Process for manufacturing a profiled steel wire

Abstract

A process for the manufacture of a profiled wire of hydrogen-embrittlement-resistant, low-alloy carbon steel for flexible pipelines for the offshore oil and gas operations sector is provided. The process includes providing a low-alloy carbon steel wire rod having a composition including, expressed in percentages by weight of the total mass 0.75<C %<0.95; 0.30<Mn %<0.85; Cr0.4%; V0.16%; and Si1.40%, the rest being iron and the inevitable impurities from smelting of the metal in the liquid state. The process further includes hot-rolling the wire rod in an austenitic region above 900 C., cooling the wire rod to ambient temperature, subjecting the wire rod to isothermal quenching to obtain a homogeneous pearlitic microstructure, subjecting the wire rod to an operation of cold mechanical transformation, carried out with a global work-hardening ratio of from approximately 50 to 80%, to give the wire rod a diameter of from approximately 5 to 30 mm and subjecting the drawn wire to a short-duration recovery heat treatment carried out below an Ac1 temperature of the steel.

Claims

1. A process for the manufacture of a profiled wire of hydrogen-embrittlement-resistant, low-alloy carbon steel for flexible pipelines for the offshore oil and gas operations sector comprising: providing a low-alloy carbon steel wire rod having a composition including, expressed in percentages by weight of the total mass:
75<C %<0.95;
30<Mn %<0.85;
Cr0.4%;
V0.16%; and
Si1.40%, the rest being iron and the inevitable impurities from smelting of the metal in the liquid state; hot-rolling the wire rod in an austenitic region above 900 C.; cooling the wire rod to ambient temperature; subjecting the wire rod to isothermal quenching to obtain a homogeneous pearlitic microstructure; subjecting the wire rod to an operation of cold mechanical transformation, carried out with a global work-hardening ratio of from approximately 50 to 80%, to give the wire rod a diameter of from approximately 5 to 30 mm; subjecting the drawn wire to a short-duration recovery heat treatment carried out below an Ac1 temperature of the steel, wherein the short-duration recovery heat treatment is carried out at a temperature from 410 to 710 C. for a duration of one minute or less.

2. The process for the manufacture of a profiled wire as recited in claim 1, wherein the isothermal quenching is a patenting operation in a molten lead bath.

3. The process for the manufacture of a profiled wire as recited in claim 2, wherein the patenting occurs at a constant temperature in a range from 520 to 600 C.

4. The process for the manufacture of a profiled wire as recited in claim 1, wherein the cold mechanical transformation includes drawing and cold rolling.

5. The process for the manufacture of a profiled wire as recited in claim 1, wherein the short-duration recovery heat treatment results in a mean tensile strength Rm of 1380 to 1920 MPa.

6. The process for the manufacture of a profiled wire as recited in claim 2, wherein the short-duration recovery heat treatment results in the profiled wire having a mean tensile strength Rm of 1380 to 1920 MPa.

7. The process for the manufacture of a profiled wire as recited in claim 1, wherein the short-duration recovery heat treatment results in the profiled wire having a mean yield strength Re of 1190 to 1730 MPa.

8. The process for the manufacture of a profiled wire as recited in claim 1, wherein the short-duration recovery heat treatment results in the profiled wire having a mean elongation at break from 9.6% to 12.0%.

9. The process for the manufacture of a profiled wire as recited in claim 1, comprising 1.4%Si0.15%.

10. The process for the manufacture of a profiled wire as recited in claim 1, wherein the composition includes Al0.06%.

11. The process for the manufacture of a profiled wire as recited in claim 1, wherein the composition includes Ni0.1%.

12. The process for the manufacture of a profiled wire as recited in claim 1, wherein the composition includes Cu0.1%.

Description

DETAILED DESCRIPTION

(1) Table I, presented on the last page of this description, shows seven examples of chemical compositions of grades conforming to the invention, identified in the first column by a nomenclature internal to the Applicant.

(2) An example of composition will now be considered in detail, taken from the steel grade referenced C88 (second-last row of Table I), the present components of which satisfy the following precise contents by weight: C: 0.861%; Mn: 0.644%, P: 0.012%, S: 0.003%, Si: 0.303%, AI: 0.47%, Ni: 0.015%, Cr: 0.032%, Cu: 0.006%, Mo: 0.003%, and V: 0.065%.

(3) Starting from a round wire rod of 12 mm diameter, a final ready-to-use wire of rectangular shape with dimensions of 9 mm4 mm is produced according to the following successive operations.

(4) It is pointed out beforehand that, in agreement with the invention, a diameter of 30 mm for the starting wire rod while cold will not be exceeded, in order to avoid pronounced work-hardening of the core of the wire during the subsequent drawing, which is carried out with a global work-hardening ratio not exceeding 80%, so as to achieve the desired final diameter of the ready-to-use profiled wire.

(5) The wire rod is a hot-rolled steel rod, i.e. in its austenitic range (traditionally above 900 C.), which is then cooled rapidly in the rolling heat before being wound in a coil to complete cooling to ambient temperature in a storage area, while awaiting delivery to the customers.

