EP3527677A1 - Hydrogen-embrittlement-resistant steel rod with high mechanical characteristics - Google Patents

Abstract

Shaft made of low-alloy carbon steel with high mechanical properties and resistant to hydrogen embrittlement, intended to be used as a component of flexible pipes for the off-shore oil sector. The yarn has the following chemical composition, expressed in percentages by weight of the total mass, 0.75 <C% <0.95 and 0.30 <Mn% <0.85 with Cr ≤ 0.4%; V ≤ 0.16%; If ≤ 1.40% and preferably ≥ 0.15%, and possibly no more than 0.06% AI, not more than 0.1% Ni, and not more than 0.1% Cu, the the remainder being iron and the inevitable impurities coming from the elaboration of the metal in the liquid state; the shape yarn has a pearlitic structure with possible traces of ferrite, without bainite or martensite; the shaped wire has a breaking strength of at least 1300 MPa.

Description

The present invention relates to the field of metallurgy dedicated to the exploitation of marine oil. It relates more particularly to steel wires which can be used as reinforcement elements or structural components or as deep water immersed structures, such as flexible offshore pipes.

It is known that a primary requirement for yarns of this type is, in parallel with high mechanical characteristics, good resistance to embrittlement by hydrogen in a sulfuric acid medium, in particular in the form of H ₂ S present in the fluids and transported hydrocarbons.

• la norme NACE TM 0284 pour la tenue à la fissuration par l'hydrogène ou "HIC" (Hydrogen Induced Cracking) en eau de mer saturée en H₂S acide;
• la norme NACE TM 0177 pour la tenue à la fissuration sous contraintes par H₂S, ou "SSCC" (Sulfide Stress Corrosion Cracking) en milieu acide. Les fils de forme, dans l'utilisation considérée ici, doivent impérativement y répondre de nos jours face à des conditions d'exploitation de plus en plus difficile (forte profondeur);
• et la norme API 17J (Spécifications for unbonded flexible pipes) pour l'évaluation des tenues HIC et SSCC sur la base d'un test sous contrainte dans un milieu acide.

It is recalled that this resistance is the subject of NACE and API standards, in particular:

• the NACE TM 0284 standard for resistance to hydrogen cracking or "HIC" (Hydrogen Induced Cracking) in seawater saturated with H ₂ S acid;
• the NACE TM 0177 standard for resistance to stress cracking by H ₂ S, or "SSCC" (Sulfide Stress Corrosion Cracking) in an acid medium. The son of form, in the use considered here, must imperatively answer them nowadays face of conditions of exploitation more and more difficult (deep depth);
• and API 17J (Specification for unbonded flexible pipes) for the evaluation of HIC and SSCC suits based on a stress test in an acid environment.

These form wires may have a round cross section, obtained by simple drawing from a wire of larger diameter. They can also, after drawing, rolling, or drawing followed by rolling, have a flat section, or profiled U, Z, T, etc.. in such a way that they can be interlocked into each other or stapled to form articulated reinforcing plies.

Today, the offer in the field of NACE quality steel wire for off shore use is mainly in low alloy steel grades offering, after quenching and tempering, around 900 MPa of breaking strength (Rm).

To manufacture these form yarns, it is customary to use, in known manner, carbon-manganese steels at 0.15-0.80% C (by weight), with a perlito-ferritic initial structure. Conventionally, after forming the initial round laminated machine wire, it is applied a relaxing heat treatment appropriate to obtain the required hardness. It is by this level of hardness that one respects the nominal criteria of use, for example the standard ISO 15156 stipulating that these nuances of steel with Mn have a holding under constraints in medium H ₂ S suitable for use "wire form" retained here, if the hardness of the wire is less than or equal to 22 HRC.

However, the form yarns obtained by the traditional processes have the reputation of being difficult to withstand the relatively severe acidity conditions encountered in deep water, those provided by the NACE TM 0177 standard with solution A (pH 2.7 to 4). in this case, due to a strong presence of H ₂ S in the hydrocarbon transported, and even more so if the target hardness levels are greater than 28 HRC (more than 900 MPa).

