RU2792989C1 - Method for the production of longitudinally welded pipes of large diameter from low alloy steel - Google Patents

Abstract

FIELD: pipelines.

SUBSTANCE: method for production of large-diameter straight-seam pipes from low-alloy steel includes milling longitudinal edges, bending them, moulding strip products into a pipe billet, its welding and subsequent expansion. After milling the longitudinal edges, the ratio of the lower chamfer to the upper chamfer is no more than 1.45, the blunting angle is no more than 9°. The height of the bending of the strip edges before forming is: Y"=(1.5-3.5)×S, where Y” – bending height, mm, S – nominal pipe wall thickness, mm. After moulding, the angles of the X-shaped cutting of the edges are at least 60°. Welding of the inner seam of the pipe billet is carried out with a heat input of 30.0-50.0 kJ/cm, and welding of the outer seam is carried out with a heat input of 32-52 kJ/cm.

EFFECT: construction and safe operation of a pipeline with an outer diameter of up to 1420 mm and a working pressure of up to 14.71 MPa is ensured.

7 cl, 2 dwg, 3 tbl

Description

The invention relates to the production of large-diameter pipes from low-alloy steel of strength class K80 (X100), intended for the construction of main oil and gas pipelines.

The invention can be used in the manufacture of large-diameter straight-seam pipes at electric pipe welding complexes by step-by-step forming or rolling of the body of the pipe billet, followed by automatic arc welding under a layer of ceramic flux and expansion.

A known method for the production of pipes of large diameter, according to which pipes are made of steel grade DNV SAWL 485 FD, the edges are cut, the root weld is performed, after which the internal seam is performed by multi-arc welding with four welding arcs in one pass. The outer seam is performed by multi-arc welding with four welding arcs in at least three passes. Between passes, the surface of the seam is cleaned of slag crust. The root weld, the inner weld and at least one pass of the outer weld are performed in the center of the weld, the last two passes of the outer weld are performed with an offset relative to the center of the weld. In this case, automatic multi-arc welding under a flux layer with a welding wire with alloying elements is used. The multi-arc welding head is oriented to a position where all electric arcs burn in one common weld pool [patent RU 2743082, IPC B23K31/02, B23K33/00, B23K9/18, 2021].

The disadvantage of this technical solution is low productivity and increased cost in mass production, due to the need to weld the outer seam in three passes.

Closest to the claimed invention is a method for manufacturing a thick-walled straight-seam pipe by submerged arc welding from X100 steel. The production method includes steel sheet ultrasonic testing, edge milling, edge pre-bending, JCO forming, pre-welding, internal welding, external welding, welding line X-ray inspection, steel pipe diameter expansion, hydrostatic pressure testing, welding line ultrasonic inspection, pipe end X-ray inspection , chamfering, testing of magnetic particles of the end of the pipe and quality control of the appearance [patent CN103521549B, IPC B21C37/08, B23K9/16, B23K9/18, C22C38/58, 2014].

The disadvantage of this technical solution is not achieving the required mechanical properties of the welded joint of pipes of strength class K80, namely: impact strength on Charpy specimens (KCV) at a test temperature of -40 ˚С - at least 87 J/cm ² , and also the strength of the welded joint is not less than 790 MPa.

The technical result of the invention is the development of a method for producing longitudinally welded pipes of large diameter of strength class K80, with a set of mechanical properties that enable the construction and safe operation of a pipeline with an outer diameter of up to 1420 mm and a working pressure of up to 14.71 MPa.

The technical result is achieved by the fact that in the method for the production of longitudinal pipes of large diameter from low-alloy steel, including milling the longitudinal edges, bending them, molding the strip into a tubular blank, welding it and subsequent expansion, according to the invention, after milling the longitudinal edges, the ratio of the value of the lower chamfer to the top one is no more than 1.45, the blunting angle is no more than 9º, the height of bending the edges of the strip steel before forming corresponds to the condition:

where Y” is the height of the hemming, mm,

S is the nominal thickness of the pipe wall, mm,

after forming, the angles of the X-shaped cutting of the edges are at least 60º, while welding the inner seam of the pipe billet is carried out with a heat input of 30.0 - 50.0 kJ/cm, and welding of the outer seam with a heat input of 32 - 52 kJ/cm.

