A method for improving the toughness of high-temperature resistant low-carbon medium and low-alloy steel

Abstract

The present invention relates to a method for improving the toughness of high-temperature resistant low-carbon medium and low-alloy steel. It successively undergoes alloy composition design, pure melting, annealing treatment, forging, second annealing treatment, solution quenching and tempering treatment; in the second annealing treatment, it is heated to 700 ± 10 °C at a heating rate of ≤ 100 °C / h, held for 15 - 18 h, cooled in the furnace after stopping the holding, and air-cooled after the temperature drops to 400 °C; the solution quenching is carried out at 1150 - 1200 °C for 20 - 30 min and quenched in water to room temperature; the tempering treatment is divided into two temperings. The first tempering temperature is 520 - 550 °C, the time is 0.5 - 2.5 h, and it is air-cooled after being taken out of the furnace. The second tempering temperature is 620 - 650 °C, the time is 1 - 3 h, and it is air-cooled after being taken out of the furnace. The method of the present invention realizes the improvement of the comprehensive properties such as the room-temperature strength and toughness and the low-temperature impact toughness while improving the high-temperature performance of the low-carbon Cr-Mo series alloy steel.

Description

Technical Field

[0001] The present invention relates to the technical field of the preparation of alloy steel, and particularly relates to a method for improving the toughness of high-temperature-resistant low-carbon medium-low alloy steel. Background Art

[0002] The quenched and tempered Cr-Mo series alloy steel has excellent comprehensive properties and is a widely used structural material, which is widely applied to machinery or equipment. It is well known in this field that subtle adjustments to the composition and content in alloy steel will affect the performance of alloy steel. By increasing the content of heat-strengthening alloying elements such as Cr and Mo, and compounding micro-alloying carbide-forming elements such as Zr, Nb, V, and W, the high-temperature performance can be improved, enabling it to be used as hot die steel, further broadening the application range of the Cr-Mo series alloy steel. Using a large amount of heat-strengthening alloying elements Cr and Mo can effectively improve the high-temperature performance of low-carbon Cr-Mo series alloy steel. However, after high alloying, the properties such as room-temperature strength and toughness, low-temperature impact toughness, and room-temperature strength of low-carbon Cr-Mo series alloy steel will decrease significantly. Compound micro-alloying with carbide-forming elements such as Nb, V, and W can also improve the high-temperature performance, but the precipitation of high-stability primary carbides affects the toughness of low-carbon Cr-Mo series alloy steel, resulting in ineffective improvement of the impact energy. Therefore, how to improve the room-temperature strength and toughness, low-temperature impact toughness, etc. while improving the high-temperature performance of low-carbon Cr-Mo series alloy steel, so as to improve the comprehensive performance of low-carbon Cr-Mo series alloy steel, is of great significance. Summary of the Invention

[0003] The purpose of the present invention is to provide a method for improving the toughness of high-temperature-resistant low-carbon medium-low alloy steel, and this preparation method can improve the comprehensive properties such as room-temperature strength and toughness, low-temperature impact toughness of low-carbon Cr-Mo series alloy steel while improving its high-temperature performance.

[0004] The purpose of the present invention is achieved through the following technical solutions:

[0005] A method for improving the toughness of high-temperature resistant low-carbon medium and low-alloy steel, characterized in that it successively undergoes alloy composition design, purified melting, annealing treatment, forging, second annealing treatment, solution quenching and tempering treatment; the chemical composition of the low-carbon medium and low-alloy steel, by weight percentage, is C: 0.20 - 0.25%, Cr: 1.0 - 3.2%, Mo: 1.0 - 3.2%, Ni: 0.5 - 2.0%, V: 0.4 - 0.8%, W: 0.05 - 0.1%, Nb: 0.025 - 0.05%, Mn: 0.2 - 0.5%, Ti: 0.1 - 0.2%, Ta: 0.1 - 0.2%, S ≤ 0.002%, P ≤ 0.003%, Al ≤ 0.015%, O ≤ 0.003%, N ≤ 0.004%, and the remaining component is Fe, wherein the mass ratio of V:W:Nb = 16:2:1, and V:Ti:Ta = 4:1:1; the second annealing treatment is to heat to 700 ± 10°C at a heating rate of ≤ 100°C / h, hold for 15 - 18 h, cool with the furnace after stopping holding, and air-cool after the temperature drops to 400°C; the solution quenching is to hold at 1150 - 1200°C for 20 - 30 min and quench in water to room temperature; the tempering treatment is divided into two temperings, the first tempering temperature is 520 - 550°C, the time is 0.5 - 2.5 h, air-cool after taking out of the furnace, the second tempering temperature is 620 - 650°C, the time is 1 - 3 h, and air-cool after taking out of the furnace.

