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Thickening process of concave edge for increasing stiffness and fatigue strength of ultra-high strength steel sheets
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Procedia Manufacturing 15 (2018) 605–611 Procedia Manufacturing 00 (2017) 000–000 www.elsevier.com/locate/procedia
17th International Conference on Metal Forming, Metal Forming 2018, 16-19 September 2018, 17th International Conference on MetalToyohashi, Forming, Metal Japan Forming 2018, 16-19 September 2018, Toyohashi, Japan
Thickening process of concave edge for increasing stiffness and
ThickeningEngineering processSociety of concave edge for increasing stiffness and Manufacturing Conference 2017, MESIC 2017, 28-30 June fatigue strength ofInternational ultra-high strength steel sheets 2017, Vigo (Pontevedra), Spain fatigue strength of ultra-high strength steel sheets
Yohei Abe*, Yuya Fujisawa, Yusuke Murai, Ken-ichiro 4.0: Mori Costing models for capacity optimization in Industry Yohei Abe*, Yuya Fujisawa, Yusuke Murai, Ken-ichiro Mori Trade-off Department of Mechanical Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan used Toyohashi capacity and operational efficiency Departmentbetween of Mechanical Engineering, University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan Abstract A. Santanaa, P. Afonsoa,*, A. Zaninb, R. Wernkeb Abstract A thickening process was applied for thea concave to increase theGuimarães, stiffness and fatigue strength of the ultra-high strength steel Universityedge of Minho, 4800-058 Portugal b A thickening wasprocess, applied the for sheet the concave edge tounder increase stiffness fatigue strength steel Unochapecó, 89809-000 Chapecó, SC,and Brazil sheets. In the process thickening was trimmed the the tension between the taper punch of andthe theultra-high step die. strength The trimmed sheets. In bent the thickening process, the sheet under the die tension between theoftaper punch and step die. The trimmed edge was by the taper of the punch intowas thetrimmed corner step of the and the surface the sheared edgethe was ironed. After filling edge wascorner bent by thethe taper of the punch intowas thesheared. corner step the die andstep the surface thedie sheared was and ironed. After filling into the step, excessive material Theofeffect of the shape inofthe on theedge stiffness fatigue strength into corner and step,tensile the excessive was sheared. The effect the step shape in the die on thethat stiffness and fatigue strength in thethe bending tests wasmaterial investigated. The stiffness of theofthickened edge was larger than without thickening in both Abstract in bending tensile tests was investigated. stiffness of the thickened edgethan wasthat larger than that withoutinthickening in both thethe bending andand tensile tests. The fatigue strengthThe of the thickened edge was higher without thickening the bending test, the bending tensileintests. The fatigue strength thickened edge due was to higher that withoutItthickening the thickening bending test, whereas, theand strength the tensile test was lowerof inthe certain conditions stressthan concentration. was foundinthat in Under the concept production processes will pushed to be and increasingly interconnected, whereas, theedge strength inofultra-high the"Industry tensile strength test4.0", was steel lower in certain conditions due tobestress It was found that thickening the concave of the sheets is effective in the increase of concentration. the stiffness fatigue strength of the part.in the concave edge of the strength is effective in the increase of theIn stiffness and fatigue strength of the part. information based on ultra-high a real time basis steel and, sheets necessarily, much more efficient. this context, capacity optimization © 2018 The Authors. Published by of Elsevier B.V.maximization, contributing also for organization’s profitability and value. goes beyond the traditional aim capacity © 2018 2018 The The Authors. Published Elsevier B.V. © Authors. Published by by B.V. committee Peer-review under responsibility of Elsevier the scientific of the 17th International Conference on Metal Forming. Indeed, lean management continuous improvement approaches suggest capacity optimization Peer-review under responsibilityand of the scientific committee of the 17th International Conference on Metal Forming. instead of Peer-review under responsibility of the scientific committee the 17thmodels International Metal Forming. maximization. The study of capacity optimization andofcosting is anConference important on research topic that deserves Keywords: Sheelfrom metal forming; strength steel sheets; SheetThis edge;paper Stiffness; Fatigue strength contributions both theThickening; practicalUltra-high and theoretical perspectives. presents and discusses a mathematical Keywords: Sheel metal forming; Thickening; Ultra-high strength steel sheets; Sheet edge; Stiffness; Fatigue strength model for capacity management based on different costing models (ABC and TDABC). A generic model has been developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s 1. Introduction value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity 1. Introduction optimization might hide operational inefficiency. The reduction in weight of automobile is important to improve the fuel efficiency. For the weight reduction of © 2017 Authors.inPublished by automobile Elsevier B.V.is important to improve the fuel efficiency. For the weight reduction of The The reduction of body-in-white parts, weight application of the high strength steel and ultra-high strength steel sheets is attractive due to the Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference body-in-white parts, application of the high strength steel and strength steel sheets is attractive to the high specific strength and cost competitiveness. The stiffness ofultra-high the parts tends to decrease because the sheetdue thickness 2017. high specific and cost competitiveness. stiffness of the tendsthe to decrease because thickness is reduced forstrength the lightweight in the applicationThe of these sheets. Inparts addition, deflection of the the partsheet is reduced by is reduced for the lightweight in the application of these sheets. In addition, the deflection of the part is reduced by Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Operational Efficiency
1. Introduction
* Corresponding author. Tel.: +81-532-44-6705; fax: +81-532-44-6690. * Corresponding author. Tel.: +81-532-44-6705; fax: +81-532-44-6690. E-mail address: [email protected] The cost of idle capacity is a fundamental information for companies and their management of extreme importance E-mail address: [email protected]
