Study of front bumper process on the basis of hot stamping

-0-Study of front bumper process on thebasis of hot stampingWang Xiaona，Xiong Fei， Jin Jianwei(Geely Automobile Research Institute, Hangzhou 311228, China)Abstract: The ftting method is used for obtaining the constitutive relation of ultra-high strength steel in this pa-per. At the same time, the forming limit of material is researched under different deformation conditions.Through experiments, the paper obtains stress- strain curve under different deformation temperatures and strainrates. On the basis of experimental data, the constitutive relation model is established which can reflect the de-formation capacity of ultra- high strength steel during process of hot stamping. Combining with finite elementsimulation results of hot stamping by the commercial software AUTOFORM, transfer path of load and match-ing law of strength, the paper determines the design criteria and forming process requirements of parts manufac-tured by hot stamping. Through stting diferent process parameters, such as punch velocity and friction coffi-cient, and blank shape, the paper finds that process parameters and blank shape have an essential effect on thequality of parts.Key words: ultra-high strength steel; hot stamping; constitutive relation; forming limitand decreases with deformation temperature increa-1 Introductionsing.To make light- weighted automobile, it is veryFrom the above analysis, it can be concludednecessary that the ultra-high strength steel parts manu-that there is an enormous influence of deformationfactured by hot stamping are applied, which offers conditions on the behavior of ultra- high strength steel.the possibility to reduce the weight of automobiles For exact analysis of the front bumper process by us-and maintain the safety requirement ". In order toing finite element method, the constitutive relationshould be deduced. During metal thermal deforma-structure of parts reasonably, which is related with tion, there are two kinds of stress- strain curves, i.e.,reasonable matching of strength. The objective of pa-dynamic recovery and dynamic recrystallization. Atper is to guide the design of parts manufactured bythe high temperature deformation stage of the ultra-hot stamping and find forming technical requirements high strength steel, dynamic recovery could be con-while taking the vehicle performance into account 12sidered as the competition between hardening mecha-nism and softening mechanism, namely the competi-2 Constitutive relationtion between dislocation accumulation and dynamicThe ultra- high strength steel BTR165 is applied recovery, which can be shown as working hardeningfor the front bumper. Through experiments, the stress-rate 0. The working hardening rate decreases linearlystrain curves are obtained, which are shown in Fig. 1.with stress increasing. When the value of workingThe flow stress increases with strain rate increasing hardening rate is close to 0，it is concluded that theReceived 10 August 2014; accepted 25 August 2014CThe Author(s) 2014. This article is published with open access at engineering.cae.cndoi: 1.1532/-ENG1-2014020中国煤化工YHCNMHGVol. 12No.5, Oct.2014 33-φ--0-500一500Cshown in Fig. 2, the degree of ccoincidence is very。400-600 Cgood. The fitting parameters are shown in Table 1 and-700CTable 2.6 300二800号σ=σ, -(σ, - σ,)exp(-Bε )(2)品200E 10000一500C 二500 Cfting ._ 700C二.700ChttingTrue strainε= 0.015/s*150.400一800C 一800 Citig(a)500 C; 300600 Cg 4006300- - 900 c.0.05_ 0.1 0.150.2占100True srain ε=0.015/s()0.05 Trmest 0.100-- 600C一600 C fting一700 C一700 C fttingt400二900二900ttm-200C起400-二 700 C300-300f-800员2000目1000.05 0.10.15True strainε=0.1/s0.05_ 0o.iTrue strain ε-0.45/s00 C- 500 C ftingig. 1 Stress strain curves from experiments500---700 C- 700 C fttingooL _ - 900C - 900 Cfitting40flow stress reaches steady state and does not increase.Therefore, the following equation could be used forderiving the constitutive relation of ultra- high strengthsteel100 Fθ= C=A- Bσ(1)0.0.5. 0.Tue sraine 0.45/*method is adopted for treatment of the Eq. (1). TheFig. 2 Stress strain ftting curvesdynamic recovery constitutive relation is concluded,namely Eq. (2)， in which σp is steady stress. AsTable 1 Parametersof σ。and σp0.015/s0.1/s0.45/sTemperature/Cσ90047.13981.75056.578112.40557.924140.47866.494137.17283.226175.27290.000188.35870090.021166.934101.110228.529117.127260.919117.661288.628129.650298.112138.458411.211151359398.876176.000400.235184.889445.333中国煤化工MHCNMH G .34 ENGINEERING-φ--0-Table 2 Fitting results of BTemperature/Cblank shape is decided. At the same time, related pro-0.015/s0.1/s0.45/scess parameters are determined, as shown in Table 3.13.02913.66814.5160012.82513.15213.41412. 13412.84213.1716009.82012.63712.1878.38211.427Fig. 4 The geometric model of the front bumper3 Forming limitThe forming limit is a very important perfor-mance index， which reflects the maximum deforma-(a) Punchtion of the sheet metal before plastic instability. Theforming limit of the ultra- high strength steel BTR165is obtained by bulging test as shown in Fig. 3.