RU2847558C2 - Protective slag for continuous casting containing kolemanite and its use - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: the invention relates to metallurgy and can be used in continuous casting. A protective slag for continuous casting is proposed, with a melting point of 1100-1130 °° C, wherein the composition of the starting material of the protective slag contains 1-25% by weight of colemanite. Colemanite is used in protective slag instead of sodium tetraborate, but colemanite can be added to the slag in larger quantities, which prevents the protective slag from loosening and dust contamination. In addition, colemanite introduces boron oxide into the protective slag, which lowers the melting point of the protective slag, cleans the slag film, reduces the viscosity of the protective slag, and ensures its necessary fluidity.

EFFECT: the protective slag acquires lubricating properties, allowing high-quality cast slabs to be obtained without the use of expensive materials.

9 cl, 1 tbl, 6 ex

Description

The invention relates to the field of metallurgy and can be used in continuous casting.

Cross-reference to related applications

[0001] The present invention claims priority over Chinese Patent Application No. CN 202211602664.6 filed with the Chinese Patent Office on December 13, 2022, entitled “Protective Slag for Continuous Casting Containing Colemanite and Its Use,” the contents of which are incorporated herein by reference in their entirety.

Field of technology to which the present invention relates

[0002] The present invention relates to the field of protective slag technology, in particular to a protective slag for continuous casting containing colemanite and to the use thereof.

Prior art of the present invention

[0003] In continuous casting protective slag, the added boron-containing material is typically sodium tetraborate. However, the excessively high proportion of crystal water contained in sodium tetraborate leads to significant water loss and to loosening to form powder during the process of converting the aqueous solution into solid particles in the protective slag granulation tower. This leads to a high dust content, thus limiting the amount that can be added.

[0004] With the gradual implementation of Tier VI vehicle emission standards in China, stricter requirements are being placed on the automotive industry for energy conservation, emission reduction, and lightweight vehicle body structures. Achieving lightweight vehicle design requires research and development of lightweight materials. The integration of high-strength steel, aluminum alloys, and various lightweight materials represents a future direction for automotive material development. A new generation of high-strength steel, which combines ultra-high strength, high formability, and low density, is a crucial R&D focus for steel companies worldwide.

[0005] Ansteel Group successfully produces lightweight dual-phase steel with all the properties required for structural applications. Its elongation coefficient significantly exceeds that of traditional dual-phase steel of the 980 MPa strength class, while simultaneously reducing the density by 5%. The global debut of lightweight dual-phase steel represents Ansteel Group's leading position in the development of high-strength lightweight automotive steel.

[0006] However, in the production process of this type of steel at Ansteel Group enterprises, the existing protective slag of the continuous casting mold often leads to alarm problems in the mold expert system during the production process, where the degree of longitudinal cracking and transverse indentation for continuously cast plates is high, up to 60%, which thus leads to the inability to meet production requirements. Therefore, there is an urgent need for development in order to obtain a protective slag that can meet the production requirements of this type of steel.

Brief summary of the present invention

[0007] According to the present invention, a protective slag for continuous casting containing colemanite is proposed. The starting materials contain colemanite, wherein the weight content of colemanite in the starting materials is 1-25%.

[0008] The mass content of colemanite in the starting materials is optionally 8-20%.

[0009] Optionally, in parts by weight, the raw materials for the protective slag contain 11-16 parts of fluorite, 2-5 parts of imported carbon black, 3-6 parts of amorphous graphite, 58-65 parts of wollastonite, 5-9 parts of calcite, 0.5-3 parts of calcined magnesite sand, 1-4 parts of bauxite, 1-4 parts of lithium carbonate, 8-20 parts of colemanite, and 1-3 parts of polyvinyl alcohol.

[0010] Optionally, the protective slag has the following mass percentage content of chemical components: CaO 35.0-39.0 wt. %, SiO ₂ 29.5-33.5 wt. %, Al ₂ O ₃ 3-5 wt. %, Fe ₂ O ₃ 0.3-1.5 wt. %, MgO 1.5-3 wt. %, Na ₂ O 0-0.5 wt. %, F 5-8.5 wt. %, Li ₂ O 0.8-1.5 wt. %, B ₂ O ₃ 4-10 wt. %, C 3-6 wt. % and inevitable impurities.

