Mold flux for austenitic 304 stainless steel square billet continuous casting and application thereof

Abstract

The application provides an austenitic 304 stainless steel square billet continuous casting mold flux and application thereof, and relates to the technical field of metallurgical auxiliary materials. The chemical components of the austenitic 304 stainless steel square billet continuous casting mold flux include: SiO2 29.0%-35.0%, CaO 33.0%-39%, Li2O 0.2%-0.6%, Al2O3 3.5%-7.5%, Na2O 4.6%-8.6%, F ‑ 6.0%-8.5%, MnO 2.0%-4.0%, Fe2O3≤2.0%, B2O3 0.18-1.0%, Cr2N 0.5%-0.8%, free carbon 0.1-0.2%. The chemical components of the mold flux only contain 0.1-0.2% of free carbon, and thus the generation rate of stainless steel black bands can be effectively reduced in the continuous casting process of the austenitic 304 stainless steel square billet.

Description

Technical Field

[0001] This invention relates to the field of metallurgical auxiliary materials technology, and in particular to a mold flux for continuous casting of austenitic 304 stainless steel billets and its application. Background Technology

[0002] Currently, the square or rectangular billets of 160-240mm austenitic 304 stainless steel continuous casting are most prone to producing stainless steel black bands.

[0003] The main reason for the formation of these stainless steel black stripes is that austenitic 304 stainless steel square or rectangular billets with a cross-section ranging from 160 to 240 mm have relatively small cross-sectional dimensions. During continuous casting, the liquid level in the crystallizer fluctuates greatly, easily producing slag inclusions and inclusions. Furthermore, the copper tubes in the crystallizer dissipate heat quickly, so these slag inclusions and inclusions easily cause wrinkles, scale build-up, and rejoining defects on the billet surface during casting. These surface defects often contain carbon from the carbon-rich layer of the protective slag, and carbon is the main cause of the black stripe. Through rolling, it becomes long strips, which are commonly referred to as stainless steel black stripes.

[0004] Currently, during the rolling process of austenitic 304 stainless steel billets, the generation rate of stainless steel black stripe is as high as 16%. A high proportion of stainless steel black stripe in the billet will result in more scrap or substandard steel, thereby increasing manufacturing costs.

[0005] Currently, methods for reducing black band rate both domestically and internationally primarily employ ultra-low carbon protective slag, which aims to minimize free carbon in the protective slag. However, without the addition of carbonaceous materials, the melting rate of the protective slag cannot be controlled, becoming a bottleneck. For example, Chinese patent CN106513607A discloses a protective slag for continuous casting crystallizers of 304 stainless steel, characterized by its chemical composition and weight percentages as follows: SiO2 27.0%–30.0%, CaO 30.0%–33%, Li2O 0.4%–1.0%, Al2O3 5%–7%, Na2O 8%–9%, ​​F - The composition is 5.0%–7.0%, MnO 2.0%–4.0%, Fe2O3 ≤2.0%, C solids 1%–2.0%, and volatile matter 11%–15%. However, the binary basicity of this protective slag is 1.04–1.14, the melting point is 1080–1110℃, and the viscosity at 1300℃ is 0.13–0.18 Pa·s. Its melting point and viscosity are too low, which can easily lead to thickening and unevenness of the slag film, resulting in uneven heat conduction. This causes stress concentration in areas where the solidified shell is thinner, increasing friction and making defects such as indentations and scale formation more likely. At the same time, using carbon black as a carbonaceous material also increases the risk of carbonization on the surface of the billet, leading to the risk of black bands during rolling.

[0006] Given the critical issue of a black band rate of up to 16% after rolling austenitic 304 stainless steel, it is urgent to develop a new mold flux for continuous casting of austenitic 304 stainless steel billets to solve the existing problems of surface depressions, scale formation, and the most prominent black band.

[0007] In view of this, the present invention is hereby proposed. Summary of the Invention

[0008] The first objective of this invention is to provide a mold flux for continuous casting of austenitic 304 stainless steel billets, wherein the chemical composition of the flux contains only 0.1 to 0.2% free carbon, thereby effectively reducing the generation rate of stainless steel black band during the continuous casting process of austenitic 304 stainless steel billets.

