Ultrahigh-Strength Steel Dual Microstructure for Cold Workability
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Solution Overview
Problem
Existing methods for manufacturing ultrahigh-strength steel with excellent cold workability and SSC resistance face challenges in securing the required strength and resistance, especially for offshore structures, due to limitations in microstructure control and dislocation density.
Innovation Solution
The development of an ultrahigh-strength steel with a specific alloy composition and controlled microstructures, including a surface layer with 90% or more polygonal ferrite and a center portion with 90% or more tempered martensite or a mixed structure of tempered martensite and tempered bainite, along with a dislocation density of 3×10^14/m^2 or less in the surface layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If ultrahigh-strength steel with yield strength of 690 MPa or more is manufactured using conventional QT heat treatment, then high strength is achieved, but cold workability deteriorates due to low uniform elongation of martensite structure
Solution Approach 1:
The patent applies local quality by creating a dual microstructure where the surface layer (0-10% thickness) contains 90% or more polygonal ferrite for ductility and cold workability, while the center portion contains 90% or more tempered martensite for high strength. This spatial differentiation of microstructure allows each region to fulfill its specific functional requirement.
Solution Approach 2:
The steel structure is segmented into two distinct regions: surface layer and center portion, each with controlled microstructure. The surface layer is optimized for cold workability with polygonal ferrite, while the center is optimized for strength with tempered martensite, resolving the contradiction between these two properties.
2Strength
If conventional ultrahigh-strength steel with tempered martensite structure is used, then high strength is achieved, but SSC resistance deteriorates due to high dislocation density facilitating hydrogen migration
Solution Approach 1:
The patent places the polygonal ferrite microstructure with low dislocation density specifically in the surface layer where corrosion and hydrogen entry occur, providing localized SSC resistance where it is most needed, while maintaining tempered martensite in the center for overall strength.
Solution Approach 2:
The patent converts the typically harmful martensite structure into a beneficial configuration by locating it in the center portion away from the corrosion-prone surface, while using polygonal ferrite at the surface to prevent hydrogen entry, thus turning the potential weakness into a strength.
3Manufacturing precision
If cold forming is used for ultrahigh-strength steel, then forming precision is improved, but cracking risk increases due to low ductility of low-temperature transformation structures
Solution Approach 1:
The patent provides a polygonal ferrite microstructure in the surface layer that offers high ductility and uniform elongation, enabling cold forming operations without cracking, while the center portion maintains tempered martensite for final strength properties after forming.
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 enhances the steel's cold workability and SSC resistance, achieving a yield strength of 690 MPa or more, while preventing surface cracking and improving hydrogen resistance, thus making it suitable for offshore structures.
Implementation Method 1
a microstructure of a surface layer portion, which is a region from a surface of the steel to 10% of a total thickness of the steel, contains 90 area % or more of polygonal ferrite, a microstructure of a region excluding the surface layer portion contains 90 area % or more of tempered martensite or 90 area % or more of a mixed structure of tempered martensite and tempered bainite
Implementation Method 2
the surface layer portion has a dislocation density of 3×10^14/m^2 or less
Data Source
AI summary
One embodiment of the present invention provides an ultrahigh-strength steel having excellent cold workability and SSC resistance, comprising, by wt %, carbon (C) in an amount of more than 0.08% and equal to or less than 0.2%, 0.05-0.5% of silicon (Si), 0.5-2% of manganese (Mn), 0.005-0.1% of aluminum (Al), 0.01% or less of phosphorus (P), 0.0015% or less of sulfur (S), 0.001-0.03% of niobium (Nb), 0.001-0.03% of vanadium (V), 0.001-0.03% of titanium (Ti), 0.01-1% of chromium (Cr), 0.01-0.15% of molybdenum (Mo), 0.01-0.5% of copper (Cu), 0.05-4% of nickel (Ni), 0.0005-0.004% of calcium (Ca), and the balance of Fe and other inevitable impurities, wherein the microstructure of a surface layer part, which is the region from the surface to 10% of the total thickness, comprises 90 area % or more of polygonal ferrite, the microstructure of the region excluding the surface layer part comprises 90 area % or more of tempered martensite or 90 area % or more of a mixed structure of tempered martensite and tempered bainite, and the dislocation density of the surface layer part is 3×10 14/m 2 or less.