Pure iron is characterized by a limited list of properties and as a base metal is not of great interest. But alloys based on it have enormous potential, you only need to determine the chemical composition and make the correct heat treatment.
The most common structural steels
All iron-based steels belong to ferrous metallurgy and have numerous classification. It is produced according to various parameters: chemical composition, purpose, content of harmful elements, strength and toughness, flexibility and many others. Structural - have become the most common in use. Some of them have universal properties and interchangeability.
Structural steel 20 belongs to the medium carbon class, has a ferrite-pearlite structure. High-quality steel, i.e., has a reduced content of harmful elements: sulfur and phosphorus. No weldability restrictions. The optimal combination of strength and ductility makes it simply a universal material for the production of rolled tubes, parts subjected to subsequent thermomechanical and thermochemical processing (cementing, galvanizing and chromium plating).
Twenty found its application
Steel 20, the properties of which can vary to a large extent with the help of chemical-thermal, thermomechanical processing, is most in demand in pipe production in the manufacture of parts with a hard surface and a soft middle. This can be shafts, sprockets, gears, bolts, crane hooks, fittings, stamping sheets (corrugated board), nuts and bolts for non-responsible fastening. Pipes made from this steel grade are used to transfer gases, steam, non-aggressive liquids that are supplied under pressure. These are pipes of superheaters, pipelines, high-pressure boilers and collectors.
Change in structure by thermochemical treatment
One and the same brand can change its characteristics by heat treatment. Steel grade 20 has good plastic characteristics, therefore, products from it are obtained by several methods: casting, cold or hot rolling or drawing. After receiving the parts by casting, chemical-thermal treatment can be applied to them. The purpose of this procedure is to obtain a hard wear-resistant layer that is not amenable to corrosion, and a plastic soft middle.
For this, the finished part is placed in an appropriate medium (coated with a dry carbon-containing substance, placed in a gaseous or liquid medium), after which it is aged from several hours to 1.5 days at high temperature. The mechanical processing of parts by this moment should be completed, since after thermochemical processing the product will already have the final structure. The element saturates the top layer of the product (from 0.3 to 3.0 mm), respectively improving its structure and properties.
Depending on the substance used, processing is called: cyanidation (zinc coating), cementation (carbon), chromium plating (chromium). Carbon gives strength, zinc - corrosion resistance, chrome, in addition to all of the above, makes the surface mirror.
Change of structure by machining
Unlike the previous processing method, which is carried out exclusively to give hardness in the upper layer of metal and flexibility in the inner, thermomechanical processing is one of the methods of shaping. Steel 20 It can be deformed both hot and cold. Each type has its advantages and disadvantages. But they are used based on the most needed properties.
Hot deformation is applied to products having a wall thickness of more than 5 mm. Since when the metal is heated, scale and decarburized microlayer are formed (undesirable structure), it is impractical to use this type of rolling for thin-walled parts. However, it has one big advantage over cold deformation.
Cold forming is applied to parts having a thickness of less than 5 mm. For cold drawing only “soft” types of steel are suitable. During rolling, the metal experiences significant deformation, or hardening. This leads to an increase in its strength and the presence of large stresses in the structure. Due to its thin walls, such a metal cannot be heated (tempering, i.e., restoring the previous structure). It is more susceptible to destruction by impacts and other dynamic loads. Structural steel pipe (steel 20) differs by the manufacturing methods and the resulting technical characteristics that affect the application. For the production of each type of pipe, there are GOSTs, norms, equipment.
Straight seam cold rolled tubes
The production process begins with the preparation of a steel strip. For this, steel sheets are cut into strips and welded into one long tape. The tape is fed to the bending rolls, where it takes the form of a pipe. The next step is welding. For any design, this is the weakest point. To eliminate the shortcomings that arise during welding (the appearance of oxides and carbon burnout) is completely impossible, but using some methods, they can be reduced. To connect steel 20, electric arc welding in a protective medium of inert gas (argon) or induction welding (high-frequency currents) is used. The pipe undergoes a mandatory inspection of the weld, after which it is cut into parts of the desired length and stored.
Cold drawn spiral welded tubes
The preparation of steel for the production of this type of pipe repeats the same process as for pipes with a direct view of the seam. Also identical: welding, inspection and trimming. Only the angle of folding of the tape is different, at which the subsequent seam bends around the pipe in a spiral curve. Due to its design features, this method is the most durable. And withstands greater tensile loads than these products with a straight seam.
Seamless pipes
Seamless pipes are particularly durable, have several advantages: they do not have welded (weak points), there are no stresses in the steel structure, the thickness of the pipes is at least 5 mm. Their production is a more complex process, and therefore expensive. Steel 20 is unique in that the pipes can be manufactured in two ways - cold and hot drawing.
Hot rolled seamless
After warming up above 1100º, the workpiece is stitched with a sleeve and forms an inner diameter. With further drawing, the pipe assumes the specified dimensions of the inner, outer diameter and wall thickness. Throughout the entire process, the temperature of the rolled stock remains high. And only after taking the final form, the tube cools. During long-term cooling, vacation takes place, all the negative effects of rolling, increased strength and brittleness are removed. With full cooling, steel 20 acquires the characteristics that were originally available. This process involves the manufacture of only pipes with walls of at least 5 mm, and the maximum thickness can reach 75 mm.
Cold drawn seamless
Unlike the previous method, this observes a small temperature nuance. The workpiece heats up, but after the initial flushing with the sleeve, the temperature is not maintained, and the workpiece is pulled in a cold state. This method differs from the hot-rolled method in that it is possible to produce strong pipes with thin walls, whereas with the hot-rolled method only thick walls are provided. For the final structure, these two methods are identical, since after cold rolling the pipes undergo normalization, in which the structure is partially restored, and stresses go away.
This is not the whole list of products based on steel 20 GOST 1050-74. The needs of the population are increasing, new ideas and production are emerging. But this brand only changes its form and purpose, leaving the right to exist.