A production method for improving slab transverse crack defects

Abstract

A production method for improving transverse crack defects in cast billets, wherein the billets are obtained by continuous casting, and protective slag is added to the crystallizer during the continuous casting process. The quaternary basicity of the protective slag satisfies Equation 1, the relationship between the viscosity of the protective slag and the casting machine speed satisfies Equation 2, and the relationship between the melting temperature of the protective slag and the liquidus temperature of the molten steel satisfies Equation 3. (CaO%+MgO%) / (SiO2%+Al2O3%)=1.0~1.2(1), 0.2≤η×Vc≤0.4(2), T L -425≤T m ≤T L -395 (3). By uniformly flowing in the protective slag liquid with high alkalinity, high viscosity and low melting temperature, the reasonable cooling intensity near the meniscus of the crystallizer is controlled to avoid transverse cracks on the surface of the billet.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy, and further to the field of continuous casting. Background Technology

[0002] Transverse cracks on the surface of cast billets are one of the most common quality defects in continuous casting production. They often occur at the edges or sides of the billets and are mostly distributed along deep vibration marks. These transverse cracks are usually very fine, often hidden 5–8 mm below the iron oxide scale on the billet surface, making them difficult to detect. This defect has long plagued the production of slabs or large billets with microalloyed or sub-peritectic compositions. For these steel grades, the greater tendency for austenite grain coarsening makes transverse cracking most pronounced. Furthermore, the prevalent presence of microalloying elements such as Al, Nb, V, and Ti in these steels, due to requirements for their microstructure and properties, further increases the difficulty of controlling transverse cracks in these steel grades.

[0003] Existing literature and patents have conducted in-depth research on transverse crack defects on the wide face and corners of cast billets, proposing various methods such as optimized composition control (e.g., controlling the content of elements such as C, N, and Al); weak cooling control in continuous casting (reducing the amount of crystallizer and secondary cooling water, and increasing the bending and straightening temperature of the cast billet); thermal insulation control of the mold flux (increasing basicity); and control of shallow vibration marks on the continuous casting billet. These methods have achieved certain results in controlling transverse cracks on the surface of continuous casting, but they all have significant limitations in actual operation, and their effect on improving cracks is very limited.

[0004] There are few reports on the research and control of transverse cracks (referred to as side cracks) on the side of cast billets in existing technologies, which is also one of the most difficult technical problems to solve in actual production. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to improve the transverse crack defect on the side of the cast billet.

[0006] To solve the above technical problems, the technical solution of the present invention is: a production method for improving transverse crack defects in cast billets, wherein the cast billet is obtained by continuous casting, and a protective slag is added to the crystallizer during the continuous casting process. The quaternary basicity of the protective slag satisfies Equation 1, the relationship between the viscosity of the protective slag and the casting machine speed satisfies Equation 2, and the relationship between the melting temperature of the protective slag and the liquidus temperature of the molten steel satisfies Equation 3.

[0007] (CaO%+MgO%) / (SiO2%+Al2O3%)=1.0~1.2 (1)

[0008] 0.2≤η×Vc≤0.4 (2)

[0009] T L -425≤T m ≤T L -395 (3)

[0010] In Formula 1, CaO%, MgO%, SiO2%, and Al2O3% are the mass percentages of CaO, MgO, SiO2, and Al2O3 in the protective slag, respectively; in Formula 2, η is the viscosity of the liquid slag at 1300℃ (Pa·S), and Vc is the casting machine speed (m / min). If the working speed is in the range of 1.0–1.4 m / min, the viscosity of the mold protective slag should be controlled within the range of 0.14–0.40 Pa·S; in Formula 3, T m To protect the melting temperature of the slag, T L The liquidus temperature of the molten steel being poured.

[0011] During the solidification process of the billet shell, the change in billet width can be mainly divided into two stages.

