KR102475025B1 - Martensitic high chromium heat-resistant steel with combined high creep rupture strength and oxidation resistance - Google Patents

Abstract

The present invention provides a martensitic heat resistant steel for boiler applications with a unique combination of improved creep strength and very good oxidation resistance when exposed to high temperatures in steam containing environments, with the following melt analysis (wt%): C: 0.10-0.16 %, Si: 0.20-0.60%, Mn: 0.30-0.80%, P ≤ 0.020%, S ≤ 0.010%, Al ≤ 0.020%, Cr: 10.50-12.00%, Mo: 0.10-0.60%, V: 0.15-0.30 %, Ni: 0.10-0.40%, B: 0.008-0.015%, N: 0.002-0.020%, Co: 1.50-3.00%, W: 1.50-2.50%, Nb: 0.02-0.07%, Ti: 0.001-0.020% . The remainder of the steel consists of iron and unavoidable impurities. The steel is normalized at a high temperature range of 1050°C to 1170°C for a time of about 10 minutes to about 120 minutes, cooled in air or water to room temperature, and then tempered at a temperature range of 750°C to 820°C for at least 1 hour. . The steel exhibits a martensitic microstructure with an average δ-ferrite content of less than 5% by volume.

Description

Martensitic high chromium heat-resistant steel with combined high creep rupture strength and oxidation resistance

The present invention relates to martensitic high chromium heat resistant steels for parts operating at elevated temperatures, i.e. 550° C. to 750° C. and high stresses. The steel according to the invention can be used in the power generation, chemical and petrochemical industries.

Ferritic/martensitic high Cr steel materials are widely used in modern power plants as reheater/superheater tubes and as steam pipes. A further improvement in the net efficiency of a thermal power plant requires an increase in the steam parameters pressure and temperature. Therefore, the realization of more efficient power plant cycles requires stronger materials with improved steam-side oxidation resistance. Existing attempts to develop new martensitic high chromium steels combining very good creep properties and good oxidation resistance have hitherto failed due to the formation of the so-called Z-phase. The Z-phase is a complex nitride that rapidly coarsens and consumes the surrounding strengthening MX (where M is Nb, V and X is C, N) precipitate.

The expression high chromium steel material generally means a steel with more than 9% Cr by weight. However, the elevated Cr content essential for good steam oxidation resistance, i.e., the Cr content containing more than 9% Cr by weight, increases the driving force for Z-phase formation as well as the coarsening rate of chromium carbide precipitates. also improve Regarding both of these, the loss of the microstructure stabilizing effect of MX and chromium carbide precipitates is responsible for the drop in long-term creep rupture strength of martensitic high Cr heat-resistant steel grades. Accordingly, a major challenge in future steel development is resolving the apparent contradiction between creep rupture strength and oxidation resistance.

Currently, for high temperature applications, applications with operating temperatures above 550°C, ASTM grades 91 and 92 are widely used, both of which contain 9% Cr by weight, and have creep rupture strength after 10 ⁵ h at 600°C. are 90 MPa and 114 MPa, respectively. The main difference between the two steels is that grade 92 contains W in the range of 1.8 weight percent and reduced Mo of 0.4 weight percent compared to 1 weight percent in the case of grade 91. Additionally, grade 92 contains a small amount of B, less than 0.005% by weight.

Both steels suffer from insufficient oxidation resistance under steam atmosphere at temperatures above 600° C., which significantly limits the temperature range of application. In particular, in boiler components with heat transfer, oxide scale acts as a thermal insulator, thereby increasing the metal temperature and consequently reducing the lifetime of the component. Additionally, the oxide scale, if broken into small pieces during operation, causes corrosion damage on subsequent steam transport parts or on turbine blades and guiding vans after entering the steam turbine. Oxide scale broken into small pieces can impede the vapor flow, often resulting in localized overheating and catastrophic failure, particularly tube clogging in the bend area.

