Influence of annealing conditions on the galvanizability and galvannealing properties of TiNb interstitial-free steels,
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I. INTRODUCTION
GALVANNEALED is a Fe-Zn alloy–coated steel sheet, produced by the interdiffusion of Zn and Fe during a short annealing process in the range from 490 8C to 540 8C. The annealing is carried out in an in-line furnace located directly after the galvanizing Zn bath, which contains a low Al content (,0.15 pct). This interdiffusion process results in a coating containing layers of intermetallic Fe-Zn phases. The layer contains, starting from the surface, the z, d, and G phases. Small amounts of Al are added to the liquid Zn bath in order to control the Fe-Zn reaction. Al is known to inhibit the development of the Fe-Zn intermetallic compounds due to the rapid formation of a thin Fe2Al5 layer containing Zn on the steel surface. This layer is commonly referred to as the inhibition layer. The inhibition layer has to break down before Fe-Zn compounds can be formed during galvannealing. High-strength interstitial-free (HS IF) steels that are solid solution–strengthened with P, Mn, or Si have slow galvannealing reaction kinetics.[1,2] Longer galvannealing times and higher galvannealing temperatures are necessary to obtain the desired alloying degree of about 10 wt pct Fe in the galvannealed coating. These conditions are, in general, not compatible with the steel strip processing windows on industrial lines during the galvannealing of standard deep-drawing low-carbon and IF steels. Compared to the average production, the lower strip reactivity results in line speeds that are up to 30 pct lower. The galvannealing temperatures required I. HERTVELDT, Graduate Student, Laboratory for Iron and Steelmaking (LISm), and B.C. DE COOMAN, Professor of Materials Science, are with the Department of Metallurgy and Materials Science, Ghent University, B9052 Zwijnaarde (Ghent), Belgium. S. CLAESSENS, Coating Research Technology Manager, is with OCAS NV, R&D Center of SIDMAR, ARBED Group Flat Rolled Products Division, B-9060 Zelzate, Belgium. Manuscript submitted February 2, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
can reach 540 8C. There is also the added risk for underalloyed strip edges. The substrate wettability by the zinc is also often unsatisfactory. Solutions for improving the galvanizability and the galvannealing properties of P-, Mn-, or Si-alloyed sheet steel must, therefore, be developed. As mentioned previously, the galvannealing kinetics are determined by two processes: the inhibition-layer breakdown and the Fe-Zn reaction. As both processes occur at the steel/coating interface region, one can expect them to be influenced by the surface state of the strip after continuous annealing. The surface state of the strip is influenced by surface segregation and selective oxidation. It is clear that the kinetics of both physical phenomena are different in the grain boundaries. Both processes are also influenced by the steel chemistry, the temperature and the annealing atmosphere. C. Wagner[3] developed a theoretical model to describe the selective oxidation of binary alloys. The selective oxidation can develop extern
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