Seamless Steel Pipe Composition for High Strength and Low Hardness
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Solution Overview
Problem
Existing high-strength seamless steel pipes for line pipes face challenges in achieving both high strength and low hardness while maintaining weldability, particularly in thick-wall products where hardness control is difficult.
Innovation Solution
A high-strength seamless steel pipe with a chemical composition that includes 0.10 to 0.18% C, 0.03 to 1.0% Si, 0.5 to 2.0% Mn, and specific amounts of other elements, satisfying the formula C + Si / 30 + Mn + Cu + Cr / 20 + Ni / 60 + Mo / 15 + V / 10 + 5 × B ≤ 0.28, which ensures high strength, low hardness, and adequate weldability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If carbon equivalent is increased to increase hardenability for high strength in thick-wall products, then strength is improved, but weldability deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.10-0.18%, Si: 0.03-1.0%, Mn: 0.50-2.00%, and other elements within specified ranges) to achieve optimal hardenability without excessive carbon equivalent. This compositional parameter optimization allows the steel to attain high yield strength (555 MPa or higher) while maintaining weldability, resolving the contradiction between strength improvement and weldability deterioration.
2Strength
If quenching is applied to increase strength in thick-wall pipes, then strength is improved, but surface layer hardness becomes too high
Solution Approach 1:
The patent uses parameter changes by optimizing the chemical composition (particularly Si: 0.03-1.0% and Mn: 0.50-2.00%) to control hardenability and tempering behavior. This allows the steel to achieve high yield strength through quenching while the controlled composition ensures that tempering reduces surface layer hardness to 250 Hv or lower, preventing the harmful effect of excessive hardness.
Solution Approach 2:
The patent creates a composite microstructure through controlled alloying, where multiple elements (C, Si, Mn, Cr, Mo, V, Ti, B) work synergistically to produce a steel with optimized hardenability and tempering response. This composite material approach enables the steel to achieve both high strength and controlled surface hardness after heat treatment.
3Stress or pressure
If wall thickness is increased to 30 mm or larger for deep-sea applications, then pressure resistance is improved, but hardness control becomes difficult
Solution Approach 1:
The patent applies parameter changes by optimizing the chemical composition parameters (particularly Si: 0.03-1.0% and Mn: 0.50-2.00%) to achieve uniform hardenability throughout the thick wall section. This compositional optimization ensures that even in thick-wall pipes (30 mm or larger), the steel achieves consistent hardness distribution after quenching and tempering, with surface layer hardness controlled at 250 Hv or lower, resolving the hardness control difficulty in thick-wall applications.
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AI summary
A high-strength seamless steel pipe providing both high strength and low hardness in a stable manner while weldability is maintained is provided. A high-strength seamless steel pipe has a chemical composition of, in mass %: 0.10 to 0.18 % C; 0.03 to 1.0 % Si; 0.5 to 2.0 % Mn; up to 0.020 % P; up to 0.0080 % S; 0.10 to 0.60 % Cr; 0.10 to 0.40 % Mo; 0.02 to 0.40 % V; 0.004 to 0.020 % Ti; 0.0005 to 0.005 % B; up to 0.10 % Al; up to 0.008 % N; 0.0004 to 0.0040 % Ca; 0.1 to 1.0 % Cu; 0.2 to 1.0 % Ni; 0 to 0.05 % Nb; and the balance being Fe and impurities, wherein the following formula, F (1), is satisfied: C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5×B≤0.28 F (1) For the element symbols in F (1), the contents of the corresponding elements in mass % are substituted.