High-Strength PC Steel Wire Microstructure Control
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Solution Overview
Problem
High-strength PC steel wires with tensile strength above 2000 MPa face a trade-off between strength and delayed fracture resistance, with existing methods being complex and inefficient in achieving both requirements.
Innovation Solution
A high-strength PC steel wire with a chemical composition and microstructure that includes a ferrite or bainitic structure in the outermost layer and a pearlite structure in the inner layer, controlled to maintain a specific hardness ratio and area fraction, enhancing delayed fracture resistance while maintaining sufficient tensile strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the tensile strength of PC steel wire is enhanced to 2000 MPa or more, then the strength is improved, but the delayed fracture resistance characteristics deteriorate
Solution Approach 1:
The patent applies local quality by creating different microstructures in different regions of the steel wire. The outer layer (to a depth of 0.01D) contains ferrite and/or bainite structures which provide delayed fracture resistance, while the inner region maintains pearlite structure for high strength. This spatial differentiation of microstructure allows simultaneous achievement of both high tensile strength and good delayed fracture resistance.
2Reliability
If a special heat treatment is performed to control hardness distribution, then the delayed fracture resistance is improved, but the production complexity increases
Solution Approach 1:
The patent uses parameter changes by controlling the cooling rate during the patenting process to achieve the desired microstructure without additional complex heat treatments. By adjusting the cooling rate parameter, the outer layer forms ferrite and/or bainite structures while the inner region forms pearlite structure, thereby obtaining both good delayed fracture resistance and high strength through a simplified single-step process.
3Strength
If the pearlite structure is formed throughout the steel wire to achieve high strength, then the tensile strength is improved, but the delayed fracture resistance deteriorates due to high hardness at the surface
Solution Approach 1:
The patent applies segmentation by dividing the steel wire into two distinct regions with different microstructures. The outer layer (depth 0.01D from surface) contains ferrite and/or bainite structures that provide low hardness and good delayed fracture resistance, while the inner region contains pearlite structure that provides high strength. This segmentation allows each region to optimize its properties for its specific function.
Data Source
AI summary
This invention provides a high-strength PC steel wire having a chemical composition containing, in mass %, C: 0.90 to 1.10%, Si: 0.80 to 1.50%, Mn: 0.30 to 0.70%, P: 0.030% or less, S: 0.030% or less, Al: 0.010 to 0.070%, N: 0.0010 to 0.010%, Cr: 0 to 0.50%, V: 0 to 0.10%, B: 0 to 0.005%, Ni: 0 to 1.0%, Cu: 0 to 0.50%, and the balance: Fe and impurities. A ratio between the Vickers hardness (HvS) at a location (surface layer) that is 0.1D [D: diameter of steel wire] from the surface of the steel wire and the Vickers hardness (HvI) of a region on the inner side relative to the surface layer satisfies the formula [1.10<HvS/HvI≤1.15]. The steel micro-structure in the region from the surface of the steel wire to 0.01D (outermost layer region) consists of, in area %, a pearlite structure: less than 80%, and the balance: a ferrite structure and/or a bainitic structure. The steel micro-structure in the region on the inner side relative to the outermost layer region contains, in area %, a pearlite structure: 95% or more. The tensile strength of the steel wire is 2000 to 2400 MPa. The method of producing this high-strength PC steel wire is simple, and the high-strength PC steel wire is excellent in delayed fracture resistance characteristics.