Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/11—Making amorphous alloys

Definitions

• the present invention relates to a method for producing a high-strength Cr—Ni alloy seamless pipe excellent in hot workability and stress corrosion cracking resistance.
• the oil wells and gas wells for mining petroleum and natural gas tend to be increased in depth.
• the materials used for such wells have been required to maintain the corrosion resistance against carbon dioxide gas, hydrogen sulfide and chloride ion, and at the same time to have further higher strength.
• Examples of the material exhibiting excellent corrosion resistance in corrosive environments include the Cr—Ni alloys disclosed in Patent Document 1, Patent Document 2 and Patent Document 3, disclosing that it is effective to increase the content of N for the purpose of increasing the strength of the Cr—Ni alloy.
• the alloy reinforced by such a method has a problem that the deformation resistance thereof is high and the hot workability thereof is poor.
• the cross roll piercing process (also referred to as the Mannesmann pipe making process) may be quoted.
• the cross roll piercing (hereinafter, simply referred to as “piercing-rolling”) is applied to a stock billet to produce a hollow material pipe (hereinafter, simply referred to as “material pipe”), the resulting material pipe is elongated by performing rolling with a rolling machine such as a plug mill or a mandrel mill, and then finally finished in shape with a sizer or stretch reducer.
• a rolling machine such as a plug mill or a mandrel mill
• the grain boundary melting phenomenon is caused by the melting of the grain boundary due to the processing-incurred heat.
• the occurrence of the grain boundary melting steeply deteriorates the ductility of the material, and hence tends to cause the lamination due to the grain boundary melting.
• the cross roll piercing process is higher in working ratio than the hot extrusion pipe-making process, and hence is provided with a larger processing-incurred heat, accordingly offering a problem that the lamination tends to be caused due to the grain boundary melting.
• Patent Document 4 discloses a technique for preventing the grain boundary melting cracking by heating a material pipe at a temperature equal to or lower than a value determine by a formula involving the circumferential velocity of the roll in the piercing-rolling of a Cr—Ni alloy and the pipe dimension.
• a value determine by a formula involving the circumferential velocity of the roll in the piercing-rolling of a Cr—Ni alloy and the pipe dimension discloses a technique for preventing the grain boundary melting cracking by heating a material pipe at a temperature equal to or lower than a value determine by a formula involving the circumferential velocity of the roll in the piercing-rolling of a Cr—Ni alloy and the pipe dimension.
• Patent Document 5 discloses a technique for preventing the grain boundary melting cracking that offers a problem in relation to austenite stainless steel, by reducing the content of P and the content of S according to the dimension of the material pipe to be subjected to piercing-rolling.
• the disclosed technique is different from a technique intended for a higher-strength Cr—Ni alloy capable of being used in an environment requiring a high corrosion resistance.
• Patent Document 6 discloses an Fe—Ni alloy seamless pipe excellent in mechanical properties and corrosion resistance in a sour gas environment wherein, in the seamless pipe, the lamination and the seam flaw are prevented by performing piercing-rolling by using a material pipe having the content of P and the content of S specified to fall within specific ranges.
• no sufficient investigation has been performed for the purpose of obtaining a higher-strength Cr—Ni alloy seamless pipe having an excellent hot workability and at the same time having an excellent stress corrosion cracking resistance.
• Patent Document 4 WO 2008/081866
• Patent Document 5 WO 2004/112977
• Patent Document 6 WO 2006/003953
• An objective of the present invention is to provide a method for producing a Cr—Ni alloy seamless pipe capable of preventing the deterioration of the hot workability and the deterioration of the stress corrosion cracking resistance caused by the actualization of the high strength, and further capable of performing the pipe-making without causing the lamination during piercing-rolling.
• the present inventors tried to prepare a higher strength material than conventional materials by increasing the content of N.
• a simple increase of the content of N deteriorates the hot workability and the stress corrosion cracking resistance to impede the production of oil well seamless pipes.
• the present inventors have focused attention on the REM (rare earth metal).
• the REM is known to be able to improve the hot workability by immobilizing the elements such as O, S and P in the alloy.
• no attention has been focused on the effect of the REM on the stress corrosion cracking resistance.
• the present inventors prepared high N alloys having various chemical compositions by melting, and evaluated the performances of the resulting alloys. Consequently, the present inventors have found that the inclusion of the REM improves the stress corrosion cracking resistance.
• the reason for the improvement of the stress corrosion cracking resistance by the REM is probably ascribable to the immobilization by the REM of P adversely affecting the stress corrosion cracking resistance.
