Heat treatment method for improving mechanical property of 40NCD7-03 structural steel

Abstract

The invention discloses a heat treatment method for improving the mechanical property of 40NCD7-03 structural steel. The heat treatment method comprises the following steps that S1, in the smelting stage, an electric furnace, AOD, LF and VD technologies are adopted for conducting primary refining on raw materials; s2, an electroslag stage, wherein the surface of the electrode blank is polished, defects are removed, and impure impurities are filtered out; s3, a forging stage, wherein an octagonal forging mode is adopted in the drawing-out process; s4, a hardening and tempering stage: the hardening and tempering stage comprises a quenching process and a tempering process, in the quenching process, cooling is performed in a water-air alternating manner, and tempering treatment is performed after the last reverse temperature is controlled to reach a proper value; s5, a finishing stage is conducted, specifically, the quenched and tempered steel is subjected to finishing; the cooling mode of the hardening and tempering process is optimized, the quenching process adopts a water-air alternating mode for cooling, the quenching cooling temperature is controlled more accurately, retained austenite is transformed and structure grains are refined through a proper cooling rate, so that the material strength is improved, and the stability of the mechanical property of the steel is improved.

Description

Technical Field

[0001] The invention relates to the technical field of heat treatment, and in particular to a heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel. Background Art

[0002] 40NCD7-03 alloy is an alloy structural steel with low cooling hardening. After quenching and tempering heat treatment, the alloy can obtain excellent strength, good plasticity and toughness, and good processing performance. It is suitable for important components such as gas turbine compressor blades and quenching tables.

[0003] The existing heat treatment process has the following disadvantages: ① 40NCD7-03 alloy steel has temper brittleness, and the existing process does not control this characteristic, so cracks often appear in the subsequent processing process, generally manifested as longitudinal cracks after heat treatment and transverse cracks during thermal deformation; ② The low-temperature impact performance of the existing heat treatment process is unstable; ③ After forging and heat treatment, the grain size distribution is uneven, and coarse grains exist on the edge, which affects the tensile properties of the bar and the stability of the flaw detection grade. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel, which can optimize the cooling mode of the quenching and tempering process, effectively refine the grains, and thus improve the stability of the mechanical properties of the steel.

[0005] In order to solve the above technical problems, the technical solutions adopted by the present invention are as follows.

[0006] A heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel comprises the following steps:

[0007] Step S1, smelting stage: using electric furnace + AOD + LF + VD process to preliminarily refine the raw materials;

[0008] Step S2, electroslag stage: polish the surface of the electrode blank to remove defects, filter out impure impurities, and send it to hot forging within 1.5 hours after the electroslag is demoulded;

[0009] Step S3, forging stage: the drawing process adopts an octagonal forging method, that is, chamfering is performed every two passes to ensure uniform deformation of the steel;

[0010] Step S4, quenching and tempering stage: the quenching and tempering stage includes a quenching process and a tempering process. The quenching process is cooled by alternating water and air, and the tempering process is performed after the last inversion temperature reaches a suitable value;

[0011] Step S5, finishing stage: finishing the quenched and tempered steel.

[0012] In the above-mentioned heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel, in step S1, the VD stage needs to evacuate the furnace after the cover is closed, and the vacuum degree is always kept below 67Pa for a period of not less than 15 minutes. During the vacuuming process, attention should also be paid to the rise of slag in the furnace to avoid slag bubbling.

[0013] In the above-mentioned heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel, in step S1, after the VD stage is completed, the air-breaking relief valve is opened to break the vacuum of the system, and then the hydrogen and oxygen are determined online to control [H]≤1.5ppm and [O]≤15ppm.

[0014] In the above-mentioned heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel, in step S1, during the VD refining process, [Al] should be controlled at 0.05% by mass percentage to increase the Al content requirement and refine the grains.

