Cold-Rolled Steel Sheet With Mn-Partitioned Microstructure for Formability
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Solution Overview
Problem
Existing high-strength steel sheets lack a balance of tensile strength, ductility, stretch flangeability, and bendability, as previous technologies have not adequately addressed these properties simultaneously.
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 and controlled cooling rates, resulting in a microstructure with finely dispersed ferrite phases and optimized hardness distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel sheet is produced with tensile strength of 980 MPa or more, then strength is improved, but ductility, stretch flangeability, and bendability deteriorate
Solution Approach 1:
The patent applies local quality by creating different Mn concentrations in different phases: high Mn concentration in retained austenite and low Mn concentration in ferrite phase. This non-uniform distribution allows the material to simultaneously achieve high strength (from martensite transformation) and excellent formability (from the soft low-Mn ferrite phase), resolving the contradiction between strength and ductility
Solution Approach 2:
The patent utilizes parameter changes by controlling the Mn concentration distribution through specific heating and cooling processes. By adjusting temperature parameters during annealing and cooling rates, the Mn partitioning between phases is controlled to achieve the optimal balance between strength and formability properties
2Strength
If Mn concentration in ferrite phase is increased to improve strength, then tensile strength is improved, but stretch flangeability deteriorates
Solution Approach 1:
The patent resolves this contradiction by creating spatial variation in Mn concentration: the ferrite phase has low Mn concentration (0.8×[%Mn] or less) providing softness and stretch flangeability, while the retained austenite phase has high Mn concentration providing strength through martensite transformation. This local quality differentiation allows simultaneous optimization of both properties
3Ease of operation
If ferrite phase is refined to improve bendability, then bendability is improved, but control of microstructure complexity increases
Solution Approach 1:
The patent achieves fine ferrite dispersion and improved bendability through controlled parameter changes during heating and cooling processes. By adjusting heating temperatures, holding times, and cooling rates, the microstructure is precisely controlled to achieve fine dispersed ferrite without requiring overly complex processing equipment
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 achieves a steel sheet with tensile strength of 980 MPa or more, excellent ductility, 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
Implementation Method 3
The ferrite high in Mn concentration is harder than the ferrite low in Mn concentration. This hard ferrite is sandwiched between the soft ferrite low in Mn concentration and the hard bainite or tempered martensite, and thus has the effect of reducing the difference in hardness between soft phase (ferrite high in Mn concentration) and hard phase (bainite or tempered martensite)
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 30% or more and 60% or less; a total area fraction of tempered martensite and bainite is 35% or more and 65% 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 40% 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.