Bainite-Ferrite Steel Sheet Isotropic Workability
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Solution Overview
Problem
Current high-strength steel sheets face challenges in achieving isotropic workability, low-temperature toughness, and cost-effectiveness while maintaining high strength and material properties such as formability and ductility, particularly in applications requiring circularity and sheet thickness uniformity.
Innovation Solution
A bainite-containing-type high-strength hot-rolled steel sheet with specific chemical compositions and a novel hot rolling method that includes multiple rolling passes, primary and secondary cooling processes, and controlled cooling rates to achieve isotropic workability and low-temperature toughness, while maintaining high strength and ductility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength is achieved through conventional steel sheet manufacturing, then strength increases, but formability and workability deteriorate
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.15-0.35%, Si: 0.01-2.5%, Mn: 1.50-3.50%, P: 0.01-0.15%, S: 0.01-0.05%, Al: 0.01-2.00%) and processing parameters (cooling rates, rolling temperatures) to achieve a microstructure containing 5-30% bainite and 70-95% ferrite, thereby simultaneously obtaining high strength and good formability
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (bainite and ferrite) with specific proportions. This composite structure combines the high strength characteristics of bainite with the good ductility and formability of ferrite, resolving the contradiction between strength and workability
2Weight of moving object
If high strength steel sheet is manufactured to reduce thickness, then weight reduction is achieved, but material properties such as ductility and formability deteriorate
Solution Approach 1:
The patent changes the microstructural parameters by controlling phase composition (5-30% bainite, 70-95% ferrite) and grain size, achieving high strength that enables thickness reduction while maintaining adequate material properties for formability and ductility
Solution Approach 2:
The multi-phase composite microstructure (bainite-ferrite mixture) provides both the high strength needed for thinning and the ductility required for forming operations, enabling weight reduction without sacrificing material performance
3Device complexity
If conventional hot rolling method is used, then manufacturing process is simple, but isotropic workability and low-temperature toughness are not achieved
Solution Approach 1:
The patent segments the hot rolling process into distinct stages with different cooling rates: first cooling at 50-150°C/s to achieve austenite transformation, then second cooling at 10-50°C/s to control ferrite and bainite formation. This segmented approach achieves isotropic workability and low-temperature toughness through controlled microstructure development
Solution Approach 2:
The patent applies preliminary action by controlling the cooling rate in the first cooling stage to achieve specific austenite grain structure before the second cooling stage. This preliminary microstructure control enables subsequent formation of isotropic multi-phase structure with good low-temperature toughness
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 provides a steel sheet with excellent isotropic workability, low-temperature toughness, and high strength, suitable for various applications including automotive and industrial components, while being manufactured at a lower cost.
Implementation Method 1
a microstructure is composed of 35% or less in a structural fraction of pro-eutectoid ferrite and a balance of a low-temperature transformation generating phase
Implementation Method 2
an average crystal grain diameter is 10 μm or less and a Charpy fracture appearance transition temperature vTrs is −20° C. or lower
Data Source
AI summary
The present invention provides a bainite-containing-type high-strength hot-rolled steel sheet. The steel sheet, containing C: greater than 0.07 to 0.2%, Si: 0.001 to 2.5%, Mn: 0.01 to 4%, P: 0.15% or less, S: 0.03% or less, N: 0.01% or less, Al: 0.001 to 2% and a balance being composed of Fe and impurities, has an average value of pole densities of the {100}<011> to {223}<110> orientation group at a sheet thickness center portion being a range of ⅝ to ⅜ in sheet thickness from the surface of the steel sheet is 4.0 or less, and a pole density of the {332}<113> crystal orientation is 4.8 or less, an average crystal grain diameter is 10 μm or less and vTrs is −20° C. or lower, and a microstructure is composed of 35% or less in a structural fraction of pro-eutectoid ferrite and a balance of a low-temperature transformation generating phase.