[0011] Optionally, the protective slag has the following mass percentage content of chemical components: CaO 36.0-39.0 wt. %, SiO ₂ 30.5-33.0 wt. %, Al ₂ O ₃ 3-4 wt. %, Fe ₂ O ₃ 0.3-0.7 wt. %, MgO 1.5-2 wt. %, Na ₂ O 0-0.3 wt. %, F 5-6.5 wt. %, Li ₂ O 1.0-1.5 wt. %, B ₂ O ₃ 5-8 wt. %, C 3-5 wt. % and inevitable impurities.

[0012] The optional binary basicity is 1.13-1.18.

[0013] The melting point is optionally 1100-1130°C.

[0014] Optionally, the viscosity at a temperature of 1300°C is 0.11-0.14 Pa⋅s.

[0015] The present invention also provides the use of any of the above-mentioned continuous casting protective slag containing colemanite for continuous casting of high-strength light dual-phase steel.

[0016] Optionally, the casting speed of high-strength light dual-phase steel is 1-1.15 m/min.

[0017] Optionally, the main components of the high-strength light dual-phase steel contain C 0.16-0.17 wt.%, Si 1.2-1.5 wt.%, Mn 1.9-2.1 wt.% and Al 0.02-0.05 wt.%.

Detailed disclosure of embodiments of the present invention

[0018] In order to ensure a clear understanding of the problem, technical solution and advantages of the embodiments of the present invention, the technical solution according to the embodiments of the present invention will be described clearly and comprehensively. Unless otherwise provided in the specific conditions according to the embodiments, standard conditions or provisions recommended by the manufacturer are used. Reagents or devices that are not specified by the manufacturer are conventional products that can be obtained from commercial sources.

[0019] According to the present invention, a casting speed of 1-1.15 m/min is provided for a high-strength light dual-phase steel that contains the following main components.

[0020]

[0021] Based on the above composition, firstly, the steel composition has a high silicon content, which is 0.4% higher than the range of conventional carbon steel. The relatively high thermal resistance of the continuously cast plate shell results in the initially produced continuously cast plate shell being very thin, which is also the main reason for frequent alarms in the crystallizer expert system during the continuous casting process. Secondly, the manganese content in the molten steel is excessively high, being less than 1.6% higher than the conventional range. This indicates that the strength of the plate shell is very high. If the fluidity of the liquid slag is poor, this may result in low filling capacity of the protective slag in the channel between the plate shell and the copper plate. Uneven slag film thickness and uneven flow may be observed, which is the cause of longitudinal cracks. In addition, if the lubricating properties of the slag film of the protective slag are unsatisfactory, this may increase the friction between the cast plate and the crystallizer, further increasing the tendency for transverse cracking of the plate.

[0022] The present inventors conduct research on the quality characteristics of steel and the continuous casting process and develop performance indicators and a protective slag composition based on target considerations.

[0023] According to one embodiment of the present invention, a protective slag for continuous casting is provided that contains colemanite. The starting materials contain colemanite, wherein the weight content of colemanite in the starting materials is 1-25%.

[0024] According to the present invention, colemanite is used instead of sodium tetraborate. The use of colemanite allows for the addition of a larger amount to the protective slag, thereby preventing problems such as loosening of the protective slag and excessive dust formation, which easily causes dust pollution. In addition, an increased amount _{of colemanite in the protective slag leads to an increased content of B2O3. B2O3 effectively lowers the} melting point of the protective slag, cleans the slag film, and reduces the viscosity of the protective slag. It has good filling fluidity and good lubricating properties. Importantly, it can replace some expensive materials such as lithium carbonate. By providing the performance characteristics of the protective slag, it reduces the cost of the protective slag.

[0025] Optionally, the weight content of colemanite can be 4-23%, 6-22%, or 7-21%, such as 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, or 25%, or any intermediate values between any two of the mentioned end point values.

[0026] According to some optional embodiments of the present invention, the weight content of colemanite in the raw materials is 8-20%. According to some optional embodiments of the present invention, in parts by weight, the raw materials for the protective slag comprise 11-16 parts fluorite, 2-5 parts imported carbon black, 3-6 parts amorphous graphite, 58-65 parts wollastonite, 5-9 parts calcite, 0.5-3 parts calcined magnesite sand, 1-4 parts bauxite, 1-4 parts lithium carbonate, 8-20 parts colemanite and 1-3 parts polyvinyl alcohol.

[0027] During the manufacturing process of high-strength steel, due to the small gap between the plate shell and the copper plate and to the formation of a uniform initial plate shell, it is most important to avoid the formation of slag blocks that may affect the uniform flow of the liquid slag. Therefore, it is necessary to use the technology of introducing a composite carbon additive to ensure a three-layer structure during the melting of the protective slag. At the same time, to ensure the rapid melting of the protective slag, the added amount of carbon should not be excessively large. It is necessary to use imported carbon black with smaller particle sizes to maintain the appropriate skeletal function and regulate the melting rate.