[0009] The second objective of this invention is to provide an application of a mold flux for continuous casting of austenitic 304 stainless steel square billets in the preparation of austenitic 304 stainless steel continuous casting square or rectangular billets.

[0010] The third objective of this invention is to provide a method for preparing austenitic 304 stainless steel continuously cast square or rectangular billets.

[0011] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0012] The present invention provides a mold flux for continuous casting of austenitic 304 stainless steel billets. The chemical composition of the mold flux, by mass percentage, includes:

[0013] SiO2 29.0%~35.0%, CaO 33.0%~39%, Li2O 0.2%~0.6%, Al2O3 3.5%~7.5%, Na2O 4.6%~8.6%, F - 6.0%–8.5%, MnO 2.0%–4.0%, Fe2O3≤2.0%, B2O3 0.18%–1.0%, Cr2N 0.5%–0.8%, free carbon 0.1%–0.2%.

[0014] Furthermore, the binary basicity of the crystallizer protective slag, CaO / SiO2, is 1.12–1.22, the melting point is 1120–1150℃, the viscosity at 1300℃ is 0.19–0.27 Pa·s, and the crystallization rate is 30–50%.

[0015] Furthermore, the raw materials for the crystallizer protective slag include: pre-melted material, wollastonite, limestone, lithium carbonate, fluorite, sodium fluoride, bentonite, cryolite, chromium nitride, binder, manganese carbonate, borax, and industrial aluminum ash slag.

[0016] Furthermore, the chromium nitride accounts for 0.5% to 0.8% of the total mass of the raw materials by weight percentage.

[0017] Furthermore, by mass percentage, the pre-melted material and wollastonite account for 43.0% to 48.0% of the total mass of the raw materials;

[0018] The mass ratio of the pre-melted material to wollastonite is 2 to 2.3:1.

[0019] Furthermore, the adhesive comprises at least one of sodium carboxymethyl cellulose, starch, and dextrin.

[0020] Furthermore, by mass percentage, the raw materials comprising the crystallizer protective slag include:

[0021] The pre-melted material consists of 43.0%–48.0% wollastonite, 16.0%–21.0% limestone, 0.5%–1.5% lithium carbonate, 3.0%–6.0% fluorite, 6%–9% sodium fluoride, 3–9% bentonite, 3.0%–5.0% cryolite, 0.5%–0.8% chromium nitride, 2.0%–4.0% binder, 3–5% manganese carbonate, 0.5%–3% borax, and 3–5% industrial aluminum ash.

[0022] The ratio of pre-melted material to wollastonite is 2 to 2.3:1.

[0023] The present invention provides the application of a mold flux for continuous casting of austenitic 304 stainless steel square billets in the preparation of austenitic 304 stainless steel continuous casting square or rectangular billets.

[0024] Furthermore, the cross-section of the austenitic 304 stainless steel continuously cast square or rectangular billet is (160~240)×(160~240)mm.

[0025] The present invention provides a method for preparing austenitic 304 stainless steel continuously cast square or rectangular billets, characterized in that the preparation is carried out using the above-mentioned mold flux and the casting speed is 1.3 to 1.6 m / min.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] The mold flux for continuous casting of austenitic 304 stainless steel billets provided in this application introduces chromium nitride into the raw materials to replace carbonaceous materials as the skeletal material for controlling the melting rate. Since chromium nitride itself does not contain carbon, the free carbon content in the mold flux is significantly reduced. The resulting mold flux contains only 0.1-0.2% free carbon in its chemical composition, thus effectively reducing the formation rate of stainless steel black bands during the continuous casting process of austenitic 304 stainless steel billets.

[0028] Meanwhile, the addition of chromium nitride in this application can also ensure a suitable slag melting rate, so that the slag has a suitable slag layer thickness, a stable slag film thickness, and good separation between the slag film and the billet shell surface, thereby ensuring smooth continuous casting and better and more stable billet surface quality.

