Rolled Steel Bar Cold Forging Grain Growth
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Solution Overview
Problem
Current rolled steel products for cold-forged components face challenges in achieving high strength, excellent cold forgeability, and grain coarsening resistance, particularly when spheroidizing annealing treatments are omitted or shortened, leading to issues like die wear and component cracking.
Innovation Solution
A rolled steel bar or wire rod with a specific chemical composition (C: 0.24-0.36%, Si: <0.40%, Mn: 0.20-0.45%, Cr: 0.70-1.45%, and Ti: 0.010-0.050%) and internal structure (ferrite-pearlite with a ferrite fraction >40%) that enhances hardenability and cold forgeability, allowing for high-strength component production without prolonged annealing treatments.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of rolled steel is increased to produce high-strength components, then component strength is improved, but cold forgeability deteriorates leading to die wear and component cracking
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.20-0.40%, Si: 0.05-0.50%, Mn: 0.50-1.50%, Cr: 0.50-2.00%, Ti: 0.010-0.050%, B: 0.0005-0.0050%) and microstructural parameters (ferrite fraction: 30-70%, grain size: 5-15 μm) to achieve a balance between strength and cold forgeability. The specific ranges are optimized to ensure high strength after quenching while maintaining adequate ductility for cold forging operations.
Solution Approach 2:
The patent creates a composite microstructure consisting of ferrite and pearlite phases with specific proportions (ferrite fraction 30-70%). This composite structure combines the ductility benefits of ferrite with the strength contributions of pearlite, enabling the steel to exhibit both good cold forgeability and high final strength after heat treatment.
2Ease of manufacture
If spheroidizing annealing treatment time is extended to improve softness and cold forgeability, then cold forgeability is improved, but manufacturing cost and energy loss increase
Solution Approach 1:
The patent applies preliminary action by incorporating alloying elements (particularly Ti and B) during steelmaking that pre-condition the microstructure to facilitate spheroidization. The Ti forms fine carbides that promote uniform spheroidizing, and B enhances hardenability, allowing the steel to achieve the desired soft microstructure with reduced annealing time compared to conventional steels.
Solution Approach 2:
The patent modifies the spheroidizing annealing parameters by optimizing the temperature range (500-700°C) and time duration based on the specific chemical composition. The controlled ranges of alloying elements enable the steel to reach the target microstructure (spheroidized carbides in ferrite matrix) faster than conventional steels, reducing energy consumption while maintaining cold forgeability.
3Ease of manufacture
If alloy element contents are reduced to lower the strength of rolled steel, then cold forgeability is improved, but hardenability deteriorates
Solution Approach 1:
The patent applies parameter changes by optimizing the specific composition ranges of alloying elements. Rather than simply reducing all alloy contents, the patent maintains C at 0.20-0.40% and Cr at 0.50-2.00% to preserve hardenability, while controlling Si and Mn within specific ranges to ensure cold forgeability. The addition of small amounts of Ti (0.010-0.050%) and B (0.0005-0.0050%) further enhances hardenability through grain refinement and carbide formation.
Solution Approach 2:
The patent creates a composite alloying strategy where multiple elements work synergistically: C and Cr provide base hardenability, Ti forms fine carbides that refine grain structure and enhance hardenability, and B significantly increases hardenability at very low concentrations. This composite approach to alloying allows the steel to achieve both good cold forgeability and high hardenability that would not be possible with single-element adjustments.
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 high-strength cold-forged components with HRC hardness of 34 or greater, while suppressing abnormal grain growth and reducing the risk of cracking during cold forging, even when spheroidizing annealing is minimized.
Implementation Method 1
after quenching and tempering
Implementation Method 2
after quenching and tempering
Implementation Method 3
abnormal grain growth during quenching can be suppressed
Data Source
AI summary
In a rolled steel bar or rolled wire rod for a cold-forged component having a predetermined chemical composition, Y1 represented by Y1=[Mn]×[Cr] and Y2 represented by Y2=0.134×(D/25.4−(0.50×√[C]))/(0.50×√[C]) satisfy Y1>Y2, the tensile strength is 750 MPa or less, an internal structure is a ferrite-pearlite structure, and the ferrite fraction in the internal structure is 40% or greater.AMOUNT IS 0.30%