RU2798180C2 - High-quality material for flexible long-dimensional pipes and method for its manufacture - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates namely to flexible steel pipes and methods for manufacturing flexible steel pipes. The flexible steel pipe comprises steel strips end-to-end butt-welded with fillet welds and formed into a pipe by welding, each strip being made of C-Mn steel, B-Ti steel or Cr-Mo steel. The pipe has areas of the base metal, welds and their heat-affected zones. The pipe has a yield strength in excess of 80 ksi and a final microstructure containing a mixture of tempered martensite and bainite. At the same time, the final microstructure of the flexible pipe in all areas of the base metal, on welds and in heat-affected zones, consisting of tempered martensite more than 90% by volume, is homogeneous with a grain size of less than 20 mcm, and has a uniform distribution of fine-grained carbides.

EFFECT: resulting flexible pipes are characterized by high strength.

17 cl, 18 dwg, 6 ex

Description

The technical field to which the invention belongs

The invention relates to materials for pipes, in particular to materials for flexible long pipes.

State of the art

In recent years, high-pressure and high-reach applications have seen more use of coiled tubing. In this regard, there is a need to produce coiled pipes with increased tensile strength so that they can withstand i) axial loads acting on suspended or lifted columns and ii) increased pressure that occurs during operation.

The standard procedure for the production of flexible pipes uses hot-rolled strip steel as a raw material, with mechanical properties achieved by microstructure refinement during rolling. This refinement is achieved by using various microalloying additives (titanium, nitrogen, vanadium) in combination with the selection of appropriate hot rolling conditions. The goal is to provide control of material recrystallization and grain growth to achieve an ultrafine microstructure. The choice of material is limited by the use of alloying elements in solid solution and precipitation hardening, since grinding is the only mechanism that allows both high strength and toughness to be achieved at the same time.

The specified raw material is specific to each supplier, and therefore different mechanical properties of hot rolled steel may be required to produce flexible pipes with different mechanical properties. However, as the mechanical properties increase, the cost of production, and hence the cost of the raw material, increases. It is known that the process of welding strips to each other, used during the assembly of a "long strip", which can be formed/welded using resistance welding (ERW) into a flexible pipe, wears out the joint area. Accordingly, high performance coiled tubing generally has relatively poor performance in the seam region of the strips to be welded. This decrease is caused by the welding breakdown of the fine grain structure obtained during hot rolling and the lack of a simple post-weld heat treatment method to restore tensile strength and toughness. In general, it is possible to restore the tensile strength, but in this case, the viscosity and, accordingly, the fatigue strength will be reduced in this area. Known industrial technology makes it possible to obtain flexible pipes with high strength, but at the same time having a high cost and low characteristics of the seams of the welded strips relative to the parameters of the pipe body.

One of the options for the production of flexible pipes is the heat treatment of the entire pipe body. This processing method is used for material formed into a pipe in the so-called "green" state, because its characteristics are not yet determined by the heat treatment conditions. In this case, the main variables that affect the properties of the final product are the chemical composition of the steel and the heat treatment conditions. Accordingly, due to the specific composition of steel, weld material and heat treatment, it is possible to obtain a flexible pipe having the same properties along the entire length, and eliminate weak points at the junction of the strips, which is critical for a standard flexible pipe with high strength. This general concept has been described previously, but so far it has not been successfully implemented in the production of flexible pipes with high strength (ie, yield strength in the range from 80 to 140 thousand psi ² ). The reason for this phenomenon is that thermal processing at a linearly increasing rate (needed to obtain high productivity) generally leads to the need for larger complex systems. This process can be simplified by choosing suitable heat treatment conditions and chemical composition.

In selecting a suitable chemistry for a normal sized industrial heat treatment system, there are several variables that affect coiled tubing performance, including but not limited to: a) axial mechanical properties, b) uniformity of microstructure and properties, c) toughness , d) fatigue resistance, e) resistance to sour media.

Disclosure of invention

The following describes a chemistry for the production of heat-treated coiled tubing that exceeds existing requirements for coiled tubing in API 5ST (maximum carbon content: 0.16%, maximum manganese content: 1.2% (CT70 -90) maximum manganese content: 1.65 (CT100-110), maximum phosphorus content: 0.02% (CT70-90) maximum phosphorus content: 0.025 (CT100-CT110), maximum sulfur content: 0.005, maximum silicon content: 0.5).

This document discloses variants of the invention for flexible steel pipes and methods for their manufacture. Options pipe has a yield strength above 80 thousand pounds/inch ² . The composition of the pipe may include 0.16-0.35 wt. % carbon, 0.30-2.00 wt. % manganese, 0.10-0.35 wt. % silicon, up to 0.005 wt. % sulfur and up to 0.018 wt. % phosphorus, the rest is iron and irremovable impurities. Also, a mixture of tempered martensite and bainite may be present in the final microstructure of the pipe, and the final microstructure of the flexible pipe consists of tempered martensite by more than 90% by volume, and the microstructure is uniform throughout the pipe body, along the linear resistance weld and at the butt joints of the strips.

The flexible steel pipes disclosed herein are formed from a plurality of welded strips, the pipe may contain base metal regions, welds, and heat affected zones, and have a yield strength greater than ⁸⁰ ksi, the pipe contains iron and 0.17- 0.35 wt. % carbon, 0.30-2.00 wt. % manganese, 0.10-0.30 wt. % silicon, 0.010-0.040 wt. % aluminum, up to 0.010 wt. % sulfur and up to 0.015 wt. % phosphorus, while the final microstructure contains a mixture of tempered martensite and bainite, and the specified final microstructure of the flexible pipe consists of more than 90% by volume of tempered martensite in the area of the base metal, on welds and in heat affected zones, and the final microstructure throughout area of the base metal, welds and in heat affected zones is uniform, and has a uniform distribution of fine-grained carbides over the entire area of the base metal, on welds and in heat affected zones.

In one embodiment, the composition additionally includes up to 1.0 wt. % chromium, up to 0.5 wt. % molybdenum, up to 0.0030 wt. % boron, up to 0.030 wt. % titanium, up to 0.50 wt. % copper, up to 0.50 wt. % nickel, up to 0.1 wt. % niobium, up to 0.15 wt. % vanadium, up to 0.0050 wt. % oxygen and up to 0.05 wt. % calcium.

In another embodiment, the composition may include from 0.17 to 0.30 wt. % carbon, from 0.30 to 1.60 wt. % manganese, from 0.10 to 0.20 wt. weight. % silicon, up to 0.7 wt. % chromium, up to 0.5 wt. % molybdenum, from 0.0005 to 0.0025 wt. % boron, from 0.010 to 0.025 wt. % titanium, from 0.25 to 0.35 wt. % copper, from 0.20 to 0.35 wt. % nickel, up to 0.04 wt. % niobium, up to 0.10 wt. % vanadium, up to 0.0015 wt. % oxygen, up to 0.03 wt. % calcium, up to 0.003 wt. % sulfur and up to 0.010 wt. % phosphorus.

Optionally, the pipe may have a minimum yield strength of 125 thousand pounds/inch ² . Optionally, the pipe may have a minimum yield strength of 140 thousand pounds/inch ² . Optionally, the pipe may have a minimum yield strength in the range of 125 thousand pounds/inch ² to 140 thousand pounds/inch ² .

Alternatively, the final microstructure may consist of at least 95% by volume of tempered martensite in the base metal region, at welds, and in heat affected zones. Optionally, the pipe may have a final grain size of less than 20 microns in the areas of the base metal, on welds and in heat-affected zones. Optionally, the pipe may have a final grain size of less than 15 microns in the areas of the base metal, on welds and in heat-affected zones.

Alternatively, the welds may be fillet welds. Alternatively, the fatigue strength at fillet welds may be at least (approximately) 80% of that for areas of the base metal. Optionally, the percentage hardness of the weld, including the heat-affected zone, may be equal to 110% or less than the hardness of the parent metal.

