High-Strength Steel Sheet Delayed Fracture Resistance
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Solution Overview
Problem
Existing high-strength steel sheets with 1,320 MPa-grade tensile strength face issues with residual stress and delayed fracture resistance, particularly at cut edge surfaces, which affect fatigue properties and coating adhesion, and existing techniques fail to adequately address microcracks of several hundred micrometers in size.
Innovation Solution
A steel sheet composition with specific element content (C, Si, Mn, P, S, Al, N, O, Nb, Ti, B, and inclusion control, along with a microstructure of martensite and bainite, and a production method involving continuous casting, hot-rolling, and continuous annealing to optimize inclusion distribution and microstructure for improved delayed fracture resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If high-strength steel sheet with 1,320 MPa-grade or higher tensile strength is used, then weight reduction is achieved, but delayed fracture resistance deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.13-0.40%, Si: 1.5% or less, Mn: 1.7-3.5%, P: 0.010% or less, S: 0.0020% or less, Nb: 0.002-0.035%, Ti: 0.002-0.10%, B: 0.0002-0.0035%) and microstructure parameters (martensite and bainite with specific area fractions) to achieve both high strength and improved delayed fracture resistance. This resolves the contradiction by finding the optimal parameter combination that prevents hydrogen-induced cracking while maintaining high tensile strength.
Solution Approach 2:
The patent employs composite material principles by creating a complex microstructure consisting of multiple phases (martensite and bainite) with specific area fractions, and by incorporating multiple alloying elements that work synergistically to control microstructure formation and inhibit hydrogen penetration. This composite approach enables simultaneous achievement of high strength and enhanced delayed fracture resistance.
2Productivity
If high-strength steel sheet is formed by cold pressing, then productivity is improved, but residual stress increases and delayed fracture resistance deteriorates
Solution Approach 1:
The patent applies preliminary action by pre-controlling the chemical composition and microstructure during steelmaking and rolling processes, so that the steel sheet is prepared in advance with optimal properties for cold press forming. The specific composition control (particularly P, S, Nb, Ti, and B content) and microstructure control (martensite and bainite fractions) are established beforehand to minimize residual stress generation during subsequent cold forming operations.
3Ease of manufacture
If conventional steel composition is used, then manufacturing cost is reduced, but delayed fracture resistance at cut edge surface is insufficient
Solution Approach 1:
The patent applies local quality by focusing control efforts on specific elements and regions that most critically affect delayed fracture resistance. Particular attention is given to controlling P, S, Nb, Ti, and B content, and to the microstructure in the cut edge surface region. This targeted approach improves reliability at the critical location without requiring uniform enhancement of all material properties, thereby managing manufacturing cost effectively.
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 provides excellent delayed fracture resistance for both base steel sheets and cut edge surfaces, enabling effective cold press forming and contributing to weight reduction and strength improvement in automotive components.
Implementation Method 1
a microstructure containing martensite and bainite, the total area fraction of the martensite and the bainite being 92% or more and 100% or less
Implementation Method 2
a microstructure containing martensite and bainite
Implementation Method 3
the total of the density of inclusion particles having a long-axis length of 20 μm or more and 80 μm or less and a minimum interparticle distance of more than 10 μm and the density of inclusion particle clusters each having a long-axis cluster length of 20 μm or more and 80 μm or less and each including two or more inclusion particles having a long-axis length of 0.3 μm or more and a minimum interparticle distance of 10 μm or less being 10 pieces/mm2 or less
Implementation Method 4
hydrogen that penetrates into the steel sheet reduces the interatomic bonding forces and causes local deformation in the steel sheet
Data Source
AI summary
A steel sheet having a specified chemical composition and a tensile strength of 1,320 MPa or more, and methods for producing the steel sheet. The steel sheet has a specified microstructure including martensite and bainite, the total area fraction of the martensite and the bainite being 92% or more and 100% or less, the balance being one or more selected from ferrite and retained austenite. The forumulae [% Ti]+[% Nb]>0.007 and [% Ti]×[% Nb]2≤7.5×10−6 are satisfied in the chemical composition.