Steel Sheet Surface Ferrite Gradient for High-Strength Bending
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Solution Overview
Problem
Existing high-strength steel sheets face a trade-off between high strength and good bendability, with existing methods either compromising on formability or collision performance during bending tests.
Innovation Solution
A steel sheet composition with specific chemical elements and microstructure, including a ferrite-rich surface layer and controlled microstructures, combined with a manufacturing process involving cold rolling, grinding, annealing, and quenching, to achieve high strength, excellent formability, and collision performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of the steel sheet is increased, then the collision performance is improved, but the bendability deteriorates
Solution Approach 1:
The invention applies different microstructures to different regions of the steel sheet thickness. The surface layer (0-20μm) contains soft ferrite phase for good bendability, while the intermediate layer (20-100μm) contains hard martensite phase for high strength. This local differentiation of material properties resolves the contradiction between strength and bendability by ensuring each region has the appropriate characteristics for its functional requirements.
Solution Approach 2:
The invention transitions from considering the steel sheet as a homogeneous material to analyzing it in the thickness dimension. By examining and controlling the microstructure at different depth positions (surface, intermediate, and other regions), the invention creates a gradient structure that simultaneously achieves high strength and good bendability through dimensional differentiation of material properties.
2Ease of operation
If the surface layer is formed into ferrite primary phase to improve bendability, then the formability is improved, but hard phases remain that may serve as crack origins
Solution Approach 1:
The invention creates a specialized surface layer (0-20μm depth) with ferrite fraction of 95% or more and fine grain size (2.0μm or less), while maintaining hard martensite phase in the intermediate layer (20-100μm). This local quality differentiation ensures the surface layer provides excellent formability without crack initiation, while the intermediate layer provides strength without compromising surface quality.
Solution Approach 2:
The invention segments the steel sheet thickness into distinct functional zones: a surface layer (0-20μm) optimized for formability with ferrite phase, and an intermediate layer (20-100μm) optimized for strength with martensite phase. This segmentation allows each zone to independently fulfill its specific functional requirement without interfering with the other.
3Ease of operation
If different bendability requirements are met for different applications, then either formability or collision performance can be optimized, but not both simultaneously
Solution Approach 1:
The invention creates a multi-functional steel sheet structure where the surface layer provides excellent formability for shaping operations, while the intermediate layer provides high strength for collision resistance. This universal design allows the same steel sheet to simultaneously satisfy both formability requirements (90° V-bending test) and collision performance requirements (VDA bending test), eliminating the need for different material selections for different applications.
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 addresses the challenge by providing a steel sheet with enhanced mechanical properties and improved mechanical properties.
Implementation Method 1
a ferrite fraction is 95% or more, and an in-plane average grain size of the ferrite in an in-plane direction is 2.0 μm or less
Implementation Method 2
quenching is performed to 300°C or lower at an average cooling rate of 0.4 °C/s or faster
Implementation Method 3
annealing the steel sheet of which the surface is ground in the grinding
Data Source
AI summary
Provided are a steel sheet and a method for manufacturing the same, the steel sheet having a predetermined chemical composition, in which a microstructure at a 1/4 thickness position that is an area centered on 1/4 position in thickness with a range of 1/8 to 3/8 of a sheet thickness of the steel sheet in a sheet thickness direction of the steel sheet from a surface of the steel sheet, includes, by area ratio, 0% to 60% of ferrite, 0% to 3% of residual austenite, and a remainder containing one or more selected from martensite, bainite, pearlite, and cementite, in a range of up to 2 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet, a ferrite fraction is 95% or more, and an in-plane average grain size of the ferrite in an in-plane direction is 2.0 µm or less, in a range of 5 to 20 µm from the surface of the steel sheet in the sheet thickness direction of the steel sheet, a ferrite fraction is less than 90%, and a tensile strength of the steel sheet is 950 MPa or more.