The Reasons, Impacts And Prevention Of The Stratification Of Square Tube Billets

Layering of the billet refers to the phenomenon where layer-like metal separation occurs parallel to the rolling direction within the billet (for seamless square tubes, it corresponds to the circular tube billet; for cold-formed and welded square tubes, it corresponds to the steel plate or strip billet). This is a concealed internal defect. The causes can be divided into two major categories: inherent metallurgical defects and induced by subsequent processes. Eventually, it expands into macroscopic layering during the plastic deformation process.

• Aggregation of non-metallic inclusions: During steelmaking, incomplete deoxidation and insufficient soft blowing in refining, as well as scale formation or erosion of refractory materials, can result in high-melting-point hard inclusions such as Al₂O₃, sulfides, and calcium-aluminate remaining in the molten steel. These inclusions have no plasticity and, during rolling, are elongated into thin, weak layers, becoming the core of the interface for delamination.
• Internal defects of continuous casting billets: During the continuous casting process, central porosity, shrinkage cavities, and subsurface bubbles may remain if they are not completely welded during subsequent rolling. They will be pressed into layered gaps; at the same time, central carbon segregation and banded structure can cause uneven mechanical properties of the cross-section, and the bonding force of the segregation zone is much lower than that of the matrix, making it prone to separation under stress.

• Inconsistent heating and improper temperature: If the tube billet is not heated thoroughly or there is a large temperature difference across the cross-section (commonly known as "yin-yang surface"), it will result in uneven metal plasticity, and during deformation, the interlayer shear stress will exceed the bonding strength, causing separation along the weak surface; if the heating temperature is too low, the overall plasticity will be insufficient, and during piercing/rolling, internal defects are more prone to be pulled apart.
• Mismatch of piercing/rolling parameters (seamless process):  During oblique rolling piercing, if the roll speed is too high and the feeding angle is too small, the tube billet will be subjected to excessive axial shear stress, causing internal minor defects to be pulled apart and extending along the axial direction, forming a layer parallel to the tube wall; excessive single-pass reduction and excessive tool wear will also exacerbate the uneven metal flow, triggering layering.
• Stress concentration in cold bending forming (welding process): The existing minor internal layering in the strip steel will expand along the thickness direction under the repeated bending stress during cold bending forming; unreasonable distribution of forming passes and excessive single-pass deformation will also directly cause shear layering in the thickness direction.
• Uncontrolled post-rolling cooling: If the cooling speed is too fast, it will simultaneously generate large thermal stress and microstructure stress, causing microcracks to form in the central segregation zone; improper stack cooling and premature unstacking will prevent the stress from being fully released, eventually developing into macroscopic layering.

The layering defect is characterized by its concealment (most of it is invisible to the naked eye) and its expansiveness. It fundamentally weakens the performance of the pipe material and poses significant risks to quality and usage safety.

• Significant deterioration of mechanical properties: Layering will cause the effective bearing wall thickness of the pipe material to decrease, and the tensile strength in the layered area will drop by 30% to 50%, the impact toughness can be reduced to below 10J, and the plasticity (elongation) and fatigue resistance will also decline simultaneously; the edges of the layering are natural stress concentration points, and they are prone to rapid expansion under external force.
• Failure of process performance: During subsequent cold bending, expansion, flattening, and bending processing, the layering area will directly crack; if there is layering at the pipe end groove, welding will cause incomplete fusion and slag inclusion during welding, directly resulting in an unqualified welding joint.
• Failure to meet standard testing requirements: When conducting ultrasonic testing (UT) according to standards such as GB/T 20490, the layering will show obvious defect waves, which are the core internal defects for determining nonconformity, directly leading to product downgrade or scrapping.