[0029] Furthermore, the protective slag for crystallizers in this application, through the reasonable combination of materials, can improve the binary basicity, melting point, basicity and crystallization rate of the protective slag, thereby reducing the two-dimensional cooling capacity of the square billet corner, preventing the square billet corner from cooling too fast, large liquid surface fluctuation, and easily leading to defects such as corner cracks, delamination, slag inclusions, and scale formation. Detailed Implementation

[0030] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and 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.

[0031] According to one aspect of the present invention, a mold flux for continuous casting of austenitic 304 stainless steel billets, wherein the chemical composition of the mold flux, by mass percentage, comprises:

[0032] SiO2 29.0%~35.0%, CaO 33.0%~39%, Li2O 0.2%~0.6%, Al2O3 3.5%~7.5%, Na2O 4.6%~8.6%, F - 6.0%–8.5%, MnO 2.0%–4.0%, Fe2O3≤2.0%, B2O3 0.18%–1.0%, Cr2N 0.5%–0.8%, free carbon 0.1%–0.2%.

[0033] The mold flux for continuous casting of austenitic 304 stainless steel billets provided in this application contains only 0.1-0.2% free carbon in its chemical composition, which can effectively reduce the generation rate of stainless steel black band during the continuous casting process of austenitic 304 stainless steel billets.

[0034] In a preferred embodiment of the present invention, the binary basicity CaO / SiO2 of the crystallizer protective slag is 1.12 to 1.22, the melting point is 1120 to 1150°C, the viscosity at 1300°C is 0.19 to 0.27 Pa·s, and the crystallization rate is 30 to 50%.

[0035] It should be noted that the continuous casting of austenitic 304 stainless steel square or rectangular billets differs from the continuous casting of slabs and round billets of the same material. This is because, for the same billet cross-sectional area, their specific surface area is larger than the latter two, and the crystallizer cooling capacity of stainless steel square or rectangular billets is stronger. Therefore, the mold flux needs to be appropriately increased by using a reasonable material combination to raise the binary basicity (CaO / SiO2) to 1.12–1.22, thereby increasing the melting point, basicity, and crystallization rate of the mold flux. This reduces the two-dimensional cooling capacity at the corners, preventing excessively rapid cooling at the corners of the billet, large liquid level fluctuations, and the resulting defects such as corner cracks, detachment, slag inclusions, and scale formation.

[0036] Meanwhile, the improvement of the binary basicity, melting point, viscosity and crystallization rate of the crystallizer protective slag in this application also solves the liquid surface fluctuation caused by the high casting speed during billet continuous casting, reduces the risk of slag entrapment and inclusion, and reduces the problems of depression and scale defects on the surface of stainless steel billets, which is extremely beneficial to reducing the black stripe rate of stainless steel plates.

[0037] In a preferred embodiment of the present invention, the raw materials of the crystallizer protective slag include: pre-melted material, wollastonite, limestone, lithium carbonate, fluorite, sodium fluoride, bentonite, cryolite, chromium nitride, binder, manganese carbonate, borax and industrial aluminum ash slag.

[0038] Note: The free carbon in the chemical composition of the mold flux in this application is obtained by the carbonization of the binder during continuous casting, which is unavoidable.

[0039] This application introduces chromium nitride (CHN) into the raw materials of the mold flux to replace carbonaceous materials as a skeletal material for controlling the melting rate. CHN is carbon-free, exhibits good high-temperature stability, a low coefficient of thermal expansion, good chemical stability, and good oxidation resistance. CHN reacts less reactively with high-chromium materials than with conventionally formulated carbonaceous materials. Furthermore, CHN is ultra-finely processed at the nanometer scale, resulting in strong dispersion and a slowing effect on the melting rate. Therefore, this application achieves the same function as carbon by using CHN instead of carbonaceous materials, but without the addition of free carbon. This significantly reduces the free carbon content in the mold flux. Except for a small amount of residual carbon left by the binder after carbonization during continuous casting, the other materials in the mold flux are almost carbon-free, effectively solving the problem of excessive black banding on the stainless steel surface. After rolling, the black banding rate on the stainless steel surface is reduced from 16% to below 2%.

[0040] Preferably, the chromium nitride is sodium-scale chromium nitride.

