Method for improving HT700T alloy shrinkage cavity structure

Abstract

The invention discloses a method for improving an HT700T alloy shrinkage cavity structure, which comprises the following steps: S1, preparing an HT700T alloy raw material, smelting by using a vacuum induction furnace, degassing, refining, and regulating and controlling components; s2, an ingot mold is prepared and assembled, and the ingot mold is preheated in a preheating station in advance; s3, a launder is prepared, and the launder is preheated in a preheating station in advance; and S4, before casting of the induction furnace, a thermocouple is used for measuring the temperature, and after the temperature reaches the steel casting temperature, power is applied for heat preservation at the last stage of casting, so that the temperature of molten steel at the last stage of casting can be increased, the liquidity of the molten steel is improved, the nodulation risk of the molten steel at a water gap during casting is reduced, and the shape of a shrinkage cavity is reduced and improved. Meanwhile, the induction cast ingot is subjected to thermal annealing, stress generated by cooling of the head of the induction cast ingot is reduced, and cracks are prevented. The electrode shrinkage cavity is improved, the cracking tendency of the electrode is prevented, and the self-consumption fluctuation problem caused by the deep shrinkage cavity of the head and the poor structure in the self-consumption smelting process is greatly solved.

Description

Technical Field

[0001] The invention relates to the technical field of high-temperature alloy smelting for ultra-supercritical coal-fired generator rotors, and in particular to a method for improving the shrinkage cavity structure of HT700T alloy. Background Art

[0002] my country's energy consumption is mainly based on coal. In 2022, coal used for electricity accounted for 54.9% of my country's total coal consumption, and coal-fired power generation accounted for 58.4% of the total power generation. It is the cornerstone of my country's power security. Coal-fired power has a decisive impact on the development of my country's "dual carbon" strategy and energy security.

[0003] In the 1950s, developed countries in Europe and the United States began researching 650°C ultra-supercritical power generation technology. In the 1980s and 1990s, increasing the temperature to 650°C became the next target, alongside the development of 580-620°C ultra-supercritical units. However, due to the failure of high-temperature materials to meet expected performance, development plans were temporarily shelved. Currently, with new advances in high-temperature alloy melting methods and technologies, (ultra)supercritical power generation technology at 650°C and above has once again attracted industry attention worldwide.

[0004] Ultra-supercritical units utilize high-temperature alloys with large finished product sizes and heavy weight, placing extremely high demands on internal purity. Because these materials operate under harsh conditions of high temperature, high pressure, and high stress, metallurgical defects can lead to cracks and premature failure. Therefore, the melting technology required for these alloys is extremely demanding, with no metallurgical defects permitted in materials used in units weighing between 2,000 and 8,000 kg.

[0005] my country has newly developed the HT700 series of materials with high service temperature, high strength and high weldability for alloy materials for ultra-supercritical units. Among them, HT700T alloy is one of them. The VIM+VAR double vacuum melting process route is adopted. The shrinkage cavities and cracks inside the induction ingot are all due to loose electrodes. Electrode blocks are prone to fall off during the consumable melting process, including isolated dendrite stems caused by dendrite element segregation at the end of solidification and electrode blocks caused by incomplete alloy shrinkage at the end of solidification, which lead to disturbances in the consumable melting process, affecting the local composition uniformity of the final consumable ingot, causing composition segregation and affecting the performance of the final product. Summary of the Invention

[0006] The object of the present invention is to provide a method for improving the shrinkage cavity structure of HT700T alloy according to claim 1, so as to solve the problems raised in the above background technology.

