A steel post-rolling slow cooling device and a slow cooling method for medium and low carbon CrMo series steel grades

Abstract

The present invention discloses a post-rolling slow cooling device for steel and a slow cooling method for medium-low carbon CrMo series steels, comprising channel side plates, including side plates A and B; a cooling bed; an upper insulation cover, which covers the cooling bed; a driving mechanism A, which is used to drive the upper insulation cover to slide up and down along the side plates A and B and stop sliding at a certain height; a lower insulation bottom, which is located below the bed, has one end hinged with the channel side plates, and the other end is a free end; a driving mechanism B, which drives the lower insulation bottom to rotate around the hinge with the channel side plates and stop at a certain angle. The slow cooling device has a simple structure, is easy to install, does not affect the original rolling equipment, and has a small investment. The slow cooling device and method of the present invention can effectively control the hardness of medium-low carbon CrMo series steels to be below 230HB, and the hardness fully meets the requirements of downstream users, and the microstructure is mainly ferrite and pearlite.

Description

Technical Field

[0001] The invention relates to the field of metal pressure processing, and relates to a method for controlling the hardness of medium- and low-carbon CrMo series steels by slow cooling, and specifically relates to a steel post-rolling slow cooling device and a medium- and low-carbon CrMo series steel slow cooling method. Background Art

[0002] Medium and low carbon CrMo series steels have the advantages of good hardenability, high strength, good plasticity and toughness, making them the most important type of steel in automotive parts manufacturing and engineering machinery manufacturing, such as gears, bolts, crankshafts, steering knuckles, hydraulic rods, etc.

[0003] With the rapid development of the machinery industry, more and more downstream parts manufacturers of steel enterprises have higher and higher automation levels. These enterprises have begun to adopt more efficient automatic shearing and cutting. This cutting method requires the hardness of the raw materials to be lower than 260HB. However, the hardness of most of the medium and low carbon CrMo series steels produced by normal processes or methods is above 260HB, especially some smaller steels. At present, domestic and foreign steel enterprises mainly adopt two ways to reduce the hardness of medium and low carbon CrMo series steels: (1) The steel is annealed to reduce the hardness. This method is also the most direct method among the current methods, but this method increases the production cost of the steel. Since the curvature of the steel cannot be guaranteed after annealing, the steel must be straightened after annealing. However, if the straightening is not properly controlled, the iron oxide scale produced by annealing is easily pressed into the steel surface, resulting in pits and pitting defects on the steel surface; (2) The second method is to achieve it by combining controlled rolling and controlled cooling, such as patent publication number CN106424158A "A rolling method for reducing the hot-rolled hardness of 42CrMo steel" and patent publication number CN106216391A "A rolling production method for reducing the hardness of 42CrMo". This method requires additional controlled rolling and controlled cooling equipment, which brings considerable construction costs to many small and medium-sized steel mills. At the same time, many steel mills did not reserve space for the construction of controlled rolling and controlled cooling devices in the early construction, so controlled rolling and controlled cooling devices cannot be implemented in such steel mills. Moreover, this method requires the intermediate billet and steel to undergo multiple water cooling to achieve a combination of controlled rolling and controlled cooling, which also makes this method only applicable to specifications ≤Φ50mm. For steels with specifications >Φ50mm, there is a temperature difference between the surface and the interior after water cooling. The larger the specification, the greater the temperature difference, resulting in a large difference in hardness at the surface, half radius and core. (3) Through the high-temperature cooling bed, timely slow cooling in the pit under high temperature, the hardness can be reduced. However, the curvature control of the medium and low carbon CrMo series steels produced by this method is poor, and they must also be straightened in the later stage. In the production process, the cooling bed fails and the steel cannot be slow cooled in the pit at high temperature, so that the hardness is well controlled. At the same time, this method is difficult to control the hardness of small-sized steel. For example, the document "Process Improvement for Reducing the Hot-Rolled Hardness of 42CrMo Steel" (Wang Yufeng, Zheng Guoyu, Yan Yan, et al. Process Improvement for Reducing the Hot-Rolled Hardness of 42CrMo Steel [J]. Hebei Metallurgy, 2016, 6 (14): 45-46), patent publication number CN106435133A "A Method for Reducing the Brinell Hardness of CrMo Series Hot-Rolled Bars", this method can make full use of the existing slow cooling pit, but this method has a difference in hardness between the two places of steel close to the slow cooling pit wall and far away from the slow cooling pit wall. Summary of the invention

[0004] The present invention provides a relatively comprehensive, economical and practical slow cooling device and method for controlling the hardness of medium-low carbon CrMo series steel grades. The slow cooling device has a simple structure, is easy to install, does not affect the original rolling equipment, and has a small investment. The slow cooling device and method of the present invention can effectively control the hardness of medium-low carbon CrMo series steel grades to below 230HB, and the hardness fully meets the requirements of downstream users, and the microstructure is mainly ferrite and pearlite.

