High-Strength Steel Sheet Microstructure for LME-Resistant Formability
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Solution Overview
Problem
Current steel sheets with high tensile strength (TS) of 1180 MPa or more lack comprehensive performance in yield stress (YS), ductility, stretch-flangeability, delayed fracture resistance, and liquid metal embrittlement (LME) resistance.
Innovation Solution
A steel sheet with a chemical composition of Si: 0.20% to 2.00% and Mn: 2.70% to 4.00%, and a microstructure comprising ferrite, bainitic ferrite, tempered martensite, and retained austenite, optimized to achieve the desired mechanical properties and resistance to LME cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the tensile strength is increased to 1180 MPa or more, then the strength is improved, but the formability (ductility and stretch-flangeability) deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.15-0.35%, Si: 0.70-1.70%, Mn: 2.00-3.50%, P: 0.030-0.090%) and microstructure parameters (ferrite area fraction: 5-30%, bainitic ferrite area fraction: 5-35%, tempered martensite area fraction: 20-50%, retained austenite volume fraction: 5-35%) to achieve TS≥1180 MPa while maintaining formability. This systematic parameter optimization resolves the contradiction between high strength and formability.
Solution Approach 2:
The patent creates a composite microstructure consisting of four phases: ferrite, bainitic ferrite, tempered martensite, and retained austenite. Each phase contributes different properties: ferrite provides ductility, tempered martensite provides strength, and retained austenite provides formability through TRIP effect. This multi-phase composite microstructure simultaneously achieves high tensile strength (≥1180 MPa) and good formability.
2Strength
If the yield stress is improved, then the strength is increased, but the formability deteriorates
Solution Approach 1:
The patent controls yield stress through parameter optimization: Ceq (0.697% or more), specific alloying element contents (Si: 0.70-1.70%, Mn: 2.00-3.50%), and microstructure parameters (ferrite area fraction: 5-30%, retained austenite volume fraction: 5-35%). This achieves YS≥1000 MPa while maintaining formability through the balanced microstructure.
Solution Approach 2:
The multi-phase composite microstructure (ferrite + bainitic ferrite + tempered martensite + retained austenite) provides both high yield stress (≥1000 MPa) and formability. The retained austenite (5-35% volume fraction) undergoes strain-induced transformation during forming, providing the TRIP effect that maintains formability even at high yield stress levels.
3Strength
If high-strength steel sheets with high C, Si, and Mn contents are used, then the tensile strength is increased to 1180 MPa or more, but liquid metal embrittlement cracking occurs during spot welding
Solution Approach 1:
The patent optimizes chemical composition parameters to achieve TS≥1180 MPa while preventing LME cracking: C: 0.15-0.35% (not excessively high), Si: 0.70-1.70%, Mn: 2.00-3.50%, P: 0.030-0.090%. This balanced composition achieves high strength without causing LME susceptibility during spot welding of galvanized sheets.
Solution Approach 2:
The patent creates local quality differences through the microstructure: ferrite regions (5-30% area fraction) with lower strength provide LME resistance, while tempered martensite regions (20-50% area fraction) provide high strength. This spatial distribution of different microstructural phases allows the material to resist LME cracking while maintaining overall high strength.
Data Source
AI summary
A steel sheet with a tensile strength (TS) of 1180 MPa or more, a member, and a method for producing them. In a region of the steel sheet within 4.9 μm in the thickness direction, a region with a Si concentration not more than one-third of the Si concentration in the chemical composition of the steel sheet and with a Mn concentration not more than one-third of the Mn concentration in the chemical composition of the steel sheet has a thickness of 1.0 μm or more. The lowest Si concentration LSi and the lowest Mn concentration LMn in the region within 4.9 μm in the thickness direction from the surface of the steel sheet and a Si concentration TSi and a Mn concentration TMn at a quarter thickness position of the steel sheet satisfy the following formula (1):LSi+LMn≤(TSi+TMn)/4 (1).