Cold Rolled Steel Strip Heat Treatment on Conventional Annealing Lines
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Solution Overview
Problem
Current continuous production lines are limited by maximum annealing temperatures, cooling rates, and overageing times, preventing the production of high strength cold rolled steel strips with desired combinations of tensile strength, total elongation, and yield strength, particularly for automotive applications.
Innovation Solution
A heat treatment method involving soaking above Ac3−60 for 1-150 seconds, cooling to a specific temperature range, and treating at Bs-T4 for 30-300 seconds, followed by cooling to ambient temperature, to achieve a microstructure comprising polygonal ferrite, acicular ferrite, and bainitic ferrite, with controlled compositions of carbon, silicon, manganese, and other elements, allowing production on conventional lines.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If complex microstructures (CP steels including ferrite, bainite, martensite and retained austenite) are used to achieve high tensile strength above 800 MPa, then tensile strength is improved, but stretch flange formability deteriorates due to deformation capability differences between phases
Solution Approach 1:
The patent changes the microstructural parameters by controlling phase compositions and transformations during heat treatment. Specifically, it transforms the complex four-phase microstructure into a simplified two-phase microstructure consisting of ferrite and martensite through controlled cooling and phase transformations, thereby improving stretch flange formability while maintaining high tensile strength
Solution Approach 2:
The patent utilizes phase transitions during heat treatment to transform retained austenite and bainite into martensite or ferrite. By controlling the cooling process and holding temperatures, the microstructure evolves from a complex multi-phase state to a simplified ferrite-martensite dual phase structure, resolving the formability issue while preserving strength
2Productivity
If current continuous production lines are used with fixed cooling rates and limited overageing time (less than 160 seconds), then production efficiency is maintained, but the desired combination of high tensile strength and high total elongation cannot be achieved
Solution Approach 1:
The patent introduces dynamic control of cooling rates and heat treatment parameters. Instead of fixed cooling rates, the process uses variable cooling rates adapted to the specific microstructural evolution requirements. The overageing time is dynamically optimized based on the desired microstructure, allowing completion of phase transformations within the production line constraints while achieving target mechanical properties
Solution Approach 2:
The patent changes the process parameters by optimizing the heat treatment temperature profile and cooling rates to match the capabilities of existing production lines. By adjusting the soaking temperature, cooling rate, and overageing time within the available window (up to 160 seconds), the process achieves the desired microstructure and mechanical properties without requiring new equipment
3Temperature
If maximum annealing temperature is limited to 890°C in current annealing lines, then equipment constraints are respected, but the expected beneficial combination of strength and ductility properties cannot be obtained
Solution Approach 1:
The patent changes the approach by optimizing the heat treatment parameters within the existing temperature limit of 890°C. Instead of requiring higher temperatures, the process achieves the desired microstructure through optimized soaking time, controlled cooling rates, and precise overageing parameters. The phase transformations are controlled to produce the target ferrite-martensite microstructure without exceeding equipment temperature capabilities
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 tensile strength ≥850 MPa, total elongation ≥14%, and yield strength ≥500 MPa, suitable for automotive applications, while utilizing existing production lines and ensuring a balanced microstructure for enhanced formability and weldability.
Implementation Method 1
a) soaking a cold rolled steel strip above (Ac3−60) for a soaking time t2 of 1-150 seconds, thereby obtaining a cold rolled steel strip having an at least partially austenitic microstructure; b) cooling the soaked steel strip resulting from step a) to a temperature T4 in the range of Bn-Ms
Implementation Method 2
a) soaking a cold rolled steel strip above (Ac3−60) for a soaking time t2 of 1-150 seconds
Implementation Method 3
b) cooling the soaked steel strip resulting from step a) to a temperature T4 in the range of Bn-Ms; d) cooling the heat treated steel strip to ambient temperature
Data Source
AI summary
A heat treatment of a high strength cold rolled steel strip includes the steps ofa) 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 include carbon, manganese, silicon and aluminium in addition to iron.