[0028] To ensure the strength of the protective slag particles, a binder is required. However, to maintain lubrication efficiency, binders containing sodium compounds, such as sodium carboxymethyl cellulose and starch, cannot be selected. Therefore, in this solution, polyvinyl alcohol is specifically selected as the binder.

[0029] Optionally, the raw material of the protective slag composition contains the following components (parts by weight). Fluorite may constitute 11 parts, 12 parts, 13 parts, 14 parts, 15 parts, or 16 parts; imported carbon black may constitute 2 parts, 3 parts, 4 parts, or 5 parts; amorphous graphite may constitute 3 parts, 4 parts, 5 parts, or 6 parts; wollastonite may constitute 58 parts, 59 parts, 60 parts, 61 parts, 62 parts, 63 parts, 64 parts, or 65 parts; calcite may constitute 5 parts, 6 parts, 7 parts, 8 parts, or 9 parts; calcined magnesite sand may constitute 0.5 parts, 1.0 part, 1.5 parts, 2.0 parts, 2.5 parts, or 3 parts; bauxite may constitute 1 part, 2 parts, 3 parts, or 4 parts; lithium carbonate may constitute 1 part, 2 parts, 3 parts, or 4 parts; colemanite may constitute 8 parts, 10 parts, 12 parts, 14 parts, 16 parts, 18 parts, or 20 parts; polyvinyl alcohol may constitute 1 part, 2 parts, or 3 parts. The weight content of the above-mentioned raw materials may take intermediate values between any two of the above-mentioned end point values within the respective ranges, and the number of weight parts of the components in the raw material composition of the protective slag according to the present invention is not limited.

[0030] According to some optional embodiments of the present invention, the protective slag has the following weight percentage of chemical components: CaO 35.0-39.0 wt. %, SiO ₂ 29.5-33.5 wt. %, Al ₂ O ₃ 3-5 wt. %, Fe ₂ O ₃ 0.3-1.5 wt. %, MgO 1.5-3 wt. %, Na ₂ O 0-0.5 wt. %, F 5-8.5 wt. %, Li ₂ O 0.8-1.5 wt. %, B ₂ O ₃ 4-10 wt. %, C 3-6 wt. % and unavoidable impurities.

[0031] In order to ensure that the protective slag has good lubricating performance, special attention should be paid to preventing the introduction of the Na element when developing the composition of the protective slag. The Na element, together with calcium oxide, has a tendency to form melilite. Melilite is prone to crystallization, which has an adverse effect on the lubricating effect. Therefore, the Na element should not be introduced into the protective slag as much as possible. According to an embodiment, lithium carbonate is intended to be used in combination with colemanite in order to reduce the use of other sodium-containing raw materials and prevent the introduction of sodium ions into the protective slag.

[0032] Optionally, the protective slag has the following chemical composition (in weight percentage): CaO can be 35.0 wt. %, 36.0 wt. %, 37.0 wt. %, 38.0 wt. %, or 39.0 wt. %; SiO ₂ can be 29.5 wt. %, 30.0 wt. %, 30.5 wt. %, 31.0 wt. %, 31.5 wt. %, 32.0 wt. %, 32.5 wt. %, 33.0 wt. %, or 33.5 wt. %; Al ₂ O ₃ can be 3 wt. %, 3.5 wt. %, 4 wt. %, 4.5 wt. %, or 5 wt. %; Fe ₂ O ₃ can be 0.3 wt. %, 0.5 wt. %, 0.7 wt. %, 0.9 wt. %, 1.2 wt. %, or 1.5 wt. %; MgO may be 1.5 wt. %, 2.0 wt. %, 2.5 wt. %, or 3 wt. %; Na ₂ O may be 0.1 wt. %, 0.2 wt. %, 0.3 wt. %, 0.4 wt. %, or 0.5 wt. %; F may be 5 wt. %, 5.5 wt. %, 6.0 wt. %, 6.5 wt. %, 7.0 wt. %, 7.5 wt. %, 8.0 wt. %, or 8.5 wt. %; Li ₂ O may be 0.8 wt. %, 0.9 wt. %, 1.0 wt. %, 1.2 wt. %, 1.4 wt. %, or 1.5 wt. %; B ₂ O ₃ may be 4 wt. %, 5 wt. %, 6 wt. %, 7 wt. %, 8 wt. %, 9 wt. %, or 10 wt. %; and C may be 3 wt. %, 4 wt. %, 5 wt. %, 6 wt. %, and inevitable impurities may also be present. The weight percentage of the above-listed starting materials may take intermediate values between any two of the end point values presented above within the respective ranges, and the weight percentage of the starting materials in the protective slag compositions according to the present invention is not limited to the presented values.

