ES2846875T3 - Heat resistant high chromium martensitic seamless steel tube or pipe with a combination of high creep resistance and oxidation resistance - Google Patents

Abstract

A seamless tube or pipe for high temperature applications made of steel that consists of the following chemical composition in weight percent: 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 between 0.001 and 0.005%, the rest of said steel being iron and unavoidable impurities.

Description

DESCRIPTION

Heat resistant high chromium martensitic seamless steel tube or pipe with a combination of high creep resistance and oxidation resistance

The invention relates to heat resistant high chromium martensitic seamless steel tubes for components operating at elevated temperatures, ie between 550 and 750 ° C and high stresses. The steel tubes or pipes according to the invention can be used in the power generation, chemical and petrochemical industries.

State of the art

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

However, the high contents of Cr, i.e. containing more than 9% by weight of Cr, which are essential for good resistance to vapor oxidation, increase the driving force for the formation of the Z-phase and also improve the rate of thickening of the chromium carbide precipitates. Both the loss of the stabilizing effect of the MX microstructure and the chromium carbide precipitates are responsible for the drop in the long-term yield strength of martensitic steels with a high heat-resistant Cr content. Therefore, the biggest challenge for future steel developments is to resolve the apparent contradiction between yield strength and oxidation resistance.

Currently, for high temperature applications, ASTM 91 and 92 grades are widely used, both contain 9% by weight of Cr with resistance to creep rupture after 105 h 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. Furthermore , grade 92 contains small amounts of B below 0.005% by weight.

Both steels suffer from sufficient oxidation resistance in vapor atmospheres at temperatures above 600 ° C, which significantly limits the application temperature range. Especially in heat transfer boiler components, the oxide scale acts as a thermal insulator, thus increasing the temperature of the metal and consequently reducing the service life of the corresponding components. Also, oxide scale, if dislodged during operation, will cause erosion damage to subsequent steam-bearing components or after entering the steam turbine on the turbine blades and guide vanes. Dislodged oxide flakes can cause tube blockage, especially in the region of bends, preventing steam flow, often resulting in local overheating and catastrophic failure.

X20CrMoV11-1 is a well established high Cr ferritic / martensitic steel for high temperature applications containing 0.20% by weight of C, 10.5-12% by weight of Cr, 1% by weight of Mo and 0, 2% by weight of V. This steel exhibits oxidation properties that are better than ASTM grades 91 and 92 steel grades due to higher Cr contents, but low creep breaking strength (creep breaking strength). after 105 h at 600 ° C being around 59MPa). Furthermore, the hot workability and weldability deteriorate due to the high C content of 0.20% by weight. ASTM 122 grade contains 10-12% Cr, 1.8% W, 1% Cu and also additions of V, Nb and N to induce precipitation of the MX reinforcing particles. The yield strength is significantly lower than that of the ASTM 92 grade, which has a yield strength of 98 MPa after 105 h at 600 ° C.

Hot workability problems also occur due to high Cu content.

There is another steel with 11 to 12% by weight of Cr. It is mainly used as a thin wall tube, and it is called VM12-SHC steel which combines good resistance to oxidation on the vapor side and resistance to creep failure. quality level ASTM 91. This concept of steel is known from patent application WO02081766 which discloses 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 at 1.80% W, Mo such that (w / 2 Mo) is not more than 1.50%, 0.50 to 2.00% Co, 0.15 to 0.35% V, 0.040 a 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 consists of iron and resulting or required impurities or residues for steel preparation or casting processes. The chemical constituent contents preferably verify a ratio such that the steel after normalizing the heat treatment between 1,050 and 1,080 ° C and tempering has a free or practically free tempered martensite structure of delta ferrite. Compared with this steel, the creep breaking strength can still be improve without affecting other properties such as corrosion resistance and mechanical properties.

