Steel Sheet Microstructure for Strength and Uniformity
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Solution Overview
Problem
Existing high strength steel sheets face challenges in achieving both high tensile strength of 1,180 MPa or more and excellent material uniformity, particularly in maintaining consistent mechanical properties across the sheet width direction.
Innovation Solution
A high strength steel sheet with a specific chemical composition, including C, Si, Mn, P, S, Al, N, and Ti, and a microstructure mainly composed of ferrite, bainite, and martensite, is developed. The chemical composition ensures a tensile strength of 1,180 MPa or more, while controlling the area fraction of non-recrystallized ferrite to 5% or less across the sheet width direction maintains material uniformity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of steel sheet is increased to reduce body weight, then tensile strength is improved, but ductility decreases
Solution Approach 1:
The steel sheet employs a composite microstructure consisting of multiple phases (ferrite, bainite, and martensite) in specific proportions. This multi-phase composite structure enables the material to achieve high tensile strength (1180 MPa or more) while maintaining adequate ductility, as each phase contributes different mechanical properties that complement each other
Solution Approach 2:
The invention controls the local distribution of microstructural phases across the sheet width by limiting the difference in non-recrystallized ferrite area fraction to 5% or less between center and edge portions. This local quality control ensures uniform mechanical properties and ductility throughout the sheet, preventing localized variations that would compromise overall performance
2Strength
If high strength steel sheet is used, then tensile strength is improved, but material uniformity deteriorates due to great variation in strength and formability
Solution Approach 1:
The invention establishes specific parameter ranges for chemical composition (C: 0.15-0.35%, Si: 0.01-3.0%, Mn: 1.0-3.5%, Ti: 0.01-0.10%) and microstructural phase fractions (ferrite: 10-50%, bainite: 10-40%, martensite: 30-60%) to achieve both high strength (1180 MPa or more) and material uniformity. These controlled parameters ensure reproducible mechanical properties across the sheet
Solution Approach 2:
The invention achieves homogeneity in material composition and microstructure by controlling the distribution of phases across the sheet width. The limitation of non-recrystallized ferrite area fraction difference to 5% or less between center and edge portions ensures uniform mechanical properties and formability throughout the sheet, eliminating the variability problem associated with high strength steels
3Strength
If high strength steel sheet is used, then tensile strength is improved, but shape fixation ability deteriorates due to low reproducibility of shape fixation
Solution Approach 1:
The invention ensures uniform shape fixation ability across the sheet by controlling the local microstructural homogeneity. The limitation of non-recrystallized ferrite area fraction difference to 5% or less between center and edge portions guarantees consistent formability and shape retention throughout the sheet, preventing localized deformation variations
Solution Approach 2:
The multi-phase composite microstructure (ferrite, bainite, martensite) provides a balance between strength and formability. The specific phase proportions and their uniform distribution enable the steel to achieve high strength while maintaining adequate ductility and shape fixation ability during forming operations
Data Source
AI summary
A steel sheet and a member and methods for manufacturing them are disclosed. The steel sheet has a certain chemical composition and a steel sheet microstructure which contains, in terms of area fraction, ferrite: 5% or more and 50% or less, and a total of martensite and bainite: 50% or more and 95% or less, non-recrystallized ferrite in the ferrite is 0% or more and 10% or less, in terms of area fraction, based on the entire microstructure, and a difference between a maximum value and a minimum value of the area fraction of the non-recrystallized ferrite in a sheet width direction of the steel sheet is 5% or less.