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**What is the purity of steel?**

After the type and grade of non-metallic inclusions in steel are tested and evaluated by microscope, and through statistical calculation, a relatively macro steel purity level (or index), that is, the purity of steel, is obtained, which provides a basis for more comprehensive evaluation of steel quality.

The method K in DIN 50 602 standard expresses a purity level.

**Calculation 1:**

Annex C of the ISO 4967 standard gives the formula of the relevant purity level *Ci*:

Where:

*fi* – weight factor, the weight factor of each inclusion level is as follows:

Grade-i | 0.5 | 1 | 1.5 | 2 | 2.5 | 3 |

weight factor-fi | 0.05 | 0.1 | 0.2 | 0.5 | 1 | 2 |

*n _{i}*

_{ }— number of field of view of level i;

*S* – total inspection area of sample, unit: mm^{2}

**Calculation 2: number point method**

The purity is calculated by dividing a certain number of grids in a field of view and detecting the number of grids occupied by inclusions.

This method is relatively objective and suitable for image processing.

This method is described in Appendix 1 of JIS G 0555.

This standard specifies the purity d (%) to detect the following three kinds of inclusions:

- Class A inclusion: inclusion with viscous deformation (sulfide, silicate, etc.) during processing.
- Class B inclusions: granular inclusions (alumina, etc.) in which the inclusions are clustered in the processing direction and are not continuously arranged.
- Class C inclusion: inclusion with irregular distribution (granular oxide, etc.) without viscous deformation.

The specimen is usually 15mm wide and 20mm high, i.e. the polishing area is 300mm^{2}.

The inclusion inspection is usually performed with a magnification of 400 times.

The glass plate with 20 vertical and horizontal grid lines is inserted into the eyepiece of the microscope.

The inspected surface is inspected on the microscope stage, and the number of grids occupied by various inclusions is counted.

The number of fields of view to be measured is 60, and at least 30 fields of view are required.

According to the total number of grids on the glass plate in the field of view, the number of fields of view and the number of grids occupied by inclusions, calculate the area percentage occupied by inclusions according to the following formula, and judge the cleanliness d (%) of the steel.

Where: p — total number of grids on the glass plate in the field of view;

f – number of field of view;

n — the number of cells occupied by all inclusions in f fields of view.

**Recording method:**

For example: dB60 × 400 = 0.08%, indicating that the content of class B inclusions is 0.08% when 60 fields of view are detected on the 300 mm^{2} test surface under 400 times.