RU2270873C1 - Method of production of skelp steel for pipes of underwater sea gas pipe lines of high parameters - Google Patents

Abstract

FIELD: metallurgy; production of low-alloyed cold-resistant steel for underwater sea gas lines at working pressure up to 19 Mpa working at low temperatures.

SUBSTANCE: proposed method includes production of steel blank, heating it to temperature above A_s3, deformation in controllable mode at specific reduction processes and at total reduction of 50-60% followed by controllable cooling; proposed steel has the following composition, mass-%: carbon, 0.05-0.9; manganese, 1.25-1.6; silicon, 0.15-0.30; chromium, 0.01-0.1; nickel, 0.3-0.6; molybdenum, 0.10-0.25; vanadium, 0.03-0.10; aluminum, 0.02-0.05; niobium, 0.01-0.06; copper, 0.2-0.4; calcium, 0.001-0.005; sulfur, 0.0005-0.005; phosphorus, 0.005-0.015; the remainder being iron; preliminary deformation of blanks is performed at temperature of 950-850°C at total reduction of 50-60%; then, blank is cooled down to temperature of 820-760°C at rate of cooling of 15°C/s on controllable cooling unit and final deformation is performed additionally at temperature of 770-740°C to required thickness of skelp at total reduction of 60-76%; further cooling is performed at higher rate of 35-55°C/s to temperature of 530-350°C, after which skelp is cooled in jacket to temperature of 150±20°C and then in the air. New stage of the proposed method makes it possible to manufacture tubes of 1067-1420 mm in diameter at thickness of walls of 24-40 mm which are used for sea gas pipe lines working at pressure of up to 19 Mpa.

EFFECT: enhanced strength, ductility and cold resistance; enhanced operational reliability; increased service life.

2 tbl, 1 ex

Description

The invention relates to metallurgy, and more particularly to the production of strip steel for offshore gas pipes with a diameter of 1067-1420 mm with a working pressure of up to 19 MPa, strength class K60.

Currently, the need for large diameter pipes for subsea pipelines has increased in connection with the development of new deposits located in areas with severe climatic conditions.

Ensuring the construction of pipelines by Russian-made pipes for underwater offshore gas pipelines of high parameters, not inferior in quality to foreign analogues, is one of the most important promising areas.

The 16G2SAF strip steel, which is close in manufacturing technology, is widely used according to TU 14-1-1950-89 using normalization, normalization and tempering, quenching and tempering, as well as controlled rolling. This steel contains a mass fraction of elements of the following composition: carbon 0.15-0.20%; manganese 1.3-1.6%; silicon 0.3-0.5%; chromium - not more than 0.3%; vanadium 0.08-0.12%, nitrogen 0.015-0.025%, sulfur - not more than 0.02%, phosphorus - not more than 0.02%, iron - the rest. Steel has the following mechanical properties: tensile strength ≥590, yield strength ≥410 MPa, elongation ≥22% and KCU at -40 ° С - 59 J.

However, strip steel 16G2SAF of strength category K60 is produced in thicknesses of 8.5-10.5 mm, which ensures operation at a working pressure in pipelines of not more than 7 MPa, and is intended for the production of pipes for onshore trunk pipelines.

Another disadvantage of this strip steel is the impossibility of using it for subsea pipelines and the failure to provide requirements for large-diameter pipes approved by Gazprom for crack resistance:

To meet the requirements for pipes for underwater gas pipelines, namely strength category K60, with a thickness of 24-40 mm to provide a working pressure in the gas pipeline of 19 MPa and Pcm below 0.22%, a method for producing strip steel using the thermomechanical processing method is required.

Closest to the manufacturing technology is a method for the production of sheet metal from steel of the following chemical composition (wt.%) Carbon - 0.05-0.15; Manganese - 1.2-2.0; silicon - 0.2-0.6; niobium - 0.01-0.10; titanium - 0.005-0.03; aluminum - 0.01-0.10; chrome 0.03-0.50; nickel - 0.03-0.50; copper - 0.03-0.50; nitrogen - 0.005-0.020, iron - the rest using the method of thermomechanical processing (RF patent 2062795, class C 21 D 9/46, 8/02, 1995 - prototype), which consists in obtaining a sheet blank, its austenization, deformation with a total the degree of compression of 50-80% to a thickness of 14 mm, cooling from a temperature of the end of deformation of 760-900 ° C at a speed of 10-60 ° C / s to a temperature of 300-20 ° C, in re-heating to a temperature of 590-740 ° C with exposure 0.2-3.0 min / mm and final cooling in air.