[0005] The technical solution disclosed in the present invention is as follows: a device for slow cooling of steel after rolling, comprising:

[0006] The channel side plate comprises a side plate A and a side plate B, wherein the side plate A and the side plate B are spaced apart and fixed on the carrier to form a channel;

[0007] A cooling bed, comprising a bed body and columns supporting the bed body, and the cooling bed is placed in the passage;

[0008] An upper heat-insulating cover is covered on the cooling bed and two sides of the upper heat-insulating cover are slidably matched with the corresponding side panels A and B;

[0009] A driving mechanism A is used to drive the upper heat-insulating cover to slide up and down along the side panels A and B and stop sliding at a certain height;

[0010] The lower insulation bottom is located below the bed, one end of which is hinged to the channel side plate and the other end is a free end;

[0011] The driving mechanism B drives the lower heat-insulating bottom to rotate around the hinge between the lower heat-insulating bottom and the channel side plate and stop at a certain angle.

[0012] On the basis of the above solution, preferably, the upper insulation cover is an arch structure.

[0013] On the basis of the above scheme, as a preferred embodiment, the upper insulation cover is composed of a three-layer structure, including an outer layer, a middle layer, and an inner layer, the outer layer and the inner layer are aluminum silicate fiberboard insulation layers, and the middle layer is a vacuum layer.

[0014] Based on the above solution, preferably, the inner wall of the upper insulation cover has a plurality of protrusions or grooves.

[0015] On the basis of the above solution, as a preferred embodiment, a linear guide is fixedly installed on the inner wall of the upper insulation cover, and a slider of the linear guide is fixedly connected to the temperature measuring gun.

[0016] On the basis of the above scheme, as a preference, the driving mechanism A is a hydraulic cylinder, the cylinder body of the hydraulic cylinder is fixed on the crossbeam above the upper insulation cover, and the piston rod of the hydraulic cylinder is fixed on the upper insulation cover.

[0017] On the basis of the above scheme, as a preferred embodiment, the lower insulation bottom is a whole plate, one end of the whole plate is hinged to the side plate A or the side plate B, the other end is a free end and the lower end surface of the end is hinged to one end of the oil cylinder, the other end of the oil cylinder is hinged to the corresponding side plate A or the side plate B, the whole plate is provided with strip holes with the same number as the rows of columns, the strip holes are connected at the free end, and each row of columns is located in a corresponding strip hole, or

[0018] There are several plates on the lower insulation bottom, one plate is between two adjacent rows of columns, one end of the plate is hinged to side plate A or side plate B, and the other end is a free end, or there are two plates between two adjacent rows of columns, one end of the two plates are respectively hinged to side plate A and side plate B, and the other end is a free end. The two plates are arranged in parallel, and the free end of each plate is hinged to the corresponding side plate A or side plate B through a cylinder.

[0019] On the basis of the above solution, as a preferred embodiment, the inner surface of the upper heat-insulating cover is an arc-shaped concave surface, the arc angle is 95°, and the arc height is 10 mm.

[0020] The slow cooling method of medium and low carbon CrMo series steel grades is carried out by using the steel post-rolling slow cooling device.

[0021] The steel is placed on a cooling bed for slow cooling, and the temperature of the steel is controlled at 790-880℃ when placed on the cooling bed;

[0022] The steel moves forward on the cooling bed during the stepping process and turns evenly during the stepping process to ensure that the steel dissipates heat evenly on the cooling bed;

[0023] The temperature measuring gun is located above the steel to detect the temperature of the steel. The linear guide rail controls the movement of the temperature measuring gun so that it moves synchronously with the steel.

[0024] The hydraulic cylinder drives the upper insulation cover to move up and down to ensure that the temperature of the steel entering the slow cooling device measured by the temperature measuring gun is controlled at 710-800℃;

[0025] In the slow cooling device, the temperature is 600-800℃, and the cooling rate is controlled at 10-14℃ / min; the temperature is 400-600℃, and the cooling rate is controlled at 5-9℃ / min; the temperature is 260-400℃, and the cooling rate is controlled at 15-20℃ / min. The temperature of the lower cooling bed is 200-260℃. After the steel comes down from the cooling bed, it is put into the pit for slow cooling. The slow cooling time is controlled to more than 24 hours.

