The correct answer is option 'C': Spheroidizing.

Spheroidizing is a heat treatment process that is used to obtain a globular form of cementite in the structure of steel. Cementite is a compound of iron and carbon, and it is a hard and brittle phase that can negatively affect the mechanical properties of steel.

Here's a detailed explanation of why spheroidizing is used to obtain a globular form of cementite in steel:

1. Introduction to cementite:
Cementite, also known as iron carbide (Fe3C), is a compound that forms in steel when the carbon content exceeds the solubility limit of carbon in ferrite. It has a distinct crystal structure and is extremely hard and brittle.

2. The problem with cementite:
The presence of cementite in steel can lead to brittleness and reduced ductility. It can act as a stress concentrator, causing localized failures and reducing the overall strength of the material. Therefore, it is desirable to modify the structure of cementite to improve the mechanical properties of steel.

3. Spheroidizing process:
Spheroidizing is a heat treatment process that involves heating the steel to a temperature just below the eutectoid temperature, which is around 723°C for plain carbon steels. The steel is then held at this temperature for a prolonged period of time to allow the cementite to transform into a more spherical shape.

4. Transformation of cementite:
During the spheroidizing process, the cementite in the steel undergoes a transformation from a lamellar (plate-like) structure to a globular form. This transformation occurs due to the diffusion of carbon atoms within the steel, which leads to the coarsening and rounding of the cementite particles.

5. Benefits of spheroidizing:
The globular form of cementite obtained through spheroidizing has several advantages over the lamellar form. It reduces the stress concentration at the cementite-ferrite interfaces, thereby improving the ductility and toughness of the steel. It also enhances the machinability of the steel and reduces the risk of cracking during processing.

In conclusion, spheroidizing is a heat treatment process used to obtain a globular form of cementite in the structure of steel. This process improves the mechanical properties of steel by reducing the brittleness associated with the lamellar form of cementite.