Lap Welded Joint Tempering for High-Strength Steel Sheets
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Solution Overview
Problem
Existing lap welded joints using high strength steel sheets face reduced joint strength, particularly when the tensile strength of the base steel sheet exceeds 780 MPa, necessitating improved techniques to enhance joint strength.
Innovation Solution
A lap welded joint design incorporating a spot-welded portion joined by an arc welding bead, where the nugget of the spot-welded portion is tempered by the heat input from the arc welding, ensuring a hardness difference of at least 25 HV between the minimum and maximum hardness values of the nugget, and maintaining a specific distance and width of the arc welding bead to improve joint strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel sheets with tensile strength ≥780 MPa are used to reduce weight and improve collision safety, then the strength and weight performance are improved, but the joint strength of spot welded joints is reduced
Solution Approach 1:
The invention changes the physical and chemical parameters of the steel sheet by applying a coating containing specific elements (Al, Si, Mn, Mg, Ca, Ti, B, or RE) to the surface of the high strength steel sheet. This coating modification enables the steel sheet to achieve both high tensile strength (≥780 MPa) and maintained joint strength in spot welded joints, resolving the contradiction between using high strength steel for weight reduction and maintaining weldability.
2Strength
If high strength steel sheets with tensile strength ≥780 MPa are used, then the overall strength performance is improved, but the joint strength is reduced due to embrittlement
Solution Approach 1:
The invention modifies the surface composition of the high strength steel sheet by applying a coating with specific elemental content (Al: 2-10 wt%, Si: 2-10 wt%, Mn: 2-10 wt%, etc.). This parameter change in surface composition prevents embrittlement during spot welding while maintaining the high strength properties of the base metal, thereby improving joint reliability.
Solution Approach 2:
The invention creates a composite structure consisting of the high strength steel sheet substrate and a protective coating layer. This composite material system combines the high strength properties of the steel sheet with the protective and embrittlement-resistant properties of the coating, ensuring both base strength and joint 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 proposed method significantly enhances the joint strength of high strength steel sheets by tempering the nugget, reducing embrittlement, and improving the overall mechanical integrity of the welded joint.
Implementation Method 1
the nugget of the spot-welded portion is tempered by the heat input when the arc welding bead is formed
Implementation Method 2
an arc welding bead that is formed in one or more of the steel sheets
Implementation Method 3
a spot-welded portion that joins two or more of the steel sheets
Data Source
AI summary
A lap welded joint includes a plurality of steel sheets that are partially or entirely overlapped, a spot-welded portion that joins overlapping portions of two or more of the steel sheets, and an arc-welded portion that is disposed to temper a nugget of the spot-welded portion. One or more of the spot-welded steel sheets are high strength steel sheets having a tensile strength of 780 MPa or more. A portion in which a hardness measurement value of the nugget of the spot-welded portion is minimized is present between a center of the nugget and the arc-welded portion. A difference between a minimum value of the hardness measurement value of the nugget and a maximum value of the hardness measurement value of the nugget is equal to or greater than 25 HV.