Experimental Study on Bending - Mechanics Characteristics and Its Application

Experimental Study on Bending - Mechanics Characteristicsof 22MnB5 and Its ApplicationRunqing Guo1,a, Liang Ying1,b and Ping Hu1,c1School of Automotive Engineering, Faculty of Vehicle Engineering and Mechanics, State Key Laboratory of Structural Analysis for Industrial Equipment,Dalian University of Technology, Dalian, China 116024a guorunqingdalian@,b yingliang_dlut@,c pinghu@ Keywords: V-shaped Heat Bending, 22MnB5, Optimal Bending Temperature, Pure Bending. Abstract. Special V-shaped heat bending test was designed for 22MnB5 high strength steel of different thickness; sheets, after staying 5 min in the furnace at around 1000℃ to austenize fully, were bended at different temperatures, the spring-back amount was investigated and microstructures of heat bending specimens are analyzed, through which the optimal temperature, 600℃-650℃, for bending forming is obtained; On the basis of research above, a real anti-collision beam was produced by hot stamping and pure bending was conducted on it, which showed qualified mechanics of the hot forming auto component.IntroductionFor growing energy shortage nowadays, it is an inevitable trend of developing fuel-efficient vehicles and eco-car. Studies have shown that lightweight is the most effective measure to achieve the goal and reducing vehicle mass by 10% can save fuel by 3%~7%[1]. Also it’s shown that steel is still the main material for vehicles, but transforming from the traditional mild steel to the ultra-high strength steel (UHSS)[2]. UHSS is the optimal path to make vehicle lightweight and meanwhile improve passive safety, because UHSS has high tensile strength, yield strength, bending strength, fatigue strength and impact energy absorption capacity. With increasing use of UHSS, the new process, hot stamping technology, is developed, through which stamping forming and quenching are achieved simultaneously. UHSS are applied comprehensively in the project of Ultra-light Steel Auto Body-Advanced Vehicle Technology Program (ULSAB-AVC) of International Iron and Steel Institute (IISI). The Body in white of the project is lightweight by 30% for using hot forming parts[2,3]. Nowadays, A pillar, B pillar, C pillar, door and roof reinforcements, bumpers and etc, are typical vehicle components formed by hot stamping[4,5]. In hot stamping process, complicated thermal- mechanical- transformation coupled relations and multi-scale problems are involved besides nonlinear friction problems of thermal boundary. And there are lots of thermal parameters affecting mechanical properties of the hot forming components.Substantially, the hot stamping process is a process of metal sheet bending so that bending properties of metal sheet influence its hot forming formability directly. For that reason, in this paper, Special V-Shaped heat bending test is designed, bending formability at different temperatures is investigated, and optimal bending forming temperature is achieved, which presents guidance function on actual hot stamping process. On the basis of above tests, a real anti-collision beam is stamped by hot forming process and its martensite content, hardness, and tensile strength are analyzed. In the end, pure bending test is conducted on the anti-collision beam, which shows its high bending strength and impact resistance.Base Material Characteristics of 22MnB5In the paper, the hot forming high strength steel investigated is a type of boron alloy steel called 22MnB5. Its composition is as shown in Table1. A small amount of boron added enhances the quenching hardenability of C-Mn steel and its strength.Table1 Composition of Boron Steel (Mass%)22MnB5 C Mn B P Cr Si Al SMax 0.240 1.290 0.0037 0.016 0.210 0.240 0.050 0.006Quenching 1000-1030 1548-1554 0.65 0.07 0.11 Special V-Shaped Heat BendingIt’s known that there is of much need to introduce hot forming process to stamp 22MnB5 HSS and quench it at the same time for ultra-high strength parts. The optimal stamping temperature (OST), a key parameter of hot stamping, affects blank formability and mechanical properties of hot forming parts directly. To get the OST, special V-shaped heat bending is designed imitating the real hot stamping, see Fig.1(a).