HRB400E square billet shear crack control method

Abstract

The invention provides an HRB400E square billet shear crack control method, and relates to the technical field of steel continuous casting. By optimizing chemical components of molten steel and continuous casting process parameters, including converter endpoint control, LF furnace refining, tundish temperature management, crystallizer cooling, vibration modes, secondary cooling section configuration, guide roller precision, withdrawal and straightening machine operation and hydraulic shearing conditions, the shear cracks of the square billet are effectively inhibited; the method has the advantages of effectively reducing square billet shear cracks, avoiding surface defects of rolled steel bars and improving product quality and yield.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of steel continuous casting, in particular to a HRB400E square billet shearing crack control method. BACKGROUND

[0002] In the process of steel continuous casting, square billet red billet cutting is one of the key links. The traditional flame cutting method has many inherent defects: obvious steel billet loss occurs during cutting, resulting in a decrease in billet yield; at the same time, a large amount of oxidized slag is generated, which needs to be cleaned regularly by manual operation, significantly increasing the labor intensity and safety risk of the operator; the cutting process takes a long time, causing a significant decrease in the temperature of the steel billet in the straight rolling process, affecting the stability of the subsequent rolling quality; in addition, in order to meet the increasingly stringent ultra-low emission environmental protection standards, a dust removal system must be additionally configured, increasing equipment investment and operating costs. In contrast, the hydraulic shearing technology has shown significant advantages: no physical cutting loss, effectively improving metal yield; no cutting slag strip and oxidized slag is generated, simplifying the production environment maintenance; fast shearing speed keeps the steel billet at a high temperature after shearing, which is more conducive to the continuous implementation of the straight rolling process; and no smoke and dust emission is needed to add a dust removal device, which has outstanding value in cost reduction and efficiency improvement and energy saving and environmental protection. Therefore, in recent years, the use of hydraulic shearing instead of flame cutting in square billet continuous casting has become a development trend in the industry, especially in steel plants with straight rolling processes. However, the hydraulic shearing faces a key problem in actual application: cracks are easily induced at the end of the square billet during the shearing process. These cracks further expand in the subsequent rolling process, resulting in serious quality problems such as surface buckling and cracking of the steel bar. Such defects not only damage the appearance and mechanical properties of the product, affecting the brand image of the enterprise, but also cause an increase in rolling scrap rate, reducing the overall yield. Although existing technologies attempt to alleviate the crack problem by optimizing the chemical composition of molten steel, adjusting the amount of secondary cooling water, and correcting the gap between the shearing blades, these methods have limited effect under complex production conditions, especially for high-speed continuous casting scenarios of specific steel grades such as HRB400E, crack control is still unstable, and it is difficult to meet the high-quality production demand. Therefore, a systematic and refined control scheme is urgently needed to fundamentally solve the crack problem caused by hydraulic shearing.

[0003] The existing technology needs to be improved in view of the above problems. SUMMARY

[0004] The purpose of the present application is to provide a HRB400E square billet shearing crack control method, which has the advantages of effectively reducing square billet shearing cracks, avoiding surface defects of the steel bar after rolling, improving product quality and yield.

[0005] In the first aspect, the present application provides a HRB400E square billet shearing crack control method using the following technical solution: A kind of HRB400E square billet shear crack control method, square billet section is 165mm × 165mm, steel grade is HRB400E, its chemical composition is as follows according to mass percentage: C:0.22~0.25%、Si:0.35~0.45%、Mn:0.90~1.00%、S:≤0.045%、P:≤0.045%、V:0.030~0.036%, the rest is Fe and the inevitable impurities during smelting; The method comprises the following steps: (1) converter liquid end point control: control C≤0.18%, P≤0.035%, S≤0.045% in the end point liquid, basicity R=2.5~3.5; when tapping, adopt slide plate to block slag, control the amount of slag ≤4 kg / t; the shape of tapping hole is good, and the tapping time is 2~5 min; (2) LF furnace refining: after adding part of alloy when converter tapping, the liquid steel enters LF furnace refining for 60~90 min, fine-tune alloy composition to make the liquid steel chemical composition reach the internal control requirement of HRB400E, and the liquid steel temperature after refining is controlled at 1550~1570 ℃ out of station; (3) tundish control: the liquid steel is injected into 40 tons T-shaped tundish, the tundish is equipped with liquid steel weighing and lifting device, and the liquid steel temperature in the tundish is controlled at 1520~1540 ℃; (4) continuous casting: through R8m / 16m six-machine six-flow arc continuous casting machine, the liquid steel is drawn into billet through crystallizer, vibrator, secondary cooling section and straightening machine; (5) crystallizer parameter: full parabolic taper tube type crystallizer is used, the length of grooved copper pipe is 1000 mm, cooling water pressure is ≥1.2 MPa, and flow is ≥170 t / h; (6) vibrator parameter: non-sine vibration mode is adopted, the vibrator is electric cylinder servo system, vibration frequency is 105~270 times / min, and amplitude is ±2.5 mm~5.0 mm; (7) foot roller section parameter: 28 nozzles are distributed in the foot roller section, and the nozzle model is 3 / 8"PZ15590; (8) secondary cooling section parameter: full water cooling is adopted in the secondary cooling section, and specific water volume is 1.2~1.4 L / kg; wherein, the length of secondary cooling first section is 2.1 m, the nozzle model is 108179-RE2-14-80 / 28 / 195, and the number is 24; the length of secondary cooling second section is 3.0 m, the nozzle model is 08179-RE2-14-80 / 28 / 195, and the number is 24; the length of secondary cooling third section is 3.2 m, the nozzle model is 107840-RE1-6-80 / 25 / 324, and the number is 28; (9) Guide roller parameters: The arc error of the guide roller is controlled within ±0.5mm, and the roller gap error is controlled within ±0.2mm; (10) Parameters of the straightening machine: The straightening machine is a five-roll progressive type with a roll diameter of Φ350mm, a billet drawing speed of 3.2~3.5m / min, a red billet pressure of 3.5~4.0MPa, and a billet temperature of 950~1050℃ before entering the straightening machine; (11) Hydraulic shearing: The working pressure of the hydraulic shear is 13.5MPa, the billet shearing temperature is 900~950℃, and the billet is sheared into 12m long pieces by the hydraulic shear, and then sent for hot delivery or direct rolling.

