Study of Transition Areas in Press-Hardened Steels in a Combined Tool for Hot and Cold Forming

Abstract

Press-hardening, also known as hot stamping, is a manufacturing process for producing car body parts that must meet the high demands of their mechanical properties and safety parameters. Moreover, these components often require different mechanical properties in different parts of the component. This work presents the press-hardening process in a special combined tool where one half of the tool is heated and the other half is cooled. The cooled part has been 3D printed due to the complexity of the internal cooling channels. The aim of this work is to investigate the variation of the microstructures in the sheet metal and the mechanical properties in relation to the cooling process in the tool and to determine the transition area where these properties cross over. Two steels were chosen for the experiment. The most commonly used steel 22MnB5, and an experimental high-strength steel with 0.2% C alloyed with manganese and aluminium. A temperature of 425 °C was set in the heated part of the tool, and different holding times in the tool were tested. In the heated part of the tool, a bainitic structure with a fraction of ferrite and retained austenite was formed, while in the quenched part of the tool, a martensitic transformation was promoted due to rapid cooling. In addition to microscopic analyses, mechanical tests and hardness measurements were also performed.

Effect of Forming and Heat Treatment Parameters on the Mechanical Properties of Medium Manganese Steel with 5% Mn

“…The lower part with lower strength and higher ductility absorbs the energy generated in a collision. The production of such tailored parts is very complex and uses various methods [ 10 , 15 , 16 , 17 ].…”

“…Another way is to change the heating or cooling parameters during the hot-pressing process. In this case, one part of the hardening tool is heated and another part of the forming tool is cooled [ 15 , 16 , 18 ]. One other possibility is the inclusion of a device allowing local annealing.…”

Mechanical Properties of High Carbon Low-Density Steels

“…Most of the existing concepts are aimed at something other than use in structural parts requiring high loads. This can only be achieved by appropriate hardening and tempering [ 13 ].…”

Experimental and Numerical Process Design for Press Partitioning of the New Q&P Steel 37SiB6

“…This also resulted in locally different mechanical properties and microstructures. In addition, a study by Votava et al [28] using a special die with differently tempered areas showed that controlled variation of sheet microstructure and mechanical properties can be achieved as a function of the cooling process in the die. These recent results clearly show that detailed knowledge of the local cooling rates throughout the part is necessary to realize the full potential of Q&P heat treatments in a forming process and to later predict part properties with a high degree of confidence.…”