Prediction of Forming Limit Diagrams for 22MnB5 in Hot Stamping Process
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JMEPEG (2013) 22:2131–2140 DOI: 10.1007/s11665-013-0491-5
Prediction of Forming Limit Diagrams for 22MnB5 in Hot Stamping Process Hongzhou Li, Xin Wu, and Guangyao Li (Submitted September 28, 2012; in revised form January 15, 2013; published online February 12, 2013) Hot stamping of ultra-high strength steels possesses many superior characteristics over conventional room temperature forming process and is fairly attractive in improving strength and reducing weight of vehicle body product. However, the mechanical and failure behavior of hot stamping boron steel 22MnB5 are both strongly affected by strain hardening, temperature, strain rate, and microstructure. In this paper, the material yield and flow behavior of 22MnB5 within the temperature and strain rate range of hot stamping are described by an advanced anisotropic yield criterion combined with two different hardening laws. The elevated temperature forming limit diagram (ET-FLD) is constructed using the M-K theoretical analysis. The developed model was validated by comparing our predicted result with experimental data in the literature under isothermal conditions. Based on the verified model, the influence of temperature and strain rate on the forming limit curve for 22MnB5 steel under equilibrium isothermal condition are discussed. Furthermore, the transient forming limit diagram is developed by performing a transient forming process simulation under non-isothermal transient condition.
Keywords
BBC2005 yield function, forming limit diagrams, hot stamping process, mechanical characterization, nonisothermal deformation
1. Introduction The requirement for reducing automobile weight and emissions, improving safety and crashworthiness qualities motivates the application of advanced light weight materials, such as aluminum alloy, magnesium alloy, and high/ultra-high strength steels in vehicle body structures. Although high/ultrahigh strength steels have high strength/weight ratio compared with mild steels, their manufacturing at room temperature is difficult due to lower ductility, more severe springback and higher press tonnage requirement. Taking advantage of the characteristics of forming processes and heat treatments of metallic material, press hardening (hot stamping) process exhibits superior features in dealing with the mentioned problems with the high/ultra-high strength steels forming. The sheet metal, used in the direct hot stamping process, is formed in a full austenite state by heating it above A3 temperature and, subsequently, quenched in the closed tools with water cooling system. Hot stamping is a complex forming process mainly due to the fact that (a) the material properties are not only strain and strain path dependent (as that in conventional forming process at room temperature), but a strong function of strain rate and temperature, being the latter Hongzhou Li and Guangyao Li State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, China; and Xin Wu Department of Mechanical Engineering, Wayne
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