Rolled metal: review of hot and cold rolling technologies

In this article:

1. Pipe drawing

2. Cold rolling of pipes

Pipes in a cold state are processed to achieve a high quality of their surface, and also when it is impossible to obtain pipes of a given size by other methods. Cold processing produces pipes with a diameter of less than 0.1 and a wall thickness of less than 0.01 mm. There are two methods of processing pipes in a cold state: cold rolling and drawing, which produces thin-walled pipes with a diameter of 1–450 and a wall thickness of 0.1–3 mm.

The blanks for the production of cold-rolled and cold-drawn pipes are mainly hot-rolled or, less commonly, electric-welded pipes.

What is cold rolled steel?

There are two main types of pressure treatment. This is hot and cold rolling. The difference between them is the temperature at which these processes are carried out.

As the name suggests, hot working involves preheating the workpiece. The heating temperature depends on the thickness of the resulting sheet. The thicker it is, the higher the temperature.

Cold rolled - rolled without preheating, which imposes some special features. But before we start discussing them, let's first clarify the production technology.

How is cold rolled steel produced?

Stage 1. At the first stage, steel is smelted. There is no fundamental difference in the production of hot-rolled steel and cold-rolled steel. In both cases, only alloys with increased ductility are suitable, the relative elongation of which should not be less than 30%. This mainly includes structural steels of normal and high quality with a carbon content of no more than 0.5%. Also, some part is accounted for by high-alloy corrosion-resistant and heat-resistant steels. The most popular among them is steel 12Х18Н10Т.

Stage 2. After melting, rolling follows. This usually happens on large industrial rollers. And here the first differences between cold rolling and hot rolling are already beginning to be visible. The thickness of cold-rolled material is limited by the drive power, size and strength of the rollers. In practice, this means that the profile wall thickness cannot exceed 4 mm.

A further increase in sheet thickness is not economically feasible due to a significant increase in cost and a reduction in the service life of the equipment. Here it is more profitable to simply increase the malleability of the metal by increasing its temperature. This is the key difference between the production technology of hot-rolled and cold-rolled steel. One is heated before pressure treatment, the other is not.

Stage 3. After receiving the workpiece of the required size, it is then cleaned. It is required solely to give the sheet a more aesthetic appearance. There are two varieties of it:

1. The mechanical method implies direct physical contact with steel. A stream of air carrying spherical metal particles is supplied to the surface of the workpiece. As a result of this effect, contaminants are peeled off and removed along with the air flow, making the appearance of cold-rolled sheets more presentable.
2. The chemical method involves purification through a chemical reaction. As a rule, steel etching is used for this. The sheet is immersed in a solution of 25% hydrochloric acid and after a certain time, all contaminants are completely corroded, making the surface smoother.

I would like to note that if the production standards are followed, cold-rolled steel does not need cleaning. Hot-rolled metal always requires foresight at this stage, because As a result of exposure to temperature, scale forms on its surface.

Heating devices

At section mills, recuperative methodical furnaces are used to heat billets, operating on a mixture of blast furnace and coke oven gases with a calorific value of 8.4 MJ/m3. Natural gas is also used as fuel. The furnaces are equipped with ceramic recuperators for heating air to 400–600 °C and metal tubular recuperators for heating the gas mixture to approximately 300 °C. The productivity of the furnaces reaches 200 t/h.

Heating furnaces with end and side landing and delivery of workpieces are used. The width of the hearth with end-fitting and dispensing reaches 7 m in accordance with the usual length of workpieces up to 6 m. Such furnaces are used to heat workpieces with a cross-section of more than 100 × 100 mm and are equipped with loading roller conveyors and pushers. In side-entry and outlet furnaces with a 13 m wide hearth, workpieces up to 12 m long and with a cross-section of less than 100×100 mm are heated. The workpieces are fed into the furnace through a side window using a trolley or retracting rollers, and are ejected from the furnace using an ejector. The workpieces are pushed along the furnace using pushers.

