Multiphase Steel Sheet Composition for Cold Press Formability
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
High-strength steel sheets with tensile strength of 1,470 MPa or more face issues with decreased ductility, cracking during cold pressing, delayed fracture, and liquid metal embrittlement cracking (LME) due to high Si content, limiting their application to complex shapes and increasing the risk of weld failure.
Innovation Solution
A steel sheet composition with specific ranges of C, Si, Mn, P, S, N, and other elements, combined with a microstructure of martensite, bainite, and retained austenite, optimized through controlled cooling and tempering, to enhance formability, delayed fracture resistance, and LME resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the tensile strength is increased to 1,470 MPa or more, then the strength is improved, but the ductility decreases causing cracking during cold pressing
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.20-0.40%, Si: 0.10-1.00%, Mn: 1.50-3.50%) and heat treatment parameters (cooling rate: 5-50°C/s, tempering temperature: 150-250°C) to achieve the optimal balance between strength and formability. This systematic parameter optimization enables the steel to attain tensile strength of 1,470 MPa or more while maintaining sufficient ductility for cold pressing operations.
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (martensite, bainite, and retained austenite) with specific area fractions (martensite: 40-78%, bainite: 20-58%, retained austenite: 2-60%). This composite microstructure combines the high strength of martensite with the ductility contribution from retained austenite and bainite, resolving the contradiction between strength and formability.
2Strength
If the tensile strength is increased to 1,470 MPa or more, then the strength is improved, but delayed fracture resistance deteriorates due to increased residual stress
Solution Approach 1:
The patent controls the tempering temperature parameter within 150-250°C to reduce residual stress while maintaining high strength. This parameter optimization prevents the deterioration of delayed fracture resistance that typically occurs with high-strength steels, achieving both high tensile strength (1,470 MPa or more) and improved delayed fracture resistance.
Solution Approach 2:
The patent creates local quality differences in the microstructure by controlling the distribution of different phases (martensite, bainite, retained austenite) and their characteristics (carbide particle size ≤0.40 μm in martensite, C content in retained austenite ≥0.50%). This heterogeneous microstructure with optimized local properties reduces stress concentration and improves delayed fracture resistance while maintaining high overall strength.
3Reliability
If the Si content is increased to improve delayed fracture resistance, then the delayed fracture resistance is improved, but liquid metal embrittlement cracking occurs during welding
Solution Approach 1:
The patent optimizes the Si content parameter within the range of 0.10-1.00% to achieve the desired balance. This controlled Si addition provides sufficient delayed fracture resistance while preventing excessive Si accumulation that would cause liquid metal embrittlement cracking during welding operations. The patent also controls other alloying elements (Mn: 1.50-3.50%, C: 0.20-0.40%) to synergistically achieve both objectives.
4Strength
If hot pressing is used to increase strength, then the strength is improved, but the cost and productivity decrease
Solution Approach 1:
The patent changes the parameter of heat treatment temperature by采用 cold pressing with tempering (150-250°C) instead of hot pressing (>Ac3 temperature). This parameter change in the thermal processing regime enables achieving high strength (1,470 MPa or more) through a simpler, faster process that improves productivity and reduces manufacturing cost while eliminating the need for high-temperature equipment and complex heating/cooling cycles.
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 a high-strength steel sheet with excellent formability, delayed fracture resistance, and LME resistance, enabling its application in complex parts and reducing part weight and failure risks in cold press forming.
Implementation Method 1
a steel microstructure containing, in area fraction, martensite: 40% or more and 78% or less, bainite: 20% or more and 58% or less, and retained austenite: 2% or more
Implementation Method 2
an average amount of C in the retained austenite is 0.5% by mass or more, a Si concentration in a region within 100 μm in a thickness direction from a surface of the steel sheet is 1.3% by mass or less
Data Source
AI summary
A steel sheet has a certain chemical composition and a steel microstructure containing, in area fraction, martensite: 40% or more and 78% or less, bainite: 20% or more and 58% or less, and retained austenite: 2% or more. Carbides in tempered martensite in the martensite have an average particle size of 0.40 μm or less, an average amount of C in the retained austenite is 0.5% by mass or more, a Si concentration in a region within 100 μm in a thickness direction from a surface of the steel sheet is 1.3% by mass or less, and a tensile strength is 1,470 MPa or more.