I don't know, to put it simply: it expands with heat and contracts with cold. When a car collides, the iron plate will stretch (deform). I want to restore the iron plate to its original length, burn a certain point of the iron plate with fire, and then quickly cool it with water or other things to achieve the effect of cold shrinkage. Although I can't see it with the naked eye, the effect is obvious. I come from Shandong, and the dialect of our sheet metal workers here is: close the iron plate (shrink)