This is probably the reason why the document PCT / FR91 / 00328 published in 1991 describes a thermomechanical process for the production of a perlito-ferritic structure-shaped yarn with a titre between 0.25 and 0.8% carbon and meets the NACE TM 0177 and TM 0284 standards with solution B (pH 4.8 to 5.4), but at the cost of a final income of relaxation of the mechanical stresses printed by the hardening of the metal which lowers the mechanical resistance to fracture (Rm) to approximately 850 MPa.

The document FR-B-2731371 published in 1996 also deals with the production of carbon steel shaped wires for the reinforcement of flexible off-shore pipes whose resistance in acidic medium with H ₂ S is sought at a high level on the basis of general knowledge. on the influence of steel microstructures on its resistance to embrittlement by hydrogen. The form wire proposed in this document, which contains from 0.05 to 0.8% of C and from 0.4 to 1.5% of Mn, has undergone, after shaping (drawing or drawing-rolling), a temper followed by an income in the end. The metal structure obtained is essentially a martensito-bainitic income. Thus, ready-to-use form wires with high mechanical characteristics would be obtained, ie an Rm at around 1050 MPa (thus in a quenched-tempered steel to reach hardness levels as high as 35 HRC, but industrially noted in rather, it is around 820 MPa) and can therefore be well above those recommended by the ISO 15156 standard, and resistant to very acidic media (pH close to 3). It is stated that, in the absence of a final income, it is possible to obtain a wire of greater hardness having even higher mechanical characteristics, but therefore with a markedly lower chemical resistance to acidic environments.

In fact, it turns out that the very high level characteristics of such wires must be satisfied only in a limited number of use cases.

In accordance with the NACE quality, a suitability in accordance with the aforementioned API 17J standard, with a H ₂ S partial pressure of up to 0.1 bar and with a pH of 3.5 to 5, would indeed be sufficient to cover most of the actual requirements, whereas the form wires manufactured by the process according to the document referred to above have a held say overqualified because meeting the high requirements of standards TM 0177 and TM 0284 established with solution A having a pH of about 3.

Furthermore, it turns out that the usual form of the market son, perlito-ferritic structure without final heat treatment are most of the time unable to meet NACE requirements, even moderate.

In addition, flexible off-shore pipes being brought to be used for immersion depths of greater and greater, a demand is now expressed in favor of an even greater resistance of a few hundred MPa, to reach say resistances of the order of 1300 MPa or more, without degrading so far NACE quality, while, it should be remembered, embrittlement of steel by hydrogen and mechanical properties are opposite properties: want to promote the one is to do it to the detriment of the other, and vice versa.

In addition, the market constraint is increasingly strong on prices, which correlatively obfuscates the usual appeal to noble alloying elements, such as chromium, niobium, etc .... or processing steps long or multiple and therefore expensive, especially if they must be conducted hot.

In this respect, the teaching of JP 59001631 A of 1984 ( DATA BASE WPI Week 198407 Thomsom Scientific, London, GB; AN 1984-039733 ) which recommends a final treatment of restoration of the long-lasting wire, in the form of an annealing which lasts several hours.

Similarly, the method described in EP 1 063 313 A1 imposes very high rates of strain hardening of the wire, of about 85%, to reach by drawing to the desired final diameter.

We will still note the existence of the EP 1 273 670 on the manufacture of steel bolts, but whose teaching emphasizes the advantage that can be expected on the resistance to stress corrosion of pearlitic bolts.

The aim of the invention is to achieve an optimal balance between a good resistance to wet embrittlement by hydrogen under the conditions of use of the shaped wire, and an increased mechanical strength thereof, and this in the framework of an industrial production which will make it possible to offer the yarn on the market in attractive economic conditions.

For this purpose, the subject of the invention is a shaped wire according to claim 1.