Recently, both in the USSR and abroad, furnaces with walking hearths have been used to heat metal. The workpieces in these furnaces do not come into contact with each other, so external defects are excluded, and heating occurs much faster and more uniformly than in furnaces with pushers. At the same time, they are less exposed to high temperatures, resulting in reduced waste and decarbonization of the metal.

Advantages of cold rolled steel over hot rolled steel

The difference in technological processes imposes some peculiarities. Among the advantages of cold-process steel are the following:

• Higher strength characteristics. First of all, this concerns bending strength, which is on average 20-25% higher than that of hot-rolled steel. This is due to the lower value and better distribution of residual stresses throughout the volume of the workpiece. Additional temperature exposure increases the likelihood of microcracks forming, which significantly reduces the metal’s resistance to mechanical loads.
• The second advantage follows from the first. The coefficient of metal utilization in the case of cold working is higher, which, other things being equal, makes this method a priority compared to the hot-rolled method.
• Cold-rolled products are distinguished by greater accuracy of profile geometry. Deviation from flatness is usually 50-55% higher. This is again due to lower residual temperature stresses, which are the cause of such deformations.
• The absence of preheating of the workpiece has a positive effect on the cleanliness of the surface. Scales and burns are not typical for cold-rolled profiles. And, as mentioned earlier, it does not require additional surface cleaning.

Types of supplies

On the metal rolling market, cold-rolled steels are found in the form of various types of profiles: coils, angles, channels, etc. The most common is the sheet profile. Its dimensions and mechanical properties are regulated by GOST 19904-90.

According to GOST, the thickness of the supplied cold-rolled steel sheets is 0.25-4 mm. The width is in the range of 510-710 mm, and the length is from 1250 to 2500 mm. GOST also allows the application of chrome coating to sheets up to 2 mm thick. The chromium layer protects the steel from aggressive environmental influences, thereby reducing the likelihood of corrosion.

Also available in rolls. It is distinguished from sheet metal by its significantly longer length, which, depending on the thickness of the profile, can reach up to 30 meters. All this puts coiled steel in a more favorable light when it comes to transportation.

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Rolled products (metalworking)

Hot rolling

Hot rolled steel coil

Hot rolling is a metalworking process that occurs above the recrystallization temperature of the material. After grains are deformed during processing, they recrystallize, which maintains an equiaxial microstructure and prevents strain hardening of the metal. The starting material is usually large pieces of metal, such as cast semi-finished products such as slabs, blooms and billets. If these products have been produced through a continuous casting process, they are usually fed directly into rolling mills at the appropriate temperature. In small operations, the material starts at room temperature and must be heated. This is done in a holding pit powered by gas or oil for larger workpieces; For smaller parts, induction heating is used. During processing of the material, the temperature must be monitored to ensure that it remains above the recrystallization temperature. In order to maintain the temperature finishing

defined above the recrystallization temperature; it is typically 50–100 °C (90–180 °F) above the recrystallization temperature. If the temperature does drop below this temperature, the material must be reheated before further hot rolling. [9]

Holding pits used to heat steel ingots before rolling.

Hot rolled metals generally have little directionality in their mechanical properties and residual stresses caused by deformation. However, in some cases non-metallic inclusions will impart some directionality, and parts less than 20 mm (0.79 in) thick often have some directionality properties. Additionally, uneven cooling causes many residual stresses, which typically occur in shapes with non-uniform cross-sections such as I-beams. Although the finished product is of good quality, its surface is coated with mill scale, which is an oxide that forms at high temperatures. It is usually removed by etching or the smooth clean surface (SCS) process, which renders the surface smooth. [10] Dimensional tolerances typically range from 2 to 5% of the total size. [eleven]

Hot rolled mild steel appears to have a wider tolerance for the amount of carbon incorporated than cold rolled steel and is therefore more difficult for the blacksmith to use. Also, for similar metals, hot-rolled steel turns out to be less expensive than cold-rolled steel. [12]

Hot rolling is used primarily to produce sheet metal or simple profiles such as rail tracks. Other typical applications of hot rolled metal: [13]

