The modern world is unthinkable without steel. This is a metal alloy of iron, carbon, and other components. Their ratio determines the brand, characteristics of the final material, and its application. Steel is supplied to production in the form of rolled products—sheets of a given thickness, produced by hot or cold methods. Technology directly affects the characteristics of the resulting material.
It is impossible to say which sheet is better, hot-rolled or cold-rolled. The answer depends on the requirements of the specific area for which the rental with the required parameters is selected.
Hot rolled
Hot rolled metal is a technology for producing sheet steel. The workpiece is first heated at a temperature of about 1000 degrees, and then rolled with rollers. Heated metal becomes more ductile, resulting in the process requiring less time. Hot-rolled sheets may have scale due to contact of hot steel with air. After production, the rolled product must be processed to remove all defects from the surface.
Iron sheets made in this way are suitable for those areas where strict requirements are not imposed on products. It has several advantages:
The disadvantage of rolled steel is that the surface is not smooth enough for some applications. After heating, the characteristics of the workpiece became heterogeneous. This is why it is impossible to ensure a perfectly smooth sheet using hot rolled metal. In addition, traces of scale remain on the surface, and the sheets require additional processing.
Cold rolled
Cold-rolled sheets are passed between rollers without preheating. This is more difficult work and the process takes a lot of time. The iron sheet has a homogeneous structure with low ductility. The result is a perfectly smooth surface. To eliminate tension in the metal, the finished sheet is crimped and fired - this no longer affects its consumer properties.
Cold-rolled sheets are in demand in construction, the space industry, mechanical engineering and shipbuilding. It has excellent external characteristics and high strength. Hot-rolled sheets do not have such properties. It is difficult to find thick sheets produced by cold rolling on sale - there is practically no demand for it. At the same time, manufacturers rarely produce hot-rolled thin sheets.
Among the advantages of the material:
- ideal geometric characteristics;
- internal homogeneity;
- smooth and flat surface.
Cold rolled metal is also used to produce steel sheets, which will then be used in the production of modern furniture. Furniture made from this material is also produced for medical purposes, the chemical industry, and laboratories. Thin cold rolled products are used to make beautiful kitchen utensils.
Which sheet should I choose?
When choosing sheet metal, you need to take into account the scope of its application. For the manufacture of rough building structures, you can choose hot-rolled sheets, which are not aesthetically pleasing, but provide high strength of shaped products. To produce steel tiles or furniture from steel sheets, it is better to use cold-rolled products that do not have scale and other defects.
If you need a thick sheet of cold rolled iron, then a batch of materials can be ordered individually. Companies extremely rarely produce such steel in series - buyers order thick sheets mainly for the manufacture of profiles and rough structures.
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Advantages and disadvantages of cold and hot rolling technologies
Cold-rolled steel, despite its higher cost, is most in demand in industry due to the following advantages:
- Smaller sheet thickness
- Smooth steel surface
- Same thickness over entire surface
- High mechanical strength and metal hardness
- Sheets do not crack when bent
The disadvantages of the method include higher cost and long production cycle.
Hot-rolled steel, among its advantages, has a lower cost and a short production cycle, but in terms of practical properties it is significantly inferior to cold-rolled products:
- Uneven thickness
- Can't get thin sheet
- Cracks occur when bent
Differences in the production of hot-rolled and cold-rolled steel
Hot deformation is carried out at temperatures exceeding the conditions for recrystallization of the metal and amounting to 60% or more of the melting temperature. The initial workpiece is a slab. At elevated temperatures, the strength of steel decreases, which reduces the force required to give the workpiece the desired dimensions. In the process, new grains are formed, preserving good ductility of the metal. It is difficult to control the final dimensions due to the presence of scale on the surface and volumetric changes during cooling.
Cold rolling is the plastic deformation of a workpiece at room temperature. The workpiece in this case is a hot-rolled sheet. Before processing, it must be cleaned of scale using a shot blasting machine, acids, or a combination of both. At deformation temperatures that are below 30% of the melting temperature, work hardening or strain hardening occurs in the sheet, increasing the strength of the rolled product and reducing ductility. For partial recrystallization of the structure, which makes it possible to slightly reduce the hardness and increase the plasticity, heat treatment is carried out - tempering.
Metal rolling temperature
The rolling temperature of metals - steel, aluminum, copper - is usually divided into two main intervals:
- cold processing;
- hot processing.
Cold rolling occurs at relatively low temperatures relative to the melting point of the metal. Hot rolling is carried out at temperatures above the recrystallization temperature of the metal. There is also a third temperature range - warm rolling, which is caused by the need to save energy and in some cases is used in industry.
The difference between the properties of hot-rolled sheet and the characteristics of cold-rolled sheet
As a result of using different technologies, sheets are obtained whose properties differ significantly. The choice of cold-rolled or hot-rolled sheet depends on the functional purpose.
