Process for accurately regulating and controlling temperature gradient in cooling process after bar rolling

Abstract

The invention relates to the technical field of cooling processes, and discloses a process for accurately regulating and controlling the temperature gradient in the cooling process after bar rolling. Comprising the steps of process preparation, 1, initial temperature detection and segmentation after rolling, 2, pre-water-cooling I-section controlled cooling (suitable for small-specification bars), 3, pre-water-cooling II-section controlled cooling (universal for all specifications), 4, post-rolling water-cooling section fine adjustment, 5, slow cooling and temperature equalization treatment and 6, quality monitoring and feedback optimization. Through sectional type cooling (pre-water-cooling I section, pre-water-cooling II section and after-rolling water-cooling section) and real-time feedback adjustment, the full-length temperature difference of the bar is controlled within + / -15 DEG C, the radial temperature gradient is smaller than or equal to 30 DEG C / mm and is reduced by 60% or above compared with a traditional process, and the deformation and cracking risks caused by internal stress are remarkably reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of cooling process, in particular to the temperature gradient accurate control process in the cooling process after bar rolling. BACKGROUND

[0002] In the field of bar rolling, the cooling process after bar rolling plays a key role in product quality. With the development of industry, the requirements for bar performance in various industries are increasingly stringent. Not only does the bar need to have high strength, but it also needs to ensure good toughness, uniform microstructure and dimensional stability.

[0003] The traditional cooling process after bar rolling has many problems. On the one hand, the temperature gradient in the cooling process is difficult to control accurately, and the cooling speed of different parts of the bar is quite different. For example, the cooling speed of the surface and the center of the bar can differ by several times due to different heat dissipation conditions, which leads to a large internal stress in the bar, and in the subsequent processing or use process, defects such as deformation and cracking are prone to occur. On the other hand, different steel grades have different cooling process requirements. For example, the phase change characteristics and microstructure transformation law of alloy steel, gear steel and bearing steel are significantly different, and the conventional cooling process cannot meet the individual cooling needs of various steel grades, resulting in uneven microstructure of the bar and inability to fully utilize the performance advantages of the material. SUMMARY

[0004] (I) Technical problems solved In view of the shortcomings of the prior art, the present application provides a temperature gradient accurate control process in the cooling process after bar rolling, which has the advantages of accurate control of temperature gradient and reduction of internal stress, and solves the problem of difficult accurate control of temperature gradient in the cooling process.

[0005] (II) Technical solutions In order to achieve the above-mentioned accurate control of temperature gradient and reduction of internal stress, the present application provides the following technical solutions: a temperature gradient accurate control process in the cooling process after bar rolling, including process preparation, step 1: initial temperature detection and segmentation after rolling, step 2: pre-water cooling Ⅰ segment controlled cooling (suitable for small size bar), step 3: pre-water cooling Ⅱ segment controlled cooling (universal for all sizes), step 4: post-rolling water cooling segment fine adjustment, step 5: slow cooling and temperature equalization treatment, and step 6: quality monitoring and feedback optimization, the process preparation; Equipment inspection and parameter presetting: Check the nozzle state of the water cooling system (including pre-water cooling Ⅰ segment, pre-water cooling Ⅱ segment and post-rolling water cooling segment) to ensure that the annular nozzles are not blocked, the angle deviation is ≤1°, and the water flow uniformity error is ≤5%; Preset cooling parameters according to steel characteristics: Low carbon steel / low alloy steel: cooling target temperature 700-750℃, pre-water cooling I section water volume 800-1000m 3 / h, water pressure 7.5bar; Bearing steel / gear steel: cooling target temperature 800-850℃, pre-water cooling II section water volume 600-800m 3 / h, water pressure 7.5bar; High carbon steel: cooling target temperature 650-700℃, post-rolling water cooling section water volume 1000-1200m 3 / h, water pressure 7.5bar; Calibration temperature detection instrument (such as infrared temperature detector), ensure measurement error ≤±5℃, data sampling frequency ≥10Hz; Cooling path planning: According to the bar specification (Φ20-100mm), select the cooling path: Small specification (Φ20-50mm): enable pre-water cooling I section + pre-water cooling II section + post-rolling water cooling section full process; Large specification (Φ50-100mm): enable pre-water cooling II section + post-rolling water cooling section, skip pre-water cooling I section to avoid excessive cooling.

