Cold-Rolled Steel Sheet Microstructure for Strength and Bendability
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Solution Overview
Problem
Existing cold-rolled steel sheets struggle to achieve high strength, uniform elongation, and hydrogen embrittlement resistance while maintaining good bendability, as previous technologies either fail to meet these requirements or compromise one or more of these properties.
Innovation Solution
A cold-rolled steel sheet with a specific chemical composition and controlled metallographic structure, including retained austenite and tempered martensite, along with controlled amounts of solute Si and ferrite, is manufactured through precise hot rolling, cold rolling, annealing, and tempering processes to enhance strength, elongation, and bendability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a hard secondary phase is provided to achieve high strength (1,310 MPa or more), then tensile strength is improved, but bendability deteriorates
Solution Approach 1:
The patent applies local quality by creating different microstructural characteristics at different locations within the steel sheet. The surface layer area (within 25 μm from the surface) is designed with specific properties (ferrite grain size, solute Si content) that differ from the interior (t/4 portion), allowing the surface to have improved bendability while the interior provides high strength through tempered martensite and retained austenite
Solution Approach 2:
The patent segments the steel sheet into distinct regions with different microstructural characteristics: a surface layer area (within 25 μm from surface) with controlled ferrite grains and solute Si, and an interior region (t/4 portion) with tempered martensite and retained austenite. This segmentation allows each region to be optimized for its specific function - surface for bendability, interior for strength
2Stability of the object's composition
If ferrite is provided to improve ductility (uniform elongation), then uniform elongation is improved, but tensile strength cannot reach 1,310 MPa or more
Solution Approach 1:
The patent applies local quality by limiting ferrite to specific regions where it is beneficial. Ferrite is present in the surface layer area to improve ductility and bendability, but is restricted in quantity and grain size in the interior region to maintain high strength. The t/4 portion has controlled ferrite content to ensure both ductility and strength requirements are met
3Strength
If cooled to near room temperature during quenching to achieve high strength, then tensile strength is improved, but retained austenite volume percentage becomes small and uniform elongation cannot be obtained
Solution Approach 1:
The patent applies parameter changes by carefully controlling the cooling rate and temperature profile during the annealing process. By controlling the cooling rate from the austenite region (820°C or higher) and the holding time, the patent achieves the desired balance between retained austenite content (for ductility) and martensite formation (for strength). The specific parameter ranges for cooling rate and holding time are optimized to achieve both high strength and sufficient uniform elongation
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 a tensile strength of 1,310 MPa or more, uniform elongation of 5.0% or more, and a limit bend radius R/t of 5.0 or less, while maintaining excellent hydrogen embrittlement resistance.
Implementation Method 1
an annealing process of heating the cold-rolled steel sheet to a soaking temperature of 820° C. or higher so that an average heating rate up to 750° C. becomes 3.0° C./sec or faster, and holding the cold-rolled steel sheet at the soaking temperature
Implementation Method 2
a tempering process of holding the cold-rolled steel sheet after the post-annealing cooling process at 200° C. or higher and 350° C. or lower for 1 second or longer
Implementation Method 3
suppression of Si partitioning during annealing by controlling cold rolling and annealing conditions
Data Source
AI summary
This cold-rolled steel sheet has a predetermined chemical composition, in which a metallographic structure of a t/4 portion, which is at a ¼ position of a sheet thickness t from a surface of the cold-rolled steel sheet in a sheet thickness direction, includes, by volume percentage, retained austenite: 2.5% or more and 10.0% or less, tempered martensite: 80.0% or more and 97.5% or less, ferrite and bainite: 0.0% or more and 15.0% or less in total, and martensite: 0.0% or more and 3.0% or less, and in a surface layer area at a position 25 μm away from the surface in the sheet thickness direction, an amount of solute Si is 0.30% or more and 1.50% or less by mass %, a volume percentage of ferrite in the metallographic structure is 0.0% or more and 20.0% or less, and a density of ferrite grains having a grain size of 15 μm or more is 0 grains/mm2 or more and 3,000 grains/mm2 or less.