High-Tensile Steel Joint Structure With Decarburized Weld Interface
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Solution Overview
Problem
Existing methods for welding high tensile strength steel sheets face challenges in achieving high cross tension strength while maintaining the mechanical properties of the base plate and ensuring good weldability, often requiring limiting carbon content or using specialized welding techniques that are costly and limit flexibility.
Innovation Solution
A joint structure is created by forming a surface soft layer, specifically a decarburized layer, on high tensile strength steel sheets before welding, which increases the carbon content in the molten-solidified portion and controls the Vickers hardness of the heat affected zone to enhance toughness and joint strength without compromising weldability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the carbon content is limited to improve weldability and cross tensile strength, then the weldability and toughness of the welded portion are improved, but the mechanical properties of the base plate are deteriorated
Solution Approach 1:
The invention applies local quality by creating a surface soft layer with lower carbon content (5-20 mass% reduction) specifically at the welding surface, while the bulk material maintains high carbon content (0.21-0.53 mass%) for strength. This localized compositional variation allows the base plate to retain high tensile strength while the welded portion achieves improved toughness and cross tensile strength through the softer, more ductile surface layer.
2Strength
If arc spot welding method is used to supply molten metal for dilution, then the hardness ratio of weld bead to base plate can be controlled, but the welding position flexibility is reduced and special equipment is required
Solution Approach 1:
The invention applies preliminary action by pre-forming the surface soft layer with reduced carbon content on the steel sheet surfaces before the welding process. This preliminary compositional modification eliminates the need for complex welding process controls or special equipment, as the favorable dilution effect is already built into the material structure. Standard resistance spot welding equipment can then be used with full welding position flexibility.
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 improves the toughness and cross tension strength of the joint without impairing the mechanical properties of the base plate, allowing for effective welding of high tensile strength steel sheets using standard equipment and maintaining good deformability.
Implementation Method 1
the surface soft layer is a decarburized layer
Implementation Method 2
a molten-solidified portion formed by melting and solidifying the first member and the second member
Implementation Method 3
a heat affected zone formed around the molten-solidified portion
Data Source
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AI summary
The present invention increases the toughness of molten-solidified portions and improves joint strength without inhibiting welding workability, even in a high-tensile steel plate in which the matrix has excellent mechanical properties and the amount of carbon in steel is large. This joined structure (100) comprises: a first member (12) made of high-tensile steel; a second member (14) made of high-tensile steel and superposed on the first member (12); a surface soft layer (20) formed on a surface of the first member (12) superposed on the second member (14) and/or a surface of the second member (14) superposed on the first member (12); a molten-solidified portion (30) formed by melting and solidifying the first member (12) and the second member (14); and a heat-affected portion (40) formed in the periphery of the molten-solidified portion (30). The total thickness of the surface soft layer (20) is from 5 µm to 200 µm. The amount of carbon in the molten-solidified portion (30) is 0.21 mass% or greater. The maximum Vickers hardness of the surface soft layer (20) in the heat-affected portion (40) is from 100 Hv to 500 Hv.