Also disclosed herein is a method for forming flexible steel pipes using strips consisting of iron and 0.17-0.35 wt. % carbon, 0.30-2.00 wt. % manganese, 0.10-0.30 wt. % silicon, 0.010-0.040 wt. % aluminum, up to 0.010 wt. % sulfur, up to 0.015 wt. % phosphorus, and the strips are welded to each other to form a pipe from the welded strips, and the pipe consists of base metal regions, welds and corresponding heat-affected zones, and the pipe is austenitized in the range of 900-1000 ° C, the pipe is quenched to create the final hardened microstructure of martensite and bainite, while the hardened microstructure contains at least 90% martensite in areas of the base metal, welds and heat affected zones; and tempering the hardened pipe at a temperature in the range of 550-720°C, wherein the tempering of the hardened pipe allows a yield strength of greater than about 80 thousand psi ² and the microstructure throughout the area of the base metal, at welds and in heat affected zones is homogeneous and has a uniform distribution of fine-grained carbides.

In one embodiment, the welding of the strips may be a fillet welding. Optionally, forming pipes may include the creation of a linear joint. According to a variant in the method, a hardened pipe is additionally wound onto a coil. According to a variant, austenization makes it possible to form a grain size of less than 20 μm in the areas of the base metal, on welds and in heat-affected zones.

In one embodiment, the composition may also include up to 1.0 wt. % chromium, up to 0.5 wt. % molybdenum, up to 0.0030 wt. % boron, up to 0.030 wt. % titanium, up to 0.50 wt. % copper, up to 0.50 wt. % nickel, up to 0.1 wt. % niobium, up to 0.15 wt. % vanadium, up to 0.0050 wt. % oxygen and up to 0.05 wt. % calcium.

In one embodiment, the composition may also include from 0.17 to 0.30 wt. % carbon, from 0.30 to 1.60 wt. % manganese, from 0.10 to 0.20 wt. % silicon, up to 0.7 wt. % chromium, up to 0.5 wt. % molybdenum, from 0.0005 to 0.0025 wt. % boron, from 0.010 to 0.025 wt. % titanium, from 0.25 to 0.35 wt. % copper, from 0.20 to 0.35 wt. % nickel, up to 0.04 wt. % niobium, up to 0.10 wt. % vanadium, up to 0.00015 wt. % oxygen, up to 0.03 wt. % calcium, up to 0.003 wt. % sulfur and up to 0.010 wt. % phosphorus.

In one embodiment, the hardened pipe may have a yield strength greater than or equal to 125 thousand pounds/inch ² . In one embodiment, the hardened pipe may have a minimum yield strength of 140 thousand pounds/inch ² . In one embodiment, the hardened pipe may have a minimum yield strength of 125 to 140 thousand pounds/inch ² .

Brief description of the drawings

In FIG. 1 A-B show CCT graphs corresponding to steels STD2 (A) and STD3 (B).

In FIG. 2A-B show CCT plots corresponding to BTi ₂ (A) and CrMoBTi ₃ (B) steels.

In FIG. 3 shows a plot of the cooling rate on the inside of the pipe as a function of wall thickness (WT) for a flexible pipe hardened with water jets from the outside.

In FIG. 4 shows a graph of the tensile strength of steel BTi ₂ as a function of the maximum tempering temperature (Tmax). Amplitude tempering cycles were used in the Gleeble® simulation: the tensile strength of the same steel grade as a function of holding time at 720°C (isothermal tempering cycles) is shown in the graph on the right.

In FIG. 5A-B show untempered martensite appearing in a central segregation band adjacent to a linear resistance weld after weld zone annealing (PWHT). In FIG. 5A-B show the characteristics of a standard Class 90 coiled tubing.

In FIG. 6A-B show localized damage in the center segregation band resulting from fatigue testing of Class 110 flexible conduit.

In FIG. 7A-B show localized damage on the center segregation band resulting from high internal pressure (9500 psig) Class ¹⁰⁰ coiled tubing fatigue testing.

In FIG. 8A-B show base metal microstructures corresponding to standard coiled tubing (A) and coiled tubing produced in accordance with the present invention (B). In both cases, the flexible conduit has a tensile strength corresponding to class 110 (yield strength from 110 thousand psi ² to 120 thousand psi ² ).

In FIG. 9A-B show the microstructures of a linear resistance weld corresponding to a standard flexible pipe (A) and a flexible pipe obtained in accordance with the present invention (B). In both cases, the flexible conduit has a tensile strength corresponding to class 110 (yield strength from 110 thousand psi ² to 120 thousand psi ² ).

In FIG. 10A-B show the microstructures of the heat affected zone (HAZ) of a resistance weld for standard coiled tubing (A) and coiled tubing produced in accordance with the present invention (B). In both cases, the flexible conduit has a tensile strength corresponding to class 110 (yield strength from 110 thousand psi ² to 120 thousand psi ² ).

In FIG. 11A-B show the microstructures of the heat affected zone (HAZ) of a fillet weld for standard coiled tubing (A) and coiled tubing produced in accordance with the present invention (B). In both cases, the flexible conduit has a tensile strength corresponding to class 110 (yield strength from 110 thousand psi ² to 120 thousand psi ² ).

In FIG. 12 shows a crack that develops during service in the fusion zone of a fillet weld (running from the inside of the pipe). The crack runs towards the large laths of the upper bainite.

In FIG. 13 shows changes in hardness (base metal hardness = 100%) on standard fillet welds obtained using standard machining and machining in accordance with the present invention. The fusion zone (FZ) is located in the region ≈+/- 5 mm from the center of the weld.

In FIG. 14A-B show microstructures corresponding to the intersection of a fillet weld and a resistance welding line weld for standard coiled tubing (A) and coiled tubing made in accordance with the present invention (B). In both cases, the flexible conduit has a tensile strength corresponding to class 110 (yield strength from 110 thousand psi ² to 120 thousand psi ² ).

In FIG. 15 is a schematic representation of fatigue testing equipment.

In FIG. 16 shows fatigue life values for fillet weld (BW) specimens versus fatigue life values for base material (BM) specimens. The results are presented as averages under different test conditions and for different classes of flexible pipes (80, 90 and 110 for standard pipes and 80, 90, 110, 125 and 140 for flexible pipes made in accordance with the present invention).

In FIG. 17 shows the increase in fatigue life in coiled tubing with the chemistry and processing conditions of the present invention. The increase is determined by comparison with the fatigue life obtained for a standard flexible conduit of the same class, which was tested under similar conditions. The results are averaged for each class under various test conditions. For grades 125 and 140, which are non-standard, fatigue life was compared with STD3 grade 110.

In FIG. 18A-B show a hemispherical ring after testing Class 80 material according to NACE TM0177 (90% of the minimum yield strength, Method A, 1 bar, H ₂ S). A: standard procedure, B: procedure according to the present invention.

Implementation of the invention

The raw material for flexible piping is produced in the workshop in the form of hot-rolled strips. Controlled rolling is used to ensure high strength and high toughness in the fine microstructure. The strip is cut longitudinally to the width required to produce pipes, and then its ends are butt-joined in a joining process (eg, plasma arc welding or friction stir welding) to produce longer strips. The strips are then shaped into tubes by resistance welding. Among other things, the characteristics of the final product are evaluated in terms of: a) axial mechanical properties, b) uniformity of microstructure and properties, c) toughness, d) fatigue resistance, e) resistance to sour environments. Using a standard processing procedure, the mechanical properties of the flexible pipe can be obtained, combining the properties of hot-rolled strip, taking into account the changes made during welding and rolling of the pipe. The properties thus obtained are limited by the characteristics of the flexible pipe mentioned above. The reason for this is that the welding process used to join the strips changes the fine-grained microstructure after rolling in such a way that even if a post-weld heat treatment is used, the final properties deteriorate. Reduced fatigue life and low resistance to acidic environments are associated with the heterogeneity of the microstructure and the presence of brittle components near the seams. It is assumed that the new procedure should at least include heat treatment of the entire pipe body. This procedure has been described previously in general terms, but has never been specifically addressed. This document describes the chemical compositions and characteristics of the raw material, as well as suitable welding methods and heat treatment conditions, which allows to obtain a quenched and tempered product with high characteristics of the pipe body and the connecting seams between the strips. This material has been developed for coiled tubing because it not only has a relatively low cost, but it also preferentially maximizes fatigue life under the specific conditions encountered when using coiled tubing (low cycle bending fatigue with simultaneous axial loading and internal pressure).