The form wire may also include the features of claims 2 to 12, taken alone or in combination.

et $$0,30<\mathrm{Mn}\%<0,85$$

avec Cr ≤ 0,4%; V ≤ 0,16%; Si ≤ 1,40% et de préférence ≥ 0,15%;
et éventuellement pas plus de 0,06% d'AI, pas plus de 0,1% de Ni, et pas plus de 0,1% de Cu, le reste étant du fer et les inévitables impuretés venant de l'élaboration du métal à l'état liquide,
et en ce que, à partir d'un fil machine, laminé à chaud dans son domaine austénitique au dessus de 900°C puis refroidi à la température ambiante, et présentant alors un diamètre de 5 à 30 mm environ, le fil de forme est obtenu en soumettant ledit fil machine de départ d'abord à un traitement thermomécanique selon deux étapes successives et ordonnées, à savoir une trempe isotherme (classiquement un patentage au plomb) qui lui confère une microstructure perlitique homogène, suivie par une opération de transformation mécanique à froid (tréfilage, ou tréfilage + laminage) menée avec un taux d'écrouissage global compris entre 50 et 80% maximum environ (et, si possible, de préférence autour de 60%) pour donner au fil sa forme définitive, et en ce que le fil de forme ainsi obtenu est alors soumis à un traitement thermique de restauration de courte durée (de moins d'une minute de préférence) mené en dessous de la température Ac1 de l'acier qui le constitue (de préférence entre 410 et 710°C), lui conférant les caractéristiques mécaniques désirées.Also described is a low-alloy carbon steel wire having high mechanical characteristics and resistant to embrittlement with hydrogen, a shaped wire for use in the off-shore oil exploitation sector, characterized in that that it has the following chemical composition, expressed in percentages by weight of the total mass, $$0,75<\mathrm{C}\%<0,95$$

and $$0,30<\mathrm{mn}\%<0,85$$

with Cr ≤ 0.4%; V ≤ 0.16%; If ≤ 1.40% and preferably ≥ 0.15%;
and optionally not more than 0.06% Al, not more than 0.1% Ni, and not more than 0.1% Cu, the balance being iron and the inevitable impurities from the elaboration of the metal in the liquid state,
and in that, from a wire rod, hot rolled in its austenitic range above 900 ° C and then cooled to room temperature, and then having a diameter of about 5 to 30 mm, the shaped wire is obtained by subjecting said starting wire rod first to a thermomechanical treatment according to two successive and ordered steps, namely an isothermal quench (conventionally a lead patenting) which gives it a homogeneous pearlitic microstructure, followed by a mechanical transformation operation to cold (wire drawing, or wire drawing + rolling) carried out with an overall work hardening rate of between approximately 50 and 80% (and, if possible, preferably around 60%) to give the wire its final shape, and in that the shaped wire thus obtained is then subjected to a short-term heat treatment (less than one minute preferably) conducted below the temperature Ac1 of the steel which constitutes it (of preferably between 410 and 710 ° C), giving it the desired mechanical characteristics.

The invention, which has just been defined above, is based on the triptique: "steel grade -treatment -application" and can be seen as an optimization of the knowledge acquired by the applicant in the field of wire metallurgy steel intended for use in the deep sea.

• une nuance d'acier simplifiée, à savoir un acier au carbone (au moins 0,75%) et au manganèse, qui s'inscrit donc en opposition avec les teneurs en carbone bien plus basses couramment retenues, et sans ajout d'éléments trempant, mais de préférence alliée avec des éléments dispersoïdes, comme le vanadium et le chrome, pour obtenir une distribution homogène de fins carbures dans toute la matrice métallique;
• cette nuance est produite à partir d'un fil machine laminé à chaud puis refroidi à l'ambiante (donc de structure ferrito-perlitique ordinaire venue de l'austénite du laminage à chaud), mais dont le diamètre (entre 5 et 30 mm environ) est réduit par rapport à la pratique habituelle. Cette disposition autorisera sa transformation en fil de forme final prêt à l'emploi par des opérations de mise en forme mécanique douce, c'est à dire sans un écrouissage trop marqué à coeur qui pourrait créer des zones d'hétérogénéité, étant précisé que c'est, bien entendu, à l'opérateur chargé du procédé de fabrication d'ajuster les paramètres de fonctionnement (réglages des paramètres opérationnels, choix des filières et des cannelures des cylindres de laminage) pour limiter les écrouissages locaux à coeur du fil.

