Method for controlling longitudinal cracks of bearing steel casting blank with 150 square cross section

Abstract

The invention discloses a method for controlling longitudinal cracks of a bearing steel casting blank with a 150-square section, and belongs to the technical field of continuous casting of ferrous metallurgy. Five core processes are used for collaborative optimization: 1) the degree of superheat of molten steel is controlled to be 25-45 DEG C, and manual and continuous temperature measurement is adopted to ensure temperature accuracy; (2) full-process protective casting is carried out, the insertion depth of a submersed nozzle ranges from 110 mm to 125 mm, and the centering deviation is smaller than or equal to 1 mm; (3) crystallizer electromagnetic stirring (M-EMS) takes the magnetic induction intensity larger than or equal to 1000 Gs as the standard, the current is set to be 380-450 A, the frequency is set to be 5-7 Hz, and the forward and reverse rotation period is set; 4, vibration parameters are set according to the pulling speed, and the cooling water amount is synchronously controlled, 5, a composite flow field is formed through a water gap of four water outlet holes with the downward inclination angle of 45 degrees from the outer wall to the inner wall, and the longitudinal crack occurrence rate is reduced to 0.5%-1% from 8%-10%. The casting blank surface quality and component uniformity are both considered, and the method is suitable for large-scale industrial production of the high-carbon bearing steel.

Description

Technical Field

[0001] This invention belongs to the field of continuous casting technology in iron and steel metallurgy, and relates to the production of high-carbon bearing steel billets. Specifically, it relates to a method for controlling longitudinal cracks in 150mm×150mm (hereinafter referred to as 150 square) cross-section bearing steel billets. By optimizing the core continuous casting process, the problem of high incidence of longitudinal cracks in bearing steel billets is solved, and the surface and internal quality of the billets are improved. Background Technology

[0002] Bearing steel, a key material for manufacturing bearing rolling elements and raceways, typically has a carbon content of 0.95%-1.10%, classifying it as a typical high-carbon steel. In the continuous casting production of 150 cubic meter small billets, longitudinal cracks are the core defect restricting billet quality. They not only lead to wire breakage and cracking in subsequent rolling processes but also reduce the fatigue life of bearing products and even cause equipment malfunctions.

[0003] The formation of longitudinal cracks is directly related to the lubrication of slag in the crystallizer, the flow characteristics of molten steel, and the uniformity of the initial billet shell growth. Existing processes have significant shortcomings in the design of key parameters: 1. Poor adaptability of vibration parameters: In order to control the depth of vibration marks, the traditional process adopts a "high frequency, low amplitude" mode throughout the casting process, without considering the impact of casting speed changes on the inflow of liquid slag. When the casting speed is ≤1.8m / min, the low amplitude makes it difficult for liquid slag to fill the air gap between the billet shell and the crystallizer wall; when the casting speed is >1.8m / min, the high frequency can easily cause liquid slag to be entrained. Both problems exacerbate the risk of longitudinal cracks.

[0004] 2. Inadequate electromagnetic stirring design: Traditional processes control compositional segregation by "reducing the intensity of electromagnetic stirring (M-EMS) in the crystallizer," with magnetic induction intensity generally below 800 Gs. This results in insufficient activity of molten steel, slow and uneven melting of the protective slag, low heat transfer efficiency, and thickness differences in the initial billet shell due to localized undercooling, which become initiation points for longitudinal cracks. At the same time, the lack of a stirring direction switching logic for different bearing steel grades makes it easy for unidirectional flow fields to form dead zones, further deteriorating compositional and temperature uniformity.

[0005] 3. Mismatched submerged entry nozzle structure: 150 cubic meter small billet continuous casting mostly adopts single-hole or double-hole submerged entry nozzles. The steel flow trajectory is simple, and the excessive local scouring force can easily damage the billet shell. In addition, the flow field disturbance is insufficient, which cannot effectively promote the melting of protective slag and cannot meet the solidification requirements of high carbon bearing steel.