Object and solution

Therefore, the object of the present invention is to provide a heat resistant martensitic seamless steel tube or pipe with a resistance to creep rupture substantially better than ASTM grade 92 steel for pipes and tubes, and with a performance of hot corrosion and vapor oxidation comparable to or better than X20CrMoV11-1 and VM12-SHC steels, described in the state of the art.

Another object of the invention is to obtain a steel that has a martensitic microstructure with a limitation of the delta ferrite content, also known as 5-ferrite, to 5% by volume on average.

Another object of the invention was to provide a steel which would allow the manufacture of small or large diameter welded and seamless tubes and pipes, forgings and plates using known and established manufacturing processes. Steel is suitable as a production material for a wide variety of components that operate under stress at elevated temperatures, particularly as seamless and welded tubes / pipes, forgings and plates in the power generation, chemical and petrochemical industries. Furthermore, the steel according to the invention is resistant to tempering, after long tempering times of up to 30 hours at 800 ° C, the elastic limit is greater than or equal to 440 MPa, the tensile stress greater than or equal to 620 MPa and the toughness at 20 ° C is greater than or equal to 40 J when tested in the longitudinal direction and 27 J when tested in the transverse direction.

According to the present invention, the object can be achieved by a seamless steel tube or a pipe for high temperature applications having the following chemical composition in weight percent:

C: 0.10 to 0.16%

Yes: 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 0.005%, the rest of said steel being iron and unavoidable impurities.

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

Preferably, the following equation is satisfied:

1.00% <Mo 0.5W <1.50% (in% by weight),

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

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

2.6 <4 ■ ( Ni Co 0.5 ■ Mn) - 20 ■ ( CN) < 11.2

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

In another preferred embodiment, the Mo content is between 0.30 and 0.60%.

Preferably, the B content is between 0.0095 and 0.013%.

In another preferred embodiment, the microstructure comprises on average at least 95% warm martensite, the remainder being delta ferrite.

In an even more preferred embodiment, the microstructure comprises on average at least 98% warm martensite, the remainder being delta ferrite.

In the most preferred embodiment, the microstructure is martensitic and free of delta ferrite.

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

- melt a steel with a chemical composition according to the invention,

- hot forming said steel,

- heating said steel and maintaining said steel for a time between 10 and 120 minutes in the temperature range between 1,050 ° C and 1,170 ° C,

- cooling said steel to room temperature,

- 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,

- cooling said steel to room temperature.

Preferably, the cooling step is carried out using air cooling or water cooling.

The invention also relates to the production of a seamless tube or pipe using the steel according to the invention or the process according to the invention.

Figure 1 shows the scheme of mass gain due to oxidation plotted against chromium content.

Object of the invention

In accordance with the present invention, a heat resistant high chromium martensitic steel is created having the following chemical composition:

(1) C: 0.10 to 0.16%,

It is necessary to add C at least 0.10% to obtain sufficient carbide precipitation. Furthermore, C is also an austenite stabilizing element. A C content below 0.10% would imply more 5-ferrite in the microstructure. The upper limit for carbon is 0.16% because excess C addition limits toughness and weldability properties.

(2) Yes: 0.20 to 0.60%,

The Si is used for deoxidation during the steelmaking process. In addition, it is one of the key elements, which determines the oxidation behavior in steels. To achieve the total oxidation enhancing effect of Si additions, an amount of at least 0.20% is necessary. The upper level of Si will preferably be limited to 0.60%, because the addition of excess Si accelerates the thickening of the precipitates and reduces the toughness. (3) Mn: 0.30 to 0.80%,

Mn is an effective deoxidation element. This binds sulfur and reduces ferrite formation 5. At least 0.30% Mn can be added. The upper limit will be 0.8%, as excessive additions reduce the strength of the steels at elevated temperatures.

(4) P <0.020%,

P is an active grain boundary element that reduces the toughness properties of steels. The content should be limited to 0.020% to avoid the negative impact of P on toughness properties.

