Slow cooling and preparation method for improving uniformity

Abstract

The application discloses a kind of preparation method and slow cooling for improving uniformity, slow cooling method includes using the slow cooling equipment with top cover to the hot coil after high carbon steel hot rolling coiling end and controls the temperature drop of the outer ring of the hot coil when being transported to enter slow cooling equipment after coiling offline within 20 ℃, and control the slow cooling speed of hot coil in slow cooling equipment is 2~4 ℃ / h, heat preservation slow cooling 70~80h.Slow cooling method is used to the hot coil after hot rolling coiling in preparation method.The application method effectively improves the slow cooling effect of high carbon steel hot coil by reasonably controlling the temperature drop of the whole process from hot coil offline to slow cooling box, and slow cooling speed and time in slow cooling equipment, to solve the problem of large strength performance fluctuation of high carbon steel, which helps to reduce the problem of large rolling force fluctuation and finished product thickness fluctuation of cold rolling.The high carbon steel 75Cr1 prepared by the slow cooling method of the application has a strength performance fluctuation of 50~85MPa, a cold rolling force fluctuation controlled within 80~100 tons, and a cold rolling thickness fluctuation of ±10 μm.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of metal materials, in particular to a slow cooling method for improving uniformity and a preparation method. BACKGROUND

[0002] High-carbon steel is a kind of carbon steel with high carbon content, and the carbon content is usually between 0.60 and 1.50%. Due to the high carbon content, the high-carbon steel has high hardness and high strength, but the plasticity and toughness are relatively low, and the welding performance is also poor. Representative steel grades of the high-carbon steel include 60, 75, 65Mn, 75Cr1, SK7, SK85 and the like.

[0003] The high-carbon steel is sensitive to temperature due to the high carbon content and alloy content. If the material is not uniformly cooled after the hot rolling is completed, the strength performance will fluctuate greatly, the cooling stress will be large, the steel strip coiling performance will be affected, and thus the cold rolling in the subsequent process is unstable, the finished product thickness fluctuates greatly, and even the strip breaking problem occurs. Especially in winter, in the low-temperature environment, the phenomenon of large fluctuation of the material strength performance is more obvious, the problem of large fluctuation of the cold rolling force is more prominent, and the finished product performance is seriously affected, and the stability of production is affected.

[0004] In order to reduce the effect of uneven cooling on the fluctuation of strength performance and improve the performance of steel strip coiling, the slow cooling method is generally used to improve the material processing performance in China. There are mainly two kinds of slow cooling methods. One is stack slow cooling, which mainly refers to the slow cooling of hot coils to room temperature after coiling by stacking in the high temperature area. For example, a Chinese invention application with application number CN201110190489.X discloses a method for improving the internal quality of steel plates. When the finished steel plates with a thickness of 18-70 mm are air-cooled to 260-500℃, the steel plates are quickly stacked and slowly cooled in the cooling bed area. When the slow cooling temperature is 260-500℃, the number of stacked steel plate blocks is not less than 8, the slow cooling time of thick 30-70 mm steel plates is greater than 48 hours, and the slow cooling time of thick 18 mm-30 mm steel plates is greater than 24 hours. This scheme is air-cooled first and then slowly cooled, and the air-cooling cooling speed is relatively high, which is easy to cause uneven cooling. This slow cooling method is simple and convenient, and the production cost is low, but the slow cooling effect is poor, and it cannot effectively solve the problem of large fluctuation of high carbon steel strength performance. The other is slow cooling in the slow cooling pit / box, and the hot coil is slowly cooled in the slow cooling pit after coiling. For example, a Chinese invention application with application number CN202010599789.2 discloses a slow cooling device and method for medium and high carbon steel hot-rolled steel strip. The application discloses that the cover plate I and the cover plate II are slid along the running tracks on both sides of the slow cooling pit by rollers, and a motor is connected to a roller on each cover plate. Each motor is controlled by a control box to control its start-stop and steering switching, so that the opening and closing of the cover plate I and the cover plate II can be realized, which facilitates the stacking of hot-rolled steel coils into the slow cooling pit, increases the safety of operation, makes the slow cooling effect better, and improves the slow cooling efficiency. This slow cooling method has relatively obvious effect, but has many shortcomings: first, the temperature drop from hot coil to slow cooling pit / box affects the slow cooling effect, and second, the initial temperature of the slow cooling pit / box also affects the slow cooling effect due to the temperature difference of hot coils with different coiling temperatures. For example, a Chinese invention application with publication number CN112063815A discloses a method for improving the uniformity of product performance by slow cooling after rolling. The application discloses a method for improving the uniformity of product performance by slow cooling after rolling, which is used in a hot-rolled coil heat preservation cover device, including: loading the finished steel coil onto the hot-rolled coil heat preservation cover device and sealing treatment; starting the internal ventilation device to discharge the high-temperature gas in the semi-circular heat preservation cover; starting the gas protection device to fill the semi-circular heat preservation cover with protective gas; starting the electric heating device to form a heat preservation atmosphere in the semi-circular heat preservation cover using the residual heat of the steel coil, and then waiting for the steel coil to slowly cool by itself, checking the gas temperature change in the semi-circular heat preservation cover detected by the thermocouple, and completing the steel coil heat treatment when the gas temperature value meets the production standard requirements, and unloading the treated steel coil from the hot-rolled coil heat preservation cover device.The patent has a short heat coil heat preservation time, the longest is only 4h, it is difficult to play the effect of slow cooling, and each coil uses a set of heat preservation device separately, which is not suitable for batch production. The strength fluctuation of the hot coil after heat preservation for 2h-4h still reaches 100-150MPa.

