BR112019000376B1 - SEAMLESS TUBE FOR HIGH TEMPERATURE APPLICATIONS, AND PRODUCTION METHOD OF THIS SEAMLESS TUBE - Google Patents

Abstract

Heat resistant martensitic steel for boiler applications with a unique combination of high creep resistance and excellent oxidation resistance in high temperature exposure to steam containing environments, having the following melt analysis (% by weight): C: 0, 10 to 0.16%, Si: 0.20 to 0.60%, Mn: 0.30 to 0.80%, P = 0.020%, S = 0.010%, Al = 0.020%, Cr: 10.50 to 12.00%, Mo: 0.10 to 0.60%, V: 0.15 to 0.30%, Ni: 0.10 to 0.40%, B: 0.008 to 0.015%, N: 0.002 to 0.020 %, Co: 1.50 to 3.00%, W: 1.50 to 2.50%, Nb: 0.02 to 0.07%, Ti: 0.001 to 0.020%. The balance of steel consists of iron and unavoidable impurities. The steel is normalized for a period of approximately 10 to approximately 120 minutes in the temperature range between 1050°C and 1170°C and cooled in air or water to room temperature and then tempered for at least one hour in the temperature range between 750°C and 820°C. It exhibits martensitic microstructure with an average ¿-ferrite content of less than 5% by vol.

Description

[0001] The invention relates to heat resistant martensitic steels of high chromium content for components operating at high temperatures, that is, between 550 and 750°C and high voltages. The steel according to the invention can be used in the power generation, chemical and petrochemical industries.

STATUS OF THE TECHNIQUE

[0002] High Cr content ferritic/martensitic steel materials are widely used in modern power plants as reheater/superheater tubes and as steam pipes. Further improvement in net efficiency of thermal power plants will require an increase in pressure and temperature of steam parameters. Therefore, performing more efficient cycles in the power plant will require stronger materials with better oxidation resistance on the steam side. Known efforts to develop a new high chromium martensitic steel that combines excellent creep properties and superior oxidation resistance have so far failed due to the formation of the so-called Z phase. consuming the surrounding strengthening MX precipitates, M being: Nb, V and X being: C, N.

[0003] High chromium steel expression material generally means steels with more than 9 wt% Cr. High Cr contents, i.e., containing more than 9 wt% Cr, which are essential for good oxidation resistance in steam, however, increase the driving force for Z-phase formation and also accentuate the rate of oxidation. thickening of chromium carbide precipitates. Both the loss of MX microstructure stabilizing effect and chromium carbide precipitates are responsible for the drop in long-term creep breaking strength of high Cr heat resistant martensitic steel grades. Therefore, the main challenge for future steel developments is to resolve the apparent contradiction between creep rupture strength and oxidation strength.

[0004] Currently, for high temperature applications, i.e. applications with service temperatures higher than 550°C, ASTM grades 91 and 92 are widely used, both containing 9 wt.% Cr with breaking strengths by creep after 105h at 600°C at 90 and 114 MPa, respectively. The main difference between the two steels is that grade 92 contains W in the range of 1.8% by weight and reduced Mo of 0.4% by weight compared to 1% by weight in the case of grade 91. Additionally, grade 92 contains small amounts of B below 0.005% by weight.

[0005] Both steels suffer from insufficient oxidation resistance in steam atmospheres at temperatures above 600°C, which is limiting the application temperature range significantly. Especially in boiler components with heat transfer, the oxide scale acts as a thermal insulator, thereby increasing the temperature of the metal and consequently reducing the lifetime of the corresponding components. Additionally, oxide scales, if chipped during operation, will cause erosion damage to steam transport components following or after entering the steam turbine at the turbine vanes and guide vanes. Chipped oxide scale can cause pipe blockage especially around bends, preventing steam flow often resulting in local overheating and catastrophic failure.

