State-of-the-Knowledge on Coating Systems for Hot Stamped Parts

• Published in: Steel research international 2012-05, Vol.83 (5), p.412-433
• Main Authors: Fan, Dong Wei, De Cooman, Bruno C.
• Format: Article
• Language: English
• Subjects: aluminized; coating; galvanized; hot stamping; intermetallics
• ISSN: 1611-3683; 1869-344X
• DOI: 10.1002/srin.201100292

In the present contribution, recent developments of coatings for hot stamped steels are reviewed. The use of bare steel in the initial hot stamping technology is discussed, including the application of lubricant oils which are used as oxidation inhibitors on bare steel surfaces. The aluminized coatings are introduced, focusing on the microstructure evolution of aluminized coatings during the hot stamping process. An analysis of the cracking of the coating, caused by the formation of brittle Fe–Al intermetallic phases and their high temperature deformation, is presented. The development of a ductile aluminide coating formed during the diffusion treatment of an aluminized coating is discussed. This aluminide coating can endure both high temperature oxidation and severe plastic deformation. The recently developed galvanized and galvannealed coatings are also reviewed and the influence of the gas atmosphere during the heating cycle on the coating stability is emphasized. The solutions which have been proposed to avoid liquid Zn‐induced embrittlement are analyzed. The use of Zn–Ni alloy coating, which is characterized by a higher melting temperature, is reviewed. The behavior of sol–gel hybrid coatings on hot stamped steels is discussed. The possible use of the recently developed Al–Zn alloy coatings, dual layer Zn–Al and Zn–Al–Mg coatings is also introduced. The application of Zn–Al–Mg post‐process galvanizing is also discussed. In each case, all available information related to the weldability, paintability, and corrosion resistance of the coating systems is also reported. Finally, the advantages and technical challenges associated with each type of coating are reviewed. The contribution is a comprehensive review of the state‐of‐the‐knowledge on coating systems developed for steel grades used in hot stamping. The properties of the widely used aluminized coatings and the Zn and Zn‐alloy coatings are discussed in detail. The alternative Zn–Ni alloy coating, the hybrid coatings, and newer types of coatings are also reviewed. The physical metallurgical mechanisms which control the microstructural evolution of the coating during the thermo‐mechanical cycle of the hot press forming process and the associated challenges related to the use of each specific coating are presented.