RU2245375C1 - Method of thermomechanical treatment of pipes - Google Patents

Abstract

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.

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Description

The invention is aimed at improving the technology of hardening oil-grade tubes of carbon and microalloyed Mb, V, Mo and Cr steels directly in the process of hot deformation.

A known method of heat treatment of pipes made of carbon and low alloy steels, which consists in the fact that the pipe is cooled with water at the exit of the last mill stand, while the cooling of the outer surface begins with 800-840 ° C for 3-5 s with an average speed of 30-40 ° C / s for 6-10 cycles, the duration of intensive cooling in the cycle is 0.2-0.3 s with pauses between cycles of 0.15-0.2 s (RF patent No. 2112052, M.cl. C 21 D 9 / 06, published on May 27, 1998).

The disadvantage of this method is that it is unsuitable for hardening pipes made of steels microalloyed by strong carbide-forming elements, since after intensive cooling in the final structure of steel upper bainite is formed. This leads to strong hardening, but at the same time the plastic characteristics and toughness are sharply reduced.

The closest analogue to the claimed invention in technical essence and the achieved result is a method of manufacturing pipes from microalloyed V and / or Nb steel, according to which the pipes after preliminary hot deformation are subjected to air cooling for 55-60 s to 735-770 ° C, heating under the final deformation, they lead to 800-850 ° C, and after deformation, water is cooled for 1.5-2 s at an average speed of 20-25 ° C / s with further cooling in air (RF patent No. 2163643, M.cl. C 21 D 8/10. Published on February 27, 2001).

The disadvantage of this method is that, as practice has shown, in the manufacture of pipes, especially with a diameter of more than 73 mm, water cooling after deformation is accompanied by the appearance of ovality, end curvature and deviation from straightness are higher than the norms allowed for threaded pipes according to GOST 633-80, API5CT and AR15D. On pipes with a diameter of 89 mm, the rejection by ovality reaches 100%, and the rejection by curvature, depending on the assortment, is 5-25%. In addition, during microalloying with molybdenum and especially the complex of Mb + Mo and Nb + Cr, the stability of austenite sharply increases and significant volumes of upper bainite appear in the structure, which leads to steel embrittlement.

The objective of the present invention is to develop a method for thermomechanical processing of oil-grade tubes made of micro-alloyed niobium, vanadium, molybdenum and chromium steels, which simultaneously with increasing the strength, ductility and cold resistance of steel provides the required geometric documents for pipe geometry in ovality and straightness.

The problem is solved in that in the method of thermomechanical treatment of pipes, including pre-deformation, exposure to air, heating, final deformation and accelerated cooling, according to the invention, heating under the final deformation is carried out to 800-870 ° C, and accelerated cooling to a temperature of 720-760 ° C is produced in the process of repeated hot deformation in a non-stressed state with an average cooling rate in the deformation zone of 40-60 ° C / s, during interdeformation pauses of 20-30 ° C / s. Under such conditions of hot deformation, the mill is not only a deforming, but also a cooling device, due to which the technological capabilities of controlling the structure and properties of pipes significantly increase. During multiple final deformation, the pipes are cooled to a temperature of 720-760 ° C in a state that the rolls of the reduction mill are in a state of disuse, which eliminates the ovalization and bending of the pipes. The cooling rate of the pipes within the specified limits is regulated by the intensity of the cooling of the rolls, the flow rate and pressure of water directed to the pipe. In the deformation zone, the pipes are cooled by contact with cooled rolls, water flows that come to its surface after cooling of the rolls, and water flows from the sprayers installed in the stands, and during interdeformation breaks, the pipes are cooled only by water.

Before the final deformation with accelerated cooling, the pipes are heated to a temperature of 800-870 ° C. The increase in the upper limit of the temperature range, compared with the prototype, is caused by the expansion of the composition of the used steels. With this heating under the final deformation, stable particles of Mb carbides and carbonitrides released during preliminary deformation and during subsequent exposure do not dissolve and inhibit the growth of austenite grains.

As a result of accelerated cooling, multiple hot deformation occurs at temperatures decreasing at each subsequent stage and the temperature of the end of the deformation is 720-760 ° C. Deformation initiates the tendency of Nb to form precipitates already in the austenitic region; therefore, its role in the process of structure formation is limited by grain refinement. The microadditives V, Mo, and Cr in the solid solution decrease the rate of diffusion redistribution of carbon and the temperature of the YYY-AAA transformation, therefore, despite grain refinement, which reduces the stability of austenite, hypereutectoid ferrite does not form upon deformation.

Cooling in the process of deformation to a temperature below 720 ° C leads to an increase in the load on the rolling equipment above the permissible and increased wear of the rolls. With an increase in the temperature of the end of deformation above 760 ° C, the degree of grinding of austenitic grains decreases, in addition, with subsequent cooling in air, up to 20% of hypereutectoid ferrite is formed.