More explicitly, this triptique is detailed as follows:

• a simplified steel grade, namely a carbon steel (at least 0.75%) and manganese, which is therefore in opposition to the much lower carbon content currently retained, and without adding soaking elements but preferably alloyed with dispersoid elements, such as vanadium and chromium, to obtain a homogeneous distribution of fine carbides throughout the metal matrix;
• this grade is produced from a hot-rolled machine wire then cooled to ambient temperature (thus of ordinary ferrito-pearlitic structure coming from the austenite of hot rolling), but whose diameter (between 5 and 30 mm approximately ) is reduced compared to usual practice. This arrangement will allow its transformation into final form wire ready for use by gentle mechanical shaping operations, ie without a hardening too marked to heart that could create areas of heterogeneity, it being understood that it is, of course, the operator responsible for the manufacturing process to adjust the operating parameters (settings of operational parameters, choice of sectors and grooves rolling rolls) to limit local hardening at the heart of the wire.

• ce fil est un fil de forme, plat, méplat ou profilé, destiné à l'exploitation pétrolière "off shore" pour constituer du fil d'armage, de frette ou de voûte entrant dans la structure des pipe-lines et autres conduites flexibles. Comme on le sait, les fils de forme en acier évoluent dans les pipe-lines entre deux couches de polymères extrudés, dans une zone appelée "annulaire". Les conditions physicochimiques prévalentes dans cette zone, lors de l'utilisation du flexible, sont aujourd'hui mieux connues. Elles dépendent de la nature de l'effluent dans le flexible (hydrocarbures liquides ou gazeux) et de la structure des différentes couches du flexible. En particulier, le pH est plus élevé qu'on ne le pensait dans les années 1990/2000 (plutôt en moyenne autour de 5,5 que de 4).

The microstructure to be created by isothermal quenching is perlite. Being easy to obtain industrially, the perlite will ensure a metallurgical structure as homogeneous as possible throughout the mass of the wire obtained and will be able to undergo the deformations applied by drawing and / or rolling

• this wire is a form of wire, flat, flat or profiled, for offshore oil exploitation to constitute wire armoring, fret or vault entering the structure of pipelines and other flexible pipes. As is known, the steel form son evolve in the pipelines between two layers of extruded polymers in an area called "annular". The physicochemical conditions prevalent in this area, when using the hose, are now better known. They depend on the nature of the effluent in the hose (liquid or gaseous hydrocarbons) and the structure of the different layers of the hose. In particular, the pH is higher than previously thought in the years 1990/2000 (on average around 5.5 rather than 4).

The invention thus finds its primary cause in the discovery of these new, less drastic conditions to be satisfied in the region of the annulus, which allow the use of form wires with higher mechanical strength.

In other words, the NACE quality of today can be expressed quite validly through test results less severe than those provided by the API standard (the applicant has had to adapt the test conditions to the API standard, especially pH, to adapt to the demand). For example, the NACE quality can be recognized for a steel wire that has withstood without breakage or internal cracking for one month under a continuous stress of 90% of the Re in an aqueous solution having a pH between 5 and 6.5 and subject to bubbling of a gas containing CO ₂ and a few millibars of H ₂ S.

The invention will be well understood and other aspects and advantages will become more apparent from the following description given by way of example.

Table I, given on the last page of this description, shows seven examples of chemical compositions of shades according to the invention, which is identified in the first column by a nomenclature internal to the applicant.

We will now consider in detail an example of a composition taken from the steel grade referenced C88 (penultimate line of Table I), whose components present have the following precise weight contents: C: 0.861%, Mn: 0.644%, P: 0.012%, S: 0.003%, Si: 0.303%, Al: 0.47%, Ni: 0.015%, Cr: 0.032%, Cu: 0.006%, Mo: 0.003%, and V: 0.065%.

From a round wire rod 12 mm in diameter, from this composition, a final wire ready for use of flat form of 9 mm × 4 mm is produced according to the following successive operations.

It is indicated in advance that, in accordance with the invention, it will not exceed 30 mm in diameter for the starting machine wire taken cold, so as not to have to wrinkle the core of the wire markedly during drawing subsequent conducted with an overall wrought rate of not more than 80% to achieve the desired final diameter of the form wire ready to use.