[0006] Preferably, the step 1: post-rolling initial temperature detection and segmentation: After the bar exits the reducing mill unit, the initial temperature (850-950℃) is detected in real time by the infrared temperature detector, and is segmented according to the specification: High temperature section (≥900℃): preferentially enter pre-water cooling I section; Medium temperature section (850-900℃): directly enter pre-water cooling II section; The roller speed adjustment (0.83-18m / s) is adopted to control the residence time of the bar in each cooling section, to ensure that the cooling time deviation of each section is ≤2s.

[0007] Preferably, the step 2: pre-water cooling I section controlled cooling (applicable to small specification bars): Cooling method: adopt sleeve type cooling pipe + ring seam nozzle to realize 360° uniform water spraying; Parameter control: Water volume: 800-1000m 3 / h (dynamically adjusted according to the initial temperature, increase 100m 3 / h for every 50℃ increase); Water temperature: ≤35℃ (adopt pure ring water, turbidity ≤10NTU); Cooling target: reduce the bar temperature to 800-850℃, with temperature drop rate controlled at 50-80℃ / s; Feedback regulation: real-time monitoring by outlet temperature detector, if the temperature deviation exceeds ±20℃, automatically adjust the water quantity (adjustment accuracy ±50m 3 / h).

[0008] Preferably, the step 3: pre-water cooling II segment controlled cooling (full specification universal): Cooling mode: 3 groups of water cooling boxes in series, 4 channels in each group, using "water spray + counterattack water + counterattack gas" combined mode to avoid the formation of steam film on the surface of the bar; Parameter control: Water quantity: 600-800m 3 / h (upper limit for small size, lower limit for large size); Water pressure: 7.5bar, nozzle and bar distance 150-200mm; Cooling target: reduce the bar temperature to 750-800℃, temperature drop rate control at 30-50℃ / s; Special control: for bearing steel prone to net carbide, use "pulse water spray" (water spray 10s, stop 5s) in this section to reduce local supercooling.

[0009] Preferably, the step 4: post-rolling water cooling segment fine adjustment: Cooling mode: 3 groups of water cooling boxes can be independently controlled, adjust the water spray intensity according to the radial temperature gradient of the bar (detected by the built-in thermocouple), when the radial temperature difference is >30℃ / mm, open the inner nozzle to enhance cooling; Parameter control: Water quantity: 1000-1200m 3 / h (upper limit for high carbon steel); Cooling target: final temperature control at 650-800℃ (preset according to steel grade), full length temperature difference ≤15℃; Bypass switching: if the detected temperature is lower than the target value by more than 50℃, automatically switch to the bypass roller and stop cooling.

[0010] Preferably, the step 5: slow cooling and temperature equalization treatment: The cooled bar enters the step-by-step cooling bed, opens the heat preservation cover (width 9m), controls the temperature gradient in the cooling bed ≤10℃ / m; Cooling bed conveying speed: 0.1-0.3m / s, adjusted according to the bar size, ensure the cooling time: small size ≥30min, large size ≥60min; Temperature equalization treatment: through the hot air circulation at the bottom of the cooling bed (temperature 500-600℃), make the internal temperature of the bar uniform, the final temperature difference ≤5℃.

[0011] Preferably, the step 6: quality monitoring and feedback optimization: Real-time monitoring: Temperature measuring points are arranged at the outlet of each cooling section, and data are uploaded to the control system in real time to generate a temperature gradient curve (sampling interval 1s); Periodically (every 2 hours), samples are taken for metallographic analysis to check the grain size and carbide distribution, and if abnormalities (such as coarse grains) occur, the cooling rate (±10℃ / s) is adjusted immediately; Parameter self-learning optimization: The system automatically records the cooling parameters and final performance data of different steel grades and specifications to establish a database; When the temperature deviation of 3 consecutive batches of the same specification product is >20℃, the initial parameters (such as water quantity and roller speed) are automatically corrected, and the correction amplitude is ≤10%; Equipment maintenance: Nozzles are cleaned daily, and temperature measuring instruments are calibrated monthly; Cooling pipes (material 2Cr13) are replaced every quarter to ensure water flow uniformity.