[0007] To achieve the above object, the present invention provides the following technical solution: The present invention provides a method for improving the shrinkage cavity structure of HT700T alloy, comprising the following steps:

[0008] Step S1: preparing HT700T alloy raw materials, and melting them in a vacuum induction furnace, and then degassing, refining, and adjusting the composition;

[0009] Step S2: prepare and assemble the ingot mold, and preheat the ingot mold in a preheating station;

[0010] Step S3: prepare the flow channel and preheat the flow channel in the preheating station in advance;

[0011] Step S4: Before pouring in the induction furnace, use a thermocouple to measure the temperature. After the pouring temperature is reached, pouring is carried out. During the pouring process, the timing is determined based on the weight of the poured molten steel. After the weight of the poured molten steel reaches the specified level, power is applied to keep the temperature constant.

[0012] Step S5: After the ingot has cooled for the specified time, the induction ingot is placed in an annealing furnace for annealing at the specified temperature. After the furnace cools to the specified temperature, the ingot is kept warm and then removed from the furnace for air cooling.

[0013] Step S6: After the surface of the induction ingot is treated, the size and charging ratio of the consumable electrode must meet the requirements before the consumable smelting can be carried out to obtain a consumable ingot;

[0014] Step S7: The obtained consumable ingot is forged and deformed, and after peeling and cutting off the rotten head, a finished Φ217 mm bar is obtained and evaluated.

[0015] Preferably, the amount of material added to the induction furnace in S1 is 6000-8000 kg.

[0016] Preferably, the ingot mold in S2 has a specification of Φ300-Φ400 mm, and the heating temperature in the preheating station is 300-500°C.

[0017] Preferably, the heating temperature of the launder in S3 in the preheating station is 700-1100°C.

[0018] Preferably, the pouring temperature of the HT700 series alloy steel in S4 is 1450-1550°C.

[0019] Preferably, the best time in S4 is when the weight of the poured molten steel reaches 60-80% of the total added amount.

[0020] Preferably, the power applied at the end of pouring in S4 is 100 to 270 kw.

[0021] Preferably, in S5, the ingot cooling time is 60 to 180 minutes, the ingot annealing temperature is 700 to 1100°C, the annealing time is 3 to 9 hours, the furnace is cooled to 400 to 600°C, and then kept warm for 2 to 4 hours, and then air-cooled out of the furnace.

[0022] Preferably, the furnace filling ratio of the consumable electrode size after treatment in S6 is 0.74 to 0.81.

[0023] As a preference, in S7, the finished bar is subjected to water immersion flaw detection to check whether there are cracks inside; and a high-magnification microstructure evaluation is performed.

[0024] The advantage of this invention lies in that by applying power insulation at the end of the pouring process, it not only raises the molten steel temperature at the end of the pouring process, improving its fluidity, but also reduces the risk of molten steel forming nodules at the nozzle during pouring, thereby reducing and improving shrinkage cavity morphology. Simultaneously, thermal annealing of the induction ingot reduces stress generated by cooling the ingot head, preventing cracks. By improving electrode shrinkage and preventing electrode cracking, it significantly reduces the consumable fluctuations caused by deep shrinkage cavities and poor structure in the head during consumable melting. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The HT700T production process provided by the embodiment of the present invention;

[0026] Figure 2 The end face morphology of the HT700T alloy head before improvement provided by the embodiment of the present invention;

[0027] Figure 3 The improved HT700T alloy head end face morphology provided in the embodiment of the present invention;

[0028] Figure 4 The shrinkage cavity structure of the HT700T alloy head provided by the embodiment of the present invention;

[0029] Figure 5 The microstructure diagram of the HT700T alloy bar provided in the embodiment of the present invention;

[0030] Figure 6 This is a flaw detection diagram of HT700T alloy provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0031] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0032] See also Figures 1-6 The present invention provides a technical solution: The present invention provides a method for improving the shrinkage cavity structure of HT700T alloy. The specific solution is as follows:

[0033] Step S1: preparing ingredients according to the HT700T alloy composition requirements, with a weight of 6000-8000 kg, and smelting in a vacuum induction furnace, and then degassing, refining, and adjusting the composition;

[0034] Step S2: prepare and assemble an ingot mold of Φ300-Φ400 mm, and preheat the ingot mold to 300-500° C. in a preheating station;

