Duplex Stainless Steel Microstructure for Strength and Pitting Resistance
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Solution Overview
Problem
Deep wells below sea level require duplex stainless steel materials with high strength, excellent low-temperature toughness, and pitting resistance, which existing technologies have not adequately addressed.
Innovation Solution
A duplex stainless steel material with a specific chemical composition and microstructure, including 30.0 to 70.0% ferrite and 50 nm or less Cu precipitates in austenite, satisfying the condition Cr+3.3(Mo+0.5W)+16N ≥ 30.0, to achieve yield strength of 586 MPa or more and excellent pitting resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the content of Cr is increased to form a passive film, then corrosion resistance is improved, but strength is not sufficiently enhanced
Solution Approach 1:
The patent employs a duplex microstructure comprising both ferrite phase and austenite phase, creating a composite material system where each phase contributes different properties. The ferrite phase provides corrosion resistance through passive film formation, while the austenite phase contributes to strength, thereby resolving the contradiction between corrosion resistance and strength.
Solution Approach 2:
The patent controls specific compositional parameters (Cr: 20-30%, Ni: 4-9%, Mo: 0.5-2%, Cu: 1-3%, N: 0.1-0.3%) and microstructural parameters (ferrite phase proportion: 30-70%, PREW ≥ 30) to simultaneously achieve high strength and excellent corrosion resistance. This parameter optimization resolves the contradiction by finding the optimal balance point.
2Strength
If the strength of duplex stainless steel is enhanced, then high strength is achieved, but low-temperature toughness deteriorates
Solution Approach 1:
The patent optimizes the compositional parameters (particularly Cr, Ni, Mo, Cu, and N content) and microstructural parameters (ferrite phase proportion and PREW value) to achieve a balance where yield strength reaches 586 MPa or more while maintaining excellent low-temperature toughness. This parameter optimization resolves the contradiction between strength and toughness.
3Strength
If the strength of duplex stainless steel is enhanced, then high strength is achieved, but pitting resistance deteriorates
Solution Approach 1:
The duplex microstructure creates a composite system where the ferrite phase provides pitting resistance through passive film formation (with PREW ≥ 30) while the austenite phase contributes to strength enhancement. This composite structure resolves the contradiction between strength and pitting resistance.
Solution Approach 2:
The patent controls specific parameters including Cr content (20-30%), Ni content (4-9%), Mo content (0.5-2%), Cu content (1-3%), and N content (0.1-0.3%) to ensure PREW ≥ 30, thereby maintaining pitting resistance while achieving high strength through the duplex microstructure.
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 proposed steel material achieves a yield strength of 586 MPa or more, maintains excellent low-temperature toughness, and retains superior pitting resistance, overcoming limitations of previous technologies.
Implementation Method 1
in the austenite, a number density of Cu precipitates having a major axis of 50 nm or less is 150 to 1500/μm3
Implementation Method 2
the content of chromium (Cr) is increased to form a passive film mainly composed of Cr oxides on the surface of the steel material
Data Source
AI summary
A duplex stainless steel material according to the present disclosure consists of, by mass %, C: 0.030% or less, Si: 0.20 to 1.00%, Mn: 0.50 to 7.00%, P: 0.040% or less, S: 0.020% or less, Al: 0.100% or less, Ni: 4.20 to 9.00%, Cr: 20.00 to 30.00%, Mo: 0.50 to 2.00%, Cu: 1.50 to 4.00%, N: 0.150 to 0.350%, and V: 0.01 to 1.50%, with the balance being Fe and impurities, and satisfies Formula (1) described in the description. A microstructure consists of, in volume ratio, ferrite in an amount of 30.0 to 70.0% with the balance being austenite. The yield strength is 586 MPa or more. A number density of Cu precipitates having a major axis of 50 nm or less in the austenite is 150 to 1500/μm3.