Superficial Soft Portion Steel Sheet for Bendability
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Solution Overview
Problem
Current ultra high strength cold rolled steel sheets with tensile strength of 1270 MPa or more face challenges in achieving both excellent bendability and delayed fracture resistance, which are crucial for lightweight and strong automobile parts.
Innovation Solution
The development of an ultra high strength cold rolled steel sheet with a specific chemical composition and microstructure, including a superficial soft portion and a tempered-martensite core, optimized to achieve a hardness ratio and thickness ratio that enhances bendability and delayed fracture resistance, while maintaining a tensile strength of 1270 MPa or more.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If dual phase steel sheets with soft ferrite and hard martensite are used to achieve high strength and high workability, then ductility is improved, but bendability deteriorates and tensile strength cannot exceed 1270 MPa
Solution Approach 1:
The patent applies local quality by creating a dual-layer microstructure with a soft superficial layer (ferrite-based) and a hard core layer (martensite-based). The soft superficial layer reduces surface stress during bending to improve bendability, while the hard core layer provides the required tensile strength exceeding 1270 MPa. This spatial differentiation of material properties resolves the contradiction between strength and bendability.
Solution Approach 2:
The patent creates a composite microstructure combining two distinct phases: a ferrite-based soft superficial layer and a martensite-based hard core layer. This composite structure integrates the advantages of both phases - the soft phase improves bendability by reducing surface stress, while the hard phase provides ultra-high tensile strength above 1270 MPa, resolving the contradiction between these two properties.
2Ease of operation
If a soft superficial layer is created to improve bendability by reducing tensile and compressive stress, then bendability is improved, but fatigue properties deteriorate due to decreased surface hardness
Solution Approach 1:
The patent applies local quality by creating a soft superficial layer with specific characteristics (ferrite-based microstructure, controlled thickness ratio of 10-30%) that improves bendability through stress reduction, while the hard core layer (martensite-based) maintains overall structural integrity and fatigue resistance. The localized softness is confined to the superficial layer, preventing widespread fatigue issues.
Solution Approach 2:
The soft superficial layer acts as a cushioning layer that absorbs and distributes surface stresses during bending operations before they reach the harder core layer. This beforehand cushioning effect protects the core structure from stress concentrations that would otherwise lead to fatigue failure, thereby maintaining fatigue properties despite the reduced surface hardness.
3Strength
If retained austenite content is increased to 10 vol% or more in the core layer to achieve high strength, then strength is improved, but martensite forms during forming causing voids and cracks that reduce bendability
Solution Approach 1:
The patent applies local quality by creating distinct microstructural zones: the core layer contains 10-20% retained austenite for high strength, while the superficial layer is ferrite-based for ductility and bendability. This spatial separation allows the core to provide strength while the superficial layer prevents crack formation during forming, resolving the contradiction between strength and bendability.
Solution Approach 2:
The patent creates a composite microstructure where the core layer (martensite with 10-20% retained austenite) provides high strength, and the superficial layer (ferrite-based) provides ductility and crack resistance. This composite structure allows the retained austenite to contribute to strength without causing the adverse effects of excessive austenite content, as the superficial ferrite layer prevents void and crack formation during forming operations.
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 enables the production of steel sheets with improved bendability and delayed fracture resistance, suitable for forming difficult parts like automobile structural members, contributing to weight reduction and safety enhancement in vehicles.
Implementation Method 1
it has been known that the provision of a soft superficial layer reduces the tensile stress and the compressive stress applied to the surface when the steel sheet is worked by bending, thereby improving bendability
Implementation Method 2
the steel sheet superficial soft portion containing tempered-martensite at a volume fraction of not less than 90%, the microstructure of the steel sheet core portion including tempered-martensite, the ultra high strength cold rolled steel sheet having a tensile strength of not less than 1270 MPa
Data Source
AI summary
The invention provides an ultra high strength cold rolled steel sheet with a small thickness which exhibits excellent bendability and delayed fracture resistance. The ultra high strength cold rolled steel sheet with excellent bendability contains C at 0.15 to 0.30%, Si at 0.01 to 1.8%, Mn at 1.5 to 3.0%, P at not more than 0.05%, S at not more than 0.005%, Al at 0.005 to 0.05% and N at not more than 0.005%, with the balance being represented by Fe and inevitable impurities, and has a steel sheet superficial soft portion satisfying the following equations: HvS/HvC≤0.8 wherein Hv(S) is the hardness of the steel sheet superficial soft portion, and Hv(C) is the hardness of a steel sheet core portion, 0.10≤tS/t≤0.30 wherein t(S) is the thickness of the steel sheet superficial soft portion, and t is the sheet thickness, the steel sheet superficial soft portion containing tempered-martensite at a volume fraction of not less than 90%, the microstructure of the steel sheet core portion including tempered-martensite, the ultra high strength cold rolled steel sheet having a tensile strength of not less than 1270 MPa.