Advantages of Hot Forming
- The ability to produce products in a wide range of thicknesses – 0.4 (actually 1.2)-200 mm.
- Relatively low cost of the process.
- Higher corrosion resistance compared to the result of cold deformation.
Disadvantages of hot rolling:
- impossibility of obtaining high-quality thin sheets;
- the surface is uneven, with a depression in the center, low quality, likely the presence of scale;
- low dimensional accuracy;
- When welding, the sheet may “lead.”
Advantages of cold rolled sheet:
- the ability to produce flat products several microns thick;
- high geometry accuracy;
- smooth, high-quality surface, the product can be used for the manufacture of products in which the aesthetic component is important;
- does not “lead” when welding.
Disadvantages of this product
- low corrosion resistance in air without anti-corrosion surface treatment;
- often - reduced plasticity;
- high manufacturing costs due to the use of a wide range of sophisticated equipment and the high energy intensity of the process.
Production of cold rolled sheets
At the beginning of the production cycle, the steel billet goes through the stage of cleaning and leveling the surface, then heating, which helps to increase the ductility of the metal and allows for hot rolling, during which the metal takes the form of rolls of thin sheet thickness.
The resulting metal sheet is subjected to etching, since the surface of the material receives scale after exposure to high temperatures, after which the sheets are welded (if it is necessary to obtain a large-sized product), as well as the longitudinal edges are cut.
Further technology involves rolling blanks through continuous machines or equipment with a reverse operating cycle, as well as annealing. The goal of production is to produce a continuous thin sheet of steel, which is then cut into sheets of the desired size.
What are the differences between the applications of cold-rolled sheets and hot-rolled sheets?
The difference in properties determines different areas of use.
Hot rolled sheet is most often used:
- in construction - for the manufacture of load-bearing coatings;
- in shipbuilding, aircraft and mechanical engineering;
- for creating structures using welded, bolted, riveted joints;
- for the production of welded pipes.
Application areas of cold rolled sheets:
- production of corrugated sheets, smooth galvanized sheets;
- automotive industry;
- obtaining tin - a thin sheet or strip with a protective, most often tin, coating, used for the manufacture of cans;
- etched annealed sheet - decapir - is used for the manufacture of enamel dishes.
Due to the high quality of the surface, cold-rolled sheets are well suited to powder coating, chrome plating, and nickel plating.
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Areas of use of hot-rolled and cold-rolled metal
| Hot-roll sheet | Cotton sheet |
Hot rolled steel sheets are used in:
This type of metal products is necessary where steel is needed, but there is no increased demand for aesthetics. For example, the construction of bridges, boilers, furnaces, structures with bolted or welded connections. | Cold rolled sheets are needed for:
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Hot-rolled and cold-rolled steel serves as the basis for the profiles of metal frame structures, which we use for the installation of prefabricated tent hangars. Depending on the project and loads on the structure, we offer 3 options for metal frames. Read more about this at the link.
Differences in the production of hot-rolled and cold-rolled steel
The production of rolled sheets is regulated by two documents: GOST 19903-2015 - for hot-rolled sheets and GOST 19903-2015 for cold-rolled sheets. Let us also mention GOST 13345-85, which establishes requirements for the production of tin - a type of cold-rolled sheet metal with a thickness of 0.18–0.36 mm. The main differences between hot-rolled and cold-rolled sheets are determined by their production technologies:
- hot rolling is performed at a billet temperature of 1,150–1,200 °C at the entrance to the first mill stand and ends at 900–950 °C;
- during cold rolling, depending on the steel grade, the temperature range is 2.5–6 times smaller.
Steel rolling: difference between hot and cold rolling technologies
In general, steel rolling is plastic processing on a rolling mill while passing between rolls.
According to the temperature of steel processing, there are hot-rolled steel (the metal is heated above the recrystallization temperature, to the level of 80% of its melting temperature) and cold-rolled steel (30% or less of the recrystallization temperature). In turn, recrystallization is the formation and growth of crystalline grains of a material (metal) due to existing crystalline grains. During hot rolling, the heating exceeds 920 degrees. For example, for low-carbon steel, the hot rolling temperature must be at least 1169 degrees.
Obtaining any rental is possible in 2 ways. The first, which is now outdated, uses steel ingots (cast in special molds), which are processed into finished rolled products in 2 stages. The resulting ingots are heated and rolled into a billet on crimping mills, and after removing surface defects (slopes, cracks, etc.), reheating and final rolling into hot-rolled sheets or shaped sections are performed.
Hot rolled sheet
In the second method, casting into molds is replaced by casting on continuous casting machines (CCMs), from where the billet is also supplied to rolling mills. In this way, such a technological link as slabs and bloomings is eliminated, quality is improved, and losses of metal and production resources when cutting the ingot are eliminated. In Ukraine, most metallurgical companies have continuous casters and casting and rolling modules, and other enterprises have plans to introduce such equipment.