[0005] Therefore, it is urgent to provide a slow cooling method with good effect and improved uniformity. SUMMARY

[0006] In order to overcome the shortcomings of the above-mentioned technology, the purpose of the present application is to provide a slow cooling and preparation method for improving uniformity, by reasonably controlling the slow cooling process, effectively improving the slow cooling effect of high carbon steel hot coil, and further solving the problem of large strength performance fluctuation of high carbon steel, which helps to reduce the problems of large rolling force fluctuation and large thickness fluctuation of finished product in cold rolling.

[0007] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0008] A slow cooling method for improving uniformity, comprising using a slow cooling device with a top cover to slow cool the hot coil after hot rolling coiling during the preparation of steel products, and the special feature is that: the outer ring temperature T1 of the hot coil is controlled to be kept at: T0-20℃≤T1≤T0 when the hot coil is transported to enter the slow cooling device after coiling off line, wherein T0 is the coiling temperature, ℃; after the hot coil enters the slow cooling device, the top cover is covered, and the heat preservation slow cooling is 70-80h, and the slow cooling speed of the hot coil is controlled to be 2-4℃ / h. By controlling the temperature drop of the outer ring of the hot coil from the coiling machine off line to the slow cooling device, the possibility of large difference between the outer ring strength and the middle part of the hot coil caused by too fast temperature drop of the outer ring is reduced.

[0009] As a preferred scheme, the slow cooling device comprises a slow cooling box or a slow cooling pit with a top cover.

[0010] As a preferred scheme, the steel product is high carbon steel.

[0011] Further, the high carbon steel is 75Cr1.

[0012] As a preferred scheme, the coiling temperature of the hot coil is 570℃-718℃.

[0013] As a preferred scheme, the method for controlling the outer ring temperature of the hot coil after coiling off line and transporting to enter the slow cooling device comprises: controlling the transportation time of the hot coil after off line to the slow cooling device within the maximum transportation time; wherein in summer, the maximum transportation time t s The following formula is obtained:

[0014] T0=20t0 2 -118t0+718;

[0015] T0=20t12 - 118t1 + 738;

[0016] t s = t1 - t0, and 0 < t s ≤ 1;

[0017] In winter, the maximum transportation time t w is obtained by the following formula:

[0018] T0= 26t0' 2 - 166t0' + 718;

[0019] T0= 26t1' 2 - 166t1' + 738;

[0020] t w = t1' - t0', and 0 < t w ≤ 1;

