Method of preparation of a high-chromium heat-resistant ferritic / martensitic steel

Abstract

The present invention relates to a method for preparing a high-chromium heat-resistant ferritic / martensitic steel. Additionally, the present invention relates to a high-chromium heat-resistant ferritic / martensitic steel obtainable by said method. The present invention is also directed to a steam contacting component made from said high-chromium heat-resistant ferritic / martensitic steel, to a steam generation equipment comprising at least one of said component, and to a thermal power plant comprising at least one of said steam generation equipment. Finally, the present invention is directed to the use of said high-chromium heat-resistant ferritic / martensitic steel in chemical and petrochemical, heat transfer, automotive, and construction industrial sectors.

Description

TECHNICAL FIELD

[0001] The present invention is encompassed within the sector of metallurgical industry. In particular, it relates to a method for preparing a high-chromium heat-resistant ferritic / martensitic steel. The present invention also relates to the high-chromium heat-resistant ferritic / martensitic steel obtainable by the said method and its uses.STATE OF THE ART

[0002] Due to current environmental regulations and in order to tackle climate change, power generation plants are obliged to increase their efficiency and reduce their emissions.

[0003] In this context, the principal activity pursues the development of ultra super-critical power plants (USC), which use elevated temperatures and pressures. Nowadays, the primary efforts are focused on improving the thermal efficiency of the plants by increasing the main steam temperature and pressure, as well as the temperature of the reheated steam. The transition to advanced ultra super-critical steam (700 to 760°C, 35 to 40 MPa) is the next step towards a 50% efficiency, beyond the efficiency factor of the plant of 45% given by the technology of the steam turbine generator based on ultra super-critical steam (28-32 MPa; 600-620°C), according to the analysis of the European Union, USA, Japan, China, South Korea, and India, interested in improving the efficiency of the electricity generation while reducing the CO 2 emissions.

[0004] The above-mentioned transition depends primarily on the availability of the construction materials with adequate properties. These materials must show high creep resistance with sufficient corrosion resistance in both the steam and exhaust gas atmosphere. At the same time, they must be capable of being processed to allow the boiler pressure part components and the main steam pipelines to be made out of them.

[0005] High-chromium steels are among the basic heat-resistant construction materials. Such steels have been used for the individual stages of steam superheating and reheating in boilers - both their chambers and coils, where the predominant destruction process is the creep. These materials are also used for the main primary and secondary steam pipelines.

[0006] The use of machines and equipment made of high-chromium steels under creep conditions results in changes in the microstructure, the effect of which is a reduction in their performance. As reported by Panait and co-workers (International Journal of Pressure Vessels and Piping, 2010 (87) 326-335; and Materials Science and Engineering. 2010 (527A) 4062-4069) and Golański et al. (Engineering Failure Analysis, 2013 (35) 692-702), for 9-12%Cr martensitic steels, the main microstructure degradation mechanisms include matrix recovery and polygonization processes, coagulation of M 23 C 6 carbides, precipitation of secondary phases: Laves phase and Z-phase, and depletion of alloying elements in matrix.

[0007] Therefore, there is a need for developing new methods for preparing steel grades with high chromium contents (9-15 wt.%) with enhanced creep and corrosion resistances to enable them to be used in steam temperatures and pressures higher than those used in today's USC (ultra super-critical) plants, which accounts for more efficient power generation plants (higher amount of energy extracted from a single unit of coal), and consequently lower CO 2 emissions produced (1% point improvement in the efficiency of a conventional pulverised coal combustion plant results in a 2% or 3% reduction of CO 2 emissions).

[0008] The present invention overcomes the drawbacks mentioned above by providing a method to prepare a high chromium ferritic / martensitic steel having improved creep strength while preserving high resistance to steam oxidation.BRIEF DESCRIPTION OF THE INVENTION

[0009] The authors of the present invention have developed a method for obtaining a high-chromium heat-resistant ferritic / martensitic steel having improved properties, which is carried out by a thermal treatment based on normalizing and tempering treatments as well as a martensitization step.

