Seamless Steel Pipe Composition for SSC Resistance and Strength Uniformity
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Solution Overview
Problem
Existing seamless steel pipes for highly corrosive wells face challenges in achieving both high yield strength and excellent sulfide stress cracking (SSC) resistance, with strength variations in the circumferential and axial directions, particularly during inline quenching processes.
Innovation Solution
A seamless steel pipe with a specific chemical composition (C: 0.21-0.35%, Si: 0.10-0.50%, Mn: 0.05-1.00%, P: 0.025% or less, S: 0.010% or less, Al: 0.005-0.100%, N: 0.010% or less, Cr: 0.05-1.50%, Mo: 0.10-1.50%, Nb: 0.010-0.050%, B: 0.0003-0.0050%, Ti: 0.002-0.050%, V: 0-0.30%, Ca: 0-0.0050%, and rare earth metal: 0-0.0050%) is produced using a method involving heating to 950-1100°C, piercing with skew rolls, quenching with an average cooling rate of 15°C/second, and tempering between 650°C and the Ac1 transformation point.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of a seamless steel pipe is increased, then the yield strength is improved, but the SSC resistance decreases
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.15-0.30%, Si: 0.10-0.50%, Mn: 0.05-1.00%, Cr: 0.05-1.50%, Mo: 0.10-1.50%, Nb: 0.010-0.050%, Ti: 0.002-0.050%, V: 0-0.30%, B: 0.0003-0.0050%) and process parameters (heating temperature: 950-1100°C, finishing temperature: 800-1000°C, cooling rate: 15°C/second or more, tempering temperature: 650-750°C) to achieve a balance between strength and SSC resistance. This systematic parameter optimization enables the steel pipe to attain yield strength of 655-862 MPa while maintaining excellent SSC resistance through controlled microstructure formation.
Solution Approach 2:
The patent employs composite material principles by creating a multi-phase microstructure consisting of martensite, bainite, and retained austenite through controlled cooling and tempering processes. This composite microstructure combines the high strength of martensite with the toughness and SSC resistance provided by bainite and retained austenite, achieving both high yield strength (655-862 MPa) and excellent SSC resistance simultaneously.
2Reliability
If offline quenching is used to enhance SSC resistance through reverse transformation, then the SSC resistance is improved, but the productivity decreases
Solution Approach 1:
The patent applies preliminary action by performing concurrent heating to Ac3 transformation point or more before quenching in the inline quenching process. This preliminary heating ensures that reverse transformation occurs during the heating phase rather than during cooling, enabling the steel to achieve refined grain structure and enhanced SSC resistance through the subsequent quenching process without requiring offline reheating operations.
Solution Approach 2:
The patent implements continuity of useful action by integrating the heating and quenching operations into a continuous inline process. The steel pipe undergoes concurrent heating to transformation temperature followed immediately by quenching without interruption or offline handling, maintaining continuous production flow and eliminating the productivity losses associated with offline quenching operations while still achieving the microstructural transformation needed for SSC resistance.
3Productivity
If inline quenching is used to enhance productivity, then the productivity is improved, but the SSC resistance and strength uniformity deteriorate
Solution Approach 1:
The patent applies feedback principles by implementing precise control of the cooling rate (15°C/second or more) and tempering temperature (650-750°C) in the inline quenching process. This controlled feedback mechanism ensures uniform cooling throughout the steel pipe, preventing localized microstructural variations and ensuring consistent SSC resistance and strength properties across the entire product while maintaining the productivity benefits of inline processing.
Solution Approach 2:
The patent uses parameter changes by optimizing the tempering temperature range (650-750°C) after inline quenching to achieve uniform microstructure transformation throughout the steel pipe. This controlled parameter adjustment during tempering ensures that the martensitic structure transforms uniformly, eliminating strength variations in circumferential and axial directions while maintaining the high productivity of the inline quenching process.
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 achieves high yield strength of 655-862 MPa with excellent SSC resistance and suppresses strength variations in the circumferential and axial directions, enhancing both productivity and energy efficiency during the inline quenching process.
Implementation Method 1
during heating before quench, reverse transformation from ferrite to austenite occurs in the steel
Implementation Method 2
quenching (rapid cooling) is carried out
Implementation Method 3
quenching is carried out from a temperature of an Ar3 transformation point or more
Implementation Method 4
thereafter, tempering is performed in a temperature range from 600° C. to an Ac1 transformation point
Data Source
AI summary
The seamless steel pipe of the present embodiment consists of in mass %, C: 0.21 to 0.35%, Si: 0.10 to 0.50%, Mn: 0.05 to 1.00%, P: 0.025% or less, S: 0.010% or less, Al: 0.005 to 0.100%, N: 0.010% or less, Cr: 0.05 to 1.50%, Mo: 0.10 to 1.50%, Nb: 0.010 to 0.050%, B: 0.0003 to 0.0050%, and Ti: 0.002 to 0.050%, the balance being Fe and impurities. In a main body region of the seamless steel pipe, a grain size number of prior-austenite grain conforming to ASTM E112 is 7.0 or more, a difference between a maximum value and a minimum value of the grain size number is 1.0 or less, yield strength is 655 to less than 862 MPa, and a difference between a maximum value and a minimum value of tensile strength is 27.6 MPa or less.