Steel Sheet Delayed Fracture Resistance via Microstructure Control
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Solution Overview
Problem
High-strength steel sheets formed by cold pressing often experience delayed fracture at sheared end surfaces due to residual stress and microcrack propagation, particularly in thin sheets used for automotive components, where existing techniques fail to adequately address the strain-affected zones and inclusion clusters that degrade delayed fracture resistance.
Innovation Solution
A steel sheet composition with specific elements like C, Si, Mn, P, S, Al, N, B, Nb, and Ti, optimized to reduce inclusion clusters and enhance microstructural properties, such as martensite and bainite content, along with controlled annealing processes to improve delayed fracture resistance at sheared end surfaces.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If high-strength steel sheets (TS≥1320 MPa) are formed by cold pressing, then productivity and cost are improved, but delayed fracture resistance deteriorates due to residual stress and microcrack propagation
Solution Approach 1:
The invention changes the chemical composition parameters of the steel sheet by strictly controlling C content (0.13-0.40%), S content (≤0.0010%), and adding specific amounts of B (0.0003-0.0035%), Nb (0.002-0.035%), and Ti (0.002-0.040%). These parameter changes modify the material properties to reduce delayed fracture susceptibility while maintaining cold pressability and high strength
Solution Approach 2:
The invention creates a composite microstructure consisting of martensite (≥80% area ratio) with dispersed fine carbide particles and specific inclusion clusters. This composite structure combines the high strength of martensite with the hydrogen-trapping capability of fine carbide particles, resolving the contradiction between strength and delayed fracture resistance
2Ease of manufacture
If sheared end surfaces are used in high-strength steel sheets, then manufacturing simplicity is improved, but delayed fracture resistance deteriorates due to strain-affected zones and inclusion clusters
Solution Approach 1:
The invention performs preliminary action by controlling the steel composition and microstructure before shearing occurs. By pre-dispersing fine carbide particles and controlling inclusion cluster distribution and size, the material is prepared to resist delayed fracture at sheared surfaces without requiring post-shearing treatments
Solution Approach 2:
The invention converts potentially harmful large inclusion clusters into beneficial fine-dispersed inclusion clusters (≤100 μm major axis) with controlled distribution. These controlled inclusions, along with fine carbide particles, serve as hydrogen traps that prevent delayed fracture, transforming what would be defect centers into protective features
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 steel sheet achieves excellent delayed fracture resistance and high tensile strength, enabling weight reduction and cost-effectiveness in component manufacturing while maintaining structural integrity.
Implementation Method 1
performing annealing of the cold-rolled steel sheet, the annealing including soaking the cold-rolled steel sheet for 150 to 600 seconds at an annealing temperature of 860° C. or more and 910° C. or less, subsequently cooling from the annealing temperature to 420° C. at an average cooling rate of 2° C./s or more
Implementation Method 2
a microstructure including martensite and bainite, the total area fraction of the martensite and the bainite to the entirety of the microstructure being more than 90%
Data Source
AI summary
A steel sheet having a specified chemical composition and a method for producing the steel sheet. The steel sheet has a microstructure including martensite and bainite. The total area fraction of the martensite and the bainite to the entirety of the microstructure is more than 90% and 100% or less. The microstructure includes inclusion clusters A and B, the content of the clusters A in the microstructure being 2 clusters/mm2 or less, and the content of the clusters B in the microstructure being 5 clusters/mm2 or less. The microstructure includes carbide particles including Fe as a main constituent which have an aspect ratio of 2.0 or less and a major axis of 0.30 μm or more and 2 μm or less. The content of the carbide particles in the microstructure is 4000 particles/mm2 or less. The microstructure includes prior γ grains having an average size of 6 to 15 μm.