[0006] Optionally, the ends of the HRB400E steel billet after hydraulic shearing are free of cracks or have only a few short and slight cracks, and no peeling or cracking defects will occur on the surface of the rolled steel bar.

[0007] Optionally, the refining time of the LF furnace in step (2) is 68 min, and the temperature of the molten steel leaving the station is 1558℃. Optionally, the temperature of the molten steel in the tundish in step (3) is 1528°C.

[0008] Optionally, the water pressure of the crystallizer in step (5) is 1.25 MPa and the flow rate is 178 t / h. Optionally, the vibration frequency of the vibrator in step (6) is 254 times / min and the amplitude is ±3.5mm.

[0009] Optionally, the specific water volume of the second cooling section in step (8) is 1.25 L / kg.

[0010] Optionally, the billet pulling speed in step (10) is 3.4 m / min, and the billet temperature before entering the straightening machine is 1020℃. Optionally, the working pressure of the hydraulic shear in step (11) is 13.0 MPa and the billet shearing temperature is 920°C.

[0011] Optionally, the HRB400E steel billet produced by the method is inspected at low magnification and found to have an internal crack grade of ≤1.5 and no cracks at the sheared end.

[0012] In summary, this application effectively suppresses billet shear cracks by optimizing the chemical composition of molten steel and continuous casting process parameters, including converter endpoint control, LF furnace refining, tundish temperature management, crystallizer cooling, vibration mode, secondary cooling section configuration, guide roller accuracy, straightener operation, and hydraulic shearing conditions. This has the advantages of effectively reducing billet shear cracks, avoiding surface defects in rolled steel bars, and improving product quality and yield. Attached Figure Description

[0013] Figure 1This is a topographical image of the end of the HRB400E square billet in this application. Detailed Implementation

[0014] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0015] Traditional flame cutting of billets in continuous casting presents several problems, including billet loss, high labor intensity in cleaning oxide slag, long cutting time leading to significant temperature drop in the billet during direct rolling, and the need for additional dust collectors. While hydraulic shearing can solve these problems, it also increases the risk of defects such as peeling and cracking at the tail of the steel bar due to shearing cracks, affecting product quality and yield.

[0016] In response, this application proposes a method for controlling shear cracks in HRB400E billets. This method targets billets with a cross-section of 165mm × 165mm and HRB400E steel grade. The chemical composition, by mass percentage, is controlled as follows: C: 0.22~0.25%, Si: 0.35~0.45%, Mn: 0.90~1.00%, S: ≤0.045%, P: ≤0.045%, V: 0.030~0.036%, with the remainder being Fe and unavoidable impurities during smelting.

[0017] The method includes the following steps: (1) End point control of molten steel in converter: C in the molten steel at the end point is controlled at ≤0.18%, P at ≤0.035%, S at ≤0.045%, and basicity R at 2.5~3.5; slag is blocked by a sliding plate during tapping, and the amount of slag is controlled at ≤4kg / t; the shape of the tapping spout is kept in good condition, and the tapping time is 2~5min; (2) LF furnace refining: After adding some alloys when tapping from the converter, the molten steel enters the LF furnace for refining for 60~90 minutes. The alloy composition is finely adjusted so that the chemical composition of the molten steel meets the internal control requirements of HRB400E. After refining, the temperature of the molten steel is controlled at 1550~1570℃ before leaving the station. (3) Tundish control: Molten steel is injected into a 40-ton T-shaped tundish, which is equipped with a molten steel weighing and lifting device. The temperature of the molten steel in the tundish is controlled at 1520~1540℃. (4) Continuous casting: Continuous casting is carried out by an R8m / 16m six-strand arc-shaped continuous casting machine with a strand spacing of 1.2m. Molten steel is drawn into billets through a crystallizer, vibrator, secondary cooling section and straightening machine. (5) Crystallizer parameters: A full parabolic tapered tube crystallizer is used, the length of the grooved copper tube is 1000mm, the cooling water pressure is set to ≥1.2MPa, and the flow rate is set to ≥170t / h; (6) Vibrator parameters: The non-sinusoidal vibration mode is adopted. The vibrator is an electric cylinder servo system. The vibration frequency is set to 105~270 times / min and the amplitude is set to ±2.5mm~5.0mm. (7) Foot roller section parameters: The foot roller section has 28 nozzles, and the nozzle model is 3 / 8" PZ15590; (8) Secondary cooling section parameters: The secondary cooling section uses full water cooling, and the specific water flow rate is set to 1.2~1.4L / kg; among which, The secondary cooling section is 2.1m long, with nozzle model 108179-RE2-14-80 / 28 / 195 and a quantity of 24 nozzles. The second cooling stage is 3.0m long, and the nozzle model is 08179-RE2-14-80 / 28 / 195, with a quantity of 24 nozzles. The second cooling section is 3.2m long, and the nozzle model is 107840-RE1-6-80 / 25 / 324, with a quantity of 28 nozzles. (9) Guide roller parameters: The arc error of the guide roller is controlled within ±0.5mm, and the roller gap error is controlled within ±0.2mm; (10) Parameters of the straightening machine: The straightening machine is a five-roll progressive type with a roll diameter of Φ350mm. The billet drawing speed is set to 3.2~3.5m / min, the red billet pressure is set to 3.5~4.0MPa, and the billet temperature is controlled at 950~1050℃ before entering the straightening machine. (11) Hydraulic shearing: The working pressure of the hydraulic shear is 13.5MPa, the billet shearing temperature is controlled at 900~950℃, the billet is sheared into 12m long / piece by the hydraulic shear, and can be red-carried or directly rolled.

