Cr-ni-mo-v martensitic heat-resistant steel and method for manufacturing the same

Abstract

The application discloses a preparation method of Cr-Ni-Mo-V martensitic heat-resistant steel, which comprises the following steps: obtaining molten steel by adopting double vacuum smelting and pouring the molten steel into ingots; forging the ingots, which comprises free forging and radial forging; the free forging process comprises multiple upsetting and elongating, the deformation of each upsetting is controlled to be 40%-50%, and the intermediate blank is produced after each elongating and reheating; the radial forging comprises forging the intermediate blank into a target size steel blank, the grain size is controlled to be 32-44 mu m, and the steel blank is air-cooled to room temperature after the forging; the steel blank after the forging is subjected to annealing treatment to eliminate residual stress; solid solution treatment and twice tempering treatment are conducted on the steel blank to prepare the Cr-Ni-Mo-V martensitic heat-resistant steel. The application further provides a Cr-Ni-Mo-V martensitic heat-resistant steel. Through accurate regulation and control of raw material proportioning and improvement of the manufacturing process, the generation of harmful phase delta ferrite is strictly controlled, and the high-temperature creep performance of the martensitic heat-resistant steel is effectively improved.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, and in particular to a Cr-Ni-Mo-V martensitic heat-resistant steel and its manufacturing method. Background Technology

[0002] Currently, my country's power structure is still dominated by thermal power generation, resulting in enormous coal consumption. Ultra-supercritical (USupercritical) units, due to their high thermal efficiency, can effectively reduce coal consumption, achieving energy conservation and emission reduction goals, and are the type of thermal power units that my country is vigorously developing. However, because my country's USupercritical technology started relatively late, it currently relies heavily on foreign technology support, and key materials for USupercritical units are mainly imported. The lagging material production technology severely restricts the development of my country's USupercritical technology. The last-stage blade is a critical component in USupercritical units, requiring high-quality materials. Cr-Ni-Mo-V martensitic heat-resistant steel, with its excellent mechanical properties and resistance to high-temperature corrosion and oxidation, is the ideal steel for the last-stage blades of USupercritical units. With the increase in installed turbine capacity, the demand for last-stage blade materials, as well as the requirements for stability of microstructure and properties at high temperatures, thermal strength, oxidation resistance, and resistance to steam corrosion, are also increasing.

[0003] High-pressure gas turbines typically operate above 400℃. Due to their very small allowable deformation, in addition to room temperature performance, they should also have high creep performance. However, the δ-ferrite in existing steels and the microstructure changes that are easily caused during high-temperature forging, such as grain inhomogeneity, affect the high-temperature performance of the steel. Therefore, the high-temperature creep performance of steel needs to be further improved.

[0004] Therefore, there is a need in the existing technology to improve Cr-Ni-Mo-V martensitic heat-resistant steel and its manufacturing methods. Summary of the Invention

[0005] In view of this, the purpose of this invention is to propose a Cr-Ni-Mo-V martensitic heat-resistant steel and its manufacturing method. By precisely controlling the raw material ratio and improving the manufacturing process, the generation of harmful δ-ferrite phase is strictly controlled, and the high-temperature creep performance of martensitic heat-resistant steel is effectively improved.

[0006] One embodiment of the present invention provides a Cr-Ni-Mo-V martensitic heat-resistant steel, comprising, by weight percentage: C: 0.08-0.1%, Si≤0.25%, Mn: 0.9-1.0%, P≤0.03%, S≤0.02%, Cr: 9-12%, Ni: 2-3%, Mo: 1-2%, V: 0.2-0.4%, N: 0.02-0.03%, with an equivalent ratio of chromium to nickel ranging from 1.6 to 1.8, and the balance being iron and unavoidable impurity elements.

