A heat treatment method for reducing δ-ferrite in heat-resistant steel 05Cr17Ni4Cu4Nb

Abstract

This invention discloses a heat treatment method for reducing δ-ferrite in heat-resistant steel 05Cr17Ni4Cu4Nb, relating to the field of steel heat treatment technology. The method includes: S1, austenitizing heat treatment: heating the heat-resistant steel electroslag ingot to 990-1010℃ at a heating rate of 50-70℃ / h and holding for 8-12h; S2, δ-ferrite coarsening heat treatment: heating the austenitized electroslag ingot to 1070-1090℃ at a heating rate of 20-30℃ / h and holding for 11-15h; S3, diffusion heat treatment: heating the coarsened electroslag ingot to 1190-1210℃ at a heating rate of 30-40℃ / h and holding for 15-20h. Through staged heat treatment research on the electroslag ingot before hot deformation, the δ-ferrite content in the ingot is reduced from over 6% to 0.35%, fully meeting the user's technical requirements and thus saving production costs.

Description

Technical Field

[0001] This invention relates to the field of steel heat treatment technology, and in particular to a heat treatment method for reducing δ-ferrite in heat-resistant steel 05Cr17Ni4Cu4Nb. Background Technology

[0002] Heat-resistant steel possesses excellent heat resistance, exhibiting not only high strength at high temperatures but also oxidation and corrosion resistance under high-temperature environments. Typically, a large amount of ferrite-forming elements are incorporated into the design of heat-resistant steel. However, these elements are difficult to balance with austenite-forming elements, leading to the formation of a harmful δ-ferrite structure after hot working. When δ-ferrite nucleates and grows primarily at the edges or corners of the original austenite grains, and the grains are large, it fails to provide grain refinement and strengthening. Even without a large amount of carbides, the area around the δ-ferrite at these locations can still become crack initiation points. When δ-ferrite nucleates and grows within the original austenite grains, the MX phase is formed within the δ-ferrite, increasing its hardness. However, a large number of coarse carbides precipitate around the δ-ferrite. These coarse carbides are generally brittle and hard phases, becoming initiation points for quasi-cleavage fracture and reducing the material's toughness.

[0003] With fierce market competition, users of 05Cr17Ni4Cu4Nb steel are increasingly stringent in their requirements for δ-ferrite content, lowering the standard from no more than 5% to no more than 3%, and recently many users have demanded that the δ-ferrite area percentage in this steel be no more than 1%. To meet these technical requirements, steel mills typically invest heavily in composition optimization design during the smelting process, but the pass rate is low. If the subsequent hot deformation process is not properly controlled, the δ-ferrite content may not meet user requirements. Summary of the Invention

[0004] The purpose of this invention is to provide a heat treatment method for reducing δ-ferrite in heat-resistant steel 05Cr17Ni4Cu4Nb. By combining a staged heat treatment mode including austenitizing heat treatment, δ-ferrite coarsening heat treatment, and diffusion heat treatment, and optimizing the process parameters at each stage, the percentage of δ-ferrite area in the electroslag ingot is effectively controlled. To achieve the above objective, this invention provides the following technical solution:

[0005] This invention provides a heat treatment method for reducing δ-ferrite in heat-resistant steel 05Cr17Ni4Cu4Nb, the method comprising the following steps:

[0006] Step S1, austenitizing heat treatment: Heat the heat-resistant steel 05Cr17Ni4Cu4Nb electroslag ingot to 990-1010 ℃ at a heating rate of 50-70℃ / h, and hold for 8-12 h;

[0007] Step S2, δ-ferrite coarsening heat treatment: The austenitized electroslag ingot is heated to 1070-1090 ℃ at a heating rate of 20-30 ℃ / h and held for 11-15 h;

[0008] Step S3, diffusion heat treatment: The roughened heat-treated electroslag ingot is heated to 1190-1210 ℃ at a heating rate of 30-40 ℃ / h and held for 15-20 h.

[0009] In a preferred embodiment, the heat-resistant steel 05Cr17Ni4Cu4Nb electroslag ingot has the following chemical composition by weight percentage: C: 0.035%–0.055%, Si: 0.10%–0.40%, Mn: 0.20%–0.50%, Cr: 15.20%–15.80%, Ni: 4.10%–4.60%, Cu: 3.10%–3.50%, N: 0.03%–0.06%, Nb: 0.15%–0.35%; the balance being Fe and unavoidable impurities.

[0010] In a preferred embodiment, the unavoidable impurities include P and S, wherein, by weight percentage, P ≤ 0.015% and S ≤ 0.0025%.

[0011] In a preferred embodiment, the heat-resistant steel 05Cr17Ni4Cu4Nb electroslag ingot has a diameter of Φ of 550–730 mm and a ferrite area percentage of ≥6.0%.

[0012] In a preferred embodiment, the δ-ferrite area percentage in the heat-resistant steel 05Cr17Ni4Cu4Nb after three-step heat treatment is <1.0%.

