High-Strength Cold-Rolled Steel Sheet for Formability Balance
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Solution Overview
Problem
Existing high-strength steel sheets do not adequately balance tensile strength, ductility, stretch flangeability, and bendability, as previous technologies neglect stretch flangeability and bendability.
Innovation Solution
A high-strength cold-rolled steel sheet with a specific chemical composition and microstructure is produced through controlled heating and cooling processes, including a ferrite-austenite dual phase region annealing, followed by controlled cooling and reheating, to achieve a microstructure with finely dispersed ferrite phases and optimized hardness distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel sheets are produced to achieve tensile strength of 1080 MPa or more, then strength is improved, but ductility, stretch flangeability, and bendability deteriorate
Solution Approach 1:
The invention applies local quality by creating distinct regions with different Mn concentrations within the steel sheet microstructure. Specifically, it forms a dual-phase structure consisting of ferrite regions (with lower Mn concentration of 0.8×[% Mn] or less) and austenite regions (with higher Mn concentration of more than 0.8×[% Mn]). This spatial differentiation of chemical composition allows different regions to contribute differently to mechanical properties: the ferrite regions provide ductility and formability, while the austenite regions contribute to strength, thereby resolving the contradiction between high tensile strength and excellent formability.
Solution Approach 2:
The invention employs composite materials principle by creating a microstructural composite of ferrite and austenite phases with distinct chemical compositions. The steel sheet contains both ferrite regions (low Mn concentration) and austenite regions (high Mn concentration) in a controlled distribution. This composite microstructure combines the advantageous properties of each phase: ferrite provides softness, ductility, and formability, while austenite provides strength and hardenability, achieving both high tensile strength (1080 MPa or more) and excellent formability simultaneously.
2Strength
If Mn concentration is increased to improve strength, then tensile strength is improved, but stretch flangeability deteriorates
Solution Approach 1:
The invention resolves this contradiction by applying local quality through spatial differentiation of Mn concentration. Instead of uniformly increasing Mn concentration throughout the steel sheet (which would improve strength but harm stretch flangeability), the invention creates localized regions with different Mn concentrations: ferrite regions with low Mn concentration (0.8×[% Mn] or less) that maintain stretch flangeability, and austenite regions with high Mn concentration (more than 0.8×[% Mn]) that provide strength. This localized compositional variation allows both properties to be optimized in their respective regions.
3Ease of operation
If ferrite grain size is reduced to improve bendability, then bendability is improved, but strength may deteriorate
Solution Approach 1:
The invention applies local quality by creating ferrite regions with specifically controlled low Mn concentration (0.8×[% Mn] or less) that promote fine grain formation. These low-Mn ferrite regions provide excellent bendability due to their fine grain structure and softness, while the overall strength is maintained by the presence of high-Mn austenite regions and the composite microstructure. Thus, bendability is improved locally in ferrite regions without sacrificing overall strength.
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 results in a steel sheet with tensile strength of 1080 MPa or more, excellent ductility, and improved stretch flangeability and bendability, suitable for automotive applications.
Implementation Method 1
When a steel sheet containing Mn is annealed in the ferrite-austenite dual phase region, distribution of element (Mn distribution) occurs in which the Mn concentration in ferrite phase decreases whereas the Mn concentration in austenite phase increases
Implementation Method 2
When the steel sheet in which the Mn distribution has occurred is cooled at an appropriate cooling rate, austenite newly transforms into ferrite around ferrite low in Mn concentration as nuclei. Since the Mn concentration of the ferrite newly formed as a result of the transformation during cooling maintains the Mn concentration of the austenite before the transformation, the ferrite formed is high in Mn concentration
Implementation Method 3
A high-strength cold-rolled steel sheet with a specific chemical composition and microstructure is produced through controlled heating and cooling processes, including a ferrite-austenite dual phase region annealing, followed by controlled cooling and reheating
Data Source
AI summary
A high-strength cold-rolled steel sheet comprises: a chemical composition that contains C, Si, Mn, P, S, N, Al, Ti, Nb, and B with a balance consisting of Fe and inevitable impurities, and satisfies [mol % N]/[mol % Ti]<1; and a steel microstructure in which: an area fraction of ferrite is 12% or more and less than 30%; a total area fraction of tempered martensite and bainite is 55% or more and 85% or less; an area fraction of quenched martensite is 15% or less; an area fraction of retained austenite is 1% or more and 10% or less; an area fraction of low-Mn ferrite having a Mn concentration of 0.8×[% Mn] or less is 5% or more and 20% or less; a result of subtracting the area fraction of the low-Mn ferrite from the area fraction of the ferrite is 10% or more; an area fraction of a residual microstructure is less than 3%; and an average grain size of the low-Mn ferrite is 10 μm or less.