The present invention relates to high strength flexible tubing (minimum yield strength ranging from 80 ^kpsi to 140 ^kpsi ) having improved low cycle fatigue compared to API 5ST standard products. In addition, the present invention can reduce sulfide stress cracking (SSC). This combination of properties is achieved through the appropriate choice of the chemical composition of the steel and the processing conditions. Industrial processing differs from the standard procedure, which includes heat treatment of the entire pipe body (FBHT), as described in US application No. US2012/0186686 A1. This FBHT procedure is performed after the coiled tubing is formed by resistance welding (ERW) and includes at least one cycle of austenitization, quenching and tempering. The embodiment mentioned above relates more to steel chemistry and processing parameters for producing quenched and tempered flexible conduit with the properties mentioned above. While it is common knowledge to achieve certain mechanical properties by thermally treating a base material with a given composition, a specific application of flexible tubing uses a raw material with a specific chemical composition to minimize the detrimental effect of individual variables, such as segregation zones, on the specific properties of this embodiment. .

One of the most important properties of coiled tubing is increased resistance to low cycle fatigue. This is due to the fact that coiled tubing is often coiled and uncoiled during standard operations, creating cyclic plastic deformations that can lead to defects. In the preferred embodiment, with low-cycle fatigue, the deformation is localized in softer areas of the material and has microscopic dimensions. In the presence of brittle components in or near these areas of strain concentration, cracks can easily form and grow in size. Therefore, reduced fatigue life is associated with heterogeneous microstructures (with softer areas in which deformation is localized) and the presence of brittle components (resulting in the formation and/or propagation of cracks). All such microstructural features occur in the heat affected zone (HAZ) of the weld. Other types of pipe body microstructure also have the above characteristics. This is due to the fact that they consist of a mixture of high and low hardness components such as ferrite, pearlite and bainite. In this case, the deformation is localized in the softer ferrite, in the region of the boundary with bainite, in which cracks form and grow in size. Currently, flexible pipes with high strength have this type of microstructure.

In order to avoid localization of deformations under low-cycle fatigue, the microstructure must not only be uniform throughout the pipe body and joints, it must be uniform on a microscopic scale. With regard to low carbon steels, the ideal microstructure is tempered martensite, which has a ferrite base with a uniform and fine-grained distribution of carbides. Thus, it is necessary to select the chemical composition and create the processing conditions described in this document in order to obtain a uniform microstructure (in the pipe body, on the fillet weld and in the resistance welding line weld) due to FBHT, consisting of at least 90% tempered martensite, preferably, this figure exceeds 95%.

In addition, tempered martensite is better suited for producing ultra-high strength materials than standard coiled tubing microstructures (including ferrite, pearlite and bainite), which require very expensive alloying additions to achieve a yield strength in excess of (approximately) 125 klb/s. inch ² .

Compared to structures containing bainite, tempered martensite also has other advantages due to increased SSC resistance.

The chemical composition of the steel is defined as the most suitable for the production of heat treated flexible conduit with FBHT and can be described as the concentration of carbon (wt.% C by wt.), manganese (wt.% Mn), silicon (wt.% Si), chromium (wt.% Cr), molybdenum (wt.% Mo), as well as concentrations of microalloying elements such as boron (wt.% B), titanium (wt.% Ti), aluminum (wt.% Al), niobium (wt. . % Nb) and vanadium (wt. % V). Also, upper limits can apply to such unavoidable impurities as sulfur (wt.% S), phosphorus (wt.% P) and oxygen (wt.% O).

The final tempered martensite structure has a steel chemistry of the present invention that is significantly different from prior art coiled tubing compositions by having a higher carbon content (e.g. compared to API 5ST specification where the maximum allowable carbon content for the flexible pipe is 0.16%), which allows to obtain the desired microstructure using FBHT, which includes at least one cycle of austenitization, quenching and tempering.

The terms "about", "about" and "practically" as used herein are intended to mean values close to the specified value, which allow you to perform the desired function or achieve the desired result. For example, the terms "about", "about", and "substantially" may refer to values within 10%, within 5%, within 1%, within 0.1%, and within 0.01% of the specified values.

Carbon is an element whose addition significantly increases the strength of steel by increasing the hardenability and precipitation of carbides during heat treatment. If the carbon content falls below 0.17%, the hardenability cannot be guaranteed, so large fractions of bainite may form during heat treatment. The presence of bainite makes it difficult to achieve a yield strength of more than 80 ksi at ^the desired level of fatigue life and SSC resistance. The prior art flexible conduit manufacturing procedure is not suitable for heat treatment since the maximum allowable carbon content according to the API5ST specification is 0.16%. Standard flex microstructures have large bainite fractions that degrade toughness, fatigue life, and SSC resistance in higher grade materials, i.e. in flexible pipelines with a minimum yield strength above 110 thousand psi ² .

On the other hand, steel with a carbon content of more than 0.35% has poor weldability, which makes it susceptible to brittle components and cracking during welding and post-weld heat treatment. In addition, a higher carbon content can lead to a significant amount of retained austenite after quenching, which becomes brittle after tempering. These brittle constituents reduce fatigue life and SSC resistance. Therefore, the carbon content of the steel ranges from (about) 0.17% to (about) 0.35%, preferably from (about) 0.17% to (about) 0.30%.

The addition of manganese improves hardenability and strength. Manganese also contributes to the deoxidation and control of sulfur content during steel production. If the manganese content is below (approximately) 0.30%, it will become more difficult to achieve the desired level of strength. However, as the manganese content increases, large segregation zones can form. In segregation zones of manganese, brittle components are likely to form during heat treatment, which deteriorates toughness and reduces durability. In addition, these segregation zones increase the material's susceptibility to sulfide stress cracking (SSC). Accordingly, the manganese content of the steel ranges from 0.30% to 2.0%, preferably from 0.30% to 1.60%, and more preferably from 0.30% to 0.80% in the case where it has place increased SSC resistance.

Silicon is an element whose addition causes deoxidation during steel production and increases the strength of the steel. In some embodiments, the viscosity may decrease when the silicon content exceeds (about) 0.30%. In addition, large segregation zones may form. Therefore, the silicon content of the steel ranges from (about) 0.10% to 0.30%, preferably from (about) 0.10% to (about) 0.20%.

The addition of chromium increases the hardenability and tempering resistance of the steel. Chromium can be used to partially replace manganese in steel to achieve high strength without the formation of large segregation zones that reduce fatigue life and SSC resistance. However, chromium is a more expensive additive, which complicates the production of flexible conduit due to the stresses that occur during hot forming. Therefore, in some embodiments, the chromium content is limited to (about) 1.0%, preferably (about) 0.7%.

Molybdenum is an element, the addition of which effectively increases the strength of steel, and also helps to slow down the softening during tempering. Temper resistance results in high-strength steel with reduced manganese content, increased fatigue life and SSC resistance. Molybdenum additions can also reduce the segregation of phosphorus to the grain boundaries, which will improve the resistance to intergranular fracture. However, this iron alloy is expensive, which makes it desirable to reduce the maximum molybdenum content of the steel. Therefore, in certain embodiments, the maximum content of molybdenum is about 0.5%.

Boron is an element whose addition is very effective in increasing the hardenability of steel. For example, boron can increase hardenability by eliminating the possibility of ferrite formation during hardening. In some embodiments, boron is used to achieve high hardenability (ie, for a hardened structure containing at least 90% martensite) of manganese-reduced steel to improve fatigue life and SSC resistance. With a boron content of less than (about) 0.0005% by weight in the steel according to the present invention, it may become difficult to achieve the desired hardenability of the steel. However, if the boron content is too high, large fractions of boron carbides can form at the grain boundaries, which negatively affects the toughness. Accordingly, in this embodiment, the boron concentration is below (about) 0.0030%, in another embodiment, the boron content is from (about) 0.0005% to 0.0025%.

Titanium is an element whose addition is effective in increasing the effectiveness of boron in steel by fixing nitrogen impurities in the form of nitrogen nitrides (TiN) and inhibiting the formation of boron nitrides. If the titanium content is too low, it may be difficult in some embodiments to achieve the desired effect of boron on the hardenability of the steel. On the other hand, if the titanium content exceeds 0.03% by weight, large fractions of titanium nitrides and carbides (TiN and TiC) may be formed, which will adversely affect the ductility and toughness. Accordingly, in certain embodiments, the titanium concentration may be limited to (about) 0.030%. In other embodiments, the titanium concentration may range from (about) 0.010% to (about) 0.025%.

