduce the tool wear. (6) A high-quality trimmed surface can be achieved with a large hot half-trimming depth in hot stamping and a small clearance in subsequent mechanical trimming. (7) Mechanical trimming using hot half-trimming, with low probability of tool chipping and reduced tool wear, can considerably decrease production time and cost for cutting ultra high strength steel sheets, as compared with conventional cold trimming and laser cutting processes. ACKNOWLEDGEMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No R1A5A ), and we applied for a patent related to this technology. REFERENCES 1. Merklein, M. and Lechler, J., Investigation of the Thermo- Mechanical Properties of Hot Stamping Steels, Journal of Materials Processing Technology, Vol. 177, No. 1, pp , Hoffmann, H., So, H., and Steinbeiss, H., Design of Hot Stamping Tools with Cooling System, CIRP Annals-Manufacturing Technology, Vol. 56, No. 1, pp , Karbasian, H. and Tekkaya, A., A Review on Hot Stamping, Journal of Materials Processing Technology, Vol. 210, No. 15, pp , of Electrically Heated Engineering Materials, Journal of Materials Processing Technology, Vol. 145, No. 2, pp , So, H., Faβmann, D., Hoffmann, H., Golle, R., and Schaper, M., An Investigation of the Blanking Process of the Quenchable Boron Alloyed Steel 22MnB5 before and after Hot Stamping Process, Journal of Materials Processing Technology, Vol. 212, No. 2, pp , Mori, K., Maeno, T., and Maruo, Y., Punching of Small Hole of Die-Quenched Steel Sheets using Local Resistance Heating, CIRP Annals-Manufacturing Technology, Vol. 61, No. 1, pp , Choi, H. S., Lim, W. S., Seo, P. K., Kang, C. G., and Kim, B. M., Local Softening Method for Reducing Trimming Load and Improving Tool Wear Resistance in Cutting of a Hot Stamped Component, Steel Research International, Special Edition, pp , Picas, I., Hernandez, R., Casellas, D., Casas, B., and Valls, I., Tool Performance in Cutting of Hot Stamped Steels, Proc. of 1st International Conference on Hot Sheet Metal Forming of High- Performance Steel, pp , Valls, I., Casas, B. and Rodriguez, N., Improving Die Durability in Hot Stamping and Hard Cutting, Proc. of the 2nd International Conference on Hot Sheet Metal Forming of High-Performance Steel, pp , Picas, I., Munoz, R., Lara, A., Hernandez, R. and Casellas, D., Effect of the Cutting Process in the Mechanical and Fatigue Properties of Press Hardened 22MnB5 Steel, Proc. of the 3rd International Conference on Hot Sheet Metal Forming of High Performance Steel, pp , Saunders, N., Guo, U., Li, X., Miodownik, A., and Schille, J. P., Using JMatPro to Model Materials Properties and Behavior, JOM, Vol. 55, No. 12, pp , Ko, D. C., Kim, B. M., and Choi, J. C., Finite-Element Simulation of the Shear Process using the Element-Kill Method, Journal of Materials Processing Technology, Vol. 72, No. 1, pp , Taupin, E., Breitling, J., Wu, W. T., and Altan, T., Material Fracture and Burr Formation in Blanking Results of FEM Simulations and Comparison with Experiments, Journal of Materials Processing Technology, Vol. 59, No. 1, pp , Eary, D. F. and Reed, E. A., Techniques of pressworking Sheet Metal: an Engineering Approach to Die Design, Prentice-Hall Englewood Cliffs, pp. 1-21, Kim, J. T., Jeon, Y. P., Kim, B. M., and Kang, C. G., Die Design for a Center Pillar Part by Process Analysis of Hot Stamping and its Experimental Verification, Int. J. Precis. Eng. Manuf., Vol. 13, No. 9, pp , Peng, X., Lu, X., and Balendra, R., FE Simulation of the Blanking