Welded Joint LME Crack Suppression via Decarburized Surface
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Solution Overview
Problem
LME cracking occurs during the production of welded joints in high strength steel sheets due to molten zinc penetrating the grain boundaries, causing brittleness and stress-induced cracking.
Innovation Solution
A welded joint design that includes a high strength steel sheet with controlled carbon concentration and cementite layer, combined with a low surface roughness and specific chemical composition, to prevent LME cracking by decarburization and strain-induced oxygen diffusion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If high strength steel sheet with Zn-based plating is used for welding, then weight reduction and fuel economy are improved, but LME cracking occurs due to molten zinc penetrating grain boundaries
Solution Approach 1:
The patent applies local quality by creating a specific microstructure in the surface layer of the steel sheet near the weld zone. By controlling the cementite area ratio to be 10% or less in the surface layer (0-100μm from surface) while maintaining overall high strength properties, the material achieves localized resistance to LME cracking without sacrificing the weight reduction benefits of high strength steel
Solution Approach 2:
The patent utilizes parameter changes by adjusting the chemical composition (C: 0.15-0.40%, Si: 0.4-2.0%, Mn: 1.5-3.5%) and heat treatment parameters (cooling rate, tempering temperature) to achieve the desired microstructure. These parameter changes enable the steel sheet to maintain high strength while developing a surface layer with reduced cementite content that resists zinc penetration
2Ease of manufacture
If the surface layer transforms to austenite during welding, then welding process is simplified, but LME cracking increases due to increased zinc penetration
Solution Approach 1:
The patent applies preliminary anti-action by pre-modifying the surface layer microstructure before welding through controlled rolling and heat treatment. By reducing the cementite area ratio to 10% or less in advance, the steel sheet is prepared to resist LME cracking during the welding process, counteracting the harmful effect of austenite transformation and zinc penetration
3Strength
If tensile stress is applied during welding, then weld strength is improved, but LME cracking is promoted due to stress acting on brittle material
Solution Approach 1:
The patent applies local quality by creating a gradient microstructure where the surface layer (0-100μm) has reduced cementite content (area ratio ≤10%) compared to the base material. This localized modification provides crack resistance at the surface where LME occurs, while allowing higher strength properties in the bulk material to maintain weld strength
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 effectively suppresses LME cracking by maintaining a low carbon concentration and controlled cementite ratio, enhancing the resistance to liquid metal embrittlement.
Implementation Method 1
the steel sheet surface layer is decarburized and further a layer with a low cementite fraction is formed
Implementation Method 2
performing high dew point annealing, the steel sheet surface layer is decarburized
Data Source
Figure 1(a)~1(b)
Figure 2
Figure 3
AI summary
The present invention has as its object the provision of a welded joint suppressing LME cracking at the time of production. The welded joint of the present invention is provided with high strength steel sheet with a tensile strength of 780 MPa or more. The high strength steel sheet has a predetermined chemical composition. At a region separated by 5 mm or more from an outside end of a nugget, in a depth direction of the high strength steel sheet, a depth with a C concentration measured by GDS of 0.01% or less is 3 µm or more and a roughness of a surface of the high strength steel sheet is an arithmetic average roughness Ra of 3.0 µm or less, and in a range of 0 to 100 µm from an outside end of the weld shoulder part to the outside, in a depth direction from the high strength steel sheet, a thickness of a layer with an area ratio of cementite of 10% or less is 5 µm or more.