Hydrogen makes better steel
13.02.2020 | Salzgitter Flachstahl GmbH
Salzgitter Flachstahl has been using hydrogen for decades in its annealing shop and hot dip galvanizing lines in order to precisely tune product characteristics
What we humans need to breathe is not always good for steel: Air and oxygen can lead to quality impairments in annealing and in hot dip galvanizing due to oxidation. So hydrogen is used in these production stages as a shield gas.
The gas is delivered in highly compressed and liquefied form by truck from Leuna and stored on the steelworks site in Salzgitter in tanks, in which its volume expands once more by a factor of ten subsequent to its transportation.
Each month some 400,000 m3 of hydrogen flow through the on-site network of pipelines to the annealing shop and nos. 1 and 2 hot dip galvanizing lines. That’s enough gas to fill 160 million balloons per month.
Of the total volume of hydrogen used in Salzgitter, each of the two galvanizing lines currently consumes 100,000 m3 per month, with the annealing shop, in which almost 100% pure hydrogen is used, accounting for the other half, or 200,000 m3. At this level of saturation, the gas must first be compressed at up to 10 bar overpressure, then dried via a pressure swing adsorption system. On the hot dip galvanizing lines, on the other hand, the hydrogen is used to replace nitrogen – on Line 1 by 20% and on Line 2 by 5%.
The annealing process is employed to ensure that all precision rolled and electro-galvanized sheet steel achieves the desired quality. Heating, holding the temperature and then cooling alters the structure of the steel and enhances properties such as yield point, tensile strength and elongation at fracture.
Steel takes an annealing tour
Precisely 81 annealing stations are available in the bell furnace for this procedure. In each shift, three to five coils are stacked one on top of another on between eight and twelve of these plinths. Together they weigh a maximum of 100 t and can be up to 5 m in height. This is when the “annealing tour” commences, and can last for 50 to 80 hours.
At the start of the tour, a crane places a protective stainless steel hood over the stack of coils. The interior is then flooded with nitrogen in order to flush out every trace of oxygen. A larger heating bell is then positioned over the protective hood. Now twelve natural gas burners ignite to heat the space between the two hoods, while at the same moment hydrogen flows into the protective hood and dispels the nitrogen. The hydrogen molecules are so small that they penetrate even between the windings of the coiled steel.
The hydrogen performs two tasks under the protective hood. Annealing Shop and Skin Pass Mill manager Dr. Jürgen Spehr, explained these as follows: “First of all, hydrogen is an outstanding conductor of heat, and it readily conducts the heat from the outer heating bell into the coil stack. Secondly, it removes the rolling oils still adhering to the material. These burn off at 200 to 500 °C – the temperature in the annealing bell is between 670 and 720 °C.”The volume within the process chamber beneath the protective hood amounts to 30 m3. Given that the hydrogen is continuously replenished in order to burn off all the oil residues, around 290 m3 of this gas are required for each annealing process. The hydrogen extracted is itself burned, helping to heat the entire process. The heat is maintained for between 6 and 35 hours, before the annealing bell is replaced by a cooling hood and the inside is cooled with air and water. After about 18 hours, the hydrogen is replaced by nitrogen.
Each “tour” is defined by an “annealing code”, which determines the annealing temperature, heating, holding and cooling times and hydrogen consumption. The parameters vary according to the grade of steel and the size and weight of the coils. There are currently around 25 different defined annealing codes. The procedure has been employed since the 1960s and has since been further optimized. Initially a gas mixture was used containing 5 to 8 % hydrogen, but since 1987 almost 100 % pure hydrogen has been employed.
The role of hydrogen in hot dip galvanizing
After annealing the steel can be galvanized – and hydrogen plays an important role in this process, too. Salzgitter Flachstahl operates two hot dip galvanizing lines. Line 2 is considered to be the most complex plant in the field of cold rolled flat products. It combines several process stages and performs the annealing as well, so it does not need to process coils from the bell furnace.
Line 2 is a mighty and imposing installation that stands more than 50 meters tall. All of the processes are fully automated, with only a few operatives required to monitor the sequences. In contrast to the annealing shop, at the start of the process here the coils are unwound, fed in and welded to the end of the preceding coil. The result is an endless strip which is cut once again when the process is finished. The schematic illustration of the plant as a whole is reminiscent of an old film projector, in which the celluloid strip is threaded in, fed up and down around numerous rollers and spools, and then coiled up again.
On Line 2 the steel strip is first pre-cleaned in several stages, then annealed in the furnace at 690 to 890 °C. The passage through the furnace takes a maximum of ten minutes. After first cooling to 210 °C, the moving strip is reheated in the induction furnace to 450 °C and immersed via an enclosed channel in a zinc bath at a temperature of approx. 420 °C
The feed channel enclosure is filled with a mixture of nitrogen and hydrogen - if the steel comes into contact with air, it oxidizes immediately. Consequently, the strip is covered in a thin layer of iron oxide which must be removed before galvanizing. “The hydrogen does the job. It reacts with the oxygen in the iron oxide to form water,” explained Dr. Frank Barcikowski, manager of the No. 2 Hot Dip Galvanizing Line. This process takes place at a temperature of at least 500 °C and lasts only until the atmosphere in the feed channel has reached maximum humidity. The gas mixture which on Line 2 contains 2 to 5 % hydrogen is therefore regularly replaced – at a rate of 1000 m3 per hour. To make sure that no air can seep in due to leaks, an overpressure is created in the feed channel. The exit lies directly in the zinc bath, so the steel has no further contact with the air and cannot oxidize once again.
As a result, hydrogen plays a decisive role just before the actual moment of galvanizing, and thereby makes a major contribution towards the quality of the hot dip galvanized metal, which is an important factor for the automobile industry in particular. One secondary aspect has an important future connotation: Hot dip galvanizing line 2 now uses the same small volume of hydrogen which is produced in the GrInHy experimental plant on the steelworks site.