Tin Free Steel Sheet refers to the surface treatment of tin-free steel plate. Generally divided into chromium-free Wuxi steel and nickel-based stainless steel plate 2. (TFS), that is, in the steel surface of the electrolytic chromate treatment, so that the surface of the steel plate deposited a layer of metal chromium (usually 5O ~ 100mg / m) and chromium hydrated oxide (generally 5 to 20 mg / m.). In foreign countries, in order to make it more clear, the use of TFS-CT or ECCS to represent the Wuxi plate. As the tin price is more expensive, and the increasingly depleted resources, therefore, can replace tin plate and low tin plate of Wuxi steel plate development prospects are increasingly broad.
Wuxi steel plate generally consists of substrate, metal chromium layer, chromium hydrated oxide layer and oil film 4 layer composition. The substrate is cold-rolled low-carbon strip, a cold-rolled sheet thickness of 0.18 ~ 0.55ram, individual manufacturers lower limit of 0.17ram, the upper limit to 0.60ram. Secondary cold-rolled thin plate thickness of 0.15 ~ 0.36ram, Baosteel branch cold-rolled thin plate mill of the second cold-rolled thinnest sheet up to 0.1mm. Metallic chromium coating according to the provisions of the JIS standard nominal coating thickness is generally 70mg / m, manufacturers generally control in about 100mg / m, chromium hydrated oxide film is generally controlled at 7 ~ 15mg / m. about. No special requirements for the oil film, the thickness of tinplate equivalent, usually 2 ~ 5mg / m.
Wuxi steel surface coating structure is mainly metal chromium and amorphous chromium hydrate oxide. But for chromium hydrated oxide. Structure has different views. And some proposed the structure of the OH-containing compound, and some proposed in the chromium hydrate oxide in the presence of sulfate, that this is an electrolyte containing anionic class of complex macromolecular composition.
Features of Wuxi steel plate
1 Advantages of Wuxi steel plate
(1) less plating: the amount of plating is only the same performance tinplate plating (2.8g / m) of about 3/9/6; low cost, the price per ton of TFS less than 150 tons of tinplate.
(2) strong adhesion, good paint: the surface oxide film was honeycomb and containing a OH and other groups, the adsorption of paint, wetting strong, for the same coating in the tinplate adhesion of 2 to 5 Times more than.
(3) good temperature resistance: tin low melting point (232 oC), 161 oC or more when brittle, heating and iron will continue to alloy, affecting the tinplate processing performance and application; In addition, tinplate low temperature conditions " Tin epidemic "phenomenon (long time at 13.2 ℃ tin will become gray tin and slowly self-powder). TFS oxide film heating more than 100 ~ C will slowly dehydration and converted to Cr O. , But does not affect the use of, in the 300 ~ 400 ℃ conditions for a short time baking is also very safe.
(4) high sulfur resistance, high alkali resistance: because chromium and chromium oxide are not with sulfur and alkali reaction, so TFS has a good anti-sulfur alkali resistance, while the atmosphere, organic solvents, oil and so on Has good corrosion resistance. The corrosion resistance of TFS depends mainly on the number of exposed pinholes and the electrochemical properties of the oxide film itself.
(5) non-toxic: TFS for food cans, its non-toxic as an important factor. The surface film of TFS is prepared by electrolysis, and its composition is chemically stable, and the chromium ions eluted from the composition of the surface film are extremely traceable. Moreover, the hexavalent chromium ions harmful to the human body are completely undissolved.
2 Wuxi steel plate shortcomings
(1) chromium layer more than 30mg / m when the poor spot welding, the need for special welding equipment or remove the chromium layer after welding. To this end, you can use very low chromium layer and chrome tin 2 steel plate method, Japan JFE company developed a more easy to weld the TFS products, welding before the need for grinding steel plate;
(2) the thickness of the oxide film is poor, so the deep processing requirements of the coating has a good resistance to deep processing performance to reduce the oxide film fracture and film mechanical damage;
(3) slightly less resistant to acid corrosion;
(4) the surface is easy to be contaminated, it is necessary to use double-sided paint;
(5) process, the chrome current efficiency is low, the corrosive electrolyte, the need for chromium wastewater treatment.