[0035] Step S3: prepare the launder and preheat the launder to 700-1100° C. in a preheating station;

[0036] Step S4: Before pouring in the induction furnace, use a thermocouple to measure the temperature. When the temperature reaches 1450-1550°C, pouring is carried out. During the pouring process, when the weight of the poured molten steel reaches 60-80% of the total added amount, apply 100-270kw of power for heat preservation;

[0037] Step S5: After the ingot has cooled for 60 to 180 minutes, the induction ingot is placed in an annealing furnace at 700 to 1100° C. for 3 to 9 hours, and after the furnace cools to 400 to 600° C., it is kept at this temperature for 2 to 4 hours and then removed from the furnace for air cooling;

[0038] Step S6: treating the surface of the induction ingot. After the treatment, the size of the consumable electrode must meet the furnace filling ratio of 0.74 to 0.81 before the consumable smelting can be carried out. After the consumable smelting, the consumable ingot is obtained.

[0039] Step S7: forging and deforming the obtained consumable ingot to obtain a finished Φ217 mm bar, and performing flaw detection evaluation.

[0040] Furthermore, the amount of induction ingot surface treatment in step S6 needs to be ≥30 kg.

[0041] The present invention is further described in detail below with reference to specific embodiments.

[0042] Example 1 comprises the following steps:

[0043] Step S1: preparing the ingredients according to the HT700T alloy composition requirements, with a weight of 7200 kg, and smelting in a vacuum induction furnace, and then degassing, refining, and adjusting the composition;

[0044] Step S2: prepare and assemble an ingot mold of Φ320 mm, and preheat the ingot mold to 430° C. in a preheating station;

[0045] Step S3: prepare the launder and preheat the launder to 920° C. in a preheating station;

[0046] Step S4: Before pouring in the induction furnace, use a thermocouple to measure the temperature. When the temperature reaches 1526°C, pouring begins. During the pouring process, when the weight of the poured molten steel reaches 67% of the total added amount, apply 220kw power for heat preservation;

[0047] Step S5: After the ingot has cooled for 150 minutes, the induction ingot is placed in an annealing furnace and annealed at 950°C for 6 hours. After the furnace cools to 550°C, the ingot is kept at this temperature for 4 hours and then removed from the furnace for air cooling.

[0048] Step S6: treating the surface of the induction ingot, measuring the size of the consumable electrode after the treatment, calculating the furnace filling ratio to be 0.76, performing consumable smelting, and obtaining a consumable ingot after consumable smelting;

[0049] Step S7: forging and deforming the obtained consumable ingot to obtain a finished Φ217 mm bar, and performing flaw detection evaluation.

[0050] In step S6, the induction ingot surface treatment amount is 43 kg;

[0051] The final Φ217mm bar was evaluated by ultrasonic water immersion testing.

[0052]

[0053] Table 1. Noise levels of each channel in ultrasonic flaw detection.

[0054] Example 2 comprises the following steps:

[0055] Step S1: preparing ingredients according to the HT700T alloy composition requirements, with a weight of 6500 kg, and smelting in a vacuum induction furnace, and then degassing, refining, and adjusting the composition;

[0056] Step S2: prepare and assemble an ingot mold of Φ350 mm, and preheat the ingot mold to 330° C. in a preheating station;

[0057] Step S3: prepare the launder and preheat the launder to 1090° C. in a preheating station;

[0058] Step S4: Before pouring in the induction furnace, use a thermocouple to measure the temperature. When the temperature reaches 1514°C, pouring begins. During the pouring process, when the weight of the poured molten steel reaches 79% of the total added amount, apply 180kw power for heat preservation;

[0059] Step S5: After the ingot has cooled for 150 minutes, the induction ingot is placed in an annealing furnace and annealed at 950°C for 6 hours. After the furnace cools to 550°C, the ingot is kept at this temperature for 4 hours and then removed from the furnace for air cooling.