[0010] Therefore, a first aspect of the present invention is directed to a method for preparing a high-chromium heat-resistant ferritic / martensitic steel, characterized in that the method comprises the following steps: a) providing a steel having a chromium content between 10.00 wt.% and 11.50 wt.%; b) heating up the steel of step a) to a temperature between 1130 °C and 1180 °C and holding it at that temperature for more than 10 minutes; c) cooling down the steel resulting from step b) to a temperature between 300 °C and 350 °C and holding it at that temperature for more than 10 minutes; d) heating up the steel resulting from step c) to a temperature between 730 °C and 800 °C and holding it at that temperature for more than 20 minutes; and e) cooling down the steel resulting from step d) to room temperature.

[0011] The method of preparation for obtaining a high-chromium heat-resistant ferritic / martensitic steel of the present invention allows obtaining a steel having improved creep strength while preserving high resistance to steam oxidation.

[0012] Thus, a second aspect of the present invention is directed to a high-chromium heat-resistant ferritic / martensitic steel obtainable by the method described herein, characterized in that it comprises: 10.00 wt.% to 11.50 wt.% of Cr; 0.30 wt.% to 0.60 wt.% of Mn; 0.10 wt.% to 0.45 wt.% of Si; 0.18 wt. % to 0.25 wt. % of V; < 0.25 wt.% of Ni; 0.40 wt.% to 0.60 wt.% of Mo; 0.07 wt.% to 0.14 wt.% of C; 0.02 wt.% to 0.07 wt.% of Nb; 0.030 wt.% to 0.075 wt.% of N; 0.0010 wt.% to 0.0050 wt.% of B; < 0.020 wt.% of P; < 0.015 wt.% of Al%; and < 0.010 wt.% of S; wherein the balance is Fe and inevitable impurity elements.

[0013] The high-chromium heat-resistant ferritic / martensitic steel obtainable by the method of the present invention can be used to manufacture steam contacting components, such as tubes. Thus, the use of steam contacting components based on the high-chromium heat-resistant ferritic / martensitic steel described herein can increase the energy efficiency and durability of steam generation equipment, such as boilers and turbines, comprising them, which are used in supercritical and ultra-supercritical power generation plants, and thus, allowing applying higher steam temperature and reducing CO 2 emissions. As consequence, the targeted applications can be extended to sectors such as chemical and petrochemical, heat transfer, automotive, and construction industries.

[0014] Therefore, another aspect of the present invention is directed to a steam contacting component made of the high-chromium heat-resistant ferritic / martensitic steel as defined above.

[0015] Another aspect of the present invention is directed to a steam generation equipment comprising at least one component as defined above.

[0016] Another aspect of the present invention is directed to a thermal power plant comprising at least one steam generation equipment as defined above.

[0017] A final aspect of the present invention is directed to the use of the high-chromium heat-resistant ferritic / martensitic steel described herein in chemical and petrochemical, heat transfer, automotive, and construction industrial sectors.

[0018] Additional advantages and features of the invention will become apparent from the detailed description that follows and will be particularly pointed out in the appended claims.DESCRIPTION OF THE INVENTION

[0019] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs.

[0020] The method of the present invention intends to solve the shortcomings of prior-art methods for obtaining high-chromium heat-resistant ferritic / martensitic steel by applying a thermal treatment based on normalizing and tempering treatments as well as a martensitization step. Thus, it is an object of the present invention to provide a method for preparing a high-chromium heat-resistant ferritic / martensitic steel, characterized in that the method comprises the following steps: a) providing a steel having a chromium content between 10.00 wt.% and 11.50 wt.%; b) heating up the steel of step a) to a temperature between 1130 °C and 1180 °C and holding it at that temperature for more than 10 minutes; c) cooling down the steel resulting from step b) to a temperature between 300 °C and 350 °C and holding it at that temperature for more than 10 minutes; d) heating up the steel resulting from step c) to a temperature between 730 °C and 800 °C and holding it at that temperature for more than 20 minutes; and e) cooling down the steel resulting from step d) to room temperature.

[0021] The term "steel" refers to an alloy of iron and carbon wherein iron is the main element and carbon contributes from about 0.008 wt.% up to about 2.14 wt.% based on the total weight of the alloy, although other elements (such as manganese, silicon, nickel, chromium, molybdenum, boron, titanium, vanadium, tungsten, cobalt, niobium, and most frequently considered undesirable impurities, such as phosphorus, sulphur, and traces of oxygen, nitrogen, and copper) may be present or added.