[0026] Medium and low carbon CrMo series steels are composed of the following elements in mass percentage: C 0.08~0.54%, Si 0.15~0.40%, Mn 0.40~0.90%, Cr0.40~1.30%, Cu≤0.20%, Ni≤0.20%, Mo0.15~0.60%, V ≤0.01%, Nb≤0.008%, As≤0.01%, Al 0.010~0.040%, P≤0.025%, S≤0.025%, B≤0.0005%, and the balance is Fe and unavoidable impurity elements.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] First, the slow cooling device of the present invention has a simple structure and few auxiliary equipment, and does not require major dismantling and reconstruction of the original equipment. As long as the steel plant has a cooling bed, the slow cooling device and method of the present invention can be put into use. Therefore, the present invention is applicable to most existing steel plants.

[0029] Second, the production method of controlling the hot-rolled hardness of medium and low carbon CrMo series steels of the present invention innovatively selects a cooling bed as a method for controlling the hot-rolled hardness of medium and low carbon CrMo series steels, utilizes the heat of the steel itself, controls the cooling rate of the steel at a specific temperature stage of the cooling bed, and enables the steel to achieve a combination of simulated high-temperature annealing and low-temperature annealing on the cooling bed, thereby achieving a method for reducing the hardness of medium and low carbon CrMo series steels.

[0030] Third, the present invention is applicable to a wide range of steel specifications, and the cross-sectional hardness of the produced medium and low carbon CrMo series steels is relatively uniform.

[0031] Fourthly, the hot-rolled hardness of the medium-low carbon CrMo series steel produced by the present invention can be stably controlled within 230HB, and the microstructure is mainly ferrite and pearlite. Moreover, according to user requirements, the specific control of the steel hardness can be achieved by controlling the heat preservation cover.

[0032] Fifth, on the basis of the present invention, according to the requirements of the thermal insulation effect, the requirements of ultra-low cooling rate and long time slow cooling can be achieved by increasing the number of thermal insulation layers in the slow cooling device of the present invention and constructing the slow cooling devices of the present invention side by side. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is a schematic diagram of the structure of the slow cooling device.

[0034] In the figure, 1-beam, 2-temperature measuring gun, 3-upper insulation cover, 4-lower insulation bottom, 5-oil cylinder, 6-inner wall of upper insulation cover, 7-round steel, 8-cooling bed, 9-wall, 10-hydraulic cylinder, 11-inner layer, 12-middle layer, 13-outer layer, 14-lifting hook, used to lift the upper insulation cover.

[0035] Figure 2 It is a partial arc-shaped structure of the inner wall of the upper insulation cover.

[0036] Figure 3 This is the 100-fold microstructure of 42CrMo hot-rolled state before the implementation of this patent in Example 1.

[0037] Figure 4 This is the 100-fold microstructure of 42CrMo hot-rolled state after the implementation of this patent in Example 1.

[0038] Figure 5 This is the 100-fold microstructure of 42CrMo hot-rolled state before the implementation of this patent in Example 2.

[0039] Figure 6 This is the 100-fold microstructure of 42CrMo hot-rolled state after the implementation of this patent in Example 2.

[0040] Figure 7 This is the 100-fold microstructure of 42CrMo hot-rolled state before the implementation of this patent in Example 3.

[0041] Figure 8 This is the 100-fold microstructure of 42CrMo hot-rolled state after the implementation of this patent in Example 3.

[0042] Fig. 9 It is a schematic diagram of the coordination between the cooling bed and the plate. DETAILED DESCRIPTION

[0043] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the specific implementation methods of the present invention will be described below with reference to the accompanying drawings. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings and other implementation methods can be obtained based on these drawings without creative work.

[0044] Take the production of medium carbon CrMo steel 42CrMo hot rolled round steel as an example:

[0045] The chemical composition of 42CrMo is shown in Table 1.

[0046] Table 1 Main chemical components (wt%)

[0047]

[0048] The rolling process is as follows: walking beam heating furnace - high pressure water dephosphorization - rough rolling unit - medium rolling unit - finishing rolling unit - double shear - hot sawing and cutting to length - cooling bed - post-rolling slow cooling device - steel collection - pit slow cooling Example 1

[0049] 1) Φ380mm billet heating

[0050] The chemical composition of the ingot is shown in Table 2.

