Hot-Rolled Steel Sheet Composition for Low-Anisotropy High Strength
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Solution Overview
Problem
High-strength steel sheets face issues with reduced formability, hole expandability, and anisotropy of strength due to hot rolling, which affect their suitability for automotive applications.
Innovation Solution
A hot rolled steel sheet with a triplex structure comprising ferrite, bainite, and martensite in specific ratios, combined with TiC precipitates in the ferrite phase, enhances strength and reduces anisotropy by precipitation strengthening.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of the steel sheet is increased, then collision safety is improved, but formability and ductility fall
Solution Approach 1:
The patent changes the microstructural parameters by controlling the formation of a triplex structure with specific phase ratios (ferrite: 60-80%, bainite: 15-30%, martensite: 3-10%) and controlling precipitate characteristics (TiC precipitates with diameter of 1.0 to 5.0 nm and number density of 1.0×10^16 to 100.0×10^16/cm³). This allows achieving high strength (tensile strength of 780 MPa or more) while maintaining good formability and ductility.
2Strength
If the strength of the steel sheet is increased, then collision safety is improved, but hole expandability falls
Solution Approach 1:
The patent optimizes the microstructural parameters including the triplex structure phase distribution and precipitate characteristics (TiC precipitates with controlled diameter and number density) to achieve high strength while maintaining hole expandability of 50% or more, resolving the contradiction between strength and hole expandability.
3Strength
If hot rolling is performed to produce high strength steel sheet, then strength is improved, but anisotropy of strength increases
Solution Approach 1:
The patent controls the microstructural parameters through precise composition control and cooling rate management to form a uniform triplex structure with specific phase ratios. This results in reduced anisotropy of strength (L direction tensile strength/C direction tensile strength ≤ 1.15) while maintaining high strength of 780 MPa or more.
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 steel sheet achieves a tensile strength of 780 MPa or more with improved hole expandability and reduced anisotropy, enhancing its workability and suitability for automotive components.
Implementation Method 1
by making TiC precipitates having a suitable diameter in the ferrite be present in a predetermined number density, it is possible to strengthen the ferrite by precipitates
Data Source
AI summary
Provided is a hot rolled steel sheet having a predetermined chemical composition, and a microstructure comprising, by area ratio, ferrite: 60 to 80%, bainite: 15 to 30%, and martensite: 3 to 10%, wherein TiC precipitates having a diameter of 1.0 to 5.0 nm are present in the ferrite in a number density of 1.0×1016 to 100.0×1016/cm3, and the hot rolled steel sheet has a tensile strength of 780 MPa or more tensile strength.