Cold heading steel wire rod of 8.8 grade with excellent quenching property and production method thereof

Abstract

This invention discloses a large-diameter cold heading steel wire rod of grade 8.8 with excellent hardenability and its production method, belonging to the technical field of steel products. The wire rod comprises the following chemical composition by mass percentage: C: 0.35%–0.39%, Si: ≤0.10%, Mn: 0.60%–0.85%, Cr: 0.20%–0.40%, Mo: 0.15%–0.25%, Ni: 0.05%–0.10%, Al: 0.020%–0.040%, Ca: 0.0040%–0.0080%, Ce: 0.0010–0.0030%, N: ≤0.0050%, with the balance being Fe and unavoidable impurities. The process route is: hot metal pretreatment – ​​converter smelting – LF refining – continuous casting – continuous rolling – billet cleaning – heating – rolling – controlled cooling. The cold heading steel wire rod produced by this invention has good cold heading performance. After quenching, the hardness of the transverse section of the wire rod is greater than 45HRC, which can be used to produce standard hexagonal flange parts such as 8.8 grade high-strength bolts, nuts, and screws.

Description

Technical Field

[0001] This invention belongs to the technical field of steel products and relates to a large-size cold heading steel wire rod of grade 8.8 with excellent hardenability and its production method. Background Technology

[0002] SWRCH35K and other carbon cold heading steels are mainly used to produce standard hexagonal flange parts such as 8.8 grade high-strength bolts, nuts, external hex bolts, and screws. These standard parts are widely used in industries such as machinery, electronics, automobiles, and construction, requiring good cold deformation capabilities and hardness after quenching to meet certain performance requirements.

[0003] As people's demands increase, the use The following quality defects occurred during the production of 8.8 grade hexagonal flange fasteners from cold heading steel wire rods of specification SWRCH35K, etc., in accordance with national standards: (1) When the cross-sectional size of the wire rod is different, due to the different degrees of hardening, although the same quenching and tempering process is used, their mechanical properties differ greatly; and the larger the cross-section, the worse the degree of hardening and the worse the mechanical properties; (2) After heat treatment, the core hardness and tensile strength of the wire rod were found to be low. These quality problems will lead to insufficient hardening in the production process of downstream users, resulting in batch scrapping, affecting the stable production and contract delivery of downstream users, etc.

[0004] CN105543675A discloses a grain-refined cold heading steel and its production process. The cold heading steel, by weight percentage, has the following composition: C 0.32–0.38%, Si 0.10–0.35%, Mn 0.60–0.90%, Cr≤0.20%, P≤0.030%, S≤0.035%, Ni≤0.20%, Al 0.010–0.040%, Ti 0.050–0.080%, Cu≤0.30%, with the balance being Fe and unavoidable impurities. The process includes smelting, continuous casting, and rolling steps. By adding Al and Ti, the material's grain size is refined, and the microstructure becomes more uniform. Furthermore, by selecting appropriate timing for Al and Ti additions and strengthening steel deoxidation, the cold heading steel's cold upsetting performance is significantly improved through increased steel purity and grain size. However, because the patented steel contains a certain amount of Ti element, it is easy to form TiN precipitate phase with the N element inherent in the steel, which deteriorates the toughness of the steel.

[0005] CN111534761A discloses a SWRCH35K hot-rolled wire rod for fire-fighting fish-tail bolts and its production method. The composition of the hot-rolled wire rod, by weight percentage, is: C 0.33-0.36, Si 0.12-0.26, Mn 0.61-0.70, P≤0.015, S≤0.010, Al 0.01-0.03, Cu≤0.1, Ni≤0.2, Cr≤0.2, As≤0.03, with the balance being Fe and unavoidable impurities. Narrow composition control is employed to meet the requirements of stable product performance, small dimensional tolerances, high output per unit time, and long service life. However, due to the narrow composition control process, high requirements are placed on steelmaking, and it is easy for the chemical composition to exceed the range, resulting in accidents such as steel scrapping or downgrading, causing significant disruption to the smooth operation of industrial production. Summary of the Invention

[0006] To address the problems of existing technologies, this invention proposes a large-size cold heading steel of grade 8.8 with excellent hardenability and its production method. By controlling the rolling process through micro-alloying elements to match strength and toughness, large-size cold heading steel wire rods are produced. The wire rods have good deformation capacity and can meet the hardenability requirements of standard parts. They can be used to produce standard hexagonal flange parts such as grade 8.8 high-strength bolts, nuts, and screws.