[0007] Another aspect of this invention provides a method for preparing Cr-Ni-Mo-V martensitic heat-resistant steel, comprising the following steps:

[0008] The steel was obtained by double vacuum smelting and cast into ingots. The chemical composition of the ingots by weight percentage is as follows: C: 0.08-0.1%, Si≤0.25%, Mn: 0.9-1.0%, P≤0.03%, S≤0.02%, Cr: 9-12%, Ni: 2-3%, Mo: 1-2%, V: 0.2-0.4%, N: 0.02-0.03%, the equivalent ratio of chromium to nickel is 1.6-1.8, and the balance is iron and unavoidable impurity elements.

[0009] Forging of ingots includes free forging and radial forging. Free forging involves multiple upsetting and drawing processes, with the deformation amount controlled at 40% to 50% for each upsetting. After each drawing process, the ingots are reheated in the furnace to produce intermediate billets. Radial forging involves forging the intermediate billets to the target size billet, controlling the grain size at 32 to 44 μm, and then air-cooling them to room temperature.

[0010] The forged steel billet undergoes annealing to eliminate residual stress;

[0011] Cr-Ni-Mo-V martensitic heat-resistant steel is prepared by solution treatment and two tempering treatments on steel billets.

[0012] In some embodiments, the forging temperature for free forging is 1140–1160°C.

[0013] In some embodiments, the forging temperature for radial forging is 1050–850°C.

[0014] In some implementations, the annealing temperature is 700°C.

[0015] In some embodiments, solution treatment is performed in a temperature range of 1040–1060°C, and tempering is performed twice in a temperature range of 550–570°C, with each tempering holding time controlled to be 0.5–1 h, and the HB hardness value after each tempering is controlled to be 350–360 HB.

[0016] In some implementations, dual vacuum smelting includes VIM vacuum induction melting and VAR consumable electrode melting furnace.

[0017] In some implementations, the free forging process includes three upsetting and drawing operations.

[0018] The present invention has at least the following beneficial technical effects:

[0019] This invention provides a method for producing Cr-Ni-Mo-V martensitic heat-resistant steel. Targeting factors affecting the high-temperature strength of Cr-Ni-Mo-V martensitic heat-resistant steel, such as δ-ferrite, grain size, and the distribution and size of precipitates, the method improves smelting purity through a VIM+VAR process while precisely controlling the composition to strictly control the formation of harmful δ-ferrite. The deformation process combines free forging and radial forging. On the one hand, high-temperature deformation increases the solution rate to further eliminate δ-ferrite; on the other hand, hot deformation controls grain size. After solution treatment, heat treatment is performed at a lower temperature, with unique secondary tempering process parameters designed to ensure dispersed distribution of precipitates and prevent coarsening of precipitates from affecting high-temperature creep performance. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other embodiments can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of an embodiment of the preparation method of Cr-Ni-Mo-V martensitic heat-resistant steel provided by the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be further described in detail below with reference to specific examples and the accompanying drawings.

[0023] The terms "comprising" and "having," and any variations thereof, used in the specification, claims, and accompanying drawings of this invention are intended to cover non-exclusive inclusion; the terms "first," "second," etc., used in the specification, claims, and accompanying drawings are used to distinguish different objects, not to describe a particular order. "A plurality of" means two or more, unless otherwise explicitly specified.

[0024] Furthermore, the reference to "embodiment" herein means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0025] To improve the high-temperature creep properties of Cr-Ni-Mo-V martensitic heat-resistant steel, this invention provides a Cr-Ni-Mo-V martensitic heat-resistant steel and its manufacturing method. By weight percentage, the Cr-Ni-Mo-V martensitic heat-resistant steel comprises: C: 0.08–0.1%, Si ≤ 0.25%, Mn: 0.9–1.0%, P ≤ 0.03%, S ≤ 0.02%, Cr: 9–12%, Ni: 2–3%, Mo: 1–2%, V: 0.2–0.4%, N: 0.02–0.03%, with an equivalent ratio of chromium to nickel ranging from 1.6 to 1.8, and the balance being iron and unavoidable impurity elements.

