Lean Duplex Stainless Steel Corrosion Resistance and Ductility
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Solution Overview
Problem
Existing duplex stainless steels face challenges in achieving both excellent corrosion resistance and formability, particularly ductility, while maintaining cost-effectiveness by reducing the use of high-priced alloy elements like nickel and molybdenum, and are prone to thermal martensite formation during cooling, which deteriorates elongation.
Innovation Solution
A lean duplex stainless steel composition is developed with adjusted amounts of alloying components such as Si, Mn, Cr, Ni, Cu, N, Mo, and W, and controlled cooling conditions during coiling and cooling after hot rolling to inhibit thermal martensite formation, enhancing the phase ratio of austenite and ferrite phases, thereby improving elongation and corrosion resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the content of high-priced alloy elements such as nickel and molybdenum is reduced to lower manufacturing costs, then cost-effectiveness is improved, but corrosion resistance may deteriorate
Solution Approach 1:
The patent applies parameter changes by precisely controlling the compositional ranges of alloying elements. It specifies Cr content at 22-27%, Ni at 2.5-4.0%, Mo at 0.20-1.0%, and adds Cu at 1.5-3.0%, while controlling C at 0.03% or less and N at 0.10-0.30%. These parameter adjustments maintain corrosion resistance by ensuring sufficient Cr and Ni for passivation while adding Cu to enhance pitting corrosion resistance, all within ranges that control manufacturing cost.
Solution Approach 2:
The patent creates a composite microstructure consisting of dual phases: ferrite phase (20-40% area ratio) and austenite phase (60-80% area ratio). This composite structure combines the corrosion resistance of ferrite with the ductility and formability of austenite, achieving both reliability and ease of manufacture by leveraging the synergistic effects of different phases rather than relying solely on expensive alloying elements.
2Ease of manufacture
If the content of nickel is reduced to compensate for price fluctuations, then price competitiveness is improved, but corrosion resistance may be compromised
Solution Approach 1:
The patent optimizes the Ni content parameter to a specific range of 2.5-4.0%, which is lower than conventional austenitic stainless steels that typically require 8% or more. This parameter change maintains adequate corrosion resistance while improving price competitiveness. The patent compensates for the reduced Ni content by increasing Cr to 22-27% and adding Cu at 1.5-3.0%, which enhances pitting corrosion resistance, thereby maintaining reliability at lower cost.
3Reliability
If duplex stainless steel is designed to enhance corrosion resistance, then reliability is improved, but ductility and formability deteriorate
Solution Approach 1:
The patent employs a composite microstructure with dual phases: ferrite (20-40% area ratio) providing corrosion resistance and austenite (60-80% area ratio) providing ductility and formability. This composite structure resolves the contradiction by allowing each phase to contribute its advantageous properties, achieving both high corrosion resistance and excellent ductility that cannot be obtained with single-phase structures.
Solution Approach 2:
The patent applies local quality by creating a heterogeneous microstructure where different phases are distributed throughout the material. The ferrite phase is locally distributed to provide corrosion resistance in specific regions, while the austenite phase is distributed to provide ductility and formability in other regions, allowing the material to exhibit both properties simultaneously at the macro level.
4Device complexity
If cooling conditions are not controlled during coiling and cooling after hot rolling, then manufacturing process is simplified, but thermal martensite forms which deteriorates elongation
Solution Approach 1:
The patent applies parameter changes by specifying precise cooling conditions: coiling temperature of 200-400°C and cooling rate of 0.5-5.0°C/s after hot rolling. These parameter changes control the phase transformation during cooling, preventing the formation of brittle thermal martensite and ensuring the formation of the desired ferrite-austenite dual-phase structure, thereby maintaining high elongation without significantly increasing process complexity.
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 results in a stainless steel with elongation of 35% or more in the hot annealed state and 40% or more in the cold annealed state, maintaining corrosion resistance equal to or better than STS 304 steels, while reducing manufacturing costs.
Implementation Method 1
a duplex stainless steel having a fine structure consisting of a mixture of an austenite phase and a ferrite phase
Implementation Method 2
inhibiting formation of thermal martensite via adjustment of cooling conditions during coiling and cooling after hot rolling
Implementation Method 3
an elongation of a hot annealed steel to 35% or more and an elongation of a cold annealed steel to 40% or more
Data Source
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AI summary
A lean duplex stainless steel and a method of manufacturing the same are provided. The lean duplex stainless steel includes, in percent (%) by weight of the entire composition, 0.08% or less of carbon (C) (excluding 0), 0.7 to 1.1% of silicon (Si), 2.4 to 3.5% of manganese (Mn), 17.9 to 20.7% of chromium (Cr), 0.05 to 1.15% of nickel (Ni), 0.18 to 0.3% of nitrogen (N), 0.4 to 2.8% of copper (Cu), and the remainder of iron (Fe) and inevitable impurities, wherein a predicted pitting potential is from 360 to 440 mV. Thus, manufacturing costs may be reduced via adjustment of components of the duplex stainless steel and both of formability and corrosion resistance may be improved by improving corrosion resistance and increasing elongation. Formability may be improved by inhibiting formation of thermal martensite and increasing elongation via adjustment of cooling conditions during coiling and cooling after hot rolling.