Ferritic Stainless Steel Composition for SOFC Separator Oxidation Resistance
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Solution Overview
Problem
Existing ferritic stainless steels used as separators in solid oxide fuel cells (SOFC) and electrolysis cells (SOEC) suffer from high thermal expansion coefficients, decreased electron conductivity, and poor oxidation resistance due to excessive Al and Si contents, which form insulating oxides, and hot workability issues from excessive REM and N contents.
Innovation Solution
A ferritic stainless steel composition with controlled limits on C, Si, Mn, P, S, Cu, Cr, V, Co, Al, Ti, N, La, W, and Mo contents, along with optional Ni, to minimize thermal expansion and improve electron conductivity and oxidation resistance without compromising hot workability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the content of Al is increased to improve oxidation resistance, then oxidation resistance is improved, but the toughness of the steel decreases and thermal expansion coefficient increases
Solution Approach 1:
The patent optimizes the Al content parameter to a specific range (0.5-2.0 mass%) rather than using excessive amounts. This parameter optimization resolves the contradiction by finding the sweet spot where oxidation resistance is sufficient while toughness is maintained. Additionally, the patent introduces Ti (0.01-0.5 mass%) and controls Si content (0.03-1.0 mass%) to work synergistically with Al, creating a balanced composition that achieves protection without the harmful side effects of excessive Al alone.
2Reliability
If the content of Si is increased to improve oxidation resistance under modification environment, then oxidation resistance is improved, but the thermal expansion coefficient increases
Solution Approach 1:
The patent optimizes Si content to a moderate range (0.03-1.0 mass%) rather than using high levels. This parameter control prevents excessive thermal expansion while maintaining adequate oxidation resistance. The patent also introduces Ti (0.01-0.5 mass%) which has a lower thermal expansion coefficient, helping to counterbalance the expansion effect of Si while still providing oxidation protection through the composite oxide film.
3Reliability
If the content of REM is increased to improve oxidation resistance, then oxidation resistance is improved, but hot workability decreases
Solution Approach 1:
The patent optimizes REM content to a moderate range (0.003-0.03 mass%) rather than using high levels. This parameter optimization maintains adequate oxidation resistance while avoiding the severe hot workability degradation that occurs with excessive REM. The patent also introduces Ti (0.01-0.5 mass%) which provides oxidation protection through oxide film formation without the hot workability penalties associated with high REM content.
4Reliability
If the content of N is increased to improve oxidation resistance, then oxidation resistance is improved, but hot workability decreases
Solution Approach 1:
The patent optimizes N content to a moderate range (0.005-0.05 mass%) rather than using high levels. This parameter control maintains sufficient oxidation resistance while avoiding the hot workability degradation caused by excessive N. The patent also introduces Ti (0.01-0.5 mass%) which can combine with N to form Ti nitrides, reducing free N content that would harm hot workability while still providing oxidation protection through the oxide film.
Data Source
AI summary
The present invention relates to a ferritic stainless steel containing: C≤0.03 mass %; Si≤0.05 mass %; 0.30 mass %≤Mn≤1.00 mass %; P≤0.05 mass %; S≤0.05 mass %; Cu≤0.10 mass %; 20.0 mass %≤Cr≤25.0 mass %; V≤0.10 mass %; Co≤0.10 mass %; Al≤0.10 mass %; 0.01 mass %≤Ti≤0.30 mass %; N≤0.03 mass %; 0.05 mass %≤La≤0.30 mass %; and Ni≤2.00 mass %; and at least one selected from the group consisting of 0.10 mass %≤W≤2.00 mass % and 0.10 mass %≤Mo≤2.00 mass %, with a balance being Fe and unavoidable impurities.