[0022] In the context of the present invention, the term "high-chromium steel" refers to a steel having a chromium content higher than 10.00 wt%, preferably between 10.00 wt.% and 11.50 wt.%.

[0023] In the context of the present invention, the term "heat-resistant steel" refers to a steel having steam oxidation resistance similar to that of T115 steel grade and thus, being suitable for working at temperatures above 600°C, that is, for application in supercritical and ultra-supercritical steam generation equipment.

[0024] In the context of the present invention, the term "ferritic / martensitic" refers to high-chromium steels characterized by a body-centered crystal structure.

[0025] The term "T115 steel grade" refers to a steel having a composition according to standard ASTM / ASME A / SA 213.

[0026] As previously mentioned, the method of the present invention is a thermal treatment based on normalizing and tempering treatments as well as an intermediate martensitization step.

[0027] The critical temperatures Ac1, Ac3, Ms, and Mf of the steel of the present invention have been measured by dilatometry.

[0028] In steel, the transformation from ferrite (α-Fe) to austenite (γ-Fe) takes place between Ac1 and Ac3. The method of the present invention comprises a normalization step (in which the steel is heated up above Ac3 for full austenitization), followed by a martensitization step (in which the steel is rapidly cooled down to a temperature between the start of martensite formation Ms and the finish of martensite formation Mf for a partial transformation, retaining part of the austenite content), and a final tempering step (in which the steel is heated up to a temperature below Ac1 to avoid any microstructural transformation). Martensite forms when austenite is cooled rapidly enough in such a way that carbon atoms do not have time to diffuse out of the austenite structure. This non-equilibrium transformation occurs in a very short time frame. Martensite is hard and brittle due to its distorted lattice structure and the high carbon content trapped within it. The properties of martensite can be altered through subsequent heat treatment processes, such as tempering. During tempering, distorted tetragonal martensite may transform into more ductile cubic martensite and expelled carbides. Martensite plays a crucial role in the mechanical properties of steel, particularly in high-strength applications. The ability to form martensite is a key factor in the heat treatment of steels to achieve desired hardness and strength.

[0029] In the method of the present invention, after full austenitization, the steel is rapidly cooled not down to room temperature but to a temperature at which 30% of austenite is retained and 70% of austenite is transformed into fresh martensite (tetragonal). Subsequent tempering softens the fresh tetragonal martensite transforming it into tempered cubic martensite, while the 30% of retained austenite can be either transformed into ferrite and carbides / carbonitrides during tempering or can be transformed into fresh martensite on cooling. This mixed microstructure is responsible for the outstanding properties observed at high temperatures.

[0030] In the method of the present invention, the larger prior-austenite grains (PAGS) obtained for higher austenitization temperature, or the higher quenching temperature after normalization, may give rise to the larger martensite block size, which accounts for a higher concentration of nanoprecipitates in a lower amount of grain boundary, which consequently may act as a more effective barrier against dislocations, thus increasing the hardness, tensile strength, and creep resistance of the steel. In this context, the high thermal stability typically exhibited by the MX carbonitrides can also contribute to the creep resistance observed. At high temperature, smaller grain sizes reduce the creep strength because they can create more triple sites that lead to crack formation. For this reason, a larger PAGS and a larger martensite block size can contribute to creep strengthening, especially at high temperature conditions.

[0031] Firstly, the method of the present invention comprises a step a) of providing a steel having a chromium content between 10.00 wt.% and 11.50 wt.%, preferably between 10.40 wt.% and 11.20 wt.%.

[0032] In a particular embodiment, the steel of step a) is a modified T115 steel grade (V1) with a boron content of 0.0010-0.0050%.

[0033] The method of the present invention further comprises a step b) of heating up the steel of step a) to a temperature between 1130 °C and 1180 °C, preferably between 1140 °C and 1160 °C, and holding it at that temperature for more than 10 minutes, preferably about 50 minutes.

