Spring Steel Composition for High-Temperature Nitriding Strength
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Solution Overview
Problem
High-strength spring materials for automobile transmissions face challenges in maintaining fatigue resistance and nitriding properties while avoiding the formation of low-temperature structures and excessive grain growth, which can lead to reduced strength and increased production time.
Innovation Solution
A wire rod and steel wire composition with specific alloying elements (C, Si, Mn, Cr, Mo, V, and Fe) and manufacturing processes that include heating, rolling, and controlled cooling to achieve a microstructure with a high pearlite content, suppressed grain size, and uniform carbide distribution, ensuring improved fatigue resistance and nitriding properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If nitriding is performed at high temperature (500°C or higher) to reduce nitriding time, then productivity is improved, but the strength of spring steel is greatly reduced because tempering heat treatment temperature is 450°C or lower
Solution Approach 1:
The patent changes the chemical composition parameters of the steel by adding specific amounts of carbide-forming elements (Cr: 0.9-1.6%, Mo: 0.1-0.25%, V: 0.1-0.25%) to enable nitriding at high temperatures without excessive strength loss. This compositional modification allows the steel to maintain sufficient strength while achieving faster nitriding
Solution Approach 2:
The patent creates a composite microstructure consisting of tempered martensite (5-20%) and pearlite (78-90%), where the tempered martensite provides strength and the pearlite provides ductility and resistance to strength deterioration during high-temperature nitriding
2Strength
If large quantities of carbide-forming elements (Cr, Mo, V) are added to prevent strength decrease during nitriding, then strength is maintained, but breakage occurs during wire rod production due to low-temperature structure formation and constant-temperature transformation time is greatly increased
Solution Approach 1:
The patent optimizes the parameters of carbide-forming element content to specific ranges (Cr: 0.9-1.6%, Mo: 0.1-0.25%, V: 0.1-0.25%) that are sufficient to prevent strength loss during nitriding but not excessive to cause low-temperature structure formation. This precise parameter control avoids breakage during wire rod production while maintaining strength
Solution Approach 2:
The patent creates local quality differences in the microstructure by controlling the formation of tempered martensite (5-20%) in specific regions, providing localized strength where needed while maintaining overall productivity
3Weight of moving object
If wire diameter is decreased to achieve weight reduction in high-strength springs, then weight is reduced, but sensitivity to inclusions increases and fatigue limit is lowered
Solution Approach 1:
The patent changes the chemical composition parameters by adding specific amounts of alloying elements (Cr, Mo, V) that enhance the material's resistance to inclusion-induced fatigue, allowing thin-wire springs to maintain high fatigue limits despite reduced diameter
Solution Approach 2:
The patent creates a composite microstructure with tempered martensite and pearlite that provides enhanced toughness and resistance to inclusion effects, compensating for the reduced wire diameter and maintaining fatigue performance
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 enhances the fatigue resistance and nitriding properties of the spring materials, maintaining strength and productivity by preventing low-temperature structure formation and optimizing carbide distribution, thereby improving the overall manufacturing efficiency and performance.
Implementation Method 1
the microstructure comprises 60% or more of a pearlite structure in the C section
Implementation Method 2
spring manufacturers increase the fatigue limit of spring materials by maintaining strength and improving surface hardness through nitriding
Implementation Method 3
heating, rolling, and controlled cooling to achieve a microstructure with a high pearlite content, suppressed grain size, and uniform carbide distribution
Data Source
AI summary
Disclosed are a wire rod and a steel wire for a spring, a spring with improved fatigue resistance and nitriding property, and methods for manufacturing same. The wire rod for a spring with improved fatigue resistance and nitriding property according to the present disclosure contains, by wt %, 0.6-0.7% of C, 2.0-2.5% of Si, 0.2-0.5% of Mn, 0.9-1.6% of Cr, 0.015% of less of P, 0.01% of less of S, 0.01% of less of Al, 0.007% of less of N, 0.1-0.25% of Mo, 0.1-0.25% of V, and Fe and inevitable impurities as the balance, wherein Cr+Mn is 1.8% or less, Mo/V is 1.5 or less, and the microstructure includes 60% or more of a pearlite structure in the C section.