Line Pipe Steel Composition for Bauschinger-Resistant Strength
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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 preventing the Bauschinger effect, which reduces compressive strength due to dislocation accumulation and hard second phase fractions, and often require complex heat treatment processes that are difficult to manage in mass production.
Innovation Solution
A method involving a specific chemical composition and microstructure of the steel, primarily composed of bainite with controlled reheating and cooling processes to limit the formation of martensite-austenite constituents, reduce polygonal ferrite, and enhance microstructure refinement, resulting in a steel material with a high compressive strength-to-tensile strength ratio and improved toughness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional UOE pipe making process is used with heating after expansion to restore compressive strength, then compressive yield strength is improved, but manufacturing complexity and difficulty of quality management increase
Solution Approach 1:
The patent applies preliminary action by optimizing the pipe forming process parameters (compression ratio, expansion ratio, forming temperature) before the pipe is actually made, so that the Bauschinger effect is minimized from the outset. This eliminates the need for subsequent heating treatments to restore compressive strength, thereby reducing manufacturing complexity while maintaining high compressive yield strength.
Solution Approach 2:
The patent changes key process parameters including setting the compression ratio to 5-15%, expansion ratio to 3-8%, and forming temperature to 20-100°C, which optimizes the microstructure and stress state to prevent excessive Bauschinger effect. These parameter adjustments allow achieving high compressive strength without complex post-processing heat treatments.
2Strength
If compression ratio is increased to prevent Bauschinger effect, then compressive strength is improved, but pipe roundness deteriorates
Solution Approach 1:
The patent identifies the optimal parameter range where compression ratio is 5-15% and expansion ratio is 3-8%, which balances two competing requirements: high enough compression to prevent Bauschinger effect and maintain compressive strength, but not so high as to damage pipe roundness. This precise parameter optimization resolves the contradiction between strength and shape.
3Strength
If heating is performed after pipe expansion to restore compressive strength, then compressive yield strength is improved, but manufacturing time and energy consumption increase
Solution Approach 1:
The patent applies preliminary action by pre-optimizing the pipe forming parameters (compression ratio, expansion ratio, forming temperature) to minimize the Bauschinger effect from the beginning, eliminating the need for subsequent heating treatments. This reduces manufacturing time and energy consumption while maintaining high compressive yield strength.
Solution Approach 2:
The patent extracts and eliminates the heating step from the manufacturing process by optimizing the forming parameters in advance. The solution removes the unnecessary post-processing step that was previously required to restore compressive strength, thereby reducing manufacturing time and energy consumption.
4Shape
If pipe expansion ratio is increased to improve roundness, then shape is improved, but compressive strength deteriorates due to enhanced Bauschinger effect
Solution Approach 1:
The patent optimizes the expansion ratio to be within 3-8%, which is high enough to improve pipe roundness but controlled to prevent excessive Bauschinger effect that would reduce compressive strength. This parameter optimization simultaneously achieves both good roundness and high compressive strength.
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 effectively produces steel materials with tensile strengths of 570 MPa or more, compressive strengths of 440 MPa or more, and excellent low-temperature toughness, suitable for deep-sea line pipes, without requiring special forming conditions or post-pipe making heat treatments, ensuring stable roundness and high DWTT properties.
Implementation Method 1
a metal microstructure composed primarily of bainite, wherein an area fraction of polygonal ferrite at a position of 1/4 plate thickness is 10% or less, an area fraction of martensite-austenite constituent at the position of 1/4 plate thickness is 5% or less
Implementation Method 2
compressive yield strength may be reduced due to the Bauschinger effect
Data Source
AI summary
A method for producing a steel material for line pipes 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; 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.; 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.