Cold Rolled Steel Strip Austempering for High Strength Formability
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Solution Overview
Problem
Current continuous production lines are limited by maximum annealing temperatures, cooling rates, and overaging times, preventing the achievement of a desired combination of high tensile strength and ductility in cold rolled steel strips, particularly for automotive applications.
Innovation Solution
A heat treatment method involving soaking, controlled cooling, and austempering within specific temperature ranges and time frames, combined with a unique steel composition, to create a microstructure comprising polygonal ferrite, acicular ferrite, and bainitic ferrite, with controlled amounts of retained austenite and martensite, suitable for existing production lines.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional annealing temperatures and overaging times are used, then production lines can operate within existing equipment limits, but the desired combination of high tensile strength and ductility cannot be achieved
Solution Approach 1:
The patent applies parameter changes by modifying the steel composition (adding specific elements like Al, Si, Mn, Cr, Mo) and adjusting heat treatment parameters (soaking time, cooling rate, austempering temperature and time) to achieve the desired microstructure and mechanical properties within existing production line capabilities
Solution Approach 2:
The patent employs preliminary action by performing controlled cooling and austempering treatments before final product delivery, creating the beneficial microstructure of bainitic ferrite with retained austenite that provides both high strength and ductility
2Manufacturing precision
If longer overaging time is provided, then complete bainitic transformation can be achieved, but production time is exceeded
Solution Approach 1:
The patent changes the composition parameters of the steel (adding alloying elements) to accelerate bainitic transformation kinetics, allowing complete transformation within shorter times (30-300 seconds) while maintaining precise microstructure control
Solution Approach 2:
The patent replaces reliance on extended time with optimized temperature-time pathways and compositional adjustments, achieving the same microstructural transformation more efficiently through controlled austempering at specific temperatures
3Ease of operation
If higher annealing temperature is applied, then better formability can be achieved, but equipment limitations are exceeded
Solution Approach 1:
The patent changes the steel composition and heat treatment parameters to achieve excellent formability through controlled microstructure formation at temperatures suitable for existing equipment, rather than requiring excessively high annealing temperatures
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 method produces a cold rolled steel strip with yield strength ≥550 MPa, tensile strength ≥980 MPa, total elongation ≥13%, hole expansion capacity ≥20%, and bending angle ≥80°, enhancing formability and weldability for automotive parts.
Implementation Method 1
a soaking step performed in a temperature range of (Ac3−20)−(Ac3+20), preferably for a soaking time t2 of 1−200 seconds, thereby obtaining a cold rolled steel strip having an austenitic microstructure; b) a cooling step performing cooling the soaked steel strip resulting from step a) to a temperature T4 in the range of Bn−Ms
Implementation Method 2
c) an austempering step consisting of heat treating the cooled strip obtained in step b) in a temperature range of Bs−T4, preferably in a temperature range of (Bs−50)−Bn, for a time t5 of 30−300 seconds
Data Source
AI summary
A method of heat treating a high strength cold rolled steel strip includinga) soaking a cold rolled steel strip,b) cooling the soaked steel stripc) heat treating the cooled strip;d) cooling the heat treated steel strip to ambient temperature range;such that the steel strip has a microstructure including various ferrites, retained austenite and martensite. The main components in the steel composition includes carbon, manganese, silicon and aluminium in addition to iron.