Spring Steel Composition for MnS Inclusion Suppression
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Solution Overview
Problem
Current spring steel technologies face challenges in achieving high tensile strength, toughness, and corrosion resistance after quenching and tempering, with existing solutions either compromising manufacturability, increasing material costs, or failing to adequately address MnS inclusion-induced corrosion.
Innovation Solution
A spring steel composition with controlled levels of C, Si, Mn, Cr, Ti, B, N, S, and optional elements like Ni, Mo, V, Cu, and Nb, where Ti and N are used to refine austenite grain size and form stable Ti-based sulfides to reduce MnS inclusions, enhancing toughness and corrosion resistance without increasing Ni or Cu content.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a large amount of Ni is added to improve corrosion resistance, then corrosion resistance is improved, but material cost increases and hot cracking risk increases
Solution Approach 1:
The invention extracts and removes Ni from the steel composition entirely, replacing its corrosion resistance function with a combination of Cr (1.00-3.00 mass%) and controlled Ti (0.010-0.100 mass%) additions that form stable sulfide inclusions, thereby achieving corrosion resistance without the manufacturing drawbacks of Ni
Solution Approach 2:
The invention uses cheaper alloying elements (Cr, Ti, Mn) in controlled amounts to replace expensive Ni, achieving comparable or superior corrosion resistance through a more cost-effective and manufacturable composition
2Strength
If Ti is added to refine grain size and suppress hydrogen embrittlement, then toughness is improved, but embrittlement risk increases requiring suppression of Ti amount
Solution Approach 1:
The invention optimizes the Ti content parameter to a specific range (0.010-0.100 mass%) that is sufficient to refine austenite grain size and trap hydrogen effectively, but low enough to avoid Ti-induced embrittlement, achieving the optimal balance between toughness and embrittlement resistance
Solution Approach 2:
The invention creates a composite microstructure where Ti forms fine dispersed precipitates (TiN, TiC, TiS) within the steel matrix, combining the grain-refining and hydrogen-trapping benefits of Ti with the embrittlement-mitigating effects of controlled composition and heat treatment
3Strength
If high strength steel is used to achieve tensile strength above 1,800 MPa, then strength is improved, but corrosion fatigue properties deteriorate due to stress concentration at corrosion pits
Solution Approach 1:
The invention takes preliminary action by adding Cr (1.00-3.00 mass%) and controlled Ti to form stable sulfide inclusions before corrosion can occur, creating a corrosion-resistant microstructure that prevents pit formation and protects the high-strength steel from corrosion fatigue
Solution Approach 2:
The invention converts the potential harm of Ti (which can cause embrittlement) into a benefit by controlling Ti content to form fine dispersed precipitates that trap hydrogen and prevent corrosion pit formation, thereby improving corrosion fatigue resistance in the high-strength steel
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 provides spring steel with tensile strength of 1,800 MPa or more, high toughness, and excellent corrosion resistance after quenching and tempering, while maintaining manufacturability and reducing material costs by suppressing MnS inclusions and hydrogen embrittlement.
Implementation Method 1
refining the grain size of a prior austenite grain whose grain boundary serves as a starting point of brittle fracture. Control of the prior austenite grain size is performed by using nitride, carbide, and carbonitride of Ti obtained by the addition of Ti
Implementation Method 2
the embrittlement due to the hydrogen penetration and the decrease in the fatigue properties are suppressed by trapping hydrogen to Ti precipitates
Implementation Method 3
improving corrosion resistance of steel with adding a large amount of Ni
Implementation Method 4
Ti and N are used to refine austenite grain size and form stable Ti-based sulfides to reduce MnS inclusions
Implementation Method 5
spring steel having high strength, high toughness, and high corrosion resistance after quenching and tempering
Implementation Method 6
after a heat treatment of quenching and tempering
Data Source
AI summary
There is provided a spring steel including predetermined chemical composition, in which ([Ti mass %]−3.43×[N mass %])/[S mass %]>4.0, and [Ni mass %]+[Cu mass %]<0.75 are satisfied, and an appearance frequency of MnS is less than 20% among inclusions having an equivalent circle diameter of 1 μm or more which are observed at a ¼ position of a diameter from a surface.