[0015] In the above-mentioned heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel, in step S2, the slag entering the electroslag stage must be baked to remove the crystal water in the slag, the baking temperature is 600-700°C, and the baking time is ≥6h.

[0016] In the above heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel, in step S3, the total forging ratio is ≥4, the holding time of the finished product is 2-3h, the deformation of the last fire should be greater than 40%, and the steel is annealed in a furnace 1.5h after forging at a temperature of 670°C.

[0017] In the above-mentioned heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel, in step S4, the quenching process heats the steel to 860°C and holds it for 2 hours after it is thoroughly burned, and then performs water-air alternating cooling, and controls the air cooling return time for each time to be 3-4 minutes. After the last return temperature reaches 50-100°C, tempering treatment is performed, and the steel is heated to 530°C and then kept warm for 10 hours, and then quickly cooled to room temperature after being taken out of the furnace.

[0018] In the above-mentioned heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel, in step S4, during the tempering treatment, the steel is heated to 530°C, burned through, kept warm for 10 hours, and then taken out of the furnace and quickly cooled to room temperature.

[0019] In the above-mentioned heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel, during the tempering treatment, the rapid cooling method is water cooling or air cooling.

[0020] Due to the adoption of the above technical scheme, the technical progress achieved by the present invention is as follows.

[0021] The invention provides a heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel, optimizes the cooling mode of the quenching and tempering process, adopts a water-air alternating cooling mode for the quenching process, controls the last inversion temperature to reach a suitable value before performing the tempering treatment, more accurately controls the quenching cooling temperature, transforms the residual austenite through an appropriate cooling rate, refines the structure grains, and thus improves the material strength. The metallographic structure of the treated steel in the invention is tempered troostite, and the grains are refined, thereby improving the stability of the mechanical properties of the steel.

[0022] Since the 40NCD7-03 alloy structural steel itself has temper brittleness, rapid cooling after tempering is performed to avoid the material from staying in the brittle temperature range for too long, thereby reducing the risk of temper brittleness and effectively reducing the temper brittleness of the steel. The present invention also controls the forging method and deformation amount to make the internal structure of the steel dense and uniform. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a process flow chart of the present invention;

[0024] Figure 2 This is the metallographic diagram obtained after water-cooling alternating quenching in Example 1 of the present invention;

[0025] Figure 3 This is the metallographic image obtained after water-cooling alternating quenching in Example 2 of the present invention;

[0026] Figure 4 This is a metallographic diagram obtained by water-cooling quenching in the prior art;

[0027] Figure 5 This is the metallographic diagram obtained by water-cooling quenching in the second prior art;

[0028] Figure 6 The following is a comparison table of experimental performances of various embodiments of the present invention and prior art. DETAILED DESCRIPTION

[0029] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0030] Example 1

[0031] A heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel, such as Figure 1 to Figure 2 As shown, the following steps are included:

[0032] Step S1, smelting stage: adopt electric furnace + AOD + LF + VD process to carry out preliminary refining of raw materials.

[0033] In the VD stage, the furnace needs to be evacuated after the lid is closed, and the vacuum degree must be kept below 67Pa at all times for no less than 15 minutes. The main purpose of vacuuming is to ensure the degassing effect and improve the purity by controlling the pressure. During the vacuuming process, attention should also be paid to the rise of slag in the furnace to avoid slag bubbling.

[0034] After the VD stage, open the air release valve to break the vacuum of the system, and then determine the hydrogen and oxygen online to control [H]≤1.5ppm and [O]≤15ppm. At the same time, during the VD refining process, [Al] should be controlled at 0.05% by mass percentage to increase the Al content requirement and refine the grains.

[0035] The raw material requirements for the smelting stage are as follows:

[0036] ① High carbon ferromanganese: FeMn68C7.0; ② Medium carbon ferromanganese: FeMn78C2.0; ④ Ferrosilicon: FeSi75-B; ⑤ High carbon ferrochrome: FeCr70C10.0; ⑥ Low carbon ferrochrome: FeCr55C0.5.

