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Better steel thanks to hydrogen
Salzgitter Flachstahl has been using hydrogen for decades in its annealing shop and hot-dip galvanizing plants to precisely refine product properties
In the annealing shop and on the two hot- dip galvanizing lines, air and oxygen impair the quality of the steel through oxidation, so hydrogen is used as a shielding gas in
these production stages. The annealing shop and the Hot-Dip Galvanizing Lines 1 and 2 consume 400,000 m3 of the gas per month.
Each of the two hot-dip galvanizing lines con- sumes 100,000 m3 per month, while consumption of the annealing shop amounts to 200,000 m3. Al- most 100 % pure hydrogen is used there now. Giv- en this high level of saturation, the gas first has to be compressed to up to 10 bar and then dried. By contrast, the nitrogen used on the hot-dip galva- nizing lines is mixed with hydrogen – 20 % on Line 1 and 5 % on Line 2. The annealing process gives all sheet metal products and electro-galvanized sheets the desired quality. Heating, temperature maintenance and cooling improve the properties of the steel such as yield strength, tensile strength and elongation at break.
The bell annealing shop has 81 stations for this purpose. On each shift, three to five coils are stacked on top of each other at eight to twelve of
these stations. Together they weigh a maximum of 100 tons and extend to heights of up to 5 meters. Then the “annealing journey” begins, as the ex- perts say. This takes 50 to 80 hours.
At the start of the journey, a crane places a protective cover made of stainless steel over the stack of coils, after which nitrogen floods the inner chamber to flush away the oxygen. A heating bell is then placed over the protective cover and twelve natural gas burners heat the space between the
two covers. At this moment, hydrogen flows into the protective cover, displacing the nitrogen. Since hydrogen molecules are very small, they even get into the ringed coils between the wound sheets.
The hydrogen performs two functions, ex- plained as follows by Dr. Jürgen Spehr, operations manager of the annealing shop and skinpass mill: “Firstly, it’s an excellent conductor of heat so it does an excellent job in transferring the heat from the outer heating cover to the coil stack. Second- ly, it removes the rolling oil from the metal. This burns at 200 to 500 °C – while the temperature in the annealing bell is between 670 and 720 °C.”
Since hydrogen is continuously flushed through,
Plant assistant Tobias Langermann (left) and plant manager Dr. Jürgen Spehr between the annealing stations in the bell annealing shop
20 HYDROGEN
Photos: Carsten Brand