A Young’s modulus of 190-210GPa has long been accepted as the natural value for iron and almost all kinds of steel. It is a structure-insensitive physical property and is hardly affected by metallurgical schemes for manufacturing high-strength steels. However, the elastic behaviour of a material is often more important in designing machine parts than its ultimate strength. This is because parts playing a pivotal role in machines have to be designed with a sufficient rigidity so as to yield only a small elastic strain against applied stress. Meanwhile, in accordance with growing concerns about low energy consumption, weight reduction has been a consistent demand imposed on all machine parts. In designing automobile parts, for instance, dimensions (thickness, diameter etc.) have already been minimized at the same time as assuring the required rigidity.