Stainless Steel: How it is Made

Metal West Metal SupplierToday we are taking a turn from carbon to discuss stainless steel. This type of steel is widely used. Chances are you own at least a few appliances that are made of stainless. I know my refrigerator is.

Stainless steel comes in many different types. At Metalwest we supply two main types of stainless-Austenitic and Ferritic.

Austenitic is the most common of the stainless steel types. This includes the 200 and 300 series. The alloy contains a minimum of 16% chromium and is also composed of a significant amount of nickel and manganese (here is a link to a periodic table in the off chance you don’t know what I am talking about).

Austenitic stainless has excellent corrosion resistance, good ductility, non-magnetic properties, strength and weldability.

The most common type of austenitic stainless is type 304 due to its corrosion resistance to a wide range of industry exposures. This makes it ideal for foodservice equipment or any application where sanitation is required.

Ferritic alloys have lower corrosion resistance than austenitic alloys due to lower chromium (10.5-27%) content. This includes some of the 400 series of stainless, including 409 and 430. Ferritic stainless has good corrosion resistance, high strength and great workability.

How Stainless Steel is Made

“OK, great. So… how is it made?”

I’m getting there dear Buzz readers. We had to cover the basics first. Here we go…

stainless steel mining cartMining

The process of making stainless steel begins with extracting chromite (an iron chromium oxide) from underground mines. It is then crushed to produce a chromite concentrate. This is what makes stainless steel strong and corrosion resistant.

You wouldn’t want a stainless steel refrigerator that doesn’t hold up and corrodes, right? The product’s ability to hold up is the basically the point of buying stainless-aside from the appealing surface finish, of course.

Melting

The next step is to melt the chromium pellets together in an electric furnace with the other raw materials (iron ore, manganese, nickel, etc.) used for the specific stainless type. After melting, the material is formed into slabs and transferred to a hot rolling mill.

stainless steel roughing millHot Rolling

Once the slab arrives at the hot rolling mill it is transferred to another furnace and the hot material is then sent through a roughing mill. At the roughing mill it is rolled forwards and backwards to reduce the thickness. As the thickness is decreased it causes the slab to become a long strip that will later be wound up into a coil.

The long strip, or band, is passed through additional rolling mills to reduce the thickness even more. At this point it is coiled then cooled by water and transferred to a cold rolling mill.

Cold Rolling

When the material arrives from the hot rolling mill it is covered in a black scale (which we defined while discussing how hot rolled is made).

At the cold rolling plant the material is uncoiled and passed through an annealing and pickling line to remove the black scale. The band is then reduced to the thickness specified by the customer and recoiled.

The coiled material is sent through another annealing and pickling process and rinsed with water.

Finishing

“OK, then what happens?”stainless steel delivery train

Flat rolled coilsWell, the stainless steel material goes through one final finishing step of removing the tension and improving the overall surface finish. The finished stainless is cut to the specified dimensions and shipped to customers around the world via train or barge.

Skip prefers the train, as he gets seasick.

When we receive the material we process it further to ensure our customers are getting the product they need.

Metal West Stainless Steel Supplier

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

cold rolled lighting fixturesNow 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.

Steel Products Metal West

Weathering Steel: What is it?

Weathering steel, commonly referred to as A606 steel, has recently gained popularity in the architectural world for its distinctive orange-brown oxide (or rust) finish. This layer of rust aids in the resistance to corrosive elements. When weathering steel is produced, it is not rusted. It gradually develops the rust-like appearance as it is exposed to the elements over time.

But, how does it develop the layer of rust and how does that actually aid in corrosion resistance? Let’s find out!

How Weathering Steel Works

Embed from Getty Images
Unlike most corrosion resistant steels that resist rust, weathering steel does rust. However, it only rusts on the outer layer and will not penetrate into the steel once the initial layer has formed. With weathering steel, the layer of rust acts as a barrier to protect the steel from corrosion, whereas with other metals the rust is porous and breaks off allowing another layer to form. The specific alloying elements in the steel produce a stable layer of rust that adheres to the base metal and isn’t as porous as typical rust.

Benefits of Weathering Steel

Weathering steel has many benefits, which make it ideal for architecture.

  • Resists further rusting and staining
  • A high strength low alloy steel (HSLA)
  • Heat and corrosion resistant
  • Ease of formability
  • Low maintenance
  • Long-term performance
  • Environmentally friendly and can be recycled

What is Weathering Steel Used For?

Embed from Getty Images
Because it is corrosion resistant, weathering style is frequently used for exposed structures. These can include bridges, building siding and roofing panels, truck and bus frames, as well as metal sculptures. It was initially used in the 1930s for ore wagons to help them resist corrosion. With its unique finish, it also eliminates the need for repainting or recoating of the steel.

There are elements that weathering steel can’t withstand, however. It shouldn’t be used for applications that will be exposed to chlorine. Chlorine will cause the rusted surface to corrode and can lead to premature failure of the material.

At Metalwest we stock A606 Type 4 in coil and sheet. For more information on product availability, contact your local sales representative as it may very between branches