[0033] According to some optional embodiments of the present invention, the protective slag has the following weight percentage of chemical components: CaO 36.0-39.0 wt. %, SiO ₂ 30.5-33.0 wt. %, Al ₂ O ₃ 3-4 wt. %, Fe ₂ O ₃ 0.3-0.7 wt. %, MgO 1.5-2 wt. %, Na ₂ O 0-0.3 wt. %, F 5-6.5 wt. %, Li ₂ O 1.0-1.5 wt. %, B ₂ O ₃ 5-8 wt. %, C 3-5 wt. % and unavoidable impurities. According to some optional embodiments of the present invention, the binary basicity is 1.13-1.18.

[0034] Optionally, the binary basicity may be 1.13, 1.14, 1.15, 1.16, 1.17, or 1.18, or an intermediate value between any two of the above endpoint values.

[0035] According to an embodiment, the binary basicity is reduced, and after the basicity is reduced, the formation of slag film crystals is reduced, and the proportion of the glassy phase increases. It can play a role in lubrication during plate casting. At the same time, a decrease in basicity simplifies heat transfer in the plate shell, thereby ensuring accelerated formation of the initial plate shell during continuous casting and preventing alarms in the crystallizer expert system. However, considering the susceptibility of this steel grade to longitudinal cracking defects, if the basicity is set at an excessively low level, the cast plate exhibits a tendency to longitudinal cracking defects. On the other hand, if the basicity is excessively high, the thickness of the hard slag film increases, which increases the tendency to initiate alarms. Using laboratory analysis, the basicity may optionally be set in the range of 1.13-1.18.

[0036] According to some optional embodiments of the present invention, the melting point is 1100-1130°C.

[0037] According to an embodiment, the melting temperature is reduced in order to enhance the rapid melting ability, increase the consumption of protective slag per ton of steel, and prevent alarms in the crystallizer expert system. In order to reduce the melting temperature, lithium oxide and colemanite are added to the protective slag. The conventional method for reducing the melting temperature is to add materials such as technical grade sodium carbonate, sodium fluoride, cryolite, etc. However, these materials contain the Na element, and after the protective slag melts, substances with a high melting point such as nepheline and melilite containing calcium and aluminum are easily formed during the crystallization process. This weakens the lubricating effect of the protective slag and triggers alarms in the crystallizer expert system. However, lithium oxide has an excellent melting temperature reduction ability and can improve the morphology of the slag film and reduce the grain size. In its liquid state, the protective slag can form smaller and more uniform crystals during the cooling process, resulting in improved control of uniform heat transfer within the protective slag. This helps prevent cracking and longitudinal cracking defects during continuous casting. However, lithium carbonate is expensive. Using lithium carbonate for large quantities of protective slag will inevitably lead to high costs, causing the product to lose its market competitiveness. Cholemanite is a mineral material that differs from sodium tetraborate. Sodium tetraborate has a low melting point but tends to form white beads under the high-temperature conditions of a granulation tower. Choosing colemanite results in a high effective yield of boron oxide, and boron oxide has improved melting point reduction compared to lithium oxide. Furthermore, colemanite has a cost advantage over sodium tetraborate and lithium carbonate. Thus, lithium carbonate is intended for use in combination with colemanite, resulting in the regulation of the composition of the protective slag, which contains 0.8-1.5 wt.% _Li2O and 4-10 _wt .% _B2O3 and provides a melting temperature in the range of 1100-1130°C.

[0038] Optionally, the melting point may be 1100°C, 1150°C, 1200°C, 1250°C, 1130°C, or an intermediate value between any two of the above end point values.

[0039] According to some optional embodiments of the present invention, the viscosity at 1300°C is 0.11-0.14 Pa⋅s.

[0040] According to the embodiment, the viscosity is reduced. The plate shell made of graded steel obtained in the continuous casting process has high strength, and the gap between the plate shell and the copper plate is small. Therefore, the plate shell experiences greater friction with the copper plate, which requires improving the lubricity of the protective slag. Considering that the casting speed in the continuous casting process of the plate is only 1-1.15 m/min, the viscosity cannot be excessively low. If it is excessively low, the consumption of liquid slag will be excessively high, leading to the phenomenon of non-uniformity during the formation of a hard slag film and increasing the likelihood of longitudinal and transverse cracking defects. Therefore, the viscosity should be in the range of 0.11-0.14 Pa⋅s.

