Macrostructure assessing method for solidification structure of continuous casting blank
1 Scope
This standard specifies the preparation of specimen for macrostructure assessment for solidification structure of continuous casting blank, and the classification, assessment and inspection report of solidification structure.
This standard is applicable to macrostructure assessment for solidification structure of continuous casting blank (e.g. square billets, slabs, round billets, rectangular billets and shaped billets, etc.).
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of this standard. For dated references, subsequent amendments (excluding corrections) to, or revisions of, any of these publications do not apply to this standard. However parties to agreements based on this standard are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. For undated references, the latest edition applies.
GB/T 226-1991 Etch test for macrostructure and defect of steels (GB/T 226-1991, neq ISO 4969:1980 Steel - Macroscopic examination by etching with strong mineral acids)
3 Preparation of specimen
3.1 The specimen shall be cut out and etched according to GB/T 226-1991.
3.2 The dendritic corrosion specimen shall be cut out and prepared according to Annex A hereto.
4 Classification and assessment of solidification structure
4.1 Fine isometric belt
4.1.1 Morphology: The structure of fine isometric belt near the surface of continuous casting blank is compact, without orientation, and the microstructure cannot be observed visually, with a layer of light-colored uniform structure shown.
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4.1.2 Cause: Near the upper meniscus of the crystallizer, the molten steel is in close contact with the inner wall of the crystallizer which has a strong heat absorption and dissipation effect. The molten steel is quenched, and the nucleation rate is greater than the growth rate of the crystal nucleus, forming fine isometric belt. The thickness of fine isometric belt depends on the superheat of molten steel and the cooling strength of the crystallizer.
4.1.3 Assessment principle: Calculate the percentage of the area of fine isometric belt in the entire inspection surface of the specimen, which is the fine isometric belt rate. The multi-point thickness of fine isometric belt may also be measured with a ruler (or computer measuring software) to provide the thickness range and average thickness of fine isometric belt.
4.2 Columnar belt
4.2.1 Morphology: Columnar crystal is a collective structure of dendritic crystal. Generally, it grows inward perpendicular to the surface of continuous casting blank, from simple to complex, from fine to coarse, from primary crystal to secondary crystal, from secondary crystal to tertiary crystal, until multiple crystals.
4.2.2 Cause: The continuous casting blank enters the secondary cooling zone, and the surface is strongly cooled by water or vapor, resulting in a large temperature gradient on the surface and liquid core of the billet, forming a unidirectional heat transfer perpendicular to the surface of the blank. The fastest growth direction of the crystal is parallel to the heat flow, which suppresses the growth of adjacent crystals and prioritizes their growth, forming columnar belt.
4.2.3 Assessment principle: Calculate the area percentage of columnar crystal (including inclined columnar crystal not intersected or inlaid with each other) to the entire inspection surface of the specimen, which is the columnar crystal rate.
4.3 Isometric belt
4.3.1 Morphology: Isometric belt (central isometric crystal) is located in the center of the continuous casting blank, presenting circular, elliptical, polygonal shapes, as well as short strip grains. It presents a non-oriented nature.
Foreword i
1 Scope
2 Normative references
3 Preparation of specimen
4 Classification and assessment of solidification structure
5 Inspection report
Annex A (Normative) Specimen preparation of dendritic corrosion
