Method of thermomechanical treatment of pipes

FIELD: metallurgy; hardening of oil grade pipes made from carbon steel and low-alloy steel (Nb, V, Mo and Cr) directly in the course of hot deformation.

SUBSTANCE: proposed method includes preliminary deformation of pipes, holding them in air, heating to 800-870 C for final deformation and accelerated adjustable cooling to 720-760 C performed during repeated hot deformation in compressed-to-solid state at average rate of cooling of 40-60 C/s in center of deformation and 20-30 C/s during inter-deformation intervals.

EFFECT: possibility of obtaining pipes of required geometric parameters in ovality and linearity at enhanced strength, ductility and heat resistance of steel.

1 tbl

 

The invention is directed to improving the technology of hardening of oil country tubular goods from carbon and micro-Mb, V, Mo and CR steels during hot deformation.

The known method of heat treatment of pipes made from carbon and low-alloy steels, namely, that the pipe is cooled by water at the exit from the last stand of the mill, while cooling the outer surface start from 800-840°C for 3-5 sec with an average speed of 30-40°C/C for 6-10 cycles, the duration of intensive cooling cycle is 0.2-0.3 s with pauses between cycles of 0.15-0.2 (patent RF №2112052, MCL 21 D 9/06, publ. 27.05.1998,).

The disadvantage of this method is that it is unsuitable for hardening of tubes made from steels, micro-strong carbidopa elements, as after intensive cooling in the final structure is formed, the upper beinit. This leads to a strong hardening, but at the same time sharply reduced plastic characteristics and impact strength.

The closest analogue to the claimed invention to the technical essence and the achieved result is a method of manufacturing tubes of the micro-V and/or Nb steel, on which the pipe after the preliminary hot deformation is subjected to cooling for 55-60 C to 735-770°C, heat is under final deformation is carried out to 800-850° From and after deformation provide cooling water for 1.5-2 with an average speed of 20-25°C/C with subsequent cooling (patent RF №2163643, MCL 21 D 8/10. publ. 27.02.2001,).

The disadvantage of this method is that, as shown, in the manufacture of pipes, especially with a diameter of 73 mm, the cooling water after deformation is accompanied by the appearance of ovality, limit curvature and deviation from linearity above are valid for threaded pipe standards GOST 633-80, API5CT and HARD. Pipe with a diameter of 89 mm marriage ovality reaches 100%, and the marriage of curvature depending on the range is 5-25%. In addition, when microregion molybdenum and especially complex MB+Mo and Nb+Cr dramatically increases the stability of austenite in the structure appear significant amounts of upper bainite, which leads to embrittlement of the steel.

The present invention is to develop a method of thermo-mechanical processing of oil country tubular goods from micro-niobium, vanadium, molybdenum and chromium steels, which along with high strength, ductility and brittle fracture of steel ensure receipt of required regulatory documents of the geometric parameters of pipe ovality and straightness.

The problem is solved in that way thermomechanical is some processing of pipes, including pre-strain, exposure to air, heat, the final deformation and accelerated cooling, according to the invention, the heating under the final deformation is carried out to 800-870°and accelerated cooling to a temperature 720-760°produced in the process of multiple hot deformation in benevolent condition with average cooling rate in the deformation zone 40-60°C/C, while mergeformatinet pauses 20-30°With a/C. Under these conditions, hot deformation mill is not only distorting, but cooling device, thanks to technological control structure and properties of pipes substantially increase. During multiple finite deformation of the pipe is cooled to a temperature 720-760°in benevolent using the reducing rolls of the mill condition that eliminates ovalization and the curvature of the pipe. The rate of cooling tubes within the specified limits is governed by the intensity of the cooling rolls, the flow and pressure of water directed on the pipe. In the deformation of the tube are cooled by contact with cooled rollers, the streams of water that come to the surface after cooling rolls, and water flows from established stands of sprayers, and during mergeformatinet pauses tubes are cooled with water only.

Before the final deformation with increased speed in the first cooling pipe is heated to a temperature 800-870° C. Increasing the upper boundary of the temperature range, in comparison with the prototype, caused by the expansion of the composition of the steels used. This heating under final deformation generated by pre-deformation and during subsequent exposure of stable particles of carbides and carbonitrides Mb dissolve and inhibit the growth of austenite grains.

In the accelerated cooling multiple hot deformation occurs when descending at each subsequent stage of the temperature and the temperature of the end of the deformation is 720-760°C. Deformation initiates the trend Nb to form the selection already in the austenitic region, so its role in the process of structure formation is limited to the grinding of grain. In the solid solution of microadditives V, Mo and CR decrease the speed of diffusion redistribution of carbon, reduce the temperature YYY-AAA transformation, so despite the grinding of grain, which reduces the stability of the austenite, zaevtektoidnyh ferrite deformation is not produced.

