A large-size continuous casting round billet isothermal annealing process utilizing residual heat

Abstract

This invention provides a waste heat isothermal annealing process for large-size continuously cast round billets. The large-size continuously cast round billets, initially heated to 500–650℃, are placed in a slow cooling pit and heated to 600–750℃ at a rate of 50–75℃ / h, then held at that temperature. After cooling, they are removed from the furnace. Compared with existing technologies, this invention utilizes the waste heat of the cast billet for isothermal annealing, significantly reducing energy consumption and improving annealing efficiency. Utilizing the waste heat for annealing improves the surface and core quality of the round billet, preventing transverse fracture of the billet as a whole. It is compatible with bogie hearth annealing processes, and compared to bogie hearth designs, the overall equipment footprint is reduced by more than 50%, saving on-site land.

Description

Technical Field

[0001] This invention belongs to the field of steel production, specifically relating to a process for isothermal annealing of large-size continuously cast round billets using residual heat. Background Technology

[0002] With the improvement of continuous casting technology both domestically and internationally, in the last five years alone, the size of continuously cast round billets has expanded from Φ800mm to Φ1200mm, and there is even a trend towards larger sizes. The impact of thermal stress and phase transformation stress changes on billet quality has become increasingly complex. Based on past experience, the process of slow cooling of most continuously cast round billets to eliminate most internal stress is no longer suitable for the current production requirements of large-scale medium-high carbon and medium-high alloy steels. Consequently, the need for timely annealing of round billets after production has increased significantly.

[0003] Currently, the industry mainly uses cold billet stress relief or incomplete annealing, and the selection of annealing processes is limited. The actual annealing temperatures are high, the cycles are long, and energy consumption is high. Tests show that under annealing temperatures of 750–850℃, if the core of the cold billet reaches 500℃, a Φ800mm round billet requires 12 hours of heating, a Φ1000mm round billet requires 15 hours, and a Φ1200mm round billet requires 18 hours. Meanwhile, some high-alloy steels undergo cold annealing, where the phase transformation stress and thermal stress from the cooling and heating processes are cyclically superimposed, resulting in a significant decrease in core quality. This easily leads to core and surface cracks, or causes existing cracks to propagate, resulting in a significant decrease in quality. If the heating rate is slightly too fast before the core temperature of the round billet is below 550℃, the round billet may fracture directly laterally under stress.

[0004] Patent CN 109652629 A, published on April 19, 2019, discloses an annealing device and method for large round alloy steel billets. The annealing device has a pool-shaped structure, with the surrounding walls consisting of an insulation layer, a heat-preserving layer, and a refractory layer from the inside out. The bottom of the device consists of a lightweight high-strength layer, a refractory layer, and a high-alumina material layer from the inside out. The top cover is a steel frame structure with a refractory fiber lining, and a hook is installed on the top of the cover. Several sub-high-speed burners are arranged on the lower part of one side wall, with a fire baffle wall in front of the burners. Several exhaust ports are located on the lower part of the wall opposite to where the burners are located, and the exhaust ports discharge the flue gas from the furnace through a flue equipped with a regulating valve. This patent mainly improves the annealing device and processes round billets with a diameter of 350-800 mm. It does not disclose how to process billets with a larger diameter. Summary of the Invention

[0005] The purpose of this invention is to provide a large-size continuous casting round billet isothermal annealing process using residual heat. The annealing process is designed using the residual heat of the casting billet and the annealing is performed using the residual heat of the casting billet, thereby improving the surface and core quality of the round billet and preventing the overall transverse fracture of the round billet.

[0006] The specific technical solution of this invention is as follows:

[0007] A large-size continuously cast round billet isothermal annealing process utilizing residual heat includes the following steps:

[0008] The hot, large-diameter continuous casting round billet is placed in a slow cooling pit, heated to 600-750℃, and held at that temperature; after cooling, it is then removed from the furnace.

[0009] The hot large-size continuous casting round billet refers to the round billet with a temperature of 500-650℃ when it is placed in the slow cooling pit.

[0010] The slow cooling pit should be located within 20 to 50 meters of the continuous casting billet cooling bed; the hoisting time should be 5 to 10 minutes; in order to ensure that the temperature of the large-sized continuous casting round billet is 500 to 650℃ when it enters the pit.

