Resistance Spot Weld Microstructure for Delayed Fracture Resistance
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Solution Overview
Problem
Existing resistance spot welding techniques for high strength steel sheets with tensile strength of 1400 MPa or more face challenges in achieving adequate cross tensile strength and delayed fracture resistance, leading to interfacial fractures and hydrogen embrittlement.
Innovation Solution
The method involves controlling the microstructure in the region from the nugget edge to the heat-affected zone by adjusting volume fractions of ferrite, martensite, and tempered martensite, and dispersing fine Nb-based and Ti-based carbides to enhance cross tensile strength and delayed fracture resistance, including a post-heat treatment process to refine crystal grains and trap hydrogen.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the tensile strength of the base metal is increased to 1400 MPa or more, then the strength of the steel sheet is improved, but delayed fracture resistance decreases due to hydrogen embrittlement
Solution Approach 1:
The invention applies parameter changes by controlling the chemical composition parameters of the steel sheet to prevent hydrogen embrittlement. Specifically, it limits phosphorus content to 0.030% or less and sulfur content to 0.005% or less, and maintains carbon content between 0.20-0.40%. These parameter controls, combined with the controlled microstructure formation through resistance spot welding, create a weld joint that resists hydrogen penetration and prevents delayed fracture, thereby maintaining high reliability even at base metal tensile strengths of 1400 MPa or more.
2Ease of manufacture
If conventional welding techniques are used for high strength steel sheets, then the welding process is simple, but the weld strength is insufficient and interfacial fracture occurs
Solution Approach 1:
The invention applies parameter changes by optimizing the resistance spot welding parameters including current (3-8 kA), time (10-500 ms), and pressure (1-10 kN/mm²). These parameter adjustments, combined with the specific steel sheet composition (C: 0.20-0.40%, Si: 1.00-2.00%, Mn: 1.50-3.50%, P: 0.030% or less, S: 0.005% or less), enable the formation of a nugget with diameter 0.5-2.0 times the sheet thickness, creating a weld joint with sufficient strength that exceeds the base metal strength without requiring complex welding processes.
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 significantly improves cross tensile strength and delayed fracture resistance, preventing interfacial fractures and hydrogen embrittlement, while maintaining the strength of the heat-affected zone.
Implementation Method 1
resistance-spot-welding a plurality of steel sheets
Implementation Method 2
the crystal grains in the microstructure are reduced in size to form fine carbides in the microstructure
Implementation Method 3
it is feared that hydrogen entering from the use environment will cause delayed fracture particularly in the resistance spot weld
Data Source
AI summary
An automotive member and a resistance spot welding method. The automotive member includes a resistance spot weld formed by resistance-spot-welding a plurality of steel sheets including at least one high strength steel sheet. The high strength steel sheet has a specific chemical composition. The microstructure in the vicinity of the edge of the nugget of the resistance spot weld is a composite microstructure containing ferrite at a volume fraction of 1 to 30%, martensite at a volume fraction of 1 to 50%, and tempered martensite at a volume fraction of 20% or more. The average number density of Nb-based precipitate grains having a grain diameter less than 0.09 μm and Ti-based precipitate grains having a grain diameter less than 0.09 μm is 10 or more grains per 100 μm2 in a sheet cross section.