[0021] wherein, T0 is the coiling temperature, ℃, T0 = 570 ~ 718 ℃; t s is the maximum transportation time in summer, h; t w is the maximum transportation time in winter, h; t0, t0' are the times when the hot coil is taken off from the coiler in summer and winter, respectively, t1, t1' are the times when the hot coil is put into the slow cooling box in summer and winter, respectively, the maximum temperature drop is 20 ℃. Since the air cooling speed is quite different in summer and winter, the transportation time needs to be controlled separately; since the air cooling speed is mainly affected by the coiling temperature of the hot coil, the longest transportation time is determined by the coiling temperature of the hot coil, and by controlling the longest transportation time, the temperature drop of the outer circle of the hot coil entering the slow cooling device is ensured to be within 20 ℃. The maximum transportation time in other seasons can refer to the maximum transportation time in winter or the average value of the maximum transportation time in summer and winter.

[0022] As a preferred solution, the slow cooling device is preheated before the hot coil enters the slow cooling device.

[0023] Further, the preheating process comprises: first, adding preheating coils to the slow cooling device for uniform distribution, covering the slow cooling device with a top cover, and preheating the slow cooling device by the preheating coils for 25 ~ 35 min; the preheating coils are other non-essential slow cooling hot rolled steel coils, and the number of preheating coils is ≥ 3 coils.

[0024] Still further, the temperature of the preheating coil is:

[0025] When the coiling temperature of the hot coil is 650 ℃ ≤ T0 ≤ 718 ℃, the temperature of the preheating coil when entering the slow cooling device is 650 ~ 718 ℃;

[0026] When the hot coil winding temperature is 600℃≤T0<650℃, the temperature of the preheated coil when entering the slow cooling equipment is 600~650℃;

[0027] When the hot coil winding temperature is 570℃≤T0<600℃, the temperature of the preheated coil when entering the slow cooling equipment is 570~600℃. Before the hot coil enters the slow cooling equipment to formally slow down, it is preheated by the preheated coil to prevent the cold slow cooling equipment environment from forming a large temperature difference with the hot coil, thereby increasing the temperature drop of the hot coil.

[0028] As a preferred solution, the preheating process comprises: covering the top cover of the slow cooling equipment, and starting the heating device arranged on the inner side wall of the slow cooling equipment to heat the slow cooling equipment to the winding temperature of the hot coil.

[0029] Further, the heating device comprises resistance wires or dense gas nozzles.

[0030] As a preferred solution, the slow cooling equipment comprises an inner side wall, and the resistance wires are arranged on the inner side wall and uniformly distributed on the inner side wall. If there is no suitable preheating condition for the hot coil, the resistance wire heating device can be started to heat the slow cooling equipment after covering the top cover to the winding temperature of the hot coil.

[0031] As a preferred solution, the inner side wall of the slow cooling equipment is provided with high-density gas nozzles, and the slow cooling uniformity and preheating speed are further improved after the gas is burned.

[0032] Further, the inner side wall of the slow cooling equipment is provided with resistance wires. If the temperature drop is too large, the resistance wires start to work to prevent the temperature drop from being too large. If the temperature drop is normal, the resistance wires stop working to reduce energy consumption. The slow cooling equipment is provided with a fan for enhancing the temperature uniformity inside the slow cooling equipment. The slow cooling equipment is provided with an infrared thermal imager for real-time monitoring of the coil temperature and feedback of the hot coil temperature drop. The resistance wires control the slow cooling cooling rate in the slow cooling box. If the slow cooling speed of the hot coil in the slow cooling box is greater than 4℃ / h, the resistance wires start to work. If the slow cooling speed of the hot coil in the slow cooling box is ≤4℃ / h, the resistance wires stop working, and the hot coil cools slowly by its own heat.

[0033] The application also provides a preparation method of high-carbon steel, which is characterized by comprising: after hot rolling and winding, the hot coil is slowly cooled by the slow cooling method, and then air-cooled, and cold-rolled after the cooling is completed; and in the winding process, the winding temperature is 570~718℃.

[0034] As a preferred solution, after the slow cooling is completed, the temperature difference of the head, middle and tail of the hot coil is 4~15℃; the strength performance fluctuation of the hot coil before cold rolling is 50~85MPa; in the cold rolling process, the rolling force fluctuation is 80~100 tons, and the thickness fluctuation is ±10μm.