(a) (b)Fig.1 (a) Special V-shaped Bending (b) Different stamping-temperatures-time curve In the test, the thermocouple, connected to the MX100 Data Acquisition Unit, is placed in the sheet middle. The sheet is transformed to V-shaped die quickly, after staying 5 min in the furnace at around 1000℃ to austenize fully. When the sheet temperature reaches the set temperature such as 500℃、600℃、700℃ and 800℃, the universal tensile testing machine is started to stamp at 500 mm/min speed and heat bending parts are formed. Also samples of 1.2mm, 1.6mm and 2.0mm are involved. The Temperature-Time curve of 1.6mm sheet is as shown in Fig.1(b). In Fig.1.(b), cooling rate reaches 508℃/s when stamping at 800℃and it decreases to 336℃/s at 500℃bending temperature, which will affect martensite forming and mechanical attributes obviously. Spring-back angles of specimens, including samples bended at room temperature, are also measured, which is as shown in Fig.2(e). It shows that the spring-back angle is smaller when bending at around 600℃-650℃, which explains that 22MnB5 has better plasticity and formability from one aspect. This conclusion agrees with Ma’s standpoint, which is “22MnB5 has better stamping formability at 650℃to 750℃ ” in [6]. For further research, microstructures of heat bending specimens are analyzed. As found in Fig.2, strip-shaped martensite is hard to find when bending at 500℃; strip-shaped martensite is the main microstructure of sheets stamped at 600℃ and 700℃ and martensite texture is more fine than that of sheets stamped at 800℃, although whose main microstructure is also martensite. From the microstructure, it’s also verified that 22MnB5 has better plasticity and formability when stamping at 600℃-650℃.Fig.2 Microstructure of sheets bended at (a)500℃ (b)600℃ (c)700℃ (d)800℃(e) Bending-temperature - spring-back angle curveThe Hot Forming Anti-Collision BeamOn the basis of above work, a real anti-collision beam is hot stamped at around 600℃-650℃, using 22MnB5 blank of 1.6mm, see Fig.3(a). Tensile strength of the beam reaches as high as 1500 Mpa; Average hardness of the six points of Fig.3(a) is 46 HRC; the main microstructure of the anti-collision beam is analyzed, as shown in Fig.3(b). It’s clear that most microstructure of the beam by hot forming is strip-shaped martensite, whose content is over 95%, and there are no cracks and other stress defects. So the hot forming process for the anti-collision beam is qualified.(a) (b)Fig.3 (a) Hot stamping for the beam (b) the main microstructure of the beam Then pure bending test is conducted on the hot stamping beam including shotpeening ones, compared with the cold stamping one, see Fig.4(a). In Fig.4(b) , the bending strength of cold stamping beams, 44 Mpa, is only one third of hot stamping ones’, 115 Mpa. The maximum bending load of hot forming specimens is as high as 11000 N, but that of cold stamping ones is only 5000 N. It reveals that hot stamping significantly enhances bending strength, energy absorption capability andpassive safety of auto parts. The test also shows that shotpeening process has little effect on mechanical properties of hot forming parts.(a) (b)Fig.4 (a) Pure bending test of the beam (b) The deformation-load curve of pure bending ConclusionsV-shaped heat bending test shows around 600 ℃ -650℃is optimal for bending forming, according to which a real anti-collision beam is hot stamped. The beam is of 1500 Mpa tensile strength, 115 Mpa bending strength, 46 HRC hardness and 95% martensite content, which is satisfying with the expected level and engineering application. Besides, the pure bending test shows shotpeening process has little effect on mechanical properties of hot forming parts.AcknowledgmentThis work was funded by the Key Project of the National Natural Science Foundation of China (No. 10932003), “863” Project of China (N o . 2009AA04Z101), and “973” National Basic Research Project of China (No. 2010CB832700).References[1] Senuma T: ISIJ International Vol. 41(6) (2001), p.520.[2] Li Wang, Xiongfei Yang, Jiangxin Lu: Iron and Steel Vol. 41(9)(2006),p.1, In Chinese[3] Hongtu Sun, Ping Hu, Ning Ma, Guozhe Shen, Bo Liu and Dinglu Zhou: Chinese Journal ofMechanical Engineering Vol. 23(2)(2010),p.252[4] Weili Xu, Shurong Guan, Jian Ai and Aihui Luo: World Iron & Steel Vol. 9(2) ( 2009),p.33, InChinese[5] Ning Ma, Ping Hu and Wei Guo: Transactions of Materials and Heat Treatment Vol. 31(11)(2010), p.33, In Chinese[6] Ping Hu, Ning Ma, Wei Guo, Guozhe Shen and Wenhua Wu. “Researching Progress of hotStamping Technique for Auto Body Load-carrying Parts with Extra-High Strength,” in Mechanics and Engineering Application Conference Proceedings, Shanghai, 2010, In Chinese。