Method for increasing continuous casting furnace number of medium carbon bearing steel

Abstract

The invention relates to a method for increasing the number of continuous casting furnaces of medium-carbon bearing steel, in order to refine grains, the mass percentage of Al in the medium-carbon bearing steel is generally required to be 0.020-0.050%, and calcium treatment is not allowed. As secondary oxidation of a casting furnace is serious, inclusions at a tundish nozzle bowl part and a stopper rod head are easy to gather, the stopper rod rises, and the continuous casting castability is influenced. According to the technical scheme, when the medium-carbon bearing steel is poured, calcium-treatable medium-carbon steel is adopted for casting a first furnace, the first furnace is subjected to Ca treatment, the second furnace is not subjected to calcium treatment, the medium-carbon bearing steel is produced after a third furnace, and the steel must contain common components of C, Si, Mn, P, S and Cr elements with the medium-carbon bearing steel. The production efficiency is improved; the production cost is reduced.

Description

TECHNICAL FIELD

[0001] The application relates to a method for increasing the continuous casting furnace number of medium-carbon bearing steel, and belongs to the technical field of metallurgy. TECHNICAL BACKGROUND

[0002] Medium-carbon bearing steel is mainly applied to manufacturing hub bearings. The main function of the hub bearing is to bear the load and provide accurate guidance for the rotation of the hub. The hub bearing not only bears the axial load but also bears the radial load, and is a very important part. At present, the new car wheel bearing unit technology has been fully adopted in the world. The hub bearing is gradually becoming an internal part integrated with the wheel, and is developing towards digitization in combination with sensors. As a new type of bearing, the automobile hub bearing unit has a large demand in the domestic and foreign markets. The hub bearing is a safety part, and the reliability requirement thereof is extremely high.

[0003] The purity of molten steel is an important index affecting the fatigue life of medium-carbon bearing steel. At present, the main difficulty is that in order to ensure the purity of molten steel, the industry usually tends to replace the VD (vacuum ladle degassing) process with the RH (vacuum circulation degassing) process. This is because the RH process is pure molten steel circulation in the vacuum treatment process, and there is no steel slag mixing and stirring. Therefore, the oxide in the slag can be effectively avoided from being involved in the molten steel, so as to significantly improve the cleanliness of the molten steel. However, when the RH process is used to produce medium-carbon bearing steel with Al content of 0.020-0.050% and without calcium treatment, a great castability challenge is faced. Unlike the VD process, due to the existence of steel slag mixing and stirring, CaO in the slag will be reduced into the molten steel, so that the high-melting-point Al2O3 inclusions are spontaneously converted into low-melting-point calcium aluminate, thereby not easily blocking the water gap. In the RH process, due to the lack of the spontaneous calcification effect of the steel slag mixing and stirring, a large amount of high-melting-point and solid Al2O3 inclusions are generated in the early stage (the first furnace) of the casting. These solid inclusions are extremely easy to adhere to the inner wall of the tundish water gap and the stopper head, resulting in rapid clogging and flocculation of the water gap, which seriously limits the continuous casting furnace number (usually the RH process can only continuously cast 3-4 furnaces), and greatly reduces the production efficiency.

[0004] Therefore, the technical personnel have carried out a lot of research. For example, CN110385411A is used to control the continuous casting clogging problem of high-aluminum cold upsetting steel. The Al content in the first 2 furnaces is reduced to control the generation of secondary oxidized Al2O3 inclusions in the casting process, which can reduce the generation of secondary oxidized Al2O3 inclusions in the casting process and improve the continuous casting furnace number. However, this measure is not applicable to this steel grade. Reducing the Al content in the steel affects the control of the grain size. The subsequent customer's heat treatment of the rolled material is easy to cause mixed grains. At the same time, reducing the Al content in the steel increases the equilibrium oxygen in the molten steel, and the purity of the molten steel is poor. It is extremely unfavorable for the service life of the safety hub bearing which has extremely high purity requirements.

[0005] To address the issue of nozzle clogging, existing technologies employ a method of continuous casting of different steel grades (e.g., first casting ordinary carbon steel that can be treated with calcium). However, in practice, the chemical composition (C, Mn, Cr, etc.) of commonly used initial casting steel grades (such as 45#, 85#, etc.) differs significantly from that of medium carbon bearing steel (such as S55C). This results in the generation of several tons or even tens of tons of substandard billets during the mixed casting stage of steel grade switching. These billets do not meet the standards for either the initial casting steel grade or the bearing steel standard and can only be disposed of as scrap. This approach is not economically feasible in mass production where cost control is extremely stringent.