[0035] Compared with the prior art, the present application has the following advantages:

[0036] The slow cooling method for improving uniformity of the present application effectively improves the slow cooling effect of high-carbon steel hot coil by reasonably controlling the temperature drop from the end of hot rolling to the slow cooling box and the slow cooling speed and time in the slow cooling equipment, thereby solving the problem of large fluctuation of strength performance of high-carbon steel and helping to reduce the problems of large fluctuation of cold rolling force and thickness of finished product. The high-carbon steel 75Cr1 prepared by the slow cooling method of the present application has a strength performance fluctuation of 50-85 MPa, a cold rolling force fluctuation controlled within 80-100 tons, and a cold rolling thickness fluctuation of ±10 μm. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 is a side view schematic diagram of the hot coil entering the slow cooling equipment in the present application;

[0038] Figure 2 is a microstructure diagram of the outer ring of the hot coil in Example 1;

[0039] Figure 3 is a microstructure diagram of the middle part of the hot coil in Example 1. DETAILED DESCRIPTION

[0040] In order to better explain the present application, the main content of the present application is further illustrated below in combination with the drawings and specific examples, but the content of the present application is not limited to the following examples only.

[0041] The slow cooling method for improving uniformity of the present application comprises, when high-carbon steel is prepared, using a slow cooling equipment with a top cover to slow cool the hot coil after the end of hot rolling, and controlling the temperature T1 of the outer ring of the hot coil when transported into the slow cooling equipment to be T0-20≤T1≤T0, wherein T0 is the coiling temperature, ℃.

[0042] Before the hot coil enters the slow cooling equipment to formally slow cool, the preheating coil is first added to the slow cooling equipment to be uniformly distributed, the top cover of the slow cooling equipment is covered, and the slow cooling equipment is preheated for 25-35 min through the preheating coil; the preheating coil is other non-essential slow cooling hot rolled steel coil, the number of hot coils is ≥3 coils; the temperature of the preheating coil is: when the hot coil coiling temperature is 650℃≤T0≤718℃, the temperature of the preheating coil when entering the slow cooling equipment is 650-718℃; when the hot coil coiling temperature is 600℃≤T0<650℃, the temperature of the preheating coil when entering the slow cooling equipment is 600-650℃; when the hot coil coiling temperature is 570℃≤T0<600℃, the temperature of the preheating coil when entering the slow cooling equipment is 570-600℃. If there is no suitable hot coil preheating condition, a heating device such as resistance wire or high-density gas nozzle can be started to heat the top cover of the slow cooling equipment to the required temperature T0.

[0043] When the hot coil enters the slow cooling equipment to be formally slow cooled after being discharged from the coiler, the top cover is covered, and the slow cooling is kept for 70-80 h in a heat preservation manner, and the slow cooling speed of the hot coil is controlled to be 2-4 ℃ / h. The coiling temperature is 570-718 ℃. The maximum transportation time of the hot coil during the transportation from the coiler to the slow cooling equipment is obtained by the following formula in summer:

[0044] T0=20t0 2 -118t0+718;

[0045] T0=20t1 2 -118t1+738;

[0046] t s =t1-t0, and 0 s ≤1;

[0047] The maximum transportation time of the hot coil during the transportation from the coiler to the slow cooling equipment is obtained by the following formula in winter:

[0048] T0=26t0' 2 -166t0'+718;

[0049] T0=26t1' 2 -166t1'+738;

[0050] t w =t1'-t0', and 0 w ≤1;

[0051] Wherein, T0 is the coiling temperature, ℃, T0=570-718 ℃; t s is the maximum transportation time in summer, h; t w is the maximum transportation time in winter, h; t0 and t0' are the times when the hot coil is discharged from the coiler in summer and winter respectively, and t1 and t1' are the times when the hot coil is calculated to enter the slow cooling box in summer and winter respectively.