in modern©production systems. In general, it isB.V. defined as unused capacity or production potential and can be measured 2351-9789 2018 The Authors. Published by Elsevier 2351-9789 2018 Authors. Published Elsevier B.V.hours of the Peer-review underThe responsibility of theby scientific committee 17th International on Metal Forming. in several©ways: tons of production, available manufacturing, etc.Conference The management of the idle capacity Peer-review under responsibility thefax: scientific committee * Paulo Afonso. Tel.: +351 253 510of 761; +351 253 604 741 of the 17th International Conference on Metal Forming. E-mail address: [email protected]
2351-9789 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 2017. 2351-9789 © 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 17th International Conference on Metal Forming. 10.1016/j.promfg.2018.07.284
Yohei Abe et al. / Procedia Manufacturing 15 (2018) 605–611 Author name / Procedia Manufacturing 00 (2018) 000–000
606 2
increasing in the stiffness of the high stress portion such as concave and hole edges in the part. On the other hand, the fatigue strength around the corner and hole edges of the part becomes low under the concentration of tensile stress. The high strength steel and ultra-high strength steel parts are generally formed by cold stamping. In stamping, bending and shearing such as punching and trimming are used. In bending, large springback and fracture are problematic due to the low ductility and high strength. The springback is reduced by overbending, stretch forming, bottoming, etc. Bottoming using a servo press was effective in the reduction in the springback in bending of ultrahigh strength steel sheets [1]. The springback in U-bending of high strength steel sheet was reduced by additional bending with a counter punch [2]. The concave bending operations are generally included in stretch-flanging. In stretch-flanging and hole expansion, the fracture tend to be caused by the tensile stress. The fracture limit is improved by controlling microstructure and heat treatment of the sheets [3]. The quality of sheared edge affected on the fracture [4, 5], and the simulation of the trimming process of the ultra-high strength steel part was carried out [6]. A smoothing process of the sheared edge [7], the punching with a humped bottom punch [8] and the punch having shear angle [9] were developed to improve the quality of sheared edge. A gradually contacting punch was proposed for reliving the concentration of tensile stress in stretch-flanging [10]. In shearing of the high strength steel sheets such as ultra-high strength steel sheets, short tool life [11], a risk of flying scraps [12], low quality of sheared edge etc. are problematic. The quality of sheared edge affected on the fracture in hole expansion, flanging and the fatigue strength of the sheet [4, 7, 13-16]. To increase the quality, plate forging [17] was effective. To improve fatigue strength of the high strength steel sheet, two-stage plate forging for thickening the hole edge of the punched sheet was introduced [18], and flanging using the step die was developed [19]. Then the thickening process was included in the punching process for the ultra-high strength steel sheets [20]. However, the developed processes were not for the trimmed sheet edges, but for the punched hole edges. The effect of the process on the concave edge for increasing stiffness and fatigue strength of the ultra-high strength steel sheets is unclear. In this study, a thickening process using the taper punch and the step die was applied for the concave edge to increase the stiffness and fatigue strength of the ultra-high strength steel sheets. The effect of the step shape in the die on the stiffness and fatigue strength was investigated. Nomenclature b h x
step width of die step depth of die position
2. Thickening process of concave edge The thickening process of the concave edge in the ultra-high strength steel sheet is shown in Fig. 1. The sheet held by the blank holder is trimmed under the tension between the taper punch and the step die. The trimmed edge is bent by the taper of the punch into the corner step of the die and the surface of the sheared edge is ironed. After filling into the corner step of the die, the excessive material is sheared. The corner step of the die has the function of forming the thickened edge and compressing the edge bottom. 20
30 Blank
Backup plate
Step die
(a) Before thickening
10°Blank holder
Filling (b) Thickening
Scrap (c) Thickened edge
Fig. 1. Thickening process of concave edge in ultra-high strength steel sheet.
10
h
Taper punch
90 40
Taper punch
Blank Backup plate
b
Step die
(a) Dimensions of blank (b) Thickening Fig. 2. Blank and thickening conditions.
Yohei Abe et al. / Procedia Manufacturing 15 (2018) 605–611 Author name / Procedia Manufacturing 00 (2018) 000–000
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The blank and the thickening conditions are shown in Fig. 2. The concave edges in the blank were thickened. The concave radii before and after thickening were 15 mm and 10 mm, respectively. The die having the depth h and width b of the step was used. The diameter and angle of the taper punch is 20 mm and 10o, respectively. The mechanical properties of the ultra-high strength steel and high strength steel sheets are shown in Table 1. The sheets are dualphase steel, and the sheet thickness is 1.2 mm. Table 1. Mechanical properties of ultra-high strength steel sheets. Sheet
1180 MPa
980 MPa
780 MPa
Tensile strength [MPa]
1287
1029
799
Reduction in area [%]
51.8
52.4
62.5
3. Thickening process and thickened edge The punch load-stroke curve and the thickened edges for 980 MPa and h = 0.9 mm are shown in Fig. 3. The punch load increases with increasing of stroke. The maximum punch load is similar to that by trimming for R = 10 mm. In the trimmed edge, the ratio of the fracture surface is high due to the low ductility of the sheet. In the thickened edges, the edge length is increased by thickening, and the ratio of the smoothed surface increases, whereas the ratio of the rollover increases. A A’
Punch load [kN]
40
1 mm
Rollover
Rollover Burnished
Smoothed
Fracture
R = 10 mm
Fracture
(b) R = 10 mm
30 20
Rollover
Rollover
10
(c) b = 0.75 mm
Smoothed
Smoothed
Fracture Fracture 4 8 12 (e) b = 1.25 mm Punch stoke [mm] (d) b = 1.0 mm (a) Punch load-stroke curve for b = 1.0 mm Fig. 3. Punch load-stroke curve and thickened edges for 980 MPa and h = 0.9 mm.
