Ultra-thick structural steel having excellent brittle crack initiation resistance, and manufacturing method therefor
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Solution Overview
Problem
Existing ultra-thick high-strength steel materials face challenges in ensuring brittle crack initiation resistance due to residual defects and inhomogeneous microstructures, particularly in welded zones and central portions, which are not adequately addressed by current methods such as adding alloying elements or refining microstructures.
Innovation Solution
A structural ultra-thick steel material with specific alloy compositions (C: 0.03-0.08%, Mn: 1.6-2.2%, Ni: 0.6-1.3%, Nb: 0.005-0.03%, Ti: 0.005-0.02%, Cu: 0.1-0.4%, P: 100 ppm or less, S: 40 ppm or less, H: 1.5 ppm or less, Fe balance) and a manufacturing process involving continuous casting, reheating, rough-rolling, finish-rolling, and controlled cooling to achieve a microstructure of 80% acicular ferrite and granular bainite with reduced cracks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel is manufactured using an ultra-thick steel plate, then strength is improved, but the structure may be coarsened and toughness deteriorates due to insufficient deformation and inhomogeneous cooling
Solution Approach 1:
The patent applies parameter changes by precisely controlling the finishing rolling temperature within 20°C to 50°C above the Ar3 transformation point, and controlling the cooling rate to 10°C/s to 100°C/s. These parameter changes ensure uniform microstructure transformation throughout the ultra-thick steel plate, preventing coarse structure formation and maintaining toughness while achieving high strength.
Solution Approach 2:
The patent employs preliminary action through advance control of the finishing rolling temperature and cooling rate before microstructure transformation occurs. By pre-setting these parameters, the patent prevents the formation of coarse low-temperature transformation phases and ensures uniform deformation throughout the steel plate, thereby maintaining both strength and toughness.
2Strength
If an ultra-thick steel plate is manufactured with high strength requirements, then yield strength is improved, but brittle crack initiation resistance deteriorates due to residual defects in the central portion
Solution Approach 1:
The patent applies parameter changes by controlling the finishing rolling temperature to be within 20°C to 50°C above the Ar3 transformation point and the cooling rate to be between 10°C/s and 100°C/s. These parameter changes promote uniform microstructure transformation throughout the ultra-thick steel plate, eliminating coarse structures and residual defects in the central portion, thereby improving brittle crack initiation resistance while maintaining high yield strength.
Solution Approach 2:
The patent employs preliminary action by pre-controlling the finishing rolling and cooling parameters before defects can form in the central portion. This advance control ensures uniform deformation and microstructure transformation throughout the steel plate, preventing the formation of residual defects that would otherwise reduce brittle crack initiation resistance.
3Length of stationary object
If the steel plate thickness is increased to achieve ultra-thick dimensions, then structural capacity is improved, but inhomogeneous microstructure forms due to different cooling rates between surface and central portions
Solution Approach 1:
The patent applies parameter changes by precisely controlling the cooling rate within 10°C/s to 100°C/s and the finishing rolling temperature within 20°C to 50°C above the Ar3 transformation point. These parameter changes ensure that the entire cross-section of the ultra-thick steel plate transforms uniformly, eliminating the inhomogeneous microstructure that would otherwise form due to different cooling rates between surface and central portions.
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 effectively reduces central defects, ensuring yield strength of 500 MPa or more, an average CTOD value of 0.4 mm or more at -10°C, and an impact transition temperature of -40°C or less, thereby enhancing brittle crack initiation resistance.
Implementation Method 1
reheating the steel slab to a temperature of 1000 to 1150°C
Implementation Method 2
cooling the hot-rolled steel material to a temperature of 300 to 600°C at a cooling rate of 3°C/sec
Data Source
AI summary
One embodiment of the present invention provides an ultra-thick structural steel having excellent brittle crack initiation resistance, and a manufacturing method therefor, the ultra-thick structural steel comprising, by wt%, 0.03-0.08% of C, 1.6-2.2% of Mn, 0.6-1.3% of Ni, 0.005-0.03% of Nb, 0.005-0.02% of Ti, 0.1-0.4% of Cu, 100 ppm or less of P, 40 ppm or less of S, 1.5 ppm or less of H, and the balance of Fe and other inevitable impurities, wherein the sum of acicular ferrite and granular bainite in the microstructure is 80% or more by area fraction, the sum of the total length of cracks having a size of 30 µm or more per unit area of 1 mm2 in a ±1 mm region on the basis of the thickness center of the steel is 130 µm or less, and the yield strength is 500 MPa or more.