The technical result of the invention is the development of a method for the production of strip steel in the thickness of 24-40 mm while providing improved weldability SECv ≤0.44% and requirements for large diameter pipes with a fracture toughness coefficient Pcm≤0.22%, as well as requirements ensuring the manufacturability of pipes, defined by the ratio σ of _0.2 / σ _in ≤0.90, with a guaranteed temporary resistance of more than 565 MPa, yield strength of more than 482 MPa and KV ^{-40 of} more than 50 J.

The technical result is achieved by the fact that in the method of producing strip steel for pipes of high-pressure subsea gas pipelines, including the preparation of a steel billet, heating to a temperature above Ac ₃ , its deformation in a controlled mode with partial reductions with a total reduction of 50-60% and the subsequent controlled cooling, according to the invention, the preform is obtained from steel of the following composition, wt.%:

Carbon 0.05-0.09 Manganese 1.25-1.6 Silicon 0.15-0.30 Chromium 0.01-0.1 Nickel 0.3-0.6 Molybdenum 0.10-0.25 Vanadium 0.03-0.10 Aluminum 0.02-0.05 Niobium 0.01-0.06 Copper 0.2-0.4 Calcium 0.001-0.005 Sulfur 0.0005-0.005 Phosphorus 0.005-0.015 Iron Rest,

preliminary deformation of the workpiece is carried out at a temperature of 950-850 ° C with a total reduction of 50-60%, then it is cooled to a temperature of 820-760 ° C with a cooling rate of 4-15 ° C / s in a controlled cooling installation (UCO), in addition, the final deformation at a temperature of 770-740 ° C to the required strip thickness with a total compression of 60-76%, further cooling is carried out accelerated on ultra-fine pulses at a speed of 35-55 ° C / s to a temperature of 530-350 ° C, then the strip is cooled in a caisson to 150 ± 20 ° С and further on air.

The use of thermomechanical treatment provides an increase in the number of ferrite nuclei and contributes to the formation of a developed substructure upon completion of deformation at a temperature close to the Ar ₃ point and uniform release of a superfine carbide phase over the entire area of ferrite grains.

Deformation at temperatures of 850-950 ° C with a total reduction of 50-60% is necessary for the study of the cast structure of the workpiece and the formation of the preliminary structure of the metal.

Cooling to temperatures of 760-820 ° C with a cooling rate of 4-15 ° C / s is carried out in order to form a solid surface layer of metal, which allows further fine deformation to obtain a fine-grained structure throughout the rolled section.

The end of deformation in the thermomechanical treatment mode at a temperature of 740-770 ° C close to Ar ₃ leads to an increase in the number of ferrite nuclei and a uniform precipitation of a finely dispersed carbide phase.

Accelerated cooling of rolled products in the temperature range from 725-750 ° C to 350-530 ° C leads to an increase in the dispersion of structural components.

Subsequent delayed cooling in the caisson helps relieve thermal stresses.

To obtain high quality and stable properties of strip steel obtained by this method, it is necessary to ensure high metallurgical quality of the workpiece, including low sulfur content (0.0005-0.005%).

The sulfur content in the declared range during the modification of calcium sulfides allows you to get a more homogeneous structure, minimize defects, thereby increasing the resistance of the metal in an acid gas environment, and increases the low temperature viscosity of steel.

A low carbon content along with an improvement in weldability and an increase in low-temperature viscosity is also favorable for reducing segregation in a continuously cast slab.

With this carbon content, manganese segregation decreases, which avoids the banded ferrite-pearlite structure.

In steel with a carbon content of less than 0.09%, a peritectic reaction does not occur during crystallization, the crystallization interval decreases, the existence interval of δ ferrite extends, which allows homogenizing a solid solution, increasing the uniformity of the distribution of elements.

The addition of manganese, molybdenum and nickel within the claimed limits provides solid-solution hardening of the metal and contributes to better hardenability during thermomechanical processing.

When microalloying with aluminum, niobium and vanadium, grain grinding is ensured due to the formation of finely dispersed carbides, which impede the growth of austenite grain upon heating, which increases the yield strength and cold resistance of strip steel.

Tests of strip steel manufactured by the proposed method showed that the selected modes and chemical composition provide, along with the required strength, high impact work at -60 ° C (at least 50 J) and the required ratio of σ _0.2 / σ _in ≤0.90.

Example. Steel was smelted in a converter with out-of-furnace treatment, refining and casting into continuously cast billets with a section of 250x1720 mm

The chemical composition is shown in table 1.

