Continuous casting square billet capable of ensuring hardenability and preparation process thereof

Abstract

The invention provides a continuous casting square billet capable of guaranteeing hardenability and a preparation process thereof, and relates to the field of steel materials. The continuous casting square billet capable of ensuring the hardenability comprises the following components in percentage by weight: 0.24 to 0.27 percent of C, 0.20 to 0.30 percent of Si, 1.10 to 1.15 percent of Mn, less than 0.015 percent of P, less than 0.005 percent of S, 0.40 to 0.45 percent of Cr, less than 0.08 percent of Ni, less than 0.08 percent of Cu, less than 0.01 percent of Mo, less than 0.01 percent of Pb, less than 0.01 percent of Sn, 0.0020 to 0.0030 percent of B, 0.025 to 0.035 percent of Ti, 0.020 to 0.030 percent of Al, 0.004 to 0.006 percent of N, less than 0.0012 percent of O, less than 0.00015 percent of H and the balance of Fe and other inevitable substances. Wherein Ti / N is more than 4.0 and less than 8.0. In the invention, when Ti / N is more than 4.0 and less than 8.0, Ti can completely fix nitrogen in the steel; at the moment, boron is added and can be stably dissolved in the steel in a solid mode to form a large amount of effective boron, then the hardenability of the steel is improved through the effective boron, and finally the continuous casting square billet meeting the hardenability requirement is obtained.

Description

Technical Field

[0001] This invention relates to the field of steel materials, specifically to a continuously cast square billet with guaranteed hardenability and its preparation process. Background Technology

[0002] Square billets that ensure hardenability are the raw material for manufacturing track steel, which is one of the chassis components of construction machinery. It is commonly used in construction machinery such as excavators, bulldozers, tracked cranes, and pavers. The market capacity of the product is about 10 million tons, and the market prospects are good.

[0003] Chinese patent CN115198166A discloses a production method for improving the cleanliness of track steel molten steel. This method involves sequentially performing converter smelting, LF refining, VD furnace vacuum treatment, and fully protected continuous casting. In the converter smelting, the carbon content is controlled to be ≥0.14% at the final stage. During tapping, steel-core aluminum, silicon-manganese, and other alloys are added sequentially for deoxidation and alloying. In the LF refining process, after slag formation and desulfurization, the sulfur content is controlled to be ≤0.01%. Then, alloys are added sequentially to complete deoxidation, denitrification, and inclusion modification. Throughout the LF refining process, bottom-blown argon gas is used for stirring to promote the flotation of inclusions. Subsequently, the steel undergoes VD vacuum treatment, during which bottom-blown argon gas further removes inclusions. Finally, continuous casting employs protective casting. The core of this method is to ensure the cleanliness of the track steel through process control, thereby reducing the levels of Class A and Class B inclusions. However, it essentially only focuses on process control of molten steel cleanliness and does not address material process control for the hardenability of track steel.

[0004] In the existing technology, there is no solution that addresses the hardenability requirements of track steel by increasing the effective boron (effective B) in the electric arc furnace steelmaking process for square billets. Summary of the Invention

[0005] This invention addresses the problem of the lack of process control for hardenable track steel materials in the prior art by providing a continuous casting billet with guaranteed hardenability and its preparation process.

[0006] The technical method of the present invention is as follows: A continuously cast square billet with guaranteed hardenability comprises, by weight percentage: C: 0.24-0.27%, Si: 0.20-0.30%, Mn: 1.10%-1.15%, P: less than 0.015%, S: less than 0.005%, Cr: 0.40-0.45%, Ni: less than 0.08%, Cu: less than 0.08%, Mo: less than 0.01%, Pb: less than 0.01%, Sn: less than 0.01%, B: 0.0020%-0.0030%, Ti: 0.025%-0.035%, Al: 0.020%-0.030%, N: 0.004-0.006%, O: less than 0.0012%, H: less than 0.00015%, with the remainder being Fe and other unavoidable substances; wherein, 4.0 < Ti / N < 8.0. The hardenability DI value of the continuously cast square billet is 33-47 mm. The B factor (i.e., the contribution of B to the increase in hardenability) of the continuously cast square billet is ≥1.6.

[0007] This invention also provides a process for preparing a continuously cast square billet that ensures hardenability, comprising: Step 1) Raw material preparation: Raw materials include pig iron and scrap steel; wherein, the sulfur content of pig iron is ≤0.05%, and the scrap steel includes, by weight percentage: Cu: less than 0.08%, Ni: less than 0.08%, Cr: less than 0.30%, Mo: less than 0.01%, Sn: less than 0.01%, Pb: less than 0.01%, P: less than 0.05%, S: less than 0.03%, B: 0.0020%-0.0030%; Step 2) Electric arc furnace smelting: Add scrap steel and pig iron to the electric arc furnace, control the gas absorption of the molten steel during the smelting process, and control the carbon content of the molten steel to ≥0.1% at the end point. Add aluminum iron to the ladle for deoxidation when tapping the steel to obtain rough molten steel. Step 3) LF Refining: The crude steel enters the refining furnace for diffusion deoxidation and slag formation. In the state of white slag, aluminum wire and ferrotitanium are fed in sequence. After adding ferrotitanium, no more electricity should be applied to obtain refined steel. The hardenability DI value of the refined steel is controlled at 33-47mm. Step 4) VD Vacuum Treatment: Refined molten steel enters the VD furnace, and a vacuum is drawn to a vacuum state of ≤67Pa. After breaking the vacuum, boron wire is fed to obtain molten steel after VD vacuum treatment. Step 5) Continuous casting: The molten steel after VD vacuum treatment is cast through a tundish and a crystallizer to obtain a continuously cast billet; Step 6) Slow cooling and finishing: The continuously cast billet is slowly cooled and finished to obtain a continuously cast billet that ensures hardenability.

[0008] In step 2), during the process of adding scrap steel and pig iron to the electric arc furnace, the furnace is energized and heated. During the heating process, slag-forming agent, carbon powder, and magnesium balls are added in stages. Steelmaking accelerator, silicon-manganese alloy, and lime are also added to the ladle.

[0009] In step 3), diffusion deoxidation is performed using calcium carbide and AD powder. The composition of the white slag is SiO2: 8.0-12.0%, CaO: 50.0-60.0%, MnO≤0.10%, Al2O3: 17.0-21.0%, MgO: 3.0-4.0%, TFe≤0.40%, and the alkalinity of the white slag is ≥4.0.

[0010] In step 5), the continuous casting process is protected throughout. This protection includes: using a covering agent in the tundish to prevent molten steel from contacting the atmosphere; using argon gas to protect the ladle casing; and using submerged entry nozzles and protective slag in the continuous casting crystallizer to prevent molten steel from being drawn into the atmosphere. The crystallizer uses electromagnetic stirring, the tundish temperature is controlled at 1535±5℃, and the casting speed is controlled at 1.60±0.05m / min.

[0011] The beneficial effects of this invention are: I. In this invention, when 4.0 < Ti / N < 8.0, Ti can completely fix the nitrogen in the steel; when boron is added at this time, boron can be stably dissolved in the steel and form a large amount of effective boron, thereby improving the hardenability of the steel through effective boron, and finally obtaining a continuously cast billet that meets the hardenability requirements.