X20CrMoV11-1 is an established ferritic/martensitic high Cr steel for high temperature applications, containing 0.20 wt% C, 10.5-12 wt% Cr, 1 wt% Mo and 0.2 wt% V. This steel exhibits better oxidation properties than those of ASTM steel grades 91 and 92 due to its higher Cr content, but poor creep rupture strength (creep rupture strength after 10 ⁵ h at 600 °C is about 59 MPa). Additionally, hot-workability and weldability deteriorate due to the high C content of 0.20% by weight. ASTM grade 122 contains 10-12% Cr, 1.8% W, 1% Cu, as well as V, Nb and N additions leading to MX reinforced particles. The creep rupture strength is significantly lower than that of ASTM grade 92 which gives a creep rupture strength of 98 MPa after 10 ⁵ h at 600°C.

In addition, there is also a hot workability problem due to the elevated Cu content.

Another steel exists with 11-12% Cr by weight. This steel is used primarily as thin-walled tubes and is named VM12-SHC steel for its combination of excellent steam side oxidation resistance and creep rupture strength at the level of ASTM grade 91. This steel concept is known from patent application WO02081766, which contains, by weight, 0.06-0.20% C, 0.1-1.00% Si, 0.1-1.00% Mn, 0.010% or less S, 10.00-13.00% Cr, 1.00% or less Ni, 1.00-1.80% W, 1.50% or less Mo with (W/2 + Mo), 0.50-2.00% Co, 0.15-0.35% V, 0.040-0.150% Nb , 0.030-0.12% N, 0.0010-0.0100% B and optionally up to 0.010% Ca, the remainder of the chemical composition being iron, and resulting from the manufacturing process or steel casting. It consists of impurities or residues resulting from or required for the process or casting. It is preferable to confirm the relationship that the chemical component content normalizes the heat treatment from 1050 ° C to 1080 ° C and allows the steel after tempering to have a tempered martensitic structure that does not contain or practically does not contain delta ferrite. . Compared to these steels, the creep rupture strength can still be improved while other properties such as corrosion resistance and mechanical properties remain unaffected.

Therefore, it is an object of the present invention for pipes and tubes to have a creep rupture strength substantially better than ASTM grade 92 steel and a high temperature similar to or better than the X20CrMoV11-1 and VM12-SHC steels described in the prior art. It is to provide a seamless tubular product of martensitic heat-resistant steel with corrosion and vapor oxidation behavior.

A further object of the present invention is to obtain a steel exhibiting a martensitic microstructure in which the delta ferrite (also known as δ-ferrite) content is limited to an average of 5% by volume.

Another object of the present invention is a steel that permits the manufacture of seamless tubular products of small or large diameter, such as seamless tubes or seamless pipes, and tubes and pipes welded using known and established manufacturing processes, It is to provide a steel suitable for the manufacture of forgings and plates.

The steel is suitable as a manufacturing material for various parts operating under stress at elevated temperatures, especially as seamless and welded tubes/pipes, forgings and plates in the power generation, chemical and petrochemical industries. In addition, the steel according to the present invention has temper resistance after a long tempering time of 30 hours or less at 800 ° C, a yield strength of 440 MPa or more, a tensile stress of 620 MPa or more, and a toughness at 20 ° C. 40 J or more when tested in the longitudinal direction, and 27 J when tested in the transverse direction.

According to the present invention, this object can be achieved by a seamless tubular product for high temperature applications of a steel having the following chemical composition in weight percent:

C: 0.10-0.16%

Si: 0.20-0.60%

Mn: 0.30-0.80%

P ≤ 0.020%

S ≤ 0.010%

Al ≤ 0.020%

Cr: 10.50-12.00%

Mo: 0.10-0.60%

V: 0.15-0.30%

Ni: 0.10-0.40%

B: 0.008-0.015%

N: 0.002-0.020%

Co: 1.50-3.00%

W: 1.50-2.50%

Nb: 0.02-0.07%

Ti: 0.001-0.020%

The remainder of the steel is iron and unavoidable impurities.

