Hot rolled and cold rolled steel are two common types of steel processing methods used to produce different types of steel products.
Like the names suggest, hot rolled steel is rolled hot, specifically above the annealing temperature of 1500°F (815°C) where cold rolled steel is produced at lower temperatures – nominally room temperature. This leads to very different crystalline atomic structures, as the difference in temperature is very significant in steel behaviors.
Here’s an overview of the differences between the two:
Differences between Hot Rolled and Cold Rolled Steel:
Surface Finish: Hot rolled steel has a rough surface finish and rounded edges, while cold rolled steel has a smooth surface finish and sharp edges.
Dimensional Tolerance: Cold rolled steel is produced with tighter dimensional tolerances than hot rolled steel, making it suitable for applications that require precise dimensions.
Strength: Hot rolled steel is generally stronger and more ductile than cold rolled steel, but this can vary depending on the specific grade and type of steel being used.
Cost: Hot rolled steel is typically less expensive than cold rolled steel because it can be produced quickly in large quantities and with less processing.
In summary, hot rolled steel is best suited for applications where strength and cost-effectiveness are the main priorities, while cold rolled steel is ideal for applications that require precise dimensions and a smooth surface finish.
I. What is Hot Rolled Steel?
Hot rolled steel is produced at high temperatures (generally above 1700°F, 927°C) and is often referred to as “hot rolled pickled and oiled” because it is treated with an acid to remove any rust or scale before being oiled to prevent further corrosion.
This process results in a steel product with a rough, black oxidized and often pitted surface finish and rounded edges. The rounded edges are a result of pinch rollers on the upper and lower faces but edges that are not constrained by rollers.
Hot rolled steel is fully normalized (i.e. not work hardened), malleable and ductile, making it highly resilient to stress and shock loading but relatively weak.
It is typically used for construction, such as in the production of I-beams, railroad tracks, and sheet metal, because it is relatively inexpensive and can be produced quickly in large quantities. It is also often used for structural components in automobiles and machinery.
Characteristics of Hot Rolled Steel
Hot rolled steel possesses these properties:
- Reduced strength and hardness, compared with more ‘technical’ steels. The heating process reduces the relative strength and hardness of the steel, making it more malleable and easier to shape.
- Improved ductility results from the annealing or normalization of heating. The normalization resulting from the process renders the steel at maximum ductility and malleability, making it more flexible and less prone to cracking and brittle behavior.
- Hot rolling allows for a wider range of sizes and shapes of steel to be produced, compared to other methods of steel production.
- Hot rolled steels are made to lower precision than cold rolled, suited to the applications they are made for.
- Hot rolled steel can have surface imperfections, such as mill scale, which is an easily removed dark oxide layer that forms on the surface of the steel during the heating process.
Advantages of Hot Rolled Steel
Hot rolled steel is the most widely produced and most commonly used. Its properties are ideally suited to large scale projects.
- The primary advantage of hot roll produced steel is that it maintains high shock and overload resilience, allowing large structures to survive difficult conditions and abuse.
- Hot rolled steel is low cost to produce, as it is fast to roll and made at relatively low precision
- The malleability and ductility of hot rolled steels makes it ideal for post roller shaping to make more complex structures. Limited work hardening at bend points increases strength, at the cost of reduced flexibility.
II. What is Cold Rolled Steel?
Cold rolled steel is produced at much lower temperatures than hot rolled steel, typically between room temperature and rarely up to 1500°F. The maximum temperature is defined by the annealing or normalization temperature for steel, above which crystal boundaries tend to dissolve into each other.
This limit prevents the process, resulting in fine crystal structures that increase the material strength and stiffness, at the cost of other properties. This process results in a steel product with a smooth surface finish and tighter dimensional tolerances than hot rolled steel.
Cold rolled steel is often used for applications that require precise dimensions, such as in the production of appliances, automobiles, and electronic components. It is also commonly used in the production of furniture and other decorative items because of its smooth surface finish.
Characteristics of Cold Rolled Steel
Cold rolled steels have quite different properties from hot rolled, despite being chemically identical. The micro structural differences have a significant impact;
- Cold rolling can increase the strength and hardness of the steel, making it more suitable for applications where ultimate tensile strength is most important, rather than ductility or malleability.
