ERW Steel Pipe Microstructure for Low Yield Ratio Buckling Resistance
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Solution Overview
Problem
Thick-walled electric resistance welded steel pipes exhibit high yield ratios and low deformability due to work hardening, making them unsuitable for large structures requiring buckling resistance, such as line pipes and building columns.
Innovation Solution
A steel pipe with a mixed microstructure of ferrite and bainite, balanced with pearlite, martensite, or austenite, and controlled grain size and residual stress, achieving a yield ratio of 90% or less and compressive residual stress of 250 MPa or less, along with specific chemical compositions and manufacturing processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If cold roll forming is performed to produce electric resistance welded steel pipes, then productivity and form accuracy are improved, but work hardening occurs causing high yield ratio and low deformability
Solution Approach 1:
The invention changes the material parameters by specifying precise chemical composition ranges (C: 0.040-0.50%, Si: 0.02-2.0%, Mn: 0.40-3.0%, etc.) and microstructure parameters (ferrite volume fraction >30%, bainite volume fraction >10%, average crystal grain size <7.0μm) to control the yield ratio and deformability of the steel pipe
Solution Approach 2:
The invention creates a composite microstructure consisting of multiple phases (ferrite, bainite, and balance microstructure of pearlite, martensite, or austenite) within the steel matrix, where each phase contributes different mechanical properties to achieve the desired balance between strength and deformability
2Strength
If wall thickness of electric resistance welded steel pipe is increased, then strength and buckling resistance are improved, but degree of work hardening increases causing excessive yield ratio
Solution Approach 1:
The invention changes the chemical composition parameters (adding Si: 0.02-2.0% and controlling Mn: 0.40-3.0%) and microstructure parameters (controlling ferrite volume fraction >30% and bainite volume fraction >10%) to adjust the yield ratio to 90% or less, enabling thick-walled pipes to maintain both strength and deformability
Solution Approach 2:
The invention employs a multi-phase composite microstructure where ferrite provides ductility and deformability, bainite contributes to strength, and the balance microstructure (pearlite, martensite, or austenite) adjusts the overall mechanical properties to achieve the target yield ratio while maintaining buckling resistance
3Ease of operation
If yield ratio is reduced to enhance deformability, then buckling resistance is improved, but manufacturing precision and microstructure control become more difficult
Solution Approach 1:
The invention establishes specific parameter ranges for chemical composition (C: 0.040-0.50%, Si: 0.02-2.0%, Mn: 0.40-3.0%, P: 0.10% or less, S: 0.050% or less, Al: 0.005-0.10%, N: 0.010% or less) and processing conditions (circumferential reduction ratio, heating temperature, cooling rate) to achieve consistent microstructure control and yield ratio of 90% or less
Solution Approach 2:
The invention implements a feedback control mechanism where the microstructure characteristics (ferrite volume fraction, bainite volume fraction, crystal grain size) are measured and used to adjust manufacturing parameters to achieve the target yield ratio and mechanical properties
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution results in a steel pipe with high strength, excellent toughness, and enhanced buckling resistance, suitable for constructing large structures like line pipes and building columns.
Implementation Method 1
both edges of the open pipe which abut to each other in the circumferential direction of the pipe are melted by high-frequency electric resistance heating and pressure-welded to each other by upset with squeeze rollers
Data Source
AI summary
An electric resistance welded steel pipe includes a base metal zone and an electric resistance welded zone. The base metal zone has a predetermined chemical composition and a microstructure including, by volume, ferrite: more than 30%, and bainite: 10% or more. The total volume fraction of the ferrite and the bainite is 70% or more and 95% or less. The balance being one or two or more phases selected from pearlite, martensite, and austenite. Further, when regions surrounded by boundaries between adjacent crystals having a misorientation of 15° or more are defined as crystal grains, the average size of the crystal grains is less than 7.0 μm, and the volume fraction of crystal grains having a size of 40.0 μm or more is 30% or less. A compressive residual stress generated in the inner and outer surfaces of the steel pipe in the axial direction is 250 MPa or less.