The cooling rate of 40-60 ° C / s in the deformation zone and 20-30 ° C / s during the inter-deformation pauses are interconnected and are consistent with the technological rolling speed so that the temperature of the end of the deformation of the pipes of the entire assortment is 720-760 ° C.

Lowering the cooling rate below the specified limits will require a reduction in rolling speed and, consequently, a decrease in mill productivity, in order to achieve the required temperature at the end of the deformation. An increase in the cooling rate is impractical, since this at a technological rolling speed will lead to a decrease in the temperature of the end of deformation below 720 ° C.

The proposed parameters of accelerated cooling during hot deformation allow, along with the required accuracy of geometric parameters, to obtain a favorable set of properties of pipes from microalloyed steel, that is, a solution to the problem posed in the invention is provided.

The proposed method of thermomechanical processing of pipes is as follows.

The billet pipe after preliminary deformation in a continuous mill and cooling in air to 735-770 ° C is heated in induction plants to 800-870 ° C. Final deformation in a multi-stand reduction mill is carried out simultaneously with accelerated cooling of the pipes in a non-worn state, which is achieved with the help of a tube compressing rolls. The cooling is carried out by contact with the surface of the cooled rolls, water flows that fall on the surface of the pipe after cooling the rolls and water flows directed by means of sprayers to the pipe. The cooling rate of the pipe is controlled by the cooling rate of the rolls themselves, the pressure and the specific volume of water directed to the pipe. The pipes are cooled to a temperature of 720-760 ° C with an average speed in the deformation zone of 40-60 ° C / s, and during inter-deformation pauses of 20-30 ° C / s.

The proposed and known method was tested in the line of the mill TPA-80 Pipe rolling shop 3 No. OAO “Sinarsky Pipe Plant” in the manufacture of tubing 60 × 5.0 mm; 73 × 5.5 mm and 89 × 6.0 mm of strength group "E" according to GOST 632-80. In the stands of the reduction mill equipped with a roll cooling system, annular sprayers were installed for additional cooling of the pipes during hot deformation. The water consumption for the roll cooling system was 30-60 m ³ / h and 30-60 m ³ / h for the sprayer. Water consumption and the number of cooling stands were selected so that the average cooling rate in the deformation zone, regardless of the assortment, was 50-55 ° C / s and 20-25 ° C / s during interdeformation pauses, and the temperature of the pipes after leaving the mill was 730- 740 ° C.

The results of the experimental rolling of steel pipes, given in the table, showed that according to the proposed technical solution, it is possible to obtain the mechanical properties of the strength group "E" and the geometric parameters for ovality and straightness, satisfying the requirements of GOST 633-80. In the production of pipes according to the prototype with cooling after deformation, the required level of properties is also achieved, but the percentage of ovalization rejects on pipes with a diameter of 89 mm reaches 100%, and for pipes with a diameter of 60 mm having the lowest longitudinal stability, the rejection by end curvature is 25- thirty%. Thus, the proposed method of thermomechanical processing allows you to get unattainable when processing according to the prototype level of accuracy of pipes in geometric parameters.

Table Way Type of cooling Pipe diameter mm Steel grades The content of elements,% Mechanical properties Marriage by geometric dimensions,% FROM Mu Cr Nb Mo σ _in , kg / mm σ_t
kg / mm δ ₅ ,% Ovality End curvature Deviations from straightness The claimed Deformation cooling
tion in reduction
onnoy camp 89 48G2B 0.45 1.25 0,038 0.08 93.5 66.5 22.0 2-3 2-3 2-3 73
60 37HGB 0.38 0.66 0.59 0.04 - 90.7
91.2 61.2
60.8 22.5
23.0 2-3 2-3 2-3 73
60 37G2S 0.36 1.42 - - - 81.2
82.5 54,2
56.8 23.5
24.0 2-3 2-3 2-3 Prototype Chilled
water after deformation
tion in reduction
onnoy camp 89 48G2B 0.45 0.82 - 0,038 0.08 94.6 68.9 21.0 100 5-10 5-10 73
60 37HGB 0.38 0.66 0.59 0.04 - 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 37G2S 0.36 1.42 - - - 79.3
81.5 52,4
55.8 23.0
22.5 3-5
2-3 10-12
25-30 7-10
10-12

Claims (1)

A method of thermomechanical processing of pipes, including pre-deformation, exposure to air, heating, final deformation and accelerated controlled cooling, characterized in that the heating under the final deformation is carried out to 800-870 ° C, and accelerated cooling to a temperature of 720-760 ° C is carried out in the process of repeated hot deformation in a non-stressed state with an average cooling rate of 40-60 ° C / s in the deformation zone and 20-30 ° C / s during inter-deformation pauses.