Cold Rolled: How it is Made
We recently discussed how hot rolled steel is made. Now it only makes sense we talk about cold rolled. Cold rolled is the next step along the line of flat-rolled steel products. The process starts with hot rolled P&O.
Hot rolled is processed to a final thickness that is heavier than most sheet products. A cold-reducing process is used, which results in a thinner thickness. The cold reduction starts with tension to the sheet. This causes the product to become thinner, obviously, but also harder and more difficult to form.
Because formability is important in fabricating sheet metal, the steel is annealed. The process of annealing consists of a heat treatment that changes the properties of the material to increase ductility. Doing this makes it a great product for the customer.
Let’s go a little more in depth shall we?
The Cold Reduction Process
So, you are looking for cold rolled coils? Well, to achieve it hot rolled P&O is sent through a series of tandem rolling mill stands (4-high or 6-high vertically stacked rolls). As the material exits each stand it is a fraction thinner than the original hot rolled P&O When it leaves the last stand the material is recoiled. However, at this point the material is hard, not easily formable and basically unusable for many applications.
OK, how do we make it usable?
The Annealing Process
To soften the steel it goes through the annealing process. The annealing process can be accomplished in two ways, batching and continuous annealing. Batch annealing involves stacking the hard coils four or five high on fixed bases or stools (hence the name “batch”). The material is covered to allow the coil to be concealed from the oxides in the atmosphere. These oxides can potentially damage the surface finish.
A large furnace is then lowered onto the stacks of coil. The space between the cover and the furnace is heated by gas while the inside of the cover is fan circulated to convey heat to the coils.
The next method of annealing is continuous. This method allows for the material to be sent through a furnace in a continuous rolled strip. It is then cooled and recoiled.
The heating of the material causes it to become softer and allows for better formability to be used in various applications. It is now usable.
Cold Rolled Finishing
Now what? The material needs to be finished to ensure it is desirable for the customer. After the annealing processes the material is kept covered to prevent oxidization while it is still at a high temperature. This also allows for the breakdown of oils and vapors that may be present after cold rolling.
The material then undergoes temper rolling. The process of temper rolling consists of cold rolling the material with a very low reduction. This finishing process helps with surface finish and flatness of the final cold rolled product.
The finished cold rolled coil is delivered to service centers, like Metalwest, for processing and then sent to customers for fabrication of metal furniture (typically office use), motorcycle exhaust pipes, lighting fixtures and many more.
Cold rolled can also be sent through further processing to create galvanized, galvannealed, electro-galvanized and GalXC. All of which we will discuss later.