[0007] A large-diameter cold heading steel wire rod of grade 8.8 with excellent hardenability comprises the following chemical composition by mass percentage (unit: wt%): C: 0.35~0.39, Si: ≤0.10, Mn: 0.60~0.85, Cr: 0.20~0.40, Mo: 0.15~0.25, Ni: 0.05~0.10, Al: 0.020~0.040, Ca: 0.0040~0.0080, Ce: 0.0010~0.0030, N: ≤0.0050, with the balance being Fe and unavoidable impurities.

[0008] The main function of the chemical composition of the above-mentioned 8.8 grade large-diameter cold heading steel wire rod with excellent hardenability is as follows:

[0009] C: C is the element with the most significant solid solution strengthening effect in steel. As the C content increases, the strength and hardness of steel increase, while the plasticity and toughness decrease. The recommended C content is 0.35% to 0.39%.

[0010] Si: Si is a ferrite solid solution strengthening element, but it also leads to a sharp increase in resistance to cold heading deformation, significantly increasing die wear and hindering cold working plastic deformation. Therefore, the range of Si is ≤0.10%;

[0011] Mn: Mn plays a role in solid solution strengthening and grain refinement strengthening, and delays the transformation of pearlite and ferrite, thereby improving the strength and work hardening properties of steel. Therefore, the range of Mn is 0.60% to 0.85%.

[0012] Cr: Cr can improve the strength and toughness of steel and has good cold heading properties. It can also effectively improve hardenability, ensuring the strength and hardness of the material after quenching. Therefore, the composition range of Cr is 0.20% to 0.40%.

[0013] Mo: Mo can significantly improve hardenability, ensuring the hardness and strength of the material after quenching. Furthermore, this element possesses extremely strong resistance to delayed fracture; therefore, the Mo content is controlled between 0.15% and 0.25%.

[0014] Ni: Ni's lattice constant is similar to that of γ-iron, so it can form a continuous solid solution. This is beneficial for improving the hardenability of steel. Ni can lower the critical point and increase the stability of austenite, so its quenching temperature can be lowered and its hardenability is good. Ni is an element that can increase the activity of carbon, which can cause carbon atoms to accumulate around dislocations, increase the stacking fault energy, thereby inhibiting crack propagation and improving the toughness of steel. Generally, large-section, large-specification steel products use Ni-added steel. When combined with Cr and Mo, hardenability can be further increased. Therefore, the Ni content is controlled at 0.05% to 0.10%.

[0015] Al: Adding a certain amount of Al (0.020%~0.040%) refines the grains and deoxidizes the steel, resulting in extremely low oxygen content and reducing the number and size of non-metallic inclusions in the steel.

[0016] The main inclusion in Ca:Al deoxidized steel is Al2O3. Al2O3 inclusions are hard and have sharp edges, which can easily scratch the matrix during subsequent steel processing. Furthermore, due to their high melting point, they tend to adhere to the inner wall of the nozzle, affecting the smooth casting process and even causing serious accidents such as nozzle blockage. To avoid the formation of pure Al2O3 inclusions, a Ca treatment process is used to react Al2O3 inclusions with CaO to form low-melting-point calcium aluminate inclusions. Simultaneously, because MnS inclusions in the steel are transformed into CaS or CaO·Al2O3·CaS composite inclusions, the isotropy of the steel plate is improved, and the performance of the steel matrix is ​​enhanced. Therefore, the Ca composition is designed to be 0.0040%–0.0080%.

[0017] When Ce is heated to austenitization, Ce atoms inevitably segregate towards the austenite grain boundaries, inhibiting the diffusion of carbon atoms to the grain boundaries and delaying the formation of cementite. This suppresses the nucleation of proeutectoid ferrite in steel, improving hardenability.

[0018] Furthermore, the dimensions of the finished wire rod are:

[0019] The production method of the above-mentioned 8.8 grade large-size cold heading steel wire rod with excellent hardenability is as follows: hot metal pretreatment - converter smelting - LF refining - continuous casting - continuous rolling - billet cleaning - heating - rolling - controlled cooling.

