The purity of steel mainly refers to the quantity, type, morphology, size and distribution of non-metallic inclusions in steel. Non-metallic inclusions usually refer to harmful impurities such as oxygen, sulfur, phosphorus, hydrogen and nitrogen remaining in mold steel. 
The smaller the percentage of these impurities in a unit volume, the higher purity of mold steel.

The purity of mold steel (the content of non-metallic inclusions) has a great influence on the strength, plasticity, fracture toughness, cutting, fatigue, hot brittleness and corrosion resistance of the steel. Experiments show that the composition, quantity, shape and distribution directly affect the physical properties of mold steel, and then directly affect the mold performance and mold lifetime.

After the same heat treatment and machining, the mold steel with the same grade (chemical composition) will show different performance under the same working conditions due to the different purity of the steel itself.

1. strength:
The bigger inclusion particles in the mold steel, the worse strength of the mold, the higher possibility of causing tread angle and scratch.

2. polishing:
Non-metallic inclusions, especially oxide inclusions, will obviously affect the cleanliness of the mold cavity surface, and the polishing grade.

3. plasticity:
The increase of banded inclusions (mainly sulfides) will lead to a significant decrease in transverse section shrinkage.

4. toughness:
The increase of sulphide content will reduce toughness of steel. The more inclusions, the worse fracture toughness of steel, and then it leads to the decrease of mold toughness, causing cracking, fracture or cracking in the mold design.

5. Fatigue resistance:
Inclusions will reduce the fatigue resistance of the mold. Generally speaking, brittle inclusions (category A) and large-scale oxide inclusions have more obvious effects, leading to the mold  scrapped.

6. Welding properties:
Sulfide inclusions and large size oxide inclusions will reduce the weldability of the mold.

7. Corrosion resistance:

The existence of composite inclusions will cause spot corrosion of the mold.