Steel Sheet Internal Oxidation Layer Suppresses LME Cracking
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
High strength TRIP steel sheets face challenges with liquid metal embrittlement (LME) cracking during spot welding, particularly when combined with galvanized or hot-dip galvanized steel sheets, leading to reduced hole expansibility and ductility, and existing solutions do not adequately address these issues while maintaining high strength and corrosion resistance.
Innovation Solution
A steel sheet composition with specific chemical elements and microstructural features, including a high volume percentage of martensite and bainite, residual austenite, and an internal oxidation layer with a controlled AlN number density, which suppresses LME cracking and enhances hole expansion rate and ductility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength TRIP steel sheet is spot-welded with galvanized or hot-dip galvanized steel sheet, then joining strength is improved, but LME cracking occurs reducing reliability
Solution Approach 1:
The patent applies preliminary action by forming an internal oxidation layer on the steel sheet surface before welding. This oxide layer is created in advance during the steel sheet manufacturing process, serving as a protective barrier that prevents molten zinc from invading grain boundaries during subsequent spot welding, thereby preventing LME cracking while maintaining joining strength
Solution Approach 2:
The patent uses an internal oxidation layer as an intermediary substance between the steel sheet and molten zinc. This oxide layer acts as a mediator that blocks the harmful interaction between zinc and the steel matrix, allowing welding to proceed successfully without causing LME cracking
2Reliability
If internal oxidation layer is formed to suppress LME cracking, then crack resistance is improved, but hole expansibility deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the thickness and oxide content of the internal oxidation layer. By optimizing these parameters, the patent achieves sufficient crack resistance while minimizing the negative impact on hole expansibility. The controlled parameters ensure the oxide layer provides protection without excessive interference with deformation processes
3Strength
If high strength is achieved through martensite and bainite structure, then tensile strength is improved, but ductility deteriorates
Solution Approach 1:
The patent uses a composite microstructure consisting of martensite, bainite, and residual austenite to achieve both high strength and ductility. The martensite and bainite provide strength, while the retained austenite contributes to ductility through transformation-induced plasticity, creating a synergistic composite structure that balances mechanical properties
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 proposed steel sheet achieves a high strength-ductility balance, effectively suppresses LME cracking, and maintains a high hole expansion rate, suitable for applications requiring both strength and formability.
Implementation Method 1
cracking (LME cracking) due to liquid metal embrittlement (LME) may occur in a spot-welding portion. The LME cracking is cracking that occurs when tensile stress acts on a welding portion in a state where zinc in a galvanized layer melts due to heat that is generated during spot welding and the molten zinc invades crystal grain boundaries of a steel sheet structure of the welding portion.
Implementation Method 2
number density of AlN is 3000 pieces/mm2 or more and less than 6000 pieces/mm2 at a depth position of 30 μm from a sheet surface
Data Source
AI summary
A steel sheet according to an aspect of the present invention has a predetermined chemical composition, in which a metallographic structure at a ¼ thickness portion includes, by volume percentage, a total of 50% or more of one or both of martensite and bainite and 8% or more of residual austenite, an average value of aspect ratios of prior austenite grains is 5.0 or more, number density of AlN is 3000 pieces/mm2 or more and less than 6000 pieces/mm2 at a depth position of 30 μm from a sheet surface, an internal oxidation layer in which at least a part of a crystal grain boundary is coated with an oxide is provided from the sheet surface to a depth of 5.0 μm or more, grain boundary coverage of the oxide is 60% or more in a region from the sheet surface to a depth of 5.0 μm, and a tensile strength is 980 MPa or more.