Iron does not deform as easily as stainless steel. Basically, it is caused by uneven blanking, or the seams are not in a straight line, or the bottom plate is uneven. Look how thick your plate is. If the internal stress is not removed by cutting, it is difficult to correct it.

You can try to knock four turns from the middle with a rubber hammer, then knock evenly from the middle to the four corners, then bake the four-sided joint with fire, and then knock the bottom joint.