A method for controlling surface quality stability of medium-carbon high-aluminum steel continuous casting round billet

Abstract

This invention specifically discloses a method for stable control of the surface quality of continuously cast round billets of medium-carbon high-alumina steel. The method involves controlling the superheat of the tundish in the first heat of 38CrMoAl refined steel to be 20-30℃; the superheat of the tundish in subsequent heats to be 10-20℃; controlling the primary cooling water flow rate to 190-200 m³ / h and the secondary cooling water flow rate to 0.10-0.11 L / kg; during the casting process, the crystallizer vibration parameters are set as follows: A1:5; A2:0; f1:80; f2:50; As:0.18; adding protective slag after the start of casting in the tundish; the casting speed to be 0.80-0.90 m / min; and cooling the cast 38CrMoAl continuously cast round billets using a slow cooling process: the temperature upon entering the slow cooling pit ≥650℃, and the slow cooling time ≥24h. The surface quality qualification rate of the 38CrMoAl continuously cast round billets is above 99.9%.

Description

Technical Field

[0001] This invention relates to the field of surface quality control technology for medium carbon high aluminum steel round billets, specifically to a method for stable control of the surface quality of φ310mm medium carbon high aluminum steel 38CrMoAl continuously cast round billets. Background Technology

[0002] 38CrMoAl is a high-aluminum steel. Due to its high aluminum (Al) content, it can form aluminum nitride (AlN) after nitriding. The dispersion strengthening effect of AlN significantly improves the surface hardness and strength of the steel. This steel exhibits good wear resistance, high strength and hardness, and good hardenability, making it significantly advantageous in manufacturing components with special requirements, such as cylinder liners, gears, high-pressure valves, worm gears, and grinding machine spindles. Therefore, developing φ310mm 38CrMoAl continuously cast round billets is of great significance for meeting the needs of high-end equipment manufacturing.

[0003] In the continuous casting process of 310 round billets (38CrMoAl), molten steel with a certain degree of superheat flows out from the submerged entry nozzle and gradually solidifies into a billet near the inner wall of the mold. During this process, the molten steel circulating above the mold is first cooled to its solidification temperature by the strong cooling effect of the mold at the meniscus, forming a weak initial billet shell. Due to the vibration of the mold, the molten protective slag near the meniscus slowly flows into the slag channel between the copper tube and the billet shell. Under the influence of the periodically changing slag channel pressure and the static pressure of the molten steel, the initial billet shell begins to deform, thus forming oscillation marks on the billet. Because the Al content in 38CrMoAl steel is high, reaching 30 times that of ordinary aluminum-containing steel, a violent aluminothermic reduction reaction occurs between the aluminum (Al) in the steel and the silicon dioxide (SiO2) in the protective slag during casting. This process changes the chemical composition and physical properties of the protective slag, causing it to lose its original function, leading to slag deterioration and ultimately cracks in the billet. The performance of the protective slag is crucial. Appropriate protective slag composition and melting characteristics should be selected to ensure that the protective slag can flow well into the slag channel, form a uniform and stable slag film, and reduce the deformation and cracking of the initial billet shell.

[0004] For 38CrMoAl, a medium-carbon, high-CrMo steel, the high content of Cr and Mo elements allows Cr and Mo to combine with C in the steel to form carbides. These carbides precipitate at grain boundaries, reducing the strength and toughness of the grain boundaries. Simultaneously, the addition of Cr and Mo increases the hardenability of the steel, making it easier for the billet to form martensite during cooling. Martensite has high hardness but poor toughness, further increasing the risk of cracking in the billet. Furthermore, 38CrMoAl steel has a relatively high liquidus temperature. When the billet enters the secondary cooling zone, improper cooling control may lead to excessively rapid cooling, causing significant thermal stress inside the billet and resulting in cracks on the surface or inside the billet.

