Steel Sheet Surface Unevenness Suppression via Localized Microstructure
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
High-strength steel sheets used for vehicle exterior panels face challenges in suppressing surface unevenness during forming, which affects both strength and surface quality, as existing methods fail to simultaneously achieve high strength and improved surface properties.
Innovation Solution
A steel sheet with a specific chemical composition and microstructure, including a dual phase structure with ferrite and a secondary phase, where the volume fraction and grain size of the secondary phase differ between the surface layer and internal regions, and a texture that suppresses inhomogeneous deformation, combined with a manufacturing process involving hot rolling, stress application, cold rolling, and controlled annealing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the C content is increased to obtain a dual phase structure for high-strengthening, then the strength is improved, but the area fraction of crystal having a crystal orientation of ±15° from {001} plane cannot be reduced, leading to surface unevenness after forming
Solution Approach 1:
The patent applies local quality by creating a dual phase structure where the surface layer region (0-20 μm from surface) has a different microstructure than the internal region. Specifically, the surface layer has a lower volume fraction of secondary phase (1-15%) compared to the internal region (5-25%), and the secondary phase in the surface layer has a smaller average grain size (0.5-4.0 μm) than in the internal region (1.0-5.0 μm). This localized microstructural differentiation suppresses surface unevenness during forming while maintaining overall high strength through the dual phase structure.
Solution Approach 2:
The patent employs parameter changes by precisely controlling the volume fraction and grain size parameters of the secondary phase in different regions, as well as controlling the texture parameter XODF{001}/{111} to be 0.70-2.50. These parameter optimizations enable the steel to achieve both high strength and suppressed surface unevenness during forming operations.
2Weight of moving object
If the thickness of steel sheet is reduced through high-strengthening, then the weight is reduced, but the surface is likely to be uneven during forming into complicated shape
Solution Approach 1:
The patent creates a localized microstructure in the surface layer region (0-20 μm from surface) with lower secondary phase volume fraction (1-15%) and smaller secondary phase grain size (0.5-4.0 μm) compared to the internal region. This local optimization suppresses surface unevenness during forming, enabling thin-gauge steel sheets to maintain excellent surface quality even when formed into complicated shapes, thus allowing weight reduction without sacrificing surface appearance.
3Manufacturing precision
If a ferritic steel sheet with low C content is used to improve surface properties after stretching, then the surface unevenness is suppressed, but the strength cannot be sufficiently increased for high-strengthening requirements
Solution Approach 1:
The patent creates a composite microstructure consisting of ferrite and secondary phase (martensite, bainite, or tempered martensite) in a dual phase structure. The surface layer region contains 1-15% volume fraction of secondary phase with 0.5-4.0 μm average grain size, while the internal region contains 5-25% volume fraction with 1.0-5.0 μm average grain size. This composite structure combines the formability advantages of ferrite with the strength benefits of the hard secondary phase, achieving both improved surface properties and high strength simultaneously.
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 suppresses surface unevenness during forming, resulting in a high-strength steel sheet with excellent surface appearance quality and enhanced design capabilities, contributing to reduced vehicle weight and improved coating vividness.
Implementation Method 1
a dual phase structure including ferrite and a secondary phase
Implementation Method 2
a texture in which an XODF{001}/{111} as the ratio of an intensity of {001} orientation to an intensity of 111} orientation in the ferrite is 0.70 to 2.50 is included
Data Source
AI summary
A steel sheet has a predetermined chemical composition, in which a metallographic structure in a surface layer region ranging from a surface to a position of 20 μm from the surface in a sheet thickness direction consists of ferrite and a secondary phase having a volume fraction of 1.0% to 15.0%, the metallographic structure in an internal region ranging from a position of more than 20 μm from the surface in the sheet thickness direction to a ¼ thickness position from the surface in the sheet thickness direction consists of ferrite and a secondary phase having a volume fraction of 5.0% to 25.0%, the volume fraction of the secondary phase in the surface layer region is less than the volume fraction of the secondary phase in the internal region, and in the surface layer region, the average grain size of the secondary phase is 0.5 μm to 4.0 μm.