Cast Strip Surface Roughness Control via Inert Atmosphere
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Solution Overview
Problem
Cast steel strips produced by twin roll casting and subsequent hot rolling often exhibit high surface roughness and micro-cracking due to oxidation and surface shearing, leading to unsatisfactory microstructure and scale formation, which is economically challenging to mitigate at standard production speeds.
Innovation Solution
The method involves shrouding the cast strip in an atmosphere with less than 5% oxygen and applying a mixture of water and oil through spray nozzles during hot rolling, which reduces surface roughness to less than 1.5 microns Ra and scale thickness to less than 10 microns, while maintaining a microstructure of polygonal ferrite, acicular ferrite, Widmanstatten, bainite, or martensite, and allowing for production rates above 80 meters per minute.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If standard casting speed of 80m/min is used with hot rolling, then productivity is maintained, but surface roughness increases to 6-8 microns Ra with micro-cracking
Solution Approach 1:
The strip is shrouded in a controlled atmosphere with less than 5% oxygen content from the moment it leaves the casting rolls through the hot rolling mill. This preliminary protective action prevents oxidation before surface defects can develop, allowing high casting speeds to be maintained without the usual surface roughness and micro-cracking problems.
Solution Approach 2:
A controlled atmosphere with reduced oxygen content (less than 5%) is maintained throughout the hot rolling process. This inert environment prevents oxidation of the hot strip surface, eliminating the root cause of surface roughness and micro-cracking that normally occur at standard production speeds.
2Manufacturing precision
If casting speed is reduced to reduce microcracking, then surface quality improves, but productivity decreases and it becomes uneconomical
Solution Approach 1:
By maintaining a controlled atmosphere with less than 5% oxygen content during hot rolling, the invention prevents oxidation-related surface defects regardless of casting speed. This allows production to proceed at economical high speeds while still achieving excellent surface quality, eliminating the need to sacrifice productivity.
Solution Approach 2:
The oxygen content parameter of the atmosphere is changed from normal atmospheric levels to less than 5% oxygen. This parameter change fundamentally alters the chemical environment during hot rolling, preventing oxidation and surface defect formation even at high casting speeds, thus resolving the contradiction between quality and productivity.
3Ease of manufacture
If hot rolling is performed in normal atmosphere, then oxidation occurs causing surface roughness, but the process is simple and economical
Solution Approach 1:
The invention implements a controlled atmosphere system with less than 5% oxygen content that envelops the strip from the casting rolls through the hot rolling mill. While this adds some system complexity, it prevents oxidation and surface roughness, achieving a balance where the improved surface quality justifies the atmospheric control mechanism.
Solution Approach 2:
A controlled atmosphere acts as an intermediary between the hot strip and the surrounding environment. This atmospheric barrier prevents direct oxidation of the strip surface during hot rolling, mediating the interaction to produce high-quality surfaces while maintaining process efficiency.
4Shape
If work roll surface texture is imprinted on strip, then surface roughness increases, but rolling process remains effective for shaping
Solution Approach 1:
The controlled atmosphere with less than 5% oxygen content prevents oxidation during hot rolling, which eliminates the surface roughness that would normally result from work roll texture imprinting. This allows the rolling process to effectively shape the strip cross-section while producing a smooth surface finish.
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
This approach significantly reduces surface roughness and scale thickness, improving the microstructure and surface quality of the cast strip, making it more economical and efficient than previous methods by controlling oxidation and surface interaction.
Implementation Method 1
the strip leaves the nip at very high temperatures on the order of 1400°C or higher. If exposed to normal atmosphere, it would suffer very rapid scaling due to oxidation at such high temperatures
Implementation Method 2
applying a mixture of water and oil through spray nozzles during hot rolling
Data Source
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AI summary
A thin cast strip is formed having at least one microstructure selected from the group consisting of polygonal ferrite, acicular ferrite, Widmanstatten, bainite and martinsite, a surface roughness of less than 1.5 microns Ra and a scale thickness of less than about 10 microns by applying a mixture of water and oil on the work rolls of the hot rolling mill, passing the thin cast strip at a temperature of less than 1100° C through the hot rolling mill while the mixture of oil and water is applied to the work rolls, and shrouding the thin cast strip from the casting rolls through the hot rolling mill in an atmosphere of less than 5 % oxygen to form the thin cast strip.