Method for controlling soft reduction cracks of 210 square bearing steel

Abstract

The invention relates to a control method for soft reduction cracks of 210 square bearing steel, which comprises the following steps: carrying out roll gap calibration before continuous casting production of the bearing steel to ensure that the roll gap error of the same flow is less than or equal to 1mm and the roll gap error of different flows is less than or equal to 2mm under the pressure of a hot blank, and the pressure control range of the hot blank is 60kN-80kN; in the whole continuous casting process, the superheat degree is controlled to be 20-35 DEG C, and constant-pulling-speed casting is achieved according to 1.10 m / min; electromagnetic stirring of the crystallizer is 350 A / 2.5 Hz, and electromagnetic stirring of the tail end is 300 A / 10 Hz; and when the M3-M7 pulling-straightening rollers execute soft reduction, the pressure is 200 to 400 kN, the pressure is 350 to 550 kN, the pressure is 450 to 650 kN, and the pressure is 300 to 500 kN in sequence. According to the 210 square bearing steel small square billet produced through the steps, the center segregation, the center porosity and the center shrinkage cavity can be controlled to be smaller than or equal to 0.5 level, the center carbon segregation ranges from 1.02 to 1.14, and a casting billet is free of surface press-down cracks and core press-down cracks.

Description

Technical Field

[0001] The invention relates to a method for controlling light-pressing cracks of 210 square bearing steel, belonging to the technical field of cast steel. Background Art

[0002] The production process of bearing steel generally has a slow drawing speed, and the position of the light reduction process is relatively late, resulting in low surface temperature of the ingot. The small square billet has a small force-bearing area, and the pressure is not easy to penetrate. The corners often produce large deformations and cracks. However, if the light reduction process is advanced to achieve light pressure at a higher temperature, internal cracks are prone to occur. The probability of cracks in 210 square bearing steel continuous casting billets is relatively high when a large amount of reduction is performed, which seriously affects the first-time pass rate of the billet.

[0003] Most patents for controlling light-pressure cracks refer to internal cracks, and only a few patents involve the control of light-pressure surface cracks. Patent publication number CN117226056A provides a thick slab continuous casting machine light-pressure guide roller and its use process method patent, which provides a roller sleeve that can avoid stress concentration cracks caused by contact. Patent publication number CN110871265A is a patent for a light-pressure method for a combination of flat rollers and convex rollers for continuous casting large square billets. The patent adopts a light-pressure method that combines a flat roller straightening machine and a convex roller straightening machine, so that the opening of the indentation shape produced on the upper surface of the billet becomes wider. The above methods all involve technical modifications to the straightening rollers, which are relatively expensive. In addition, the current continuous casting straightening machine still uses flat rollers as the mainstream. When the concave rollers are bent due to uneven secondary cooling during the production of small square billets, it may cause more serious consequences. Summary of the invention

[0004] In order to solve the above problems, the present invention discloses a method for controlling light pressure cracks of 210 square bearing steel, which optimizes the continuous casting process and prevents the occurrence of surface or internal light pressure cracks while ensuring the center quality of the small square billet of the bearing steel. The specific technical scheme is as follows: A method for controlling cracks of 210 square bearing steel under light pressure, comprising the following steps: Step 1: Roll gap calibration: Before continuous casting of bearing steel, the roll gap calibration is carried out to ensure that the roll gap error of the same flow under hot billet pressure is ≤±1mm, and the roll gap error of different flows is ≤±2mm. The roll gap error refers to the deviation from the average roll gap of the same flow. The hot billet pressure control range is 60kN~80kN; Step 2: Continuous casting speed control: Continuous casting speed control is performed according to the superheat of the molten steel in the tundish. When the superheat is less than 20°C, the pulling speed is 1.15m / min; when the superheat is 20-40°C, the pulling speed is 1.10m / min; when the superheat is greater than 40°C, the pulling speed is 1.05m / min; Step 3: Electromagnetic stirring of the crystallizer at 350A / 2.5Hz, and electromagnetic stirring of the end at 300A / 10Hz; Step 4: The second cooling zone below the crystallizer is 1 (full roller zone), 2, 3, and 4, with lengths of 0.42m, 1.79m, 1.79m, and 1.43m, respectively. The corresponding water flow densities are 0.0138L / cm 2 、0.0030L / cm 2 、0.0014L / cm 2 、0.0013L / cm 2 , the total specific water volume is 0.36L / kg; Step 5: The total reduction at the end of solidification is 17mm. Each stream has 7 straightening rollers, which are numbered M1-M7 from upstream to downstream. When performing light reduction, only straightening rollers M3-M7 are enabled. The distances between straightening rollers M3-M7 and the meniscus of the crystallizer are 13.5m, 14.6m, 15.7m, 16.8m, and 17.9m, respectively. The reductions at straightening rollers M3-M7 are 2.5mm, 3.5mm, 4.5mm, 4.5mm, and 2mm, respectively. Step 6: Straightening roller pressure control: When performing light pressure, the pressure of the straightening rollers M3-M7 are 200~400kN, 350~550kN, 450~650kN, and 300~500kN respectively; if the pressure of the straightening roller exceeds the control range, adjust the roller gap calibration offset in steps of 0.5mm until the pressure falls within the corresponding control range of each straightening roller.

