阅读说明：本技术 一种低碳低合金钢用超高拉速薄板坯连铸保护渣 (Ultra-high pulling speed sheet billet continuous casting covering slag for low-carbon low-alloy steel ) 是由 朱立光 袁志鹏 肖鹏程 王杏娟 刘增勋 张洪波 高永春 张军国 单庆林 王彬 王 于 2020-04-23 设计创作，主要内容包括：本发明公开了一种低碳低合金钢用超高拉速薄板坯连铸保护渣,该保护渣的化学成分重量百分比为：CaO：28～30％,SiO<Sub>2</Sub>：24～26％,Al<Sub>2</Sub>O<Sub>3</Sub>：5～7％,MgO：5～7％,Na<Sub>2</Sub>O：7～9％,K<Sub>2</Sub>O：0～1％,CaF<Sub>2</Sub>：7～9％,Fe<Sub>2</Sub>O<Sub>3</Sub>：0～1％,Li<Sub>2</Sub>O：1～2％,C：5～7％,余量为不可避免的杂质。通过对该保护渣组分的调整,特别是通过调整助熔剂Li<Sub>2</Sub>O以及单质碳的含量,降低保护渣的熔点和转折温度,并且提高熔化速度,使其在连铸浇注过程中保持足够的液渣层厚度,同时改善其润滑性能,使保护渣的熔化速度和流入速度达到稳定的平衡状态,增大了保护渣的熔化区间,在提高保护渣流动性的同时改善传热,从而有效降低在浇注过程中裂纹和夹渣的发生率,对超高拉速下低碳低合金钢薄板坯生产稳定顺行及铸坯质量的提高提供保障。(The invention discloses a super-high pulling speed sheet billet continuous casting covering slag for low-carbon low-alloy steel, which comprises the following chemical components in percentage by weight: CaO: 28-30% of SiO 2 ：24～26％，Al 2 O 3 ：5～7％，MgO：5～7％，Na 2 O：7～9％，K 2 O：0～1％，CaF 2 ：7～9％，Fe 2 O 3 ：0～1％，Li 2 O: 1-2%, C: 5-7%, and the balance of inevitable impurities. By adjusting the composition of the mold flux, in particular by adjusting the Li content of the flux 2 The contents of O and simple substance carbon, the melting point and the transition temperature of the covering slag are reduced, the melting speed is increased, the enough thickness of a liquid slag layer is kept in the continuous casting and pouring process, the lubricating property of the covering slag is improved, the melting speed and the inflow speed of the covering slag reach a stable balance state, the melting interval of the covering slag is increased, the fluidity of the covering slag is improved, the heat transfer is improved, the occurrence rate of cracks and slag inclusion in the pouring process is effectively reduced, and the ultra-high tension is realizedThe stable and smooth production of the low-carbon low-alloy steel sheet billet at the high speed and the improvement of the quality of the casting blank provide guarantee.)

1. The ultrahigh-pulling-speed sheet billet continuous casting mold flux for the low-carbon low-alloy steel comprises the following chemical components in percentage by weight: CaO: 28-30% of SiO₂：24～26％，Al₂O₃：5～7％，MgO：5～7％，Na₂O：7～9％，K₂O：0～1％，CaF₂：7～9％，Fe₂O₃：0～1％，Li₂O: 1-2%, C: 5-7%, and the balance of inevitable impurities.

2. The ultra-high pulling speed thin slab continuous casting mold flux for the low-carbon low-alloy steel according to claim 1, wherein the mold flux comprises the following chemical components in percentage by weight: CaO: 28.5 to 29.5 percent of SiO₂：24.5～25.5％，Al₂O₃：6.1～6.8％，MgO：5.9～6.3％，Na₂O：7.8～8.6％，K₂O：0.1～0.3％，CaF₂：7.9～8.2％，Fe₂O₃：0.4～0.6％，Li₂O：1.1～1.3％，C：5.9～6.8％。

3. The ultra-high pulling speed thin slab continuous casting mold flux for low carbon low alloy steel according to claim 1 or 2, wherein the continuous casting pulling speed of the ultra-high pulling speed thin slab for low carbon low alloy steel is 5.0m/min to 6.5 m/min.

