Introduction:
The process of steel-making involves the reduction of iron oxide to pig iron using carbon, followed by the oxidation of excess carbon. This is then followed by the addition of more carbon to produce steel. Each step in this process has its own significance and serves a specific purpose.

Reason for the middle step:
The middle step, which involves the oxidation of excess carbon, is necessary for the following reasons:

1. Removing impurities: During the reduction of iron oxide to pig iron, carbon is used as a reducing agent. However, this process also results in the formation of impurities such as silicon, manganese, and phosphorus. These impurities need to be removed in order to produce high-quality steel. The oxidation of excess carbon helps in removing these impurities by forming oxides that can be easily separated from the molten metal.

2. Controlling the carbon content: The carbon content in steel needs to be carefully controlled in order to achieve the desired properties. Too much carbon can make the steel brittle, while too little carbon can make it too soft. By oxidizing the excess carbon, the carbon content can be precisely adjusted to the desired level, allowing for the production of steel with the desired properties.

3. Improving the overall quality: The oxidation of excess carbon also helps in improving the overall quality of the steel by reducing the presence of other impurities. The oxides formed during this process can be easily removed, resulting in a cleaner and more refined steel.

Why it is not combined with the first or third step:
While it may seem logical to combine the oxidation of excess carbon with either the reduction of iron oxide or the addition of carbon, there are specific reasons why each step is performed separately:

1. Separate reduction of iron oxide: The reduction of iron oxide to pig iron requires a high temperature and a reducing agent such as carbon. Combining this step with the oxidation of excess carbon would result in a complex and inefficient process, as the temperature and reaction conditions required for each step are different.

2. Controlled addition of carbon: The addition of carbon to produce steel is a carefully controlled process. By adding carbon in a separate step, the carbon content can be precisely adjusted to achieve the desired properties. Combining this step with the oxidation of excess carbon would make it difficult to control the carbon content accurately.

3. Efficient removal of impurities: By having a separate step for the oxidation of excess carbon, the formed oxides can be easily separated from the molten metal, allowing for more efficient removal of impurities. Combining this step with either the reduction of iron oxide or the addition of carbon would complicate the separation process and may result in a lower-quality steel.

Conclusion:
The middle step in the steel-making process, which involves the oxidation of excess carbon, is necessary to remove impurities, control the carbon content, and improve the overall quality of the steel. This step is performed separately from the reduction of iron oxide and the addition of carbon to ensure efficient and controlled production of high-quality steel.