Spot Welded Joint Tempering for High-Carbon Steel Strength
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Solution Overview
Problem
The strength of spot welded joints using high carbon equivalent steel sheets decreases due to increased carbon content, leading to reduced joint strength and toughness, especially when using single energization methods in resistance spot welding.
Innovation Solution
A method involving a spot welded joint with steel sheets having a carbon content of 0.280% to 0.700% by mass, where the average aspect ratio of prior austenite grains in the melting boundary region is between 1.0 to 1.5, and the number density of iron-based carbides with an equivalent circle diameter of 30 nm or more is 3.0×10^6 per mm², along with a specific energization and tempering process to improve joint strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If rapid cooling is performed after welding to achieve martensitic structure, then strength is improved, but toughness decreases
Solution Approach 1:
The invention changes the microstructural parameters by controlling the cooling rate within a specific range (10°C/s to 1000°C/s) and applying post-heating treatment at Ac1 transformation point or higher temperature for a controlled duration (0.1 seconds to 10 seconds), which transforms the martensitic structure into a structure with improved toughness while maintaining high strength
Solution Approach 2:
The invention converts the harmful effect of rapid cooling (which creates brittle martensite) into a beneficial process by subsequently applying controlled post-heating and rapid cooling. This secondary thermal treatment transforms the brittle martensitic structure into a tougher structure, turning the initial harm into a benefit for improving joint toughness
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 enhances the cross tensile strength of the spot welded joint by regulating grain sizes and carbide distribution, improving toughness and joint strength compared to single energization methods.
Implementation Method 1
resistance spot welding method in which a sheet set in which two or more steel sheets are overlapped is clamped between a pair of electrodes, and energized to be joined while being pressurized
Implementation Method 2
the high-strength steel sheet has a large carbon equivalent (Ceq) of a base material in order to achieve its strength, and in spot welding, a welded portion is rapidly cooled after being heated, so that the welded portion has a martensitic structure
Implementation Method 3
as a post-tempering heat treatment step, after cooling for a cooling time tct (ms) represented by the formula (1), energization is performed for an energization time tt (ms) represented by the formula (3) at a current value It (kA) represented by the formula (2)
Data Source
AI summary
A spot welded joint is provided in which, in a cross section in a sheet thickness direction of a sheet set in which two or more steel sheets including at least one steel sheet having a C content of 0.280% by mass or more and 0.700% by mass or less overlap, an average of ratios of major axes to minor axes (major axes/minor axes) of prior austenite grains in a melting boundary region, which is up to 1 mm inside a melting boundary of a nugget end portion, is in a range of 1.0 to 1.5, and the number density of iron-based carbides having an equivalent circle diameter of 30 nm or more in the melting boundary region is 3.0×106×C content (mass %) or more per 1 mm2. The sheet set, in which the C content of at least one steel sheet is 0.280% by mass or more and 0.700% by mass or less, is spot welded under a specific condition, and then tempered under a specific condition.