Spring Steel Decarburization Resistance via Sb Sn Alloying
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Solution Overview
Problem
The challenge is to develop high-strength steel for springs that maintains excellent decarburization resistance and scale exfoliation properties, as conventional methods lead to surface hardness deterioration and inadequate compressive residual stress due to decarburization during the manufacturing process, affecting fatigue resistance and scale exfoliation.
Innovation Solution
Optimizing the added amounts of C, Si, Mn, Cr, Sb, and Sn, along with controlling the ratios of these elements through specific formulas (A, B, and C values), to suppress decarburization and improve scale exfoliation, ensuring better decarburization resistance and scale exfoliation properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Shape
If steel for springs is heated for formation into a spring-like shape, then the spring can be formed into the desired shape, but decarburization occurs in the surface layer causing hardness deterioration
Solution Approach 1:
The patent applies parameter changes by optimizing the chemical composition parameters (C: 0.45-0.65%, Si: 0.15-0.70%, Mn: 0.10-1.00%, Cr: 0.20-1.50%, Sb: 0.010-0.030%, Sn: 0.010-0.030%) and controlling the ratios between these elements to suppress decarburization during heating while maintaining formability. This resolves the contradiction by changing the material parameters to prevent carbon loss during the heating process required for shaping.
Solution Approach 2:
The patent creates a composite chemical composition system combining multiple alloying elements (C, Si, Mn, Cr, Sb, Sn) that work synergistically to suppress decarburization. The specific combination and ratios of these elements form a composite material system that provides both decarburization resistance and adequate surface hardness, resolving the contradiction between formability and surface property maintenance.
2Reliability
If compressive residual stress is applied via shot peening to improve fatigue resistance, then fatigue resistance improves, but decarburization causes insufficient compressive residual stress application
Solution Approach 1:
The patent applies preliminary action by suppressing decarburization during the heating and forming processes before shot peening is applied. By controlling the chemical composition and preventing carbon loss in advance, the surface layer maintains adequate carbon content and hardness, ensuring that subsequent shot peening can effectively apply compressive residual stress and improve fatigue resistance.
3Stability of the object's composition
If the added amount of Sb is increased to suppress decarburization, then decarburization resistance improves, but scale exfoliation property deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the Sb content within a narrow range (0.010-0.030%) and optimizing its ratio with other elements, particularly Sn. This controlled parameter change provides sufficient decarburization resistance while preventing excessive scale formation, resolving the contradiction between composition stability and manufacturability.
Solution Approach 2:
The patent creates a composite alloying system where Sb works synergistically with Sn and other elements. The combined effect of multiple alloying elements at optimized ratios provides decarburization suppression without the adverse scale exfoliation effects that would occur with excessive Sb alone, resolving the contradiction between composition stability and ease of manufacture.
4Weight of moving object
If high strength material is used to achieve post quenching-tempering strength of about 1800 MPa or more, then weight can be reduced, but decarburization causes hardness deterioration affecting spring properties
Solution Approach 1:
The patent applies parameter changes by optimizing the base chemical composition parameters (higher C content 0.45-0.65%, controlled Si 0.15-0.70%, Mn 0.10-1.00%, Cr 0.20-1.50%) to achieve high strength while adding decarburization-suppressing elements (Sb 0.010-0.030%, Sn 0.010-0.030%). This allows the use of high-strength material for weight reduction while preventing surface hardness deterioration through controlled composition parameters.
Solution Approach 2:
The patent creates a composite high-strength steel system combining multiple alloying elements that work together to achieve both high strength (for weight reduction) and decarburization resistance. The synergistic effect of C, Si, Mn, Cr, Sb, and Sn in specific ratios enables the material to maintain surface hardness despite heating processes, resolving the contradiction between weight reduction and property maintenance.
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 optimized chemical composition and processing method result in high-strength steel with enhanced decarburization resistance and scale exfoliation properties, improving the fatigue resistance and durability of springs.
Implementation Method 1
carbon in the surface layer is reduced, and total decarburization occurs
Implementation Method 2
despite rapid cooling after heating, the layer transforms into ferrite, and a ferrite phase forms
Implementation Method 3
compressive residual stress is applied to the surface of the spring via shot peening after quenching-tempering
Implementation Method 4
subjected to quenching-tempering processes
Implementation Method 5
subjected to quenching-tempering processes
Data Source
AI summary
Provided is high strength steel for springs that, compared to conventional high strength steel for springs, has excellent decarburization resistance and scale exfoliation property, by optimization of the added amounts of C, Si, Mn, and Cr as well as of Sb and Sn. The steel for springs contains C: more than 0.45 mass% and less than 0.65 mass%, Si: 0.15 mass% or more and 0.70 mass% or less, Mn: 0.10 mass% or more and 1.00 mass% or less, Cr: 0.20 mass% or more and 1.50 mass% or less, P: 0.025 mass% or less, S: 0.025 mass% or less, O: 0.0015 mass% or less, Sb: 0.010 mass% or more and less than 0.030 mass%, and Sn: 0.010 mass% or more and 0.030 mass% or less, under predetermined conditions.