JP4389057B2 - Mold flux for continuous casting of steel - Google Patents

Description

  The present invention relates to a mold flux which is added and melted on molten steel in a mold in continuous casting of steel to perform a lubricating action between the mold and the cast piece.

  In general, in continuous casting of steel, molten steel insulation in the mold (mold), oxidation prevention of the molten steel, absorption of non-metallic inclusions floating from the molten steel, lubrication between the mold and the slab, heat removal control of the slab, etc. For the purpose, powder or granular mold flux is used, but mold flux for continuous casting plays an important role in stabilizing the operation and improving the quality of the slab, but it is caught in steel. It has a defect that tends to cause inclusion defects.

  Therefore, as the means for preventing defects caused by entrainment of molten mold flux into molten steel, as well as increasing needs for cleaning molten steel in strict quality materials such as automotive outer plates and deep drawing steel plates, increasing the viscosity of mold flux Has been oriented. However, it has been pointed out that simply increasing the viscosity tends to increase operational troubles such as breakout due to reduced lubricity and surface defects such as cracks due to non-uniform inflow. The high-viscosity mold flux is in a situation where it is difficult to apply it to actual operations because there are insufficient measures to prevent harmful effects associated with increasing the viscosity.

As countermeasures, many proposals have been made to set certain limits on the physical property values such as the solidification temperature in parallel with the increase in viscosity, but there are optimum values for the physical properties of the mold flux according to the steel type and operating conditions. For this reason, since the degree of freedom in selecting physical property values is reduced, the scope of application to steel types and operating conditions is restricted, and it cannot be a general-purpose measure. In JP-A-2002-113561, a coarse organic substance and / or a coarse carbonaceous substance that is larger than the average particle diameter of the main raw materials constituting the granules are blended, and formed into granules by a spray drying method. There is disclosed a granular mold flux excellent in heat retention effect, characterized by accumulating coarse organic substances and / or coarse carbonaceous substances inside the granules. However, if the viscosity is less than 8 poise, good slab quality can be obtained. However, if the viscosity becomes higher than 8 poise, cracks that may be caused by non-uniform inflow depending on the raw material composition and the amount of heat retained in the mold flux. Surface defects may occur, and it is difficult to apply to a high viscosity mold flux. The main reasons for this are that high viscosity requires consideration not only for the heat retention effect but also the raw material composition and homogeneity of the mold flux, and per unit time that the mold flux gives to the molten steel when the consumption unit decreases due to the increase in viscosity. It is conceivable that a larger amount of heat needs to be applied because the amount of heat is relatively reduced. In JP 2000-280051 A, a high-viscosity mold flux of 3 poise or more can be stably cast by setting the molten steel flow velocity near the meniscus to 8 cm / sec or more by electromagnetic stirring in the mold. This is an improvement of the negative effects of the operation by the operation method, and it is difficult to be a general-purpose measure applicable to all continuous casting machines.
JP 2002-113561 A JP 2000-280051 A

  The present invention was made in order to prevent adverse effects caused by high viscosity of the mold flux (operation troubles such as breakout due to reduced lubricity and occurrence of surface defects such as cracks due to uneven inflow) By proposing a mold flux that is excellent in homogenous meltability and excellent inflow properties even at a high viscosity of 8 poise or higher, it eliminates physical property selection and operational constraints, and improves good quality and productivity. The purpose is to contribute.

The present invention has been made to solve the above-described problems, and the gist thereof is as follows.
(1) Alumina (Al ₂ O ₃ ) having a viscosity at 1300 ° C. of 8 Poise or higher and a melting point of 1800 ° C. or higher in a mold flux used for continuous casting of steel for automobile outer plates and deep-drawn steel plates , Among magnesia (MgO), lime (CaO), zircon (ZrSiO ₄ ), and strontium oxide (SrO), the mass ratio of one or more high melting point raw materials is 8 mass in terms of oxide in the slag state. % Or less and 4 mass% or more, containing 3.0 to 4.2 mass% of fluorine (F), blending an organic substance and 1 mass% of a carbonaceous substance to give a calorific value of 200 cal / g or more. A mold flux for continuous casting of steel, characterized by being granulated.

