The reasons are due to the high permeability of the steel. It could be magnetized due to the pipe being stored near power lines. If it is FBE coated, it could be from the coating process. It could even be from being stored in a North and South direction for an extended period of time and the earth's magnetic field. There could be a number of reasons. I've seen magnetic fields in pipe that would hold a large magna-lite flashlight. The magnetization causes what is referred to as "arc blow".

Demagnetization would be difficult, expensive and would probably only remove a portion of the magnetic field for a short period of time.

The solution is to wrap coils around each magnetized piece to be welded and run AC current through them during welding. That should neutralize or at least minimize the effects of the magnetic field.

When you say that the weld edges become "strongly magnetic", are these weld edges magnetic before you start welding, or does it happen while you are welding?

If the magnetism originates while welding, then I assume that it is due to the "ordinary" arc blow problems that can be found on any magnetic material. Generally this will be due to the position of the earth cable relative to the arc, as well as the weld prep.

If this is the case, welding with AC will eliminate the problems, especially if the welding cable is "wrapped" around the workpiece. This is not always practical. Otherwise, just keep moving the earth cable till you get to a place where it works.

Regards
Niekie