Retained Austenite Steel Sheet for Cold-Formable High Strength
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Solution Overview
Problem
Existing ultra-high strength steels face limitations in workability and formability due to low elongation, leading to restricted applications in complex-shaped components and increased manufacturing costs, while reducing steel sheet thickness compromises passenger safety and stiffness.
Innovation Solution
A steel composition with specific elemental ratios and microstructure, including 20% retained austenite with an aspect ratio of 2.0 or higher, combined with controlled annealing processes, achieves a tensile strength of 1,400 MPa, yield ratio of 0.7 or greater, and a product of tensile strength and elongation of 22,000 MPa%, suitable for cold forming.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If ultra-high strength steel is used to reduce weight and secure safety, then strength and safety are improved, but elongation decreases and workability deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.1-0.3%, Mn: 6-10%, Si: 2% or less, Al: 0.5% or less) and microstructural parameters (retained austenite content: 20% or more, aspect ratio: 2.0 or higher) to achieve both high strength and improved ductility. This resolves the contradiction by finding the optimal parameter range where strength is maximized while elongation is maintained at 10% or more.
Solution Approach 2:
The patent creates a composite microstructure consisting of retained austenite and martensite, where the retained austenite (with aspect ratio 2.0 or higher) acts as a ductile phase that can transform during deformation, while the martensite provides strength. This composite structure enables the steel to achieve both high yield strength (≥1000 MPa) and adequate elongation (≥10%), resolving the strength-ductility contradiction.
2Ease of manufacture
If hot-formed steel is used to improve workability and strength, then formability and strength are improved, but manufacturing cost increases due to equipment investment and heat treatment
Solution Approach 1:
The patent replaces expensive hot-forming equipment and complex heat treatment processes with a simpler cold-rolling based manufacturing process. By developing a steel composition that achieves high strength and formability through controlled cold processing and simple annealing, the patent eliminates the need for costly hot-forming facilities while maintaining or improving performance.
Solution Approach 2:
The patent substitutes the thermal-mechanical hot-forming process with a cold-rolling and controlled annealing process. Instead of heating and forming the steel at high temperatures, the invention uses controlled cold deformation followed by specific annealing treatments to achieve the desired microstructure and properties, thereby replacing complex thermal processing with simpler mechanical and thermal steps.
3Weight of moving object
If steel sheet thickness is reduced to reduce weight, then weight is reduced, but stiffness decreases and passenger safety is compromised
Solution Approach 1:
The patent changes the material parameters by developing ultra-high strength steel with yield strength ≥1000 MPa and tensile strength ≥1300 MPa. This dramatic improvement in strength parameters allows the use of thinner steel sheets while maintaining or improving structural stiffness and safety, thereby reducing overall vehicle weight without compromising passenger protection.
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 an ultra high-strength and high-ductility steel sheet with improved formability and safety, enabling the production of complex automotive components without the need for costly hot-forming equipment.
Implementation Method 1
the ultra high-strength and high-ductility steel sheet comprises 20 area % or more of retained austenite as a microstructure
Implementation Method 2
a method for manufacturing an ultra high-strength and high-ductility steel sheet... includes heating a slab... preparing a hot-rolled steel sheet by finish hot rolling the heated slab... winding the hot-rolled steel sheet... preparing a cold-rolled steel sheet by cold rolling the wound hot-rolled steel sheet... and selectively annealing the cold-rolled steel sheet
Data Source
AI summary
An ultra high strength and high ductility steel sheet having an excellent yield ratio contains, in weight %, 0.1-0.3% of carbon (C), 2% or less of silicon (Si), 6-10% of manganese (Mn), 0.05% or less of phosphorus (P), 0.02% or less of sulfur (S), 0.02% or less of nitrogen (N), 0.5% or less (excluding 0%) of aluminum (Al), and the balance Fe and inevitable impurities, and further contains at least one selected from the group consisting of 0.1% or less of titanium (Ti), 0.1% or less of niobium (Nb), 0.2% or less of vanadium (V), and 0.5% or less of molybdenum (Mo), wherein the ultrahigh-strength and high-ductility steel sheet comprises 20 area % or more of residual austenite as a microstructure, the average aspect ratio of the residual austenite being 2.0 or higher.