Spring Steel Microstructure Control for Delayed Fracture Resistance
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Solution Overview
Problem
Existing high-strength spring steel for suspension systems in automobiles faces challenges in achieving both high tensile strength and delayed fracture resistance while maintaining cold formability, often requiring the addition of expensive alloy elements.
Innovation Solution
The development of spring steel with controlled crystal orientations of martensite structures, specifically by hot rolling and tempering, to enhance both delayed fracture resistance and cold formability without the need for expensive alloy elements, achieving a tensile strength of 1800 MPa or more.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high strength spring steel is made by adding alloy elements such as Ni or Cu, then delayed fracture resistance is improved, but cost rises
Solution Approach 1:
The invention changes the microstructural parameters of the steel by controlling crystal orientations of martensite structures. Specifically, it increases the fraction of <100> orientations and suppresses <111> orientations through controlled hot rolling and tempering processes, achieving improved delayed fracture resistance without adding expensive alloy elements
Solution Approach 2:
The invention replaces expensive alloy elements (Ni, Cu, V, Mo) with a cost-effective microstructural control approach using standard hot rolling and tempering processes. This achieves the same functional improvement (delayed fracture resistance) through process control rather than material composition changes
2Reliability
If high strength spring steel is made by adding alloy elements such as V or Mo, then delayed fracture resistance is improved by forming precipitates, but cost rises
Solution Approach 1:
Instead of adding alloy elements to form precipitates, the invention changes the crystal orientation parameters of existing martensite structures. The controlled hot rolling and tempering processes produce a specific texture with increased <100> orientations that provides hydrogen trapping capability without requiring additional alloying elements
3Strength
If the strength of spring steel is raised to 1800 MPa or more, then tensile strength is improved, but sensitivity to hydrogen penetration and corrosion pits increases, making the spring liable to break due to delayed fracture
Solution Approach 1:
The invention changes the crystallographic orientation parameters of martensite structures to simultaneously achieve high tensile strength and delayed fracture resistance. The specific texture control (increasing <100> and suppressing <111> orientations) creates a microstructure that is both strong and resistant to hydrogen embrittlement
Solution Approach 2:
The invention creates a composite-like microstructure within the martensite phase by establishing a specific distribution of crystal orientations. This textured microstructure exhibits combined properties of high strength and high delayed fracture resistance, similar to how composite materials combine different phases to achieve superior properties
4Reliability
If alloy elements are added to improve delayed fracture resistance, then critical hydrogen content is increased, but cold formability is not improved
Solution Approach 1:
The invention changes the crystal orientation parameters of martensite structures, which simultaneously affects both delayed fracture resistance and cold formability. The increased <100> orientations provide both hydrogen trapping capability and improved ductility, unlike alloy element addition which only addresses hydrogen resistance
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
This approach results in spring steel with improved tensile strength, delayed fracture resistance, and cold formability, enabling the production of lightweight suspension springs for automobiles without the use of costly alloy elements.
Implementation Method 1
90% or more of the metal microstructures by area fraction is tempered martensite
Implementation Method 2
stretching of the prior austenite grains at the surface in the rolling direction
Implementation Method 3
quench hardening it
Implementation Method 4
tempering it
Data Source
Figure 1
AI summary
Provided is spring steel for suspension suppressing or not requiring addition of expensive alloy elements, having a large tensile strength, and excellent in cold formability and delay fracture resistance, wherein at a cross-section parallel to a rolling direction, 90% or more of the metal microstructures by area fraction is tempered martensite, and at a cross-section parallel to the rolling direction, in a range of 10% of diameter or thickness from the surface, a ratio of a length in a long axis direction of prior austenite grains and a length in a direction perpendicular to the long axis direction of the prior austenite grains is 1.5 or more and a ratio of <011> fraction/<111> fraction of martensite texture as seen from the rolling direction is 3.0 or more.