[0006] The present invention realizes a significant improvement in the high toughness of low-carbon Cr-Mo series alloy steel through technical measures such as reasonably optimizing and allocating the contents of strong carbide-forming elements such as Nb, V, W, Ti, Ta, etc., reducing the content of high-temperature ultra-high-stability carbides, increasing the proportion and content of elements that can remelt strong carbides at high temperatures, using low-diffusion elements and low-temperature pre-tempering processes to control the diffusion of primary carbides to grain boundaries, and methods such as significantly increasing the solution temperature, shortening the solution time, strictly controlling the contents of residual Al and O, and significantly reducing the content of N. Specifically, the present invention simultaneously adds V, W, Nb, Mn, Ti, Ta, adjusts and controls their addition amounts and their proportion relationships, strictly controls the contents of residual Al and O, significantly reduces the content of N, and combines with solution at an ultra-high temperature of 1150-1200°C for a short time to achieve full re-solution of the alloy, promote the change in the morphology of high-melting-point and high-stability NbC, and the diffusion and dissolution of carbides such as WC, VC, TiC, TaC and V(C, N) carbonitrides in the matrix and their contents, increases the precipitation temperature of each element in the solution and quenched martensite, achieves high strength in a high-temperature environment, and the increase in the precipitation temperature inhibits the growth of matrix grains at high temperatures, ensuring high toughness in its high-temperature environment; its higher solution temperature and shorter holding time can, on the one hand, increase the solubility of alloying elements, thereby increasing the room-temperature strength of the material, and can also remelt strong carbide and nitride, primary carbides, and carbonitrides that affect brittleness, improving the toughness of the material. In addition, the contents of S, P, O, Al and N are restricted, reducing the influence of the precipitation of brittle substances such as AlN at grain boundaries on toughness and avoiding the influence on the toughness of the material.

[0007] Due to the slow diffusion of alloying elements, especially the slow diffusion rate of W in the matrix, the present invention limits the pre-tempering temperature (i.e., the first tempering) in the tempering treatment, so that during the pre-tempering process at a lower temperature, some new phases of carbides, nitrides and carbonitrides preferentially nucleate nearby rather than quickly diffuse to the grain boundaries, restricting their precipitation at the grain boundaries, effectively reducing the brittleness caused by grain boundary precipitation, and then further growing during the secondary tempering process to promote the generation of secondary hardening.

[0008] As a further optimization, the above-mentioned purification melting adopts processes such as vacuum induction melting + electroslag remelting / vacuum consumable melting to meet the purification requirements of S≤0.002%, P≤0.003%, Al≤0.015%, O≤0.003%, N≤0.004%.

[0009] As a further optimization, in the above-mentioned vacuum induction melting process, the vacuum degree is better than 3 Pa (i.e., the vacuum degree is less than 3 Pa).

[0010] Most specifically, a method for improving the toughness of high-temperature-resistant low-carbon medium-low alloy steel is characterized by successively including the following steps:

[0011] (1) Alloy composition design:

[0012] The chemical composition of the low-carbon medium-low alloy steel is, by weight percentage: C: 0.20 - 0.25%, Cr: 1.0 - 3.2%, Mo: 1.0 - 3.2%, Ni: 0.5 - 2.0%, V: 0.4 - 0.8%, W: 0.05 - 0.1%, Nb: 0.025 - 0.05%, Mn: 0.2 - 0.5%, Ti: 0.1 - 0.2%, Ta: 0.1 - 0.2%, S ≤ 0.002%, P ≤ 0.003%, Al ≤ 0.015%, O ≤ 0.003%, N ≤ 0.004%, and the remaining component is Fe. Among them, the mass ratio of V:W:Nb = 16:2:1, and V:Ti:Ta = 4:1:1;