• Truck frames
• Automotive clutch discs, wheels and rims
• Pipes and tubes
• Water heaters
• Agricultural machinery
• Belts
• Stampings
• Compressor housings
• Metal buildings
• Railway hopper cars and car components
• Doors and shelving
• Discs
• Guardrails for streets and highways

Profiled rolling design

Rolling mills are often divided into stripping, intermediate and finishing separators. During profile rolling, a stock (round or square) with an edge diameter typically in the range of 100–140 mm is continuously deformed to produce a specific finished product with smaller cross-sectional dimensions and geometry. Various sequences may be adopted to produce a specific end product from a given workpiece. However, since each rolling mill costs significantly more (up to 2 million euros), a typical requirement is to reduce the number of rolling passes. Various approaches have been achieved, including empirical knowledge, the use of numerical models and artificial intelligence techniques. Lambiase et al. [14] [15] validated a finite element (FE) model to predict the final shape of a rolled bar in a round-flat pass. One of the main challenges when designing rolling mills is to reduce the number of passes. A possible solution to such requirements is to pass

slotted

, also called
split-pass
, which splits the incoming rod into two or more parts, thereby effectively increasing the cross-sectional reduction ratio per pass, according to Lambiase . [16] Another solution to reduce the number of passes in rolling mills is the use of automated systems for roll design, proposed by Lambias and Langella. [17] Subsequently, Lambiase developed an automated system based on artificial intelligence and, in particular, an integrated system including an inference engine based on genetic algorithms, a knowledge database based on an artificial neural network trained with a parametric finite element model, and for optimization and automatic design. rolling mills. [18]

Cold rolling

Cold rolling of a metal occurs at a temperature below its recrystallization temperature (usually at room temperature), which increases strength due to strain hardening by up to 20%. It also improves surface quality and produces tighter tolerances. Typically, cold rolled products include sheets, strips, rods and rods; these products are usually smaller than the same hot-rolled ones. Due to the smaller size of the workpieces and greater strength compared to hot-rolled steel, four-roll or cluster mills are used. [2] Cold rolling cannot reduce the thickness of the workpiece as much as hot rolling in a single pass.

Cold rolled sheets and strips come in different states: full hard

,
semi-hard
,
quarter-hard
and
rolled
.
Fully hard rolled products reduce the thickness by 50%, while in other cases the reduction is less. The cold rolled steel is then annealed to impart ductility to the cold rolled steel, which is known as cold rolled and annealed
.
A curl, also known as a skin pass
, involves the smallest reduction: 0.5–1%. It is used to obtain a smooth surface, uniform thickness and reduce the yield stress phenomenon (preventing the formation of Luders bands during subsequent processing). It blocks dislocations on the surface and thereby reduces the possibility of the formation of Lüders bands. To avoid the formation of Lüders bands, it is necessary to create a significant density of unpinned dislocations in the ferrite matrix. It is also used to break up glitter in galvanized steel. Sheathed rolled products are typically used in downstream cold working processes where good ductility is required.

Other shapes can be cold rolled if the cross-section is relatively uniform and the cross-section is relatively small. Cold rolling of profiles requires a series of forming operations, usually such as sizing, dismantling, roughing, semi-finishing, semi-finishing and finishing.

When processed by a blacksmith, the smoother, more stable and lower level of carbon encapsulated in the steel makes it easier to process, but at the expense of higher cost. [19]

Typical applications for cold rolled steel include metal furniture, desks, filing cabinets, tables, chairs, motorcycle exhaust pipes, computer cabinets and equipment, appliances and components, shelving, lighting fixtures, hinges, pipes, steel drums, lawn mowers, electronics. cabinets, water heaters, metal containers, fan blades, frying pans, wall and ceiling mount kits, and a variety of building supplies. [20]

Hot-rolled and cold-rolled steel sheets: differences and scope of application

One option for steelmaking is the production of cold-rolled sheets. They have medium or large length and width. But the thickness can be small - from 0.35 to 5 millimeters. Cold-rolled sheets are produced by cold rolling in metallurgical plants. It is used for the production of various small products and spare parts, as well as for the manufacture of external fences and hanging structures. What physical properties does the material have? How is cold rolled sheet produced? And how is it different from hot-rolled products? These issues will be discussed in the article.

