CN100443215C - Protective slag for ultralow carbon steel continuous casting - Google Patents

Abstract

The invention discloses a process method for improving the comprehensive performance of M42 high-speed steel. In terms of ingredients, silicon and cobalt are respectively controlled at 0.5%-0.55% and 7.9%-8.1% of the total weight of the ingredients, and at the same time nitrogen in the steel is The content is 250-800ppm; and a certain amount of composite deoxidizer and rare earth elements are respectively added before tapping and during tapping. Microalloying, deoxidation and inoculation treatment by the above means can improve the purity of M42 steel, purify grain boundaries, improve crystallization structure and refine carbides. The quality and performance of the M42 high-speed steel produced by this process are obviously improved. Even compared with imported materials, it is not inferior in main indicators such as quenching and tempering hardness value, red hardness, impurity element content and carbide particle size. Therefore, the service life of the cutting tool processed by the high-speed steel manufactured by this process can be significantly improved.

Description

Mold flux for continuous casting of ultra-low carbon steel

technical field

The invention relates to a mold flux for a steelmaking continuous casting crystallizer, in particular to a mold flux for continuous casting of ultra-low carbon steel whose carbon content is less than or equal to 50ppm.

Background technique

The continuous casting mold slag, which is uniformly mixed with the base material, flux and melting speed adjustment material, by chemical or mechanical methods, mainly plays a role in the continuous casting process: to protect the molten steel surface of the mold from air oxidation; to protect the molten steel surface Heat insulation and heat preservation; absorption of floating inclusions in molten steel; acting as a lubricant in the casting process; controlling the heat transfer from the casting slab to the mold. Among them, the base material (mainly containing CaO, SiO ₂ , Al ₂ O ₃ ) and flux (Na ₂ O, F ^- , B ₂ O ₃ , Li ₂ O) mainly adjust the melting temperature, viscosity, absorption of inclusions, and crystallization of mold flux. Performance and other physical parameters control the lubrication and uniform heat transfer function of the mold flux; while carbonaceous materials (carbon black, graphite, coke, etc.) control the melting model and melting speed of the mold flux. Therefore, carbonaceous materials also It can be called a melting rate regulator. It plays a very important role in preventing mold slag from crusting on the molten steel surface of the mold and ensuring that the molten slag flows into the gap between the billet and the mold wall evenly and stably. It can be said that the carbonaceous material is the typical compositional feature of continuous casting mold slag that is different from other metallurgical slags. The mechanism of carbonaceous materials controlling the melting process of mold flux is: on the one hand, the carbonaceous powder dispersed between the base material and the flux mixture powder isolates each mixture powder particle, hinders and delays the heating of the mixture The sintering reaction that may occur during the heating process can reduce or eliminate the slag rings that appear in the crystallizer due to over-developed sintering; on the other hand, carbonaceous materials with high melting points are mixed in the slag droplets of the mixture in the form of solid phase particles and skeletons , which slows down the rate at which slag droplets gather to form a slag layer. Carbonaceous materials can effectively control the melting process of mold slag through the combined effect of isolation and skeleton. However, due to the low saturation solubility of carbon in molten slag, there is often a layer of rich carbon with a thickness of about 0.1 to 2 mm between the mold slag slag layer and the unmelted sintered layer or semi-molten layer on the molten steel surface of the mold. carbon layer. Studies have shown that even if the mold flux product contains about 2% carbon, the carbon in the carbon-rich layer can still be as high as 5-25%. When this mold slag is used to cast ultra-low carbon steel with [C]≤50ppm, the carbon-rich layer is in contact with molten steel or the primary meniscus billet shell, which is likely to cause severe carbonation of the billet, reducing continuous casting productivity and steel product quality.

