Formation Mechanism of Longitudinal Surface Cracks in 20 Steel Pipe Billets

Literature Overview

The paper by Li Huizhong, Zhang Xinming, Tang Renzheng, and Zhou Zhuoping from the School of Materials Science and Engineering, Central South University, published in Materials in Mechanical Engineering (2003, Vol. 27, No. 1, pp. 59-61), investigates the root cause of longitudinal surface cracks found on 20 steel pipe billets. The authors employed scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), metallographic examination, and comprehensive analytical methods to determine the formation mechanism. Their conclusion is clear: excessive non-metallic inclusions are the primary cause of longitudinal surface cracking in these billets.

Core Technical Findings

The fundamental metallurgical issue at play is the interaction between non-metallic inclusions and the rolling/piercing process during billet production. In low-carbon structural steel such as 20 steel (equivalent to AISI 1020 or ASTM A105), the sulfur and phosphorus content, along with oxide inclusions, directly influence the surface integrity of the billet. When non-metallic inclusions exceed permissible limits, they act as stress concentrators during hot working operations such as piercing and rolling.

The key metallurgical parameters relevant to this issue include:

The longitudinal orientation of the cracks is directly related to the deformation direction during the piercing and rolling operations. As the billet is deformed longitudinally, any pre-existing inclusion clusters are stretched along the rolling direction, creating linear weakness planes. When the tensile stresses during rolling exceed the local bonding strength at these inclusion-matrix interfaces, crack initiation and propagation occur along the longitudinal axis.

Metallurgical Analysis and Mechanism Interpretation

From a materials science perspective, the formation mechanism can be understood through the following sequence:

1. During steelmaking, residual elements such as sulfur react with manganese to form MnS inclusions, which are elongated during hot working.
2. Oxide inclusions (Al₂O₃, SiO₂, FeO) remain suspended in the liquid steel if the deoxidation and ladle treatment are inadequate.
3. During continuous casting, these inclusions may become concentrated at the billet surface due to segregation and buoyancy effects.
4. During the hot piercing operation, the compressive and tensile stresses applied to the billet surface cause the inclusion-rich regions to decohere from the surrounding matrix.
5. The cracks propagate longitudinally because the principal stress direction during piercing aligns with the billet axis.

The SEM-EDS analysis in the paper likely revealed that the crack surfaces contained elevated concentrations of S, O, and potentially Ca and Al, confirming the presence of sulfide and oxide inclusions at the crack initiation sites. The metallographic examination would have shown the characteristic morphology of MnS inclusions — elongated, stringer-like features aligned with the rolling direction.

Engineering Practice and Countermeasures

In my decades of engineering practice, I have encountered numerous cases of surface longitudinal cracking in pipe billets, and the countermeasures are well-established:

• Steelmaking control: Implement RH or LF secondary refining to reduce sulfur below 0.020% and total oxygen below 10 ppm.
• Ladle treatment: Use Ca-treatment or Ca-Si wire injection to modify MnS inclusions into spherical CaS-MnS-Al₂O₃ composite inclusions that are less detrimental.
• Continuous casting optimization: Control the casting speed, mold powder composition, and electromagnetic stirring to minimize surface inclusion accumulation.
• Heat treatment prior to piercing: Ensure the billet reheating temperature is sufficient (typically 1200–1250°C) to allow partial dissolution or redistribution of fine inclusions.
• Inspection protocols: Implement eddy current testing (ECT) and ultrasonic testing (UT) on billets prior to piercing to reject defective material.

Study Insights and Reflections

This paper, though published in 2003, remains highly relevant to modern pipe manufacturing. The fundamental metallurgical principle — that inclusion control governs surface integrity — has not changed. In contemporary practice, the emphasis has shifted toward total oxygen control and inclusion modification rather than merely reducing sulfur content. The concept of "clean steel" production, where inclusion size, shape, and distribution are all optimized, is now standard in high-quality pipe billet production.

The lesson for engineers is that surface quality issues in finished pipes often trace back to the very beginning of the manufacturing chain — the steelmaking and casting stages. A systematic approach following the PDCA cycle (Plan-Do-Check-Act) should be applied: plan by setting strict chemical composition limits, execute through process control, verify through NDT and metallographic sampling, and act by implementing corrective measures when deviations are detected.