(6) Once delivered to the processing shop, this starting wire rod, which is unwound from its coil, is first subjected to isothermal quenching from room temperature. Traditionally this involves patenting at constant temperature around 520-600 C. by passage through a molten lead bath, before cooled. This patenting confers on the steel wire a pearlitic microstructure, with possible traces of ferrite but without bainite or martensite, which structure it will retain until the end.

(7) The wire is then drawn (round or already rectangular) in gentle manner, which means, as already mentioned hereinabove, in such a way as to limit to the maximum the level of stresses at the core, which will confer thereon the work-hardening of the metal. The reason for this is that it is advisable to limit the damage to the microstructure at the core, which damage would create sites favorable to preferential accumulation of hydrogen. It will then be possible to subject the wire to cold-rolling to achieve the final dimensions, its being clarified that the global work-hardening ratio (drawing+rolling) will be between 50 and 80% maximum, and preferably around 60% if possible.

(8) The intermediate wire obtained in this way has an Rm of approximately 1900 MPa.

(9) It still has to be softened to facilitate its subsequent shaping and to confer its properties of resistance to hydrogen-induced embrittlement, since these are little altered by the work-hardening. For this purpose, a simple final, rapid recovery heat treatment, therefore at a temperature below its Ac 1 value between 410 and 710 C. for the entire range of steel grades used), lasting less than one minute, will confer on it the desired final Rm, the exact value of which will of course depend on the operating conditions of this recovery treatment.

(10) In this regard, Table II hereinafter presents the final mechanical characteristics obtained for a profiled wire that has been subjected to a rapid recovery heat treatment under the following operating conditions, identified by rows A to E: dwell time of 5 seconds at a temperature below the Ac 1 temperature of the steel grade under consideration and given in the second column of the table, before quench cooling in water.

(11) The other columns respectively indicate the mean tensile strength Rm, the mean yield strength Re, the mean elongation at break A % of the treated wire resulting from the applied thermomechanical operations, and the Re/Rm ratio.

(12) It will be noted, as could have been expected, that both the Rm and the Re decrease regularly when the recovery temperature becomes higher (rows from A to E). The Re/Rm ratio remains constant and the percentage elongation A % increases in the same sense.

(13) TABLE-US-00001 TABLE II Recovery Mean Rm Mean Re temp. ( C.) (MPa) (MPa) Mean A % Re/Rm A 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

(14) The NACE tests of the RIC (Hydrogen-Induced Cracking) and SSC (Sulfide Stress Cracking) types were carried out on each of the wires obtained after these different recovery treatments. The data and results are presented in Table III below. It is seen that all the samples analyzed respond positively to the tests: after ultrasonic inspection, no internal cracking of the blister type, which would be evidence of hydrogen-induced corrosion embrittlement, is observed.

(15) TABLE-US-00002 TABLE III Rm Applied (in NACE test Duration stress in US MPa) type (in days) H2S % pH SSC results A 1920 HIC + SSC 30 0.1 5.8 90% Re OK B 1760 HIC + SSC 30 0.1 5.8 90% Re OK C 1550 HIC + SSC 30 0.22 5.6 90% Re OK D 1480 HIC + SSC 30 0.22 5.6 90% Re OK E 1380 HIC + SSC 30 0.22 5.6 90% Re OK

(16) It is self-evident that the invention would not be limited to the described examples but instead extends to multiple variants and equivalents that fall within its definition as given by the attached claims.

(17) TABLE-US-00003 TABLE I code of C % Mn % P % S % Si % Al % Ni % grade Mini Maxi Mini Maxi Mini Maxi Mini Maxi Mini Maxi Mini Maxi Mini Maxi C 78 D2 0.75 0.80 0.50 0.70 0.02 0.02 0.15 0.30 0.02 0.06 0.08 C 82D2 0.80 0.85 0.50 0.70 0.02 0.02 0.15 0.30 0.02 0.06 0.08 C82 0.77 0.85 0.65 0.85 C 86 D2 B 0.83 0.88 0.50 0.70 0.02 0.02 0.15 0.30 0.005 0.10 C 86 D2 0.82 0.88 0.65 0.85 0.02 0.02 0.15 0.30 0.02 0.06 0.10 C88 0.80 0.90 0.50 0.70 0.02 0.02 0.20 0.35 0.02 0.06 0.10 C92 0.88 0.95 0.30 0.60 0.015 0.015 1.00 1.40 0.005 0.10 code of Cr % Cu % Mo % V % B % N2% grade Mini Maxi Mini Max Mini Maxi Mini Maxi Mini Maxi Mini Maxi C 78 D2 0.10 0.08 0.02 0.007 C 82D2 0.10 0.10 0.02 0.007 C82 0.02 0.10 0.03 0.16 0.007 C 86 D2 B 0.10 0.12 0.025 0.002 0.007 0.007 C 86 D2 0.10 0.10 0.02 0.007 C88 0.10 0.10 0.01 0.05 0.10 0.008 C92 0.10 0.30 0.10 0.007