Fe-Based Electroplated Steel Sheet for LME-Resistant Welding
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Solution Overview
Problem
Existing methods for manufacturing hot-dip galvanized steel sheets fail to adequately address the issue of liquid metal embrittlement (LME) cracking during resistance welding, particularly in high-strength steel sheets with added Si, which forms oxides that degrade wettability and increase residual stresses, leading to intergranular cracking.
Innovation Solution
Forming an Fe-based electroplating layer on cold-rolled steel sheets with a specific coating weight and annealing conditions (A+B ≥ 3.0 for CRs and GIs, A+B ≥ 5.0 for GAs) to create internal oxides within the electroplating layer, reducing zinc penetration and refining crystal grains, thereby enhancing resistance to welding cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If Si is added to increase the strength of steel sheets, then the strength increases, but oxides form on the steel sheet surface during annealing which degrade chemical convertibility and wettability
Solution Approach 1:
The patent applies Fe-based electroplating before annealing to pre-form a protective layer on the steel sheet surface. This preliminary action prevents oxide formation during subsequent annealing, thereby maintaining both the strength-enhancing Si content and the chemical convertibility/wettability required for hot-dip galvanizing
Solution Approach 2:
The Fe-based electroplating layer serves as an intermediary between the Si-containing steel sheet and the annealing atmosphere. This intermediate layer prevents direct oxidation of Si on the steel sheet surface while allowing the strength benefits of Si to be retained, thus resolving the contradiction between strength and surface reactivity
2Reliability
If Fe-based electroplating is applied before annealing to ensure proper chemical convertibility and coating appearance, then chemical convertibility and coating appearance improve, but the process complexity increases
Solution Approach 1:
The patent combines the electroplating and annealing processes into an integrated manufacturing flow where Fe-based electroplating is performed immediately before annealing. This merging ensures that the electroplated layer is freshly formed and optimally positioned to prevent oxidation during annealing, achieving both chemical convertibility and coating appearance improvement without requiring separate, complex post-treatment steps
3Strength
If Si is added to increase strength, then the strength increases, but residual stresses increase during resistance welding leading to LME cracking
Solution Approach 1:
The Fe-based electroplating layer is formed before resistance welding to create a protective barrier that prevents zinc penetration during welding. This preliminary protective action allows the use of high-strength Si-containing steel while preventing the LME cracking that would otherwise occur during resistance welding
Solution Approach 2:
The electroplating layer acts as an intermediary barrier between the molten zinc during resistance welding and the Si-containing steel substrate. This intermediate layer prevents zinc from penetrating into the steel and causing liquid metal embrittlement, thereby enabling the use of high-strength steel in welded automotive components
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 proposed method results in steel sheets with excellent chemical convertibility, coating appearance, and significantly improved resistance to cracking in resistance welding, ensuring robust performance in automotive applications.
Implementation Method 1
annealing conditions (A+B ≥ 3.0 for CRs and GIs, A+B ≥ 5.0 for GAs) to create internal oxides within the electroplating layer
Implementation Method 2
annealing conditions (A+B ≥ 3.0 for CRs and GIs, A+B ≥ 5.0 for GAs) to create internal oxides within the electroplating layer
Implementation Method 3
annealing conditions (A+B ≥ 3.0 for CRs and GIs, A+B ≥ 5.0 for GAs) to create internal oxides within the electroplating layer, reducing zinc penetration and refining crystal grains
Data Source
AI summary
To provide an Fe-based electroplated steel sheet that not only has excellent chemical convertibility or excellent coating appearance when subjected to hot-dip galvanizing, but also has excellent resistance to cracking in resistance welding. Disclosed is an Fe-based plated steel sheet including: a cold-rolled steel sheet having a chemical composition containing Si in an amount of 0.1 mass % or more and 3.0 mass % or less; and an Fe-based electroplating layer formed on one or both surfaces of the cold-rolled steel sheet with a coating weight per surface of 1.0 g/m2 or more, in which a thickness of an internal oxidation layer is 2.00 μm or less, and an average value of C concentration in a range of 10 μm to 20 μm in a thickness direction from the surface of the Fe-based electroplating layer is 0.10 mass % or less.