In order to solve the problem of mold slag adding carbon to the slab, a lot of research has been done, the main work includes:

(1) The mold slag is mixed with carbonaceous materials that are not easy to add carbon to the slab. For example, under the premise of controlling the melting characteristics of mold slag, choose carbonaceous materials with low ignition point, let the carbon burn out during the melting process of mold slag as much as possible, and reduce the carbon content of carbon-rich layer; use carbonaceous materials with large specific surface area Instead of carbonaceous materials with small specific surface area to reduce the total carbon content in slag, etc. However, so far, the better ones of this type of mold flux still lead to more than 10ppm of carburization on the surface of the slab when casting ultra-low carbon steel.

(2) Substitute carbonaceous materials with nitrides. For example, BN, Si ₃ N ₄ , MnN, Ca ₂ N and other nitrides are added to the mold flux to replace carbonaceous materials to control the melting speed of the mold flux. In particular, the crystal structure of BN is similar to that of graphite, and it is not easy to bond with the mold slag base material in a strong reducing atmosphere. However, nitrides are easily oxidized by air to form oxides and nitrogen. Oxides such as B ₂ O ₃ sharply reduce the melting temperature and viscosity of slag, and N ₂ is wrapped in slag to cause bubbling and expansion of the slag layer. Very detrimental to stable casting process. In order to suppress the occurrence of this phenomenon, it was proposed to add reducing agents such as Al and Si alloys to the mold flux. However, in actual production, during the heating and melting process of the mold powder, there are many opportunities for partial contact with air, and the effect of the above method is not obvious. Moreover, BN ultrafine powder is expensive, about 300,000-500,000 yuan/ton in my country. If 3% BN is added, the cost of mold flux will increase by 9,000-15,000 yuan/ton. Applied to industrial production.

(3) Substitute carbonaceous materials with carbides. For example, replacing carbon black with carbides such as SiC, WC, CaC ₂ , TiC, ZrC, Mo ₂ C or NbC in whole or in part can inhibit the carburization of carbon black to the slab and achieve the purpose of controlling the melting rate of mold slag . However, due to the interaction characteristics of these carbides and the mold slag base material, such as the strong wettability of SiC solid particles and slag, it is easy to increase the viscosity of the mold slag, which hinders the normal performance of other metallurgical properties of the mold slag.

Contents of the invention

The object of the present invention is to provide a mold slag which does not affect the metallurgical performance of the mold slag itself and can better prevent the carbonization of the continuous casting slab of ultra-low carbon steel.

The aim is to achieve such a mold flux for continuous casting of ultra-low carbon steel. The components (weight %) of the mold flux are: 1.41 carbonaceous material, 2.38 metal melting rate modifier, 4.67 MgO, 6.82 Na ₂ O, 4.76 F ^- , 3.89 Al ₂ O ₃ , 6.51 MnO, 1.69 Fe ₂ O ₃ , the rest are CaO and SiO ₂ ; among them, CaO/SiO ₂ is 1.02.

A further feature is that the carbonaceous material is carbon black.

A further feature is that the metal melting rate modifier is any one of Ca-Si alloy, Ca-Ba-Si alloy, Mn-Si alloy, Fe-Mn alloy, or the sum of any two, or any The sum of three, or the sum of four.

It is not difficult to see from the scheme that in the mold flux of the present invention, the component ratio of carbonaceous materials is very low (2% lower than the existing lower ratio), that is to say, compared with the prior art Therefore, the present invention directly reduces the source of carbon addition to the ultra-low carbon steel continuous casting slab from the aspect of the carbonaceous material content of the mold slag. In order not to reduce the reasonable amount of melting rate regulating materials, the present invention uses a metal melting rate regulating agent with the function of "skeleton isolation" that has never been used in this type of mold flux in the prior art to supplement the reduced carbonaceous materials. This melt rate regulator. At the same time, the present invention adds Fe ₂ O ₃ , MnO and other components to the mold flux to allow them to react with the carbon in the carbon-rich layer to further reduce its carbon content. Therefore, compared with the prior art, the present invention integrates the above-mentioned technical measures into the mold slag, thereby not only avoiding the carbon addition of the mold slag to the continuous casting slab, but also ensuring the coordination of other metallurgical functions of the mold slag. .

