Cold-Rolled Steel Microstructure for Edge Cracking Resistance
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Solution Overview
Problem
The production of high-strength, cold-formable flat steel products for vehicle components is challenging due to sensitivity to edge cracks during hot rolling and difficulties in forming complex shapes, especially on an industrial scale.
Innovation Solution
A cold-rolled flat steel product with a specific composition (C: 0.08-0.25%, Al: 3-5.4%, Mn: 9-14%, and a microstructure of 10-60% austenite and 40-90% ferrite) is developed, optimized for high formability and strength, using a method involving controlled hot rolling and annealing processes to achieve a high n-value of at least 0.21 and austenite grain size of 0.85-3 µm.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel composition is used to achieve high tensile strength, then strength is improved, but edge cracking sensitivity increases during hot rolling
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.08-0.25%, Al: 3-5.4%, Mn: 9-14%, B: 0.003-0.1%, Cr: 0.01-2%, Si: 0.01-0.5%) and processing parameters (hot rolling temperature 850-1050°C, cold rolling reduction 20-80%, annealing temperature 900-1100°C) to achieve the desired microstructure and mechanical properties while minimizing edge cracking
Solution Approach 2:
The patent creates a composite microstructure consisting of austenite and ferrite phases with specific proportions (austenite: 10-60%, ferrite: 40-90%) and controlled grain sizes (austenite grain size: 0.85-3 µm) to achieve both high strength and resistance to edge cracking during hot rolling
2Adaptability or versatility
If high degree of deformation is applied to form complex shapes, then formability is improved, but edge cracking occurs during hot rolling
Solution Approach 1:
The patent changes the composition parameters by adding specific alloying elements (Al: 3-5.4%, Mn: 9-14%, B: 0.003-0.1%, Cr: 0.01-2%, Si: 0.01-0.5%) that modify the steel's mechanical properties and resistance to cracking, enabling high degree of deformation without edge cracking
Solution Approach 2:
The patent creates a composite microstructure with austenite and ferrite phases that provides both formability and cracking resistance, allowing the steel to be deformed into complex shapes without developing edge cracks during hot rolling
3Ease of manufacture
If conventional steel composition is used, then production is simpler, but mechanical properties are insufficient for vehicle components
Solution Approach 1:
The patent systematically adjusts multiple composition parameters (C, Al, Mn, B, Cr, Si) and processing parameters to achieve the optimal balance between manufacturability and mechanical properties, transforming conventional steel into high-performance cold-rolled steel suitable for vehicle components
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 provides a reliable, high-strength, and formable flat steel product with reduced edge cracking and enhanced elongation, suitable for complex component formation in vehicle and other applications, while maintaining a balance of mechanical properties.
Implementation Method 1
the microstructure of the flat steel product consists of 10 - 60 area% austenite and 40 - 90 area% ferrite
Implementation Method 2
Mn: 9 - 14%, Al: 3 - 5.4%... consists of 10 - 60 area% austenite and 40 - 90 area% ferrite
Data Source
AI summary
The invention provides a high-strength, cold-rolled flat steel product which is highly reliable in satisfying the practical requirements for its usability, has an n value of at least 0.21, and can be reliably produced on an industrial scale. To this end, the flat steel product is made of a steel which consists of (in wt%) C: 0.08 - 0.25 %, Al: 3 - 5.4 %, Mn: 6 - 14 %, B: 0 - 0.1 %, Cr: 0 - 2 %, Si: 0 - 0.4 %, P: 0 - 0.1 %, S: 0 - 0.3 %, Ta: 0 - 0.5 %, W: 0 - 0.5 %, Ni: 0 - 2 %, Cu: 0 - 2 %, Ca: 0 - 0.15 %, N: 0 - 0.02 % Co: 0 - 2 % and one or more elements from the group "Ti, Nb, V and Mo", with the proviso that the sum of the contents of these elements is 0.05 - 1 %, and/or one or more elements from the group "Zr, La, Ce and Y", with the proviso that the sum of the contents of these elements is 0.05 - 0.3 %, and consists of residual iron and unavoidable impurities, wherein %Mn/%Al > 1.2 applies to the ratio %Mn/%Al, and 3 < Aleq < 8 applies to Aleq = %Al + 0.4 x (%Si)3 - 3 x (%Si)2 + 8.3 x %Si, where %Mn, %Al and %Si are the Mn-, Al-, and Si-contents of the steel, and the microstructure of 10 - 60 area% of the flat steel product is austenitic and 40 - 90 area% is ferritic with an austenitic particle mean size of 0.85 - 3 μm. In addition, the invention provides a method for producing such a flat steel product, said method comprising final annealing as the fundamental work step, carried out as continuous annealing over 20 s - 10 min at 950 - 1070°C or as box annealing over 0.5 - 60 h at > 850 - 950°C.