• the Cr—Ni alloy having a high content of N for increasing the strength is high in deformation resistance, and hence tends to cause the grain boundary melting due to the processing-incurred heat in the piercing-rolling which is high in working ratio.
• the occurrence of the grain boundary melting deteriorates the ductility of the material, leading to a problem that the lamination of the material pipe is caused during piercing-rolling.
• the present inventors prepared, by melting, Cr—Ni alloys high in the content of N, having various chemical compositions, and examined the pipe workability during piercing-rolling
• the present inventors further made a continuous study on the basis of such new findings as described above, and consequently, obtained the following findings (a) to (g).
• the content of N is required to be set at as high as 0.10 to 0.30%, and for the purpose of ensuring the hot workability, the content of Al is required to be set at 0.03 to 0.30%.
• the content of REM can be determined from the viewpoint that the content of REM is the content necessary for immobilizing P as P-compounds.
• the ratio of the content of P to the content of REM [P/REM] is important.
• P, N and REM in formula (1) represent the contents (mass %) of P, N and REM, respectively.
• the reduction of the content of P leads to a significant effect to increase the grain boundary melting temperature.
• the additional reduction of the content of Si and the content of Mn results in an effect to further increase the grain boundary melting temperature and makes further difficult cause the grain boundary melting.
• the content of Si is preferably set at 0.3% or less.
• the content of Mn is set preferably at 0.7% or less and more preferably at 0.6% or less.
• the present invention has been perfected on the basis of the above-described findings, and the gist of the present invention is as described in the following items (1) to (8) associated with the method for producing a Cr—Ni alloy seamless pipe.
• the following items (1) to (8) are referred to as the inventions (1) to (8), respectively.
• the inventions (1) to (8) may be collectively referred to as the present invention.
• a method for producing a high-strength Cr—Ni alloy seamless pipe comprising: preparing an alloy billet that has a chemical composition consisting, by mass %, of C: 0.05% or less, Si: 1.0% or less, Mn: less than 3.0%, P: 0.005% or less, S: 0.005% or less, Cu: 0.01 to 4.0%, Ni: 25% or more and less than 35%, Cr: 20 to 30%, Mo: 0.01% or more and less than 4.0%, N: 0.10 to 0.30%, Al: 0.03 to 0.30%, O (oxygen): 0.01% or less, REM (rare earth metal): 0.01 to 0.20%, and the balance being Fe and impurities, and satisfying the following formula (1);
• P, N and REM in formula (1) represent the contents (mass %) of P, N and REM, respectively.
• Second group Ti: 0.5% or less, Nb: 0.5% or less, V: 0.5% or less and Zr: 0.5% or less,
• Third group Ca: 0.01% or less and Mg: 0.01% or less.
• Second group Ti: 0.5% or less, Nb: 0.5% or less, V: 0.5% or less and Zr: 0.5% or less,
• Third group Ca: 0.01% or less and Mg: 0.01% or less.
• the present invention it is possible to produce a high-strength Cr—Ni alloy seamless pipe, excellent in hot workability and stress corrosion cracking resistance, without causing the lamination during piercing-rolling, although the seamless pipe has a high strength due to a high content of N of the Cr—Ni alloy.
• C is an impurity contained in the alloy; when the content of C exceeds 0.05%, the stress corrosion cracking accompanied by the grain boundary fracture due to the precipitation of an M 23 C 6 type carbide (M: element such as Cr, Mo or Fe) tends to occur, and accordingly, the content of C is set at 0.05% or less.
• M element such as Cr, Mo or Fe
• the content of C is preferably 0.03% or less.
• Si is an element to decrease the grain boundary melting temperature and to cause the lamination during piercing-rolling. Even with a reduced content of P, when the content of Si exceeds 1.0%, the lamination occurs during piercing-rolling. Accordingly, the content of Si is set at 1.0% or less. For the purpose of reducing the high deformation resistance during piercing-rolling, it is preferable to perform the piercing at further higher temperatures. In this case, for the purpose of preventing the lamination, it is preferable to further increase the grain boundary melting temperature, and the content of Si is preferably set at 0.3% or less and more preferably at 0.2% or less. The smaller content of Si is the more preferable, and the lower limit of the content of Si is not particularly specified. However, when Si is contained for deoxidation, Si is preferably contained in a content of 0.01% or more.
