Effects of rapid conductive heating and passive cooling with cold dies on hot-forming formability and final mechanical properties of boron steel sheets

This paper is focused on the assessment of hot-forming limits and post-process properties of the 22MnB5 steel sheets heated to 950°C using rapid conductive heating, which is proposed as a fast and efficient method. An experimental axisymmetric bulging die set appropriate for conductive heating was designed and manufactured. Stretch-forming tests were conducted on rectangular specimens with three different widths until failure using dies at room temperature. The die set at room temperature performed passive cooling as well as deformation. The tests showed the limits of formability. Microhardness and microstructure analyses of the formed steel proved the bainitic–martensitic nature obtained at the end of the process. The results show good agreement with the published data collected by conventional furnace heating, which is a slower and more inefficient process.

Conductive Heating during Press Hardening by Hot Metal Gas Forming for Curved Complex Part Geometries

Climate targets set by the EU, including the reduction of CO2, are leading to the increased use of lightweight materials for mass production such as press hardening steels. Besides sheet metal forming for high-strength components, tubular or profile forming (Hot Metal Gas Forming—HMGF) allows for designs that are more complex in combination with a lower weight. This paper particularly examines the application of conductive heating of the component for the combined press hardening process. The previous Finite-Element-Method (FEM)-supported design of an industry-oriented, curved component geometry allows the development of forming tools and process peripherals with a high degree of reliability. This work comprises a description regarding the functionality of the tools and the heating strategy for the curved component as well as the measurement technology used to investigate the heat distribution in the component during the conduction process. Subsequently, forming tests are carried out, material characterization is performed by hardness measurements in relevant areas of the component, and the FEM simulation is validated by comparing the resulting sheet thickness distribution to the experimental one.

Investigation on the mechanical properties of press-hardened boron steel sheets using the conductive heating technique

An aluminized 22MnB5 (Boron) steel sheet, used for structural parts in the automotive industry, was subjected to press-hardening followed by austenitizing, both in a conventional furnace and via the conductive (electric resistance) heating method, an innovative technique based on the Joule’s principle for fast heating of the sheet metal. Conductive heating presents a number of advantages over the in-furnace heating method. These include a more efficient use of energy, as well as the requirement of less time and space for heating, thus lowering costs. After press-hardening was performed using both methods, the microstructural and mechanical characterizations of both specimens were examined for optical microscopy, hardness, tensile strength, and high-speed impact tests. The results showed that the press-hardening process transformed the ferritic–pearlitic microstructure in the as-received state into martensite after die quenching and caused a substantial increase in hardness and strength at the expense of ductility and impact toughness. On the other hand, no significant difference was observed in either the microstructure or mechanical properties with respect to the heating method used. The results obtained in the present investigation concur with the findings of current literature.