4. The ultra-high drawing speed thin slab continuous casting mold flux for low carbon low alloy steel according to claim 1 or 2, wherein the basicity of the mold flux is CaO/SiO₂The temperature is controlled within the range of 1.1-1.3.

5. The mold flux for ultra-high pulling speed thin slab continuous casting for low carbon low alloy steel according to claim 1 or 2, wherein the melting point is 1050 to 1070 ℃ and the viscosity at 1300 ℃ is 0.03 to 0.05 Pa-s.

Technical Field

The invention relates to the technical field of ferrous metallurgy, in particular to a metallurgical auxiliary material for continuous casting, and more particularly relates to continuous casting covering slag suitable for a sheet billet with ultrahigh casting speed, which is used in a low-carbon low-alloy steel continuous casting process.

Background

The continuous casting covering slag is a functional material which takes silicate as a base and contains a plurality of fluxes and framework materials. With the continuous improvement of the continuous casting speed and the use of electromagnetic braking equipment, the environment of the casting powder in the crystallizer and the expressed metallurgical behavior are greatly changed, and particularly the melting uniformity and the lubricating performance are difficult to ensure. In addition, due to the complicated structure of the funnel-type crystallizer, the crack sensitivity of the continuous casting billet is more and more prominent, so that the performances of controlling heat transfer of the casting powder and guaranteeing uniform growth of a primary billet shell need to be optimized at the same time.

In addition, along with the increase of the pulling speed, the molten steel updating speed at the liquid level of the crystallizer is accelerated, conditions are created for steel slag reaction and foreign material adsorption of the covering slag from the aspect of mechanics, and adverse effects are caused on the stability of the covering slag in the using process. The protective slag has excellent lubricating property and strong capability of controlling heat transfer so as to ensure that a blank shell and the wall of a crystallizer have small friction force and can grow uniformly and avoid cracks, thereby providing new and more complex technical requirements for the performance of the protective slag.

The electromagnetic braking can not only influence the flow field of the molten steel in the crystallizer, but also has great influence on the temperature field in the crystallizer, thereby further influencing a series of actions such as melting, crystallization, inflow, lubrication and the like of the casting powder.

Compared with a medium-thick plate blank and a square blank crystallizer, the difference of the surface temperature of the thin plate blank shell at different positions is larger. In addition, as the pulling rate increases, the amount of consumption of mold flux per unit time increases, and the vibration frequency increases accordingly, so that the inflow of mold flux into the mold tends to be difficult. Meanwhile, the flow velocity of molten steel in the crystallizer and the turbulence of a meniscus are intensified, so that molten mold flux on the liquid surface is easily drawn into the molten steel, thereby causing steel leakage and casting blank quality accidents. The introduction of the electromagnetic braking equipment can effectively reduce the flowing speed of the molten steel in the crystallizer, reduce the fluctuation of the liquid level of the crystallizer and further reduce the slag entrapment probability of the crystallizer. However, the electromagnetic braking causes the temperature field in the crystallizer to be greatly changed, thereby causing the melting and inflow uniformity of the casting powder to be reduced, the quality of the continuous casting billet to be damaged, and the incidence rate of cracks and slag inclusion to be increased. Therefore, various performances of the mold flux at different drawing speeds (particularly ultra-high drawing speeds) need to be systematically researched, and the applicability of the mold flux in the ultra-high drawing speed thin slab continuous casting process under electromagnetic braking needs to be comprehensively evaluated.

Despite the change in the equilibrium phase diagram of this steel grade after addition of alloying elements compared to the classical Fe-C phase diagram, it is still in the category of mild steels. The steel grade is mainly ferrite and ferrite in the solidification process, and has very little austenite transformation, so the steel grade is considered to have small volume shrinkage in the solidification process. For the casting process of the steel in the crystallizer, the problems of lubrication and friction between the blank shell and the copper wall need to be paid more attention.

