Phase Transformations in the Al-Si Coating during the Austenitization Step

Abstract

Nowadays more and more hot stamped steel sheets dedicated for the automotive body-in-white structure are pre-coated to prevent the steel surface against iron oxidation and decarburization during the austenitization step. For these applications, the coating is deposited by continuous hot-dipping the steel in an Al-Si bath. The Al-Si coating, at the delivery state, contains Al-grains, Al-Fe-Si ternary phases, Al-Fe binary phases. During the austenitization, the Al-Si coating transforms completely by inter-diffusion and solidification reactions. The mechanisms of Al-Fe-Si phase transformations at high temperature are almost unknown. The phase transformations occurring during austenitization define the final coating microstructure responsible for the in use properties of the product like spot welding, painting adherence or corrosion behavior. It is the aim of this paper to propose a new way of understanding the mechanisms of phase transformation in the Al-Si coating during the austenitization step (between 900 and 930°C) before the transfer into the hot-stamping press.

Delayed Fracture in Automotive Advanced High Strength Steel: A New Investigation Approach

“…The microstructure of the steel is typically ferrite‐perlite and it results in a fully martensitic after press hardening process, while the Al–Si coating transforms into Fe x –Al y (Si) compounds …”

“…Since diffusion is governed by the gradient of concentration, and since the hydrostatic component of the stress tensor enhances the hydrogen solubility where its value is higher, SSRT seems to be useful to study the phenomenon by increasing the gradient of concentration and so favoring a further accumulation of hydrogen. In addition, the test leaves hydrogen time enough to diffuse towards the critical points in the material, giving rise to the embrittlement effect . SSRT laboratory tests were used to determine a threshold diffusible hydrogen concentration for the material.…”

Tribological interactions and modelling of friction in hot stamping

“…A multi-layered structure forms and its thickness depends on temperature and time. The diffusion process is rather complex and a large number of phases can be obtained depending on process conditions (Grigorieva, et al 2011) (Fan and de Cooman 2012). Typically after heat treatment a five FeAl-Si layered structure is obtained (Grigorieva, et al 2011), (Suehiro, et al 2003) (Fan and de Cooman 2012) (Ghiotti, Bruschi and Borsetto 2011).…”

“…The layers are distinct due to variances in aluminium and iron percentages. Typically the first layer (from the substrate interface) contains Al5Fe2 (Windmann, Rottger and Theisen 2014) (Fan and de Cooman 2012) (Grigorieva, et al 2011) (Suehiro, et al 2003) or FeAl2 (Fan, Kim, et al 2010). The second layer contains Si-enriched FeAl (Suehiro, et al 2003) (Windmann, Rottger and Theisen 2014) or Fe2SiAl2 (Grigorieva, et al 2011) (Fan, Kim, et al 2010).…”

“…Typically the first layer (from the substrate interface) contains Al5Fe2 (Windmann, Rottger and Theisen 2014) (Fan and de Cooman 2012) (Grigorieva, et al 2011) (Suehiro, et al 2003) or FeAl2 (Fan, Kim, et al 2010). The second layer contains Si-enriched FeAl (Suehiro, et al 2003) (Windmann, Rottger and Theisen 2014) or Fe2SiAl2 (Grigorieva, et al 2011) (Fan, Kim, et al 2010). The third layer contains Al5Fe2 (Windmann, Rottger and Theisen 2014) (Grigorieva, et al 2011) or FeAl2 (Fan, Kim, et al 2010), (Suehiro, et al 2003).…”