Thin Steel Plate Composition for Low-Temperature Toughness and CTOD
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Solution Overview
Problem
Existing technologies struggle to provide thin steel plates with high strength and low-temperature toughness suitable for offshore and floating structures, particularly in demanding environments like deep sea and polar regions.
Innovation Solution
A thin steel plate with specific alloy composition (0.05 to 0.1% C, 0.05 to 0.3% Si, 1.0 to 2.0% Mn, 0.005 to 0.04% Sol. Al, 0.005 to 0.03% Nb, 0.005 to 0.02% Ti, 0.05 to 0.4% Cu, 0.3 to 1.0% Ni, 0.001 to 0.08% N, 0.01% P, 0.003% S, balance Fe) and microstructure (30 to 50% acicular ferrite, 50 to 70% polygonal ferrite) is manufactured through heating, rough rolling, finish rolling, air-cooling, and controlled water-cooling to achieve thicknesses of 8 to 30 mm.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strengthening and post-materialization are applied to steel materials for offshore structures, then strength is improved, but low-temperature toughness deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.05-0.10%, Si: 0.05-0.30%, Mn: 1.00-2.00%, etc.) and processing parameters (heating temperature, rolling temperatures, cooling rates) to achieve a balance between strength and low-temperature toughness. The specific parameter ranges are optimized to prevent embrittlement while maintaining high strength through controlled microstructure formation.
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (acicular ferrite, polygonal ferrite, and bainite) within the steel material. This composite microstructure combines the high strength of martensitic phases with the good toughness of ferritic phases, resolving the contradiction between strength and low-temperature toughness through microstructural design.
2Weight of moving object
If thin materials are used for offshore structures, then weight reduction and usability are improved, but high strength and low-temperature toughness become harder to secure
Solution Approach 1:
The patent uses parameter changes by optimizing the chemical composition (adding microalloying elements like Nb and Ti) and processing parameters (controlled cooling rates of 10-30°C/s, specific temperature ranges for phase transformation) to achieve superior mechanical properties in thin plates that would be difficult to obtain in conventional thick sections.
Solution Approach 2:
The patent transitions from conventional thick-plate design to thin-plate design by changing the dimensional parameter (thickness), and compensates for the potential loss of strength and toughness through microstructural control and alloying, effectively moving the solution from a thickness-dependent approach to a microstructure-dependent approach.
3Ease of manufacture
If conventional cooling methods are used after hot rolling, then manufacturing simplicity is maintained, but the required microstructure for excellent low-temperature toughness cannot be achieved
Solution Approach 1:
The patent segments the cooling process into distinct stages: air cooling from finishing temperature to Ar3 transformation point, then water cooling through the critical transformation range (550-650°C), followed by air cooling to room temperature. This segmented approach creates the desired mixed microstructure of acicular ferrite, polygonal ferrite, and bainite phases.
Solution Approach 2:
The patent applies periodic action through the two-stage cooling process with different cooling rates applied at different time periods. The first stage uses slower air cooling to form polygonal ferrite, while the second stage uses faster water cooling to form acicular ferrite and bainite, creating a time-dependent microstructural evolution that achieves the target properties.
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 results in a thin steel plate with yield strength of 460 MPa or more, elongation of 17% or more, impact toughness of 50 J or more at -40°C, and CTOD value of 0.4 mm or more at -20°C, ensuring excellent low-temperature toughness and fatigue properties.
Implementation Method 1
heating a steel slab satisfying the alloy composition described above to 1200° C. or higher
Implementation Method 2
air-cooling the hot-rolled steel plate
Implementation Method 3
cooling the hot-rolled steel plate at a cooling speed of 10 to 30° C./s, wherein the cooling is water cooling
Data Source
AI summary
The present invention relates to structural steel that can be desirably used in offshore structures and the like, more specifically, to a thin steel plate having excellent low-temperature toughness and CTOD properties, and to a method for manufacturing the same.