Steel Pipe Surface Hardness Control via Local Quality
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Solution Overview
Problem
Existing methods for manufacturing steel pipes for line pipes in corrosive environments require special equipment and processes, which are inefficient, and tempering at high temperatures for long times is also inefficient in terms of production efficiency, while maintaining the necessary hardness and corrosion resistance.
Innovation Solution
A seamless steel pipe with a specific chemical composition and microstructure, including tempered martensite, tempered bainite, and ferrite, is produced through quenching and tempering processes that control the cooling rate and tempering parameters to reduce the surface layer hardness without compromising yield strength and corrosion resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If quenching is performed to increase hardness and strength, then yield strength is improved, but surface layer hardness becomes too high causing sulfide stress corrosion cracking
Solution Approach 1:
The invention applies different hardness requirements to different regions of the pipe wall. The center portion maintains high hardness (≥350 Hv) for strength, while the surface layer (0.3mm depth) is controlled to have lower hardness (≤250 Hv) to prevent sulfide stress corrosion cracking. This local differentiation of material properties resolves the contradiction between overall strength and surface corrosion resistance.
Solution Approach 2:
The invention uses excessive quenching action (water quenching from 950-1050°C) to achieve very high center hardness, then applies partial correction by controlling the surface layer through specific chemical composition (Si: 0.10-0.50%, Mn: 1.50-2.50%, Cr: 0.10-0.50%, Mo: 0.05-0.30%, V: 0.03-0.15%) and tempering treatment to reduce surface hardness to the acceptable range while maintaining center hardness.
2Object-affected harmful factors
If tempering is performed at high temperature for long time to reduce surface hardness, then corrosion resistance is improved, but production efficiency decreases
Solution Approach 1:
The invention changes the chemical composition parameters (specifically Si: 0.10-0.50%, Mn: 1.50-2.50%, Cr: 0.10-0.50%, Mo: 0.05-0.30%, V: 0.03-0.15%) to enable effective surface hardness control during standard tempering processes. This compositional adjustment allows the steel to achieve appropriate surface hardness (≤250 Hv) without requiring extended tempering times, thus maintaining production efficiency while ensuring corrosion resistance.
3Object-affected harmful factors
If surface decarburization is promoted to reduce surface hardness, then corrosion resistance is improved, but manufacturing complexity increases
Solution Approach 1:
The invention enables the steel material itself to control its surface hardness through its chemical composition (Si, Mn, Cr, Mo, V content) and standard heat treatment parameters. The material's inherent properties and the standard tempering process work together to automatically achieve the desired surface hardness (≤250 Hv) without requiring additional decarburization equipment or complex surface treatment processes, thus maintaining manufacturing simplicity.
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 effectively reduces the surface layer hardness of steel pipes while maintaining a yield strength of at least 415 MPa and ensuring corrosion resistance, improving production efficiency by avoiding the need for specialized equipment and prolonged high-temperature tempering.
Implementation Method 1
a microstructure containing tempered martensite and/or tempered bainite
Implementation Method 2
a quenching and tempering treatment is conducted to adjust the yield strength to be higher than 450 MPa and adjust the wickers hardness
Data Source
Figure 1~2
Figure 3A~3B
Figure 4A~4B
AI summary
A steel pipe for line pipe with reduced hardness of a surface layer portion is provided. A steel pipe for line pipe has a chemical composition of, in mass%: C: 0.02 to 0.11 %; Si: 0.05 to 1.0 %; Mn: 0.30 to 2.5 %; P: up to 0.030 %; S: up to 0.006 %; Cr: 0.05 to 0.36 %; Mo: 0.02 to 0.33 %; V: 0.02 to 0.20 %; Ti: 0.001 to 0.010 %; Al: 0.001 to 0.100 %; N: up to 0.008 %; Ca: 0.0005 to 0.0040 %; and other elements, the chemical composition satisfying Equation (1) below, the steel pipe having a microstructure containing tempered martensite and/or tempered bainite and further containing ferrite in at least one of a portion between an outer surface of the steel pipe and a depth of 1 mm from the outer surface, and a portion between an inner surface of the steel pipe and a depth of 1 mm from the inner surface, Cr+Mo+V≤0.40 wherein each of the chemical symbols in equation (1) is substituted for by the content of the corresponding element in mass%.