2
(a) R = 10 mm
0 0.5 1 1.5 2
Smoothed
b = 0.75 mm
1.25 mm
Position from top surface [mm]
1.5
1180 MPa
1
Fracture
780 MPa
0.5
Position from top surface [mm]
Burnished
Rollover 0
980 MPa
Position from top surface [mm]
0
0
Rollover
0.5 1 1.5 2
(b) h = 0.9 mm Fig. 4. Ratio of trimmed and thickened edges.
Smoothed Fracture h = 0.3mm 0.6 mm (c) b = 1.0 mm
0.9 mm
Yohei Abe et al. / Procedia Manufacturing 15 (2018) 605–611 Author name / Procedia Manufacturing 00 (2018) 000–000
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The ratio of the trimmed and the thickened edges are shown in Fig. 4. In the trimmed edge, the ratio of the fracture surface increases with increasing of the tensile strength of sheet due to low ductility. The edge length increases with increasing of the step depth of die. In the thickened edges, the ratio of the smoothed surface increases instead of the fracture surface. The distribution of hardness in the thickened edge for b = 1.0 mm and h = 0.9 mm is shown in Fig. 5. The hardness was measured along the sheared edge from the bottom corner. The hardness in the thickened edge is higher than that before forming. 4. Increase of stiffness and fatigue strength of thickened sheet in bending and tensile tests The effect of the step shape on the bending stiffness of the thickened sheet is shown in Fig. 6. The bending stiffness was measured in elastic deformation of the three-point bending test. For comparison, the bending stiffness of the trimmed edges for R = 15 mm and 20 mm is shown. The bending stiffness increases with increasing of the step width and depth of the die. The bending stiffness of the thickened sheet is higher than that of the trimmed sheet for R = 15 mm. 500 1180 MPa
300 780 MPa(Blank)
0
6
5
780 MPa 980 MPa(Blank)
2
1 x
150 100
R = 15 mm R = 10 mm
50
0.2
0 1
2
3 4 Position x [mm]
5
Fig. 5. Distribution of hardness in thickened edge for b = 1.0 mm and h = 0.9 mm.
(a) R =10 mm
1.0 0.5 Step width b [mm] (a) h = 0.9 mm
6
(c) b = 1.0 mm
(b) b = 0.75 mm (d) b = 1.25 mm Fig. 7. Fracture after fatigue bending test for 980 MPa and h = 0.9 mm.
1.5
R = 20 mm 150
R = 15 mm R = 10 mm
100 50 0
0.6 0.3 Step depth h [mm] (b) b = 1.0 mm
0.9
Fig. 6. Effect of step shape on bending stiffness of thickened sheet.
Number of cycles to fracture [104]
100
4
3
0.2
0.5
200
R = 20 mm
Bending stiffness [N/mm]
Bending stiffness [N/mm]
980 MPa
14 12 10 8 6 4 2 0
Polished R = 10 mm b = 0.75 mm 1.0 mm 1.25 mm
1180 MPa (Blank)
0.2
Hardness [HV1]
400
40 200
200
780 MPa
980 MPa
1180 MPa
Fig. 8. Number of cycles to fracture in bending test for h = 0.9 mm.
The fracture after the fatigue bending test for 980 MPa and h = 0.9 mm is shown in Fig. 7. The fracture occurs in the center of the sheet for the trimmed sheet and b = 0.75 mm. For b = 1.0 mm and 1.25 mm, the fracture portion shifts from the center due to the thickened edge.
Yohei Abe et al. / Procedia Manufacturing 15 (2018) 605–611 Author name / Procedia Manufacturing 00 (2018) 000–000
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The number of cycles to the fracture in the bending test for h = 0.9 mm is shown in Fig. 8. For comparison, the number for the polished sheet was measured. The number for the trimmed sheet is lower than that for the polished sheet due to plastic deformation of the edges in trimming. The number for the thickened sheet increases with increasing of the step width of the die, and is higher than those for the polished and trimmed sheets. The distribution of equivalent stress in the bending test for 980 MPa are shown in Fig. 9. The equivalent stress in the bending test was calculated using the finite element simulation. In the simulation, the effect of the plastic deformation in the trimming and thickening process was not considered. Although the high equivalent stress is observed on the sheet surface for R = 10 mm, the equivalent stress for the thickened sheet is eased without stress concentration. Therefore, it seems that the number of cycles for the thickened sheet increases by ease of the stress in Fig. 8. The effect of the thickened edge on the bending fatigue strength, bending stiffness and weight for 980MPa is shown in Fig. 10. The weight in the central portion is shown. The fatigue strength for R = 15 mm and the strength for b = 1.0 mm and h = 0.9 mm are almost same. The bending stiffness for b = 1.0 mm and h = 0.9 mm is higher than that for R = 15 mm. The weight for b = 1.0 mm and h = 0.9 mm is lighter than that for R = 15 mm, and thus the use of the thickened edge instead of the large corner radius is effective in the reduction in weight.
5
Step side
10
(b) b = 1 mm, h = 0.9 mm
Fig. 9. Distributions of equivalent stress in bending for 980 MPa.
8 6 4
4.9 4.8
4.7 4.6
Weight
2
4.5
0
5
200
4.9
150 100
4.8
Stiffness
12
Weight of central portion [g] Bending stiffness [N/mm]
5.1
Number
1.1 N・m
(a) R = 10 mm
14
Number of cycles to fracture [104]
Equivalent stress [MPa]
Fixed
50
4.7 4.6 4.5
4.4 4.4 0 10 15 20 1.0 1.25 0.75 10 15 1.0 1.25 0.75 0.9 0.9 0.9 0.9 0.9 0.9 R [mm] R [mm] b [mm] b [mm] Thickened Thickened h [mm] h [mm] (a) Fatigue strength (b) Bending stiffness Fig. 10. Effect of thickened edge on bending fatigue strength, bending stiffness and weight for 980 MPa.
