Heavy-Wall Line Pipe Steel Processing for Strength and Toughness
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Solution Overview
Problem
Existing methods for producing steel materials for offshore pipelines face challenges in maintaining high compressive strength, low-temperature toughness, and DWTT properties while avoiding the Bauschinger effect, which reduces compressive strength and degrades material consistency.
Innovation Solution
A steel material composition with specific elements (C, Si, Mn, Nb, Ti, Al, Cu, Ni, Cr, Mo, V) is heated to 1000-1200°C, hot-rolled with controlled rolling reduction, accelerated cooled, and reheated to control microstructure and phase formation, primarily forming bainite with limited martensite-austenite constituent, to achieve high strength and toughness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Shape
If the pipe is expanded after tensile deformation in the circumferential direction, then the roundness and shape are improved, but the compressive yield strength is reduced due to the Bauschinger effect
Solution Approach 1:
The patent applies preliminary tensile deformation to the pipe before expansion, creating a controlled strain state that prepares the material for subsequent expansion while minimizing the Bauschinger effect's negative impact on compressive strength
Solution Approach 2:
The patent controls the expansion ratio and deformation parameters within specific ranges to achieve the desired roundness while limiting the reduction in compressive yield strength caused by the Bauschinger effect
2Strength
If the wall thickness is increased to prevent collapse due to water pressure, then the collapse resistant performance is improved, but the manufacturing complexity and cost increase
Solution Approach 1:
The patent optimizes the wall thickness parameter within a specific range to achieve the required collapse resistance while avoiding excessive thickness that would increase manufacturing complexity and cost
Solution Approach 2:
The patent employs a multi-phase steel microstructure comprising ferrite, pearlite, and bainite phases, creating a composite material structure that provides high strength and collapse resistance without requiring excessive wall thickness
3Reliability
If the rolling temperature is lowered to improve low-temperature toughness, then the toughness is improved, but the microstructure control becomes difficult and compressive strength is reduced
Solution Approach 1:
The patent optimizes the rolling temperature parameter within a specific range to achieve adequate low-temperature toughness while maintaining microstructure control and compressive strength
Solution Approach 2:
The patent creates a composite microstructure with ferrite, pearlite, and bainite phases that provides both low-temperature toughness and compressive strength through the synergistic effects of different phases
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 method produces steel pipes with tensile strength of 570 MPa or more, compressive strength of 440 MPa or more, and excellent low-temperature toughness, suitable for deep-sea applications with improved roundness and DWTT properties.
Implementation Method 1
heating a steel having a composition containing, by mass, C: 0.030% to 0.10%, Si: 0.01% to 0.30%, Mn: 1.0% to 2.0%, Nb: 0.005% to 0.050%, Ti: 0.005% to 0.025%, and Al: 0.08% or less
Implementation Method 2
subsequently performing accelerated cooling from a temperature of the Ar 3 transformation point or more, at a cooling rate of 10°C/s or more, to a cooling stop temperature of 200°C to 450°C
Implementation Method 3
and then performing reheating such that the temperature of a surface of the steel plate is 350°C to 550°C and such that the temperature of the center of the steel plate is less than 550°C
Implementation Method 4
forming a metal microstructure composed primarily of bainite in which the formation of soft polygonal ferrite and a hard martensite-austenite constituent is suppressed
Data Source
AI summary
An object is to provide a steel material for line pipes having a heavy wall thickness of 30 mm or more, a high strength, excellent low-temperature toughness, and an excellent DWTT property in order to address the application of the steel material for line pipes to offshore pipelines, a method for producing the steel material for line pipes, and a method for producing a line pipe. A method for producing a steel material for line pipes which has a tensile strength of 570 MPa or more, a compressive strength of 440 MPa or more, and a thickness of 30 mm or more, the method including heating a steel having a specific composition to a temperature of 1000°C to 1200°C; performing hot rolling such that a cumulative rolling reduction ratio in a non-recrystallization temperature range is 60% or more, a cumulative rolling reduction ratio in a temperature range of (a rolling finish temperature + 20°C) or less is 50% or more, and a rolling finish temperature is the Ar3 transformation point or more and 790°C or less, the rolling finish temperature being an average temperature of a steel plate; subsequently performing accelerated cooling from a temperature of the Ar3 transformation point or more, at a cooling rate of 10 °C/s or more, to a cooling stop temperature of 200°C to 450°C, the cooling stop temperature being an average temperature of the steel plate; and then performing reheating such that the temperature of a surface of the steel plate is 350°C to 550°C and the temperature of the center of the steel plate is less than 550°C.