Disclosure of Invention

The invention aims to provide the ultra-high pulling speed sheet billet continuous casting protective slag for the low-carbon low-alloy steel under the electromagnetic braking condition, which can ensure the thickness of a sufficient liquid slag layer under the conditions of ultra-high pulling speed, electromagnetic braking equipment and a funnel-shaped crystallizer, simultaneously improve the lubricating capability, improve the heat transfer, reduce the occurrence rate of casting blank cracks and slag inclusion and provide guarantee for stable and efficient production. In other words, the invention designs the ultrahigh-pulling-speed thin slab continuous casting covering slag for the low-carbon low-alloy steel under the electromagnetic braking condition on the basis of the original conventional continuous casting covering slag for the low-carbon low-alloy steel of the low-carbon thin slab with the pulling speed of less than 5.0m/min, thereby further improving the lubricating property and reducing the defects of casting blanks.

In order to achieve the aim, the ultrahigh-pulling-speed sheet billet continuous casting mold flux for the low-carbon low-alloy steel, which is designed by the invention, comprises the following chemical components in percentage by weight: CaO: 28-30% of SiO₂：24～26％，Al₂O₃：5～7％，MgO：5～7％，Na₂O：7～9％，K₂O：0～1％，CaF₂：7～9％，Fe₂O₃：0～1％，Li₂O: 1-2%, C: 5-7%, and the balance of inevitable impurities.

Further, the preferable chemical components of the mold flux are as follows: CaO: 28.5 to 29.5 percent of SiO₂：24.5～25.5％，Al₂O₃：6.1～6.8％，MgO：5.9～6.3％，Na₂O：7.8～8.6％，K₂O：0.1～0.3％，CaF₂：7.9～8.2％，Fe₂O₃：0.4～0.6％，Li₂O：1.1～1.3％，C：5.9～6.8％。

Further, the basicity of the mold flux is preferably controlled to be in the range of 1.1 to 1.3, the melting point of the mold flux is preferably controlled to be in the range of 1050 to 1070 ℃, and the viscosity of the mold flux at 1300 ℃ is preferably controlled to be in the range of 0.03 to 0.05Pa · s.

The action mechanism and the limiting reason of various chemical components in the ultrahigh-pulling-speed sheet billet continuous casting protective slag for the low-carbon low-alloy steel are as follows:

CaO: is one of the main components of the mold powder, is related to crystallization temperature and belongs to an oxide outside a network. Therefore, the content of CaO in the covering slag is increased, the viscosity of the slag can be obviously reduced, and oxide inclusions, particularly Al, in steel can be absorbed₂O₃And TiO₂. In the invention, the alkalinity of the casting powder is controlled to be 1.1-1.3, and the casting powder is ensured to have good crystallization performance, so the weight percentage content of CaO is controlled to be 28-30%, preferably 28.5-29.5%.

SiO₂: belongs to a network former and improves SiO in the casting powder₂The content of the (B) can obviously improve the viscosity of the slag, and simultaneously, the covering slag can generate a glass phase, so that the casting blank is easy to lubricate. However, if too high, it is pointed out in patent CN101954464 that chain-like structures [ SiO ] are formed₃]_nThe viscosity of the casting powder is over high, and the lubricating effect is reduced; if too low, the glass phase of the mold flux is not formed, and the lubricating requirement of the continuous casting billet cannot be met. In the invention, the alkalinity of the casting powder is controlled to be 1.1-1.3, and the casting powder is ensured to have certain lubricating property, so that the SiO in the invention₂The content is controlled within the range of 24-26%, preferably within the range of 24.5-25.5%.

Al₂O₃: adding Al to the slag₂O₃The viscosity of the slag is increased, but it may lower the solidification point of the slag, thereby improving the crystallizer lubrication. However, a large amount of the calcium aluminate yellow stone (2 CaO. Al) enters the slag and is easy to form high melting point₂O₃·SiO₂) And nepheline (Na)₂O·Al₂O₃·2SiO₂) The lubrication action is deteriorated. In the invention, in order to keep lower viscosity and ensure that the casting powder has certain lubricating property, Al in the invention₂O₃The content is controlled within the range of 5-7%, preferably6.1-6.8%.

MgO: in the case of high-speed continuous casting, MgO is a preferable component of the mold flux, and the viscosity and melting point of the mold flux can be remarkably reduced. The addition of MgO can increase the fluidity of the slag and increase the slag consumption when the same viscosity and softening point of the protective slag are maintained. However, the amount of the additive is too high, which may deteriorate the melting property of the mold flux. In the invention, in order to improve the flow property of the low-carbon low-alloy steel, the MgO content is controlled within the range of 5-7%, preferably within the range of 5.9-6.3%.