By making the viscosity of the mold flux high (viscosity at 1300 ° C. is 8 poise or more), the mold flux is prevented from being caught in the molten steel. And the homogeneous meltability of molten slag is ensured by restrict | limiting the use ratio of the high melting-point raw material which has melting | fusing point of 1800 degreeC or more to a total of 8 mass% or less by the oxide conversion value in a slag state. In addition, by blending organic substance and 1% by mass carbonaceous substance so that the calorific value is 200 cal / g or more and granulate, uniform flowability between molten slag mold and solidified shell is ensured. Is done. As a result, while ensuring the advantages of increasing the viscosity of the mold flux, the adverse effects of increasing the viscosity, i.e., avoiding operational troubles and quality troubles due to reduced homogenous meltability and uniform inflow properties, and stable operation It has become possible to provide a new high-viscosity mold flux that can contribute to quality improvement.

  In continuous casting of steel, mold flux is added on the surface of molten steel injected into the mold. The mold flux added on the molten steel surface is melted once to form a slag pool layer, and then flows between the mold and the solidified shell and is consumed in order to maintain good slab quality. It is essential that the molten slag flows uniformly in a homogeneous state. Considering this, as a reason that a good result cannot be obtained by simply increasing the viscosity, these behavior characteristics, that is, the behavior characteristics of flowing uniformly in a uniform state are insufficient with a high viscosity mold flux. Presumed to be. That is, in a general mold flux manufactured by mixing different component raw materials, non-uniformity (micro component segregation or local melting temperature difference) due to the raw material remains in the slag immediately after melting. The higher the viscosity, the more difficult it is to homogenize the molten slag by flow and diffusion in the slag pool. Further, since the inflow amount decreases as the viscosity increases, it is considered that the inflow becomes non-uniform by being easily affected by the temperature drop or temperature change of the meniscus portion that is the inflow site. Therefore, the inventors obtained the following knowledge as a result of various investigations for maintaining and improving these behavioral characteristics, while focusing on homogeneous meltability and uniform inflowability, while developing high-viscosity mold fluxes. This completes the present invention.

First, regarding the homogenous meltability, it has been found that the viscosity of the slag immediately after melting is extremely unstable, the viscosity stabilization time of the molten slag tends to increase due to the increase in viscosity, and the quality of the slab deteriorates accordingly. Therefore, as a result of investigating the correspondence relationship between the viscosity stabilization time of the molten slag and the slab quality using a plurality of samples whose raw material composition and product viscosity were changed, alumina (Al ₂ O ₃₎ was used as a condition for ensuring homogeneous meltability. ), Magnesia (MgO), lime (CaO), zircon (ZrSiO ₄ ), strontium oxide (SrO), etc., the total use ratio of high melting point materials having a melting point of 1800 ° C. or higher in terms of oxides in the slag state It has been found that it is necessary to limit the amount to 8 % by mass or less.

Here, the reason for limiting to the high-melting-point raw material having a melting point of 1800 ° C. or higher is that all the different-component raw materials other than the base raw material cause a hindrance to homogeneous melting, but only the high-melting-point raw material of 1800 ° C. or higher is special. This is because the homogeneous meltability of the entire product can be almost evaluated by the total amount. The reason why the total use ratio of the high melting point raw material is limited to 8 % by mass or less is that, as shown in FIG. 2 showing the measurement result of the relationship between the use ratio of the high melting point raw material and the viscosity stabilization time, the high viscosity of 8 poise or more. This is because with mold flux, the viscosity stabilization time suddenly increases above 8 % regardless of the product viscosity.

Next, with regard to uniform inflowability, it was found that a tendency to improve the quality of the slab by adding heat to the high-viscosity mold flux to increase the heat retention, so in the sample that changed the exothermic raw material and product shape, As a result of performing a casting test and evaluating the uniform inflow by the quality of the slab, as a condition to ensure the uniform inflow, an organic substance and 1% by mass of a carbonaceous substance are blended to give a calorific value of 200 cal / g or more. And it has been found that it is necessary to granulate into granules.