10 0
160 120 80
40 30
R = 10 mm
20 10
0 1.5 0.9 1.0 0.5 0.6 0.3 Step depth h [mm] Step width b [mm] (b) b = 1.0 mm (a) h = 0.9 mm Fig. 11. Effect of step shape on tensile stiffness for 980 MPa.
40 0
Polished R=10 mm b = 0.75 mm 1.0 mm 1.25 mm
20
R = 10 mm
60 50
780 MPa
104
980 MPa (a) h = 0.9 mm
1180 MPa
200 160 120 80 40 0
Polished R=10 mm h = 0.3 mm 0.6 mm 0.9 mm
30
Number of cycles to fracture [103]
40
200
Number of cycles to fracture [103]
Tensile stiffness [kN/mm]
60
Tensile stiffness [kN/mm]
104
50
Weight of central portion [g]
Central portion
780 MPa
980 MPa 1180 MPa (b) b = 1.0 mm Fig. 12. Number of cycles to fracture in tensile test.
Yohei Abe et al. / Procedia Manufacturing 15 (2018) 605–611 Author name / Procedia Manufacturing 00 (2018) 000–000
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The effect of the step shape on the tensile stiffness for 980 MPa is shown in Fig. 11. The tensile stiffness of thickened sheet is higher than that of the trimmed sheet for R = 10 mm. The tensile stiffness increases with increasing of the step width of die, whereas the effect of step depth on the stiffness is not observed. The number of cycles to fracture in the tensile test are shown in Fig. 12. The number for the polished sheet is the highest. The number for the trimmed sheet is reduced by trimming with the large plastic deformation. The number for the thickened sheet with the small step width and depth of the die is larger than that for the trimmed sheet. The distribution of equivalent stress in the tension test for 980 MPa is shown in Fig. 13. Although the equivalent stress on the sheet surface for R = 10 mm is low, the stress concentration occurs around the rollover for the thickened sheet. It seems that the stress concentration brings early fracture in the fatigue test.
Equivalent stress [MPa] 1080 990 900 810 720 630 540 450 360 270 180 90 0
4.6 kN
1100 MPa
40
600 MPa
Fixed
i) Rollover side ii) Step side (a) R = 10 mm (b) b = 1.25 mm, h = 0.9 mm Fig. 13. Distributions of equivalent stress in tension for 980 MPa.
120 80 40
4.7 4.6 4.5
60 50 40 30 20 10 Weight
Tensile stiffness
160
4.8
70 Tensile stiffness [kN/mm]
200
4.9
Weight of central portion [g]
240
Number
Number of cycles to fracture [103]
280
5
80
4.9 4.8 4.7 4.6 4.5
Weight of central portion [g]
5
104
4.4 0 10 15 20 0.75 1.0 1.0 4.4 10 15 20 0.75 1.0 1.0 R [mm] 0.9 0.3 0.6 R [mm] 0.9 0.3 0.6 b [mm] Thickened b [mm] Thickened h [mm] h [mm] (a) Fatigue strength (b) Tensile stiffness Fig. 14. Effect of thickened edge on tensile fatigue strength, tensile stiffness and weight for 980 MPa. 0
The effect of the thickened edge on the tensile fatigue strength, tensile stiffness and weight for 980 MPa is shown in Fig. 14. The fatigue strength and stiffness in the tensile test for b = 1.0 mm and h = 0.3 mm are almost similar to those for R = 15 mm. The use of the thickened edge for b = 1.0 mm and h = 0.3 mm instead of trimming for R = 15 mm is effective in the reduction in the weight.
Yohei Abe et al. / Procedia Manufacturing 15 (2018) 605–611 Author name / Procedia Manufacturing 00 (2018) 000–000
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5. Conclusions In this study, a thickening process was applied for the concave edge to increase the stiffness and fatigue strength of the ultra-high strength steel sheets. The following results were obtained: 1) The length of the sheared edge was increased by the thickening process, and the ratio of the smoothed surface increased instead of fracture surface. 2) The bending stiffness increased with increasing of the step width and depth of the die. The number of cycles in the bending test for the thickened sheet increased with increasing of the step width of die, and was higher than those for the polished and trimmed sheets due to ease of stress. 3) The tensile stiffness of thickened sheet was higher than that of the trimmed sheet, and the number of cycles for the thickened sheet with the small step width and depth of die was higher. 4) The use of the thickened edge for the optimum condition instead of the large corner radius was effective in the reduction in the weight of the part. References [1] K. Mori, K. Akita, Y. Abe, Springback behaviour in bending of ultra-high-strength steel sheets using CNC servo press, International Journal of Machine Tools & Manufacture, 47-2 (2007) 321–325. [2] L. Komgrit, H. Hamasaki, R. Hino, F. Yoshida, Elimination of springback of high-strength steel sheet by using additional bending with counter punch, Journal of Material Processing Technology, 229 (2016) 199–206. [3] K. Hasegawa, K. Kawamura, T. Urabe, Y. Hosoya, Effects of microstructure on stretch-flange-formability of 980 MPa grade cold-rolled ultra high strength steel sheets, Iron and Steel Institute of Japan International, 44-3 (2004) 603–609. [4] P. Sartkulvanich, B. Kroenauer, R. Golle, A. Konieczny, T. Altan, Finite element analysis of the effect of blanked edge quality upon stretch flanging of AHSS, CIRP Annals - Manufacturing Technology, 59 (2010) 279–282. [5] T. Matsuno, Y. Kuriyama, H. Murakami, S. Yonezawa, N. Kanamaru, Effects of punch shape and clearance on hole expansion ratio and fatigue properties in punching of high strength steel sheets, Steel Research International, 81-9 (2010) 853–856. [6] X. Han, K. Yang, Y. Ding, S. Tan, J. Chen, Numerical and experimental investigations on mechanical trimming process for hot stamped ultrahigh strength part, Journal of Material Processing Technology, 234 (2016) 158–168. [7] K. Mori, Y. Abe, Y. Suzui, Improvement of stretch flangeability of ultra high strength steel sheet by smoothing of sheared edge, Journal of Material Processing Technology, 210-4 (2010) 653–659. [8] Y. Takahashi, O. Kawano, S. Horioka, K. Ushioda, Improvement of stretch flangeability of high-tensile-strength steel sheets by piercing under tension using humped bottom punch, SAE Technical Papers, SAE-2013-01-0609. [9] T. Matsuno, K. Sato, R. Okamoto, M. Mizumura, M. 