[0026] like Figure 1 The flowchart shown is an embodiment of the preparation method of Cr-Ni-Mo-V martensitic heat-resistant steel provided by the present invention, including the following steps:

[0027] S1. The steel is obtained by double vacuum smelting and cast into ingots. The chemical composition of the ingots by weight percentage is as follows: C: 0.08-0.1%, Si≤0.25%, Mn: 0.9-1.0%, P≤0.03%, S≤0.02%, Cr: 9-12%, Ni: 2-3%, Mo: 1-2%, V: 0.2-0.4%, N: 0.02-0.03%, the equivalent ratio of chromium to nickel is 1.6-1.8, and the balance is iron and unavoidable impurity elements.

[0028] S2. Forging the ingot includes free forging (S2-1) and radial forging (S2-2). The free forging process includes multiple upsetting and drawing operations. The deformation amount of each upsetting operation is controlled at 40% to 50%. After each drawing operation, the ingot is reheated in the furnace to produce an intermediate billet. Radial forging includes forging the intermediate billet to the target size billet, controlling the grain size to 32 to 44 μm, and then air cooling to room temperature after forging.

[0029] S3. The forged steel billet is annealed to eliminate residual stress.

[0030] S4. Solution treatment (S4-1) and two tempering treatments (S4-2) are performed on the steel billet to prepare Cr-Ni-Mo-V martensitic heat-resistant steel.

[0031] Furthermore, in S1, the content of the austenite-forming element Mn is carefully controlled, and the chromium-nickel equivalent ratio is adjusted to a range of 1.6–1.8 to reduce the formation of δ-ferrite. Dual vacuum smelting includes VIM vacuum induction melting and VAR consumable electrode melting furnace.

[0032] Furthermore, in S2, an intermediate billet is first produced by free forging at a heating temperature of 1140–1160℃. The upsetting deformation is controlled at 40%–50% each time. After each drawing, the billet is reheated in the furnace. This high-temperature deformation increases the surface area of ​​δ-ferrite, causing dynamic recrystallization and increasing the number of grain boundaries within the δ-ferrite. This improves the overall diffusion rate of alloying elements, thereby increasing the solid solution rate of δ-ferrite and further reducing its density. After free forging, the intermediate billet is radially forged to the required dimensions at a forging temperature of 1050–850℃, and then air-cooled to room temperature. The grain size is controlled at 32–44 μm.

[0033] Furthermore, in S3, the forging process is followed by annealing at 700°C to eliminate residual stress.

[0034] Furthermore, in step S4, heat treatment is performed, specifically including solution treatment at 1040–1060℃, followed by two tempering treatments at 550–570℃, with each tempering holding time controlled between 0.5 and 1 hour. The purpose of the two temperings is to ensure the dispersed distribution of the precipitated phase, and to control the tempering time to prevent the precipitated phase from coarsening and affecting high-temperature performance. After each tempering, the HB hardness value is controlled between 350 and 360 HB.

[0035] The present invention will be further explained below with reference to specific embodiments.

[0036] Example 1:

[0037] The ingots were smelted using a double vacuum process (VIM+VAR) and cast into ingots. The chemical composition of the ingots by weight percentage was: C: 0.09%, Si: 0.15%, Mn: 0.95%, P≤0.03%, S≤0.02%, Cr: 9.5%, Ni: 2.5%, Mo: 1.5%, V: 0.21%, N: 0.022%, and the chromium-nickel equivalent ratio was 1.77.

[0038] First, the intermediate billet is produced by free forging at a heating temperature of 1150℃. Each upsetting is controlled at 50%. After each upsetting and drawing, it is reheated in the furnace. After free forging, it is transferred to radial forging at a forging temperature of 1050~850℃. After forging, it is air-cooled to room temperature. The average grain size after forging is 35.2μm.

[0039] After forging, it is annealed at 700℃.

[0040] The solution was subjected to a solution treatment at 1050℃ for 40 minutes, followed by a first tempering at 570℃ for 1 hour, and a second tempering at 560℃ for 0.5 hours. The hardness value after the first tempering was 358 HB, and the hardness value after the second tempering was 352 HB.

[0041] High-temperature creep performance tests were conducted at 450℃, and the total plastic strain was 0.12–0.14%.

