Spot Welded Joint Phosphorus Segregation Control
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Solution Overview
Problem
High-strength steel sheets with tensile strengths above 750 MPa pose challenges in achieving stable cross-tension strength (CTS) in spot welded joints due to stress concentration and reduced toughness, leading to potential peeling fractures and compromised joint strength.
Innovation Solution
A spot welding process involving main energizing followed by post-heating to solidify the nugget edge and then applying a secondary energizing current to heat and modify the nugget edge, reducing phosphorous and sulfur segregation, thereby enhancing the CTS and joint reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If quench hardening is applied to increase the strength of the nugget, then the tensile strength increases, but the toughness of the nugget decreases leading to peeling fracture
Solution Approach 1:
The invention creates different microstructural zones within the nugget: the center region undergoes quench hardening for high strength, while the outer layer region (50 μm to 150 μm from center) is specifically treated to have controlled carbide precipitation and finer microstructure that provides both strength and toughness, preventing peeling fracture
Solution Approach 2:
The invention precisely controls the carbide particle size (0.5 μm to 2.0 μm average) and number density (5×10^18 to 20×10^18 particles/m³) in the outer layer region, along with the microstructure average crystal grain size (5 μm to 15 μm), to achieve the optimal balance between toughness and strength
2Reliability
If post heating energizing is performed to improve toughness, then the toughness increases, but productivity deteriorates due to lengthened energizing time
Solution Approach 1:
The invention performs the heat treatment action during the normal welding cycle by utilizing the inherent heat generation from the welding current itself, rather than as a separate post-process. The controlled cooling rate (10°C/s to 100°C/s) achieved through specific welding parameters effectively performs the heat treatment needed for toughness improvement within the original welding time frame
Solution Approach 2:
The invention uses the welding current itself to generate the heat needed for the heat treatment of the nugget. The electrical resistance heating during the welding process automatically provides the thermal energy required to control the microstructure and improve toughness, eliminating the need for external heating equipment or additional time
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 process improves the CTS of spot welded joints in high-strength steel sheets by mitigating brittle element segregation, resulting in a more reliable and stable joint strength, particularly for steel sheets with tensile strengths between 750 MPa and 2500 MPa.
Implementation Method 1
a two-step energizing method in which heating energizing is performed after main energizing
Implementation Method 2
post heating energizing is performed with a current value that is equal to or higher than a main energizing current value
Implementation Method 3
forming a nugget by main energizing
Implementation Method 4
the nugget edge is solidified and then heated during post-energizing
Implementation Method 5
tempering energizing is performed after the passage of a predetermined amount of time from completion of the main energizing, and a nugget portion and a heat-affected zone of the spot welded joint are annealed to decrease hardness
Data Source
AI summary
In a spot welded joint according to an aspect of the present invention, with respect to a rectangular planar region of 100 μm×100 μm which is perpendicular to a sheet surface of the plurality of the steel sheets and which centers around an inner point spaced away by 100 μm in a direction parallel with the sheet surface from an edge of a nugget, in a case of measuring a P concentration at a pitch of 1 μm to obtain measured values of the P concentration at 100×100 of measurement points, and in a case of repetitively calculating an average value of the measured values of the P concentration at 20 of the measurement points which are adjacent to each other and which are arranged in a row in the direction parallel with the sheet surface among the 100×100 of the measurement points while shifting each one point to obtain 81×100 of the average values, the number of the average values which are more than two times an average P concentration is 0 to 100.