High-Strength Steel Sheet With Uniform Widthwise Microstructure
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Solution Overview
Problem
Existing high-strength steel sheets with tensile strength of 590 MPa or higher face challenges in achieving concurrent excellent formability and stability of mechanical properties due to variations in dimensions and properties across the width, leading to issues like spring back, local cracking, and decreased productivity.
Innovation Solution
A steel sheet with a specific chemical composition and controlled microstructure, including ferrite, bainitic ferrite, as-quenched martensite, and retained austenite, is manufactured through a controlled heat treatment process to ensure uniform mechanical properties across the width, using a combination of hot rolling, cold rolling, and annealing steps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the tensile strength of steel sheet is increased to 590 MPa or higher, then the strength is improved, but the formability and stability of mechanical properties deteriorate due to variations in strength, ductility, and stretch flange formability
Solution Approach 1:
The patent applies local quality by creating distinct microstructure regions: a first region with 15-30% retained austenite for high ductility and formability, and a second region with 0-10% retained austenite for high strength. This spatial differentiation of microstructural characteristics allows different areas of the steel sheet to exhibit optimized properties, resolving the contradiction between overall strength enhancement and localized formability maintenance.
Solution Approach 2:
The patent employs composite material principles by combining multiple microstructural phases (ferrite, bainite, martensite, and retained austenite) in specific proportions and distributions. The composite microstructure leverages the strengths of each phase: ferrite and bainite provide ductility, while martensite provides strength, and retained austenite contributes to both through TRIP effect. This multi-phase composite approach enables simultaneous achievement of high tensile strength (≥590 MPa) and excellent formability.
2Strength
If the tensile strength is increased to 590 MPa or higher, then the strength is improved, but the formability decreases due to the inverse relationship between strength and formability
Solution Approach 1:
The patent utilizes phase transition mechanisms, specifically the TRIP (Transformation Induced Plasticity) effect, where retained austenite transforms to martensite during deformation. This phase transition absorbs energy and delays necking, significantly improving formability. The first region with 15-30% retained austenite is designed to undergo extensive TRIP effect during forming operations, enabling excellent formability even with high overall tensile strength of 590 MPa or higher.
Solution Approach 2:
The composite microstructure combining multiple phases (ferrite, bainite, martensite, and retained austenite) resolves the strength-formability trade-off by leveraging the complementary properties of each phase. Ferrite and bainite provide ductility and formability, while martensite provides strength, and retained austenite contributes to both through TRIP effect. This multi-phase composite approach enables simultaneous achievement of high tensile strength (≥590 MPa) and excellent formability.
3Weight of moving object
If high strength steel sheets are used to reduce automobile weight, then the weight reduction is achieved, but the productivity decreases due to variations in dimensions and properties leading to spring back and local cracking
Solution Approach 1:
The patent applies local quality by creating distinct microstructure regions: a first region with 15-30% retained austenite for high ductility and formability, and a second region with 0-10% retained austenite for high strength. This spatial differentiation of microstructural characteristics allows different areas of the steel sheet to exhibit optimized properties, resolving the contradiction between overall strength enhancement and localized formability maintenance.
Solution Approach 2:
This principle is not applicable to the patent as it deals with metal microstructure control through heat treatment and rolling processes, not pneumatic or hydraulic systems.
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 results in a steel sheet with a tensile strength of 590 MPa or higher, exhibiting excellent formability and stability of mechanical properties, enabling weight reduction in automotive parts without increasing costs due to material defects.
Implementation Method 1
it is considered that ferrite transformation occurs in a cooling process depending on the chemical composition of a steel sheet. In this case, since it is not possible to promote transformation from austenite to bainite as described in Patent Literature 1, it is not possible to achieve high ductility while ensuring the stability of mechanical properties
Implementation Method 2
by dividing a cooling step after annealing has been performed into two stages, that is, a rapid cooling stage and a gradual cooling stage, since transformation from austenite to bainite rapidly progresses
Implementation Method 3
by allowing element partitioning from ferrite to austenite to occur by performing dual-phase annealing in a temperature range of higher than 800° C. and lower than the Ac3 transformation temperature
Data Source
AI summary
A steel sheet has a specified chemical composition and a specified steel microstructure. An average grain size of ferrite and/or bainitic ferrite is 7.0 μm or less. On the basis of a distribution in the width direction of the steel sheet, a ratio of a deviation of the grain size of the ferrite and/or the bainitic ferrite to the average grain size of the ferrite and/or the bainitic ferrite is 10% or less. On the basis of a distribution in the width direction of the steel sheet, a ratio of a deviation of the area fraction of as-quenched martensite to the area fraction of the as-quenched martensite is 10% or less. On the basis of a distribution in the width direction of the steel sheet, a ratio of a deviation of the area fraction of retained austenite to the area fraction of the retained austenite is 10% or less.