ANS:C - return of sharp yield point in temper rolled low carbon sheet after a period of time.

Ageing of steel typically refers to the phenomenon where certain changes occur in the material properties of steel over time, often due to internal structural changes or environmental factors. Among the options provided:

• Degradation of steel by corrosion: While corrosion can certainly occur over time, the term "ageing" typically refers to changes in material properties rather than degradation through corrosion.
• Failure of steel structure by overstressing: This option refers to failure due to mechanical stress rather than ageing-related changes in material properties.
• Decarburisation of steel by overheating: Decarburisation is a process where the carbon content in steel is reduced due to exposure to high temperatures in the presence of oxygen. While this can affect the properties of steel, it doesn't encapsulate the broader concept of ageing.

• Return of sharp yield point in temper rolled low carbon sheet after a period of time: This option describes a specific phenomenon associated with the tempering process in steel, where the material may exhibit changes in its yield point after a period of time. This is a form of ageing in steel where the material's mechanical properties evolve over time.