Hot-Rolled Steel Sheet Microstructure for Stable Shearing
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Solution Overview
Problem
Existing hot-rolled steel sheets struggle to achieve high strength and stable shearing workability, particularly in applications requiring high-strength materials like vehicle body components, where shearing workability and end surface accuracy are critical but often unstable due to variations in clearance.
Innovation Solution
A hot-rolled steel sheet with a specific chemical composition and microstructure, including controlled Mn segregation, periodic microstructure, and targeted heat treatment processes, such as slab heating, hot rolling, and cooling, to achieve a tensile strength of 780 MPa or more with stable shearing workability.
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 may be compromised
Solution Approach 1:
The patent changes the chemical composition parameters (C: 0.050-0.250%, Si: 0.05-3.00%, Mn: 1.00-4.00%, P: 0.100% or less, S: 0.0300% or less) and microstructural parameters (ferrite grain size ratio ds/db ≤ 0.95, area percentages of martensite/bainite ≥ 80%) to achieve both weight reduction and collision resistance. This allows the steel sheet to maintain high strength while enabling thinner gauges for vehicle bodies.
Solution Approach 2:
The patent creates a composite microstructure consisting of martensite and bainite phases with specific area ratios (martensite: 70-95%, bainite: 5-30%). This composite structure provides both high strength and excellent formability, resolving the contradiction between weight reduction and collision resistance by optimizing the combination of different phases.
2Productivity
If steel sheet thickness is reduced to achieve weight reduction, then productivity is improved, but manufacturing precision deteriorates due to unstable shearing workability
Solution Approach 1:
The patent controls the ferrite grain size ratio (ds/db ≤ 0.95) and chemical composition parameters to stabilize the microstructure during shearing working. This ensures that even with reduced thickness and varying clearances, the end surface accuracy remains stable with minimal shear droop, maintaining manufacturing precision while enabling thinner sheets for higher productivity.
Solution Approach 2:
The patent accepts and controls the formation of shear droop as a temporary, acceptable feature during the shearing process rather than attempting to eliminate it completely. By stabilizing the microstructure to control shear droop proportion, the process becomes more predictable and reliable, allowing thinner sheets to be processed efficiently without compromising end surface quality.
3Manufacturing precision
If ferrite grain size is controlled to improve shearing workability, then end surface accuracy is improved, but strength may be reduced
Solution Approach 1:
The patent uses a composite microstructure of martensite and bainite where the martensite phase provides high strength (tensile strength ≥ 780 MPa) and the controlled ferrite grain size ratio (ds/db ≤ 0.95) provides stable shearing workability. The specific area ratios (martensite: 70-95%, bainite: 5-30%) ensure both strength and formability are achieved simultaneously.
Solution Approach 2:
The patent applies different grain size characteristics to different regions: the surface layer has controlled ferrite grain size (ds) while the interior has grain size (db), with the ratio ds/db ≤ 0.95. This local quality control stabilizes shear droop during shearing while the overall microstructure maintains high strength through the martensite-bainite composite structure.
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 hot-rolled steel sheet with enhanced strength and shearing workability, ensuring consistent shearing end surface accuracy and resistance to inside bend cracking, suitable for vehicle and mechanical structural applications.
Implementation Method 1
the remainder in microstructure is one or two or more of bainite, martensite, and tempered martensite
Implementation Method 2
it is important that Mn segregation does not occur much, the microstructural morphology is periodic
Data Source
AI summary
This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure, in terms of area %, residual austenite is less than 3.0%, ferrite is less than 15.0%, and pearlite is less than 5.0%, an E value that indicates periodicity of the microstructure is less than 10.7, and an I value that indicates uniformity of the microstructure is less than 1.020, a standard deviation of a Mn concentration is 0.60 mass % or less, and a tensile strength is 780 MPa or more.