The wire rod is a hot-rolled steel wire, ie in its austenitic range (typically above 900 ° C) which is then rapidly cooled in the hot rolling mill before winding it into a coil to finish cool to room temperature on a storage area awaiting delivery to the customer.

Once delivered to the transformer, this starting wire, which is unrolled from its coil, is first subjected, from the ambient temperature, isothermal quenching. Conventionally, it will be a patenting at constant temperature around 520-600 ° C by passing in a bath of molten lead, before cooling. This patenting gives the steel wire a pearlitic microstructure, with possible traces of ferrite, but without bainite or martensite, and that it will keep until the end.

The wire is then drawn (round or already flat) in a "soft" manner, that is to say, as already mentioned above, so as to limit as much as possible the level of core stresses that the metal wrought will confer on it. The reason for this is that damage to the core microstructure should be limited, which would create sites favorable to a preferential accumulation of hydrogen. The wire may then be subjected to a cold rolling final setting, it being specified that the rate of overall hardening (drawing + rolling) will be between 50 and 80% maxi, and, if possible preferably around 60%.

The intermediate yarn thus obtained has an Rm of about 1900 MPa.

It remains to soften to facilitate its subsequent shaping and confer its properties of resistance to embrittlement by hydrogen, somewhat impaired by hardening. For this purpose, a simple final heat treatment of fast restoration, therefore at a temperature below its value of Ac1 (ie between 410 and 710 ° C for the whole range of grades of steel used) and less of a minute, him will confer desired final Rm, whose exact value will of course depend on the operating conditions of this restoration treatment.

In this respect, Table II below gives the final mechanical characteristics obtained for a shaped wire having undergone a fast heat treatment of restoration under the following operating conditions, indicated by the lines A to E: stay of a duration of 5 seconds , at a temperature below the Ac1 temperature of the steel grade considered and given in the second column of the table, before brutal cooling with water.

The other columns indicate respectively the average ultimate limit Rm, the average yield strength Re, the average elongation rate at break A% of the treated wire resulting from the thermomechanical operations applied, and the Re / Rm ratio.

It should be noted, as one might expect, that the Rm, like the Re, decreases regularly as the recovery temperature increases (lines from A to E). The ratio Re / Rm remains constant and the rate of elongation A% increases in the same direction. <B> Tab. It </ b> Temp. Restoration (° C) Rm avg (MPa) Re avg. (MPa) A% avg. Re / Rm AT 410 1920 1730 9.6 0.90 B 500 1760 1530 9.7 0.86 C 600 1550 1360 11.0 0.87 D 635 1480 1280 12.0 0.86 E 675 1380 1190 11.6 0.86

The NACE tests, according to the HIC (Hydrogen Induced Cracking) and SSC (Sulfide Stress Cracking) type, were carried out on each of the yarns obtained after these different restoring treatments. The data and results are shown in Table III below.

We see that all the samples analyzed respond positively to the tests: after ultrasonic testing, we do not observe internal blister-type cracks, which would reflect hydrogen embrittlement. <B> Tab. III </ b> Rm (in MPa) NACE test type Duration (in days) H ₂ S% pH Constraint applied in SSC Results in the US AT 1920 HIC + SSC 30 0.1 5.8 90% Re RAS B 1760 HIC + SSC 30 0.1 5.8 90% Re RAS C 1550 HIC + SSC 30 0.22 5.6 90% Re RAS D 1480 HIC + SSC 30 0.22 5.6 90% Re RAS E 1380 HIC + SSC 30 0.22 5.6 90% Re RAS

It goes without saying that the invention can not be limited to the examples described, but that it extends to multiple variants and equivalents to the extent that its definition is fulfilled by the appended claims.