[0034] In step b) or "normalizing step" of the method of the present invention, the temperature is increased to reduce un-dissolved MX type carbide-nitride precipitates. In the normalization, temperature is raised as compared to usually used temperatures in order to reduce un-dissolved MX type carbide-nitride precipitates, where M and X denote metallic elements, and C and / or N, respectively. Thermodynamic phase calculations, as reported by Sundman et al (CALPHAD 9 (1985) 153-190), have shown that the amount of MX can be reduced to 0.049% from 0.132% and the amount of Nb in MX, major constituent element, can be reduced to 41% from 80% in total Nb content by raising normalizing temperature from 1050 to 1150°C. In parallel, the normalization temperature is increased to enlarge the prior austenite grain size since this trigger the formation of a final microstructure of larger grain size that contributes to creep strength.

[0035] In a particular embodiment, step b) is performed at 1150 °C.

[0036] The method of the present invention further comprises a step c) of cooling down the steel resulting from step b) to a temperature between 300 °C and 350 °C, preferably between 320 °C and 325 °C, and holding it at that temperature for more than 10 minutes, preferably for about 50 minutes.

[0037] In step c) or "martensitization step" of the method of the present invention, a mixed austenite / martensite microstructure is formed.

[0038] Therefore, in a particular embodiment, step c) is performed at about 323 °C.

[0039] The authors of the present invention have observed that the mixed austenite / martensite microstructure is optimum when step c) is applied at a temperature at which 30% of austenite is retained, together with a 70% of martensite. Without being bound to any theory in particular, it is believed that this mixed microstructure is transformed during tempering and final cooling into an optimum combination of fresh martensite, tempered martensite, and ferrite together with carbides (M 23 C 6 ) and carbonitrides (MCN) that, together with the large grain size formed, strengthen the microstructure against creep.

[0040] The method of the present invention further comprises a step d) of heating up the steel resulting from step c) to a temperature between 730 °C and 800 °C, preferably between 740 °C and 760 °C, and holding it at that temperature for more than 20 minutes, preferably for about 90 minutes.

[0041] In step d) or "tempering step" of the method of the present invention, the temperature is reduced and maintained below Ac1 to avoid any microstructural transformation.

[0042] In a particular embodiment, step d) is performed at about 750 °C.

[0043] As previously mentioned, the method of the present invention comprises a step e) of cooling down the steel resulting from step d) to room temperature.

[0044] In the context of the present invention, the term "room temperature" refers to a temperature of from 0° to 25°C, that is suitable for human occupancy and at which laboratory experiments are usually performed.

[0045] According to a further aspect of the present invention, it is provided a high-chromium heat-resistant ferritic / martensitic steel obtainable by the method of the present invention, wherein the steel comprises: 10.00 wt.% to 11.50 wt.% of Cr; preferably 10.40 wt.% to 11.20 wt.% of Cr; 0.30 wt.% to 0.60 wt.% of Mn; preferably 0.35 wt.% to 0.55 wt.% of Mn; 0.10 wt.% to 0.45 wt.% of Si; preferably 0.20 wt.% to 0.40 wt.% of Si; 0.18 wt.% to 0.25 wt.% of V; preferably 0.19 wt.% to 0.24 wt.% of V; < 0.25 wt.% of Ni; preferably 0.10 wt.% to 0.20 wt.% of Ni; 0.40 wt.% to 0.60 wt.% of Mo; preferably 0.40 wt.% to 0.50 wt.% of Mo; 0.07 wt.% to 0.14 wt.% of C; preferably 0.08 wt.% to 0.12 wt.% of C; 0.02 wt.% to 0.07 wt.% of Nb; preferably 0.03 wt.% to 0.06 wt.% of Nb; 0.030 wt.% to 0.075 wt.% of N; preferably 0.035 wt.% to 0.065 wt.% of N; 0.0010 wt.% to 0.0050 wt.% of B; < 0.020 wt.% of P; < 0.015 wt.% of Al%, and < 0.010 wt.% of S wherein the balance is Fe and inevitable impurity elements.

[0046] In the context of the present invention, the term "inevitable impurity elements" refers to other elements, such as traces of oxygen, copper, etc.