- Smoother surface finish: The cold rolling process produces a smoother surface finish on the steel, which is ideal for applications where appearance is important.
- Cold rolling can improve the dimensional accuracy of the steel, making it easier to achieve tight tolerances. Relaxation and recovery after cold rolling is very small, allowing the pinch rollers to define final dimensions with precision.
- Cold rolled steel can be produced in thinner gauges than hot rolled steel.
- Cold rolling reduces ductility of the steel, making it more prone to cracking or breaking.
- Cold rolling is a more expensive process than hot rolling, due to the additional processing required to achieve the desired characteristics.
Advantages of Cold Rolled Steel
Cold rolled steel is critically important in many industrial and engineering applications;
- It offers higher intrinsic tensile and flexural strength, allowing machines that can be built with and retain their precision, despite high loading
- It requires little or no finishing to make high precision parts
- Cold rolled (alloy and medium Carbon) steels are easily heat treated to deliver some extreme strength and hardness properties
- Cold rolling allows the production of thinner gages in sheet steel, for reduced weight applications that require high strength , such as automotive bodies
- Cold rolled steels are sometimes thought to be less prone to corrosion because of the altered crystalline structure
- The energy costs in cold rolling ae said to be lower, although this difference may be outweighed by greater processing effort
III. Hot Rolled vs Cold Rolled Steel: A Detailed Comparison Between the Two Popular Metal Choices
The two steel materials produced by hot rolling and cold rolling are very different in their properties and applications, so it’s important that those who are using the steels to build structures and machines have a thorough understanding of the nature of the choice that must be made in specifying materials in the initial stages of any development/project.
A ship needs very different material properties in its structure than a life-boat davit and an anchor chain capstan – and understanding these differences is key to making products that function and operate safely.
Similarly, a truck chassis has very different duties than a drive shaft, so getting the material details right is key to outcomes that serve.
1. Material Properties Differences Between Hot Rolled Steel and Cold Rolled Steel
In general, the properties of the two material families have been introduced above, we now want to introduce you to more specific details that can help in correctly specifying/selecting materials.
Not all steels are created equal to all tasks.
Both hot and cold rolled steels are, of course, steel – but steel is a big family of materials that ranges from I-beam girders (RSJs, rolled steel joists) through street lamp posts to ball bearings.
‘Elemental’ steel is Iron with a tightly controlled Carbon level.
- Low Carbon steels are generally unable to be heat treated, but they will work-harden. They are tough and ductile, malleable for easier manufacture of complex shapes and highly weldable.
- Medium Carbon steels are harder, tougher and more responsive to heat treatment (hardening and tempering)
- High Carbon steels are harder and stronger again, and highly responsive to heat treatment
Most hot rolled steel is low or medium Carbon, with low alloying elements, relying on the native properties of the steel for strength, ductility, malleability and weldability
‘Alloy’ steels diverge from their elemental steel origins, with additions of small amounts of more costly metals (and some non metals) which alter their properties. Metals such as Molybdenum, Chromium, Nickel etc and non metals such as silicon can hugely affect a range of properties
Commonly, cold rolled steels are elemental steel and hot rolled steels are a mix of elemental and alloyed steels, depending on specifics of the applications they are made for.
Hot rolled steel is not generally considered to be more corrosion-resistant than cold rolled steel. In fact, in some cases, cold rolled steel can be more corrosion-resistant than hot rolled steel. In particular, the black scale on hot rolled steel can quickly turn from black oxide to red oxide, giving the appearance of rapid corrosion.
The corrosion resistance of steel depends on several factors, including the composition of the steel, the presence of impurities or other elements, and the surface finish of the steel.
Hot rolled steel may have a rougher surface finish and may contain more impurities than cold rolled steel, which can make it more prone to corrosion. Cold rolled steel often includes alloying elements that improve the corrosion resistance
Additionally, the heating and cooling processes used in hot rolling can sometimes cause the formation of scale on the surface of the steel, which can act as a barrier to further corrosion. However, this scale can also contain impurities and may not provide long-term protection against corrosion.
In terms of durability, both hot rolled and cold rolled steels can be highly durable materials when correctly specified and utilized in real world applications. The specific characteristics of each type of steel can affect their durability in different ways.
