Cold Rolled Steel Coiling Temperature Optimization
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Solution Overview
Problem
Current high strength steel sheets and strips used in the automotive industry face limitations in formability and bending properties, particularly in achieving tensile strengths above 950 MPa, while maintaining excellent formability and reducing susceptibility to grain-boundary oxidation, liquid metal embrittlement, and hydrogen embrittlement.
Innovation Solution
A cold rolled steel composition with specific alloying elements (C, Mn, Cr, Si, Al, Mn+Cr, and optional elements like Mo, Nb, V, Ti, Ca, and controlled impurities) and microstructure (40% tempered martensite, 10-30% fresh martensite, 2-20% retained austenite, and 10-35% polygonal ferrite) that allows for industrial-scale production in Continuous Annealing and Hot Dip Galvanizing Lines, enhancing bending properties and phosphatability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a low coiling temperature of 450°C is used to achieve good phosphonation coverage, then phosphonation coverage is improved, but cold rolling forces increase
Solution Approach 1:
The patent changes the coiling temperature parameter to a specific range (450-550°C) to optimize phosphonation coverage while managing cold rolling forces through controlled microstructure development
Solution Approach 2:
The patent utilizes phase transition during coiling and subsequent annealing to create a multi-phase microstructure (ferrite, bainite, martensite, retained austenite) that balances phosphonation coverage with acceptable rolling forces
2Strength
If high strength steel with tensile strength ≥950 MPa is produced, then strength is improved, but formability and bending properties deteriorate
Solution Approach 1:
The patent creates a composite microstructure containing four distinct phases (ferrite, bainite, martensite, and retained austenite), where each phase contributes different properties: ferrite provides ductility, bainite provides strength, martensite provides hardness, and retained austenite provides TRIP effect for improved formability
Solution Approach 2:
The patent creates local variations in microstructure with specific phase distributions and grain sizes that optimize both strength and formability in different regions of the steel sheet
3Strength
If conventional high strength steels are used, then strength is achieved, but TRIP effect and work hardening capability are lost
Solution Approach 1:
The patent utilizes the TRIP (Transformation Induced Plasticity) effect by maintaining metastable retained austenite (10-30% volume fraction) that transforms to martensite during deformation, providing exceptional work hardening and delaying necking
Solution Approach 2:
Instead of fully transforming austenite to martensite for maximum strength, the patent inverts the approach by retaining a significant portion of austenite in a metastable state that transforms during deformation, achieving both strength and formability
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 achieves a tensile strength of at least 950 MPa with improved formability, reduced grain boundary oxidation, and enhanced bending properties, along with reduced susceptibility to embrittlement and improved phosphatability, facilitating the production of complex high-strength structural members.
Implementation Method 1
When the steel is deformed, the austenite transforms into martensite, which results in remarkable work hardening
Implementation Method 2
The hot rolled strip is thereafter coiled. The coiling resistance is reduced with increasing temperature. Commonly a coiling temperature of 600° C. is employed
Implementation Method 3
The coiled strip is thereafter batch annealed, followed by cold rolling
Data Source
AI summary
A cold roll strip or sheet includes in (wt %): C 0.12-0.20; Mn 1.9-2.6; Cr 0.15-0.3; Si 0.3-0.8; Al 0.8-1.2; Mn+Cr 1.8-5; Nb≤0.008; Ti≤0.02; Mo≤0.08; Ca≤0.005; V≤0.02; and balance Fe apart from impurities. The steel is within the area defined by the coordinates A, B, C, D, where Ri/t (y-axle) is plotted vs TS (MPa)/YR (x-axle), and where A is [2200, 3.5], B is [2600, 4.5], C is [2600, 3], and D is [2200, 2].