Spring Steel Wire High Strength Cold Coilability
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current spring steel technologies face challenges in achieving both high strength and workability, particularly in cold coiling processes, leading to issues with spring shape formation and increased capital costs due to variations in heat treatment and processing efficiency.
Innovation Solution
The development of spring steel with specific chemical compositions and heat treatment processes that control the distribution of oxides, sulfides, and carbides, including the use of elements like Zr, Cr, and Mo, to achieve a tensile strength of 2000 MPa or more while maintaining coilability, by limiting the area ratio of certain carbide sizes and residual austenite.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of spring-use steel wire for cold coiling is increased, then the hardness and durability are improved, but the wire breaks during cold coiling and cannot be formed into spring shape
Solution Approach 1:
The invention changes the chemical composition parameters of the steel wire, specifically controlling carbon content at 0.45-0.70%, silicon at 1.00-3.00%, and limiting harmful elements like aluminum to 0.01% or less and titanium to 0.003% or less. This parameter optimization allows the steel to achieve tensile strength of 2000 MPa or more while maintaining sufficient ductility for cold coiling without breakage
Solution Approach 2:
The invention creates a composite microstructure consisting of tempered martensite matrix with controlled carbide distribution. By controlling the area ratio of carbides with diameter of 0.2 μm or more to be 7% or less, the material achieves both high strength from the martensite and improved ductility from the controlled carbide distribution, enabling both high strength and coilability
2Ease of manufacture
If hot coiling is used to prevent breakage during coiling, then workability is improved, but variations in spring dimensions occur due to heat treatment and processing efficiency drops
Solution Approach 1:
The invention performs preliminary strengthening through controlled quenching and tempering before coiling, creating a microstructure with tempered martensite and controlled carbide distribution. This preliminary action provides sufficient strength and ductility for cold coiling, eliminating the need for subsequent heating and thermal refining, thereby preventing dimensional variations and maintaining high processing efficiency
Solution Approach 2:
The invention replaces the thermal field (heating) with a controlled chemical composition and microstructure design. By optimizing the chemical composition and creating a specific microstructure with controlled carbide distribution, the steel gains sufficient cold coiling capability without requiring thermal softening, thus substituting thermal processing with material design
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 enables the production of high-strength spring steel wires with improved fatigue strength and coilability, reducing the risk of breakage during coiling and enhancing the overall processing efficiency and cost-effectiveness.
Implementation Method 1
steel wire quenched and tempered on-line, so-called oil tempered steel wire
Implementation Method 2
adding V, Nb, Mo, or another element, dissolving this by quenching, forming fine carbides precipitated by tempering
Data Source
AI summary
The present invention provides spring steel used for spring steel wire achieving both high strength and cold coilability and spring steel wire, that is, spring steel containing, by mass %, C: 0.45 to 0.70%, Si: 1.0 to 3.0%, Mn: 0.05 to 2.0%, P: 0.015% or less, S: 0.015% or less, N: 0.0015 to 0.0200%, and t-O: 0.0002 to 0.01 and further limiting Al≤0.01% and Ti≤0.003%. Further, it is characterized by satisfying the following regarding the cementite-based spherical carbides present at an observed plane, an occupied area ratio of grains with a circle equivalent diameter of 0.2 μm or more of 7% or less, a density of presence of grains with a circle equivalent diameter of 0.2 to 3 μm of 1/μm2 or less, and a density of presence of grains with a circle equivalent diameter 3 μm or more of 0.001/μm2 or less, having an prior austenite grain size number of #10 or higher and a residual austenite of 15 mass % or less, and having an area ratio of poor regions with a small density of presence of cementite-based carbides of a circle equivalent diameter of 2 μm or more of 3% or less.