(5) S <0.010%,

S forms sulfides and reduces the toughness and hot workability of steels. Limiting the S content higher than 0.010 prevents the formation of defects during hot work operation and negative impact on toughness.

(6) Al <0.020%,

Al is a powerful deoxidation element used during the steelmaking process. Excess Al addition above 0.02% can induce the formation of AIN, thus reducing the amount of reinforcement MX (where M: Nb, V and where X: C, N) the nitride precipitates in the steel and , consequently, the creep resistance properties.

(7) Cr: 10.5 to 12.00%,

Cr forms carbides that form at the limits of the martensitic microstructure. Chromium carbides are essential for the stabilization of the martensitic microstructure during exposure to elevated temperatures. Cr improves the high temperature oxidation behavior of steels. Contents of at least 10.5% are needed to develop the full oxidation enhancing effect of Cr additions. Cr contents above 12% result in increased ferrite formation 5.

(8) Mo: 0.10 to 0.60%,

Mo is an important element for the improvement of the resistance to the creep rupture that is also responsible for the strengthening of the solid solution. This element is also incorporated in carbides and intermetallic phases. A Mo content of 0.10% can be added. Mo additions above 0.60% will deteriorate the toughness and induce an increase in the ferrite content 5. It should be noted that the contents of M and W must comply with the relationship (in% by weight) 1 <Mo 0.5 x W <1.5, to ensure sufficient precipitation of carbides and intermetallic phases. (9) V: 0.15 to 0.30%,

V combines with N to form coherent nitrides MX (where M: Nb, V and where X: C, N), which contribute to improved long-term creep properties. Contents below 0.15% are not sufficient to achieve this long-term creep enhancing property effect, while contents above 0.30% decrease toughness and increase the danger for ferrite contents. 5 above 5% by volume mean. (10) Ni: 0.10 to 0.40%,

Ni is an important element to improve toughness. Therefore, a minimum content of 0.10% is necessary. However, it reduces the temperature A ^c1 and tends to reduce the resistance to breakage by creep, if it is added in contents higher than 0.40%.

(11) B: 0.008 to 0.015%,

B is a decisive element responsible for stabilizing M ²³ C ⁶ carbides and delaying the recovery of the martensitic microstructure. Strengthens grain boundaries and improves long-term stability of creep resistance. Furthermore, B is responsible for a remarkable improvement in the yield break ductility. To achieve the maximum reinforcing effect, additions of at least 0.008% are necessary. However, contents greater than 0.015% substantially reduce the maximum processing temperature of steels and are considered harmful. The additions of B and N must satisfy the B / N ratio <1.5 to allow transformation using known hot work processes. In fact, this B / W ratio allows the manufacture of small or large diameter welded and seamless tubes, pipes and plates using the manufacturing process according to the invention. Preferably, the content of B should be between 0.0095 and 0.0130 (% by weight).

(12) N: 0.002 to 0.020%,

Nitrogen is necessary for the formation of MX (with M: Nb, V and X: C, N) being nitrides and carbonitrides responsible for achieving resistance to creep failure. At least 0.002% can be added. However, excessive additions of N, i.e., above 0.020%, result in improved BN formation, which reduces the strengthening effect of additions of B.

Preferably, the B and N contents (in% by weight) should satisfy the following relationship:

(13) Co: 1.50 to 3.00%,

Co is a very effective and useful austenite-forming element in limiting ferrite 5 formation. Furthermore, it only has a weak effect on Ac1 temperature. In addition, it is an element that improves creep resistance properties by reducing the size of the initial precipitates after heat treatment. Therefore, a content will be added minimum of 1.50%. However, Co in excessive additions can induce embrittlement due to increased precipitation of the intermetallic phases during high temperature operation. At the same time, the Co is very expensive. Therefore, a limitation of the additions to 3.00%, preferably 2.50%, is necessary.