Given that the production of flexible tubing with low mechanical properties is preferred due to low temper resistance, additions of boron and titanium increase hardenability without increasing temper resistance. Accordingly, this will allow the production of pipes with a yield strength equal to 80 thousand ^psi , without a significant increase in the holding time during tempering, and significantly increase productivity. Since one of the limitations regarding the production of flexible tubing with a heat treatment line is the length of the line to achieve adequate material holding time during tempering, the use of boron and titanium will greatly affect the production of flexible tubing with low yield strength.

Copper is an element which, in certain embodiments, is optional in the production of steel. However, in some coiled tubing applications, copper may be required to improve atmospheric corrosion resistance. Thus, in an embodiment, the copper content of the steel may be limited to (about) 0.50%. In other embodiments, the copper concentration may range from (about) 0.25% to (about) 0.35%.

Nickel is an element whose addition increases the strength and toughness of steel. In the presence of copper in the steel composition, nickel can be used to avoid defects that occur during hot rolling, known as hot brittleness. However, nickel is very expensive and in certain embodiments, the nickel content of the steel is limited to (about) 0.50%. In other embodiments, the nickel concentration may range from (about) 0.20% to (about) 0.35%.

Niobium is an element whose addition to steel can reduce the austenitic grain size of the steel during reheating in the austenitic region and increase the strength and toughness accordingly. Niobium can also be released during tempering, increasing strength due to quenching in the layer of dispersed particles. In one embodiment, the niobium content of the steel may range from (about) 0% to (about) 0.10%, preferably from (about) 0% to (about) 0.04%.

Vanadium is an element whose addition can be used to increase the strength of steel by precipitating carbides during tempering. However, if the content of vanadium in the steel exceeds (about) 0.15%, a large fraction of vanadium carbide particles may be formed, which will reduce the toughness of the steel. Therefore, in certain embodiments, the vanadium content of the steel is limited to (about) 0.15%, preferably (about) 0.10%.

Aluminum is an element whose addition promotes deoxidation during steel production and makes it possible to reduce the grain size of the steel. In one embodiment, when the aluminum content of the steel is less than (about) 0.010%, the steel may be susceptible to oxidation due to high levels of impurities. In other embodiments, when the aluminum content of the steel is more than (about) 0.040%, large fractions of impurities can be formed that reduce the toughness of the steel. Therefore, the aluminum content of the steel can range from (about) 0.010% to (about) 0.040%.

Sulfur is an element, the addition of which leads to a decrease in the toughness and machinability of steel. Accordingly, in some embodiments, the sulfur content of the steel is limited to a maximum of (about) 0.010%, preferably (about) 0.003%.

Phosphorus is an element whose addition leads to a decrease in the toughness of steel. Accordingly, the phosphorus content of the steel is limited to a maximum of (about) 0.015%, preferably (about) 0.010%.

Oxygen can be an impurity in the steel, which is present mainly in the form of oxides. In one embodiment of the steel, as the oxygen content increases, the impact properties of the steel deteriorate. Accordingly, in certain steel embodiments, a relatively low oxygen content of less than or equal to (about) 0.0050 wt. %; preferably less than or equal to (about) 0.0015 wt. %.

Calcium is an element whose addition to steel can increase the toughness by changing the shape of the sulfide inclusions. In one embodiment, the steel composition may contain calcium and sulfur in a ratio of at least Ca/S>1.5. In other embodiments of the steel, a high calcium content is unnecessary, whereby the steel may have a maximum calcium content of (about) 0.05%, preferably (about) 0.03%.

The content of unwanted impurities, including but not limited to nitrogen, lead, tin, arsenic, antimony, bismuth, etc., should preferably remain as low as possible. However, the properties (eg, strength, toughness) of the steel, which depend on the composition of the steel, as described herein, may vary slightly when the content of impurities is within the selected limits. In one embodiment, the nitrogen content of the steel may be less than (about) 0.010%, preferably less than or equal to (about) 0.008%. In another embodiment, the lead content of the steel may be less than or equal to (about) 0.005%. In yet another embodiment, the tin content of the steel may be less than or equal to (about) 0.02%. In yet another embodiment, the arsenic content of the steel may be less than or equal to (about) 0.012%. In another embodiment, the antimony content of the steel may be less than or equal to (about) 0.008%. In yet another embodiment, the bismuth content of the steel may be less than or equal to (about) 0.003%.

The choice of a particular steel chemistry in accordance with the present invention will depend on the characteristics of the final product and the limitations of industrial equipment (for example, in induction heat treatment lines, it is difficult to obtain a long holding time during tempering). Where possible, the addition of manganese can be reduced as it reduces fatigue life and SSC resistance by creating a large segregation zone. Chromium and, to a lesser extent, molybdenum will be used to replace manganese, while the heat treatment of the entire case will remain as simple as possible. Both elements increase the stability of the carbides and resistance to softening, which could lead to an increase in holding time during tempering. Thus, these features are preferred for stronger products (e.g. Grade 110 and above) for which tempering resistance is preferred, and undesirable for lower grade products (Grade 80) where long and impractical industrial heat treatment lines.

In the case of steel of a lower grade (class 80), it is preferable to use microalloyed additions of boron and titanium and provide an appropriate carbon content. These elements will allow to obtain good hardenability without the use of a large amount of manganese additives. In addition, boron and titanium do not improve tempering resistance. Thus, a simple and short tempering procedure can be used to obtain the required level of strength.

The following sections describe the industrial processing procedure according to the present invention, with particular attention to the full body heat treatment (FBHT) conditions.

The raw material for the flexible conduit is produced in a steel mill as hot rolled strips with a wall thickness that can vary from (about) 0.08 inches to (about) 0.30 inches. To reduce the grain size of the microstructure after rolling, steel suppliers can use controlled rolling. However, the refinement of the microstructure of the strips after rolling is not important, since in this document the microstructure and mechanical properties are determined mainly at the final FBHT. This flexibility in the hot rolling process allows the cost of raw materials to be reduced and the chemical composition of the steel to be used, which cannot be achieved with complex hot rolling procedures (in general, controlled rolling can be used for low carbon microalloy steel).

For the production of pipes, steel strips are cut in the longitudinal direction in width. The pipes are then joined end-to-end by welding (eg plasma arc welding or friction stir welding) to increase the length of the strips, which will allow the required length of the pipes to be obtained. These welded strips are shaped into pipes, for example by resistance welding. The outer diameter of standard flexible pipes ranges from 1 inch to 5 inches. The length of the pipe is (about) 15,000 feet, however, the length can vary from (about) 10,000 feet to (about) 40,000 feet.

After tube forming, whole body heat treatment (FBHT) is used. The aim of this heat treatment is to produce a uniform final microstructure consisting of at least 90% tempered martensite, the remainder being bainite. This microstructure, having a uniform distribution of carbides and a grain size of less than 20 µm, preferably less than 15 µm, guarantees a good combination of strength, ductility, toughness and low cycle fatigue life. In addition, as mentioned above, due to the appropriate choice of the chemical composition of the steel, this type of microstructure is suitable for increasing the resistance to sulfide stress cracking (SSC) compared to standard structures, including ferrite, pearlite and coarse upper bainite.

The FBHT contains at least one cycle (Q) of austenitization and quenching (Q) followed by tempering (T). Austenization is carried out at temperatures in the range between 900°C and 1000°C. At this stage, it is necessary to choose the total duration of the steady state after exceeding the equilibrium temperature Ae3, in order to ensure the total dissolution of the iron carbides without an excessive increase in the size of the austenite grains. The target grain size is below 20 µm, preferably below 15 µm. Quenching must be carried out at a controlled minimum cooling rate to achieve a final microstructure after quenching that includes at least 90% martensite throughout the pipe.

Tempering is carried out at temperatures from 550°С to 720°С. Heat treatment at temperatures above 720°C can lead to partial transformation of martensite into austenite with a high carbon content. The formation of this component must be avoided as it tends to become brittle components, which can lead to reduced toughness and fatigue life. On the other hand, when tempering is performed at a temperature below 550°C, the process of restoring the damaged structure after hardening will not be complete. Accordingly, the viscosity may decrease again. It is necessary to carry out a tempering cycle at temperatures in the above range, which will allow obtaining the necessary mechanical properties. The minimum yield strength may vary from 80 thousand pounds/inch ² to 140 thousand pounds/inch ² . An appropriate steady state duration at a given temperature must be chosen to ensure a uniform distribution of carbide in the base metal area of the pipe and the weld area (linear resistance weld and joint between strips). In some cases, more than one cycle of austenitization, quenching and tempering may be performed to obtain the optimum combination of strength and toughness. After FBHT, the pipe can be subjected to grain size classification procedures to ensure that the specified tolerances for dimensions, stress relief and coiling are met.

