High-Strength Steel Sheet Microstructure Control
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Solution Overview
Problem
Current high-strength steel sheets for automobile parts fail to simultaneously achieve high tensile strength, yield ratio, strength-ductility balance, hole expansion ratio, and cross tensile strength of spot welded portions, which are essential for weight reduction and collision safety.
Innovation Solution
A high-strength steel sheet with a specific microstructure comprising a ferrite fraction of 5% or less, 60% or more tempered martensite and tempered bainite, 10% or more retained austenite, and a Mn concentration distribution variation, achieved through a multi-step austempering heat treatment process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If high-strength steel sheets are made thinner for weight reduction, then weight decreases, but strength and impact resistance properties deteriorate
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.20-0.40%, Si: 0.70-1.50%, Mn: 1.00-2.50%, Al: 0.05-0.50%) and heat treatment parameters (austenite holding temperature: 700-900°C, cooling rate: 10-100°C/sec) to achieve a specific microstructure with 10-30% retained austenite, thereby simultaneously achieving high strength and weight reduction
Solution Approach 2:
The patent creates a composite microstructure combining multiple phases (tempered martensite, tempered bainite, and retained austenite) within the steel sheet. This composite structure at the microlevel provides both high strength from the tempered phases and ductility/impact resistance from the retained austenite, enabling thin-gauge sheets to maintain high strength
2Strength
If tensile strength is increased to 980 MPa or higher, then collision safety improves, but yield ratio and strength-ductility balance deteriorate
Solution Approach 1:
The patent uses parameter changes by controlling the ratio of alloying elements (particularly Si and Mn) and heat treatment parameters to achieve a specific microstructure composition where tempered martensite provides high tensile strength while retained austenite maintains ductility, achieving TS≥980 MPa with YR≥0.75 and TS×EL≥20,000 MPa%
Solution Approach 2:
The patent applies local quality by creating a microstructure where different phases are distributed at the microscopic level: hard tempered martensite and bainite provide local strength, while softer retained austenite regions provide local ductility and energy absorption, achieving both high tensile strength and good strength-ductility balance
3Ease of operation
If deep drawability and hole expansion ratio are improved for formability, then stretch formability increases, but tensile strength and collision resistance deteriorate
Solution Approach 1:
The patent applies parameter changes by optimizing the chemical composition (particularly Si content at 0.70-1.50% which suppresses carbide precipitation and maintains austenite stability) and heat treatment parameters to achieve 10-30% retained austenite, enabling both excellent formability (hole expansion ratio ≥20%, maximum forming height ≥16 mm) and high tensile strength (≥980 MPa)
Solution Approach 2:
The patent creates a composite microstructure where retained austenite regions act as soft phases that facilitate plastic deformation and hole expansion during forming operations, while the surrounding tempered martensite and bainite provide the strength framework, achieving both high formability and high tensile strength
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 exhibits tensile strength of 980 MPa or higher, yield ratio of 0.75 or more, TS × EL of 20,000 MPa% or more, hole expansion ratio of 20% or more, and cross tensile strength of 6 kN or higher, enhancing both formability and collision resistance.
Implementation Method 1
a ferrite fraction is 5 volume% or less, a total fraction of tempered martensite and tempered bainite is 60 volume% or more, and an amount of retained austenite is 10 volume% or more
Implementation Method 2
the amount of retained austenite is 10 volume% or more, MA has an average size of 1.0 μm or less
Data Source
Figure 1
AI summary
Disclosed is a high-strength sheet including: C: 0.15% by mass to 0.35% by mass, total of Si and Al: 0.5% by mass to 3.0% by mass, Mn: 1.0% by mass to 4.0% by mass, P: 0.05% by mass or less, and S: 0.01% by mass or less, with the balance being Fe and inevitable impurities, wherein the steel structure satisfies that: a ferrite fraction is 5% or less, the total fraction of tempered martensite and tempered bainite is 60% or more, the amount of retained austenite is 10% or more, MA has an average size of 1.0 µm or less, a half-width of the concentration distribution of Mn in the carbon-concentrated region that is equal to the amount of retained austenite is 0.3% by mass or more, and a scattering intensity at the q value of 1 nm-1 in X-ray small angle scattering is 1.0 cm-1 or less.