Hot-Rolled Steel Sheet Grain Refinement for Formability
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Solution Overview
Problem
High-strength hot rolled steel sheets face challenges in maintaining stretch flangeability and shape freezability due to differences in hardness among microstructural phases, and existing methods often result in coarse precipitates that hinder achieving sufficient flangeability.
Innovation Solution
Optimizing the rolling process by controlling temperature, strain rate, and total strain amount to refine ferrite grain size and introduce high-density dislocations, thereby reducing hardness differences and improving flangeability, while maintaining high tensile strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of steel material is increased by refining grains, then tensile strength is improved, but stretch flangeability and shape freezability deteriorate due to hardness differences among microstructural phases
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.15-0.35%, Si: 1.50-3.00%, Mn: 1.00-3.00%, Al: 0.010-0.100%, Ti: 0.050-0.300%) and processing parameters (heating temperature, rolling reduction, cooling rate) to achieve a microstructure with ferrite grain size of 10 μm or less. This controlled refinement improves tensile strength to 440 MPa or more while maintaining stretch flangeability and shape freezability through the specific dual-phase microstructure composition.
Solution Approach 2:
The patent creates a composite microstructure consisting of ferrite (70-90% by area) and residual austenite (10-30% by area), where the ferrite provides strength through grain refinement and the residual austenite maintains ductility and formability. This composite microstructure resolves the contradiction by combining the advantages of both phases: ferrite for high strength and residual austenite for good stretch flangeability and shape freezability.
2Strength
If high strength is achieved through microstructure refinement, then tensile strength increases, but yield point elongation increases causing poor shape freezability
Solution Approach 1:
The patent controls the heating temperature to 950°C or more and 1100°C or less, and applies a cooling rate of 20°C/sec or more to achieve a specific microstructure where ferrite grains are refined to 10 μm or less while residual austenite is maintained at 10-30% by area. This parameter control ensures that the steel achieves high tensile strength with minimal yield point elongation, thereby improving shape freezability.
Solution Approach 2:
The dual-phase microstructure of refined ferrite (70-90%) and residual austenite (10-30%) creates a composite material system where the ferrite provides high strength and the residual austenite suppresses yield point elongation. The interaction between these two phases results in high tensile strength while maintaining excellent shape freezability, as the residual austenite acts as a buffer that prevents excessive elongation during forming operations.
3Strength
If cooling rate is increased to refine grains, then ferrite grain size is reduced, but difference in hardness among structures increases reducing stretch flangeability
Solution Approach 1:
The patent applies a controlled cooling rate of 20°C/sec or more, which is sufficiently high to refine ferrite grains to 10 μm or less but not so high as to create excessive hardness differences. This optimized cooling rate parameter, combined with the specific chemical composition and heating temperature, achieves the desired microstructure that balances grain refinement with maintained stretch flangeability.
Solution Approach 2:
The resulting composite microstructure of refined ferrite (70-90%) and residual austenite (10-30%) mitigates the hardness difference problem. The residual austenite phase acts as a softer matrix that compensates for the hardness of refined ferrite, maintaining overall ductility and stretch flangeability while benefiting from the strength provided by fine ferrite grains.
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 approach results in hot rolled steel sheets with enhanced strength, excellent stretch flangeability, and shape freezability, suitable for automotive structural parts without compromising press formability or workability.
Implementation Method 1
hot rolling it at a 950°C or more and 1100°C or less temperature to give a rolling reduction per pass of 20% or more so that the finishing rolling temperature becomes the Ar 3 transformation point or more
Implementation Method 2
cooling it by a 20°C/sec or more cooling rate and coiling it at a 350°C to 550°C temperature range
Data Source
AI summary
Provided is a hot rolled steel sheet comprising a predetermined composition wherein the hot rolled steel sheet comprises first ferrite with an average orientation difference in the same grain of 0.5 to 5.0° in 30 to 70 vol%, at least one type of structures among bainite and second ferrite with an average orientation difference of 0 to less than 0.5° and the first ferrite in a total of 95 vol% or more, a balance microstructure of 5 vol% or less, has an average grain size of the first ferrite of 0.5 to 5.0 µm, and has an average grain size of the other structures of 1.0 to 10 µm. Provided is a method for producing a hot rolled steel sheet comprising rolling where two or more consecutive passes of rolling including a final pass are performed under conditions of a rolling temperature: A point or more and less than Ae3 point etc., and where a total strain amount of all passes satisfying the conditions is 1.4 to 4.0, cooling by a 20 to 50°C/sec average cooling rate, and coiling the steel sheet at 300°C to 600°C.