[0051] Table 2 Main chemical components (wt%)

[0052]

[0053] The casting is heated in a walking beam heating furnace, which is divided into four sections: a preheating section, a heating section, a second heating section and a soaking section, wherein the preheating section has no combustion nozzle; the temperature of the preheating section is 810-850°C, the temperature of the heating section is 1000-1053°C, the temperature of the heating section is 1235-1258°C, and the temperature of the soaking section is 1240-1270°C;

[0054] 2) Steel rolling

[0055] The ingot is rolled in the rolling lines of the rough rolling unit, the intermediate rolling unit and the finishing rolling unit. The starting rolling temperature is 1155°C and the final rolling temperature is 1003°C. The steel shall not be cooled by water during and after the rolling process.

[0056] 3) Cooling bed cooling

[0057] Slow cooling device for steel after rolling, such as Figure 1-2 As shown, it includes channel side panels, including side panel A and side panel B. Side panel A and side panel B are spaced apart and fixed on a carrier to form a channel. Side panel A and side panel B can be walls, and the carrier is the ground.

[0058] The cooling bed comprises a bed body and columns supporting the bed body. The cooling bed is placed in the passage, and the columns are supported on the ground.

[0059] The upper insulation cover is covered on the cooling bed and the two sides of the upper insulation cover are slidably matched with the corresponding side panels A and B; the upper insulation cover is an arch structure and consists of a three-layer structure, including an outer layer, a middle layer, and an inner layer. The outer layer and the inner layer are several layers of aluminum silicate fiberboard insulation layers bonded by high-temperature resistant sealant with a thickness of 150mm. The middle layer is a vacuum layer with a thickness of 70mm. The inner surface of the upper insulation cover is an arc-shaped concave surface (can also be convex), the arc angle is 95°, and the arc height is 10mm.

[0060] The driving mechanism A is used to drive the upper insulation cover to slide up and down along the side panels A and B and stop sliding at a certain height; specifically, it is controlled by the hydraulic cylinder system on both sides (driving mechanism A), the cylinder body of the hydraulic cylinder is fixed on the beam above the upper insulation cover, and the piston rod of the hydraulic cylinder is fixed on the upper insulation cover, thereby controlling the up and down movement of the upper insulation cover.

[0061] The lower insulation bottom is located below the bed, one end of which is hinged to the channel side plate and the other end is free. There are several forms of the lower insulation bottom, such as:

[0062] 1. The lower insulation bottom is a whole plate, one end of the whole plate is hinged to the side plate A or side plate B, and the other end is a free end and the lower end surface of the end is hinged to one end of the oil cylinder, and the other end of the oil cylinder is hinged to the corresponding side plate A or side plate B. The whole plate is provided with strip holes with the same number as the rows of columns. The strip holes are connected at the free end, and each row of columns is located in a corresponding strip hole;

[0063] Second, the lower insulation bottom has several boards, there is a board between two adjacent rows of columns, one end of the board is hinged to the side board A or side board B, and the other end is a free end, or there are two boards between two adjacent rows of columns 400 Fig. 9 As shown, one end of the two plates 300 is hinged to the side plate A100 and the side plate B200 respectively, and the other end is a free end. The two plates are arranged in parallel, and the free end of each plate is hinged to the corresponding side plate A or side plate B through a cylinder.

[0064] The oil cylinder is a driving mechanism B, which is used to drive the lower thermal insulation bottom to rotate around the hinge between it and the channel side plate and stop at a certain angle.

[0065] A linear guide is fixedly installed on the inner wall of the upper insulation cover, and the slider of the linear guide is fixedly connected to the temperature gun. It is used to measure the temperature of a certain steel. The linear guide, such as a screw linear guide, drives the temperature gun to move forward and backward. The temperature measuring range of the temperature gun is 500℃~900℃. The temperature gun moves to track the temperature of a specific steel entering and exiting the slow cooling device. After the temperature gun touches the receiver at the end of the slow cooling device, the real-time tracking of the temperature of a steel entering and exiting the slow cooling device is completed. Then the temperature gun will automatically return to the starting position through the linear guide to prepare for the next cycle of temperature measurement. At the same time, the measured temperature of the steel entering and exiting the insulation device is transmitted to the PLC. The PLC system calculates the temperature drop rate of the steel in the insulation device, and the calculated temperature drop rate is compared with the set temperature drop rate. According to the comparison result, the upper insulation cover is controlled to move up and down. If the temperature drop rate is higher than the set value, the upper insulation cover will move downward, otherwise, the upper insulation cover will move upward. Each time the upper insulation cover descends or rises, the height is controlled to be 10mm;

[0066] The stepping rate of the cooling bed is controlled by the temperature drop rate of the steel to ensure that the temperature of the steel leaving the insulation device meets the process requirements.

