Structural Steel Brittle Crack Resistance via Localized Microstructure
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Solution Overview
Problem
Structural steels used in extreme environments, such as shipbuilding and offshore construction, face challenges in maintaining brittle crack propagation resistance as thickness increases, with existing techniques like grain-refining being ineffective for high-strength steels above 570 MPa tensile strength and difficult to control in thick plates.
Innovation Solution
A structural steel composition optimized with 0.02-0.12% C, 0.01-0.8% Si, 1.7-2.5% Mn, and specific inclusion of elements like Al, P, and Nb, combined with a manufacturing process involving reheating, rough rolling, heat recuperation, and second cooling to create a microstructure with tempered bainite, fresh martensite, and acicular ferrite, enhancing brittle crack resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the thickness of steel sheet increases to provide high strength for extreme environments, then the strength and toughness characteristics are improved, but the performance of stopping propagation of brittle cracks decreases
Solution Approach 1:
The patent applies local quality by creating a non-uniform microstructure where the surface part contains tempered bainite as base structure with fresh martensite and austenite, while the center part contains acicular ferrite. This localized microstructural differentiation allows the surface to have high strength and toughness while the overall thick plate maintains brittle crack propagation resistance through the acicular ferrite structure in the center.
Solution Approach 2:
The patent creates a composite microstructure within the steel plate, combining different phases (tempered bainite, fresh martensite, austenite, and acicular ferrite) in specific regions. This composite approach allows the material to simultaneously achieve high strength from the martensite and bainite regions while maintaining crack resistance through the acicular ferrite structure in the center part.
2Reliability
If grain-refining is performed on the surface part of steel to suppress brittle crack propagation, then the crack propagation resistance is improved, but the technique cannot be applied to high-strength steel having tensile strength of 570 MPa or higher
Solution Approach 1:
The patent changes the microstructural parameters by controlling the volume fractions and particle diameters of different phases. The surface part has tempered bainite with average particle diameter of 3 μm or less and high angle grain boundary fraction of 45% or more, while maintaining tensile strength of 570 MPa or higher through the combination of multiple phases including fresh martensite and austenite.
Solution Approach 2:
The patent creates a composite microstructure in the surface part combining tempered bainite as base structure with fresh martensite and residual austenite. This composite approach allows achieving both high strength (570 MPa or higher) and improved brittle crack propagation resistance through the refined tempered bainite structure with high angle grain boundaries.
3Shape
If rolling process is performed in the middle of recuperating heat in the surface part to grain-refine, then the surface structure is refined, but the rolling process becomes difficult to control
Solution Approach 1:
The patent applies preliminary action by performing rough rolling before heat recuperation to establish the basic surface structure, then performing finish rolling after heat recuperation when the surface part has been reheated. This sequencing allows the surface part to be in a more favorable temperature state during finish rolling, making the process easier to control while still achieving the desired grain refinement and microstructure.
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 steel exhibits improved brittle crack propagation resistance with a tensile strength of 570 MPa or higher, achieving a high angle grain boundary fraction and Kca value of 6000 N/mm3/2 at -10°C, effectively addressing the limitations of existing techniques.
Implementation Method 1
heat recuperation, and second cooling to create a microstructure with tempered bainite, fresh martensite, and acicular ferrite
Implementation Method 2
reheating, rough rolling, heat recuperation, and second cooling to create a microstructure
Data Source
AI summary
A structural steel having excellent brittle crack propagation resistance, according to one aspect of the present invention, comprises, by wt %, 0.02-0.12% of C, 0.01-0.8% of Si, 1.7-2.5% of Mn, 0.005-0.5% of Al, and the balance of Fe and inevitable impurities, wherein an outer surface part and an inner center part thereof are microstructurally distinguished in the thickness direction, and the surface part comprises tempered bainite as a base structure, comprises fresh martensite as a second structure and can comprise austenite as a residual structure.