The preforms were heated above Ac ₃ , which corresponded to 1130 ° C, then subjected to preliminary deformation at a temperature of 950 and 850 ° C with a total compression of 56 and 60%, then cooled to a temperature of 820 and 760 ° C at a speed of 4 and 15 ° C / s, the final deformation on strip steel with a thickness of 24, 31 and 40 mm was carried out at a temperature of 770 and 740 ° C with a total compression of 60 and 76%, followed by accelerated cooling in ultra-fine steel to temperatures of 530 and 350 ° C, and the steel was transferred to a caisson for cooling to 150 ± 20 ° C. Further cooling was performed in air.

The mechanical properties of strip steel rolled products were determined on transverse samples: tensile type Ш No. 4 according to GOST 1497, impact type 11 according to GOST 9454. The results are presented in table 2.

The technical efficiency of the invention is expressed in the possibility of manufacturing domestic trunk pipes for an offshore gas pipeline with a diameter of 1067-1420 mm, in increasing their reliability and durability by increasing the impact work and crack resistance of strip steel, as well as reducing the complexity of manufacturing welded pipes by increasing the manufacturability of steel.

Table 1 The chemical composition of steel Steel Conventional number of heats Mass fraction of elements,% Pcm C _equiv FROM Si Mn R S Cr Ni Cu Al V Nb Mo Ti Sa N Fe one 0,07 0.30 1,6 0.010 0.005 0.01 0.30 0.30 0.05 0,065 0,035 0.10 - 0.001 - REST 0.20 0.41 Proposed 2 0.05 0.15 1.43 0.015 0.0025 0.10 0.60 0.20 0.02 0,030 0.010 0.25 - 0.005 - 0.17 0.42 3 0.09 0.23 1.25 0.005 0,0005 0.06 0.45 0.40 0,035 0.10 0,060 0.17 - 0.003 - 0.21 0.42 Famous four 0.11 0.30 1,60 - - 0.20 0.50 0.40 0.004 - 0.06 - 0.005 - 0.005 - -

table 2 Method parameters and mechanical properties of the car Steel Service no. Thick cabbage soup on pro kata Method Parameters Mechanical properties Preliminary rolling UCO cooling Final rolling UCO cooling Cooling temperature in caisson, ° С Temporary resistance σ _in , MPa Conditional yield strength σ _0.2 , MPa σ _0.2 / σ _in Elongation δ ₃ Work hit KV ^-20 , J Work hit KV ^-60 , J Temper Atura, ° С Compression,%, not less Temperature, ° С Speed, ° C / s Temperature, ° С Compression% Temperature ° C Speed, ° C / s Proposed one 40 950 fifty 760 four 770 76 530 35 150 590 487 0.82 25 288 273 40 850 60 820 fifteen 740 60 350 55 150 610 502 0.82 27 269 268 2 31 950 fifty 820 four 770 60 530 55 150 580 490 0.84 27 299 285 31 850 60 760 fifteen 740 76 350 35 150 610 498 0.82 26 291 287 3 24 950 fifty 820 four 770 76 530 35 150 570 476 0.84 28 299 253 24 850 60 760 fifteen 740 60 350 55 150 615 500 0.81 27 277 264 Famous four fourteen 950 fifty - - 760 60 300 thirty - 596 455 - 33 78 85 Note. 1. The values of the mechanical properties are given by the results of testing three samples per point.
2. Mechanical properties determined after thermomechanical treatment.

Claims (1)

A method for the production of strip steel for pipes of high-pressure subsea gas pipelines, including the preparation of a steel billet, heating above Ac ₃ temperature, its deformation in a controlled mode with partial reductions with a total reduction of 50-60%, controlled cooling, characterized in that the preform is obtained from steel of the following composition, wt.%: Carbon 0.05-0.09 Silicon 0.15-0.30 Manganese 1.25-1.6 Chromium 0.01-0.1 Nickel 0.3-0.6 Molybdenum 0.10-0.25 Copper 0.2-0.4 Calcium 0.001-0.005 Aluminum 0.02-0.05 Niobium 0.01-0.06 Vanadium 0.03-0.10 Sulfur 0.0005-0.005 Phosphorus 0.005-0.015 Iron Rest preliminary deformation is carried out at a temperature of 950-850 ° C with a total compression of 50-60%, then cooled to a temperature of 820-760 ° C at a speed of 4-15 ° C / s, the final deformation is carried out at a temperature of 770-740 ° C to the required thickness strip with a total compression of 60-76%, further cooling is carried out accelerated at a speed of 35-55 ° C / s to a temperature of 530-350 ° C, then the strip is cooled in a caisson to a temperature of (150 ± 20) ° C and then in air.