[0006] X20CrMoV11-1 is a well-established high Cr ferritic/martensitic steel for high temperature applications containing 0.20 wt% C, 10.5 to 12 wt% Cr, 1 wt% Cr. Mo and 0.2 wt% V. This steel exhibits oxidation properties that are better than those of ASTM grades 91 and 92 steel due to higher Cr contents, but poor creep rupture strength creep failure after 105h at 600°C being around 59 MPa). Additionally, the hot workability and weldability are deteriorated due to the high C content of 0.20% by weight. ASTM grade 122 contains 10 to 12% Cr, 1.8% W, 1% Cu and also additions of V, Nb and N to induce precipitation of MX strengthening particles. The creep breaking strength is significantly below that of ASTM grade 92 which has a creep breaking strength of 98 MPa after 105h at 600°C.

[0007] Also, hot workability concerns due to high Cu contents are present.

[0008] Another steel with 11 to 12 wt% Cr exists. It is mainly used as thin wall pipe and is called VM12-SHC steel which combines good oxidation resistance on the steam side and creep rupture strength at ASTM quality 91 level. Such a steel concept is known from patent application WO02081766 disclosing a steel for high temperature use containing by weight: 0.06 to 0.20% C, 0.10 to 1.00% Si, 0.10 to 1.00% Mn, not more than 0.010% S, 10.00 to 13.00% Cr, not more than 1.00% Ni, 1.00 to 1.80% W, Mo such that (W/2+Mo) is not greater than 1.50%, 0.50 to 2.00% Co, 0.15 to 0.35% V, 0.040 to 0.150% Nb, 0.030 to 0.12% N, 0.0010 to 0.0100% B and optionally up to 0.0100% Ca, the rest of the chemical composition consisting of iron and impurities or residues resulting from or required for the preparation or smelting processes of steel. The chemical constituent contents preferably show a relationship such that the steel after normalizing heat treatment between 1050 and 1080°C and tempering has a tempered martensite structure free or substantially free of delta ferrite. Compared with this steel, creep rupture strength can be further improved while keeping other properties such as corrosion resistance and mechanical properties unaffected.

PURPOSE AND SOLUTION

[0009] The object of the present invention is therefore to present a seamless tubular product in a heat resistant martensitic steel with creep rupture strength substantially better than ASTM grade 92 steel for pipes and tubes and with corrosion. and steam oxidation behavior comparable or better than the X20CrMoV11-1 and VM12-SHC steels, described in the prior art.

[0010] A further objective of the invention is to obtain a steel exhibiting martensitic microstructure with a limitation of delta ferrite, also known as δ-femta, content at 5% by vol. average.

[0011] Another objective of the invention was to present a steel that allows the manufacture of seamless tubular products of small or large diameter, such as seamless tubes or seamless pipes, and a steel suitable for the manufacture of welded tubes and pipes, forgings and sheets using known and established manufacturing processes.

[0012] Steel is suitable as a production material for a full range of components operating under stress at elevated temperatures, particularly such as seamless and welded tubes/pipes, forgings and sheet metal in the power generation, chemical and petrochemical industries. Furthermore, the steel according to the invention is resistant to tempering, after long tempering times up to 30 hours at 800°C, the strength intensity is above or equal to 440 MPa, the tensile stress is above or equal to 620 MPa and toughness at 20°C is above or equal to 40 J when tested in the longitudinal direction and 27 J when tested in the transverse direction.

[0013] According to the present invention, the objective can be achieved by a seamless tubular product for high temperature applications in a steel having the following chemical composition in weight percentage:

[0014] C: 0.10 to 0.16%

[0015] Si: 0.20 to 0.60%

[0016] Mn: 0.30 to 0.80%

[0017] P <0.020%

[0018] S <0.010%

[0019] Al <0.020%

[0020] Cr: 10.50 to 12.00%

[0021] Mo: 0.10 to 0.60%

[0022] V: 0.15 to 0.30%

[0023] Ni: 0.10 to 0.40%

[0024] B: 0.008 to 0.015%

[0025] N: 0.002 to 0.020%

[0026] Co: 1.50 to 3.00%

[0027] W: 1.50 to 2.50%

[0028] Nb: 0.02 to 0.07%.

[0029] Ti: 0.001 to 0.020%, the balance of said steel being iron and unavoidable impurities.

[0030] Preferably, the boron and nitrogen ratio is such that: B/N < 1.5 to achieve hot workability.

[0031] Preferably, the following equation is satisfied:

[0032] 1.00% <Mo+0.5W <1.50% (% by weight),

[0033] In another preferred embodiment, the following equation is satisfied (% by weight):

[0034] In another preferred embodiment, the following equation is satisfied (% by weight):

[0035] In a preferred embodiment, the carbon content is between 0.13 and 0.16%.