[0005] In the continuous casting production of 310 round billets of 38CrMoAl steel, the cooling of the crystallizer, the protective slag, and the secondary cooling all have a significant impact on the quality of the billets. By optimizing these process parameters, crack defects in the billets can be effectively reduced, and the yield rate of the billets can be improved. Summary of the Invention

[0006] The purpose of this invention is to provide a method for stable control of the surface quality of medium-carbon high-alumina steel continuously cast round billets, so as to solve the problems of crack defects and low product qualification rate in round billets produced by existing processes.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A method for stable control of the surface quality of continuously cast round billets of medium-carbon high-alumina steel is disclosed. The method involves casting refined molten medium-carbon high-alumina steel (38CrMoAl). The superheat of the tundish in the first heat of the molten medium-carbon high-alumina steel (38CrMoAl) is 20-30°C; the superheat of the tundish in subsequent heats is 10-20°C. Choosing a lower superheat can shorten the solidification time, accelerate the billet shell growth rate, increase the initial billet shell thickness and uniformity, reduce solidification shrinkage, alleviate tensile stress at the solidification front, and reduce the sensitivity to longitudinal and transverse surface cracks (38CrMoAl is a medium-carbon alloy steel with high crack sensitivity). After the intermediate ladle is opened for casting, a protective slag is added. The chemical composition of the protective slag, by mass percentage, is: 29-36 wt% CaO, 20-30 wt% SiO2, 1-4 wt% Al2O3, 1.0-3.0 wt% Li2O, 1.0-3.8 wt% MgO, 7-12 wt% F, and 5-9 wt% TC. The viscosity range of the protective slag is 0.10-0.16 Pa·s, the basicity R is controlled within the range of 1.0-1.20, the melting point is designed to be 1070-1120℃, and the moisture content is ≤0.30%. CaO and SiO determine the basicity R of the protective slag, forming the basic slag system. When the basicity of the protective slag is too low, the lubrication effect is good, but the Al2O3 absorption capacity is insufficient, easily leading to inclusions. When the basicity is too high, the tendency for crystallization increases, and uneven heat transfer causes longitudinal cracks. To balance lubrication and inclusion adsorption effects, the optimal range for basicity R is selected as 1.0–1.2, correspondingly determining the range of CaO and SiO2. The Al2O3 content in the original slag needs to be controlled at a low level to allow space for absorbing floating Al2O3 from the steel; the Al2O3 content in the slag is selected as 1–4% through design and verification. Li2O, as a powerful flux, improves the fluidity of high-viscosity slag, lowers the crystallization temperature, promotes the formation of a glassy slag film, and ensures uniform heat transfer; the Li2O content in the slag is selected as 1–3% through design and verification. A low viscosity (0.10–0.16) is chosen. Pa·s) can reduce the concentration of thermal stress in the crystallizer, making the billet shell more uniformly stressed during the cooling process, thereby reducing the risk of surface cracks; the melting point is designed to be 1070-1120℃, which is lower than the solidification temperature of the molten steel, to ensure the timely formation of the liquid slag layer; when the melting point is too high, the liquid slag layer is too thin, and insufficient lubrication will cause friction cracks. The casting speed is 0.80~0.90m / min; according to the characteristics of this steel grade, the principle of prioritizing surface quality must be followed, and it must be ensured that the temperature of the straightening point avoids the brittle zone (750-900℃), and the surface temperature of the straightening point is controlled at 950–1050℃. In combination with the characteristics of the protective slag, the thickness of the billet shell at the crystallizer outlet and the length of the liquid core, the casting speed of 0.80~0.90m / min is selected. The primary cooling water flow rate is controlled at 190-200 m³ / h, and the secondary cooling water flow rate is 0.10-0.11 L / kg. The selection of primary cooling (crystallizer cooling) and secondary cooling (secondary cooling) for 38CrMoAl continuous casting round billets should follow the core principle of "strong primary cooling to stabilize the billet shell, and weak secondary cooling to prevent cracking." It should be refined by combining the high aluminum content of the steel grade, crack sensitivity, and solidification characteristics of large cross sections. Primary cooling needs to meet the heat flux density to avoid the risk of uneven billet shell thickness caused by low water volume; secondary cooling needs to maintain the temperature gradient of the round billet cross section to prevent excessive thermal stress. During the casting process, the crystallizer vibration parameters are set as follows: A1:5; A2:0; f1:80; f2:50; As:0.18, where A1 represents stroke constant parameter 1; A2 represents stroke constant parameter 2; f1 represents frequency constant parameter 1; f2 represents frequency constant parameter 2; and As represents a non-sinusoidal factor. The core of the crystallizer vibration parameter setting is to enhance the lubrication of the protective slag by using a composite vibration of high-frequency micro-amplitude and low-frequency large amplitude. The continuous casting billet is cooled by a slow cooling process: the temperature in the slow cooling pit is ≥650℃ and the slow cooling time is ≥24h. AlN precipitates fastest at 550-750℃. Holding at 650℃ can coarsen the precipitates, reduce brittleness, control the cooling rate of the core, balance the structural stress, and thus reduce cracks.

