This report shows the results of weldability, contact resistance, rust resistance, corrosion behavior and lacquer adhesion on the Lightly Tin Coated Steel (LTS) having 0.3 and 0.6g/m2 of tin compared with tinplate having 1.28g/m2 of tin and Tin Free Steel. Preliminary test on the iron-tin alloy composition of heavily coated tinplate indicated that at 220°C-380°C, iron-tin alloys were almost FeSn2 and there were some differences in shape and size of the alloy. Weldability at the welding speed of 30m/min and 48m/min was improved with increase of tin coating weight; Available current range of reflowed 0.3g/m2 LTS was less than 100A at 30m/min, but that of non-reflowed 0.6g/m2 LTS reached 190A which would be sufficient for commercial welding at 30m/min. Contact resistance of 0.6g/m2 LTS was almost similar to that of tinplate, however fully alloyed LTS had relatively high resitance. Polarization characteristics of iron, tin, iron-tin alloy and metallic chromium indicated that in tomato juice, corrosion behavior of LTS was similar to that of tinplate, however underfilm corrosion of LTS was less than that of tinplate and iron pick up tendency of LTS was also less than TFS. Lacquer adhesion of fully alloyed LTS was better than that of tinplate and other LTS.
Masking coat consisting of sodium silicate, sodium borate, magnesium oxide, titanium oxide and alminium oxide for hot dipped one side galvanizing has been investigated. (1) Silicate and borate in glass matrix melt and glassify over 800°C and steel sheet can be tightly covered, resulting in keeping bare side of hot dipped one side galvanized steel sheet masked by coat from molten zinc and oxidation. The glass matrix seems to be reinforced by magnesium oxide which disperses in the matrix as fine particles. (2) Acicular crystals formed on masking coat containing titanium oxide and aluminum oxide in glass matrix baked at 800 to 900°C have repelling effect against molten zinc. The acicular crystals mainly consist of titanium oxide that is recrystalized phase during annealing, but it is transformed to magnesium titanate over 900°C. (3) Masking coat exfoliates from steel sheet because of difference of thermal expansion ratio between masking coat and steel through quenching at 200-400°C after hot dipping. Bending of steel sheet accelerates removing the masking coat. (4) The elements enriched in surface layer of base steel are absorbed into masking coat during annealing, resulting in cleanness of steel surface and good corrosion resistance against SST after painting. (5) It has been confirmed that hot dipped one side galvanized steel sheet with deep drawing quality or high strength deep drawing quality having r value of over 2.0 can be obtained by using extra low carbon steel containing Nb or Nb-P in CGL without overaging.
Workability and adhesion of the plated film were studied after plastic working of Zn-Fe/Zn-Ni double layer electroplated steel sheet. The steel sheet is a newly developed one and excellent in wet adhesion of paint film and corrosion resistance as painted. After plastic working, many cracks were observed in the plated film of the double layer electroplated steel sheet. Occurrence of metal powder during working, however, was extremely slight because of the excellent adhesion of plated film to the substrate. Corrosion resistance of the specimens painted after working was also studied. Double layer electroplated steel sheet showed better corrosion resistance than galvannealed steel sheet. This is attributed to a large percentage of phosphophyllite in the phosphate coating precipitated by conversion treatment of double layer electroplated steel sheet even after plastic working.
Recently, the can manufacturing technology by welded method has been developed and taking the place of soldered can. With the advances of technology, it has been desired to study the new economical materials for welded can. Authors have developed the nickel plated type and LTS type (low tin coated steel) metal finished sheet and investigated the properties of them. The results of the experiments, showed that the weldability is sufficient for the high speed can making and the corrosion resistance after enamel coating is possible to use as containers.
Effect of increasing Al content of Zn coating bath on the durability of hot dip galvanizing steel sheet is studied. After investigating the corrosion mechanism, easiness of coating operation and economical merit of Zn-Al coating, Zn-4.5%Al-0.1%Mg alloy coated steel sheet has been developed. It has an excellent corrosion performance in various atmosphere and a superior coating adherence to the conventional galvanized steel sheet. On the basis of long period exposure test results, it is expected to decelerate the development of red rust on the painted sheet. Therefore, it has a bright future as a resource-savable coated steel sheet.