• Mn is an element to decrease the grain boundary melting temperature and to cause the lamination during piercing-rolling. Even with a reduced content of P, when the content of Mn is 3.0% or more, the lamination occurs during piercing-rolling. Accordingly, the content of Mn is set at less than 3.0%, and is preferably less than 1.0%. For the purpose of reducing the high deformation resistance during piercing-rolling, it is preferable to perform the piercing at further higher temperatures. In this case, for the purpose of preventing the lamination, it is preferable to further increase the grain boundary melting temperature, and the content of Mn is more preferably set at 0.7% or less and furthermore preferably at 0.6% or less.
• the content of Mn is still furthermore preferably 0.3% or less.
• the content of Mn is preferably 0.01% or more.
• P is an important element.
• P is an impurity contained in the alloy; when piercing-rolling is performed, a high content of P tends to cause the lamination. Accordingly, the content of P is set at 0.005% or less and is preferably 0.003% or less. Additionally, as described below, the content of P is required to satisfy formula (1), in relation to the content of N and the content of REM.
• the allowable content of S is required to be 0.005% or less, and the smaller content of S is the more preferable.
• the content of S is preferably 0.002% or less and more preferably 0.001% or less.
• Cu is effective in stabilizing the passive film formed on the surface of the alloy, and is necessary for improving the pitting resistance and the general corrosion resistance.
• the content of Cu is set at 0.01 to 4.0%.
• the content of Cu is preferably 0.1 to 2.0% and more preferably 0.6 to 1.4%.
• Ni is made to be contained as an austenite stabilizing element. From the viewpoint of the corrosion resistance, the content of Ni is required to be 25% or more. The content of Ni of 35% or more leads to the increase of the cost. Accordingly, the content of Ni is set at 25% or more and less than 35%. The content of Ni is preferably 28% or more and less than 33%
• Cr is a component to remarkably improve the stress corrosion cracking resistance.
• the content of Cr is set at 20 to 30%.
• the content of Cr is preferably 23 to 28%.
• Mo is effective, like Cu, in stabilizing the passive film formed on the surface of the alloy, and is effective in improving the stress corrosion cracking resistance.
• the content of Mo is set at 0.01% or more and less than 4.0%.
• the content of Mo is preferably 0.1% to 3.5%.
• N is effective to increase the strength of the alloy.
• the content of N is set at 0.10 to 0.30%.
• a preferable range of the content of N is 0.16 to 0.25%.
• the content of N is required to satisfy formula (1), in relation to the content of P and the content of REM.
• Al fixes the O (oxygen) in the alloy to improve the hot workability, and is also effective in preventing the oxidation of REM.
• O oxygen
• REM oxygen
• Al it is essential for Al to be contained together.
• the content of Al is less than 0.03%, the effect of Al is not sufficient.
• the content of Al exceeds 0.30%, the hot workability is deteriorated. Accordingly, the content of Al is set at 0.03 to 0.30%.
• the content of Al is preferably 0.05 to 0.30% and more preferably more than 0.10% and 0.20% or less.
• O (oxygen) is an impurity contained in the alloy, and remarkably deteriorates the hot workability. Accordingly, the content of O (oxygen) is set at 0.01% or less. The content of O (oxygen) is preferably 0.005% or less.
• REM rare earth metal
• REM is effective in improving the hot workability and the stress corrosion cracking resistance, and hence is required to be contained.
• REM tends to be oxidized, and hence it is essential to contain REM along with Al.
• the content of REM is set at 0.01 to 0.20%.
• the content of REM is preferably 0.02 to 0.10%.
• REM is a generic name for the 17 elements which include 15 lanthanoid elements and Y and Sc, and one or more of these elements can be contained.
• the content of REM means the sum of the contents of these elements.
• the method for containing REM may be such that one or more of these elements are added or industrially added in the form of a mish metal.
• the content of REM is required to satisfy the following formula (1) in relation to the content of N and the content of P:
• P, N and REM in formula (1) represent the contents (mass %) of P, N and REM, respectively.
• the Cr—Ni alloy according to the present invention may further contain, in addition to the above-described alloying elements, one or more elements selected from at least one group of the following first group to third group.
• Second group Ti, Nb, V, Zr: 0.5% or less
• W is an optionally contained element.
• W has an effect to improve the stress corrosion cracking resistance. Accordingly, when it is intended to improve the stress corrosion cracking resistance, W can be contained if necessary. However, when the content of W is 8.0% or more, the hot workability and the economic efficiency are deteriorated, and hence, when W is contained, the upper limit of the content of W is set at 8.0%. For the purpose of certainly developing the improvement effect of the stress corrosion cracking resistance, it is preferable to contain W in a content of 0.01% or more. The content of W is more preferably 0.1 to 7.0%.