[0036] In another preferred embodiment, the Mo content is between 0.20 and 0.60%.

[0037] Preferably, the content of B is between 0.0095 and 0.013%.

[0038] In a preferred embodiment, the Ti content is between 0.001 and 0.005%.

[0039] In another preferred embodiment, the microstructure comprises on average at least 95% tempered martensite, the equilibrium being delta ferrite.

[0040] In an even more preferred embodiment, the microstructure comprises on average at least 98% tempered martensite, the equilibrium being delta ferrite.

[0041] In the most preferred embodiment, the microstructure is martensitic and delta ferrite free.

[0042] The invention also relates to a production method comprising the following steps:

[0043] - melting a steel with a chemical composition according to the invention,

[0044] - hot form said steel,

[0045] - heat said steel and keep said steel for a time between 10 and 120 minutes in the temperature range between 1050°C and 1170°C,

[0046] - cool said steel to room temperature,

[0047] - reheat said steel and keep said steel up to a tempering temperature TT that is between 750°C and 820°C for at least one hour,

[0048] - cool said steel to room temperature.

[0049] Preferably, the cooling step is done using air cooling or water cooling.

[0050] The cooling step after the reheating step can be done using water cooling.

[0051] The cooling step after the heating step can be done using water cooling.

[0052] The invention may also relate to the production of a welded tube, pipe or sheet using the same steel as that according to the seamless tubular product of the invention or the process according to the invention.

[0053] Figure 1 shows the schematic of the mass gain due to oxidation marked against the chromium content.

EXPOSED MATTER OF THE INVENTION

[0054] In accordance with the present invention, a high chromium heat resistant martensitic steel is created having the following chemical composition: (1) C: 0.10 to 0.16%,

[0055] C needs to be added at least 0.10% to obtain sufficient carbide precipitation. Additionally C is also an austenite stabilizing element. C contents below 0.10% would imply more δ-femta in the microstructure. The upper limit for carbon is 0.16% because the excessive addition of C limits toughness and weldability properties.

[0056] (2) Si: 0.20 to 0.60%,

[0057] Si is used for deoxidation during the steel making process. Additionally, it is one of the key elements that determines the oxidation behavior of steels. In order to achieve complete oxidation by improving the effect of Si additions, an amount of at least 0.20% is required. The upper level of Si will preferably be limited to 0.60%, because the excessive addition of Si accelerates the thickening of the precipitates and decreases the toughness. Preferably, the lower limit is 0.25%.

[0058] (3) Mn: 0.30 to 0.80%,

[0059] Mn is an effective deoxidation element. It deters sulfur and reduces the formation of δ-femta. At least 0.30% Mn can be added. The upper limit will be 0.8%, as excessive additions reduce the strength of steels at elevated temperatures.

[0060] (4) P <0.020%,

[0061] P is an active grain boundary element, which reduces the toughness properties of steels. The content has to be limited to 0.020% in order to avoid the negative impact of P on toughness properties. P may be present in an amount equal to or greater than 0.00%, as it may be unavoidable as an impurity.

[0062] (5) S <0.010%,

[0063] S forms sulfides and reduces the toughness and hot workability properties of steels. Limiting the upper S content to 0.010 prevents defect formation during hot work operation and negative impact on toughness. S may be present in an amount equal to or greater than 0.00%, as it may be unavoidable as an impurity.

[0064] (6) Al <0.020%,

[0065] Al is a potent deoxidizing element used during the steel making process. Excessive addition of Al above 0.02% can induce the formation of AlN, thereby reducing the amount of nitride precipitate from the strengthening MX (M being: Nb, V and X being: C, N) in the steel and consequently creep resistance properties. Al may be present in an amount equal to or greater than 0.00%, as it may be unavoidable as an impurity.

[0066] (7) Cr: 10.5 to 12.00%,

[0067] Cr forms carbides that form at the limits of the martensitic microstructure. Chromium carbides are essential for stabilizing the martensitic microstructure during exposure to elevated temperatures. Cr improves the high temperature oxidation behavior of steels. Contents of at least 10.5% are required to reveal complete oxidation enhancing the effect of Cr additions. Cr contents above 12% result in greater formation of δ-femta.

