High-Strength Steel Sheet Microstructure for Delayed Fracture Resistance
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Solution Overview
Problem
High-strength steel sheets with tensile strengths of 1180 MPa or more require excellent part strength, stretch flangeability, bendability, and delayed fracture resistance, but existing materials struggle to simultaneously achieve these properties effectively.
Innovation Solution
A high-strength steel sheet with a specific composition (C: 0.090-0.390%, Si: 0.01-2.00%, Mn: 2.00-4.00%, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, O: 0.0100% or less) and microstructure (martensite area ratio of 70% or more, ferrite area ratio of 10% or less, retained austenite area ratio of 10% or less, and metastable carbides present in martensite blocks) is developed, along with a manufacturing method involving hot rolling, pickling, cold rolling, annealing, and post-heating processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel sheet with tensile strength of 1180 MPa or more is used to increase part strength, then yield strength and yield ratio are improved, but delayed fracture resistance deteriorates due to hydrogen intrusion
Solution Approach 1:
The patent changes the chemical composition parameters by strictly controlling C (0.090-0.390%), Si (0.01-2.00%), Mn (2.00-4.00%), P (0.100% or less), S (0.0200% or less), Al (1.000% or less), N (0.0100% or less), and O (0.0100% or less) to achieve the desired balance between strength and fracture resistance
Solution Approach 2:
The patent creates a composite microstructure consisting of martensite (70% or more), ferrite (10% or less), retained austenite (10% or less), and metastable carbides (2% or more of martensite blocks) to simultaneously achieve high strength and improved delayed fracture resistance
2Strength
If high-strength steel sheet is used to achieve large impact absorption energy, then yield strength is improved, but stretch flangeability and bendability deteriorate
Solution Approach 1:
The patent optimizes composition parameters including C (0.090-0.390%), Si (0.01-2.00%), Mn (2.00-4.00%), and controlled impurities (P, S, Al, N, O) to achieve a microstructure that balances strength with formability
Solution Approach 2:
The patent employs a composite microstructure with martensite (70% or more) for strength, ferrite (10% or less) for ductility, retained austenite (10% or less) for transformation-induced plasticity, and metastable carbides (2% or more) for toughness, thereby achieving both high impact absorption energy and good stretch flangeability and bendability
3Strength
If high-strength steel sheet is used to increase tensile strength, then yield ratio is improved, but delayed fracture resistance deteriorates under stress during pressing or assembling
Solution Approach 1:
The patent严格控制 composition parameters including C (0.090-0.390%), Si (0.01-2.00%), Mn (2.00-4.00%), and critical impurities (P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, O: 0.0100% or less) to prevent hydrogen-induced delayed fracture while maintaining high yield ratio
Solution Approach 2:
The patent creates a composite microstructure where martensite (70% or more) provides high yield ratio, ferrite (10% or less) provides ductility, retained austenite (10% or less) provides transformation-induced plasticity, and metastable carbides (2% or more of martensite blocks) provide toughness and resistance to delayed fracture under stress during pressing or assembling
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 a tensile strength of 1180 MPa or more while maintaining excellent part strength, stretch flangeability, bendability, and delayed fracture resistance, making it suitable for automotive structural members to improve fuel efficiency and industrial utility.
Implementation Method 1
in the microstructure, an area ratio of martensite is 70% or more
Implementation Method 2
a proportion of the number of martensite blocks in which metastable carbides are present to the number of martensite blocks is 2% or more
Data Source
AI summary
There is provided a high-strength steel sheet having a tensile strength of 1180 MPa or more. The high-strength steel sheet having a component composition containing C: 0.090 mass % or more and 0.390 mass % or less, Si: 0.01 mass % or more and 2.00 mass % or less, Mn: 2.00 mass % or more and 4.00 mass % or less, P: 0.100 mass % or less, S: 0.0200 mass % or less, Al: 1.000 mass % or less, N: 0.0100 mass % or less, and O: 0.0100 mass % or less, with a remaining part consisting of Fe and inevitable impurities; and a microstructure. In the microstructure, an area ratio of martensite is 70% or more, an area ratio of ferrite is 10% or less, an area ratio of retained austenite is 10% or less, and a proportion of the number of martensite blocks in which metastable carbides are present to the number of martensite blocks is 2% or more.