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Kanamaru, Effects of punch shape and clearance on hole expansion ratio and fatigue properties in punching of high strength steel sheets, Steel Research International 81 Supplement Metal Forming, 2010 (2010) 853–856. [14] D.J. Thomas, M.T. Whittaker, G. Bright, Y. Gao, The influence of mechanical and CO2 laser cut-edge characteristics on the fatigue life performance of high strength automotive steels, Journal of Materials Processing Technology, 211-2 (2011) 263–274. [15] D.J. Thomas, Effect of mechanical cut-edges on the fatigue and formability performance of advanced high-strength steels, Journal of Failure Analysis and Prevention, 12 (2012) 518–531. [16] P. Kadarno, K. Mori, Y. Abe, Two-stage plate forging for thickening hole edge of punched high strength steel sheet, Proceedings of 14th International Conference on Metal Forming, Krakow, (2012) 899–902. [17] M. Merklein, J.M. Allwood, B.A. Behrens, A. Brosius, H. Hagenah, K. Kuzman, K. Mori, A.E. Tekkaya, A. 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Procedia Manufacturing 15 (2018) 605–611 Procedia Manufacturing 00 (2017) 000–000 www.elsevier.com/locate/procedia
17th International Conference on Metal Forming, Metal Forming 2018, 16-19 September 2018, 17th International Conference on MetalToyohashi, Forming, Metal Japan Forming 2018, 16-19 September 2018, Toyohashi, Japan
Thickening process of concave edge for increasing stiffness and
ThickeningEngineering processSociety of concave edge for increasing stiffness and Manufacturing Conference 2017, MESIC 2017, 28-30 June fatigue strength ofInternational ultra-high strength steel sheets 2017, Vigo (Pontevedra), Spain fatigue strength of ultra-high strength steel sheets
Yohei Abe*, Yuya Fujisawa, Yusuke Murai, Ken-ichiro 4.0: Mori Costing models for capacity optimization in Industry Yohei Abe*, Yuya Fujisawa, Yusuke Murai, Ken-ichiro Mori Trade-off Department of Mechanical Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan used Toyohashi capacity and operational efficiency Departmentbetween of Mechanical Engineering, University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan Abstract A. Santanaa, P. Afonsoa,*, A. Zaninb, R. Wernkeb Abstract A thickening process was applied for thea concave to increase theGuimarães, stiffness and fatigue strength of the ultra-high strength steel Universityedge of Minho, 4800-058 Portugal b A thickening wasprocess, applied the for sheet the concave edge tounder increase stiffness fatigue strength steel Unochapecó, 89809-000 Chapecó, SC,and Brazil sheets. In the process thickening was trimmed the the tension between the taper punch of andthe theultra-high step die. strength The trimmed sheets. In bent the thickening process, the sheet under the die tension between theoftaper punch and step die. The trimmed edge was by the taper of the punch intowas thetrimmed corner step of the and the surface the sheared edgethe was ironed. After filling edge wascorner bent by thethe taper of the punch intowas thesheared. corner step the die andstep the surface thedie sheared was and ironed. After filling into the step, excessive material Theofeffect of the shape inofthe on theedge stiffness fatigue strength into corner and step,tensile the excessive was sheared. The effect the step shape in the die on thethat stiffness and fatigue strength in thethe bending tests wasmaterial investigated. The stiffness of theofthickened edge was larger than without thickening in both Abstract in bending tensile tests was investigated. stiffness of the thickened edgethan wasthat larger than that withoutinthickening in both thethe bending andand tensile tests. The fatigue strengthThe of the thickened edge was higher without thickening the bending test, the bending tensileintests. The fatigue strength thickened edge due was to higher that withoutItthickening the thickening bending test, whereas, theand strength the tensile test was lowerof inthe certain conditions stressthan concentration. was foundinthat in Under the concept production processes will pushed to be and increasingly interconnected, whereas, theedge strength inofultra-high the"Industry tensile strength test4.0", was steel lower in certain conditions due tobestress It was found that thickening the concave of the sheets is effective in the increase of concentration. the stiffness fatigue strength of the part.in the concave edge of the strength is effective in the increase of theIn stiffness and fatigue strength of the part. information based on ultra-high a real time basis steel and, sheets necessarily, much more efficient. this context, capacity optimization © 2018 The Authors. Published by of Elsevier B.V.maximization, contributing also for organization’s profitability and value. goes beyond the traditional aim capacity © 2018 2018 The The Authors. Published Elsevier B.V. © Authors. Published by by B.V. committee Peer-review under responsibility of Elsevier the scientific of the 17th International Conference on Metal Forming. Indeed, lean management continuous improvement approaches suggest capacity optimization Peer-review under responsibilityand of the scientific committee of the 17th International Conference on Metal Forming. instead of Peer-review under responsibility of the scientific committee the 17thmodels International Metal Forming. maximization. The study of capacity optimization andofcosting is anConference important on research topic that deserves Keywords: Sheelfrom metal forming; strength steel sheets; SheetThis edge;paper Stiffness; Fatigue strength contributions both theThickening; practicalUltra-high and theoretical perspectives. presents and discusses a mathematical Keywords: Sheel metal forming; Thickening; Ultra-high strength steel sheets; Sheet edge; Stiffness; Fatigue strength model for capacity management based on different costing models (ABC and TDABC). A generic model has been developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s 1. Introduction value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity 1. Introduction optimization might hide operational inefficiency. The reduction in weight of automobile is important to improve the fuel efficiency. For the weight reduction of © 2017 Authors.inPublished by