Projection Welded Nut Joint for High-Strength Steel Fracture Resistance
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Solution Overview
Problem
Existing methods for joining high strength steel sheets with nuts using projection welding face challenges in achieving both satisfactory peeling strength and delayed fracture resistance, particularly in automotive applications where bolt fastening is common.
Innovation Solution
The solution involves controlling the projection welding conditions and chemical composition of the steel sheet to ensure a hardened region on both the steel sheet and nut sides extends continuously for at least 500 µm, with the nut side hardness exceeding the steel sheet side, and forming a martensite microstructure in the weld heat-affected zone to enhance peeling strength and delayed fracture resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If projection welding is performed to join high strength steel sheet and nut, then peeling strength is improved, but delayed fracture resistance deteriorates
Solution Approach 1:
The patent applies local quality by creating distinct microstructural zones with different properties: a martensite microstructure in the weld heat-affected zone on the steel sheet side for high hardness and peeling strength, while controlling the overall hardness distribution to maintain delayed fracture resistance. The hardened region is localized to specific areas rather than uniformly distributed throughout the entire joint.
Solution Approach 2:
The patent changes physical and chemical parameters including controlling the tensile strength of the steel sheet within 980-1500 MPa, controlling the hardness distribution with maximum hardness of 400-650 HV in the weld heat-affected zone, and ensuring the hardened region extends continuously for 500 µm or more. These parameter changes optimize both peeling strength and delayed fracture resistance.
2Weight of moving object
If steel sheet thickness is decreased to reduce weight, then fuel efficiency is improved, but structural strength deteriorates
Solution Approach 1:
The patent changes the material parameter by using high strength steel sheet with tensile strength of 980 MPa or higher, allowing thinning of the steel sheet while maintaining sufficient structural strength. The controlled hardness distribution and martensite microstructure further enhance the strength-to-weight ratio of the joined structure.
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 method results in a welded joint with peeling strength of 9.0 kN or higher and excellent delayed fracture resistance, even under corrosive conditions, by ensuring specific hardness distributions and microstructural properties.
Implementation Method 1
projection welding
Implementation Method 2
resistance welding
Implementation Method 3
forming a martensite microstructure in the weld heat-affected zone
Data Source
Figure 1(a)~1(c)
Figure 2(a)~2(c)
Figure 3
AI summary
Provided are a projection welded joint and a projection welding method. The present invention is a projection welded joint formed by joining a steel sheet having a tensile strength of 980 MPa or higher and a nut by performing projection welding, in which, in a hardness distribution from the steel sheet to the nut through a weld metal zone, a hardened region exists on each of the steel sheet side and the nut side, in which the hardness of each hardened region is higher than that of the base metal on the corresponding side, in which each hardened region extends continuously for a distance of 500 µm or more, in which the maximum hardness NHmax on the nut side is higher than the maximum hardness SHmax on the steel sheet side, and in which a region where the maximum hardness SHmax on the steel sheet side exists is formed of a martensite microstructure having a prior austenite grain size of 20 µm or less.