Depending on the relative position of the metal strip and the axes of the rolls, rolling can be longitudinal (the strip is perpendicular to the rolls), transverse (the strip is parallel to the rolls) and cross-helical or “oblique” (the rolls are at an angle to each other and to the axis of the strip). This allows you to obtain a product of various shapes. The main types of rolled products in terms of shape and size are hot-rolled sheet, coil or tin, and sectional (plain or shaped).
Comparison of hot-rolled and cold-rolled steel
Today, about 80% of the total volume of rolled metal is produced by hot rolling in the world. It requires less production effort and therefore less electricity. Increased plasticity during such rolling makes it possible to obtain a significant reduction in the cross-sectional area of the workpiece in one process. Ingots, which are largely heterogeneous structurally and chemically, can only be processed hot.
Steel grade 09G2S is largely used for hot-rolled (hot-rolled) products, and 08PS for cold-rolled (cold-rolled) products. Cold-rolled steel goes through a longer processing path - it is pickled, and only then is transferred to the mill. Therefore, for example, the thickness of a cold-rolled sheet is no more than 5 mm, and its surface is much smoother.
Hot rolled
The cotton sheet is usually of the same thickness over the entire area, without scale, which does not need to be knocked down before painting. In addition, during cold rolling the surface of the metal is strengthened, and this improves its mechanical properties; The cotton sheet does not crack when bent and is more resistant to tension and tearing. Whereas a hot-rolled sheet with a thickness of 3 mm or more has a rather large error in thickness over the sheet area, has an uneven surface, and after heat treatment it warps and requires straightening.
TABLE. Comparison of properties of cold-rolled and hot-rolled sheets
| Cold rolled sheet | Hot rolled sheet | |
| Surface | Smooth, therefore used where it matters in the final product, incl. for metal products (hardware) and in instrument making | Uneven, most often the middle of the sheet “sinks”, therefore it is mainly used in construction and welding of metal structures |
| Sheet stress | Uniform. This is an important property of metal for any purpose, especially for instrument and automotive manufacturing. | Uneven. However, it is used in mechanical engineering |
| Raw material | Usually made of low carbon steel and has high ductility (can be bent repeatedly and strongly) | Made from carbon, low alloy steel. Alloy steel is also used for the production of boilers and pressure vessels. |
| Application | — auto and shipbuilding — instrument making — in construction (for obtaining corrugated sheets, roofing sheets) – pipe production | – construction (mainly in the form of load-bearing coverings) — mechanical engineering, incl. shipbuilding and aircraft construction |
In Ukraine, a significant part of the rolling capacity of metallurgical enterprises needs to be modernized in terms of both the production efficiency of the equipment and the available range.
Production of hot rolled steel
“Hot” mills are capable of producing slabs with a thickness of 50-350 mm, sheets with a thickness of 3-50 mm and strips (coiled into rolls) with a thickness of up to 20 mm. Mills for thick sheets include 1-2 stands of the “duo” or “quarto” type (i.e., with one or two pairs of rolls), sometimes with additional vertical stands for pressing the side edges. For strips, mills with 10-15 quarto stands and stands with vertical rolls are used.
At the exit from the rolls, output roller tables and other auxiliary equipment for further processing are installed: for plate mills - straightening machines, shears, thermal ovens, etc., for strip mills - winders for winding into rolls, a conveyor for rolls, equipment for unwinding, straightening, cutting
In turn, high-quality mills are quite different in their properties. Their main types are
- universal mills for wide-flange beams, usually from 3 or 5 consecutive stands;
- step-type rail and beam mills of 2 or several lines with “trio” and “duo” stands;
- large-section mills of stepped and semi-continuous type from 2 or several lines with “trio” and “duo” stands;
- medium-grade step-type mills with 2 or 3 lines;
- small-section mills (continuous or semi-continuous);
- continuous wire mills.
Overview of Hot Rolled Steel Specifications
Hot-rolled steel is divided, among other things, into:
- “thick sheet” of carbon steel, as well as high-quality structural steel;
- thin sheet of high strength steel;
- sheets of carbon, low-alloy or alloy steel for boilers and pressure vessels;
- sheet for cold stamping made of high-quality structural steel;
- for shipbuilding;
- low-alloy structural for bridge construction;
- high-quality hot-rolled steel (round, square, hexagonal);
- sectional and shaped from non-alloy steel;
- sections made of structural unalloyed or alloyed steel for cold die forging;
- graded from precision alloys with high electrical resistance.
Hot-rolled sheet can be classified according to accuracy (A - increased accuracy, B - normal accuracy), flatness (PO - especially high, PV - high, PN - normal flatness), according to the nature of the edge (NO - with an uncut edge, O - with cut edge), etc.