[0012] II. In the preparation process of this invention, the order of adding aluminum (Al), titanium (Ti), and boron (B) is strictly required: Al must be added first to fully deoxidize, then Ti is added to fix nitrogen (i.e., nitrogen fixation), and finally B is added to dissolve it in the steel to enhance hardenability. This is because Ti has a stronger affinity for nitrogen than B has for nitrogen. Adding Ti can preferentially bind nitrogen to change the form of boron, reduce the ineffective reaction between boron and nitrogen, and ensure that most of the boron exists in a solid solution state in the steel, thereby guaranteeing hardenability. Attached Figure Description

[0013] Figure 1 This is a graph showing the hardenability as a function of Ti / N. Detailed Implementation

[0014] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0015] The concept of this invention: Track steel is generally boron-containing low-carbon manganese steel. This type of steel has the characteristics of low alloy cost and excellent performance, especially its hardenability can be significantly improved. However, boron-containing steel is an extremely difficult steel to smelt and has the greatest performance fluctuation. The form in which boron exists is crucial to its performance: boron in steel mainly exists as solid solution boron (Bsolid), boron nitride (BN), boron oxide (B2O3), iron-boron cementite [Fe3(CB)], and iron-boron acid compounds [Fe2]. 23 [(CB)6] Boron exists in five forms. Among them, boron in boron oxide and boron nitride is acid-insoluble boron, while the other forms of boron are acid-soluble boron; and boron oxide, boron nitride, iron boron cementite, and iron borate compounds (the latter two are collectively referred to as boron phases) are all detrimental to the hardenability of steel. Therefore, even boron-containing steels with the same boron content may have vastly different hardenability, leading to significant fluctuations in steel properties. Therefore, the core of this application is to improve hardenability stability by reducing the boron nitride content and increasing the acid-soluble boron content through process optimization.

[0016] In the smelting process of boron-containing track steel, the order in which aluminum (Al), titanium (Ti), and boron (B) are added is strictly required: Al must be added first for thorough deoxidation, then Ti is added to fix nitrogen (nitrogen fixation), and finally B is added to dissolve it in the steel, thereby enhancing hardenability. Since Ti has a stronger affinity for nitrogen than B, the core purpose of adding Ti to boron-containing steel is to change the form in which boron exists, ensuring that most of the boron exists in a solid solution state, thus guaranteeing hardenability.

[0017] This invention provides a continuously cast square billet with guaranteed hardenability, comprising, by weight percentage: C: 0.24-0.27%, Si: 0.20-0.30%, Mn: 1.10%-1.15%, P: less than 0.015%, S: less than 0.005%, Cr: 0.40-0.45%, Ni: less than 0.08%, Cu: less than 0.08%, Mo: less than 0.01%, Pb: less than 0.01%, Sn: less than 0.01%, B: 0.0020%-0.0030%, Ti: 0.025%-0.035%, Al: 0.020%-0.030%, N: 0.004-0.006%, O: less than 0.0012%, H: less than 0.00015%, with the remainder being Fe and other unavoidable substances; wherein, 4.0 < Ti / N < 8.0.

[0018] Depend on Figure 1 The hardenability variation curves shown indicate that the hardenability of the experimental steel exhibits a three-stage change with increasing Ti / N ratio: firstly, it increases with increasing Ti / N ratio; then, it remains relatively stable (forming a plateau region) with further increases in Ti / N ratio; and finally, it decreases with further increases in Ti / N ratio. Therefore, the optimal Ti addition should satisfy the condition 4.0 < Ti / N < 8.0.

[0019] Specifically: When Ti / N < 4.0, Ti cannot completely fix the nitrogen in the steel to form stable TiN. In this case, after adding boron, the remaining nitrogen will react with some of the boron to form BN, making it difficult to increase the effective boron content. When 4.0 < Ti / N < 8.0, Ti can completely fix the nitrogen in the steel. After the boron is added, it can be stably dissolved in the steel to form a large amount of effective boron. When Ti / N > 8.0, although Ti can still completely fix the nitrogen in the steel and convert the added boron into effective boron, excessive Ti will increase the probability of forming large refractory TiN particles. These particles will become the nucleation cores of ferrite during the phase transformation. At the same time, excessive Ti is also easy to combine with C to form TiC, which leads to a decrease in the C content in austenite. Both of these have an adverse effect on the hardenability of the steel.

[0020] In this invention, the DI value (hardenability value) is calculated from the chemical analysis measurement data of the smelting process. The target DI (excluding B) is 33-47 mm, and its calculation formula is as follows: DI (excluding B) = 0.54C × (0.7Si+1) × (3.333Mn+1) × (2.16Cr+1) × (3Mo+1) × (0.363Ni+1) × (0.365Cu+1) × (1.73V+1) × 25.4mm (Applicable condition: C < 0.4%).

[0021] Low-magnification inspection is required for continuously cast billets. The etching method must meet the requirements of GB / T226, and the low-magnification level of the billet must be ≤1.0. Samples must be taken from the beginning and end of each heat of continuously cast billets for hardenability and composition testing. After reforging, the samples are tested according to GB / T225, with a normalizing temperature of 900±10℃ and a quenching temperature of 870±10℃. Hardenability must meet the following requirements: J1.5=46-52HRC, J13≥34HRC, J25≤32HRC. Non-metallic inclusions must meet the following requirements: Class A, Class B, Class C, and Class D (including coarse and fine inclusions) must each be ≤1.0.

[0022] Furthermore, the B factor (i.e., the contribution of B to the increase in hardenability) ≥ 1.6, and the actual B factor (abbreviated BF) is calculated as follows: BF = Measured DI (hardenability data and C content) / Calculated DI (composition excluding B). Here, the DI value represents the ideal critical diameter of the steel, that is, the critical diameter when quenched in a hypothetical quenching medium with infinite quenching intensity. The magnitude of the DI value directly characterizes the hardenability of the steel; the larger the value, the better the hardenability. The DI value is an inherent property of the steel itself, mainly determined by its chemical composition.

[0023] This invention also provides a process for preparing a continuously cast square billet that ensures hardenability, comprising: Step 1) Raw material preparation: Raw materials include pig iron and scrap steel; wherein, the sulfur content of pig iron is ≤0.05%, and the scrap steel, by weight percentage, includes: Cu: less than 0.08%, Ni: less than 0.08%, Cr: less than 0.30%, Mo: less than 0.01%, Sn: less than 0.01%, Pb: less than 0.01%, P: less than 0.05%, and S: less than 0.03%. For example, scrap steel, by weight percentage, includes: Cu: 0.04%, Ni: 0.02%, Cr: 0.04%, Mo: 0.006%, Sn: 0.004%, Pb: 0.002%, P: 0.03%, S: 0.02%, and B: 0.0020%-0.0030%.

[0024] Step 2) Electric Arc Furnace Smelting: Scrap steel and pig iron are added to the electric arc furnace. During the smelting process, the gas absorption of the molten steel is controlled, and the carbon content of the molten steel is controlled to be ≥0.1% at the end point. When tapping, aluminum-iron is added to the ladle for deoxidation to obtain rough molten steel. Here, aluminum-iron, by weight percentage, includes: Al 38-42%, with the remainder being Fe.

[0025] This invention involves adding pig iron during the electric arc furnace smelting process to increase the carbon content of the molten steel, reduce impurities, and enhance the degassing capacity of the electric arc furnace. Simultaneously, pure scrap steel is selected to optimize raw material purity. During tapping, ferrosilicon is added to the ladle for deoxidation, thereby reducing the oxygen content in the steel.

[0026] During the process of adding scrap steel and pig iron to the electric arc furnace, the furnace is energized and heated. During this heating process, 25 kg / t of slag-forming agent, 20 kg / t of carbon powder, and 8 kg / t of magnesium balls are added in stages. The mass ratio of scrap steel to pig iron is 70-80:20-40. For example, a mass ratio of 75:35. Here, the purpose of the slag-forming agent is to create electric arc furnace slag with high basicity. The purpose of the carbon powder is to react with oxygen in the molten steel, reducing the oxygen content in the steel; simultaneously, the carbon-oxygen reaction releases a large amount of heat, which can replace electrical energy and reduce costs. Magnesium balls are used for furnace protection.