[0012] (Three) beneficial effects Compared with the prior art, the present application provides a rod rolling post-cooling process for precise control of temperature gradient, which has the following beneficial effects: 1. The rod rolling post-cooling process for precise control of temperature gradient, which is precisely controllable in temperature gradient, reduces internal stress, and through segmented cooling (pre-water cooling Ⅰ section, Ⅱ section and post-rolling water cooling section) and real-time feedback adjustment, the full-length temperature difference of the rod is controlled within ±15℃, and the radial temperature gradient is ≤30℃ / mm, which is reduced by more than 60% compared with the traditional process, significantly reducing the risk of deformation and cracking caused by internal stress.

[0013] 2. The rod rolling post-cooling process for precise control of temperature gradient, which is suitable for multiple steel grades and specifications, improves performance consistency, and presets differentiated cooling parameters (such as target temperature and water quantity) for different steel grades such as low-carbon steel, bearing steel and high-carbon steel, and avoids the precipitation of network carbide through special control such as "pulse water spraying", so that the mechanical property compliance rate of the product is improved to more than 98%, and the cooling path is differentiated for small and large specifications, small specifications are cooled throughout the process to ensure rapid cooling, and large specifications skip pre-water cooling Ⅰ section to avoid excessive cooling, and the adaptability is improved by 40%.

[0014] 3、The temperature gradient accurate regulation process during the rod rolling and cooling process, the process intelligently self-optimizes, reduces quality fluctuations, through real-time temperature monitoring (sampling interval 1s), metallographic analysis feedback and parameter self-learning system, when the temperature deviation of 3 batches of products in succession >20℃, the water quantity, roller speed and other parameters are automatically corrected (correction amplitude ≤10%), so that the process stability is improved by 50%, and the manual intervention frequency is reduced by 60%.

[0015] 4、The temperature gradient accurate regulation process during the rod rolling and cooling process, the process prolongs the equipment life and reduces the operation and maintenance cost, through daily nozzle cleaning, monthly instrument calibration, quarterly cooling pipe replacement (2Cr13 material) and other maintenance mechanisms, the cooling system water flow uniformity error is ensured to be ≤5%, the equipment failure downtime is shortened to <2 hours per month, and the operation and maintenance cost is reduced by 20%.

[0016] 5、The temperature gradient accurate regulation process during the rod rolling and cooling process, the process strengthens the uniformity of the heat treatment processed structure, the hot air circulation (500-600℃) of the cooling bed and the time control (small size ≥30min, large size ≥60min) of the staged cooling, so that the final temperature difference of the rod is ≤5℃, the grain size deviation is <1 level, the carbide distribution uniformity is improved by 30%, and high-quality billets are provided for subsequent processing. DETAILED DESCRIPTION

