Continuous annealing lines
A continuous annealing line (CAL) restores the mechanical properties by recrystallisation of the steel after cold rolling by heating the strip up to the annealing temperature. It processes either very thin strips used for tin plate applications or cold rolled coils subsequently transformed into sheets. It can also be designed in such a way that processing coils for both tinplate and sheets is possible on the same Continuous Annealing Line.
Drever International furnaces provide a perfect control of the heating and of the cooling rates requested to perform the heat treatment of the most advanced steel grades, for both sheets and tin plate applications.
Continuous annealing line (radiant tube furnace)
These lines have been developed for higher production rate than batch annealing furnaces. The throughput of a Continuous Annealing Line reaches up to 280 t/h at a strip speed of about 800 m/min. The whole annealing process including heating, soaking, slow and rapid cooling, overaging and final cooling to ambient air needs only a few minutes.
Cold rolled strip widths between 600 mm and 1850 mm and strip thicknesses from 0.15 mm to 3.0 mm can be handled. In the Continuous Annealing Lines carbon-manganese steels, high strength low alloyed steels (HSLA), dual-phase (DP) steels and annealed coils for tinplate can be processed.
The Advanced high strength steels (AHSS) for automotive applications are not only annealed, but they are also heated to a higher temperature to reach the (partial) austenitization of their microstructure. Their production requests the perfect control of the temperature to reach the suitable level of austenite while travelling through the furnace. Our new X-CAP® technology allows the measurement of austenite inside the furnace. Very high cooling rates are also possible with our Ultra-Fast Cooling technology UFCplus.
Our mathematical model as well as our dedicated control system drive the temperature of the different zones of the furnace to reach the requested microstructure and the aimed mechanical properties at the end of the line.
The usual configuration for the Continuous Annealing Lines is vertical. Some lines can be designed with a galvanization pot, so they can be used also as a continuous galvanizing line (CGL mode).
Drever International supplies mainly furnaces heated by radiant tubes (RTF). The atmosphere of the furnace is made of HNx, a non-oxidizing mixture of nitrogen and 5% hydrogen.
A longitudinal induction booster can be used before the radiant tube furnace to increase the productivity of the line or to decrease its length.
The heating is provided by gas fired tubes (either with natural gas, coke oven gas or a mixture of coke oven and blast furnace gas).
The burners and the recuperators are specially designed to have a high energy efficiency, and to limit the emission of NOx.
To increase further the energy efficiency of the furnace, the exhaust fumes are collected and go through a heat exchanger to reheat a flow of HNx gas. This reheated HNx gas is then blown directly on the cold strip to preheat it.
Besides increasing the energy efficiency, this HNx preheating decreases the risk of heat buckles on the strip and also increases the life time of the first deflection rolls by avoiding thermal shocks.
The strip is held at the annealing temperature for a few seconds: this is the soaking. The temperature of the furnace is maintained either by gas-fired tubes, or with electric resistance.
Transversal inductive heating
In option, a transversal inductive heating can be installed to reach the highest austenitizing temperature used in the latest generation of AHSS steels for automotive.
After the annealing and the soaking, the strip is cooled down following various heat cycles. Several technologies are used:
- Very low cooling rate with « lyra » cold tubes cooled down by air
- Slow cool with slotted tubes blowing HNx
- Fast cooling with plenums blowing cold HNx
- Ultra-fast cooling by cold HNx with hydrogen injection. Drever International masters the technologies needed to confine the hydrogen injection inside the ultra-fast cooling section
- Possibility to reheat the strip by induction heating after the quench of AHSS steel grades to reach the temperature of tempering.
The strip is often held at an intermediate temperature of 350°C to 450°C before the final cooling: the overaging. As the time requested can be very long, some lines include several consecutive overaging chambers.
The strip is further cooled down in a protective HNx atmosphere before a final cooling with water.
Drever International delivers
- The annealing furnace
- The control system and a mathematical model of the furnace
- Patented equipment dedicated to the manufacturing of advanced high strength steels (AHSS): X-CAP®
Our parent company, SMS group and its affiliates deliver all the other equipment of the line.
More information on SMS group offer