The present invention will be further described below in conjunction with the accompanying drawings.

Description of drawings

Figure 1 shows the effect of carbon black content in mold flux on the sintering index of mold flux and carburization of steel samples

Figure 2 shows the non-wetting state of metal melting rate modifier and mold slag slag at 1350 °C

Figure 3 shows the change of mold slag layer thickness with time in the continuous casting production process

In Figure 2, 1 is mold slag slag; 2 is metal melting rate regulator

Detailed ways

An ultra-low carbon steel continuous casting mold flux. The components (weight %) of the mold flux are: 0.5-1.6 carbonaceous material, 2-8 metal melting rate modifier, 2-7 MgO, 6-12 _Na2O , 2-8 ^F- , 2-7 Al ₂ O ₃ , 3-8 MnO, 1-3 Fe ₂ O ₃ , and the rest are CaO and SiO ₂ ; among them, CaO/SiO ₂ is 0.75-1.05.

In the mold flux of the present invention, CaO and _SiO2 are base materials, and the basicity of the mold flux is controlled by their ratio.

When casting ultra-low carbon electrical steel with high [Si] content, due to the weak heat transfer capacity of the slab shell, in order to avoid steel breakout accidents, the alkalinity of mold slag CaO/SiO ₂ is set at 0.75-0.85, which can ensure the slag film It is a glass body and has good lubricating properties for the billet;

When casting [Ti] or [Nb]-containing non-atomic space punching steel, the basicity CaO/SiO ₂ is 0.85-1.05, which is conducive to the absorption of Al 2 O 3 and Al ₂ O ₃ at the mold steel-slag interface. Inclusions such as TiO ₂ reduce the surface and subcutaneous defects of the slab, and at the same time stabilize the solidification temperature and viscosity of the slag to achieve stable high-speed continuous casting.

Al ₂ O ₃ and Fe ₂ O ₃ in the mold flux of the present invention are mainly brought into the mold flux by raw materials. According to the CaO-SiO ₂ -Al ₂ O ₃ ternary phase diagram, a certain content of Al ₂ O ₃ is conducive to stabilizing the physical and chemical properties of the mold flux. When casting steel with less Al ₂ O ₃ inclusions, the Al ₂ O 3 in the mold flux ₃ content can be controlled at (weight %) 4-7, and when casting steel types with more Al ₂ O ₃ inclusions, the content of Al ₂ O ₃ in mold flux should be controlled at (weight %) 2-4.

The main purpose of adding MgO within the content range to the mold flux of the present invention is to stabilize the melting temperature and viscosity of the mold flux. Because when the mold flux does not contain MgO, the inclusions containing CaO and MgO floating in the molten steel will reduce the viscosity of the mold flux and increase the melting temperature of the mold flux, so that the performance of the mold flux no longer matches the continuous casting process parameters and induces casting Billet defects; when the mold flux contains a certain amount of MgO, the influence of the above-mentioned inclusions on the performance of the mold flux is weakened, and the performance of the mold flux tends to be stable. However, too high MgO content is likely to promote the precipitation of high melting point components such as forsterite and deteriorate the lubricating properties of mold flux, so MgO is preferably 2-7 (wt%).

Na ₂ O and F ^- in the mold flux of the present invention are used to work together with CaO and SiO ₂ to control the melting temperature of the mold flux to be 1070-1150°C and the viscosity at 1300°C to be 0.10-0.35Pa·S. Because Na ₂ O itself has a low melting point (920°C), it can form a low-melting eutectic with CaO and SiO ₂ in the mold flux, thereby reducing the melting temperature of the mold flux, and F ^- breaks the complex by replacing the bridge oxygen site. The silicon oxide anion group structure makes the size of the slag micro-elements smaller and easier to migrate, thereby reducing the viscosity of the mold slag. When the casting speed determined by the continuous casting process is 0.65 ~ 1.4m/min, the melting temperature and viscosity of the mold slag are taken as the upper limit within the above range, and the _Na2O and F ^- contents are taken between the middle limit and the lower limit specified in the present invention. value, and when the casting speed determined by the continuous casting process is 1.4 to 2.0m/min, the melting temperature and viscosity of the mold slag are within the above-mentioned range and take the middle and lower limits, and the Na ₂ O and F ^- contents are within the middle and upper limits specified in the present invention value between.

