Steel Sheet Str Control for Ghost Line Suppression
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing high-strength steel sheets used for vehicle outer panels, such as roofs, hoods, and doors, suffer from surface irregularities known as 'ghost lines' during forming due to the anisotropic properties of dual phase steel, which affect appearance and design freedom.
Innovation Solution
A steel sheet composition with controlled aspect ratio (Str) of 0.35 to 0.75 and a difference in Str (ΔStr) of 0.15 or less, combined with a microstructure of 70-97% ferrite and 3-30% hard phase, is developed to maintain high strength and suppress ghost lines.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If dual phase steel with soft ferrite and hard martensite is used to achieve high strength, then tensile strength is improved, but surface irregularities called ghost lines form during press-forming
Solution Approach 1:
The invention changes the surface property parameters by controlling the aspect ratio Str to 0.35 to 0.75 and the difference ΔStr to 0.15 or less. This parameter optimization resolves the contradiction by enabling the steel sheet to maintain high tensile strength through its dual phase microstructure while suppressing ghost line formation through controlled surface anisotropy characteristics
Solution Approach 2:
The invention utilizes a composite microstructure consisting of soft ferrite phase (70-97% area ratio) and hard phase mainly consisting of martensite (3-30% area ratio). This composite structure provides both high strength from the hard phase and good formability from the soft ferrite, while the controlled surface aspect ratio prevents ghost line formation during forming operations
2Weight of moving object
If steel sheet thickness is reduced to achieve lighter weight, then vehicle weight is reduced, but strength and surface quality become more difficult to maintain
Solution Approach 1:
The dual phase composite microstructure with controlled aspect ratio enables thin steel sheets to achieve high strength-to-weight ratio. The hard martensite phase provides strength while the soft ferrite phase maintains ductility, allowing weight reduction without sacrificing mechanical properties or surface quality
3Weight of moving object
If steel sheet thickness is reduced to achieve lighter weight, then vehicle weight is reduced, but surface quality deteriorates due to increased sensitivity to forming irregularities
Solution Approach 1:
By optimizing the surface aspect ratio parameters (Str: 0.35 to 0.75, ΔStr: 0.15 or less), the invention makes thin steel sheets resistant to ghost line formation during forming. This parameter control compensates for the increased sensitivity of thin sheets to forming irregularities, maintaining excellent surface quality even at reduced thickness
Data Source
AI summary
Provided is a steel sheet having a chemical composition comprising C: 0.020 to 0.100%, Mn: 1.00 to 2.50%, P: 0.100% or less, S: 0.0200% or less, Al: 0.005 to 0.700%, N: 0.0150% or less, O: 0.0100% or less, etc., and a balance of Fe and impurities, and a microstructure comprising, by area %, ferrite: 70 to 97% and a hard phase: 3 to 30%, wherein an Str of the surface is 0.35 to 0.75, and a difference ΔStr of the Str and the Str after imparting 5% tensile strain is 0.15 or less.