[0037] Step S2, electroslag stage: polish the surface of the electrode blank to remove defects and filter out impure impurities.

[0038] The slag entering the electroslag stage must be baked at a temperature of 600-700°C for a time of ≥6 hours to remove the crystallized water in the slag.

[0039] Slag system: CaF2:Al2O3=70%:30%, φ480mm ingot slag weight 71~73kg / furnace (target 72kg / furnace).

[0040] After the electroslag is demolded, it will be sent to hot forging within 1.5 hours.

[0041] Step S3, forging stage: the drawing process adopts an octagonal forging method, that is, chamfering is performed every two passes to ensure that the deformation of the steel is uniform.

[0042] The total forging ratio is ≥4, the holding time of the finished product is 2-3h, the deformation of the last fire should be greater than 40%, and the product is annealed in the furnace within 1.5h after forging, and the temperature is controlled at 670℃.

[0043] Step S4, quenching and tempering stage: the quenching and tempering stage includes a quenching process and a tempering process. The quenching process is cooled by alternating water and air, and the tempering process is performed after the last inversion temperature is controlled to reach an appropriate value.

[0044] In the quenching process, the steel is heated to 860℃ and kept for 2 hours, and then cooled alternately by water and air. The air cooling time is controlled to be 3-4 minutes each time. After the last temperature reaches 50-100℃, tempering treatment is carried out. The steel is heated to 530℃ and kept for 10 hours after being burned through, and then cooled to room temperature by water.

[0045] Specifically, when water cooling, the water entry time is determined according to the specifications of the finished product. When the steel comes out of water for the first time, the surface temperature of the steel is measured with a temperature gun. The returning temperature is recorded after the steel returns to temperature. The second water entry time is determined based on the returning temperature. The temperature is measured again when the steel comes out of water for the second time. After 3-4 minutes of returning to temperature, the steel is determined to enter water again based on the returning temperature. When the last returning temperature is within the range of 50-100°C, the next heat treatment can be carried out.

[0046] During the quenching process, alternating water-air cooling can more accurately control the quenching cooling temperature, which can improve the tensile strength. At the same time, rapid cooling during the tempering process can reduce the precipitation of carbides between grain boundaries, which is conducive to eliminating temper brittleness.

[0047] Step S5, finishing stage: finishing the quenched and tempered steel.

[0048] In this embodiment, φ480 electroslag ingots are used to forge φ170 finished products, and the heating temperature during the forging process is 1180°C.

[0049] Through the implementation scheme of this embodiment, the metallographic image of the 40NCD7-03 structural steel finally produced is as follows: Figure 2 As shown, Figure 2 The metallographic image in the figure is the metallographic structure observed under the ZEISS Scope.A1 metallographic microscope.

[0050] Example 2

[0051] On the basis of the above implementation scheme, φ480 electroslag ingots are used to forge φ170 finished products, and the heating temperature during the forging process is 1180°C.

[0052] Tempering stage: The quenching process heats the steel to 860℃ and holds it for 2 hours, then performs alternating water-air cooling, and controls the air cooling time to be 3-4 minutes each time. After the last temperature reaches 50-100℃, it is tempered. The steel is heated to 530℃ and held for 10 hours. After being taken out of the furnace, it is cooled to room temperature by air cooling.

[0053] Through the implementation scheme of this embodiment, the metallographic image of the 40NCD7-03 structural steel finally produced is as follows: Figure 3 As shown, Figure 3 The metallographic image in the figure is the metallographic structure observed under the ZEISS Scope.A1 metallographic microscope.