[0041] It should be noted that chromium nitride materials do not contain carbon and do not react with the high chromium content in the slag layer. This application uses nanometer-sized chromium nitride with a large specific surface area, which can effectively replace the original carbonaceous materials, providing a good framework and effectively slowing down the melting rate of the protective slag and increasing its melting time. This reduces the problems of excessively fast melting rate and excessively thick liquid slag layer caused by the reduction of carbonaceous materials. Simultaneously, chromium nitride has a low coefficient of thermal expansion, good high-temperature performance, and excellent self-lubricating properties. Furthermore, the ultrafine particle size reduction of chromium nitride improves the flowability and spreading properties of the protective slag.

[0042] In the preferred embodiment described above, the chromium nitride accounts for 0.5% to 0.8% of the total mass of the raw materials by weight percentage.

[0043] In a preferred embodiment, the chromium nitride content is 0.5% to 0.8% of the total raw material mass. Production practice has shown that when the chromium nitride content is below 0.5%, the material cannot control the melting rate of the protective slag, leading to excessively rapid melting, a gradual increase in the slag layer, and an imbalance between melting and consumption, which is detrimental to continuous casting. Conversely, when the chromium nitride content is above 0.8%, it easily leads to a decrease in melting rate, a reduction in slag layer thickness, an increase in slag entrapment, an increase in slag adhesion on the billet surface, and an increase in the black band rate. Therefore, the scheme using chromium nitride at a total raw material mass of 0.5% to 0.8% yields better results.

[0044] In the above preferred embodiment, the pre-melted material and wollastonite account for 43.0% to 48.0% of the total mass of the raw materials by mass percentage;

[0045] The mass ratio of the pre-melted material to wollastonite is 2 to 2.3:1.

[0046] In a preferred embodiment, the mass ratio of premelted material to wollastonite in this application is 2–2.3:1. Since the premelted material is a calcined clinker with uniform composition, a mass ratio of 2–2.3:1 for the premelted material and wollastonite ensures the stability of the protective slag composition and the stability of the crystallizer liquid level during use. Furthermore, this range also satisfies the heat transfer requirements and the appropriate crystallization rate determined by suitable amounts of sodium oxide and fluoride.

[0047] It should be noted that pre-melted materials are materials such as quartz, calcite, and glass that have been melted and then quenched in water to become granular materials. Different pre-melted materials have different chemical compositions and contents due to the different raw materials added. Pre-melted materials can be selected by those skilled in the art based on their needs within the existing technology. After selecting a pre-melted material, those skilled in the art can add other raw materials to make up for any missing parts in the pre-melted material according to the composition ratio of the protective slag, thus obtaining a protective slag with a specified ratio. The advantage of using pre-melted materials is that through melting, various different substances are fully integrated, the chemical composition is more uniformly distributed, and this is beneficial to the overall stability of the protective slag's performance.

[0048] In this application, conventional premelted materials in the field can be used, and will not be elaborated further.

[0049] In the preferred embodiment described above, the adhesive comprises at least one of sodium carboxymethyl cellulose, starch, and dextrin.

[0050] In a preferred embodiment of the present invention, the raw materials comprising the crystallizer protective slag, by mass percentage, include:

[0051] The pre-melted material consists of 43.0%–48.0% wollastonite, 16.0%–21.0% limestone, 0.5%–1.5% lithium carbonate, 3.0%–6.0% fluorite, 6%–9% sodium fluoride, 3–9% bentonite, 3.0%–5.0% cryolite, 0.5%–0.8% chromium nitride, 2.0%–4.0% binder, 3–5% manganese carbonate, 0.5%–3% borax, and 3–5% industrial aluminum ash.

[0052] The ratio of pre-melted material to wollastonite is 2 to 2.3:1.

[0053] Typical, but not limiting, preferred embodiments of the above-mentioned premelted material + wollastonite content are: 43%, 43.5%, 44%, 44.5%, 45.0%, 45.5%, 46.0%, 46.5%, 47.0%, 47.5%, 48.0%, or any value within the range of 43.0% to 48.0%.

[0054] Typical, but not limiting, preferred embodiments of the above-mentioned limestone content are: 16%, 17%, 18%, 19%, 20%, 21%, or any value within the range of 16% to 21%.