[0060] Step S6: treating the surface of the induction ingot, measuring the size of the consumable electrode after the treatment, calculating the furnace filling ratio to be 0.795, performing consumable smelting, and obtaining a consumable ingot after consumable smelting;

[0061] Step S7: forging and deforming the obtained consumable ingot to obtain a finished Φ217 mm bar, and performing flaw detection evaluation.

[0062] In step S6, the induction ingot surface treatment amount is 48 kg;

[0063] The final Φ217mm bar was evaluated by ultrasonic water immersion testing.

[0064]

[0065] Table 2. Noise levels of each channel in ultrasonic flaw detection.

[0066] Although the specific embodiments of the present invention have been described in a particular order, it should be understood that such actions or steps are required to be performed in the particular order shown or in a sequential order, or that all illustrated actions or steps are required to be performed to obtain the desired result. Under certain circumstances, multitasking and parallel processing may be advantageous. Similarly, although some specific implementation details have been included in the above discussion, these should not be construed as limiting the scope of the present disclosure. Certain features described in the context of a separate embodiment may also be implemented in a single implementation in combination.

[0067] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for improving the shrinkage structure of HT700T alloy, characterized in that: The specific steps include: Step S1: preparing the ingredients according to the HT700T alloy composition requirements, with a weight of 6000-8000 kg, and smelting in a vacuum induction furnace, and then degassing, refining, and adjusting the composition; Step S2: preparing and assembling an ingot mold of Φ300-Φ400 mm, and preheating the ingot mold to 300-500° C. in a preheating station; Step S3: prepare the launder and preheat the launder to 700-1100° C. in a preheating station; Step S4: Before pouring in the induction furnace, use a thermocouple to measure the temperature. When the temperature reaches 1450-1550°C, pouring is carried out. During the pouring process, when the weight of the poured molten steel reaches 60-80% of the total added amount, apply 100-270 kW of power for heat preservation. Step S5: After the ingot has cooled for 60 to 180 minutes, the induction ingot is placed in an annealing furnace at 700 to 1100° C. for 3 to 9 hours. When the furnace cools to 400 to 600° C., the ingot is kept warm for 2 to 4 hours and then air-cooled. Step S6: treating the surface of the induction ingot. After the treatment, the size of the consumable electrode must meet the furnace filling ratio of 0.74 to 0.81 before the consumable smelting can be carried out. After the consumable smelting, the consumable ingot is obtained. Step S7: The obtained consumable ingot is forged and deformed to obtain a finished Φ217 mm bar, which is then evaluated.

2. The method for improving the shrinkage cavity structure of HT700T alloy according to claim 1, characterized in that: The preheating of the ingot mold in step S2 is to slow down the cooling of the edge of the molten steel in the ingot mold and reduce the temperature gradient.

3. The method for improving the shrinkage cavity structure of HT700T alloy according to claim 1, characterized in that: The preheating of the launder in step S3 is to reduce the temperature drop of the molten steel in the launder.

4. The method for improving the shrinkage cavity structure of HT700T alloy according to claim 1, characterized in that: In step S4, power insulation is applied at the end of pouring to maintain the temperature of the molten steel, improve the fluidity of the molten steel, reduce the depth of the head shrinkage cavity and improve the shrinkage cavity morphology.

5. The method for improving the shrinkage cavity structure of HT700T alloy according to claim 1, characterized in that: In step S5, two-stage annealing is adopted in the induction ingot thermal annealing. On the one hand, high-temperature annealing releases the stress generated by the molten steel during solidification shrinkage and reduces the tendency of electrode cracking. On the other hand, low-temperature annealing reduces the surface hardness of the ingot and improves the machinability.

6. The method for improving the shrinkage cavity structure of HT700T alloy according to claim 1, characterized in that: The size of the electrode after surface treatment in step S6 must meet the filling ratio requirement, which is beneficial to reducing the occurrence of edge arcing and glow discharge.

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