1.What are the principles and applicable scenarios of the comparative method (optical microscope)?
Principle: The prepared sample is compared with the standard rating chart under a 100x microscope.
Applicable scenarios: The most commonly used and standardized method. Suitable for production inspection, incoming material acceptance, and process monitoring.
2.What are the principles and applicable scenarios of the intercept method/area method (optical microscope)?
Principle: By statistically analyzing the number of grain boundaries within a given length or area, the grain size can be accurately calculated.
Applicable scenarios: arbitration analysis, precision measurement, scientific research.
3.How is the sampling location determined?
Samples should be taken from the head, middle, and tail of the cold-rolled coil, as well as from the edge, quarter-length, and middle sections along the width (according to inspection requirements). This is because cold rolling and annealing processes can cause differences in properties and microstructure along length and width.
A transverse section (i.e., a section perpendicular to the rolling direction) is typically taken to observe the grain distribution along the thickness direction.
Sampling direction: Specify whether it is a longitudinal, transverse, or normal section. Grain size is usually determined by observing the transverse section.
Sampling size: Typical sample size is 20mm × 20mm × plate thickness.
4.How to observe and rate?
Microscopic Observation:
Observe and rate at 100x magnification (10x objective, 10x eyepiece). This is the standard magnification.
Choose a representative field of view, avoiding atypical areas such as shear bands and abnormally large grains.
Grain Size Rating:
Standard Basis: Chinese Standard GB/T 6394-2017 "Metallic Materials - Determination of Average Grain Size" or American Standard ASTM E112-13.
Comparative Method: Compare the observed image with the rating chart in the standard to find the best matching level. The relationship between grain size level (G) and grain size is: n = 2^(G-1), where n is the number of grains per square inch at 100x magnification. The higher the G value, the finer the grains.
For example: a grain size of level 8 indicates very fine grains and high strength; a grain size of level 5 indicates relatively coarse grains, which may have better plasticity but lower strength.
Report results: The average grain size level should be reported, and the grain size distribution range should be noted if necessary (e.g., "level 7, locally level 5-8").
5.What are the special precautions for cold-rolled coils?
Surface-to-Core Difference: Due to the unique characteristics of cold rolling deformation and annealing processes, grain size may differ from the surface to the core (typically, the surface grains are finer). The observation location must be noted during inspection, or reported separately.
Annealed State: The inspection must be conducted on the finished coil after final annealing. If it is a "semi-finished" or "hardened" cold-rolled coil, its microstructure consists of elongated deformed grains, and the equiaxed grain size rating method is not applicable here.
"Mixed Grain" Phenomenon: Under certain processes (such as uneven annealing), a "mixed grain" microstructure with significantly different grain sizes may occur. In this case, the grades of the largest and smallest grains must be recorded, and their potential impact on performance (such as the formation of "orange peel" defects during stamping) must be assessed.
Influence of Texture: Grains have not only size but also orientation. A high grade (fine grains) accompanied by a strong, favorable texture (such as {111} texture) can simultaneously achieve high strength and excellent deep-drawing performance.