High-Strength Steel Sheet Microstructure for Hydrogen Fracture Resistance
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Solution Overview
Problem
Existing high strength steel sheets with tensile strength of 1310 MPa or more face challenges in maintaining delayed fracture resistance, leading to potential fractures when exposed to hydrogen environments.
Innovation Solution
A steel sheet with a chemical composition of C: 0.12% to 0.40%, Si: 1.5% or less, Mn: 1.8% to 4.0%, and specific contents of Nb and Ti, along with a microstructure of 95% martensite or more, where 90% of Nb and Ti are present as carbonitrides with diameters of 100 nm or more, and a coating layer, to enhance delayed fracture resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If high strength steel sheets with 1310 MPa or more tensile strength are formed by cold pressing, then weight saving and productivity are improved, but residual stress increases and delayed fracture resistance deteriorates
Solution Approach 1:
The invention changes the chemical composition parameters of the steel sheet by precisely controlling the content ranges of C, Si, Mn, P, S, N, and Al, and adding specific amounts of Nb and Ti. These parameter changes enable the steel to achieve both high strength and improved delayed fracture resistance through controlled precipitation of fine carbides that trap hydrogen
Solution Approach 2:
The invention creates a composite microstructure consisting of tempered martensite (50% or more area fraction) combined with fine carbide precipitates formed by Nb and Ti additions. This composite structure provides both the high strength needed for cold pressing and the hydrogen trapping capability to prevent delayed fracture
2Weight of moving object
If high strength steel sheets with 1310 MPa or more tensile strength are used, then weight saving is achieved, but hydrogen enters the steel sheet causing microcracks and delayed fracture
Solution Approach 1:
The invention converts the harmful effect of hydrogen into a beneficial one by using Nb and Ti to form fine carbide precipitates that act as hydrogen trapping sites. These precipitates capture hydrogen atoms before they can cause embrittlement, transforming hydrogen from a harmful factor into a trapped, harmless state within the steel matrix
Solution Approach 2:
The fine carbide precipitates formed by Nb and Ti act as intermediary elements between hydrogen and the steel matrix. These precipitates serve as hydrogen trapping sites that intercept hydrogen atoms, preventing them from reaching critical concentrations that would cause delayed fracture
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 solution significantly improves delayed fracture resistance, maintaining strength while reducing the risk of hydrogen-induced fractures, suitable for automotive parts and weight-saving applications.
Implementation Method 1
90% of Nb and Ti are present as carbonitrides with diameters of 100 nm or more
Implementation Method 2
precipitation of fine carbides serving as hydrogen trapping sites improves the delayed fracture resistance
Implementation Method 3
The steel sheet has a microstructure in which an area fraction of martensite relative to the entire microstructure is 95% or more
Data Source
AI summary
Provided are a steel sheet having high strength and high delayed fracture resistance and a method for manufacturing the steel sheet. The steel sheet has a specific chemical composition and a microstructure in which the area fraction of martensite is 95% to 100%, with the balance being one or more of bainite, ferrite, and retained austenite. In the steel sheet, prior-austenite grains have an average grain size of 18 μm or less, 90 mass % or more of the total content of Nb and Ti contained is present as a carbonitride having an equivalent circular diameter of 100 nm or more, and a Nb carbonitride and a Ti carbonitride, having an equivalent circular diameter of 1.0 μm or more, are present at a rate of 800 pieces/mm2 or less in total. The steel sheet has a tensile strength of 1310 MPa or more.