What GOST 550 Steel 20 Seamless Pipe Is
Seamless pipe made from Steel 20 is one of the standard plain carbon tube products used in Chile and CIS practice for steam, water, gas and general engineering service. Steel 20 is the material definition: it is a low carbon structural steel whose chemical composition and mechanical properties are set out in GOST 1050, and it is the grade against which the pipe's strength and ductility are verified.
The manufacturing route is the same seamless route used for carbon steel tube everywhere: a solid billet is turned into a hollow shell by piercing, the shell is elongated and sized, and the finished tube is heat treated, straightened and tested. What distinguishes a Steel 20 order is the material control at each step, because the grade's low carbon and low alloy content give it good formability and weldability but leave the process parameters to carry the whole burden of achieving the required dimensional accuracy and surface quality.
The Material Basis: Steel 20 to GOST 1050
Steel 20 is an unalloyed structural steel with a nominal carbon content of about one fifth of one percent, which is why it is specified by number rather than by a strength class. A gentle carbon level keeps the steel ductile and weldable while silicon and manganese provide the strength and the deoxidation.
| Element | Typical range to GOST 1050 |
|---|---|
| Carbon | 0.17 to 0.24 percent |
| Silicon | 0.17 to 0.37 percent |
| Manganese | 0.35 to 0.65 percent |
| Phosphorus | 0.030 percent maximum |
| Sulfur | 0.035 percent maximum |
| Chromium, nickel and copper | Residual limits fixed by the standard |
| Property | Typical requirement to GOST 1050 |
|---|---|
| Tensile strength | 410 MPa minimum |
| Yield strength | 245 MPa minimum |
| Elongation | 25 percent minimum |
Because the grade is ductile and weldable, it is used for steam and hot water piping, boiler and heat exchanger tubes in low and medium pressure service, gas and water lines, and general mechanical tubing. Dimensional requirements for the finished tube follow the applicable GOST seamless tube standards for hot deformed and cold deformed product.
The Hot Route: Heating, Piercing and Rolling
The process begins with the selection of steel billets or ingots of the correct grade, which are cut to the required weight and inspected for surface defects. The billet is then heated in a reheat furnace to the temperature range at which the steel becomes forgeable. Heating must be uniform through the section and the soaking time must be long enough to equalise the temperature, because a cold core will not pierce cleanly and a burnt surface will not roll out.
Once the billet has reached the required temperature it is pierced. A piercing mill forces a mandrel through the centre of the heated steel, opening a hollow shell and simultaneously elongating it. The pierced shell is then elongated and formed by hot rolling, using a plug mill, a mandrel mill or an Assel or Diescher type mill depending on the size range and the wall thickness. During rolling the outside diameter is reduced, the wall thickness is brought to the required value and the length is increased. Rolling also refines the grain structure and improves the mechanical properties of the steel.
After rolling, the tube is reheated or held for sizing, passed through a sizing and reducing mill to bring the diameter within tolerance, cooled on a cooling bed, straightened and cut to length. Each of these steps leaves its own mark on the finished product: sizing controls the diameter, straightening controls the bow, and the cooling practice influences the final microstructure.
The Cold Route: Drawing, Annealing and Finishing
Cold drawing is used where tighter tolerances, a thinner wall or a superior surface finish is required. The hot rolled shell or tube is first pickled to remove the oxide scale, then coated with a lubricant carrier so that the drawing lubricant will adhere. The prepared tube is then pulled through a die and over a mandrel at room temperature, which reduces both the diameter and the wall thickness and improves the surface finish and the dimensional accuracy.
Because cold working hardens the steel, the tube must be annealed between drawing passes. The number of passes, the reduction per pass and the annealing schedule are chosen to reach the finished size without exceeding the ductility of the material. After the final pass the tube is straightened, cut, and where necessary given a final bright annealed or pickled finish. Cold drawn tube is normally supplied with closer dimensional tolerance than hot finished product, which is why it is preferred for heat exchanger tubes, instrument lines and precision mechanical applications.
Heat Treatment, Testing and Release
The final stage of production is heat treatment. Normalising, quenching and tempering are used to achieve the required strength, hardness and ductility, and the specific scheme depends on the intended use of the pipe and on the requirements of the purchase specification. For plain carbon Steel 20 tube, normalising is the usual treatment, because it produces a uniform ferrite and pearlite structure with predictable properties and relieves the stresses left by rolling or drawing.
Quality control runs through the whole route rather than sitting at the end of it. It includes chemical verification of the heat, dimensional checks of diameter and wall thickness, visual inspection of the internal and external surfaces, and non-destructive testing such as ultrasonic or eddy current examination to detect internal defects. Hydrostatic pressure testing confirms pressure integrity, and mechanical testing of specimens from the finished tube verifies tensile strength, yield strength and elongation against the grade requirement. Only after these checks are complete is the tube cut to final length, marked with the heat number and released.
FAQ
Q: What is Steel 20?
Steel 20 is a plain carbon structural steel grade specified by GOST 1050, with a carbon content of roughly 0.17 to 0.24 percent and minimum values of 410 MPa tensile strength and 245 MPa yield strength.
Q: Why is the billet heated before piercing?
Heating makes the steel forgeable so that the piercing mandrel can open a clean hollow shell. Uniform through section heating is essential, because a cold core causes piercing defects and an overheated surface will not roll out properly.
Q: What is the difference between hot rolled and cold drawn tube?
Hot rolling shapes the tube at high temperature and is economical for normal sizes and tolerance. Cold drawing pulls the tube through a die at room temperature to achieve tighter tolerance, thinner walls and a better surface finish.
Q: Why is annealing needed between cold drawing passes?
Cold working hardens the steel and reduces its ductility. Annealing restores the ductility so that further reduction is possible without cracking the tube.
Q: What heat treatment is normally applied to Steel 20 tube?
Normalising is the usual treatment. It produces a uniform ferrite and pearlite structure, gives predictable strength and ductility, and relieves the internal stresses introduced by rolling or drawing.
Q: What tests are carried out before the tube is released?
Chemical verification of the heat, dimensional inspection of diameter and wall thickness, visual examination of the surfaces, ultrasonic or eddy current testing, a hydrostatic pressure test and mechanical testing for tensile strength, yield strength and elongation.