[0067] After the steel is sawn to length, it is placed on a cooling bed for slow cooling. The temperature of the upper cooling bed is controlled at 790-880°C (the steel is located on the cooling bed outside the slow cooling device at this time), and the temperature of the steel entering the slow cooling device is controlled at 710-800°C; the steel can be evenly turned over during the stepping process of the cooling bed to ensure uniform heat dissipation of the steel on the cooling bed; the upper insulation cover is moved by the driving mechanism A, and the lower insulation bottom is opened or closed synchronously (or asynchronously) to ensure that the temperature of the steel entering the slow cooling device is controlled at 710-800°C; in the slow cooling device, the temperature is >600-800°C, the cooling rate is controlled at 10-14°C / min, the temperature is >400-600°C, the cooling rate is controlled at 5-9°C / min, and the temperature is 260-400°C, The cooling rate is controlled at 15-20°C / min. The temperature of the lower cooling bed is 200-260°C. After the steel is lowered from the cooling bed, it is slowly cooled in the pit. The slow cooling time is controlled at 24 hours to prevent the occurrence of white spot defects.

[0068] The hardness comparison of hot rolling state before and after using the patent of this invention is shown in Table 3 and the microstructure comparison is shown in Table 3. Figure 3 and Figure 4 .

[0069] Table 3 Comparison of hardness in hot rolled state

[0070]

[0071] Example 2

[0072] 1) Φ500mm billet heating

[0073] The chemical composition of the ingot is shown in Table 4.

[0074] Table 4 Main chemical components (wt%)

[0075]

[0076] The casting is heated in a walking beam heating furnace, which is divided into four sections: a preheating section, a heating section, a second heating section and a soaking section, wherein the preheating section has no combustion nozzle; the temperature of the preheating section is 810-850°C, the temperature of the heating section is 1000-1053°C, the temperature of the heating section is 1235-1258°C, and the temperature of the soaking section is 1240-1270°C;

[0077] 2) Steel rolling

[0078] The ingot is rolled in the rolling lines of the rough rolling unit, the intermediate rolling unit and the finishing rolling unit. The starting rolling temperature is 1148°C and the final rolling temperature is 1010°C. The steel shall not be cooled by water during and after the rolling process.

[0079] 3) Cooling bed cooling

[0080] The device for slow cooling steel after rolling includes channel side plates, including side plate A and side plate B. Side plate A and side plate B are arranged at intervals and fixed on a carrier to form a channel. Side plate A and side plate B can be walls, and the carrier is the ground.

[0081] The cooling bed comprises a bed body and columns supporting the bed body. The cooling bed is placed in the passage, and the columns are supported on the ground.

[0082] The upper insulation cover is covered on the cooling bed and the two sides of the upper insulation cover are slidably matched with the corresponding side panels A and B; the upper insulation cover is an arch structure and consists of a three-layer structure, including an outer layer, a middle layer, and an inner layer. The outer layer and the inner layer are several layers of aluminum silicate fiberboard insulation layers bonded by high-temperature resistant sealant with a thickness of 150mm. The middle layer is a vacuum layer with a thickness of 70mm. The inner surface of the upper insulation cover is an arc-shaped concave surface (can also be convex), the arc angle is 95°, and the arc height is 10mm.

[0083] The driving mechanism A is used to drive the upper insulation cover to slide up and down along the side panels A and B and stop sliding at a certain height; specifically, it is controlled by the hydraulic cylinder system on both sides (driving mechanism A), the cylinder body of the hydraulic cylinder is fixed on the beam above the upper insulation cover, and the piston rod of the hydraulic cylinder is fixed on the upper insulation cover, thereby controlling the up and down movement of the upper insulation cover.

[0084] The lower insulation bottom is located below the bed, one end of which is hinged to the channel side plate and the other end is free. There are several forms of the lower insulation bottom, such as:

[0085] 1. The lower insulation bottom is a whole plate, one end of the whole plate is hinged to the side plate A or side plate B, and the other end is a free end and the lower end surface of the end is hinged to one end of the oil cylinder, and the other end of the oil cylinder is hinged to the corresponding side plate A or side plate B. The whole plate is provided with strip holes with the same number as the rows of columns. The strip holes are connected at the free end, and each row of columns is located in a corresponding strip hole;

[0086] 2. The lower insulation bottom has several plates, there is a plate between two adjacent rows of columns, one end of the plate is hinged to the side plate A or the side plate B, and the other end is a free end, or there are two plates between two adjacent rows of columns, one end of the two plates are hinged to the side plate A and the side plate B respectively, and the other end is a free end. The two plates are arranged in parallel, and the free end of each plate is hinged to the corresponding side plate A or side plate B through a cylinder.

