Process for isothermal annealing of large-size continuous casting round billets by using waste heat

Abstract

The invention provides a waste heat isothermal annealing process for a large-size continuous casting round billet, which comprises the following steps: putting the large-size continuous casting round billet at 500-650 DEG C into a slow cooling pit, heating to 600-750 DEG C according to 50-75 DEG C / h, and preserving heat; and after cooling, discharging from the furnace. Compared with the prior art, the isothermal annealing is performed by using the waste heat of the casting blank, so that the energy consumption is greatly reduced, and the annealing efficiency is improved; casting blank waste heat is used for annealing, the quality of the surface and the core of the round blank is improved, and overall transverse breakage of the round blank is prevented; and compared with the design of the trolley furnace, the occupied area of the whole equipment is reduced by more than 50%, and the field land is saved.

Description

Technical Field

[0001] The invention belongs to the field of steel production, and in particular relates to a large-size continuous casting round billet isothermal annealing process utilizing residual heat. Background Art

[0002] With the improvement of continuous casting technology at home and abroad, in the past five years alone, the specifications of continuous casting round billets have expanded from Φ800mm to Φ1200mm, and there is even a trend towards larger specifications. The impact of thermal stress and phase change stress changes on the quality of the billet is becoming more and more complex. According to past experience, most of the processes for continuous casting round billets that can eliminate most of the internal stress by slow cooling are no longer suitable for the current large-scale medium-high carbon and medium-high alloy steel production requirements. The demand for timely annealing of round billets after they come off the production line has increased significantly.

[0003] At present, the industry mainly adopts cold billet stress relief or incomplete annealing, and the annealing process is also limited. The actual annealing temperature is high, the cycle is long, and the energy consumption is high. According to the test, under the annealing temperature of 750-850℃, if the core of the cold billet reaches 500℃, the Φ800mm round billet needs to be heated for 12h, the Φ1000mm round billet needs to be heated for 15h, and the Φ1200mm round billet needs to be heated for 18h. At the same time, some high-alloy steels are cold annealed, and the phase change stress and thermal stress cycle of the cooling and heating process are superimposed, the core quality is significantly reduced, and core and surface cracks are easily generated, or the original cracks are extended, resulting in a significant reduction in quality. If the heating speed is slightly faster before the core temperature of the round billet is less than 550℃, the round billet will be directly transversely fractured under stress.

[0004] The patent with the publication number CN 109652629 A published on April 19, 2019, discloses an annealing device and annealing method for large round billets of alloy steel. The annealing device is a pool-type structure, and the surrounding walls are insulation layer, thermal insulation layer and refractory layer from inside to outside; the bottom of the device is a lightweight high-strength layer, refractory layer and high-aluminum material layer from inside to outside; the top cover of the device is a steel structure frame, with refractory fiber lining inside, and a hook is set on the top of the cover; a number of sub-high-speed burners are arranged at the lower part of one side wall, and a fire baffle is set in front of the burner; a number of exhaust ports are arranged at the lower part of the wall opposite to the burner, and the exhaust ports discharge the smoke in the furnace through the flue, and the flue is provided with a regulating valve. This patent is mainly for the improvement of the annealing device, and the object of treatment is round billets with a diameter of 350-800mm. It does not disclose how to treat billets with larger diameters. Summary of the invention

[0005] The purpose of the present invention is to provide a large-scale continuous casting round billet isothermal annealing process using residual heat, design the annealing process using the residual heat of the billet, and use the residual heat of the billet for annealing, so as to improve the surface and core quality of the round billet and prevent the occurrence of overall transverse fracture of the round billet.

[0006] The specific technical solutions of the present invention are as follows:

[0007] A large-size continuous casting round billet isothermal annealing process using residual heat, comprising the following steps:

[0008] The hot large-sized continuous casting round billet is placed in a slow cooling pit, heated to 600-750℃, and kept warm; after cooling, it is taken out of the furnace.

