Microstructural Evaluation and Property Change
of 5 Wt Pct Al-Zn Coating on Press Hardening Steel
JUN-KAI CHANG, CHAO-SUNG LIN, WOEI-REN WANG, and WEI-JEN CHENG
Microstructural evolution of a 5 wt pct Al-Zn coating on press hardening steel was studied in
comparison to hot-dip galvanized (GI) and galvannealed (GA) coatings. The results show that
the presence of 5 wt pct Al eﬀectively suppresses oxidation during austenitization; meanwhile,
the presence of Fe resulting from galvannealing accelerates oxidation. Alloying with Al or Fe in
the coating prior to austenitization reduces the susceptibility to liquid metal embrittlement
(LME). The presence of Al in the as-coated Zn coating enhances the corrosion resistance in HCl
solution and reduces the cathodic kinetics in NaCl solution. However, for sacriﬁcial protection,
the austenitized GI steel outperforms the other austenitized-coated steels. Nevertheless, the 5
wt pct Al-Zn coating exhibits better overall performance including high-temperature oxidation
resistance, less LME susceptibility, and cathodic protection.
Ó The Minerals, Metals & Materials Society and ASM International 2018
high-strength steels (AHSS), including
dual-phase steel, twinning-induced plasticity steel, com-
plex phase steel, and martensitic ultrahigh-strength steel
(MART steel), have found many applications in the
automobile industry for developing lightweight vehicles
to meet demands of reduction in fuel consumption and
carbon dioxide emissions.
Speciﬁcally, the AHSS
sheets are applied in car cage parts, such as body-pillars,
bumpers, door beams, as well as roof rails, and
passenger survival in crash events is better ensured.
Hot stamping process is used to fabricate MART steel
parts to avoid springback associated with the conven-
tional cold forming technology.
steel, like MnB steel, has been increasingly used in the
automobile industry because its strength can be raised
from 400 MPa to 1500 ~ 1600 MPa after rapid cooling
in the forming die.
Prior to press forming, the press
hardening steel is generally heat treated at 880 ~ 950 °C
for 3 ~ 10 minutes,
when severe oxidation and
Therefore, surface modiﬁ-
cation to form a protective coating is essential for press
Hot-dip aluminizing and galvanizing are commonly
employed to fabricate corrosion protective coatings on
Al-10 wt pct Si coating is the widely
used coating on press hardening steel and has been
patented as Usibor by ArcelorMittal Corporation.
This coating can withstand oxidation and decarburiza-
tion during the austenitization treatment via the forma-
tion of an Al
Although the coating
formed after hot stamping oﬀers barrier protection, it
only imparts limited sacriﬁcial protection aﬀorded by
the resulting Fe-Al intermetallic phases.
tion of such a Fe-Al diﬀusion layer is not suﬃcient
under harsh corrosion environments.
Zn-based coating on steel is known to provide both
barrier protection and sacriﬁcial protection.
However, the process temperature of austenitization is
markedly higher than the melting point of Zn
(419.5 °C). The molten Zn layer suﬀers extensive oxi-
dation and evaporation.
The ZnO scale is evident
and deteriorates the resistance spot weldability of the
Molten Zn(Fe) also quickly reacts
with the steel substrate, leading to the formation of
Zn-saturated a-Fe [a-Fe(Zn)] and, if present, few
amounts of the Fe
(C) phase. The Zn content of
the coating layer has been substantially reduced and the
sacriﬁcial protection capability also decreases although
the coating layer still oﬀers barrier protection.
Moreover, molten Zn (usually with dissolved Fe)
contacting the steel substrate can cause liquid metal
JUN-KAI CHANG and CHAO-SUNG LIN are with the
Department of Materials Science and Engineering, National Taiwan
University, 1, Roosevelt Road, Section 4, Taipei 106, Taiwan. Contact
e-mail: email@example.com WOEI-REN WANG is with the Division of
Metallic Materials Research, Material and Chemical Research
Laboratories, Department of Additive Manufacturing Materials &
Applications, Industrial Technology Research Institute, Tainan 709,
Taiwan. WEI-JEN CHENG is with the Department of Iron & Steel
Research and Development, China Steel Corporation, 1, Chung Kang
Road, Kaohsiung 820, Taiwan.
Manuscript submitted September 27, 2017.
Article published online May 31, 2018
METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, AUGUST 2018—3715