Welded Joint LME Cracking Prevention via Decarburized Surface Layer
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Solution Overview
Problem
LME cracking occurs during the production of welded joints in high strength steel sheets due to the penetration of molten zinc into the grain boundaries, causing brittleness, especially in Zn-based plated steel sheets.
Innovation Solution
The solution involves pre-treating the steel sheet with an abrasive to impart strain and decarburizing the surface layer through high dew point annealing, forming a low cementite fraction to suppress LME cracking.
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 improvement are achieved, but LME cracking occurs due to molten zinc penetrating grain boundaries
Solution Approach 1:
The patent applies preliminary action by performing abrasive blasting and high dew point annealing treatments on the steel sheet surface before welding. These pre-treatments create a decarburized surface layer with reduced cementite content, which prevents LME cracking during the subsequent welding process by blocking molten zinc penetration paths at grain boundaries
Solution Approach 2:
The patent changes physical and chemical parameters of the steel sheet surface through controlled decarburization. By adjusting the depth of the decarburized layer (3-20 μm) and controlling the cementite area ratio (≤10%) in the surface layer through high dew point annealing, the material creates optimal surface conditions that prevent LME while maintaining bulk high strength properties
2Reliability
If the surface layer is decarburized to suppress LME, then weldability improves, but the surface layer structure becomes more complex requiring additional processing steps
Solution Approach 1:
The patent merges multiple functions into a single high dew point annealing process. This one-step heat treatment simultaneously achieves decarburization of the surface layer, formation of a low cementite surface structure, and preparation of the surface for welding, thereby simplifying the overall process despite the complexity of the surface structure created
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
This approach effectively reduces LME cracking by controlling the surface layer's C concentration and cementite area ratio, enhancing the joint's resistance to liquid metal embrittlement.
Implementation Method 1
by firing an abrasive at the steel sheet before annealing under suitable conditions, it is possible to impart strain and obtain a suitable surface state
Implementation Method 2
by performing high dew point annealing, the steel sheet surface layer is decarburized and further a layer with a low cementite fraction is formed
Implementation Method 3
the steel sheet surface layer is decarburized
Implementation Method 4
LME cracking is believed to occur due to the surface layer part of steel sheet transforming to austenite at the time of welding, the molten zinc penetrating the grain boundaries causing the steel sheet to become brittle
Implementation Method 5
the surface layer part of steel sheet transforming to austenite at the time of welding
Data Source
Figure 1(a)~1(b)
Figure 2~3
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 non heat affected zone separated by 5 mm or more from an outside end of the spot welded part, 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 more than 3.0 µm, and, in a heat affected zone at a distance of 0 to 100 µm from a weld shoulder part, 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 8 µm or more.