High-Strength Steel Sheet Microstructure for Stable Hole Expansion
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Solution Overview
Problem
Conventional high-strength steel sheets used in automobile parts fail to achieve a balance of high tensile strength, yield ratio, uniform elongation, total elongation, and hole expansion ratio, leading to potential cracking and uneven deformation.
Innovation Solution
A high-strength steel sheet with specific microstructural and chemical compositions, including controlled fractions of martensite-austenite constituent (MA), soft α-phase, and retained austenite, along with a controlled heat treatment process to optimize the standard deviation of retained austenite diameter, ensuring improved mechanical properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If the steel sheet is thinned to reduce vehicle body weight, then fuel efficiency is improved, but the strength of the parts deteriorates
Solution Approach 1:
The invention changes the microstructural parameters of the steel sheet by controlling the fraction of MA constituent (5-20%) and retained austenite (15-40%), and by controlling the standard deviation of retained austenite diameter (0.155 μm or more). These parameter changes enable the steel sheet to achieve both high strength (TS≥780 MPa) and high ductility (EL≥21%), allowing thinning of the steel sheet while maintaining part strength.
2Strength
If the tensile strength is increased to ensure collision safety, then the yield strength increases, but the total elongation and uniform elongation deteriorate
Solution Approach 1:
The invention creates a composite microstructure consisting of multiple phases: martensite-austenite (MA) constituent (5-20%), retained austenite (15-40%), and other microstructures. This composite microstructure combines the high strength characteristics of martensite with the high ductility characteristics of retained austenite, achieving TS≥780 MPa and EL≥21% simultaneously. The controlled standard deviation of retained austenite diameter (0.155 μm or more) ensures uniform distribution and prevents localized deformation.
3Ease of manufacture
If the steel sheet is formed into complex shapes, then the workability is improved, but cracks may occur due to uneven deformation
Solution Approach 1:
The invention changes the microstructural parameters by controlling the fraction of MA constituent (5-20%), retained austenite (15-40%), and the standard deviation of retained austenite diameter (0.155 μm or more). These parameter changes ensure uniform deformation characteristics during forming, achieving hole expansion ratio ≥20% and total elongation ≥21%, which prevents cracks during complex shape formation while maintaining high workability.
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 solution results in a steel sheet with enhanced tensile strength, yield ratio, uniform elongation, total elongation, and hole expansion ratio, providing stable deformation and formability for complex shapes.
Implementation Method 1
microstructures of the steel sheet satisfies that: a fraction of MA in all steel microstructures is more than 0% and 15.0% or less by area ratio, a fraction of a soft α-phase microstructure in the all steel microstructures is 0% or more and 50% or less by area ratio, and a standard deviation of an equivalent circle diameter of retained austenite is greater than 0.155 μm
Data Source
AI summary
Disclosed is a high-strength steel sheet having predetermined chemical components, and microstructures of the steel sheet satisfies that: a fraction of MA in all steel microstructures being more than 0% and 15.0% or less by area ratio; a fraction of a soft α-phase microstructure in the all steel microstructures being 0% or more and 50% or less by area ratio; and a standard deviation of an equivalent circle diameter of retained austenite being greater than 0.155 μm.