[0041] Optionally, the viscosity at 1300°C may be 0.11 Pa⋅s, 0.12 Pa⋅s, 0.13 Pa⋅s, 0.14 Pa⋅s, or an intermediate value between any two of the above end point values.

[0042] According to some optional embodiments of the present invention, the degree of recrystallization is less than 1%.

[0043] Optionally, the degree of recrystallization may be 0%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, etc.

[0044] Embodiments of the present invention also provide for the use of any of the above embodiments of a continuous casting protective slag containing colemanite for the purpose of continuously casting high-strength lightweight dual-phase steel.

[0045] According to the present invention, colemanite is used instead of sodium tetraborate. The use of colemanite allows for the addition of a larger amount to the protective slag, thereby preventing problems such as loosening of the protective slag and excessive dust formation, which easily causes dust pollution. In addition, an increased amount _{of colemanite in the protective slag leads to an increased content of B2O3. B2O3 effectively lowers the} melting point of the protective slag, cleans the slag film, and reduces the viscosity of the protective slag. It has good filling fluidity and good lubricating properties. Importantly, it can replace some expensive materials such as lithium carbonate. By providing the performance characteristics of the protective slag, it reduces the cost of the protective slag.

[0046] Examples

[0047] The features and operational characteristics of the present invention are now described in detail with the provision of examples.

[0048] Example 1

[0049] The protective slag for continuous casting of high-strength light dual-phase steel contains fluorite powder, imported carbon black, amorphous graphite powder, wollastonite powder, calcite powder, calcined magnesite sand powder, bauxite powder, lithium carbonate, colemanite powder and polyvinyl alcohol, the mass percentage of which was 13%, 2%, 5%, 55%, 6%, 1%, 3%, 3%, 10%, and 2%, respectively.

[0050] A protective slag for continuous casting of high-strength light dual-phase steel was produced, having the following composition and the exact amount of each ingredient: CaO: 37.2%; SiO ₂ : 31.7%; Al ₂ O ₃ : 3.5%; Fe ₂ O ₃ : 0.6%; MgO: 1.9%; Na ₂ O: 0.0%; F: 5.8%; Li ₂ O: 1.2%; B ₂ O ₃ : 5%; C: 5%, wherein the binary basicity (weight percentage of CaO/SiO ₂ ) was 1.17; the melting point was 1110°C; the viscosity was 0.12 Pa⋅s at a temperature of 1300°C; and the recrystallization degree was 0%.

[0051] The experiment was conducted at an Ansteel group plant. First, the last two heatings for the study were carried out, then the last half of the casting sequence was studied, and finally, the full casting sequence was studied. The protective slag used in the experiment was type A. The steel grade cast during the experiment was DP980, with a cross-section of 230 × 1250, and a casting speed of 1.1 m/min. The experiment was conducted in three casting sequences. During the experiment, the type A protective slag for continuous casting exhibited good spreading and fluidity in the mold, with the total slag layer thickness being 50-70 mm, and the liquid slag thickness being 10-13 mm. Effective insulation of the molten steel from the powdered slag layer was ensured. The flame on the mold surface was moderate, and the slag consumption was moderate, ranging from 0.5 to 0.55 kg/t. This ensured good lubrication. Minimal slag block formation was observed during the experiment, and no alarms were generated in the mold. The surfaces of the cast plates were smooth and free of any defects, thus achieving the experimental objective.

[0052] Example 2

[0053] The protective slag for continuous casting of high-strength light dual-phase steel contained fluorite powder, imported carbon black, amorphous graphite powder, wollastonite powder, calcite powder, calcined magnesite sand powder, bauxite powder, lithium carbonate, colemanite powder and polyvinyl alcohol, the mass percentages of which were 14%, 2.5%, 3.5%, 52%, 5%, 1.5%, 3%, 2%, 15% and 1.5%, respectively.

[0054] A protective slag for continuous casting of high-strength light dual-phase steel was produced, having the following composition and the exact amount of each ingredient: CaO: 36.9%; SiO ₂ : 32.1%; Al ₂ O ₃ : 3.5%; Fe ₂ O ₃ : 0.5%; MgO: 1.6%; Na ₂ O: 0.2%; F: 6.2%; Li ₂ O: 0.9%; B ₂ O ₃ : 7.5%; C: 4%, wherein the binary basicity (weight percentage of CaO/SiO ₂ ) was 1.15; the melting point was 1125°C; the viscosity was 0.11 Pa⋅s at a temperature of 1300°C; and the recrystallization degree was 0%.