[0068] (8) Mo: 0.10 to 0.60%,

[0069] Mo is an important element for improving creep rupture strength which is also responsible for the solid strengthening of the solution. This element is incorporated into carbides and intermetallic phases as well. Mo content of 0.10% can be added. Mo additions above 0.60% will deteriorate toughness and induce an increase in δ-ferrite content. It is observed that the contents of M and W must satisfy the ratio (by weight %) 1 <Mo+0.5 x W <1.5, in order to guarantee sufficient precipitation of the carbides and intermetallic phases.

[0070] (9) V: 0.15 to 0.30%,

[0071] V combines with N to form coherent MX nitrides (M being: Nb, V and X being: C, N), which contribute to the enhancement of creep properties in the long term. Contents below 0.15% are not sufficient to achieve this effect on the long-term creep improvement property while contents above 0.30% decrease toughness and increase the danger of δ-ferrite contents above 5% by volume. medium.

[0072] (10) Ni: 0.10 to 0.40%,

[0073] Ni is an important toughness enhancing element. Therefore, a minimum content of 0.10% is required. However, it reduces the Ac1 temperature and tends to reduce the creep breaking strength if added at contents above 0.40%.

[0074] (11) B: 0.008 to 0.015%,

[0075] B is a decisive element responsible for the stabilization of M23C6 carbides and delay in the recovery of the martensitic microstructure. It strengthens the grain boundaries and improves the long-term stability of creep failure strength. In addition, B is responsible for the remarkable improvement in creep failure ductility. To obtain the maximum strengthening effect, additions of at least 0.008% are required. Contents above 0.015%, however, substantially reduce the maximum processing temperature of steels and are considered harmful. Additions of B and N must satisfy the ratio B/N<1.5 to enable transformation using known hot work processes. In fact, this B/N ratio allows the fabrication of seamless and welded small or large diameter tubes, pipes and sheets using the manufacturing process according to the invention. Preferably, the content of B should be between 0.0095 and 0.0130% (% by weight).

[0076] (12) N: 0.002 to 0.020%,

[0077] Nitrogen is necessary for the formation of MX nitrides (M being: Nb, V and X being: C, N) and carbonitrides responsible for obtaining the creep rupture strength. At least 0.002% can be added. Excessive additions of N, ie above 0.020%, however, result in marked formation of BN, thus reducing the strengthening effect of the additions of B. Preferably, the contents of B and N (by weight %) should satisfy the following relationship:

[0078] (13) Co: 1.50 to 3.00%,

[0079] Co is a very effective austenite forming element and useful in limiting the formation of δ-ferrite. Furthermore, it has only a weak effect on the Ac1 temperature. Additionally, it is an element that improves creep strength properties by reducing the size of the initial precipitates after heat treatment. Therefore, a minimum content of 1.50% must be added. Preferably, the minimum content is 1.75%. However, Co in excessive additions can induce embrittlement due to the accentuated precipitation of intermetallic phases during high temperature operation. At the same time Co is very expensive. Therefore, a limitation of the additions to 3.00%, preferably to 2.50%, is necessary.

.[0080] It is preferable that the contents of Ni, Co, Mn, C and N (by weight %) are in accordance with the following equation:

.

[0081] (14) W: 1.50 to 2.50%,

[0082] W is known as an effective solution strengthener. At the same time, it is incorporated into carbides and forms the Laves C14 phase, which can contribute to the enhancement of creep resistance as well. Therefore, a minimum content of 1.50% is required. However, this element is expensive, segregating heavily during the steel making and smelting process, and it forms intermetallic phases that lead to significant breakability. Therefore, the upper limit for W additions can be set to 2.50%. It is observed that the contents of Mo and W (by weight %) must satisfy the ratio 1.00 <Mo+0.5W <1.50, in order to guarantee sufficient precipitation of carbides and intermetallic phases.

[0083] (15) Nb: 0.02 to 0.07%.

[0084] Nb forms stable MX carbonitrides important not only for creep properties, but also for grain size control of austenite. A minimum content of 0.02% can be added. Nb contents above 0.07% result in the formation of coarse Nb carbides which can reduce creep resistance properties. Therefore, the upper limit is set to 0.07%.

