Hot-Pressed Steel Sheet With Ni Diffusion for Welded Fracture Resistance
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Solution Overview
Problem
High-strength steel sheets with tensile strength of 1780 MPa or greater used in automotive structural members exhibit low ductility, leading to cracking and spring back during cold press forming, and are prone to delayed fracture due to residual stress and hydrogen embrittlement, especially after projection welding.
Innovation Solution
Inhibiting Mn segregation and creating a Ni diffusion region in the surface layer of hot-pressed members to reduce potential differences between welded portions and bolts/nuts, thereby preventing hydrogen absorption and microcrack formation, which enhances delayed fracture resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If high-strength steel sheet (TS≥1780 MPa) is used to reduce vehicle weight, then weight reduction is achieved, but delayed fracture resistance deteriorates due to hydrogen embrittlement and residual stress
Solution Approach 1:
The patent applies local quality by creating a dual-layer surface structure: a Fe-Al alloy layer (5-15 μm thick) providing corrosion resistance and a Ni diffusion region (0.5-5 μm thick) providing hydrogen barrier properties. This localized modification of surface composition and structure addresses the delayed fracture issue at the surface level while maintaining the high strength of the bulk material, thereby resolving the contradiction between weight reduction and delayed fracture resistance.
2Ease of manufacture
If cold press forming is used to form high-strength steel sheet, then forming capability is achieved, but dimensional accuracy deteriorates due to spring back and cracking
Solution Approach 1:
The patent employs parameter changes by modifying the surface composition through controlled addition of Al (0.01-2.0 mass%) and Ni (0.01-1.0 mass%), and by controlling the heating temperature during hot pressing (Ac3 transformation temperature to 1100°C). These parameter changes enable the steel sheet to achieve both formability during cold press forming and dimensional accuracy after forming, while the subsequent heat treatment maintains the high strength required for the application.
3Productivity
If projection welding is used to attach nuts to hot-pressed member, then assembling efficiency is improved, but delayed fracture resistance deteriorates at welded portions due to hydrogen absorption
Solution Approach 1:
The patent uses the Ni diffusion region as an intermediary barrier between the external environment (source of hydrogen) and the welded portion of the steel sheet. The Ni-rich surface layer (0.5-5 μm thick) acts as a hydrogen trap and diffusion barrier, preventing hydrogen generated during projection welding from penetrating into the steel substrate. This intermediary layer protects the welded portion from hydrogen embrittlement while allowing the projection welding process to proceed efficiently.
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 approach results in hot-pressed members with improved delayed fracture resistance and high tensile strength, reducing vehicle weight and enhancing fuel efficiency while maintaining dimensional accuracy and reducing residual stress.
Implementation Method 1
creating a Ni diffusion region in the surface layer of hot-pressed members
Implementation Method 2
a steel sheet having a microstructure in which a prior austenite average grain diameter is 8 μm or less, and martensite is present in a volume fraction of 95% or greater
Data Source
AI summary
A hot-pressed member has a predetermined chemical composition. In the hot-pressed member, a steel sheet has a microstructure in which a prior austenite average grain diameter is 8 μm or less, and martensite is present in a volume fraction of 95% or greater in a region within 30 μm of a surface; a Ni diffusion region having a thickness of 0.5 μm or greater exists in a surface layer; a standard deviation of Vickers hardness values is 35 or less; Mndif (mass %) in a sheet thickness direction ≤0.20, where Mndif (mass %) is a degree of Mn segregation; and a tensile strength is 1780 MPa or greater.