[0011] Large-sized continuous casting round billets are placed in a slow cooling pit, using a grid pattern or a single mold arrangement;

[0012] After the hot, large-sized continuous casting round billets are placed into the slow cooling pit, they are heated to 600-750°C after all the billets have reached a certain temperature. During batch isothermal annealing, since there are many billets in the slow cooling pit, the billets that enter the pit first will cool down during the process of the billets entering the pit later. In order to ensure that all the billets reach the same temperature, the slow cooling pit needs to be heated. After all the billets have reached a temperature of 500-650°C, they are heated together to 600-750°C.

[0013] The slow cooling pit has a heating function and is insulated with a full fiber furnace lining structure; the slow cooling pit is equipped with a pit cover; the heating control of the slow cooling pit is as follows: burners are staggered in the lower part of the furnace, controlled in 6 to 8 zones; the maximum heating temperature of the slow cooling pit is 900℃; the furnace temperature uniformity of the slow cooling pit is ±20℃; the heating rate is 10-80℃ / h; the slow cooling pit is heated by fuel gas: mixed coal gas, with a calorific value of ~9.6MJ / Nm³. 3 ;

[0014] The diameter of the large-size continuous casting round billet is 800-1200 mm, preferably 1000-1200 mm; the steel grade of the large-size continuous casting round billet includes GCr15SiMnMo, P91, P92 or 42CrMo.

[0015] The heating rate to 600–750°C is 50–75°C / h. When the steel is heated above 650°C, the heat dissipation from the slow cooling pit increases, and the heating rate slows down as the temperature rises. If the heating rate exceeds 75°C / h, the heating efficiency decreases, and energy consumption increases accordingly. Furthermore, for alloy steel round billets larger than 1000mm, the thermal stress is significant when the heating rate exceeds 75°C / h, and there is tensile stress in the core. If defects such as cracks or shrinkage cavities exist in the core, they may propagate. Conversely, a heating rate less than 50°C / h results in a longer heating time and lower production efficiency. Therefore, this invention controls the heating rate to 50–75°C / h.

[0016] The temperature is raised to 600-750℃ and then held for 1-2.5 hours per 100mm, depending on the diameter. For example, for a diameter of 800mm, the holding time is 8 × (1-2.5) hours.

[0017] The cooling refers to cooling the steel in the furnace after the heating is turned off; or cooling at a controlled rate of 30-60℃ / h; further, cooling to below 350℃ before unloading. After isothermal treatment, the steel's microstructure has transformed into an equilibrium state, and the phase transformation stress has been eliminated. However, due to the large size of the billet and the excessively rapid cooling rate, the cooling stress is relatively large, resulting in a relatively large tensile stress in the core. This stress also has a certain time-dependent effect. If there are certain cracks or porosity defects in the core, the stress will be released after the billet has been left in the furnace for a certain period of time, leading to the expansion of defects and the formation of more serious defects.

[0018] The large-size continuous casting round billets can be batch isothermal annealed using the residual heat isothermal annealing process.

[0019] During the casting process of a round billet, the different cooling rates of various parts create thermal stress due to the temperature difference between the inside and outside. Phase transformation causes volume changes, and due to the unequal cooling time, structural stress is generated. The superposition of thermal stress and structural stress constitutes the internal stress of the round billet, which generally increases with the increase of alloy element content in the steel and the weight of the billet. For some air-quenched martensitic steels, such as high-chromium steel and die steel, even with slow cooling after casting, if internal stress is not relieved by timely annealing, there is a risk of spontaneous cracking or even explosion during storage. Round billets are generally sawn into sections later; therefore, it is necessary to reduce the surface hardness of the billet to facilitate sawing. In short, most round billets require annealing to eliminate internal stress, prevent cracking, and reduce hardness. Traditional annealing processes include: ordinary annealing or two-stage annealing with heating slightly above Ac3 and Ac1 to Acm, and low-temperature annealing below Ac1 without phase transformation and recrystallization, to achieve stress relief and softening. Its disadvantages include: 1. High heating temperature, long heating time, and high energy consumption, which can easily damage the internal quality of the round billet. The round billet generally undergoes a cooling, phase transformation, heating, and then another phase transformation process. Especially during the cooling process, due to the superposition of phase transformation stress and cooling stress, defects may occur in the round billet, especially those over 800mm and those made of medium and high alloy steel, or the original defects may be aggravated. If annealing is not carried out in time, the defects in the round billet will be aggravated, or even crack or explode. For steels with slightly poor thermal conductivity, if the heating rate is slightly faster than 650℃, it may also cause the round billet to break directly. For general steels, if heating is carried out between 700 and 850℃, it is the initial formation region of austenite, which is a process of large-scale heat absorption. In this range, the heating rate is fast, the heating efficiency is low, and the energy consumption is high. 2. The phase transformation process takes a relatively long time and has low efficiency. Cooling from the holding temperature to around 500℃ will result in the precipitation of proeutectoid ferrite or carbides and the decomposition of supercooled austenite into pearlite, requiring slow cooling; or in the low-temperature annealing process at slightly below Ac1, where no phase transformation recrystallization occurs, the internal stress of the billet will be relaxed as the carbides focus and grow and the ferrite recovers and recrystallizes, but the transformation time is relatively longer and the hardness is not easily reduced.

