A continuous casting method to prevent high alloy steel billet warping

Abstract

This invention discloses a continuous casting method to prevent high-alloy steel billet head tilting, belonging to the field of metallurgical technology. The method includes: inserting a dummy bar into the mold inlet; injecting molten steel from the tundish into the mold; and pulling down the dummy bar when the high-alloy steel molten level reaches a specified height to begin billet pulling. High pressure is used during dummy bar pulling; after the dummy bar pulls the billet head through the first straightening machine, the pressure is reduced to low pressure to decrease the adhesion resistance between the billet head and the straightening machine. Furthermore, the water flow rate in the mold and the initial pulling speed are increased during the pulling process to shorten the time required to adjust the initial pulling speed to the target pulling speed, thus reducing the risk of billet head tilting due to excessive billet shrinkage. Using the above-mentioned continuous casting method provided by this invention can eliminate the problem of high-alloy steel billet head tilting and reduce various production and equipment accidents caused by billet head tilting.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, and more specifically, to a continuous casting method for preventing high-alloy steel billets from warping. Background Technology

[0002] If the billet head is too high during continuous casting, it will prevent the billet from passing through the gap between the fan-shaped section / straightening machine rolls at the straight section. In severe cases, it may even block the upper roll of the fan-shaped section / straightening machine, damaging the equipment. Patent CN104889355B provides a continuous casting method to prevent billet head from tilting, which includes the processes of preparing molten steel, packing, crystallization, continuous casting, straightening, and cutting. By controlling the starting speed of the continuous casting machine and the cooling water in the secondary cooling zone, the upward tilting of the billet head during the steel pouring process can be suppressed. Patent CN110666125B provides a method for eliminating billet warping in continuous casting. By logically controlling the opening sequence of the water spray valves in each cooling circuit of the secondary cooling zone, the amount of cooling water on the inner and outer arc sides of the billet is adjusted, eliminating the temperature difference between the inner and outer arcs of the billet, reducing the billet's resistance to deformation, and suppressing billet warping. Simultaneously, when the billet enters the straightening zone, a water spray cooling device is arranged on the lower side of the billet. Due to the temperature difference between the upper and lower sides of the billet, existing billet warping can also be effectively eliminated, preventing the billet warping from damaging rollers and cutting torches, ensuring the smooth operation of continuous casting production. Both of these patents only eliminate billet warping by changing the secondary cooling method. However, in actual production, billet warping is not only caused by uneven secondary cooling but also by other factors. How to reduce the probability of high-alloy steel billet warping while considering other factors is an urgent problem to be solved. Summary of the Invention

[0003] The purpose of this invention is to overcome the defects of the prior art and provide a continuous casting method to prevent high alloy steel billets from warping.

[0004] The technical problem solved by this invention is achieved by the following technical solution.

[0005] This invention provides a continuous casting method to prevent high-alloy steel billets from warping, comprising: sending the dummy bar into the mold inlet, then injecting molten steel from the tundish into the mold; when the high-alloy steel molten steel level reaches a specified height, pulling down the dummy bar to start billet pulling; wherein, high pressure is used during the dummy bar pulling process, and after the dummy bar drives the billet head through the first straightening machine, the pressure is switched to low pressure to reduce the adhesion resistance between the billet head and the straightening machine; and during the billet pulling process, the water flow rate in the mold and the initial pulling speed are increased to shorten the time for adjusting the initial pulling speed to the target pulling speed, thereby reducing the billet head warping caused by excessive billet shrinkage.

[0006] The present invention has the following beneficial effects:

[0007] This invention provides a continuous casting method to prevent high-alloy steel billets from warping. The method mainly includes: rationally controlling the pressure during the drawing process to reduce the adhesion resistance between the billet head and the straightening machine; increasing the drawing speed and crystallizer water flow rate during the drawing process; increasing the initial drawing speed of the high-alloy steel billet and shortening the time required to increase the drawing speed to the target speed after initial casting; thereby eliminating the problem of billet head warping caused by excessively rapid billet shrinkage due to mismatch between the drawing speed and cooling speed when the drawing speed is low or the initial drawing speed takes too long to adjust to the target speed. Detailed Implementation

[0008] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0009] The following is a detailed description of a continuous casting method for preventing high-alloy steel billets from warping, provided by an embodiment of the present invention.

