OneSteel manufactures and distributes more than 40 000 products to some 30 000 customers in the construction, manufacturing, housing, mining and agricultural industries. The Australia-based company's products range from thick beams to thin steel wires. OneSteel discovered its Laverton facility was producing rods with excessive variations in mill scale - the iron oxides formed during processing. While some scale is inevitable, the scale must be removed to prevent wear on expensive equipment, avoid defects in the final product, and limit yield loss. To reduce the scale on rods to acceptable levels, a OneSteel team turned to Minitab Statistical Software to guide their efforts.
The challenge
During processing, OneSteel's raw metals are shipped between facilities. At the Laverton Rod Mill, steel is heated to temperatures over 1000°C and rolled into rods that receive initial cooling in water boxes. Next, the rods are formed into a pattern of rings, placed on a conveyor to be cooled further, and finally coiled together.
High-temperature processing at Laverton caused iron oxides, or mill scale, to form on the rod's surface. Scale levels vary depending on the type of rolling mill used, the temperature of the steel, and how long it's heated. Scale is abrasive and reduces the life of the drawing die which, in turn, can reduce quality.
To remove the scale, the Geelong Wire Mill uses reverse bend descaling, a mechanical process of passing the rods over a series of sheaves. This bending removes the brittle scale without harming the flexible steel.
However, scale levels on the Laverton rods often exceeded the desired level for mechanical descaling. The result was decreased yield and costs including:
- Clean up and disposal.
- Operation and maintenance of the scale extraction system.
- Lower run rates.
- Shorter die life.
- Increased lubricant usage.
- Lower product quality.
In one year, the Geelong Wire Mill spent over $800 000 for housekeeping, lubricant, and dies, so reducing scale on the Laverton rods could potentially create enormous savings for OneSteel.
Team members from the rod and wire mills included a Six Sigma Black Belt, technical managers, process engineers, quality professionals, a metallurgist, and equipment operators. For their comprehensive study using the DMAIC method, the team defined defective rods as rods with descalability lower than 95 per cent. Next, they sampled the rods arriving at Geelong from Laverton.
The team measured scale weight as a proportion of the total product (steel rod coils) in kg/tonne. They found that 62 per cent of samples exceeded 7kgs of scale per tonne, with variations up to 3kg per tonne. This results in a significant yield loss since variation of just 1kg per tonne equals $75 000 in yield. Geelong loss estimates were around $220 000 per year.
Throughout the project, team members turned to Minitab to analyse data and to communicate their findings. For example, Minitab's 2-sample t-test proved the difference in scale weights between grades of steel rods, while a box plot clearly displayed the information.
In addition, they used Minitab's powerful regression analysis to reveal the relationship between laying head temperature and scale weight, and a scatterplot displayed the relationship.
The team also applied Minitab's Probability Density Function to actual sampling data and compared it to a desired result to determine the potential savings accrued through reduced variation and scale weights.
Pareto analysis determined which rod grades caused delays on the wire drawing machines. Changing the characteristics of the feed used to create the steel rods at the Laverton Rod Mill was not an option, so the team looked for other ways to reduce the percentage of rod scale and increase the wire mill's yield. They ran experiments to test the cooling processes used at the rod mill, the descaling and drawing practices used at the wire mill, and operational parameters at both mills.
First, they ran a test to evaluate the effects of cooling conveyer parameters on the amount of scale. They also measured the temperatures of the wire drawing block and the laying head at the rod mill. Minitab analysis revealed a significant difference when the reform temperature on the cooling conveyer was reduced by 30 per cent.
Next, they tested the effects of drawing methods on yield. For the control run and the test run, the team set up production lines with new dies, new lubricant for each die, new brushes in the descaler unit, and kept the run speed consistent.
The run using their trial parameters resulted in lower temperatures, cleaner product, lower die wear, and less visible scale. A strength test showed no statistical difference between the quality of the trial product and the control product.
They measured die wear on the wire drawing machine and found the new parameters also dramatically affected the life of the drawing dies.
In one test, the diameter of wire from a die was 6.22mm at the start of the run. After running 15tonnes of wire using the control rod, the die had reached a diameter of nearly 6.32mm. Die wear for the line that used the trial rod was significantly less - after 15tonnes, the diameter was below 6.24mm.
After implementing various improvements, the team used Minitab to analyse scale weight and laying head temperature for a recent rolling of 7mm wire. Minitab's analysis confirmed that the rod mill could produce rods suitable for mechanical descaling, with a much lower yield loss than in the past.
Using Minitab, the team created control charts to clearly show the effect of trials undertaken at the mill and effectively communicate their results.
Results
Team efforts have improved the cooling process at the Laverton Rod Mill. Operators can better control laying head temperature, and Minitab analysis has confirmed that the improved cooling process significantly reduced scale weight.
The Minitab histograms show how improvements to the rod mill's calibration procedure reduced the gap between the estimated true temperature and the temperature reported by the laying head. This reduced real laying temperatures compared to previous rollings, and lower laying temperatures reduce yield loss due to mill scale for rod mill customers.
The new rod from Laverton is a much cleaner, visibly superior product for the Geelong Wire Mill. By reducing the amount of scale, increasing yield, and realizing savings in labour and consumables, OneSteel anticipates saving approximately $275 000 annually. The reduction in scale also offers additional downstream processing benefits.
With the improvements confirmed, OneSteel's project team entered the Control phase of their DMAIC process. They used Minitab's control charts to lock in the changes they made to procedures and maintenance activities. That made it easy to sustain the improvements and ensure that the savings continue in future years.l
Enter √ at www.engineerlive.com/asia
Eston Martz is with Minitab Inc, State College, PA, USA. www.minitab.com