Determination of interfacial heat transfer coefficient and analysis of influencing factors in warm forming the third-generation automotive medium-Mn steel

The third-generation automotive medium-Mn steel (TAMM steel) was newly developed. The medium-Mn steel holds the advantages of fine and even microstructure, low austenizing temperature and wide quenching rate range, and thereby it is possible to contribute to excellent properties, high efficiency and low cost in warm/hot forming. However, research on the forming of this TAMM steel has just started. It is necessary to understand the relation between forming process and heat transfer, and further apply the heat transfer characteristics to numerical simulation. Therefore, this paper explored the interfacial heat transfer rules during warm forming the TAMM steel and determined the interfacial heat transfer coefficient (IHTC) using a self-designed experimental platform and Beck's inverse estimation method. The results indicate that the IHTC is not constant but varies nonlinearly during quenching. Moreover, the influences of several factors, including stamping pressure, stamping temperature and surface coating, on the IHTC were investigated. The IHTC increases with the increase of stamping pressure and stamping temperature. However, it decreases greatly with the increase of coating thickness. It is also found that the microstructure and mechanical properties are not sensitive to the coating and quenching rate, which is beneficial to the achievement of even mechanical properties of the stamped TAMM part, and helpful to the flexible design of quenching method and coating application in actual warm stamping process.