Galvanized Steel Sheet Surface Decarburization for LME Resistance
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Solution Overview
Problem
The issue of liquid metal embrittlement (LME) cracking during welding of high strength galvanized steel sheets, particularly due to the transformation of the surface layer to austenite and the penetration of molten zinc along grain boundaries, leading to brittleness, is not adequately addressed by existing technologies.
Innovation Solution
A plated steel sheet with a specific chemical composition and surface treatment, including high amounts of Si and Al, combined with high dew point annealing, to stabilize a ferrite phase and suppress LME cracking by decarburizing the surface layer, ensuring a low C concentration and high ferrite area ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel sheet is used to increase tensile strength, then strength is improved, but LME cracking resistance deteriorates due to austenite transformation and zinc penetration
Solution Approach 1:
The invention changes the chemical composition parameters of the steel sheet by adding specific amounts of Si (0.01-3.0 mass%) and Al (0.01-2.0 mass%), and controlling C (0.05-0.40 mass%), Mn (0.10-5.0 mass%), and other elements. These parameter changes modify the surface layer microstructure to stabilize ferrite phase and reduce LME sensitivity while maintaining high strength through the base steel composition and heat treatment
Solution Approach 2:
The invention creates a localized decarburized layer at the surface with different chemical composition and microstructure from the base steel. The surface layer has lower C concentration and stabilized ferrite phase, while the base steel maintains high strength martensite or bainite structure, achieving both high strength and LME resistance through local property differentiation
2Reliability
If the surface layer is decarburized to stabilize ferrite phase, then LME resistance is improved, but manufacturing complexity increases due to specific heat treatment requirements
Solution Approach 1:
The invention performs decarburization and ferrite stabilization during the annealing process before plating. By conducting high dew point annealing (dew point 80-150°C) in the annealing furnace before plating, the surface layer is pre-treated to have low C concentration and stabilized ferrite phase, eliminating the need for additional post-annealing steps and simplifying the overall manufacturing process
3Reliability
If Si and Al content is increased to stabilize ferrite phase, then LME resistance is improved, but manufacturing precision requirements increase to control composition within specific ranges
Solution Approach 1:
The invention defines specific parameter ranges for Si (0.01-3.0 mass%), Al (0.01-2.0 mass%), and other elements that balance LME resistance improvement with manufacturing feasibility. These ranges are optimized to ensure sufficient ferrite stabilization and decarburization effect while allowing normal fluctuations in steelmaking processes without requiring excessive precision control
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 solution effectively enhances the LME resistance of the plated steel sheet, preventing brittleness and improving weldability by stabilizing the ferrite phase and reducing C concentration through controlled annealing and alloying.
Implementation Method 1
the surface layer of the steel sheet is decarburized and the ferrite (a) phase stabilizes
Implementation Method 2
the ferrite (a) phase stabilizes, the surface layer of the steel sheet is covered by a ferrite phase
Implementation Method 3
LME becomes able to be suppressed in plated steel sheet using this steel sheet
Data Source
AI summary
A plated steel sheet having a high LME resistance is provided. The plated steel sheet having predetermined chemical composition, having a depth with a C concentration, measured by GDS, of 0.05% or less in a depth direction of the base steel sheet starting from an interface of the base steel sheet and plating layer of 10 μm or more, having a thickness of a layer with an area ratio of a ferrite phase of 90% or more in a depth direction from the base steel sheet surface of 20 μm or more, and having a surface roughness of the interface of the base steel sheet and plating layer of an Ra of 3.0 μm or less.