Plated Steel Sheet Mn Gradient for Bendability
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Solution Overview
Problem
Existing steel sheets with high tensile strength face challenges in bendability and plating capabilities, particularly for those with tensile strength over 980 MPa, due to micro cracks and voids caused by hard microstructures, which are not adequately addressed by existing techniques.
Innovation Solution
A steel sheet composition with specific elements (C, Si, Mn, P, Al, N, Ti, Nb, Mo, and others) and microstructure optimization, along with a hot-dip galvanized or hot-dip galvannealed plating layer, is used to control the Mn content gradient and microstructure distribution, ensuring a Mn content ratio of 1.5 or less in the surface and bulk regions, thereby enhancing bendability and plating capabilities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel sheet with tensile strength over 980 MPa is used, then strength is improved, but bendability deteriorates due to hard microstructure causing voids and micro cracks
Solution Approach 1:
The patent applies local quality by creating a non-uniform Mn distribution where the surface layer (0-20μm) has lower Mn content (≤1.5 times bulk content) to reduce hardness and prevent void formation during bending, while the bulk material maintains higher Mn content for overall strength. This localized compositional gradient resolves the contradiction between surface bendability and bulk strength.
Solution Approach 2:
The patent changes the Mn concentration parameter through controlled distribution, specifying that Mn content in the surface layer should not exceed 1.5 times the bulk Mn content. This parameter control transforms the microstructure to reduce hardness at the surface while maintaining high strength in the bulk, enabling both excellent bendability and high tensile strength of 980 MPa or more.
2Strength
If large amount of Si and Mn is contained to inhibit plating capabilities, then strength is improved, but plating capabilities deteriorate due to non-plating occurrence
Solution Approach 1:
The patent applies local quality by creating a compositional gradient where Si and Mn content varies through the thickness. The surface layer (0-20μm) has restricted Mn content (≤1.5 times bulk) to ensure plating capability, while the bulk material can contain higher Mn (1.5-4.0%) for strength. This localized control resolves the contradiction between overall strength and surface plating quality.
Solution Approach 2:
The patent resolves the plating capability issue by transitioning from uniform compositional control to three-dimensional compositional grading through the thickness direction. By controlling Mn distribution in the thickness dimension (surface vs. bulk), the patent enables high bulk strength while maintaining surface plating capability, effectively adding a dimensional aspect to compositional design.
3Strength
If hard microstructure is present in surface layer to increase strength, then tensile strength is improved, but bendability deteriorates due to void formation
Solution Approach 1:
The patent applies local quality by creating distinct microstructural zones: the surface layer (0-20μm) has controlled Mn content to prevent excessive hardness and void formation during bending, while the bulk material maintains higher Mn content for overall strength. This localized compositional control creates a gradient structure that enables both surface bendability and bulk strength.
Solution Approach 2:
The patent creates a composite microstructure with different compositional characteristics through the thickness. The surface layer acts as a ductile zone with lower Mn content, while the bulk serves as a strong zone with higher Mn content. This micro-composite structure, achieved through controlled Mn distribution, enables the material to exhibit both high strength and excellent bendability.
Data Source
AI summary
A steel sheet, a plated steel sheet, and methods for producing a hot-rolled steel sheet, a cold-rolled full hard steel sheet, and a steel sheet. The steel sheet has a specified composition and a microstructure including 0 to 80% of polygonal ferrite and 20 to 100% of a total of martensite, bainite, and residual austenite in terms of an area ratio within 20 μm of the steel sheet surface. The content of Mn in martensite present within 20 μm of the steel sheet surface ([Mn]SM) and the content of Mn in a bulk ([Mn]B) satisfy [Mn]SM/[Mn]B≤1.5. At a location 300 μm from the steel sheet surface, an area ratio of the martensite is in a range of 20 to 50%.