[0020] The inventors discovered that using residual heat isothermal annealing of round billets can eliminate the aforementioned drawbacks. The main reasons are: the invention features low heating temperature, short phase transformation time, and high annealing efficiency. At 500–650℃, the round billet, except for a small amount transforming into a proeutectoid ferrite + pearlite softening phase, retains most of its austenite state. Heating the steel to the nose temperature point of the isothermal transformation curve (TTT curve) (where the pearlite transformation time is shortest) allows for isothermal treatment. Depending on the steel grade, the heating temperature is generally within the specific range of 600–750℃ for homogenization. The pearlite transformation is completed within minutes to 1–2 hours, far less than the several hours required by conventional annealing processes. Simultaneously, within this temperature range, for 800–1200mm round billets, a heating rate of 50–75℃ / h is highly efficient and energy-saving. If heating efficiency and energy consumption are not considered, rapid heating is possible without the risk of billet breakage due to heating stress. The round billet of this invention exhibits good core quality. Compared to conventional annealing, the round billet only undergoes an isothermal phase transformation softening process to eliminate phase transformation stress. Subsequent cooling is controlled at 30–60°C / h, and the billet is removed from the furnace at 350°C. This effectively eliminates the influence of thermal stress, thus minimizing the generation of internal stress and reducing hardness, thereby ensuring the quality of the round billet, especially the core quality. This invention utilizes available site conditions and related facilities, fully considering production efficiency and energy conservation. During the round billet production process, reheating is unnecessary to ensure that the majority, or even all, of the billet remains in a single austenitic structure, reducing the phase transformation process. Reheating to a slightly lower isothermal temperature rapidly transforms the billet into a pearlitic structure (except for very rare high-Ni steels), thus completely eliminating phase transformation stress and simultaneously reducing hardness.

[0021] Existing technologies do not fully utilize the residual heat of cast billets for annealing, while this invention utilizes the residual heat of cast billets for isothermal annealing, resulting in high production efficiency, relatively good core quality, and no transverse fracture. This invention provides a highly efficient and energy-saving annealing process for large-diameter continuously cast round billets (Φ800-1200mm) to eliminate internal stress, reduce hardness, prevent internal defects and defect propagation, and facilitate subsequent processes such as cutting, forging, and heat treatment. This also improves production efficiency, product quality, and energy conservation. This invention utilizes the residual heat of large-diameter round billets for batch isothermal annealing of over 450 tons, increasing annealing efficiency by more than 25%, while also considering cold billet annealing. This invention uses lower annealing temperatures and shorter annealing times for high-alloy steel, reducing energy consumption by 30%, while avoiding the risks of core defect propagation and transverse fracture during billet heating.

[0022] Compared with existing technologies, this invention utilizes the residual heat of the cast billet for isothermal annealing, which significantly reduces energy consumption and improves annealing efficiency; it utilizes the residual heat of the cast billet and a slow cooling pit with heating function to design annealing process parameters; it uses the residual heat of the cast billet for annealing, which improves the surface and core quality of the round billet and prevents the overall transverse fracture of the round billet; it is compatible with bogie hearth furnace annealing process, and compared with bogie hearth furnace design, the entire equipment occupies more than 50% less floor space, saving on-site land. Attached Figure Description

[0023] Figure 1 The residual heat annealing process curve for Φ800mm round billet GCr15SiMnMo;

[0024] Figure 2 The residual heat annealing process curve for a Φ1000mm round billet using P91;

[0025] Figure 3 The residual heat annealing process curve for a Φ1000mm round billet using P92;

[0026] Figure 4 The residual heat annealing process curve for 42CrMo round billet with a diameter of 1200mm;