[0042] Example 2:

[0043] The ingots were smelted using a double vacuum process (VIM+VAR) and cast into ingots. The chemical composition of the ingots by weight percentage was: C: 0.1%, Si: 0.15%, Mn: 1.0%, P≤0.03%, S≤0.02%, Cr: 11.0%, Ni: 2.9%, Mo: 1.8%, V: 0.38%, N: 0.028%, and the chromium-nickel equivalent ratio was 1.8.

[0044] First, the intermediate billet is produced by free forging at a heating temperature of 1160℃. Each upsetting is controlled at 50%. After each upsetting and drawing, it is reheated in the furnace. After free forging, it is transferred to radial forging at a forging temperature of 1050~850℃. After forging, it is air-cooled to room temperature. The average grain size after forging is 40.2μm.

[0045] After forging, it is annealed at 700℃.

[0046] The solution was subjected to a solution treatment at 1050℃ for 30 minutes, followed by a first tempering at 560℃ for 1 hour, and a second tempering at 550℃ for 0.5 hours. The hardness value after the first tempering was 360 HB, and the hardness value after the second tempering was 354 HB.

[0047] Creep performance tests were conducted at 450℃, and the total plastic strain was 0.11–0.13%.

[0048] Example 3

[0049] The ingots were smelted using a double vacuum process (VIM+VAR) and cast. The chemical composition of the ingots by weight percentage was: C: 0.09%, Si: 0.2%, Mn: 0.8%, P≤0.03%, S≤0.02%, Cr: 11.5%, Ni: 2.5%, Mo: 2.0%, V: 0.21%, N: 0.023%, and the chromium-nickel equivalent ratio was 2.19.

[0050] First, the intermediate billet is produced by free forging at a heating temperature of 1150℃. Each upsetting is controlled at 50%. After each upsetting and drawing, it is reheated in the furnace. After free forging, it is transferred to radial forging at a forging temperature of 1050~850℃. After forging, it is air-cooled to room temperature. The average grain size after forging is 38.5μm.

[0051] After forging, it is annealed at 700℃.

[0052] The solution was subjected to solution treatment at 1050℃ for 40 min, followed by tempering at 570℃ for 1 h, and then tempering at 560℃ for 0.5 h. The hardness value after the first tempering was 354 HB, and the hardness value after the second tempering was 350 HB.

[0053] In this embodiment, the Cr / Ni equivalent ratio was not controlled within the range specified in this application. High-temperature creep performance tests were conducted at 450°C, and the total plastic strain was 0.15–0.17%.

[0054] Comparative Example 1:

[0055] The ingots were smelted using a double vacuum process (VIM+VAR) and cast. The chemical composition of the ingots by weight percentage was: C: 0.1%, Si: 0.15%, Mn: 1.0%, P≤0.03%, S≤0.02%, Cr: 11%, Ni: 2.9%, Mo: 1.8%, V: 0.38%, N: 0.028%, and the chromium-nickel equivalent ratio was 1.8.

[0056] First, the intermediate billet is produced by free forging at a heating temperature of 1100℃. Each upsetting is controlled at 50%. After each upsetting and drawing, it is reheated in the furnace. After free forging, it is transferred to radial forging at a forging temperature of 1050~850℃. After forging, it is air-cooled to room temperature. The average grain size after forging is 25μm.

[0057] After forging, annealing is performed at 700℃;

[0058] The solution was subjected to a solution treatment at 1050℃ for 30 minutes, followed by a first tempering at 560℃ for 1 hour, and a second tempering at 550℃ for 0.5 hours. The hardness value after the first tempering was 362 HB, and the hardness value after the second tempering was 364 HB.

[0059] Compared to Example 2, Comparative Example 1 did not control the average grain size and conducted creep performance tests at 450°C, with a total plastic strain of 0.17–0.18%.

[0060] Comparative Example 2:

[0061] The ingots were smelted using a double vacuum process (VIM+VAR) and cast into ingots. The chemical composition of the ingots by weight percentage was: C: 0.1%, Si: 0.15%, Mn: 1.0%, P≤0.03%, S≤0.02%, Cr: 11%, Ni: 2.9%, Mo: 1.8%, V: 0.38%, N: 0.028%, and the chromium-nickel equivalent ratio was 1.8.

