Hot-Rolled Steel Sheet Microstructure Control
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Solution Overview
Problem
Existing hot-rolled steel sheets struggle to achieve a balance between high strength, excellent ductility, fatigue property, and shearing property, with previous techniques either improving ductility or end surface accuracy but not simultaneously addressing all these requirements.
Innovation Solution
A hot-rolled steel sheet with a specific chemical composition and microstructure, including C: 0.050% to 0.250%, Si: 0.05% to 3.00%, Mn: 1.00% to 4.00%, and controlled amounts of Ti, Nb, and V, along with a microstructure comprising residual austenite less than 3.0%, ferrite 15.0% to 60.0%, and alloy carbides with specific size and density, to achieve a tensile strength of 980 MPa or more.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If high-strength steel sheet is used to reduce vehicle body weight, then weight reduction is achieved, but collision resistance and safety may be compromised
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.050-0.250%, Si: 0.05-3.00%, Mn: 1.00-4.00%, Ti+Nb+V: 0.060-0.500%, sol. Al: 0.001-2.000%) and microstructural parameters (ferrite area ratio: 15.0-60.0%, residual austenite: <3.0%, pearlite: <5.0%, alloy carbide density: ≥3.5×10^16/cm³, alloy carbide radius: 0.5-5.0 nm) to achieve tensile strength of 980 MPa or more while maintaining excellent formability with total elongation of 10.0% or more
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (ferrite, residual austenite, pearlite, and alloy carbides) where each phase contributes different properties: ferrite provides ductility and formability, residual austenite contributes to strength through TRIP effect, and fine alloy carbides provide precipitation hardening. This multi-phase composite structure achieves both high strength and excellent formability simultaneously
2Stability of the object's composition
If techniques focus on improving ductility, then formability is enhanced, but end surface accuracy after shearing working deteriorates
Solution Approach 1:
The patent resolves this contradiction by changing multiple parameters simultaneously: controlling chemical composition (particularly Mn content at 1.00-4.00% and sol. Al at 0.001-2.000%) and microstructural parameters (ferrite area ratio at 15.0-60.0%, residual austenite at <3.0%, and alloy carbide characteristics) to achieve both high ductility (total elongation ≥10.0%) and excellent shearing property without secondary sheared surfaces
Solution Approach 2:
The patent applies local quality by creating a controlled microstructure where alloy carbides with specific size (0.5-5.0 nm radius) and high density (≥3.5×10^16/cm³) are distributed within the ferrite matrix. This localized control of carbide distribution and size provides both the ductility needed for forming and the surface quality needed for shearing, as the fine carbides prevent crack initiation while maintaining material plasticity
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 resulting steel sheet exhibits high strength, excellent ductility, fatigue property, and shearing property, making it suitable for vehicle and mechanical structural members.
Implementation Method 1
an average sphere equivalent radius of alloy carbides in the ferrite is 0.5 nm or more and less than 5.0 nm, an average number density of the alloy carbides in the ferrite is 3.5×10^16/cm³ or more
Data Source
AI summary
This hot-rolled steel sheet has a desired chemical composition and microstructure, an average sphere equivalent radius of alloy carbides in the ferrite is 0.5 nm or more and less than 5.0 nm, an average number density of the alloy carbides in the ferrite is 3.5×1016/cm3 or more, an E value that indicates periodicity of the microstructure is 10.7 or more, and an I value that indicates uniformity of the microstructure is 1.020 or more, a standard deviation of a Mn concentration is 0.60 mass % or less, and a tensile strength is 980 MPa or more.