PHASE DEVELOPMENT IN POST-DIP ANNEALED ZINC COATINGS ON STEEL SHEETS

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1 METAL , Ostrava, Czech Republic PHASE DEVELOPMENT IN POST-DIP ANNEALED ZINC COATINGS ON STEEL SHEETS Mária Kollárová Andrej Leško Iveta Sinaiová Ľudovít Parilák IMR SAS, Watsonova 7, 3 3 Košice, Slovak Republic, kollarova@imrnov.saske.sk, lesko@imrnov.saske.sk, parilak@imrnov.saske.sk Abstract The influence of annealing on the morphology development of hot dip galvanized steels has been investigated. Two industrially manufactured hot dip galvanized steel sheets of various chemical compositions were annealed at different temperatures for various holding times with aim to investigate morphology and kinetics of intermetallic phases development. Titanium stabilized IF steel and low carbon Al-killed steel were used. Temperatures of,, C were chosen for annealing with respect to the parameters of galvannealing, industrially used treatment after galvanizing. For shorter times of seconds annealing the iron content in zinc layer was higher for low carbon steel due to the better developed phase after hot dip galvanizing and the phosphorous retarding effect on intermetallic phases formation in IF steel. Longer times of annealing exhibited higher diffusivity of iron into the zinc layer for IF steels especially at C. In both steel the highest temperature, C, and the longest holding time, seconds converted the entire zinc layer to the most iron rich intermetallic phase.. INTRODUCTION Zinc-coated steel sheets are widely used in many branches of industry due to their corrosion protection. Hot dip galvanizing is one of the most well-known processes to produce Zn coatings. Between zinc coatings, Fe-Zn alloyed coatings found large use in automotive industry, because of their excellent weldability, paintability and cosmetic corrosion resistance in comparison to pure Zn coatings. Generally, Fe-Zn coatings are typically produced by annealing the steel coated sheets after hot dip galvanizing, electrogalvanizing or zinc vapour deposition processes []. Coating properties strongly depend on the intermetallic phases presence, resulted from diffusion processes during galvanizing and galvannealing. By this treatment almost pure zinc η(,3 wt% Fe) phase is transformed to a fully alloyed coating, consisting of various intermetallic phases with different properties (bbc, brittle, hard), (fcc, brittle, very hard), (hexagonal, ductile, hard), (hcp, ductile, hard), (monoclinic, brittle, soft)[]. In recent years demand for excellent deep drawing steels resulted in the IF steel production increase. IF steels being free from interstitial atoms, do not need overaging to stabilize the mechanical properties in comparison to continuously cast Al-killed steels and can be galvanized without additional heat process [3]. The steel chemistry, mainly the C and P content can influence the individual Fe-Zn (- Fe 3 Zn, -Fe Zn, -FeZn 7, FeZn, -FeZn 3 ) intermetallic phases growth characteristics from pure zinc η phase in the coating through diffusion parameters modification during galvanizing or galvannealing [,]. IF steel ultralow interstitial elements content amount can lead to difficulties in iron diffusion control, overalloying of zinc layer and coating properties

2 METAL , Ostrava, Czech Republic impairing. Additions of phosphorus in IF steel sheets reduces the formation of brittle intermetallic phase and decreases the sensitivity of the coating to the decohesion []. On the other hand, the phosphorus atoms segregation to the ferritic grain boundaries, can lead to the brittle intergranular failure [7,], especially apparently assumed at low temperatures or very high deformation rates. In the presented work the phase development in post-dip annealed zinc coatings on Ti-added IF steel sheet and low carbon Al-killed steel sheet is introduced.. EXPERIMENTAL PROCEDURE Two different types of hot dip galvanized steel sheets, Ti-added IF steel (IF) and Al-killed low carbon (AK) steels manufactured for automobile applications were selected to investigate the phase development during galvannealing (additional annealing after hot-dip galvanizing). The chemical compositions and coated sheets thicknesses of investigated steels are listed in Tab.. In the case of coating thickness, the given values are only averaged. Table Chemical compositions and coated sheets thicknesses of experimental steels thickness of the chemical composition [%] sheet [mm] coating [µm] C Mn S P N Si Al Ti IF,,3,,,3,,,, <, A,7,7,,3,7,9,3 <,,,9 For annealing of samples with dimensions x mm, small tube furnace was used and the treatment was carried out in the protective nitrogen atmosphere. The used temperatures of,, C and the time were monitored and recorded during the whole process. Cross sectional samples were prepared via classical metallographic method, polished with diamond paste and slightly etched using mixture of % picric acid in amyl alcohol and % HNO 3 in amyl alcohol with aim to reveal the overall coatings microstructures on the investigated steel sheets. For coatings observations by means of scanning electron microscope (SEM) TESLA BS 3 with LINK analysator, specimens were cleaned in methanol using ultrasound method. Energy dispersive X-ray microanalysis (EDX) was conducted across the coating from coating/steel substrate interface at kv accelerating voltage to determine the microstructure and composition of the intermetallic compounds on samples in as produced state and after annealing. 3. RESULTS AND DISCUSSION The cross sectional views of both galvanized steels are documented on Fig. and Fig.. The zinc layer on low carbon steel, Fig. consists of developed, continuous phase layer and crystals of covered by η phase. The zinc layer on IF steel is characterized by locally developed islands of phase with crystals of phase covered by η phase upperlayer. The difference can be explained by different hot dip galvanizing conditions or by retarding effect of phosphorous on the IF steel surface. The low carbon steel during the annealing responded with intensive diffusional processes and growth of intermetallic phases in accordance to the used temperature and holding time. The

