Ferrite-Austenite Steel Sheet Strain-Induced Martensitic Transformation
Solution Overview
Problem
Current stainless steel sheets with a ferrite phase as the parent phase lack sufficient impact absorption energy during high-speed deformation, compromising collision safety performance while also facing challenges in formability and weld toughness.
Innovation Solution
A ferrite-austenite stainless steel sheet is developed with a metastable austenite phase that undergoes strain-induced transformation to martensite during deformation, enhancing strength, work-hardening rate, and impact absorption energy, achieved by adjusting element contents and employing specific annealing processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If ferrite stainless steel sheets are used to reduce cost, then manufacturing cost is reduced, but impact absorption energy during high-speed deformation is insufficient
Solution Approach 1:
The invention changes the microstructural parameters of the steel by controlling the volume fraction of austenite phase (10-50%) and adjusting chemical composition parameters (C: 0.01-0.10%, Si: 0.01-2.00%, Mn: 0.10-10.00%, Ni: 0.50-5.00%, Cr: 10.00-25.00%). This creates a ferrite-austenite dual-phase structure that maintains low cost while enabling strain-induced martensitic transformation to improve impact absorption energy.
Solution Approach 2:
The invention creates a composite microstructure combining ferrite matrix with metastable austenite phase (10-50% volume fraction). This dual-phase composite structure allows the material to exhibit both the cost advantage of ferrite steel and the high impact absorption capability through strain-induced transformation of the austenite phase during high-speed deformation.
2Use of energy by moving object
If austenite stainless steel sheets are used to improve impact absorption, then impact absorption energy is enhanced, but manufacturing cost increases
Solution Approach 1:
The invention optimizes the chemical composition parameters to maintain Ni content at 0.50-5.00% (lower than conventional austenite steels) while controlling C and Si contents. This parameter optimization reduces material cost while preserving the strain-induced transformation capability that provides high impact absorption energy.
Solution Approach 2:
The invention creates local quality differentiation by maintaining a ferrite matrix (providing cost efficiency and ductility) with dispersed metastable austenite phases (10-50% volume fraction) that locally transform during impact. This local transformation mechanism provides high impact absorption energy without requiring the entire material to be expensive austenite steel.
3Weight of moving object
If the thickness of sheet steel is reduced to lower vehicle body weight, then vehicle body weight is reduced, but rigidity and collision safety performance deteriorate
Solution Approach 1:
The invention changes the material's mechanical parameters through controlled strain-induced martensitic transformation. During collision, the metastable austenite phase transforms to martensite, dramatically increasing strength and rigidity. This allows thinner sheets to achieve the same collision safety performance as thicker conventional sheets.
Solution Approach 2:
The invention introduces dynamic strengthening through strain-induced phase transformation. The material's microstructure dynamically changes during impact - the austenite phase transforms to martensite under high strain rate conditions, providing on-demand strength enhancement that allows weight reduction while maintaining collision safety.
4Strength
If high-strength steel sheets are used to enhance collision safety, then collision safety is improved, but corrosion resistance deteriorates
Solution Approach 1:
The invention creates a composite microstructure with ferrite matrix and austenite phase. The ferrite phase provides excellent corrosion resistance, while the austenite phase provides strength through strain-induced transformation. This composite structure achieves both high collision safety and superior corrosion resistance.
Solution Approach 2:
The invention optimizes chemical composition parameters including Cr (10.00-25.00%) for corrosion resistance, while controlling C (0.01-0.10%) and Si (0.01-2.00%) to enable strain-induced transformation. This parameter optimization achieves both high strength for collision safety and excellent corrosion resistance.
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 ferrite-austenite stainless steel sheet exhibits improved impact absorption properties, formability, and weld toughness, effectively enhancing collision safety and reducing production costs compared to austenite stainless steels, while maintaining excellent corrosion resistance.
Implementation Method 1
contains a ferrite phase as a main phase and 10% or more of an austenite phase, wherein a work-hardening rate in a strain range of up to 30% is 1000MPa or more which is measured by a static tensile testing and a difference between static and dynamic stresses which occur when 10% of deformation is caused is 150MPa or more
Implementation Method 2
a method for producing the sheet includes annealing a cold-rolled steel sheet
Data Source
Figure 1~2
Figure 3
Figure 4
AI summary
This stainless steel sheet includes, in terms of mass %, C: 0.001 to 0.1%, N: 0.01 to 0.15%, Si: 0.01 to 2%, Mn: 0.1 to 10%, P: 0.05% or less, S: 0.01 % or less, Ni: 0.5 to 5%, Cr: 10 to 25%, and Cu: 0.5 to 5%, with a remainder being Fe and unavoidable impurities, and contains a ferrite phase as a main phase and 10% or more of an austenite phase, wherein a work-hardening rate in a strain range of up to 30% is 100MPa or more which is measured by a static tensile testing and a difference between static and dynamic stresses which occur when 10% of deformation is caused is 150MPa or more. This method for producing a stainless steel includes annealing a cold-rolled steel sheet under conditions where a holding temperature is set to be in a range of 950 to 1150°C and a cooling rate until 400°C is set to be in a range of 3°C/sec or higher.