• Structural bearing fracture risk: When used in structural scenarios such as building grid structures and mechanical supports, layering under static loads will gradually expand, leading to section fracture; under alternating loads, it will quickly induce fatigue cracks, causing sudden brittle fractures and resulting in structural safety accidents.
• Pressure-bearing pipeline leakage and burst: When used for fluid transportation and pressure pipelines, the expansion of layering towards the inner wall will cause pipe wall leakage. Under high-pressure conditions, it may directly burst; if transporting sulfur-containing hydrogen or acidic media, the layering gaps will trigger hydrogen-induced cracking (HIC) and crevice corrosion, accelerating the failure of the pipe material.
• Durability and appearance deterioration: After the corrosive medium enters the layering gaps, it will cause internal concealed corrosion, gradually destroying the pipe material from the inside out, significantly shortening the service life; severe layering will also cause the pipe surface to bulge, size deviations, and bubble formation and peeling after galvanizing or coating.

• Improve the purity of molten steel and the quality of continuous casting:

1. Strengthen LF/VD refining processes to ensure the duration of soft argon blowing, promoting the flotation and discharge of inclusions; use calcium treatment to modify hard Al₂O₃ inclusions, converting them into spherical low-melting-point inclusions, reducing the source of sheet-like stratification.
2. Use a mold + end electromagnetic stirring throughout the continuous casting process to reduce central segregation and central porosity; control the pulling speed steadily to avoid roll slag and subsurface bubbles.

• Strictly inspect raw materials upon entry to the factory: For billets/plates entering the factory, 100% ultrasonic pre-inspection must be conducted to detect internal stratification, inclusions, and porosity defects. Unqualified billets must be directly rejected; simultaneously, surface cracks and scars must be ground down to avoid the extension of surface defects during rolling.

• Standardize billet heating preparation: Based on the steel type, formulate precise heating curves, control the heating rate to ensure uniform heat penetration; plan the centralized loading of billets of the same specification and steel type in advance to avoid uneven heating due to size differences.

• Precisely control the heating process: Ensure the holding time of the equalizing section, control the temperature difference of the tube section within the allowable range, eliminate "dual faces" and low-temperature rolling; strictly control the upper limit of the heating temperature to avoid the decrease in intergranular bonding force due to overheating.

• Optimize the piercing and rolling parameters (seamless process):

1. Control the roll speed below the critical layering speed, appropriately increase the feeding angle, reduce the axial shear stress during piercing, and avoid the generation of layering from the deformation mechanism.
2. Reasonably distribute the reduction amount of each pass to avoid excessive deformation in a single pass; regularly check the wear of rolls, guide plates, and heads, replace them in time if the wear exceeds the limit, and ensure uniform metal flow.

• Optimize the cold bending and welding process (welding process): Refine the forming passes, reduce the single-pass deformation angle, decrease the shear stress in the thickness direction, avoid the expansion of internal layering of the strip; match the high-frequency welding power and extrusion amount to ensure the weld fusion quality, avoid layering of the weld.

• Standardize post-rolling cooling and slow cooling: 

1. Control the post-rolling cooling speed at a stable rate of 3 to 5 °C/s, avoid the formation of brittle bainite in the core of thick-walled billets, and reduce the organizational stress.
2. Implement constant-temperature stacking and slow cooling: for thicknesses ≤ 50mm, slow cooling should be ≥ 12 hours; for thicknesses > 50mm, slow cooling should be ≥ 24 hours. The starting temperature of the stack should be ≥ 500°C. Do not allow early unstacking; for plates with similar length and width, stack them together, and if necessary, wrap them with asbestos cloth for insulation to prevent the edges from cooling too quickly. 

• Full coverage of non-destructive testing for finished products: Comply with the GB/T 20490 standard to conduct automatic ultrasonic testing, covering the entire pipe wall at 100%, accurately locating layering defects; add manual re-inspection at the end of the pipe to avoid defective products from being released.
• Heat treatment and performance verification: High-grade steel, thick-walled square and rectangular tubes undergo normalizing + tempering treatment to eliminate residual stress, homogenize the structure, and improve segregation for toughness; sample pressure flattening, expansion, and cold bending process tests to verify no layering cracking under processing loads.
• Defect traceability and closed-loop improvement: When a layering defect is detected, reverse trace the steelmaking, casting, and rolling process parameters based on the defect shape and distribution, identify the cause and promptly adjust the process; minor layering can have the defective section removed and downgraded for use, while severe layering results in the entire section being scrapped.