It is preferable that the Ni, Co, Mn, C and N contents (in% by weight) agree with the following equation: 2.6 <4 ■ (Ni Co + 0.5 ■ Mn) - 20 ■ (C N) < 11.2.

(14) W: 1.50 to 2.50%,

W is known as an effective dissolution strengthener. At the same time, it is incorporated into carbides and forms the C14 Laves phase, which can also help improve creep resistance. Therefore, a minimum content of 1.50% is required. However, this element is expensive, it segregates strongly during the steelmaking and casting process and forms intermetallic phases that lead to significant embrittlement. Therefore, the upper limit for W additions can be set at 2.50%. Note that the Mo and W contents (in% by weight) must comply with the ratio 1.00 <Mo 0.5 W <1.50 to guarantee sufficient precipitation of carbides and intermetallic phases. (15) Nb: 0.02 to 0.07%.

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

(16) Ti: 0.001-0.005%

Ti is a strong nitride-forming element. It is useful to protect free B by forming nitrides. A minimum content of 0.001% is needed for this purpose. However, excessive Ti content above 0.020% can reduce toughness properties due to the formation of large blocky TiN precipitates.

The remainder of the steel comprises iron and ordinary residual elements from the steelmaking and smelting process. By impurities we mean elements such as tantalum, zirconium and any other element that cannot be avoided. It should be mentioned that tantalum and zirconium are not intentionally added to steel, however they can be present in less than 50 ppm total as unavoidable impurities.

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

Cu can be present in a content equal to or less than 0.20%.

Element A can be present in a content equal to or less than 150 ppm; Sn can be present in a content equal to or less than 150 ppm; Sb can be present in a content equal to or less than 50 ppm; The Pb can be present in 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.

Steel is normalized over a period of approximately 10 to approximately 120 minutes in the temperature range of 1,050 ° C to 1,170 ° C and cooled in air or water at room temperature, and then annealed for at least one hour in the range temperature between 750 ° C and 820 ° C.

The resulting steel has been found to possess remarkable and absolutely excellent resistance to elevated temperatures and superior resistance to steam oxidation. Furthermore, it was found that because the Cr ^eq / Ni ^eq. is less than 2.3, the average content of ferrite 5 can be limited to less than 5% by volume to avoid toughness problems, where Cr ^eq and Ni ^eq. they are defined as Cr 6Si 4Mo 1.5W 11V 5Nb 8Ti and 40C 30N 2Mn 4Ni 2Co Cu, respectively. Surprisingly, it was found that the B / N ratio equal to or less than 1.5 must be maintained to allow hot work operation with known transformation processes.

The delta ferrite content should not exceed 5% by volume as contents above 5% by volume will affect toughness properties.

By hot forming processes, we mean: hot rolling, pilger rolling, hot drawing, forging, plug milling, push bench process where the mandrel rod pushes the elongated hole through various roller supports in-line to produce a continuous hollow laminate, and other known lamination processes.

Examples

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) and also the steels according to the comparative examples (Steel 3, Steel 4), having the chemical composition indicated in Table 1, were cast into ingots of 100 kg using a vacuum induction melting furnace, then hot rolled to plates (13-25 mm thick) and subsequently normalized and tempered. The normalization heat treatment was carried out in the temperature range 1,060 ° C to 1,100 ° C for 30 minutes, followed by air cooling to room temperature. Tempering was carried out at 780 ° C for 120 minutes, followed again by air cooling.

The steels of Comparative Examples 3 and 4 have B contents below 0.008 and are therefore not according to the invention.

[In the case of steel 3, the additions of Ni, Co, Mn, C and N do not comply with the equation

2.6 <4- ( Ni + Co + 0.5 ■ Mn) - 20 ■ ( C + N) <11.2 (in% by weight).

Steel 4 does not meet the following formula:

Table 1

For the two example steels (Steel 1, Steel 2), the results presented in Table 2 were obtained at room temperature for tensile strength, yield strength, elongation, area reduction, and Charpy V notch impact energy.

