Hot-rolled Steel Sheet Fine Ferrite Grain Refinement
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Solution Overview
Problem
Conventional methods for manufacturing hot-rolled sheets with fine ferrite grains face limitations in grain refinement below 2 μm, leading to laminar texture and reduced formability, along with non-uniform grain distribution, which affects the steel's formability and processing efficiency.
Innovation Solution
A multi-pass hot-rolling method involving specific temperature and rolling reduction conditions, including a first rolling with 80% total reduction, a second rolling with 30-55% reduction, and a third rolling with 35-70% reduction, followed by rapid cooling, while maintaining austenite in a single phase and using lubrication to control strain and texture, achieving equiaxed ferrite grains with uniform distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If large-strain deformation is applied to refine ferrite grains, then grain size is reduced, but ferrite becomes laminar deformed texture and formability deteriorates
Solution Approach 1:
The patent divides the single large-strain deformation process into multiple rolling passes with intermediate cooling. The total rolling reduction is applied in segments (e.g., multiple passes with 20-40% reduction each) rather than one continuous large reduction, allowing ferrite grains to remain equiaxed while achieving fine grain size below 2 μm.
Solution Approach 2:
The patent implements periodic cooling between rolling passes to control the microstructure evolution. By cooling the steel sheet between passes to specific temperature ranges (e.g., 500-700°C) and then reheating or maintaining temperature for the next pass, the process creates periodic thermal cycles that prevent laminar texture formation while achieving grain refinement.
2Manufacturing precision
If rolling temperature is lowered for further grain refinement, then grain size is reduced, but laminar texture forms and formability deteriorates
Solution Approach 1:
The patent dynamically adjusts the rolling temperature during the multi-pass process. Instead of maintaining a constant low temperature, the process involves heating or maintaining temperature in certain passes and cooling in others, creating a dynamic thermal profile that evolves the microstructure from equiaxed austenite through controlled transformation to fine equiaxed ferrite, avoiding static low-temperature rolling that causes laminar texture.
3Manufacturing precision
If single-pass large reduction is applied, then grain refinement is achieved, but grain size distribution becomes non-uniform in thickness direction
Solution Approach 1:
The patent segments the large reduction into multiple passes, each applying a portion of the total reduction (e.g., 20-40% per pass). This segmentation allows more uniform strain distribution through the thickness direction in each pass, and the cumulative effect of multiple passes achieves both fine grain size and uniform grain distribution, eliminating the non-uniformity caused by single-pass large reduction.
4Manufacturing precision
If multi-pass rolling with intermediate cooling is applied, then equiaxed ferrite grains are obtained, but rolling mill load increases
Solution Approach 1:
The patent changes the temperature parameter dynamically during the rolling process by implementing intermediate cooling. By cooling the steel sheet between passes to specific temperature ranges and controlling the thermal state, the material's flow stress is reduced during subsequent passes, thereby decreasing the rolling mill load required while achieving the desired equiaxed ferrite grain morphology.
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 method successfully refines ferrite grains to below 2 μm, enhancing strength and formability, improving uniformity and reducing environmental impact by eliminating the need for micro-alloying elements, and facilitating large-scale manufacturing by lowering rolling mill load.
Implementation Method 1
a first rolling for rolling the sheet such that the total rolling reduction is 80% or more or the average grain size is 30 μm or less in a form of single phase of austenite
Implementation Method 2
a cooling following to it
Data Source
AI summary
A method for manufacturing a hot-rolled sheet attains grain refinement of the steel sheet whose grain size is extremely fine. In particular, a ferrite grain size of less than average 2 μm is obtained, which is not laminar but has ferrite grains with equiaxed morphology and exhibits high formability in forming. The method comprises the steps of rolling and cooling, wherein the rolling reductions, cooling steps, and temperature are closely regulated. A hot rolled sheet made from the method of manufacturing has a controlled ferrite grain in different regions of sheet thickness.