High-Strength Steel Sheet Microstructure for Ghost Line Prevention
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Solution Overview
Problem
Ghost lines, or minute unevenness, are produced on the surface of high-strength steel sheets during press forming due to the preferential deformation of soft phases around hard phases, leading to poor appearance quality, especially in automotive panel components that require high strength, reduced thickness, and complex shapes.
Innovation Solution
A steel sheet with a specific chemical composition and microstructure, including 70-95% ferrite and 5-30% hard phases, controlled Mn segregation through direct rolling after solidification, and limited Mn concentration variations to prevent banded microstructures, ensuring a standard deviation of Mn concentrations is 0.025 or less in the rolling direction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If the steel sheet is made with high strength and reduced thickness for weight reduction, then the fuel efficiency is improved, but the surface tends to produce unevenness and ghost lines during forming
Solution Approach 1:
The invention changes the chemical composition parameters (C: 0.15-0.35%, Si: 0.70-1.50%, Mn: 1.00-3.00%, P: 0.010-0.050%, S: 0.005-0.020%, Al: 0.010-1.000%, Cr: 0.010-1.000%, Ti: 0.001-0.100%, Nb: 0.001-0.100%, V: 0.001-0.100%, B: 0.0005-0.0050%) and microstructure parameters (ferrite volume fraction: 60-90%, hard phase volume fraction: 10-40%, ferrite grain diameter: 5-30 μm) to achieve high strength while preventing ghost lines during forming
Solution Approach 2:
The invention creates a composite microstructure consisting of soft ferrite phases and hard phases (martensite, bainite, or tempered martensite) in specific proportions, where the ferrite provides formability and the hard phases provide strength, achieving both high strength and good surface quality after forming
2Strength
If the steel sheet includes hard phases and soft phases for high strength, then the strength is improved, but the soft phases are preferentially deformed during press forming producing ghost lines
Solution Approach 1:
The invention creates local quality differences by distributing hard phases and soft phases in a specific pattern where hard phases are dispersed throughout the microstructure with controlled spacing, creating regions with different mechanical properties that resist preferential deformation during forming
Solution Approach 2:
The invention optimizes the volume fraction of hard phases (10-40%) and soft ferrite phases (60-90%), along with ferrite grain diameter (5-30 μm), to achieve a balance where the material has sufficient strength while the soft phases do not deform preferentially during press forming
Data Source
AI summary
An objective of the present invention is to provide a steel sheet having a high strength which can provide excellent appearance quality. The steel sheet has a chemical composition including, in mass %, C: more than 0.030% to 0.145%, Si: 0% to 0.500%, Mn: 0.50% to 2.50%, P: 0% to 0.100%, S: 0% to 0.020%, Al: 0% to 1.000% or less, N: 0% to 0.0100%, and the like, wherein a metal micro-structures consisting of 70 to 95% of ferrite in volume fraction and 5 to 30% of hard phases in volume fraction, and a value X1 obtained by dividing a standard deviation of average Mn concentrations in a rolling direction at ¼ sheet-thickness positions in a sheet thickness direction by an average Mn concentration at the ¼ sheet-thickness positions is 0.025 or less.