[0006] In summary, existing processes have not formed a systematic solution for bearing steel with a cross-section of 150 cubic meters, and the incidence of longitudinal cracks has remained at 8%-10% for a long time. There is an urgent need for innovative processes to achieve a breakthrough. Summary of the Invention

[0007] The purpose of this invention is to overcome the defects of existing continuous casting production of 150 cubic meter cross-section bearing steel and to provide a method that uses "composite vibration parameters of phased casting speed + precise electromagnetic stirring + special four-hole nozzle" for coordinated control. This method simultaneously optimizes three dimensions: slag lubrication, molten steel flow, and heat transfer uniformity, thereby significantly reducing the incidence of longitudinal cracks.

[0008] The technical solution adopted in this invention is: A method for controlling longitudinal cracks in a 150mm square section bearing steel billet includes the following steps: Step 1). Steel temperature and temperature control: After the steel ladle has passed the LF or RH treatment, it is hoisted to the continuous casting ladle turret. After the ladle starts pouring, manual temperature measurement is used first. After the tundish is full of molten steel (tonnage of 28-32 tons), it is switched to continuous temperature measurement to ensure that the deviation between manual and continuous temperature measurement is ≤±2℃ and the superheat of the continuous casting molten steel is stable at 25-45℃.

[0009] Step 2). Full-process protective casting and nozzle control: After the ladle starts casting, implement full-process protective casting to avoid contact between molten steel and air; adjust the centering accuracy of the tundish and immersion nozzle to ensure that after the nozzle is inserted into the crystallizer, the front-to-back and left-to-right distance between it and the copper tube wall is ≤1mm, and the nozzle insertion depth is 110-125mm.

[0010] Step 3). Crystallizer electromagnetic stirring (M-EMS) parameter settings: With a magnetic induction intensity ≥1000Gs as the core control standard, set the M-EMS current intensity to 380-450A and the stirring frequency to 5-7Hz; at the same time, set the forward and reverse rotation cycles according to the steel grade. GCr15 steel executes "forward rotation for 12 seconds → stop for 2 seconds → reverse rotation for 12 seconds", 9SiCr steel executes "forward rotation for 9 seconds → stop for 2 seconds → reverse rotation for 9 seconds", and 100D steel executes "forward rotation for 15 seconds → stop for 2 seconds → reverse rotation for 15 seconds", all of which are cyclical.

[0011] Step 4). Setting vibration and cooling parameters for casting speed: Install the crystallizer vibration control system and dynamically adjust the vibration parameters according to the continuous casting speed: when the casting speed is ≤1.8m / min, the vibration frequency is 250-300cpm and the amplitude is 9mm; when the casting speed is >1.8m / min, the vibration frequency is 200-250cpm and the amplitude is 11mm. At the same time, set the primary cooling water flow rate to 130-140m3 / h and the secondary cooling water flow rate to 0.35-0.40L / kg. Use constant casting speed to cast until the tundish casting is completed.

[0012] Step 5). Use of a dedicated four-hole submersible nozzle: A four-hole submersible nozzle is used. Four outlet holes are evenly arranged around the bottom of the nozzle. That is, with the center point of the nozzle as the vertex, straight lines drawn from the vertex through the center points of two adjacent outlet holes form a right triangle. The outlet holes are 10cm away from the bottom of the nozzle. The four outlet holes are inclined downward at 45° from the outer wall to the inner wall of the nozzle. The diameter of the outlet holes is 18-22mm, which ensures that the molten steel forms a "rotation-convection" composite flow field in the crystallizer.

[0013] Furthermore, in step 1), the temperature of the ladle after LF or RH treatment is raised to 65-85°C above the liquidus temperature of the bearing steel to ensure that the superheat of the molten steel is stable at 25-45°C during continuous casting.

[0014] Furthermore, in step 3), the stirring area of ​​M-EMS covers the critical solidification area 50-200mm below the surface of the molten steel in the crystallizer, and the measured magnetic induction intensity reaches 1050-1200Gs.

[0015] Furthermore, in step 4), the adjustment of vibration parameters is synchronized with the change in pulling speed, with a response delay of ≤5s, to avoid untimely inflow of liquid slag caused by pulling speed fluctuations.

[0016] Furthermore, in step (4), when the steel grade is GCr15 steel, the forward and reverse rotation cycle is "forward rotation for 12 seconds → stop for 2 seconds → reverse rotation for 12 seconds", when the steel grade is 9SiCr steel, the forward and reverse rotation cycle is "forward rotation for 9 seconds → stop for 2 seconds → reverse rotation for 9 seconds", and when the steel grade is 100D steel, the forward and reverse rotation cycle is "forward rotation for 15 seconds → stop for 2 seconds → reverse rotation for 15 seconds", all of which are cyclical.

