Cold-Rolling And Hot-Rolling Steel Pipe
Steel products vary in grade, shape, size, specification, and finish, many of which have unique properties. Steel pipe manufacturing has a significant impact on how it will be used across a wide range of industries. For prefabricated steel products, cold-rolled and hot-rolled determines the useability of steel pipe and make up the key differences between the two end products. Steel with differing grades or specifications can be manufactured through either hot-rolled or cold-rolled processes.
Hot-Rolled Steel Manufacturing Process
Hot-Rolling steel is the process of steel manufacturing through heat, generally separated into low, medium, and high-pressure boiler steel pipes. Hot-rolled steel has been roll-pressed at high temperatures of over 1,700 degrees which rises above re-crystallization threshold temperatures for most steel. Through this process, the steel is easier to form and work with. Manufacturing begins with a large, rectangular piece of metal called a billet. This billet is heated and sent over to be flattened into a roll. Keeping it at a high temperature, it is run (at high speeds) through a series of rollers until the desired dimensions are reached. Hot-rolled steel is cooled after it has been processed and will shrink slightly while it cools, meaning its final form is not as precise as cold-rolling is.
Ex. Hot-Rolling Seamless Steel Pipe Process
Round Billet → Heating → Perforation → Extrusion → Sizing → Hot Rolling → Cooling → Straightening → Hydrostatic Test → Signing → Storage
How To Identify Hot-Rolled Steel
- Scaled Surface - cooling from extreme temperatures
- Slightly Rounded Edges Or Corners - due to cooling process and shrinkage
- Distortions - due to cooling process and shrinkage
Cold-Rolled Steel Manufacturing Process
Cold-rolling steel is a process done near or at room temperature, but essentially is hot-rolled steel that has been taken further into processing. After the steel has been hot-rolled and cooled, it is re-rolled at room temperature for better specification of dimensions and higher quality surface qualities. Technically speaking, cold-rolled refers to sheets that undergo compression between rollers. However, cold-roll loosely describes a wide range of finishing processes in the steel industry, including turning, grinding, and polishing (furthering a hot-rolled item into a better and more refined end product).
Ex. Cold-Rolling Seamless Steel Pipe Process
Round Billet → Heating → Perforation → Annealing → Pickling → Anoint → Cold Rolling → Heating → Straightening → Hydrostatic Test → Signing → Storage
How To Identify Cold-Rolled Steel
- Finished Surface With Close Tolerances
- Oily Smooth Surface
- Uniformity And Straightness (Noticeable For Tubing)
- True Sizing, Defined Edges
Key Differences Between Cold-Rolled Pipe And Hot-Rolled Pipe
| Hot-Rolled | Cold-Rolled |
|---|---|
| low, medium, high-pressure boiler steel pipe, stainless steel pipe, oil cracking pipe, geological steel pipe, and other | low, medium pressure boiler tube, high-pressure boiler tube, alloy steel pipe, stainless steel pipe, and oil cracking tube, alloy thin wall steel tube, thin-walled stainless steel pipe, and special-shaped steel tube |
| Does Not Allow Local Buckling | Allows Local Buckling (Bending Bearing Capacity) |
| Distribution Of Residual Stress Is Film Type | Distribution Of Residual Stress Is Bending |
| Higher Free Torsional Rigidity (Better Torsion Resistance) | Lower Free Torsional Rigidity |
| Less Control Over End Result, Lower Dimensional Accuracy | High Dimensional Accuracy |
| Lower Quality Surface Characteristics | Smoother, Oily/Greasy Surface Characteristics, Sharp Edges |
| High Temperature Process, Then Cool Down (Less Control Over Shape) | Room Temperature Process Furthering Hot-Rolling (More Control Over Shape) |
| Highly Used In Agricultural Industry | Used For More Technically Precise Applications (Aesthetics) |
| Cheaper | More Expensive (Stronger Properties) |
Benefits Of Cold-Rolled Steel: Summary
Greater pressure is required to roll and form steel at room temperature or during the cold-rolling process. Manipulating steel below the recrystallization threshold offers several benefits.
Cold-Rolled Steel Strength
Significantly stronger than hot-rolled steel, cold-rolled steel is compressed by rollers at room temperature, which allows the steel to become stronger through strain hardening. Typically, cold-rolled steel is about 20% stronger than hot-rolled steel.
Tolerances Of Cold-Rolled Steel
The word "tolerance" in metalworking refers to the overall thickness of the metal material. Hot-rolled steel is typically thicker, while cold-rolled steel is thinner without comprising the strength. Because of this, cold-rolled steel has tighter tolerances that are unbeatable.
A Better Surface Finish
Cold-rolled steel has an improved, smooth surface compared to hot-rolled steel. Easy to differentiate from hot-rolled steel, cold-rolled steel has a smooth and often greasy or oily finish and fewer surface imperfections. More suitable for certain application, cold-rolled steel is often used because it is aesthetically nicer.