[0008] Preferably, the consumption of the protective slag is controlled at 1.0-1.1 kg / t, and the thickness of the liquid slag layer of the protective slag is 12-15 mm.

[0009] Preferably, the diameter of the medium-carbon high-alumina steel 38CrMoAl continuously cast round billet is 310mm.

[0010] Compared with the prior art, the beneficial effects of the present invention are: the present invention provides a production method with stable quality control and no surface cracks by controlling the continuous casting start-up, primary cooling water, casting speed, secondary cooling water volume, and protective slag optimization. The qualified rate of the produced φ310mm medium carbon high aluminum steel 38CrMoAl continuous casting round billet is as high as 99.9% or more. Attached Figure Description

[0011] Figure 1 A schematic diagram of the surface of the round blank produced by the method of the embodiment is shown. Detailed Implementation

[0012] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below through examples.

[0013] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0014] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments. Example

[0015] like Figure 1 As shown in the figure, this embodiment provides a method for stable control of the surface quality of medium carbon high alumina steel continuously cast round billets. The method is as follows: 1) Confirm and set the process parameters: Based on the characteristics and cross-sectional dimensions of 38CrMoAl steel, the primary cooling water flow rate is set to 190-200 m³ / h, using a weak cooling process, and the secondary cooling water specific volume is 0.10-0.11 L / kg; Based on the characteristics of 38CrMoAl steel, the crystallizer vibration parameters are set as follows: A1: 5; A2: 0; f1: 80; f2: 50; As: 0.18, where A1 represents stroke constant parameter 1; A2 represents stroke constant parameter 2; f1 represents frequency constant parameter 1; f2 represents frequency constant parameter 2; As represents the non-sinusoidal factor; Based on the chemical composition of 38CrMoAl steel and the crystallizer cooling conditions, the chemical composition of the protective slag is optimized. The chemical composition of the protective slag, by mass percentage, is: 29-36 wt% CaO, 20-30 wt% SiO2, 1-4 wt% Al2O3, and 1.0-3.0 wt%... The composition includes % Li2O, 1.0-3.8wt% MgO, 7-12wt% F, and 5-9wt% TC; viscosity range 0.10-0.16 Pa·s; R control range 1.0-1.50; melting point design 1070-1120℃; moisture ≤0.30%. This composition increases the basicity of the protective slag, reduces its viscosity, and helps it form a stable slag film within the crystallizer. Adding a certain amount of Li2O to the protective slag helps reduce the negative impact of high-Al steel on the protective slag.

[0016] 2) Before each refining process, the steel temperature is determined based on the condition of the ladle and the temperature drop during the process to ensure that the tundish superheat is not too high. Specifically, the tundish superheat for the first heat is 20~30℃, and the tundish superheat for subsequent heats is 10~20℃.

[0017] 3) Before pouring, the steel pourer in the tundish should check and confirm the status of the ingot, and whether there is any slippage or improper sealing. If there is any abnormality, it should be dealt with in time.

[0018] 4) Once the confirmation is complete, notify the operator to stop the fire. Then, the operators at each post should raise the baking device, raise the baffle, and lower the stopper. Check the tundish nozzle for any debris and deal with it promptly. At the same time, install the tundish argon blowing pipe and perform the argon blowing operation. The argon blowing time should be ≥10 minutes. Argon gas should be turned on 4 minutes before the fire is stopped. After the tank is cleaned, pouring can begin directly.