[0018] For ease of understanding, the following explains some key terms in this embodiment: HRB400E billet refers to a billet with a cross-sectional size of 165mm×165mm and a steel grade of HRB400E. Its chemical composition must meet specific mass percentage requirements, including elements such as C, Si, Mn, S, P, and V, with the remainder being Fe and unavoidable impurities during smelting.

[0019] End-point control of molten steel in converters refers to the precise control of the carbon, phosphorus, sulfur content and basicity of molten steel at the end of the converter smelting process, and the adoption of measures such as slag blocking by sliding plates, to ensure that the quality of molten steel meets the requirements of subsequent refining.

[0020] LF furnace refining refers to the secondary refining of molten steel after it has been tapped from the converter into an LF (Ladle Furnace) furnace. Through the addition of alloys and heating, the chemical composition and temperature of the molten steel are finely adjusted to achieve the desired metallurgical effect.

[0021] Tundish control refers to the process of injecting molten steel into the tundish during continuous casting and controlling the temperature and level of the molten steel in the tundish to ensure that the molten steel flows into the crystallizer in a stable and uniform manner.

[0022] Continuous casting refers to the process of continuously casting liquid steel into solid steel billets, which typically includes equipment such as crystallizers, vibrators, secondary cooling sections, and straightening machines.

[0023] The crystallizer is a key component of continuous casting equipment, used to cool and solidify molten steel to form a billet with a solidified shell of a certain thickness.

[0024] A vibrator is a device used in conjunction with a crystallizer. Through periodic vibration, it prevents the billet from sticking to the crystallizer wall and improves the surface condition of the billet.

[0025] The foot roll section is located below the crystallizer and is used to support and guide the initially solidified billet and to perform preliminary cooling on it.

[0026] The secondary cooling section is a key cooling area in the continuous casting process. Water is sprayed to cool the billet a second time, causing it to solidify and reach the required temperature distribution.

[0027] Guide rollers are used to support and guide the billet during continuous casting, ensuring that the billet runs smoothly along the set arc.

[0028] A straightening machine is a component of continuous casting equipment, used to pull out and straighten curved billets into straight steel billets.

[0029] Hydraulic shearing refers to the use of hydraulically driven shearing equipment to cut continuously cast steel billets to length, which is characterized by high efficiency and zero loss.

[0030] The method for controlling shear cracks in the HRB400E billet in this embodiment is achieved through the following means: Regarding the final control of molten steel in converters, the C, P, and S contents, as well as the basicity R, can be controlled. For example, the C content can be controlled below 0.20%, the P content below 0.040%, the S content below 0.050%, and the basicity R between 2.0 and 4.0. During tapping, slag-blocking balls or manual slag blocking can be used, and the slag discharge is controlled to within 5 kg / t. The tapping spout shape can be circular or elliptical, and the tapping time can be controlled between 1 and 6 minutes.

[0031] In terms of LF furnace refining, after tapping from the converter, the molten steel can enter the LF furnace for refining. The refining time can be set to 50-100 minutes, during which the alloy composition is adjusted to ensure that the chemical composition of the molten steel meets the requirements of HRB400E steel grade. After refining, the temperature of the molten steel can be controlled at 1540-1580℃ before it leaves the station.

[0032] In terms of tundish control, molten steel can be poured into tundishes of different capacities, such as 30-ton or 50-ton tundishes, which can be equipped with liquid level detection devices. The temperature of the molten steel in the tundish can be controlled between 1510 and 1550℃.

[0033] In continuous casting, different models of arc-shaped continuous casting machines can be used, such as R7m / 14m or R9m / 18m continuous casting machines, and the flow spacing can be set to 1.0m or 1.5m. Molten steel passes through equipment such as a crystallizer, vibrator, secondary cooling section, and tension leveler in sequence to finally form a steel billet.

[0034] Regarding crystallizer parameters, tubular crystallizers with different tapers can be used, and the length of the grooved copper tube can be 900mm or 1100mm. The cooling water pressure can be set to 1.0MPa or 1.5MPa, and the flow rate can be set to 160t / h or 180t / h.

[0035] Regarding the vibrator parameters, sinusoidal or non-sinusoidal vibration modes can be used, and the vibrator can be mechanical or hydraulic. The vibration frequency can be set to 90~280 times / min, and the amplitude can be set to ±2.0mm~6.0mm.

[0036] Regarding the parameters of the foot roller section, the foot roller section can be distributed with different numbers of nozzles, such as 20 or 30 nozzles, and the nozzle model can be selected according to the cooling requirements.

[0037] Regarding the parameters of the secondary cooling section, it can employ either air-water mixed cooling or atomized cooling, with a specific water flow rate set to 1.0~1.5L / kg. The lengths, nozzle types, and quantities of the first, second, and third secondary cooling sections can be adjusted according to actual production needs.

[0038] Regarding the guide roller parameters, the guide roller arc error can be controlled within ±0.8mm, and the roller gap error can be controlled within ±0.3mm.

[0039] Regarding the parameters of the tension leveler, it can be a three-roll or seven-roll progressive type, with roll diameters of Φ300mm or Φ400mm. The billet drawing speed can be set to 3.0~3.6m / min, and the billet pressure can be set to 3.0~4.5MPa. The billet temperature before entering the tension leveler can be controlled at 900~1100℃.

[0040] In hydraulic shearing, the working pressure of the hydraulic shear can be set to 12.0~14.0MPa, and the billet shearing temperature can be controlled between 880~960℃. The billet is sheared into different lengths by the hydraulic shear, such as 10m or 15m long / piece, and can be either hot-feeded or directly rolled.

[0041] This application achieves systematic control of parameters in key stages of the HRB400E billet production process, including the converter steelmaking endpoint, LF furnace refining, tundish, continuous casting, crystallizer, vibrator, foot roll section, secondary cooling section, guide rolls, tension leveler, and hydraulic shearing. This enables the suppression of shear cracks generated during hydraulic shearing, thereby avoiding defects such as peeling and cracking on the surface of the reinforcing bars, and improving product quality and yield.