[0047] The inventors have surprisingly found that the method of the present invention as defined above allows obtaining a high-chromium ferritic / martensitic steel with steam oxidation resistance at 650°C similar to that of the T115 grade and enhanced creep behavior at 650°C in comparison to the T115 grade and even T92 grade. The most competitive ferritic / martensitic steel grades available in the market for the targeted application (high creep resistance) are T91, T92, T93, VM12, X20, and T115, out of which the T92 grade presents the highest creep strength (both at 600°C and 650°C) whereas the T115, VM12, and X20 grades possess the highest steam oxidation resistance (at 600 °C and 650 °C).

[0048] Without being bound to any theory in particular, it is believed that the content of B in the steel stabilizes the carbides M 23 C 6 in the form of M 23 BC 6 , and increases hardenability when B is in solution.

[0049] The high-chromium heat-resistant ferritic / martensitic steel obtainable by the method of the present invention can be used to manufacture steam contacting components, such as tubes. Thus, the use of steam contacting components based on the high-chromium heat-resistant ferritic / martensitic steel described herein can increase the energy efficiency and durability of steam generation equipment, such as boilers and turbines, comprising them.

[0050] Therefore, another aspect of the present invention refers to a steam contacting component made from the high-chromium heat-resistant ferritic / martensitic steel as defined above.

[0051] In addition, another aspect of the present invention is directed to a steam generation equipment comprising at least one component as defined above.

[0052] As consequence, the obtained steel having high creep strength allows a better performance in advanced power generation plants, heat recovery steam generators, refinery furnaces. For example, it enables to increase temperature conditions in boiler equipment used to generate process steam to feed petrochemical industrial processes, thus improving energy efficiency of the steam generation equipment.

[0053] Therefore, another aspect of the present invention is directed to a thermal power plant comprising at least one steam generation equipment as defined above.

[0054] In a particular embodiment, thermal power plant is a supercritical power generation plant (15-20 MPa and 560-570 °C), or an ultra-supercritical power generation plant (28-32 MPa and 600-620 °C).

[0055] Moreover, the targeted applications can be extended to sectors such as chemical and petrochemical, heat transfer, automotive, and construction industries.

[0056] Therefore, another aspect of the present invention is directed to the use of the high-chromium heat-resistant ferritic / martensitic steel described herein in chemical and petrochemical, heat transfer, automotive, and construction industrial sectors.

[0057] In the context of the present invention, it is to be understood as designating any value lying within the range defined by the number ±5 %, more preferably a range defined by the number ±2 %. For example, "10" should be construed as "within the range of 9.5 to 10.5", preferably "within the range of 9.8 to 10.2".

[0058] Through the description and the claims, the word "comprises" and variations thereof are not intended to exclude other technical features, components or steps. Additional advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention without undue burden.EXAMPLES

[0059] In the following, the invention will be further illustrated by means of Examples. The Examples should in no case be interpreted as limiting the scope of the invention, but only as an illustration of the invention.Example 1 - Thermal treatment of T115 commercial steel grade according to a standard method (TT1) versus modified T115 (V1) according to the method of the present invention (TT2)

[0060] A T115 commercial steel grade was subjected to a standard thermal treatment (TT1) through a normalization step at 1050 °C, followed by air cooling and final tempering at 765 °C.

[0061] A modified T115 steel grade (V1) (boron content of 0.0010-0.0050%) was subjected to the thermal treatment according to the present invention (TT2), which comprises a normalization step at 1150 °C, followed by an intermediate step called martensitization in which the material is not cooled down to room temperature but to 323 °C, and finally a tempering step at the temperature of 750 °C.

[0062] The samples cast and forged were treated accordingly, using a combination of muffle-type furnaces and, in particular, a salt bath installation for the martensitization step.Example 2 - Creep tests

[0063] The creep behaviour of the steels obtained in Example 1 ("T115-TT1" and "V1-TT2") at their application temperature (650 °C) was analysed on cylindrical samples (90 mm × 45 mm). In Table 1, the conditions of the creep tests performed, together with the creep response obtained for both materials, are shown. The stress values applied were adjusted according to the time to rupture observed. As can be observed, V1-TT2 presents a longest creep lifetime in comparison to T115-TT1, with a creep performance 60% improved. Table 1 - Laboratory cylinders creep tests Steel grade Thermal treatment (TT) σ (MPa) Time (h) T115TT1115591.5T115TT1883726V1TT21158846V1TT28817318.25 Example 3 - Oxidation tests

