Hot and cold rolling. Cold rolled and hot rolled sheet: what is the difference and which is better

For high-speed steel cylinders, the maximum values ​​of normal thermal stresses are located on the surface of the cylinder, and the thermal shear stresses are maximum at the interface. In the case of high-speed steel cylinders, normal thermal stresses have a maximum on the outer surface. On the other hand, tensile stresses are maximum at the interface. The shear stress values ​​are negligible compared to normal thermal stresses.

For normal thermal stresses along the axis, the maximum values ​​of tension and compression are located on the active part of the cylinder. The extreme values ​​of thermal shear stresses are located on the boundary part related to the envelope. Thus, it can be concluded that at the level of the surface part, the risk of worsening thermal fatigue is high.

The maximum values ​​of the obtained thermal stresses are those that are directed along the axis. For the four types of materials used in the manufacture of hot rolling rolls, the areas in which normal thermal stresses are extreme are the outer surface and interface, and shear stresses are negligible.

In most of the cases considered, the normal thermal stresses along the axis are much greater than the stresses with respect to the axis. We conclude that the thermal fatigue behavior of these cylinders is constrained. For the remaining three materials, there is little difference in the change in thermal stresses.

The extreme values ​​of thermal stresses on the near part of the interface are small compared to those measured on the surface. Measured on each material, as well as their normalized values ​​in relation to the maximum tensile stresses in tension. We have also determined the Eickelberg factor, which will determine the limits of coiled roll cracking.

From the numerical analysis of thermal stresses in hot rolling cylinders, we can conclude the following. For the four materials used; The concentration of thermal stresses is observed in two zones of the cylinder, which are the outer surface of the cylinder and the boundary between the core and the shell.

:. Of the four materials used; only high speed steel cylinders generate low normalized normal stresses. Comparing the Eikelberg coefficients of the four materials used, only high-speed steel cylinders have the best resistance to thermal cracking.

On the manufacturing enterprises finishing devices have been developed that bring the steel to a perfect surface condition for its use and sale. In the first stage, at the plant, plates, blanks and flowers are obtained from the steelmaking process by continuous casting. These semi-finished products pass through the rolling mill to the required size. It is the raw material for making sheets and cartridges, which are finished products for sale in the market.

The cold drawing operation consists in passing a rod from a rolled, hot-rolled state into a mold, reducing its diameter in order to stretch it and obtain a rod with rigid dimensions. His appearance modified with smooth surface. Its mechanical characteristics are also changed by this stretching. Hardening is the most obvious change mechanical properties. The bars are straightened so that they are as straight as possible. At the end of the draw line are checked, weighed, ribbed, marked and shipped.

Examples: Stainless steels are often stretched to a diameter. Rolled or rolled products from crushed steels. The peeling operation is a chip cleaning operation that aims to remove its surface layer from steel in a hot rolling state. There are two purposes for fastening.

To obtain a surface finish without skin defects resulting from casting or rolling of the product. obtaining geometric and dimensional characteristics that exceed the geometric and dimensional characteristics of raw materials. Thus, the peeling operation is a sizing operation that aims to reduce ovalization defects that may result from the rolling operation.

Before crushing, bars are first installed. They pass through lathe for peeling, where the bar develops without turning inside a rotating head containing several cutting tools. They are then crushed by passing through a roller chest to improve their straightness. This operation aims to reduce ovalization and waviness resulting from peeling.

In addition, the waves, which are stripes on the rods, are a means of recognizing this production process. Unlike tension, this method does not cause hardening of the surface, but can cause repeated howls, which make the rods less permanent than tension, but leave the material free of impurities or surface defects.

Grinding is a machining operation designed to remove material by grinding to obtain a surface finish with very hard dimensions. We are talking about cylindrical rectification between points or without centering and rectification on flat surfaces.