[0055] The experiment was conducted on an in-house setup. First, the last two heating runs were tested, then the last half of the casting sequence was tested, and finally, the entire casting sequence was tested. The protective slag used in the experiment was type B. The steel grade cast during the experiment was DP980, with a cross-section of 230×1300 and a casting speed of 1.1 m/min. The experiment was conducted in three casting sequences. During the experiment, type B continuous casting protective slag exhibited good spreading and fluidity in the mold, with the total slag layer thickness being 50-70 mm and the liquid slag thickness being 9-12 mm. Effective insulation of the molten steel from the powdered slag layer was ensured. The flame on the mold surface was moderate, and the slag consumption was moderate, ranging from 0.55 to 0.6 kg/t. This ensured good lubrication. Minimal slag block formation was observed during the experiment, and no alarms were generated in the mold. The surfaces of the cast plates were smooth and free of any defects, thus achieving the experimental objective.

[0056] Comparative Example 1

[0057] The protective slag for continuous casting of high-strength light dual-phase steel contained fluorite powder, imported carbon black, amorphous graphite powder, wollastonite powder, sodium carbonate, calcite powder, calcined magnesite sand powder, bauxite powder, lithium carbonate, colemanite powder and polyvinyl alcohol, the mass percentages of which were 14%, 2%, 4%, 51%, 4%, 8%, 1%, 3%, 3%, 8%, and 2%, respectively.

[0058] A protective slag for continuous casting of high-strength light dual-phase steel was prepared, having the following composition and the exact amount of each ingredient: CaO: 38.1%; SiO ₂ : 29.3%; Al ₂ O ₃ : 3.5%; Fe ₂ O ₃ : 0.6%; MgO: 1.9%; Na ₂ O: 2.4%;

F: 6.2%; _Li2O : 1.2%; _B2O3 : ₄ %; C: 5%, wherein the binary basicity (weight percentage of CaO/ _SiO2 ) was 1.3; the melting point was 1140°C; the viscosity was 0.13 Pa⋅s at 1300°C; and the recrystallization degree was 30%.

[0059] The experiment was conducted at an Ansteel Group enterprise. First, the last four heatings for the study were carried out. The protective slag used in the experiment was type C. The steel grade cast during the experiment was DP980, with a cross-section of 230×1250, and a casting speed of 1.0 m/min. During the experiment, type C continuous casting protective slag showed good spreading and fluidity in the mold, with the total slag layer thickness being 50-70 mm and the liquid slag thickness being 8-11 mm. Effective insulation of the molten steel from the powdered slag layer was ensured. The flame on the mold surface was moderate, and the slag consumption was at a moderate level, ranging from 0.5 to 0.53 kg/t. This ensured good lubrication effects. During the experiment, slag blocks formed rapidly, triggering three alarms. The surfaces of the cast plates exhibited adhesion-type vibration marks due to a reduction in casting speed to 0.1 m/min associated with the alarms. The experimental results were less satisfactory than those for types A/B. The main reason for this was that the addition of sodium carbonate increased the melilite content of the liquid slag film, achieving high basicity and recrystallization, thus ensuring rapid slag block formation.

[0060] Comparative example 2

[0061] The protective slag for continuous casting of high-strength light dual-phase steel contained fluorite powder, imported carbon black, amorphous graphite powder, wollastonite powder, calcite powder, calcined magnesite sand powder, bauxite powder, lithium carbonate, colemanite powder, and polyvinyl alcohol, the mass percentages of which were 15%, 3%, 3%, 42%, 2%, 2%, 3%, 4%, 18%, and 3%, respectively.

[0062] A protective slag for continuous casting of high-strength light dual-phase steel was produced, having the following composition and the exact amount of each ingredient: CaO: 36.1%; SiO ₂ : 33.8%; Al ₂ O ₃ : 3.9%; Fe ₂ O ₃ : 0.7%; MgO: 2.6%; Na ₂ O: 0.0%; F: 6.5%; Li ₂ O: 1.9%; B ₂ O ₃ : 9%; C: 4.5%, wherein the binary basicity (weight percentage of CaO/SiO ₂ ) was 1.07; the melting point was 1070°C; the viscosity was 0.09 Pa⋅s at a temperature of 1300°C; and the recrystallization degree was 0%.

