Plated Steel Sheet Decarburized Ferrite Layer Strength Formability
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Solution Overview
Problem
Conventional plated steel sheets face challenges in achieving high strength while maintaining good elongation and bendability, as improving formability often compromises strength and vice versa.
Innovation Solution
A plated steel sheet configuration featuring a decarburized ferrite layer and a base material with specific chemical compositions, including C, Si, Mn, and retained austenite, which enhances the formation of island martensite and tempered martensite to improve elongation and bendability while maintaining high strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If hard martensite is contained in a steel sheet for the purpose of high-strengthening, then strength is improved, but formability deteriorates
Solution Approach 1:
The invention creates a dual-phase microstructure where hard martensite regions provide strength while softer retained austenite regions provide formability. The steel sheet contains both martensite (3-20 volume%) and retained austenite (5-20 volume%) phases, allowing different regions to contribute different properties - the martensite provides high strength while the retained austenite provides TRIP effect for improved formability and elongation
Solution Approach 2:
The invention effectively creates a composite microstructure by combining two distinct phases - martensite and retained austenite - within the same steel sheet. This composite structure allows the material to exhibit both the high strength of martensite and the ductility/TRIP effect of retained austenite, resolving the contradiction between strength and formability
2Ease of manufacture
If tempering is performed to improve formability, then formability is improved, but strength is reduced
Solution Approach 1:
The invention carefully controls the tempering parameters (temperature and time) to achieve a specific microstructure. By tempering at temperatures that preserve retained austenite (avoiding complete transformation to pearlite or bainite), the invention maintains both the strength contribution from tempered martensite and the formability contribution from retained austenite TRIP effect
Solution Approach 2:
The invention utilizes phase transition control during tempering - specifically, controlling the transformation of martensite while preserving retained austenite. The tempering process transforms as-quenched martensite to tempered martensite while maintaining a controlled amount of retained austenite (5-20 volume%), and this phase transition control is key to achieving both improved formability and maintained strength
3Stability of the object's composition
If elongation property is improved by utilizing TRIP effect of retained austenite, then elongation is improved, but excessive hard martensite deteriorates bendability
Solution Approach 1:
The invention creates a dual-phase microstructure where hard martensite regions provide strength while softer retained austenite regions provide formability. The steel sheet contains both martensite (3-20 volume%) and retained austenite (5-20 volume%) phases, allowing different regions to contribute different properties - the martensite provides high strength while the retained austenite provides TRIP effect for improved elongation and bendability
Solution Approach 2:
The invention carefully controls the volume fraction of martensite to be within 3-20%, avoiding excessive martensite content that would deteriorate bendability. By controlling the martensite volume fraction and using tempering to reduce its hardness while preserving retained austenite, the invention achieves both high elongation (12% or more) and good bendability
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 proposed solution achieves a balance of high tensile strength, yield strength, and total elongation, along with excellent bendability, as demonstrated by achieving tensile strength of 780 MPa or more, yield strength of 420 MPa or more, and total elongation of 12% or more, with no cracks or constriction in 90-degree V-shaped bending tests.
Implementation Method 1
the M-A indicates a region of complex of martensite and retained austenite generated in martensite transformation during cooling after concentration of C in non-transformed austenite is caused in ferrite transformation or bainite transformation
Implementation Method 2
after the generation of M-A, the M-A is tempered at a temperature at which the retained austenite is remained
Data Source
Figure 1~2
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AI summary
A base material (13) included in a plated steel sheet (1) includes a structure, at a 1/4 sheet thickness position, represented by, in volume fraction: tempered martensite: 3.0% or more; ferrite: 4.0% or more; and retained austenite: 5.0% or more. An average hardness of the tempered martensite in the base material (13) is 5 GPa to 10 GPa, and a part or all of the tempered martensite and the retained austenite in the base material form an M-A. A volume fraction of ferrite in a decarburized ferrite layer (12) included in the plated steel sheet (1) is 120% or more of the volume fraction of the ferrite in the base material (13) at the 1/4 sheet thickness position, an average grain diameter of the ferrite in the decarburized ferrite layer (12) is 20 µm or less, a thickness of the decarburized ferrite layer (12) is 5 µm to 200 µm, a volume fraction of tempered martensite in the decarburized ferrite layer (12) is 1.0 volume% or more, a number density of the tempered martensite in the decarburized ferrite layer (12) is 0.01/µm2 or more, and an average hardness of the tempered martensite in the decarburized ferrite layer (12) is 8 GPa or less.