[0012] ① Solidification and shrinkage stage near the meniscus of the crystallizer: This stage occurs when the molten steel is subjected to intense cooling by the crystallizer, forming a shell of a certain thickness, and the width of the cast billet is in the shrinkage stage. During this stage, the greater the solidification shrinkage of the molten steel, the greater the shrinkage of the cast billet width.

[0013] ②The thermal expansion and extension stage in the lower part and outlet of the crystallizer: that is, the molten steel in the billet that has not yet solidified exerts static pressure on the solidified billet shell, thereby producing an expansion effect, which increases the width of the billet. The billet width is in the extension stage. In this stage, the thinner the initial billet shell, the greater the extension of the billet width.

[0014] Whether it's shrinkage or elongation, the greater the deformation, the greater the external force on the billet, leading to cracks at the weakest point. Therefore, it's crucial to ensure both minimal shrinkage deformation near the meniscus inside the crystallizer and minimal elongation deformation in the lower part of the crystallizer and at the outlet, in order to reduce the stress on the billet shell.

[0015] By uniformly flowing in a protective slag liquid with high alkalinity, high viscosity, and low melting temperature, the appropriate cooling intensity near the meniscus of the crystallizer can be controlled. This can avoid strong contraction caused by strong cooling inside the crystallizer and ensure that the billet shell in the lower part of the crystallizer and the outlet has sufficient thickness, thereby reducing the width extension of the cast billet.

[0016] The purpose of using a higher alkalinity in the protective slag is to control the appropriate heat transfer of the crystallizer protective slag and ensure a reasonable cooling intensity of the crystallizer.

[0017] The relationship between the viscosity of the protective slag and the casting speed of the casting machine satisfies Equation 2. The purpose is to control the uniform flow of the protective slag in the crystallizer, ensure lubrication throughout the process, and avoid the accumulation of liquid slag at the bottom of the negative slip.

[0018] The relationship between the melting temperature of the protective slag and the liquidus temperature of the molten steel satisfies Equation 3. The purpose is to control the appropriate melting rate and consumption of the protective slag in the crystallizer to form a multi-layered molten structure.

[0019] Furthermore, the production method for improving transverse crack defects in cast billets is characterized in that the chemical composition and mass percentage of the protective slag are SiO2: 24-28%, CaO: 25-30%, MgO: 4-6%, Al2O3: 3-6%, Na2O: 7-13%, F: 9-13%, C: 4-6%, with the remainder being unavoidable impurities.

[0020] Furthermore, the production method for improving transverse crack defects in cast billets is characterized in that the physical and chemical properties of the protective slag are: melting temperature 1080-1130℃, viscosity η 1300℃ The pressure is 0.14-0.40 Pa·s, and the bulk density is 0.4-0.8 g / cm³. 3 .

[0021] Furthermore, the production method for improving transverse crack defects in cast billets is characterized in that the carbon content of the protective slag is 4-6%, wherein the carbon black content is 1.5-2.0%, and the specific surface area of ​​the carbon black is 100-150 m². 2 / g; the particle size of the protective slag base material is 0.01-0.1mm.

[0022] During continuous casting, protective slag is continuously added to the surface of the molten steel in the crystallizer. Because the temperature of the molten steel is higher than the melting temperature of the protective slag, a liquid slag layer, a sintered layer, and a powdery slag layer are formed sequentially on the surface of the molten steel. When sintering is severe, on the one hand, the sintered solid slag can be drawn into the primary billet shell or into the gap between the crystallizer and the billet shell, causing slag inclusions and pitting cracks on the surface of the cast billet. On the other hand, the protective slag forms a slag ring on the crystallizer wall. A severe slag ring can not only block the flow channel of the liquid slag, but also cause uneven flow of liquid slag, leading to cracks on the surface of the cast billet and even inducing leakage. For steel grades sensitive to billet cracking, high-basicity protective slag is often used to control heat transfer in the crystallizer. Because of its high basicity, high-basicity protective slag is more sensitive to temperature changes and is more prone to severe sintering during the heating and melting process.

