Hot-Rolled Steel Sheet Composition for Strength-Ductility Balance
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Solution Overview
Problem
Existing hot-rolled steel sheets lack high strength, adequate sheet thickness reduction at critical fracture, excellent ductility, shearing property, and corrosion resistance, which are essential for vehicle and mechanical structural applications.
Innovation Solution
A hot-rolled steel sheet with a specific chemical composition and microstructure, including controlled residual austenite, ferrite, and pearlite ratios, along with optimized concentrations of elements like Ti, Nb, V, and Si—Al—Cr oxides, enhances strength, ductility, and corrosion resistance.
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 formability may be compromised
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.15-0.35%, Si: 0.60-1.00%, Mn: 1.50-2.50%, Cr: 1.00-2.00%, Ti: 0.050-0.200%, Nb: 0.050-0.200%, V: 0.050-0.200%) and microstructural parameters (ferrite 30-60%, bainite 40-60%, martensite 5-20%, residual austenite <3%) to achieve high tensile strength (1320-1530 MPa) and total elongation (10-14%), thereby reducing vehicle body weight while maintaining collision resistance
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (ferrite, bainite, martensite, and residual austenite) within the steel sheet. This multi-phase composite structure combines the ductility of ferrite, the strength of bainite and martensite, and the formability contribution of residual austenite, achieving both high strength and excellent formability simultaneously
2Weight of moving object
If sheet thickness is reduced to achieve weight reduction, then weight reduction is achieved, but sheet thickness reduction at critical fracture and ductility deteriorate
Solution Approach 1:
The patent controls chemical composition parameters (particularly C: 0.15-0.35% and Si: 0.60-1.00%) and microstructural parameters (ferrite 30-60%, bainite 40-60%, martensite 5-20%, residual austenite <3%) to achieve high total elongation (10-14%) and sheet thickness reduction at critical fracture (55-70%), enabling thin sheets to maintain adequate formability
Solution Approach 2:
The multi-phase composite microstructure (ferrite + bainite + martensite + residual austenite) provides a balance between strength and ductility, allowing the steel to undergo significant plastic deformation (55-70% thickness reduction) before fracture, which is critical for forming thin-walled vehicle components
3Productivity
If shearing working is applied to manufacture blank sheets, then productivity is improved, but end surface accuracy deteriorates due to secondary sheared surfaces
Solution Approach 1:
The patent controls the chemical composition parameters (C: 0.15-0.35%, Si: 0.60-1.00%, Mn: 1.50-2.50%, Cr: 1.00-2.00%, Ti: 0.050-0.200%, Nb: 0.050-0.200%, V: 0.050-0.200%) to optimize the microstructure and reduce brittleness, thereby suppressing the formation of secondary sheared surfaces during shearing working and improving end surface accuracy while maintaining high productivity
4Ease of manufacture
If conventional hot-rolled steel sheet composition is used, then manufacturing cost is controlled, but corrosion resistance is insufficient
Solution Approach 1:
The patent optimizes the chemical composition parameters (particularly Cr: 1.00-2.00%, Si: 0.60-1.00%, and Mn: 1.50-2.50%) to enhance corrosion resistance. The controlled Cr content forms protective oxide layers, while Si and Mn contribute to corrosion resistance, achieving improved reliability without excessive cost increase
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 steel sheet achieves high strength, improved ductility, excellent shearing properties, and enhanced corrosion resistance, making it suitable for vehicle and mechanical structural members.
Implementation Method 1
Ti, Nb, and V are elements that are finely precipitated in steel as a carbide and a nitride and improve the strength of steel by precipitation hardening
Implementation Method 2
Si has an action of making steel sound by deoxidation (suppressing the occurrence of a defect such as a blowhole in steel)
Data Source
AI summary
This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure at a position of ¼ from a surface in a sheet thickness direction, in terms of area %, residual austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, and pearlite is less than 5.0%, an E value is 10.7 or more, an I value is 1.020 or more, a CS value is −8.0×105 to 8.0×105, a standard deviation of Mn concentrations is 0.60 mass % or less, at the surface, a number density of Si—Al—Cr oxides having a sphere equivalent radius of 0.500 μm or more is 2.0×103 pieces/cm2 or less, and a number density of Si—Al—Cr oxides having a sphere equivalent radius of 0.005 μm to 0.050 μm is 1.0×105 pieces/cm2 or more.