Recovered TWIP Steel Sheet Balancing Strength and Formability
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Solution Overview
Problem
Existing TWIP steels face challenges in achieving high strength and formability, with mechanical properties such as Ultimate Tensile Strength (UTS) and yield stress (YS) not meeting automotive application requirements, while maintaining sufficient elongation and formability.
Innovation Solution
A cold-rolled and recovered TWIP steel sheet with an austenitic matrix, comprising specific chemical compositions and a manufacturing process involving reheating, hot rolling, coiling, multiple cold-rolling stages, and recovery heat treatment, to enhance mechanical properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steels are used to improve mechanical properties, then strength increases, but elongation and formability decrease
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.15-0.50%, Si: 0.01-3.00%, Mn: 1.00-5.00%, P: 0.01-0.05%, S: 0.01-0.03%, Al: 0.01-0.050%, Ti: 0.003-0.100%, V: 0.005-0.500%, Nb: 0.005-0.500%, B: 0.0005-0.0050%) and processing parameters (recrystallized fraction >75%, mean grain size <18 μm, coiling temperature 200-580°C) to achieve a balance between strength and formability, resolving the contradiction through optimized material parameters
Solution Approach 2:
The patent creates a composite microstructure consisting of recrystallized austenitic grains with precipitated carbides dispersed throughout. This composite structure combines the ductility of recrystallized grains with the strength contribution from carbide precipitates, achieving both high strength and good formability simultaneously
2Ease of operation
If TWIP steels are used to improve formability, then elongation increases, but Ultimate Tensile Strength and yield stress decrease
Solution Approach 1:
The patent creates a composite microstructure consisting of recrystallized austenitic grains with precipitated carbides dispersed throughout. This composite structure combines the ductility of recrystallized grains with the strength contribution from carbide precipitates, achieving both high strength and good formability simultaneously
Solution Approach 2:
The patent applies local quality by creating regions with different properties: recrystallized grains provide ductility and formability, while locally distributed carbide precipitates provide strength. The non-uniform distribution of carbides throughout the matrix allows different regions to contribute different properties, resolving the strength-formability contradiction
3Ease of operation
If recrystallized fraction is increased to improve formability, then elongation increases, but manufacturing complexity increases
Solution Approach 1:
The patent applies preliminary action by performing hot rolling at high temperature (above 850°C) to ensure complete recrystallization occurs during the rolling process itself, rather than requiring a separate post-processing step. The slab is reheated to 1000-1300°C before rolling, and hot rolling is completed at ≥850°C, ensuring the recrystallized fraction exceeds 75% directly from the hot rolling operation
Solution Approach 2:
The patent merges the recrystallization process with the hot rolling operation. By controlling the hot rolling temperature to be at least 850°C and the final coiling temperature to be 200-580°C, the recrystallization occurs during the rolling process itself, combining two operations (recrystallization heat treatment and rolling) into one continuous process, thereby reducing manufacturing complexity
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 TWIP steel sheet with improved mechanical properties, including high UTS and elongation, suitable for automotive applications.
Implementation Method 1
the recrystallized fraction of the steel being greater than 75%
Implementation Method 2
having an austenitic matrix comprising by weight
Data Source
AI summary
A cold rolled and recovered TWIP steel sheet is provided having an austenitic matrix including by weight: 0.71<C<1.2%, 13.0≤Mn<25.0%, S≤0.030%, P≤0.080%, N≤0.1%, 0.1≤Si≤3.0%, 0.1≤V≤2.50%, and on a purely optional basis, one or more elements such as Cu≤5.0%, Al≤4.0%, Nb≤0.5%, B≤0.005%, Cr≤1.0%, Mo≤0.40%, Ni≤1.0%, Ti≤0.5%, 0.06≤Sn≤0.2%, the remainder of the composition being made of iron and inevitable impurities resulting from elaboration.