Study on temperature distribution of HSS in hot FLD test

In this paper, we study forming limit diagram (FLD) of high strength steel (HSS) under high temperature condition to simulate the hot stamping process. We investigate the sheet specimen temperature distribution in hot FLD test. First, we propose a simple model of heat transfer process in sheet specimen, in which the sheet specimen is separated into two sections due to different mechanism. In thermal radiation, the convection and thermal contact conductance are included in two partial differential equations (PDEs) to express the temperature distribution in these two sections (S₁ and S₂ sections). Second, hot FLD experiments (Nakazima test) are performed and temperature distributions of sheet specimen are investigated by thermal infrared imager. Furthermore, the experimental temperature distributions are compared with theoretical ones, and these two measurements make good agreement between each other. This conclusion validates the feasibility of proposed thermal model in hot stamping process. In addition, an uneven sheet temperature distribution is examined in expansion process experiment, we observe that the temperature of specimen increases from center to edge until it reaches the temperature of punch. The further theoretical analysis proves that this phenomenon is mainly caused by the punch through thermal contact conductance between punch and S₁ section, which indicates that a hot punch is indispensable to sustain the temperature of the deforming sheet. Third, the theoretical temperature distribution on S₁ section under different die height is calculated. The radiation heat flux of the sheet decreases as the die height is increasing, but the sheet temperature increment is small, which implies that the height of die affects the sheet temperature slightly. - Graphical abstract: Two PDEs are proposed to represent the sheet temperature distribution in the hot FLD test process with the heat transfer considered, such as convection, conductance and radiation heat transfer. The test about sheet temperature distribution in hot FLD test is carried out, the experimental data is compared with the theoretical one. The heat flux of different areas of the sheet is calculated and the result proves that the contact heat flux between sheet and punch is the major factor to affectthe sheet temperature distribution. Meanwhile, the temperature distribution of the area which contacts with the punch is calculated by using the die height as a parameter, the results represent that the die height affectsthe sheet temperature slightly. - Highlights: • An uneven sheet temperature distribution is found in hot FLD test. • The theoretical way is proposed to analyze the sheet temperature distribution. • The theoretical data agree well with the experimental data. • The thermal conductance is proved as a major factor affecting sheet temperature. • The die height affects the sheet temperature distribution slightly.