Examples:

Example A: Choice of Chemical Composition to Improve Hardenability

As mentioned above, the microstructure according to the present invention is at least 90% tempered martensite and has a uniform distribution of fine-grained carbide, the rest is bainite. This microstructure allows the production of coiled tubing with the desired combination of high strength, extended low cycle fatigue life and improved SSC resistance.

The tempered martensite can be obtained by at least one heat treatment cycle comprising quenching and tempering performed after the tube is formed by resistance welding. The heat treatment may be repeated two or more times if the grain size needs to be further reduced to increase the SSC resistance. This is due to the fact that subsequent cycles of austenitization and hardening reduce not only the size of the former austenite grain, but also the size of blocks and groups of martensite.

To obtain a target microstructure with good hardenability, it is necessary that at least 90% martensite is formed at the end of hardening. The choice of an appropriate chemical composition is important to achieve a given volume content of martensite. The selection of the appropriate steel composition was based on the results of experiments performed using the Gleeble® 3500 thermomechanical simulator. After that, industrial tests were carried out to confirm the results of laboratory studies.

Table A1 lists some of the chemical compositions of the steel analyzed in the laboratory. For all of these chemical compositions, dilatometric tests were carried out using the Gleeble® complex, which made it possible to plot the transformation under continuous cooling (CCT). The CCT plots were used in conjunction with the results of metallographic analysis of the simulation samples to determine the minimum cooling rate that allows a martensite content of more than 90%. This critical cooling rate, mainly depending on the chemical composition of the steel, will be designated as CR90,

Table A1: Experimentally obtained chemical compositions of steel. Element concentrations are given in percent by weight (wt %).

Examples of the resulting CCT plots are shown in FIG. 1, 2. In all cases, to achieve a low austenite grain size (AGS) of 10–20 µm, austenitization was performed at a temperature of 900–950°C. Steel grades STD1, STD2 and STD3 have a chemical composition according to API 5ST specifications, but not according to the present invention due to the low carbon addition rate (Table A1). The critical cooling rate of CR90 exceeds 100°C/s for STD1 and STD2 and 50°C/s for STD3.

In FIG. 1A-B show CCT graphs corresponding to steel types STD2 (A) and STD3 (B). Bold indicates critical cooling conditions to obtain a final microstructure of approximately 90% martensite with the remainder being bainite. In FIG. 2A-B show CCT graphs corresponding to steel types BTi ₂ and CrMoBTi ₃ . Bold indicates critical cooling conditions to obtain a final microstructure of approximately 90% martensite with the remainder being bainite. The first type of steel is C-Mn microalloyed with B-Ti (see Table A1). CrMoBTi2 is a medium carbon steel with chromium and molybdenum additions, also microalloyed with B-Ti. The measured critical cooling rates (corresponding to the cooling curves shown in bold in the CCT plots) are 25°C/s and 15°C/s for steel types BTi ₂ g and CrMoBTi ₃ , respectively.

In FIG. 3 shows the average cooling rate for pipes processed with industrial hardening heads (cooling water jets on the outside of the pipe). Values are given as a function of pipe wall thickness (WT). The shaded area on the graph corresponds to the range of wall thicknesses encountered when using coiled tubing. It should be understood that when choosing the chemical composition of the steel to obtain a tempered martensite content of more than 90%, the critical cooling rate of the alloy must be equal to or less than 30 ° C / s. Otherwise, the bainite content will exceed 10%, as a result, the pipe wall will be larger ( WT=0.3 inch) in the above systems.

Steel types STD1, STD2 and STD3 have critical cooling rates above 30°C/s, so they are not in accordance with the present invention. On the other hand, the hardenability is suitable for steel type BTi ₂ and CrMoBTi ₃ . The increase in hardenability is due to the increased carbon content and the addition of B-Ti.

Table A2 shows the values of the critical cooling rate measured for steel types from table A1. Steel grades STD1, STD2 and STD3 have the currently used chemistry for Class 80, 90 and 110 coiled tubing and meet the requirements of the API 5ST specification. However, even the higher alloy STD3 steel has a critical cooling rate to achieve a tempered martensite content in pipes above 90% with the desired wall thickness WT. It should be understood that standard materials are not suitable for obtaining the desired microstructure according to the present invention, and therefore it is necessary to increase the hardenability of the steel. The main element affecting the hardenability of low-alloy steel is carbon. Thus, in order to obtain a critical cooling rate not exceeding 30°C/s, in this case it was necessary to increase the carbon content, which exceeds the maximum value specified in the API 5ST specification (0.16% by weight). In this embodiment, the carbon content is in the range of 0.17% to 0.35% (the maximum level was chosen to ensure high weldability and toughness). As mentioned above, the content of other elements was changed so that the CR90 was not less than 30°C/s.

Table A2: Critical cooling rates for the martensite content to exceed 90% (CR90) corresponding to the analyzed steel types. Values obtained by dimetric tests using the Gleeble® complex and metallographic analysis.

Below are guidelines for choosing the appropriate chemical composition of the steel, which were obtained after analyzing the experimental data presented in table A2.

C-Mn steel: hardenability mainly depends on the addition of carbon and manganese. To achieve the required hardenability, (approximately) 2% manganese can be used when the carbon content is close to the lower limit (CMn1 steel). However, manganese is an element that creates high segregation zones, which can lead to reduced fatigue life. Accordingly, the amount of manganese addition decreases with increasing carbon content. For example, at a carbon concentration of about 0.25%, it is sufficient to use 1.6% manganese (CMn2 steel) to achieve the required hardenability.

B-Ti Steel: These alloys are plain carbon steel microalloyed with boron and titanium. By increasing the hardenability caused by the presence of boron, the manganese content can be further reduced. With a value of carbon content close to the lower limit, it is sufficient to use about 1.6% manganese to achieve the required hardenability. At a carbon concentration of about 0.25%, 1.3% manganese (BTi ₂ steel) is sufficient to achieve the required hardenability.

Cr-Mo steel: This type of steel contains chromium and molybdenum additives that increase temper resistance, making it suitable for ultra-high strength steel grades. In addition, chromium and molybdenum are elements that increase hardenability; which makes it possible to further reduce the amount of manganese addition. However, chromium and molybdenum are expensive additives and reduce the hot workability of steel, their maximum content is limited to 1% and 0.5%, respectively. In one example, when the carbon content is close to the lower limit, about 1% manganese can be used to achieve CR90 (CrMo1). If the steel is also microalloyed with B-Ti, the manganese content can be further reduced to 0.6% (CrMoBTi1).

Example B: Choice of Chemical Composition for Different Classes of Coiled Tubing For tempering analysis, the values presented in Table A1 were used in an industrial heat treatment simulation using Gleeble®. Modeling consisted of austenitization at 900–950°C, quenching at 30°C/s, and tempering. With regard to steel types STD1, STD2 and STD3, a higher cooling rate was used to obtain a martensite content of at least 90% during quenching. For types STD1 and STD2, a hardening rate of about 150°C/s was used, and steel STD3 was cooled at a rate of 50°C/s. Similar increased cooling rates can be achieved in small samples using the Gleeble® complex and external water cooling. After quenching, the samples were tempered using two types of cycles:

- amplitude cycle: heating at a rate of 50°C/s to a maximum temperature (Tmax), which can be in the range from 550°C to 720°C. Cooling is carried out at a rate of about 1.5°C/s to room temperature. These cycles were designed to simulate real tempering conditions in induction furnaces, which are characterized by high heating rates, no holding time at maximum temperature, and the use of air cooling,

- isothermal cycle: heating at a rate of 50°C/s to 710°C, holding at a temperature for some time, from 1 minute to 1 hour, and cooling at a rate of about 1.5°C/s. This cycle has been used to simulate tempering in industrial lines with multiple soaking inductors or with a tunnel kiln.

In all cases, the tempering temperature is in the range from 550°C to 720°C. Temperatures above 720° C. were not considered, since undesired re-austenization occurs in this case. On the other hand, when tempering is carried out at a temperature below 550°C, it is not possible to completely restore the dislocated structure, and brittle components appear in the material, which can reduce fatigue life.