[0013] The technical effects and advantages of this invention are as follows:

[0014] This invention combines previous research on smelting and hot deformation processes. By conducting staged heat treatment research on electroslag ingots before hot deformation, the δ-ferrite content in the electroslag ingots is reduced from over 6% to 0.35%, fully meeting the user's technical requirements and thus saving production costs incurred in controlling δ-ferrite during the hot deformation process.

[0015] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. Detailed Implementation

[0016] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0017] To address the shortcomings of existing technologies, this invention discloses a heat treatment method for reducing δ-ferrite in heat-resistant steel 05Cr17Ni4Cu4Nb, the method comprising the following steps:

[0018] Step S1, austenitizing heat treatment:

[0019] The heat-resistant steel 05Cr17Ni4Cu4Nb electroslag ingot was heated to 990–1010℃ at a heating rate of 50–70℃ / h and held for 8–12 h. The purpose of this process was to aggregate the randomly distributed δ-ferrite along the grain boundaries into regular, elongated δ-ferrite. This heating temperature was chosen to partially eliminate compositional inhomogeneity, allowing alloy carbides to dissolve into austenite, strengthening the solid solution. Upon cooling, supersaturated martensite was obtained, preparing for the precipitation of strengthening phases during subsequent aging.

[0020] In step S1 of the present invention, the heat-resistant steel 05Cr17Ni4Cu4Nb electroslag ingot has the following chemical composition by weight percentage: C: 0.035%~0.055%, Si: 0.10%~0.40%, Mn: 0.20%~0.50%, Cr: 15.20%~15.80%, Ni: 4.10%~4.60%, Cu: 3.10%~3.50%, N: 0.03%~0.06%, Nb: 0.15%~0.35%; the balance being Fe and unavoidable impurities.

[0021] Furthermore, the aforementioned unavoidable impurities include P and S, wherein, by weight percentage, P ≤ 0.015% and S ≤ 0.0025%.

[0022] In step S1 of the present invention, the specification Φ of the heat-resistant steel 05Cr17Ni4Cu4Nb electroslag ingot is 550~730mm; its δ ferrite area percentage is ≥6.0%.

[0023] Step S2, δ-ferrite coarsening heat treatment:

[0024] The austenitized electroslag ingot was heated to 1070–1090 °C at a heating rate of 20–30 °C / h and held for 11–15 h. The purpose of choosing this temperature is to allow the aggregated elongated δ-ferrite to form a bamboo-like structure (i.e., cracking) due to the dissolution of some chemical elements, thus preparing for the subsequent diffusion of chemical elements.

[0025] Step S3, Diffusion Heat Treatment:

[0026] The coarsened heat-treated electroslag ingot was heated to 1190–1210 ℃ at a heating rate of 30–40 ℃ / h and held for 15–20 h. The purpose of choosing this temperature is to allow the high concentration of alloying element Cr in the fractured δ-ferrite to diffuse to a lower concentration, thereby reducing the overall δ-ferrite area percentage.

[0027] Furthermore, the δ-ferrite area percentage in the heat-resistant steel 05Cr17Ni4Cu4Nb after the three-step heat treatment is <1.0%.

[0028] Example 1:

[0029] Exemplary Example 1 of the present invention provides a heat treatment method for reducing δ-ferrite in heat-resistant steel 05Cr17Ni4Cu4Nb. Specifically, taking an electroslag ingot with a specification of Φ550 mm as an example, the heat treatment process is as follows:

[0030] Step S1, austenitizing heat treatment:

[0031] The heat-resistant steel 05Cr17Ni4Cu4Nb electroslag ingot was placed in a bogie-type gas furnace and heated to 1000℃ at a heating rate of 70℃ / h, and held at that temperature for 8 h. The purpose of this process is to aggregate the irregularly distributed δ-ferrite along the grain boundaries in the electroslag ingot into regular, elongated δ-ferrite. This heating temperature was chosen to partially eliminate compositional inhomogeneity, allowing alloy carbides to dissolve into austenite, strengthening the solid solution, and resulting in supersaturated martensite after cooling, preparing for the precipitation of strengthening phases during subsequent aging processes.

[0032] The heat-resistant steel 05Cr17Ni4Cu4Nb electroslag ingot, by weight percentage, has the following chemical composition: C: 0.038%; Si: 0.304%; Mn: 0.315%; Cr: 15.32%; Ni: 4.20%; Cu: 3.14%; N: 0.035%; Nb: 0.282%; P: 0.013%; S≤0.0020%; balance Fe.

[0033] Step S2, δ-ferrite coarsening heat treatment:

[0034] The austenitized electroslag ingot was heated to 1080 ℃ at a heating rate of 30 ℃ / h and held for 11 h. The purpose of choosing this temperature is to cause the agglomerated elongated δ-ferrite to produce a bamboo-like structure (i.e., cracking) due to the dissolution of some chemical elements, in preparation for the subsequent diffusion of chemical elements.

[0035] Step S3, Diffusion Heat Treatment:

[0036] The coarsened heat-treated electroslag ingot was heated to 1200 ℃ at a heating rate of 40 ℃ / h and held for 15 h. The purpose of choosing this temperature was to allow the high concentration of alloying element Cr in the fractured δ-ferrite to diffuse to a lower concentration, thereby reducing the overall δ-ferrite area percentage.

