Introduction
A 4-Hi 750mm reversible cold rolling mill is a type of rolling mill that uses four rolls: two work rollers and two backup rollers. The "750" refers to the maximum width (in millimeters) of the material that can be processed. Reversible rolling mills are designed to allow rolling in both directions, providing flexibility and energy efficiency. This case study explores the implementation of a 4-Hi 750mm reversible cold rolling mill in a steel manufacturing plant to improve production efficiency and product quality.
Background
A mid-sized steel plant, located in an industrial hub, was facing challenges in producing high-quality cold-rolled steel sheets for the construction industries. The company was using an older single-direction rolling mill that required multiple passes and manual adjustments, leading to inefficiencies, increased labor costs, and inconsistent product thickness.
To address these issues, the company decided to upgrade its rolling mill operations by investing in a 4-Hi 750 reversible rolling mill. The primary objectives were:
- Improve product quality: Achieve tighter tolerances on thickness and surface finish.
- Increase productivity: Reduce the number of passes required by utilizing both forward and reverse rolling.
- Enhance operational efficiency: Automate the process to minimize manual intervention.
Technical Specifications
The newly installed 4-Hi 750 reversible rolling mill had the following specifications:
- Type: 4-Hi configuration with two work rolls and two backup rolls.
- Roll Diameter: Work rolls with a diameter of 150 mm and backup rolls with a diameter of 500 mm.
- Width Capacity: Maximum width of 750 mm.
- Rolling Speed: Capable of rolling at speeds up to 500 meters per minute.
- Material: Designed to roll cold steel sheets, with a thickness range from 0.2 mm to 4 mm.
- Reversibility: The mill could roll in both forward and reverse directions, reducing the need for manual handling between passes.
- Automation: Equipped with a modern PLC (Programmable Logic Controller) system for precise control over roll gap and pressure.
Implementation Process
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Site Preparation: The plant reconfigured its rolling line to accommodate the new 4-Hi 750 reversible rolling mill. This involved modifying the layout and ensuring proper foundation and mechanical support for the new equipment.
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Installation: The mill was installed over a period of two weeks, with assistance from the equipment supplier’s technical team. Alongside the mill installation, the electrical systems and PLC controls were integrated into the existing production management software.
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Training: Operators were trained to utilize the automated features of the rolling mill, including setting parameters for thickness, speed, and roll pressure, as well as understanding safety protocols for reversible rolling.
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Testing and Calibration: Before full-scale production, the mill underwent calibration tests to ensure accurate rolling thickness and smooth reversibility. Initial tests showed a significant improvement in the consistency of the rolled sheets’ thickness across the full width.
Results
The implementation of the 4-Hi 750 reversible cold rolling mill led to several positive outcomes:
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Improved Quality: The mill’s ability to control thickness within ±0.01 mm resulted in highly consistent cold-rolled sheets. Surface finish also improved due to the better control of roll pressure and speed.
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Increased Productivity: By utilizing the reversible feature, the number of passes required to achieve the final thickness was reduced by 25%. This not only sped up production but also reduced wear and tear on the rolls.
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Reduced Labor Costs: The automation of roll gap adjustment and the ability to roll in both directions significantly reduced manual intervention during the rolling process. Fewer operators were required on the production line, allowing the company to reallocate labor to other areas.
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Enhanced Efficiency: The upgraded mill allowed for faster changeovers between different sheet thicknesses and materials, enabling the plant to serve a wider range of customer orders without lengthy downtimes.