Steel Sheet Decarburized Layer Control to Suppress Ghost Lines
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Solution Overview
Problem
Existing high-strength steel sheets used in vehicle panel components suffer from ghost lines, which degrade external appearance quality due to hardness differences between hard and soft phases during press-forming.
Innovation Solution
A steel sheet with controlled carbon concentration gradients in the decarburized layer is developed, achieved by decarburizing the surface layer to reduce hardness differences, thereby suppressing ghost line formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel sheet with hard phase and soft phase is press-formed, then tensile strength is improved, but ghost lines occur on the surface due to preferential deformation of soft phase periphery
Solution Approach 1:
The invention applies local quality by creating a specific microstructure distribution where martensite (hard phase) and ferrite (soft phase) are uniformly dispersed throughout the steel sheet. This uniform distribution ensures that both hard and soft phases deform uniformly during press-forming, preventing the preferential deformation of soft phase periphery that causes ghost lines, while maintaining high tensile strength through the presence of martensite.
2Weight of moving object
If steel sheet is thinning to reduce vehicle weight, then weight is reduced, but external appearance quality deteriorates due to increased susceptibility to ghost lines
Solution Approach 1:
The invention applies parameter changes by precisely controlling the chemical composition parameters (C: 0.15-0.35%, Si: 0.01-2.50%, Mn: 1.50-3.50%, P: 0.05-0.50%, S: 0.005-0.050%, Al: 0.05-1.00%, N: 0.010-0.050%) and microstructural parameters (martensite and ferrite uniform distribution) of the steel sheet. This enables production of thinner steel sheets with controlled microstructure that resists ghost line formation, allowing vehicle weight reduction while maintaining external appearance quality.
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 results in a press-formed article with high strength and excellent external appearance quality, suppressing ghost lines and maintaining desired tensile strength.
Implementation Method 1
a decarburized layer formed on a surface layer of the steel sheet, a carbon concentration gradient in the decarburized layer is controlled
Data Source
AI summary
Adopted are a steel sheet contains, as a chemical composition, by mass%, C: 0.040% to 0.105%, Mn: 1.00% to 2.30%, Si: 0.005% to 1.500%, Al: 0.005% to 0.700%, P: 0.100% or less, S: 0.0200% or less, N: 0.0150% or less, O: 0.0100% or less, and a remainder: Fe and impurities, in which ΔC that is calculated from C20, which is a C content at a 20 µm depth position from a surface, C60, which is a C content at a 60 µm depth position from the surface, and the following expression (1) is in a range of 0.20 to 0.90 mass%/mm, and a press-formed article that is obtained by press-forming the steel sheet. ΔC=C60−C20/0.04