Analysis and Process Control of External Folding Defects in 37Mn5 Steel Pipes

Literature Overview

This study, published in Special Steel (Vol. 31, Issue 1, 2010, pp. 30-32) by Wei Jun and colleagues from University of Science and Technology Beijing and Chengde Jianlong Special Steel Co., Ltd., investigates the root cause of external folding defects in 37Mn5 steel pipes with dimensions Φ85 mm × 8 mm. The research employs metallographic analysis combined with process parameter optimization to identify and mitigate the formation mechanism of this critical surface defect. As a metallurgist with decades of experience in steel pipe production quality, I find this paper particularly valuable for its clear linkage between inclusion morphology and rolling/forging defect formation.

Core Technical Findings

The fundamental conclusion of this research is that brittle inclusions—specifically silicates and SiO₂ particles of approximately 20 μm in size located at grain boundaries—are the primary cause of external folding in 37Mn5 steel pipes. The steel composition (0.39% C, 1.30% Mn) positions it as a medium-carbon, manganese-strengthened structural steel commonly used in high-pressure pipe applications. During hot deformation processes such as piercing and rolling, these brittle inclusion clusters act as crack initiation sites, and the resulting micro-cracks propagate along the pipe surface during subsequent forming operations, manifesting as external folds.

Inclusion Characteristics and Defect Mechanism

Parameter	Value / Description
Steel grade	37Mn5 (0.39% C, 1.30% Mn)
Pipe dimensions	Φ85 mm × 8 mm
Inclusion type	Silicates and SiO₂
Inclusion size	~20 μm
Inclusion location	Grain boundaries
Defect type	External folding
Defect mechanism	Inclusion-induced micro-crack propagation during hot deformation

The mechanism is straightforward yet critical: during the hot working of the steel blank (piercing, rolling, or bending), the brittle inclusions at grain boundaries cannot accommodate the plastic deformation imposed on the ductile matrix. This mismatch generates localized stress concentrations that initiate micro-cracks. As the material continues to deform, these micro-cracks coalesce and propagate along the outer surface, producing the characteristic external fold morphology.

Process Control Measures

The authors propose a comprehensive set of metallurgical process controls targeting inclusion reduction at every stage from steelmaking through continuous casting:

Steelmaking and Refining Controls

Process Stage	Control Parameter	Target Value	Purpose
Converter tapping	Ladle top slag thickness	≤ 50 mm	Reduce slag re-oxidation
LF refining	(FeO) content in slag	≤ 1%	Minimize oxygen potential
LF refining	White slag time	≥ 15 min	Promote inclusion modification
Ladle treatment	Soft argon stirring	≥ 10 min	Promote inclusion flotation
Continuous casting	Full protection casting	Throughout	Prevent re-oxidation

Resulting Steel Quality

Quality Indicator	Achieved Value
Dissolved oxygen [O]	(11.6–17.1) × 10⁻⁶
A-type inclusion level	0–2.0
B-type inclusion level	0
C-type inclusion level	0
D-type inclusion level	0.5–1.0

Engineering Practice Implications

From a practical standpoint, this study demonstrates a classic PDCA (Plan-Do-Check-Act) cycle applied to steel pipe quality improvement. The "Plan" phase involves identifying the root cause through metallographic examination; the "Do" phase implements the multi-stage inclusion control strategy; the "Check" phase verifies through inclusion rating and defect monitoring; and the "Act" phase standardizes the successful parameters.

The key insight for pipe manufacturers is that external folding is not primarily a mechanical forming problem but a metallurgical cleanliness issue. This distinction is crucial because it redirects corrective actions from rolling mill adjustments to steelmaking and continuous casting optimization. The requirement for soft argon stirring of at least 10 minutes is particularly important—this allows sufficient time for modified inclusions to float out of the liquid steel under buoyancy forces before solidification locks them in place.

Practical Recommendations for Similar Applications

For any medium-carbon manganese steel pipe susceptible to folding, implement inclusion analysis as a standard incoming inspection for billets.
Maintain (FeO) ≤ 1% in LF slag consistently—this is often overlooked when production schedules are tight.
Full protection during continuous casting (tundish shroud, immersion nozzle, and powder) is non-negotiable for clean steel production.
The target dissolved oxygen of approximately 12–17 ppm is achievable but requires integrated control across all stages.

Study Insights and Reflections

After reviewing this paper, I reflect on how the fundamental metallurgical principle—that inclusions are the enemy of ductility under deformation—manifests differently across pipe production processes. In seamless pipe piercing, internal folds are more common due to the inward compression forces; however, in ERW and HFW processes where the pipe body undergoes bending and welding, external folds become the dominant concern because the outer surface experiences tensile stresses during the bending operation.

The 20 μm size threshold for critical inclusions is significant. Inclusions smaller than this can often be accommodated by the surrounding matrix without causing cracking, but those exceeding this size create stress concentrations sufficient to initiate damage during moderate plastic deformation. This finding aligns with classical metallurgical theory regarding the critical inclusion size for crack initiation, which depends on the ratio of inclusion size to grain size and the applied strain.

This research carries clear reference value for manufacturers producing pipes from medium-carbon steels where manganese content exceeds 1.0%. The systematic approach to inclusion control—addressing each processing stage individually and then verifying the integrated result—provides a replicable methodology for tackling similar surface quality issues in other steel products.