[0006] Therefore, the technical problem to be solved by this invention is to develop a production method that can not only ensure the top purity of bearing steel by utilizing the RH process, but also overcome the unique problem of flocculation in the initial casting of RH, and achieve zero rejection in the mixed casting section. Summary of the Invention

[0007] In order to overcome the technical problems existing in the prior art, the present invention provides a method for increasing the number of consecutive castings of medium carbon bearing steel. By using this method, while ensuring that the purity of molten steel meets the requirements of wheel hub bearings, the castability of medium carbon bearing steel is significantly improved, the yield is increased, and the production cost is reduced.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is: a method for increasing the number of consecutive castings of medium carbon bearing steel, the method comprising: The first batch of medium carbon steel that can be treated with calcium is the first batch of steel to be cast. The first batch is treated with calcium to liquefy the high-melting-point inclusions remaining after vacuuming. The inclusions are floated to the surface as much as possible by using a long soft blowing time, while ensuring that there is a certain amount of calcium in the molten steel, so that the inclusions generated during the first batch of steel can be liquefied. The second heat will not undergo calcium treatment; it will serve as transition steel to eliminate the impact of the first heat's calcium treatment. Soft blowing for ≥35 minutes will improve the purity of the molten steel. Then, it will be used for casting medium and low grade steels. "No calcium treatment" means that no active calcium feeding line operation will be performed. For the third and subsequent heats (following continuous casting), no calcium treatment is performed, and the soft blowing time is ≥30 min, casting medium carbon wheel hub bearing steel. The steel grades in the second and third heats contain at least the common components of C, Si, Mn, P, S, and Cr. During continuous casting, the mixed casting portion of the billets is reclassified as medium-low grade calcium-treated medium carbon steel; the Ca content in the first heat is >0.0005%; the Ca content in the second heat is ≤0.0003%; and the mass percentage of Ca in the medium carbon bearing steel in the third heat is ≤0.00015%.

[0009] Preferably, the mass percentage of each common component in the low-aluminum steel is as follows: In the transition steel, the mass percentage of each common component is: C: 0.53~0.57%, Si: 0.15~0.35%, Mn: 0.60~0.90%, P≤0.025%, S≤0.020%; In the medium-carbon bearing steel, the mass percentage of each common component is: C: 0.54~0.58%, Si: 0.17~0.30%, Mn: 0.75~0.85%, P≤0.020%, S≤0.010%; The overlapping portion of the common component content ranges in their respective steel grades is: C: 0.54~0.57%, Si: 0.17~0.30%, Mn: 0.75~0.85%, P≤0.020%, S≤0.010%. Furthermore, the production process is as follows: converter smelting - ladle refining furnace (LF) refining - RH vacuum circulation degassing treatment - billet continuous casting; wherein the RH vacuum treatment process uses pure molten steel circulation and does not involve steel slag mixing.

[0010] Furthermore, the cross-section of the continuously cast billet is a 220mm × 260mm square billet.

[0011] Furthermore, the method achieves a continuous casting number of ≥10 furnaces, and a continuous casting time of ≥8 hours in the tundish.

[0012] Furthermore, the method for reclassifying the mixed-cast billets includes: starting from the beginning of the mixed casting in the continuous casting mold, the first 3 fixed-length billets of each flow are judged as calcium-treated medium carbon steel, and subsequent billets are judged as medium carbon bearing steel.

[0013] Compared with existing technologies, the present invention has the following advantages: (1) This invention addresses the problem of the inability to spontaneously modify inclusions in the RH process by using a first heat of high-calcium (Ca > 0.0005%) steel for liquid preparation. Utilizing the high calcium potential of the first heat, the high-melting-point Al2O3 generated during the initial casting stage due to secondary oxidation is rapidly converted into liquid calcium aluminate, forming a low-melting-point protective film on the inner wall of the nozzle. Subsequently, a second heat of calcium-free transition and a cleaning system ensure that the third heat of medium-carbon bearing steel is cast in a clean and unobstructed channel. Actual measurements show that the method of this invention increases the continuous casting time in the tundish under the RH process from the conventional 3-4 hours to 8-9 hours, and the number of consecutive heats from approximately 4 to over 10.