The reason why the exothermic raw materials (combustion substances) are limited to organic substances and 1% by mass carbonaceous substances is that the combustion substances such as metal powder that remains as a part of the slag component remain, and the viscosity of molten slag is stable. This is because the homogenization time is extremely increased and the homogeneous meltability is inhibited.
Moreover, the reason for limiting the amount of heat to 200 cal / g or more is that if it is less than 200 cal / g, the amount of heat is insufficient and the uniform inflow due to the heat retention of the meniscus portion is hindered.
Furthermore, the reason for granulating in a granular form is that in a powder product, carbonaceous substances and organic substances are burnt and burned out at an early stage, and the sustainability of the heat retaining effect cannot be obtained. Here, the granular form means granules granulated by any method such as rolling, extruding, stirring, spray drying and the like. In JP-A-2002-113561, it is limited to granules granulated by a spray drying method for the purpose of suppressing the aggregate effect (melting rate adjusting function) by extremely reducing the amount of combustion substances exposed on the surface of the granules. However, in the present invention, there is no limitation on the granulation method because the main purpose is to improve the stability and stability of combustion by confining the carbonaceous substance or organic substance, which is a heat source (heating material), inside the granules. .
Also, the melting rate can be adjusted by changing the particle size and combination of carbonaceous materials and organic materials. However, more preferably, as the heat-generating raw material, a coarse powdery organic material larger than the average particle diameter of the main raw material constituting the granule and 1% by weight of the coarse powdery carbonaceous material are blended and granulated by spray drying. Ideally granulate. The reason for this is that in the spray-drying granulation method, when the slurry droplets instantaneously evaporate and dry in a high-temperature atmosphere during the granulation process of the granule, the fine substance also moves to the surface at the same time as the moisture, and the intragranular structure As a result of the redistribution of the substance, it becomes possible to accumulate most of the combustion substance having an average particle size larger than the average particle size of the main raw material constituting the granule inside the granular mold flux, and stable combustion sustainability This is because better results can be obtained.

  Produces multiple samples with different types and amounts of raw materials other than base materials, product viscosity, types and amounts of exothermic materials (heat generation amount), and product shapes, and measures uniform viscosity by measuring viscosity stabilization time A typical slab continuous caster with a mold size (200-280) x (800-1800) mm and a casting speed of 1.0-1.8 m / min, and at least 3 casts for each casting condition A comparative casting test (3-6 charges / cast) was conducted to investigate the correspondence with the slab quality.

  The viscosity stabilization time of the mold flux was set in a vertical tubular furnace (Elema furnace) in which a measurement sample previously decarburized at 700 ° C. for 60 minutes and placed in an iron crucible was directly held at 1300 ° C., and the sample was melted After confirming this, the rotor of the E-type viscometer is immersed in the slag, and when the sample temperature reaches 1300 ° C., the viscosity resistance value of the rotor is continuously measured to determine the CV value (variation coefficient) of the viscosity resistance. That is, the time until the standard deviation / average value × 100) was stabilized was measured. (In this experiment, the time taken until the CV value obtained from 10 measurements before and after the time at the center is 0.2% at intervals of 3 minutes) FIG. 1 shows a measurement example at this time. In FIG. 1, the horizontal axis represents the elapsed time (min.) After reaching 1300 ° C., and the vertical axis represents the CV value (%) of the viscous resistance. In addition, the sample used for this viscosity stabilization time measurement is No. 1 in Tables 1 to 3 described later. 3 and 8.

When the viscosity stabilization time was measured for various mold fluxes (details of the mold flux used were omitted) by the above method, the mold flux with a viscosity of less than 8 poise was stabilized in a relatively short time of about 10 minutes. However, when the viscosity becomes higher than 8 poise, it may appear that the molten material is homogeneously melted in the initial melting stage (that is, the slag stage immediately after melting). It took a long time to homogenize and stabilize the viscosity. Similar results were obtained in repeated experiments, and reproducibility was confirmed. Regarding this, the initial lag time of about 10 minutes may affect the time lag until the temperature of the entire measurement sample becomes completely uniform, but the subsequent delay is clearly homogeneous meltability. This is thought to be due to the decline in If it is assumed that the initial 10 minutes of viscosity stabilization time corresponds to the residence time in the slag pool in actual casting, further delay in viscosity stabilization time will promote non-uniform flow of heterogeneous slag. As a result, the slab quality is adversely affected.

The difference in viscosity stabilization time as described above can be attributed to the difference in the blending of the mold flux. Therefore, the blending of the mold flux (type and amount of different component raw materials other than the base raw material) and the viscosity stabilization time. As a result of investigating the relationship, a result as shown in FIG. 2 was obtained. That is, when the mold flux has a viscosity of less than 8 poise (as in the example of changing the use ratio of the high melting point raw material in the 6 poise product in FIG. 2), it stabilizes in a relatively short time of about 10 minutes, and the use ratio of the high melting point raw material However, in the case of a mold flux having a high viscosity of 8 poise or more, the high melting point raw material is 8 % (as in the example of changing the use ratio of the high melting point raw material in 10, 20, and 50 poise products in FIG. 2). The result that the viscosity stabilization time increased rapidly by adding excessively was obtained. From the results shown in FIG. 2, when the viscosity becomes 8 poise or higher, the molten slag is homogenized and the viscosity becomes uniform depending on the blending (whether the blending of the high melting point raw material), even though it appears to have melted homogeneously in the initial melting stage. It has been confirmed that it takes a long time to stabilize.
2 has a melting point of 1800 ° C. or higher, such as alumina (Al ₂ O ₃ ), magnesia (MgO), lime (CaO), zircon (ZrSiO ₄ ), strontium oxide (SrO). The high-melting-point raw material has the same effect that the viscosity stabilization time increases abruptly if the total high-melting-point raw material exceeds 8 % regardless of whether it is used alone or in combination. . Even if the high-melting-point raw materials are uniformly evaluated by the total amount, there is no big problem because these raw materials have the melting and homogenization of "extremely higher melting point than calcium silicate which is the base component of mold flux". This is thought to be due to sharing the cause of inhibition.

