Defects in Hot Rolling Products (Plates / Coils)

 

Introduction:

Hot rolled steel products can exhibit various types of defects that may affect their surface quality, dimensional accuracy, and mechanical properties. Some common types of defects found in hot rolled products include:

 

Surface Defects:

Scale: Excessive scales, also known as mill scale or iron oxide scale, can form on the surface of hot rolled steel products during the hot rolling process. While some amount of scale formation is inevitable due to the exposure of steel to high temperatures in the rolling mill, excessive scales can be detrimental to the quality and appearance of the steel products. Several reasons can contribute to the formation of excessive scales in hot rolling steel products.

Cracks: Cracks in hot rolled steel products can significantly compromise their integrity, performance, and suitability for various applications. These cracks can occur during the hot rolling process or as a result of subsequent handling, cooling, or storage conditions. Understanding the reasons for cracks in hot rolling steel products is crucial for implementing preventive measures to minimize their occurrence.

Blisters: Blisters in hot rolled steel products are localized bulges or raised areas on the surface of the steel that can compromise the quality and appearance of the product. These blisters are typically caused by internal defects or imperfections within the steel material, as well as certain processing conditions during hot rolling. Some common reasons for blisters in hot rolled plates and coils include.

Surface scratches: Abrasions or marks on the surface of the steel caused by handling or processing.

Pits: Small depressions or cavities on the surface of the steel.

Rust spots: Surface corrosion or oxidation resulting in localized discoloration.

 

 

Edge Defects:

Edge cracks: Edge cracks in hot rolling steel products can occur due to various factors related to material composition, processing conditions, and handling practices. Understanding the reasons for edge cracks is essential for implementing preventive measures to minimize their occurrence and ensure the quality of the finished steel products.

Edge waves: Undulations or distortions along the edges of the steel caused by uneven cooling or rolling.

 

Internal Defects:

Laminations: Delamination or separation of layers within the steel plate due to non-metallic inclusions or imperfections.

Inclusions: Non-metallic particles or impurities embedded within the steel matrix, affecting mechanical properties and surface quality.

Segregation: Non-uniform distribution of alloying elements or impurities within the steel, leading to variations in composition and properties.

Internal Cracks: Internal cracks in hot rolling steel products can be particularly problematic as they are not readily visible on the surface and can significantly compromise the integrity and performance of the steel. These cracks can propagate internally within the steel material and may only become apparent during subsequent processing or service.

 

 

Dimensional Defects:

Camber: Camber in hot rolling steel products refers to a longitudinal curvature or bending along the length of the steel material. Camber defects can significantly impact the dimensional accuracy and flatness of the steel products, leading to challenges in downstream processing and fabrication.

Wedge: Wedge-type defects in hot rolling steel products refer to elongated or tapered defects that appear as wedge-shaped protrusions in the cross section of the rolling area from top to bottom in full width of the steel material.

Crown: This is opposite to Wedge defect and in this defect rolled material is negative side rolled in the both edges and positive side rolled in the centre of width which looks like crown shape in the full cross section and full width of the hot rolled products.

Buckling: Deformation or distortion of the steel product, resulting in waves or wrinkles on the surface either in the center of rolled product width or on both edges and any one edge.

Twist: Twisting or torsional deformation of the steel product, causing misalignment or irregularities.

Short Length / Short Width: This type of defects reasons either can be from the rolling slab dimensions side or wrong planning during hot rolling or wrong cutting during finishing of hot rolled products.

Thickness Variation: This type of defects happened either from wrong planning or some issue during hot rolling.

 

Metallurgical Defects:

Grain growth: Abnormal enlargement of grain size in the steel microstructure, affecting mechanical properties and toughness.

Decarburization: Loss of carbon content at the surface of the steel due to high-temperature exposure, leading to reduced hardness and strength.

Processing Defects:

Roll marks: Impressions or patterns on the surface of the steel caused by irregularities in the rolling process.

Slivers: Thin strips or flakes of steel separated from the main body of the steel product during rolling or processing.

Delayed fracture or crack: Delayed fracture/ crack or hydrogen embrittlement, is a type of defect that occurs in hot rolling steel products after quenching and tempering, or solely after quenching. This phenomenon is characterized by cracks that develop over time, often hours or days after the quenching process, rather than immediately. The main reasons for delayed cracking in hot rolling steel products will discussed in our next discussion in this blog.

 

These are just some of the common defects that can occur in hot rolled steel products. Each defect may have various causes, ranging from process parameters and material properties to handling and storage conditions. Proper quality control measures and preventive actions are essential to minimize defects and ensure the quality and performance of hot rolled steel products.

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