The value of blunting of the longitudinal edges corresponds to the condition:

where B is the value of the lower chamfer in mm.

The forming of the pipe billet is carried out in 17-21 stages.

Carry out automatic arc welding with a consumable electrode under a layer of ceramic flux, while the composition of the electrode consists of the following elements, wt.%:

nickel 1.5-2.4 manganese 1.2–2.0 chromium 0.05-0.7 titanium no more than 0.03 nitrogen no more than 0.012 inevitable impurities rest

The value of plastic deformation during expansion is 0.5 - 1.6%.

The pipe is made from strip products containing elements in the following ratio, wt.%:

carbon 0.03–0.07 silicon 0.10–0.35 manganese 1.70–2.10 sulfur no more than 0.004 phosphorus no more than 0.015 chromium no more than 0.30 nickel 0.40–1.00 copper no more than 0.50 aluminum 0.02–0.08 titanium 0.001–0.03 molybdenum 0.10–0.50 vanadium no more than 0.10 niobium 0.02 - 0.10 nitrogen no more than 0.008 boron no more than 0.001 calcium 0.0005–0.006

if necessary

REM no more than 0.002 iron and inevitable impurities rest,

The characteristics of the pipe correspond to the following values:

Yield strength in the longitudinal / transverse direction - 630-840 / 690-840 MPa;

Ultimate strength in the longitudinal / transverse direction - 755-940 / 790-940 MPa;

Impact strength at a temperature of -40 °C - not less than 250 J/cm ² for the base metal and not less than 87 J/cm ² for the welded joint;

Falling weight test (DCL) at -40 °C - not less than 85%;

Crack resistance (CTOD) at -20 °C - not less than 0.20 mm for the base metal, not less than 0.15 for the welded joint.

The essence of the invention.

Compliance with the ranges of the above parameters and welding materials makes it possible to obtain the required pipe geometry, as well as to provide a set of mechanical properties of the welded joint.

The ratio of the value of the lower chamfer to the upper one should be no more than 1.45, and the value of blunting C \u003d (B-1.0) ± 2.0 mm, where B is the value of the lower chamfer in mm, otherwise when welding internal or external seams it will be necessary to increase welding currents to fill the gap, which will increase the total heat input and reduce the impact strength (KCV) along the fusion line at -40 °C.

The blunting angle should be no more than 9°, otherwise it will not be possible to provide the required geometry of the workpiece shape.

It has been experimentally established that the height of the edge bending (Y”) should be within (1.5 ÷ 3.5)xS in order to avoid the formation of the shape of the welded edges with a “heart” (overformed) or “house” (underformed), since in the future this leads to a change in the size of the chamfers on the tubular billet after assembly. At the same time, the coefficients (1.5 ÷ 3.5) were obtained experimentally and their choice in the stated range allows to provide the necessary cutting of edges for welding and the required shape of the pipe blank during assembly.

Welding of the inner seam of the pipe billet is carried out with a heat input of 30.0–50.0 kJ/cm, and welding of the outer seam with a heat input of 32–52 kJ/cm. The declared heat input of welding is required to fill the groove, obtain the necessary structure of the weld and the heat-affected zone, and ensure the above mechanical properties of the welded joint.

The corners of the X - figurative cutting of the edges must be at least 60 °. The specified value of the angles is determined experimentally and is necessary to obtain the required shape (geometrical parameters) of the weld.

The chemical composition of the welding wires should be as follows.

Nickel content in the amount of 1.5-2.4% allows to increase the impact strength of the welded joint. Nickel refines the grain without compromising weldability.

The content of manganese in the amount of 1.2–2.0% increases the strength properties of the welded joint, but at a higher concentration it worsens the weldability of the steel and increases the tendency to form cold cracks.