[0052] The slow cooling equipment includes a slow cooling box or a slow cooling pit with a top cover, and can control the slow cooling speed. As a specific embodiment, the slow cooling equipment is a cubic structure, includes an inner side wall, a heating device such as a resistance wire or a high-density gas nozzle is installed on the inner side wall, and the heating device is uniformly distributed on the four inner side walls. If the temperature drop is too large, the resistance wire starts to work to heat and prevent the temperature drop from being too large. If the temperature drop is normal, the resistance wire stops working to reduce energy consumption. Two fans are arranged in the slow cooling equipment, and the two fans are oppositely arranged on the inner side walls or in front of the inner side walls at opposite ends in the slow cooling equipment, for enhancing the temperature uniformity in the slow cooling equipment. The slow cooling equipment is provided with an infrared thermal imager for real-time monitoring of the coil temperature and feedback of the hot coil temperature drop.

[0053] The high-carbon steel in the examples and comparative examples is 75Cr1; the slow cooling equipment adopted is a slow cooling box, such asFigure 1 As shown, the hot coil is uniformly placed in the slow cooling device after being discharged from the coiler. In the preparation of high-carbon steel 75Cr1 in the examples, the hot coil is cooled by the above slow cooling method after being coiled, and then air-cooled. After cooling, cold rolling is performed, and the coiling temperature is 570-718°C. The difference between the examples and the comparative examples is the difference in process parameters during slow cooling. The temperature and time control parameters during the process of discharging the hot coil from the coiler to the slow cooling box are shown in Tables 1 and 2. In Table 2, the number of preheated coils is 3, which are uniformly placed at the head, middle and tail of the slow cooling box, and the preheating time is 30 min.

[0054] The through-coil strength, cold rolling force and through-coil thickness of the hot coil in each example and comparative example were obtained, wherein the through-coil strength was detected by GB / T 228.1-2010 "Metallic Materials Tensile Testing Part 1: Room Temperature Test Method", and the results are shown in Tables 2 and 3. The microstructure diagrams of the outer circle and the middle part of the hot coil after cooling in Example 1 are shown in Figure 2 and Figure 3 .

[0055] Table 1: Part of the process parameters of the examples and the comparative examples

[0056]

[0057] Table 2: Part of the process parameters of the examples and the comparative examples

[0058]

[0059] Table 3: Through-coil performance fluctuation of the examples and the comparative examples

[0060]

[0061] As shown in Table 2 and Table 3, in the Comparative Example 1 and the Comparative Example 3, the outer circle temperature T1 of the hot coil after the hot coil is discharged and transported to enter the slow cooling box meets the requirement of the present application, the temperature drop control is normal, but the slow cooling speed of the hot coil in the slow cooling box is too large (under the condition of no preheating, no covering and no other means to control the slow cooling speed), the temperature difference of the head, the middle and the tail of the hot coil is large, the tension fluctuation of the coil is as high as 196 MPa and 153 MPa, which causes the rolling force fluctuation of the cold rolling to be large and the thickness fluctuation of the finished product to be large. In the Comparative Example 2 and the Comparative Example 4, the outer circle temperature T1 of the hot coil after the hot coil is discharged and transported to enter the slow cooling box does not meet the requirement of the present application, the time of entering the slow cooling box is too long, which leads to the temperature drop being too large, the slow cooling speed of the hot coil in the slow cooling box is controlled normally, but the temperature difference of the head, the middle and the tail of the hot coil is still large, the tension fluctuation of the coil is large, reaching 145 MPa and 137 MPa, which causes the rolling force fluctuation of the cold rolling to be large and the thickness fluctuation of the finished product to be large. The tension fluctuation of the coil produced by using the method of the prior art for post-rolling heat preservation and slow cooling to improve the uniformity of the finished product performance also reaches 100-150 MPa.

[0062] The high-carbon steel 75Cr1 prepared in the Examples 1-5 by using the scheme of the present application has the temperature difference of the head, the middle and the tail of the hot coil after the slow cooling is finished not more than 15℃, the tension fluctuation of the coil before the cold rolling is only 50-85 MPa, the rolling force fluctuation of the cold rolling of the coil is not more than 100 tons, and the thickness of the coil after the cold rolling is within ±10 μm. Figure 2 and Figure 3 As shown in the microstructure diagrams of the outer circle and the middle part of the hot coil in the Examples 1-5, the microstructure of the outer circle and the middle part of the hot coil in the Example 1 has no obvious difference.