[0062] First, the intermediate billet is produced by free forging at a heating temperature of 1150℃. Each upsetting is controlled at 50%. After each upsetting and drawing, it is reheated in the furnace. After free forging, it is transferred to radial forging at a forging temperature of 1050~850℃. After forging, it is air-cooled to room temperature. The grain size after forging is 35.2μm.

[0063] After forging, it is annealed at 700℃.

[0064] The sample was solution treated at 1050℃ for 1 hour, then tempered once at 600℃ for 1 hour, and then tempered a second time at 550℃ for 0.5 hours. The hardness value after the first tempering was 320 HB, and the hardness value after the second tempering was 335 HB.

[0065] Compared to Example 2, Comparative Example 2 changed the tempering temperature and conducted high-temperature creep performance tests, with a total plastic strain of 0.16–0.19%.

[0066] The above are exemplary embodiments disclosed in this invention. However, it should be noted that various changes and modifications can be made without departing from the scope of the embodiments of this invention as defined by the claims. The functions, steps, and / or actions of the methods according to the disclosed embodiments described herein do not need to be performed in any particular order. Furthermore, although the elements disclosed in the embodiments of this invention may be described or claimed individually, they may be understood as multiple unless explicitly limited to a singular number.

[0067] It should be understood that, as used herein, the singular form “a” is intended to include the plural form as well, unless the context clearly supports an exception. It should also be understood that, as used herein, “and / or” refers to any and all possible combinations of one or more of the associated listed items.

[0068] The embodiment numbers disclosed in the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0069] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention (including the claims) is limited to these examples. Within the framework of the invention, technical features of the above embodiments or different embodiments can be combined, and many other variations of different aspects of the invention exist, which are not provided in the details for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the protection scope of the invention.

Claims

1. A method for preparing Cr-Ni-Mo-V martensitic heat-resistant steel, characterized in that, include: The steel is obtained by double vacuum smelting and cast into ingots. The chemical composition of the ingots by weight percentage is as follows: C: 0.08-0.1%, Si≤0.25%, Mn: 0.9-1.0%, P≤0.03%, S≤0.02%, Cr: 9-12%, Ni: 2-3%, Mo: 1-2%, V: 0.2-0.4%, N: 0.02-0.03%, the equivalent ratio of chromium to nickel is in the range of 1.6-1.8, and the balance is iron and unavoidable impurity elements. The ingot is forged, including free forging and radial forging. The free forging process includes multiple upsetting and drawing operations, with the deformation amount of each upsetting operation controlled at 40% to 50%. After each drawing operation, the ingot is reheated in the furnace to produce an intermediate billet. The radial forging process includes forging the intermediate billet to a target size billet, controlling the grain size to be 32 to 44 μm, and then air-cooling it to room temperature. The forged steel billet undergoes annealing to eliminate residual stress; The steel billet is subjected to solution treatment and two tempering treatments to prepare Cr-Ni-Mo-V martensitic heat-resistant steel; The dual vacuum smelting process includes VIM vacuum induction melting and VAR consumable electrode melting; solution treatment is performed in the temperature range of 1040 to 1060°C, and tempering is performed twice in the temperature range of 550 to 570°C, with each tempering holding time controlled at 0.5 to 1 hour, and the HB hardness value after each tempering is controlled at 350 to 360 HB.

2. The method for preparing Cr-Ni-Mo-V martensitic heat-resistant steel according to claim 1, characterized in that, The forging temperature for free forging is 1140–1160°C.

3. The method for preparing Cr-Ni-Mo-V martensitic heat-resistant steel according to claim 1, characterized in that, The radial forging temperature is 1050–850°C.

4. The method for preparing Cr-Ni-Mo-V martensitic heat-resistant steel according to claim 1, characterized in that, The annealing temperature is 700℃.

5. The method for preparing Cr-Ni-Mo-V martensitic heat-resistant steel according to claim 1, characterized in that, The free forging process includes three upsetting and drawing processes.

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