[0085] (16) Ti: 0.001 to 0.020%

[0086] Ti is a strong nitride forming element. It is useful to protect free B by forming nitrides. A minimum content of 0.001% is required for this purpose. Excessive Ti content above 0.020%, however, can reduce toughness properties due to the formation of large precipitates of blocking TiN.

[0087] Steel balance comprises iron and ordinary residual elements from the steel making and smelting process. The casting techniques used are those known to the skilled person. By impurities are known elements such as tantalum, zirconium and any other elements that cannot be avoided. It should be mentioned that tantalum and zirconium are not intentionally added to steel, however they may be present at less than 50 ppm overall as unavoidable impurities.

[0088] In one embodiment of steel, the unavoidable impurities may comprise one or more of copper (Cu), arsenic (As), tin (Sn), antimony (Sb) and lead (Pb).

[0089] Cu may be present at a content equal to or less than 0.20%.

[0090] The As element may be present in a content equal to or less than 150 ppm; Sn may be present at a content equal to or less than 150 ppm; Sb may be present at a content equal to or less than 50 ppm; Pb can be present at a content equal to or less than 50 ppm and the total content of As + Sn + Sb + Pb is equal to or less than 0.04% by mass.

[0091] Steel is normalized for a period of approximately 10 to approximately 120 minutes in the temperature range between 1050°C and 1170°C and cooled in air or water to room temperature, and then tempered for at least one hour in the range temperature between 750°C and 820°C.

[0092] The resulting steel has been found to have outstanding and absolutely excellent high temperature strength and superior steam oxidation resistance. Furthermore, it was verified that by the ratio Creq./Nieq. is less than 2.3, the average δ-femta content can be limited to less than 5% by vol. to avoid toughness problems, where Creq. and Nieq. are defined as Cr+6Si+4Mo+1.5W+11V+5Nb+8Ti and 40C+30N+2Mn+4Ni+2Co+Cu, respectively. Surprisingly, it has been found that a B/N ratio equal to or less than 1.5 has to be maintained in order to enable hot working operation with known transformation processes.

[0093] The content of delta ferrite will not exceed 5% by vol., as contents above 5% by vol. will impair the toughness properties.

[0094] By hot forming processes, it is planned: hot rolling, “pilgering”, hot drawing, forging, plug mill, thrust bench processes where the chuck shank pushes the elongated cavity through several platforms of in-line roll to produce cavity, continuous lamination and other known lamination processes. The steel according to the invention is capable of being formed in the form of tubes and pipes. Numerous attempts have been made with steels exhibiting satisfactory properties, such as oxidation behavior, creep resistance, but these steels did not provide a satisfactory formed product through these hot forming processes. In particular, even sometimes it was not possible to obtain seamless tubes or pipes. The steel of the invention makes it possible to have seamless tubular products with satisfactory properties and the possibility of obtaining seamless tubular products or sheets by hot forming processes, these products being in dimensional requirements.

EXAMPLES

[0095] The benefits of the steel of the present invention will be explained in more detail based on the following examples. The steels according to the present invention (steel 1, steel 2, steel 3) and also the steels of the comparative example (steel 4, steel 5), having the chemical composition indicated in table 1, were cast into 100 kg ingots using vacuum induction melting furnace, then hot rolled to sheet (13 to 25 mm thick) and subsequently normalized and tempered. Normalizing heat treatment was performed in the temperature range of 1060°C to 1100°C for 30 minutes, followed by air-cooling to room temperature. Tempering was done at 780°C for 120 minutes, again followed by air-cooling.

[0096] Steels 4 and 5 of the comparative example have B contents below 0.008 and therefore are not in accordance with the invention.

.[0097] In the case of steel 4, the additions of Ni, Co, Mn, C and N are not in accordance with the equation

.