[0064] The steam oxidation behaviour of the two steels obtained in Example 1 (T115-TT1 and V1-TT2) at their application temperature (650 °C) was studied on specimens of 20 mm × 10 mm × 3 mm taken from the heat-treated materials. The specimens were subjected to oxidation testing in water steam atmosphere for 1000 h. The average thickness of the scale formed on the surface of the samples was measured by means of an optical microscope to evaluate the steam oxidation resistance. In Table 2, the conditions of the oxidation tests performed, together with the response obtained for both materials T115-TT1 and V1-TT2, are shown. In the case of T115-TT1, the oxide layer formed on the surface of the sample reaches a thickness of 16,5 µm, while in the case of V1-TT2, a better behavior is observed with an oxide layer of 13,5 µm in thickness.Steel grade Thermal treatment (TT) Steam oxidation test - Average Scale Thickness (µm) T115TT116.5V1TT213.5

Claims

1. A method for preparing a high-chromium heat-resistant ferritic / martensitic steel, characterized in that the method comprises the following steps: a) providing a steel having a chromium content between 10.00 wt.% and 11.50 wt.%; b) heating up the steel of step a) to a temperature between 1130 °C and 1180 °C and holding it at that temperature for more than 10 minutes; c) cooling down the steel resulting from step b) to a temperature between 300 °C and 350 °C and holding it at that temperature for more than 10 minutes; d) heating up the steel resulting from step c) to a temperature between 730 °C and 800 °C and holding it at that temperature for more than 20 minutes; and e) cooling down the steel resulting from step d) to room temperature.

2. Method for preparing a high-chromium heat-resistant ferritic / martensitic steel according to claim 1, wherein the steel of step a) is a modified T115 steel grade (V1) with a boron content in the range of 0.0010-0.0050 wt.%.

3. Method for preparing a high-chromium heat-resistant ferritic / martensitic steel according to any one of claims 1 or 2, wherein step b) is performed at about 1150 °C.

4. Method for preparing a high-chromium heat-resistant ferritic / martensitic steel according to any one of claims 1 to 3, wherein step c) is performed at about 323 °C.

5. Method for preparing a high-chromium heat-resistant ferritic / martensitic steel according to any one of claims 1 to 4, wherein step d) is performed at about 750 °C.

6. A high-chromium heat-resistant ferritic / martensitic steel obtainable by the method according to claims 1 to 5, characterized in that it comprises: - 10.00 wt.% to 11.50 wt.% of Cr; - 0.30 wt.% to 0.60 wt.% of Mn; - 0.10 wt.% to 0.45 wt.% of Si; - 0.18 wt.% to 0.25 wt.% of V; - < 0.25 wt.% of Ni; - 0.40 wt.% to 0.60 wt.% of Mo; - 0.07 wt.% to 0.14 wt.% of C; - 0.02 wt.% to 0.07 wt.% of Nb; - 0.030 wt.% to 0.075 wt.% of N; - 0.0010 wt.% to 0.0050 wt.% of B; - < 0.020 wt.% of P; - < 0.015 wt.% of Al%; and - < 0.010 wt.% of S; wherein the balance is Fe and inevitable impurity elements.

7. A steam contacting component made from the high-chromium heat-resistant ferritic / martensitic steel according to claim 6.

8. A steam contacting component according to claim 7, wherein the component is a tube.

9. A steam generation equipment comprising at least one component according to any one of claims 7 or 8.

10. The steam generation equipment according to claim 9, wherein the equipment is a pressure boiler or a turbine.

11. A thermal power plant comprising at least one steam contacting component according to any one of claims 7 or 8, or at least one steam generation equipment according to any one of claims 9 or 10.

12. A thermal power plant according to claim 11, wherein the thermal power plant is a supercritical power generation plant, or an ultra-supercritical power generation plant.

13. Use of the high-chromium heat-resistant ferritic / martensitic steel according to claim 6 in chemical and petrochemical, heat transfer, automotive, and construction industrial sectors.

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