[0027] Figure 5 For Φ800mm GCr15SiMnMo, low magnification after residual heat annealing;

[0028] Figure 6 For Φ800mm GCr15SiMnMo cold blanks, annealed at low magnification;

[0029] Figure 7 It is a low magnification sample of Φ1000mm P91 after residual heat annealing;

[0030] Figure 8 It is a low-magnification product of annealed Φ1000mm P91 cold blank;

[0031] Figure 9 For Φ1000mm P92 residual heat annealing low magnification (after drilling sampling);

[0032] Figure 10 It is a low-magnification product of annealed Φ1000mm P92 cold blank;

[0033] Figure 11 For Φ1200mm 42CrMo low magnification annealing with residual heat;

[0034] Figure 12 Φ1200mm 42CrMo cold-annealed low-magnification billet;

[0035] Figure 13 Transverse cracking and fracture occurred in the annealed Φ800mm GCr15SiMnMo cold billet. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] Unless otherwise specified in the embodiments, the techniques or conditions described in the literature in this field or in accordance with the product manual may be followed.

[0038] This invention provides a process for isothermal annealing of large-size continuously cast round billets using residual heat, comprising the following processes:

[0039] 1) Slow-cooling pit configuration (set up according to the standard slow-cooling pit with heating function):

[0040] Location of the slow cooling pit: within 20-50m from the location of the continuous casting billet cooling bed; hoisting time: 5-10 minutes;

[0041] Clearance dimensions: 5000~9000mm×12000~150000mm×4~6000mm, pit bottom elevation: -3000~-4000mm, pit top elevation: +1~2000mm;

[0042] Pit structure: All-fiber furnace lining structure for heat insulation;

[0043] The slow-cooling pit is equipped with a pit cover, a steel frame structure, clamps and disk lifting points, and includes insulation material;

[0044] Heating control: Burners are staggered in the lower part of the furnace and controlled in 6 to 8 zones;

[0045] Heating parameters: Maximum heating temperature: 900℃;

[0046] Furnace temperature uniformity: ±20℃; Heating rate: 10-80℃ / h;

[0047] Gas: Mixed coal gas, calorific value: ~9.6 MJ / Nm³ 3 ;

[0048] Stacking in the pit: Place the pads and billets according to the spacing of different specifications of lifting tools, with a spacing of 600-1200mm.

[0049] Charging capacity: ~500t; height not exceeding the flue gas inlet of the furnace.

[0050] 2) Annealing process:

[0051] 2-1) Place large-size continuous casting round billets in the heating and slow cooling pit, using a grid pattern or a single mold arrangement;

[0052] 2-2) The temperature of large-size continuous casting round billets entering the pit: 500~650℃; batch isothermal annealing of billets. Since there are many billets in the slow cooling pit, the billets that enter the pit first will cool down during the process of the billets entering the pit later. In order to ensure that all round billets reach the same temperature, the slow cooling pit needs to be heated. After all billets reach the same temperature of 500~650℃, they are heated together to 600~750℃.

[0053] 2-3) Heat the heating and slow cooling pit at a rate of 50-75℃ / h;

[0054] 2-4) Holding temperature and time: Refer to the TTT curve (isothermal transformation curve of supercooled austenite) of the steel grade to set the holding temperature. The holding temperature is ±10~30℃ of the TTT curve nose temperature (i.e. 600~750℃). The holding time is 1~2.5h / 100mm after the billet reaches the temperature.

[0055] 2-5) Cooling: Pit cooling or controlled cooling at 30-60℃ / h until below 350℃, then uncover and remove from the furnace.

[0056] This invention utilizes the above method to rapidly anneal large-diameter round billets with a diameter of Φ800–1200 mm. It primarily utilizes the residual heat of the cast billet, preserving the majority of the microstructure in an austenitic state. Employing an isothermal transformation process at 600–750℃, compared to conventional annealing processes, the annealing temperature is reduced by 50–200℃, the annealing cycle is shortened by 25%, and energy consumption is reduced by 30%. This significantly improves the surface and core quality of medium and high alloy steels, while also being compatible with bogie hearth annealing processes.

[0057] The following are several specific embodiments and comparative examples of the present invention, which further illustrate the technical solution of the present invention.

