Steel Sheet and Plated Steel Sheet with Internal Oxide Traps
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Solution Overview
Problem
High strength steel sheets used in corrosive environments face issues with hydrogen embrittlement cracking and liquid metal embrittlement due to penetration of hydrogen and zinc from plating layers, which degrade plateability and fatigue properties.
Innovation Solution
Forming an internal oxidation layer with fine granular oxides inside the steel sheet, controlled by adjusting the composition and structure to trap hydrogen and zinc, and creating a surface depleted layer with low silicon and high aluminum content to enhance interdiffusion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an external oxidation layer is formed on the steel sheet surface, then the steel sheet gains protection from oxidation, but the oxides form a film on the surface that impedes interdiffusion between steel and plating constituents, resulting in insufficient plateability
Solution Approach 1:
Instead of forming oxides on the external surface of the steel sheet, the invention inverts the oxidation process to form an internal oxidation layer within the steel substrate. This is achieved by controlling the oxidation conditions during heat treatment to promote oxygen diffusion into the steel and formation of oxide precipitates inside the material rather than on the surface, thereby eliminating the harmful external oxide film while maintaining oxidation protection
Solution Approach 2:
The invention extracts the harmful oxide film formation from the steel sheet surface by promoting internal oxidation instead. The oxidation process is redirected to occur within the steel matrix, extracting the problematic surface oxide layer formation and replacing it with internal oxide precipitates that do not interfere with plating adhesion
2Strength
If high strength steel sheet is used in corrosive environments, then the strength requirement is met, but hydrogen generated from corrosion penetrates into the steel and causes hydrogen embrittlement cracking
Solution Approach 1:
The internal oxidation layer acts as an intermediary barrier within the steel structure. The oxide precipitates created during controlled oxidation serve as trapping sites that intercept and hold hydrogen atoms, preventing them from reaching and accumulating at critical locations such as grain boundaries where they would cause embrittlement and cracking
Solution Approach 2:
The invention converts the potentially harmful oxidation process into a beneficial one by controlling oxide formation to occur internally rather than externally. The oxidation that would normally create harmful surface films is redirected to create internal oxide precipitates that serve as hydrogen traps, transforming a harmful process into a protective mechanism
3Reliability
If a Zn-based plating layer is applied to high strength steel sheet, then corrosion resistance is improved, but during hot stamping or welding the molten Zn penetrates into the steel and causes liquid metal embrittlement
Solution Approach 1:
The internal oxidation layer with oxide precipitates serves as an intermediary barrier that intercepts molten zinc during hot stamping or welding processes. The oxide particles act as physical obstacles and trapping sites that prevent zinc from penetrating deep into the steel substrate, thereby avoiding liquid metal embrittlement while maintaining the protective plating layer
Solution Approach 2:
The oxidation process is performed in advance during heat treatment to create the internal oxidation layer with oxide precipitates before the plating or hot forming processes. This preliminary action prepares the steel substrate with a built-in protective structure that will prevent zinc penetration during subsequent manufacturing operations
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
Improves plateability, hydrogen embrittlement resistance, and liquid metal embrittlement resistance by effectively trapping hydrogen and zinc, preventing their penetration and maintaining structural integrity.
Implementation Method 1
forming an internal oxidation layer with fine granular oxides inside the steel sheet, controlled by adjusting the composition and structure to trap hydrogen and zinc
Implementation Method 2
forming an internal oxidation layer with fine granular oxides inside the steel sheet, controlled by adjusting the composition and structure to trap hydrogen and zinc
Implementation Method 3
the molten Zn will sometimes penetrate into the steel and cause cracks inside the steel sheet
Data Source
Figure 1~2
AI summary
High strength steel sheet and plated steel sheet having high plateability, LME resistance, and hydrogen embrittlement resistance, that is, steel sheet containing C: 0.05 to 0.40%, Si: 0.2 to 3.0%, Mn: 0.1 to 5.0%, and sol. Al: 0.4 to 1.50%, having an internal oxidation layer including fine granular oxides in a surface layer of the steel sheet, a number density of fine granular oxides in the internal oxidation layer being 4.0/µm2 or more, and including a surface depleted layer with a steel composition not including oxides which satisfies, by mass%, Si≤0.6% and Al≥0.05% at a depth of 1/2 of the average depth of the internal oxidation layer calculated from the cross-sectional SEM image of the steel sheet, and a plated steel sheet using the same.