[0009] The hot large-size continuous casting round billet refers to the large-size continuous casting round billet being placed in the slow cooling pit at a temperature of 500-650°C;

[0010] The slow cooling pit is within 20 to 50 meters from the continuous casting billet cooling bed; the lifting time is 5 to 10 minutes; to ensure that the temperature of large-sized continuous casting round billets when entering the pit is 500 to 650°C.

[0011] Large-sized continuous casting round billets are placed in the slow cooling pit in a tic-tac-toe or one-shaped arrangement;

[0012] After placing the hot large-sized continuous casting round billets into the slow cooling pit, when all the round billets reach a certain temperature, they are heated to 600-750℃; for 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 following billets entering the pit. In order to ensure that all the round billets reach the same temperature, the slow cooling pit needs to be heated to ensure that all the billets reach a temperature of 500-650℃, and then the temperature is raised to 600-750℃ together.

[0013] The slow cooling pit has a heating function and adopts a full fiber lining structure for insulation; the slow cooling pit is provided with a pit cover; the heating control of the slow cooling pit: the burners are arranged alternately at the lower part of the furnace, and are controlled in 6 to 8 zones; the maximum heating temperature of the slow cooling pit: 900°C; the furnace temperature uniformity of the slow cooling pit: ±20°C; the heating rate: 10-80°C / h; the slow cooling pit is heated by gas: mixed coal gas, calorific value: ~9.6MJ / Nm 3 ;

[0014] The diameter of the large-size continuous casting round billet is 800-1200 mm, preferably 1000-1200 mm; the steel type 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 to above 650°C, the heat dissipation of the slow cooling pit increases, and the higher the temperature, the slower the heating rate. If the heating rate exceeds 75°C / h, the heating efficiency decreases and the energy consumption increases accordingly. At the same time, when the heating rate exceeds 75°C / h, for alloy steel round billets above 1000mm, the thermal stress is large, and there is a certain tensile stress in the core. If there are defects such as cracks and shrinkage holes in the core, it may cause the defect to expand; if the heating rate is less than 50°C / h, the heating time is long and the production efficiency is low. Therefore, the present invention is controlled at 50-75°C / h.

[0016] The temperature is raised to 600-750° C. and then kept warm. The holding time is calculated based on the diameter size and is 1-2.5 h / 100 mm. For example, if the diameter is 800 mm, the holding time is 8×(1-2.5) h.

[0017] The cooling means cooling with the furnace after turning off the heating; or controlling the cooling at 30-60℃ / h; further, cooling to below 350℃ before taking out of the furnace. After isothermal treatment, the structure of the steel has been transformed into an equilibrium structure, and the phase change stress has been eliminated. However, due to the large size of the round billet, the cooling speed is too fast, the cooling stress is large, and the final tensile stress in the core is also large, and there is a certain time-effect. For example, there are certain cracks and loose defects in the core. After the round billet is placed for a certain period of time, the stress is released, resulting in the expansion of defects and the formation of more serious defects.

[0018] The large-size continuous casting round billet can realize batch isothermal annealing of large-size continuous casting round billets of 450 tons to 500 tons by utilizing the residual heat isothermal annealing process.