In this embodiment, the carbonaceous material used is carbon black. It is mainly used to meet the most basic isolation between the mold powder matrix particles, so as to prevent excessive sintering between the mold powder particles and endanger the use characteristics of the mold powder. After melting the steel samples with a carbon content of 42ppm in the laboratory induction furnace, add mold slag with different carbon black content on the molten steel surface, and test the amount of carbon added to the steel sample and the sintered agglomerate of mold slag after holding for a certain period of time , the results are shown in Figure 1. From the results, it can be seen that when the carbon black content in the mold slag exceeds 1.7%, it is easy to cause carburization of the slab, but if the carbon black content is too low, the sintering degree of the mold slag is serious. When the sintering index is greater than 1.5, the mold slag Severe agglomeration cannot be used normally in the continuous casting mold. Therefore, the content of mold slag carbon black in the present invention takes a value (weight %) of 0.5-1.6.

In this specific embodiment, the metal melting rate modifier is any one of Ca-Si alloy, Ca-Ba-Si alloy, Mn-Si alloy, Fe-Mn alloy, or the sum of any two, Or the sum of any three, or the sum of four. The purpose of using the metal melting rate regulator is to replace some carbonaceous materials. The metal melting rate regulator exerts the function of "isolation skeleton" to control and stabilize the melting process of the mold slag, so that the melting characteristics of the mold slag can meet the requirements of the continuous casting process. The speed regulator does not contain carbon and therefore does not carburize the strand. After the metal melting rate regulator of the mold flux of the present invention is treated, it is non-wetting with the mold flux slag at 1250-1350°C (the wetting angle is greater than 90°, see Figure 2), which is the effective metal melting rate regulator. The root cause of controlling the flux melting process.

Further, in the metal melting rate regulator, the molar numbers of each component are equal. For example: the combination of Ca-Si alloy and Ca-Ba-Si alloy, then, in the combined metal melting rate regulator, the moles of Ca and Si are still equal to the moles of Ba.

In the mold flux of the present invention, MnO passes the following formula

MnO+C=Mn+CO

It reacts with C in the carbon-rich layer, and the thermodynamic reaction temperature is 1410°C, which is equivalent to the temperature of the carbon-rich layer between the slag layer and the semi-molten layer. That is to say, when the temperature is lower than 1410°C, MnO is not easy to react with the carbonaceous material in the mold flux, so that the carbonaceous material can fully play the role of "skeleton isolation" in the powder slag or granular slag layer and the sintered layer To adjust the melting process of mold slag, only when the temperature is higher than 1410°C, that is, in the carbon-rich layer and slag layer, MnO will oxidize C and consume it, avoiding the carbon-rich layer with high carbon content on the slab. . Moreover, because MnO is weak in oxidation, it will not contaminate molten steel and deteriorate the physical properties of mold slag after reacting with some residual metal melting rate modifiers.

Furthermore, the mold slag is pre-melted hollow granular slag with a particle size of 0.1-1.0mm. The specific production is similar to the existing ones, that is, after melting, condensing, crushing and grinding the materials such as base material and flux in the pre-melting furnace, they are mixed together with carbonaceous materials and metal melting rate modifiers in a rotating stirring device, and sprayed It is atomized and dried in the drying tower to make pre-melted hollow particle mold flux with a particle size of 0.1-1.0mm.

In view of the fact that those skilled in the art have done a lot of work in the research of ultra-low carbon steel continuous casting mold flux, and have accumulated a lot of rich experience in theory and practice. Therefore, after conscientiously reading this specific embodiment and the corresponding analysis thereof, one must be able to, according to other specific conditions, within the scope of the components proposed in the present invention and their corresponding proportions, (do several limited routines at most) Test) to specifically select a group of mold powder formulations that meet other conditions to realize the present invention and achieve the technical effects described in the present invention. Therefore, the following only cites a best verification embodiment.