3 METAL , Ostrava, Czech Republic temperature of C and seconds time allowed the growth of phase layer, and formation of new phase crystals on expense of pure zinc η phase, Fig.3. With further increase of Fig. A steel - as galvanized Fig. IF steel - as galvanized temperature and holding time the phase continued to grow through the whole η phase. This process was completed after seconds at C and after seconds at C. The phase on the interface became visible and detectable and continued to grow into the phase, Figs. and. With the longer holding time, the phase continues in its growth, consuming the Fig.3 A steel, C, sec. Fig. A steel, C, sec. Fig. A steel, C, sec. Fig. A steel, C, sec.

4 METAL , Ostrava, Czech Republic phase [], Fig.. The phase is more sensitive to the used etchant and therefore on the steel substrate surface the etched out groove can be seen. The formation and growth of intermetallic phases on the IF steel substrate was similar, with slight differences in morphology and quantity of individual phases. The intermetallic phases development is documented on selected micrographs on Figs. 7-. On Fig. the entire zinc layer was converted into the phase and due to its very good etchability the front layer of zinc coating cross section is etched out. This state was reached at C after seconds of annealing. The most extreme temperature and time i.e. C and seconds exhibited a serious damage of the zinc coating layer even it was treated in protective atmosphere. Only few localities on the samples remained for chemical composition microanalyses, which showed very high iron content in whole zinc layer and proved the high reactivity of iron and zinc especially et elevated temperatures. Fig.7 IF steel, C, sec. Fig. IF steel, C, sec. Fig.9 IF steel, C, sec. Fig. IF steel, C, sec. The detailed chemical microanalyses on the annealed samples were carried out and the values of iron content in dependence on the distance from the iron zinc interface were plotted for different materials, temperatures and holding times, Figs. and. The data of as galvanized states are included for better comparison and the regions for individual iron zinc phases are marked as well. The relatively big scatter of iron content values for IF steels comes from nonhomogeneous and noncontinuous intermetallic layer after hot dip galvanizing. In all cases the iron content in the zinc layer increased after heat treatment. The results for both steels after seconds holding time shows higher iron content in zinc layer on low carbon steel.

5 METAL , Ostrava, Czech Republic This may be caused by differences in starting composition or in more effective phosphorous retarding effect on intermetallic phases formation in IF steels. The values plotted for C at seconds holding time of A steel exhibit certain scatter, which is caused by the occurrence of local areas in the compact phase. The opposite results were recorded for seconds holding time, especially for C temperature. During the longer treatment the higher reactivity of IF steel can explain this phenomenon. The phosphorous retarding effect on the intermetallic phases formation is more efficient for shorter holding times. 3 C C C C 3 C C C a) Fig. A steel, a) seconds, b) seconds b) 3 C C C 3 C C C a) Fig. IF steel, a) seconds, b) seconds b). CONCLUSIONS The additional heat treatment of two hot dip galvanized steel sheet samples was carried out at three different temperatures and for and seconds holding time to monitor the intermetallic zinc iron phases development in the zinc coating. The following results can be formulated from obtained and evaluated results.. The shorter seconds holding time resulted in average higher iron content in low carbon steel zinc coatings for all annealing temperatures. This was caused by the presence of continuous layer of phase in as galvanized state and by the retardation effect of phosphorous on the intermetallic phases formation in IF steel.

6 METAL , Ostrava, Czech Republic. The transformation of zinc η phase to the phase was completed after seconds at C or after seconds at C for low carbon steel and after seconds at C or seconds at C for IF steel. 3. The longer seconds holding time especially for C temperatures showed higher iron content in zinc coating for IF steel. The longer time allowed to prove the higher reactivity of IF steel material with the zinc layer.. The highest temperature of C and longest holding time of seconds converted in both cases the entire zinc layer into the iron very rich intermetallic phase. Acknowledgement The authors are grateful to the Slovak Grant Agency for Science (grant /7/) for support of this work. REFERENCES [] Jordan, C.E.- Goggins, K.M.- Marder, A.R. Metall.and Mat. Trans. A, Vol. A, May, 99, pp. -9 [] Lin, C.S.- Meshii, M.- Cheng, C.C. ISIJ Int., Vol. 3 (99), No. pp. 9- [3] Takechi, H. ISIJ Int.., Vol.3, 99, No., pp. - [] Lin, C.S.- Meshii, M.- Cheng, C.C. ISIJ Int., Vol. 3 (99), No. pp. 3- [] Verma, A.R.B.- Vanooij, W.J., Surf. and Coat. Technology 9: -, 997, pp. 3- [] Jordan, C.E.- Marder, A.R. Journal of Mater.Science 3, 997, pp. 93- [7] Lejček, P.- Hofmann, S.- Krajnikov, A. Mater. Science and Eng. A 3-3, 997, pp. 3- [] Kollárová, M.- Leško, A.- Sinaiová, I.- Parilák, Ľ. In: Proc. Fractography,.-.., Stará Lesná, pp. -7