High-Carbon Cold-Rolled Steel Sheet for Quenching Hardness and Workability
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Solution Overview
Problem
High carbon cold rolled steel sheets used for knitting needles require improved impact characteristics and wear resistance without compromising secondary workability, as they often break prematurely due to increased load from higher knitting machine speeds.
Innovation Solution
A high carbon cold rolled steel sheet with a composition of 0.85 to 1.10% C, 0.005 to 0.020% Nb, and controlled Mn and Cr levels, combined with a manufacturing process involving multiple cold rolling and spheroidizing annealing, to achieve a balanced hardness, impact characteristics, and wear resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the C content is increased to improve hardenability and quenching hardness, then the wear resistance and durability are improved, but the cold workability deteriorates due to increased carbide formation
Solution Approach 1:
The patent optimizes the C content parameter to a specific range (0.60-1.30%) to achieve the desired balance between quenching hardness and cold workability. This parameter change resolves the contradiction by finding the optimal value that satisfies both requirements simultaneously.
Solution Approach 2:
The patent creates a composite microstructure consisting of martensite, retained austenite, and carbides through controlled composition and heat treatment. This composite structure provides both the hardness from martensite and the workability from retained austenite, resolving the contradiction between strength and manufacturability.
2Strength
If Mn and Cr contents are increased to improve hardenability, then the quenching hardness is improved, but the impact characteristics deteriorate due to insufficient tempering in the martensite phase
Solution Approach 1:
The patent optimizes the Mn and Cr content parameters within specific ranges and combines them with controlled tempering temperature parameters. This multi-parameter optimization achieves both high hardness and good impact characteristics by preventing excessive carbide precipitation that would harm toughness.
Solution Approach 2:
The patent utilizes phase transition control through tempering heat treatment to transform the martensite structure into a more ductile state. By controlling the tempering process, the patent achieves sufficient toughness while maintaining the hardness provided by the martensitic structure formed during quenching.
3Adaptability or versatility
If the thickness is reduced to meet small diameter yarn knitting requirements, then the adaptability to circular knitting machines is improved, but the strength and wear resistance deteriorate
Solution Approach 1:
The patent changes the material parameters (composition and microstructure) to compensate for the reduced thickness. By optimizing the steel composition and controlling the microstructure to contain martensite, retained austenite, and fine carbides, the patent achieves high strength in thin sections, enabling use in circular knitting machines with small diameter yarns.
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 sheet maintains excellent secondary workability and tool life while achieving impact values of 9 J/cm² or more, hardness of 600 to 750 HV, and wear resistance, suitable for severe use environments like knitting needles.
Implementation Method 1
repeatedly performing cold rolling and spheroidizing annealing
Implementation Method 2
when the material is subjected to a quenching and tempering treatment
Implementation Method 3
when the material is subjected to a quenching and tempering treatment
Data Source
Figure 1
Figure 2~3(b)
Figure 4~5
AI summary
Provided is a high carbon cold rolled steel sheet which can have good impact characteristics and hardness characteristics and excellent wear resistance after rapid cooling (quenching) treatment after short-time solution treatment and low-temperature tempering treatment (quenching and tempering treatment), has little decrease in secondary workability before the quenching and tempering treatment, and has a sheet thickness of less than 1.0 mm. The high carbon cold rolled steel sheet has a steel sheet chemical composition consisting of, by mass%, C: 0.85% to 1.10%, Mn: less than 0.60%, Si: 0.10% to 0.35%, P: 0.030% or less, S: 0.030% or less, Cr: less than 0.60%, Mn + Cr satisfying less than 1.0%, Nb: 0.005% to 0.020%, and the balance being Fe and inevitable impurities. Thereby, compared with conventional steel materials, there is little decrease in the secondary workability before quenching and tempering. In addition, by adopting a steel sheet structure with an average particle diameter of carbide of 0.2 to 0.7 (µm) and a spheroidization rate of 90% or more, even with a quenching and tempering treatment in such a short time as 3 to 15 min, it is possible to provide a machine part having excellent impact characteristics with an impact value of 9 J/cm2, sufficient hardness characteristics in a range of 600 to 750 HV, and excellent wear resistance.