This verification example is carried out in the slab continuous casting mold, and comparative example 1 and comparative example 2 are two kinds of existing mold fluxes.

In this embodiment, the ultra-low carbon steel with carbon content less than or equal to 50 ppm is cast, and the casting speed is 0.80-1.40 m/min. Take molten steel samples in the tundish and take steel samples at the corresponding positions of the slab at the corresponding time, analyze the carbon content in the steel samples, and subtract the carbon content of the tundish steel samples from the carbon content of the slab steel samples to obtain the slab carburization quantity. Table 1 shows the chemical composition of the mold flux of the embodiment, comparative example 1 and comparative example 2. Fig. 3 shows the thickness of the molten slag layer during casting of the mold slag of the present invention and the mold slag of Comparative Example 1 and Comparative Example 2. It can be seen from Figure 3 that after the mold slag of Comparative Example 1 is added to the mold, the liquid slag layer increases continuously with the prolongation of the casting time. Not only does the mold slag have poor thermal insulation on the molten steel surface, but also an excessively thick liquid slag layer easily affects the liquid level of the mold. The steel breakout accident was induced due to the accurate control of the furnace, so the slag was forced to be removed at 45 minutes (about the casting time of one furnace of steel), so that the liquid level was seriously disturbed, and the carbon increase of the slab was also serious, which was 7-12ppm; in comparative example 2, due to Contains relatively high carbonaceous materials, and the thickness of the liquid slag layer is stable during the casting process, but the thickness of the liquid slag layer is 5-9mm, which is poorly adaptable to the high-speed continuous casting process, and the non-cleaning rate on the surface of the slab is 86.58%. , and the slab carbon increase is more, which is 8-15ppm; while the mold slag of the present invention not only has a stable melting process, but also has a stable slag layer thickness of 9-12mm in multi-furnace continuous casting, which meets the continuous casting process with high casting speed Conditions, the non-cleaning rate of the slab surface reaches 97.2%, the amount of carbon added is 2-6ppm, with an average of 4.6ppm, and all the slabs are used for red delivery.

Table 1 Example and Comparative Example Mold Flux Chemical Composition

Claims (7)

1, a kind of protective slag for continuous super low carbon steel casting is characterized in that, the component of this covering slag (weight %) is: 1.41 carbonaceous material, 2.38 metal melting rate modifier, 4.67 MgO, 6.82 Na ₂O, 4.76 F ^-, 3.89 Al ₂O ₃, 6.51 MnO, 1.69 Fe ₂O ₃, all the other are CaO and SiO ₂Wherein, CaO/SiO ₂Be 1.02.

2, protective slag for continuous super low carbon steel casting according to claim 1 is characterized in that, described carbonaceous material is a carbon black.

3, protective slag for continuous super low carbon steel casting according to claim 1 and 2; it is characterized in that described metal melting rate modifier is any or any two sum or wantonly three kinds of sums or the four kinds of sums in Ca-Si alloy, Ca-Ba-Si alloy, Mn-Si alloy, the Fe-Mn alloy.

According to the protective slag for continuous super low carbon steel casting described in claim 1 or 2, it is characterized in that 4, this covering slag is a pre-melted type hollow bead slag, its granularity is 0.1～1.0mm.

According to the protective slag for continuous super low carbon steel casting described in the claim 3, it is characterized in that 5, this covering slag is a pre-melted type hollow bead slag, its granularity is 0.1～1.0mm.

According to the protective slag for continuous super low carbon steel casting described in the claim 3, it is characterized in that 6, in described metal melting rate modifier, the molal quantity of each component equates.

According to the protective slag for continuous super low carbon steel casting described in the claim 5, it is characterized in that 7, in described metal melting rate modifier, the molal quantity of each component equates.

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