[0010] This invention provides a continuous casting method to prevent high-alloy steel billets from warping, comprising: sending the dummy bar into the mold inlet, then injecting molten steel from the tundish into the mold; when the high-alloy steel molten steel level reaches a specified height, pulling down the dummy bar to start billet pulling; wherein, high pressure is used during the dummy bar pulling process, and after the dummy bar drives the billet head through the first straightening machine, the pressure is switched to low pressure to reduce the adhesion resistance between the billet head and the straightening machine; and during the billet pulling process, the water flow rate in the mold and the initial pulling speed are increased to shorten the time for adjusting the initial pulling speed to the target pulling speed, thereby reducing the billet head warping caused by excessive billet shrinkage.

[0011] Most current methods for eliminating billet warping in continuous casting involve altering the secondary cooling process. However, in actual production, billet warping is not only caused by uneven secondary cooling but also by factors such as billet (molten steel) composition, dummy ingot position, the sticking force between the billet head and the dummy ingot head, the initial casting speed, and the crystallizer water flow rate. Based on optimizing the secondary cooling water and considering the above factors, this invention proposes a continuous casting method to prevent high-alloy steel billet warping. This method mainly includes: rationally controlling the pressure during the dummy ingot drawing process to reduce the adhesion resistance between the billet head and the straightening machine; and shortening the time required to increase the casting speed to the target speed after initial casting. This eliminates billet warping caused by excessively rapid billet shrinkage due to mismatch between casting speed and cooling rate when the casting speed is low or the initial casting speed takes too long to reach the target speed. This is because the billet head lifting during the drawing and billet pulling process after secondary cooling is mainly due to: (1) High alloy steel has a high alloy content, poor thermal conductivity, and good thermal strength and toughness. When the billet pulling speed is low and the time to adjust the starting speed to the target speed is too long, the billet pulling speed and cooling speed are not compatible, causing the billet to shrink too quickly after the last stand, resulting in billet head lifting. Therefore, this method increases the initial billet pulling speed and the amount of water in the crystallizer; (2) The driving force of the straightening machine is generated by the friction between the billet pulling roller surface and the billet surface. The surface adhesion resistance of high alloy steel billets is large, and when the pressure of the straightening machine is low, it is easy to cause the billet head to lift. Therefore, this method delays the time for the straightening machine to switch to low pressure. Through the above methods, the problem of high alloy steel billet head lifting is eliminated, and various production and equipment accidents caused by billet head lifting are reduced.

[0012] In an optional implementation, the drawing process includes the following steps: high pressure is used during the drawing process. After the blank is detected to have entered the drawing straightener, the high pressure is maintained and the drawing straightener rolls continue to run for 2 to 4 revolutions depending on their radius, so that 500 to 900 mm of the blank continues to bear the high pressure after entering the drawing straightener. After this part of the blank leaves, the drawing straightener switches to normal pressure. The drawing straightener consists of multiple drawing straighteners starting from the first drawing straightener in the drawing direction, and the drawing method of each subsequent drawing straightener is the same as that of the first drawing straightener.

[0013] The driving force of the straightening machine is generated by the friction between the drawing rollers and the billet surface. High-alloy steel billets have significant surface adhesion resistance, and when the straightening machine pressure is low, the billet head is prone to tilting. Currently, steel mills use two types of hydraulic pressure in straightening machines during continuous casting: one uses high pressure during the drawing process and switches to low pressure after the drawing rod has completely exited the straightening machine, thereby improving billet quality. The solution provided in this invention adjusts the pressure of the straightening machine during the drawing process. The pressure does not switch to low pressure immediately upon detecting the first billet entering, but only after the first billet has passed the first straightening machine. This reduces the likelihood of the first billet tilting due to low pressure when entering the straightening machine, thus improving billet quality.