• Second group one or more selected from Ti: 0.5% or less, Nb: 0.5% or less, V: 0.5% or less and Zr: 0.5% or less, with the total content of one or more elements of 0.5% or less
• Ti, Nb, V and Zr are optionally contained elements. These elements have an effect to make grains fine and to improve the ductility. Accordingly, when a further better ductility is demanded, one or more of these elements can be contained, if necessary. However, when the total content of one or more of these elements exceed 0.5%, inclusions are produced in large amounts to cause the ductility deterioration phenomenon, and hence, when one or more of these elements are contained, the upper limit of the content of such elements is set at a total content of such elements of 0.5%. For the purpose of certainly developing the improvement effect of the ductility, it is preferable to contain such elements in a total content of 0.005% or more. The content of such elements is more preferably 0.01 to 0.5% and furthermore preferably 0.05 to 0.3%.
• Third group either or both of Ca: 0.01% or less and Mg: 0.01% or less
• Ca and Mg are optionally contained elements. These elements have an effect to improve the hot workability, and accordingly either or both of these elements can be contained if necessary.
• the upper limit of the total content is set at 0.01%.
• the total content of such elements is more preferably 0.0003 to 0.01% and furthermore preferably 0.0005 to 0.005%.
• the Cr—Ni alloy seamless pipe according to the present invention contains the above-described essential elements or further the above-described optional elements, and the balance is Fe and impurities.
• the “impurities” as referred to herein mean the substances that contaminate the materials when the Cr—Ni alloys are industrially produced, due to the raw materials such as ores and scraps, and due to various other factors in the production process, and are allowed to contaminate within the ranges not adversely affecting the present invention.
• the melting of the Cr—Ni alloy of the present invention there can be used an electric furnace, an AOD furnace, a VOD furnace or the like.
• the molten alloy obtained by melting is cast into ingots, the ingots can be converted by subsequent forging into slabs, blooms and billets.
• the molten alloy can be converted by a continuous casting method into slabs, blooms and billets.
• a seamless material pipe is made by hot working on the basis of a cross roll piercing pipe-making process.
• the cross roll piercing pipe-making process is also referred to as the Mannesmann pipe making process.
• This is a process in which the billet as the stock is subjected to cross roll piercing by using a piercing mill (cross roll piercing machine) to produce a hollow material pipe, the material pipe is rolled to be elongated with a rolling machine such as a mandrel mill or a plug mill, and finally the resulting pipe is finished in shape by using a sizer or a stretch reducer.
• the cross roll piercing includes the cross roll piercing with a toe angle.
• the yield strength of a Cr—Ni alloy seamless pipe suitable for use in deep oil wells and deep gas wells is 900 MPa or more in terms of the 0.2% yield stress.
• the concerned yield strength is more preferably 964 MPa or more.
• a Cr—Ni alloy seamless pipe having a yield strength of 900 MPa or more is produced by the production process in which the seamless material pipe for cold working, made into a pipe by the above-described cross roll piercing process, is subjected to a solution treatment and further subjected to a cold working.
• the seamless material pipe for cold working subjected to hot working with the cross roll piercing process is subjected, after the solution heat treatment to a cold working based on the cold rolling such as cold drawing or pilger rolling.
• the cold working may be performed once or a plurality of times, or alternatively, if necessary, after heat treatment, the cold working may be performed once or a plurality of times.
• the high-strength Cr—Ni alloy pipe obtained by the cold working after the solution treatment having a yield strength of 900 MPa or more, is suitable for the oil well seamless pipe for use in deep oil wells or deep gas wells.
• the cold working ratio is preferably set at 10 to 40% in terms of the reduction of area. When the cold working ratio is less than 10%, no intended high strength may be obtained. On the other hand, when the cold working ratio exceeds 40%, the strength is made high, but the ductility or the toughness may be deteriorated.
• the cold working ratio is more preferably set at 20 to 35%.
• the cold working is preferably performed with the cold working ratio of 30 to 80% in terms of the reduction of area.
• the cold working ratio is less than 30%, no intended high strength may be obtained.
• the cold working ratio exceeds 80%, the strength is made high but the ductility or the toughness may be deteriorated.
• Table 1 shows the chemical compositions (mass %) of Invention Examples (Test Nos. 1 to 23) and the Comparatives (Test Nos. A to J).
• the alloys according to Invention Examples were melted and cast into 30 kg ingots by using a vacuum induction melting furnace. The resulting ingots were subjected to hot forging to be molded into billets of 100 mm in outer diameter. The billets heated at 1240° C. and 1260° C. were subjected to piercing-rolling with a small sized cross roll piercing apparatus to be produced into pipes of 116 mm in outer diameter and 20 mm in wall thickness.