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.

Brief information

Cold rolled sheet (C/C) is a flat metal product that looks like long strips. C/C products are usually made of steel alloys, but sheets of other metals (aluminum, copper, duralumin alloys, brass) are also found. Cold sheets are produced in metallurgical shops by pressing hot-rolled sheets. C/C sheets are used for the production of car frames, cans and boxes, metal utensils, and corrugated sheets.

Cold steel is produced in large, long rolls, and to obtain an individual rectangle, the sheet metal is cut into individual pieces using industrial shears. Most cold-rolled strips have a small thickness - from 0.35 to 2 millimeters. Although there are also thick products, the thickness of which can reach up to 5 millimeters. The width of cold steel sheets usually ranges from 1.5 to 5 meters, although wider products are also available. C/C products, if necessary, can undergo additional processing - heating, tempering, galvanizing, mechanical deformation, applying a protective layer, painting.

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 sheets

• high accuracy and reduction of material losses due to the absence of excess;
• strength and corrosion resistance;
• fairly good weldability indicators, which allows the use of all types of welding, as well as high quality welds;
• the ability to use all anti-corrosion treatment technologies - galvanizing, powder painting;
• plasticity, thanks to which stamped parts are made.

• the main disadvantage is the limited sheet thickness of 5 mm;
• high cost compared to hot-rolled sheet.

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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Features of the material

• High strength. Cold-rolled sheet metal undergoes a number of technological treatments that eliminate internal tension in the alloy and improve the strength of the material.
• Small thickness. The C/C product is produced by repeated pressing, which makes it possible to obtain a product of small thickness (less than 1 millimeter).
• Nice view. The surface of cold rolled products is clean (no scale, burning, or traces of thermal effects). Therefore, the material can be used for aesthetic finishing of surfaces or products.
• Low risk of rust formation. At the end of processing of C/C products, the material is usually heated for recrystallization, which minimizes the risk of corrosion during long-term use.

Please note that in addition to cold-rolled sheets, there are also hot-rolled sheets. These materials have much in common, and C/C products are produced from hot-rolled sheets using pressing.

The main differences between cold-rolled and hot-rolled steel

Pipe drawing

During drawing, pipes are produced in several passes with a gradual reduction in diameter and wall thickness. Before drawing the pipes, a number of preparatory operations are performed: inspection, stripping, cutting into pieces, hammering the ends, etching, applying technological coatings and lubricants.

Pipe drawing (Fig. 206) is carried out in several ways: without a mandrel, on a short mandrel, on a long mandrel and on a deformable core.

Drawing without a mandrel is used when it is necessary to reduce only the diameter of the pipe. The deformation is carried out using a die, fixedly fixed in the stop (rest) of the drawing machine. Short mandrel drawing is used to simultaneously reduce the diameter and wall thickness of the pipe.

Currently, the method of drawing on a floating mandrel, a variation of the method of drawing on a short mandrel, has become widespread. The design of all drawing mills is based on a common principle: the use of continuously moving chains to move the pipe during the drawing process. On modern drawing mills, all operations are mechanized. The length of the pipes after drawing is limited by the length of the drawing machine. The most advanced and productive are drawing units, where the pipe production process is carried out on a drum in coils. The length of the pipes can reach 100 mm or more, and the drawing speed is 100 - 200 m/min.

Drawing on continuous drawing machines is a very advanced method of pipe production.

Properties of hot rolled steel

Hot rolled sheets have a rough surface. The finished sheet has rounded, imprecise corners. If they are trimmed, the material is marked with the index O; if it is supplied without trimming, it receives the index HO. The thickness of this type of steel is uneven and is characterized by a high percentage of scale content.