[0063] In summary, according to the method for improving the uniformity provided by the present application, the slow cooling effect of the high-carbon steel hot coil is effectively improved by reasonably controlling the temperature drop from the hot coil being discharged to the slow cooling in the slow cooling box and the slow cooling speed and time in the slow cooling equipment, and then the problem of large strength performance fluctuation of the high-carbon steel is solved. The Examples of the present application have the small tension fluctuation of the coil, and the problems of large rolling force fluctuation of the cold rolling and large thickness fluctuation of the finished product in the subsequent cold rolling are better solved.

[0064] The above examples are only the best examples, and are not a limitation on the embodiments of the present application.

Claims

1. A method of improving the uniformity of a slow cooling process, comprising slow cooling a hot coil after the end of hot rolling and coiling using a slow cooling device with a roof during the production of a steel product, characterized in that: The outer coil temperature T1 of the hot coil is controlled to be kept at: T0-20℃≤T1≤T0, wherein T0 is the coiling temperature, and the hot coil is covered with a top cover and is kept for 70-80 hours for slow cooling at a speed of 2-4℃ / h after entering the slow cooling device; and the steel product is high carbon steel. The method for controlling the outer coil temperature of the hot coil after being coiled offline and transported to the entry of the slow cooling device comprises: controlling the transportation time of the hot coil after being coiled offline and transported to the slow cooling device within a maximum transportation time; wherein, in summer, the maximum transportation time t s is obtained by the following formula: T0=20t0 2 - 118t0+ 718; T0 = 20t1 2 - 118t1 + 738; t s = t1- t0, and 0 < t s ≤ 1; In winter, the maximum transport time t w is obtained by the equation: T0= 26t0' 2 - 166t0' + 718; T0= 26t1' 2 - 166t1' + 738; t w = t1' - t0', and 0 < t w ≤ 1; wherein T0 is the coiling temperature, °C, T0 = 570-718 °C; t s is the maximum transportation time in summer, h; t w is the maximum transportation time in winter, h; t0, t0' are the times when the hot coil is taken off the coiler in summer and winter, respectively, and t1, t1' are the times when the hot coil is put into the slow cooling box in summer and winter, respectively.

2. The slow cooling method according to claim 1, characterized by: The slow cooling device comprises a slow cooling box or a slow cooling pit with a top cover.

3. The slow cooling method according to claim 1, characterized by: The coiling temperature of the hot coil is 570-718℃.

4. The slow cooling method according to claim 1, characterized by: The slow cooling device is preheated before the hot coil enters the slow cooling device.

5. The slow cooling method according to claim 4, characterized by: The preheating process comprises: first, preheating coils are added to the slow cooling device for uniform distribution, and the slow cooling device is preheated for 25-35 minutes by the preheating coils after being covered with a top cover; and the number of the preheating coils is ≥3.

6. The slow cooling method according to claim 5, characterized by: The temperature of the preheating coils is: When the coiling temperature of the hot coil is 650℃≤T0≤718℃, the temperature of the preheating coils is 650-718℃ when entering the slow cooling device; When the coiling temperature of the hot coil is 600℃≤T0<650℃, the temperature of the preheating coils is 600-650℃ when entering the slow cooling device; When the coiling temperature of the hot coil is 570℃≤T0<600℃, the temperature of the preheating coils is 570-600℃ when entering the slow cooling device.

7. The slow cooling method according to claim 4, characterized by: The preheating process comprises: covering the slow cooling device with a top cover, and heating the slow cooling device to the coiling temperature of the hot coil by a heating device arranged on the inner side wall of the slow cooling device.

8. A method of producing a high carbon steel, characterized by: The preheating process comprises: After the slow cooling of the hot coil by the slow cooling method of any one of claims 1-7 during the cooling process after hot rolling and coiling, the hot coil is air-cooled, and then cold rolling is performed after the cooling is completed; In the coiling process, the coiling temperature is 570-718℃.

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