A pipe cutter works great with softer metals like Al or copper - somewhat more laborious with steel, although it'll work eventually. What's wrong with simply sawing it, with a tool designed to cut steel? The biggest challenge will be keeping the cut straight and perpendicular (this is where the pipe cutter will excel - do a light score all the way around, ensure that it meets up with itself, and succeeding deeper cuts will track the score perfectly). Last time I cut a steerer, I used an old/cheap treadless stem - clamped it to the steerer and use the top as the guide. The saw blade scratched the stem top up a bit, but resulted in a perfect cut.