Amplitude tempering cycles are preferred due to line length reduction and productivity increase. Thus, the possibility of obtaining a given quality with a particular chemical composition of the steel was mainly determined by the tempering curve, which allows the use of this type of cycles. If, after amplitude tempering at 720°C, the strength remains high for the grade of steel used, holding at the maximum temperature can be performed. However, as the exposure time increases, a larger, more expensive, and less productive production line may be needed.

In FIG. 4 (left inset) shows the tempering curve for BTi ₂ steel. Tensile strength is shown as a function of maximum tempering temperature. Amplitude thermal cycles were used in modeling. The figure shows that steel grades 90-125 can be obtained by changing the maximum peak temperature from about 710°C to 575°C, respectively. With this chemical composition, it is impossible to obtain a yield strength of 140 thousand ^psi without lowering the tempering temperature below 550°C. With regard to steel grades with a lower yield strength, to obtain grade 80, it is necessary to hold at 710°C for 3 minutes (inset on the right side of Fig. 4).

Table B1 was compiled based on the results obtained using the simulation on the Gleeble® complex. In this table, for each type of steel analyzed, the possibility of producing steel of various grades with a minimum yield strength from 80 thousand psi ² to 140 thousand psi ² is indicated. For example, for BTi _2, steel grades 90-125 can be obtained using amplitude tempering cycles. However, for grade 80 steel, a 2-minute hold at 720°C can be performed, and therefore the word “hold” is in the corresponding cell.

Table B1: Possibility of industrial production of steel grades 80-140 using the analyzed steel chemistry. If the cell contains the word "exposure", this means that it is necessary to perform an exposure at a temperature of 720 ° C for 1 minute to obtain the specified steel grade.

Based on the results obtained, it becomes clear that in order to obtain a higher class steel, it is necessary to use additives of carbon and chromium with molybdenum. In particular, due to the low carbon content, it is not possible to obtain grade 140 steel using standard API5ST specification chemistries. On the other hand, it is best not to use chromium additives and molybdenum. In this case, B-Ti microalloys can be used to obtain good hardenability (for example, BTi ₁ chemistry is a suitable alternative).

It should be noted that in order to produce martensitic structures in standard steels (STD1, STD2 and STD3) in the laboratory, it was necessary to use a higher quenching rate than can be achieved in the mill. Accordingly, if the cooling rate is limited to values achievable in industrial conditions, using standard steel and the FBHT procedure, it will not be possible to obtain any of the grades of flexible pipes.

Example C: Choice of Chemical Composition to Reduce the Negative Effects of Segregation Occurring During Solidification

During the solidification of the steel, the alloying elements tend to remain dissolved in the liquid due to the increased dissolution ability compared to solid particles (eg, ferrite and austenite). After solidification, the areas saturated with the solution form two zones of heterogeneous chemical compositions: the microsegregation zone and the macrosegregation zone.

Microsegregation occurs as a result of freezing of a liquid saturated with a solution in the interdendritic space. This phenomenon is not a major problem since the effect of microsegregation can be eliminated during subsequent hot working. On the other hand, macrosegregation is a larger scale chemical composition heterogeneity in a cast billet. It cannot be completely eliminated by exposure to high temperatures and/or hot working. From the point of view of the present invention, of interest is the situation in which a continuous plate is cast, which leads to the formation of a central segregation band.

The formation of a central segregation band should be avoided because:

- as a result of welding (when creating a fillet weld and a contact weld, see the example from Fig. 5 A-B), brittle components in the form of untempered martensite can form in this area. Such undesirable components can be eliminated during the subsequent heat treatment of the entire body. However, when bending between welding and heat treatment, plastic deformation may occur, which is a manufacturing defect;

- after FBHT, the remaining part of the central segregation band is an area enriched in substitutional solutions (eg manganese, silicon, molybdenum) with a higher density of coarse-grained carbides compared to the rest of the material. This region is susceptible to cracking during low cycle fatigue as seen in FIG. 6-7. The figures also show segregation bands, which are associated with low SSC resistance.

Although macrosegregation cannot be avoided, its negative effect on toughness, fatigue life and SSC resistance can be reduced by selecting the appropriate steel chemistry.

Based on the results of measurements of EDX (energy dispersive x-ray radiation), corresponding to a wide range of chemical compositions of steel, saturation factors for various alloying elements were obtained for the central segregation band. The results are presented in Table C1. Saturation factors (EF) are the ratios between the concentration of each element in the central band and the average value in the matrix. These values mainly depend on the coefficient of thermodynamic separation between liquid and solids; as well as from diffusion abilities during hardening.

Table C1: Saturation factors (EF) for the center segregation band corresponding to different substituting alloying elements.

Table C1 clearly shows that some elements have a strong tendency to segregate during solidification, such as silicon and copper. On the other hand, chromium and nickel have low saturation factors. Nickel is an expensive additive, however, if hardenability and/or temper resistance is to be increased without large segregation zones, chromium can be used.

Saturation factors allow you to determine the degree of concentration increase that can be expected from each element in the central segregation band. However, not all of these elements have the same effect on the material's tendency to form brittle components during welding or heat treatment. Obviously, the higher the increase in hardenability, the higher the tendency for brittle components to form during processing. It is important to note that elements with high diffusion coefficients, such as carbon and boron, can delaminate during solidification and become homogeneous during hot rolling. Thus, they do not tend to form brittle components localized on the segregation band.

After analyzing the CCT graphs (example A), we can conclude that manganese leads to a strong increase in hardenability. This is independent of carbon and boron, which are not present in large segregation zones after hot rolling. On the other hand, silicon and copper, which have a strong segregation tendency, do not have much effect on the hardenability. Due to the high saturation factor and the strong effect on hardenability, the manganese content must be reduced as much as possible when attempting to eliminate the negative effect on macrosegregation by reducing low cycle fatigue.

Usually, a large amount of manganese is added to steel due to its effect on hardenability. In accordance with the present invention, hardenability is mainly achieved by a higher addition of carbon, whereby the concentration of manganese can be reduced. An additional reduction in the manganese content is achieved by adding boron and (or) chromium. Table C2 shows examples in which the critical cooling rate (CR90) for various types of steel is taken from the CCT charts (data from the previous example A). To achieve hardenability of steel with a carbon content of about 0.25%, the amount of manganese can be reduced from 1.6% to 1.3% by adding boron and to 0.4% by using Cr-Mo.

Table C2: Critical cooling rate to ensure a martensite content of more than 90% (CR90) measured for the analyzed steel type. Values were determined by dilatometric testing and metallographic analysis using the Gleeble® package.

Example D: Uniformity of the microstructure

As mentioned above, the fatigue life of a flexible conduit is highly dependent on microscopic features such as microstructural heterogeneity. The combination of soft and hard structural components makes it possible to localize plastic deformation, which is the main cause of the formation and propagation of cracks. This section will compare the microstructure of flexible conduit obtained using a standard manufacturing method using chemistries conforming to API 5ST specification and microstructure obtained using chemistries and processing conditions conforming to this document.

The reference material used was standard 110 grade steel flexible tubing (yield strength 110 ^ksi to 120 ^ksi ) with a chemistry named STD2 from Table A1, which complies with API 5ST specification. This reference material has been compared to the same class of flexible tubing produced with BTi ₂ chemistry using FBHT.

In this comparison, various pipe sections were taken into account:

- base metal (BM): the microstructure of the flexible pipeline, located separately from the linear resistance weld and fillet weld, while the word "separate" means that it is not included in the heat-affected zone (HAZ) formed during welding of any type and possibly post-weld heat treatment (PWHT) used;

fillet weld (BW): microstructural area corresponding to the junction of the strips, which can be obtained using plasma arc welding (PAW), friction stir welding (FSW) or any other welding technology. This area also has a corresponding heat affected zone generated during welding and PWHT;

- linear resistance weld: a microstructure formed by longitudinal resistance welding during pipe forming and localized PWHT, which in general is the annealing of the weld zone. As in the previous cases, this area also includes the corresponding heat-affected zone.

