Formation Mechanism of Longitudinal Surface Cracks in 20 Steel Tube Billets

Literature Overview

Li Huizhong, Zhang Xinming, Tang Renzheng, and Zhou Zhuoping (2003), from the School of Materials Science and Engineering at Central South University, published their findings in Materials in Mechanical Engineering (Vol. 27, No. 1, pp. 59–61). This study investigates the root cause of longitudinal surface cracks in 20 steel (a low-carbon structural steel, approximately 0.20% C) tube billets using scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and metallographic examination.

Core Findings

Primary Cause Identification

The investigation conclusively identifies excessive non-metallic inclusions as the primary cause of longitudinal surface cracks in 20 steel tube billets. This finding has significant implications for steelmaking process control and quality assurance.

Inclusion Characteristics

Inclusion Type	Morphology	Distribution	Effect on Crack Formation
MnS (Manganese sulfide)	Elongated, stringer-like	Along rolling direction	Primary crack initiation sites
Al₂O₃ (Aluminum oxide)	Angular, sharp-edged	Random or aligned	Stress concentration points
Silicate inclusions	Globular or elongated	Clustered	Reduce ductility locally
Calcium sulfide	Spherical	Dispersed	Beneficial (modifies MnS)

Metallurgical Analysis

Crack Initiation and Propagation Mechanism

The formation mechanism follows a well-defined sequence:

Inclusion accumulation: During steelmaking and casting, non-metallic inclusions (primarily MnS) form and become elongated along the deformation direction during hot rolling.
Stress concentration: The elongated inclusions create localized stress concentrations at the billet surface, particularly at the inclusion-matrix interface.
Interface debonding: Under rolling stresses, the weak inclusion-matrix interface undergoes microcracking.
Crack coalescence: Individual microcracks along the inclusion stringers coalesce to form continuous longitudinal cracks.
Surface exposure: As the billet surface is finished, the internal cracks become surface defects.

5W2H Analysis of the Defect

Question	Answer
What	Longitudinal surface cracks in 20 steel tube billets
Where	Billet surface, parallel to rolling direction
When	During or after hot rolling process
Why	Excessive non-metallic inclusions (primarily MnS)
Who	Affected steelmaking and rolling operations
How	Inclusion-induced interface debonding and crack propagation
How much	Depends on inclusion content and morphology

Process Control and Countermeasures

Steelmaking Process Optimization

Deoxidation control: Ensure adequate deoxidation with aluminum to reduce dissolved oxygen and minimize oxide inclusion formation.
Sulfur content reduction: Maintain sulfur levels below 0.030% through basic oxygen steelmaking and ladle refining.
Calcium treatment: Apply calcium wire injection to modify MnS inclusions from elongated to spherical morphology, reducing their harmful effect.
Ladle refining: Implement LF (Ladle Furnace) and RH (Vacuum Degasser) treatments to achieve clean steel with low inclusion content.

Rolling Process Control

Temperature control: Maintain proper rolling temperatures to avoid excessive inclusion elongation.
Reduction ratio optimization: Avoid excessive single-stand reduction ratios that promote inclusion stringering.
Surface inspection: Implement online eddy current or ultrasonic surface inspection to detect and reject cracked billets.

Engineering Practice Integration

Quality Assurance Protocol

Based on this research, the following quality assurance measures should be implemented:

Incoming steel quality verification including inclusion level testing per ASTM E545 or equivalent standards.
Metallographic examination of suspect billets with emphasis on inclusion content and morphology.
SEM-EDS analysis for definitive identification of inclusion chemistry.
Statistical process control of sulfur and oxygen levels in the steelmaking process.

Industry Relevance

For steel pipe manufacturers using 20 steel as a raw material, this research highlights the critical importance of raw material quality control. Surface cracks in tube billets can propagate during subsequent forming operations (pilgering, mandrel drawing, or rotary piercing), leading to catastrophic product rejection or, worse, in-service failures in pressure-containing applications.

Study Insights and Implications

This study provides a clear metallurgical explanation for a common manufacturing defect in steel tube production. The identification of non-metallic inclusions as the primary crack initiation mechanism enables targeted process improvements at the steelmaking stage rather than attempting to remediate defects after they have formed. Engineers should recognize that the quality of the final steel pipe product is fundamentally determined by the cleanliness of the raw steel, and that investment in steelmaking process control yields far greater returns than post-production inspection and rejection.