[0023] The amount of carbon and the type and characteristics of carbonaceous materials have a crucial impact on the sintering properties of protective slag. The mechanism by which carbonaceous materials inhibit sintering is that they can coat the base material. With increasing carbon content, the solid-phase reaction temperature can be correspondingly increased, thus inhibiting the sintering of the protective slag. Carbonaceous materials with a large specific surface area play a major role in inhibiting sintering, forming a tight first layer of coating on the base slag particles. When the amount of such carbonaceous materials is insufficient, the sintering performance of the protective slag is severely affected at higher temperatures. With a sufficient amount of finer carbonaceous materials, carbonaceous materials with smaller specific surface areas can then act as a second layer of isolation, inhibiting the generation and aggregation of droplets in the base material, thereby suppressing sintering. Therefore, the specific surface area of ​​the carbonaceous materials in the protective slag directly affects the sintering performance, and the amount of high specific surface area carbonaceous materials should not be insufficient.

[0024] The carbon content in the protective slag is 4-6%, and as much high specific surface area carbonaceous material as possible is added to coat the base slag particles, but not exceeding the threshold of 150 μm of the specific surface area of ​​the carbonaceous material. 2 / g (above this threshold, melting temperature becomes the primary factor). For example, when a large amount of highly dispersed carbon black (specific surface area 100-150m²) is added to the protective slag... 2 At a concentration of 4% (g / cm³), carbon black has a large specific surface area, resulting in good coating of the protective slag and strong inhibition of sintering. Furthermore, granular slag is more effective than powdered slag because when the content reaches 4%, sufficient carbon black particles are densely dispersed around the slag particles, which is an ideal condition for inhibiting sintering. Under these conditions, as the temperature increases, more and more components in the slag will begin to form a liquid phase. However, due to the sufficient amount of small-diameter carbon black between the slag particles, the rate of droplet formation in the slag will slow down. On the other hand, the required bulk density of the protective slag is 0.4-0.8 g / cm³. 3 The protective slag base material has a particle size in the range of 0.01-0.1mm. This prevents the molten slag from wetting and binding other particles through the carbonaceous material. It can prevent small droplets from agglomerating and forming a liquid phase, which to some extent reduces the segregation phenomenon of the protective slag and effectively inhibits the sintering effect of the protective slag.

[0025] Furthermore, to fully utilize the function of the protective slag, a stable and suitable melting structure is required. When the carbon black content is between 1.5% and 2.0%, a multi-layered structure will be formed, consisting of a primary slag layer, a sintered layer, a semi-molten layer, and a molten layer. A multi-layered structure with a semi-molten layer is superior to other structures. The sintered layer in a multi-layered structure is thinner than in a three-layered structure, which reduces the likelihood of slag adhering to the mold. The presence of the semi-molten layer allows the protective slag to significantly contribute slag to the slag layer. Therefore, a multi-layered structure is more adaptable to changes in casting parameters.

[0026] The side surface of the cast billet mentioned in this invention refers to the narrow side of the cast billet.

[0027] The viscosity of the protective slag mentioned in this invention refers to the viscosity of the liquid slag at 1300℃.

[0028] The carbon content and carbonaceous material content mentioned in this invention are all mass percentages.

[0029] The beneficial effects of adopting the above technical solution are as follows:

[0030] This invention effectively improves transverse crack defects on the side of the billet by controlling the shrinkage and elongation deformation of the initial billet shell. It ensures minimal shrinkage deformation near the meniscus inside the crystallizer and minimal elongation deformation in the lower part of the crystallizer and at the outlet, thereby reducing stress on the billet shell. By effectively controlling the sintering properties of the mold flux, it prevents sintered solid slag from being drawn into or extruding the initial billet shell. This has yielded unexpectedly good results, improving product quality and enhancing the company's economic benefits and brand image. Attached Figure Description

[0031] Figure 1 The image shows a photograph of a transverse crack on the side of a cast billet. The crack indicated by the arrow in the image is a transverse crack on the side of the cast billet. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0033] Example 1

[0034] The composition and mass percentage of 380CL peritectic microalloyed steel produced are shown in Table 1 below:

[0035] Table 1

[0036]

[0037] The continuous casting process is controlled as follows:

[0038] The mass percentage of each component and its physicochemical properties in the protective slag used are shown in Table 2.

[0039] Table 2

[0040]

[0041] ① Protective slag alkalinity:

[0042] The quaternary basicity R = (CaO% + MgO%) / (SiO2% + Al2O3%) = 1.0

[0043] ② Viscosity of protective slag:

[0044] The casting speed of this steel grade casting machine is Vc = 1.0~1.4m / min. Therefore, the viscosity of the protective slag liquid is controlled to be η = 0.14~0.4Pa.S to ensure that the product of casting speed and viscosity is η×Vc = 0.2~0.4.

[0045] ③ Melting temperature of protective slag: Liquidus temperature T of steel grade L =1522℃, the melting temperature Tm range of the protective slag is =1097~1127℃;

[0046] The inverted taper of the crystallizer is 1.10%; the gaps between the four pairs of foot rollers on the narrow face of the crystallizer and the vertical line of the narrow face copper plate are 0, 0.4 mm, 0.9 mm, and 1.4 mm respectively, controlling the mechanical alignment error to be 0-0.1 mm; the protective slag for the crystallizer is hollow granular slag, with a carbon content of 6%, of which the content of high specific surface area carbon black is 2%, the content of fine coke is 4%, and the specific surface area of ​​the carbon black is 100-120 m². 2 / g; the protective slag base material has a particle size of 0.01-0.08mm, the content of which is ≥90%, and the remaining particle size is 0.081-0.1mm.

[0047] After using the above process, the side cracks of the billet were basically eliminated, and the occurrence rate of peeling on the surface of the hot-rolled plate was reduced from 13% to less than 0.2%.

[0048] Example 2

[0049] The steel composition and its mass percentage for FL590X duplex cold-rolled microalloyed steel are shown in Table 3 below:

[0050] Table 3

[0051]

[0052] The continuous casting process is controlled as follows:

[0053] The mass percentage of each component and its physicochemical properties in the protective slag used are shown in Table 4.

[0054] Table 4

[0055]

[0056] ① Protective slag alkalinity:

[0057] The quaternary basicity R = (CaO% + MgO%) / (SiO2% + Al2O3%) = 1.2

[0058] ② Viscosity of protective slag:

[0059] The casting speed of this steel grade casting machine is Vc = 1.1~1.3m / min. Therefore, the viscosity of the protective slag liquid is controlled to be η = 0.17~0.31Pa.S to ensure that the product of casting speed and viscosity is η×Vc = 0.22~0.34.

[0060] ③ Melting temperature of protective slag: Liquidus temperature T of steel grade L =1517℃, the melting temperature Tm range of the protective slag is =1092~1122℃;

[0061] The inverted taper of the crystallizer is 1.30%; the gaps between the four pairs of foot rollers on the narrow face of the crystallizer and the vertical line of the narrow face copper plate are 0, 0.4 mm, 0.9 mm, and 1.4 mm respectively, controlling the mechanical alignment error to 0-0.1 mm; the protective slag for the crystallizer is hollow granular slag, with a carbon content of 4%, of which the content of high specific surface area carbon black is 1.8%, the content of fine coke is 2.2%, and the specific surface area of ​​the carbon black is 120-140 m². 2 / g; the protective slag base material has a particle size of 0.01-0.08mm, the content of which is ≥90%, and the remaining particle size is 0.081-0.1mm.

[0062] After using the above process, the side cracks of the billet were basically eliminated, and the occurrence rate of peeling on the surface of the hot-rolled plate was reduced from 18% to less than 0.1%.

