What are the differences between hot-rolled and cold-rolled seamless steel tubes?

The term “hot-rolled steel tube” refers to a steel tube that has been rolled in a furnace at a high temperature, usually in the range of 1,200 to 1,500 degrees Celsius. There are two parts to a hot-rolled steel tube: the outer layer and the inner core. The outer layer is formed by heating molten metal (usually iron) to temperatures exceeding 1,200°C. In the inner core, molten metal is mixed with impurities such as carbon or sulfur, which are not present in the outer layer. 

It is necessary to heat these two layers together at very high temperatures (1,500 °C or higher) in order to reduce friction during welding or cutting processes. In order to weld or cut the tube parts. When welding at high temperatures, the inner layer is drawn into a long cylindrical shape. Pipes, tubing, and other products requiring strength in order to resist stresses are generally made from hot-rolled steel tubes.

Various industries use steel tubes, which are a common product. cooteel tubes come in different types and have different properties. Compared to cold rolled steel tubes, hot rolled coil steel tubes have a higher tensile strength, but they are also more prone to damage. Hot Rolled Vs Cold Rolled.

Hot-rolled steel tubes are seamless steel tubes that have been rolled in a process define rolling process (or hot-rolling process). Hot-rolled steel tubes are manufactured by passing sheet metal through an extrusion press at 1,800°C before being pressed into shape. The cold-rolled or cold-forming process involves passing the sheet metal through a die that cools in water for about one week. 

In addition to general cold formed steel pipes, low-pressure boiler pipes, high-pressure boiler pipes, alloy steel pipes, stainless steel pipes, petroleum cracking pipes, and other steel pipes, cold-rolled seamless steel pipes include carbon thin-walled steel pipes, alloy thin-walled steel pipes, stainless steel pipes, profiled steel pipes.

The outer diameter of the hot-rolled seamless pipe is generally larger than 32mm, and the wall thickness is 2.5-75mm. Cold-rolled seamless steel pipe can reach a diameter as large as 6mm with a thickness of 0.25mm, and thin-walled pipes can reach a diameter of 5mm with a thickness of less than 0.25mm. Hot rolling and cold rolling is less accurate than cold rolling.

Cold-rolled seamless steel pipes have small diameters, and hot-rolled seamless steel pipes have large diameters. Cold-rolled seamless steel pipe is more accurate and more expensive than hot-rolled seamless steel pipe.

1) Uses of different kinds

Seamless steel pipes come in a variety of types, including general steel pipes, boiler steel pipes, alloy steel pipes, stainless steel pipes, petroleum cracking pipes, and geological steel pipes.  Seamless steel pipes used in industrialized or industrial applications are medium- and high-pressure steel pipes, as well as alloy steel pipes. These pipes have different properties and use, but they all have the same basic design. 

Their main hot working and cold working difference is their design; some are simpler while others are more complex. Medium- and high-pressure steel pipes are commonly used in industrial applications as well as medium- and high-pressure steel pipes in different diameters, lengths, thicknesses, and materials. The main physical properties of the medium- and high-pressure steel pipe are tubular; it is a fluid or solid object that has no thickness but is made up of a set of tubes. In this case, it is important to take the pipe’s design and materials into account, as well as its operating temperature and pressure levels. 

2) Size differences

Hot-rolled seamless pipes usually have an outer diameter of greater than 32mm and a wall thickness of 2.5-75mm. A cold-rolled seamless pipe can have a diameter of 6mm and a wall thickness of 0.25mm. A thin-walled pipe has an outer diameter of up to 5mm and a wall thickness of less than 0.25mm. The dimensional accuracy of cold rolling is higher than that of hot rolling.

3) Differences in processes

  1. Cold-rolled formed steel allows local buckling, which allows full use of the bar’s bearing capacity after buckling; hot-rolled steel does not allow local buckling.
  2. Since hot-rolled steel and cold-rolled steel have different sources of residual stress, their distributions are also very different. Compared with cold-formed thin-walled steel, which has a curved residual stress distribution, hot-rolled steel or welded steel has a thin-film residual stress distribution.Hot rolling process & Cold rolling process.
  3. Since hot-rolled steel has a higher free torsional stiffness than cold-rolled steel, its torsion resistance is better.

4) Advantages and disadvantages of each

Steel plates or steel strips are cold drawn, cold bent, cold drawn, and other cold processing at room temperature to form cold-rolled seamless pipes.

The advantages include:

A variety of cross-sectional forms can be formed with a fast forming speed, high output, and no damage to the coating; cold rolling can cause large plastic deformation of steel, improving its yield.

The disadvantages include:

Cold-rolled steel typically has an open section, which reduces its free torsional stiffness. 

Torsion buckling occurs when it is bent, and bending and buckling occur when it is compressed. Steel strips (or rods) must be thin enough to have no free torsional stiffness in order to avoid these problems. Due to their low weight, strength, and electrical conductivity, steel strips with a small cross-sectional area are commonly used in certain applications.

Metal strips can be punched and formed into various shapes. Hammers and/or other tools are commonly used to bend and twist the metal strip. Depending on the application, the bend angle of the steel can also vary from 0° to 180°.

In cold-rolled steel, the wall thickness is small, there is no thickening at the corners, and local concentrated loads cannot be absorbed efficiently.

Cold-rolled seamless tubes are relative to hot-rolled seamless tubes. A cold-rolled seamless tube is rolled below the recrystallization temperature, while a hot-rolled seamless tube is rolled above it.

The advantages include:

The process destroys the casting structure of the steel ingot, refines the grain structure of the steel, and eliminates defects in the microstructure, resulting in a dense steel structure with improved mechanical properties. As a result of this improvement, the steel is no longer isotropic to some extent; bubbles, cracks, and looseness that form during casting can also be welded under high temperature and pressure.

The disadvantages include:

Non-metallic inclusions (mainly sulfides, oxides, and silicates) inside steel are pressed into thin sheets, leading to delamination (interlayer). When the weld shrinks, delamination greatly deteriorates the tensile properties of the steel in the thickness direction. 

As a result of weld shrinkage, the local strain often reaches several times the yield point strain, which is much greater than the strain caused by the load;

Uneven cooling causes residual stress. There are residual stresses in hot-rolled steel sections of various cross-sections. Although the residual stress is self-balanced, it still has a certain influence on the performance of steel components under external force. 

As a result, there will be a negative difference between hot rolling and cold rolling the actual dimensions and those calculated by the engineer. We have to stabilize the thickness of each product with a certain degree of tolerance. “Is there anything you can do about your products?” he replied. When the cutting edge is thin, I can improve their performance, but I have to maintain a certain tolerance. It is important to consider the width of the edge as well as the thickness of the products. In terms of thickness and surface finish, they will be difficult to control if they are not flexible enough.

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