Cold drawn steel is fundamentally the hot rolled steel drawn passing through dies to achieve the final shape. The dies apply pressure with the help of some press machines, and after passing this steel through these dies several times, the steel will have the desired dimensions. This process is known as cold drawn, due to the fact that it occurs at room temperature (below the re-crystallization temperature), enhancing with this the accuracy of the dimensions (tolerances) and shapes, the tensile strength and the external appearance of the material, by giving to the surface a smooth and polished finish.
The cold drawn steel typically comes in round, hexagonal, rectangular or square shapes.
Advantages:
- The cold drawn steel has better accuracy in dimensions and roundness.
- Cold drawn steel has better mechanical properties than the hot rolled steel.
- The cold drawn steel surface finish is smooth and perfect for projects that demand polished surface in the material.
Disadvantages:
- More expensive to produce, as it requires multiple processes for achieving the final dimensions and shapes.
- Less manageable than hot rolled.
How is cold drawn steel made?
In a nutshell, cold drawing is the pulling of a hot rolled steel bar through a die that is slightly smaller in size. So how is cold drawn steel made? The process path consists of first shot blasting the hot rolled bar, then cold drawing it through the die, then rotary (roll) straightening, and finally cutting to length.
Shot blasting:
This initial operation will clean the surface of the hot rolled bar, essentially removing all scale and rust. This operation also creates a bar surface which will help carry the drawing lubricant into the die.
Cold drawing:
The main operation, which actually produces the cold drawn bars. We use a heavy frame drawn bench which can draw out over 40′ long finished cold drawn bars. The die itself is mounted securely so as not to move during operation. A pusher mechanism will grab the bar and advance the first 12″ of the bar through the die. Next a buggy carriage will grab the portion of the bar that’s sticking out through the die. The buggy is attached to a large endless chain that is power driven and able to pull the buggy and bar all the way through the die.
Cold drawing draft, size reduction:
The standard “draft” is a 0.063″ reduction in diameter. A “heavy draft” is considered anything over 0.125″ in size reduction, and is used in 100K min yield strength products like ASTM A311-B (ASTM A311 / A311M-04(2015), Standard Specification for Cold-Drawn, Stress-Relieved Carbon Steel Bars Subject to Mechanical Property Requirements, ASTM International, West Conshohocken, PA, 2015, www.astm.org). There are also certain grades of product which will undergo two or more passes, and depending on the grade would require an annealing process in between draws.
Rotary (roll) straightening:
After the drawing operation itself is completed, the bars will undergo a straightening operation. Straightness tolerances will vary depending on grade and size, and tolerances are called out in the ASTM A108 specification (see link and citation above). We can also hold tighter than standard straightness depending on the material upon request. The rotary straightening operation also improves surface finish and will help control size slightly.
Saw cutting:
The final operation before packaging is saw cutting the bars to length. The end gripper marks left from the drawing operation are cut off. The bars are then cut to the required length. Standard cold drawn lengths are 12′, 20′, and 24′ long. We can provide up to 40′ cut lengths upon request.
Packaging:
Cold drawn bars are packaged in bundles of 2,000# and 4,000# quantities. The bars also receive a coating of rust preventative and all bars are traceable and identifiable with tagging showing heat number, size, length, grade, product, weight, and Northlake lot number.
