Steel Sheet Dislocation Density Control for Shape Uniformity
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Solution Overview
Problem
High-strength steel sheets used in automotive parts face challenges in achieving both high strength and excellent shape uniformity and shape fixability due to martensite transformation during forming, leading to dimensional accuracy issues.
Innovation Solution
A steel sheet with a microstructure containing 20% to 100% martensite and 0% to 80% ferrite, where the dislocation density ratio at the widthwise edge to the center is controlled between 100% to 140% on the surface and 100% to 140% at the thicknesswise center, along with a specific chemical composition and processing methods involving hot rolling, annealing, and water quenching with restraining forces to reduce transformation strain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If martensite is formed to obtain sufficient steel sheet strength, then strength is improved, but uniformity of sheet shape deteriorates due to transformation strain
Solution Approach 1:
The invention controls the dislocation density ratio parameter (edge/center) within 100-140% range and martensite volume fraction within 1-25%, transforming the approach from attempting to eliminate transformation strain to controlling the parameters that govern its distribution and impact
Solution Approach 2:
The invention addresses the non-uniformity by controlling the dislocation density distribution across the sheet width, ensuring the ratio between edge and center dislocation densities remains within 100-140%, thereby achieving uniform shape characteristics despite martensite transformation
2Shape
If levelling or skin pass rolling is applied to obtain desired dimensional accuracy, then shape uniformity is improved, but shape fixability deteriorates due to introduced strain
Solution Approach 1:
The invention performs preliminary control of dislocation density distribution before forming operations, ensuring the edge-to-center ratio is maintained within 100-140%, which pre-prevents the non-uniformity that would otherwise require corrective levelling that would harm shape fixability
3Strength
If the volume fraction of martensite is increased to achieve high strength, then strength is improved, but shape uniformity deteriorates due to increased transformation strain
Solution Approach 1:
The invention identifies and controls two key parameters simultaneously: martensite volume fraction (1-25%) and dislocation density ratio (100-140%), demonstrating that high strength can be achieved while maintaining shape uniformity through proper parameter coordination rather than maximizing a single parameter
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 steel sheet with high tensile strength (≥750 MPa), excellent shape uniformity (≤15 mm warpage), and improved shape fixability (ΔYR between -3% to 3%), enhancing the performance of automotive components by balancing strength and dimensional accuracy.
Implementation Method 1
many steel sheets utilize martensite, which is a hard phase. However, when martensite is formed, the uniformity of the sheet shape deteriorates due to transformation strain
Implementation Method 2
processing methods involving hot rolling, annealing, and water quenching
Data Source
AI summary
The steel sheet of the present invention has a steel microstructure containing, in area fraction, martensite: 20% to 100%, ferrite: 0% to 80%, and another metal phase: 5% or less, in which, on a surface of the steel sheet, a ratio of dislocation density in metal phases at a widthwise edge of the steel sheet to dislocation density in the metal phases at a widthwise center of the steel sheet is 100% to 140%, and, at a thicknesswise center of the steel sheet, a ratio of dislocation density in the metal phases at the widthwise edge of the steel sheet to dislocation density in the metal phases at the widthwise center of the steel sheet is 100% to 140%. The maximum amount of warpage of the steel sheet when the steel sheet is sheared to a length of 1 m in a rolling direction is 15 mm or less.