[0027] Steelmaking accelerators, ferrosilicon alloys, and lime can also be added to the ladle. Specifically, after opening the pallet, aluminum ferrophosphate and steelmaking accelerators are added; when the ladle reaches 1 / 4 of its weight of molten steel, ferrosilicon alloys and lime are added sequentially. Before tapping is finished, all slag (lime CaO) and alloy (ferrosilicon alloy, specifically Mn68Si18) must be added to the ladle, and the added alloy and lime must be thoroughly mixed with the steel stream. Here, the steelmaking accelerator, by weight percentage, includes: CaO: 30-35%, Al2O3: 40-45%, Al: 25-30%, N≤0.05%. This invention achieves deoxidation and alloying of molten steel by adding steelmaking accelerators, ferrosilicon alloys (specifically Mn68Si18), lime, etc., to meet the steel composition requirements. Simultaneously, the addition of Mn68Si18 is to adjust the chemical composition of elements Mn and Si to the lower limit.

[0028] In one embodiment, step 2) further includes: temperature measurement and sampling to detect the chemical composition of the molten steel. The electric arc furnace adopts full-process foamed slag smelting and uses argon bottom blowing for stirring, with slight fluctuations in the slag surface to avoid air intake. The electric arc furnace uses eccentric bottom tapping.

[0029] In one embodiment, step 2) further includes: to avoid the steel slag reaction reducing Si and causing an increase in Al2O3 inclusions in the molten steel, the tapping time is controlled at 3-4 minutes; when the weight of the molten steel in the ladle reaches 3 / 4, all the alloy slag material is added.

[0030] Step 3) LF Refining: The crude steel enters the refining furnace for slag diffusion deoxidation. In the state of white slag, aluminum wire and ferrotitanium are fed in sequence. After adding ferrotitanium, no more electricity should be applied to obtain refined steel. The hardenability DI value of the refined steel is controlled at 33-47mm.

[0031] In one embodiment, diffusion deoxidation uses 2 kg / t of calcium carbide and 2 kg / t of AD powder. Here, the AD powder composition by weight percentage includes: metallic Al: 12%-18%, Al2O3: 60%-75%, SiO2 ≤ 8%, moisture ≤ 0.5%, and particle size ≤ 100 mesh. The Al2O3 in the AD powder undergoes special processing and is in a critical state, exhibiting strong activity. It readily combines with CaO to form a calcium aluminate slag system, which has a high sulfur capacity. Furthermore, the presence of metallic aluminum in the AD powder reduces oxygen activity, thus further facilitating sulfur removal from molten steel (iron).

[0032] In one embodiment, the white slag comprises, by weight percentage: SiO2: 8.0-12.0%, CaO: 50.0-60.0%, MnO≤0.10%, Al2O3: 17.0-21.0%, MgO: 3.0-4.0%, TFe≤0.40%. Here, the basicity of the white slag is ≥4.0, TFe+MnO≤0.50%, and Al2O3 is controlled at 17.0-21.0%. The white slag has good fluidity and a high sulfur capacity, making it suitable for steel refining and vacuum treatment.

[0033] In one embodiment, the aluminum wire feeding step includes: feeding aluminum wire in the state of white slag, detecting the aluminum content in the molten steel, and replenishing the aluminum wire.

[0034] Step 4) VD Vacuum Treatment: Refined molten steel enters the VD furnace and is evacuated to a vacuum level ≤67 Pa. After breaking the vacuum, boron wire is fed in to obtain VD vacuum-treated molten steel. Here, the vacuum state is maintained for 15-20 minutes to ensure that the gas content in the molten steel meets the requirements. Feeding in boron wire after breaking the vacuum is crucial to eliminating interfering factors that affect boron recovery, ensuring a high boron recovery rate and stable boron content. Step 4 mainly involves removing gases from the molten steel through vacuum, reducing the hydrogen (H) and oxygen content in the molten steel.

[0035] Step 5) Continuous casting: The molten steel after VD vacuum treatment is cast through a tundish and a crystallizer to obtain a continuously cast billet.

[0036] During continuous casting, full-process protective casting is employed to significantly reduce secondary oxidation. 25MnTiB steel is not permitted to be continuously cast with other non-B steels. The ladle casing is protected with argon gas, and the tundish is protected with a covering agent to prevent the molten steel from contacting the atmosphere. The covering agent composition, by weight percentage, includes: MgO: 80.0%±1.0%, CaO: 12.0%±1.0%, SiO2: 3.0%±0.5%, Al2O3: 1.8%±0.5%, FeO: 1.2%±0.5%. In the continuous casting crystallizer (also known as a dedicated crystallizer), immersion nozzles and special protective slag are used to prevent the molten steel from being drawn into the atmosphere. The crystallizer employs electromagnetic stirring at 350A / 3.5Hz, the tundish temperature is 1535±5℃, and the casting speed is 1.60±0.05m / min. Simultaneously, hardenability and composition tests are conducted on the continuously cast billets at the beginning and end of each heat. The composition of the special protective slag, by weight percentage, includes: CaO: 30.5%±3.0%, SiO2: 28.5%±3.0%, Al2O3: 4.8%±2.0%, B2O3: 4.3%±1.5%, Na2O: 5.0%±1.5%, MgO: 5.5%±1.5%, FC: 11.0%±2.0%, TC: 13.5%±2.5%. The physical properties of the special protective slag are: melting point 1140±30℃, viscosity (1300℃) 0.35±0.09 Pa·S, H2O≤0.5%, CaO / SiO2 ratio 1.07±0.06.

[0037] The key process parameters for continuous casting are controlled as follows: the crystallizer uses electromagnetic stirring (parameter 350A / 3.5Hz), the tundish temperature is controlled at 1535±5℃, and the casting speed is controlled at 1.60±0.05m / min. Simultaneously, samples are taken from the beginning and end of each hot continuous casting billet for hardenability and composition testing to ensure the billet quality meets standards.

[0038] Step 6) Slow cooling and finishing: The continuously cast billet is slowly cooled and finished to obtain a continuously cast billet that ensures hardenability.

[0039] In one embodiment, the slow cooling of the continuously cast billet is achieved using a stacked slow cooling tank. The slow cooling time is 24-72 hours. Preferably, the time is 24 hours.

[0040] In one embodiment, finishing includes: inspecting the surface quality of the continuously cast billet and finishing the surface defects that can be cleaned, wherein cracks, pinholes, inclusions, HIC, and white spots are not allowed on the surface of the billet.

[0041] In this invention, unless otherwise specified, all raw materials used in the preparation are commercially available products well known to those skilled in the art.

[0042] The present invention will be described in detail below through embodiments and experimental examples. However, these are merely examples and do not limit the present invention in any way.

[0043] Example 1 This embodiment 1 provides a process for preparing a continuously cast square billet that ensures hardenability, including the following steps: Step 1, Raw Material Preparation: Due to the special requirements of the steel grades that ensure hardenability, a special crystallizer protective slag and submerged entry nozzle are used to guarantee product quality. The sulfur content in the pig iron must be ≤0.05%. Alloys, lime, magnesium spheres, carbon powder, steelmaking accelerators, AD powder, calcium carbide, and the special protective slag must be kept dry. The results of impurity element testing in the classified scrap steel are as follows: Cu: 0.04%, Ni: 0.02%, Cr: 0.04%, Mo: 0.006%, Sn: 0.004%, Pb: 0.002%, P: 0.03%, S: 0.02%, B: 0.0002%.