automobile Elsevier B.V.is important to improve the fuel efficiency. For the weight reduction of The The reduction of body-in-white parts, weight application of the high strength steel and ultra-high strength steel sheets is attractive due to the Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference body-in-white parts, application of the high strength steel and strength steel sheets is attractive to the high specific strength and cost competitiveness. The stiffness ofultra-high the parts tends to decrease because the sheetdue thickness 2017. high specific and cost competitiveness. stiffness of the tendsthe to decrease because thickness is reduced forstrength the lightweight in the applicationThe of these sheets. Inparts addition, deflection of the the partsheet is reduced by is reduced for the lightweight in the application of these sheets. In addition, the deflection of the part is reduced by Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Operational Efficiency
1. Introduction
* Corresponding author. Tel.: +81-532-44-6705; fax: +81-532-44-6690. * Corresponding author. Tel.: +81-532-44-6705; fax: +81-532-44-6690. E-mail address: [email protected] The cost of idle capacity is a fundamental information for companies and their management of extreme importance E-mail address: [email protected]
in modern©production systems. In general, it isB.V. defined as unused capacity or production potential and can be measured 2351-9789 2018 The Authors. Published by Elsevier 2351-9789 2018 Authors. Published Elsevier B.V.hours of the Peer-review underThe responsibility of theby scientific committee 17th International on Metal Forming. in several©ways: tons of production, available manufacturing, etc.Conference The management of the idle capacity Peer-review under responsibility thefax: scientific committee * Paulo Afonso. Tel.: +351 253 510of 761; +351 253 604 741 of the 17th International Conference on Metal Forming. E-mail address: [email protected]
2351-9789 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 2017. 2351-9789 © 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 17th International Conference on Metal Forming. 10.1016/j.promfg.2018.07.284
Yohei Abe et al. / Procedia Manufacturing 15 (2018) 605–611 Author name / Procedia Manufacturing 00 (2018) 000–000
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increasing in the stiffness of the high stress portion such as concave and hole edges in the part. On the other hand, the fatigue strength around the corner and hole edges of the part becomes low under the concentration of tensile stress. The high strength steel and ultra-high strength steel parts are generally formed by cold stamping. In stamping, bending and shearing such as punching and trimming are used. In bending, large springback and fracture are problematic due to the low ductility and high strength. The springback is reduced by overbending, stretch forming, bottoming, etc. Bottoming using a servo press was effective in the reduction in the springback in bending of ultrahigh strength steel sheets [1]. The springback in U-bending of high strength steel sheet was reduced by additional bending with a counter punch [2]. The concave bending operations are generally included in stretch-flanging. In stretch-flanging and hole expansion, the fracture tend to be caused by the tensile stress. The fracture limit is improved by controlling microstructure and heat treatment of the sheets [3]. The quality of sheared edge affected on the fracture [4, 5], and the simulation of the trimming process of the ultra-high strength steel part was carried out [6]. A smoothing process of the sheared edge [7], the punching with a humped bottom punch [8] and the punch having shear angle [9] were developed to improve the quality of sheared edge. A gradually contacting punch was proposed for reliving the concentration of tensile stress in stretch-flanging [10]. In shearing of the high strength steel sheets such as ultra-high strength steel sheets, short tool life [11], a risk of flying scraps [12], low quality of sheared edge etc. are problematic. The quality of sheared edge affected on the fracture in hole expansion, flanging and the fatigue strength of the sheet [4, 7, 13-16]. To increase the quality, plate forging [17] was effective. To improve fatigue strength of the high strength steel sheet, two-stage plate forging for thickening the hole edge of the punched sheet was introduced [18], and flanging using the step die was developed [19]. Then the thickening process was included in the punching process for the ultra-high strength steel sheets [20]. However, the developed processes were not for the trimmed sheet edges, but for the punched hole edges. The effect of the process on the concave edge for increasing stiffness and fatigue strength of the ultra-high strength steel sheets is unclear. In this study, a thickening process using the taper punch and the step die was applied for the concave edge to increase the stiffness and fatigue strength of the ultra-high strength steel sheets. The effect of the step shape in the die on the stiffness and fatigue strength was investigated. Nomenclature b h x
step width of die step depth of die position
2. Thickening process of concave edge The thickening process of the concave edge in the ultra-high strength steel sheet is shown in Fig. 1. The sheet held by the blank holder is trimmed under the tension between the taper punch and the step die. The trimmed edge is bent by the taper of the punch into the corner step of the die and the surface of the sheared edge is ironed. After filling into the corner step of the die, the excessive material is sheared. The corner step of the die has the function of forming the thickened edge and compressing the edge bottom. 20
30 Blank
Backup plate
Step die
(a) Before thickening
10°Blank holder
Filling (b) Thickening
Scrap (c) Thickened edge
Fig. 1. Thickening process of concave edge in ultra-high strength steel sheet.
10
h
Taper punch
90 40
Taper punch
Blank Backup plate
b
Step die
(a) Dimensions of blank (b) Thickening Fig. 2. Blank and thickening conditions.