[0005] Furthermore, as the best implementation state, the parameters of the entire continuous casting process are preferably set as follows: the superheat is controlled at 20-35°C, and the casting is performed at a constant casting speed of 1.10m / min.

[0006] Furthermore, the tensioning and straightening roller pressure control method of step 6 is: the operating system of the continuous casting machine presses down with reference to the roll gap value of the previous roll that has been pressed down and the current roll that has not been pressed down. Since the actual roll gap of the tensioning and straightening roller that has not been pressed down = the current measured roll gap + the calibrated offset + the deformation of the tensioning and straightening machine - the temperature drop shrinkage, the initial value of the calibrated offset is obtained by calibration in step 1, and the corrected roll gap calibrated offset is subsequently obtained according to step 3. The deformation of the tensioning and straightening machine is 0.2 mm, and the temperature drop shrinkage is 0.6 mm.

[0007] A small square billet of 210 square bearing steel is produced by the above-mentioned method for controlling cracks of 210 square bearing steel by light pressing.

[0008] Furthermore, the center segregation, center porosity and center shrinkage of the 210 square bearing steel billet are all ≤0.5, the center carbon segregation is 1.02-1.14, and the billet has no surface depression cracks and core depression cracks.

[0009] The beneficial effects of the present invention are: The present invention does not involve equipment modification. When a flat roll is used for a relatively high reduction amount, it ensures that no light reduction cracks occur inside and on the surface of a 210 square bearing steel billet. At the same time, the central shrinkage cavity, central segregation and central porosity of the billet are all controlled below level 0.5, and the central carbon segregation is controlled within a range of 1.02-1.12. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 This is a low-power image of a 210 square bearing steel billet according to an embodiment of the present invention. Figure 2 This is the carbon segregation diagram of the GCr15 ingot core of the embodiment of the present invention. Figure 3 This is a low-power image of a 210 square bearing steel billet of the comparative example of the present invention. Figure 4 This is a crack diagram of a 210 square bearing steel billet in the comparative example of the present invention. Figure 5 It is the carbon segregation diagram of the GCr15 ingot core of the comparative example of the present invention. DETAILED DESCRIPTION

[0011] The present invention is further explained below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the following specific embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention.

[0012] Combined with Figure 1-2 , an embodiment of the specific application of the present invention is given below: 1) Roll gap calibration should be carried out before continuous casting of bearing steel. The specific process of roll gap calibration is: when not in production, place the standard block (hard alloy steel with a thickness of 200mm) horizontally between the upper and lower rollers of a certain straightening roller, and the two rollers are pressed onto the standard block with a pressure of 60-80kN. The operating system of the continuous casting machine believes that the roll gap value at this time is 200mmm and accepts this calibration result. The straightening roll distribution of the straightening machine of this patent is: there are 7 straightening rolls in each flow that can perform light pressure, numbered M1-M7 from upstream to downstream, and the 3-7 rolls (i.e., M3-M7 rolls) involved in this patent are 13.5m, 14.6m, 15.7m, 16.8m, and 17.9m away from the meniscus of the crystallizer. After the roll gap calibration, the calibration results are verified during the production of 45 steel, and the roll gap error of the same flow under hot billet pressure is ≤±1mm, and the roll gap error of different flows is ≤±2mm. The specific results are shown in Table 1.