[0042] In some embodiments described above, a method for controlling shear cracks in HRB400E billets is proposed. This method aims to reduce the generation of shear cracks in billets by optimizing multiple aspects, including converter steel endpoint control, LF furnace refining, tundish control, continuous casting, crystallizer parameters, vibrator parameters, foot roll section parameters, secondary cooling section parameters, guide roll parameters, tension leveler parameters, and hydraulic shearing. However, in actual production, even with these measures, micro-cracks may still appear at the sheared ends of the billets. These cracks risk evolving into surface quality defects such as peeling and crazing during subsequent rolling processes, affecting the quality and performance of the final product.

[0043] This application further proposes that the ends of HRB400E steel billets after hydraulic shearing should be free of cracks or have only a few short, slight cracks, and that no peeling or cracking defects should occur on the surface of the rolled steel bars. "Free of cracks" means that the sheared surface of the steel billet is smooth and intact, without any visible macroscopic or microscopic fractures, indicating minimal damage to the material during the shearing process. "A few short, slight cracks" means that even if cracks exist, their size is extremely small, with limited depth and length, and their number is scarce, so as not to significantly affect the overall performance of the steel billet or subsequent processing. This state is a key indicator for measuring shearing quality, directly reflecting the level of control over material damage during the shearing process, and is the foundation for ensuring the quality of subsequent products. Furthermore, "peeling" refers to the localized peeling or bulging of the metal layer on the surface of the steel bar, usually caused by internal defects or poor bonding between the surface oxide layer and the matrix. "Cracking" refers to an irregular network of fine cracks on the surface of the steel bar, often related to insufficient plasticity of the material, stress concentration, or uneven cooling. These defects seriously affect the mechanical properties and appearance quality of the steel bars.

[0044] Through the above technical solutions, the shear crack control method for HRB400E billets proposed in this application, based on the optimization of steel smelting, continuous casting, and shearing process parameters, further clarifies the ultimate goal of shearing quality. Specifically, by precisely controlling various parameters in the hydraulic shearing process and combining it with refined management of preceding processes, the initiation and propagation of shear cracks can be effectively suppressed. When the billet end reaches a state with no cracks or only a few short and slight cracks, it means that the shear stress concentration has been effectively alleviated, and the toughness of the material is fully maintained in the shearing area, thereby avoiding stress concentration points caused by shear damage. These small, controllable cracks will not expand or deepen due to thermal stress, deformation stress, and other factors during subsequent high-temperature rolling, and will not transform into macroscopic peeling or cracking defects. Therefore, this solution fundamentally improves the shearing quality of HRB400E billets, ensures the surface integrity and internal quality of subsequently rolled steel bars, significantly reduces the scrap rate, and improves the product qualification rate and market competitiveness.

[0045] In the HRB400E billet shear crack control method, LF furnace refining is a crucial step to ensure that the chemical composition and temperature of the molten steel meet the requirements of subsequent continuous casting. However, if the refining time and the temperature of the molten steel exiting the station are controlled within too wide a range, it may lead to insufficient uniformity of the molten steel composition or large fluctuations in the molten steel temperature, thereby affecting the stability of the continuous casting process and the quality of the final billet, and even increasing the risk of shear cracks.

[0046] In response, this application further proposes an optimized LF furnace refining scheme, wherein the LF furnace refining time in step (2) is 68 min and the molten steel temperature at the outlet is 1558℃.

[0047] LF furnace refining time refers to the total duration of heating, stirring, deoxidation, desulfurization, and alloying operations in the LF furnace. Precisely controlling the refining time to 68 minutes means that operators or automated systems must strictly monitor the start and end of the refining process to ensure the molten steel remains in the LF furnace for the preset duration. This is typically achieved through timers, process control systems, and real-time monitoring of the molten steel's condition. Refining time is a key parameter affecting the purity, compositional uniformity, and temperature control of the molten steel. Too short a refining time may lead to incomplete reactions and inadequate impurity removal; too long a refining time increases energy consumption and may cause secondary oxidation of the molten steel or excessive temperature drop.

[0048] The outlet temperature of molten steel refers to the actual temperature of the molten steel after refining in the LF furnace, before it enters the tundish or continuous casting machine. Precisely controlling the outlet temperature at 1558℃ requires adjusting heating power and argon stirring intensity during the final stages of refining to achieve this target value. This is typically achieved through real-time temperature measurement using immersion thermocouples, combined with precise adjustments using temperature prediction models and feedback control systems. The outlet temperature directly affects the temperature stability of the subsequent tundish and the smooth progress of the continuous casting process. Temperatures that are too high or too low can lead to abnormalities in continuous casting production, such as crystallizer sticking, difficulty in billet pulling, and decreased billet quality.

[0049] By precisely setting the LF furnace refining time to 68 minutes using the above technical solution, the molten steel can be fully refined, ensuring homogenization of the alloy composition and effectively removing inclusions, significantly improving the purity of the molten steel. Simultaneously, the temperature of the molten steel exiting the furnace is precisely controlled at 1558℃, ensuring a more stable temperature as it enters the tundish and continuous casting machine, avoiding instability in the continuous casting process caused by temperature fluctuations. This precise temperature and time control helps optimize the superheat of the molten steel, reducing the formation of internal defects in the billet, thereby significantly reducing the tendency for the billet to crack during shearing, and providing high-quality billets for subsequent rolling, avoiding quality defects such as peeling and cracking.

[0050] In the HRB400E billet shear crack control method, precise control of the tundish molten steel temperature is crucial for the stability of the continuous casting process and the final billet quality. Although the basic scheme provides a control range for the tundish molten steel temperature, in actual production, failure to precisely control the molten steel temperature at the optimal point may lead to instability in molten steel fluidity, crystallization behavior, and solidification structure, thereby affecting the internal quality and surface condition of the billet and increasing the risk of cracks during shearing.

