High Strength Steel Sheet Cu Precipitation Shapeability
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Solution Overview
Problem
Conventional high strength steel sheets with tensile strength of 900 MPa or more suffer from insufficient workability and are prone to brittle fracture under high-speed tension, with phase transformation during annealing occurring too quickly, leading to unstable properties and varying quality.
Innovation Solution
The solution involves efficiently precipitating Cu in the steel sheet during annealing by imparting strain, which improves stretch flangeability and tensile strength, achieving a microstructure with specific volume fractions of ferrite, bainite, tempered martensite, and residual austenite phases, and optimizing the composition and processing conditions to maintain coherence between Cu particles and the bcc iron phase.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of high strength steel sheet is increased to 900 MPa or more, then the collision safety is improved, but the shapeability becomes insufficient and local deformation becomes difficult
Solution Approach 1:
The invention changes the microstructural parameters by controlling the volume ratios of different phases (ferrite: 10-75%, bainitic ferrite/bainite: 10-50%, tempered martensite: ≤50%, fresh martensite: ≤15%, residual austenite: ≤20%) and controlling Cu particle characteristics (density, size, coherence state) to achieve both high strength and improved shapeability
Solution Approach 2:
The invention creates a composite microstructure consisting of multiple phases (ferrite, bainite, martensite, austenite) with Cu particles distributed throughout, where each phase contributes different properties: ferrite provides ductility, martensite provides strength, and Cu particles provide precipitation hardening, achieving a balance between strength and shapeability
2Strength
If the tensile maximum stress is increased to 900 MPa or more, then the collision safety is improved, but the fracture mode easily changes from ductile fracture to brittle fracture under high speed tension
Solution Approach 1:
The invention changes the microstructural parameters by controlling the volume ratios of different phases and Cu particle characteristics to maintain ductile fracture mode even at high strength levels and under high-speed tension conditions
Solution Approach 2:
The Cu particles serve multiple functions: they provide precipitation hardening for strength, and when coherent or semi-coherent with the bcc phase, they act as obstacles to crack propagation, maintaining ductile fracture behavior under high-speed loading conditions
3Productivity
If phase transformation during annealing is accelerated to improve productivity, then the production efficiency is improved, but the properties become unstable and quality varies
Solution Approach 1:
The invention changes the kinetic parameters of phase transformation by controlling temperature, time, and applying strain during annealing to achieve stable microstructural parameters (volume ratios of phases, Cu particle density and size) even with accelerated transformation rates
Solution Approach 2:
The invention applies strain during annealing as a feedback mechanism to control and stabilize the phase transformation process, ensuring consistent microstructural parameters and property stability even when transformation occurs rapidly
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
This approach results in high strength steel sheets with excellent stretch flangeability and tensile properties, maintaining stability and quality while preventing brittle fracture, and can be applied to high strength galvanized steel sheets as well.
Implementation Method 1
by making Cu efficiently precipitate in steel sheet
Implementation Method 2
a high strength steel material which was hardened by causing the fine precipitation of Cu
Implementation Method 3
it is sufficient to impart strain to the steel sheet during annealing of the steel sheet
Implementation Method 4
maintaining stability and quality while preventing brittle fracture... maintaining coherence between Cu particles and the bcc iron phase
Data Source
AI summary
High strength steel sheet which secures tensile maximum strength 900 MPa or more high strength while having excellent shapeability, which high strength steel sheet which is excellent in shapeability characterized by having a predetermined composition of ingredients, by the steel sheet structure including a ferrite phase and martensite phase, by the ratio of Cu particles incoherent with the bcc iron being 15% or more with respect to the Cu particles as a whole, by a density of Cu particles in the ferrite phase being 1.0×1018/m3 or more, and by an average particle size of Cu particles in the ferrite phase being 2.0 nm or more.