Hot rolled steel can be though less durable than cold rolled steel because the heating and cooling processes used in hot rolling can lead to internal stresses and imperfections such as cracks and voids.
These imperfections can weaken the steel and make it more prone to failure over time, particularly in applications where strength and durability are important.
Cold rolled steel, on the other hand, is generally more durable than hot rolled steel because it undergoes a more controlled and precise manufacturing process that reduces the likelihood of internal stresses and imperfections.
Additionally, the cold rolling process increases the strength and hardness of the steel, making it more resistant to wear and damage over time.
In applications where cyclic and shock loading are regular occurrences, hot rolled steels additional ductility allows for greater structural survivability, despite the equal weight/thickness strength of cold rolled steels being higher.
The specific type and grade of steel, as well as the conditions in which it is used will greatly influence durability. Both hot rolled and cold rolled steels can be vulnerable to corrosion if they are not properly protected or maintained.
The densities of hot rolled and cold rolled steels are virtually identical.
- Hot rolled steels being weaker by core properties, a larger weight of material is required to achieve the same strength
- Cold rolled steels can be produced in thinner sections, allowing weight reductions in structures where either material could serve
- The increased potential for work hardening in cold rolled steels (where they are not fully work hardened by manufacture) can allow increased strength in structures, reducing the total weight of material required for real world applications
Hot rolled steel typically has a rougher surface finish than cold rolled steel, and often contains inclusions and small voids which add to machining difficulties. Additionally, the heating and cooling processes used in hot rolling can result in internal stresses and irregularities in the steel that can affect its response to machining.
Cold rolled steel typically has a smooth surface finish and a uniform microstructure, so it is considered easier to machine.
The cold rolling process generally increases the strength and hardness of the steel, making it more resistant to deformation and wear during machining operations, which allows for more precision in machining operations.
The specific type and grade of steel, as well as the machining process and conditions, can also play a significant role in the machinability of both hot rolled and cold rolled steels. Alloying elements, Carbon levels and the use of better quality cutters and cutting fluids have more effect on the machinability of both types of steel.
The weldability of hot and cold rolled steels varies considerably because of several factors, primarily the type and grade of steel, the welding process and conditions.
Hot rolled steel is considered more weldable than cold rolled steel because it usually has a lower carbon content and is easier to flow during welding.
However, hot rolled steel can also have internal stresses and irregularities that can affect its weldability, and may require preheating or other measures to reduce the risk of cracking or distortion.
Cold rolled steel often has a higher carbon content and a harder, more brittle surface that can make it more difficult to weld than hot rolled steel. But the uniformity of cold rolled steels composition and structure can make it easier to predict and control the welding process.
Overall, the weldability of both hot and cold rolled steels will depend on the specific type and grade of steel more than the initial forming process.
Proper selection of welding materials and techniques, including preheating, post-weld heat treatment, and use of appropriate filler metals, can help to improve the weldability and overall quality of both types of steel.
Note that cold rolled steels will lose their work hardening at the weld points, which can significantly affect overall structural properties.
2. Strength Differences Between Hot Rolled Steel and Cold Rolled Steel
For identical alloying and Carbon levels, the properties vary widely between hot and cold rolled steel;
The tensile strength of hot rolled steel is generally around 67,000 psi (4.62×108 pascal), whereas cold rolled steels usually rate around 85,000 psi (5.86×108 pascal). That’s a 26% increase in strength attributed to the production process
Yield strengths are even more divergent at 70,000 psi (4.83×108 pascal) for cold rolled and 45,000 (3.1×107 pascal). That’s a 55% difference attributed to the manufacturing process.
3. Ability to Form
Hot and cold rolled steels offer quite different .
Before melting, the steel rises to the normalizing temperature, so by the time the melting point of 2,500°F (1370°C) is reached there is no ‘structural’ difference between the two types of steel. Both melt at around 2,500°F (1370°C).
4. Appearance and Touch
It is apparent at a glance which material is which – the feel and look are very different.
Hot rolled steel is:
- Rough surfaced
- Coated with black iron oxide, in film and flakes
- Rounded at the edges
Cold rolled steel is:
- Bright silver (often called silver steel)
- Smooth surfaced
- Oxide free
- Square cornered
Hot rolled steel is considered soft. It is malleable and ductile and has a Brinell hardness of 137.
