This first stress-strain diagram show mild steel.


This second diagram shows a hardened steel curve and a soft steel curve on the same diagram. You can see the stress and strain track exactly on top of each other (extreme left side of the diagram) until the soft steel starts to yield. The hard steel curve continues upward and you can see that it doesn't have much plastic region (more brittle). This is what you'd see in two identical flat springs, one soft and the other heat treated.