[0013] (2) Purified melting:

[0014] Take purified and dried raw materials, and use vacuum induction melting to prepare a vacuum ingot with a vacuum degree better than 3 Pa; then use electroslag remelting or vacuum consumable melting for purified melting, with argon protection throughout the process, and perform shroud cooling treatment after ingot stripping, and the treatment time is 24 - 30 h;

[0015] (3) Annealing treatment:

[0016] Heat at a heating rate of ≤ 100 °C / h to 700 ± 10 °C, hold for 15 - 18 h, cool with the furnace after stopping holding, and take out and air-cool after the temperature drops to 400 °C;

[0017] (4) Forging:

[0018] It includes blooming forging and finish forging; blooming forging is to hold the steel ingot at a heating temperature of 1220 - 1260 °C for more than 15 h, the forging start temperature is 1060 - 1100 °C, the final forging temperature ≥ 850 °C, and the total forging ratio ≥ 9; finish forging is to hold the forging at a heating temperature of 1130 - 1150 °C for more than 1.5 h, the forging start temperature ≥ 1050 °C, and the final forging temperature ≥ 850 °C;

[0019] (5) Second annealing treatment:

[0020] Heat at a heating rate of ≤ 100 °C / h to 700 ± 10 °C, hold for 15 - 18 h and then stop holding, cool with the furnace, and take out and air-cool after the temperature drops to 400 °C;

[0021] (6) Solution quenching:

[0022] Hold at 1150 - 1200 °C for 20 - 30 min, and quench to room temperature in water;

[0023] (7) Tempering treatment:

[0024] The first tempering temperature is 520 - 550 °C, the time is 0.5 - 2.5 h, air cooling after taking out of the furnace; the second tempering temperature is 620 - 650 °C, the time is 1 - 3 h, air cooling after taking out of the furnace.

[0025] The present invention has the following beneficial effects:

[0026] The present invention provides a method for improving the toughness of high-temperature resistant low-carbon medium-low alloy steel. The prepared low-carbon medium-low alloy steel has a room-temperature tensile strength of more than 1320 MPa and a maximum high-temperature strength of more than 650 MPa at 700 °C. At the same time, the room-temperature impact energy is not less than 100 J and the impact energy at -40 °C is not less than 89 J, having excellent comprehensive properties, that is, while improving the high-temperature performance of low-carbon Cr-Mo series alloy steel, its room-temperature strength and toughness, low-temperature impact toughness and other comprehensive properties are improved. Specific embodiments

[0027] The present invention will be specifically described below through examples. It is necessary to point out here that the following examples are only used to further illustrate the present invention and cannot be understood as limiting the protection scope of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention according to the above content of the present invention.

[0028] Example 1

[0029] A method for improving the toughness of high-temperature resistant low-carbon medium-low alloy steel, which successively includes the following steps:

[0030] (1) Alloy composition design:

[0031] By weight percentage: C: 0.20%, Cr: 1.0%, Mo: 2.0%, Ni: 1.0%, V: 0.4%, W: 0.05%, Nb: 0.025%, Mn: 0.3%, Ti: 0.1%, Ta: 0.1%, S ≤ 0.002%, P ≤ 0.003%, Al ≤ 0.015%, O ≤ 0.003%, N ≤ 0.004%, and the remaining components are Fe;

[0032] (2) Purified melting:

[0033] Take purified and dried raw materials, and prepare a vacuum ingot by vacuum induction melting with a vacuum degree of 2 Pa; then carry out purified melting by electroslag remelting, with full argon protection throughout the process, and carry out shroud cooling treatment after ingot stripping, and the treatment time is 25 h;

[0034] (3) Annealing treatment:

[0035] Heat at a heating rate of 60 °C / h to 700 °C, hold for 16 h, cool with the furnace after stopping holding, and air cool after the temperature drops to 400 °C;

[0036] (4)Forging:

[0037] It includes blooming forging and finish forging; for blooming forging, the ingot is held at a heating temperature of 1250 °C for 18 h, the forging start temperature is 1080 °C, the forging end temperature is 850 °C, and the total forging ratio is 12; for finish forging, the forging is held at a heating temperature of 1140 °C for more than 2 h, the forging start temperature is 1050 °C, and the forging end temperature is 850 °C.