(% em peso) tampouco. Tabela 1

[0098] Steel 5 does not satisfy the following formula:

(% by weight) either. Table 1

[0099] For the two exemplary steels (steel 1, steel 2, steel 3), the results presented in table 2 were obtained at room temperature for tensile strength, yield stress, elongation, area reduction and notch impact energy Charpy V. Table 2

[00100] Creep tests carried out in accordance with ISO DIN EN 204 on specimens of the three exemplary steels also showed a notable improvement in creep breaking strength. This is reflected in the rupture times being at least almost twice as long as that of prior art steels such as P91, P91, VM12-SHC, P122 and X20CrMoV11-1 during the long-term creep test at 130MPa and 100MPa. The results are shown in Table 3. Also the steels of the comparative example did not reach the creep breaking strength of the steels according to the invention. Table 3

[00101] Figure 1 shows the schematic of mass gain due to oxidation in water vapor atmosphere at elevated temperatures marked against chromium content. The basis for the construction of the schematic is the water vapor atmosphere oxidation tests performed in accordance with ISO 21608:2012.

[00102] In figure 1, three regions exhibiting different vapor oxidation behavior have been defined as follows:

[00103] (I.) Non-protective behavior for mass gain above 10mg/cm2 after 5,000h

[00104] (II.) Intermediate behavior for mass gain in the range of 5 to 10mg/cm2

[00105] (III.) Protective behavior for mass gains below 5mg/cm2.

[00106] Correspondingly, the classification of different high Cr-content heat resistant martensitic steels with respect to oxidation behavior was performed in Table 4 below. Regions I, II and III correspond with mass gains as described in figure 1. The three exemplary steels clearly outperform P91, P92, P122 and X20CrMoV11-1 with respect to steam oxidation resistance. The invention exhibits behavior comparable to VM12-SHC. Table 4

[00107] According to the invention, it is possible to provide a high chromium heat resistant martensitic steel with enhanced creep properties and resistance to steam oxidation which can be used to produce tubes, forgings, pipes and plates operating at high temperature in power generation, chemical and petrochemical industries.

Claims (13)

1. Seamless tube for high temperature applications, characterized in that it is made of a steel having the following chemical composition in weight percentage: C: 0.10 to 0.16% Si: 0.20 to 0.60% Mn: 0.30 to 0.80% P<0.020% S <0.010% Al <0.020% Cr: 10.50 to 12.00% Mo: 0.10 to 0.60% V: 0.15 to 0.00 30% Ni: 0.10 to 0.40% B: 0.008 to 0.015% N: 0.002 to 0.020% Co: 1.50 to 3.00% W: 1.50 to 2.50% Nb: 0.02 to 0.07% Ti: 0.001 to 0.005%, the balance of said steel being iron and unavoidable impurities. 2. Seamless tube, according to claim 1, characterized in that: B/N <1.5. 3. Seamless tube, according to claim 1 or 2, characterized in that, % by weight: 1.00% <Mo+0.5W <1.50%. 4. Seamless tube, according to any one of claims 1 to 3, characterized in that % by weight:

. 5. Seamless tube, according to any one of claims 1 to 4, characterized in that % by weight:

. 6. Seamless tube, according to any one of claims 1 to 5, characterized in that the carbon content is between 0.13 and 0.16%. 7. Seamless tube, according to any one of claims 1 to 6, characterized in that the Mo content is between 0.30 and 0.60%. 8. Seamless tube, according to any one of claims 1 to 7, characterized in that the content of B is between 0.0095 and 0.013%. 9. Seamless tube, according to any one of claims 1 to 8, characterized in that the microstructure comprises at least 95% tempered martensite, the balance being delta ferrite. 10. Seamless tube, according to any one of claims 1 to 9, characterized in that the microstructure comprises at least 98% tempered martensite, the equilibrium being delta ferrite. 11. Seamless tube, according to any one of claims 1 to 10, characterized in that the microstructure is martensitic and free of delta ferrite. 12. Method of producing a seamless pipe, as defined in any one of claims 1 to 11, characterized in that it comprises the following steps: - melting a steel with a chemical composition as defined in any one of claims 1 to 11, - hot forming said steel, - heating said steel and keeping said steel for a time between 10 and 120 minutes in the temperature range between 1050°C and 1170°C, - cooling said steel to ambient temperature, - reheating said steel and keeping said steel to a tempering temperature TT which is between 750°C and 820°C for at least one hour, - cooling said steel to room temperature. 13. Method of producing a seamless steel tube, according to claim 12, characterized in that the cooling steps are carried out using air cooling or water cooling.

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