[0020] Furthermore, in LF refining, the main tasks are composition fine-tuning, temperature control, and slag formation. After LF alloying treatment, calcium alloy cored wire is fed into the ladle. After feeding the wire, argon gas is used for weak stirring. The argon gas soft blowing flow rate is 10-30 NL / min, which causes slight creeping of the slag and prevents the molten steel from being exposed. The ladle is then removed from the station after weak stirring for 5-10 minutes.

[0021] Furthermore, the calcium alloy cored wire is a double-layer pure calcium cored wire.

[0022] Furthermore, during continuous casting, the tundish superheat is controlled at 20–30℃, and the casting speed is constant (the billet casting speed is controlled at 0.75±0.02m / min); the secondary cooling water volume is controlled at 0.9–1.2L / kg; the crystallizer is electromagnetically stirred with a stirring voltage of 300–500V, a current intensity of 300–500A, and a frequency of 3–8Hz. The electromagnetic stirring process promotes the flotation of non-metallic inclusions in the molten steel, reduces the size of inclusion particles in the steel, and ensures uniform distribution of inclusions; it reduces elemental segregation in the steel, thereby homogenizing the microstructure and mitigating banded defects; it reduces the number and depth of surface defects such as transverse cracks, longitudinal cracks, and corner cracks in the billet, providing high-quality raw materials for the production of scratch-free wire rod.

[0023] Furthermore, the cross-sectional dimensions of the continuously cast billet are (250~350)mm × (350~450)mm.

[0024] Furthermore, during the continuous casting process, protective casting is adopted, and a covering agent is added to the molten steel in the ladle and tundish; protective sleeves are used from the ladle to the tundish and from the tundish to the crystallizer, and argon blowing is used for protection; protective slag is added to the crystallizer.

[0025] Furthermore, in the rolling process, the initial rolling temperature is controlled at 1000–1040℃, the finishing rolling inlet temperature at 950–990℃, and the wire drawing temperature at 910–930℃.

[0026] Furthermore, in the controlled cooling process, the Stellmore air-cooled line cooling method is adopted: the temperature is cooled to 680-770°C at a cooling rate of 6-12°C / s before being placed into the hood. The insulation hood is completely closed, the cooling rate inside the hood is 0.5-5.0°C / s, and the temperature exiting the hood is 460-600°C. The cooling bed is air-cooled.

[0027] Furthermore, by controlling the heating, rolling, and controlled cooling processes, wire rods with ideal ferrite and pearlite structures can be obtained.

[0028] Furthermore, the coils after controlled cooling undergo finishing, inspection, and warehousing.

[0029] Furthermore, the cold heading steel wire rod is subjected to heat treatment, which is quenching treatment, with a temperature of 880-890℃ and water or oil cooling.

[0030] The beneficial effects of this invention are:

[0031] The invention produces Cold heading steel wire rod has a ferrite area ratio of 40-50% and a pearlite area ratio of 50-60%; longitudinal banded structure ≤ grade 2; A and B type non-metallic inclusions ≤ grade 2; C and D type non-metallic inclusions ≤ grade 2; after hot rolling, the wire rod has a tensile strength greater than 580 MPa, an elongation greater than 22%, and a reduction of area greater than 45%; it has good cold heading performance, with a 1 / 2 cold upsetting pass rate of over 90% and a 1 / 3 cold upsetting pass rate of over 60%; after quenching, the hardness of the transverse section of the wire rod is greater than 45 HRC, and it can be used to produce 8.8 grade high-strength bolts, nuts, screws, and other hexagonal flange standard parts. Attached Figure Description

[0032] Figure 1 The transverse microstructure morphology of the wire rod prepared in Example 1 (×100).

[0033] Figure 2 The transverse microstructure morphology of the wire rod prepared in Example 1 (×500). Detailed Implementation

[0034] The following non-limiting embodiments are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the invention in any way.

[0035] Example 1

[0036] A production method for large-diameter cold heading steel wire rod of grade 8.8 with excellent hardenability, the process route is as follows: hot metal pretreatment – ​​converter smelting – LF refining – continuous casting – continuous rolling – billet cleaning – heating – rolling – controlled cooling. Wherein:

[0037] LF refining: After the molten steel is alloyed by LF, double-layer pure calcium cored wire is fed into the ladle. After feeding the wire, argon gas is used for weak stirring. The soft blowing flow rate is 20NL / min, which makes the steel slag slightly creep and the molten steel does not leak out. After weak stirring for 7 minutes, the ladle is removed from the station.

