Electric Resistance Welded Steel Pipe Yield Ratio Reduction
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Solution Overview
Problem
Existing electric resistance welded steel pipes for linepipes face challenges in achieving a decreased yield ratio while maintaining excellent toughness and strength, with existing methods either increasing production costs, requiring additional heat treatments, or being limited by specific chemical compositions and microstructures.
Innovation Solution
An electric resistance welded steel pipe with a specific chemical composition and microstructure, including a ferrite-rich base metal portion and a bainite or pearlite-rich second phase, optimized for a yield ratio of 0.90 or less, tensile strength of 465 to 760 MPa, and Charpy absorbed energy of 100 J or more, without the need for post-pipe-making heat treatments.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a repeated strain is applied to a steel strip by a bending-unbending process before pipe-making to induce Bauschinger effect, then the yield ratio in the pipe axis direction decreases, but the number of steps increases and production cost increases
Solution Approach 1:
The steel strip is subjected to preliminary bending-unbending processing before pipe-making to induce Bauschinger effect and create favorable residual stress distribution. This preliminary action modifies the material properties in advance, allowing the final pipe to achieve reduced yield ratio without requiring additional complex processing steps after pipe formation.
Solution Approach 2:
The invention changes the physical state and stress parameters of the steel strip through controlled bending-unbending cycles. By adjusting bending radius, number of cycles, and timing, the material undergoes plastic deformation that creates beneficial residual stresses, transforming the stress-strain characteristics to achieve the desired yield ratio reduction.
2Strength
If the base metal portion has a microstructure composed of ferrite structure and martensite with area ratio of 1 to 20%, then the yield ratio in the pipe axis direction decreases, but the manufacturing precision requirements increase
Solution Approach 1:
The invention precisely controls chemical composition parameters (C: 0.080-0.120%, Mn: 0.30-1.00%, Ti: 0.005-0.050%, Nb: 0.010-0.100%, N: 0.001-0.020%, Si: 0.010-0.450%, Al: 0.010-0.100%) and processing parameters to achieve the target microstructure. By optimizing these parameters, the ferrite-martensite microstructure with controlled area ratios is obtained, which delivers the desired yield ratio reduction while maintaining manufacturing feasibility.
3Strength
If post-pipe-making heat treatments are applied to achieve decreased yield ratio, then the yield ratio decreases, but the productivity decreases and production cost increases
Solution Approach 1:
The bending-unbending processing is performed on the steel strip before pipe-making, rather than requiring heat treatment after pipe formation. This preliminary mechanical processing achieves the yield ratio reduction goal without interrupting the continuous pipe-making process, thereby maintaining high productivity and avoiding additional production costs associated with post-manufacturing heat treatment facilities and operations.
4Reliability
If the base metal portion has high tensile strength and yield strength with decreased yield ratio, then the crack resistance improves, but the manufacturing complexity increases
Solution Approach 1:
The invention achieves the optimal balance of tensile strength (490-780 MPa), yield strength (380-680 MPa), and yield ratio (0.85-0.95) by controlling chemical composition and applying bending-unbending processing. This parameter optimization enhances crack resistance through improved material properties while the processing method itself remains relatively simple, avoiding complex multi-step manufacturing sequences.
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 achieves a balance of high tensile and yield strength with excellent toughness and reduced yield ratio, minimizing production costs and avoiding the need for additional heat treatments, thereby reducing buckling and enhancing crack resistance in linepipe applications.
Implementation Method 1
a repeated strain is applied to a steel strip as a material, for example, by a bending-unbending process, before pipe-making, to induce Bauschinger effect, thereby achieving a decrease in the yield ratio in the pipe axis direction of the resulting electric resistance welded steel pipe
Data Source
AI summary
The present invention provides an electric resistance welded steel pipe for a linepipe, in which a base metal portion includes, in terms of % by mass, 0.03% or more and less than 0.10% of C, from 0.30 to 1.00% of Mn, from 0.010 to 0.100% of Nb, from 0.010 to 0.500% of Si, and a balance including Fe and impurities, in which a value of CNeq is from 0.12 to 0.25, a ratio Mn/Si is 2.0 or more, and a value of LR is 0.25 or more; in which the base metal portion has a metallographic microstructure which has a ferrite ratio of from 80 to 98%, with a balance structure including pearlite and/or bainite, and which has a difference in hardness (balance structure - ferrite) of from 50 to 100 Hv; in which the electric resistance welded steel pipe satisfies a YS of 360 MPa or more, a TS of 465 MPa or more, and a YR of 0.90 or less; and in which each of the base metal portion and an electric resistance welded portion has a Charpy absorbed energy at 0°C of 100 J or more. CNeq=C+Mn/6+Cr/5+Ni+Cu/15+Nb+Mo+V LR=2.1C+Nb/Mn