Assortment of hot-smoked sheets according to GOST

Accepted standards divide hot-rolled sheet metal into types according to thickness and area of ​​use. Currently there are GOST standards with numbers:

• 14637-89, which describes the parameters of hot-rolled thick sheets with a thickness of 4-160 mm of normal quality, produced from carbon steels;
• 16523-97, which determines the properties of thin hot-rolled steel up to 3.9 mm;
• 5520-79, which normalizes the production of hot-rolled plate products with a thickness of 4-160 mm, the basis of which is carbon, low-alloy, alloy steel, used for the manufacture of boiler equipment and industrial vessels;
• 5521-93, describing sheets of normal and increased strength with a thickness of 4-60 mm, produced from steel with an increased proportion of manganese for use in shipbuilding;
• 6713-91, which specifies the parameters of rolled plates of low-alloy grades for the construction of bridges (conventional and northern);
• R 55374-2012, responsible for bridge structures made of alloy steels.

Hot rolled sheet dimensions

Hot rolled steel is cut into sheets according to dimensions regulated by GOST 19903-2015. This standard specifies a thickness of 0.4-160 mm. It takes ordinary unalloyed or low-alloy steel as a basis. If the metal is wrapped in rolls, its thickness should be 1.2-12 mm.

The edge may or may not be trimmed. Accuracy is described as improved or normal for thicknesses up to 12 mm. Flatness options: normal, improved, high, extra high.

Rolls are formed from a single piece of fabric or two welded pieces. Acceptable fragment lengths are no less than 1:5. Other parameters for the lengths and number of pieces are discussed with the customer.

Practical use

Cold-rolled sheets, individual sheets and strips are widely used in various industries. Products of this type do not require special care and do not rust if stored properly. Another major advantage is a pleasant appearance (hot rolled products lack this advantage due to technological processing features). Producers of cold steel are the People's Republic of China, Russia, USA, Germany, France, Japan.

Areas of application

• Cold-rolled sheets with a low carbon content are used in mechanical engineering (therefore often called auto sheets). A metal car frame, as well as various parts (motor elements, tank, moving parts), are made from this material on robotic conveyor lines. For the needs of the automotive industry, they are usually smelted in strips that have an average thickness (from 1 to 2.5 millimeters).
• Cold rolled steel is also used to produce tin. For production, the original sheet is cold deformed using heavy-duty rolling presses, which help produce material less than 0.5 millimeters thick. For convenience, the resulting tin is cut with industrial scissors into small strips, the width of which is up to 1.5 meters. The resulting strips can be used for decorative purposes, as well as for the production of tin cans, metal fences, and partitions.
• Cold steel can also be used to produce a material called decapir, which is used to make homemade enamel cookware. To make decapir, tin is annealed in high-temperature furnaces to make the material more ductile. After annealing, the dishes are formed using presses. During annealing, metal scale and various debris are formed on the surface of the alloy. To remove harmful substances, decapir is cleaned with acids using the etching method in baths. After cooling, we receive high-quality metal utensils without scale, which can be used for cooking or storing any products.
• Cold-rolled steel sheets are also used for the production of corrugated sheets. This material is a thin metal sheet on which an additional zinc layer is applied. Corrugated sheets are used to create fences or fences, as well as for exterior decoration of houses (roofs, external walls, partitions, inclined surfaces). The zinc layer acts as a protective element that prevents contact of the material with the external environment. After all, the profiled sheet, due to the nature of its operation, will often come into contact with precipitation, and zinc will prevent the formation of rust and harmful compounds. If necessary, the corrugated sheet can be made with additional stiffening ribs, which will protect the material from mechanical damage.

Rolling of tool steel

Steel grades X12, X12M, as well as high-speed steel, belong to the ledeburite class. The ledeburite eutectic of this steel melts at low temperatures (about 1200°C), and in high-speed steel at temperatures above 1300°C. During rolling, the ledeburite eutectic is destroyed; in this case, ledeburite carbides behave like foreign inclusions, forming a line structure. For this reason, the heating temperature should not exceed 1140–1160 °C; the temperature at the end of rolling must not be lower than 850 °C.