[0019] During the casting process of the round billet, due to the different cooling rates of various parts, the temperature difference between the inside and outside will form thermal stress; phase change will cause volume changes, and due to the unequal cooling, tissue stress will be generated. The superposition of thermal stress and tissue stress constitutes the internal stress of the round billet, which generally increases with the increase of the alloy element content in the steel and the mass of the round billet. For some air-quenched martensitic steels, such as high-chromium steel and die steel, even if they are slowly cooled after casting, if they are not annealed in time after cooling to eliminate the internal stress, there is a risk of self-cracking or even explosion during storage. Generally, the round billet will be sawed into sections later, so it is necessary to reduce the surface hardness of the round billet for sawing. In short, most round billets need to be annealed to eliminate internal stress, prevent cracking, and reduce hardness. The traditional annealing processes include: ordinary annealing or two-stage annealing with heating slightly above Ac3 and Ac1~Acm, and low-temperature annealing with no phase change and recrystallization slightly below Ac1 to achieve the purpose of stress relief and softening. The disadvantages are: 1. The heating temperature is high, the time is long, the energy consumption is large, and the internal quality of the round billet is easily damaged. The round billet generally undergoes cooling, phase change, heating, and phase change again. Especially in the cooling process, due to the superposition of phase change stress and cooling stress, the round billet, especially the round billet above 800mm and the core of medium and high alloy steel, may have defects or cause the deterioration of the original defects. If it cannot be annealed in time, it will cause the defects of the round billet to worsen or even crack or explode. For steels with slightly poor thermal conductivity, if the heating speed is slightly faster before 650℃, it will also cause the round billet to break directly; for general steels, if heated between 700 and 850℃, it is the initial formation area of ​​austenite, which is a large amount of heat absorption process. The heating speed in this interval is fast, the heating efficiency is low, and the energy consumption is large. 2. The phase change process takes a relatively long time and is inefficient. When cooling from the holding temperature to about 500°C, the precipitation process of proeutectoid ferrite or carbide and the decomposition of supercooled austenite into pearlite will occur, and slow cooling is required; or in the low-temperature annealing process slightly below Ac1 where no phase transformation and recrystallization occurs, the internal stress of the round billet will be relaxed as the carbides grow in focus and the ferrite recovers and recrystallizes, but the transformation time is relatively longer and the hardness is not easy to reduce.

[0020] The inventors found that the above-mentioned drawbacks can be eliminated by isothermal annealing of round billets with residual heat. The main reasons are: the heating temperature of the present invention is low, the phase transformation time is short, and the annealing efficiency is high. When the round billet is at 500-650°C, except for a small amount of transformation into the proeutectoid ferrite + pearlite softening phase, most of it remains in the austenite state. At this time, the steel is heated to the nose temperature point of the isothermal transformation curve-TTT curve (the pearlite transformation time is the shortest) isothermally. Depending on the nose temperature point of each steel type, the heating temperature is generally uniform within a specific temperature range of 600-750°C, and the pearlite transformation is completed within a few minutes to 1-2 hours, which is much lower than the several hours of conventional annealing process. At the same time, in this temperature range, for 800-1200mm round billets, the heating rate of 50-75°C / h is highly efficient and has low energy consumption. If the heating efficiency and energy consumption are not considered, it can be heated quickly, and the round billet will not be broken due to the influence of heating stress. The core of the round billet of the present invention has good quality. Compared with conventional annealing, the round billet only undergoes isothermal phase transformation softening to eliminate phase transformation stress, and then controls the cooling rate to 30-60℃ / h, and takes out of the furnace at 350℃, which can effectively eliminate the influence of thermal stress, thereby minimizing the generation of internal stress and reducing hardness, thereby ensuring the quality of the round billet, especially the core quality. The present invention utilizes on-site conditions and configures related facilities, fully considers production efficiency and energy conservation, and can ensure that most or even all of the round billet is in a single austenite without reheating during the round billet production process, reducing the phase transformation process, and then heating to a slightly lower temperature isothermally, and quickly transforming into a pearlite structure (except for very few high-Ni steels, etc.), so that the phase transformation stress can be completely eliminated and the hardness can be reduced at the same time.

[0021] The prior art does not fully utilize the residual heat of the ingot for annealing, while the present invention utilizes the residual heat of the ingot for isothermal annealing, which has high production efficiency, relatively good core quality, and no transverse fracture. The present invention provides an efficient and energy-saving production annealing process, which anneals large-scale continuous casting round billets with a diameter of Φ800-1200mm to eliminate the internal stress of the round billet, reduce the hardness, prevent internal defects and defect expansion, and facilitate the cutting, forging and heat treatment of the next process, while improving production efficiency, product quality and energy saving. The present invention utilizes the residual heat of large-scale round billets to perform batch isothermal annealing of more than 450 tons, and the annealing efficiency is increased by more than 25%, taking into account cold billet annealing; the present invention uses a lower annealing temperature and a shorter time to anneal high-alloy steel, which reduces energy consumption by 30%, while avoiding the risk of core defect expansion and round billet heating cross-section.