• N Al Ca O REM others Invention 1 0.190 0.12 0.0019 0.003 0.026 Nd Examples 2 0.192 0.12 0.0020 0.003 0.019 Nd 3 0.200 0.12 0.0021 0.003 0.020 Nd 0.02Nb 4 0.205 0.10 0.0019 0.002 0.028 Nd 0.01Zr 5 0.197 0.12 0.0018 0.003 0.013 Nd 6 0.195 0.11 0.0018 0.003 0.034 Nd 0.03V 7 0.198 0.13 0.0019 0.001 0.031 Nd 8 0.198 0.10 — 0.003 0.021 Nd 9 0.203 0.10 0.0021 0.003 0.023 Nd 0.02Ti 10 0.203 0.12 0.0021 0.002 0.028 Nd 11 0.200 0.13 0.0019 0.001 0.035 Nd 12 0.124 0.11 0.0022 0.004 0.011 Nd 13 0.261 0.13 0.0017 0.003 0.033 Nd 14 0.201 0.12 0.0019 0.003 0.037 Nd + 0.016 Ce
• the seamless material pipes after piercing-rolling were cut perpendicularly to the longitudinal direction, at a position of 50 mm in the longitudinal direction from the rear end of the pipe, and observed whether the lamination of the material pipe was caused or not. As a result, the mark ⁇ shows that no lamination was caused, and the mark ⁇ shows that the lamination was caused.
• the seamless material pipes heated at 1240° C. and subjected to piercing-rolling were then subjected to a solution treatment in which the material pipes were heated and maintained at 1050° C. for 1 hour, and then water cooled.
• the material pipes were subjected to a cold drawing with a reduction of area of 30%, which are the seamless pipes according to Invention Examples and Comparatives. It is found that, in Invention Examples, omitting the subsequent hot elongation rolling process and hot shaping rolling process after piercing-rolling causes no adversely affection to the mechanical properties and the corrosion resistance. Accordingly, in a more simplified manner, the seamless material pipes that were subjected to piercing-rolling with a small size cross roll piercing apparatus and directly solution-treated and cold worked were to be used for evaluation.
• Room-temperature tensile test specimens 6 mm in diameter and 40 mm in length in the parallel portion, were cut out to the longitudinal direction from the seamless pipes after the cold working, and subjected to a tensile test at room temperature in the air to measure the 0.2% yield stress. Further, for the purpose of evaluating the stress corrosion cracking resistance, test specimens, 3.81 mm in diameter and 25.4 mm in length in the parallel portion, were cut out to the longitudinal direction from the same pipes after the cold working, and the low strain rate tensile test was performed.
• the ratio of the reduction of area in the corrosive environment to the reduction of area in the inert environment was used as the index for the stress corrosion cracking resistance; when the ratio was 0.8 or more, the stress corrosion cracking resistance was determined to be satisfactory ( ⁇ ), and when the ratio was less than 0.8, the stress corrosion cracking resistance was determined to be poor ( ⁇ ).
• Test Nos. B to F the lamination was caused, and hence the measurements of the 0.2% yield stress and the stress corrosion cracking resistance were not conducted.
• Table 2 shows the test results and the N ⁇ P/REM values.
• Comparative A neither the heating at 1240° C. nor the heating at 1260° C. resulted in causing the lamination, and the stress corrosion cracking resistance was satisfactory. In Comparative A, however, the content of N was outside the range restricted by the present invention, and hence the 0.2% yield stress was low.
• P was contained excessively, and hence both of the heating at 1240° C. and the heating at 1260° C. resulted in causing the lamination.
• Mn was contained excessively, and hence both of the heating at 1240° C. and the heating at 1260° C. resulted in causing the lamination.
• Comparative F Si was contained excessively, and hence both of the heating at 1240° C.
• Comparative G REM was not contained, and hence the stress corrosion cracking resistance was poor.
• Comparative H to J the chemical composition of the alloy was within the range specified by the present invention, but did not satisfy formula (1), and hence the stress corrosion cracking resistance was poor.
• the present invention it is possible to produce a high-strength Cr—Ni alloy seamless pipe, excellent in hot workability and stress corrosion cracking resistance, without causing the lamination during piercing-rolling, although the seamless pipe has a high strength provided by a high content of N.
• the high-strength Cr—Ni alloy seamless pipe obtained by the present invention can be used for mining of petroleum and natural gas in deep and harsh corrosive environments, having hitherto been unmineable, and hence significantly contributes to stable supply of energy.

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• 2010
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