[0044] The second step is electric arc furnace smelting: 78 tons of scrap steel and 31 tons of pig iron are added to a 100-ton electric arc furnace. The furnace is heated in stages, with 25 kg / t of slag-forming agent, 20 kg / t of carbon powder, and 8 kg / t of magnesium balls for furnace protection added in stages. The electric arc furnace uses an eccentric hearth tapping method, with a tapping temperature of 1624℃, a chemical composition of C: 0.11%, P: 0.012%, a tapping volume of 101 tons, and a residual volume of 41 tons. Aluminum-iron deoxidizer is added to the ladle during tapping to reduce the oxygen content of the steel. The amount of aluminum-iron added is 4.8 kg / t; the amount of steelmaking accelerator added is 4.9 kg / t. Alloying during tapping involves adding 17.3 kg / t of Mn68Si18 to adjust the chemical composition of Mn and Si to the lower limit. 7.8 kg / t of lime is added during tapping.

[0045] The order of adding alloys and auxiliary materials is as follows: after opening the pallet, add ferroaluminum and steelmaking accelerator; when the ladle reaches 1 / 4 of its weight, add ferrosilicon manganese alloy Mn68Si18 and lime in sequence. Before the end of tapping, all slag and ferrosilicon manganese alloy must be added to the ladle, and the added alloy and lime must be thoroughly mixed with the steel stream. To avoid the reduction of Si by the steel slag, which would lead to an increase in Al2O3 inclusions in the molten steel, the tapping time is 3 minutes and 31 seconds, and the slag layer thickness is 26 mm. When the ladle reaches 3 / 4 of its weight, all alloy slag is added. The oxygen content of the molten steel sample is 28 ppm, which is relatively low compared to the oxygen content obtained by the above manufacturing method. Temperature sampling was conducted. The temperature of the crude steel molten steel was 1564℃. The composition was: C: 0.23%, Si: 0.24%, Mn: 1.12%, P: 0.011%, S: 0.026%, Cu: 0.04%, Ni: 0.02%, Cr: 0.04%, Mo: 0.006%, Sn: 0.004%, Pb: 0.002%.

[0046] In electric arc furnace smelting, temperature measurement and sampling are conducted to analyze the chemical composition of the molten steel. The electric arc furnace employs full-process foamed slag smelting and uses argon bottom blowing for stirring, resulting in slight slag surface fluctuations to prevent air intake.

[0047] The third step, LF refining: The LF refining process is as follows: temperature measurement and sampling at the designated location → power supply and heating → slag formation → argon blowing → adjusting alloy chemical composition → temperature adjustment → wire feeding operation → temperature fine-tuning, alloy fine-tuning, temperature measurement and sampling → LF end ladle lifting.

[0048] During slag formation and wire feeding operations, calcium carbide and AD powder are used as deoxidizers for diffusion deoxidation in refining. After maintaining the white slag for 20 minutes, samples are taken and 4 m / t of aluminum wire is fed. Here, the composition of the white slag in the refining furnace is: SiO2: 9.2%, CaO: 56.4%, MnO: 0.06%, Al2O3: 19.1%, MgO: 3.5%, TFe: 0.24%. The basicity of the white slag is ≥4.0, and TFe: 0.24% fully meets the requirements. The Al2O3 content is controlled at 20%±0.1%. The white slag has good fluidity and is particularly suitable for steel refining and vacuum treatment.

[0049] Before refining, aluminum wire is fed a second time to achieve the target aluminum content of 0.04%, and FeTi40 alloy is added. Before adding FeTi40, Si is controlled between 0.20-0.25%. The temperature is increased by 15℃, and after adding ferrotitanium at a rate of 4 kg / t, no further power is applied. Simultaneously, samples are taken for chemical composition analysis. The chemical composition of the molten steel sample is as follows: C: 0.26%, Si: 0.27%, Mn: 1.14%, P: 0.012%, S: 0.0028%, Cr: 0.42%, Ni: 0.02%, Cu: 0.04%, Ti: 0.040%, Al: 0.040%, B: 0.0002%, Sn: 0.003%, Pb: 0.001%, TO content 0.0016%, N: 0.0054%. The calculated Ti / N ratio is 7.41. The chemical composition of the molten steel obtained through the above manufacturing method is very stable. The calculated DI value is 41 mm.

[0050] The LF refining temperature is 1625℃, and the molten steel in the ladle is hoisted to the VD position.

[0051] Step 4, Vacuum Treatment (VD): After the ladle containing molten steel reaches the VD position, the temperature is measured at 1637℃. During the VD vacuum process, the VD vacuum cover is opened and then tightly closed. The vacuum pumps are then turned on in stages to perform vacuum treatment. The molten steel is held at a vacuum degree ≤67Pa for 25 minutes to ensure that the gas content in the molten steel meets the requirements. After breaking the vacuum, boron wire is fed in. The key is to eliminate interfering factors that affect the boron recovery rate and ensure a high boron recovery rate and stable boron content. After the vacuum treatment is completed, the VD furnace cover is opened to break the vacuum. The amount of argon gas blown into the ladle is appropriately increased to blow a small opening in the slag surface covering the molten steel. Using the theoretical calculation of 95% recovery rate, boron wire is fed into the molten steel through the small opening using a wire feeder, adjusted to 0.0028%. If there is residual boron, the amount added is reduced. After the vacuum was broken, nitrogen and oxygen samples were taken for analysis, and hydrogen content was determined. The H content was 0.00012%. After the analysis results were obtained, titanium wire was quickly fed to the ladle until the Ti content reached 0.040%, and then calcium wire was fed. After soft blowing with argon for 10 minutes, the ladle was hoisted. The chemical composition of the molten steel sample was analyzed as follows: C: 0.26%, Si: 0.26%, Mn: 1.12%, P: 0.012%, S: 0.0023%, Cr: 0.41%, Ni: 0.02%, Cu: 0.04%, Ti: 0.032%, Al: 0.025%, B: 0.0022%, Sn: 0.003%, Pb: 0.001%, TO content 0.0011%, N: 0.0054%, H: 0.00012%. The calculated Ti / N ratio was 5.93. The chemical composition and B content of the molten steel obtained by the above manufacturing method are very stable. The calculated DI value is 39 mm. The VD temperature reading is 1564℃, and the molten steel is hoisted into the continuous casting ladle.

[0052] Step 5, Continuous Casting: Before production, check the equipment accuracy. The tundish is baked for 2.5 hours, and must be cleaned and kept clean before being put into operation. Full-process protective casting is used during continuous casting. The ladle is protected with an argon gas sleeve, and the tundish is protected with a covering agent to prevent the molten steel from contacting the atmosphere. Submerged entry nozzles and special protective slag are used in the continuous casting crystallizer to prevent the molten steel from being drawn into the atmosphere. The continuous casting specification is a 200mm × 200mm square billet. The tundish temperature is 1537℃, the casting speed is 1.6m / min, the crystallizer electromagnetic stirring parameters are 350A / 4Hz, the crystallizer water flow rate is 130t / h, and the chemical composition analysis sampling uses a main sample + a final 10T sample + a NO analysis sample. 200mm long low-magnification and hardenability samples are taken from the first and last ends of the second cast billet.

[0053] Step 6, slow cooling: The continuously cast billets are stacked and slow cooled in a slow cooling tank for 48 hours.

[0054] Step 7, finishing: After the continuously cast billet is slowly cooled in the stacking slow cooling tank for 48 hours, the surface quality of the continuously cast billet is inspected and surface defects that can be cleaned are finished. Cracks, pinholes, slag inclusions, HIC, and white spots are not allowed on the surface of the billet.

[0055] Example 2 This embodiment 2 provides a process for preparing a continuously cast square billet that ensures hardenability, including the following steps: Step 1, Raw Material Preparation: Due to the special requirements of the steel grades that ensure hardenability, a special crystallizer protective slag and submerged entry nozzle are used to guarantee product quality. The sulfur content in the pig iron must be ≤0.05%. Alloys, lime, magnesium spheres, carbon powder, steelmaking accelerators, AD powder, calcium carbide, and the special protective slag must be kept dry. The results of impurity element testing in the classified scrap steel are as follows: Cu: 0.04%, Ni: 0.02%, Cr: 0.04%, Mo: 0.006%, Sn: 0.004%, Pb: 0.002%, P: 0.03%, S: 0.02%, B: 0.0002%.