[0013] Table 1 Roll gap of 45 steel straightening machine under 60kN pressure

[0014] 2) The overheat temperature of the continuous casting tundish is 35°C, the pulling speed is 1.10m / min, and the entire furnace continuous casting process is cast at a constant pulling speed of 1.10m / min.

[0015] 3) The crystallizer copper tube is 0.9m long. Below the crystallizer are the second cooling zone 1 (full roller zone), zone 2, zone 3, and zone 4, with lengths of 0.42m, 1.79m, 179m, and 143m respectively. Then there is the air cooling zone (only radiation heat release and a small amount of air convection heat transfer, no spray water), then the end electromagnetic stirring, and finally the 1-7 straightening rollers. The crystallizer electromagnetic stirring is surrounded by the copper tube. The crystallizer electromagnetic stirring is 350A / 2.5Hz, and the end electromagnetic stirring is 300A / 10Hz.

[0016] 4) The water flow density of the secondary cooling zone 1 (foot roller zone), zone 2, zone 3, and zone 4 is 0.0138L / cm 2 、0.0030L / cm 2 、0.0014L / cm 2 、0.0013L / cm 2 , total specific water volume is 0.36L / kg.

[0017] 5) The total reduction at the end of continuous casting solidification is 17mm. The reductions at 13.5m, 14.6m, 15.7m, 16.8m, and 17.9m (i.e., M3-M7 rollers) are 2.5mm, 3.5mm, 4.5mm, 4.5mm, and 2mm, respectively. The pressure of the straightening rollers for reduction should be controlled at 200~400kN, 350~550kN, 450~650kN, and 300~500kN, respectively. See Table 2 for details.

[0018] Table 2 The tension roller pressure of 45 steel in the light reduction process

[0019] 6) When the pressure of the M6 ​​and M7 rollers of the 3-stream and the M3 roller of the 5-stream exceeds the control range, the roller gap calibration offset should be adjusted in steps of 0.5mm. The adjustment ranges are -0.5mm, +0.5mm, and -0.5mm respectively. The pressures of the three rollers after adjustment are 329kN, 550kN, and 400kN respectively.

[0020] The method of adjusting the roll gap calibration offset is: for example, the control pressure is 200-400kN, and the roll gap offset is originally 3mm. If the pressure exceeds 400, it means that there is a positive error in the actual pressing amount. At this time, the roll gap offset is adjusted to 2.5mm, which will essentially reduce the pressing amount by 0.5mm, thereby reducing the pressure. Conversely, the roll gap offset is adjusted to 3.5mm. The reason why the pressing amount setting is not changed is because it is caused by equipment error.

[0021] The center segregation, center porosity and center shrinkage of the 210 square bearing steel billet produced according to the above steps can be controlled to ≤0.5 level. The center carbon segregation is 1.02-1.14, and the billet has no surface depression cracks and core depression cracks.

[0022] Combined with Figure 3-5 The following is a conventional process in the industry before the present invention is proposed as a comparative example: 1) The roll gap calibration was not performed before continuous casting of bearing steel. The roll gap size results during the production of 45 steel are shown in Table 3; Table 3 Roll gap of 45 steel straightening machine under 60kN pressure

[0023] 2) The continuous casting tundish has an overheat temperature of 30°C and a casting speed of 1.10 m / min. The entire continuous casting process is cast at a constant casting speed of 1.10 m / min; 3) Crystallizer electromagnetic stirring 350A / 2.5Hz, end electromagnetic stirring 300A / 10Hz; 4) The water flow density of the secondary cooling zone 1 (foot roller zone), zone 2, zone 3, and zone 4 is 0.0138L / cm 2 、0.0030L / cm 2 、0.0014L / cm 2 、0.0013L / cm 2 , total specific water volume 0.36L / kg; 5) The total reduction at the end of continuous casting solidification is 15mm, and the reductions at 13.5m, 14.6m, 15.7m, 16.8m, and 17.9m are 1.0mm, 3.0mm, 4.0mm, 5.0mm, and 2.0mm respectively. The specific pressure of the straightening rollers for the reduction is shown in Table 4; Table 4 The tension and straightening roller pressure of 45 steel in the light reduction process

[0024] 6) The 210 square bearing steel billet produced according to the above steps has a central shrinkage cavity of about 1.0. The carbon segregation of medium carbon is unstable, with individual values ​​above 1.20, and there are light pressure cracks on the surface of the billet.

