Spring Steel Wire Rod Composition for High Strength With Low Segregation
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Solution Overview
Problem
Existing spring steel for motorbike suspension lacks sufficient strength and fatigue resistance, and current methods for enhancing strength, such as using tempered martensite structure, are difficult to manufacture and expensive, while induction heat treatment leads to severe material deviations with high strength steel.
Innovation Solution
A wire rod and steel wire with specific alloy compositions (0.5-0.7% C, 0.4-0.9% Si, 0.3-0.8% Mn, 0.2-0.6% Cr, and controlled segregation) are manufactured using continuous casting and heat treatment processes to achieve high strength and reduced material deviation, including a surface ferrite decarburized layer and high-pressure water cooling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If tempered martensite structure is used for high strength, then strength is improved, but manufacturing difficulty and cost increase
Solution Approach 1:
The patent changes the microstructure type from tempered martensite to upper bainite by adjusting cooling parameters and alloy composition. This parameter change achieves comparable strength (1770-2200 MPa tensile strength) while significantly improving manufacturability and reducing cost, as upper bainite can be obtained through controlled cooling without the complex heat treatment required for tempered martensite
Solution Approach 2:
The patent creates a composite microstructure consisting of upper bainite as the matrix with controlled carbide precipitation. This composite structure, achieved through specific alloying (C: 0.23-0.40%, Si: 0.70-1.50%, Mn: 0.50-2.00%, Cr: 0.10-0.60%) and cooling rates (0.5-5.0°C/s), provides both high strength and improved ductility compared to conventional tempered martensite
2Ease of manufacture
If alloy content is reduced and tempering temperature is lowered, then cost is reduced, but ductility becomes insufficient
Solution Approach 1:
The patent optimizes the alloy composition parameters within specific ranges (C: 0.23-0.40%, Si: 0.70-1.50%, Mn: 0.50-2.00%, Cr: 0.10-0.60%) to achieve the desired balance between strength and ductility. This controlled parameter adjustment allows for reduced alloy content compared to conventional steels while maintaining adequate ductility through the upper bainite microstructure formation
Solution Approach 2:
The patent creates local quality differences through controlled carbide precipitation and segregation patterns during cooling. The upper bainite structure exhibits local variations in carbide distribution and microhardness that enhance overall ductility while maintaining high strength, allowing the material to exhibit different properties in different regions to optimize both strength and formability
3Strength
If induction heat treatment is applied for high strength, then strength is improved, but material deviation due to segregation becomes severe
Solution Approach 1:
The patent performs preliminary microstructure control during the cooling process before final heat treatment. By forming the upper bainite structure and controlling carbide precipitation during cooling (at rates of 0.5-5.0°C/s), the material achieves a more uniform microstructure that is less susceptible to segregation during subsequent induction heat treatment, thereby reducing material deviation
Solution Approach 2:
The patent modifies the cooling parameters (rate: 0.5-5.0°C/s) and alloy composition to create a microstructure that is more homogeneous and resistant to segregation. The upper bainite structure with controlled carbide distribution achieves this uniformity, allowing induction heat treatment to produce more consistent results with reduced material deviation across different batches
4Strength
If cross-section reduction ratio is increased for high strength, then strength is improved, but material deviation increases
Solution Approach 1:
The patent performs preliminary microstructure uniformization during the cooling process before drawing operations. By establishing the upper bainite structure and controlling carbide precipitation early in the processing sequence, the material achieves a more homogeneous microstructure that maintains consistency even during subsequent cross-section reduction, thereby reducing material deviation despite high reduction ratios
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 provides high-strength wire rods and steel wires with excellent cross-section reduction ratios and small material deviation, suitable for ultrahigh strength springs with reduced alloy content, applicable to products requiring low spring index.
Implementation Method 1
preparing a bloom by continuous casting of molten steel
Implementation Method 2
rolling the bloom into billets and then into wire rods
Implementation Method 3
with the recent development of induction heat treatment (IT heat treatment) technology
Implementation Method 4
sufficient hardenability may be secured even with the use of water cooling
Data Source
AI summary
Disclosed in the present specification are: a wire rod for ultrahigh-strength springs, which can be applied to motorcycle suspension springs; a steel wire; and a manufacturing method therefor. According to one embodiment of the disclosed wire rod for ultrahigh-strength springs, the wire rod comprises, by wt %, 0.5-0.7% of C, 0.4-0.9% of Si, 0.3-0.8% of Mn, 0.2-0.6% of Cr, 0.015% or less of P, 0.010% or less of S, 0.01% or less of Al, 0.01% or less of N, 0.005% or less of O, and the balance of Fe and inevitable impurities, wherein in 1 mm2 area of the center of the cross-section perpendicular to the longitudinal direction, the proportion of the area satisfying at least one from among C>0.8%, Si>0.9%, Cr>0.8% and Mn>0.8% by wt % can be 5% or less.