Thick-Walled Seamless Steel Pipe Grain Uniformity
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Solution Overview
Problem
Seamless steel pipes with thick walls for undersea oil and gas pipelines face challenges in achieving both high strength and toughness, particularly due to variations in crystal grain size across the wall thickness, which can lead to brittle fractures and reduced toughness.
Innovation Solution
A seamless steel pipe composition with specific chemical elements (C, Si, Mn, P, S, Al, Cr, Ni, Mo, N, Ca, Nb, Ti, V, and Cu) and a production method involving hot working, accelerated cooling, reheating to 990-1100°C, quenching, and tempering to control prior austenite grain size and distribution, ensuring uniform toughness across the wall thickness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the wall thickness and strength of the steel pipe are increased, then the pipe can withstand high pressure and external loads, but the toughness is reduced and brittle fracture becomes more likely
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.15-0.35%, Si: 0.01-0.20%, Mn: 1.50-3.00%, P: ≤0.030%, S: ≤0.010%, Al: 0.010-0.100%, Cr: 0.10-2.00%, Ni: 0.10-2.00%, Mo: 0.10-1.00%, N: 0.003-0.015%, Ca: 0.0005-0.0050%, Nb: 0.010-0.100%, Ti: 0.010-0.100%, V: 0.010-0.100%, Cu: 0.10-1.00%) and heat treatment parameters (cooling rate ≥80°C/min, quenching and tempering) to achieve a microstructure with fine prior austenite grain size (≤80μm) that provides both high strength and high toughness, resolving the contradiction between strength and toughness in thick-walled pipes
Solution Approach 2:
The patent creates a composite microstructure by combining multiple alloying elements that work synergistically: Nb and Ti form fine precipitates for strengthening, Cr and Ni provide solid solution strengthening and improve toughness, Mo enhances hardenability, and Al provides grain refinement. This composite approach at the microstructural level allows the steel to achieve both high strength and high toughness simultaneously
2Strength
If a thick-walled seamless steel pipe is produced by conventional hot rolling and quenching and tempering, then high strength can be achieved, but the pipe may be reduced in toughness due to variations in crystal grain size across the wall thickness
Solution Approach 1:
The patent applies parameter changes by controlling the cooling rate at ≥80°C/min during hot rolling and specifying precise quenching and tempering parameters, which ensures uniform transformation of austenite to martensite throughout the thick wall section, achieving fine and uniform prior austenite grain size (≤80μm) across the entire wall thickness, thereby resolving the contradiction between strength and grain size uniformity
Solution Approach 2:
The patent applies local quality by ensuring that the chemical composition and microstructure are uniformly distributed throughout the thick-walled pipe section. The controlled cooling rate and heat treatment parameters ensure that every region of the pipe wall achieves the same fine grain structure and mechanical properties, eliminating local variations that would compromise overall performance
3Strength
If Nb is added to increase strength in thick-walled seamless steel pipes, then high strength can be achieved, but the pipe may be further reduced in toughness in the near surface portion and the scatter of toughness between near surface and central portions increases
Solution Approach 1:
The patent applies parameter changes by precisely controlling the Nb content within 0.010-0.100% and combining it with specific cooling rates (≥80°C/min) and quenching and tempering parameters. This controlled approach ensures that Nb forms fine, uniformly distributed precipitates that strengthen the steel without causing excessive grain coarsening or localized embrittlement, maintaining toughness uniformity across the wall thickness
Solution Approach 2:
The patent applies local quality by ensuring uniform distribution of Nb and other alloying elements throughout the thick-walled pipe section through controlled hot rolling and cooling. The uniform chemical composition and microstructure ensure that Nb strengthens the steel consistently across all regions without creating localized zones of reduced toughness, particularly at the near surface portion
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 method produces seamless steel pipes with high yield strength and excellent toughness, minimizing variations in toughness between the near surface and central portions, thus enhancing the pipe's resistance to stress and corrosion.
Implementation Method 1
a seamless steel pipe produced by hot rolling is cooled to a temperature not more than an Ar3 transformation point at a cooling rate not less than 80° C./min and then is quenched and tempered
Implementation Method 2
a step of heating a starting material consisting of: in mass %, C: 0.03 to 0.08%, Si: not more than 0.25%, Mn: 0.3 to 2.0%
Implementation Method 3
a step of cooling the hot worked material pipe in an accelerated manner; a step of quickly cooling the soaked material pipe to thereby quench the material pipe
Data Source
AI summary
There is provided a seamless steel pipe having high strength and high toughness even if having a thick wall. A seamless steel pipe according to the present embodiment consists of: in mass %, C: 0.03 to 0.08%, Si: not more than 0.25%, Mn: 0.3 to 2.0%, P: not more than 0.05%, S: not more than 0.005%, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni: 0.02 to 1.0%, Mo: 0.02 to 0.8%, N: 0.002 to 0.008%, Ca: 0.0005 to 0.005%, and Nb: 0.01 to 0.1%, the balance being Fe and impurities, and has a wall thickness of not less than 50 mm. In a cross section perpendicular to an axial direction of the seamless steel pipe, an average crystal grain size of prior austenite grains in a near surface portion is less than 80 μm, the near surface portion being a 500 μm×500 μm area centered at a position of a depth of 2 mm from a surface, and a difference between the average crystal grain size of the prior austenite grains in the near surface portion and an average crystal grain size of prior austenite grains in a central portion of a wall thickness of the cross section is less than 50 μm, the central portion being a 500 μm×500 μm area centered at a center position of the wall thickness.