[0055] Typical, but non-limiting, preferred embodiments of the lithium carbonate content are 0.5%, 1.0%, and 1.5%, or any value within the range of 0.5% to 1.5%.

[0056] Typical, but not limiting, preferred embodiments of the fluorite content are: 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, 5.5%, 6%, or any value within the range of 3.0% to 6.0%.

[0057] Typical, but non-limiting, preferred embodiments of the sodium fluoride content are: 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, or any value within the range of 6.0% to 9.0%.

[0058] Typical but non-limiting preferred embodiments of the bentonite content are: 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, or any value within the range of 3.0% to 9.0%.

[0059] Typical, but not limiting, preferred embodiments of the cryolite content are: 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, or any value within the range of 3.0% to 5.0%.

[0060] Typical, but non-limiting, preferred embodiments of the chromium nitride content are: 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, or any value within the range of 0.5% to 0.8%.

[0061] Typical, but not limiting, preferred embodiments of the above-mentioned adhesive content are 2.0%, 2.5%, 3.0%, 3.5%, and 4.0%, or any value within the range of 2.0% to 4.0%.

[0062] Typical, but not limiting, preferred embodiments of the above-mentioned manganese carbonate content are: 3%, 3.5%, 4.0%, 4.5%, 5.0%, or any value within the range of 3% to 5%.

[0063] Typical, but not limiting, preferred embodiments of the borax content are: 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, or any value within the range of 0.5% to 3%.

[0064] Typical, but not limiting, preferred embodiments for the content of the aforementioned industrial aluminum ash slag are: 3%, 3.5%, 4.0%, 4.5%, and 5.0%, or any value within the range of 3% to 5%.

[0065] According to one aspect of the present invention, a mold flux for continuous casting of austenitic 304 stainless steel square billets is used in the preparation of austenitic 304 stainless steel continuous casting square or rectangular billets.

[0066] The mold flux for continuous casting of austenitic 304 stainless steel square billets described in this application can be widely used in the preparation process of continuous casting square or rectangular billets of austenitic 304 stainless steel. The resulting continuous casting square or rectangular billets of austenitic 304 stainless steel have no or few black bands, and the surface is free of depressions and scabs.

[0067] In a preferred embodiment of the present invention, the cross-section of the austenitic 304 stainless steel continuously cast square or rectangular billet is (160-240) × (160-240) mm.

[0068] According to one aspect of the present invention, a method for preparing austenitic 304 stainless steel continuously cast square or rectangular billets is provided, wherein the above-mentioned mold flux is used for preparation and the casting speed is 1.3 to 1.6 m / min.

[0069] This invention provides a method for preparing austenitic 304 stainless steel continuously cast square or rectangular billets, which are prepared by continuous casting with the above-mentioned mold flux at a casting speed of 1.3 to 1.6 m / min.

[0070] The binary basicity (CaO / SiO2) of the mold flux in this application is 1.12–1.22, the melting point is 1120–1150℃, the viscosity at 1300℃ is 0.19–0.27 Pa·s, and the crystallization rate is 30–50%. The aforementioned relatively high binary basicity, melting point, viscosity, and crystallization rate can effectively alleviate the liquid surface fluctuations caused by high casting speeds of 1.3–1.6 m / min during continuous casting of billets, reduce the risk of slag entrapment and inclusions, and reduce surface defects such as depressions and scale on stainless steel billets. This is highly beneficial for reducing the black stripe rate of stainless steel plates. The technical solution of this invention will be further explained below with reference to embodiments and comparative examples.

[0071] Example 1

[0072] A mold flux for continuous casting of austenitic 304 stainless steel billets comprises the following components:

[0073] The pre-melted material consists of 46% wollastonite, 16.0% limestone, 1.5% lithium carbonate, 3.0% fluorite, 6.5% sodium fluoride, 9% bentonite, 3.0% cryolite, 0.8% chromium nitride, 4.0% binder, 4.2% manganese carbonate, 3% borax, and 3% industrial aluminum ash.

[0074] The ratio of pre-melted material to wollastonite is 2.3:1.