In FIG. 8A-B show a base metal microstructure corresponding to a standard coiled tubing (A) and a microstructure obtained in accordance with the present invention (B). In the first case, a ferrite base with a fine-grained distribution of carbides is visible. This matrix and fine-grained structure are formed as a result of controlled hot rolling. The microstructure obtained in accordance with the present invention (FIG. 8B) mainly consists of tempered martensite. In this case, the volume content of bainite is below 5%. The tempered martensite structure also has a fine-grained distribution of iron carbides in the ferritic matrix. The main difference between the standard and new structures relates to the morphology of the ferrite grains and subgrains and to the concentration of dislocations. However, in terms of refinement and uniformity, both structures are similar.

In FIG. 9A-B show scanned electron microscope images corresponding to linear resistance welds. It can be seen that two structural components appear in the standard structure: soft ferrite grains and hard blocks consisting of a mixture of troostite, martensite and a small amount of retained austenite. In this structure, plastic deformation is localized in ferrite, and cracks can form and propagate in neighboring brittle components (in untempered martensite and high-carbon retained austenite). On the other hand, the microstructure of a linear resistance weld obtained with the chemical composition and processing conditions specified herein is uniform and very similar to the corresponding structure of the base metal.

In FIG. 10A-B show microstructures corresponding to the HAZ zone in resistance welding. In the standard material, there are signs of the presence of remnants of the central segregation band, which partially turns into untempered martensite after annealing the weld zone. Brittle components are also visible, localized along the linear resistance welding seam, while the formation and propagation of cracks can occur during processing. The risk of a defect increases compared to the previous case due to the larger size of the components mentioned above. On the other hand, in the quenched and tempered flexible pipeline, the structure next to the linear resistance weld is homogeneous, while the remains of the central segregation band are not observed.

In FIG. 11A-B are several scanning electron microscope images that correspond to the fillet weld HAZ in both standard coiled tubing and tubing made in accordance with the present invention. For standard materials, the microstructure of this zone is very different from the microstructure of the base metal (VM). It mainly consists of upper bainite and has a large grain size (50 microns, and not less than 15 microns for VM). This type of coarse grain structure is not suitable for low cycle fatigue due to the easy propagation of cracks along the bainite laths. FIG. 12 shows an example of fatigue cracks running in the region of coarse bainite in a fillet weld. These are secondary cracks located near the area of the main defect that appears during the processing of standard class 110 flexible conduit.

On the other hand, the microstructure of the fillet weld in the present invention is also very similar to that of the base metal. In this case, grains of upper bainite are not visible. It should be noted that some amount of bainite may be formed after heat treatment of the whole body, but due to the choice of suitable processing conditions and chemical composition, the corresponding volume content of this component will be below 10%. This is the main reason for achieving good hardenability of products with the chemistries described herein. In addition, due to the upper limit of the austenitization temperature, the final grain size is small (less than 20 microns) compared to large bainite laths, which completely eliminates the propagation of cracks.

Other examples of microstructural uniformity achieved by combining steel chemistry and processing conditions according to the present invention are shown in FIGS. 13, 14. In Figs. 13 shows the standard change in fillet weld hardness for coiled tubing manufactured in the standard manner compared to tubing produced using a new chemistry and manufacturing procedure. Obviously, when using this invention, the change in hardness is significantly reduced. As a consequence, the tendency of the material to collect deformations at localized locations (in this case, the fillet weld HAZ) is also reduced, and fatigue life is increased.

In FIG. 14A-B show some microstructures corresponding to the intersection of a fillet weld and a resistance welding line weld. Obviously, when using the standard procedure, a large microstructural inhomogeneity is achieved. This heterogeneity is successfully eliminated due to the chemical composition and processing conditions corresponding to the present invention.

Example E: Coiled Tubing Fatigue Test

A series of tests were carried out in the laboratory to compare the characteristics of the flexible conduit produced in accordance with the present invention with the characteristics of standard products. Coiled tubing specimens were tested in a durability test facility, which is shown schematically in FIG. 15. This machine can simulate bending deformations during winding and unwinding while generating internal pressure. Thus, these tests make it possible to rank materials in terms of low-cycle durability close to real operating conditions.

During the endurance test, specimens (pipes 5 or 6 feet long) are fixed at one end with a hydraulically actuated counter force applied to the opposite end. Deformation cycles created during testing by bending specimens around a bent mandrel of constant radius, followed by straightening in the opposite direction. Metal plugs are welded to the ends of the sample and connected to a hydraulic pump, which makes it possible to periodically fill the sample with water at a constant internal pressure until it bursts. The test ends when a drop in internal pressure is detected due to crack propagation along the wall.

The tests were carried out on flexible pipes with various chemical compositions and classes, as shown in Table E1. The geometric shape of the pipe is the same in all cases (OD 2ʺ, WT 0.19ʺ). STD1, STD2 and STD3 are steel grades meeting the limits specified in the API 5ST specification and manufactured according to a standard procedure. BTi ₁ , BTi ₂ and CrMoBTi ₄ are chemical compositions selected and made in accordance with the present invention. It should be noted that CrMoBTi ₄ steel was used to produce two non-standard grades with minimum yield strengths of 125 ksi and 140 ^ksi (the highest value described in the API 5ST specification has an SMYS of 110 ^ksi ). .lb/in ² ). The tests were carried out with pipe sections with or without a fillet weld (in all cases, the samples had a longitudinal linear resistance weld). The severity of the test conditions mainly depended on two parameters: bending radius and internal pressure. In this case, the bend radius was 48 inches, which corresponds to a plastic deformation of about 2%. The internal pressure values were assumed to be 1600 psi and 13500 psi ² , resulting ⁱⁿ hoop stresses ranging from (approximately) 10% to 60% of the minimum yield strength for these classes.

Table E1: Steel chemistry and grades of flexible pipes analyzed during this test.

In FIG. 16 presents some results regarding the comparison of fatigue life obtained in specimens with or without fillet weld (BW). The values indicated in the figure are the average values obtained from testing standard and non-standard classes of flexible pipes. When using a standard material, there is a noticeable reduction in fatigue life in the case of a fillet weld in the samples. On the other hand, coiled tubing made in accordance with the present invention does not show significant changes in fatigue life when tested on specimens with BW. This is a consequence of the homogeneity of the structure, which has practically no differences in mechanical properties for the base metal, the linear resistance weld and the fillet weld.

In FIG. 17 shows the increase in coiled tubing fatigue life achieved by the steel chemistry and processing conditions of the present invention. Grades 80, 90 and 110 were compared with equivalent pipes made using a standard procedure. As for non-standard grades 125 and 140, they were compared with steel grade STD3 grade 110, which was tested under similar conditions (pipe geometry, bending radius and internal pressure). The results presented in the figures correspond to the average values for each class, the "whiskers" of errors represent the distributions obtained using different values of internal pressure.

In FIG. 17 shows that the expected increase in fatigue life is achieved using the chemistry and processing conditions of the present invention. For example, for class 110, an increase equal to (approximately) 100% of fatigue life was obtained. This is because in standard coiled tubing life is limited by the fillet weld value (a common weak point in terms of low cycle fatigue due to microstructural heterogeneity and the presence of brittle components). In flexible pipes made in accordance with the present invention, there is no significant reduction in fatigue life at the fillet weld, which greatly increases the overall performance of the pipe. As for the non-standard grades, the large increase in fatigue life is due to the fact that they were compared to the standard grade 110 tested under similar machining conditions. However, at similar pressures, the generated hoop stresses were close to the minimum yield strength of the lower class numbers, and the severity of the test conditions for class 110 is higher than for classes 125 and 140. These results indicate that due to the use of steel with a higher class number (which cannot be achieved with the standard method) under similar machining conditions, the fatigue life increases significantly.

Example F: Sulfide Stress Crack Resistance

The hydrogen embrittlement of a material in an _H2S containing environment refers to the combined action of corrosive environments, the presence of deposits (e.g. precipitation and dislocations) which can lead to a local increase in hydrogen concentration, as well as the formation of brittle regions in which cracks can also easily propagate. . A potential source of critical areas of increased brittleness in standard coiled tubing materials is the segregation of substitutional elements, such as manganese, in the raw material. Areas of varying concentration are generated by other thermal cycles performed during fillet welding, PWHT, resistance welding, and weld zone annealing, and can lead to localized brittle components. In particular, when the material has annealed seam zones after resistance welding, the pipe body quickly releases heat from the seam zone. At sufficiently high segregation, elongated regions of increased hardness may form, in which martensite may be present, which is a consequence of the cooling conditions. These areas remain in the pipe, providing easy pathways for crack propagation. The fact that this process is used at the end of production makes it possible to minimize areas of increased hardness. Other significant differences include: a) no new product dislocations formed during cold forming of pipes, b) fewer carbides in the new product and their isolation compared to standard long regions of brittle pearlite/bainite carbides. As a result, flexible pipes made using the chemistry and processing conditions of the invention have improved crack resistance in H ₂ S containing environments.

