Structural Steel Composition for High-Heat-Input Welding Toughness
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Solution Overview
Problem
Existing techniques for high heat input welding in structural steel plates face challenges in maintaining toughness due to coarse austenite grains and martensite-austenite (MA) formation, with issues such as titanium oxide dispersion difficulties and increased alloy costs, particularly for high-strength grades and high heat input welding above 300 kJ/cm.
Innovation Solution
Combining the pinning effects of titanium oxide and TiN, and minimizing phosphorus content while increasing manganese, to reduce coarse austenite grains and MA formation, thereby enhancing the toughness of the heat-affected zone (HAZ) without significant strength loss or increased alloy costs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If high heat input welding is applied to increase welding efficiency, then welding productivity is improved, but HAZ toughness deteriorates due to coarse austenite grains and MA formation
Solution Approach 1:
The patent changes chemical composition parameters (C: 0.03-0.08%, Si: 0.01-0.15%, Mn: 1.80-2.60%, P: ≤0.008%, S: 0.0005-0.0040%, Al: ≤0.005%, Ti: 0.005-0.020%, B: 0.0003-0.0025%, N: 0.0050-0.0080%) to control microstructure formation during high heat input welding, preventing coarse austenite grain growth and MA formation while maintaining base plate strength
Solution Approach 2:
The patent uses boron as an intermediary element that forms boron nitride with nitrogen, preventing nitrogen from embrittling the matrix microstructure. Boron acts as a mediator to fix solute nitrogen and prevent harmful interactions while maintaining HAZ toughness
2Shape
If TiN is added to reduce coarse austenite grains, then austenite grain size is reduced, but solute titanium and solute nitrogen embrittle the matrix microstructure and decrease toughness
Solution Approach 1:
Boron acts as an intermediary that binds with nitrogen to form boron nitride, preventing nitrogen from embrittling the matrix. This resolves the contradiction by removing the harmful nitrogen while maintaining the grain-refining effect of titanium
Solution Approach 2:
The patent optimizes titanium content to 0.005-0.020% and nitrogen content to 0.0050-0.0080%, with boron at 0.0003-0.0025%, to achieve fine austenite grain size while preventing embrittlement through controlled composition
3Shape
If titanium oxide is used to reduce coarse austenite grains, then austenite grain size is reduced, but uniform and fine dispersion of oxide is difficult to achieve
Solution Approach 1:
The patent creates a composite system where titanium oxide and TiN work together. Titanium oxide provides fine dispersion and grain refinement, while TiN provides additional pinning effect. The combination leverages the advantages of both materials to achieve uniform fine dispersion and effective grain size reduction
4Reliability
If carbon content is reduced to inhibit MA formation, then MA content is reduced, but base plate strength decreases requiring additional alloying elements
Solution Approach 1:
The patent optimizes carbon content to 0.03-0.08% and combines it with increased manganese (1.80-2.60%) and controlled alloying elements (Cr, Mo, V) to achieve both low MA formation and sufficient base plate strength (yield strength ≥460 N/mm²). The synergistic composition allows reduced carbon while maintaining strength
Solution Approach 2:
The patent creates a composite strengthening system combining moderate carbon with manganese, chromium, molybdenum, and vanadium to achieve strength without relying solely on high carbon content, thereby reducing MA formation while maintaining base plate strength
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 significantly improves the toughness of the HAZ adjacent to the fusion line after high heat input welding, ensuring excellent weld quality and structural integrity for large steel structures, with improved austenite grain size and MA fraction control.
Implementation Method 1
Combining the pinning effects of titanium oxide and TiN, to reduce coarse austenite grains
Implementation Method 2
a larger amount of manganese is added to increase the drive force of ferrite transformation
Implementation Method 3
to create ferrite nucleation sites
Implementation Method 4
by adding boron to the HAZ through diffusion from the weld metal in the minimum amount required to fix solute nitrogen generated by the dissolution of TiN
Data Source
Figure 1
AI summary
Provided is a structural steel plate suitable for high heat input welding with a welding heat input of more than 300 kJ/cm and suitable for various structures in fields such as shipbuilding, architecture, and civil engineering. Specifically, the structural steel plate has a chemical composition containing, in percent by mass, 0.03% to 0.08% carbon, 0.01% to 0.15% silicon, 1.8% to 2.6% manganese, 0.008% or less phosphorus, 0.0005% to 0.0040% sulfur, 0.005% or less aluminum, 0.003% to 0.03% niobium, 0.005% to 0.030% titanium, 0.0050% to 0.0080% nitrogen, 0.0003% to 0.0025% boron, and optionally at least one of vanadium, copper, nickel, chromium, molybdenum, calcium, magnesium, zirconium, and a rare earth metal, and having a Ceq (IIW) of 0.33 to 0.45, the balance being iron and incidental impurities. The structural steel plate has a prior austenite grain diameter of 200 µm or less and a martensite-austenite constituent area fraction of 1.0% or less in a heat-affected zone microstructure adjacent to a fusion line after high heat input welding with a welding heat input of more than 300 kJ/cm.