[0075] The chemical composition (mass percentage) of the protective slag is as follows: SiO2 32.0%, CaO 39%, Li2O 0.6%, Al2O 33.7%, Na2O 5.2%, F - 4.4%, MnO 2.2%, Fe2O3 2.0%, B2O3 1.44%, Cr2N 0.8%, free carbon 0.193%, the balance being volatile gases, such as CO2.

[0076] The protective slag has a binary basicity of CaO / SiO2 of 1.22, a melting point of 1146℃, a viscosity of 0.27 Pa·s at 1300℃, and a crystallization rate of 34%.

[0077] Effect verification:

[0078] The protective slag of this embodiment was used to conduct a continuous casting experiment of austenitic 304 stainless steel square billet. The operation was carried out in accordance with the current specifications for continuous casting steel. The steel grade used in the experiment was austenitic 304 stainless steel square billet with a cross section of 240*240mm and a casting speed of 1.3m / min.

[0079] During the experiment, the protective slag spread well within the crystallizer, with virtually no slag streaks. The molten steel surface fluctuated minimally, and the slag layer was 5–7 mm thick, indicating a balance between melting and consumption, appropriate melting rate control, and good lubrication. Simultaneously, the temperature difference in the copper tubes of the crystallizer remained between 5 and 7°C, and no transverse concavity appeared in the cast billet, indicating suitable heat transfer. The billet quality was normal, with no slag inclusions or carbon enrichment, and the black stripe rate after rolling was 1.2%.

[0080] Example 2

[0081] A mold flux for continuous casting of austenitic 304 stainless steel billets comprises the following components:

[0082] The pre-melted material consists of 43% wollastonite, 16.5% limestone, 0.5% lithium carbonate, 4.0% fluorite, 8% sodium fluoride, 13% bentonite, 3.0% cryolite, 0.6% chromium nitride, 2.0% binder, 3% manganese carbonate, 1.4% borax, and 5% industrial aluminum ash.

[0083] The ratio of pre-melted material to wollastonite is 2.15:1.

[0084] The chemical composition (mass percentage) of the protective slag is as follows: SiO2 32.0%, CaO 37%, Li2O 0.2%, Al2O 35.5%, Na2O 6.4%, F - 6.8%, MnO 1.7%, Fe2O3 1.3%, B2O3 0.7%, Cr2N 0.6%, free carbon 0.13%, balance being volatile gases.

[0085] The protective slag has a binary basicity of CaO / SiO2 of 1.16, a melting point of 1146℃, a viscosity of 0.23 Pa·s at 1300℃, and a crystallization rate of 43%.

[0086] Effect verification:

[0087] The protective slag of this embodiment was used to conduct a continuous casting experiment of austenitic 304 stainless steel square billet. The operation was carried out in accordance with the current specifications for continuous casting steel. The steel grade used in the experiment was austenitic 304 stainless steel square billet with a cross section of 240*240mm and a casting speed of 1.6m / min.

[0088] During the experiment, the protective slag spread well during use, there were no flames on the slag surface of the molten steel, the molten steel surface fluctuated little, the slag layer was 6-9mm, the slag strips were relatively small, the cast billet had no defects such as depressions, scale, slag inclusions, or carbon increase, and the black strip rate after rolling was basically zero.

[0089] Example 3

[0090] A mold flux for continuous casting of austenitic 304 stainless steel billets comprises the following components: pre-melted material + 48% wollastonite, 16.9% limestone, 1% lithium carbonate, 3% fluorite, 7% sodium fluoride, 6.2% bentonite, 5.0% cryolite, 0.5% chromium nitride, and 3.0% binder, 3% manganese carbonate, 2.4% borax, and 4% industrial aluminum ash.

[0091] The ratio of premelted material to wollastonite is 2.0:1.

[0092] The chemical composition (mass percentage) of the protective slag is as follows: SiO2 31.25%, CaO 35.0%, Li2O 0.4%, Al2O 36.0%, Na2O 7.6%, F - 6.1%, MnO 1.7%, Fe2O3 0.8%, B2O3 1.2%, Cr2N 0.5%, free carbon 0.11%, balance being volatile gases.

