TRIP Steel Sheet Composition for High Strength and Cold Formability
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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 microstructural features, including 20% retained austenite with an aspect ratio of 2.0 or higher, combined with controlled annealing processes, to achieve 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 improve strength, then strength is improved, but elongation 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: 2-5%, Al: 0.5-2.0%) and microstructural parameters (retained austenite content: 20-50%, aspect ratio: 1.5-3.0) to achieve both high strength and improved formability. This resolves the contradiction by finding the optimal parameter combination that balances strength and ductility.
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (ferrite, retained austenite, and martensite) within the steel sheet. This composite structure allows the material to exhibit both high strength (from martensite) and good formability (from retained austenite that can transform during deformation), resolving the strength-ductility contradiction.
2Ease of operation
If hot-formed steel is used to improve workability and strength, then formability is improved, but manufacturing cost deteriorates
Solution Approach 1:
The patent uses a cost-effective cold forming process with controlled annealing instead of expensive hot forming equipment and high-temperature processing. By achieving the required formability through compositional design and controlled microstructure at lower temperatures, the manufacturing cost is significantly reduced while maintaining formability.
Solution Approach 2:
The patent changes the processing temperature parameter from high temperature (hot forming) to low temperature (cold forming with controlled annealing). By controlling the annealing temperature and time parameters, the steel achieves the necessary formability without requiring expensive hot forming equipment, thus reducing manufacturing cost.
3Weight of moving object
If steel sheet thickness is reduced to reduce weight, then weight is reduced, but stiffness deteriorates
Solution Approach 1:
The patent changes the material strength parameters by optimizing the steel composition and microstructure to achieve ultra-high strength (tensile strength ≥1400 MPa) with improved yield ratio (≥0.7). This allows the use of thinner steel sheets that maintain adequate stiffness and safety, resolving the weight-stiffness contradiction.
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 significant cost increases, replacing hot-formed steel in structural members.
Implementation Method 1
the ultra high-strength and high-ductility steel sheet has 20 area % or more of retained austenite as a microstructure
Implementation Method 2
utilizing hot-formed steel has been proposed. Hot-formed steel is formed by heating a steel sheet provided by a steel manufacturer at a high temperature and cooling the same, followed by introducing a low-temperature transformation phase into the steel sheet
Implementation Method 3
a method of utilizing hot-formed steel has been proposed. Hot-formed steel is formed by heating a steel sheet provided by a steel manufacturer at a high temperature and cooling the same
Data Source
AI summary
Provided is a method for manufacturing an ultra high-strength and high-ductility steel sheet having an excellent yield ratio. The method includes: heating a slab in a temperature range of 1050 to 1300° C.; finish hot rolling the heated slab in a temperature range of 800° C. to 1000° C.; winding the hot-rolled steel sheet in a temperature range of 50° C. to 750° C.; cold rolling the wound hot-rolled steel sheet at a reduction rate of 15% or higher after pickling; and annealing the cold-rolled steel sheet in a temperature range of 600° C. to 720° C. for 10 sec to 3,600 sec, or annealing in a temperature range of higher than 720° C. and 900° C. or lower for 10 sec to 3,600 sec and cooling, followed by a second annealing in a temperature range of 480° C. to 700° C. for 10 sec to 3,600 sec.