[0063] The experiment was conducted at the Ansteel Group plant in the last four heatings for the study. The protective slag used in the experiment was type E. The steel grade cast during the experiment was DP980, with a cross-section of 230 × 1250, and a casting speed of 1.15 m/min. During the experiment, type E continuous casting protective slag showed good spreading and fluidity in the mold, with the total slag layer thickness being 50-70 mm and the liquid slag thickness being 6-8 mm. The liquid slag layer was relatively thin, and the flame on the mold surface was at a higher level. The slag consumption was at a moderately average level, ranging from 0.55 to 0.6 kg/t, although on the higher side. During the experiment, a smaller number of slag blocks were observed, and no alarms were generated in the mold. However, inspection of the cast plates revealed that the surface was not smooth, exhibiting deep vibration marks, and the degree of longitudinal and transverse cracking _was high, reaching 10%. The experimental results were not entirely satisfactory. The main reason for this was the relatively large addition of _Li2O and _B2O3 , which resulted in lower melting point and viscosity values, thereby increasing liquid slag consumption.

[0064] Comparative Example 3

[0065] According to the specified composition of the protective slag components, the protective slag contained fluorite powder, imported carbon black, sodium carbonate, amorphous graphite powder, wollastonite powder, calcite powder, calcined magnesite sand powder, bauxite powder and sodium carboxymethyl cellulose. The mass percentage content of these components was 15%, 2%, 7%, 4%, 53%, 13%, 1%, 3%, and 2%, respectively.

[0066] The slag had the following composition and the exact amount of each ingredient: CaO: 39.5%; SiO ₂ : 28.3%; Al ₂ O ₃ : 4.3%; Fe ₂ O ₃ : 0.6%; MgO: 1.7%; Na ₂ O: 4.5%; F: 6.7%; C: 5%, wherein the binary basicity (weight percentage CaO/SiO ₂ ) was 1.38; the melting point was 1150°C; the viscosity was 0.10 Pa s at a temperature of 1300°C; and the degree of recrystallization was 100%.

[0067] The experiment was conducted at the Ansteel Group plant in the last four heatings for the study. The protective slag used in the experiment was type E. The steel grade cast during the experiment was DP980, with a cross-section of 230×1250, and a casting speed of 1.1 m/min. During the experiment, type E protective slag for continuous casting showed good spreading and fluidity in the mold, with the total slag layer thickness being 50-70 mm and the liquid slag thickness being 12-15 mm. Effective insulation of the molten steel from the powdered slag layer was ensured. The flame on the surface of the mold was moderate, and the slag consumption was at a moderate level, ranging from 0.38 to 0.44 kg/t. This ensured good lubrication effects. During the experiment, slag blocks formed rapidly, and alarms frequently occurred in the mold. The casting surface of the studied casting plates exhibited vibrational marks of an adhesive type due to a reduction in casting speed to 0.1 m/min associated with the alarms, resulting in unsatisfactory experimental results. The primary cause was high alkalinity, which led to the rapid formation of slag blocks. Liquid slag could not be consumed through the gap between the mold and the copper plate, resulting in a high degree of recrystallization and poor lubrication.

[0068] Comparative Example 4

[0069] The protective slag for continuous casting obtained in the comparative example contained fluorite powder, imported carbon black, amorphous graphite powder, wollastonite powder, calcite powder, calcined magnesite sand powder, bauxite powder, lithium carbonate, colemanite powder, polyvinyl alcohol, the weight percentage of which was 12%, 3%, 3%, 64.5%, 6%, 3%, 4%, 1.5%, 2%, and 1%, respectively.

[0070] The slag had the following composition and the exact amount of each ingredient: CaO: 36.9%; SiO ₂ : 33.6%; Al ₂ O ₃ : 4.2%; Fe ₂ O ₃ : 0.7%; MgO: 2.1%; Na ₂ O: 0.0%; F: 5.5%; Li ₂ O: 0.5%; B ₂ O ₃ : 2%; C: 4.5%, wherein the binary basicity (weight percentage CaO/SiO ₂ ) was 1.1; the melting point was 1140°C; the viscosity was 0.13 Pa⋅s at a temperature of 1300°C; and the degree of recrystallization was 40%.

