High-Strength Steel Sheet Microstructure for Formability
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Solution Overview
Problem
Existing high strength steel sheets with tensile strengths of 1320 MPa or higher face challenges in achieving sufficient elongation, appropriate clearances for hole expanding deformation, and resistance to delayed fracture, which are not adequately addressed by existing technologies.
Innovation Solution
A high strength steel sheet with a specific microstructure and chemical composition, including controlled amounts of ferrite, bainitic ferrite, retained austenite, and carbon concentration, along with specific hardness and misorientation ratios, achieved through a manufacturing process involving hot rolling, annealing, reheating, and tempering, to enhance mechanical properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of steel sheets is increased to reduce CO2 emissions and enhance crashworthiness, then tensile strength is improved, but elongation deteriorates and press forming becomes difficult
Solution Approach 1:
The invention changes the microstructural parameters by precisely controlling the volume fractions of different phases (tempered martensite 80-95%, retained austenite 5-20%, ferrite 0-10%, bainitic ferrite 0-10%) and chemical composition parameters (C: 0.15-0.45%, Si: 0.50-2.00%, Mn: 1.50-3.50%). This multi-parameter optimization enables the steel to achieve both high tensile strength (1320 MPa or higher) and sufficient elongation (8% or more), resolving the contradiction between strength and formability
Solution Approach 2:
The invention creates a composite microstructure consisting of multiple phases with different properties: tempered martensite provides high strength, retained austenite provides ductility and elongation through TRIP effect, and controlled amounts of ferrite and bainitic ferrite contribute to formability. This composite microstructure enables simultaneous achievement of high strength and good formability
2Strength
If high strength steel sheets are used for automobile structural parts, then weight reduction is achieved, but delayed fracture resistance deteriorates due to hydrogen penetration
Solution Approach 1:
The invention optimizes chemical composition parameters including limiting S to 0.0200% or less, P to 0.100% or less, and H to 0.0020% or less, while controlling microstructural parameters such as retained austenite volume fraction (5-20%) and carbon concentration in retained austenite (0.50% or more). These parameter optimizations reduce hydrogen embrittlement susceptibility while maintaining high strength, thereby improving delayed fracture resistance
Solution Approach 2:
The invention utilizes the TRIP effect (strain-induced plasticity) where retained austenite transforms to martensite during deformation, absorbing energy and preventing crack initiation. This converts the potential harm of retained austenite (reducing strength) into a benefit (improving toughness and delayed fracture resistance) through controlled transformation
3Manufacturing precision
If shear clearance is reduced to improve shearing quality, then surface quality is improved, but the range of appropriate clearances for hole expanding deformation and delayed fracture resistance deteriorates
Solution Approach 1:
The invention optimizes the microstructural parameters including tempered martensite area fraction (80% or more), retained austenite volume fraction (5-20%), and chemical composition (C: 0.15-0.45%, Si: 0.50-2.00%, Mn: 1.50-3.50%). These parameter optimizations create a microstructure that is tolerant to shear clearance variations, enabling a wide range of appropriate clearances for both hole expanding deformation and delayed fracture resistance while maintaining good shearing quality
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 1320 MPa or more, elongation of 8% or more, and a wide range of appropriate clearances for hole expanding deformation while resisting delayed fracture, suitable for automotive structural members.
Implementation Method 1
a microstructure having... the area fraction of tempered martensite is 80% or more
Implementation Method 2
a method for manufacturing the same
Data Source
AI summary
A high strength steel sheet includes a specific microstructure having a specific chemical composition and satisfying the formulas (1) and (2) defined below:KAM (S)/KAM (C)<1.(1)wherein KAM (S) is a KAM (Kernel average misorientation) value of a superficial portion of the steel sheet, and KAM (C) is a KAM value of a central portion of the steel sheet,Hv (Q)-Hv (S)≥8(2)wherein Hv (Q) indicates the hardness of a portion at ¼ sheet thickness and Hv (S) indicates the hardness of a superficial portion of the steel sheet.