[0051] In this regard, this application further proposes a tundish molten steel temperature of 1528℃. The tundish molten steel temperature is a key process parameter in continuous casting, and its precise control directly affects the superheat, fluidity, and heat transfer and solidification behavior of the molten steel within the mold. Setting the tundish molten steel temperature precisely at 1528℃ aims to provide an optimized superheat for the continuous casting process. This temperature ensures good fluidity of the molten steel in the tundish and mold, effectively preventing solidification or blockage caused by excessively low temperatures, while also preventing problems such as an excessively thin solidified shell and billet bulging caused by excessively high temperatures. Precise temperature control helps stabilize the solidification front within the mold, promoting the formation of a uniform and dense billet solidification structure, thereby providing billets with excellent internal quality for subsequent shearing processes.

[0052] By precisely controlling the molten steel temperature in the tundish at 1528℃, a stable superheat can be ensured during continuous casting, thereby optimizing the fluidity of the molten steel and reducing temperature fluctuations in the tundish and crystallizer. This precise temperature control helps stabilize the formation and growth of the solidified shell in the crystallizer, effectively suppressing the generation of internal defects in the billet, such as porosity and segregation, and reducing thermal stress generated during solidification. When the internal structure of the billet is uniform and dense, and the thermal stress distribution is reasonable, its ability to resist crack initiation and propagation during hydraulic shearing is significantly enhanced, thereby effectively avoiding or greatly reducing the occurrence of shear cracks and further improving the overall quality of the HRB400E billet.

[0053] In some embodiments described above in this application, the cooling water pressure and flow rate of the crystallizer during continuous casting are set within a certain range to ensure the initial solidification of the steel billet. However, in actual production of HRB400E square billets, if the crystallizer cooling parameters fail to precisely match the steel grade characteristics and billet size, uneven solidification shell growth may occur, resulting in excessive thermal stress, which in turn affects the surface quality of the billet and increases the risk of cracking during subsequent shearing.

[0054] In this regard, this application further proposes that in the above method, the water pressure of the crystallizer in step (5) is 1.25 MPa and the flow rate is 178 t / h.

[0055] Specifically, the crystallizer water pressure is one of the key parameters affecting the crystallizer's cooling efficiency. During continuous casting, cooling water exchanges heat with the steel billet through the crystallizer copper tubes, carrying away the heat released during solidification. Precisely controlling the crystallizer water pressure at 1.25 MPa ensures that the cooling water flows within the copper tubes at an appropriate velocity and pressure, maintaining a stable heat transfer coefficient and preventing boiling or vaporization of the cooling water within the tubes, which would affect cooling uniformity. This pressure value helps form a uniform solidification shell, reducing thermal stress concentration caused by uneven cooling. Simultaneously, the cooling water flow rate directly determines the crystallizer's ability to remove heat. Sufficient cooling water flow rate is essential for efficient cooling and stable solidification during continuous casting. Precisely controlling the flow rate at 178 t / h ensures that the crystallizer can continuously and effectively remove solidification heat during the continuous casting of 165mm × 165mm HRB400E square billets, maintaining good heat conduction between the crystallizer copper tubes and the steel billet. This flow rate helps prevent overheating or insufficient cooling of the billet surface, thereby optimizing the morphology of the solidification front and reducing the probability of surface defects in the billet.

[0056] By precisely setting the crystallizer water pressure to 1.25 MPa and the cooling water flow rate to 178 t / h, the crystallizer can cool the HRB400E billet in a more stable and uniform manner. This precisely controlled cooling condition helps optimize the growth of the solidified shell on the billet, reduces thermal stress generated during solidification, and effectively suppresses the formation of surface cracks. When the billet is sheared in subsequent steps, the incidence of shear cracks is significantly reduced, or even completely avoided, due to the significantly improved surface quality, thereby improving the overall quality and production efficiency of the HRB400E billet.

[0057] In some of the embodiments described above in this application, the vibrator parameters during continuous casting have a critical impact on the surface quality and internal solidification structure of the billet. Improper selection of vibration parameters may lead to insufficient lubrication between the molten steel and the solidified shell of the billet in the mold, or cause localized tearing of the solidified shell, resulting in severe vibration marks, cracks, and other defects on the billet surface, thereby affecting the product quality during subsequent shearing and rolling processes.

[0058] In this regard, this application further proposes to optimize the vibrator parameters in the continuous casting step (4). Specifically, the vibration frequency of the vibrator in step (6) is 254 times / min and the amplitude is ±3.5mm.

[0059] Vibration frequency refers to the number of vibration cycles a vibrator completes per unit time. In continuous casting, vibration frequency affects the flow state of molten steel within the mold, the melting and penetration of the protective slag, and the formation and demolding of the solidified shell of the billet. A higher vibration frequency helps promote the penetration of the protective slag into the gap between the solidified shell and the mold wall, forming a uniform lubricating layer and reducing friction between the billet and the mold wall. Simultaneously, a suitable vibration frequency can effectively suppress the depth and unevenness of vibration marks on the billet surface, avoiding stress concentration and microcrack initiation caused by excessively deep vibration marks. Amplitude refers to the maximum distance the vibrator deviates from its equilibrium position during vibration. The magnitude of the amplitude directly affects the billet's trajectory and stress state within the mold. A moderate amplitude ensures that the solidified shell of the billet maintains periodic contact and separation from the mold wall during descent, which is beneficial for the uniform distribution of the protective slag and the stable formation of the lubricating film. Too small an amplitude may lead to insufficient lubrication and increase the risk of adhesion; while too large an amplitude may cause excessive deformation or tearing of the solidified shell, exacerbating surface defects. Therefore, precise amplitude control is crucial for maintaining good surface quality of the cast billet.

[0060] By precisely setting the vibrator frequency to 254 times / min and controlling its amplitude within ±3.5mm, the flow behavior of molten steel and the lubrication effect of the protective slag within the mold can be optimized. This specific combination of vibration parameters effectively reduces the friction between the solidified shell of the billet and the mold wall, while promoting uniform penetration of the protective slag, thereby significantly reducing the depth and inhomogeneity of vibration marks on the billet surface. Furthermore, precise vibration control prevents excessive stretching or tearing of the solidified shell during demolding, effectively suppressing the formation of surface microcracks. This optimized vibration mode ensures that the HRB400E billet achieves excellent surface quality and a dense solidification structure during the continuous casting stage, providing high-quality base material for subsequent shearing processes, thus reducing the probability of shear cracks from the source and ultimately improving the overall performance of the product.