Na₂O: belongs to a network exo-oxide, can destroy the network structure of silicate, plays a role in reducing the melting point and the viscosity in the casting powder, and can form nepheline (Na) when being added excessively₂O·Al₂O₃·2SiO₂) The amount of the catalyst added should be limited, which is disadvantageous for the crystallizer lubrication. In the invention, in order to ensure that the casting powder has certain lubricating property, Na is added₂The content of O is controlled to be 7-9%, preferably 7.8-8.6%.

CaF₂: the effect of increasing the content within the range of less than 10% on the viscosity reduction of the mold flux is large, and the effect is not obvious when the content is increased. The large amount of the additive can form the gunite (3 CaO. SiO)₂·CaF₂) And the like, thereby deteriorating the vitrifying property of the slag and deteriorating the lubricating conditions. In addition, F^-Too high can erode the nozzle. In the invention, CaF is added to make the casting powder have proper viscosity and certain glass property₂The content is controlled within the range of 7-9%, preferably 7.9-8.2%.

Li₂O: is a stronger fluxing agent even if Li in slag is added₂When the content of O is low, the melting temperature is also greatly affected. Li₂Addition of O in minute amounts (Li)₂O < 2%), improved vitrification of the mold powder, reduced crystallization rate, but excessive addition thereof (Li)₂O is more than 4 percent), but the vitrification degree of the melilite is greatly reduced because a large amount of melilite crystals are separated out. Thus, Li₂The proper amount of O is added to obtain the protective slag with low melting point, low viscosity and good glass property, and the proper amount is Li₂O is less than 2 percent. In the present invention, in order to suitably reduce the mold fluxMelting point and transition temperature, improving lubricating property of the protective slag, and adding Li₂The content of O is controlled to be 1-2%, preferably 1.1-1.3%.

C: in the physicochemical properties of the covering slag, the carbon elements mainly play a role in regulating the melting rate, and the carbon content is generally controlled within 10% in order to obtain higher melting rate by the composite carbon blending method generally adopted for preparing the covering slag at present. In the present invention, in order to form a molten slag layer having a sufficient thickness after the mold flux is introduced into the mold, the melting rate of the mold flux should be further increased so that the mold flux can better flow into the gap between the mold and the cast slab, the lubrication performance is improved, and the C content is controlled to be in the range of 5 to 7%, preferably 5.9 to 6.8%.

Alkalinity: the important indexes of the control heat transfer capability and the capability of absorbing inclusions in molten steel of the covering slag are reflected, and the quality of the lubricating property of the covering slag is also reflected. The alkalinity is too large, the capability of absorbing inclusions is also large, but the crystallization temperature is large, and the heat transfer and the lubricating performance are not facilitated; the alkalinity is too small, the viscosity of the casting powder is higher, and the lubricating effect is reduced. In the invention, in order to ensure that the viscosity of the casting powder is within a reasonable range and ensure that the casting powder has certain lubricating and heat transfer properties, the alkalinity of the casting powder is controlled within the range of 1.1-1.3.

Melting point: slag entrapment and casting blank defects are easily caused due to low temperature, and the slag consumption is increased, so that the casting blank forming is not facilitated; too high a content of the compound tends to solidify and crystallize, resulting in poor lubricity. According to the invention, the melting point of the casting powder is controlled to be 1050-1070 ℃ by adjusting the content of each component, so that the casting powder can form a liquid slag layer with enough thickness in the crystallizer, and the casting blank can be lubricated in the crystallizer in the whole process.

Viscosity (1300 ℃): is one of the main physical and chemical properties of the covering slag, represents the relative viscous force between molecular layers when the covering slag is melted, and is also one of important parameters of apparent covering slag lubricating property. When the viscosity is too high, the fluidity of the protective slag is deteriorated, and the lubricating effect is reduced; the low viscosity can increase the slag consumption, the casting blank is difficult to form, and the slag is easy to roll and clamp. According to the special process conditions of ultrahigh pulling speed and thin slab, the viscosity of the low-carbon low-alloy steel covering slag is controlled to be 0.03-0.05 Pa.s, so that slag entrapment can be effectively prevented, and good lubrication can be ensured.

