Resistance-Welded Automotive Member for Plug Fracture Strength
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Solution Overview
Problem
High-strength steel sheets with tensile strength exceeding 900 MPa face challenges in maintaining ductility and productivity due to limitations in carbon content and increased steelmaking costs, along with a high likelihood of interface fractures during welding, which decreases the strength of welds and prolongs assembly processes.
Innovation Solution
Adjusting the steel sheet composition to optimize manganese content and adding Ti or Nb to refine the microstructure, reducing stress concentration and preventing interface fractures by ensuring higher hardness in the heat-affected zone, thereby achieving a plug fracture in cross tensile tests.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the C content is limited to maintain carbon equivalent within a predetermined range, then the weld strength is improved, but the ductility of the steel sheet deteriorates
Solution Approach 1:
The invention changes the composition parameters by limiting C content to 0.23% or less while ensuring Si content is 0.70% or more and Mn content is 1.50% or more. This parameter adjustment maintains carbon equivalent within a range that ensures weld strength while preserving the ductility of the steel sheet, achieving both strong welds and adequate formability.
2Strength
If low-P or low-S processing is implemented to improve weld quality, then the weld strength is improved, but the steelmaking cost increases
Solution Approach 1:
The invention changes the composition parameters to Si: 0.70% or more and Mn: 1.50% or more, which inherently improve weld quality and prevent interface fracture. This approach achieves strong welds without requiring stringent low-P or low-S processing, thereby avoiding the increased steelmaking costs associated with such restrictive composition control.
3Strength
If the welding time is prolonged to ensure adequate weld strength, then the weld quality is improved, but the assembly productivity decreases
Solution Approach 1:
The invention changes the base material composition to Si: 0.70% or more and Mn: 1.50% or more, which creates a steel sheet that forms strong welds with plug fracture mode under conventional welding conditions. This eliminates the need for prolonged welding times, thereby maintaining high assembly productivity while ensuring adequate weld strength and quality.
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 optimized composition ensures sufficient strength and ductility in high-strength steel sheets, preventing interface fractures and achieving a plug fracture in cross tensile tests, enhancing the resistance weld's reliability and productivity.
Implementation Method 1
adding Ti or Nb to refine the microstructure
Implementation Method 2
resistance welding
Data Source
Figure 1~2
Figure 3~4
AI summary
Provided is an automotive member having a resistance weld for which the fracture form in a cross tensile test is a plug fracture, the automotive member being a high-strength steel sheet, or particularly, a steel sheet having a tensile strength exceeding 900 MPa. The automotive member has a resistance weld for fixing two or more steel sheets including at least one high-strength steel sheet containing a predetermined composition, the balance being Fe and unavoidable impurities, in which a maximum hardness (HVBM) in a heat-affected zone of the resistance weld is at least 1.1 times hardness (HVW) of a nugget in the resistance weld formed in a softest steel sheet of a sheet set during resistance welding, and furthermore, an average grain size of a steel sheet structure of the heat-affected zone within 2 mm in a direction at a right angle to a sheet thickness from an end part of the nugget of the high-strength steel sheet is 3 µm or less, and a minimum hardness (HVmin) in the heat-affected zone is at least 90% of hardness (HVα) of the high-strength steel sheet before the resistance welding.