[0056] The second step is electric arc furnace smelting: 70% scrap steel and 30% pig iron are added to a 100-ton electric arc furnace. The furnace is energized and heated in stages, with 25 kg / t of slag-forming agent, 20 kg / t of carbon powder, and 8 kg / t of magnesium balls for furnace protection added in stages. The electric arc furnace uses an eccentric hearth tapping method, with a tapping temperature of 1622℃, a chemical composition of C: 0.010%, P: 0.012%, a tapping volume of 102 tons, and a residual volume of 42 tons. Ferroaluminum is added to the ladle for deoxidation to reduce the oxygen content in the steel. 5.1 kg / t of 40 ferroaluminum is added; 4.8 kg / t of steelmaking accelerator is added. Alloying is achieved by adding 17.2 kg / t of Mn68Si18 to adjust the chemical composition of Mn and Si to the lower limit. 8.1 kg / t of lime is added at the tapping stage.

[0057] The order of adding alloys and auxiliary materials is as follows: after opening the pallet, add ferroaluminum and steelmaking accelerator; when the ladle reaches 1 / 4 of its weight, add ferrosilicon manganese alloy Mn68Si18 and lime in sequence. Before the end of tapping, all slag and ferrosilicon manganese alloy must be added to the ladle, and the added alloy and lime must be thoroughly mixed with the steel stream. To avoid the reduction of Si by the steel slag, which would lead to an increase in Al2O3 inclusions in the molten steel, the tapping time is 3 minutes and 31 seconds, and the slag layer thickness is 28 mm. When the ladle reaches 3 / 4 of its weight, all alloy slag is added. The oxygen content of the molten steel sample is 25 ppm, which is relatively low compared to the oxygen content obtained by the above manufacturing method. Temperature sampling was conducted. The temperature of the crude steel was 1561℃. The composition was: C: 0.22%, Si: 0.24%, Mn: 1.11%, P: 0.010%, S: 0.024%, Cu: 0.03%, Ni: 0.02%, Cr: 0.04%, Mo: 0.007%, Sn: 0.004%, Pb: 0.003%.

[0058] In electric arc furnace smelting, temperature measurement and sampling are conducted to analyze the chemical composition of the molten steel. The electric arc furnace employs full-process foamed slag smelting and uses argon bottom blowing for stirring, resulting in slight slag surface fluctuations to prevent air intake.

[0059] The third step, LF refining: The LF refining process is as follows: temperature measurement and sampling at the designated location → power supply and heating → slag formation → argon blowing → adjusting alloy chemical composition → temperature adjustment → wire feeding operation → temperature fine-tuning, alloy fine-tuning, temperature measurement and sampling → LF end ladle lifting.

[0060] During slag formation and wire feeding operations, calcium carbide and AD powder are used as deoxidizers for diffusion deoxidation in refining. After maintaining the white slag for 20 minutes, samples are taken and 4 m / t of aluminum wire is fed. Here, the composition of the white slag in the refining furnace is: SiO2: 9.7%, CaO: 56.0%, MnO: 0.02%, Al2O3: 19.6%, MgO: 3.8%, TFe: 0.15%. The basicity of the white slag is ≥4.0, and TFe: 0.15% fully meets the requirements. The Al2O3 content is controlled at 20%±0.1%. The white slag has good fluidity and is particularly suitable for steel refining and vacuum treatment.

[0061] Before refining, aluminum wire is fed a second time to achieve the target aluminum content of 0.04%, and FeTi40 alloy is added. Before adding FeTi40, Si is controlled between 0.20-0.25%. The temperature is increased by 15℃, and the addition is at a rate of 4 kg / t. After adding the ferrotitanium, no further electricity is applied, and samples are taken for chemical composition analysis. The chemical composition of the molten steel sample is as follows: C: 0.26%, Si: 0.28%, Mn: 1.15%, P: 0.012%, S: 0.0022%, Cr: 0.43%, Ni: 0.02%, Cu: 0.03%, Ti: 0.041%, Al: 0.036%, B: 0.0002%, Sn: 0.004%, Pb: 0.003%, TO content 0.0012%, N: 0.0056%. The calculated Ti / N ratio is 7.32. The chemical composition of the molten steel obtained through the above manufacturing method is very stable. The calculated DI value is 42 mm.

[0062] The LF refining temperature is 1628℃, and the molten steel in the ladle is hoisted to the VD position.

[0063] Step 4, VD Vacuum Treatment: After the ladle containing molten steel reaches the VD position, the temperature is measured at 1637℃. During the VD vacuum process, the VD vacuum cover is opened and then tightly closed. The vacuum pumps are then turned on in stages to perform vacuum treatment. The molten steel is held at a vacuum degree ≤67Pa for 25 minutes to ensure that the gas content in the molten steel meets the requirements. After breaking the vacuum, boron wire is fed in. The key is to eliminate interfering factors that affect the boron recovery rate and ensure a high boron recovery rate and stable boron content. After the vacuum treatment is completed, the VD furnace cover is opened to break the vacuum. The amount of argon gas blown into the ladle is appropriately increased to blow a small opening in the slag surface covering the molten steel. Using the theoretical calculation amount of 95% recovery, boron wire is fed into the molten steel through the small opening using a wire feeder and adjusted to 0.0028%. If there is residual boron, the amount added is reduced. After breaking the vacuum, nitrogen and oxygen samples are taken for analysis and hydrogen determination. After the analysis results are obtained, titanium wire is quickly fed in according to the N content until the Ti content is 0.040%, and then calcium wire is fed in. After soft blowing argon for 10 minutes, the ladle is lifted. The chemical composition of the molten steel sample was analyzed as follows: C: 0.26%, Si: 0.27%, Mn: 1.14%, P: 0.012%, S: 0.0016%, Cr: 0.42%, Ni: 0.02%, Cu: 0.03%, Ti: 0.031%, Al: 0.024%, B: 0.0023%, Sn: 0.004%, Pb: 0.003%, TO content: 0.0006%, N: 0.0056%, H: 0.00010%. The calculated Ti / N ratio was 5.54. The chemical composition and B content of the molten steel obtained by the above manufacturing method are very stable. The calculated DI value was 41 mm. The VD temperature was 1567℃. The molten steel was hoisted from the ladle to the continuous casting.

[0064] Step 5, Continuous Casting: Before production, check the equipment accuracy. The tundish is baked for 2.5 hours, and must be cleaned and kept clean before being put into operation. Full-process protective casting is used during continuous casting. The ladle is protected with an argon gas sleeve, and the tundish is protected with a covering agent to prevent the molten steel from contacting the atmosphere. Submerged entry nozzles and special protective slag are used in the continuous casting crystallizer to prevent the molten steel from being drawn into the atmosphere. The continuous casting specification is a 200mm × 200mm square billet. The tundish temperature is 1534℃. The casting speed is 1.6m / min. The crystallizer electromagnetic stirring parameters are 350A / 4Hz. The crystallizer water flow rate is 130t / h. Chemical composition analysis sampling includes a main sample + a final 10T sample + a NO analysis sample. 200mm long low-magnification and hardenability samples are taken from the first and last ends of the second cast billet.

[0065] Step 6, slow cooling: The continuously cast billets are stacked and slow cooled in a slow cooling tank for 48 hours.

[0066] Step 7, finishing: After the continuously cast billet is slowly cooled in the stacking slow cooling tank for 48 hours, the surface quality of the continuously cast billet is inspected and surface defects that can be cleaned are finished. Cracks, pinholes, slag inclusions, HIC, and white spots are not allowed on the surface of the billet.

