Steel cylinders are not recommended for dispensing beer on your kegerator

Steel cylinders rust (some even have rust when they are brand new), and are twice the weight of aluminium cylinders. If moisture and CO2 mix they form carbonic acid which actually eats away steel, so your steel cylinder could be corroding from the inside without you knowing about it. Aluminum cylinders have a much longer working life than steel cylinders due to the steel cylinders corrosion issues, so it works out much better in the long run

Make sure the co2 cylinder is certified with the standards in your country when you purchase a kegerator.