[0061] In some embodiments of this application, the specific water content in the secondary cooling section during continuous casting is controlled within the range of 1.2 to 1.4 L / kg. However, within this relatively wide range, improper selection of the specific water content may lead to uneven cooling or excessive cooling intensity of the billet, thereby increasing the internal thermal stress of the billet and affecting its internal quality, and even inducing or exacerbating shear cracks during subsequent shearing. In this regard, this application further proposes that the specific water content in the secondary cooling section in step (8) of the above method is 1.25 L / kg.

[0062] The specific water volume in the secondary cooling section refers to the amount of cooling water received by a unit mass of steel billet in the secondary cooling section during continuous casting. This parameter is a key indicator for controlling the cooling intensity of the steel billet, directly affecting its solidification behavior, temperature distribution, and thermal stress state. Precisely setting the specific water volume in the secondary cooling section to 1.25 L / kg means that while ensuring sufficient cooling and solidification of the steel billet, overcooling is effectively avoided. Specifically, by precisely adjusting the water supply pressure and flow rate of each nozzle in the secondary cooling section, combined with the billet drawing speed, fine control of the surface temperature and internal solidification front of the steel billet can be achieved, ensuring the uniformity and appropriateness of the cooling process. This specific water volume helps reduce the thermal stress generated in the steel billet during cooling, thereby reducing the risk of internal defects and surface cracks caused by thermal stress concentration.

[0063] By precisely controlling the water content in the secondary cooling section to 1.25 L / kg, the cooling process of HRB400E billets can be effectively optimized. This water content, at the lower end of the range, helps avoid internal thermal stress concentration in the billet caused by excessive cooling intensity, thereby reducing the risk of internal defects during solidification and cooling. This moderate cooling strategy ensures the uniformity of the billet's temperature field, reduces the thermal stress gradient, and makes it less likely for shear cracks to form or worsen at the billet's ends during subsequent hydraulic shearing. Simultaneously, this refined cooling control also helps improve the uniformity of the billet's microstructure, further enhancing its overall quality and laying the foundation for the subsequent rolling of reinforcing bars free of surface defects such as peeling and cracking.

[0064] In continuous casting, the billet drawing speed and the billet temperature before entering the straightening machine are key parameters affecting billet quality and production efficiency. Improper selection of these parameters can lead to uneven internal structure, stress concentration, and even quality defects such as cracks during subsequent shearing or rolling, thus affecting the performance and yield of the final product.

[0065] In this regard, this application further proposes that in the above method, the billet pulling speed in step (10) is 3.4 m / min and the billet temperature before entering the straightening machine is 1020℃.

[0066] Specifically, the billet drawing speed refers to the speed at which the continuous casting machine pulls the solidified steel billet from the crystallizer and conveys it downwards. It directly affects the solidification process of the billet, the formation of its internal structure, and production efficiency. During continuous casting, the setting of the billet drawing speed needs to comprehensively consider the characteristics of the steel grade, the cooling intensity of the crystallizer, the cooling mode of the secondary cooling section, and the requirements of subsequent straightening and shearing processes. An excessively fast drawing speed may result in an excessively large incompletely solidified area inside the billet, increasing the risk of center segregation and internal cracks; an excessively slow speed will reduce production efficiency and may cause the billet to remain in the high-temperature zone for too long, resulting in coarse grains. This application sets the billet drawing speed to 3.4 m / min, aiming to optimize production efficiency while ensuring the internal quality of the billet.

[0067] Meanwhile, the billet temperature before entering the straightening mill refers to the surface and internal temperature of the billet when it enters the straightening mill for the straightening operation. This temperature is crucial for the billet's plastic deformation capacity, straightening effect, and subsequent shearing and rolling processes. Billets exhibit better plasticity at higher temperatures, which helps reduce the tendency to crack during straightening. However, excessively high temperatures may lead to over-softening of the billet, increasing the difficulty of deformation or causing surface oxidation; excessively low temperatures will reduce the billet's plasticity, increase straightening stress, and easily generate surface or internal cracks. This application controls the billet temperature before entering the straightening mill at 1020℃ to ensure that the billet has good plasticity, facilitating straightening, while avoiding the negative effects of excessively high temperatures, thus providing good initial conditions for subsequent shearing and rolling.

[0068] By precisely setting the billet drawing speed to 3.4 m / min and strictly controlling the billet temperature to 1020℃ before entering the straightening machine, this application effectively balances production efficiency and billet internal quality during continuous casting. This optimized parameter combination ensures that the billet forms a uniform and fine microstructure during solidification, reducing center segregation and internal stress. Simultaneously, the billet is in its optimal plasticity state upon entering the straightening machine, reducing the risk of cracking during straightening. This not only improves the overall quality of the billet but also provides a more stable semi-finished product for subsequent hydraulic shearing and rolling processes, significantly reducing the probability of shear cracks in HRB400E square billets and helping to avoid quality defects such as peeling and cracking on the surface of rolled steel bars.

[0069] In some embodiments described above in this application, a method for controlling shear cracks in HRB400E billets is proposed, which includes a hydraulic shearing step that specifies the range of hydraulic shearing working pressure and billet shearing temperature. However, in actual production, if the hydraulic shearing parameters are not properly selected, such as excessively high working pressure or billet temperature deviating from the optimal range, excessive stress concentration may occur at the billet ends during shearing, thereby inducing or exacerbating the formation of shear cracks and affecting the surface quality and subsequent rolling performance of the billet.

[0070] In this regard, this application further proposes that the hydraulic shearing steps include: the hydraulic shear working pressure is 13.0 MPa and the billet shearing temperature is 920℃.

