TY - BOOK

T1 - Fundamentals and prevention of zinc induced embrittlement in steel sheets during the direct press hardening process

AU - Tomasch, Melanie

N1 - embargoed until 21-12-2025

PY - 2021

Y1 - 2021

N2 - Direct press hardening of low alloy steel sheets is an efficient way of manufacturing high strength automotive components relevant for passenger safety in case of a crash. The direct press hardening process combines the advantages of good formability at elevated temperatures as well as high strength and good form accuracy after quenching in the closed die. To guarantee corrosion protection of the components during the time in service a galvannealed zinc coating is applied on the entire coil surface by hot dip galvanizing and a following annealing step prior to the manufacturing process. On the one hand this ensures a continuous coating thickness but on the other hand the coating has to withstand the austenitization, forming and quenching cycle of the subsequent press hardening process. Zinc has compared to steel a higher thermal expansion coefficient as well as a different metallographic structure leading to different forming, expansion and shrinking behavior during heating, forming and cooling which consequently could induce significant stresses and strains in the coating near region. Furthermore certain amounts of non solidified zinc phases either in a liquid or gaseous state might be present in the coating during forming depending on the present temperature and local iron-zinc content. These phases are made responsible for the propagation of coating cracks into the steel surface during the press hardening process due to their embrittling effect on the steel sheet. These phenomena are named Liquid Metal Embrittlement (LME) as well as Vapor Metal Embrittlement (VME) according to the phase of the embrittler metal zinc. The present work focuses on the effects of VME since the selected process temperatures and coating composition prevent the occurrence of significant amounts of liquid zinc. To be able to investigate the mechanisms of the crack formation in detail, press hardening experiments were conducted in a first step. Metallographic investigations of the press hardened specimens were subsequently carried out on the formed cracks as well as the material microstructure by means of light optical microscopy and scanning electron microscopy. Subsequently a thermo mechanically coupled numerical model of the conducted press hardening experiments was developed for the simulation software ABAQUS®. Accurate results for the prevalent conditions within the component during the manufacturing process and for the properties during the operating service time require the knowledge of temperature dependent thermal and mechanical material properties as well as occurring interactions between the sheet and the die. Since the final high strength of the component is achieved by a martensite phase transformation during quenching in the closed die, its effect on the material was taken into account in the simulation as well. This is accomplished by means of several user subroutines. The required input data for the numerical model was determined in a series of experiments. Consequently, the numerical model allows to analyze the forming and quenching process in the closed die in detail throughout the entire press hardening process cycle. The wall thickness distribution as well as the flange width of the components and the press forces during forming are used as parameters to compare the simulation results with forming experiments and to validate the chosen material model. By means of the conducted investigations the mechanisms and factors influencing the crack formation could be analyzed. Using the obtained data a postprocessing routine was developed including a physically based damage indicator to be able to predict the occurrence of cracks induced by zinc in a gaseous phase during direct press hardening.

AB - Direct press hardening of low alloy steel sheets is an efficient way of manufacturing high strength automotive components relevant for passenger safety in case of a crash. The direct press hardening process combines the advantages of good formability at elevated temperatures as well as high strength and good form accuracy after quenching in the closed die. To guarantee corrosion protection of the components during the time in service a galvannealed zinc coating is applied on the entire coil surface by hot dip galvanizing and a following annealing step prior to the manufacturing process. On the one hand this ensures a continuous coating thickness but on the other hand the coating has to withstand the austenitization, forming and quenching cycle of the subsequent press hardening process. Zinc has compared to steel a higher thermal expansion coefficient as well as a different metallographic structure leading to different forming, expansion and shrinking behavior during heating, forming and cooling which consequently could induce significant stresses and strains in the coating near region. Furthermore certain amounts of non solidified zinc phases either in a liquid or gaseous state might be present in the coating during forming depending on the present temperature and local iron-zinc content. These phases are made responsible for the propagation of coating cracks into the steel surface during the press hardening process due to their embrittling effect on the steel sheet. These phenomena are named Liquid Metal Embrittlement (LME) as well as Vapor Metal Embrittlement (VME) according to the phase of the embrittler metal zinc. The present work focuses on the effects of VME since the selected process temperatures and coating composition prevent the occurrence of significant amounts of liquid zinc. To be able to investigate the mechanisms of the crack formation in detail, press hardening experiments were conducted in a first step. Metallographic investigations of the press hardened specimens were subsequently carried out on the formed cracks as well as the material microstructure by means of light optical microscopy and scanning electron microscopy. Subsequently a thermo mechanically coupled numerical model of the conducted press hardening experiments was developed for the simulation software ABAQUS®. Accurate results for the prevalent conditions within the component during the manufacturing process and for the properties during the operating service time require the knowledge of temperature dependent thermal and mechanical material properties as well as occurring interactions between the sheet and the die. Since the final high strength of the component is achieved by a martensite phase transformation during quenching in the closed die, its effect on the material was taken into account in the simulation as well. This is accomplished by means of several user subroutines. The required input data for the numerical model was determined in a series of experiments. Consequently, the numerical model allows to analyze the forming and quenching process in the closed die in detail throughout the entire press hardening process cycle. The wall thickness distribution as well as the flange width of the components and the press forces during forming are used as parameters to compare the simulation results with forming experiments and to validate the chosen material model. By means of the conducted investigations the mechanisms and factors influencing the crack formation could be analyzed. Using the obtained data a postprocessing routine was developed including a physically based damage indicator to be able to predict the occurrence of cracks induced by zinc in a gaseous phase during direct press hardening.

KW - direct press hardening

KW - zinc coating

KW - zinc induced embrittlement

KW - micro cracks

KW - martensite phase transformation

KW - process simulation

KW - damage indicator

KW - direktes Presshärten

KW - Zinkbeschichtung

KW - zinkinduzierte Versprödung

KW - Mikrorisse

KW - Martensit Phasenumwandlung

KW - Prozesssimulation

KW - Schädigungsindikator

M3 - Doctoral Thesis

ER -