The ratio of boron and nitrogen is preferably such that B/N ≤ 5 in order to achieve hot workability.

Preferably, the following equation (where % is weight %) is satisfied:

1.00% ≤ Mo+0.5W ≤ 1.50%

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

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

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

In another preferred embodiment, the Mo content is 0.20-0.60%.

Preferably, the B content is 0.0095-0.013%.

In a preferred embodiment, the Ti content is 0.001-0.005%.

In another preferred embodiment, the microstructure comprises, on average, at least 95% tempered martensite and the balance delta ferrite.

In an even more preferred embodiment, the microstructure comprises, on average, at least 98% tempered martensite and the balance delta ferrite.

In a most preferred embodiment, the microstructure is martensitic and does not contain delta ferrite.

In addition, the present invention includes the following steps:

- casting a steel having a chemical composition according to the invention;

- hot forming the steel;

- heating the steel and holding the steel at a temperature range of 1050 ° C to 1170 ° C for a time of 10 minutes to 120 minutes,

- cooling the steel to room temperature;

- reheating the steel and maintaining the steel at a tempering temperature (TT) between 750 ° C and 820 ° C for at least 1 hour;

- cooling the steel to room temperature

It relates to a manufacturing method comprising a.

Preferably, the cooling step is performed using air cooling or water cooling.

The cooling step after the reheating step may be performed using water cooling.

The cooling step after the heating step may be performed using water cooling.

The invention may also relate to the manufacture of welded tubes, pipes or plates using the process according to the invention or the same steel as according to the seamless tubular product of the invention.

Figure 1 shows a schematic diagram of the mass gain due to oxidation plotted against chromium content.

According to the present invention, a martensitic high chromium heat-resistant steel having the following chemical composition is formed:

(1) C: 0.10-0.16%

C needs to be added at 0.10% or more to obtain sufficient carbide precipitation. Additionally, C is also an austenite stabilizing element. A C content of less than 0.10% contains more δ-ferrite in the microstructure. The upper limit for carbon is 0.16% because the addition of excess C limits roughness and weldability properties.

(2) Si: 0.20-0.60%

Si is used for deoxidation during the steel manufacturing process. Additionally, it is one of the main elements and determines the oxidation behavior in steel. In order to achieve full oxidation improving the effect of Si addition, an amount of 0.20% or more is required. The upper limit Si level is preferably limited to 0.60%, because the addition of excess Si accelerates the coarsening of the precipitate and reduces the roughness. The lower limit is preferably 0.25%.

(3) Mn: 0.30-0.80%

Mn is an effective deoxidizing element. It binds sulfur and reduces δ-ferrite formation. More than 0.30% Mn may be added. The upper limit should be 0.8%, since excess addition reduces the strength of the steel at elevated temperatures.

(4) P ≤ 0.020%

P is a grain-boundary active element, which reduces the roughness characteristics of steel. Its content should be limited to 0.020% to avoid negative effects of P on roughness characteristics. Since P may inevitably exist as an impurity, it may be present in an amount of 0.00% or more.

(5) S ≤ 0.010%

S forms sulfides to reduce the roughness and hot workability characteristics of steel. Restricting the upper limit content of S to 0.010% prevents defect formation during hot working operation and negative effects on roughness. Since S may inevitably exist as an impurity, it may be present in an amount of 0.00% or more.

(6) Al ≤ 0.020%

Al is a strong deoxidizing element used during the steel manufacturing process. Addition of excess Al above 0.02% can lead to AlN formation, thereby reducing the amount of reinforced MX (where M is Nb, V and X is C, N) nitride precipitates in the steel and consequently creep strength reduce the characteristic. Since Al may inevitably exist as an impurity, it may be present in an amount of 0.00% or more.