[0022] Compared with the prior art, the present invention utilizes the residual heat of the ingot for isothermal annealing, which greatly reduces energy consumption and improves annealing efficiency; utilizes the residual heat of the ingot and a slow cooling pit with a heating function to design annealing process parameters; utilizes the residual heat of the ingot for annealing to improve the surface and core quality of the round ingot and prevent the occurrence of overall transverse fracture of the round ingot; is compatible with the trolley furnace annealing process, and relative to the trolley furnace design, the floor space of the entire equipment is reduced by more than 50%, saving on-site land. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is the residual heat annealing process curve of Φ800mm round billet GCr15SiMnMo;

[0024] Figure 2 This is the residual heat annealing process curve of Φ1000mm round billet P91;

[0025] Figure 3 This is the residual heat annealing process curve of Φ1000mm round billet P92;

[0026] Figure 4 This is the residual heat annealing process curve of Φ1200mm round billet 42CrMo;

[0027] Figure 5 It is Φ800mmGCr15SiMnMo residual heat annealing low multiple;

[0028] Figure 6 It is Φ800mmGCr15SiMnMo cold billet annealing low multiple;

[0029] Figure 7 It is the low multiple of Φ1000mmP91 after residual heat annealing;

[0030] Figure 8 It is the low multiple of Φ1000mmP91 cold billet after annealing;

[0031] Fig. 9 It is Φ1000mmP92 residual heat annealing low multiple (after drilling sampling);

[0032] Fig.10 It is the low multiple of Φ1000mmP92 cold billet after annealing;

[0033] Fig.11 It is Φ1200mm 42CrMo residual heat annealing low multiple;

[0034] Fig.12 Φ1200mm 42CrMo cold billet annealing low multiple;

[0035] Fig.13 Transverse cracking and fracture of Φ800mm GCr15SiMnMo cold billet during annealing. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in combination with the embodiments of the present invention. Obviously, the described embodiments are 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 creative work are within the scope of protection of the present invention.

[0037] If no specific techniques or conditions are specified in the examples, they can be carried out according to the techniques or conditions described in the literature in the art or according to the product instructions.

[0038] The present invention provides a large-size continuous casting round billet isothermal annealing process using residual heat, including the following processes:

[0039] 1) Slow cooling pit configuration (according to the conventional slow cooling pit with heating function):

[0040] Slow cooling pit location: within 20 to 50 meters from the continuous casting billet cooling bed; lifting time 5 to 10 minutes;

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

[0042] Pit structure: full fiber lining structure insulation;

[0043] The slow cooling pit is equipped with a pit cover, which adopts a steel frame structure with clamps and disk hanging points, and contains insulation materials;

[0044] Heating control: The burners are arranged alternately in the lower part of the furnace and are 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: ~9.6MJ / Nm 3 ;

[0048] Stacking in the pit: Place shims and blanks according to the spacing of different specifications of hoists, with a spacing of 600 to 1200mm.

[0049] Charging tonnage: ~500t; height not exceeding the furnace smoke outlet.

[0050] 2) Annealing process:

[0051] 2-1) Place large-sized continuous casting round billets in the heating and slow cooling pit in a tic-tac-toe or uniform arrangement;

[0052] 2-2) The pit temperature for large-size continuous casting round billets is 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 following billets entering the pit. In order to ensure that all round billets reach the same temperature, the slow cooling pit needs to be heated. After ensuring that all billets reach the same temperature of 500-650℃, they are heated up to 600-750℃ together.

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

[0054] 2-4) Holding temperature and time: refer to the TTT curve (isothermal transformation curve of supercooled austenite) of the steel grade, set the holding temperature, the holding temperature is ±10-30℃ of the nose temperature of the TTT curve as the holding 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. When the temperature drops below 350℃, remove the lid and take the product out of the furnace.

