Ultra-High-Strength Steel Sheet With Manganese-Rich Hydrogen Traps
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Solution Overview
Problem
Existing high strength steel sheets with tensile strengths of 1,500 MPa or more suffer from hydrogen embrittlement due to hydrogen accumulation at grain boundaries, particularly in microstructures comprising martensite and tempered martensite, with existing technologies failing to provide effective solutions for improved hydrogen embrittlement resistance under high stress.
Innovation Solution
A steel sheet with a controlled chemical composition and microstructure, including regions with a higher manganese content than the average, dispersed uniformly at predetermined intervals, optimized through rolling, coiling, and annealing conditions to prevent hydrogen accumulation at austenite grain boundaries.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strengthening is applied to increase tensile strength to 1,500 MPa or more, then vehicle body weight is reduced and fuel efficiency is improved, but hydrogen embrittlement susceptibility increases and cracking occurs
Solution Approach 1:
The invention creates a dual-phase microstructure where martensite regions provide high strength while retained austenite regions (5-20% area ratio) act as hydrogen traps and stress relief zones. This local differentiation of material properties within the steel sheet structure allows simultaneous achievement of high tensile strength (1,500 MPa or more) and improved hydrogen embrittlement resistance through the heterogeneous distribution of phases with different functional characteristics.
Solution Approach 2:
The invention employs a composite microstructure consisting of martensite and retained austenite phases coexisting in specific proportions. The martensite phase contributes to high strength while the retained austenite phase (containing 1.0-3.0% austenite-forming elements) provides hydrogen embrittlement resistance. This composite microstructural design enables the steel sheet to achieve both high tensile strength (1,500 MPa or more) and superior hydrogen embrittlement resistance that cannot be obtained by single-phase structures.
2Strength
If martensite microstructure is used to achieve high strength, then tensile strength of 1,500 MPa or more is obtained, but hydrogen segregates to grain boundaries and causes grain boundary embrittlement
Solution Approach 1:
The invention creates a dual-phase microstructure where martensite regions provide high strength while retained austenite regions (5-20% area ratio) act as hydrogen traps and stress relief zones. This local differentiation of material properties within the steel sheet structure allows simultaneous achievement of high tensile strength (1,500 MPa or more) and improved hydrogen embrittlement resistance through the heterogeneous distribution of phases with different functional characteristics.
Solution Approach 2:
The retained austenite phase acts as an intermediary between hydrogen and martensite grain boundaries. Instead of allowing hydrogen to directly attack martensite grain boundaries and cause embrittlement, the retained austenite regions serve as intermediate hydrogen trapping zones, absorbing hydrogen atoms before they can reach and weaken the martensite grain boundaries, thereby preventing grain boundary embrittlement while maintaining high strength.
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 enhances hydrogen embrittlement resistance properties while maintaining high strength, effectively preventing cracking and improving the steel's overall performance.
Implementation Method 1
in a cross section in a sheet thickness direction, regions having an Mn content of 1.1×[Mnave] or more, where the value of [Mnave] is an average Mn content throughout the sheet thickness direction, have a number density of 5.0×10−4 pieces/μm2 or more and are present so that an average interval between closest regions having an Mn content of 1.1×[Mnave] or more is 10.0 μm or less
Implementation Method 2
The solution enhances hydrogen embrittlement resistance properties while maintaining high strength, effectively preventing cracking
Data Source
AI summary
What is provided is a steel sheet having a predetermined chemical composition, in which a microstructure includes, by area ratio, ferrite: 5.0% or less, martensite and tempered martensite: more than 90.0% in total, and a remainder: one or two or more of bainite, pearlite, and residual austenite, in a cross section in a sheet thickness direction, regions having an Mn content of 1.1×[Mnave] or more, where the [Mnave] is an average Mn content throughout the sheet thickness direction, have a number density of 5.0×10−4 pieces/μm2 or more and are present so that the average interval between closest regions having an Mn content of 1.1×[Mnave] or more is 10.0 μm or less, and a tensile strength is 1,500 MPa or more.