Linepipe Steel Microstructure for Compressive Strength and Toughness
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Solution Overview
Problem
Offshore pipelines require high compressive strength and fracture toughness to resist collapse under water pressure, but existing methods like Joule heating and accelerated cooling often compromise productivity, roundness, or fracture toughness due to the Bauschinger effect, which is challenging to control, especially in mass production.
Innovation Solution
Optimizing the metal microstructure of steel pipes by reducing ferrite-bainite interfaces and hard second phases, controlling alloy elements like C, Nb, and Si, and using reheating treatments to decompose MA into cementite, while maintaining a bainite microstructure, to enhance compressive strength and fracture toughness without requiring specific forming conditions or heat treatments.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Shape
If a pipe expanding process is performed in cold forming to manufacture UOE steel pipe, then the pipe can be formed into the desired shape, but the compressive strength is lowered by the Bauschinger effect
Solution Approach 1:
The patent applies heating treatment after the pipe expanding process to change the temperature parameter of the steel pipe. This thermal parameter change restores the compressive strength that was reduced by the Bauschinger effect during cold forming, while maintaining the desired pipe shape.
2Strength
If Joule heating is applied to eliminate dislocation after pipe expansion, then the yield point increases, but productivity is deteriorated due to prolonged heating time
Solution Approach 1:
The patent employs periodic heating treatment with controlled duration after pipe expansion. By applying heat treatment for a specific period rather than continuous prolonged heating, the method eliminates dislocation to restore yield point while minimizing the impact on production efficiency.
3Strength
If the outer surface is heated to a higher temperature than the inner surface to increase compressive yield strength on the outer surface, then the compressive strength distribution is improved, but the manufacturing complexity increases
Solution Approach 1:
The patent applies differential heating where the outer surface is heated to a higher temperature than the inner surface. This creates a non-uniform temperature distribution that results in improved compressive yield strength distribution across the pipe wall thickness, with the outer surface having higher strength to counteract external pressure.
4Strength
If accelerated cooling is performed to enhance strength, then the tensile strength increases, but the fracture toughness is deteriorated
Solution Approach 1:
The patent performs heating treatment after accelerated cooling and pipe expansion. This preliminary heating action restores the compressive strength that was reduced by the Bauschinger effect while maintaining the microstructure achieved through accelerated cooling, thereby preserving both tensile strength and fracture toughness.
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 approach results in steel pipes with high compressive strength and excellent fracture toughness, suitable for deep-sea applications, without the need for complex heat treatments or forming conditions, ensuring improved collapse resistance and low-temperature fracture toughness.
Implementation Method 1
a steel plate is heated to a temperature (austempering temperature) of Ar 3 to (Ar 3+70°C)
Implementation Method 2
the steel plate is accelerated cooled at a cooling rate of 10°C/sec or more
Implementation Method 3
a steel pipe is heated by Joule heating and, after the steel pipe is expanded, a temperature is held for a fixed time or more
Implementation Method 4
compressive strength becomes lower than tensile yield strength. To consider a factor which causes such lowering of compressive strength, a UOE steel pipe is subjected to a pipe expanding process in a final step of pipe making so that the UOE steel pipe receives compression after tensile deformation is imparted to the pipe in the circumferential direction of the pipe whereby the compressive strength is lowered by a Bauschinger effect
Data Source
Figure 1~2
AI summary
To provide a steel pipe for a linepipe having a heavy wall thickness and having excellent fracture toughness in a base material and a welded heat affected zone by suppressing lowering of yield stress caused by a Bauschinger effect by optimizing the metal microstructure of a steel plate. To be more specific, provided are a welded steel pipe for a linepipe having high compressive strength and high fracture toughness which is a steel pipe having the composition which contains by mass% 0.03 to 0.08% C, 0.10% or less Si, 1.00 to 2.00% Mn, 0.010% or less P, 0.0030% or less S, 0.06% or less Al, 0.005 to 0.020% Nb, 0.005 to 0.025% Ti, 0.0010 to 0.0060% N, and Fe and unavoidable impurities as a balance, wherein Ti(%)/N(%) is a value which falls within a range of 2 to 4, a Ceq value is 0.30 or more, and fractions and the like of bainite, M-A constituent (MA) and cementite in the metal microstructure of a base material and the metal microstructure of the welded heat affected zone are specified, and method of producing the same.