Steel Sheet Surface Soft Part for Bendability and Hydrogen Resistance
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Solution Overview
Problem
Existing steel sheets face challenges in simultaneously achieving high bendability and hydrogen embrittlement resistance, as improvements in strength often compromise ductility and formability.
Innovation Solution
A steel sheet design with a central part composed of 60% tempered martensite, less than 30% ferrite, bainite, and pearlite, and less than 5% as-quenched martensite, combined with a surface soft part exceeding 10 μm thickness and containing carbides with controlled particle size and density, to enhance both bendability and hydrogen embrittlement resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of steel sheet is improved, then tensile strength increases, but ductility and stretch flangeability deteriorate
Solution Approach 1:
The invention creates a dual-layer structure where the surface layer and central part have different microstructures and properties. The surface layer contains 60% or more ferrite for ductility, while the central part contains 60% or more tempered martensite for strength, allowing both high strength and good formability to coexist in different regions of the same material
Solution Approach 2:
The steel sheet is divided into two distinct functional zones: a surface layer optimized for formability and a central part optimized for strength. This segmentation allows each region to perform its specialized function without compromising the other, resolving the contradiction between overall strength and local ductility
2Strength
If the strength of steel sheet is improved, then tensile strength increases, but bendability deteriorates
Solution Approach 1:
The surface layer is designed with high ferrite content (60% or more) which provides excellent bendability, while the central part maintains high strength through tempered martensite (60% or more). This local differentiation allows the material to be bent easily at the surface while maintaining overall structural strength
Solution Approach 2:
The steel sheet structure is segmented into a surface layer for bendability and a central part for strength. The surface layer's ferrite-rich microstructure enables easy bending operations, while the central part's martensite structure ensures the bent component maintains its strength after forming
3Strength
If the strength of steel sheet is improved, then tensile strength increases, but hydrogen embrittlement resistance deteriorates
Solution Approach 1:
The surface layer with high ferrite content (60% or more) provides hydrogen embrittlement resistance as ferrite is less susceptible to hydrogen embrittlement than martensite. The central part maintains high strength through tempered martensite, creating a gradient where the surface protects against hydrogen while the core provides strength
Solution Approach 2:
The steel sheet is segmented into a surface layer for hydrogen embrittlement resistance and a central part for strength. The ferrite-rich surface layer acts as a barrier to hydrogen penetration and accumulation, while the martensite-rich central part maintains the required 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 achieves a tensile strength of 1180 MPa or more with improved bendability and hydrogen embrittlement resistance, suitable for automotive applications.
Implementation Method 1
a microstructure of the central part of sheet thickness comprises, by volume ratio, 60% or more of tempered martensite
Implementation Method 2
the surface soft part includes carbides in a number density of 1×104/mm2 or more... improved in hydrogen embrittlement resistance
Data Source
AI summary
The present invention provides steel sheet having both bendability and hydrogen embrittlement resistance. The steel sheet of the present invention includes a central part of sheet thickness and a surface sort part formed at one side or both sides of the central part of sheet thickness. The microstructure of the central part of sheet thickness comprises, by volume ratio, 60% or more of tempered martensite, respectively less than 30% of ferrite, bainite, pearlite, and retained austenite, and less than 5% of as-quenched martensite. A thickness of the surface soft part is more than 10 μm per side and 15% or less of a thickness of the central part of sheet thickness, an average hardness of the surface soft part is 0.90 time or less of an average hardness of the central part of sheet thickness, the surface soft part includes carbides in a number density of 1×104/mm2 or more, an average particle size of the carbides is 0.250 μm or less, and a standard deviation of a log of a particle size is 0.05 or less.