[0067] Example 3 This embodiment 3 provides a process for preparing a continuously cast square billet that ensures hardenability, including the following steps: Step 1, Raw Material Preparation: Due to the special requirements of the steel grades that ensure hardenability, a special crystallizer protective slag and submerged entry nozzle are used to guarantee product quality. The sulfur content in the pig iron must be ≤0.05%. Alloys, lime, magnesium spheres, carbon powder, steelmaking accelerators, AD powder, calcium carbide, and the special protective slag must be kept dry. The results of impurity element testing in the classified scrap steel are as follows: Cu: 0.04%, Ni: 0.02%, Cr: 0.04%, Mo: 0.006%, Sn: 0.004%, Pb: 0.002%, P: 0.03%, S: 0.02%, B: 0.0002%.

[0068] The second step is electric arc furnace smelting: 70% scrap steel and 30% pig iron are added to a 100-ton electric arc furnace. The furnace is energized and heated in stages, with 25 kg / t slag-forming agent, 20 kg / t carbon powder, and 8 kg / t magnesium balls for furnace protection added in stages. Steel is tapped when the furnace temperature reaches 1620℃, the chemical composition is C: 0.08%-0.12%, and P≤0.010%, with a tapping rate controlled at 100-105 tons. The electric arc furnace uses an eccentric hearth tapping method, with a tapping temperature of 1624℃ and 40% of the steel remaining during operation. Ferroaluminum is added to the ladle for deoxidation to reduce the oxygen content in the steel. 5 kg / t of 40 ferroaluminum is added; 5 kg / t of steelmaking accelerator is added. Alloying is achieved by adding 17 kg / t of Mn68Si18 to adjust the chemical composition of Mn and Si to the lower limit. 8 kg / t of lime is added during tapping.

[0069] The order of adding alloys and auxiliary materials is as follows: after opening the pallet, add ferroaluminum and steelmaking accelerator; when the weight of molten steel in the ladle reaches 1 / 4, add ferrosilicon manganese alloy Mn68Si18 and lime in sequence. Before the end of tapping, all slag and ferrosilicon manganese alloy must be added to the ladle, and the added alloy and lime must be thoroughly mixed with the steel stream. To avoid the reduction of Si by the steel slag, which would lead to an increase in Al2O3 inclusions in the molten steel, the tapping time is 3 minutes and 31 seconds, and the slag layer thickness is 28 mm. When the weight of molten steel in the ladle reaches 3 / 4, all alloy slag is added. The oxygen content of the molten steel sample is 30 ppm, which is relatively low compared to the oxygen content of the molten steel obtained by the above manufacturing method. Temperature sampling was conducted. The temperature of the crude steel molten steel was 1564℃. The composition was as follows: C: 0.23%, Si: 0.24%, Mn: 1.10%, P: 0.008%, S: 0.026%, V: 0.005%, Cu: 0.04%, Ni: 0.02%, Cr: 0.04%, Mo: 0.006%, Sn: 0.004%, Pb: 0.002%.

[0070] In electric arc furnace smelting, temperature measurement and sampling are conducted to analyze the chemical composition of the molten steel. The electric arc furnace employs full-process foamed slag smelting and uses argon bottom blowing for stirring, resulting in slight slag surface fluctuations to prevent air intake.

[0071] The third step, LF refining: The LF refining process is as follows: temperature measurement and sampling at the designated location → power supply and heating → slag formation → argon blowing → adjusting alloy chemical composition → temperature adjustment → wire feeding operation → temperature fine-tuning, alloy fine-tuning, temperature measurement and sampling → LF end ladle lifting.

[0072] During slag formation and wire feeding operations, calcium carbide and AD powder are used as deoxidizers for diffusion deoxidation in refining. After maintaining the white slag for 20 minutes, samples are taken and 4 m / t of aluminum wire is fed. The composition of the white slag in the refining furnace is: SiO2: 9.9%, CaO: 57.1%, MnO: 0.03%, Al2O3: 20.2%, MgO: 3.4%, TFe: 0.18%. The basicity of the white slag is ≥4.0, and TFe: 0.18% fully meets the requirements. The Al2O3 content is controlled at 20%±0.1%. The white slag has good fluidity and is particularly suitable for steel refining and vacuum treatment.

[0073] Before refining, aluminum wire is fed a second time to achieve a target aluminum content of 0.04%, and FeTi40 alloy is added. Before adding FeTi40, the Si content is controlled between 0.20-0.25%, and the temperature is increased by 15℃, with an addition rate of 4 kg / t. No further power is applied after adding the ferrotitanium. Simultaneously, samples are taken for chemical composition analysis, with a target DI of 33-47 mm², ensuring a DI ratio of 0.9-1.1 between the two heats. The chemical composition of the molten steel sample was analyzed and found to be: C: 0.26%, Si: 0.28%, Mn: 1.15%, P: 0.010%, S: 0.0023%, Cr: 0.45%, Ni: 0.03%, Cu: 0.04%, Ti: 0.041%, Al: 0.035%, B: 0.0002%, Sn: 0.004%, Pb: 0.001%, TO content 0.0014%, N: 0.0052%, with a calculated Ti / N ratio of 7.88. The chemical composition of the molten steel obtained by the above manufacturing method is very stable. The calculated DI value is 43 mm.

[0074] The LF refining temperature is 1623℃, and the molten steel in the ladle is hoisted to the VD position.

[0075] Step 4, VD Vacuum Treatment: After the ladle containing molten steel reaches the VD position, the temperature is measured at 1637℃. During the VD vacuum process, the VD vacuum cover is opened and then tightly closed. The vacuum pumps are then turned on in stages to perform vacuum treatment. The molten steel is held at a vacuum degree ≤67Pa for 25 minutes to ensure that the gas content in the molten steel meets the requirements. After breaking the vacuum, boron wire is fed in. The key is to eliminate interfering factors that affect the boron recovery rate and ensure a high boron recovery rate and stable boron content. After the vacuum treatment is completed, the VD furnace cover is opened to break the vacuum. The amount of argon gas blown into the ladle is appropriately increased to blow a small opening in the slag surface covering the molten steel. Using the theoretical calculation amount of 95% recovery rate, boron wire is fed into the molten steel through the small opening using a wire feeder and adjusted to 0.0028%. After breaking the vacuum, nitrogen and oxygen samples are taken for analysis and hydrogen determination. After the analysis results are obtained, titanium wire is quickly fed in according to the N content until the Ti content is 0.040%, and then calcium wire is fed in. After soft blowing argon for 10 minutes, the ladle is lifted. The chemical composition of the molten steel sample was analyzed as follows: C: 0.26%, Si: 0.27%, Mn: 1.14%, P: 0.010%, S: 0.0019%, Cr: 0.44%, Ni: 0.03%, Cu: 0.04%, Ti: 0.033%, Al: 0.029%, B: 0.0023%, Sn: 0.004%, Pb: 0.001%, TO content: 0.0009%, N: 0.0053%, H: 0.00011%. The calculated Ti / N ratio was 6.23. The chemical composition and B content of the molten steel obtained by the above manufacturing method are very stable. The calculated DI value is 42 mm. The VD temperature was 1566℃. The molten steel was hoisted from the ladle to the continuous casting.

[0076] Step 5, Continuous Casting: Before production, check the equipment accuracy. The tundish is baked for 2.5 hours, and must be cleaned and kept clean before being put into operation. Full-process protective casting is used during continuous casting. The ladle is protected with an argon gas sleeve, and the tundish is protected with a covering agent to prevent the molten steel from contacting the atmosphere. Submerged entry nozzles and special protective slag are used in the continuous casting crystallizer to prevent the molten steel from being drawn into the atmosphere. The continuous casting specification is a 200mm × 200mm square billet. The tundish temperature is 1536℃. The casting speed is 1.6m / min. The crystallizer electromagnetic stirring parameters are 350A / 4Hz. The crystallizer water flow rate is 130t / h. Chemical composition analysis sampling includes a main sample + a final 10T sample + a NO analysis sample. 200mm long low-magnification and hardenability samples are taken from the first and last ends of the second cast billet.

