Line Pipe Steel Bainite Processing for Compressive Strength
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Solution Overview
Problem
Existing methods for producing steel materials for offshore pipelines face challenges in maintaining high compressive strength and low-temperature toughness while avoiding the Bauschinger effect, which reduces compressive strength due to dislocation accumulation and hard second phase fractions, and often require complex heat treatments or difficult temperature management.
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 high cumulative rolling reduction, and accelerated cooled to form a microstructure primarily composed of bainite, reducing the area fraction of polygonal ferrite and martensite-austenite constituents, and expanded with a controlled pipe expansion ratio to achieve high tensile and compressive strengths.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Shape
If pipe expansion is performed to improve roundness and fit, then the pipe shape is improved, but compressive yield strength is reduced due to the Bauschinger effect
Solution Approach 1:
The patent applies preliminary tensile deformation through pipe expansion before the final forming process. By pre-straining the pipe in tension, the material develops a hardened state that resists subsequent compressive deformation during forming, thereby maintaining compressive yield strength while achieving the required roundness and shape.
Solution Approach 2:
The patent changes the stress state parameter from compression to tension during the pipe expansion process. By subjecting the pipe to tensile stress before forming, the material's mechanical properties are modified through work hardening, which prevents the Bauschinger effect and maintains high compressive strength in the final product.
2Strength
If heating is performed subsequent to pipe expansion to restore compressive yield strength, then compressive strength is improved, but production time and energy consumption increase
Solution Approach 1:
The patent extracts the heat treatment step from the production process by using cold forming techniques that inherently maintain high compressive strength without requiring post-expansion heating. This eliminates the time-consuming thermal processing step while still achieving the required mechanical properties through controlled plastic deformation and work hardening.
Solution Approach 2:
The patent replaces the thermal field (heating process) with a mechanical field solution. Instead of using heat to restore compressive strength, the invention uses controlled mechanical deformation and work hardening during the forming process itself to achieve the same strength restoration, thereby eliminating the need for additional heating equipment and process time.
3Strength
If compression ratio is increased during forming to maintain high compressive strength, then compressive strength is improved, but pipe expansion ratio must be increased which affects roundness
Solution Approach 1:
The patent applies preliminary tensile deformation through pipe expansion before the final forming process. By pre-straining the pipe in tension, the material develops a hardened state that resists subsequent compressive deformation during forming, thereby maintaining compressive yield strength while achieving the required roundness and shape.
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 without requiring special forming conditions or post-pipe making heat treatments.
Implementation Method 1
heating to 1000-1200°C, hot-rolled with high cumulative rolling reduction, and accelerated cooled to form a microstructure primarily composed of bainite
Implementation Method 2
hot-rolled with high cumulative rolling reduction ratio
Data Source
AI summary
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; and subsequently performing accelerated cooling from a cooling start temperature of the Ar3 transformation point or more, at a cooling rate of 10° C./s or more, until the temperature of a surface of a steel plate reaches 300° C. to 500° C.