According to the test conditions, the thickness of the liquid slag layer needs to be increased on the original basis, the lubricating property of the casting powder is improved, the melting point is reduced, and the melting interval is increased. Therefore, as described above, Li in the slag is appropriately increased₂The content of O reduces the melting point and improves the lubricating property of the casting powder; meanwhile, the carbon distribution amount is reduced, the melting speed is improved, and the thickness of a liquid slag layer after the casting powder is added into the crystallizer is increased, so that the requirement on the performance of the casting powder under the special condition is met.

Drawings

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the invention and not to limit the invention. In the drawings:

fig. 1A and 1B illustrate a schematic structural view of a crystallizer in an embodiment of the present invention;

fig. 2A to 2D are diagrams illustrating a comparison of surface sticking of a thin slab to which mold flux according to an embodiment of the present invention is applied and a thin slab to which mold flux according to an embodiment of the present invention is not applied;

fig. 3A to 3D are graphs illustrating the comparison of surface cracks of a thin slab to which the mold flux according to the embodiment of the present invention is applied and a thin slab to which the mold flux according to the embodiment of the present invention is not applied;

fig. 4A and 4B are photographs illustrating a comparison of red hot cast slab sticking of a thin slab to which mold flux according to an embodiment of the present invention is applied and a thin slab to which mold flux according to an embodiment of the present invention is not applied.

Detailed Description

The mold flux for ultra-high casting speed thin slab continuous casting of low-carbon low-alloy steel according to the present invention will be described in detail with reference to the accompanying drawings and specific examples.

[ high-speed thin slab continuous casting mold according to an embodiment of the present invention ]

A typical high-speed thin slab continuous casting mold is shown in fig. 1A and 1B. In order to ensure that the submerged nozzle has enough working space, the central part of the upper part of the crystallizer is provided with a funnel-shaped area which is completely different from the traditional slab crystallizer, so that the special complex inner cavity structure of the continuous casting thin slab crystallizer is formed.

The continuous casting process of the thin slab still faces multiple difficulties in industrial production, and mainly shows that the steel leakage risk is increased under the condition of ultrahigh pulling speed operation, longitudinal cracks on the surface of the continuous casting slab, the quality defect of slag inclusion of the casting slab is high, and the like. The fundamental reason is that complex, transient and multiphase metallurgical physical and chemical reactions (including crystallizer vibration, heat transfer, casting powder lubrication, molten steel flow, blank shell solidification and stress deformation and the like) occur in a funnel-type crystallizer of a thin slab caster, and the physical and chemical reactions are closely related to the performance characteristics of continuous casting powder.

With the increase of the drawing speed of the continuous casting machine, the superposition effect of multiple problems of heat transfer, molten steel flow, blank shell stress-strain and the like in the funnel-type crystallizer is further enhanced, and the dominance of the influence effect of the protective slag is more and more prominent.

Therefore, the important function of various properties of the casting powder in the continuous casting process is deeply understood, the synergistic action mechanism of different properties in the continuous casting process is accurately analyzed, the key factor for ensuring the high-speed smooth running of the sheet billet is realized, and the important basic condition for realizing the high-quality production of the sheet billet continuous casting is realized.

The continuous casting covering slag is a functional material which takes silicate as a base material and contains various fluxes and framework materials, and the performance design of the functional material is one of the most critical technologies in the continuous casting process. The performance of the casting powder in the crystallizer mainly has two difficulties: the contradiction of viscosity characteristics between the requirements of slag entrapment resistance and liquid slag inflow and the contradiction of slag film characteristics between blank shell lubrication and heat transfer control. The continuous casting has steel leakage accidents, surface cracks of casting blanks and slag inclusion defects, and most of the defects are closely related to the two pairs of contradictions.

[ mold flux according to example of the invention ]

Table 1 shows the chemical composition percentages by weight (the balance being unavoidable impurities) of examples 1 to 4 of the mold flux for ultra-high casting speed thin slab continuous casting for low-carbon low-alloy steel of the present invention.

Table 1:

examples 1 to 4 in the above Table 1 will be described in detail.