The chromium content in the amount of 0.05-0.7% makes it possible to increase the strength properties of the welded joint. At elevated concentrations, chromium can cause the formation of refractory oxides, as well as a sharp increase in hardness in the heat-affected zone due to the formation of chromium carbides.

Titanium in an amount of not more than 0.03% forms carbides, preventing the formation of hard chromium carbides, and also increases the strength properties of the weld.

Nitrogen is a harmful impurity. Its content should be limited to 0.012%. With its increased content, there is a decrease in impact strength in the weld zone.

The chemical composition of steel should be as follows:

carbon 0.03 - 0.07 silicon 0.10 - 0.35 manganese 1.70 - 2.10 sulfur no more than 0.004 phosphorus no more than 0.015 chromium no more than 0.30 nickel 0.40 - 1.00 copper no more than 0.50 aluminum 0.02 - 0.08 Titanium 0.001 - 0.03 molybdenum 0.10 - 0.50 vanadium no more than 0.10 niobium 0.02 - 0.10 nitrogen no more than 0.008 boron no more than 0.001 calcium 0.0005 - 0.006

if necessary

REM no more than 0.002 iron and inevitable impurities rest

To obtain the required strength, the carbon content must be at least 0.03%, while its addition in an amount of more than 0.07% leads to a deterioration in the plastic properties of steel.

The addition of silicon is necessary for the deoxidation of steel during smelting. To ensure the required level of deoxidation, its content should be at least 0.10%, but not more than 0.35%, to limit the amount of silicate inclusions that worsen impact strength and crack resistance.

Manganese increases the degree of ferrite saturation with dissolved elements involved in the precipitation hardening mechanism. To ensure the required mechanical properties of steel (characterizing strip steel of strength category K80), the manganese content must be at least 1.70%. The content of manganese in the amount of more than 2.1% is not economically feasible.

The chromium content is limited to a concentration of 0.3%. In the claimed range, chromium increases the hardenability of steel. At more than 0.3% chromium can lead to the formation of brittle structural components that reduce the ability of the steel to resist the development of cracks.

To increase the stability of austenite, nickel and copper are added to steel. To obtain the desired effect, the nickel content should not be less than 0.40%. Nickel content in the amount of more than 1.0% is not economically feasible.

Steel contains copper in an amount of not more than 0.50%. The presence of copper in steel increases its strength, but, at the same time, when the stated value is exceeded, it reduces ductility and impact strength, weakening grain boundaries during slow cooling with an enriched phase.

Vanadium, niobium and titanium, in the stated ranges, are strong carbonitride forming elements. At the same time, they contribute to the production of a cellular dislocation microstructure of steel, which provides a combination of high strength characteristics and high impact strength.

To use the additional mechanism of dispersion strengthening, the steel must be with the addition of titanium, vanadium and niobium, in an amount not less than 0.021% in total and not more than 0.23%. When the total content of these elements in an amount of less than 0.021% is not achieved the desired effect of hardening.

Molybdenum additives give the steel a fine-grained structure, increase strength with equal ductility. Molybdenum in an amount of less than 0.10% does not significantly affect the properties of steel. Its content of more than 0.50% significantly increases the cost of steel, which is not economically feasible.

Nitrogen is needed to precipitate fine nitrides and to restrain the growth of austenite grains. When the nitrogen content is above 0.008%, its concentration in the solid solution increases, which worsens the impact strength and crack resistance of steel at low temperatures.

Aluminum deoxidizes and modifies steel, binds nitrogen into nitrides. To reduce the oxygen content in molten steel, it is necessary to add at least 0.02% aluminum. When its content is more than 0.08%, the viscoplastic properties of steel decrease.

To improve low-temperature impact strength in the heat-affected zone, as well as to increase the ability to hardenability, boron is added in an amount of not more than 0.001%.