[0014] (2) Unlike existing methods for joining dissimilar steel grades, this invention designs the chemical composition of the initial casting steel grade (e.g., 55#) and the target steel grade (e.g., S55C) to have a common overlapping range of C, Si, Mn, P, S, and Cr. During the continuous casting mixing stage, although the steel composition gradually changes, it always remains within the acceptable range of overlap between the two. This allows the mixed casting billet, which could originally only be treated as scrap, to be directly judged as a low-grade, calcium-treated medium carbon steel (55#) for sale. This invention transforms the unavoidable scrap in traditional dissimilar steel continuous casting into qualified products, significantly improving the metal yield. This invention avoids large particle inclusions of type Ds, which are crucial for the fatigue life of bearing steel. Under the premise of ensuring the purity of the molten steel, it can effectively improve the quality of medium carbon wheel hub bearing steel, resulting in a stable casting process, increased yield, and reduced production costs. Detailed Implementation

[0015] This invention is not limited to the specific embodiments listed below. Those skilled in the art can implement this invention using various other specific embodiments based on the content disclosed herein. Any modifications or alterations made to the design structure and concept of this invention fall within the protection scope of this invention. It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other.

[0016] The present invention will be further described in detail below with reference to the embodiments: Example 1 Calcium-treated medium carbon steel, taking 55# as an example, and medium carbon bearing steel, taking S55C as an example, can be used. Production process: Blast furnace molten iron → 120-ton converter → 120-ton ladle refining furnace (LF) → 120-ton vacuum degassing furnace (RH) → 8-strand continuous casting machine (220mm) 260mm square billet).

[0017] The required percentage content of components for 55# is as follows: C: 0.53~0.57%, Si: 0.15~0.35%, Mn: 0.60~0.90%, P≤0.025%, S≤0.020%; the required percentage content of components for S55C is as follows: C: 0.54~0.58%, Si: 0.17~0.30%, Mn: 0.75~0.85%, P≤0.020%, S≤0.010%; the designed common components for 55# and S55C are: C: 0.54~0.57%, Si: 0.17~0.30%, Mn: 0.75~0.85%, P≤0.020%, S≤0.010%. A method for increasing the number of consecutive castings of medium-carbon bearing steel; the specific steps are as follows: Step 1, the final composition of the first 55# converter: C: 0.051%, Si: 0.001%, Mn: 0.063%, P: 0.018%, S: 0.024%, Cr: 0.023%, tapping temperature 1621℃.

[0018] After RH soft blowing for 42 minutes, the composition of molten steel (mass percentage) is as follows: C: 0.56%, Si: 0.19%, Mn: 0.818%, P: 0.019%, S: 0.003%, Al: 0.0252%, Ca: 0.0019%, and the initial pouring temperature is 1567℃. Step 2, final composition of the second 55# converter: C: 0.045%, Si: 0.001%, Mn: 0.081%, P: 0.015%, S: 0.028%, Cr: 0.031%, tapping temperature 1637℃.

[0019] After RH soft blowing for 35 minutes, the composition of molten steel (mass percentage) is as follows: C: 0.553%, Si: 0.214%, Mn: 0.824%, P: 0.015%, S: 0.003%, Al: 0.0222%, Ca: 0.0002%, and the initial pouring temperature is 1550℃. Step 3, the final composition of the third S55C converter: C: 0.081%, Si: 0.001%, Mn: 0.09%, P: 0.014%, S: 0.026%, Cr: 0.024%, tapping temperature 1633℃.

[0020] After RH soft blowing for 30 minutes, the composition of molten steel (mass percentage) is as follows: C: 0.557%, Si: 0.195%, Mn: 0.814%, P: 0.015%, S: 0.002%, Al: 0.0241%, Ca: 0.0001%, and the initial pouring temperature is 1539℃. Step 4: After the third heat of S55C was cast, mixed casting began. Starting from the crystallizer, three 10-meter-long billets were taken from each run and identified as mixed casting billets, i.e., 55#. After taking the first three billets, the remaining billets in the third heat were identified as medium carbon bearing steel S55C. The mass percentage of common components in the mixed casting billets was: C: 0.555%, Si: 0.201%, Mn: 0.819%, P: 0.015%, S: 0.002%, Al: 0.0229%, Ca: 0.0002%. During the initial casting process of the medium carbon bearing steel, the stopper rod and liquid level curves were stable, the casting was normal, the tundish casting time was 8 hours, and the number of continuous casting heats was 11. The non-metallic inclusion rating is shown in Table 1.

[0021] Example 2

[0022] Calcium-treated medium carbon steel, taking 55# as an example, and medium carbon bearing steel, taking S55C as an example, can be used. Production process: Blast furnace molten iron → 120-ton converter → 120-ton ladle refining furnace (LF) → 120-ton vacuum degassing furnace (RH) → 8-strand continuous casting machine (220mm) 260mm square billet).