[0014] In an optional implementation, the high pressure is 32-36t.

[0015] In an optional implementation, the traction rod used in the traction process is a chain traction rod, and the detachment position is set within 0.6 to 1 meter of the outlet of the last straightening machine. This is to reduce the stress caused by the top force pulling the billet head upward when the billet is detached due to excessive length.

[0016] In an optional implementation, the temperature of the billet head detachment is controlled at 780–830°C.

[0017] In an optional implementation, the billet pulling process includes the following steps: controlling the initial pulling speed to be 0.3 m / min, the speed increase rate to be 0.01 m / s, increasing the initial flow rate of the crystallizer to 3200 NL / min, and then restoring the crystallizer water flow rate to 2800 NL / min after the billet exits the crystallizer.

[0018] Taking the production of 320×425mm H13 steel at Shaogang Steel Plant as an example, the normal starting casting speed is 0.15m / min, the speed increment is 0.03m / 5s, the normal pouring speed is 0.65m / min, and the crystallizer water flow rate is 2800NL / min. By adjusting the starting casting speed to 0.3m / min and the speed increment to 0.01m / s, the initial crystallizer flow rate is increased to 3200NL / min, significantly shortening the time required to adjust the starting casting speed to the target speed. After the billet exits the crystallizer, the crystallizer water flow rate returns to 2800NL / min. This method, by increasing the starting casting speed for high-alloy steel and shortening the time required to increase the casting speed to the target speed after initial pouring, eliminates the problem of excessive billet shrinkage and billet warping caused by a mismatch between the casting speed and cooling rate when the casting speed is low or the time required to adjust the starting casting speed to the target speed is too long.

[0019] In an optional implementation, during the process of injecting molten steel into the crystallizer through the tundish via an immersion nozzle, the time from the molten steel falling into the crystallizer to the start of casting is controlled to be 12–18 seconds. This time depends primarily on the position of the dummy bar within the crystallizer. Taking the production of H13 steel at Shaoguan Steel Plant as an example, if the dummy bar is inserted 300mm into the crystallizer with a cross-section of 320×425mm, then maintaining an 8mm opening on the stopper rod during casting ensures a start-up time between 12 and 18 seconds. This time control ensures that the molten steel evenly fills the dummy bar during casting, preventing billet shape defects caused by inconsistent steel flow.

[0020] In an optional embodiment, the tundish immersion nozzle is an immersion nozzle with side holes, wherein the side holes are two or four holes.

[0021] In an optional embodiment, before the dummy ingot head is fed into the crystallizer inlet, it is coated entirely with a mixture of rapeseed oil and graphite powder in a mass ratio of 1:9-4:6, with the coating thickness controlled to be 3-5 mm, to reduce the biting force between the billet and the dummy ingot head. At the point where the dummy ingot head enters the crystallizer inlet, the entire dummy ingot head is coated with a mixture of rapeseed oil and graphite powder in a ratio of approximately 1:9-4:6, with a coating thickness of 3-5 mm. This serves to create a certain gap between the molten steel and the dummy ingot head, preventing the molten steel from completely penetrating and adhering to the inside of the dummy ingot head, thereby reducing the biting force between the billet and the dummy ingot head.

[0022] In an optional implementation, the high-alloy steel is H13 steel.

[0023] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0024] Example 1

[0025] Taking the production of H13 steel at Shaogang Steel Plant as an example, the continuous casting method to prevent high-alloy steel billets from warping includes the following steps:

[0026] Before producing S1. high alloy steel, the ingot head is sent into the crystallizer inlet, and a mixture of rapeseed oil and graphite powder is applied to the entire ingot head in a ratio of approximately 2:8, with a coating thickness of 5mm.

