Multiphase Steel Sheet Microstructure for High-Strength Forming
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing high-strength steel sheets with tensile strengths of 1180 MPa or more face challenges in achieving high yield stress and yield ratio, press formability, ductility, flangeability, bendability, and fracture resistance, limiting their application in impact energy absorbing members of automobiles.
Innovation Solution
A steel sheet with a specific microstructure composition and controlled phase fractions, including controlled area fractions of ferrite, bainite, tempered bainite, and tempered martensite, along with controlled grain sizes and carbide densities, combined with a hot rolling and annealing process, to enhance strength and formability characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel sheets with tensile strength of 1180 MPa or more are used, then strength is improved, but press formability and ductility deteriorate
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.15-0.35%, Si: 0.01-2.50%, Mn: 1.50-3.50%, Al: 0.01-2.00%) and microstructural parameters (area fractions of ferrite, bainite, tempered bainite, and tempered martensite) to achieve a balanced state where high tensile strength (1180 MPa or more) is obtained while maintaining adequate press formability for automotive applications
Solution Approach 2:
The patent employs composite materials by creating a multi-phase microstructure consisting of ferrite, bainite, tempered bainite, and tempered martensite phases. This composite microstructure combines the ductility contribution from ferrite and bainite with the strength contribution from tempered martensite, achieving both high strength and press formability simultaneously
2Strength
If high-strength steel sheets with tensile strength of 1180 MPa or more are used, then strength is improved, but ductility and flangeability deteriorate
Solution Approach 1:
The patent applies parameter changes by optimizing the chemical composition (particularly C: 0.15-0.35% and Si: 0.01-2.50%) and controlling the microstructural parameters (area fractions of different phases and grain sizes) to achieve a balance where tensile strength reaches 1180 MPa or more while maintaining ductility and flangeability at acceptable levels for automotive structural members
Solution Approach 2:
The patent uses composite materials by forming a multi-phase microstructure where ferrite and bainite phases contribute to ductility and flangeability, while tempered martensite provides high strength. The synergistic combination of these phases resolves the contradiction between strength and ductility
3Strength
If high-strength steel sheets with tensile strength of 1180 MPa or more are used, then strength is improved, but bendability and fracture resistance deteriorate
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.15-0.35%, Si: 0.01-2.50%, Mn: 1.50-3.50%) and microstructural parameters (area fractions of phases, grain sizes, and carbide characteristics) to achieve tensile strength of 1180 MPa or more while maintaining bendability and fracture resistance necessary for forming bent portions in automotive frame structural members
Solution Approach 2:
The patent employs composite materials by creating a multi-phase microstructure where the softer ferrite and bainite phases provide ductility and bendability, while the tempered martensite phase provides high strength. This composite structure prevents brittle fracture during bending operations
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 1180 MPa or more, with improved yield stress and yield ratio, high ductility, flangeability, bendability, and fracture resistance, suitable for automotive structural members and impact energy absorbing components.
Implementation Method 1
a steel microstructure, as a microstructure at a quarter thickness position of the base steel sheet, in which an area fraction of ferrite: less than 20.0%, an area fraction of fresh martensite: 15.0% or less, an area fraction of retained austenite: 3.0% or less, an area fraction of bainite and tempered bainite: more than 10.0% and 70.0% or less, an area fraction of tempered martensite: 30.0% or more and 80.0% or less
Implementation Method 2
a hot rolling and annealing process, to enhance strength and formability characteristics
Data Source
AI summary
A base steel sheet has a specified chemical composition and has a steel microstructure at a quarter thickness position containing specified ranges of ferrite, fresh martensite, retained austenite, bainite, tempered bainite, and tempered martensite, island-like fresh martensite and island-like retained austenite in bainite grains and in tempered bainite grains have an average grain size of 2.00 μm or less, carbides in the bainite grains and in the tempered bainite grains have an average particle size of 500 nm or less, carbides with a particle size of 300 nm or more in the bainite grains and in the tempered bainite grains have a number density of 3.0/μm2 or less, and the amount of diffusible hydrogen in the base steel sheet is 0.50 ppm by mass or less.