Cold rolling is a process of hardening the material that is 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 range of applications in medical engineering, aerospace, and automotive. Keep reading to get more information about what cold-rolled steel is, how it is produced, and what it can be used for.
What is cold-rolled stainless steel?
Cold-rolled steel, sometimes abbreviated as CRS (Cold Rolled Steel), is known to be an extremely ductile material and is ideal for applications where precision is required. It is used in many applications, such as appliances, furniture, closets, and filing cabinets. CRS in construction applications is commonly used to build 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, can only occur when a metal is below its recrystallization temperature. When hot rolling is done using high temperatures, cold rolling is done at room temperature. Instead of heat, mechanical stress is used to change the structure of the metal. Strain hardening can increase the strength of the metal by up to 20% and can also improve the surface finish of the metal.
During the cold rolling process, when the metal is subjected to mechanical stress, it causes a permanent change in the crystal structure of the metal. This causes an increase in its strength and often improves corrosion resistance. In addition to improving its surface finish, another advantage of cold rolling is better dimensional accuracy.
A coil of cold-rolled stainless steel can be manufactured with precision, allowing the metal to be produced with extremely tight tolerances. The act of hardening the stainless steel material through cold rolling allows us to achieve various levels of tempering, such as quarter, half, and hard. The hardness level depends on how much cold work has been done on the steel. Quarter, half, and full hard materials have higher amounts of reduction (sometimes up to 50%). This increases the yield limit but decreases the ductility of the steel. Quarter hard steel can be bent 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. Cold-rolled metal is often used in applications where the metal needs to be bent without the risk of breaking.
The different grade families of stainless steel that are often cold-rolled are:
- Stainless steel austenitic
- Stainless steel ferritic
- Stainless steel martensitic
- Stainless Steel precipitation hardened
How does a cold rolling mill work?
The process of cold rolling a metal alloy begins with a sheet of metal or a coil. These materials are placed in large rollers, which compress and squeeze them at high pressure just below their final 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 strips through a sequence of tandem rolling stands. The rolls in these stands are stacked vertically and operate with huge motors. The motors work hard to apply extreme compression to the metal.
These mills take coils of pickled, hot-rolled products and pass them through the rollers, making them thinner. Once the metal has passed through the rollers and it has reached the desired thickness, it is finished winding, but it is not ready for use. At this point in the process, the metal is still highly cold worked, and although it is high strength, it can be brittle. It must be annealed at a higher temperature to soften the steel and make it less difficult to work with. Once the metal has been annealed, it is easier to use it in many applications as it can be bent and formed.
Another method of cold rolling uses a reversing mill, which passes the strip back and forth between the mandrels. This reduces the thickness of the strip during each pass until the desired thickness is reached. The metal can be reduced between 60 and 80 percent through cold rolling, and then it 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?
Although cold rolling is most often used in steel, many alloys and metals can be cold-rolled to change their crystalline composition. When rolled at temperatures below their recrystallization point, permanent defects occur. These defects reduce the ability of the crystals to move within the metal structure, improving both the tensile strength and hardness of the metal.
Metals such as titanium, aluminum, and nickel alloys, along with stainless steel, can be cold-rolled. Although cold rolling of metals like a roll of stainless steel increases the strength of the metal and its surface finish, it in turn decreases the ductility. The same applies to metal wire that has been stretched or cold-rolled. However, once the metal is annealed, it is ready to be used in multiple ways.
Cold working methods for manufacturing
Deep drawing
When stretching the metal in depth, 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, allowing for passage and clearance. The raw material is forced into the die through compression force. Each drawing operation is a separate step, and with each step there is a reduction in diameter, which increases the height or depth of the piece. This is done to ensure that as the metal changes shape, recrystallization also occurs. Products manufactured by deep drawing include battery boxes, implantable medical devices, aerospace and defense components for aircraft.
Cold extrusion
Extrusion is an important and versatile manufacturing process, although it remains a relatively new metallurgy technique. Cold extrusion forces a piece of material through a die at 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.
Precision stamping
Metal stamping is another manufacturing process that is ideal for producing metal products where strict tolerances are required. In metal stamping, a flat metal strip is converted into a variety of shapes. This complex process includes several different metal forming techniques, such as punching, coining, drilling, sheet and/or plate making, and bending, among others.
Profiling
Roll forming involves bending a long strip of metal (usually rolled steel) that is passed through sets of rollers, each of which performs an incremental part of the bend. The finished product is bent until it reaches the desired cross-sectional profile.
As you can see, cold-rolled steel can be used in many ways and in many industries. If you want to learn more about cold-rolled metals, contact one of the metallurgical experts at Ulbrinox who can help you find the perfect alloy, grade, and properties to meet your needs. Ulbrinox has over 20 years of experience working with cold metals and takes pride in supplying customers within the nuclear, aerospace, and medical fields.
