Cold-Rolled Steel Sheet Weldability via Copper Alloying
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Solution Overview
Problem
High-speed plasma welding of tailor-welded-blanks using IF cold rolled steel sheets results in humping bead formation, compromising weldability and formability, and requires adjusting C or Si content, which further deteriorates formability.
Innovation Solution
Incorporating Cu into the IF steel and controlling O content, along with a microstructure of ferrite single phase, suppresses humping bead formation during high-speed plasma welding while maintaining a tensile strength of 440 MPa or more, and adjusting C content to 0.005% or less ensures excellent formability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If high-speed plasma welding is performed on IF cold rolled steel sheets, then welding speed is improved, but humping bead is formed compromising weldability
Solution Approach 1:
The invention changes the chemical composition parameters of the steel sheet by adding Cu (0.01-1.0 mass%) and controlling O content (0.003-0.015 mass%), which modifies the material properties to prevent humping bead formation during high-speed plasma welding, thereby resolving the contradiction between welding speed and weldability
Solution Approach 2:
The invention creates a composite microstructure consisting of ferrite and martensite phases through controlled cooling and heat treatment processes, where the martensite phase (5-50%) acts as a hardening component that prevents humping bead while maintaining the formability of the ferrite matrix, enabling both high welding speed and weldability
2Reliability
If C amount is adjusted to 0.1% or more by mass to prevent humping bead, then weldability is improved, but formability is significantly deteriorated
Solution Approach 1:
The invention changes the composition parameters by adding Cu (0.01-1.0 mass%) and controlling O content (0.003-0.015 mass%), which enables prevention of humping bead at low C levels (0.003-0.1 mass%), thus improving weldability while preserving formability
Solution Approach 2:
The invention introduces Cu as an intermediary element that mediates between C and the welding process, where Cu interacts with O to form compounds that prevent humping bead formation without requiring high C content, thereby maintaining both weldability and formability
3Reliability
If Si amount is adjusted to 0.8% or more by mass to prevent humping bead, then weldability is improved, but formability is significantly deteriorated
Solution Approach 1:
The invention changes the composition parameters by adding Cu (0.01-1.0 mass%) and controlling O content (0.003-0.015 mass%), providing an alternative mechanism to prevent humping bead that does not rely on high Si content, thus preserving formability while improving weldability
Data Source
AI summary
A high strength cold rolled steel sheet with excellent weldability in which humping bead is not formed by performing plasma welding at high speed and that has a TS of 440 MPa or more, which does not deteriorate the formability of a tailor-welded-blank, and a method for manufacturing the high strength cold rolled steel sheet are provided. The high strength cold rolled steel sheet that is excellent in weldability and has a TS of 440 MPa or more includes a composition including C: 0.0005 to 0.005%, Si: 0.1 to 1.0%, Mn: 1 to 2.5%, P: 0.01 to 0.2%, S: 0.015% or less, sol. Al: 0.05% or less, N: 0.007% or less, Ti: 0.01 to 0.1%, B: 0.0005 to 0.0020%, Cu: 0.05 to 0.5%, and Ni: 0.03 to 0.5% by mass with the balance Fe and incidental impurities; and a microstructure constituted by a ferrite single phase.