1.What are the core standards and rating methods?
Chinese National Standard GB/T 38813-2020 "Determination of Recrystallization Grain Size of Cold-Rolled Steel Sheets"
This is the most directly relevant standard, detailing how to prepare samples, visualize grain boundaries, and compare with standard rating charts to determine the grain size level.
International Standard ASTM E112 "Standard Test Method for Determination of Average Grain Size"
Consistent with the principles of GB/T 38813, it is a globally accepted benchmark.
Grain Size Level (G): The standard defines the grain size level index G, which is related to grain size as: n = 2^(G-1) (the average number of grains observed under a 100x microscope within a 1 square inch area). A larger G value indicates finer grains.
2.What are the common steel grades?
Low-carbon aluminum-killed steel (e.g., SPCC, DC01); deep-drawing/ultra-deep-drawing steel (e.g., SPCD/DC04, SPCE/DC05, 08Al); high-strength steel (e.g., bake-hardening steel BH, duplex steel DP); electrical steel (silicon steel).
3.What are the key control requirements and common problems?
Uniformity: The grain size of the entire steel roll must be uniform in both length and width. Non-uniformity leads to fluctuations in mechanical properties and problems such as slip lines and wrinkling during stamping.
Avoid mixed grains: This refers to the presence of grains with significantly different sizes within the same field of view. This severely degrades the material's formability.
Prevent abnormal grain growth (secondary recrystallization): In ordinary low-carbon steel, the presence of a few abnormally large grains (such as "pancake-shaped grains") is a defect that disrupts material uniformity, resulting in an "orange peel" appearance on the surface.
4.How to control grain size after annealing?
Cold rolling reduction: This is the driving force for recrystallization. Generally, a higher reduction (more severe deformation) results in finer recrystallized grains after annealing. However, the reduction must be matched with the annealing regime.
Annealing temperature and time: Above the recrystallization temperature, higher temperatures and longer times tend to promote grain growth. The target grain size can be obtained by precisely controlling the temperature profile of a continuous annealing furnace or a bell-type furnace.
Steel chemical composition and second-phase particles:
Fine compound particles formed by elements such as Al, Nb, and Ti (e.g., AlN, NbC) can pin grain boundaries, inhibiting grain growth and thus obtaining a fine-grained structure (used in high-strength steels).
For deep-drawing steel, it is necessary to control the coarsening and distribution of these particles to allow for moderate grain growth.
5.The standard for grain size after annealing of cold-rolled coils is not a fixed value, but rather refers to what?
Complies with the rating methods of national standards (GB/T 38813 or ASTM E112).
Meets the implicit requirements for microstructure and properties in specific product standards (such as automotive sheet standards, deep-drawing sheet standards).
Achieves the target grain size level expected in material design (e.g., deep-drawing steel aims for coarse grains of G5-G7, and high-strength steel aims for fine grains of G10 or higher).