[0017] Furthermore, in step 5), the material of the four-hole nozzle is selected as zirconium-containing refractory material to ensure good corrosion resistance during the casting process of high carbon bearing steel, with a service life of ≥8 hours / piece.

[0018] The beneficial effects of this invention are: First, the longitudinal crack control effect is significant: through the combination of "partial tension vibration + 1000Gs-level EMS + four-hole nozzle", the longitudinal crack incidence of 150 cubic section bearing steel billet has been reduced from the traditional 8%-10% to 0.5%-1%, and the billet qualification rate has been increased by more than 9 percentage points.

[0019] Secondly, it has strong process adaptability: the vibration parameters of different drawing speeds solve the problem of insufficient slag lubrication under different drawing speeds. The 1000Gs-grade EMS strength ensures the activity of molten steel, while the forward and reverse cycle process of different steel grades avoids the deterioration of compositional segregation, achieving the dual goals of "inhibiting cracks" and "controlling segregation".

[0020] Third, the steel grades are well-targeted: the electromagnetic stirring forward and reverse rotation cycles are designed to address the differences in solidification characteristics of typical bearing steels such as GCr15, 9SiCr, and 100D, avoiding quality fluctuations caused by a "one-size-fits-all" process and adapting to the production needs of multiple grades of bearing steel.

[0021] Fourth, a balance between cost and practicality: No new continuous casting equipment is required; it can be achieved simply by optimizing parameters and improving the nozzle structure. After optimization, the consumption of protective slag is stabilized at 0.35-0.40 kg / t, meeting the needs of large-scale industrial production. Detailed Implementation

[0022] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0023] A method for controlling longitudinal cracks in a 150mm square section bearing steel billet includes the following steps: 1. Steel temperature and temperature control: After the steel ladle has passed the LF or RH treatment, it is hoisted to the continuous casting ladle turret. After the ladle starts pouring, manual temperature measurement is used first. After the molten steel in the tundish reaches 28-32 tons (full liquid level), it is switched to continuous temperature measurement to ensure that the deviation between manual and continuous temperature measurement is ≤±2℃ and the superheat of the continuous casting steel is stable at 25-45℃.

[0024] 2. Full-process protective casting and nozzle control: After the ladle starts casting, full-process protective casting is implemented to prevent molten steel from coming into contact with air; the alignment accuracy of the tundish and immersion nozzle is adjusted to ensure that the difference between the nozzle and the copper tube wall in front and behind and left and right is ≤1mm after the nozzle is inserted into the crystallizer, and the nozzle insertion depth is controlled at 110-125mm.

[0025] 3. Crystallizer electromagnetic stirring (M-EMS) parameter settings: With a magnetic induction intensity ≥1000Gs as the core control standard, the M-EMS current intensity is set to 380-450A and the stirring frequency to 5-7Hz. At the same time, the forward and reverse rotation cycles are designed according to the differences in steel grades. GCr15 steel executes "forward rotation for 12 seconds → stop for 2 seconds → reverse rotation for 12 seconds", 9SiCr steel executes "forward rotation for 9 seconds → stop for 2 seconds → reverse rotation for 9 seconds", and 100D steel executes "forward rotation for 15 seconds → stop for 2 seconds → reverse rotation for 15 seconds", all of which are cyclical.

[0026] 4. Setting of Vibration and Cooling Parameters Based on Casting Speed: Install a crystallizer vibration control system and dynamically adjust the vibration parameters according to the continuous casting speed: when the casting speed is ≤1.8m / min, the vibration frequency is 250-300cpm and the amplitude is 9mm; when the casting speed is >1.8m / min, the vibration frequency is 200-250cpm and the amplitude is 11mm. Simultaneously, set the primary cooling water flow rate to 130-140m³ / min. 3The secondary cooling water volume is 0.35-0.40 L / kg, and constant casting speed is used until the casting of the intermediate ladle is completed.

[0027] 5. Use of dedicated four-hole submerged entry nozzle: The four-hole submerged entry nozzle is adapted to a 150mm square cross section. All four outlet holes are inclined at a 45° angle along the horizontal direction, and the included angle between the centers of adjacent outlet holes is 90°. The outlet hole diameter is 18-22mm, ensuring that the molten steel forms a "rotation-convection" composite flow field in the crystallizer.