(7) Cr: 10.5-12.00%

Cr forms carbides formed at the boundaries of the martensitic microstructure. Chromium carbides are essential for stabilization of the martensitic microstructure during exposure to elevated temperatures. Cr improves the high temperature oxidation behavior of steel. A content of 10.5% or more is required to reveal complete oxidation which improves the effect of Cr addition. A Cr content greater than 12% results in increased δ-ferrite formation.

(8) Mo: 0.10-0.60%

Mo is an important element for improving creep rupture strength, which also causes solid solution strengthening. Moreover, this element is incorporated into carbides and intermetallic phases. A Mo content of 0.10% may be added. Mo addition of more than 0.60% deteriorates the roughness and induces an increase in the δ-ferrite content. The M and W contents (wt%) must satisfy the relationship: 1 ≤ Mo + 0.5 x W ≤ 1.5 to ensure sufficient precipitation of carbide and intermetallic phases.

(9) V: 0.15-0.30%

V combines with N to form a cohesive MX (where M is Nb, V and X is C, N) nitride, which contributes to the improvement of long-term creep properties. A content of less than 0.15% is not sufficient to achieve this long-term creep improving property effect, while a content of more than 0.30% reduces the roughness and increases the risk that the δ-ferrite content will exceed 5% by volume on average.

(10) Ni: 0.10-0.40%

Ni is an important roughness improving element. Therefore, a minimum content of 0.10% is required. However, when it is added in an amount greater than 0.40%, A _c1 Decreasing temperature tends to reduce creep rupture strength.

(11) B: 0.008-0.015%

B is a critical element that causes stabilization of M ₂₃ C ₆ carbide and delay in recovery of martensitic microstructure. It strengthens grain boundaries and improves long-term stability of creep rupture strength. Moreover, B is responsible for a notable improvement in creep rupture ductility. In order to achieve the maximum strengthening effect, an addition of 0.008% or more is necessary. However, contents greater than 0.015% substantially reduce the maximum processing temperature of the steel and are considered detrimental. B and N additions must satisfy the relation: B/N ≤ 1.5 to allow transformation using known hot working processes. In practice, this B/N relationship permits the manufacture of small or large diameter, seamless tubes, pipes and plates, and welded tubes, pipes and plates using the manufacturing process according to the present invention. Preferably, the B content should be 0.0095-0.0130 (% by weight).

(12) N: 0.002-0.020%

Nitrogen is required for the formation of MX (where M is Nb, V and X is C, N) nitrides and carbonitrides responsible for achieving creep rupture strength. 0.002% or more may be added. However, addition of excess N, i.e., N addition of more than 0.020%, results in enhanced BN formation, thereby reducing the reinforcing effect of B addition.

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

(13) Co: 1.50-3.00%

Co is a very effective austenite forming element and is useful in limiting δ-ferrite formation. Moreover, it has only a very weak effect on the A _c1 temperature. Additionally, it is an element that improves creep strength properties by reducing the size of initial precipitates after heat treatment. Therefore, a minimum content of 1.50% should be added. Preferably, the minimum content is 1.75%. However, addition of excess Co can lead to brittleness due to enhanced precipitation of intermetallic phases during high temperature operation. At the same time, Co is very expensive. For this reason, it is necessary to limit the addition to 3.00%, preferably 2.50%.

The Ni, Co, Mn, C and N contents (wt%) follow the equation:

(14) W: 1.50-2.50%

W is known as an effective solution strengthening agent. At the same time, it is incorporated into the carbide and forms a C14 lave phase, which may also contribute to creep rupture enhancement. Therefore, a minimum content of 1.50% is required. However, this element is expensive and segregates strongly during steel making and casting processes, and it forms intermetallic phases that cause significant brittleness. For this reason, the upper limit for W addition may be set at 2.50%. The Mo and W content (wt%) must satisfy the relationship: 1.00 ≤ Mo + 0.5W ≤ 1.50 to ensure sufficient precipitation of carbides and intermetallic phases.

