Research method

Control of Conditions of Finished Surface ofAlloyed Hot-Dip Galvanized Steel Sheets

X-ray diffraction/scattering

Elucidation of alloying reaction inside zinc coatingRole of SPring-8

Clarification of growth behavior of an iron-zinc (Fe-Zn) alloyed layer in an alloyedhot-dip galvanized steel sheet* by real-time in situ observation during thealloying process
Establishment of guidelines for creating a smooth and durable coating surface
R&D facility: Sumitomo Metal Industries, Ltd.

*Alloyed hot-dip galvanized steel sheet: Hot-dip galvanized steel sheets are obtained by coating zinc (Zn) over a steel sheet surface. Thesesheets have been used for many years because of their excellent corrosion resistance. However, small swellings are generated under the Zn coating
after galvanization, causing the delamination of the coating and the deterioration of weldability. These problems can be solved by changing the Zn
coating into an Fe-Zn alloy through heat treatment after hot-dip galvanization. The thus-obtained alloyed hot-dip galvanized steel sheet has been
widely used in automobiles in recent years.

BackgroundResults

The condition of the finished surface of alloyedhot-dip galvanized steel sheets is considerably
affected by the initial formation behavior of Fe-Zn
intermetallic compounds in the Zn coating.
Hence, it was necessary to understand the entire
process of the formation of Fe-Zn intermetalliccompounds including the initial formation.
Thus far, the observation and measurement
had been carried out after temporarily cooling aheat-treated hot-dip galvanized steel sheet to stop
the formation of intermetallic compounds in theZn coating. Therefore, the initial formationbehavior of intermetallic compounds had notbeen clarified.

We succeeded in the real-time in situ observationof the process by which Fe-Zn intermetallic
compounds are grown in the Zn coating of analloyed hot-dip galvanized steel sheet from theinitial stage using the high-brilliance X-ray at
SPring-8.
The analysis result of X-ray diffraction profiles
obtained in real time revealed that the growth rateof Fe-Zn intermetallic compounds depends on thediffusion velocity at which Fe atoms are mixed
with Zn atoms, rather than the rate at which Featoms react with Zn atoms to form Fe-Zn
intermetallic compounds, even at the initial stageof the growth. From this result, we establishedguidelines for technologies to control the initialstage of the formation of intermetallic compounds,
which is important for creating smooth coating
surfaces.

X-ray diffraction profile associated with heating of hot-dip Zn coating

Hot-dip galvanizing line
Zn is attached to a continuously moving steel sheet.

δ1 phase: Intermetallic compound with composition of FeZn7 or FeZn10
ζ phase: Intermetallic compound with composition of FeZn13

Structure inside coating of alloyed hot-dip
galvanized steel sheet (SEM image)δ1 phase

The figure below shows an SEM cross-sectional image of a Zn
coating that was cooled in the middle of an alloying reaction.
Fe-Zn intermetallic compounds are grown on the interface
between the Zn coating and steel sheet.

Steel sheet

X-ray intensity (arb. unit)

Zncoating
Fe-Znintermetalliccompoundζ phase not generated

Heating duration or time elapsed (s)Al added (Zn-0.13%Al/Fe)

10 s

Suppression of alloy phase deposition

7s
1s

Diffraction angle

Formation of the Fe-Zn intermetallic compounds is prevented for 7s
after the Zn coating melts; then, Fe-Zn intermetallic compounds areformed. This indicates that a small amount of aluminum (Al) added toZn forms an Fe-Al intermetallic compound layer on the interface
between the Zn coating and steel sheet, preventing the reaction
between the two; subsequently, Fe-Zn intermetallic compounds form
after this Fe-Al intermetallic compound layer disappears.

Formation behavior of Fe-Zn intermetallic compound
The thickness of the Fe-Zn intermetallic compound layer increases
in proportion to the square root of time after the melting of the Zncoating during the formation process. This indicates that thegrowth rate of the Fe-Zn intermetallic compounds is determined by
the diffusion velocity of Fe atoms being mixed with Zn atoms.
Thickness of Fe-Zn intermetallic
compound layer (μm)

Achievements

Latent period

Heating duration in hot-dip galvanization (s)4041