In recent years, steel makers have been promoting new plating lines and the additional equipment according to the rapidly increasing demand of the surface treatd steel sheet for the automobile body. Especially concerning the electroplating equipment, the structure of the electroplating cell has influence on the expense of the equipment and the running cost. On this report, our purpose is to increase the current density and reduce the voltage for developing the high efficient electroplating cell of horizontal type. In order to achieve above purpose, the electrode is devided into two parts, then the gas which evolves by the electrolysis is removed efficiently from the cell, at the same time, the practical current density increases drastically. And more, the catenary and the irregular shape of strip are prevented by setting the liquid cushion nozzle in the cell due to the static pressure, then the distance between strip and the electrode is reduced. Therefore the voltage drop by electroplating is reduced and it becomes possible to apply the high current density. By this development, the high efficient electroplating becomes possible.
Iron-zinc alloy electroplating has been known to show anomalous codeposition, that is, zinc preferentially deposits in spite of its less noble nature compared with iron. One of the reasons for this behavior is considered that a Zn (OH)2 film formed on the cathode surface prevents the depositon of iron. However, in some cases when using chloride baths, iron was found to deposit preferentially and the effects of plating variables were the opposite of the case of anomalous codepostion. In these cases, the Zn (OH)2 film on the cathode surface was considered to be very thin and unstable. The appearence, crystal size and crystal structure of the deposits obtained from chloride baths varied according to the iron content in the deposit. The crystal structure was difficult to determine because of the lattice strain by electrodeposition. However, it was estimated by an X-ray diffraction method using specimens as-plated and heated after plating. It was found to be η phase when the iron content was below 2%, η+δ1+Γ phases when 2-30%, Γ+α phases when 30-90% and α phase when above 90%.
Low-carbon rimmed steel sheets were dipped into pure Al and Al-Si alloys by a gas reduction type aluminizing instrument to study the effects of Si on the reaction between steel sheets and molten Al under short time dipping. The growth of an alloy layer at steel-coating interface, the phase change of intermetallic compounds in the alloy layer and the wetting characteristics between steel sheets and molten metals were investigated by means of an observation of cross-sectional microstructure, an X-ray diffraction technique, an X. M. A. and a meniscograph measurement. The results obtained are as follows. 1) The alloy layer in aluminizing with pure Al bath was composed of η-phase (Al5Fe2) and θ-phase (Al3Fe). Increase of the alloy layer thickness with dipping time was devided into two parabolic growth steps. It was considered that the growth of the alloy layer is controlled by an internal diffusion of Al through θ-phase in the first step and through η-phase in the second step respectively. 2) When the steel was dipped in Al-2.5%Si bath, the alloy layer was composed of η-phase, θ-phase and (Al, Fe, Si) H-phase. The growth rate of the alloy layer with Al-2.5 %Si was much lower than that with pure Al bath. 3) The alloy layer formed by dipping into Al-8%Si bath was composed of only (Al, Fe, Si) H-phase and it did not grow for dipping time up to 30 seconds. 4) The wetting force between steel sheets and pure molten Al in which the alloy layer formation was vigorous, was as high as 400 dyn/cm, but that between steel sheets and molten Al-8% Si in which the growth of the alloy layer was not detected, was as low as about 50 dyn/cm. From these results, it was considered that the (Al, Fe, Si) H-phase, an outermost alloy layer, has the inhibition effects on the reaction between steel sheets and molten Al.
In order to electroplate the Ni-Zn alloy with high corrosion resistance onto the steel sheet, the behavior of electrodeposition of Ni-Zn alloy at high current densities has been studied. Because of raising the pH in the vicinity of the cathode in plating, the layer of zinc hydroxide is formed. Electrodeposition of Ni-Zn occurs through this layer. So electrodeposition of Ni-Zn alloy is effected strongly on the pH in the vicinity of cathode surface. In order to obtain the γ-phase Ni-Zn alloy film with high corrosion resistance, it is effective to maintain the condition in the vicinity of the cathode surface. Therefore, it is important not only to maintain the operating condition, such as bath composition, current density and pH, but also to give the steel strip a certain of flow rate, for obtaining a uniform alloy coated steel sheet.