ERW Steel Pipe Tempering for Low Yield Ratio and Buckling Resistance
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Solution Overview
Problem
Existing electric resistance welded steel pipes face challenges in achieving high strength, toughness, and buckling resistance, particularly for thick-walled pipes, limiting their application in large structures like line pipes and building columns due to high yield ratios and low deformability.
Innovation Solution
A specific chemical composition and microstructure are combined with controlled processing steps, including hot rolling, cooling, coiling, electric resistance welding, tempering, and sizing, to produce a steel pipe with a yield ratio of 85% or less and compressive residual stress of 150 MPa or less, enhancing deformability and buckling resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If cold roll forming is performed to produce electric resistance welded steel pipes, then productivity and shape accuracy are improved, but work hardening occurs causing higher yield ratio and lower deformability
Solution Approach 1:
The patent applies tempering heat treatment at 500-700°C for 10-1000 seconds to change the thermal parameters of the steel pipe, reducing the yield ratio from above 90% to 85% or less. This parameter change in temperature and time enables the steel pipe to achieve sufficient deformability for bending operations while maintaining the productivity benefits of continuous cold roll forming manufacturing
Solution Approach 2:
The patent creates a composite microstructure consisting of ferrite (50-98% area fraction) and tempered bainite (2-50% area fraction) through controlled cooling after hot rolling. This composite material structure combines the ductility of ferrite with the strength of tempered bainite, achieving both high deformability and adequate strength required for structural applications
2Strength
If wall thickness of electric resistance welded steel pipe is increased, then strength and buckling resistance are improved, but work hardening increases causing higher yield ratio and lower deformability
Solution Approach 1:
The patent applies tempering heat treatment at 500-700°C for 10-1000 seconds to thick-walled steel pipes (17mm or more), reducing the yield ratio from above 90% to 85% or less. This thermal parameter change enables thick-walled pipes to achieve both high strength and sufficient deformability, making them suitable for large structures requiring earthquake resistance
Solution Approach 2:
The patent creates local microstructural quality differences through controlled cooling rates (10-40°C/s) after hot rolling, producing a microstructure with ferrite and tempered bainite in specific proportions. This local quality control in the microstructure enables the material to simultaneously exhibit high strength and good deformability even in thick-walled configurations
3Ease of operation
If tempering is performed to reduce yield ratio, then deformability is improved, but yield elongation occurs making the pipe susceptible to local deformation
Solution Approach 1:
The patent optimizes the tempering parameters (temperature: 500-700°C, time: 10-1000 seconds) to achieve a precise balance where the yield ratio is reduced to 85% or less for adequate deformability, while the microstructure remains sufficiently refined to prevent excessive yield elongation and local deformation susceptibility
Solution Approach 2:
The patent creates a dynamic microstructure through controlled cooling rates (10-40°C/s) after hot rolling, producing a mixture of ferrite and tempered bainite that provides both ductility and resistance to local deformation. This dynamic microstructural composition allows the pipe to deform uniformly without localized yielding
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 resulting steel pipe achieves a yield stress of 450 MPa or more, Charpy absorbed energy of 70 J or more at -40°C, and buckling start strain exceeding 40*t/D, suitable for large structures requiring earthquake resistance.
Implementation Method 1
a steel microstructure of a wall-thickness center of the base metal zone includes ferrite and bainite
Implementation Method 2
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
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 chemical composition containing, by mass, predetermined amounts of C, Si, Mn, P, S, Al, N, Nb, V, and Ti, with the balance being Fe and incidental impurities. The steel microstructure of the wall-thickness center of the base metal zone includes ferrite and bainite such that the total volume fraction of the ferrite and the bainite in the steel microstructure is 70% or more, with the balance being one or more selected from pearlite, martensite, and austenite. The steel microstructure has an average grain size of 7.0 m or less and a dislocation density of 1.0×1014 to 6.0×1015 m−2. The residual stress generated in the inner and outer surfaces of the pipe in the axial direction is 150 MPa or less.