Cooling during deformation to a temperature below 720°results in increased loads on the rental equipment above the permissible and increases the wear of the rolls. When the temperature of the end of deformation above 760°With decreasing particle size of austenitic grains, in addition when you follow the eat the cooling air is formed up to 20% commecting ferrite.

The cooling rate of 40-60°C/C in the deformation zone and 20-30°C/C during mergeformatinet pauses interrelated and consistent with the process rolling speed so that the temperature of the end of the warp pipes of all sizes was 720-760°C.

Lowering the cooling rate below the specified limits will require to achieve the desired temperature of the end of the deformation reducing the rolling speed, and hence the performance of the mill. Increasing the cooling rate is impractical because it is in process of rolling speed will reduce the temperature of the end of deformation below 720°C.

The proposed parameters of the accelerated cooling during hot deformation allow along with the required accuracy of the geometric parameters to obtain a favorable combination of properties of pipes made of micro steel, there is provided a solution in the invention of the task.

The proposed method of thermo-mechanical processing of tubes is carried out as follows.

Pipe-blank after preliminary deformation in a continuous rolling mill and air cooling to 735-770°With heat in the induction units to 800-870°C. Final deformation in mnogoletija the reducing mill is made simultaneously with accelerated cooling the pipes in benevolent condition, which is achieved by squeezing the pipe rolls. Cooling is carried out by contact with the cooled surface of the rolls, the streams of water that fall on the surface of the pipe after cooling rolls and streams of water directed through the sprayers on the pipe. The rate of cooling pipes is governed by the intensity of the cooling rolls themselves, pressure and specific volume of water directed on the pipe. The cooling pipes to a temperature 720-760°with an average speed in the deformation zone 40-60°C/C, and during mergeformatinet pauses 20-30°With a/C.

The proposed and known method were tested in the line of mill -80 Pipe rolling mill 3 No. of OAO “Sinarsky pipe plant” in the manufacture of tubing size 60×5.0 mm; 73×5.5 mm and 89×6.0 mm strength group "E" according to GOST 632-80. In the stands of the reducing mill, equipped with a cooling system for rolls were installed ring sprayer for additional cooling of the pipes during hot deformation. Water consumption for the cooling rolls was 30-60 m3per hour and 30-60 m3/h on the sprayer. The flow of water and the quantity of cooling stands were selected so that the average cooling rate in the deformation zone, regardless of the mix was 50-55°With a/C and 20-25°C/C during mergeformatinet pauses, and tempera is ur pipe after leaving the mill 730-740° C.

The results of the pilot pochatok of steel pipes shown in the table, showed that the proposed technical solution can be obtained mechanical properties strength group "E" and geometrical parameters on ovality and straightness that meets the requirements of GOST 633-80. In the manufacture of pipes of the prototype cooling after deformation also achieved the required level of properties, but the percentage of marriage ovalization on pipes with a diameter of 89 mm reaches 100%, and on pipes with a diameter of 60 mm, with the lowest longitudinal stability, the marriage of the end curvature is 25-30%. Thus, the proposed method of thermomechanical processing allows to obtain unavailable when processing the prototype level of precision tubes according to the geometric parameters.

Table
MethodCoolingPipe diameter, mmSteel gradeThe content of elements, %Mechanical propertiesMarriage geometric size %
    MuCrNbMoσinkg/m the σt

kg/mm
δ5, %OvalityLeaf curvatureDeviations from straightness
DeclareThe cooling process is in fact deformation

tion in reducti

concentration camp
89GB0,451,25 0,0380,0893,566,522,02-32-32-3
  73

60
GB0,380,660,590,04-90,7

91,2
61,2

60,8
22,5

23,0
2-32-32-3
  73

60
GS0.361,42---81,2

82,5
54,2

56,8
23,5

24,0
2-32-32-3
The placeholderThe cooling

of water after the fact deformation

tion in reducti

concentration camp
89GB0,450,82 -0,0380,0894,668,921,01005-105-10
  73

60
GB0,380,660,590,04-89,3

91,5
59,4

62,8
22,0

20,3
3-5

2-3
10-12

25-30
7-10

10-12
  73

60
GS0,361,42---79,3

81,5
52,4

55,8
23,0

22,5
3-5

2-3
10-12

25-30
7-10

10-12

The method of thermo-mechanical processing of tubes, including pre-strain, exposure to air, heat, the final deformation and accelerated controlled cooling, characterized in that the heating under the final deformation is carried out to 800-870°and accelerated cooling to a temperature 720-760°produced in the process of multiple hot deformation in benevolent condition with average cooling rate of 40-60°C/C in the deformation zone and 20-30°C/C during mergeformatinet PAH is.



 

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