Cortized steels supplied with transport protection packaging are often protected on the surface with paper or oil that coats them. This provides effective protection from corrosion. It is made with a so-called rolling mill, a device consisting of at least two very strong iron or steel cylinders arranged horizontally on parallel axes, contained in fixed fittings called cages, which rotate in the opposite direction from the tractor.

If a relatively intermediate material is introduced into the space between the two cylinders, such as hot iron, it will be frictionally dragged and forced to take the shape of the space between the two cylinders. The oldest design of the mill is by Leonardo da Vinci, and therefore it is worth recognizing that he first identified the importance of this tool for metalworking: it is a rolling mill for tin plates, starting from dishes about 30 cm wide, with hard bronze cylinders that have an "iron soul" .

As can be seen from the figure, Leonardo also had a principle that was practically applied only in last years, that is, a laminate with four overlapping cylinders, but only two plants will be used to transfer the material, while the two outer ones will serve to withstand the forces.

First practical use rolling mills were made for metals other than iron, and especially for lead, pond, etc. Which could be easily deformed even in cold weather. Others follow animals or hydraulic power. Later, too, the cutting of sheet metal was carried out by a special mill, in which the cylinders, not planes, consisted of a series of chess discs that were joined together: the sheet was inserted between these discs.

At that time, the mills were improved. For smooth cylinders intended only for the manufacture of sheets, shaped cylinders have been replaced, capable of producing pieces of the most different forms. Lamination of ferrous metals. Hot lamination. - Carried out in accordance with the general procedure mentioned above. The mill rollers carry grooves dug in the normal direction to their longitudinal axis, reproducing the contour you want. Under pressure it comes to practice, it occupies exactly the entire space it has to pass through, and it also spreads.

After repeated passes and subsequent transformations of the sections, it takes its final form. As can be seen, the first purpose of laminating an ingot is to reduce its section and simultaneously increase its length in order to obtain a certain profile faster and more accurately than would be possible with forging under a hammock. The two operations are actually similar, but when digging, the pressure applied to the rod and its progression is intermittent, the pressure and feed are continuous and constant.

The smaller the shape of the mold that forces the iron into forging or cylinder diameter when rolling, the greater the bar deformation pressure. The more acute a fencer's tool is and the smaller the cylinders, the more horizontal component that can be used to lengthen the rod in terms of its length and the less power needed to change the section. Therefore, small and fast cylinders contribute to elongation, large and with a small expansion rate. When laminating, the elongation and expansion to which the rod is subjected must be adjusted accordingly, since if the elongation is excessive, the iron does not fill the entire molded channel, if in a defect it tends to cause transverse bursts.

Cylinder sizes as their resistance transverse deformation varies in proportion to the diameter of the cube, they must be remarkable, because the forces they are forced to withstand, and sometimes more than expected, for the passage of a cold ingot. In addition, the pressure value that can be given to the bar to deform it depends on the size of the cylinders and therefore, depending on their diameter, can provide more or less economy in the number of steps with which a side can obtain a particular profile.

Also, when two cylinders cannot move in the vertical direction, the rod thickness should not be more than half the distance between the two centers. Increasing the diameter of the cylinders increases their peripheral speed and the higher the abrasion stress acting on the rod. The more the laminate is thin and the longer it must be at its speed to avoid excessive cooling.

The average speed, expressed in meters per second, for non-reversible rolling mills varies within the following limits as the cylinder diameter changes. In continuous rolling mills and automatic rolling mills, the speed can be even higher because the iron in the individual steps is automatically set in motion.

At each pass in the channels, the rod, due to the transverse pressure exerted on it, decreases in length and elongates, but this reduction is a percentage of previous section cannot exceed a certain limit, which ranges from 20 to 30% in the first channels. In the last cell, and especially in the last channel, this value does not exceed 15%, because in this you want to get contour accuracy, and then try not to suppress excess cylinders. It is good practice that the initial portion of the ingot is at least 10 times larger than that of the finished laminate so that the raw material can be compressed and machined to give the laminate the required mechanical properties.