Rolled Wire Rod for Spring Steel Strength Toughness
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Solution Overview
Problem
Current methods for producing spring steel with high tensile strength and toughness, such as those used in vehicle suspension springs, fail to effectively balance strength and toughness, particularly in achieving tensile strengths of 2,000 MPa or greater while maintaining sufficient drawability and preventing wire breakage during drawing.
Innovation Solution
A rolled wire rod with a specific chemical composition and microstructure, including a carbon equivalent of 0.75% to 1.00%, an area fraction of 90% or greater bainite and tempered martensite, and a tensile strength of 1,350 MPa or less, is developed to achieve high strength and toughness through controlled quenching and tempering processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steels with tensile strength greater than 1.800 MPa are used for spring manufacturing, then the strength of the spring is improved, but the toughness deteriorates making the spring vulnerable to damage from impact loads
Solution Approach 1:
The invention changes the microstructure parameters of the rolled wire rod by controlling the area fraction of bainite and martensite to be 90% or greater, and controlling tensile strength to be 1.350 MPa or less before heat treatment. This parameter control enables the steel to achieve both high strength (2.000 MPa or greater) and high toughness after quenching and tempering, resolving the contradiction between strength and toughness in spring steel
Solution Approach 2:
The invention performs preliminary microstructure control during the rolling process before heat treatment, creating a specific microstructure with 90% or greater area fraction of bainite and martensite while controlling tensile strength to 1.350 MPa or less. This preliminary action prepares the steel for subsequent quenching and tempering to achieve both high strength and high toughness, preventing the strength-toughness trade-off that occurs when starting with already high-strength material
2Strength
If the microstructure of the rolled wire rod is optimized for high strength, then the tensile strength after heat treatment is improved, but the drawability deteriorates causing wire breakage during drawing
Solution Approach 1:
The invention changes the microstructure parameters by controlling the area fraction of bainite and martensite to be 90% or greater and controlling the tensile strength to be 1.350 MPa or less in the rolled wire rod before heat treatment. This specific parameter range maintains adequate drawability during wire drawing while enabling the steel to achieve tensile strength of 2.000 MPa or greater after quenching and tempering
Solution Approach 2:
The invention performs preliminary microstructure control during rolling to create a specific microstructure with 90% or greater area fraction of bainite and martensite while maintaining tensile strength at 1.350 MPa or less. This preliminary preparation ensures both adequate drawability for subsequent wire drawing and the potential for high strength after heat treatment, resolving the contradiction between current manufacturability and final properties
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 spring steel with a tensile strength of 2,000 MPa or greater and high toughness, ensuring adequate drawability and preventing wire breakage, while maintaining a Charpy impact value of 60.0 J/cm2 or greater, suitable for high-strength spring applications.
Implementation Method 1
a rolled wire rod for spring steel suitable for a spring steel having a tensile strength of 2,000 MPa or greater and high toughness after a heat treatment such as quenching and tempering
Data Source
AI summary
A rolled wire rod for spring steel contains, as a chemical composition, by mass %: C: 0.42% to 0.60%; Si: 0.90% to 3.00%; Mn: 0.10% to 1.50%; Cr: 0.10% to 1.50%; B: 0.0010% to 0.0060%; N: 0.0010% to 0.0070%; Mo: 0% to 1.00%; V: 0% to 1.00%; Ni: 0% to 1.00%; Cu: 0% to 0.50%; Al: 0% to 0.100%; Ti: 0% to 0.100%; Nb: 0% to 0.100%; P: limited to less than 0.020%; S: limited to less than 0.020%; and a remainder including Fe and impurities, the carbon equivalent (Ceq) is 0.75% to 1.00%, the area fraction of tempered martensite and bainite included in a microstructure is 90% or greater, the tensile strength is 1,350 MPa or less, and the reduction of area is 40% or greater.