  Tables 1 to 3 show typical examples of the results of performing a casting test by making a prototype of a mold flux in which the heat generation amount (that is, the type and amount of the heat generation raw material) and the product shape are changed in addition to the raw material composition and viscosity. . Table 1 shows the composition of each mold flux and loss on ignition (1000 ° C. × 1 hour heating loss), Table 2 shows the ratio of main composition and refractory raw material in the slag state of each mold flux, and Table 3 shows The characteristics and casting results of each mold flux 1 to 6 are examples of the present invention, No.1. 7 to 12 are comparative examples. The main composition in the slag state is converted to a percentage obtained by excluding the loss of ignition from the product composition including the loss of ignition. Further, the calorific value is a product total calculated by calculating the calorific value accompanying the endothermic heat of the used raw material from the standard heat of formation of the single substance or compound.

As a result of the casting test, the comparative material 7 having a product viscosity of 6 poise has an entrainment defect, and the comparative materials 8 and 9 having a blending ratio of high melting point raw materials of 16% and 12% have a cracking defect, a breakout warning, and a heat generation amount. In comparison materials 10 and 11 of 152 and 63 cal / g, a pinhole type defect and also a cracking defect occurred in the powder type comparison material 12, but the viscosity is 8 poise or more and the blending ratio of the high melting point raw material is 8 mass% or less. In the present invention examples 1 to 6, which were granulated in a granule shape with a calorific value of 200 cal / g or more, the slab surface quality was greatly improved.
This is because the use ratio of the high-melting-point raw materials having a melting point of 1800 ° C. or higher is limited to 8 % by mass or less to prevent the viscosity stabilization time from increasing, and the homogeneous meltability is maintained. The result of improving the uniform inflow and improving the stable sustainability of the combustion heat retention effect by adding a calorific value of 200 cal / g or more by blending the substance and 1% by mass of carbonaceous substance and making it granular. Conceivable.
In addition, the above-mentioned organic substances include wood powder, wood chips, bark, large sawdust, pulp, paper, bamboo, plant stems, seeds and grains, and so on, as long as the combustion gas during use does not harm the work environment. In addition to the powder such as the outer skin, dextrin, CMC (carboxymethylcellulose), lignin, α starch and the like used as a binder for granulation can be used.
Carbonaceous materials include natural or artificial black smoke, coke, coal, charcoal, and other common carbons with a fixed carbon content of about 70% by mass or more as long as combustion residues and impurities do not adversely affect slag components. Coarse coarse powder can be used.

It is a figure which shows the relationship between the elapsed time after sample temperature reaches | attains 1300 degreeC, and the CV value (variation coefficient (standard deviation / average value x100)) of the viscous resistance by a rotating cylinder method. It is a figure which shows the relationship between the use ratio of a high melting-point raw material with a melting temperature of 1800 degreeC or more, and a viscosity stabilization time. In addition, the use ratio of the high melting point raw material indicated the addition amount from the high melting point raw material as an oxide conversion value in a slag state.

Claims (1)

Mold flux used for continuous casting of steel for automobile outer plates and deep-drawn steel plates has a viscosity at 1300 ° C. of 8 poise or higher and a melting point of 1800 ° C. or higher, alumina (Al ₂ O ₃ ), magnesia (MgO ), Lime (CaO), zircon (ZrSiO ₄ ), and strontium oxide (SrO), the use ratio of the high melting point raw material consisting of one or two or more is 8 mass% or less in terms of oxide in the slag state and 4 mass% or more, containing 3.0 to 4.2 mass% of fluorine (F), blending an organic substance and 1 mass% of a carbonaceous substance, giving a calorific value of 200 cal / g or more, and granule Mold flux for continuous casting of steel, characterized in that it is shaped into a shape.

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