Table 2

Creep tests, carried out in accordance with ISO DIN EN 204, on the specimens of the two example steels also showed a notable improvement in the resistance to creep failure. This is reflected in that the failure times are at least three times greater than those of the latest generation steels such as P91, P91, VM12-SHC, P122 and X20CrMoV11-1 during long-term creep tests at 130 MPa and 100 MPa. The results are shown in Table 3. Furthermore, the comparative example steels do not reach the resistance to creep failure of the steels according to the invention.

Table 3

Figure 1 shows the scheme of the mass gain due to oxidation in an atmosphere of water vapor at elevated temperatures represented as a function of the chromium content. The basis for the construction of the scheme are the oxidation tests in a steam atmosphere carried out in accordance with ISO 21608: 2012.

In Figure 1, three regions have been defined that show different vapor oxidation behaviors as follows:

(I.) Non-protective behavior for mass gain greater than 10 mg / cm2 after 5,000 h

(II.) Intermediate behavior for mass gain in the range of 5-10 mg / cm2

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

Consequently, in the following Table 4 the classification of different heat-resistant martensitic steels with high Cr content with respect to oxidation behavior was carried out. Regions I, II and III correspond to mass gains as described in Figure 1. The two example steels clearly outperform P91, P92, P122 and X20CrMoV11 -1 with respect to resistance to steam oxidation. The invention has a behavior comparable to VM12-SHC.

Table 4

According to the invention, it is possible to provide a heat resistant high chromium martensitic steel with improved creep properties and resistance to steam oxidation which is used to produce tubes, pipes operating at high temperature in the generating industry energy, chemical and petrochemical.

Claims (13)

1. A seamless tube or pipe for high temperature applications made of steel that consists of the following chemical composition in percent by weight: C: 0.10 to 0.16% Yes: 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 between 0.001 and 0.005% the rest of said steel being iron and unavoidable impurities. 2. A seamless tube or pipe according to claim 1, wherein: B / ⁿ < 1.5. 3. A seamless tube or pipe according to claim 1 or 2, wherein, in% by weight: 1.00 % < Mo + 0.5W <1.50% 4. A seamless tube or pipe according to any one of claims 1 to 3, wherein in% by weight:

5. A seamless tube or pipe according to any one of claims 1 to 4, wherein, in% by weight: 2.6 <4 ■ ( Ni Co 0.5 ■ Mn) - 20 ■ ( C + N) < 11.2 6. A seamless tube or pipe according to any one of claims 1 to 5, wherein the carbon content is between 0.13 and 0.16%. 7. A seamless tube or pipe according to any one of claims 1 to 6, wherein the Mo content is between 0.30 and 0.60%. 8. A seamless tube or pipe according to any one of claims 1 to 7, wherein the B content is between 0.0095 and 0.013%. 9. A seamless tube or pipe according to any of claims 1 to 8, wherein the microstructure comprises at least 95% tempered martensite, the remainder being delta ferrite. 10. A seamless tube or pipe according to any of claims 1 to 9, wherein the microstructure comprises at least 98% tempered martensite, the remainder being delta ferrite. A seamless tube or pipe according to any one of claims 1 to 10, wherein the microstructure is martensitic and free of delta ferrite. 12. Production method of a seamless tube or pipe according to any of claims 1 to 11, comprising the following steps: - melt a steel with a chemical composition according to any of claims 1 to 8, - hot forming said steel, - heating said steel and maintaining said steel for a time between 10 and 120 minutes in the temperature range between 1,050 ° C and 1,170 ° C, - cooling said steel to room temperature, - reheat said steel and keep said steel at a tempering temperature TT that is between 750 ° C and 820 ° C for at least one hour, - cooling said steel to room temperature. 13. Production method of a seamless steel tube or pipe according to claim 12, wherein the cooling step is carried out using air cooling or water cooling.