[0058] Example 1

[0059] A large-size continuously cast round billet is subjected to a residual heat isothermal annealing process, using the aforementioned slow cooling pit configuration. Specifically, the Φ800mm round billet is subjected to residual heat annealing of GCr15SiMnMo, with a pit temperature of 500-550℃. The chemical composition of the round billet is: C: 1.02%, Si: 0.95%, Mn: 1.87%, P: 0.01%, S: 0.002%, Cr: 1.5%, Mo: 0.23%, with the remainder being Fe and unavoidable impurities.

[0060] The specific annealing process includes the following steps:

[0061] 1) Loading: Use clamps to lift the GCr15SiMnMo Φ800mm round billet into the slow cooling pit with heating function, and place it in a line to ensure the temperature of entering the pit is 500~550℃;

[0062] 2) Ignition: After the material is loaded, the slow cooling pit is ignited. After the temperature in the pit reaches 500-540℃, it is held at that temperature for 120 minutes.

[0063] 3) Heating: After the isothermal period ends, increase the temperature by 50-65℃ / h to 680-720℃;

[0064] 4) Heat preservation: Keep warm for 1020 minutes;

[0065] 5) Cooling: After the heat preservation is completed, the furnace is extinguished and cooled to 450°C. The lid is then removed and cooled to below 350°C. The furnace is then removed from the pit and the annealing cycle is 32 hours.

[0066] Example 1: The annealed round billet was inspected, and low-magnification inspection was performed as follows: Figure 5 The hardness results are shown in Table 1.

[0067] Table 1. Comparison of hardness before and after preheating and annealing of Φ800mm GCr15SiMnMo.

[0068] Hardness (HBW) 387 393 370 388 392 386 Before annealing Hardness (HBW) 236 239 232 231 237 235 After annealing

[0069] Table 1 shows that the hardness decreases significantly after annealing, with an average hardness of 235 HB, which is suitable for subsequent sawing. If the hardness is greater than 270 HB, subsequent sawing will be very difficult.

[0070] Comparative Example 1: The same billet as in Example 1 was annealed using cold billet annealing. Since the aforementioned heating and slow cooling pit device was not available on-site, an annealing furnace was used. The specific annealing process was as follows:

[0071] 1) Charging: The billets are laid out in a straight line inside the furnace, with layers separated by pads; the furnace temperature of the billets is 200℃.

[0072] 2) Ignition and heating: After the material is loaded, ignite and heat up to 500-525℃ at a rate of 25-35℃ / h, then hold at that temperature for 90 minutes.

[0073] 3) Heating: After the isothermal period ends, the temperature is increased to 700-740℃ at a rate of 40℃ / h;

[0074] 4) Heat preservation: 700~740℃ for 1440 minutes;

[0075] 5) Cooling: After the heat preservation is completed, control the cooling rate at ≤30℃ / h, cool to 300℃, remove from the pit, and the annealing cycle is 52h.

[0076] 40% of the round billets cracked and fractured laterally, such as Figure 13 For areas without cracks, take a transverse low-magnification observation as shown. Figure 6 There are core cracks, but no cracking or breakage occurred during hot billet annealing, and no core cracks were found at low magnification.

[0077] Example 2

[0078] A large-size continuously cast round billet utilizes a residual heat isothermal annealing process, specifically a Φ1000mm round billet P91 residual heat annealing, with a billet entry temperature of 550±50℃. The chemical composition of the round billet is as follows: C: 0.10%, Si: 0.30%, Mn: 0.40%, P: 0.01%, S: 0.002%, Cr: 8.5%, Mo: 0.85%, Ni: 0.16%, V: 0.15%, Al: 0.008%, Nb: 0.075%, [N]: 550ppm, [H]: 0.8ppm, with the remainder being Fe and unavoidable impurities.

[0079] The specific annealing process includes the following steps:

[0080] 1) Loading: Use clamps to lift the material into the pit and place it in a line to ensure the temperature inside the pit is 550±50℃;

[0081] 2) Ignition: After the material is loaded, ignite and hold at 550±20℃ for 120 minutes.

[0082] 3) Heating: After the isothermal period ends, increase the temperature by 50-75℃ / h to 710-750℃;

[0083] 4) Heat preservation: 710~750℃ for 1200min;

[0084] 5) Cooling: After the heat preservation is completed, the furnace is extinguished and cooled to 450°C. The lid is then removed, and the furnace is cooled to below 350°C. The furnace is then removed from the pit. The annealing cycle is 57 hours.

[0085] Example 2: The annealed round billet was inspected, and low-magnification inspection was performed as follows: Figure 7 The hardness results are shown in Table 2.

