INDUCTIVE REFLOW ON AN ELECTROLYTIC TINNING LINE

Steel for packaging is produced by three Sollac plants. One of these is Basse-Indre plant, located near Nantes, produces at once tin steel and tin free steel. Basse-Indre production is around 360 000 tons per year. As an answer to the steel for packaging market evolution, Sollac decided in 1991 a new organization of its production. For this new organization, it was necessary to modernize the two electrolytic lines of Basse-Indre to increase quality and production. On the tinning electrolytic line, it was decided to increase strip quality to install an inductive heating to complete the existing conductive reflow section. On a tinning line, reflow section is necessary to obtain, by reflow and water quench, a bright surface. Thermal treatment in this section creates an iron tin alloy which is necessary in order to prevent cans corrosion. In our plant, we have experienced induction heating for reflow on an other tinning line. The existing equipment is a 1 000 kW power at 150 kHz frequency with a valves generator. For our new application, we also need 1 000 kW power but studies with generators designer conclude that the best way was the solid states generators and that, in this case, two little generators are better than a big one. Solid state generators advantages compared to valves generators are better efficiency, lower electrical tension on simplest inductor, high time life of components... At 1991 end, we ordered two 500 kW generators based on semi conductor technology. Supplier was Elva France. Elva France is a subsidiary of a Norwegian company (Elva Induksjon a.s.) which is specialized in induction heating technology. Each heating system consist of a thyristor rectifier, a high frequency (160 kHz) converter, a transmission line and the matching and compensation unit with heating coil. A water cooling unit delivers water to both systems in parallel. Control of reflow section was revamped using existing computer and PLC. For one strip, we try to have a constant power induction and to compensate speed variations with conduction power. A radiation pyrometer is used to regulate the strip temperature at quench entry tank. Induction heating system started on the beginning of 1992, and after some tests to adjust the frequency matching, we obtain good results An important tin plate surface quality improvement was obtained. The thermal efficiency is around 77%. The success of this first installation of semi conductor inductive heating generators in steel industry opens a lot of new inductive heating application in the steel for packaging production.