High-Strength Steel Sheet Composition for Formability and Energy Absorption
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Solution Overview
Problem
Existing steel sheets with high tensile strength (TS) and yield stress (YS) for automotive impact energy absorbing members suffer from decreased press formability, particularly ductility, flangeability, and bendability, leading to edge cracking and insufficient energy absorption characteristics.
Innovation Solution
A steel sheet with a specific chemical composition and microstructure, including a surface soft layer, controlled grain boundary lengths, and a galvanized layer, is produced through hot rolling, annealing, and cooling processes to achieve TS of 1180 MPa to 1470 MPa, high YS, and improved formability and energy absorption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the tensile strength and yield stress of the steel sheet are increased to improve impact energy absorption, then the strength and energy absorption characteristics are improved, but the press formability including ductility, flangeability, and bendability decreases, leading to edge cracking and insufficient formability
Solution Approach 1:
The steel sheet employs a dual-layer microstructure with a soft ferrite phase (50-90% area ratio) providing ductility and formability in the bulk material, while a hard martensite phase (10-50% area ratio) provides high strength at critical locations. This local differentiation of material properties allows simultaneous achievement of high tensile strength (1180-1470 MPa) and good press formability
Solution Approach 2:
The invention creates a composite microstructure consisting of two distinct phases: soft ferrite and hard martensite. This composite structure at the micro-scale enables the steel sheet to exhibit both high strength characteristics from the martensite phase and good formability from the ferrite phase, resolving the contradiction between strength and formability
2Use of energy by moving object
If the tensile strength is increased to 780 MPa grade or more for impact energy absorbing members, then the energy absorption capability is improved, but the press formability decreases resulting in low yield due to variations at the time of forming
Solution Approach 1:
The steel sheet employs a dual-layer microstructure with a soft ferrite phase (50-90% area ratio) providing ductility and formability in the bulk material, while a hard martensite phase (10-50% area ratio) provides high strength at critical locations. This local differentiation of material properties allows simultaneous achievement of high tensile strength (1180-1470 MPa) and good press formability
Solution Approach 2:
The invention creates a composite microstructure consisting of two distinct phases: soft ferrite and hard martensite. This composite structure at the micro-scale enables the steel sheet to exhibit both high strength characteristics from the martensite phase and good formability from the ferrite phase, resolving the contradiction between strength and formability
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 exhibits high tensile strength, high yield strength, press formability, and excellent energy absorption characteristics, suitable for automotive impact energy absorbing members.
Implementation Method 1
increase absorbed energy at the time of impact
Implementation Method 2
annealing, and cooling processes
Data Source
Figure 1(a)~1(b)
AI summary
A steel sheet and a member with a TS of 1180 MPa or more and less than 1470 MPa, a high YS, a high YR, high press formability (bendability and ductility of the steel sheet), and good energy absorption characteristics, and methods for producing the steel sheet and the member are provided. A base steel sheet has a predetermined chemical composition, a surface soft layer of 20 µm or more is provided on a surface layer of the base steel sheet, ferrite, retained austenite, fresh martensite, bainitic ferrite, and tempered martensite are set to predetermined ranges in a steel microstructure at a quarter thickness position of the base steel sheet, and LF-MA/LF is 0.70 or less.