Analysis of the Causes of Layering in Square Tube Billets
Analysis of the causes of layering in square tube billets:
Layering (Delamination/Layering) refers to the occurrence of layered metal separation within the billet along the rolling direction, which is a concealed internal defect. According to its causes, it can be classified into two major categories: inherent metallurgical defects of the raw materials (the fundamental cause) and induced by subsequent processing techniques (the expansion condition) - the former is the "seed", and the latter is the "amplifier", which eventually expands into macroscopic layering during plastic deformation.
1. Inherent Metallurgical Defects (Fundamental Cause)
| Defect Type | Formation Mechanism | Effect On Layering |
| Non-metallic inclusions aggregation | Insufficient deoxidation during steelmaking, insufficient refining soft blowing, erosion and peeling of refractory materials, residual high-melting-point hard inclusions (Al₂O₃, sulfides, calcium-aluminate) | These inclusions have no plasticity and are elongated into thin sheet-like weak interfaces during rolling, becoming the "core" of layering |
|
Center porosity/ voids in continuous casting billets |
Insufficient central secondary cooling during continuous casting, leaving pores |
If the subsequent rolling fails to fully weld them together, they are pressed into layered gaps |
| Subsurface bubbles/ holes | Extrusion slag in the mold, gas precipitation | They are pressed into linear layers along the rolling direction |
|
Central carbon segregation/ strip structure |
Unstable casting speed, uneven secondary cooling | Slag segregation areas have a bonding force much lower than the base material, and they separate under stress |
|
Continuous casting out-of-roundness/ angle cracks |
Improper mold cone shape, strong secondary cooling | Internal cracks that have not been welded together in the mold do not close during punching and expand |
Academic research also indicates: Chain/rod-shaped primary carbide and nitride precipitates (such as Nb(C,N)) distributed along the axial direction in high-alloy steels (such as stainless steel, high-temperature alloys) do not deform in coordination with the matrix, resulting in concentrated interface stress, which is also an important metallurgical cause of oblique rolling perforation layering.
II. Defects Induced by Processing Techniques (Extended Conditions)
1. Inappropriate Heating Conditions
- Inconsistent heating / "opposite sides": Large temperature difference across the cross-section → Unequal plasticity of each layer → Interlayer shear stress exceeds bonding strength → Separation along the weak plane
- Insufficient heating temperature: Insufficient overall plasticity, internal minor defects are torn during piercing/rolling
- Excessive heating / Overheating and Overburning: Decreased grain boundary bonding strength, making it more prone to intergranular cracking
2. Mismatch of piercing/rolling parameters (for seamless tubes)
Mechanical characteristics during two-roll inclined rolling with piercing:
- Excessive roll speed and too small feeding angle → Excessive axial shear stress → Internal micro-defects are elongated axially, forming layers parallel to the tube wall
- Excessive reduction in a single pass → Uneven metal flow, mismatch between the surface and the center of deformation, and the transition layer is subjected to additional tensile stress + shear stress and is thus cracked
- Excessive wear of the punch/ guide plate → Disordered metal flow, accumulation and bulging, accelerating layering
- Larger wall thickness, higher steel grade, and greater deformation resistance → More significant tendency for layering (thick-walled tubes, high-alloy steels are high-risk areas)
3. Stress Concentration in Cold Bending Forming (ERW Pipe Process)
- Minor laminar defects originally present in the strip steel expand along the thickness direction under repeated bending forming stress
- Inappropriate distribution of forming passes and excessive deformation angle per pass → Directly causing shear layering in the thickness direction
- Inconsistent power of high-frequency welding / extrusion volume → Weld seam area not fully fused, resulting in weld seam layering
4. Uncontrolled Post-Rolling Cooling
- Excessive cooling rate (especially in the thick-walled pipe core) → Thermal stress + microstructural stress superimposition, generating microcracks in the central segregation area
- Improper pile-up cooling, premature unstacking → Residual stress not fully released → Later develops into macroscopic layering
- Excessive cooling rate also generates brittle bainite structure, further reducing the core's toughness
III. On-site Rapid Judgement: How to Preliminarily Determine the Source
| Layered characteristics | High-probability source |
| Continuous and long-distance distribution along the rolling direction, in the form of sheets | Inclusions/central defects in continuous casting (metallurgical root cause) |
| Concentrated in the middle transition layer of the wall thickness, related to perforation deformation | Improper slitting perforation deformation parameters |
| Appears only near the weld seam | Unfused ERW welding or forming stress |
| Defect waves are observed in the end groove by ultrasonic testing | Post-rolling cooling stress or pile-up cooling problem |
| Visible inclusions can be seen at the layering location | Insufficient steel water purity |