[0017] The technical solutions in the embodiments of the present application will be clearly and completely described below in combination with the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0018] Process preparation: Equipment inspection and parameter presetting: Check the nozzle state of the water cooling system (including pre-water cooling I section, pre-water cooling II section and post-rolling water cooling section) to ensure that the annular nozzle has no blockage, the angle deviation is ≤1°, and the water flow uniformity error is ≤5%; Preset the cooling parameters according to the steel characteristics: Low carbon steel / low alloy steel: cooling target temperature 700-750℃, pre-water cooling I section water quantity 800-1000m 3 / h, water pressure 7.5bar; Bearing steel / gear steel: cooling target temperature 800-850℃, pre-water cooling II section water quantity 600-800m 3 / h, water pressure 7.5bar; High carbon steel: cooling target temperature 650-700℃, post-rolling water cooling section water quantity 1000-1200m3 Water pressure: 7.5 bar Calibrate the temperature detection instrument (such as infrared thermometer) to ensure measurement error ≤±5℃, data sampling frequency ≥10Hz; Cooling path planning Select the cooling path according to the bar specifications (Φ20-100mm): Small specifications (Φ20-50mm): enable pre-water cooling I + pre-water cooling II + full process of post-rolling water cooling; Large specifications (Φ50-100mm): enable pre-water cooling II + post-rolling water cooling, skip pre-water cooling I to avoid excessive cooling; Specific process steps Step 1: Initial temperature detection and segmentation after rolling After the bar exits the reducing mill unit, the initial temperature (850-950℃) is detected in real time by the infrared thermometer, and segmented according to specifications: High temperature section (≥900℃): preferentially enter pre-water cooling I; Medium temperature section (850-900℃): directly enter pre-water cooling II; Use roller speed adjustment (0.83-18m / s) to control the residence time of the bar in each cooling section, ensuring that the cooling time deviation of each section is ≤2s; Step 2: Pre-water cooling I section control cooling (applicable to small size bars) Cooling method: use sleeve cooling pipe + ring seam nozzle to achieve 360° uniform water spraying; Parameter control: Water quantity: 800-1000m 3 / h (adjust dynamically according to initial temperature, increase by 100m 3 / h for every 50℃ increase); Water temperature: ≤35℃ (use clean ring water, turbidity ≤10NTU); Cooling target: reduce the bar temperature to 800-850℃, with a temperature drop rate of 50-80℃ / s; Feedback adjustment: monitor in real time through the outlet temperature detector, if the temperature deviation exceeds ±20℃, automatically adjust the water quantity (adjustment accuracy ±50m 3 / h); Step 3: Pre-water cooling II section control cooling (universal for all specifications) Cooling method: 3 groups of water cooling boxes in series, each group has 4 channels, using "spraying + counterattack water + counterattack gas" combined mode to avoid the formation of steam film on the surface of the bar; Parameter control: Water quantity: 600-800m3 / h (upper limit for small sizes, lower limit for large sizes); Water pressure: 7.5 bar, nozzle-to-bar distance: 150-200 mm; Cooling target: reduce bar temperature to 750-800°C at a rate of 30-50°C / s; Special control: for steels prone to network carbides such as bearing steel, use "pulse water spraying" (10s spraying, 5s pause) to reduce local supercooling; Step 4: Post-rolling water cooling section fine adjustment Cooling method: 3 groups of water cooling boxes can be independently controlled, adjust water spraying intensity according to radial temperature gradient (detected by built-in thermocouple), when radial temperature difference > 30°C / mm, open inner nozzle to enhance cooling; Parameter control: Water flow: 1000-1200 m 3 / h (upper limit for high carbon steel); Cooling target: control final temperature to 650-800°C (preset according to steel type), full-length temperature difference ≤ 15°C; Bypass switching: if temperature is detected to be 50°C below target value, automatically switch to bypass roller and stop cooling; Step 5: Slow cooling and temperature equalization treatment After cooling, the bar enters the step-by-step cooling bed, opens the heat preservation cover (width 9m), controls the temperature gradient in the cooling bed ≤ 10°C / m; Cooling bed conveying speed: 0.1-0.3 m / s, adjust according to bar size to ensure cooling time: small size ≥ 30 min, large size ≥ 60 min; Temperature equalization treatment: use hot air circulation at the bottom of the cooling bed (temperature 500-600°C) to homogenize the internal temperature of the bar, final temperature difference ≤ 5°C; Step 6: Quality monitoring and feedback optimization Real-time monitoring: Set temperature measurement points at the outlet of each cooling section, data is uploaded to the control system in real time to generate temperature gradient curves (sampling interval 1s); Regularly (every 2 hours) take samples for metallographic analysis to check grain size and carbide distribution, if abnormalities (such as coarse grains) occur, adjust the cooling rate (±10°C / s) immediately; Parameter self-learning optimization: The system automatically records cooling parameters and final performance data for different steel types and sizes, establishing a database; When temperature deviation > 20°C occurs for 3 consecutive batches of the same size product, automatically correct the initial parameters (such as water flow, roller speed), correction amplitude ≤ 10%; Equipment maintenance: Clean the nozzle of debris every day, and calibrate the temperature measuring instrument every month; Replace the cooling pipe (material 2Cr13) every quarter to ensure the uniformity of water flow.