[0038] (5)Second annealing treatment:

[0039] Heat it at a heating rate of 60 °C / h to 700 °C, hold for 16 h, then stop holding and cool in the furnace. After the temperature drops to 400 °C, take it out of the furnace and air-cool.

[0040] (6)Solution quenching:

[0041] Hold at 1200 °C for 30 min, then quench in water to room temperature.

[0042] (7)Tempering treatment:

[0043] The first tempering temperature is 520 °C and the time is 2.5 h. Take it out of the furnace and air-cool. The second tempering temperature is 650 °C and the time is 3 h. Take it out of the furnace and air-cool.

[0044] Example 2

[0045] A method for improving the toughness of high-temperature resistant low-carbon medium-low alloy steel, which successively includes the following steps:

[0046] (1)Alloy composition design:

[0047] By weight percentage: C: 0.25%, Cr: 3.2%, Mo: 3.2%, Ni: 2.0%, V: 0.8%, W: 0.1%, Nb: 0.05%, Mn: 0.5%, Ti: 0.2%, Ta: 0.2%, S≤0.002%, P≤0.003%, Al≤0.015%, O≤0.003%, N≤0.004%, and the remaining components are Fe.

[0048] (2)Purified melting:

[0049] Use purified and dried raw materials, prepare a vacuum ingot by vacuum induction melting, with a vacuum degree of 3 Pa; then use vacuum consumable for purified melting, with full argon protection throughout the process. After ingot stripping, perform shroud cooling treatment for 30 h.

[0050] (3)Annealing treatment:

[0051] Heat it at a heating rate of 40 °C / h to 710 °C, hold for 15 h, then stop holding and cool in the furnace. After the temperature drops to 400 °C, take it out of the furnace and air-cool.

[0052] (4) Forging:

[0053] It includes cogging forging and finish forging; Cogging forging is to keep the ingot at a heating temperature of 1260 °C for 20 h, the forging start temperature is 1100 °C, the final forging temperature is 850 °C, and the total forging ratio is 9; Finish forging is to keep the forgings at a heating temperature of 1150 °C for more than 2 h, the forging start temperature is 1150 °C, and the final forging temperature is 950 °C;

[0054] (5) Second annealing treatment:

[0055] Heat it at a heating rate of 100 °C / h to 690 °C, keep it warm for 18 h, then stop keeping warm and cool it in the furnace. After the temperature drops to 400 °C, take it out of the furnace and air-cool it;

[0056] (6) Solution quenching:

[0057] Keep it at 1150 °C for 20 min, then quench it in water to room temperature;

[0058] (7) Tempering treatment:

[0059] The first tempering temperature is 550 °C, the time is 0.5 h, take it out of the furnace and air-cool it. The second tempering temperature is 620 °C, the time is 1 h, take it out of the furnace and air-cool it.

[0060] Example 3

[0061] A method for improving the toughness of high-temperature resistant low-carbon medium and low alloy steel, which successively includes the following steps:

[0062] (1) Alloy composition design: [[ID=z2]]

[0063] By weight percentage: C: 0.22%, Cr: 2.0%, Mo: 2.2%, Ni: 1.2%, V: 0.48%, W: 0.06%, Nb: 0.03%, Mn: 0.2%, Ti: 0.12%, Ta: 0.12%, S≤0.002%, P≤0.003%, Al≤0.015%, O≤0.003%, N≤0.004%, and the remaining component is Fe;

[0064] (2) Purified melting:

[0065] Use purified and dried raw materials, and prepare a vacuum ingot by vacuum induction melting, with a vacuum degree of 2.5 Pa; Then use electroslag remelting for purified melting, with full argon protection throughout the process. After ingot stripping, carry out shroud cooling treatment, and the treatment time is 28 h;

[0066] (3) Annealing treatment:

[0067] Heat it at a heating rate of 50 °C / h to 710 °C, keep it warm for 15 h, then stop keeping warm and cool it in the furnace. After the temperature drops to 400 °C, take it out of the furnace and air-cool it;

[0068] (4) Forging:

[0069] It includes ingot breakdown forging and finish forging; Ingot breakdown forging is to keep the steel ingot at a heating temperature of 1260 °C for 20 h, the starting forging temperature is 1100 °C, the final forging temperature is 900 °C, and the total forging ratio is 9; Finish forging is to keep the forging at a heating temperature of 1150 °C for 2 h, the starting forging temperature is 1100 °C, and the final forging temperature is 900 °C;