[0028] Example 1: Production of GCr15 steel (carbon content 1.00%) 1. The GCr15 steel ladle, after LF treatment, with a liquidus temperature of +70℃ and a superheat of 35℃ for continuous casting, is hoisted to the ladle turret. 2. After centering the immersion nozzle, insert to a depth of 120mm. Set the M-EMS current to 420A and frequency to 6Hz. The forward and reverse rotation cycle is "forward rotation 12 seconds → stop 2 seconds → reverse rotation 12 seconds". 3. Continuous casting speed: 1.7 m / min; crystallizer vibration parameters: 270 cpm, amplitude: 9 mm; cooling water: 135 m³ / min. 3 / h, secondary cooling water ratio 0.38L / kg; 4. Production results: The thickness of the liquid slag layer is 12mm, the consumption of protective slag is 0.38Kg / t, 1000 casting billets were randomly selected for shot blasting and magnetic particle testing, the incidence of longitudinal cracks was 0.8%, and the central segregation was grade 0.5.

[0029] Example 2: Production of 9SiCr steel (carbon content 0.98%) 1. The 9SiCr steel ladle, after VD treatment, with a liquidus temperature of +65℃ and a superheat of 30℃ for continuous casting, is hoisted to the ladle turret. 2. After centering the immersion inlet, the insertion depth is 115mm. The M-EMS is set with a current of 400A and a frequency of 5.5Hz. The forward and reverse rotation cycle is "forward rotation for 9 seconds → stop for 2 seconds → reverse rotation for 9 seconds". 3. Continuous casting speed: 2.0 m / min; crystallizer vibration parameters: 230 cpm, amplitude: 11 mm; cooling water: 140 m³ / min. 3 / h, secondary cooling water ratio 0.35L / kg; 4. Production results: The thickness of the liquid slag layer is 12mm, the consumption of protective slag is 0.40Kg / t, 1000 casting billets were randomly selected for shot blasting and magnetic particle testing, the incidence of longitudinal cracks was 0.5%, and the center segregation level was 0.5.

[0030] Example 3: Production of 100D steel (carbon content 1.02%) 1. A 100D steel ladle treated with VD, with a liquidus temperature of +80℃ and a superheat of 40℃ for continuous casting, is hoisted to the ladle turret. 2. After centering the immersion nozzle, insert to a depth of 125mm. M-EMS setting current 440A, frequency 6.5Hz, forward and reverse cycle "forward 15 seconds → stop 2 seconds → reverse 15 seconds"; 3. Continuous casting speed: 1.6 m / min; crystallizer vibration parameters: 250 cpm, amplitude: 9 mm; cooling water: 135 m³ / min. 3 / h, secondary cooling water ratio 0.35L / kg; 4. Production results: The thickness of the liquid slag layer is 10mm, the consumption of protective slag is 0.35Kg / t, 1000 billets were randomly selected for shot blasting and magnetic particle testing, the longitudinal crack incidence rate was 0.5%, and the center segregation level was 0.5.

[0031] This invention provides a technical method to improve the lubrication effect of 150 cubic bearing steel during casting, thereby reducing surface longitudinal cracks. By setting a higher electro-stirring intensity in the crystallizer and coordinating with a forward and reverse cyclic process, the flow of protective slag is promoted to be more uniform. By setting composite vibration parameters in stages with different casting speed ranges, the vibration requirements at low and high casting speeds are met, expanding the slag inflow channel and achieving a dynamic balance between slag generation and consumption during production. A "rotation-convection" composite flow field is formed by a four-hole nozzle with a horizontal 45° inclination angle (the outlet hole is inclined downwards at 45° from the outer wall to the inner wall of the nozzle), blocking the path of longitudinal crack initiation at its source. Through the implementation of the measures in this invention, the phenomenon of surface longitudinal cracks in 150 cubic bearing steel is eliminated, thereby improving the surface quality of the cast billet and reducing finishing costs.

[0032] Those skilled in the art will understand that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as herein.