Sulfur and phosphorus are harmful impurities, therefore, the indicated values of the sulfur content (not more than 0.004%) and phosphorus (not more than 0.015%) are necessary to obtain high values of impact strength at low temperatures.

With a sulfur content of more than 0.004%, sulfide inclusions are formed in the steel, which significantly reduce the impact strength and crack resistance.

Phosphorus is one of the elements with the highest tendency to segregation and segregation along grain boundaries, and, as a result, negatively affecting the impact strength of steel and crack resistance. In this regard, the upper limit of the phosphorus content is set in an amount of not more than 0.015%.

To increase the ability to hardenability, boron is added to steel in an amount of not more than 0.001%.

Calcium and rare earth metals (REMs) are elements used to control the shape of sulfides. They make it possible to suppress the formation of MnS joints stretched in the rolling direction and improve the properties of the steel in the thickness direction of the sheet, in particular, increase the resistance to longitudinal cracking. On the other hand, to reduce the amount of oxides, the upper limit of the content of calcium and REM in steel is set at no more than 0.006% and 0.002%, respectively.

The technical solution is illustrated in Fig. 1 and FIG. 2.

Fig. 1

A - upper chamfer, mm

B - bottom chamfer, mm

C - dullness, mm

S – wall thickness, mm

α ₁ - angle of inclination of the lower chamfer, deg.

α ₂ - the angle of inclination of the upper chamfer, deg.

α ₃ - blunting angle, deg.

Fig. 2

D – sheet width, mm

S - sheet thickness

x ₃ - the value of the displacement of the lower tool, relative to the upper one, mm

Y” – edge bending height, mm.

In this case, the upper corner of the X-shaped cutting of the edges is determined by the formula:

a, the lower corner of the X-shaped cutting of the edges is determined by the formula:

Example.

The proposed method was tested in the manufacture of a batch of pipes with an outer diameter of 1220 mm and a wall thickness of 20 mm from steel of strength class K80.

The preparation of edges for welding for this standard size is made based on the execution of assembly, internal and external seams in one pass. The parameters of bending the edges and forming the pipe blank are selected in such a way as to provide the necessary geometry of the pipe and maintain the required shape of the X-shaped chamfer for welding.

To ensure high productivity and satisfactory formation of the seam, welding was carried out with four arcs. Submerged arc welding mode parameters are determined in such a way as to ensure the optimal formation of the welded joint, as well as the necessary and sufficient heat input necessary to obtain the required structure and mechanical characteristics of the welded joint.

8 experiments were carried out, during which 4 combinations of technical parameters were tested in the production of pipes (Table 1), as well as 2 combinations of wires that provide the required chemical composition of the weld (Table 2). The results of mechanical tests are given in table.3.

The results of mechanical tests show that compliance with the declared technical parameters of production (experiments 2 - 6) makes it possible to achieve the required level of mechanical properties of a pipe of strength class K80.

Table 1

Technical parameters in the production of pipes.

Parameter combination B, mm C, mm A, mm α _{1 α2} α ₃ Yʺ,mm Q VSH, kJ/cm Q NSH, kJ/cm Number of molding steps Expanding amount 1 7.5 6.5 6.0 37.5 37.5 2 29 33.7 43.5 21 1.0 2 (according to the invention) 7.0 6.5 6.5 35 37.5 2 46 39.9 40.8 21 1.0 3 (according to the invention) 7.8 6.5 5.7 37.5 40 2 66 35.4 41.7 19 1.0 4 9.5 6.5 4.0 40 45 5 53 30.4 54.3 19 1.0

table 2

Mass fraction of chemical elements in welding consumables.