Claims (12)

Low-alloy carbon steel wire with high mechanical properties and resistance to hydrogen embrittlement, shaped wire for use as a component of flexible pipes for the off-shore oil exploitation sector, characterized in that it has the following chemical composition, expressed in percentages by weight of the total mass,
0.75 <C% <0.95 and
0.30 <Mn% <0.85
with Cr ≤ 0.4%; V ≤ 0.16%; If ≤ 1.40% and preferably ≥ 0.15%,
and optionally not more than 0.06% Al, not more than 0.1% Ni, and not more than 0.1% Cu, the balance being iron and the inevitable impurities from the elaboration of the metal in the liquid state;
in that the shaped wire has a pearlitic structure with possible traces of ferrite, without bainite or martensite;
and in that the shaped wire has a breaking strength (Rm) of at least 1300 MPa. A shape yarn according to claim 1, wherein the form yarn is obtained from a hot rolled wire in its austenitic range above 900 ° C and then cooled to room temperature and then having a diameter of 5 to 30 mm, by subjecting said wire rod firstly to a thermomechanical treatment carried out in two successive and ordered steps, namely an isothermal quench which gives it a homogeneous pearlitic microstructure, followed by a cold mechanical transformation operation carried out with a rate of overall hardening of between 50 and 80% maximum to give the wire its final shape, and in that the shaped wire thus obtained is then subjected to a short-term heat treatment carried out below the temperature Ac1 of the steel forming it, giving it said breaking strength of at least 1300 MPa. Shaped wire according to any one of claims 1 or 2, wherein the shaped wire is able to resist without breakage or internal cracking for a month under continuous stress of 90% of the limited elasticity (Re) in a solution aqueous having a pH between 5 and 6.5 and bubbled with a gas containing CO ₂ and a few millibars of H ₂ S. Shaped wire according to any one of claims 1 or 2, wherein the shaped wire is able to resist without breakage or internal cracking for a month under continuous stress of 90% of the limited elasticity (Re) in a solution aqueous having a pH between 5 and 6.5 and bubbled with a gas containing CO ₂ and between 0.1% and 0.22% H ₂ S. A shaped wire according to any one of claims 1 to 4, the shaped wire has a breaking strength (Rm) of less than 1900 MPa. A shaped wire according to any one of claims 1 to 5, wherein the chemical composition comprises between 0.15% and 0.30% Si. A form yarn according to claim 6, wherein the chemical composition comprises between 0.75 and 0.88% C and between 0.50 and 0.70% Mn. A form yarn according to claim 6, wherein said chemical composition comprises, in weight percentages of the total mass: 0.75 <C% <0.85 0.50 <Mn% <0.70 P% ≤ 0.02% S% ≤ 0.02 0.15 ≤ Si% ≤ 0.30 0.02 ≤ Al% ≤ 0.06 Ni% ≤ 0.08 Cr% ≤ 0.10 Cu% ≤ 0.10 the rest being iron and the inevitable impurities coming from the development of the metal in the liquid state. The form yarn of claim 8, wherein: 0.75 <C% <0.80 and Cu% ≤ 0.08; or 0.80 <C% <0.85 and Cu% ≤ 0.10. A form yarn according to claim 6, wherein said chemical composition comprises, in weight percentages of the total mass: 0.82 <C% <0.88 0.65 <Mn% <0.85 P% ≤ 0.02 S% ≤ 0.02 0.15 ≤ Si% ≤ 0.30 0.02 ≤ Al% ≤ 0.06 Ni% ≤ 0.10 Cr% ≤ 0.10 Cu% ≤ 0.10 the rest being iron and the inevitable impurities coming from the development of the metal in the liquid state. A shaped wire according to any one of claims 1 to 5, wherein said chemical composition comprises, in weight percentages of the total mass: 0.77 <C% <0.85 0.65 <Mn% <0.85 0.02 ≤ Cr% ≤ 0.10 Cu% ≤ 0.10 0.03 ≤ V% ≤ 0.16, the rest being iron and the inevitable impurities coming from the development of the metal in the liquid state. A shaped wire according to any one of claims 1 to 5, wherein said chemical composition comprises, in weight percentages of the total mass: 0.80 <C% <0.90 0.50 <Mn% <0.70 P% ≤ 0.02 S% ≤ 0.02 0.20 ≤ Si% ≤ 0.35 0.02 ≤ Al% ≤ 0.06 Ni% ≤ 0.10 Cr% ≤ 0.10 Cu% ≤ 0.10 0.05 ≤ V% ≤ 0.10 the rest being iron and the inevitable impurities coming from the development of the metal in the liquid state.