Hot-Rolled Steel Sheet Cold Formability Hardening Balance
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Solution Overview
Problem
Existing hot-rolled steel sheets for automotive parts face challenges in achieving both excellent cold formability and hardenability, leading to issues such as decreased cold formability, increased die wear, and non-uniform hardness, particularly during severe cold forming processes like fine blanking and cold forging.
Innovation Solution
A hot-rolled steel sheet with a carbon content of 0.18% to 0.29% and specific adjustments in Mn, Al, Ti, and B levels, combined with a microstructure of ferrite and pearlite, along with controlled hot rolling and cooling processes, to achieve a balance of cold formability and hardenability, ensuring uniform tensile strength across the width direction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the carbon content is increased to improve hardenability, then the hardenability is improved, but the cold formability deteriorates
Solution Approach 1:
The patent optimizes the carbon content to a specific range (0.18-0.29%) rather than simply increasing it, and combines it with controlled amounts of alloying elements (Mn: 1.0-2.5%, Ti: 0.005-0.05%, B: 0.0005-0.0050%) to achieve the desired balance between hardenability and cold formability
Solution Approach 2:
The patent creates a composite microstructure consisting of ferrite and pearlite phases through controlled hot rolling and cooling processes, where the ferrite provides ductility for cold forming while the pearlite contributes to hardenability
2Strength
If the ferrite grain diameter is reduced to improve strength, then the strength is improved, but the cold formability deteriorates
Solution Approach 1:
The patent specifies an optimal ferrite grain diameter range (5.0-15.0 μm) that balances strength and cold formability, and controls the volume fraction of ferrite (30-80%) to achieve the desired properties
3Strength
If the pearlite fraction is increased to improve hardenability, then the hardenability is improved, but the cold formability deteriorates
Solution Approach 1:
The patent optimizes the pearlite fraction to a specific range (20-70%) that provides sufficient hardenability while maintaining adequate cold formability through the presence of ferrite phase
4Manufacturing precision
If the coiling temperature is reduced to refine the microstructure, then the microstructure is refined, but the cold formability deteriorates
Solution Approach 1:
The patent specifies an optimal coiling temperature range (500-750°C) that achieves sufficient microstructure refinement while maintaining cold formability, and controls the cooling rate (5-50°C/s) to achieve the desired balance
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 results in a steel sheet with improved cold formability, reduced die wear, and uniform hardness, enabling efficient and cost-effective manufacturing of automotive parts with high yield and reduced maintenance costs.
Implementation Method 1
a microstructure in which the fraction of ferrite and pearlite with respect to the whole microstructure is 95% or more in terms of the sum of the volume fraction of both components, in which the mean grain diameter of the ferrite is 7.0 μm or more and 15.0 μm or less, and in which the volume fraction of the ferrite with respect to the whole microstructure is 50% or more
Data Source
AI summary
Hot-rolled steel has a chemical composition containing, by mass %, C: 0.18% or more and 0.29% or less, N: 0.0050% or less, Ti: 0.002% or more and 0.05% or less, B: 0.0005% or more and 0.0050% or less, and appropriately controlled amounts of Si, Mn, P, S, Al, and a tensile strength of 500 MPa or less with a variation in tensile strength of 60 MPa or less throughout a region including the edges in the width direction of the steel sheet, and having excellent cold formability and hardenability.