[0023] The required percentage content of components for 55# is as follows: C: 0.53~0.57%, Si: 0.15~0.35%, Mn: 0.60~0.90%, P≤0.025%, S≤0.020%; the required percentage content of components for S55C is as follows: C: 0.54~0.58%, Si: 0.17~0.30%, Mn: 0.75~0.85%, P≤0.020%, S≤0.010%; the designed common components for 55# and S55C are: C: 0.54~0.57%, Si: 0.17~0.30%, Mn: 0.75~0.85%, P≤0.020%, S≤0.010%. A method for increasing the number of consecutive castings of medium-carbon bearing steel; the specific steps are as follows: Step 1, the final composition of the first heat of 55# converter: C: 0.062%, Si: 0.001%, Mn: 0.052%, P: 0.016%, S: 0.026%, Cr: 0.020%, tapping temperature 1622℃.

[0024] After RH soft blowing for 40 minutes, the composition of molten steel (mass percentage) is as follows: C: 0.55%, Si: 0.20%, Mn: 0.821%, P: 0.016%, S: 0.002%, Al: 0.0243%, Ca: 0.0015%, and the initial pouring temperature is 1565℃. Step 2, final composition of the second 55# converter: C: 0.055%, Si: 0.001%, Mn: 0.063%, P: 0.015%, S: 0.026%, Cr: 0.030%, tapping temperature 1625℃.

[0025] After RH soft blowing for 36 minutes, the composition of molten steel (mass percentage) is as follows: C: 0.56%, Si: 0.21%, Mn: 0.820%, P: 0.015%, S: 0.002%, Al: 0.0233%, Ca: 0.00025%, and the initial pouring temperature is 1551℃. Step 3, the final composition of the third S55C converter: C: 0.072%, Si: 0.001%, Mn: 0.075%, P: 0.012%, S: 0.031%, Cr: 0.030%, tapping temperature 1627℃.

[0026] After RH soft blowing for 30 minutes, the composition of molten steel (mass percentage) is as follows: C: 0.57%, Si: 0.20%, Mn: 0.83%, P: 0.012%, S: 0.002%, Al: 0.0237%, Ca: 0.0001%, and the initial pouring temperature is 1535℃. Step 4: After the third heat of S55C was cast, mixed casting began. Starting from the crystallizer, three 10-meter-long billets were taken from each run and identified as mixed casting billets, i.e., 55#. After taking the first three billets, the remaining billets in the third heat were identified as medium carbon bearing steel S55C. The mass percentage of common components in the mixed casting billets was: C: 0.555%, Si: 0.201%, Mn: 0.821%, P: 0.014%, S: 0.002%, Al: 0.0231%, Ca: 0.0002%. During the initial casting process of the medium carbon bearing steel, the stopper rod and liquid level curves were stable, the casting was normal, the tundish casting time was 9 hours, and the number of continuous casting heats was 13. The non-metallic inclusion rating is shown in Table 1.

[0027] Comparative Example 1

[0028] Medium carbon bearing steel S55C is used for casting, without transitional steel grades. Production process: Blast furnace hot metal → 120-ton converter → 120-ton ladle refining furnace (LF) → 120-ton vacuum degassing furnace (RH) → 8-strand continuous casting machine (220mm) (260mm square billet). The specific steps are as follows: Step 1, the final composition of the third S55C converter: C: 0.077%, Si: 0.001%, Mn: 0.076%, P: 0.016%, S: 0.023%, Cr: 0.028%, tapping temperature 1641℃.

[0029] After RH soft blowing, the steel composition (mass percentage) is as follows: C: 0.560%, Si: 0.213%, Mn: 0.827%, P: 0.017%, S: 0.002%, Al: 0.0209%, Ca: 0.0001%, and the initial pouring temperature is 1565℃. Step 2: The continuous casting process begins. The stopper rod curve shows an upward trend. Starting from the later stage of the third heat, the liquid level fluctuations intensify, and the liquid level in each flow crystallizer frequently exceeds the ±3mm fluctuation range. More and more scrap billets are being picked out, and casting is stopped after 5 consecutive heats. The non-metallic inclusion rating is shown in Table 1.

[0030] Comparative Example 2

[0031] The main difference between Comparative Example 2 and Example 2 is that a VD device was used for vacuum treatment, while the other target control parameters (such as endpoint temperature and endpoint composition control targets) remained the same as in Example 2.

[0032] Comparative Example 2 had 18 continuous casting furnaces, and the production casting was stable, but there was an over-standard Ds2.5 grade inclusion, and the rating of spherical oxides increased significantly. The rating of non-metallic inclusions is shown in Table 1.

[0033] Comparative Example 3

[0034] The same RH process and identical steps as in Example 1 were used. The difference was that ordinary 45# steel (C: 0.42~0.50%, with no compositional overlap with S55C) was used for the first heat and the second transition heat, instead of the 55# steel of this invention. Since there is no overlap in composition, the billets need to be scrapped at 1.2 times the weight of the tundish capacity (45t), resulting in 54t of billets being scrapped and recycled. The non-metallic inclusion rating is shown in Table 1.