Ingots of high-chromium tool steel are subjected to forging, although rolling is also possible. The billet for long products is deformed according to the same regime as high-speed steel.

High-chromium tool steels are prone to the formation of quench cracks, so they should be subjected to slow cooling after rolling and forging under the same conditions as high-speed steel.

HVG steel is used for the manufacture of measuring gauges and other precision instruments, since it has the property of little change in its dimensions after heat treatment. Long instruments are also made from it. This steel is sensitive to the stresses generated by heating, which must therefore be carried out quite slowly. This equally applies to the cooling rate, which must be slow to avoid the formation of cracks.

Due to the high carbon content, the heating temperature of this steel should be quite low: the steel is prone to the formation of carbide segregation and a carbide (cementite) network. To prevent the formation of a cementite network, the temperature at the end of rolling should be kept as low as possible with mandatory rapid cooling to 650 °C.

Main properties of cold-rolled products

Advantages of cold rolling:

• the possibility of obtaining a very thin sheet - from a few microns - which is unattainable with hot deformation;
• higher dimensional accuracy;
• good surface quality.

The main disadvantages of cold deformation:

• the need for a wide range of sophisticated equipment;
• a large number of redistributions;
• significant energy consumption, including due to the need for mandatory annealing.

Longitudinal mills

When rolling any profile according to the scheme adopted at the mill, it is necessary to ensure a stable technological rolling process, compliance with the dimensional accuracy of the specified conditions using the minus tolerance field in accordance with GOST, with the delivery of rolled profiles according to theoretical weight.

The stability of the rolling process is characterized by the constancy of the profile dimensions during rolling. The optimal rolling scheme provides for the ease of setting up the mill to obtain this high-precision schedule. At the same time, the roll reinforcement should also be as simple and interchangeable as possible. These requirements are determined by the calibration of profiles and rolls, which determines the optimal rolling scheme. The rolling scheme for groups of profiles or their individual types included in the mill’s assortment is also based on the maximum simplification of mill maintenance.

The production of long products is characterized by an expansion of the product range and an increase in product quality. Section mills are classified according to the following criteria: the diameter of the finishing stand rolling rolls, the location of the stands, the range of rolled profiles, the specification of the mill, the principle of the rolling technological process.

Based on the location of the stands, mills are divided into linear, which are usually used in cases where there is not enough space to accommodate them; with a sequential arrangement of stands on several parallel lines; with a sequential arrangement of stands on the same line.

Section mills are divided into free rolling and continuous mills. Free-rolling mills are those in which the roll during the rolling process is located in only one stand, while in one stand two strips can be rolled simultaneously, for example, in roughing three-roll stands of large-section and rail and beam mills.

Continuous mills are those in which, with a sequential arrangement of stands during the rolling process, the strip is simultaneously located in several stands (at least two). Modern continuous mills are equipped with individual electric motors, the presence of which simplifies the setup of the mill due to the possibility of changing the constant by varying the rotation speed of the rolls in any case. This leads to a reduction in the roll fleet.

To classify sectional mills, round and square steel profiles are conventionally accepted, and then, according to their cross-sectional areas, other profiles are accepted, including shaped ones.

Production technology

From a technological point of view, the production of C/C steel is a labor-intensive process that requires additional equipment, electricity, and skilled workers. Typically, cold-rolled steel is made in special workshops at metallurgical plants, and home production of such materials is almost impossible. For convenience, the production of C/C materials can be divided into three stages - pre-processing, rolling and pressing, and final processing. Below we will look at the steps in more detail.

Initial processing

For the production of cold-rolled steel, hot-rolled sheets are used, the thickness of which ranges from 2 to 10 millimeters. After smelting and processing, hot rolled products undergo a natural cooling stage, and then they are packaged into rolls and supplied to the cold rolling workshop. In the workshop, the material is unpacked and placed on a conveyor table. At the initial stage of processing, hot rolled products are cleaned, as well as the oxide film-scale is removed. This stage is very important, since in the case of rolling a material with scale, the oxide film will be pressed into the material, which will seriously reduce its physical properties, and can also damage the press rolls.