[0086] Table 2. Hardness comparison of Φ1000mm P91 before and after preheat annealing

[0087] Hardness (HBW) 284 300 276 308 292 292 Before annealing Hardness (HBW) 206 199 202 200 207 203 After annealing

[0088] As can be seen from Table 2, the hardness decreased significantly after annealing, with an average hardness of 203 HB, which is suitable for subsequent sawing and cutting.

[0089] Comparative Example 2: Using the same cast billet as in Example 2, cold billet annealing was performed. If the aforementioned heating and slow cooling pit device was not available on site, the specific annealing process was as follows:

[0090] 1) Loading: Use clamps to lift the material into the pit and place it in a line;

[0091] 2) Ignition: Ignite after loading is complete;

[0092] 3) Heating: Increase the temperature by 25-35℃ / h to 610-650℃;

[0093] 4) Heat preservation: 600-650℃ for 1000 minutes;

[0094] 5) Heating: Increase the temperature by 50-60℃ / h to 790-830℃

[0095] 6) Insulation: Insulate at 790~830℃ for 2000min

[0096] 7) Cooling: After the heat preservation is completed, the furnace is extinguished and cooled to 350℃. The furnace is then removed from the pit, and the annealing cycle is 80 hours.

[0097] The average hardness was 207 HB, the annealing cycle was 80 hours, and the low magnification test results were as follows: Figure 8 Compared to not using the residual heat annealing process, the residual heat annealing process increases production efficiency by 28%, reduces energy consumption by more than 35%, and reduces the core defect area by 3 times.

[0098] Example 3

[0099] A large-size continuously cast round billet utilizes a residual heat isothermal annealing process, specifically a Φ1000mm round billet undergoing P92 residual heat annealing. The billet's initial temperature is 550±50℃, and the chemical composition of the billet is as follows:

[0100] C: 0.10%, Si: 0.30%, Mn: 0.40%, P: 0.01%, S: 0.002%, Cr: 8.7%, Mo: 0.55%, Ni: 0.16%, V: 0.22%, Al: 0.008%, Nb: 0.075%, W: 1.72%, B: 0.0040%, [N]: 650ppm, [H]: 0.8ppm, with the remainder being Fe and unavoidable impurities.

[0101] The annealing process includes the following steps:

[0102] 1) Loading: Use clamps to lift the material into the pit and place it in a line to ensure the temperature inside the pit is 550±50℃;

[0103] 2) Ignition: After the material is loaded, ignite and hold at 550±20℃ for 120 minutes.

[0104] 3) Heating: After the isothermal period ends, increase the temperature by 50-75℃ / h to 700-740℃;

[0105] 4) Heat preservation: 700~740℃ for 1500min;

[0106] 5) Cooling: After the heat preservation is completed, the furnace is extinguished and cooled to below 400℃. The furnace is then opened and removed from the pit at below 350℃. The annealing cycle is 60 hours.

[0107] Example 3: The annealed round billet was inspected, and low-magnification inspection was performed as follows: Figure 9 The hardness results are shown in Table 3.

[0108] Table 3. Comparison of hardness before and after residual heat annealing of Φ1000mm P92 steel.

[0109] Hardness (HBW) 295 312 287 320 300 303 Before annealing Hardness (HBW) 212 215 205 210 215 211 After annealing

[0110] As shown in Table 3, the hardness decreased significantly after annealing, with an average hardness of 211 HB, which is suitable for subsequent sawing and cutting.

[0111] Comparative Example 3: Using the same cast billet as in Example 3, cold billet annealing was performed. If the aforementioned heating and slow cooling pit device was not available on site, the specific annealing process was as follows:

[0112] 1) Loading: Use clamps to lift the material into the pit and place it in a line;

[0113] 2) Ignition: Ignite after loading is complete;

[0114] 3) Heating: Increase the temperature by 25-35℃ / h to 600-640℃;

[0115] 4) Heat preservation: 600~640℃ for 1200min;

[0116] 5) Heating: Increase the temperature by 50-60℃ / h to 800-840℃;

[0117] 6) Insulation: Insulate at 810~840℃ for 3000min;

[0118] 7) Cooling: Reduce temperature by 20-30℃ / h to 600℃;

[0119] 8) Heat preservation: 600~620℃ for 1000min;

[0120] 7) Cooling: After the heat preservation is completed, the furnace is extinguished and cooled to 350℃. The furnace is then removed from the pit, and the annealing cycle is 120 hours.