Table F1: Steel chemistry and grades of flexible pipes analyzed during this test.

To perform the first SSC crack resistance analysis, Grade 80 coiled tubing specimens were obtained using: i) a standard process and ii) a new chemistry - the processes were evaluated using Method C (semi-circular profile ring), NACE TM0177. The chemical compositions of the steel are shown in Table F1. Both materials (3 samples for each case) were tested by performing a resistance weld in the center of a semi-circular profile ring under the following conditions:

load: 90% of 80 ksi, procedure A, 1 bar, H ₂ S, test duration: 720 hours ^.

When testing a standard flexible pipe, all 3 samples failed the test. In contrast, 3 samples corresponding to the new chemistry and procedure passed the tests (FIGS. 5A-B and images of semi-circular profile rings). Although tests were carried out to evaluate the embrittlement resistance of the various grades as well as the effect of the fillet weld, this first result showed a clear improvement over the standard conditions found in most homogeneous base metal microstructures and resistance weld line welding using the new procedure. production.

As shown in FIG. 18A-B, the hemi-ring produced by the standard procedure has more cracks in the middle compared to the hemi-circular ring produced by the embodiments of the disclosed process which did not crack.

In some embodiments, B-Ti and Cr-Mo additions may reduce the maximum manganese content. In some embodiments, classes above 110 are difficult to obtain using the standard method.

Features, materials, characteristics, or groups specified for various options should be considered as applicable to any other suitable option described in this document. All features discussed in this specification (including the claims, abstract, and accompanying drawings) and/or all disclosed steps or processes may be combined in various combinations, except combinations of at least some of the features and (or) steps are mutually exclusive. The scope of protection is not limited to the detailed description of the following embodiments. Protection extends to all new features or combinations thereof disclosed in this specification (including the claims, abstract and accompanying drawings) or to all new steps of the method or process disclosed herein, or combinations thereof.

Although specific options have been discussed above, they are presented here by way of example only and should not be construed as limiting the scope of protection. In addition, various omissions, substitutions, and changes may be made to the methods, compositions, and apparatuses described herein. Those skilled in the art will appreciate that, in some embodiments, some of the steps shown and/or disclosed in the processes may differ from those depicted in the figures. Depending on the options, some of the steps described above may be excluded or added. In addition, the features and attributes of the particular embodiments disclosed above may be combined in various forms to form additional variations that will be part of the gist of the present invention.

Although specific embodiments, examples, and uses are presented herein, those skilled in the art should understand that the present disclosure is outside the scope of the specific embodiments disclosed and includes alternative implementations and/or uses, as well as obvious combinations of changes. and equivalents, including options that do not have all of the features and benefits described herein.

Claims (94)

1. A flexible steel pipe comprising steel strips end-to-end butt-welded with fillet welds and formed into a tube by welding, wherein each strip is made of C-Mn steel, B-Ti steel, or Cr-Mo steel; moreover, the pipe has areas of the base metal, welds and their heat-affected zones, while the pipe has a yield strength in excess of 80 thousand pounds/inch ² ; the final microstructure containing a mixture of tempered martensite and bainite; moreover, the final microstructure of the flexible pipe in all areas of the base metal, on welds and in heat-affected zones, more than 90% by volume consists of tempered martensite, is homogeneous with a grain size of less than 20 μm and has a uniform distribution of fine-grained carbides. 2. Flexible steel pipe according to claim 1, characterized in that each strip is made of steel containing the following components, wt.%: carbon 0.17-0.35; manganese 0.30-2.00; silicon 0.10-0.30; aluminum 0.010-0.040; sulfur up to 0.010; phosphorus up to 0.015; chromium up to 1.0; molybdenum up to 0.5; boron up to 0.0030; titanium up to 0.030; copper up to 0.50; nickel up to 0.50; niobium up to 0.1; vanadium up to 0.15; oxygen up to 0.0050; calcium up to 0.05; iron and impurities - the rest. 3. Flexible steel pipe according to claim 2, in which each strip is made of steel containing the following components, wt.%: carbon from 0.17 to 0.30; manganese from 0.30 to 1.60; silicon from 0.10 to 0.20; chromium up to 0.7; molybdenum up to 0.5; boron from 0.0005 to 0.0025; titanium from 0.010 to 0.025; copper from 0.25 to 0.35; nickel from 0.20 to 0.35; niobium up to 0.04; vanadium up to 0.10; oxygen up to 0.0015; calcium up to 0.03; sulfur up to 0.003; phosphorus up to 0.010; iron and impurities - the rest. 4. Flexible steel pipe according to claim. 1, characterized in that it has a minimum yield strength equal to 125 thousand psi ² . 5. Flexible steel pipe according to claim. 1, characterized in that it has a minimum yield strength equal to 140 thousand psi ² . 6. Flexible steel pipe according to claim. 1, characterized in that it has a minimum yield strength in the range from 125 thousand pounds/inch ² to 140 thousand pounds/inch ² . 7. Flexible steel pipe according to claim 1, characterized in that it has a final microstructure in areas of the base metal, at welds and in heat affected zones, consisting of at least 95% tempered martensite by volume. 8. Flexible steel pipe according to claim 1, characterized in that it has a microstructure with a grain size of less than 15 microns in the areas of the base metal, on welds and in heat affected zones. 9. Flexible steel pipe according to claim 1, characterized in that it has a fillet weld fatigue strength of at least about 80% of the base metal fatigue strength. 10. Flexible steel pipe according to claim. 1, characterized in that it has a hardness in the welds, including the heat-affected zone, 110% or less of the hardness of the base metal. 11. A method of manufacturing a flexible steel pipe according to any one of paragraphs. 1-10, including the production of strips of C-Mn steel, B-Ti steel or Cr-Mo steel, butt-welding with fillet welds, each of the welded strips having a base metal region, a weld, and a heat-affected zone; forming a pipe from welded strips by welding; austenization of the pipe at a temperature of 900-1000°C; quenching the pipe by cooling at a rate to produce a hardened microstructure containing at least 90% martensite, the balance being bainite, in areas of the base metal, at welds, and in heat affected zones; tempering the hardened pipe to provide a final microstructure containing at least 90% tempered martensite, the rest bainite, in all areas of the base metal, at welds and in heat affected zones, and a yield strength above 80 thousand psi ² ; moreover, the microstructure of the flexible pipe in all areas of the base metal, on welds and in heat-affected zones is homogeneous with a grain size of less than 20 microns and has a uniform distribution of fine-grained carbides. 12. The method according to p. 11, characterized in that after the tempering, the pipe is wound onto the bay. 13. The method according to p. 11, characterized in that strips are obtained from steel containing the following components, wt.%: carbon 0.17-0.35; manganese 0.30-2.00; silicon 0.10-0.30; aluminum 0.010-0.040; sulfur up to 0.010; phosphorus up to 0.015; chromium up to 1.0; molybdenum up to 0.5; boron up to 0.0030; titanium up to 0.030; copper up to 0.50; nickel up to 0.50; niobium up to 0.1; vanadium up to 0.15; oxygen up to 0.0050; calcium up to 0.05; iron and impurities - the rest. 14. The method according to p. 13, in which strips are obtained from steel containing the following components, wt.%: carbon from 0.17 to 0.30; manganese from 0.30 to 1.60; silicon from 0.10 to 0.20; chromium up to 0.7; molybdenum up to 0.5; boron from 0.0005 to 0.0025; titanium from 0.010 to 0.025; copper from 0.25 to 0.35; nickel from 0.20 to 0.35; niobium up to 0.04; vanadium up to 0.10; oxygen up to 0.00015; calcium up to 0.03; sulfur up to 0.003; phosphorus up to 0.010; iron and impurities - the rest. 15. The method according to p. 11, characterized in that the pipe is made with a yield strength of at least 125 thousand psi ² . 16. The method according to p. 11, characterized in that the pipe is made with a yield strength of at least 140 thousand psi ² . 17. The method according to p. 11, characterized in that the pipe is made with a yield strength in the range from 125 to 140 thousand psi ² .

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