[0093] The protective slag has a binary basicity of CaO / SiO2 of 1.12, a melting point of 1125℃, a viscosity of 0.27 Pa·s at 1300℃, and a crystallization rate of 38%.

[0094] Effect verification:

[0095] The protective slag of this embodiment was used to conduct a continuous casting experiment of austenitic 304 stainless steel square billet. The operation was carried out in accordance with the current specifications for continuous casting steel. The steel grade used in the experiment was austenitic 304 stainless steel square billet with a cross section of 240*240mm and a casting speed of 1.3m / min.

[0096] During the experiment, the protective slag spread well, the liquid slag was 7-9mm, there were no flames on the surface of the molten steel, the liquid surface was relatively stable, the slag consumption was 0.48kg / T, the heat transfer was fast, which met the purpose of high casting speed and fast heat transfer, the quality of the billet was normal, and no transverse dents, cracks, scabs, slag inclusions or carbon increase were found. The black strip rate after rolling was 0.95%.

[0097] Comparative Example 1

[0098] A mold flux for continuous casting of austenitic 304 stainless steel billets comprises: pre-melted material + 40.8% wollastonite, 17.4% limestone, 1.0% lithium carbonate, 7.2% fluorite, 4% sodium fluoride, 14.9% bentonite, 4.0% cryolite, 1.2% chromium nitride, 3.0% binder, 3.5% manganese carbonate, 2% borax, and 1% aluminum ash.

[0099] The ratio of premelted material to wollastonite is 2.0:1.

[0100] Its chemical composition is: SiO2 27.0%, CaO 39%, Li2O 0.4%, Al2O3 3.7%, Na2O 4.6%, F - 7.2%, MnO 2.0%, Fe2O3 1.12%, B2O3 1.0%, Cr2N 1.2%, free carbon content 0.52%, balance is volatile gases.

[0101] The protective slag has a binary basicity of CaO / SiO2 of 1.44, a melting point of 1164℃, a viscosity of 0.14 Pa·s at 1300℃, and a crystallization rate of 63%.

[0102] Effect verification:

[0103] The protective slag of this embodiment was used to conduct a continuous casting experiment of austenitic 304 stainless steel square billet. The operation was carried out in accordance with the current specifications for continuous casting steel. The steel grade used in the experiment was austenitic 304 stainless steel square billet with a cross section of 240*240mm and a casting speed of 1.3m / min.

[0104] During the experiment, the controlled melting rate of the comparative protective slag was too slow. Under the process conditions of a cross-section of 240*240mm and a casting speed of 1.3m / min, the slag layer thickness was only about 3mm, which was too thin. This easily led to fluctuations in the liquid surface, flames on the molten steel surface, and slag consumption of 0.6-0.8kg / T. The billet quality exhibited defects such as depressions, scale, and slag inclusions. Due to the excessively thin slag layer, the experiment was stopped for safety reasons. The black stripe rate after billet rolling was 7-30%, which did not meet the requirements for field use.

[0105] Comparative Example 2

[0106] A mold flux for continuous casting of austenitic 304 stainless steel billets comprises: pre-melted material + 57.0% wollastonite, 5.45% limestone, 1.0% lithium carbonate, 3.2% fluorite, 7% sodium fluoride, 13.5% bentonite, 0% cryolite, 0.45% chromium nitride, 3.0% binder, 3% manganese carbonate, 2.4% borax, and 4% aluminum ash.

[0107] The ratio of premelted material to wollastonite is 2.0:1.

[0108] Its chemical composition is as follows: SiO2 40.0%, CaO 36.0%, Li2O 0.4%, Al2O3 2.1%, Na2O 5.2%, F - 4.2%, MnO 1.7%, Fe2O3 0.8%, B2O3 1.2%, Cr2N 0.45%, free carbon content 0.09%, balance is volatile gases.

[0109] The protective slag has a binary basicity of CaO / SiO2 of 0.90, a melting point of 1097℃, a viscosity of 0.33 Pa·s at 1300℃, and a crystallization rate of 13%.

[0110] Effect verification:

[0111] The protective slag of this embodiment was used to conduct a continuous casting experiment of austenitic 304 stainless steel square billet. The operation was carried out in accordance with the current specifications for continuous casting steel. The steel grade used in the experiment was austenitic 304 stainless steel square billet with a cross section of 240*240mm and a casting speed of 1.3m / min.