[0077] Step 6, slow cooling: The continuously cast billets are stacked and slow cooled in a slow cooling tank for 48 hours.

[0078] Step 7, finishing: After the continuously cast billet is slowly cooled in the stacking slow cooling tank for 48 hours, the surface quality of the continuously cast billet is inspected and surface defects that can be cleaned are finished. Cracks, pinholes, slag inclusions, HIC, and white spots are not allowed on the surface of the billet.

[0079] Example 4 This embodiment 4 provides a process for preparing a continuously cast square billet that ensures hardenability, including the following steps: Step 1, Raw Material Preparation: Due to the special requirements of the steel grades that ensure hardenability, a special crystallizer protective slag and submerged entry nozzle are used to guarantee product quality. The sulfur content in the pig iron must be ≤0.05%. Alloys, lime, magnesium spheres, carbon powder, steelmaking accelerators, AD powder, calcium carbide, and the special protective slag must be kept dry. The results of impurity element testing in the classified scrap steel are as follows: Cu: 0.04%, Ni: 0.02%, Cr: 0.04%, Mo: 0.006%, Sn: 0.004%, Pb: 0.002%, P: 0.03%, S: 0.02%, B: 0.0002%.

[0080] The second step is electric arc furnace smelting: 70% scrap steel and 30% pig iron are added to a 100-ton electric arc furnace. The furnace is energized and heated in stages, with slag-forming agents, carbon powder, and magnesium balls for furnace protection added in stages. Steel is tapped when the furnace temperature reaches the tapping temperature of 1620℃, the chemical composition C: 0.08%-0.12%, and P≤0.010%, with a tapping rate controlled at 100-105t. The electric arc furnace uses an eccentric hearth tapping method, with a tapping temperature of 1624℃ and 40% of the steel remaining during operation. Ferroaluminum is added to the ladle for deoxidation to reduce the oxygen content in the steel. The amount of ferroaluminum added is 5 kg / t; the amount of steelmaking accelerator added is also 5 kg / t. Alloying is achieved by adding 16.9 kg / t of Mn68Si18 to adjust the chemical composition of Mn and Si to the lower limit. 8 kg / t of lime is added during tapping.

[0081] The order of adding alloys and auxiliary materials is as follows: after opening the pallet, add ferroaluminum and steelmaking accelerator; when the weight of molten steel in the ladle reaches 1 / 4, add ferrosilicon manganese alloy and lime in sequence. Before the end of tapping, all slag and alloys must be added to the ladle, and the added alloys and lime must be thoroughly mixed with the steel stream. To avoid the reduction of Si by the steel slag, which would lead to an increase in Al2O3 inclusions in the molten steel, the tapping time is 3 minutes and 31 seconds, and the slag layer thickness is 28 mm. When the weight of molten steel in the ladle reaches 3 / 4, all alloy slag is added. The oxygen content of the molten steel sample is 30 ppm, which is relatively low for the molten steel obtained by the above manufacturing method. Temperature sampling was conducted. The temperature of the crude steel molten steel was 1564℃. The composition was as follows: C: 0.23%, Si: 0.24%, Mn: 1.09%, P: 0.008%, S: 0.026%, V: 0.005%, Cu: 0.04%, Ni: 0.02%, Cr: 0.04%, Mo: 0.006%, Sn: 0.004%, Pb: 0.002%.

[0082] In electric arc furnace smelting, temperature measurement and sampling are conducted to analyze the chemical composition of the molten steel. The electric arc furnace employs full-process foamed slag smelting and uses argon bottom blowing for stirring, resulting in slight slag surface fluctuations to prevent air intake.

[0083] The third step, LF refining: The LF refining process is as follows: temperature measurement and sampling at the designated location → power supply and heating → slag formation → argon blowing → adjusting alloy chemical composition → temperature adjustment → wire feeding operation → temperature fine-tuning, alloy fine-tuning, temperature measurement and sampling → LF end ladle lifting.

[0084] During slag formation and wire feeding operations, calcium carbide and AD powder are used as deoxidizers for diffusion deoxidation in refining. After maintaining the white slag for 20 minutes, samples are taken, and 4 m / t of aluminum wire is fed. The composition of the white slag in the refining furnace is: SiO2: 9.5%, CaO: 56.8%, MnO: 0.05%, Al2O3: 19.9%, MgO: 3.7%, TFe: 0.20%. The white slag basicity ≥4.0 and TFe: 0.20% fully meet the requirements. Al2O3 is controlled at 20% ± 0.1%. The white slag has good fluidity and is particularly suitable for steel refining and vacuum treatment. The LF refining temperature is 1622℃, and the molten steel in the ladle is hoisted to the VD position.

[0085] Before refining, aluminum wire is fed a second time to achieve a target aluminum content of 0.04%, and FeTi40 alloy is added. Before adding FeTi40, Si is controlled between 0.20-0.25%. The temperature is increased by 15℃, and the addition is at a rate of 4 kg / t. After adding the ferrotitanium, no further electricity is applied, and samples are taken for chemical composition analysis. The chemical composition of the molten steel sample is as follows: C: 0.27%, Si: 0.25%, Mn: 1.13%, P: 0.012%, S: 0.0025%, Cr: 0.43%, Ni: 0.01%, Cu: 0.02%, Ti: 0.038%, Al: 0.032%, B: 0.0003%, Sn: 0.007%, Pb: 0.001%, TO content 0.0011%, N: 0.0054%. The calculated Ti / N ratio is 7.04. The chemical composition of the molten steel obtained through the above manufacturing method is very stable. The calculated DI value is 42 mm.

[0086] Step 4, VD Vacuum Treatment: After the ladle containing molten steel reaches the VD position, the temperature is measured at 1637℃. During the VD vacuum process, the VD vacuum cover is opened and then tightly closed. The vacuum pumps are then turned on in stages to perform vacuum treatment. The molten steel is held at a vacuum degree ≤67Pa for 25 minutes to ensure that the gas content in the molten steel meets the requirements. After breaking the vacuum, boron wire is fed in. The key is to eliminate interfering factors that affect the boron recovery rate and ensure a high boron recovery rate and stable boron content. After the vacuum treatment is completed, the VD furnace cover is opened to break the vacuum. The amount of argon gas blown into the ladle is appropriately increased to blow a small opening in the slag surface covering the molten steel. Using the theoretical calculation amount of 95% recovery, boron wire is fed into the molten steel through the small opening using a wire feeder and adjusted to 0.0024%. If there is residual boron, the amount added is reduced. After breaking the vacuum, nitrogen and oxygen samples are taken for analysis and hydrogen determination. After the analysis results are obtained, titanium wire is quickly fed in according to the N content until the Ti content is 0.040%, and then calcium wire is fed in. After soft blowing argon for 10 minutes, the ladle is lifted. The chemical composition of the molten steel sample was analyzed as follows: C: 0.27%, Si: 0.24%, Mn: 1.12%, P: 0.012%, S: 0.002%, Cr: 0.42%, Ni: 0.01%, Cu: 0.02%, Ti: 0.030%, Al: 0.026%, B: 0.0024%, Sn: 0.007%, Pb: 0.001%, TO content 0.0007%, N: 0.0055%, H: 0.00011%. The calculated Ti / N ratio was 5.45. The chemical composition and B content of the molten steel obtained by the above manufacturing method are very stable. The calculated DI value is 41 mm. The VD temperature was measured at 1565℃. The molten steel was then transferred from the ladle to the continuous casting machine.

