Thick Steel Plate Low-Temperature Toughness
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Solution Overview
Problem
Current steel materials used in offshore structural facilities face challenges in maintaining excellent low temperature impact toughness and CTOD properties, particularly at -60°C and -80°C, with existing methods struggling to balance strength and toughness effectively.
Innovation Solution
A thick steel sheet composition of 0.02 to 0.055% C, 0.005 to 0.08% Si, 1.0 to 2.0% Mn, 0.01% P or less, 0.003% S or less, 0.001 to 0.01% Al, 0.5 to 2.0% Ni, 0.001 to 0.02% Ti, 0.005 to 0.03% Nb, 0.05 to 0.4% Cu, and 0.002 to 0.006% N, with a microstructure of 95% ferrite and 2% or less MA and cementite, is developed, along with a manufacturing process involving recrystallization-region rolling and tempering to achieve yield strength of 420MPa or higher and impact toughness of 200J or higher at -80°C and CTOD of 0.5mm or higher at -60°C.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength and thickness of steel material are increased to meet larger facility sizes, then the structural strength is improved, but the low temperature impact toughness and CTOD properties deteriorate
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.02-0.055%, Si: 0.005-0.08%, Mn: 1.0-2.0%, Ni: 0.5-2.0%, etc.) and processing parameters (reduction ratios of final three passes, rolling temperatures, cooling rates) to achieve a microstructure with 95% or more ferrite and 2% or less MA and cementite, thereby simultaneously achieving high strength (yield strength 420MPa or more) and excellent low temperature impact toughness (200J or more at -80°C).
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (ferrite, MA, and cementite) with specific proportions, where ferrite provides toughness and the controlled amounts of MA and cementite contribute to strength, achieving a balance between strength and low temperature impact toughness that neither single-phase material could provide alone.
2Reliability
If conventional rolling methods are used to maintain CTOD properties, then the manufacturing process is simple, but sufficient low temperature toughness and CTOD properties cannot be achieved
Solution Approach 1:
The patent changes multiple processing parameters including maintaining specific reduction ratios (15-25%) for the final three rolling passes, controlling rolling temperature ranges (finishing at 850-950°C), and applying specific cooling rates (2-15°C/sec) to achieve the desired microstructure and properties, representing a more controlled approach than conventional methods.
Solution Approach 2:
The patent applies preliminary action by carefully controlling the composition and microstructure during manufacturing (achieving 95% or more ferrite before delivery), so that the steel inherently possesses the required low temperature toughness and CTOD properties without requiring additional post-processing treatments.
3Reliability
If existing steel compositions are used to achieve strength, then the manufacturing cost is lower, but excellent impact toughness at -80°C cannot be secured
Solution Approach 1:
The patent changes the chemical composition parameters to specific ranges (C: 0.02-0.055%, Si: 0.005-0.08%, Mn: 1.0-2.0%, Ni: 0.5-2.0%, Ti: 0.001-0.02%, Nb: 0.005-0.03%, Cu: 0.05-0.4%, N: 0.002-0.006%) that enable the steel to achieve impact toughness of 200J or more at -80°C while maintaining yield strength of 420MPa or more, representing an optimization of composition for extreme low temperature performance.
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 provides a steel sheet with enhanced yield strength, impact toughness, and CTOD properties at extremely low temperatures, ensuring the steel's suitability for harsh offshore environments while maintaining strength and toughness without degrading properties.
Implementation Method 1
a microstructure consists of ferrite of 95 area% or higher, and a sum of martensite-austenite MA and cementite of 2 area% or lower
Implementation Method 2
an elongation rate of 25% or higher, and impact toughness of 400J or higher at -60°C
Data Source
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AI summary
An aspect of the present invention relates to a thick steel plate having excellent low-temperature impact toughness and CTOD characteristics, the thick steel plate comprising, by weight %: 0.02-0.06% of C, 0.005-0.08% of Si, 1.0-2.0% of Mn, 0.01% or less of P, 0.003% or less of S, 0.001-0.01% of Al, 0.5-2.0% of Ni, 0.001-0.02% of Ti, 0.005-0.03% of Nb, 0.05-0.4% of Cu, 0.002-0.006% of N, and a balance of Fe and inevitable impurities with the proviso of satisfying the following equations, wherein the thick steel plate has a microstructure including ferrite in an amount of 95 area % or greater and a sum of MA and cementite in an amount of 2 area % or less : Equation 1: 3.0≤Mn+2Ni≤4.3 and Equation 2: 0.05≤C+Si+10Al≤0.25 (wherein element symbols each represent contents thereof by weight %) .