[0071] The hydraulic shear working pressure refers to the force applied by the hydraulic shears when shearing a steel billet, which is precisely controlled by the hydraulic system. During the billet shearing process, the working pressure is one of the key parameters affecting the shearing quality. If the working pressure is too high, it may cause significant plastic deformation of the sheared surface, even leading to tearing and increasing the risk of crack formation; if the working pressure is too low, it may fail to effectively shear the billet, resulting in incomplete shearing or an uneven sheared surface. Setting the hydraulic shear working pressure to 13.0 MPa aims to ensure that the billet receives sufficient shearing force for effective separation during shearing by precisely controlling the shearing force, while avoiding excessive plastic deformation and stress concentration caused by excessive pressure, thereby reducing the probability of shearing cracks. This pressure value has been optimized to suit the mechanical properties and billet dimensions of HRB400E steel, ensuring the stability of the shearing process and the quality of the sheared surface.

[0072] The billet shearing temperature refers to the temperature at which the billet is subjected to hydraulic shearing. The billet temperature significantly affects its plasticity, toughness, and deformation behavior during shearing. At high temperatures, the billet exhibits better plasticity and is less prone to brittle fracture during shearing; however, excessively high temperatures may lead to severe oxidation or excessive deformation of the sheared surface. Conversely, excessively low temperatures will harden the billet, reduce its plasticity, and make it more susceptible to cracking during shearing. Controlling the billet shearing temperature at 920℃ is based on the characteristics of HRB400E steel and the need for shear crack control. At this temperature, the HRB400E billet exhibits good plasticity, can withstand shear stress without easily cracking, and also ensures the smoothness and quality of the sheared surface. This temperature value helps optimize the internal microstructure of the billet, enabling it to exhibit optimal crack resistance during shearing and effectively suppressing the initiation and propagation of shear cracks.

[0073] By precisely setting the hydraulic shear working pressure to 13.0 MPa and strictly controlling the billet shearing temperature at 920℃, this application effectively optimizes the stress state and plastic deformation behavior of the billet during the shearing process. Specifically, the working pressure of 13.0 MPa ensures moderate shearing force, which can thoroughly shear the billet while avoiding local stress concentration and plastic tearing caused by excessive pressure. At the same time, the shearing temperature of 920℃ puts the HRB400E billet in a state of good plasticity and is not prone to brittle fracture, significantly improving the billet's resistance to shear stress. This precise combination of parameters works synergistically to minimize the generation and propagation of microcracks at the billet end during shearing, thereby significantly improving the quality of the sheared end of the billet and reducing the risk of surface defects such as peeling and cracking during subsequent rolling.

[0074] In some embodiments described above, a method for controlling shear cracks in HRB400E billets is proposed. This method aims to reduce the generation of shear cracks in billets by precisely controlling parameters at multiple stages, including the converter steelmaking endpoint, LF furnace refining, tundish, continuous casting, crystallizer, vibrator, foot roll section, secondary cooling section, guide rolls, tension leveler, and hydraulic shearing. However, in actual production, even with strict control of the above process parameters, there is still a risk of unsatisfactory internal crack levels in the billet or the appearance of micro-cracks at the shearing end, which can affect the quality and yield of subsequent rolled products.

[0075] To address this, this application further proposes low-magnification inspection of the produced HRB400E steel billets to ensure that the internal crack grade is no higher than 1.5 and that there are no cracks at the sheared end. Low-magnification inspection of the HRB400E steel billets refers to using macroscopic inspection methods to examine the internal structure of the billet to assess its internal defects. An internal crack grade ≤1.5 indicates that the macroscopic crack defects inside the billet are very minor, even negligible. This is typically achieved by optimizing the solidification and cooling conditions during continuous casting, such as precisely controlling the cooling intensity and uniformity of the secondary cooling zone, and the billet drawing speed of the straightening machine (3.2~3.5 m / min) and the red billet pressure (3.5~4.0 MPa), to effectively reduce residual stress inside the billet, thereby inhibiting the formation and propagation of internal cracks. No cracks at the sheared end means that after hydraulic shearing, there are no macroscopic cracks visible to the naked eye on the cut surface of the billet. The occurrence of sheared end cracks is often closely related to factors such as shearing temperature, shearing force, shearing tool condition, and the plasticity of the billet. By controlling the billet shearing temperature within a suitable range of 900~950℃ and using a hydraulic shear working pressure of 13.5MPa, it is possible to ensure that the billet maintains good plasticity during shearing, thus preventing the initiation and propagation of cracks caused by shear stress concentration. Furthermore, regularly checking and maintaining the sharpness and clearance of the shearing blades also helps to obtain a smooth, crack-free sheared end face.

[0076] Through the above technical solution, this application can effectively solve the problems of unstable internal quality of steel billets and easy cracking at the shearing ends in traditional methods. This quality control method not only ensures the overall internal density of HRB400E steel billets, avoiding the expansion of internal defects into surface quality defects such as peeling and cracking during subsequent rolling, but also eliminates end cracks caused by shear stress concentration by optimizing shearing process parameters. This significantly improves the yield of HRB400E steel billets and the surface quality of subsequent rolled products, providing a solid foundation for the production of high-quality steel bars.

[0077] The following example will provide a more detailed explanation of the above technical solution: In the preparation process of 165×165mm HRB400E steel billets: The final control parameters for the converter molten steel were: C: 0.12%, P: 0.028%, S: 0.042%; the molten steel underwent 68 minutes of refining in the LF furnace; the chemical composition of the molten steel leaving the station was: C: 0.23%, Si: 0.38%, Mn: 0.93%, P: 0.030%, S: 0.032%, V: 0.032%, with the remainder being Fe and unavoidable impurities during smelting; the molten steel temperature was 1558℃ when it left the station. The molten steel in the tundish was at 1528℃ and was drawn into billets through an R8m / 16m six-strand arc-shaped continuous casting machine, passing through a crystallizer, vibrator, secondary cooling section, and straightening machine. The crystallizer water pressure was 1.25 MPa, and the flow rate was 178 t / h; the vibration frequency was 254 times / min, and the amplitude was ±3.5 mm; the specific water content was 1.25 L / Kg; the drawing speed was 3.4 m / min; the straightening machine pressure was 3.8 MPa; the billet temperature entering the straightening machine was 1020℃; the hydraulic shear working pressure was 13.0 MPa, and the billet shearing temperature was 920℃. After the billet cooled to room temperature, there were no shear cracks at the end, and the morphology was as follows. Figure 1 As shown.