[0027] S2. The tundish immersion nozzle has 2 or 4 side holes. During the start-up operation, the steel flow is kept constant until the start-up. The time from the molten steel falling into the crystallizer to the start-up operation is 16 seconds.

[0028] S3. The drawing process uses high pressure, 36t. Once the billet head enters the straightening machine, the high pressure is maintained for 2-4 rotations depending on the straightening roller radius, ensuring that approximately 700mm of the billet head continues to bear high pressure after entering the straightening machine. After this portion of the billet leaves, the straightening machine switches to normal pressure, 7-9t (here, "straightening machine" refers to all straightening machines starting from the drawing direction). The dummy bar is chain-type, and the dummy bar removal position is located within 0.6 meters of the outlet of the last straightening machine. The dummy bar removal temperature of the cast billet head is controlled at 780℃.

[0029] S4. Control the initial casting speed to 0.3 m / min, the speed increase rate to 0.01 m / s, the initial flow rate of the crystallizer to 3200 NL / min, and after the billet exits the crystallizer, the crystallizer water flow rate to 2800 NL / min.

[0030] Example 2

[0031] Taking the production of H13 steel at Shaogang Steel Plant as an example, the continuous casting method to prevent high-alloy steel billets from warping includes the following steps:

[0032] Before producing S1. high alloy steel, the ingot head is fed into the crystallizer inlet, and a mixture of rapeseed oil and graphite powder is applied to the entire ingot head in a ratio of approximately 2:8, with a coating thickness of 3mm.

[0033] S2. The tundish immersion nozzle has 2 or 4 side holes. During the start-up operation, the steel flow is kept constant until the start-up. The time from the molten steel falling into the crystallizer to the start-up operation is 12 seconds.

[0034] S3. The drawing process uses high pressure, 36t. Once the billet head enters the straightening machine, the high pressure is maintained for 2-4 rotations depending on the straightening roller radius, ensuring that approximately 900mm of the billet head continues to bear high pressure after entering the straightening machine. After this portion of the billet leaves, the straightening machine switches to normal pressure, 7-9t (here, "straightening machine" refers to all straightening machines starting from the drawing direction). The dummy bar is chain-type, and the dummy bar removal position is located within 0.8 meters of the outlet of the last straightening machine. The temperature of the billet head after dummy bar removal is controlled between 800℃ and 800℃.

[0035] S4. Control the initial casting speed to 0.3 m / min, the speed increase rate to 0.02 m / s, the initial flow rate of the crystallizer to 3200 NL / min, and after the billet exits the crystallizer, the crystallizer water flow rate to 2870 NL / min.

[0036] Example 3

[0037] Taking the production of H13 steel at Shaogang Steel Plant as an example, the continuous casting method to prevent high-alloy steel billets from warping includes the following steps:

[0038] Before producing S1. high alloy steel, the ingot head is sent into the crystallizer inlet, and a mixture of rapeseed oil and graphite powder is applied to the entire ingot head in a ratio of approximately 2:8, with a coating thickness of 5mm.

[0039] S2. The tundish immersion nozzle has 2 or 4 side holes. During the start-up operation, the steel flow is kept constant until the start-up. The time from the molten steel falling into the crystallizer to the start-up operation is 16 seconds.

[0040] S3. The drawing process uses high pressure, 32t. Once the billet head enters the straightening machine, the high pressure is maintained, and the machine continues to run for 2-4 revolutions depending on the straightening roller radius, ensuring that approximately 700mm of the billet head remains under high pressure after entering the machine. After this portion of the billet leaves, the straightening machine switches to normal pressure, 7-9t (here, "straightening machine" refers to all straightening machines starting from the drawing direction). The dummy bar is chain-type, and the dummy bar removal position is located within 1 meter of the outlet of the last straightening machine. The dummy bar removal temperature of the cast billet head is controlled at 830℃.

