High-Strength Steel Sheet Microstructure for Bendability and Flatness
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Solution Overview
Problem
Existing high strength steel sheets used in automobiles face challenges in achieving a tensile strength of 1180 MPa or higher while maintaining excellent bendability, flatness in the width direction, and working embrittlement resistance, as previous technologies do not adequately address these properties simultaneously.
Innovation Solution
A high strength steel sheet with a specific chemical composition and manufacturing process, including controlled annealing, bending, and cooling rates, to achieve 1180 MPa tensile strength, enhanced bendability, and improved flatness and embrittlement resistance by limiting the area fraction of martensite, volume fraction of retained austenite, and average grain size of prior austenite.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the tensile strength is increased to 1180 MPa or higher, then the strength is improved, but the bendability deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the volume fraction of retained austenite (3-15%) and the area fraction of martensite (80% or more), along with controlling packet structure (proportion of largest packet ≤70%). These parameter adjustments enable the steel to achieve tensile strength of 1180 MPa or higher while maintaining excellent bendability, resolving the contradiction between strength and formability.
2Strength
If the tensile strength is increased to 1180 MPa or higher, then the strength is improved, but the working embrittlement resistance deteriorates
Solution Approach 1:
The patent resolves this contradiction through parameter changes by controlling the volume fraction of retained austenite (3-15%) and the packet structure (proportion of largest packet ≤70%). The retained austenite acts as a buffer that prevents embrittlement during forming operations, while the overall microstructure maintains high strength. This enables simultaneous achievement of 1180 MPa tensile strength and excellent working embrittlement resistance.
3Strength
If the tensile strength is increased to 1180 MPa or higher, then the strength is improved, but the flatness in the width direction deteriorates
Solution Approach 1:
The patent applies parameter changes by controlling the packet structure where the proportion of the packet with the largest area in prior austenite grains is limited to 70% by area or less. This packet structure control, combined with martensite area fraction of 80% or more, achieves both high tensile strength (1180 MPa or higher) and excellent flatness in the width direction (steepness of 0.02 or less), resolving the contradiction between strength and flatness.
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 1180 MPa tensile strength, excellent bendability, and improved flatness and embrittlement resistance, suitable for automotive structural members, contributing to weight reduction and enhanced fuel efficiency.
Implementation Method 1
annealing, bending, and cooling rates
Implementation Method 2
volume fraction of retained austenite
Implementation Method 3
bending and unbending the steel sheet 1 to 15 times in total with a roll
Implementation Method 4
cooling the steel sheet at an average cooling rate of 20° C./s or more
Data Source
AI summary
A high strength steel sheet having 1180 MPa or higher tensile strength and a method for manufacturing the same are disclosed. The high strength steel sheet has a specific chemical composition and is such that in a region at ¼ sheet thickness, the area fraction of martensite is 80% or more, the volume fraction of retained austenite is 3% or more and 15% or less, the area fraction of the total of ferrite and bainitic ferrite is 10% or less, the average grain size of prior austenite is 20 μm or less, and the average of the proportions of packets having the largest area in prior austenite grains is 70% by area or less of the prior austenite grain.