Combination of welding consumables Ni,% Mn,% Cr% Ti,% N, % 1 1.9 1.8 0.6 0.02 0.010 2 2.2 1.3 0.12 0.04 0.009

Table 3

The results of mechanical tests of the weld

experiment number (Combination of parameters) -(welding consumable) σ _in , MPa (across) σ _in , MPa (along) KCV ^-20 , seam center, J / cm ² KCV ^-20 , fusion line J/ ^cm2 KCV ^-40 , seam center, J / cm ² KCV ^-40 , fusion line J/ ^cm2 CTOD^-20, seam center, mm 1 1-1 843 851 91 135 63 76 0.09 2 (according to the invention) 1-2 821 835 212 254 109 187 0.26 3 (according to the invention) 2-1 837 846 134 261 103 250 0.21 4 (according to the invention) 2-2 807 829 228 317 137 269 0.37 5 (according to the invention) 3-1 829 835 168 259 115 196 0.18 6 (according to the invention) 3-2 823 830 198 287 147 210 0.24 7 4-1 744 751 76 84 44 63 0.03 8 4-2 782 795 83 91 34 57 0.04 Note:
σ _in - ultimate strength, MPa;
KCV ^-20 - impact strength at a test temperature of minus 20 ^o C, the average value for three samples is given;
KCV ^-40 - impact strength at a test temperature of minus 20 ^o C, the average value for three samples is given;
CTOD ^-20 - crack tip opening at a test temperature of minus 20 ^o C, the minimum value is given for three samples.

Claims (22)

1. A method for the production of longitudinally welded pipes of large diameter from low-alloy steel, including milling the longitudinal edges, bending them, forming the strip into a tubular blank, welding it and subsequent expansion, characterized in that after milling the longitudinal edges, the ratio of the value of the lower chamfer to the upper one is no more than 1.45, the blunting angle is not more than 9°, the height of bending the edges of the strip steel before forming corresponds to the condition: Y"=(1.5-3.5)×S, where Y” – edge bending height, mm; S is the nominal thickness of the pipe wall, mm; after molding, the angles of the X-shaped cutting of the edges are at least 60º, while welding the inner seam of the pipe billet is carried out with a heat input of 30.0-50.0 kJ/cm, and welding of the outer seam is carried out with a heat input of 32-52 kJ/cm. 2. The method according to p. 1, characterized in that the value of the blunting of the longitudinal edges corresponds to the condition: С=(B–1.0)±2.0 mm, where B is the value of the lower chamfer, mm. 3. The method according to p. 1, characterized in that the molding of the tubular billet is carried out in 16-22 stages. 4. The method according to p. 1, characterized in that automatic arc welding is carried out with a consumable steel electrode under a layer of ceramic flux, while the steel electrode contains nickel, manganese, chromium, titanium, nitrogen in the following ratio, wt.%: nickel 1.5-2.4 manganese 1.2-2.0 chromium 0.05-0.7 titanium no more than 0.03 nitrogen no more than 0.012 inevitable impurities rest 5. The method according to p. 1, characterized in that the amount of plastic deformation during expansion is 0.5-1.6%. 6. The method according to p. 1, characterized in that the pipe is made from strip products containing elements in the following ratio, wt.%: carbon 0.03-0.07 silicon 0.10-0.35 manganese 1.70-2.10 sulfur no more than 0.004 phosphorus no more than 0.015 chromium no more than 0.30 nickel 0.40-1.00 copper no more than 0.50 aluminum 0.02-0.08 titanium 0.001-0.03 molybdenum 0.10-0.50 vanadium no more than 0.10 niobium 0.02-0.10 nitrogen no more than 0.008 boron no more than 0.001 calcium 0.0005-0.006 if necessary REM no more than 0.002 iron and inevitable impurities rest 7. The method according to claim 1, characterized in that the characteristics of the pipe correspond to the following values: yield strength in the longitudinal / transverse direction - 630-840 / 690-840 MPa; tensile strength in the longitudinal / transverse direction - 755-940 / 790-940 MPa; impact strength at -40°C - not less than 250 J/ ^cm2 for the base metal and not less than 87 J/ ^cm2 for the welded joint; falling weight test (DCL) at -40°С – not less than 85%; crack resistance (CTOD) at -20°C - not less than 0.20 mm for the base metal, not less than 0.15 mm for the welded joint.

Images