[0070] (5) Second annealing treatment:

[0071] Heat it at a heating rate of 100 °C / h to 690 °C, keep it warm for 18 h, then stop keeping warm and cool it in the furnace. After the temperature drops to 400 °C, take it out of the furnace and air-cool it;

[0072] (6) Solution quenching:

[0073] Keep it at 1150 °C for 25 min, then quench it in water to room temperature;

[0074] (7) Tempering treatment:

[0075] The first tempering temperature is 530 °C, the time is 2.5 h, take it out of the furnace and air-cool it. The second tempering temperature is 630 °C, the time is 2 h, take it out of the furnace and air-cool it.

[0076] Comparative example 1

[0077] Keep other steps in Example 1, only replace the alloy composition design in step (1) of Example 1 with:

[0078] C: 0.20%, Cr: 2.2%, Mo: 1.2%, Ni: 2.0%, W: 0.1%, V: 0.7%, Nb: 0.10%, S ≤ 0.002%, P ≤ 0.003%, Al ≤ 0.015%; O ≤ 0.003%, N ≤ 0.004%, and the remaining components are Fe.

[0079] Comparative example 2

[0080] Keep other steps in Example 1, only replace the alloy composition design in step (1) of Example 1 with:

[0081] C: 0.25%, Cr: 3.0%, Mo: 2.0%, Ni: 2.0%, W: 0.9%, V: 0.5%, Nb: 0.15%, Mn: 0.8%, and the remaining components are Fe.

[0082] Comparative example 3

[0083] Keep other steps in Example 1, only replace the solution quenching in step (6) of Example 1 with:

[0084] First, hold at 900 °C for 1 h, then raise the solution temperature to 1100 °C and hold for 1 h. Subsequently, quench in water first. After coming out of the water, the temperature is 150 °C. Finally, quench in oil until the temperature drops below 100 °C.

[0085] Comparative Example 4

[0086] Retain other steps in Example 1, and only replace the solution quenching in step (6) of Example 1 with:

[0087] First, hold at 850 °C for 2 h, then raise the solution temperature to 950 °C and hold for 1 h. Subsequently, quench in water first. After coming out of the water, the temperature is 120 °C. Finally, quench in oil until the temperature drops below 100 °C.

[0088] Comparative Example 5

[0089] Retain other steps in Example 1, and at the same time replace the alloy composition design in step (1) of Example 1 with:

[0090] C: 0.25%, Cr: 3.0%, Mo: 2.0%, Ni: 1.5%, W: 0.9%, V: 0.5%, Nb: 0.10%, S ≤ 0.002%, P ≤ 0.003%, Al ≤ 0.015%; O ≤ 0.003%, N ≤ 0.004%, and the remaining components are Fe;

[0091] Replace the solution quenching in step (6) of Example 1 with:

[0092] First, hold at 800 °C for 2 h, then raise the solution temperature to 1060 °C and hold for 1 h. When quenching, quench in water first, and the water outlet temperature is 100 °C. Then quench in oil to reduce the temperature to 90 °C.

[0093] The performance indexes of the low-carbon medium-low alloy steels prepared in Examples 1, 2, and 3 of the present invention and Comparative Examples 1-5 are shown in Table 1.

[0094] Table 1: Statistical results of the performance indexes of Examples 1-3 and Comparative Examples 1-5.

[0095]

[0096] It can be seen that in the present invention, by reasonably optimizing and adjusting the contents of elements such as Nb, V, W, Mn, Ti, Ta, etc., using processes such as low-diffusion elements and low-temperature pre-tempering, and by significantly increasing the solution temperature, shortening the solution time, strictly controlling the contents of residual Al and O, and significantly reducing the content of N, etc., the high toughness of the low-carbon Cr-Mo series alloy steel is greatly improved.

