Met. Mater. Int., Vol. 16, No. 2 (2010), pp. 185~195 doi: 10.1007/s12540-010-0405-0 Published 26 April 2010 Thermo-Mechanical Finite Element Analysis Incorporating the Temperature Dependent Stress-Strain Response of Low Alloy Steel for Practical Application to the Hot Stamped Part Hyun-Ho Bok 1, * , Myoung-Gyu Lee 2 , Hoon-Dong Kim 1 , and Man-Been Moon 1 1 Technical Research Center, Hyundai Hysco, 313, Donggok-ri, Songsan-myeon, Dangjin-gun, Chungnam 343-831, Korea 2 Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, Korea (received date: 17 March 2009 / accepted date: 20 July 2009) The isothermal stress-strain responses of a low alloy steel sheet (0.2C-0.1Si-1.4Mn-0.5Cr-0.01Mo-0.002B steel, 1.2t) at elevated temperature, which simulates the material response in the hot stamping process, were measured by the Gleeble3500 thermo-mechanical simulator. The measured stress-strain responses fitted to the Swift equations discretized within the deformation temperature were used for the practical finite element simulation of the hot stamping process, in which the complicated effects of the volumetric mismatch between phases and phase transformation inducing plasticity were effectively ignored due to the significant constraints by the press stamping. The material parameters for the thermo-mechanical FE simulations were determined by considering the effects of plastic work and phase transformation on the temperature history. A mini-sized b-pillar reinforcing part was used as the simulation model. In spite of the simplified approach adopted in this paper, the finite element procedure could provide important information on the temperature distribution, martensitic phase distribution (or final product hardness), and the effect of process parameters such as punch force on the performance of the final hot stamped product. Keywords: thermomechanical processing, mechanical properties, hot stamping, computer simulation 1. INTRODUCTION In an effort to save energy and reduce environmental pol- lution, advanced high strength steel (AHSS) has garnered increasing attention in the automotive steel industry. How- ever, the limited formability of AHSS leads to the develop- ment of a process controlled manufacturing technique known as hot stamping, in which both thin-gauge and ultra high strength are attainable by high temperature deformation and the emergence of a hard phase by transformation, respec- tively. For example, hot-stamped crash safety parts such as the b-pillar reinforcing part, bumper beam, and roof rail could be good replacements for parts using conventional HSLA or AHSS without sacrificing structural stiffness and crashworthiness. A conventional hot stamping process consists of heating a blank up to over 900 ° C followed by soaking for several minutes in a furnace for a full autenitization, transferring the blank to a press machine, and rapid forming followed by quenching with cooled stamping tools. Ultimate tensile strengths can reach up to 1500 MPa due to the formation of hard martensite from the austenite phase during the rapid cooling of the sheets inside the stamping tools. Therefore, the hot stamping process consisting of sheet stamping and unsteady heat transfer between the blank sheet and tools, and a phase transformation can be classified as metallo-thermo- mechanical problem. The unstable contact status between the blank sheet and tools caused by the geometric complexity of automotive parts may be one of the reasons for highly non-uniform tem- perature distribution and the cooling history during the pro- cess. The unexpected temperature distribution may cause the formation of unfavorable phases by diffusional phase trans- formations. To suppress the formation of soft phases and enhance athermal martensitic transformation, the hardenabil- ities of the steels should be increased so that a larger fraction of martensite can be transformed even at a moderate cooling speed. Boron steels are good examples that exhibit marten- site transformation under a cooling rate of -30 ° Cs -1 [1] and -25 ° Cs -1 [2] by the addition of boron, chromium, and man- ganese. In addition to the metallurgical aspect, an austenitic *Corresponding author: sky1975@hysco.com KIM and Springer