Cold Rolled Steel with Retained Austenite for High Strength Formability
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Solution Overview
Problem
Conventional high strength steels lack sufficient formability to produce complex structural members in automotive applications, and existing TRIP steels face issues with weldability due to high carbon content, while requiring specific processing routes to stabilize austenite, which can be impractical for industrial-scale production.
Innovation Solution
A cold rolled steel with a composition of C0.15-0.25%, Si0.7-1.4%, Mn2.3-3.2%, and controlled microstructure, including retained austenite in a tempered martensite matrix, optimized for high tensile strength, formability, and weldability, processed in a Continuous Annealing Line to achieve a Hole Expanding Ratio (HER) of at least 40% and suitable for Resistance Spot Welding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high carbon content (approximately 1 wt. %) is used to stabilize austenite at room temperature, then formability and ductility are improved through the TRIP effect, but weldability is impaired
Solution Approach 1:
The patent applies local quality by concentrating carbon specifically within the retained austenite phase rather than having uniform high carbon content throughout the steel. This is achieved through controlled cooling and transformation processes that partition carbon into the austenite during phase transformation, allowing high carbon content (0.8-1.5 wt. %) to be localized where it stabilizes austenite for the TRIP effect, while the overall steel maintains weldability because the carbon is not uniformly distributed throughout the matrix
Solution Approach 2:
The patent utilizes parameter changes by controlling the cooling rate and transformation temperature to achieve specific microstructural conditions. By adjusting the cooling rate (1-100°C/s) and holding temperature (150-450°C), the patent transforms the material's phase composition dynamically, creating a multi-phase microstructure with retained austenite (10-40 vol. %) that provides both high formability through TRIP effect and acceptable weldability, thus changing the physical state parameters to resolve the contradiction
2Weight of moving object
If conventional high strength steels are used to reduce car body mass, then fuel consumption is reduced, but formability is insufficient for producing complex structural members
Solution Approach 1:
The patent applies composite materials by creating a multi-phase microstructure consisting of tempered martensite matrix with dispersed retained austenite islands (10-40 vol. %). This composite microstructure combines the high strength of martensite with the ductility and TRIP effect of retained austenite, enabling the steel to achieve both high tensile strength (≥1380 MPa) and excellent formability, thus allowing production of complex structural members at reduced car body mass
Solution Approach 2:
The patent utilizes spheroidality by controlling the morphology of retained austenite to be spherical or near-spherical islands dispersed in the martensite matrix. This spherical morphology is critical for optimizing the TRIP effect during deformation, as spherical austenite particles transform more uniformly and provide better work hardening compared to irregular shapes, thereby improving formability while maintaining high strength for lightweight construction
3Weight of moving object
If high strength levels are achieved in steel, then car body mass can be reduced, but local elongation and workability deteriorate in advanced forming operations
Solution Approach 1:
The patent applies dynamics by creating a microstructure that dynamically transforms during deformation. The retained austenite (10-40 vol. %) remains metastable until deformation occurs, at which point it transforms to martensite through the TRIP effect. This dynamic phase transformation provides progressive work hardening that maintains high local elongation (≥10 %) even in high strength steel (≥1380 MPa), enabling advanced forming operations like bending and roll forming to be performed successfully
Solution Approach 2:
The patent applies preliminary action by pre-establishing the multi-phase microstructure with controlled retained austenite content (10-40 vol. %) and distribution before the forming operation. The steel is processed through controlled cooling and isothermal holding to create the optimal phase composition and morphology in advance, so that during subsequent forming operations, the TRIP effect can immediately activate to provide the necessary local elongation and workability, preventing edge tearing and forming defects
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 achieves a tensile strength of at least 1380 MPa with excellent formability and weldability, enabling the production of complex structural members with high local elongation and improved workability, particularly in advanced forming operations like bending and roll forming, while maintaining good weldability.
Implementation Method 1
When the steel is deformed, the austenite transforms into martensite, which results in remarkable work hardening. This hardening effect acts to resist necking in the material and postpones failure in sheet forming operations.
Implementation Method 2
The invention aims at providing a steel having a composition and microstructure that can be processed to complicated high strength structural members, where the local elongation is of importance. The steel strip or sheet is processed in a Continuous Annealing Line.
Data Source
AI summary
The invention relates to a cold rolled steel strip or sheet having a composition consisting of (in wt. %): balance Fe apart from impurities, the cold rolled steel has a multiphase microstructure comprising a matrix mainly composed of tempered martensite and has a tensile strength (Rm) of at least 1380 MPa.