Welded Steel CTOD Improvement via Oxygen Control
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Solution Overview
Problem
Current steel compositions for welded structures, particularly in low to medium heat input welding, face challenges in maintaining sufficient CTOD properties in the IC zone at harsh temperatures, such as -60°C, while ensuring strength and toughness in both the FL and IC zones, especially when oxygen levels and non-metallic inclusions are high.
Innovation Solution
A steel composition with reduced oxygen content and specific alloy element limitations, including C, Si, Mn, Ni, Ti, N, P, S, Al, Nb, Cu, and V, within defined ranges, combined with a thermo-mechanical control process during continuous casting and rolling, to enhance CTOD properties in both the FL and IC zones by minimizing non-metallic inclusions and hardness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If oxygen content and non-metallic inclusions are increased to improve manufacturing processability and base metal strength, then the CTOD property of the IC zone deteriorates at harsh temperatures
Solution Approach 1:
The patent applies parameter changes by precisely controlling the oxygen content within 0.0015 to 0.0035 mass% and limiting non-metallic inclusions to 0.003 mm² or less. This optimization of chemical composition parameters resolves the contradiction by finding the optimal balance point where base metal strength is sufficient while IC zone CTOD property is improved at harsh temperatures.
Solution Approach 2:
The patent applies local quality by differentiating the requirements for different zones: the base metal allows higher strength through controlled alloying, while the IC zone requires lower oxygen and inclusion content to ensure CTOD property. The selective control of oxygen content and inclusion levels creates different local properties in different regions of the welded structure.
2Strength
If alloy element content is increased to improve base metal toughness, then the CTOD property of the IC zone deteriorates due to increased hardness
Solution Approach 1:
The patent applies parameter changes by establishing an optimized alloy composition formula with specific ranges: C: 0.015-0.045%, Si: 0.05-0.20%, Mn: 1.50-2.00%, Ni: 0.10-1.50%, Ti: 0.005-0.015%, and limiting P, S, Al, Nb, Cu, and V to 0.008% or less each. This balanced composition achieves base metal toughness while controlling IC zone hardness to maintain CTOD property.
Solution Approach 2:
The patent applies composite materials by creating a multi-element alloy system where different elements serve complementary functions: C and Mn provide strength, Ni improves toughness, Ti controls inclusions, and Si deoxidizes. The synergistic interaction of these elements achieves both base metal toughness and IC zone CTOD property.
3Reliability
If oxygen content is reduced to improve IC zone CTOD property, then the amount of transformation nuclei for intragranular ferrite decreases
Solution Approach 1:
The patent applies parameter changes by optimizing the oxygen content to 0.0015-0.0035 mass%, which is higher than ultra-low oxygen steels but lower than conventional steels. This optimized oxygen level provides sufficient transformation nuclei for intragranular ferrite while maintaining improved IC zone CTOD property compared to conventional steels.
Solution Approach 2:
The patent applies copying by using Ti-oxides as transformation nuclei that replicate the function of oxygen-based nuclei. The controlled Ti content (0.005-0.015%) ensures adequate Ti-oxide formation to serve as transformation nuclei for intragranular ferrite, compensating for the reduced oxygen content while maintaining microstructure stability.
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 effectively improves the CTOD properties in both the FL and IC zones at -60°C, ensuring high-strength and high-toughness steel suitable for harsh environments, like offshore and seismically resistant structures, by stabilizing the microstructure and reducing oxide-based inclusions.
Implementation Method 1
a steel for a welded structure... excellent in a CTOD (Crack Tip Opening Displacement) property... transformation nuclei for the generation of an intragranular ferrite (IGF)... making a microstructure fine after welding
Data Source
Figure 1
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Figure 4A~4B
AI summary
A steel for a welded structure includes the following composition: by mass%, C at a C content [C] of 0.015 to 0.045%; Si at a Si content [Si] of 0.05 to 0.20%; Mn at a Mn content [Mn] of 1.5 to 2.0%; Ni at a Ni content [Ni] of 0.10 to 1.50%; Ti at a Ti content [Ti] of 0.005 to 0.015%; O at an O content [O] of 0.0015 to 0.0035%; and N at a N content [N] of 0.002 to 0.006%, and a balance composed of Fe and unavoidable impurities. In the steel for a welded structure, the P content [P] is limited to 0.008% or less, the S content [S] is limited to 0.005% or less, the Al content [Al] is limited to 0.004% or less, the Nb content [Nb] is limited to 0.005% or less, the Cu content [Cu] is limited to 0.24% or less, the V content [V] is limited to 0.020% or less, and a steel composition parameter PCTOD is 0.065% or less, and a steel composition hardness parameter CeqH is 0.235% or less.