[0033] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for controlling longitudinal cracks in a 150mm square section bearing steel billet, characterized in that, Includes the following steps: Step (1) Steel temperature and temperature control: After the steel ladle has passed the LF or RH treatment, it is hoisted to the continuous casting ladle turntable. After the ladle starts casting, manual temperature measurement is used first. After the molten steel in the tundish reaches the full liquid level, it is switched to continuous temperature measurement to ensure that the deviation between manual and continuous temperature measurement is ≤±2℃ and the superheat of the continuous casting steel is stable at 25-45℃. Step (2) Full-process protective casting and nozzle control: After the ladle starts casting, full-process protective casting is implemented to avoid contact between molten steel and air; adjust the centering accuracy of the tundish and immersion nozzle to ensure that after the nozzle is inserted into the crystallizer, the front-to-back and left-to-right distance between it and the copper tube wall is ≤1mm, and the nozzle insertion depth is 110-125mm. Step (3) Setting the parameters of the crystallizer electromagnetic stirring (M-EMS): With a magnetic induction intensity ≥1000Gs as the core control standard, set the M-EMS current intensity to 380-450A and the stirring frequency to 5-7Hz; at the same time, set the forward and reverse rotation cycles according to the differences in steel grades. Step (4) Setting vibration and cooling parameters for casting speed: Install the crystallizer vibration control system and dynamically adjust the vibration parameters according to the continuous casting speed. When the casting speed is ≤1.8m / min, the vibration frequency is 250-300cpm and the amplitude is 9mm. When the casting speed is >1.8m / min, the vibration frequency is 200-250cpm and the amplitude is 11mm. At the same time, set the primary cooling water flow rate to 130-140m³ / min. 3 / h, the secondary cooling water ratio is 0.35-0.40L / kg, and constant casting speed is used until the casting of the tundish is completed; Step (5) Use of four-hole nozzle: Four-hole immersion nozzles are used. Four outlet holes are evenly arranged around the bottom of the nozzle. The outlet holes are 10cm away from the bottom of the nozzle. The four outlet holes are inclined downward at 45° from the outer wall to the inner wall of the nozzle. The diameter of the outlet holes is 18-22mm, which ensures that the molten steel forms a "rotation-convection" composite flow field in the crystallizer.

2. The method for controlling longitudinal cracks in a 150mm square cross-section bearing steel billet according to claim 1, characterized in that, In step (1), the ladle after LF or RH treatment raises the temperature of the molten steel to 65-85°C above the liquidus temperature of the bearing steel, so as to ensure that the superheat of the molten steel is stable between 25-45°C during continuous casting.

3. The method for controlling longitudinal cracks in a 150mm square cross-section bearing steel billet according to claim 1, characterized in that, In step (3), the stirring area of ​​M-EMS covers the solidification area 50-200mm below the surface of the molten steel in the crystallizer, and the magnetic induction intensity is 1050-1200Gs.

4. The method for controlling longitudinal cracks in a 150mm square cross-section bearing steel billet according to claim 1, characterized in that, In step (4), the adjustment of vibration parameters is synchronized with the change in tension speed, and the response delay is ≤5s.

5. The method for controlling longitudinal cracks in a 150mm square cross-section bearing steel billet according to claim 1, characterized in that, In step (4), when the steel grade is GCr15 steel, the forward and reverse rotation cycle is "forward rotation for 12 seconds → stop for 2 seconds → reverse rotation for 12 seconds", when the steel grade is 9SiCr steel, the forward and reverse rotation cycle is "forward rotation for 9 seconds → stop for 2 seconds → reverse rotation for 9 seconds", and when the steel grade is 100D steel, the forward and reverse rotation cycle is "forward rotation for 15 seconds → stop for 2 seconds → reverse rotation for 15 seconds", all of which are cyclical.

6. The method for controlling longitudinal cracks in a 150mm square cross-section bearing steel billet according to claim 1, characterized in that, In step (5), the material of the four-hole water nozzle is zirconium-containing refractory material, with a service life of ≥8 hours / piece.

7. The method for controlling longitudinal cracks in a 150mm square cross-section bearing steel billet according to claim 1, characterized in that, It is suitable for continuous casting production of high-carbon bearing steel with a carbon content of 0.95%-1.10%, and the consumption of protective slag is 0.35-0.40Kg / t.

8. A 150 square section bearing steel billet prepared based on the method for controlling longitudinal cracks in a 150 square section bearing steel billet according to any one of claims 1-7.