[0054] Figure 4 and Figure 5 This is the metallographic diagram of the product obtained by water cooling quenching under the same observation conditions, compared with the group cooled by water and air alternately (reference Figure 2 and Figure 3) The structure is dense and uniform, without segregation, which is better than the groups with only water cooling and air cooling (reference Figure 4 and Figure 5 ), the main reason is that the residual austenite is transformed through an appropriate cooling rate, the grain size is refined, and the strength of the material is improved. After tempering, rapid cooling is used to prevent the material from staying in the brittle temperature range for too long, thereby reducing the risk of tempering brittleness.

[0055] The invention provides a heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel, optimizes the cooling mode of the quenching and tempering process, adopts a water-air alternating cooling mode for the quenching process, controls the last inversion temperature to reach a suitable value before performing the tempering treatment, more accurately controls the quenching cooling temperature, transforms the residual austenite through an appropriate cooling rate, refines the structure grains, and thus improves the material strength. The metallographic structure of the treated steel in the invention is tempered troostite, and the grains are refined, thereby improving the stability of the mechanical properties of the steel.

[0056] Since the 40NCD7-03 alloy structural steel itself has temper brittleness, rapid cooling after tempering is performed to avoid the material from staying in the brittle temperature range for too long, thereby reducing the risk of temper brittleness and effectively reducing the temper brittleness of the steel. The present invention also controls the forging method and deformation amount to make the internal structure of the steel dense and uniform.

Claims

1. A heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel, characterized in that: The following steps are involved: Step S1, smelting stage: using electric furnace + AOD + LF + VD process to preliminarily refine the raw materials; Step S2, electroslag stage: polish the surface of the electrode blank to remove defects, filter out impure impurities, and send it to hot forging within 1.5 hours after the electroslag is demoulded; Step S3, forging stage: the drawing process adopts an octagonal forging method, that is, chamfering is performed every two passes to ensure uniform deformation of the steel; Step S4, quenching and tempering stage: the quenching and tempering stage includes a quenching process and a tempering process. The quenching process is cooled by alternating water and air, and the tempering process is performed after the last inversion temperature reaches a suitable value; Step S5, finishing stage: finishing the quenched and tempered steel.

2. A heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel according to claim 1, characterized in that: In step S1, the VD stage needs to evacuate the furnace after the cover is closed, and keep the vacuum degree below 67Pa for not less than 15 minutes. During the vacuuming process, attention should be paid to the rise of slag in the furnace to avoid slag bubbling.

3. A heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel according to claim 1, characterized in that: In step S1, after the VD stage is completed, the vacuum relief valve is opened to break the vacuum of the system, and then the hydrogen and oxygen are determined online to control [H]≤1.5ppm and [O]≤15ppm.

4. A heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel according to claim 1, characterized in that: In step S1, during the VD refining process, [Al] should be controlled at 0.05% by mass to increase the Al content requirement and refine the grains.

5. A heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel according to claim 1, characterized in that: In step S2, the slag entering the electroslag stage must be baked to remove the crystal water in the slag, the baking temperature is 600-700°C, and the baking time is ≥6h.

6. A heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel according to claim 1, characterized in that: In step S3, the total forging ratio is ≥4, the heat preservation time of the finished product is 2-3h, the deformation of the last fire should be greater than 40%, and the product is annealed in a furnace 1.5h after forging, and the temperature is controlled at 670℃.

7. A heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel according to claim 1, characterized in that: In step S4, the quenching process heats the steel to 860°C and holds it for 2 hours, then performs water-air alternating cooling, controls the air cooling return time for 3-4 minutes each time, and performs tempering after the last return temperature reaches 50-100°C. The steel is heated to 530°C and held for 10 hours, then taken out of the furnace and quickly cooled to room temperature.

8. A heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel according to claim 1, characterized in that: In step S4, during the tempering treatment, the steel is heated to 530°C, burned thoroughly, kept warm for 10 hours, and then taken out of the furnace and quickly cooled to room temperature.

9. A heat treatment method for improving the mechanical properties of 40NCD7-03 structural steel according to claim 8, characterized in that: During the tempering process, the rapid cooling method is water cooling or air cooling.

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