I n s i t u O b s e r v a t i o n o f F o r m a t i o n o f Fe-Z n I n t e r m e t a l l i c C o m p o u n d s D u r i n g G a l v a n n e a l i n g P r o c e s s b y X - r a y D i f f r a c t i o n Fig. 1. Schematic illustration of the in situ observation system . Galvannealed steel sheets are widely used in the automotive industry to protect car bodies from corrosion. In the industrial process of galvannealed steel sheets, steel substrates are dipped in molten zinc containing a small amount of aluminum, then annealed in a furnace at about 500 C. Since th e c oating of t he galvannealed steel s heets mainly consis t s of Fe-Zn intermetallic compounds, such as FeZn 7 ( δ 1 phase) and FeZn 13 ( ζ phase), it is very i mportant to study t he growth behavior of t hos e c ompounds during a galvannealing process to understand t he r oughness or t he mechanica l properties of the coating after the process. As summarized by Horstmann [1], many report s on hot-dip galvanizing r eactions have been presented. However, because t hose r eactions occur in a short period up to 60 seconds, it is very difficult to detect t hem with static analyses using the specimen quenched after annealin g, such as a cross-sectional observation of the coating with an electron microscope or the measurement of the iron content of the coating. T herefore, a rapid detection system is r equired to observe t hose r eactions dynamically, i.e. , “ in situ observation.” In order to perform the “ in situ observation,” penetration dept h of t he X -ray and time definition of t he detecto r are important factors because it is necessary to observe t he w hole c oating with a 10 ~ 20 μ m thickness, and to observe t he r eaction finishin g within 60 seconds. An easy way to achiev e good time definition of the detector is to increase th e s ource intensity. T herefore, t he X -ray s ource , being able to penetra te deeply, i.e. , having high energy and high intensity, is necessary for the “ in situ observation. ” In this study, synchrotron radiation was used as 100 Q u a r t z h o l d e r I n f r a r e d b e a m P l a t i n g Q u a r t z r o d I 2 θ o r 0 . 2 d e g . / s S c i n t i l l a t i o n c o u n t e r I m a g i n g p l a t e I n f r a r e d b e a m h e a t e r S a m p l e c h a m b e r S l i t 2 T h e r m o c o u p l e Q u a r t z h o l d e r S l i t 1 I 0 2 o r 5 d e g . I o n c h a m b e r C o m p u t e r S a m p l e Fig. 1 Figures 2(a) 2(b) δ 1 (330) 177 s 155 s 133 s 111 s 89 s 67 s 44 s 22 s 1 s δ 1 (330) Intensity I (arb. units ) Diffraction Angle 2 θ (degree) Diffraction Angle 2 θ (degree) Intensity I (arb. units ) 8. 0 8 .2 8. 4 8 .6 8. 8 11.5 11.6 11.7 11.8 11. 9 15 s 14 s 13 s 12 s 11 s 10 s 9 s 8 s 7 s 4 s 1 s (b) (a ) Fig. 2. D iffraction profiles obtained from galvanized steel sheet durin g annealing. (a ) Wavelength of incident X-ray: 0.0319 nm; thickness of the coating: 30 μ m ; detector: scintillation counter. (b ) Wavelength of incident X-ray: 0.0443 nm; thickness of the coating: 10 μ m; detector: imaging plate . Fig. 2(a) Fig. 2(b) t he X -ray s ource and t he experiments were performed at beamlin e BL19B 2 [2]. T he schematic illustration of t he in situ observation system is s hown in . T he galvanized steel s heet sample was m ounted on a quartz holder in th e sample chamber filled with N 2 gas, and was heated f rom t he polished side with t he infrared beam heater (Thermo Riko: GA152) mounted on t he 8- ax is goniometer. We obtained heating time- dependent diffraction peak profiles with either a scintillation counter or an imaging plate. W hen th e scintillation c ounter was used, t he peak profile s were m easured at intervals of 5 seconds while being scanned at an angle velocity of 0.2 degrees per second. When the imaging plate was used, th e peak profiles were measured every second. and s how diffraction peak profiles obtained with a scintillation counter and an imaging plate, respectively. T he samples had z inc coatings containing small amounts of aluminum . Heating time shown in the figures was started to be counted when the coating fully melted. An increas e in the diffraction peak intensity identified as δ 1 (330) was successfully observed in t he profiles. Th e diffraction peak could not be observed in profiles up to 20 seconds after the coating melted in , therefore, it is considered that the 30- μ m coatin g, was t oo thick to a llow detection of the diffraction peaks of the δ 1 phase growing near the interfac e between the coating and the steel substrate at th e beginning of the annealing process. On the other hand, the diffraction peak could not be observed up to 7 seconds a fter t he c oating melted in . In this measurement, th e thickness of t he c oating does not a ffect th e observation of the diffraction peaks of th e δ 1 phas e growing near t he interface as m entioned above, because the steel substrate’s diffraction peak wa s obtained clearly. It is well known that aluminum in t he c oating forms an Al-rich layer between the 101 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 100 0 5 10 15 20 25 30 35 Thickness of δ 1 Phase ( μ m) Time After Zinc Melting (s ) 10 μ m 30 μ m μ (t - t inc ) 1/2 t inc F ig. 3. R elationship between estimated thickness of the δ 1 phase and annealing time. Fig. 3 References [1] D. Horstmann and F.K. Peters, Stahl u. Eise n 90 (1970) 1106. [2] A. Taniyama, T. Takayama, M. Arai, H. Kawata , M. Sato, I. Hirosawa, T. Fukuda and J. M izuki, Proc . Int. C onf. on Designing of Interfacia l Structures in Advanced Materials and their Joints, Osaka, Japan (2002) 385. [3] H. Bablik, Galvanizing (Hot Dip), 3rd ed. , Spon Ltd., London (1950) 204. coating and the steel substrat e, retarding the growth of Fe-Zn intermetallic compounds [3]. T herefore, the variation of peak profiles indicates that th e Al - rich layer prevents the growth of δ 1 phase at th e beginning of t he annealing process. T he perio d from the beginning of annealing to the beginning of Fe-Zn intermetallic compound growth is called an “incubation period. ” A ssuming that th e δ 1 phase grows in a layer-by - layer manner, t he time dependence of thicknes s was estimated as shown in . T he estimate d thickness of the δ 1 phase increased by the parabolic l aw with annealing time, taking into a ccount th e incubation period (t in c ). T hese results suggest that the growth of δ 1 phase is dominated by a diffusion of Fe atoms and Zn atoms in the coating. Akir a Taniyama Corporate R&D Laboratories, Sumitomo Meta l Industries, Ltd. E-mail: akir a@ pca.amaken.sumitomometals.co.j p 102
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