Seamless Steel Pipe Duplex Microstructure High Strength
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Solution Overview
Problem
Existing seamless steel pipes for oil wells lack a yield strength of 862 MPa or more while maintaining excellent low-temperature toughness.
Innovation Solution
A seamless steel pipe with a chemical composition of C: 0.050% or less, Si: 0.50% or less, Mn: 0.01 to 0.20%, P: 0.025% or less, S: 0.0150% or less, Cu: 0.09 to 3.00%, Cr: 15.00 to 18.00%, Ni: 4.00 to 9.00%, Mo: 1.50 to 4.00%, Al: 0.040% or less, N: 0.0150% or less, Ca: 0.0010 to 0.0040%, Ti: 0.020% or less, Nb: 0.020% or less, and V: 0 to 0.20%, combined with a duplex microstructure of ferrite and martensite, and a layered structure extending in both the L-direction and C-direction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional 13Cr or 17Cr steel material is used to achieve corrosion resistance in high-temperature environments, then corrosion resistance is improved, but yield strength of 862 MPa or more cannot be achieved
Solution Approach 1:
The patent optimizes the chemical composition parameters by precisely controlling the content ranges of Cr (15-18%), Ni (4-9%), Mo (1.5-4%), and other alloying elements. This parameter optimization enables the steel to achieve both high corrosion resistance in high-temperature environments and high yield strength of 862 MPa or more, resolving the contradiction between these two properties
Solution Approach 2:
The patent creates a composite microstructure consisting of ferrite and martensite phases. This dual-phase composite structure combines the corrosion resistance and toughness of ferrite with the high strength of martensite, while the layered distribution pattern further enhances both properties simultaneously
2Strength
If high-strength steel pipe is produced to meet yield strength requirements, then strength is improved, but low-temperature toughness deteriorates
Solution Approach 1:
The patent controls the chemical composition parameters, particularly keeping C content at 0.05% or less and optimizing Ni and Cr contents, to prevent excessive hardening and brittleness. This parameter control enables the steel to achieve high yield strength while maintaining excellent low-temperature toughness
Solution Approach 2:
The ferrite-martensite composite microstructure with layered distribution provides both high strength and good toughness. The ferrite phase contributes to ductility and toughness, while the martensite phase provides high strength, and their layered arrangement creates a synergistic effect that maintains toughness even at high strength levels
3Reliability
If Cr content is increased from 13% to 17% to improve corrosion resistance, then corrosion resistance is improved, but manufacturing complexity and cost increase
Solution Approach 1:
The patent optimizes the Cr content to a specific range of 15-18%, which is slightly lower than the conventional 17Cr specification. This optimized parameter range, combined with balanced additions of Ni (4-9%) and Mo (1.5-4%), achieves equivalent or superior corrosion resistance while improving manufacturability and reducing costs
Data Source
Figure 1~2
Figure 3
Figure 4~5
AI summary
A seamless steel pipe which can achieve both a yield strength of 862 MPa or more and excellent low-temperature toughness is provided. The chemical composition of the seamless steel pipe contains Cr: 15.00 to 18.00% in mass% and satisfies Formulae (1) and (2). Furthermore, in the microstructure, (I) a total volume ratio of ferrite and martensite is 80% or more, with the balance being retained austenite of a volume ratio of 20% or less, (II) the number of intersections NTL in the L-direction observation field of view is 38 or more and NTL/NL is 1.80 or more, and further (III) the number of intersections NTc in the C-direction observation field of view is 30 or more and NTc/NC is 1.70 or more. 156Al+18Ti+12Nb+11Mn+5V+328.125N+243.75C+12.5S≤12.5 Ca/S≥4.0 NTL/NL≥1.85 NTC/NC≥1.80