Steel Pipe Structural Member With Weld-Softening-Resistant Ends
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing methods for manufacturing long structural members for large vehicles like buses face challenges in maintaining high load-bearing performance and impact energy absorption due to heat input softening during welding of quenched steel pipes, making it difficult to identify and achieve optimal transition portions for strength changes.
Innovation Solution
The use of high strength steel pipes with specific microstructural compositions and welding techniques, including quenched and non-quenched portions, to prevent heat input softening and ensure consistent strength across connections.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If quenched steel pipes are welded to manufacture long structural members, then the length requirement is satisfied, but heat input softening occurs and strength is reduced
Solution Approach 1:
The steel pipe is designed with local quality variation through selective quenching: the pipe center portion has a quenched microstructure (martensite/bainite) for high strength, while the pipe end portions have a non-quenched microstructure (ferrite/pearlite) for lower strength and weldability. This local differentiation allows welding at end portions without heat input softening while maintaining high strength in the center portion that bears the load.
Solution Approach 2:
The microstructure at the pipe end portions is preliminarily prepared to be non-quenched (ferrite/pearlite) before welding occurs. This preliminary preparation ensures that when welding is performed at the end portions, the material is already in a state resistant to heat input softening, eliminating the need for post-weld heat treatment or strength recovery measures.
2Strength
If transition portions are created for strength change, then load-bearing performance is improved, but identification and manufacturing precision are difficult
Solution Approach 1:
The invention utilizes phase transition of steel microstructure through controlled quenching: the pipe center portion undergoes quenching to form martensite/bainite phases for high strength, while the pipe end portions remain in ferrite/pearlite phases for lower strength and better weldability. This phase transition approach creates distinct, identifiable regions with different microstructures and properties, making transition portions easily distinguishable and manufacturable.
Data Source
AI summary
This structural member is a structural member including a plurality of high strength steel pipes, in which the high strength steel pipe has a quenched portion in a pipe center portion and a non-quenched portion extending over a whole circumference of at least one pipe end portion, in the quenched portion, an area ratio of a martensite is 90% or more, in the non-quenched portion, an area ratio of a ferrite is 30% or more and 100% or less, an area ratio of a pearlite is 0% or more and 70% or less, and a total area ratio of a martensite and a bainite is 0% or more and 10% or less, and the non-quenched portion has a welded portion that is welded to another member.