Cold rolling is a work hardening process used to change the structure of metals and is often used to process stainless steel. Cold rolled stainless steel as a raw material has a wide variety of applications in medical, aerospace and automotive engineering. Read on to learn more about what cold rolled steel is, how it's produced, and what it can be used for.
What is Cold Rolled Stainless Steel?
Cold rolled steel, sometimes abbreviated to CRS, is well-known for being an extremely ductile material, and is ideal for applications where precision is necessary. It is used in many applications, like household appliances, furniture, lockers, and filing cabinets. In construction applications, CRS is commonly used for building garages, steel sheds, and other industrial buildings.
In simple terms, cold rolling is the process of strengthening steel by changing its shape without using heat. Cold rolling, unlike hot rolling such as with hot rolled steel, can only occur when a metal is below its re-crystallization temperature. Where hot rolling is performed using high temperatures, cold rolling is performed at room temperature. Instead of heat, mechanical stress is used to change the structure of metal. Strain hardening can then increase the metal's strength by up to 20%, and can also improve a metal's surface finish.
During the cold rolling process, when the metal is put under mechanical stress, it causes a permanent change to the crystalline structure of the metal. This causes an increase in its strength and often improves corrosion resistance. Along with improving its surface finish, another advantage of cold rolling is better dimensional accuracy.
A cold rolled stainless steel coil can be precision manufactured, allowing the metal to be produced to extremely tight tolerances. The act of work hardening the stainless steel material through cold rolling allows us to achieve various levels of temper, such as quarter-, half- and full hard. The hardness level depends on how much cold work has been done on the steel. Quarter-, half-, and full hard stocks have greater amounts of reduction (sometimes up to 50%). This increases the yield point but decreases the ductility of the steel. Quarter-hard steel can be bent back over itself without breaking, while half-hard can be bent at a 90-degree angle, and full hard can be bent at a 45-degree angle without breaking it. Cold rolled metal is often used in applications where the metal needs to be bent without the risk of breaking.
The different families of stainless steel grade that are often cold rolled are:
- Austenitic Stainless Steel
- Ferritic Stainless Steel
- Martensitic Stainless Steel
- Precipitation Hardening Stainless Steel
How Does a Cold Rolling Mill Work?
The process of cold rolling a metal alloy starts with either sheet metal or strip coil. These materials are placed into large rollers, which compress it down and squeeze it under high pressure just below its ultimate tensile strength. Depending on the amount of compression, different mechanical properties and hardness properties are achieved in the finished product. Through cold reduction, the thickness of the metal can be reduced by processing steel strip through a sequence of tandem rolling mill stands. The rolls on these stands are stacked vertically and powered by huge motors. The motors work hard to apply extreme compression to the metal.
These mills take coils of hot-rolled, pickled products and pass them through, making them thinner. Once the metal has passed through the rolls and has reached its desired thickness, it's done being rolled but it is not quite ready for use. At this point in the process, the metal is still highly cold-worked, and while it is high strength, it can be brittle. It needs to be annealed at a higher temperature to soften the steel so it is less difficult to work with. Once the metal has been annealed, it's easier to use it in many applications, because it's able to be bent and formed.
Another method of cold rolling uses a reversing mill, which passes the strip back and forth between mandrels. This reduces the thickness of the strip during each pass until the desired thickness is reached. Metal can be reduced by between 60 and 80 percent through cold rolling, and then can be used in the creation of consumer goods or for use in other industries.
Can Metals other than Steel Benefit from a Cold Rolling Mill?
Though cold rolling is most often used on steel, many alloys and metals can be cold rolled to change their crystalline makeup. When they are rolled at temperatures below their re-crystallization point, permanent defects are caused. These defects reduce the crystals' abilities to move within the structure of the metal, improving both the metal's tensile strength and hardness.
Metals like titanium, aluminum, and nickel alloys, along with stainless steel can all be cold rolled. Though cold rolling metals such as stainless steel coil increases the strength of the metal and its surface finish, it does decrease the ductility. The same applies to metal wire which has been cold drawn or rolled. However, once the metal is annealed, it's ready to be used in multiple ways.
Cold Working Methods for Manufacturing
When deep drawing metal, a rolled coil is punched through a hole or die. The punch is used to achieve the desired shape. The die cavity matches the punch but is slightly wider which allows for passage and clearance. The raw material is forced into the die via the compression force. Each draw operation is a separate step, and with each step there is a reduction in diameter, increasing the height or depth of the part. This is done to ensure that, as the metal changes shape, re-crystallization occurs as well. Products made by deep drawing include battery enclosures, implantable medical devices, aerospace and defense components for aircraft.
Extrusion is an important and versatile manufacturing process, though it is still a relatively new metalworking technique. Cold extrusion forces a slug of material through a die at either room temperature or slightly elevated temperature. This produces a product of constant cross-section. Many metals can be extruded, including tin, aluminum alloys, copper, lead, steel, and others.
Metal stamping is another manufacturing process that is ideal for producing metal products where tight tolerances are required. In metal stamping, a flat metal strip coil is converted into a variety of shapes. This complex process includes a few different metal forming techniques, like punching, coining, piercing, blanking, and bending, among others.
Roll forming involves bending a long strip of metal (usually coiled steel) that is passed through sets of rolls, with each performing an incremental part of the bend. The finished product is bent until the desired cross-section profile is reached.
As you can see, cold rolled steel can be used in a lot of ways and in many industries. If you'd like to know more about cold rolled metals, contact one of Ulbrich's metallurgical experts who can help you find the perfect alloy, grade, and properties to meet your needs. Ulbrich has nearly 100 years of experience cold working metals and is proud to supply customers within the nuclear, aerospace, and medical fields.