Seamless Steel Pipe Surface Hardness Control via Scale
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Solution Overview
Problem
Existing methods for producing seamless steel pipes for line pipes struggle to maintain low surface hardness while achieving high strength and toughness, particularly in harsh environments with high H2S content, as surface hardness often exceeds the recommended 250 HV10 due to variations in cooling rates and microstructure differences.
Innovation Solution
A method involving specific chemical compositions and heat treatment processes, including the use of Ni or Cu metal particles to enhance scale adhesion and control cooling rates, combined with controlled heat treatment and alloy element management to maintain low surface hardness and high strength, ensuring the steel pipe's chemical composition and microstructure are optimized for improved SSC resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If quenching followed by tempering is performed to achieve high strength, then strength is improved, but surface hardness becomes excessively high
Solution Approach 1:
The invention applies different properties to different regions of the steel pipe. The surface region is treated to have low hardness (250 HV10 or lower) through controlled cooling and scale formation, while the interior region maintains high strength through the same heat treatment process. This spatial differentiation of properties resolves the contradiction between overall strength and surface hardness.
Solution Approach 2:
The invention changes the cooling rate parameter during heat treatment to control microstructure formation. By using a controlled cooling rate that prevents excessive hardening at the surface while maintaining strength in the interior, the contradiction between strength and surface hardness is resolved. The specific cooling parameters are optimized to achieve the desired property distribution.
2Strength
If alloying elements are increased to improve strength, then strength is improved, but surface hardness increases due to higher cooling rate sensitivity
Solution Approach 1:
The invention optimizes the composition parameters of alloying elements within specific ranges. By controlling the amounts of C, Si, Mn, Cr, Ni, Mo, and other elements within defined limits, the steel achieves high strength while reducing sensitivity to cooling rate variations at the surface, thereby preventing excessive surface hardness.
Solution Approach 2:
The invention creates a composite microstructure through controlled alloying and heat treatment. The resulting microstructure combines bainite and martensite phases with controlled distribution, where the alloying elements are strategically selected to provide strength enhancement without causing excessive surface hardening during cooling.
3Strength
If cooling rate is increased to achieve high strength through martensite formation, then strength is improved, but microstructure becomes non-uniform between surface and interior
Solution Approach 1:
The invention changes the cooling rate parameter from a high value that causes non-uniform microstructure to a controlled moderate value that produces uniform bainitic microstructure throughout the pipe wall. This parameter optimization ensures both high strength and microstructural uniformity between surface and interior regions.
Solution Approach 2:
The invention achieves homogeneous microstructure distribution throughout the steel pipe by controlling the cooling process. The resulting uniform bainitic microstructure eliminates the non-uniformity between surface and interior that would otherwise occur with faster cooling rates, while still achieving the required strength level.
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 a yield strength of 550 MPa or higher and toughness at -80°C or lower, with surface hardness restrained to 250 HV10 or lower, effectively addressing the challenge of maintaining sulfide stress cracking resistance in harsh environments.
Implementation Method 1
heated in an atmosphere with the temperature of Ac 3 +50°C or higher
Implementation Method 2
subjected to quenching treatment in which accelerated cooling is performed at a rate of 10°C/s or higher
Implementation Method 3
accelerated cooling is performed at a rate of 10°C/s or higher
Implementation Method 4
subsequently being tempered at a temperature of Ac 1 -50°C or lower
Data Source
AI summary
There is provided a seamless steel pipe for line pipe having a chemical composition consisting, by mass percent, of C: 0.03-0.15%, Si: ≤0.50%, Mn: 1.0-2.0%, P: ≤0.050%, S: ≤0.005%, Cr: 0.1-1.0%, Al: 0.001-0.10%, N: ≤0.01%, Ni: 0.05-2.0%, B: 0.0003-0.0015%, Ca: 0.0002-0.0050%, Mo: 0.10-0.50%, Ti: 0.001-0.05%, Cu: 0-2.0%, Nb: 0-0.05%, V: 0-0.10%, the balance: Fe and impurities, and satisfying the conditions of 2Nb+4V+Mo≤0.50, wherein a metal micro-structure of the steel pipe contains 50% or more of bainite, in an area fraction, a wall thickness of the steel pipe is 25 mm or larger, and in a scale formed on the surface of the steel pipe, metal particles consisting mainly of Ni or Cu having an average circle-equivalent diameter of 0.1-5 µm exist, and a distance from a boundary between the base metal of the steel pipe and the scale to a region in which the metal particles do not exist is 20 µm or longer.