(15) Nb: 0.02-0.07%

Nb forms stable MX carbonitrides that are important for controlling creep properties as well as austenite grain size. A minimum content of 0.02% may be added. Nb contents greater than 0.07% result in the formation of coarse Nb carbides which may reduce creep strength properties. Therefore, the upper limit should be set at 0.07%.

(16) Ti: 0.001-0.020%

Ti is a strong nitride forming element. It helps protect Glass B by forming nitrides. A minimum content of 0.001% is required for this purpose. However, an excess Ti content of more than 0.020% may reduce the roughness characteristics due to the formation of large blocks of TiN precipitates.

The remainder of the steel includes iron and common residual elements derived from the steel making and casting process. The casting techniques used are known to those skilled in the art. By impurities we mean elements such as tantalum, zirconium and any other elements which cannot be avoided. Tantalum and zirconium are stated not to be intentionally added to the steel, but may be present in total less than 50 ppm as unavoidable impurities.

In embodiments of the steel, the unavoidable impurities may include one or more of copper (Cu), arsenic (As), tin (Sn), antimony (Sb), and lead (Pd).

Cu may be present in a content of 0.20% or less.

The element As may be present in an amount of 150 ppm or less, Sn may be present in an amount of 150 ppm or less, Sb may be present in an amount of 50 ppm or less, Pb may be present in an amount of 50 ppm or less, and the total The content As + Sn + Sb + Pb is 0.4% by mass or less.

The steel is normalized at a temperature range of 1050°C to 1170°C for a time of about 10 minutes to about 120 minutes, cooled to room temperature in air or water, and then tempered at a temperature range of 750°C to 820°C for at least one hour.

The resulting steel was found to possess remarkably absolute very good elevated temperature strength and very good steam oxidation resistance. Besides, Cr _{eq .} /Ni _{eq .} It has been found that the ratio is less than 2.3 and the average δ-ferrite content can be limited to less than 5% by volume to avoid roughness problems, where Cre _{eq .} and Ni _{eq .} 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 of 1.5 or less must be maintained in order to enable hot work operation by known deformation processes.

The delta ferrite content should not exceed 5% by volume, because a content of more than 5% by volume impairs the roughness properties.

The hot forming process is hot rolling, pilgering, hot drawing, forging, plug mill, mandrel rod forming an elongated hollow through several in-line roll stands. means the push-bench process in which hollows are produced by pushing, continuous rolling and other rolling processes known in the art. The steel according to the present invention can be formed into the shape of tubes and pipes. Numerous attempts have been made with respect to steels exhibiting satisfactory properties such as oxidation behavior and creep resistance, but these steels have failed to produce satisfactory shaped articles through such hot forming processes. In particular, it was not even possible to obtain seamless tubes or pipes in some cases. The steel of the invention makes it possible to have a seamless tubular product with satisfactory properties and the possibility of obtaining a seamless tubular product by means of a hot forming process, which product meets the dimensional requirements.

Example

Advantages of the steel of the present invention will be explained in more detail on the basis of examples to be described later. Steels according to the present invention (Steel 1, Steel 2, Steel 3) as well as Comparative Example steels (Steel 4, Steel 5) having the chemical compositions shown in Table 1 below were prepared using a vacuum induction melting furnace to produce 100 kg ingots (ingot). ), then hot rolled into plates (13-25 mm thick), then normalized and tempered. Normalization heat treatment was performed at a temperature range of 1060° C. to 1100° C. for 30 minutes, followed by cooling to room temperature in air. Tempering was carried out at 780° C. for 120 minutes, followed by cooling in air again.

Comparative Examples Steel 4 and Steel 5 have a B content of less than 0.008% and therefore are not according to the present invention.

In the case of steel 4, the Ni, Co, Mn, C and N additions do not follow the equation (wt %):

Steel 5 does not satisfy the following formula (% by weight):

[Table 1]

For the two example 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 Charpy V notch impact energy.