[0037] Example 2:

[0038] Exemplary Example 2 of the present invention provides a heat treatment method for reducing δ-ferrite in heat-resistant steel 05Cr17Ni4Cu4Nb. Specifically, taking an electroslag ingot with a specification of Φ730 mm as an example, the heat treatment process is as follows:

[0039] Step S1, austenitizing heat treatment:

[0040] The heat-resistant steel 05Cr17Ni4Cu4Nb electroslag ingot was placed in a bogie-type gas furnace and heated to 1000℃ at a heating rate of 50℃ / h, and held at that temperature for 12 h. The purpose of this process is to aggregate the irregularly distributed δ-ferrite along the grain boundaries in the electroslag ingot into regular, elongated δ-ferrite. The purpose of choosing this heating temperature is to partially eliminate the inhomogeneity of the composition, allowing the alloy carbides to dissolve into the austenite, strengthening the solid solution, and obtaining supersaturated martensite after cooling, which prepares for the precipitation of strengthening phases during the subsequent aging process.

[0041] The heat-resistant steel 05Cr17Ni4Cu4Nb electroslag ingot, by weight percentage, has the following chemical composition: C: 0.038%; Si: 0.304%; Mn: 0.315%; Cr: 15.32%; Ni: 4.20%; Cu: 3.14%; N: 0.035%; Nb: 0.282%; P: 0.013%; S≤0.0020%; balance Fe.

[0042] Step S2, δ-ferrite coarsening heat treatment:

[0043] The austenitized electroslag ingot was heated to 1080 ℃ at a heating rate of 20 ℃ / h and held for 15 h. The purpose of choosing this temperature is to cause the agglomerated elongated δ-ferrite to produce a bamboo-like structure (i.e., cracking) due to the dissolution of some chemical elements, in preparation for the subsequent diffusion of chemical elements.

[0044] Step S3, Diffusion Heat Treatment:

[0045] The coarsened heat-treated electroslag ingot was heated to 1200 ℃ at a heating rate of 30 ℃ / h and held for 20 h. The purpose of choosing this temperature was to allow the high concentration of alloying element Cr in the fractured δ-ferrite to diffuse to a lower concentration, thereby reducing the overall δ-ferrite area percentage.

[0046] The percentage of δ-ferrite area in the heat-resistant steel 05Cr17Ni4Cu4Nb electroslag ingot obtained according to exemplary embodiments 1-2 of the present invention is shown in Table 1:

[0047] Table 1. δ-ferrite content of each embodiment after heat treatment

[0048]

[0049] As shown in Table 1, this invention reduces the δ-ferrite content in the electroslag ingot from over 6% to 0.35%~0.85% by conducting staged heat treatment research on the electroslag ingot before hot deformation, which fully meets the user's technical requirements and saves the production cost incurred by controlling δ-ferrite in the hot deformation process.

[0050] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A heat treatment method for reducing δ-ferrite in heat-resistant steel 05Cr17Ni4Cu4Nb, characterized in that, The method includes the following steps: Step S1, austenitizing heat treatment: Heat the heat-resistant steel 05Cr17Ni4Cu4Nb electroslag ingot to 990-1010 ℃ at a heating rate of 50-70 ℃ / h, and hold for 8-12 h; Step S2, δ-ferrite coarsening heat treatment: The austenitized electroslag ingot is heated to 1070-1090 ℃ at a heating rate of 20-30 ℃ / h and held for 11-15 h; Step S3, diffusion heat treatment: The roughened heat-treated electroslag ingot is heated to 1190-1210 ℃ at a heating rate of 30-40 ℃ / h and held for 15-20 h.

2. The heat treatment method for reducing δ-ferrite in heat-resistant steel 05Cr17Ni4Cu4Nb according to claim 1, characterized in that, The heat-resistant steel 05Cr17Ni4Cu4Nb electroslag ingot, by weight percentage, has the following chemical composition: C: 0.035%~0.055%, Si: 0.10%~0.40%, Mn: 0.20%~0.50%, Cr: 15.20%~15.80%, Ni: 4.10%~4.60%, Cu: 3.10%~3.50%, N: 0.03%~0.06%, Nb: 0.15%~0.35%; the balance being Fe and unavoidable impurities.

3. The heat treatment method for reducing δ-ferrite in heat-resistant steel 05Cr17Ni4Cu4Nb according to claim 2, characterized in that, The unavoidable impurities include P and S, wherein, by weight percentage, P ≤ 0.015% and S ≤ 0.0025%.

4. The heat treatment method for reducing δ-ferrite in heat-resistant steel 05Cr17Ni4Cu4Nb according to claim 1, characterized in that, The specifications of the heat-resistant steel 05Cr17Ni4Cu4Nb electroslag ingot are Φ 550~730 mm; its δ ferrite area percentage is ≥6.0%.

5. The heat treatment method for reducing δ-ferrite in heat-resistant steel 05Cr17Ni4Cu4Nb according to claim 1, characterized in that, The percentage of δ-ferrite area in the heat-resistant steel 05Cr17Ni4Cu4Nb after three-step heat treatment is <1.0%.

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