[0056] The present invention adopts the above method to quickly anneal large-size round billets with a diameter of Φ800-1200mm. Mainly using the residual heat of the billet, most of the structure remains in the austenite state, and adopts a 600-750℃ isothermal transformation process. Compared with the conventional annealing process, the annealing temperature is reduced by 50-200℃, the annealing cycle is shortened by 25%, and the energy consumption is reduced by 30%. It can significantly improve the surface core quality of medium and high alloy steels, and is compatible with the trolley furnace annealing process.

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

[0058] Example 1

[0059] A large-size continuous casting round billet is annealed using a residual heat isothermal annealing process and the above-mentioned slow cooling pit configuration, specifically a Φ800mm round billet GCr15SiMnMo residual heat annealing, the pit entry temperature is 500-550°C, and the chemical composition content 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%, and the rest is Fe and unavoidable impurities.

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

[0061] 1) Loading: GCr15SiMnMoΦ800mm round billets are hoisted into a slow cooling pit with heating function by clamps, and placed in a straight line to ensure that the pit temperature is 500-550℃;

[0062] 2) Ignition: After loading, ignite the slow cooling pit, and keep the temperature in the pit at 500-540℃ for 120 minutes;

[0063] 3) Heating: After the isothermal treatment, the temperature is raised to 680-720°C at 50-65°C / h;

[0064] 4) Insulation: Insulation 1020min;

[0065] 5) Cooling: After the insulation is completed, turn off the furnace and cool it to 450℃, then open the lid, cool it to below 350℃, and then take it out of the pit. The annealing cycle is 32h.

[0066] Example 1 The round billet after annealing was tested, and the low-power detection was as follows Figure 5 The hardness results are shown in Table 1.

[0067] Table 1 Hardness comparison of Φ800mm GCr15SiMnMo before and after preheating annealing

[0068] Measurement location 1 / 4r 1 / 2r center 1 / 2r 1 / 4r average Remark Hardness(HBW) 387 393 370 388 392 386 Before annealing Hardness(HBW) 236 239 232 231 237 235 After annealing

[0069] As can be seen from Table 1, the hardness decreases significantly after annealing. The average hardness after annealing is 235HB, which is suitable for subsequent sawing and cutting. If the hardness is greater than 270HB, subsequent sawing will be very difficult.

[0070] Comparative Example 1: The same casting as Example 1 was annealed by cold slab, but there was no heating and slow cooling pit device on site. The annealing furnace was used for annealing. The specific annealing process was as follows:

[0071] 1) Loading: Lay out the billets in a line in the furnace, and separate the layers with iron pads; Billet temperature in the furnace: 200℃;

[0072] 2) Ignition and heating: After loading, ignite, heat to 500-525℃ at 25-35℃ / h, and keep isothermal for 90 minutes;

[0073] 3) Heating: After isothermal treatment, heat to 700-740°C at 40°C / h;

[0074] 4) Insulation: 700-740℃ for 1440min;

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

[0076] 40% of the round billets have transverse cracks and fractures, such as Fig.13 . Take a low-power horizontal observation of the uncracked area. Figure 6 , there are core cracks, hot billet annealing does not show cracking or breakage, and there are no core cracks at low magnification.

[0077] Example 2

[0078] A large-size continuous casting round billet is annealed using a residual heat isothermal annealing process, specifically a Φ1000mm round billet P91 residual heat annealing, a billet entering pit temperature of 550±50°C, and a chemical composition content of the round billet of: 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, and the rest is Fe and unavoidable impurities.

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

[0080] 1) Loading: Use clamps to lift into the pit, place in a straight line, and ensure that the temperature entering the pit is: 550±50℃;

[0081] 2) Ignition: After loading, ignite and keep isothermal at 550±20℃ for 120min;

[0082] 3) Heating: After isothermal treatment, heat to 710-750°C at 50-75°C / h;

[0083] 4) Insulation: 710-750℃ for 1200min;

[0084] 5) Cooling: After the insulation is completed, turn off the furnace and cool it to 450℃, remove the cover, and then cool it to below 350℃, take it out of the pit, and the annealing cycle is 57h.

