YB 4002-91

Continuous Cast Steel Billet — Macrostructure Defect Rating Chart

Issued: 1991-05-04  |  Implemented: 1992-01-01

Issued by: Ministry of Metallurgical Industry, People’s Republic of China

1 Subject Content and Scope of Application

This standard specifies the macrostructure and defect morphology characteristics, causes, and rating principles for continuous cast steel billets.

This standard is applicable to the rating of defects in acid-etched macrostructure of carbon steel and low-alloy steel billets produced by continuous casting process. The cross-sectional dimension range of billets is 90~200 mm in side length.

Whether each type of defect in the rating charts is permitted and the acceptable grade shall be specified in the corresponding technical standards or bilateral agreements.

2 Referenced Standards

GB 226 — Acid etching test method for macrostructure and defects of steel

3 Classification and Rating of Macrostructure and Defects of Continuous Cast Steel Billets

3.1 Specimen Display Method

Performed in accordance with GB 226.

3.2 Morphology Characteristics, Causes, and Rating Principles of Each Type of Defect

3.2.1 Center Pipe/Core Porosity

Morphology: Voids and dark spots concentrated at the center region on the etched specimen surface.

Cause: Caused by the final stage of solidification where shrinkage cavities and segregation concentrate.

Rating Principle: According to Rating Chart No. 1 (Appendix A), rated based on the quantity, size, and concentration of dark spots and voids.

3.2.2 Center Segregation

Morphology: Dark bands showing deeper corrosion at the center region of the etched specimen.

Cause: During solidification of molten steel, due to the influence of solidification patterns and slower cooling at the center of the billet, compositional segregation occurs in the core.

Rating Principle: According to Rating Chart No. 2 (Appendix A), rated based on the size of darker corroded areas at the center.

3.2.3 Shrinkage Cavity

Morphology: Irregular cavities at the center of the specimen.

Cause: Produced by volume concentration shrinkage during solidification of molten steel.

Rating Principle: According to Rating Chart No. 3 (Appendix A), rated based on the cavity size.

3.2.4 Internal Cracks

3.2.4.1 Corner Cracks

Morphology: Cracks at the corners of the specimen, at a certain depth from the surface and perpendicular to the surface. In severe cases, they extend diagonally inward.

Cause: Formed by local stress caused by corner surface depression and severe diamond deformation of the billet.

Rating Principle: According to Rating Chart No. 4a (Appendix A), rated based on the number, size, and distance from the surface.

3.2.4.2 Edge Cracks

Morphology: Cracks at the edge of the specimen, generated at the boundary between equiaxed and columnar crystals and extending inward along the columnar crystals.

Cause: Caused by straightening deformation of bulged billets passing through guide rolls.

Rating Principle: According to Rating Chart No. 4b (Appendix A), rated based on the number and size of cracks.

3.2.4.3 Intermediate Cracks

Morphology: Cracks generated in the columnar crystal zone and extending along the columnar crystals. These cracks are generally perpendicular to both side surfaces of the billet. In severe cases, cracks exist simultaneously in all four directions (up, down, left, right) from the center point.

Cause: Caused by thermal stress from inadequate forced cooling and subsequent reheat of the billet surface when passing through the spray cooling zone.

Rating Principle: According to Rating Chart No. 4c (Appendix A), rated based on the number and size of cracks.

3.2.4.4 Center Cracks

Morphology: Cracks generated in the columnar crystal zone near the center and perpendicular to the billet cross-section. Generally formed on the upper arc surface, and may penetrate through the center in severe cases.

Cause: Caused by tensile stress during bending and straightening of the casting billet.

Rating Principle: According to Rating Chart No. 4d (Appendix A), rated based on the number and size of cracks.

3.2.5 Subcutaneous Bubbles

Morphology: Pores of varying sizes located beneath the surface of the billet.

Cause: Caused by gas entrapped during casting process.

Rating Principle: According to Rating Chart No. 5 (Appendix A), rated based on the distance from the surface, length, and number of bubbles.

3.2.6 Non-Metallic Inclusions

Morphology: Non-metallic particles of different shapes and colors or voids left after non-metallic inclusions fall off during etching. Generally located at one-quarter of the side length beneath the upper arc surface.

Cause: Deoxidation products during smelting and inclusions formed by secondary oxidation of molten steel enter the mold and are difficult to float and separate.

Rating Principle: According to Rating Chart No. 6 (Appendix A), rated based on the number and size of inclusions. Large particle inclusions are not permitted.

3.3 Rating Levels

All defect rating charts are divided into 5 levels. Level 0 chart (Appendix A) is a common chart shared by all defect types.

3.4 Rating Method

Each type of defect is rated according to its degree by comparison with the rating charts, limited to visible defects to the naked eye. When the degree is between two adjacent levels, a half level may be assigned.

3.5 Reference Dimensions

The actual dimension of the chart side length is 100 mm. When comparing and rating defects of billets of other dimensions, the rating charts should be scaled down or enlarged proportionally.

4 Test Report

The inspection report shall include the following items:

a) Commissioning unit;

b) Steel grade being inspected;

c) Melting number of the inspected sample;

d) Inspection method;

e) Inspection results: defect type, rating grade, and other remarks;

f) Inspector and inspection date.

Appendix A — Rating Charts for Macrostructure and Defects of Continuous Cast Steel Billets

The appendix contains the following rating charts:

• Fig. A1 — Level 0 Chart (No Defect)
• Fig. A2 — Rating Chart No. 1: Center Porosity
• Fig. A3 — Rating Chart No. 2: Center Segregation
• Fig. A4 — Rating Chart No. 3: Shrinkage Cavity
• Fig. A5 — Rating Chart No. 4a: Corner Cracks
• Fig. A6 — Rating Chart No. 4b: Edge Cracks
• Fig. A7 — Rating Chart No. 4c: Intermediate Cracks
• Fig. A8 — Rating Chart No. 4d: Center Cracks
• Fig. A9 — Rating Chart No. 5: Subcutaneous Bubbles
• Fig. A10 — Rating Chart No. 6: Non-Metallic Inclusions

(Note: The visual rating charts are presented in the original PDF document below. Each rating chart is divided into 5 levels (0-4) with half-level ratings allowed for intermediate cases.)

WhatsApp: +86-18658777855
Email: 1007788887@qq.com