[0038] Continuous casting: The cross-sectional dimensions of the continuously cast billet are 320mm×410mm. The superheat of the tundish is controlled at 25℃, and the casting speed is constant (the casting speed of the billet is controlled at 0.75m / min). The secondary cooling water ratio is controlled at 1.0L / kg. Protective casting (a covering agent is added to the molten steel in the ladle and tundish; a protective sleeve is used from the ladle to the tundish and from the tundish to the crystallizer, and argon blowing is used for protection; protective slag is added to the crystallizer). Electromagnetic stirring of the crystallizer: The electromagnetic stirring voltage of the crystallizer is 350V, the current intensity is 350A, and the frequency is 6Hz.

[0039] Steel billets are rolled on high-speed wire rod mills The wire rod rolling process is as follows: the initial rolling temperature is controlled at 1025℃, the finishing rolling inlet temperature is 960℃, and the wire exit temperature is 915℃; the controlled cooling process is as follows: the wire rod is cooled to 690℃ at a cooling rate of 7℃ / s before being placed in the cooling hood, the insulation hood is completely closed, the cooling rate inside the hood is 1.5℃ / s, the exit temperature is 500℃, and the wire rod is air-cooled on the cooling bed.

[0040] Example 2

[0041] A production method for large-diameter cold heading steel wire rod of grade 8.8 with excellent hardenability, the process route is as follows: hot metal pretreatment – ​​converter smelting – LF refining – continuous casting – continuous rolling – billet cleaning – heating – rolling – controlled cooling. Wherein:

[0042] LF refining: After the molten steel is alloyed by LF, double-layer pure calcium cored wire is fed into the ladle. After feeding the wire, argon gas is used for weak stirring. The soft blowing flow rate is 25NL / min, which makes the steel slag slightly creep and the molten steel does not leak out. After weak stirring for 8 minutes, the ladle is removed from the station.

[0043] Continuous casting: The cross-sectional dimensions of the continuously cast billet are 320mm×410mm. The superheat of the tundish is controlled at 27℃, and the casting speed is constant (the casting speed of the billet is controlled at 0.76m / min). The secondary cooling water ratio is controlled at 0.95L / kg. Protective casting is used (a covering agent is added to the molten steel in the ladle and tundish; a protective sleeve is used from the ladle to the tundish and from the tundish to the crystallizer, and argon blowing is used for protection; protective slag is added to the crystallizer). The electromagnetic stirring voltage of the crystallizer is 450V, the current intensity is 400A, and the frequency is 7Hz.

[0044] Steel billets are rolled on high-speed wire rod mills The wire rod rolling process is as follows: the initial rolling temperature is controlled at 1010℃, the finishing rolling inlet temperature is 970℃, and the wire exit temperature is 925℃; the controlled cooling process is as follows: the wire rod is cooled to 720℃ at a cooling rate of 7℃ / s before being placed in the cooling hood, the insulation hood is completely closed, the cooling rate inside the hood is 2.0℃ / s, the exit temperature is 490℃, and the wire rod is air-cooled on the cooling bed.

[0045] Example 3

[0046] A production method for large-diameter cold heading steel wire rod of grade 8.8 with excellent hardenability, the process route is as follows: hot metal pretreatment – ​​converter smelting – LF refining – continuous casting – continuous rolling – billet cleaning – heating – rolling – controlled cooling. Wherein:

[0047] LF Refining: After LF alloying treatment, double-layer pure calcium cored wire is fed into the ladle. Argon gas is then used for gentle stirring at a flow rate of 30 NL / min, causing slight slag movement and preventing molten steel from leaking out. The ladle is then removed from the station after 6 minutes of gentle stirring. (Note:)

[0048] Continuous casting: The cross-sectional dimensions of the continuously cast billet are 320mm×410mm. The superheat of the tundish is controlled at 25℃, and the casting speed is constant (the casting speed of the billet is controlled at 0.75m / min). The secondary cooling water ratio is controlled at 1.15L / kg. Protective casting (a covering agent is added to the molten steel in the ladle and tundish; a protective sleeve is used from the ladle to the tundish and from the tundish to the crystallizer, and argon blowing is used for protection; protective slag is added to the crystallizer). Electromagnetic stirring in the crystallizer: The electromagnetic stirring voltage of the crystallizer is 500V, the current intensity is 480A, and the frequency is 4Hz.

[0049] Steel billets are rolled on high-speed wire rod mills The wire rod rolling process is as follows: the initial rolling temperature is controlled at 1005℃, the finishing rolling inlet temperature is 960℃, and the wire exit temperature is 925℃; the controlled cooling process is as follows: the wire rod is cooled to 710℃ at a cooling rate of 9℃ / s before being placed in the cooling hood, the insulation hood is completely closed, the cooling rate inside the hood is 2.0℃ / s, the exit temperature is 510℃, and the wire rod is air-cooled on the cooling bed.

[0050] Example 4

[0051] A production method for large-diameter cold heading steel wire rod of grade 8.8 with excellent hardenability, the process route is as follows: hot metal pretreatment – ​​converter smelting – LF refining – continuous casting – continuous rolling – billet cleaning – heating – rolling – controlled cooling. Wherein:

[0052] LF Refining: After LF alloying treatment, double-layer pure calcium cored wire is fed into the ladle. Argon gas is then used for weak stirring at a flow rate of 15 NL / min, causing slight slag movement and preventing molten steel from leaking out. The ladle is then removed from the station after 9 minutes of weak stirring. (Note:)

[0053] Continuous casting: The cross-sectional dimensions of the continuously cast billet are 320mm×410mm. The superheat of the tundish is controlled at 25℃, and the casting speed is constant (the casting speed of the billet is controlled at 0.74m / min). The secondary cooling water ratio is controlled at 1.10L / kg. Protective casting (a covering agent is added to the molten steel in the ladle and tundish; a protective sleeve is used from the ladle to the tundish and from the tundish to the crystallizer, and argon blowing is used for protection; protective slag is added to the crystallizer). Electromagnetic stirring in the crystallizer: The electromagnetic stirring voltage of the crystallizer is 380V, the current intensity is 450A, and the frequency is 7Hz.

[0054] Steel billets are rolled on high-speed wire rod mills The wire rod rolling process is as follows: the initial rolling temperature is controlled at 1005℃, the finishing rolling inlet temperature is 960℃, and the wire exit temperature is 925℃; the controlled cooling process is as follows: the wire rod is cooled to 715℃ at a cooling rate of 7℃ / s before being placed in the cooling hood, the insulation hood is completely closed, the cooling rate inside the hood is 3.0℃ / s, the exit temperature is 490℃, and the wire rod is air-cooled on the cooling bed.

[0055] Comparative Example

[0056] A production method for 8.8 grade cold heading steel wire rod, the process route is as follows: hot metal pretreatment – ​​converter smelting – LF refining – continuous casting – continuous rolling – billet cleaning – heating – rolling – controlled cooling. Wherein:

[0057] Continuous casting: The cross-sectional dimensions of the continuously cast billet are 320mm×410mm, and the casting speed is constant (the casting speed is controlled at 0.74m / min); from the ladle to the tundish, and from the tundish to the crystallizer, a protective sleeve is used and argon blowing is used for protection;

[0058] Steel billets are rolled on high-speed wire rod mills The wire rod rolling process is as follows: the initial rolling temperature is controlled at 990℃, the finishing rolling inlet temperature is 940℃, and the wire exit temperature is 925℃; the controlled cooling process is as follows: the wire rod is cooled to 715℃ at a cooling rate of 7℃ / s before being placed in the cooling hood, the insulation hood is completely closed, the cooling rate inside the hood is 3.0℃ / s, the exit temperature is 490℃, and the wire rod is air-cooled on the cooling bed.

[0059] The chemical composition, content, and method of the wire rods were designed according to Examples 1 to 4 and the comparative examples of the present invention. The measured chemical composition is shown in Table 1; the test results of non-metallic inclusions in the wire rods are shown in Table 2; the mechanical properties of the hot-rolled wire rods, and the hardness index after cold forging and quenching heat treatment are shown in Table 3. Among them, the heat treatment process adopted for the wire rods of the comparative examples and Examples 1 to 4 in Table 3 is shown in Table 4.

[0060] The transverse microstructure of the hot-rolled wire rod obtained in Example 1 is shown in the figure. Figure 1 , Figure 2 Its metallographic structure is ferrite + pearlite.

[0061] Large-size cold-heading steel wire rods produced using the above method have good cold-heading performance; the hardness of the transverse section of the wire rod after quenching is greater than 45HRC, and it can be used to produce standard hexagonal flange parts such as 8.8 grade high-strength bolts, nuts, and screws.

[0062] Table 1 Chemical composition (wt%)

[0063]

[0064] Table 2. Inspection Results of Non-metallic Inclusions in Steel

[0065]

[0066] Table 3 Test data of mechanical properties, cold forging, and quenching hardness of steel.

[0067]

[0068] Table 4. Process Regulations for Quenching and Heat Treatment of Wire Rod

[0069]

Claims

1. A large-size cold-upset steel wire rod of 8.8 grade having excellent hardenability, characterized in that, The chemical composition includes the following percentages by mass: C: 0.35%–0.39%, Si: ≤0.10%, Mn: 0.60%–0.85%, Cr: 0.20%–0.40%, Mo: 0.15%–0.25%, Ni: 0.05%–0.10%, Al: 0.020%–0.040%, Ca: 0.0040%–0.0080%, Ce: 0.0010–0.0030%, N: ≤0.0050%, with the balance being Fe and unavoidable impurities. The production method of the 8.8 grade large-size cold heading steel wire rod with excellent hardenability includes: hot metal pretreatment - converter smelting - LF refining - continuous casting - continuous rolling - billet cleaning - heating - rolling - controlled cooling; In the rolling process, the initial rolling temperature is controlled at 1000-1040℃, the finishing rolling inlet temperature is 950-990℃, and the wire drawing temperature is 910-930℃. In the controlled cooling process, the Stellmore air-cooled line cooling method is adopted: the temperature is cooled to 680-770°C at a cooling rate of 6-12°C / s before being placed into the hood. The insulation hood is completely closed, the cooling rate inside the hood is 0.5-5.0°C / s, and the temperature outside the hood is 460-600°C. The cooling bed is air-cooled. During the LF refining process, after the molten steel undergoes LF alloying treatment, calcium alloy cored wire is fed into the ladle. After feeding the wire, argon gas is used for weak stirring. The argon gas flow rate is 10-30 NL / min. After weak stirring for 5-10 minutes, the ladle is removed from the station.

2. The 8.8 grade large-size cold heading steel wire rod with excellent hardenability according to claim 1, characterized in that, The dimensions of the wire rod are:

3. The 8.8 grade large-size cold heading steel wire rod with excellent hardenability according to claim 1, characterized in that, The wire rod has a microstructure of ferrite + pearlite, with ferrite accounting for 40-50% of the area and pearlite accounting for 50-60% of the area; longitudinal banded structure ≤ grade 2; A and B type non-metallic inclusions ≤ grade 2; C and D type non-metallic inclusions ≤ grade 2; the wire rod has a tensile strength greater than 580 MPa, elongation greater than 22%, and reduction of area greater than 45%; the pass rate of 1 / 2 cold upsetting is over 90%, and the pass rate of 1 / 3 cold upsetting is over 60%; after quenching, the hardness of the transverse section of the wire rod is greater than 45 HRC; the quenching process is: temperature 880-890℃, water cooling or oil cooling.

4. The 8.8 grade large-size cold heading steel wire rod with excellent hardenability according to claim 1, characterized in that, The calcium alloy cored wire is a double-layer pure calcium cored wire.

5. The 8.8 grade large-size cold heading steel wire rod with excellent hardenability according to claim 1, characterized in that, During continuous casting, the superheat of the tundish is controlled at 20-30℃, the billet casting speed is controlled at 0.75±0.02m / min, the secondary cooling water volume is controlled at 0.9-1.2L / kg, and electromagnetic stirring of the crystallizer is adopted: the stirring voltage is 300-500V, the current intensity is 300-500A, and the frequency is 3-8Hz.

6. The 8.8 grade large-size cold heading steel wire rod with excellent hardenability according to claim 5, characterized in that, During continuous casting, the cross-sectional dimensions of the continuously cast billet are (250~350)mm×(350~450)mm; protective casting is adopted, and a covering agent is added to the molten steel in the ladle and tundish; protective sleeves are used from the ladle to the tundish and from the tundish to the crystallizer, and argon blowing is used for protection; protective slag is added to the crystallizer.

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