Low-Alloy Steel Pipe Resolving Strength-Corrosion Trade-Off
Solution Overview
Problem
Existing high-strength thick-walled seamless steel pipes for oil country tubular goods face challenges in achieving excellent sulfide stress corrosion cracking resistance, particularly in hydrogen sulfide-containing sour environments, with existing technologies struggling to maintain high yield strength and stability in KISSC values.
Innovation Solution
The development of a low alloy high-strength thick-walled seamless steel pipe with specific chemical compositions and microstructures, including controlled Mo segregation and quenching processes, to achieve a yield strength of 758 MPa or more and stable KISSC values of 26.4 MPa√m or more in a hydrogen sulfide-saturated environment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength is achieved through increased carbon content and alloying, then yield strength improves, but sulfide stress corrosion cracking resistance deteriorates
Solution Approach 1:
The invention changes the chemical composition parameters by strictly limiting carbon content to 0.23% or less and controlling specific alloying element ranges (Cr: 1.00-2.50%, Mo: 0.10-1.00%, B: 0.0005-0.0100%). These parameter changes enable achieving yield strength of 758 MPa or more while maintaining excellent sulfide stress corrosion cracking resistance, resolving the contradiction between strength and corrosion resistance.
Solution Approach 2:
The invention creates a composite microstructure consisting of martensite as the primary phase with controlled precipitates. This composite structure at the micro level provides both high strength through martensite and excellent SSC resistance through the specific precipitate distribution and composition control, effectively resolving the strength-corrosion resistance contradiction.
2Strength
If wall thickness is increased to 40 mm or more for thick-walled applications, then structural integrity improves, but manufacturing precision and material quality control become more difficult
Solution Approach 1:
The invention changes manufacturing parameters by specifying precise chemical composition ranges and microstructural characteristics that must be achieved regardless of wall thickness. For thick-walled pipes (40 mm or more), these controlled parameters ensure uniform material quality throughout the wall thickness, making the steel suitable for both thin-walled and thick-walled applications with consistent performance.
3Strength
If yield strength is increased to 758 MPa or more, then load-bearing capacity improves, but stability of KISSC values in sour environments deteriorates
Solution Approach 1:
The invention changes the chemical composition parameters to achieve a specific microstructural state. By controlling C ≤ 0.23%, Cr: 1.00-2.50%, Mo: 0.10-1.00%, and B: 0.0005-0.0100%, the material achieves yield strength ≥ 758 MPa while maintaining stable KISSC values in hydrogen sulfide-saturated environments, resolving the contradiction between strength and environmental stability.
Solution Approach 2:
The invention applies local quality control at the microstructural level by ensuring specific precipitate distribution and phase composition throughout the material. This localized microstructural control ensures that every region of the steel, regardless of overall size or wall thickness, exhibits consistent KISSC value stability in sour environments while maintaining high yield strength.
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 steel pipe with enhanced sulfide stress corrosion cracking resistance and stable high KISSC values, effectively addressing the limitations of existing technologies by regulating the stress-strain curve and Mo segregation to inhibit plastic deformation and improve corrosion resistance.
Implementation Method 1
excellent sulfide stress corrosion cracking resistance (SSC resistance) especially in a hydrogen sulfide-containing sour environment
Implementation Method 2
controlled Mo segregation and quenching processes
Data Source
Figure 1
Figure 2~3
Figure 4~5
AI summary
A low alloy high strength thick-walled seamless steel pipe for oil country tubular goods having excellent SSC resistance is provided. The steel pipe of the present invention is a low alloy high strength thick-walled seamless steel pipe for oil country tubular goods having a wall thickness of 40 mm or more and a yield strength of 758 MPa or more, the steel pipe including a composition containing, in terms of mass%, C: 0.25 to 0.31%, Si: 0.01 to 0.35%, Mn: 0.55 to 0.70%, P: 0.010% or less, S: 0.001% or less, O: 0.0015% or less, Al: 0.015 to 0.040%, Cu: 0.02 to 0.09%, Cr: 0.8 to 1.5%, Mo: 0.9 to 1.6%, V: 0.04 to 0.10%, Nb: 0.005 to 0.05%, B: 0.0015 to 0.0030%, Ti: 0.005 to 0.020%, and N: 0.005% or less, and having Ti/N of 3.0 to 4.0, with the balance being Fe and inevitable impurities, wherein a cumulative frequency rate at a measurement point at which a Mo segregation degree by a predetermined expression is 1.5 or more is 1% or less, and the steel pipe has σο.7/σ0.4 of 1.02 or less in the stress-strain curve.