Galvanized Steel Sheet Coating for Weld Crack Resistance
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Galvanized steel sheets with high strength face significant challenges in resisting cracking during resistance welding, particularly due to liquid metal embrittlement, which is exacerbated by the presence of Si and increased residual stresses.
Innovation Solution
Forming an Fe-based electroplating layer on a Si-containing cold-rolled steel sheet before galvanization, followed by a controlled heating process and annealing to create internal oxides, refining crystal grains and reducing solute Si, thereby improving resistance to cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel sheets are used to achieve lightweight and high-strength automotive bodies, 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 steel sheet surface before galvanization. This pre-plating layer is then subjected to controlled heating and annealing to create internal oxides that modify the surface structure in advance, preventing zinc penetration and liquid metal embrittlement during subsequent welding operations.
Solution Approach 2:
The patent applies local quality by creating a non-uniform internal oxidation layer with specific grain boundary coverage (60-90%) at the steel sheet surface. The oxidation depth is controlled to 3.0 μm or less, and the structure is localized to the Fe-based electroplating layer interface, providing targeted protection against zinc penetration while maintaining overall material properties.
2Strength
If Si is added to increase the strength of steel sheets without significantly compromising formability, then strength is improved, but resistance to cracking in resistance welding deteriorates due to increased susceptibility to liquid metal embrittlement
Solution Approach 1:
The patent converts the harmful effect of Si (increased susceptibility to liquid metal embrittlement) into a benefit by controlling the oxidation of Si during annealing. The Si oxidation creates internal oxides that form a protective barrier at grain boundaries, preventing zinc penetration. The Si content is optimized at 0.1-3.0% to balance strength enhancement with controlled oxidation behavior.
3Reliability
If the depth of the internal oxidation layer is increased to suppress cracking during resistance welding, then resistance to cracking is improved, but manufacturing complexity increases and excessive oxidation may occur
Solution Approach 1:
The patent applies parameter changes by precisely controlling the annealing conditions (temperature range of 400-650°C, dew point above -30°C) and the composition of the Fe-based electroplating layer. These parameter changes enable controlled formation of internal oxides with optimal grain boundary coverage (60-90%) and limited oxidation depth (3.0 μm or less), achieving crack resistance without excessive oxidation.
4Reliability
If conventional surface layer control methods are used to improve resistance to cracking, then some improvement is achieved, but a high level of resistance to cracking in resistance welding cannot be satisfied
Solution Approach 1:
The patent applies composite materials by creating a multi-layered structure consisting of the steel sheet substrate, Fe-based electroplating layer with controlled internal oxidation, and outer galvanized layer. This composite structure combines the strength of the steel sheet with the protective properties of the oxidized Fe-based layer, achieving high-level crack resistance while maintaining manufacturability through established plating and annealing processes.
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 method enhances the steel sheet's resistance to cracking, especially internal cracking, by delaying zinc penetration and reducing residual stresses, resulting in improved weld integrity.
Implementation Method 1
Si that diffuses from the cold-rolled steel sheet to the Fe-based electroplating layer during annealing is caused to form an oxide inside the Fe-based electroplating layer
Implementation Method 2
form internal oxides on crystal grain boundaries in the Fe-based electroplating layer
Implementation Method 3
subjecting the pre-annealing Fe-based electroplated steel sheet to a heating process with an average heating rate of 10° C./sec or higher in a temperature range from 400° C. to 650° C.
Implementation Method 4
annealing process in an atmosphere with a dew point above −30° C., to form internal oxides on crystal grain boundaries in the Fe-based electroplating layer
Data Source
AI summary
To provide a steel sheet with excellent resistance to cracking in resistance welding at a welded portion. Disclosed is a galvanized steel sheet including: a Si-containing cold-rolled steel sheet containing Si in an amount of 0.1 mass % or more and 3.0 mass % or less; 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 5.0 g/m2 or more; and a galvanized layer formed on the Fe-based electroplating layer, in which in an intensity profile measured by glow discharge optical emission spectrometry, ISi,Fe/ISi,bulk is 0.50 or more, and an average value of C concentration in a region ranging from 10 μm to 20 μm from an interface between the galvanized layer and the Fe-based electroplating layer towards the Fe-based electroplating layer is 0.10 mass % or less.