TinMan, you don't seem to be understanding the actual mechanics of the issue. The stress is already in the steel, some remains even when the steel has been thermally stress relieved, it has just been significantly reduced. One could say that the steel is forced into an unnatural or "sprung" state during its manufacture. The movement due to heating and cooling and external forces like rolling as well as the forces produced internally by heating being uneven throughout the steel produce these sort of "locked in" or frozen stresses.
When you cut away some of the steel, you unbalance these forces that had been balanced against each other, and movement occurs. From what I understand, thermal stress relief brings the temperature of the steel up high enough that the steel molecules all push against each other enough to yield the steel somewhat, helping to equalize the internal forces. Then the steel is slowly cooled so that it evenly cools throughout such that forces aren't going to be locally higher in some areas than in other areas. There still may be stress/locked-in "spring" in the steel, it will just be much reduced.