Advantages of Hot Rolled Steel, Common Applications
The main advantage of hot-rolled steel is its cost, which allows the use of larger volumes in areas where surface quality or appearance are not so important. For example, during the construction of large objects, the “beauty” of load-bearing metal structures is not important; the same applies to large internal components of machines and equipment. Accordingly, hot-rolled steel is used primarily in such “heavy” sectors as the construction industry and heavy engineering.
Production of cold rolled steel
“Cold” mills can be sheet (for single sheets), broadband (for rolls), strip rolling (for reeled tape with a thickness of 1 micron to 4 mm and a width of 20 to 600 mm), foil rolling (for strips less than 0.1 mm thick) , flattening (for crimping wire into a narrow strip), for cold rolling of pipes.
Cold rolled
During coil rolling, winding and tension drums - winders - are installed on both sides of the working stand to unwind the coil before feeding it into the rolls and wind it up when exiting them. The most efficient sheet metal mills are continuous ones.
Overview of cold rolled steel specifications
Cold-rolled steel is divided, in particular, into:
- thin sheet of carbon steel of high quality or ordinary quality for general purpose;
- thin sheet of low-carbon quality steel for cold stamping;
- thin sheet of electrical isotropic or anisotropic steel;
- tape made of corrosion-resistant and heat-resistant steel, carbon steel, thermobimetals, etc.
Advantages of Cold Rolled Steel, Common Applications
The advantage of cold-smoked metal is its strength characteristics and appearance. There is no scale on it. The stress in the sheet is distributed evenly, which is fundamentally important for rolled products for any purpose. Most often it is made of low-carbon steel and has high ductility (that is, it can be bent repeatedly and strongly). Therefore, it is used in various fields of mechanical engineering (including automobile and shipbuilding), instrument making, and construction (corrugated sheets, roofing sheets, finishing materials). It is the cold-rolled sheet that is galvanized and painted to produce coated metal, which has improved protective properties.
conclusions
The choice between hot-rolled and cold-rolled steel (and their various types) depends on the needs of the consumer: whether it is the construction of load-bearing structures or, on the contrary, a finishing material for an external facade, the manufacture of large heavy machines - or the production of precision electronics. Hot rolled steel wins in price and variety of variations, cold rolled steel wins in quality and “appearance”. At the same time, let us note that long products are hot-rolled, while sheet sizes that are similar to each other can be either hot- or cold-rolled.
The difference between the properties of hot-rolled sheet and the characteristics of cold-rolled sheet
The characteristics of rolled steel are determined by the properties of the alloy from which it is made. In addition, the quality of the finished product is affected by its production technology. The differences between hot-rolled sheets and cold-rolled sheets are manifested in such properties as:
- plasticity of the metal - it is higher for a cold sheet;
- weldability is also better for cold-rolled steel;
- fracture strength is higher for cold-rolled sheets.
However, in addition to the differences caused by the properties of the metal itself, the difference between hot-rolled and cold-rolled sheets continues in the production technology of these types of rolled products. It would be more correct to talk about the possibilities of cold and hot processing of metals. They determine most of the differences between cold-rolled and hot-rolled sheets.
What is the difference between cold rolled sheet and hot rolled sheet?
As the temperature of the metal rises, its flexibility in processing rapidly increases. At the same time, new opportunities arise, but at the same time technological limitations arise for the manufacture of certain types of products. For example, it is impossible to roll rails using “cold” technology, while using hot rolling machines it is impossible to produce sheets with a thickness of less than 0.4 mm . From a technological point of view, the difference between cold-rolled and hot-rolled sheets is expressed in such parameters as:
- thickness – for cold-rolled sheets it is 0.35–5 mm, and for hot-rolled products – 0.4–160 mm;
- quality surface characteristics, which are higher for cold rolled products;
- precision manufacturing - tolerances for cold-rolled sheets are much tighter.
The combination of properties and characteristics determines the main areas of application of different types of rolled metal. Often they are interchangeable materials, but most often rolled products are intended for the manufacture of certain types of products or other purposes. This division is explained, among other things, by financial reasons, since cold-rolled metal products of the same range are more expensive . In the following summary table we will show in more detail how cold-rolled sheet differs from hot-rolled products.
Warm rolling
Warm rolling of metals is between hot rolling and cold rolling. It occurs in the temperature range of 30-60% of the melting temperature of the metal on an absolute scale. The forces required to deform the metal during warm rolling are higher than during hot rolling. The final surface quality and dimensional tolerances are higher than with hot rolling, but not as good as with cold rolling. Although warm rolling has disadvantages, the main incentive for its use is economics. Heating metal for hot working is very, very expensive. By lowering the rolling temperature, significant funds are released, which can be used to improve product quality.
