Steel Plate Microstructure Refinement for Low-Temperature Welding
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Solution Overview
Problem
Conventional techniques fail to adequately improve joint CTOD properties at low temperatures for steel plates used in multilayer welding, particularly in regions with Coarse Grain Heat Affected Zones (CGHAZ) and Inter-Critically reheated Coarse Grain Heat Affected Zones (ICCGHAZ), which are critical for marine structures and other applications.
Innovation Solution
The solution involves optimizing the chemical composition of the steel plate, including controlling carbon, phosphorus, and alloying elements, and refining the microstructure through specific rolling processes to achieve a grain size of 20 µm or less, along with precise hot rolling and cooling conditions to enhance toughness and crack resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional techniques (TiN, REM, Ca additions) are used to prevent austenite grain coarsening in CGHAZ, then grain growth is suppressed, but joint CTOD properties at extremely low temperatures (-60°C or lower) remain insufficient
Solution Approach 1:
The patent changes the chemical composition parameters by adding specific amounts of Ti (0.005-0.030%), REM (0.002-0.020%), and Cu (0.05-2.0%) to create a combined effect that suppresses austenite grain coarsening more effectively than conventional single-element additions, achieving both grain size control and improved low-temperature CTOD properties
Solution Approach 2:
The patent creates a composite microstructure by combining multiple alloying elements (Ti, REM, Cu) that work synergistically: TiN particles prevent grain coarsening, REM-based acid sulfides further suppress grain growth, and Cu strengthens the base metal, resulting in a refined microstructure with superior joint CTOD properties at -60°C
2Strength
If multilayer welding is used to join steel plates, then structural assembly is achieved, but ICCGHAZ microstructure with low toughness is generated at the boundary between layers
Solution Approach 1:
The patent applies local quality by targeting the heat-affected zone specifically with combined Ti and REM additions that preferentially refine austenite grains in the CGHAZ and ICCGHAZ regions during welding, while maintaining the base metal composition for overall strength, thus improving local toughness in the critical welding zone
Solution Approach 2:
The patent incorporates Ti and REM elements into the steel composition before welding, so that during the welding process these elements automatically precipitate as fine particles (TiN and REM-based acid sulfides) that suppress austenite grain coarsening in the heat-affected zone, preparing the microstructure in advance to resist the grain-growing effect of welding heat
3Strength
If alloying elements are added to improve base metal strength, then tensile strength increases, but joint CTOD properties deteriorate due to increased hardness and brittleness
Solution Approach 1:
The patent optimizes the concentration parameters of alloying elements, specifically limiting Cu to 0.05-2.0% and combining it with small amounts of Ti and REM, to achieve the right balance between base metal strength and joint CTOD properties, avoiding excessive hardness and brittleness while maintaining adequate strength
Solution Approach 2:
The patent creates a composite alloying system where Cu provides base metal strength, while Ti and REM provide grain refinement and toughness enhancement in the heat-affected zone, resulting in a multi-functional composition that simultaneously improves both base metal strength and joint CTOD properties without the trade-off
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
This approach results in steel plates with excellent joint CTOD properties, achieving a crack opening displacement of 0.30 mm or more at -60 °C in both CGHAZ and SC/ICHAZ, meeting special low-temperature specifications.
Implementation Method 1
refining the microstructure through specific rolling processes to achieve a grain size of 20 µm or less
Implementation Method 2
a zone near a weld line having had a coarse microstructure through a preceding welding pass (CGHAZ: Coarse Grain Heat Affected Zone) is reheated into a ferrite-austenite dual phase region
Data Source
AI summary
Excellent CTOD properties for multilayer welding joint is provided for a steel plate. The steel plate comprises a specific chemical composition with Ceq of 0.45 % or less where Ceq%=C+Mn/6+Cu+Ni/15+Cr+Mo+V/5 and Pcmof0.22%orlesswherePcm%=C+Si/30+Mn+Cu+Cr/20+Ni/60+Mo/15+V/10+5B an average effective grain size of 20 µm or less at a mid-thickness part of the steel plate; and porosities having an equivalent circular diameter of 200 µm or more, the number of the porosities per mm2 being 0.1/mm2 or less.