Yohei Abe et al. / Procedia Manufacturing 15 (2018) 605–611 Author name / Procedia Manufacturing 00 (2018) 000–000
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The blank and the thickening conditions are shown in Fig. 2. The concave edges in the blank were thickened. The concave radii before and after thickening were 15 mm and 10 mm, respectively. The die having the depth h and width b of the step was used. The diameter and angle of the taper punch is 20 mm and 10o, respectively. The mechanical properties of the ultra-high strength steel and high strength steel sheets are shown in Table 1. The sheets are dualphase steel, and the sheet thickness is 1.2 mm. Table 1. Mechanical properties of ultra-high strength steel sheets. Sheet
1180 MPa
980 MPa
780 MPa
Tensile strength [MPa]
1287
1029
799
Reduction in area [%]
51.8
52.4
62.5
3. Thickening process and thickened edge The punch load-stroke curve and the thickened edges for 980 MPa and h = 0.9 mm are shown in Fig. 3. The punch load increases with increasing of stroke. The maximum punch load is similar to that by trimming for R = 10 mm. In the trimmed edge, the ratio of the fracture surface is high due to the low ductility of the sheet. In the thickened edges, the edge length is increased by thickening, and the ratio of the smoothed surface increases, whereas the ratio of the rollover increases. A A’
Punch load [kN]
40
1 mm
Rollover
Rollover Burnished
Smoothed
Fracture
R = 10 mm
Fracture
(b) R = 10 mm
30 20
Rollover
Rollover
10
(c) b = 0.75 mm
Smoothed
Smoothed
Fracture Fracture 4 8 12 (e) b = 1.25 mm Punch stoke [mm] (d) b = 1.0 mm (a) Punch load-stroke curve for b = 1.0 mm Fig. 3. Punch load-stroke curve and thickened edges for 980 MPa and h = 0.9 mm.
2
(a) R = 10 mm
0 0.5 1 1.5 2
Smoothed
b = 0.75 mm
1.25 mm
Position from top surface [mm]
1.5
1180 MPa
1
Fracture
780 MPa
0.5
Position from top surface [mm]
Burnished
Rollover 0
980 MPa
Position from top surface [mm]
0
0
Rollover
0.5 1 1.5 2
(b) h = 0.9 mm Fig. 4. Ratio of trimmed and thickened edges.
Smoothed Fracture h = 0.3mm 0.6 mm (c) b = 1.0 mm
0.9 mm
Yohei Abe et al. / Procedia Manufacturing 15 (2018) 605–611 Author name / Procedia Manufacturing 00 (2018) 000–000
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The ratio of the trimmed and the thickened edges are shown in Fig. 4. In the trimmed edge, the ratio of the fracture surface increases with increasing of the tensile strength of sheet due to low ductility. The edge length increases with increasing of the step depth of die. In the thickened edges, the ratio of the smoothed surface increases instead of the fracture surface. The distribution of hardness in the thickened edge for b = 1.0 mm and h = 0.9 mm is shown in Fig. 5. The hardness was measured along the sheared edge from the bottom corner. The hardness in the thickened edge is higher than that before forming. 4. Increase of stiffness and fatigue strength of thickened sheet in bending and tensile tests The effect of the step shape on the bending stiffness of the thickened sheet is shown in Fig. 6. The bending stiffness was measured in elastic deformation of the three-point bending test. For comparison, the bending stiffness of the trimmed edges for R = 15 mm and 20 mm is shown. The bending stiffness increases with increasing of the step width and depth of the die. The bending stiffness of the thickened sheet is higher than that of the trimmed sheet for R = 15 mm. 500 1180 MPa
300 780 MPa(Blank)
0
6
5
780 MPa 980 MPa(Blank)
2
1 x
150 100
R = 15 mm R = 10 mm
50
0.2
0 1
2
3 4 Position x [mm]
5
Fig. 5. Distribution of hardness in thickened edge for b = 1.0 mm and h = 0.9 mm.
(a) R =10 mm
1.0 0.5 Step width b [mm] (a) h = 0.9 mm
6
(c) b = 1.0 mm
(b) b = 0.75 mm (d) b = 1.25 mm Fig. 7. Fracture after fatigue bending test for 980 MPa and h = 0.9 mm.
1.5
R = 20 mm 150
R = 15 mm R = 10 mm
100 50 0
0.6 0.3 Step depth h [mm] (b) b = 1.0 mm
0.9
Fig. 6. Effect of step shape on bending stiffness of thickened sheet.
Number of cycles to fracture [104]
100
4
3
0.2
0.5
200
R = 20 mm
Bending stiffness [N/mm]
Bending stiffness [N/mm]
980 MPa
14 12 10 8 6 4 2 0
Polished R = 10 mm b = 0.75 mm 1.0 mm 1.25 mm
1180 MPa (Blank)
0.2
Hardness [HV1]
400
40 200
200
780 MPa
980 MPa
1180 MPa
Fig. 8. Number of cycles to fracture in bending test for h = 0.9 mm.
The fracture after the fatigue bending test for 980 MPa and h = 0.9 mm is shown in Fig. 7. The fracture occurs in the center of the sheet for the trimmed sheet and b = 0.75 mm. For b = 1.0 mm and 1.25 mm, the fracture portion shifts from the center due to the thickened edge.
Yohei Abe et al. / Procedia Manufacturing 15 (2018) 605–611 Author name / Procedia Manufacturing 00 (2018) 000–000
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The number of cycles to the fracture in the bending test for h = 0.9 mm is shown in Fig. 8. For comparison, the number for the polished sheet was measured. The number for the trimmed sheet is lower than that for the polished sheet due to plastic deformation of the edges in trimming. The number for the thickened sheet increases with increasing of the step width of the die, and is higher than those for the polished and trimmed sheets. The distribution of equivalent stress in the bending test for 980 MPa are shown in Fig. 9. The equivalent stress in the bending test was calculated using the finite element simulation. In the simulation, the effect of the plastic deformation in the trimming and thickening process was not considered. Although the high equivalent stress is observed on the sheet surface for R = 10 mm, the equivalent stress for the thickened sheet is eased without stress concentration. Therefore, it seems that the number of cycles for the thickened sheet increases by ease of the stress in Fig. 8. The effect of the thickened edge on the bending fatigue strength, bending stiffness and weight for 980MPa is shown in Fig. 10. The weight in the central portion is shown. The fatigue strength for R = 15 mm and the strength for b = 1.0 mm and h = 0.9 mm are almost same. The bending stiffness for b = 1.0 mm and h = 0.9 mm is higher than that for R = 15 mm. The weight for b = 1.0 mm and h = 0.9 mm is lighter than that for R = 15 mm, and thus the use of the thickened edge instead of the large corner radius is effective in the reduction in weight.