[0035] Table 1. Rating of Non-metallic Inclusions

[0036] This invention effectively prevents nozzle clogging in the tundish of carbon bearing steel during the initial casting process in the RH process, ensuring a stable casting process, improving yield, reducing production costs, and fully meeting product quality requirements. Continuous casting time in the tundish can reach 8-9 hours, enabling 10-11 heats to be cast consecutively. Furthermore, this invention can be extended to similar steel grades such as carburized bearing steel.

[0037] Benefit Calculation: The benefits mainly come from reducing the costs of cutting off the top and bottom ends, downgrading intermediate billets, and saving on refractory materials in the tundish and the three main components of continuous casting. Taking a certain factory's 220mm... With a 260mm head cut of 6m and a tail cut of 4.5m, and 8 machines, 8 streams, and 10 heats of continuous casting with double tundish, this invention can achieve 8 heats of continuous casting (excluding transition heats), increasing efficiency by 60%. The cost of S55C is calculated at 4650.74 yuan / ton, the price of scrap steel is calculated at 2619.05 yuan / ton, and the cost of tundish refractory materials and the three major components of continuous casting is calculated at 50,000 yuan.

[0038] Weight after removing the head and tail = (6 + 4.5) × 8 × 0.22 × 0.26 × 7.85 = 37.718t;

[0039] Benefits from cutting off the head and tail = 37.718 × (4650.74 - 2619.05) × 60% = 45,979 yuan; The benefit from reducing intermediate billet downgrading = 54 × (4650.74 - 2619.05) = 109,711 yuan; Total benefit = 4.5979 + 10.9711 + 5 × 60% = 18.569 million yuan / pouring cycle; The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for increasing the number of consecutive castings of medium-carbon bearing steel, characterized in that: The method includes using calcium-treated medium carbon steel for the first and second heats, and using medium carbon bearing steel for the third heat and subsequent consecutive heats. The specific pouring control steps are as follows: The first heat undergoes calcium treatment, controlling the calcium content in the steel to be >0.0005%; the second heat does not undergo calcium treatment, controlling the calcium content in the steel to be ≤0.0003%; the third heat and subsequent heats do not undergo calcium treatment, producing medium carbon bearing steel, controlling the calcium content in the steel to be ≤0.00015%. Among them, the calcium-treated medium carbon steel and medium carbon bearing steel have common components, which include at least C, Si, Mn, P, S and Cr. The content range of the common components in their respective steel grades overlaps. During the continuous casting process, the mixed casting billets between the second and third heats are reclassified as qualified medium carbon steel products based on the overlapping area of ​​the common components.

2. The method for increasing the number of continuous castings of medium carbon bearing steel according to claim 1, characterized in that: The mass percentage of each common component in the calcium-treated medium-carbon steel is as follows: C: 0.53~0.57%, Si: 0.15-0.35%, Mn: 0.60~0.90%, P≤0.025%, S≤0.020%; the mass percentage of each common component in the medium-carbon bearing steel is as follows: C: 0.54~0.58%, Si: 0.17~0.30%, Mn: 0.75~0.85%, P≤0.020%, S≤0.010%; the overlapping portion of the content range of the common components in their respective steel grades is as follows: C: 0.54~0.57%, Si: 0.17~0.30%, Mn: 0.75~0.85%, P≤0.020%, S≤0.010%.

3. The method for increasing the number of continuous castings of medium carbon bearing steel according to claim 1, characterized in that: The soft blowing time for the first furnace is ≥40 min.

4. The method for increasing the number of continuous castings of medium carbon bearing steel according to claim 1, characterized in that: The second furnace is not treated with calcium, and the soft blowing time of the second furnace is ≥35min.

5. The method for increasing the number of continuous castings of medium carbon bearing steel according to claim 1, characterized in that: The soft blowing time for the third and subsequent furnaces shall be ≥30 min.

6. The method for increasing the number of continuous castings of medium carbon bearing steel according to claim 1, characterized in that: The method enables a continuous casting number of ≥10 furnaces, and the continuous casting time of the tundish is ≥8 hours.

7. The method for increasing the number of continuous castings of medium carbon bearing steel according to claim 1, characterized in that: The method for reclassifying mixed-cast billets includes: starting from the beginning of mixed casting in the continuous casting mold, the first 3 fixed-length billets of each flow are judged as calcium-treated medium carbon steel, and subsequent billets are judged as medium carbon bearing steel.

Images