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The effects of different galvannealed (GA) coatings, containing Fe varying from 7.0 to 11.4 mass%, on steel sheets on the electrode life in resistance spot welding (RSW) have been investigated with metallurgical analysis of the coating microstructures and properties, and the surfaces and cross-sections of failed electrodes. The results showed that the electrode life in RSW of GA steel with 11.4 mass% Fe in coating was 110% higher than that with coatings containing 7.0 or 9.6 mass% Fe. The improvement was believed to be caused by the build-up of a Fe-rich alloy layer on the electrode surface, which could serve as a barrier to prevent copper loss from the electrode surfaces to the steel sheets, thus reducing the growth rate of the electrode tip face diameters. In addition, higher Fe content in the coating resulted in increased contact resistance and hence a lower welding current needed in RSW. [doi:10.2320/matertrans.M2010239]

Hot-dip galvannealed sheet (GA) were developed from the automotive industry. It has high strength of 590 MPa grade in tensile strength. However, in order succeeds from automobile industry, the microstructure and galvannealing behavior of galvannealed TRIP steel sheets must be strictly controlled. The metal where the silicon content is high has problems with weld-ability, zinc coating and reduction of retained austenite volume fraction after galvannealing process. The main purpose of this study is to solve the problem as indicated above.

In order to investigate the structure of Fe-Zn r phase in a commercial galvannealed IF steel sheet, the electron diffraction and composition of r phase in a commercial galvannealed IF steel sheet were studied by transmission electron microscope (TEM) and X-ray energy dispersive spectroscopy (EDX). The cross-sectional TEM specimen was prepared using focused ion beam (FIB) technique which was also introduced briefly. The results show that there exist superlattice reflections in the selected area diffraction patterns (SADPs) of r phase which indicate that the r phase is a kind of ordered intermetallic compound. The results of computer simulation of electron diffraction patterns based on the single crystals X-ray diffraction of previous researchers give a advice to the atomic coordinates of r phase in a commercial galvannealed IF steel sheets.

Calcium fluoride solid electrolyte has been extensively used in galvanic cells for the measurement of fluorine potential to obtain the activity of alloys and compounds, because the ionic transference number of the electrolyte is about unity even at P F2 /P 0 = 10 -62 above 550°C. So far there were no literature data available for the measurement below 500°C by use of the calcium fluoride solid electrolyte. The purpose of this work is to develop the sensors for the control of aluminum concentration in a hot dip galvanizing bath, which is of industrial importance for producing high quality galvannealed steel sheets. In this report, a trial has been made to measure the fluorine potential in a molten zinc bath artificially equilibrated with ZnF 2 at the temperatures between 450 ∼ 500°C using CaF 2 cells with some kinds of reference electrode. By use of Bi, BiF 3 or Zn, ZnF 2 as the reference electrode, the cells respond quickly and stable emf values which agree fairly well with the literature data are obtained. Subsequently, a trial has been made to measure the fluorine potential in molten Zn-Al baths artificially equilibrated with AlF 3 at the temperatures between 450-500°C using CaF 2 cells with the above reference electrodes. The initial response of these cells after immersion of the cells into these baths is relatively slow. However, once emf of the cell becomes stable, each cell responds quickly to the aluminum concentration change. The response time is not longer than 5 min. From the analysis of such emf measurements, the emf values are found to increase linearly with the logarithm of the aluminum concentration. The possibility of the present method has been ascertained in Zn-Al baths.