Steel Sheet Mn Gradient for Bendability and Plating
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing high-strength steel sheets with tensile strength over 1,180 MPa face challenges in bendability and plating properties due to micro cracks and voids, particularly at the surface layer, which affect their performance in automotive components.
Innovation Solution
A steel sheet composition with specific microstructural and chemical optimizations, including controlled Mn content gradients and microstructure ratios, along with a hot-dip galvanized or galvannealed plating layer, to enhance bendability and plating properties while maintaining high tensile strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel sheet with tensile strength over 1,180 MPa is produced, then collision safety and fuel efficiency are improved, but bendability deteriorates due to hard microstructure at surface layer causing voids and cracking
Solution Approach 1:
The patent applies local quality by creating distinct microstructure zones at different depths from the surface. The surface layer (0-20μm) contains martensite with controlled Mn content to prevent excessive hardness, while the bulk material maintains high strength. This gradient structure allows the surface to be more ductile for bending while the interior provides overall strength.
Solution Approach 2:
The patent changes chemical composition parameters (Mn content gradient from surface to bulk), microstructural parameters (martensite fraction, crystal grain size), and processing parameters (cooling rates, plating conditions) to achieve the desired balance between strength and bendability while preventing non-plating defects.
2Strength
If large amount of Si and Mn is contained to improve strength, then tensile strength increases, but plating properties deteriorate due to inhibition of plating and generation of non-plating areas
Solution Approach 1:
The patent applies local quality by creating a Mn content gradient where the surface layer has controlled Mn content (not exceeding bulk content by more than 1.5 times) to ensure good plating properties, while the bulk material contains higher Mn content (2.0-4.0%) to provide high strength. This spatial variation in composition resolves the contradiction between strength and plating.
Solution Approach 2:
The patent resolves the contradiction by adding a compositional dimension (Mn content gradient through thickness) to the material design. Instead of using uniform high Mn content throughout, the Mn content varies through the thickness direction, allowing simultaneous achievement of high strength in bulk and good plating at surface.
3Strength
If hard microstructure is formed at surface layer to increase strength, then tensile strength improves, but voids occur during bending due to hardness difference between martensite and other phases
Solution Approach 1:
The patent creates a surface layer with controlled martensite fraction (40-100% in outer 20μm) and controlled Mn content to achieve moderate hardness that prevents excessive hardness difference with adjacent phases. This local optimization reduces stress concentration and prevents void formation during bending while maintaining overall high strength through the bulk martensite structure.
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 60% of polygonal ferrite and 40 to 100% of a total of martensite, bainite, and residual austenite in terms of an area ratio within 20 μm from 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 40 to 80%, and the polygonal ferrite and the bainite have an average crystal grain size of less than 15 μm.