[0085] Example 2 The round billet after annealing was tested, and the low-power detection was as follows Figure 7 The hardness results are shown in Table 2.

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

[0087] Measurement location 1 / 4r 1 / 2r center 1 / 2r 1 / 4r average Remark 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 decreases significantly after annealing, and the average hardness after annealing is 203HB, which is suitable for subsequent sawing and cutting.

[0089] Comparative Example 2: The same casting as in Example 2 is subjected to cold annealing. If there is no heating and slow cooling pit device on site, the specific annealing process is as follows:

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

[0091] 2) Ignition: After loading, ignite;

[0092] 3) Heating: 25-35°C / h to 610-650°C;

[0093] 4) Insulation: 600-650℃ for 1000min;

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

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

[0096] 7) Cooling: After the insulation is completed, turn off the furnace to cool to 350℃, remove from the pit, and the annealing cycle is 80h.

[0097] The average hardness test is 207HB, the annealing cycle is 80h, and the low magnification test is as follows Figure 8 Compared with the process without the use of the residual heat annealing process, the production efficiency of the residual heat annealing process is increased by 28%, the energy consumption is reduced by more than 35%, and the core defect area is reduced by 3 times.

[0098] Example 3

[0099] A large-size continuous casting round billet is annealed using residual heat isothermal annealing process, specifically Φ1000mm round billet P92 residual heat annealing, billet pit temperature: 550±50℃, round billet chemical composition content 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, the rest are Fe and unavoidable impurities.

[0101] The annealing process includes the following steps:

[0102] 1) Loading: Use clamps to lift into the pit, place in a straight line, and ensure that the temperature entering the pit is: 550±50℃;

[0103] 2) Ignition: After loading, ignite and keep isothermal at 550±20℃ for 120min;

[0104] 3) Heating: After the isothermal treatment, the temperature is raised to 700-740°C at 50-75°C / h;

[0105] 4) Insulation: 700-740℃ for 1500min;

[0106] 5) Cooling: After the insulation is completed, turn off the furnace and cool it down. When it cools down to below 400℃, remove the lid and when it cools down to below 350℃, remove the furnace. The annealing cycle is 60h.

[0107] Example 3 The round billet after annealing was tested, and the low magnification test was as follows Fig. 9 The hardness results are shown in Table 3.

[0108] Table 3 Hardness comparison of Φ1000mm P92 before and after residual heat annealing

[0109] Measurement location 1 / 4r 1 / 2r center 1 / 2r 1 / 4r average Remark Hardness(HBW) 295 312 287 320 300 303 Before annealing Hardness(HBW) 212 215 205 210 215 211 After annealing

[0110] As can be seen from Table 3, the hardness decreases significantly after annealing, and the average hardness after annealing is 211HB, which is suitable for subsequent sawing and cutting.

[0111] Comparative Example 3: The same casting as in Example 3 is subjected to cold annealing. If there is no heating and slow cooling pit device on site, the specific annealing process is as follows:

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

[0113] 2) Ignition: After loading, ignite;

[0114] 3) Heating: 25-35°C / h to 600-640°C;

[0115] 4) Insulation: 600-640℃ for 1200min;

[0116] 5) Heating: 50-60°C / h to 800-840°C;

[0117] 6) Insulation: 810-840℃ for 3000min;

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

[0119] 8) Insulation: 600-620℃ for 1000min;

[0120] 7) Cooling: After the insulation is completed, turn off the furnace to cool to 350℃, remove from the pit, and the annealing cycle is 120h.

[0121] Its low magnification detection Fig.10 Compared with the process without the use of the residual heat annealing process, the production efficiency is increased by 50%, the energy consumption is reduced by more than 45%, and the core defect area is reduced by 5 times.

[0122] Example 4

[0123] A large-size continuous casting round billet is annealed using a waste heat isothermal annealing process, specifically: Φ1200mm round billet 42CrMo waste heat annealing, billet pit entry temperature: 600±50℃, round billet chemical composition content 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 rest is Fe and unavoidable impurities.

