Leaf Spring Steel Fatigue Strength TiC Grain Refinement
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Solution Overview
Problem
Conventional spring steel technologies fail to provide optimal fatigue strength and hydrogen embrittlement resistance for leaf springs, particularly when subjected to high-strength shot peening, and are prone to decarburization and early breakage due to large bainite structures and low cross-sectional area changes during rolling.
Innovation Solution
A steel composition with specific ranges of C, Si, Mn, Cr, Ti, B, and N is developed, where Ti is added between 0.07% and 0.15% to inhibit bainite formation, and Ti/N ≥ 10 to create fine TiC and austenite grains, enhancing fatigue life and hydrogen embrittlement resistance, while maintaining high hardness and toughness through high-strength shot peening.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If tensile strength is increased by increasing hardness, then fatigue strength is improved in ordinary environment, but fatigue strength is significantly decreased in corrosive environment
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters of the steel (C: 0.35-0.50%, Si: 0.70-1.50%, Mn: 0.50-1.50%, Cr: 1.00-2.00%, Ti: 0.030-0.100%, B: 0.0005-0.0050%) to achieve a balance between hardness and corrosion resistance. This compositional parameter optimization allows the steel to maintain high tensile strength while resisting hydrogen embrittlement in corrosive environments, thereby improving fatigue strength without the adverse effects of conventional high-hardness steels.
Solution Approach 2:
The patent creates a composite microstructure consisting of martensite as the primary phase with fine-grained characteristics. This composite material approach combines the high strength of martensite with the corrosion resistance provided by specific alloying elements (particularly Si and Cr), resulting in a material that exhibits both high tensile strength and improved resistance to hydrogen embrittlement, thus resolving the contradiction between strength improvement and fatigue strength degradation in corrosive environments.
2Strength
If high-strength shot peening is applied to improve fatigue strength, then surface fatigue resistance is improved, but early breakage occurs due to large bainite structures
Solution Approach 1:
The patent applies parameter changes by controlling the Ti content (0.030-0.100%) and Ti/N ratio (≥5) to refine the austenite grain size before transformation, and by optimizing the tempering temperature (150-250°C) to suppress bainite formation. These parameter changes result in a fine-grained martensitic microstructure that can withstand high-strength shot peening without forming large bainite structures, thereby preventing early breakage while maintaining improved fatigue strength.
Solution Approach 2:
The patent applies preliminary action by performing tempering treatment at 150-250°C after quenching to transform and refine the microstructure before the leaf spring undergoes service or further processing. This preliminary tempering suppresses the formation of large bainite structures and creates a fine-grained martensitic structure that is resistant to early breakage, even when subjected to high-strength shot peening treatment.
3Strength
If hardness is increased to achieve higher strength, then tensile strength is improved, but toughness is degraded in the high hardness range
Solution Approach 1:
The patent applies parameter changes by optimizing the chemical composition (particularly Si: 0.70-1.50% and Cr: 1.00-2.00%) and heat treatment parameters (tempering temperature: 150-250°C) to achieve a fine-grained martensitic microstructure. This parameter optimization allows the steel to maintain high tensile strength (HV510 or more) while preserving toughness through the fine grain structure, which prevents the typical degradation of toughness that occurs with high hardness in conventional steels.
Solution Approach 2:
The patent applies local quality by creating a fine-grained microstructure throughout the steel, where the grain size is controlled at a microscopic level. This local refinement of the microstructure ensures that even in the high hardness range, the material maintains adequate toughness because the fine grains prevent the propagation of cracks and maintain ductility, thus resolving the contradiction between high strength and toughness.
4Area of stationary object
If cross-sectional area is large in final product, then leaf spring structural requirements are met, but decarburization occurs during rolling
Solution Approach 1:
The patent applies parameter changes by increasing the Si content (0.70-1.50%) which enhances the steel's resistance to decarburization during the rolling process. The higher Si content forms protective oxides on the surface and reduces carbon diffusion, thereby preventing decarburization even in large-cross-sectional leaf springs during hot rolling, thus maintaining manufacturing precision while meeting structural requirements.
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 steel exhibits improved fatigue strength, resistance to hydrogen embrittlement, and increased toughness in the high hardness range, preventing early breakage and maintaining strength even after high-strength shot peening, with remarkable effects at a Vickers hardness of at least HV510.
Implementation Method 1
Ti is added between 0.07% and 0.15% to inhibit bainite formation, and Ti/N ≥ 10 to create fine TiC and austenite grains
Implementation Method 2
create fine TiC and austenite grains, enhancing fatigue life and hydrogen embrittlement resistance
Implementation Method 3
the steel exhibits improved fatigue strength, resistance to hydrogen embrittlement, and increased toughness in the high hardness range, preventing early breakage and maintaining strength even after high-strength shot peening
Implementation Method 4
maintaining high hardness and toughness through high-strength shot peening, with remarkable effects at a Vickers hardness of at least HV510
Data Source
AI summary
Disclosed is steel for a leaf spring with high fatigue strength containing, in mass percentage, C: 0.40 to 0.54%, Si: 0.40 to 0.90%, Mn: 0.40 to 1.20%, Cr: 0.70 to 1.50%, Ti: 0.070 to 0.150%, B: 0.0005 to 0.0050%, N: 0.0100% or less, and a remainder composed of Fe and impurity elements. Also disclosed is a high fatigue-strength leaf spring part obtained by forming the steel. The steel for a leaf spring is prepared to have a Ti content and a N content to satisfy a relation of Ti/N≥10. Preferably, the leaf spring part is subjected to a shot peening treatment in a temperature range of the room temperature through 400°C with a bending stress of 650 to 1900 MPa being applied to it.