[0019] Further, the process temperature gradient is precisely controllable, reducing internal stress. Through segmented cooling (pre-water cooling Ⅰ section, Ⅱ section, and post-rolling water cooling section) and real-time feedback adjustment, the full-length temperature difference of the bar is controlled within ±15℃, and the radial temperature gradient is ≤30℃ / mm, which is reduced by more than 60% compared to the traditional process, significantly reducing the risk of deformation and cracking caused by internal stress; Further, the process is suitable for multiple steel types and specifications, improving performance consistency. Differentiated cooling parameters (such as target temperature, water quantity) are preset for different steel types such as low-carbon steel, bearing steel, and high-carbon steel, and special control such as "pulse water spraying" is used to avoid the precipitation of network carbides, so that the mechanical property compliance rate of the product is improved to more than 98%, and the cooling path is differentiated for small and large specifications. Small specifications ensure rapid cooling through full-flow cooling, and large specifications skip pre-water cooling Ⅰ section to avoid excessive cooling, with a 40% increase in adaptability; Further, the process is intelligently self-optimized, reducing quality fluctuations. Through real-time temperature monitoring (sampling interval 1s), metallographic analysis feedback, and parameter self-learning system, when the temperature deviation of 3 consecutive batches of products is >20℃, the water quantity, roller speed, and other parameters are automatically corrected (correction amplitude ≤10%), which improves the process stability by 50% and reduces the frequency of manual intervention by 60%; Further, the process prolongs the service life of the equipment and reduces the operation and maintenance cost. Through daily nozzle cleaning, monthly instrument calibration, quarterly cooling pipe replacement (2Cr13 material), and other maintenance mechanisms, the cooling system water flow uniformity error is ≤5%, the equipment failure downtime is shortened to <2 hours per month, and the operation and maintenance cost is reduced by 20%; Further, the process enhances the uniformity of the treated uniform temperature, and the hot air circulation (500-600℃) of the cooling bed and the graded cooling time control (small specifications ≥30min, large specifications ≥60min) make the final temperature difference of the bar ≤5℃, the grain size deviation <1 level, and the carbide distribution uniformity improved by 30%, providing high-quality billets for subsequent processing.

[0020] Although embodiments of the present application have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made therein without departing from the principles and spirit of the application, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A process for precisely controlling the temperature gradient during the cooling of a bar after rolling, comprising process preparation, Step 1: initial temperature detection and segmentation after rolling, Step 2: pre-water cooling I segment controlled cooling (applicable to small size bars), Step 3: pre-water cooling II segment controlled cooling (universal for all sizes), Step 4: fine adjustment of the water cooling segment after rolling, Step 5: slow cooling and temperature equalization treatment, and Step 6: quality monitoring and feedback optimization, characterized in that: The process preparation; Equipment inspection and parameter presetting: Check the nozzle state of the water cooling system (including pre-water cooling I section, pre-water cooling II section, and post-rolling water cooling section) to ensure that the annular nozzle is not blocked, the angle deviation is ≤1°, and the water flow uniformity error is ≤5%; Preset cooling parameters according to the characteristics of the steel grade: Low carbon steel / low alloy steel: cooling target temperature 700-750°C, pre-water cooling Ⅰ section water volume 800-1000m 3 / h, water pressure 7.5 bar; Bearing / gear steel: cooling target temperature 800-850℃, pre-water cooling Ⅱ section water amount 600-800m 3 / h, water pressure 7.5 bar; High carbon steel: cooling target temperature 650-700°C, water quantity in post-rolling water cooling section 1000-1200 m 3 / h, water pressure 7.5 bar; Calibrate the temperature detection instrument (such as infrared temperature detector) to ensure that the measurement error is ≤±5℃ and the data sampling frequency is ≥10Hz; Cooling path planning: Select the cooling path according to the bar specifications (Φ20-100mm): Small specifications (Φ20-50mm): enable pre-water cooling I section + pre-water cooling II section + post-rolling water cooling section full process; Large specifications (Φ50-100mm): enable pre-water cooling II section + post-rolling water cooling section, skip pre-water cooling I section to avoid excessive cooling.

2. The process for precise temperature gradient control during post-rolling cooling of a bar according to claim 1, characterized in that: Step 1: Post-rolling initial temperature detection and segmentation: After the bar exits the reducing mandrel unit, the initial temperature (850-950℃) is detected in real time by an infrared temperature detector, and is segmented according to specifications: High temperature section (≥900℃): preferentially enter pre-water cooling I section; Medium temperature section (850-900℃): directly enter pre-water cooling II section; Use roller speed adjustment (0.83-18m / s) to control the residence time of the bar in each cooling section to ensure that the cooling time deviation of each section is ≤2s.

