Fe-Plated Galvanized Steel Sheet for LME-Resistant Resistance Welding
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Solution Overview
Problem
High-strength galvanized steel sheets are prone to liquid metal embrittlement (LME) cracking during resistance welding, especially when the welding electrode is angled, leading to increased residual stresses and intergranular cracking.
Innovation Solution
Forming an Fe-based electroplating layer with a coating weight of more than 20.0 g/m2 on a cold-rolled steel sheet before annealing, which acts as a soft layer to reduce stress and suppress the decrease in toughness due to solid dissolution of Si, thereby improving resistance to cracking in resistance welding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength galvanized steel sheets are used to improve strength, then strength is improved, but resistance to cracking in resistance welding deteriorates due to liquid metal embrittlement
Solution Approach 1:
The patent applies preliminary action by forming an Fe-based electroplating layer on the cold-rolled steel sheet before annealing and galvanization. This pre-plating layer is created in advance to establish a protective barrier that prevents zinc penetration during subsequent resistance welding operations, thereby resolving the cracking issue while maintaining high strength
Solution Approach 2:
The Fe-based electroplating layer serves as an intermediary substance between the steel sheet and the galvanized layer. This intermediate layer acts as a mediator that blocks the direct contact and interaction between molten zinc and the Si-containing steel substrate during resistance welding, preventing liquid metal embrittlement while allowing the high-strength properties to be maintained
2Strength
If Si is added to increase strength through solid solution strengthening, then strength is improved, but formability deteriorates
Solution Approach 1:
The patent applies local quality by creating a Fe-based electroplating layer with specific local properties at the surface of the steel sheet. This localized Fe-rich layer has different characteristics from the bulk Si-containing steel, providing a surface layer that is less susceptible to zinc penetration while the bulk material maintains its high strength and formability properties
3Ease of operation
If the welding electrode is angled during resistance welding, then welding operation is simplified, but residual stresses increase leading to increased cracking
Solution Approach 1:
The patent applies beforehand cushioning by introducing the Fe-based electroplating layer in advance as a protective cushion or buffer. This pre-formed layer cushions against the harmful effects of residual stresses generated during angled resistance welding operations, absorbing or mitigating the stress concentration that would otherwise lead to cracking at the welded portion
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 solution effectively enhances the resistance to cracking in resistance welding, particularly internal cracking, by delaying the penetration of molten zinc into the steel sheet and maintaining the toughness of the welded portion.
Implementation Method 1
forming a soft Fe-based electroplating layer with a coating weight of more than 20.0 g/m2 per surface of the cold-rolled steel sheet reduces the stress applied to the steel sheet surface during welding
Implementation Method 2
the Fe-based electroplating layer can act as a layer deficient in solute Si to suppress the decrease in toughness due to solid dissolution of Si
Implementation Method 3
there is concern that liquid metal embrittlement (LME) may occur during resistance welding when residual stresses are generated in the vicinity of a welded portion and the zinc in the coated or plated layer melts and diffuses into crystal grain boundaries
Data Source
AI summary
Provided is a galvanized steel sheet with excellent resistance to cracking in resistance welding at a welded portion, even if crystal orientations of an Fe-based electroplating layer and a cold-rolled steel sheet are integrated at a high ratio at the interface between the Fe-based electroplating layer and the cold-rolled steel sheet. The galvanized steel sheet has a Si-containing cold-rolled steel sheet containing Si in an amount of 0.1 mass % to 3.0 mass %; an Fe-based electroplating layer formed on at least one surface of the Si-containing cold-rolled steel sheet with a coating weight per surface of more than 20.0 g/m2, and a galvanized layer formed on the Fe-based electroplating layer, where crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet are integrated at a ratio of more than 50 % at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet.