Al-coated Steel Sheet Surface Control for Anodizing
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Solution Overview
Problem
Hot-dip Al based alloy plated steel sheets face challenges in achieving high total reflectivity and corrosion resistance, especially when subjected to anodizing treatment, due to the detrimental effects of silicon content in the plating bath and the formation of Al-Fe based intermetallic compounds on the surface.
Innovation Solution
An Al-coated steel sheet is developed with a modified hot-dip Al based alloy plated layer, where the silicon content in the plating bath is controlled between 1.5% to 3.0% by mass, and a post heat treatment is applied to reduce the silicon concentration in the surface layer to 1.07% by mass or less, and limit the area ratio of Al-Fe based intermetallic compounds to 10% or less, resulting in an Al-Fe-Si based alloy layer with enhanced properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a plating bath containing Si is used to reduce bath temperature and alloy layer thickness, then heat resistance is improved, but total reflectivity and corrosion resistance deteriorate due to high Si content in the plated layer
Solution Approach 1:
The patent optimizes the Si content parameter in the plating bath to a specific range (5-12 mass%) rather than using high Si content, and controls the plated layer Si content to 3 mass% or less. This parameter optimization resolves the contradiction by achieving adequate heat resistance while preventing excessive Si accumulation that would harm corrosion resistance and reflectivity.
Solution Approach 2:
The patent creates a non-uniform Si distribution within the plated layer, with lower Si content (3 mass% or less) in the surface region and controlled Si content in the underlying alloy layer. This local quality differentiation allows the surface to maintain high reflectivity and corrosion resistance while the underlying structure provides heat resistance.
2Temperature
If Si content in the plated layer is increased to maintain heat resistance, then heat resistance is improved, but total reflectivity deteriorates due to light absorption
Solution Approach 1:
The patent creates a surface region with controlled Si content (3 mass% or less) that maintains high optical reflectivity, while the underlying alloy layer contains Si for heat resistance. This spatial differentiation of Si content resolves the contradiction between heat resistance and total reflectivity.
Solution Approach 2:
The patent optimizes the Si content parameter in the plated layer to 3 mass% or less, which is the threshold value that maintains high total reflectivity while providing adequate heat resistance through the underlying alloy layer structure.
3Temperature
If a post heat treatment is applied to form AIN barrier layer for enhanced high temperature durability, then heat resistance is improved, but the process complexity increases
Solution Approach 1:
The patent incorporates N elements (such as AlN-forming elements) into the base steel sheet before plating, so that the AIN barrier layer can form during the subsequent heat treatment. This preliminary action reduces process complexity compared to adding N after plating, while still achieving the desired high temperature durability.
4Reliability
If anodizing treatment is applied to improve corrosion resistance, then corrosion resistance is improved, but appearance deteriorates when Al-Fe based intermetallic compounds are present on the surface
Solution Approach 1:
The patent creates a surface region with controlled composition (Si content 3 mass% or less and limited Al-Fe intermetallic compounds) that provides both good anodizing response for corrosion resistance and acceptable appearance. This local quality control resolves the contradiction between corrosion resistance and appearance.
Solution Approach 2:
The patent optimizes the surface composition parameters, specifically limiting Si content to 3 mass% or less and controlling Al-Fe intermetallic compound formation, which enables successful anodizing treatment that achieves both corrosion resistance and acceptable appearance.
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 Al-coated steel sheet exhibits improved total reflectivity, corrosion resistance, and appearance after anodizing, making it suitable for applications requiring heat resistance and light reflection, while maintaining the durability of the original hot-dip Al based alloy plated steel sheets.
Implementation Method 1
a post heat treatment is applied to reduce the silicon concentration in the surface layer to 1.07% by mass or less
Implementation Method 2
anodizing treatment
Data Source
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AI summary
To provide an Al-coated steel sheet that is improved in total reflection characteristics, corrosion resistance, and appearance after subjecting to an anodizing treatment. [Means for Resolution] An Al-coated steel sheet containing a base steel sheet having on a surface thereof an Al-coated layer having an average thickness of 7 µm or more with an Al-Fe-Si based alloy layer intervening therebetween, a surface layer portion of the Al-coated layer extending from the surface thereof to a depth of 3 µm having an average Si concentration of 2.0% by mass or less, and preferably 1.3% by mass or less, and an area ratio of an Al-Fe based intermetallic compound phase occupying the surface of the Al-coated layer being 10% or less.