[0041] S4. Control the initial casting speed to 0.25 m / min, the speed increase rate to 0.01 m / s, the initial flow rate of the crystallizer to 3000 NL / min, and after the billet exits the crystallizer, the crystallizer water flow rate to 2730 NL / min.

[0042] Comparative Example 1

[0043] Similar to the steps in Example 1, the only difference is that the high pressure of the drawing ingot is 7t, resulting in a billet warping rate of 8%.

[0044] Comparative Example 2

[0045] Similar to the steps in Example 1, the only difference is that about 300mm of the blank continues to be subjected to high pressure after entering the straightening machine, resulting in a blank warping rate of 6%.

[0046] Comparative Example 3

[0047] The steps were similar to those in Example 1, except that the starting pulling speed was 0.15 m / min, and the result was a billet lift rate of 5.5%.

[0048] Comparative Example 4

[0049] The steps were similar to those in Example 1, except that the crystallizer water flow rate was 2700 NL / min, resulting in a billet lift rate of 4.5%.

[0050] Comparative Example 5

[0051] Similar to the steps in Example 1, the only difference is that the slab head temperature is below 780°C, resulting in a head head warping rate of 2.3%.

[0052] As can be seen from the above embodiments and comparative examples, the billet warping rate of the scheme provided by the embodiments of the present invention is between 0 and 3%, preferably 0%, while the billet warping rate is higher in the comparative examples when the continuous casting conditions are different.

[0053] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A continuous casting method for preventing high-alloy steel billets from warping, characterized in that, include: The ingot head is fed into the crystallizer inlet, and then molten steel from the tundish is poured into the crystallizer. When the high-alloy steel molten steel level reaches the specified height, the ingot rod is pulled down to begin billet pulling. High pressure is used during the ingot pulling process. Once the billet head is detected entering the straightening machine, the high pressure is maintained, and the machine continues to run for 2-4 revolutions depending on the straightening roller radius, ensuring that 500-900mm of the billet head continues to bear high pressure after entering the straightening machine. After this portion of the billet head leaves, the straightening machine switches to normal pressure. The straightening machines are multiple straightening machines starting from the first straightening machine in the billet pulling direction, and the ingot pulling method of each subsequent straightening machine is the same as the first straightening machine. The high pressure is 32-36t, and the normal pressure is 7-9t to reduce the adhesion resistance between the billet head and the straightening machine. During the billet pulling process, the crystallizer water flow rate and the initial pulling speed are increased to shorten the time required to adjust the initial pulling speed to the target pulling speed, reducing the risk of billet head warping caused by excessive billet shrinkage. The billet pulling process includes the following steps: controlling the initial pulling speed to 0.3 m / min, increasing the initial flow rate of the crystallizer to 3200 NL / min, and then restoring the crystallizer water flow rate to 2800 NL / min after the billet exits the crystallizer. The traction rod used in the traction process is a chain traction rod, and the detachment position of the traction rod is set within 0.6 to 1 meter of the outlet of the last straightening machine.

2. The continuous casting method according to claim 1, characterized in that, The temperature of the first billet detachment from the slab is controlled at 780~830℃.

3. The continuous casting method according to claim 1, characterized in that, During the process of injecting molten steel into the crystallizer through the tundish via the submerged entry nozzle, the time from the molten steel falling into the crystallizer to the start of casting is controlled to be 12-18 seconds.

4. The continuous casting method according to claim 3, characterized in that, The immersion inlet is an immersion inlet with side holes, wherein the side holes are two or four holes.

5. The continuous casting method according to claim 1, characterized in that, Before the dummy bar is fed into the crystallizer inlet, a mixture of rapeseed oil and graphite powder with a mass ratio of 1:9-4:6 is used to coat the dummy bar completely, and the coating thickness is controlled to be 3-5mm to reduce the biting force between the billet and the dummy bar.

6. The continuous casting method according to claim 1, characterized in that, The high-alloy steel is H13 steel.