Descaling is carried out using two methods:

• Mechanical cleaning. In this case, scale cleaning is carried out using a stream of metal particles that are sprayed over the entire surface of the sheet under high pressure. For processing large sheets, processing is done using hand tools (the worker puts on a suit, picks up the tool and performs the stripping). In the case of compact products, rolled products can be placed in special chambers, where processing will be carried out automatically.
• Chemical cleaning. In this case, processing is performed using strong acids (hydrochloric, sulfuric, nitric, phosphoric). The processing is carried out in large pickling baths with acids, where rolled steel is placed for a short time. After etching, the metal surface is cleaned of acid residues, and then the rolled sheet is wiped dry and dried.

Assortment

The most common type of rolled products are long profiles. Section steel is divided into large-, medium- and small-grade. Accordingly, rolling mills are divided into large-, medium- and small-section. In table Figure 14 shows the assortment of section mill profiles.

How to determine the mass of rolled products?

In plants or factories, industrial electronic scales are used to determine the weight of cold-rolled strip. The weighing procedure is carried out in a standard way - a strip is placed on the scales, and after a few seconds the weight of the object in kilograms or tons is displayed on the display. There are also indirect methods for determining the weights of sheet metal. The most popular of them is the use of a calculation formula that looks like this: W = P x L x W x H. The formula is deciphered as follows:

• P is the density of the steel product. The density value directly depends on the composition of the metal alloy. Depending on the amount of alloying additives, the density of steel ranges from 7700 to 7900 kg per cubic meter. For convenience of calculations, we will take the average density value - 7800 kilograms per cubic meter (calculation error - less than 3%).
• L (Length) — length of the product. Please note that the length must be taken in meters. If you have a length in centimeters, then this value must be divided by 100.
• W (Width) - the width of the product, expressed in meters. If you have a width value expressed in centimeters, then this value must also be divided by 100.
• H (Height) - sheet height, converted to meters. Height is often indicated in centimeters or millimeters. To convert this value to meters, you need to divide by 100 (centimeters) or 1000 (millimeters).

To understand this formula, let's look at an example. Let us have cold rolling; its length is 500 centimeters, width 100 centimeters, height 25 millimeters. It is necessary to determine its mass, taking the value of 7800 kilograms per cubic meter as the density parameter. To solve the problem, you first need to convert the length/width/height into meters - this will be 5 meters, 1 meter and 0.025 meters, respectively. Now let’s substitute the values ​​into the formula: W = P x L x W x H = 7800 x 5 x 1 x 0.025 = 975 kilograms. Don't forget - this formula is suitable for determining the mass of rectangular sheets. If you have a cold-rolled sheet that has a non-standard shape or strongly rounded edges, then this formula cannot be used.

Differences between hot rolled and cold rolled steel

It is important to note that their main difference is the production process. Hot refers to a process that takes place under high heat, while cold refers to a process that takes place at normal temperatures. While these methods have an impact on performance as well as general application, they should not be confused with the formal specifications and grades of steel that determine metallurgical composition and characteristics. Steels come in a variety of grades and specifications, both hot-rolled and cold-rolled, including basic carbon and other steel alloys.

It may seem obvious, but certain types of steel are more suitable for certain specific applications. Therefore, understanding which one to use will help prevent waste of raw materials. And you can also save money and time spent on additional processing. Understanding the differences between hot and cold rolled steel products makes it easier to choose one or the other.

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Cold rolling

Cold rolling usually refers to the plastic deformation of metal at room temperature. The phenomena that are associated with cold working occur when a metal is deformed at temperatures around 30% and below its melting point on an absolute scale of measurement. During cold working, an increasing number of dislocations appear in the metal. The entanglement of these dislocations causes hardening of the metal - cold hardening or strain hardening. During strain hardening, the strength of the metal increases with increasing deformation.