[0078] It should be noted that the workflow described above is merely illustrative and does not limit the scope of protection of this application. In practical applications, those skilled in the art can select some or all of it to achieve the purpose of this embodiment according to actual needs, and no restrictions are imposed here.

[0079] In addition, for technical details not described in detail in this embodiment, please refer to the method for controlling shear cracks in HRB400E billets provided in any embodiment of this application, which will not be repeated here.

[0080] Furthermore, it should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or system that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or system. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or system that includes that element.

[0081] The sequence numbers of the embodiments in this application are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0082] Through the above description of the embodiments, those skilled in the art can clearly understand that the methods of the above embodiments can be implemented by means of software plus necessary general-purpose hardware platforms. Of course, they can also be implemented by hardware, but in many cases the former is a better implementation method. Based on this understanding, the technical solution of this application, in essence, or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product is stored in a storage medium (such as read-only memory (ROM) / RAM, magnetic disk, optical disk), and includes several instructions to cause a terminal device (which may be a mobile phone, computer, server, or network device, etc.) to execute the methods of the various embodiments of this application. The above are only preferred embodiments of this application and do not limit the patent scope of this application. All equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A method for controlling shear cracks in HRB400E billets, wherein the billet cross-section is 165mm×165mm, the steel grade is HRB400E, and its chemical composition by mass percentage is: C: 0.22~0.25%, Si: 0.35~0.45%, Mn: 0.90~1.00%, S: ≤0.045%, P: ≤0.045%, V: 0.030~0.036%, with the remainder being Fe and unavoidable impurities during smelting; Its features are, The method includes the following steps: (1) End point control of molten steel in converter: control the C in the molten steel at the end point to be ≤0.18%, P ≤0.035%, S ≤0.045%, and basicity R = 2.5~3.5; use a sliding plate to block slag during tapping, and control the amount of slag to be ≤4kg / t; ensure the shape of the tapping port is good, and the tapping time is 2~5min; (2) LF furnace refining: After adding some alloy when tapping from the converter, the molten steel enters the LF furnace for refining for 60~90 minutes. The alloy composition is finely adjusted so that the chemical composition of the molten steel meets the internal control requirements of HRB400E. After refining, the temperature of the molten steel is controlled at 1550~1570℃ before leaving the station. (3) Tundish control: Molten steel is injected into a 40-ton T-shaped tundish, which is equipped with a molten steel weighing and lifting device, and the molten steel temperature in the tundish is controlled at 1520~1540℃; (4) Continuous casting: Continuous casting is carried out by an R8m / 16m six-strand arc-shaped continuous casting machine with a strand spacing of 1.2m. Molten steel is drawn into billets through a crystallizer, vibrator, secondary cooling section and straightening machine. (5) Crystallizer parameters: A full parabolic tapered tube crystallizer is used, with a grooved copper tube length of 1000mm, cooling water pressure ≥1.2MPa, and flow rate ≥170t / h; (6) Vibrator parameters: The non-sinusoidal vibration mode is adopted. The vibrator is an electric cylinder servo system with a vibration frequency of 105~270 times / min and an amplitude of ±2.5mm~5.0mm. (7) Foot roller section parameters: The foot roller section has 28 nozzles, and the nozzle model is 3 / 8"PZ15590; (8) Secondary cooling section parameters: The secondary cooling section uses full water cooling, with a specific water flow rate of 1.2~1.4 L / kg; among which, The secondary cooling section is 2.1m long, with nozzle model 108179-RE2-14-80 / 28 / 195 and a quantity of 24 nozzles. The second cooling stage is 3.0m long, with nozzle model 08179-RE2-14-80 / 28 / 195 and a quantity of 24 nozzles. The secondary cooling and three-stage section is 3.2m long, with nozzle model 107840-RE1-6-80 / 25 / 324 and a quantity of 28 nozzles. (9) Guide roller parameters: The arc error of the guide roller is controlled within ±0.5mm, and the roller gap error is controlled within ±0.2mm; (10) Parameters of the straightening machine: The straightening machine is a five-roll progressive type with a roll diameter of Φ350mm, a billet drawing speed of 3.2~3.5m / min, a red billet pressure of 3.5~4.0MPa, and a billet temperature of 950~1050℃ before entering the straightening machine; (11) Hydraulic shearing: The working pressure of the hydraulic shear is 13.5MPa, the billet shearing temperature is 900~950℃, and the billet is sheared into 12m long pieces by the hydraulic shear, and then sent for hot delivery or direct rolling.

2. The method according to claim 1, characterized in that, The ends of HRB400E steel billets after hydraulic shearing are free of cracks or have only a few short and slight cracks, and will not produce peeling or cracking defects on the surface of the rolled steel bars.

3. The method according to claim 1, characterized in that, The refining time in the LF furnace in step (2) is 68 min, and the temperature of the molten steel leaving the station is 1558℃.

4. The method according to claim 1, characterized in that, The temperature of the molten steel in the tundish mentioned in step (3) is 1528℃.

5. The method according to claim 1, characterized in that, The water pressure of the crystallizer in step (5) is 1.25 MPa and the flow rate is 178 t / h.

6. The method according to claim 1, characterized in that, The vibration frequency of the vibrator in step (6) is 254 times / min and the amplitude is ±3.5mm.

7. The method according to claim 1, characterized in that, The specific water volume of the second cooling section mentioned in step (8) is 1.25 L / kg.

8. The method according to claim 1, characterized in that, The billet pulling speed in step (10) is 3.4 m / min, and the billet temperature before entering the straightening machine is 1020℃.

9. The method according to claim 1, characterized in that, The working pressure of the hydraulic shear in step (11) is 13.0 MPa, and the billet shearing temperature is 920℃.

10. The method according to claim 1, characterized in that, The HRB400E steel billet produced by the method was inspected at low magnification and found to have an internal crack grade of ≤1.5 and no cracks at the sheared end.

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