5
Step side
10
(b) b = 1 mm, h = 0.9 mm
Fig. 9. Distributions of equivalent stress in bending for 980 MPa.
8 6 4
4.9 4.8
4.7 4.6
Weight
2
4.5
0
5
200
4.9
150 100
4.8
Stiffness
12
Weight of central portion [g] Bending stiffness [N/mm]
5.1
Number
1.1 N・m
(a) R = 10 mm
14
Number of cycles to fracture [104]
Equivalent stress [MPa]
Fixed
50
4.7 4.6 4.5
4.4 4.4 0 10 15 20 1.0 1.25 0.75 10 15 1.0 1.25 0.75 0.9 0.9 0.9 0.9 0.9 0.9 R [mm] R [mm] b [mm] b [mm] Thickened Thickened h [mm] h [mm] (a) Fatigue strength (b) Bending stiffness Fig. 10. Effect of thickened edge on bending fatigue strength, bending stiffness and weight for 980 MPa.
10 0
160 120 80
40 30
R = 10 mm
20 10
0 1.5 0.9 1.0 0.5 0.6 0.3 Step depth h [mm] Step width b [mm] (b) b = 1.0 mm (a) h = 0.9 mm Fig. 11. Effect of step shape on tensile stiffness for 980 MPa.
40 0
Polished R=10 mm b = 0.75 mm 1.0 mm 1.25 mm
20
R = 10 mm
60 50
780 MPa
104
980 MPa (a) h = 0.9 mm
1180 MPa
200 160 120 80 40 0
Polished R=10 mm h = 0.3 mm 0.6 mm 0.9 mm
30
Number of cycles to fracture [103]
40
200
Number of cycles to fracture [103]
Tensile stiffness [kN/mm]
60
Tensile stiffness [kN/mm]
104
50
Weight of central portion [g]
Central portion
780 MPa
980 MPa 1180 MPa (b) b = 1.0 mm Fig. 12. Number of cycles to fracture in tensile test.
Yohei Abe et al. / Procedia Manufacturing 15 (2018) 605–611 Author name / Procedia Manufacturing 00 (2018) 000–000
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The effect of the step shape on the tensile stiffness for 980 MPa is shown in Fig. 11. The tensile stiffness of thickened sheet is higher than that of the trimmed sheet for R = 10 mm. The tensile stiffness increases with increasing of the step width of die, whereas the effect of step depth on the stiffness is not observed. The number of cycles to fracture in the tensile test are shown in Fig. 12. The number for the polished sheet is the highest. The number for the trimmed sheet is reduced by trimming with the large plastic deformation. The number for the thickened sheet with the small step width and depth of the die is larger than that for the trimmed sheet. The distribution of equivalent stress in the tension test for 980 MPa is shown in Fig. 13. Although the equivalent stress on the sheet surface for R = 10 mm is low, the stress concentration occurs around the rollover for the thickened sheet. It seems that the stress concentration brings early fracture in the fatigue test.
Equivalent stress [MPa] 1080 990 900 810 720 630 540 450 360 270 180 90 0
4.6 kN
1100 MPa
40
600 MPa
Fixed
i) Rollover side ii) Step side (a) R = 10 mm (b) b = 1.25 mm, h = 0.9 mm Fig. 13. Distributions of equivalent stress in tension for 980 MPa.
120 80 40
4.7 4.6 4.5
60 50 40 30 20 10 Weight
Tensile stiffness
160
4.8
70 Tensile stiffness [kN/mm]
200
4.9
Weight of central portion [g]
240
Number
Number of cycles to fracture [103]
280
5
80
4.9 4.8 4.7 4.6 4.5
Weight of central portion [g]
5
104
4.4 0 10 15 20 0.75 1.0 1.0 4.4 10 15 20 0.75 1.0 1.0 R [mm] 0.9 0.3 0.6 R [mm] 0.9 0.3 0.6 b [mm] Thickened b [mm] Thickened h [mm] h [mm] (a) Fatigue strength (b) Tensile stiffness Fig. 14. Effect of thickened edge on tensile fatigue strength, tensile stiffness and weight for 980 MPa. 0
The effect of the thickened edge on the tensile fatigue strength, tensile stiffness and weight for 980 MPa is shown in Fig. 14. The fatigue strength and stiffness in the tensile test for b = 1.0 mm and h = 0.3 mm are almost similar to those for R = 15 mm. The use of the thickened edge for b = 1.0 mm and h = 0.3 mm instead of trimming for R = 15 mm is effective in the reduction in the weight.
Yohei Abe et al. / Procedia Manufacturing 15 (2018) 605–611 Author name / Procedia Manufacturing 00 (2018) 000–000
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5. Conclusions In this study, a thickening process was applied for the concave edge to increase the stiffness and fatigue strength of the ultra-high strength steel sheets. The following results were obtained: 1) The length of the sheared edge was increased by the thickening process, and the ratio of the smoothed surface increased instead of fracture surface. 2) The bending stiffness increased with increasing of the step width and depth of the die. The number of cycles in the bending test for the thickened sheet increased with increasing of the step width of die, and was higher than those for the polished and trimmed sheets due to ease of stress. 3) The tensile stiffness of thickened sheet was higher than that of the trimmed sheet, and the number of cycles for the thickened sheet with the small step width and depth of die was higher. 4) The use of the thickened edge for the optimum condition instead of the large corner radius was effective in the reduction in the weight of the part. 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