3. The process for precise temperature gradient control during post-rolling cooling of a bar according to claim 1, characterized in that: Step 2: Pre-water cooling I section controlled cooling (applicable to small size bars): Cooling method: use sleeve type cooling pipe + ring gap nozzle to achieve 360° uniform water spraying; Parameter control: Water quantity: 800-1000 m 3 / h (dynamically adjusted according to initial temperature, increasing 100 m 3 / h for every 50°C increase) Water temperature: ≤35℃ (use clear ring water, turbidity ≤10NTU); Cooling target: reduce the bar temperature to 800-850℃, with a temperature drop rate controlled at 50-80℃ / s; Feedback regulation: real-time monitoring by outlet temperature meter, if temperature deviation exceeds ±20℃, automatically adjust water quantity (adjustment accuracy ±50m 3 / h).

4. The process for precise temperature gradient control during post-rolling cooling of a bar according to claim 1, characterized in that: Step 3: Pre-water cooling II section controlled cooling (universal for all specifications): Cooling method: 3 groups of water cooling boxes are connected in series, each group has 4 channels, and uses a "water spraying + counterattack water + counterattack gas" combined mode to avoid the formation of steam film on the surface of the bar; Parameter control: Water quantity: 600-800 m 3 / h (upper limit for small size, lower limit for large size) Water pressure: 7.5bar, nozzle to bar distance is maintained at 150-200mm; Cooling target: reduce the bar temperature to 750-800℃, with a temperature drop rate controlled at 30-50℃ / s; Special control: for steel grades prone to reticular carbides in bearing steel, use "pulse water spraying" (spray water for 10s and stop for 5s) in this section to reduce local overcooling.

5. The process for precise temperature gradient control during post-rolling cooling of a bar as claimed in claim 1, wherein: Step 4: Post-rolling water cooling section fine adjustment: Cooling method: 3 groups of water cooling boxes can be independently controlled, adjust the water spraying intensity according to the radial temperature gradient of the bar (detected by the built-in thermocouple), and when the radial temperature difference is >30℃ / mm, open the inner nozzle to enhance cooling; Parameter control: Water quantity: 1000-1200 m 3 / h (upper limit for high carbon steel); Cooling target: control the final temperature at 650-800℃ (preset according to steel grade), and the full length temperature difference is ≤15℃; Bypass switching: if the detected temperature is lower than the target value by 50℃ or more, automatically switch to the bypass roller to stop cooling.

6. The process for precise temperature gradient control during post-rolling cooling of a bar as claimed in claim 1, wherein: Step 5: Slow cooling and temperature equalization treatment: After cooling, the bar enters the step-by-step cooling bed, opens the heat preservation cover (width 9m), and controls the temperature gradient in the cooling bed to ≤10℃ / m; Cooling bed conveying speed: 0.1-0.3 m / s, adjusted according to bar size, ensure cooling time: small size ≥ 30 min, large size ≥ 60 min; Uniform temperature treatment: through hot air circulation at the bottom of the cooling bed (temperature 500-600℃), the internal temperature of the bar is homogenized, and the final temperature difference is ≤5℃.

7. The process for precise temperature gradient control during post-rolling cooling of a bar as claimed in claim 1, wherein: Step 6: quality monitoring and feedback optimization: Real-time monitoring: Temperature measurement points are set at the outlet of each cooling section, and data is uploaded to the control system in real time to generate a temperature gradient curve (sampling interval 1s); Periodically (every 2 hours), samples are taken for metallographic analysis to check grain size and carbide distribution. If abnormalities occur (such as coarse grains), adjust the cooling rate immediately (±10℃ / s); Parameter self-learning optimization: The system automatically records the cooling parameters and final performance data of different steel grades and sizes, establishing a database; When the temperature deviation of the same size product is >20℃ for 3 consecutive batches, automatically correct the initial parameters (such as water quantity, roller speed), with a correction amplitude of ≤10%; Equipment maintenance: Clean the nozzle of debris daily and calibrate the temperature measuring instrument every month; Replace the cooling pipe (material 2Cr13) every quarter to ensure water flow uniformity.