Claims

1. A method for improving the toughness of high-temperature resistant low-carbon medium and low alloy steel, characterized in that: It successively undergoes alloy composition design, purification melting, annealing treatment, forging, secondary annealing treatment, solution quenching and tempering treatment; the chemical composition of the low-carbon medium-low alloy steel, by weight percentage, is C: 0.20 - 0.25%, Cr: 1.0 - 3.2%, Mo: 1.0 - 3.2%, Ni: 0.5 - 2.0%, V: 0.4 - 0.8%, W: 0.05 - 0.1%, Nb: 0.025 - 0.05%, Mn: 0.2 - 0.5%, Ti: 0.1 - 0.2%, Ta: 0.1 - 0.2%, S ≤ 0.002%, P ≤ 0.003%, Al ≤ 0.015%, O ≤ 0.003%, N ≤ 0.004%, and the remaining component is Fe. Among them, the mass ratio of V:W:Nb = 16:2:1, and V:Ti:Ta = 4:1:1; the secondary annealing treatment is to heat at a heating rate of ≤ 100 °C / h to 700 ± 10 °C, hold for 15 - 18 h, cool with the furnace after stopping holding, and air-cool after the temperature drops to 400 °C; the solution quenching is to hold at 1150 - 1200 °C for 20 - 30 min and quench in water to room temperature; the tempering treatment is divided into two temperings. The first tempering temperature is 520 - 550 °C, the time is 0.5 - 2.5 h, air-cool after taking out of the furnace, and the second tempering temperature is 620 - 650 °C, the time is 1 - 3 h, air-cool after taking out of the furnace.

2. The method for improving the toughness of high-temperature resistant low-carbon medium and low alloy steel according to claim 1, characterized in that: The purification melting adopts vacuum induction melting + electroslag remelting / vacuum consumable melting.

3. The method for improving the toughness of high-temperature resistant low-carbon medium and low alloy steel according to claim 2, characterized in that: In the vacuum induction melting process, the vacuum degree is better than 3 Pa.

4. A method for improving the toughness of high-temperature resistant low-carbon medium and low alloy steel, characterized in that, It successively includes the following steps: (1) Alloy composition design: The chemical composition of the low-carbon medium-low alloy steel, by weight percentage, is C: 0.20 - 0.25%, Cr: 1.0 - 3.2%, Mo: 1.0 - 3.2%, Ni: 0.5 - 2.0%, V: 0.4 - 0.8%, W: 0.05 - 0.1%, Nb: 0.025 - 0.05%, Mn: 0.2 - 0.5%, Ti: 0.1 - 0.2%, Ta: 0.1 - 0.2%, S ≤ 0.002%, P ≤ 0.003%, Al ≤ 0.015%, O ≤ 0.003%, N ≤ 0.004%, and the remaining component is Fe. Among them, the mass ratio of V:W:Nb = 16:2:1, and V:Ti:Ta = 4:1:1; (2) Purification melting: Take purified and dried raw materials, use vacuum induction melting to prepare a vacuum ingot with a vacuum degree better than 3 Pa; then use electroslag remelting or vacuum consumable melting for purification melting, with argon protection throughout the process, and perform shroud cooling treatment after ingot stripping, and the treatment time is 24 - 30 h; (3) Annealing treatment: Heat at a heating rate of ≤ 100 °C / h to 700 ± 10 °C, hold for 15 - 18 h, cool with the furnace after stopping holding, and air-cool after the temperature drops to 400 °C; (4) Forging: It includes cogging forging and finish forging; for cogging forging, the ingot is heated to a temperature of 1220 - 1260°C and held for over 15 h, the cogging temperature is 1060 - 1100°C, the finishing temperature is ≥850°C, and the total forging ratio is ≥9; for finish forging, the forging is heated to a temperature of 1130 - 1150°C and held for over 1.5 h, the cogging temperature is ≥1050°C, and the finishing temperature is ≥850°C. (5) The second annealing treatment: It is heated at a heating rate of ≤100°C / h to 700 ± 10°C, held for 15 - 18 h, then cooled in the furnace. After the temperature drops to 400°C, it is taken out of the furnace and air-cooled. (6) Solution quenching: It is held at 1150 - 1200°C for 20 - 30 min and quenched in water to room temperature. (7) Tempering treatment: The first tempering temperature is 520 - 550°C and the time is 0.5 - 2.5 h, then taken out of the furnace and air-cooled. The second tempering temperature is 620 - 650°C and the time is 1 - 3 h, then taken out of the furnace and air-cooled.

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