High-Strength Sheet Microstructure for Bendability and Embrittlement Control
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Solution Overview
Problem
High-strength steel sheets with a tensile strength of 1,180 MPa face challenges in bendability and are prone to liquid metal embrittlement during welding due to microstructural issues, particularly in the surface layers, which limits their application and strength.
Innovation Solution
A high-strength steel sheet with a specific chemical composition and microstructure, including controlled grain sizes and orientations, combined with a zinc-coated layer, to enhance bendability and prevent liquid metal embrittlement during welding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If high-strength steel sheets with tensile strength of 1,180 MPa are used for weight reduction, then weight reduction effect is improved, but bendability deteriorates due to steep strain gradient and cracking in bent portions
Solution Approach 1:
The patent applies local quality by creating distinct microstructure zones at different depths from the surface. The surface layer (0-50μm) has fine prior austenite grains (10μm or less) for liquid metal embrittlement resistance, while the inner portion has coarser grains for ductility. This spatial variation in microstructure properties resolves the contradiction between strength and bendability by optimizing each zone for its specific function.
Solution Approach 2:
The patent employs preliminary action through controlled cooling processes that establish the desired microstructure before welding or forming operations. By pre-establishing fine grain structures in the surface layer and controlling precipitate distribution, the steel sheet is prepared in advance to resist liquid metal embrittlement and accommodate deformation, preventing cracking during subsequent bending operations.
2Reliability
If zinc coating is applied to prevent corrosion, then corrosion resistance is improved, but liquid metal embrittlement occurs during resistance spot welding due to zinc liquefaction and grain boundary permeation
Solution Approach 1:
The patent applies parameter changes by precisely controlling the microstructural parameters of the steel sheet to resist liquid metal embrittlement. By maintaining fine prior austenite grain sizes (10μm or less) in the surface layer and controlling the distribution and size of precipitates (1.0μm or less), the material parameters are optimized to prevent zinc grain boundary permeation during welding, thereby resolving the contradiction between corrosion resistance and welding integrity.
Solution Approach 2:
The patent effectively creates a composite microstructure with multiple phases (ferrite, martensite, bainite, tempered martensite, and retained austenite) distributed throughout the steel sheet. This composite microstructure, with specific phase fractions and spatial distributions, provides both the base material strength and resistance to liquid metal embrittlement, allowing the zinc coating to provide corrosion protection without causing welding failures.
3Reliability
If grain size is reduced to improve liquid metal embrittlement resistance, then welding reliability is improved, but manufacturing complexity increases due to precise microstructure control requirements
Solution Approach 1:
The patent applies segmentation by dividing the steel sheet into distinct depth zones with different microstructural characteristics. The surface layer (0-50μm) is segmented with fine prior austenite grains for welding reliability, while the inner portion has different grain characteristics. This segmentation allows targeted microstructure control in critical areas without requiring uniform complex control throughout the entire material, thereby reducing manufacturing complexity while maintaining welding reliability.
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 1,180 MPa with improved bendability and resistance to liquid metal embrittlement, ensuring structural integrity and reliability in welded joints.
Implementation Method 1
80% or less of grain boundaries of the prior austenite grains are high-angle grain boundaries with a misorientation of 15° or more
Implementation Method 2
in a region extending 200 μm from the surface of the steel sheet in the through-thickness direction, the average size of precipitates is 1.0 μm or less
Data Source
AI summary
A high-strength steel sheet with a tensile strength of 1,180 MPa or more has a predetermined chemical composition and a steel microstructure in which the area fraction of ferrite is 5% or less, the area fraction of martensite is 2% to 10%, the area fraction of bainite is 5% to 37%, the area fraction of tempered martensite is 42% to 65%, the volume fraction of retained austenite is 3% to 15%, the average grain size of ferrite and bainite is 3 μm or less, in a region extending 50 μm from a surface of the steel sheet in a through-thickness direction, and the average grain size of prior austenite grains is 10 μm or less, the average grain size of the prior austenite grains in the through-thickness direction is 0.9 or less of the average grain size thereof in a rolling direction.