The steering tube is brazed to the fork crown, so it is one piece. If you mean the stem is stuck, it is probably corroded to the inside of the steerer tube.
The stem should have an expander bolt that when loosened eases the grip of the quill wedge (or expander cone) against the inside wall of the steerer tube and you should be able to pull it (the stem) out of the steerer tube once the expander bolt is loose. Sometimes, if the wedge isn't too badly stuck to the steerer from corrosion, you can loosen the expander bolt and use a scrap of 2 x 4 lumber on top of the bolt head and sharply hit the 2 x 4 with a hammer a couple of times. If that fails, read the Sheldon Brown article I linked to in my post above.
The stem should look like this:
It could have either a wedge or an expander cone to secure it in the steerer tube:
