Steel Sheet Microstructure for High Strength and Formability
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Solution Overview
Problem
Current technologies face challenges in producing high-strength steel sheets with tensile strength of 980 MPa or higher that also exhibit excellent elongation and hole expansibility, essential for vehicle components, as increasing strength typically deteriorates formability.
Innovation Solution
A cold-rolled steel sheet with a specific chemical composition and microstructure, including ferrite, granular bainite, and martensite, controlled within specific ranges, along with limited upper bainite, lower bainite, residual austenite, and pearlite, to achieve the desired strength and formability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strengthening is applied to increase tensile strength to 980 MPa or higher, then strength is improved, but formability including elongation and hole expansibility is degraded
Solution Approach 1:
The steel sheet's microstructure is segmented into multiple distinct phases (ferrite, granular bainite, martensite) with specific area ratios. This segmentation allows different regions to contribute different properties: ferrite provides ductility and elongation, martensite provides strength, and granular bainite provides a balance between both, achieving TS≥980 MPa while maintaining E1≥10% and λ≥35%
Solution Approach 2:
The patent applies local quality control by specifying different area ratio ranges for each microstructural phase. The ferrite phase (10-40 area%) provides local ductility, martensite (20-60 area%) provides local strength, and granular bainite (5-40 area%) provides local toughness. This localized distribution of microstructural phases enables the steel to achieve both high strength and good formability simultaneously
2Strength
If a composite structure of ferrite and martensite is used to enhance strength and elongation, then strength and elongation are improved, but hole expansibility is degraded due to significant hardness difference
Solution Approach 1:
Granular bainite acts as an intermediary phase between ferrite and martensite. With intermediate hardness and specific microstructural characteristics, granular bainite (5-40 area%) serves as a buffer that reduces the hardness contrast between the soft ferrite and hard martensite phases. This intermediary phase enables hole expansibility λ≥35% while maintaining the strength benefits of the ferrite-martensite composite structure
3Ease of manufacture
If upper bainite or lower bainite structure is used to reduce hardness difference and improve hole expansibility, then hole expansibility is improved, but elongation is degraded due to dislocations and hard cementite
Solution Approach 1:
The patent changes the microstructural parameters by specifying granular bainite with controlled area ratio (5-40%) and controlling the total of upper bainite, lower bainite, residual austenite, and pearlite to less than 15 area%. This parameter control ensures the steel achieves hole expansibility λ≥35% while maintaining total elongation E1≥10% and tensile strength TS≥980 MPa
Data Source
AI summary
A steel sheet according to an aspect of the present invention has a chemical composition within a predetermined range; in which a metallographic structure at a thickness ¼ portion includes, by unit area %, ferrite: 10% or more and less than 50%, granular bainite: 5% or more and less than 50%, and martensite: 20% or more and less than 60%; in the metallographic structure at the thickness ¼ portion, a total of upper bainite, lower bainite, residual austenite, and pearlite is 0% or more and less than 15% by unit area %; at the thickness ¼ portion, a product of an area ratio Vm of the martensite and an average hardness Hv of the martensite is 12,000 to 34,000; and a tensile strength is 980 MPa or higher.