Cause Analysis of External Folding Defects in 34Mn5 Steel Pipes

Literature Overview

This paper by Wang Zhiyi, Ma Lijun, and Fu Changliang, published in Tianjin Metallurgy (2014, No. 4), investigates the root cause of external folding defects found on the surface of 34Mn5 steel pipes after rolling from round billet. The authors employed comprehensive metallurgical examination techniques including chemical composition analysis, defect morphology observation, metallographic examination, and thermal acid etching of the cast billet. The study traces the defect origin to slag groove defects on the continuous cast round billet surface and demonstrates how adjusting ladle protection slag properties effectively eliminates the root cause.

Technical Background

34Mn5 is a low-carbon manganese steel grade commonly used for structural applications where good weldability and moderate strength are required. The steel is typically produced as continuous cast round billets, which are then hot-rolled into seamless steel pipes. External folding is a surface defect that manifests as raised, fold-like features on the pipe surface, which can significantly compromise the mechanical properties and service life of the pipe.

Material Specification

Property	34Mn5 Typical Values
Carbon (C)	0.30–0.38%
Manganese (Mn)	1.40–1.70%
Silicon (Si)	≤0.40%
Sulfur (S)	≤0.035%
Phosphorus (P)	≤0.035%
Application	Structural steel, pipes, mechanical parts

Defect Analysis Methodology

The investigation followed a systematic approach to identify the root cause:

Step 1: Chemical Composition Verification

The chemical composition of the defective pipes was analyzed to rule out material segregation or composition-related causes. The results confirmed that the composition was within specification, eliminating compositional inhomogeneity as a root cause.

Step 2: Defect Morphology Examination

Visual and macroscopic examination of the external folding defects revealed:

Fold-like raised features on the pipe outer surface
The folds contained oxide inclusions and non-metallic inclusions
The defect depth and width varied along the pipe length
The distribution pattern suggested a surface-origin defect rather than an internal defect

Step 3: Metallographic Examination

Cross-sectional metallographic examination of the defective areas showed:

The folding extended from the surface inward
The fold material contained decarbonized layers and oxide particles
The surrounding base material showed normal ferrite-pearlite structure
The interface between the fold and base material showed evidence of plastic deformation

Step 4: Thermal Acid Etching of Cast Billet

The critical diagnostic step involved thermal acid etching of the continuous cast round billet surface. This technique revealed:

Slag grooves on the billet surface, which are linear depressions or grooves formed by interaction between the molten steel and the protection slag in the mold
The slag grooves contained entrapped slag inclusions and decarbonized material
The groove pattern corresponded to the location and orientation of the external folding defects in the finished pipe

Root Cause Mechanism

The defect formation mechanism can be described as follows:

Origin in continuous casting: During continuous casting, the interaction between the molten steel and the mold protection slag creates slag grooves on the billet surface. These grooves contain entrapped slag, decarbonized steel, and oxide particles.
Propagation during hot rolling: As the round billet is reheated and hot-rolled into pipe, the complex stress state (tensile, compressive, and shear stresses) causes the slag grooves to extend and deepen. The decarbonized, weakened material at the groove bottom is susceptible to cracking and folding.
Formation of external folding: Under the combined effects of rolling deformation and surface tension, the weakened groove material folds outward, creating the characteristic external folding defect on the pipe surface.

Contributing Factors

Factor	Role in Defect Formation
Slag groove depth	Deeper grooves provide more material for folding
Slag groove length	Longer grooves create more extensive folding
Decarbonization extent	Greater decarbonization weakens the groove material
Rolling reduction	Higher reduction increases stress on groove material
Billet surface temperature	Higher temperature increases plasticity and folding tendency

Corrective Measures

The study identifies the adjustment of ladle protection slag properties as the primary corrective measure. By optimizing the following slag parameters:

Slag Parameter	Original Condition	Optimized Condition	Effect
Basicity (CaO/SiO₂)	Not optimized	Adjusted for better surface protection	Reduces slag groove formation
Al₂O₃ content	Variable	Controlled for optimal viscosity	Improves slag film stability
Melting point	Not controlled	Lowered for better fluidity	Reduces surface interaction
Surface tension	Not optimized	Reduced for better coverage	Minimizes slag groove depth

The optimized protection slag effectively eliminated slag groove defects on the cast billet surface, thereby reducing the external folding defect rate in the finished steel pipes.

Engineering Practice Recommendations

This case study provides valuable lessons for steel pipe manufacturers:

Upstream quality control: Surface defects in finished pipes often originate from casting defects. Investment in continuous casting quality directly reduces downstream defect rates.
Systematic root cause analysis: The multi-step investigation approach (composition → morphology → metallography → billet examination) is essential for identifying true root causes rather than treating symptoms.
Protective slag optimization: The protection slag is a critical but often underappreciated variable in continuous casting. Regular monitoring and optimization of slag properties should be part of standard practice.
Thermal acid etching as a diagnostic tool: This relatively simple technique can reveal surface defects that are invisible to conventional inspection methods, making it valuable for root cause investigation.

Key Reflections

The 34Mn5 steel pipe external folding case demonstrates the importance of tracing defects back to their origin in the production chain. The slag groove, a seemingly minor casting surface imperfection, becomes a significant quality issue after hot rolling. This underscores the principle that defect prevention is most effective at the earliest stage of production. Manufacturers should establish systematic protocols for investigating surface defects, including mandatory examination of the source billets, and should maintain detailed records of casting parameters to facilitate correlation between process variables and defect occurrence.