[0121] Its low-magnification detection, such as Figure 10 Compared to not using the residual heat annealing process, the residual heat annealing process increases production efficiency by 50%, reduces energy consumption by more than 45%, and reduces the core defect area by 5 times.

[0122] Example 4

[0123] A large-size continuously cast round billet utilizes a residual heat isothermal annealing process, specifically: Φ1200mm round billet 42CrMo residual heat annealing, billet entry temperature: 600±50℃, the chemical composition of the round billet is as follows: C: 0.43%, Si: 0.30%, Mn: 0.87%, P: 0.01%, S: 0.002%, Cr: 1.16%, Mo: 0.23%, the remainder being Fe and unavoidable impurities.

[0124] Specific heat treatment methods:

[0125] 1) Loading: Use clamps to lift the material into the pit and place it in a line. The temperature inside the pit should be 600±50℃.

[0126] 2) Ignition: After the loading is completed, ignite and hold at 600±20℃ for 180 minutes.

[0127] 3) Heating: After the isothermal period ends, increase the temperature by 50-75℃ / h to 670-710℃;

[0128] 4) Heat preservation: After the heating is completed, keep warm at 670-710℃ for 1200 minutes;

[0129] 5) Cooling: After the heat preservation is completed, the furnace is extinguished and cooled to below 450℃. The lid is then removed and cooled to below 350℃. The furnace is then removed from the pit and the annealing cycle is 40 hours.

[0130] Example 4: The annealed round billet was inspected, and low-magnification inspection was performed as follows: Figure 11 The hardness results are shown in Table 4.

[0131] Table 4. Hardness comparison of Φ1200mm 42CrMo before and after residual heat annealing

[0132] Hardness (HBW) 324 300 296 318 332 314 Before annealing Hardness (HBW) 206 199 202 200 207 203 After annealing

[0133] The hardness decreases significantly after annealing, with an average hardness of 203 HB, which is suitable for subsequent sawing and cutting.

[0134] Comparative Example 3: Using the same cast billet as in Example 3, cold billet annealing was performed. If the aforementioned heating and slow cooling pit device was not available on site, the specific annealing process was as follows:

[0135] 3) Loading: Use clamps to lift the material into the pit and place it in a line;

[0136] 4) Ignition: Ignite after loading is complete;

[0137] 3) Heating: Increase the temperature by 25-30℃ / h to 660-700℃;

[0138] 4) Heat preservation: 660~700℃ for 1800min;

[0139] 7) Cooling: After the heat preservation is completed, the furnace is extinguished and cooled to 350℃. The furnace is then removed from the pit, and the annealing cycle is 55 hours.

[0140] Its average hardness is 192 HB, and its low magnification test results are as follows: Figure 12 .

[0141] Compared to not using the waste heat annealing process, the waste heat annealing process increases production efficiency by 44%, reduces energy consumption by more than 35%, and no defects were found during low-magnification observation.

[0142] The above description of the embodiments is intended to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A process for isothermal annealing of large size continuously cast round billets utilizing waste heat, characterized by, The large-size continuous casting round billet isothermal annealing process utilizing residual heat is specifically as follows: The hot, large-diameter continuous casting round billet is placed in a slow cooling pit, heated to 600~750℃, and held at that temperature; after cooling, it is then removed from the furnace. The hot large-size continuous casting round billet refers to the round billet with a temperature of 500~650℃ when it is placed in the slow cooling pit. The heating rate to 600~750℃ is 50~75℃ / h; The heat preservation time is calculated based on the billet diameter, and the time is 1~2.5h / 100mm; The cooling is controlled at 30~60℃ / h, and the temperature is cooled to below 350℃ before being removed from the furnace; The diameter of the large-size continuous casting round billet is 800~1200mm; The slow cooling pit is located within 20-50m from the continuous casting billet cooling bed; the hoisting time is 5-10 minutes. The steel grades for the large-size continuously cast round billets include GCr15SiMnMo, P91, P92, or 42CrMo.

2. The sized continuously cast round billet utilizing a waste heat isothermal annealing process according to claim 1, characterized by, The furnace temperature uniformity of the slow cooling pit is ±20℃; the heating rate is 10-80℃ / h.

3. The isothermal annealing process using residual heat for continuously cast round billets according to claim 1, characterized in that, The diameter of the large-size continuous casting round billet is 1000~1200mm.

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