[0019] 5) Ladle opening operation: Pull the ladle into position at once to receive the flowing sand. After the steel flows down, control the flow to fall 150-300mm into the ladle. If the impact zone causes severe steel overturning, the opening degree of the ladle cylinder can be appropriately reduced. After the steel overturning is reduced, the ladle cylinder can be fully opened.

[0020] 6) Start-up and pouring operation of the intermediate tank: Set the pouring speed before pouring: 0.30-0.40m / min (target 0.30-0.35m / min).

[0021] 7) Casting start requirements: After the ladle starts casting, the weight of molten steel in the tundish reaches 22t, and the pressure bar starts casting. After the molten steel has passed the side hole of the nozzle, protective slag is then pushed into the crystallizer. The amount of protective slag added is controlled at 1.1-1.2kg / t, and the thickness of the liquid slag layer is controlled at 12~15mm.

[0022] 8) After receiving the flow start instruction from the machine operator, the intermediate operator starts the flow using the pressure bar. After starting the flow, control the injection size to match the set pull speed, and raise the liquid level in the crystallizer to about 180mm from the top in 10-20 seconds. Then, power on the straightening machine, start the vibration, and start the pull speed. Gradually raise the liquid level to 90-110mm from the top of the crystallizer. Smoothly increase the pull speed to 1.4-1.8 times the initial set pull speed, with a target of 1.6 times (this operation should be completed in about 30 seconds). Manually stabilize the liquid level in the crystallizer for more than 10 seconds before putting it into automatic liquid level control.

[0023] 9) After the liquid level is automatically controlled, the casting speed continues to increase steadily. After all the casting streams have started flowing, the tundish is lowered and the casting speed is gradually adjusted to 0.80~0.90m / min.

[0024] 10) The billet is cut to length to obtain a medium carbon high alumina round billet with a diameter of 310mm. After cutting, it is quickly hoisted into the slow cooling pit. The temperature of the billet in the slow cooling pit is ≥650℃, and the slow cooling time is controlled at ≥24h.

[0025] 11) Then, the surface of the billet is inspected by shot blasting. By implementing the above measures, the first-pass inspection rate of the billet can be increased to 99.9%.

[0026] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for stable control of the surface quality of continuously cast round billets of medium carbon high alumina steel, characterized in that: The method involves casting molten medium-carbon high-alumina steel 38CrMoAl refined steel. The superheat of the tundish in the first heat of the molten medium-carbon high-alumina steel 38CrMoAl refined steel is 20~30℃; the superheat of the tundish in subsequent heats is 10~20℃. After the intermediate ladle is opened for casting, a protective slag is added. The chemical composition of the protective slag, by mass percentage, is: 29-36 wt% CaO, 20-30 wt% SiO2, 1-4 wt% Al2O3, 1.0-3.0 wt% Li2O, 1.0-3.8 wt% MgO, 7-12 wt% F, and 5-9 wt% TC. The consumption of the protective slag is controlled at 1.0-1.1 kg / t, and the thickness of the liquid slag layer is 12-15 mm. The viscosity range of the protective slag is 0.10-0.16 Pa·s, the basicity R is controlled within the range of 1.0-1.20, the melting point is designed to be 1070-1120℃, and the moisture content is ≤0.30%. The primary cooling water flow rate is controlled at 190-200 m³ / h, and the secondary cooling water flow rate is 0.10-0.11 L / kg; The crystallizer vibration parameters are set as follows: A1:5; A2:0; f1:80; f2:50; As: 0.18, where A1 represents stroke constant parameter 1; A2 represents stroke constant parameter 2; f1 represents frequency constant parameter 1; f2 represents the frequency constant parameter 2; As represents the non-sinusoidal factor. The casting speed is 0.80~0.90m / min; The continuous casting billet is cooled by a slow cooling process: the temperature in the slow cooling pit is ≥650℃ and the slow cooling time is ≥24h.

2. The method for stable control of surface quality of medium-carbon high-alumina steel continuously cast round billets according to claim 1, characterized in that, The diameter of the medium-carbon high-alumina steel 38CrMoAl continuously cast round billet is 310mm.

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