[0025] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as generally understood by those skilled in the art to which this application belongs. It should also be understood that terms such as those defined in common dictionaries should be understood to have meanings consistent with the meanings in the context of the prior art, and will not be interpreted with idealized or overly formal meanings unless defined as herein.

[0026] Based on the above ideal embodiments of the present invention, the relevant staff can make various changes and modifications without departing from the technical concept of the present invention through the above description. The technical scope of the present invention is not limited to the contents of the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A method for controlling cracks in 210 square bearing steel under light pressure, characterized in that: The following steps are involved: Step 1: Roll gap calibration: Before continuous casting of bearing steel, the roll gap calibration is carried out to ensure that the roll gap error of the same flow under hot billet pressure is ≤±1mm, and the roll gap error of different flows is ≤±2mm. The roll gap error refers to the deviation from the average roll gap of the same flow. The hot billet pressure control range is 60kN~80kN; Step 2: Continuous casting speed control: Continuous casting speed control is performed according to the superheat of the molten steel in the tundish. When the superheat is less than 20°C, the pulling speed is 1.15m / min; when the superheat is 20-40°C, the pulling speed is 1.10m / min; when the superheat is greater than 40°C, the pulling speed is 1.05m / min; Step 3: Electromagnetic stirring of the crystallizer at 350A / 2.5Hz, and electromagnetic stirring of the end at 300A / 10Hz; Step 4: The second cooling zone below the crystallizer is 1 (full roller zone), 2, 3, and 4, with lengths of 0.42m, 1.79m, 1.79m, and 1.43m, respectively. The corresponding water flow densities are 0.0138L / cm 2 、0.0030L / cm 2 、0.0014L / cm 2 、0.0013L / cm 2 , the total specific water volume is 0.36L / kg; Step 5: The total reduction at the end of solidification is 17mm. Each stream has 7 straightening rollers, which are numbered M1-M7 from upstream to downstream. When performing light reduction, only straightening rollers M3-M7 are enabled. The distances between straightening rollers M3-M7 and the meniscus of the crystallizer are 13.5m, 14.6m, 15.7m, 16.8m, and 17.9m, respectively. The reductions at straightening rollers M3-M7 are 2.5mm, 3.5mm, 4.5mm, 4.5mm, and 2mm, respectively. Step 6: Straightening roller pressure control: When performing light pressure, the pressure of the straightening rollers M3-M7 are 200~400kN, 350~550kN, 450~650kN, and 300~500kN respectively; if the pressure of the straightening roller exceeds the control range, adjust the roller gap calibration offset in steps of 0.5mm until the pressure falls within the corresponding control range of each straightening roller.

2. The method for controlling the light-pressing cracks of 210 square bearing steel according to claim 1 is characterized in that: During the entire continuous casting process, the superheat is controlled at 20-35°C, and the casting is carried out at a constant pulling speed of 1.10m / min.

3. The method for controlling the light-pressing cracks of 210 square bearing steel according to claim 1 is characterized in that: The method for controlling the straightening roller pressure in step 6 is as follows: the operating system of the continuous casting machine presses down with reference to the roll gap value of the previous roll that has been pressed down and the current roll that has not been pressed down. Since the actual roll gap of the straightening roller that has not been pressed down = the current measured roll gap + the calibrated offset + the deformation of the straightening machine - the temperature drop shrinkage, the initial value of the calibrated offset is obtained by calibration in step 1, and the corrected roll gap calibrated offset is subsequently obtained according to step 3, the deformation of the straightening machine is 0.2mm, and the temperature drop shrinkage is 0.6mm.

4. A 210 square bearing steel billet produced by the method for controlling light compression cracks of 210 square bearing steel as described in any one of claims 1 to 3.

5. The 210 square bearing steel billet according to claim 4, characterized in that: The center segregation, center porosity and center shrinkage of the 210 square bearing steel billet are all ≤0.5, the center carbon segregation is 1.02-1.14, and the billet has no surface depression cracks and core depression cracks.

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