Cold Rolled TBF Steel Sheet for High Strength and Formability
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional high strength steels are not suitable for producing complex automotive body parts due to low formability, and existing TRIP steels face issues with weldability and surface quality, making it difficult to achieve a tensile strength of at least 980 MPa in a conventional industrial annealing line.
Innovation Solution
A cold rolled TBF steel sheet with a specific composition (C: 0.15-0.18%, Mn: 2.2-2.4%, Si: 0.7-0.9%, Cr: 0.1-0.35%, Si + 0.8Al + Cr: 0.5-1.8%, and optional elements) that stabilizes austenite and optimizes transformation temperatures for improved formability and processability in industrial annealing lines.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high carbon content (approximately 1 wt. %) is used to stabilize austenite at room temperature, then the formability and tensile strength are improved, but the weldability is impaired
Solution Approach 1:
The patent changes the chemical composition parameters by reducing carbon content from approximately 1 wt. % to 0.23-0.35 wt. % and adjusting alloying element contents (Mn: 1.50-2.50 wt. %, Si: 0.10-1.00 wt. %, Cr: 0.05-1.00 wt. %, Mo: 0.05-0.50 wt. %) to achieve austenite stabilization through alternative mechanisms, thereby maintaining formability while improving weldability
Solution Approach 2:
The patent creates a multi-phase composite microstructure consisting of retained austenite (5-20 vol. %), bainitic ferrite (70-80 vol. %), and martensite (5-15 vol. %), where each phase contributes different properties: austenite provides TRIP effect for formability, bainitic ferrite provides strength, and the composite structure achieves both high tensile strength (≥980 MPa) and good weldability
2Strength
If high silicon content (at least 1 wt. %) is used to inhibit austenite decomposition during bainite transformation, then the formability is improved, but the surface quality of hot rolled steel and coatability of cold rolled steel deteriorate
Solution Approach 1:
The patent reduces silicon content from at least 1 wt. % to 0.10-1.00 wt. % (preferably 0.15-0.50 wt. %) and compensates by optimizing other alloying elements (Mn: 1.50-2.50 wt. %, Cr: 0.05-1.00 wt. %, Mo: 0.05-0.50 wt. %) to maintain austenite stability and TRIP effect while eliminating surface quality degradation and coatability issues
Solution Approach 2:
The patent introduces chromium (0.05-1.00 wt. %) and molybdenum (0.05-0.50 wt. %) as intermediary elements that can substitute for silicon's role in stabilizing austenite during bainite transformation, thereby achieving the desired microstructure without the harmful surface effects associated with high silicon content
3Object-affected harmful factors
If aluminium is used to replace silicon for stabilizing austenite, then the surface quality and coatability are improved, but the transformation temperature (Ac3) increases making full austenitizing in conventional industrial annealing lines very difficult or impossible
Solution Approach 1:
The patent limits aluminium content to 0.01-0.60 wt. % (avoiding high Al additions that raise Ac3) and instead uses optimized combinations of Mn (1.50-2.50 wt. %), Cr (0.05-1.00 wt. %), and Mo (0.05-0.50 wt. %) to control transformation temperatures, enabling full austenitizing in conventional industrial annealing lines (850-950°C) while maintaining good surface quality and coatability
Solution Approach 2:
The patent applies different alloying strategies to different functional requirements: uses low Al content for surface quality, uses Mn-Cr-Mo combination for transformation temperature control, and uses optimized C content (0.23-0.35 wt. %) for austenite stability, achieving multiple objectives through localized compositional optimization
4Strength
If conventional high strength steels are used to achieve high tensile strength, then the strength is improved, but the formability for complex structural parts is too low
Solution Approach 1:
The patent creates a multi-phase composite microstructure with retained austenite (5-20 vol. %), bainitic ferrite (70-80 vol. %), and martensite (5-15 vol. %), where the retained austenite provides TRIP effect for excellent formability during complex shape forming, while the bainitic ferrite and martensite provide high tensile strength (≥980 MPa), achieving both requirements simultaneously
Solution Approach 2:
The patent utilizes the TRIP (Transformation Induced Plasticity) effect where retained austenite transforms to martensite during deformation, providing remarkable work hardening that resists necking and postpones failure in sheet forming operations, thereby enabling excellent formability for complex automotive body parts
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 at least 980 MPa with excellent formability, elongation, and hole expansion ratio, suitable for industrial production and automotive applications.
Implementation Method 1
When the steel is deformed, the austenite transforms into martensite, which results in remarkable work hardening
Implementation Method 2
heating the steel strip to an austenite transformation temperature
Implementation Method 3
cooling the steel strip at a controlled cooling rate
Data Source
AI summary
The present invention relates to high strength cold rolled steel sheet suitable for applications in automobiles, construction materials and the like, specifically high strength steel excellent in formability. In particular, the invention relates to cold rolled steel sheets having a tensile strength of at least 980 MPa and a method for producing such steel sheet.