(b) Blank holder0.(2) Female dieA T-7000■7-800CFig. 5 The die surface design of the front bumper*昏复Table 3 Initial process parameters0.4-ParameterValueInitial temperature/C9100.2-W aiting time/sPunch velocity(mm.s")120Blankholder stroke/mm0b.1 0.90.00.TPunch stroke/mmMinor strainDie temperaturerCrig. 3 Forming limit of the ultra-high strengthHolding time/ssteel BTR165Holding pressure/N00 000From Fig. 3, it can be seen that the forming lim-5 Finite result analysisit value increases with the deformation temperature in-Fig. 6 shows the forming result of the front bum-creasing on the left. And at the right, there is diffe-per. There is extreme thinning in the reverse formingrent trend when the deformation temperature isregion. The maximum reduction ratio is 34.38 %. Du-600 C, 700 C, and 800 C, respectively. When the ring actual production process, cerack will appear indeformation temperature is 600 C and 700 C，the this region, which results in mechanical performanceforming limit value increases with minor strain in-defect. Therefore， the extreme thinning should becreasing. Comparing with the improving rate of theavoided in order to ensure the quality of the parts.forming limit under 700 C, it is more obvious under600 C. When the deformation temperature is 800 C，)the forming limit value decreases with minor strain in-creasing.(a4 Geometric model of the front bumperand parametersFig. 4 shows the geometric model of the frontbumper. It can be seen that there is reverse forming re-(b)gion which is difficult to form. Therefore, attentionshould be paid to this region when die is designed.Sphitisin ofspieInsur Comprss Thicking(C)According to the characteristics of the front bumper,Fig. 6 The form中国煤化工YHCNMHGVol." 12No.5, Oct. 2014 35-φ--0-6 Improving measuresOn the basis of above analysis，the maximum re-duction ratio occurs in groove. The groove is reverse(a) Deforming resultforming region. Before forming the groove，the mate-0.rial is drawn. Therefore, there is extreme thinning inthis region for lack of supplement of the material. Inorder to solve the problem, cold stamping is first ad-0,opted to form the groove. And hot stamping is adop-ted to form the other characteristics of the front bum-per. Fig. 7 shows the cold stamping die for formingthe groove.Fig.7 The cold stamping die for forming the groove0.20-0.15- 0.10-0.05 0.00 0.0; 0.10(b)FLDThe drawing velocity is 130 mm/s and blank0128775holder force is 50 000 N. The result after cold stam-ping is shown in Fig. 8. Because of high strength22MnB5 under room temperature and low ductility,(C) Maximum reduction ratiothe maximum reduction ratio is 17.93 % after coldFig. 9 The result after hot stampingstamping.ultra-high strength steel BTR165 under defferent defor-mation conditions of deformation temperautre andstrain rate. And the exact constitutive relation equa-050120305tion is selcted to obtain the formula by using fittingmethod. In comparision with the experimental data,Fig. 8 The result after cold stampingthe constitutive relation equation can exactly describethe trend of ultra- high strength steel under defferentThe hot stamping is applied to the front bumperdeformation conditions. At the same time, formingafter cold stamping of the groove. The structure of thelimit of ultra- high strength steel is also studied. Ondie is the same as that shown in Fig. 4. The parame-ters are defined as the followings: The drawing veloc-front bumper is researched by finite element method.ity is 120 mm/s, the temperature of the die is 25 9CAccording to the results， the quality defects areand the stroke of the blank holder is 90 mm. The re-found and the solution is proposed.sult after hot stamping is shown in Fig. 9. FromFig. 9, it can be seen that the maximum reduction ra-Referencestio decreases significantly with the cold stamping forI] Karbasian H, Tekkaya A E. A review on hot stamping [J]. Jour-the groove. With the method, the maximum reduc-nal of Materials Processing Technology, 2010, 210(15): 2103-18.8tion rate is 17.66 %，which satisfies the engineering[2] Merklein M, Lechler J. Investigation of the thermo- mechanicaldemand.properties of hot stamping steels []. Journal of Materials Proces-7 Conclusionssing Technology, 2006, 177(1-3): 452-455.The paper studies the stress-strain curves of中国煤化工YHCNMH G .36 ENGINEERING-φ--0-AuthorWang Xiaona, received her PhD degree in material science from Northwestern Polytechnical University in2010. Now she is working on automotive light weight technology as a principal engineer at Geely Automobile Re-search Insititute. She can be reached by E-mail: wangxiaona78@ 163.comFoundation items: National Science & Technology Pillar Program during the Twelfth Five- year Plan Period(No. 2011 BAG03B02, No. 2011BAG03B06)中国煤化工YHCNMHGVol. 12No.5, Oct.2014 37