[0071] The experiment was conducted at the Ansteel Group plant in the last four heatings for the study. The protective slag used in the experiment was type F. The steel grade cast during the experiment was DP980, with a cross-section of 230×1250, and a casting speed of 1.0 m/min. During the experiment, type F continuous casting protective slag showed good spreading and fluidity in the mold, with the total slag layer thickness being 50-70 mm and the liquid slag thickness being 12-15 mm. Effective insulation of the molten steel from the powdered slag layer was ensured. The flame on the mold surface was moderate, and the slag consumption was at an average level, ranging from 0.42 to 0.50 kg/t. However, during the experiment, larger slag blocks formed, triggering alarms in the crystallizer's expert system. The longitudinal cracking rate was high, reaching 30%, and unsatisfactory experimental results were obtained.

[0072] Based on this, it can be seen that the protective slag for continuous casting of high-strength dual-phase light steel proposed according to the embodiments of the present invention is superior in all respects to the protective slag for continuous casting of high-strength dual-phase light steel in the comparative example in its performance characteristics. This shows that the protective slag for continuous casting of high-strength dual-phase light steel proposed according to the embodiments of the present invention can effectively ensure the smooth casting process of high-strength dual-phase light steel and prevent cracking defects.

[0073] Thus, the use of the protective slag proposed according to the present invention for continuous casting of high-strength light dual-phase steel effectively prevents longitudinal and transverse cracking defects that occur during the continuous casting process of high-strength light dual-phase steel. These defects occur due to the thin initial shell of the steel plate, the high strength of the shell of the plate, and significant friction between the shell of the plate and the copper plate.

[0074] The above is only an optional embodiment of the present invention, which is not intended to limit the present invention, and various changes and variations can be made in the present invention by those skilled in the art. Any modifications, equivalent replacements, improvements, etc. made within the idea and principles of the present invention should be included in the scope of legal protection of the present invention.

Practical industrial applicability

[0075] The present invention provides a protective slag for continuous casting containing colemanite and its use. According to the present invention, colemanite is used instead of sodium tetraborate. The use of colemanite allows for the addition of a larger amount to the protective slag, thereby preventing problems such as loosening of the protective slag and excessive dust formation, which easily causes dust pollution. In addition, an increased amount of colemanite in the protective slag leads to an increased content of B ₂ O ₃ . B ₂ O ₃ effectively lowers the melting point of the protective slag, cleans the slag film and reduces the viscosity of the protective slag. It has good filling fluidity and good lubricating properties. Importantly, it can replace some expensive materials. By providing the performance characteristics of the protective slag, it reduces the cost of the protective slag. Thus, it has excellent practical characteristics and very diverse application prospects.

Claims (13)

1. Protective slag for continuous casting, wherein the protective slag has the following component content, wt. %: Sao 35.0-39.0 SiO ₂ 29.5-33.5 Al ₂ O ₃ 3-5 Fe ₂ O ₃ 0.3-1.5 MgO 1.5-3 Na ₂ O up to 0.5 F 5-8.5 Li ₂ O 0.8-1.5 In ₂ O ₃ 4-10 WITH 3-6 inevitable impurities, the melting point of the protective slag is 1100-1130°C, and the content of colemanite in the composition of the starting material of the protective slag is 1-25 wt.%. 2. A protective slag for continuous casting according to claim 1, wherein the content of colemanite in the composition of the starting material of the protective slag is 8-20 wt.%. 3. A protective slag for continuous casting according to item 1 or 2, wherein the protective slag has the following component content, wt. %: Sao 36.0-39.0 SiO ₂ 30.5-33.0 Al ₂ O ₃ 3-4 Fe ₂ O ₃ 0.3-0.7 MgO 1.5-2 Na ₂ O up to 0.3 F 5-6.5 Li ₂ O 1.0-1.5 B ₂ O ₃ 5-8 WITH 3-5 and inevitable impurities. 4. A protective slag for continuous casting according to claim 1 or 2, wherein the binary basicity is 1.13-1.18, wherein the binary basicity is the mass percentage ratio CaO/SiO ₂ . 5. A protective slag for continuous casting according to claim 1 or 2, in which the viscosity at a temperature of 1300°C is 0.11-0.14 Pa⋅s. 6. A protective slag for continuous casting according to claim 1 or 2, in which the degree of recrystallization is less than 1%. 7. The use of a protective slag for continuous casting according to any of paragraphs 1-6 as a protective slag in continuous casting of high-strength two-phase steel. 8. The use according to paragraph 7, wherein the casting speed of high-strength two-phase steel is 1-1.15 m/min. 9. The use according to claim 7 or 8, wherein the high-strength dual-phase steel contains the following main components, wt. %: WITH 0.16-0.17 Si 1.2-1.5 Mn 1.9-2.1 A1 0.02-0.05