[0112] During the experiment, the protective slag in the comparative sample exhibited excessively rapid heat transfer and melting, causing the slag thickness to increase from 7-9 mm to approximately 15 mm within about 5 minutes. No flame was observed on the molten steel surface, and the surface fluctuated significantly. Later, signs of adhesion appeared, necessitating the cessation of the experiment. Calculations showed a slag consumption of 0.35 kg / T. The current billet quality exhibits transverse concavity, scale buildup, and slag inclusions. The black stripe rate after rolling ranges from 5% to 30%, failing to meet the requirements for field use.

[0113] In summary, the mold flux for continuous casting of austenitic 304 stainless steel billets in this application introduces chromium nitride into the raw materials to replace carbonaceous materials as the skeleton material for controlling the melting rate. Simultaneously, with a reasonable combination of raw materials, the resulting mold flux contains only 0.1–0.2% free carbon and exhibits a better melting rate. Therefore, it can effectively reduce the formation rate of stainless steel black bands during the continuous casting process of austenitic 304 stainless steel billets.

[0114] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A mold flux for continuous casting of austenitic 304 stainless steel billets, characterized in that, The chemical composition of the crystallizer protective slag, by mass percentage, includes: SiO229.0%~35.0%, CaO 33.0%~39%, Li2O 0.2%~0.6%, Al2O33.5%~7.5%, Na2O 4.6%~8.6%, F - 6.0%–8.5%, MnO 2.0%–4.0%, Fe2O3≤2.0%, B2O30.18%–1.0%, Cr2N0.5%–0.8%, free carbon0.1%–0.2%; The crystallizer protective slag has a binary basicity of CaO / SiO2 of 1.12–1.22, a melting point of 1120–1150℃, a viscosity of 0.19–0.27 Pa•s at 1300℃, and a crystallization rate of 30–50%.

2. The mold flux for continuous casting of austenitic 304 stainless steel billets according to claim 1, characterized in that, The raw materials for the crystallizer protective slag include: pre-melted material, wollastonite, limestone, lithium carbonate, fluorite, sodium fluoride, bentonite, cryolite, chromium nitride, binder, manganese carbonate, borax, and industrial aluminum ash.

3. The mold flux for continuous casting of austenitic 304 stainless steel billets according to claim 2, characterized in that, By mass percentage, the pre-melted material and wollastonite account for 43.0% to 48.0% of the total mass of the raw materials; The mass ratio of the pre-melted material to wollastonite is 2~2.3:

1.

4. The mold flux for continuous casting of austenitic 304 stainless steel billets according to claim 2, characterized in that, The binder includes at least one of sodium carboxymethyl cellulose, starch, and dextrin.

5. The mold flux for continuous casting of austenitic 304 stainless steel billets according to claim 2, characterized in that, The raw materials comprising the crystallizer protective slag, by mass percentage, include: The composition includes: pre-melted material + wollastonite 43.0%–48.0%, limestone 16.0%–21.0%, lithium carbonate 0.5%–1.5%, fluorite 3.0%–6.0%, sodium fluoride 6%–9%, ​​bentonite 3–9%, cryolite 3.0%–5.0%, chromium nitride 0.5%–0.8%, binder 2.0%–4.0%, manganese carbonate 3–5%, borax 0.5%–3%, and industrial aluminum ash 3–5%. The ratio of pre-melted material to wollastonite is 2 to 2.3:

1.

6. The application of a mold flux for continuous casting of austenitic 304 stainless steel square billets according to any one of claims 1 to 5 in the preparation of austenitic 304 stainless steel continuous casting square or rectangular billets.

7. The application according to claim 6, characterized in that, The cross-section of the austenitic 304 stainless steel continuously cast square or rectangular billet is (160~240)×(160~240)mm.

8. A method for preparing austenitic 304 stainless steel continuously cast square or rectangular billets, characterized in that, The crystallizer protective slag as described in any one of claims 1 to 5 is used for preparation, and the pulling speed is 1.3 to 1.6 m / min.