[0087] The oil cylinder is a driving mechanism B, which is used to drive the lower thermal insulation bottom to rotate around the hinge between it and the channel side plate and stop at a certain angle.

[0088] A linear guide is fixedly installed on the inner wall of the upper insulation cover, and the slider of the linear guide is fixedly connected to the temperature gun. It is used to measure the temperature of a certain steel. The linear guide, such as a screw linear guide, drives the temperature gun to move forward and backward. The temperature measuring range of the temperature gun is 500℃~900℃. The temperature gun moves to track the temperature of a specific steel entering and exiting the slow cooling device. After the temperature gun touches the receiver at the end of the slow cooling device, the real-time tracking of the temperature of a steel entering and exiting the slow cooling device is completed. Then the temperature gun will automatically return to the starting position through the linear guide to prepare for the next cycle of temperature measurement. At the same time, the measured temperature of the steel entering and exiting the insulation device is transmitted to the PLC. The PLC system calculates the temperature drop rate of the steel in the insulation device, and the calculated temperature drop rate is compared with the set temperature drop rate. According to the comparison result, the upper insulation cover is controlled to move up and down. If the temperature drop rate is higher than the set value, the upper insulation cover will move downward, otherwise, the upper insulation cover will move upward. Each time the upper insulation cover descends or rises, the height is controlled to be 10mm;

[0089] The stepping rate of the cooling bed is controlled by the temperature drop rate of the steel to ensure that the temperature of the steel leaving the insulation device meets the process requirements.

[0090] After the steel is sawn to length, it is placed on a cooling bed for slow cooling. The temperature of the upper cooling bed is controlled at 790-880°C (the steel is located on the cooling bed outside the slow cooling device at this time), and the temperature of the steel entering the slow cooling device is controlled at 710-800°C; the steel can be evenly turned over during the stepping process of the cooling bed to ensure uniform heat dissipation of the steel on the cooling bed; the upper insulation cover is moved by the driving mechanism A, and the lower insulation bottom is opened or closed synchronously (or asynchronously) to ensure that the temperature of the steel entering the slow cooling device is controlled at 710-800°C; in the slow cooling device, the temperature is >600-800°C, the cooling rate is controlled at 10-14°C / min, the temperature is >400-600°C, the cooling rate is controlled at 5-9°C / min, and the temperature is 260-400°C, The cooling rate is controlled at 15-20°C / min. The temperature of the lower cooling bed is 200-260°C. After the steel is lowered from the cooling bed, it is slowly cooled in the pit. The slow cooling time is controlled at 24 hours to prevent the occurrence of white spot defects.

[0091] The hardness comparison of hot rolling state before and after using the patent of this invention is shown in Table 5 and the microstructure comparison is shown in Table 5. Figure 5 and Figure 6 .

[0092] Table 5 Hardness comparison of hot rolled state

[0093]

[0094] Example 3

[0095] 1) Φ500mm billet heating

[0096] The chemical composition of the ingot is shown in Table 6.

[0097] Table 6 Main chemical components (wt%)

[0098]

[0099] The casting is heated in a walking beam heating furnace, which is divided into four sections: a preheating section, a heating section, a second heating section and a soaking section, wherein the preheating section has no combustion nozzle; the temperature of the preheating section is 810-850°C, the temperature of the heating section is 1000-1053°C, the temperature of the heating section is 1235-1258°C, and the temperature of the soaking section is 1240-1270°C;

[0100] 2) Steel rolling

[0101] The ingot is rolled in the rolling lines of the rough rolling unit, the intermediate rolling unit and the finishing rolling unit. The starting rolling temperature is 1136°C and the final rolling temperature is 998°C. The steel shall not be cooled by water during and after the rolling process.

[0102] 3) Cooling bed cooling

[0103] The device for slow cooling steel after rolling includes channel side plates, including side plate A and side plate B. Side plate A and side plate B are arranged at intervals and fixed on a carrier to form a channel. Side plate A and side plate B can be walls, and the carrier is the ground.

