Galvannealed Steel Sheet with Soft Interface Layer
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Solution Overview
Problem
Existing high-strength galvannealed steel sheets face challenges in achieving both high fatigue durability and formability, with surface cracking issues and hydrogen embrittlement being significant problems, particularly in automotive applications where high tensile strength and resistance to bending are critical.
Innovation Solution
A galvannealed steel sheet composition with specific alloy elements (C, Si, Mn, Al, Ni, Cu, Cr, Mo, B, Ti, Nb, V, REM, and Ca) and a manufacturing process involving controlled annealing conditions and a direct current magnetic field during continuous casting, resulting in a microstructure with a high ferrite area ratio and a soft layer at the interface, enhances fatigue durability and hydrogen embrittlement resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of steel sheet is increased to protect passengers and reduce weight, then the fatigue durability and tensile strength are improved, but the formability deteriorates
Solution Approach 1:
The invention creates a dual-phase microstructure where hard martensite or residual austenite phases are dispersed within a soft ferrite matrix. This local quality differentiation allows the steel to exhibit high strength through the hard phases while maintaining good formability through the soft ferrite background structure, resolving the contradiction between strength and formability.
Solution Approach 2:
The invention produces a composite microstructure consisting of multiple phases (ferrite, martensite, and/or residual austenite) with specific area ratios. This composite structure combines the high strength characteristics of martensite/austenite with the ductility and formability of ferrite, achieving both improved strength and maintained formability.
2Reliability
If hard structures such as martensite or residual austenite are increased to suppress fatigue cracking propagation, then the strength is improved, but the fatigue limit does not increase further
Solution Approach 1:
The invention localizes the hard phases (martensite and/or residual austenite) as dispersed structures within the soft ferrite matrix, rather than making the entire structure hard. This allows the hard phases to effectively block fatigue cracking propagation paths while the soft ferrite matrix maintains overall ductility and fatigue limit, resolving the contradiction between fatigue durability and fatigue limit.
3Ease of manufacture
If Si or Al is added to prevent carbide precipitation during thermal treatment, then the formability is improved, but oxides containing Si or Al are formed on the surface
Solution Approach 1:
The invention carefully controls the concentration parameters of Si and Al within specific ranges (Si: 0.01-2.0%, Al: 0.003-0.5%) to achieve the optimal balance. At these controlled concentrations, Si and Al effectively delay carbide precipitation and maintain formability while minimizing oxide formation on the surface, resolving the contradiction between formability improvement and surface quality.
4Reliability
If long alloying treatment with high temperature is applied to steel sheets containing Si or Al, then the wetting properties are improved, but the productivity degrades and austenite is decomposed
Solution Approach 1:
The invention optimizes the alloying treatment parameters by controlling the temperature range (400-600°C) and time duration to achieve adequate plate adhesiveness without excessive treatment. This parameter optimization ensures good wetting properties and plate adhesiveness while maintaining productivity and preventing austenite decomposition, resolving the contradiction between plate adhesiveness and productivity.
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 provides excellent fatigue durability and hydrogen embrittlement resistance, preventing surface cracking and maintaining formability, thus suitable for high-strength applications like automotive reinforcing members without degrading productivity.
Implementation Method 1
a direct current magnetic field that traverses the thickness of a slab is applied so as to form a direct current electric field zone during continuous casting
Implementation Method 2
annealing is carried out in a two-phase region of ferrite (α) + austenite (γ)
Implementation Method 3
a thermal treatment is carried out in a temperature region of approximately 300°C to 450°C, thereby using a bainite transformation and obtaining residual austenite even at room temperature
Implementation Method 4
the alloying reaction between Zn and Fe
Data Source
Figure 1
AI summary
Provided is a galvanized steel sheet having a tensile strength of 770 MPa or more including a steel sheet portion, and a plated layer formed on the surface of the steel sheet portion, in which the plated layer is a galvanized plated layer or an galvannealed plated layer, the steel sheet portion has a soft layer that directly adjoins the interface with the plated layer and an inside layer that is other than the soft layer, the thickness D of the soft layer is 0.001 % to 5% of the thickness t of the steel sheet portion, and, when the hardness of the soft layer measured by the nano-indentation method is indicated by H1, and the representative hardness of the steel sheet portion measured by the nano-indentation method is indicated by Ha in a cross section that goes along the thickness direction of the steel sheet portion, H1 is 5% to 75% of Ha.