[0087] Step 5, Continuous Casting: Before production, check the equipment accuracy. The tundish is baked for 2.5 hours, and must be cleaned and kept clean before being put into operation. Full-process protective casting is used during continuous casting. The ladle is protected with an argon gas sleeve, and the tundish is protected with a covering agent to prevent the molten steel from contacting the atmosphere. Submerged entry nozzles and special protective slag are used in the continuous casting crystallizer to prevent the molten steel from being drawn into the atmosphere. The continuous casting specification is a 200mm × 200mm square billet. The tundish temperature is 1535℃. The casting speed is 1.6m / min. The crystallizer electromagnetic stirring parameters are 350A / 4Hz. The crystallizer water flow rate is 130t / h. Chemical composition analysis sampling includes a main sample + a final 10T sample + a NO analysis sample. 200mm long low-magnification and hardenability samples are taken from the first and last ends of the second cast billet.

[0088] Step 6, slow cooling: The continuously cast billets are stacked and slow cooled in a slow cooling tank for 48 hours.

[0089] Step 7, finishing: After the continuously cast billet is slowly cooled in the stacking slow cooling tank for 48 hours, the surface quality of the continuously cast billet is inspected and surface defects that can be cleaned are finished. Cracks, pinholes, slag inclusions, HIC, and white spots are not allowed on the surface of the billet.

[0090] Performance evaluation: The hardenability-guaranteed billets obtained in Examples 1-4 were subjected to performance testing. The testing methods were as follows: chemical composition, N, O, H analysis, low magnification level, inclusions A, B, C, and D (coarse and fine series), and hardenability at point S of the head and tail samples were determined. The DI value and B factor were calculated. The test results are shown in Tables 1-6. Table 1. Chemical composition test results of 25MnTiB billets prepared by the method of the present invention. Note: Sample A and Sample Z represent the head and tail samples of the continuously cast square billet, respectively.

[0091] Table 2: Test results of N, O, and H in the square billet prepared by the method of the present invention to ensure hardenability Table 3: Low-magnification test results of square billets with guaranteed hardenability prepared by the method of the present invention Table 4: Test results of inclusions in the billet prepared by the method of the present invention to ensure hardenability Table 5: End-quench test results of square billets prepared by the method of the present invention to ensure hardenability Note: Point S is the distance from the center of the first point on each side to the end face.

[0092] Table 6: Calculation results of B factor for 25MnTiB billet prepared by the method of the present invention As shown in Tables 1 and 2, the chemical composition of the hardenable billets produced by the methods provided in Examples 1-4 of this invention precisely falls within the designed chemical composition range. The gaseous element content is well controlled, with oxygen content ≤12ppm, nitrogen content ≤56ppm, and hydrogen content ≤1.5ppm. The boron content in the hardenable billet steel produced using this invention is very stable, with a maximum boron content of 24ppm, a minimum boron content of 21ppm, and an average boron content of 22ppm.

[0093] As shown in Table 3, the low-magnification test results of the square billets with guaranteed hardenability produced by the methods provided in Examples 1-4 of this invention are ≤0.5 grade. As shown in Table 4, through process control and a series of measures in the electric arc furnace smelting process, embodiments 1-4 of the present invention can achieve a coarse and fine level of A-class inclusions, B-class inclusions, C-class inclusions, and D-class inclusions of ≤1.0 in the finished track steel products, thus ensuring the microscopic cleanliness requirements of the track steel products.

[0094] As shown in Tables 5 and 6, the square billet samples manufactured using continuous casting and rolling technology provided by this invention, after being forged, exhibit excellent hardenability with fluctuation ranges of: J1.5: 46-50 HRC, J13: 35-41 HRC, J25: 20-27 HRC; DI value (without considering the influence of B) 39-41 mm; and B factor 1.9-2.1, all of which meet the target values.

[0095] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A continuously cast square billet that ensures hardenability, characterized in that, Included by weight percentage: C: 0.24-0.27%, Si: 0.20-0.30%, Mn: 1.10%-1.15%, P: below 0.015%, S: below 0.005%, Cr: 0.40-0.45%, Ni: below 0.08%, Cu: below 0.08%, Mo: below 0.01%, Pb: below 0.01%, Sn: below 0.01%, B: 0.0020%-0.0030%, Ti: 0.025%-0.035%, Al: 0.020%-0.030%, N: 0.004-0.006%, O: below 0.0012%, H: below 0.00015%, with the remainder being Fe and other unavoidable substances; where 4.0 < Ti / N < 8.

0.

2. The continuously cast square billet with guaranteed hardenability according to claim 1, characterized in that, The hardenability DI value of the continuously cast billet is 33-47 mm.

3. The continuously cast square billet with guaranteed hardenability according to claim 1, characterized in that, The B factor of the continuously cast billet is ≥1.

6.

4. A process for preparing a continuously cast square billet with guaranteed hardenability according to any one of claims 1-3, characterized in that, include: Step 1) Raw material preparation: Raw materials include pig iron and scrap steel; wherein, the sulfur content of pig iron is ≤0.05%, and the scrap steel is calculated by weight percentage. Including: Cu: less than 0.08%, Ni: less than 0.08%, Cr: less than 0.30%, Mo: less than 0.01%, Sn: less than 0.01%, Pb: less than 0.01%, P: less than 0.05%, S: less than 0.03%, B: 0.0020%-0.0030%; Step 2) Electric arc furnace smelting: Add scrap steel and pig iron to the electric arc furnace, control the gas absorption of the molten steel during the smelting process, and control the carbon content of the molten steel to ≥0.1% at the end point. Add aluminum iron to the ladle for deoxidation when tapping the steel to obtain rough molten steel. Step 3) LF Refining: The crude steel enters the refining furnace for diffusion deoxidation and slag formation. In the state of white slag, aluminum wire and ferrotitanium are fed in sequence. After adding ferrotitanium, no more electricity should be applied to obtain refined steel. The hardenability DI value of the refined steel is controlled at 33-47mm. Step 4) VD Vacuum Treatment: Refined molten steel enters the VD furnace, and a vacuum is drawn to a vacuum state of ≤67Pa. After breaking the vacuum, boron wire is fed to obtain molten steel after VD vacuum treatment. Step 5) Continuous casting: The molten steel after VD vacuum treatment is cast through a tundish and a crystallizer to obtain a continuously cast billet; Step 6) Slow cooling and finishing: The continuously cast billet is slowly cooled and finished to obtain a continuously cast billet that ensures hardenability.

5. The preparation process according to claim 4, characterized in that, In step 2), during the process of adding scrap steel and pig iron to the electric arc furnace, the electric arc furnace is powered on and heated. During the heating process, slag-forming agent, carbon powder and magnesium balls are added in stages.

6. The preparation process according to claim 4, characterized in that, In step 2), steelmaking accelerator, silicon-manganese alloy, and lime are also added to the ladle.

7. The preparation process according to claim 4, characterized in that, In step 3), diffusion deoxidation uses calcium carbide and AD powder.

8. The preparation process according to claim 4, characterized in that, The white slag comprises, by weight percentage: SiO2: 8.0-12.0%, CaO: 50.0-60.0%, MnO≤0.10%, Al2O3: 17.0-21.0%, MgO: 3.0-4.0%, TFe≤0.40%, and the alkalinity of the white slag is ≥4.

0.

9. The preparation process according to claim 4, characterized in that, In step 5), during the continuous casting process, the casting is protected throughout; the protection during the continuous casting process includes: using a covering agent in the tundish to protect the molten steel from contact with the atmosphere, using argon gas to protect the ladle casing, and using submerged entry nozzles and protective slag in the continuous casting crystallizer to prevent the molten steel from being drawn into the atmosphere.

10. The preparation process according to claim 9, characterized in that, In step 5), the crystallizer is electromagnetically stirred, the temperature of the continuous casting tundish is controlled at 1535±5℃, and the continuous casting speed is controlled at 1.60±0.05m / min.

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