Heat Transfer in Hot Stamping of High-Strength Boron Steel Sheets
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mping (HS) technology of high-strength boron steel sheets is widely recognized as the best manufacturing solution for producing structural components of car body-in-white. This technology offers a considerable potential for minimizing the weight of components by reducing the thickness of the sheet metal used and by reducing the component numbers needed.[1] The principle of HS technology is to austenize boron steel blanks in a furnace and then simultaneously form and quench them within a cooled tool. As the blank is quenched within the tool at a rate greater than 30 K/s, the austenitic microstructure transforms into martensite. A fully martensitic microstructure is generally desired due to very high tensile strengths of approximately 1600 MPa and Vickers hardness values in excess of 480 HV.[2,3] In a hot-stamped part with distributed properties, precise control of the final microstructure requires accurate knowledge of the thermal history within the blank. Commercial finite element method (FEM) necessitates a thorough understanding of the heat transfer between the quenched blank and the cooled tools.[4–7] Interface heat transfer coefficient (IHTC) between the blank and the tool is crucial to modeling blank temperature and predicting microstructure.
ZHIQIANG ZHANG, Associate Professor, and XIANSHUANG LI, Master Degree Candidate, are with the School of Material Science and Engineering, Jilin University, Changchun, P.R. China. Contact e-mail: [email protected] YONG ZHAO and XIANGJI LI, Associate Professors, are with the Roll Forging Research Institute, Jilin University, Changchun, P.R. China. Manuscript submitted February 15, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
Merklein and Lechler[8] studied the heat transfer between Usibor 1500P blanks and flat water-cooled dies. Using a lumped capacitance approach, the heat transfer coefficient (HTC) was inferred from Newton’s law of cooling. HTCs measured at contact pressures between 5.0 and 40.0 MPa were found to reach a maximum when the blank is within 673 K to 873 K (400 °C to 600 °C), reflecting the fact that the HTC should depend on both applied pressure and interface temperature. Further HS tests were conducted by Merklein et al.[6] to maintain higher dies temperatures using heating cartridges. Results for die temperatures of 373 K and 573 K (100 °C and 300 °C) and contact pressures from 0 MPa up to 30 MPa indicate that the HTC between the blank and the die increases with the die temperature and contact pressure. Abdul Hay et al.[9,10] measured the HTC between Usibor 1500P blank and a U-shaped die. The blank and tools were instrumented with subsurface thermocouples and Beck’s function specification method[11] was used to infer the heat flux at the blank/die interface. The blank surface temperature was not measured directly; instead, it was inferred by solving a direct heat conduction problem with the measured blank temperature as a Dirichlet boundary condition. The calculated HTC was found to increase rapidly and reach a maximum after approximately 10.0 seconds. A local mi
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