Hot-rolled steel sheet with bainite and residual austenite
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Solution Overview
Problem
Current hot-rolled steel sheets face challenges in achieving high strength, excellent ductility, stretch flangeability, and low temperature toughness while maintaining uniform material properties across the sheet width direction, particularly in vehicle applications.
Innovation Solution
A hot-rolled steel sheet with a chemical composition of C: 0.100% to 0.250%, Si: 0.05% to 3.00%, Mn: 1.00% to 4.00%, and specific inclusion of Nb, Al, and other elements, featuring a metallographic structure with 77.0% to 97.0% bainite and tempered martensite, 0% to 5.0% ferrite, 0% to 5.0% pearlite, and 3.0% or more residual austenite, along with controlled cooling and coiling processes to stabilize austenite and refine grain size.
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 are compromised
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.15-0.35%, Si: 0.01-2.50%, Mn: 1.50-3.50%, etc.) and processing parameters (cooling rate: 10-50°C/s, coiling temperature: 350-500°C) to achieve a metallographic structure with 77-97% bainite and 3-20% residual austenite, resulting in tensile strength of 980 MPa or more while maintaining excellent formability
Solution Approach 2:
The patent creates a composite metallographic structure combining bainite (77-97%) and residual austenite (3-20%), where bainite provides high strength and residual austenite provides ductility through transformation-induced plasticity (TRIP), achieving both collision resistance and formability in the same material
2Stability of the object's composition
If residual austenite is added to improve ductility through TRIP, then elongation is enhanced, but hole expansibility deteriorates due to hard martensite formation
Solution Approach 1:
The patent controls the carbon content (0.15-0.35%) and carbon equivalent (3.50-4.50%) to regulate the transformation behavior of residual austenite, ensuring it transforms into martensite at controlled rates during forming operations, thereby maintaining both ductility (total elongation 10% or more) and hole expansibility (maximum hole expansion ratio 1.25 or more)
3Strength
If rapid cooling is applied to form bainite and martensite for high strength, then tensile strength is improved, but material property uniformity across sheet width deteriorates
Solution Approach 1:
The patent applies local quality by implementing position-dependent cooling rates: the center portion is cooled at 10-50°C/s while the end portions are cooled at 5-30°C/s, ensuring uniform material properties across the sheet width while maintaining high tensile strength (980 MPa or more) throughout the entire sheet
4Stability of the object's composition
If incubation time is extended to stabilize residual austenite, then ductility is improved, but productivity decreases due to longer processing time
Solution Approach 1:
The patent optimizes the incubation time parameter to 1-20 seconds at 500-720°C, which is sufficient to achieve 3-20% residual austenite content and excellent ductility (total elongation 10% or more) while maintaining rapid overall cooling rates (10-50°C/s) that ensure high productivity and efficient mass production
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 hot-rolled steel sheet with enhanced strength, ductility, stretch flangeability, and low temperature toughness, ensuring consistent material properties and suitability for vehicle components.
Implementation Method 1
steel sheets containing residual austenite exhibit excellent ductility by transformation-induced plasticity (TRIP), and therefore many investigations have been conducted so far
Implementation Method 2
cooling a steel sheet to a temperature range of 720° C. or lower within 1 second after the completion of hot rolling
Implementation Method 3
retaining the steel sheet in a temperature range of higher than 500° C. and 720° C. or lower for an incubation time of 1 to 20 seconds
Data Source
AI summary
This hot-rolled steel sheet has a predetermined chemical composition. The metallographic structure at a sheet thickness ¼ depth from a surface and at a center position in a sheet width direction in a sheet width cross section parallel to a rolling direction contains, by area %, 77.0% to 97.0% of bainite and tempered martensite in total, 0% to 5.0% of ferrite, 0% to 5.0% of pearlite, 3.0% or more of residual austenite, and 0% to 10.0% of martensite. The average grain size of the metallographic structure excluding the residual austenite is 7.0 μm or less. The C concentration in the residual austenite is 0.5 mass % or more. The number density of iron-based carbides having a diameter of 20 nm or more is 1.0×106 carbides/mm2 or more.