[0124] Specific heat treatment method:

[0125] 1) Loading: Use clamps to lift into the pit and place in a straight line. The pit temperature is 600±50℃;

[0126] 2) Ignition: After loading, ignite and keep isothermal at 600±20℃ for 180min;

[0127] 3) Heating: After isothermal treatment, heat to 670-710°C at 50-75°C / h;

[0128] 4) Insulation: After heating, keep at 670-710℃ for 1200min;

[0129] 5) Cooling: After the insulation is completed, turn off the furnace and cool it down to below 450℃, then open the cover, cool it down to below 350℃, and then take it out of the pit. The annealing cycle is 40h.

[0130] Example 4 The round billet after annealing was tested, and the low magnification test was as follows Fig.11 The hardness results are shown in Table 4.

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

[0132] Measurement location 1 / 4r 1 / 2r center 1 / 2r 1 / 4r average Remark 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, and the average hardness after annealing is 203HB, which is suitable for subsequent sawing and cutting.

[0134] Comparative Example 3: The same casting as in Example 3 is subjected to cold annealing. If there is no heating and slow cooling pit device on site, the specific annealing process is as follows:

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

[0136] 4) Ignition: After loading, ignite;

[0137] 3) Heating: 25-30°C / h to 660-700°C;

[0138] 4) Insulation: 660-700℃ for 1800min;

[0139] 7) Cooling: After the insulation is completed, turn off the furnace to cool to 350℃, remove from the pit, and the annealing cycle is 55h.

[0140] The average hardness test is 192HB, and the low-power test is Fig.12 .

[0141] Compared with the process without the use of the residual heat annealing process, the production efficiency of the residual heat annealing process is increased by 44%, the energy consumption is reduced by more than 35%, and no defects are found under low-power observation.

[0142] The description of the above embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and 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 present invention should be within the protection scope of the present invention.

Claims

1. A large-scale continuous casting round billet isothermal annealing process using residual heat, characterized in that: The isothermal annealing process of the large-size continuous casting round billet using residual heat is specifically as follows: The hot large-sized continuous casting round billet is placed in a slow cooling pit, heated to 600-750℃, and kept warm; after cooling, it is taken out of the furnace.

2. The isothermal annealing process for continuous casting round billets using residual heat according to claim 1 is characterized in that: The hot large-size continuous casting round billet refers to a large-size continuous casting round billet having a temperature of 500-650° C. when the round billet is placed in the slow cooling pit.

3. The isothermal annealing process for continuous casting round billets using residual heat according to claim 1 is characterized in that: The slow cooling pit is within 20 to 50 meters from the continuous casting billet cooling bed; the lifting time is 5 to 10 minutes.

4. The isothermal annealing process for continuous casting round billets using residual heat according to claim 1 is characterized in that: The furnace temperature uniformity of the slow cooling pit is: ±20°C; the heating rate is: 10-80°C / h.

5. The isothermal annealing process for standard continuous casting round billets using residual heat according to claim 1 is characterized in that: The diameter of the large-size continuous casting round billet is 800-1200 mm.

6. The isothermal annealing process for standard continuous casting round billets using residual heat according to claim 1 or 5, characterized in that: The diameter of the large-size continuous casting round billet is 1000-1200 mm.

7. The isothermal annealing process for standard continuous casting round billets using residual heat according to claim 1 or 5, characterized in that: The heating rate to 600-750° C. is 50-75° C. / h.

8. The isothermal annealing process for standard continuous casting round billets using residual heat according to claim 1 or 7, characterized in that: The holding time is calculated based on the diameter of the ingot, and the time is 1 to 2.5 h / 100 mm.

9. The isothermal annealing process for standard continuous casting round billets using residual heat according to claim 1, characterized in that: The cooling is controlled at 30-60°C / h.

10. The isothermal annealing process of standard continuous casting round billet using residual heat according to claim 1 or 9, characterized in that: Cool to below 350℃ and then take out of the furnace.

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