Tinned steel sheet having a high degree of corrosion resistance and a method of producing the same

#1. Field of the Invention

#This invention relates to tinned steel sheet of high corrosion resistance having only a very small amount of iron dissolved through the edges, or any defective portion of its coating in a corrosive environment and a method of producing the same.

#2. Description of the Prior Art

#FIG. 1 is a graph showing the corrosive coupling electric current as measured between chromium-containing steel sheet having a 0.002 micron thick undercoating layer of nickel formed thereon and a layer of tin formed thereon in an aqueous solution containing 1.5% of citric acid and 1.0% of sodium chloride, which was employed to accelerate corrosion on the inner surface of a vessel formed from the coated steel sheet, in relation to its chromium content;

#FIG. 2 is a graph showing the corrosive coupling electric current as measured between chromium-containing steel sheet having a 0.05 micron thick undercoating layer of a nickel alloy containing 20% of cobalt and a layer of tin formed thereon, and also between Cr--Ni steel sheet containing 1% of nickel and having a 0.05 micron thick undercoating layer of a Ni-20% Co alloy and a layer of tin formed thereon, in a 0.3% aqueous solution of phosphoric acid, which was employed to accelerate corrosion on the inner surface of each coated steel sheet, in relation to its chromium content;

#FIG. 3 is a graph showing the corrosive coupling electric current as measured between chromium-containing steel sheet having a 0.11 micron thick undercoating layer of nickel formed thereon and subjected to 60 seconds of thermal diffusion treatment at 750.degree. C. and a layer of tin formed thereon, and also between Cr-Ni steel sheet containing 0.5% of nickel and having a 0.11 micron thick undercoating layer of nickel formed thereon and subjected to the same thermal diffusion treatment and a layer of tin formed thereon, in a 5% aqueous solution of sodium chloride, which was employed to accelerate corrosion on the outer surface of a vessel formed from each coated steel sheet and also simulate a substance containing salt or impurities contained in fuel, in relation to the chromium content of each coated steel sheet;

#The tinplate of this invention comprises a sheet of steel containing not more than 0.1% by weight of carbon, 0.005 to 0.08% by weight of acid-soluble aluminum and 0.2 to 20% by weight of chromium. It may also contain a total of 0.03 to 0.5% by weight of at least one of titanium, niobium, zirconium and vanadium. It may further contain not more than 3% by weight of nickel. The steel sheet can be manufactured by a customary process which includes melting in a converter, electric or other melting furnace, slab forming either by continuous casting or by ingot blooming, hot rolling, cold rolling and annealing.

#The steel can contain up to a maximum of 0.1% by weight of carbon. It is preferable to reduce its carbon content as far as possible. An increase in the amount of carbon gives rise to an increase in the precipitation of chromium carbide. It not only lowers the mechanical properties and corrosion resistance of the steel, but also disables the formation of a uniform coating layer. If the steel contains titanium, niobium, zirconium or vanadium, it is preferable to limit its carbon content to a maximum of 0.02% by weight in order to ensure its good workability and the formation of a uniform coating layer which would be prevented by the precipitation of a carbide, such as titanium carbide.

#If the steel contains only less than 0.005% by weight of soluble aluminum, it is difficult to prevent during the melting of the steel the generation of blowholes by oxidizing gas which are very likely to develop surface defects on the steel and thereby lower its corrosion resistance. If the steel contains over 0.08% by weight of soluble aluminum, the scattering of aluminum oxide on the steel surface disables the formation of a sound coating layer which is free from uncoated spots, or pinholes, and which ensures the high corrosion resistance of the steel sheet. Therefore, it is essential to use steel containing 0.005 to 0.08% by weight of acid-soluble aluminum.