[Table 2]

In addition, creep tests performed according to ISO DIN EN 204 on specimens of the two example steels showed a notable improvement in creep rupture strength. This reflected times to failure that were at least nearly twice as great as prior art steels such as P91, P92, VM12-SHC, P122 and X20CrMoV11-1 during long-term creep tests at 130 MPa and 100 MPa. The results are shown in Table 3 below. In addition, the comparative steel did not reach the creep rupture strength of the steel according to the present invention.

[Table 3]

Figure 1 shows a schematic diagram of the mass gain due to oxidation in a steam atmosphere at elevated temperatures plotted against the chromium content. The basis for the construction of the schematic is an oxidation test in a water vapor atmosphere performed according to ISO 21608:2012.

In Figure 1, three regions exhibiting different vapor oxidation behavior are defined as follows:

(I.) Unprotected behavior for mass gains greater than 10 mg/cm ² after 5,000 h

(II.) 5-10 mg/cm ² Intermediate behavior for mass gains in the range

(III.) 5 mg/cm ² Protection behavior for mass gains of less than

Correspondingly, the classification of the martensitic high Cr heat-resistant steels with respect to oxidation behavior was performed according to Table 4 below. Regions I, II and III correspond to mass gains as described in FIG. 1 . The two example steels clearly outperform P91, P92, P122 and X20CrMoV11-1 for vapor oxidation resistance. The present invention exhibits similar behavior to VM12-SHC.

[Table 4]

According to the present invention, a martensitic high chromium heat-resistant steel with improved creep properties and steam oxidation resistance can be used to manufacture tubes, forgings, pipes and plates operating under high temperatures in the power generation, chemical and petrochemical industries. make it possible to

Claims (15)

Seamless tubular products for high-temperature applications, made of steel having the following chemical composition in weight percent, obtained by a hot forming process:
C: 0.10-0.16%
Si: 0.20-0.60%
Mn: 0.30-0.80%
P ≤ 0.020%
S ≤ 0.010%
Al ≤ 0.020%
Cr: 10.50-12.00%
Mo: 0.10-0.60%
V: 0.15-0.30%
Ni: 0.10-0.40%
B: 0.008-0.015%
N: 0.002-0.020%
Co: 1.50-3.00%
W: 1.50-2.50%
Nb: 0.02-0.07%
Ti: 0.001-0.020%
The remainder of the steel is iron and unavoidable impurities,
B/N ≤ 1.5. delete The seamless tubular product according to claim 1, having the formula:
1.00% ≤ Mo+0.5W ≤ 1.50%
(Where, % is % by weight) The seamless tubular product according to claim 1, having the formula:

(Where, % is % by weight) The seamless tubular product according to claim 1, having the formula:

(Where, % is % by weight) The seamless tubular product according to claim 1, wherein the carbon content is 0.13-0.16%. The seamless tubular product according to claim 1, wherein the Mo content is 0.30-0.60%. The seamless tubular product according to claim 1, wherein the B content is 0.0095-0.013%. The seamless tubular product according to claim 1, wherein the Ti content is 0.001-0.005%. The seamless tubular product according to claim 1, wherein the product has a microstructure, the microstructure comprising at least 95% tempered martensite and the balance delta ferrite. The seamless tubular product according to claim 1, wherein the product has a microstructure, the microstructure comprising at least 98% tempered martensite and the balance delta ferrite. The seamless tubular product according to claim 1, wherein the product has a microstructure, the microstructure is martensitic and does not contain delta ferrite. The seamless tubular product according to claim 1 , which is a seamless tube. A method for producing a seamless tubular product according to claim 1,
- casting a steel having the chemical composition according to claim 1;
- hot forming the steel;
- heating the steel and maintaining the steel at a temperature range of 1050 ° C to 1170 ° C for a time of 10 minutes to 120 minutes;
- cooling the steel to room temperature;
- reheating the steel to maintain the steel at a tempering temperature (TT) between 750°C and 820°C for at least 1 hour;
- cooling the steel to room temperature
Manufacturing method comprising a. 15. The method of claim 14, wherein the cooling steps are performed using air cooling and water cooling.

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