What is the purity of steel?
After testing and evaluating the type and degree of non-metallic inclusions in the steel under a microscope and performing statistical calculations, a relatively macro level of purity (or index) of the steel is obtained. This level of purity serves as the basis for a more comprehensive assessment of the quality of the steel.
The purity level expressed in DIN 50 602 is determined by the K method.
Calculus 1:
Annex C of ISO 4967 provides the formula for 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 | two | 2.5 | 3 |
| weight factor- fi | 0.05 | 0.1 | 0.2 | 0.5 | 1 | two |
n me — field of view number of level i;
S – total sample inspection area, unit: mm 2
Calculation 2: number point method
Purity is calculated by dividing the number of occupied grids by the total number of grids in a field of view.
This method is relatively objective and suitable for image processing. It is described in Appendix 1 of JIS G 0555.
This standard specifies the purity (d%) necessary to detect three types 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 grouped in the processing direction and are not arranged continuously.
- Class C inclusion: inclusion with irregular distribution (granular oxide, etc.) without viscous deformation.
The sample typically measures 15mm wide and 20mm high, resulting in a polishing area of 300mm².
Inclusion inspection is usually performed at 400x magnification.
A glass plate with 20 vertical and horizontal grid lines is placed in the eyepiece of the microscope.
The surface under inspection is examined on the microscope stage and the number of grids occupied by various inclusions is counted.
A total of 60 fields of view must be measured, with at least 30 fields of view required.
Using the number of grids on the glass plate within the field of view, the number of fields of view, and the number of grids occupied by inclusions, calculate the percentage area occupied by inclusions using the formula below to determine the cleanliness d (%) of the steel.
Where: p — total number of grids on the glass plate in the field of view;
f – field of view number;
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 less than 400 times.