[0063] Example 3

[0064] The composition and mass percentage of W980X high-strength microalloyed steel are shown in Table 5 below:

[0065] Table 5

[0066]

[0067] The continuous casting process is controlled as follows:

[0068] The mass percentage of each component and its physicochemical properties in the protective slag used are shown in Table 6.

[0069] Table 6

[0070]

[0071] ① Protective slag alkalinity:

[0072] The quaternary basicity R = (CaO% + MgO%) / (SiO2% + Al2O3%) = 1.1

[0073] ② Viscosity of protective slag:

[0074] The casting machine for this steel grade operates at a speed Vc = 1.0–1.2 m / min; therefore, controlling the viscosity of the protective slag is crucial.

[0075] η = 0.23~0.39 Pa·s, ensuring that the product of pulling speed and viscosity η×Vc = 0.28~0.39;

[0076] ③ Melting temperature of protective slag: Liquidus temperature T of steel grade L =1512℃, the melting temperature Tm range of the protective slag is =1087-1117℃;

[0077] The inverted taper of the crystallizer is 1.20%; the gaps between the four pairs of foot rollers on the narrow face of the crystallizer and the vertical line of the narrow face copper plate are 0, 0.4 mm, 0.9 mm, and 1.4 mm respectively, controlling the mechanical alignment error to 0-0.1 mm; the protective slag for the crystallizer is hollow granular slag, with a carbon content of 5%, of which the content of high specific surface area carbon black is 1.5%, the content of fine coke is 3.5%, and the specific surface area of ​​the carbon black is 130-150 m². 2 / g; the protective slag base material has a particle size of 0.01-0.08mm, the content of which is ≥90%, and the remaining particle size is 0.081-0.1mm.

[0078] After using the above process, the side cracks of the billet were basically eliminated, and the occurrence rate of peeling on the surface of the hot-rolled plate was reduced from 15% to less than 0.1%.

[0079] The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A production method for improving transverse crack defects in cast billets, wherein the cast billet is obtained by continuous casting, and protective slag is added to the crystallizer during the continuous casting process, characterized in that, The quaternary basicity of the protective slag satisfies Equation 1; the relationship between the viscosity of the protective slag and the casting machine speed satisfies Equation 2; and the relationship between the melting temperature of the protective slag and the liquidus temperature of the molten steel satisfies Equation 3. (CaO% + MgO%) / (SiO2% + Al2O3%) = 1.0 ~ 1.2 (1) 0.2≤η*Vc≤0.4 (2) T L -425≤T m ≤T L -395 (3) In Equation 1, CaO%, MgO%, SiO2%, and Al2O3% are the mass percentages of CaO, MgO, SiO2, and Al2O3 in the protective slag, respectively; in Equation 2, η is the viscosity of the liquid slag at 1300℃ (Pa·s), and Vc is the casting machine speed (m / min); in Equation 3, T m To protect the melting temperature of the slag, T L The liquidus temperature of the molten steel being poured; The carbon content of the protective slag is 4-6%, of which the carbon black content is 1.5-2.0%, and the specific surface area of ​​the carbon black is 100-150 m². 2 / g; The particle size of the protective slag base material is 0.01-0.1 mm, and the bulk density of the protective slag is 0.4-0.8 g / cm³. 3 ; The protective slag forms a multi-layered structure consisting of a raw slag layer, a sintered layer, a semi-molten layer, and a molten layer.

2. The production method for improving transverse crack defects in cast billets according to claim 1, characterized in that, The chemical composition and mass percentage of the protective slag are as follows: SiO2: 24-28%, CaO: 25-30%, MgO: 4-6%, Al2O3: 3-6%, Na2O: 7-13%, F: 9-13%, C: 4-6%, with the remainder being unavoidable impurities.

3. The production method for improving transverse crack defects in cast billets according to claim 1, characterized in that, The physical and chemical properties of the protective slag are as follows: melting temperature 1080-1130℃, viscosity η 1300℃ The value is 0.14-0.40 Pa·s.

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