Electric Resistance Welded Steel Pipe Ferrite Decarburized Layer
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Solution Overview
Problem
Conventional techniques fail to produce electric resistance welded steel pipes or tubes with high carbon content that prevent quench cracks while maintaining excellent fatigue strength, as methods intended for steel bars impair weld quality or require precise cooling control.
Innovation Solution
A ferrite decarburized layer of specific depth (20 μm to 50 μm) is formed on the surface of electric resistance welded steel pipes or tubes before quenching, preventing quench cracks and optimizing fatigue strength by maintaining the steel in an austenite-ferrite dual phase region during hot diameter-reducing rolling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the carbon content is increased to enhance strength after quenching, then the strength is improved, but quench cracks occur more frequently
Solution Approach 1:
The patent applies local quality by creating a ferrite decarburized layer specifically at the surface of the steel pipe or tube. This surface layer has different microstructure (ferrite) and composition (lower carbon content) compared to the interior, providing crack resistance at the surface while maintaining high strength in the interior through martensite transformation during quenching.
Solution Approach 2:
The patent implements preliminary action by forming the ferrite decarburized layer before the quenching process. This pre-formed protective layer prevents quench cracks from occurring during the subsequent rapid cooling, addressing the crack issue before it can manifest during the harmful phase of the process.
2Reliability
If more than 2 mass % Si is added to enhance temper softening resistance, then the temper softening resistance is improved, but electric resistance weldability is impaired
Solution Approach 1:
The patent changes the silicon content parameter from more than 2 mass % (as in PTL 1) to a controlled range of 0.10 mass % to 1.0 mass %. This parameter adjustment maintains sufficient temper softening resistance while preserving electric resistance weldability, resolving the contradiction between these two properties.
3Object-affected harmful factors
If forced cooling is applied to suppress prior austenite grain coarsening, then quetch crack resistance is improved, but the process requires highly accurate controlled cooling
Solution Approach 1:
The patent forms the ferrite decarburized layer as a preliminary protective structure before quenching, which inherently provides quench crack resistance without requiring complex real-time cooling control systems. The protective effect is built into the material structure in advance.
Solution Approach 2:
The ferrite decarburized layer serves as a self-protective mechanism that automatically prevents quench cracks during the quenching process, eliminating the need for externally controlled forced cooling systems and associated complexity.
4Object-affected harmful factors
If a ferrite decarburized layer is formed to prevent quench cracks, then quench crack resistance is improved, but the layer depth must be precisely controlled to maintain fatigue strength
Solution Approach 1:
The patent defines a specific depth range (20 μm to 50 μm) for the ferrite decarburized layer, optimizing the balance between crack prevention and fatigue strength. This parameter specification provides clear manufacturing guidance while ensuring both protective function and mechanical performance.
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 effectively prevents quench cracks in steel pipes or tubes with carbon content of 0.40% or more, ensuring excellent fatigue strength and productivity, making them suitable for automotive parts without compromising weld quality.
Implementation Method 1
When the steel material is cooled to a temperature range of Ms (martensite transformation start) point or less, however, the steel material undergoes volume expansion as a result of martensite transformation
Implementation Method 2
By providing a ferrite decarburized layer (also referred to as 'complete decarburized layer') of a specific depth in a surface layer of a steel pipe or tube before quenching
Data Source
AI summary
Provided is an electric resistance welded steel pipe or tube that develops no quench cracks despite having carbon content of 0.40% or more and has excellent fatigue strength. An electric resistance welded steel pipe or tube comprises: a chemical composition containing, in mass %, C: 0.40% to 0.55%, Si: 0.10% to 1.0%, Mn: 0.10% to 2.0%, P: 0.10% or less, S: 0.010% or less, Al: 0.010% to 0.100%, Cr: 0.05% to 0.30%, Ti: 0.010% to 0.050%, B: 0.0005% to 0.0030%, Ca: 0.0001% to 0.0050%, and N: 0.0005% to 0.0050%, with a balance consisting of Fe and inevitable impurities; and a ferrite decarburized layer at each of an outer surface and an inner surface, the ferrite decarburized layer having a depth of 20 μm to 50 μm from the surface.