[0104] The cooling bed comprises a bed body and columns supporting the bed body. The cooling bed is placed in the passage, and the columns are supported on the ground.

[0105] The upper insulation cover is covered on the cooling bed and the two sides of the upper insulation cover are slidably matched with the corresponding side panels A and B; the upper insulation cover is an arch structure and consists of a three-layer structure, including an outer layer, a middle layer, and an inner layer. The outer layer and the inner layer are several layers of aluminum silicate fiberboard insulation layers bonded by high-temperature resistant sealant with a thickness of 150mm. The middle layer is a vacuum layer with a thickness of 70mm. The inner surface of the upper insulation cover is an arc-shaped concave surface (can also be convex), the arc angle is 95°, and the arc height is 10mm.

[0106] The driving mechanism A is used to drive the upper insulation cover to slide up and down along the side panels A and B and stop sliding at a certain height; specifically, it is controlled by the hydraulic cylinder system on both sides (driving mechanism A), the cylinder body of the hydraulic cylinder is fixed on the beam above the upper insulation cover, and the piston rod of the hydraulic cylinder is fixed on the upper insulation cover, thereby controlling the up and down movement of the upper insulation cover.

[0107] The lower insulation bottom is located below the bed, one end of which is hinged to the channel side plate and the other end is free. There are several forms of the lower insulation bottom, such as:

[0108] 1. The lower insulation bottom is a whole plate, one end of the whole plate is hinged to the side plate A or side plate B, and the other end is a free end and the lower end surface of the end is hinged to one end of the oil cylinder, and the other end of the oil cylinder is hinged to the corresponding side plate A or side plate B. The whole plate is provided with strip holes with the same number as the rows of columns. The strip holes are connected at the free end, and each row of columns is located in a corresponding strip hole;

[0109] 2. The lower insulation bottom has several plates, there is a plate between two adjacent rows of columns, one end of the plate is hinged to the side plate A or the side plate B, and the other end is a free end, or there are two plates between two adjacent rows of columns, one end of the two plates are hinged to the side plate A and the side plate B respectively, and the other end is a free end. The two plates are arranged in parallel, and the free end of each plate is hinged to the corresponding side plate A or side plate B through a cylinder.

[0110] The oil cylinder is a driving mechanism B, which is used to drive the lower thermal insulation bottom to rotate around the hinge between it and the channel side plate and stop at a certain angle.

[0111] A linear guide is fixedly installed on the inner wall of the upper insulation cover, and the slider of the linear guide is fixedly connected to the temperature gun. It is used to measure the temperature of a certain steel. The linear guide, such as a screw linear guide, drives the temperature gun to move forward and backward. The temperature measuring range of the temperature gun is 500℃~900℃. The temperature gun moves to track the temperature of a specific steel entering and exiting the slow cooling device. After the temperature gun touches the receiver at the end of the slow cooling device, the real-time tracking of the temperature of a steel entering and exiting the slow cooling device is completed. Then the temperature gun will automatically return to the starting position through the linear guide to prepare for the next cycle of temperature measurement. At the same time, the measured temperature of the steel entering and exiting the insulation device is transmitted to the PLC. The PLC system calculates the temperature drop rate of the steel in the insulation device, and the calculated temperature drop rate is compared with the set temperature drop rate. According to the comparison result, the upper insulation cover is controlled to move up and down. If the temperature drop rate is higher than the set value, the upper insulation cover will move downward, otherwise, the upper insulation cover will move upward. Each time the upper insulation cover descends or rises, the height is controlled to be 10mm;

[0112] The stepping rate of the cooling bed is controlled by the temperature drop rate of the steel to ensure that the temperature of the steel leaving the insulation device meets the process requirements.

[0113] After the steel is sawn to length, it is placed on a cooling bed for slow cooling. The temperature of the upper cooling bed is controlled at 790-880°C (the steel is located on the cooling bed outside the slow cooling device at this time), and the temperature of the steel entering the slow cooling device is controlled at 710-800°C; the steel can be evenly turned over during the stepping process of the cooling bed to ensure uniform heat dissipation of the steel on the cooling bed; the upper insulation cover is moved by the driving mechanism A, and the lower insulation bottom is opened or closed synchronously (or asynchronously) to ensure that the temperature of the steel entering the slow cooling device is controlled at 710-800°C; in the slow cooling device, the temperature is >600-800°C, the cooling rate is controlled at 10-14°C / min, the temperature is >400-600°C, the cooling rate is controlled at 5-9°C / min, and the temperature is 260-400°C, The cooling rate is controlled at 15-20°C / min. The temperature of the lower cooling bed is 200-260°C. After the steel is lowered from the cooling bed, it is slowly cooled in the pit. The slow cooling time is controlled at 24 hours to prevent the occurrence of white spot defects.

