 Remelting and Scrap Recycling
|
|
Approximately two thirds of the metal used by Hulamin is purchased from the local Bayside smelter in either pig (casting ingot) or rolling ingot form. The balance comprises of recycled process scrap. Hulamin utilizes approx. 50 alloy variants in its existing operation, cast between its three casting centres. All scrap from the manufacturing processes is segregated by alloy, and recycled into ingots and billets. During this process, metal is filtered, treated and where necessary, re-alloyed to the required alloy specification. |
|
Casting |
|
Hulamin operates 60 ton melting furnaces to heat the metal to between 660 and 700° C. After melting, the molten metal is transferred to holding furnaces, where the liquid metal is treated and impurities are allowed to settle out at the correct temperature. After settling the molten metal is filtered, skimmed and cleaned to remove any impurities. The furnace is then tilted, allowing the liquid aluminium to flow into water-cooled rectangular moulds of either 490 or 600mm thickness and in a range of widths, weighing between 6 and 16 tons, in a process known as “Direct Chill” casting. During casting, the metal is further treated and filtered. |
|
Scalping |
|
The surface of a rolling ingot contains oxides formed during the solidification process. These oxides are detrimental to the product and the ingots are therefore machined in a milling operation to remove the surface layer from the rolling surfaces and sides. |
|
Preheating
|
|
Scalped ingots are initially preheated to ensure a homogeneous structure is created from the cast product which is important for uniformity in the final product, whilst also utilizing the heat required for this operation to assist with hot rolling. After scalping, the ingots are charged in gas-fired pre-heating furnaces at 500-620°C. These are pusher type furnaces where ingots are “pushed” through the furnaces sequentially, through five heating zones. Automated control systems regulate the temperature for precise homogenization. |
|
Hot Rolling |
|
The hot rolling process comprises of two stages.
In the first stage, at the end of the preheating cycle, the ingots are automatically loaded onto the hot rolling line. The ingot is then processed through the reversing hot roughing mill also known as a breakdown mill. This initial hot rolling process reduces its thickness by up to 95% and increases its length by 24 times, through a series of precisely controlled reversing rolling passes. Lead and tail ends of the strip are cropped during the process to ensure uniformity along the length.
In the second hot rolling stage, the slab, which is now approximately 140 metres in length, is then taken along the run-out table to the hot finishing mill, where precise control of many variables; including temperature, emulsion, speed, tension and force; ensures that it is rolled to a tight tolerance intermediate thickness. In this process, the strip is rolled and coiled on a twin coiler in each of three passes, after which it is removed from the hot line and cooled to ambient temperature from a temperature of 300- 360° C.
|
|
Cold Rolling |
|
Hot rolled coils are transported to the cold mill area after hot rolling for rolling down to the customers’ required thickness. Cold rolling derives its name from the fact that coils are at ambient temperature when starting the rolling process, unlike hot rolling which requires preheating. Cold rolling is a single pass operation, whereby coils are loaded onto the uncoiler, rolled, removed from the recoiler and cooled (from approximately 100° C.). Cold rolling pass schedules to achieve the required tight tolerances and mechanical properties are designed by experienced metallurgical and control engineers. |
|
Annealing |
|
After one or more cold rolling passes, some alloys must be softened through reheating or annealing to regulate the mechanical properties or to permit further reduction in thickness during cold rolling. Coils are heated at varying temperatures of between 300 and 360° C, with varying cycle times depending on the alloy and end use. Certain specific customer requirements are obtained by partially annealing the end product rather than cold rolling to the correct temper, which is the most common process route. |
|
Foil |
|
Hulamin also has a stand-alone foil mill capable of further reducing cold rolled products to gauges down to 6.0 microns. |
|
Plate Processing |
|
Some thicker products do not require cold rolling. This group of products is known as plate and generally covers sheets thicker than 6mm. Most of the plate produced by Hulamin is produced from heat-treatable alloys. Heat treatment in this instance imparts strength to the final products and involves heating and tightly controlled rapid water cooling, also known as quenching and artificial hardening by exposing to temperatures between 150-190° C. After achieving the required mechanical properties in these automated processes, the plates are processed for width, length, flatness and surface finish. Automated shears, saws and cut-to-length lines cut the plates to the required customer order specification. |
|
Coil Coating |
|
Many customers do not require the width, length or coil size that is ideal for cold rolling, and hence further processing is required to achieve the required dimensions. Also, flatness and metal surface cleanliness can be mechanically improved. This is achieved by processing the cold rolled coils through any of the various precision slitters, shears, tensions levelling, degreasing and packing lines designed to meet the tight market requirements. |
|
Inspection |
|
Hulamin carries out ongoing inspection of finished products. Various tests are performed to verify the properties and finish of the final products. The products are only released against agreed international standards. |
|
Shipping |
Hulamin exports to in excess of 60 countries around the world and on all continents. Hulamin has considerable expertise in tackling the shipping and packing issues that emerge from this complex operation.
|