Cold rolled steel is harder, tougher, less ductile and malleable and has a Brinell hardness of 167.
5. Pricing & Options
Cold and hot rolled finished steel products such as plate, sheet, bar and tube have very different pricing structures, but much depends on the type and size of cross section, the alloying elements, the precision of the product and the volume purchased.
Finished steels range from $2000 per ton and upwards, depending on purity, alloying elements and scrap content.
Ingot steel has a general commodity price varying between US$1700 and US$2500. This depends on purity and scrap levels. Steel is among the most recycled materials in all industries, and scrap steel has a value of US$200 per tonne and requires considerable energy to process into a useful and consistent material.
Most steel has a proportion of scrap in it, and this can affect purity to a high degree. Some scrap suffers from higher sulfur contamination, making it corrode much faster. Vacuum arc remelt can be used for purification, but this pushes up the price considerably.
IV. Hot Rolled Steel vs Cold Rolled Steel : Applications
These two families of steels serve a wide range of purposes across many industries. Selection of steels for a product or application can be a difficult process, as different grades and processes can have large effects on properties, which in turn feeds through into the design process.
Often where weight AND strength are critical, higher grades must be selected. As a rule, stationary systems and devices (bridges, buildings, pylons, rebar etc) can use the lowest grades of cold rolled steels and simply use more to compensate for the weakness.
Applications of Cold Rolled Steel
Cold rolled steel is used for precision parts, weight sensitive applications and high strength components.
Many home appliances use more complex alloys such as stainless steels (Nickel and Chromium containing alloys of steel) and HSLA (high strength low alloy) steels, to reduce panel thicknesses and weight while achieving the requisite strength.
Much furniture design makes use of steel either as cosmetic structures or hidden components. There is a strong tendency to employ higher quality and cold rolled steels, as material costs are a lower part of the total product price than is the case for some other product types. Also furniture can mostly be thought of as being more weight sensitive than price sensitive, in general.
Aerospace Structural Components
There is very limited demand for steel in aerospace structures, as the strength to weight ratio is not as favorable as it is for Aluminum or Magnesium alloys and the sector has overwhelming weight sensitivity.
Auto parts are commonly made from cold rolled material, particularly monocoque chassis and body panels. This is due to weight considerations being greater than cost.
An exception to this is in heavy vehicle chassis parts, where the weight sensitivity gives way to cost in some cases, allowing use of lower cost materials to be beneficial.
However, in military vehicles, weight sensitivity will generally overwhelm cost issues, so higher value materials that deliver higher performance are the norm.
V. Hot Rolled vs Cold Rolled Steel: Which Metal Should You Use?
As a rule, the broad decision about whether to use cold rolled or hot rolled steels for a particular application is not difficult.
Hot rolled steels are lower cost, often by a significant factor. Cold rolled steels are preferred where weigh/strength/precision over ride cost decisions.
Hot rolled steels are most used for things that don’t have to move on land, where weight is therefore less important. They are also better suited to survivability overload applications, such as ships (surviving long term wave impacts) and building frames (surviving seismic and vehicle motion/vibration)
Cold rolled steels are reserves for precision applications such as ICE and machine parts, higher stress uses such as cranes, moving vehicles (other than ships) and more weight sensitive applications such as domestic appliances, furniture etc.
Cold rolled steels are considered more machinable than holt rolled, although this mostly depends on the quality of the hot rolled steel – if it is free of inclusions and porosity, it is still highly machinable and somewhat softer cutting than cold rolled steel.
Hot rolled steel is largely unsuited to aesthetic applications – it is not smooth, not shiny and more corroding than cold rolled steel.
VI. Hot Rolled vs Cold Rolled Steel: Which is Cheaper?
Hot rolled steel is considerably less expensive than hot rolled. Hot rolled steel comes from a very competitive and non specialist supply chain, so it is sold at commodity pricing. It is often made with a high percentage of scrap.
Cold rolled steels are more specialist, have higher alloy content and they are much more precisely made, making them considerably more costly and generally used for higher cosmetic, higher stress or weight sensitive applications.
VII. Kemal: Manufacturing Metal Parts and Prototypes for You
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