[0114] The hardness comparison of hot rolling state before and after using the patent of this invention is shown in Table 7 and the microstructure comparison is shown in Table 7. Figure 7 and Figure 8 .

[0115] Table 7 Comparison of hardness in hot rolled state

[0116]

[0117] It should be noted that the above embodiments can be freely combined as needed. The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered as the protection scope of the present invention.

Claims

1. A device for slow cooling of steel after rolling, characterized in that: include: The channel side plate comprises a side plate A and a side plate B, wherein the side plate A and the side plate B are spaced apart and fixed on the carrier to form a channel; A cooling bed, comprising a bed body and columns supporting the bed body, and the cooling bed is placed in the passage; The upper insulation cover is an arch-shaped structure, which is covered on the cooling bed and the two sides of the upper insulation cover are slidably matched with the corresponding side panels A and B. The inner wall of the upper insulation cover is fixedly installed with a linear guide rail, and the slider of the linear guide rail is fixedly connected to the temperature measuring gun; A driving mechanism A is used to drive the upper heat-insulating cover to slide up and down along the side panels A and B and stop sliding at a certain height; The lower insulation bottom is located below the bed, one end of which is hinged to the channel side plate and the other end is a free end; The lower insulation bottom is a whole plate, one end of the whole plate is hinged to the side plate A or side plate B, the other end is a free end and the lower end surface of the end is hinged to one end of the cylinder, the other end of the cylinder is hinged to the corresponding side plate A or side plate B, or The lower insulation bottom is composed of several plates, one plate is arranged between two adjacent rows of columns, one end of the plate is hinged to the side plate A or the side plate B, and the other end is a free end, or two plates are arranged between two adjacent rows of columns, one end of the two plates is hinged to the side plate A and the side plate B respectively, and the other end is a free end, the two plates are arranged in parallel, and the free end of each plate is hinged to the corresponding side plate A or the side plate B through an oil cylinder; The driving mechanism B drives the lower heat-insulating bottom to rotate around the hinge between the lower heat-insulating bottom and the channel side plate and stop at a certain angle.

2. The device for slow cooling of steel after rolling according to claim 1, characterized in that: The upper insulation cover is composed of a three-layer structure, including an outer layer, a middle layer and an inner layer. The outer layer and the inner layer are aluminum silicate fiberboard insulation layers, and the middle layer is a vacuum layer.

3. The device for slow cooling of steel after rolling according to claim 1, characterized in that: The inner wall of the upper heat-insulating cover is provided with a plurality of protrusions or grooves.

4. The device for slow cooling of steel after rolling according to claim 1, characterized in that: The driving mechanism A is a hydraulic cylinder, a cylinder body of the hydraulic cylinder is fixed on the crossbeam above the upper thermal insulation cover, and a piston rod of the hydraulic cylinder is fixed on the upper thermal insulation cover.

5. The device for slow cooling of steel after rolling according to claim 1, characterized in that: The inner surface of the upper insulation cover is an arc-shaped concave surface, the arc angle is 95 degrees, and the arc height is 10mm.

6. A method for slow cooling of medium and low carbon CrMo series steels, using the steel post-rolling slow cooling device as claimed in any one of claims 1 to 5, characterized in that: The steel is placed on a cooling bed for slow cooling, and the temperature of the steel is controlled at 790-880℃ when placed on the cooling bed; The steel moves forward on the cooling bed during the stepping process and turns evenly during the stepping process to ensure that the steel dissipates heat evenly on the cooling bed; The temperature measuring gun is located above the steel to detect the temperature of the steel. The linear guide rail controls the movement of the temperature measuring gun so that it moves synchronously with the steel. The hydraulic cylinder drives the upper insulation cover to move up and down to ensure that the temperature of the steel entering the slow cooling device measured by the temperature measuring gun is controlled at 710-800℃; In the slow cooling device, the temperature is 600-800℃, and the cooling rate is controlled at 10-14℃ / min; the temperature is 400-600℃, and the cooling rate is controlled at 5-9℃ / min; the temperature is 260-400℃, and the cooling rate is controlled at 15-20℃ / min. The temperature of the lower cooling bed is 200-260℃. After the steel comes down from the cooling bed, it is put into the pit for slow cooling, and the slow cooling time is controlled at more than 24 hours.

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