Hot Press Forming behavior of Zn, Al-Si and Al-Zn alloy coatings on 22MnB5 Press Hardening Steel

Abstract

In present study, the metallurgical evolution of 22MnB5 steel and the interaction with the Al-Si alloy, pure Zn and Al-Zn alloy coating were systematically investigated for the press hardening thermo-mechanical cycle. The study analyzed the effect of the heating method, the heating rate, the soaking time and the coating type on 22MnB5 press hardening steel.

Various rapid heating methods have been developed to increase the productivity of press hardening steel. One of these methods is direct resistance Joule heating. This heating method results in the melting of the surface coating and the formation of a persistent liquid trail as a result of the high thermal conductivity and low melting temperature of the Al-10% Si alloy coating. The present work showed that this issue can be addressed by an alloying preheating treatment prior to the press hardening process.

The coatings on press hardening steel are needed to suppress high temperature oxidation and decarburization. Additional corrosion protection is provided by Zn coatings which provide cathodic protection to press hardened parts. Due to the low melting temperature of Zn and Zn-Fe intermetallic compounds, the Zn-coated PHSs are susceptible to LME during the die-quenching process. In the present work, the mechanical properties of Zn coated PHS were evaluated in terms of tensile properties and bending properties. A deterioration of the room temperature bendability, due to microcrack formation and propagation, was observed. The presence of Γ-Fe3Zn10 observed in the microcracks at room temperature indicated the presence of liquid Zn at the die-quenching temperature, making it possible to trace the progress of the liquid Zn phase during microcrack formation. The results suggest that Zn-grain boundary diffusion causes the phase transformation of the austenite grain boundary region to ferrite. This results in intergranular cracking due to the lower strength of ferrite. The microcrack formation is most severe in areas where the applied stress and the friction are highest during the forming process, making the occurrence of LME-mitigated microcrack propagation dependent on the local deformation conditions.

During the conventional die-quenching processing of a galvanized PHS steel, a thick ZnO layer is formed at the surface. In the present study, the characterization of the surface oxide formation was analysed for different heating rates. When the hating rate is increased, the oxide at the surface is a thin Al2O3 layer. This remarkable change in surface oxide during rapid heating is due to the partial melting of the coating instead of the solidification of the coating by Fe-Zn intermetallics formation.

Press hardening is increasingly being used to produce ultra-high strength steel parts for passenger cars. Al-Si alloy, pure Zn and Zn-alloy coatings have been used to provide corrosion protection to press hardening steel grades. The use of coatings has drawbacks such as coating delamination or liquid metal induced embrittlement. In the present work, the microstructural evolution of Al-Zn coating during press hardening was studied. The 55 wt. % Al-Zn coating can in principle provide both Al barrier protection and Zn cathodic protection to press hardened steel. During the heat treatment associated with the press hardening, the 55 wt. % Al-Zn alloy coating is converted to an intermetallic surface layer of Fe2Al5 and a FeAl intermetallic diffusion layer. The Zn is separated from both intermetallic compounds and accumulates at grain boundaries and at the surface. This Zn separation process is beneficial in terms of providing cathodic protection to Al-Zn coated press hardening steel. The use of an increased heating rate during the austenitization cycle resulted in a high volume fraction of Zn at FeAl grain boundaries, and the presence of Zn islands in Fe2Al5 grains. These microstructural features suppressed the high temperature oxidation and evaporation of Zn. The alloyed coating did not cause liquid metal induced embrittlement.