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 challenges in achieving the desired strength and formability, particularly in bending properties, while also being susceptible to grain boundary oxidation, liquid metal embrittlement, and hydrogen embrittlement, and have limitations in phosphatability and recyclability.
Innovation Solution
A cold rolled steel with a specific composition and microstructure, including 50%+ tempered martensite and bainite, 2-20% retained austenite, and limited fresh martensite and polygonal ferrite, carefully selected alloying elements (C, Mn, Cr, Si, Al, and controlled process parameters to reduce grain boundary oxidation and enhance phosphatability, bendability, and resistance to embrittlement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a low coiling temperature of 450°C is employed to achieve good phosphatation coverage, then phosphatability is improved, but cold rolling forces increase
Solution Approach 1:
The patent optimizes the coiling temperature parameter to a specific range (450-550°C) that balances phosphatability improvement with acceptable cold rolling forces. This parameter change resolves the contradiction by finding the optimal temperature window where phosphatation coverage is sufficient while avoiding excessive rolling forces.
2Strength
If high strength levels are achieved through conventional high strength steels, then strength is improved, but formability deteriorates due to too low stretch flangability
Solution Approach 1:
The patent creates a multi-phase microstructure comprising martensite, bainite, and retained austenite. This composite microstructure combines the high strength of martensite with the formability enhancement from retained austenite that undergoes TRIP effect during deformation, thereby resolving the contradiction between strength and formability.
Solution Approach 2:
The patent utilizes the TRIP effect where retained austenite transforms into martensite during plastic deformation. This phase transition during forming operations provides remarkable work hardening that resists necking and postpones failure, thereby improving formability while maintaining high strength levels.
3Productivity
If conventional hot rolling and coiling processes are used, then production efficiency is maintained, but grain boundary oxidation occurs reducing bendability
Solution Approach 1:
The patent employs controlled atmosphere conditions during hot rolling and coiling processes to prevent grain boundary oxidation. By creating an inert or reducing environment, the harmful oxidation is suppressed while maintaining production efficiency, thereby resolving the contradiction between productivity and prevention of harmful factors.
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 susceptibility to embrittlement, and enhanced phosphatability, allowing for the production of complex structural members with superior bending properties and recyclability.
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 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. The cold rolled strip is thereafter continuously annealed.
Data Source
AI summary
A cold roll strip or sheet includes in (wt %) C 0.08-0.28; Mn 1.4-4.5; Cr 0.01-0.5; Si 0.01-2.5; Al 0.01-0.6; Si+Al≥0.1; Si+Al+Cr≥0.4; Nb≤0.008; Ti≤0.02; Mo≤0.08; Ca≤0.005; V≤0.02; 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 [1200, 2), B is [2000, 4], C is [2000, 3], and D is [1200, 1].