JP6354411B2 - Mold flux for continuous casting - Google Patents

Description

  The present invention relates to a mold flux used for continuous casting of a high-temperature molten metal (hereinafter referred to as molten steel) such as steel, and particularly when the slag layer is formed by melting on the surface of the molten steel. The present invention relates to a mold flux that can be prevented from being caught in molten steel.

  In continuous casting of steel or the like, the mold flux plays an important role in lubrication in the mold, heat insulation of the molten steel surface, prevention of molten steel oxidation, heat flux control of cooling in the mold, and the like. On the other hand, the mold flux may be caught in molten steel and become a quality defect.

  In order to suppress the phenomenon that the mold flux is caught in the molten steel, it is necessary to keep the tension (interface tension) at the interface between the molten steel and the slag high when the mold flux melts on the molten steel surface to form molten slag. It is valid.

In the case of a general mold flux mainly composed of CaO and SiO ₂ , the interfacial tension with the molten steel can be increased by increasing the basicity expressed by CaO / SiO _2. Hard mold fluxes are required to have high basicity.

  On the other hand, the mold flux with high basicity deteriorates the lubricity in the mold. The lubrication in the mold is carried out by a film formed by the slag formed by melting the mold flux on the molten steel surface flowing into the gap between the mold and the slab. In the case of a mold flux having a high basicity, the glass layer in the film decreases and the crystal layer increases (crystallization progresses), which is a cause of deterioration in lubricity.

Conventionally, an invention that clearly shows the property of being difficult to crystallize while having a high basicity has not been made. However, as a similar one, for example, in Patent Document 1, ZrO ₂ , SrO ₂ , There has been proposed a mold flux in which physical properties are adjusted by adding an appropriate amount of F or the like.

  Moreover, in patent document 2, the mold flux which made it hard to be caught in molten steel by making interface tension with molten steel high is proposed.

  However, these Patent Documents 1 and 2 do not clearly point out the aspect of high basicity or high interfacial tension that is difficult to be caught in molten steel, and at the same time impairs lubricity, thus improving lubricity. Therefore, the examination from the viewpoint of controlling crystallization is insufficient.

  That is, when trying to obtain a characteristic that mold flux is difficult to be caught in molten steel, there is a problem that the lubricity in the mold is impaired by the characteristic, and no means for solving the trade-off has been found so far. .

Japanese Patent No. 4460463 Japanese Patent No. 4486878

  The problem to be solved by the present invention is that, when trying to obtain a characteristic that the mold flux is difficult to be caught in molten steel, there is a problem that the lubricity in the mold is impaired by the characteristic, and thus the trade-off has been solved so far. The means to do was not found.

  An object of the present invention is to solve the above problems. That is, the present invention provides a mold flux that is highly basic and difficult to crystallize, and that achieves a mold flux that has good lubricity and is not easily caught in molten steel.

A general mold flux contains CaO and SiO ₂ as main components, contains F as a physical property adjusting component, and further contains Na ₂ O, Al ₂ O ₃ , MgO and the like as physical property adjusting or inevitable impurities. Further, the ratio of the main component, that is, the basicity represented by CaO ₂ / SiO ₂ falls within the range of about 0.6 to 2.0 in terms of physical property adjustment. The physical properties mentioned here refer to viscosity and coagulation temperature.

In such a mold flux, when CaO ₂ / SiO ₂ becomes high (approximately more than 1.0), the main crystal that crystallizes or precipitates (hereinafter referred to as crystal precipitation) when it is cooled once melted is caspidine ( 3CaO · 2SiO ₂ · CaF ₂ ) is generally used.

  It is known that caspidyne often crystallizes as a main crystal even if it is a mold flux having a composition out of the primary crystal region in the equilibrium diagram. That is, with a high basicity mold flux, crystallization of caspidine is likely to proceed, and the glass layer of the film in the gap between the mold and the slab becomes insufficient, resulting in a loss of lubricity.

  The inventor pays attention to Sr, which is an element of Ca, and Ca in caspodyne does not replace Sr. Therefore, if Ca of high basicity mold flux is replaced with Sr, crystallization of caspodyne is suppressed. I found it.

The present invention has been made based on the above findings by the inventors,
A mold flux used for continuous casting of molten steel,
CaO, SrO, SiO ₂ as the main components, and CaO in (CaO + SrO ₂ ) / SiO ₂ and SrO ₂ / (CaO + SrO 2) converted from all Ca concentrations contained in CaO, CaF ₂ etc. to CaO concentration In this case, (CaO + SrO ₂ ) / SiO ₂ as an atomic ratio is 1.1 to 3.5, and SrO 2 / (CaO 2 + SrO 2) is 0.22 to 0.55 .

The present invention also provides:
Does the XRD peak intensity of the caspodyne crystal that precipitates when the mold flux of the present invention is once melted and cooled down fall below 90% of the XRD peak intensity when the total atomic concentration of SrO is replaced with CaO? Alternatively, the solidification temperature of the mold flux is 30 ° C. or more lower than the solidification temperature when the total atomic concentration of SrO is replaced with CaO, and the FeO and MnO concentrations are both 3 atomic% or less. .

  Since Sr is an element of the same family as Ca, even if a part of Ca in the mold flux is replaced with Sr, the influence on the interfacial tension with the molten steel is small. That is, in the present invention, by replacing a part of Ca in the high basicity mold flux with Sr, crystallization of caspodyne is suppressed and the lubricity in the mold is improved while maintaining a high interfacial tension with molten steel. can do.

  According to the present invention, it is possible to obtain a mold flux having both high lubricity and characteristics that are difficult to be caught in molten steel.

  The object of the present invention is to obtain a mold flux that is highly basic but difficult to crystallize by replacing part of Ca in the mold flux with Sr.

In particular,
The present invention is a mold flux used for continuous casting of molten steel,
CaO, SrO, SiO ₂ as the main components, and CaO in (CaO + SrO ₂ ) / SiO ₂ and SrO ₂ / (CaO + SrO 2) converted from all Ca concentrations contained in CaO, CaF ₂ etc. to CaO concentration If it is, in which of the atomic ratio (CaO + SrO) / SiO ₂ is at 1.1 to 3.5, and SrO / (CaO + SrO) is a 0.22 to 0.55.

  In addition, the XRD peak intensity of the caspodyne crystal that precipitates when the mold flux of the present invention is once melted and cooled is reduced to 90% or less of the XRD peak intensity when the total atomic concentration of SrO is replaced with CaO. Or the solidification temperature of the mold flux is reduced by 30 ° C. or more from the solidification temperature when the total atomic concentration of SrO is replaced with CaO, and the FeO and MnO concentrations are both 3 atomic% or less. Is.

In the mold flux of the present invention, containing CaO ₂ , SrO ₂ , and SiO ₂ as main components means that a part of Ca is replaced with Sr with respect to a general mold flux mainly composed of CaO and SiO ₂ . Common mold fluxes are mainly composed of CaO and SiO ₂ because the solidification temperature is easy to adjust to an appropriate value (approximately 1000 to 1250 ° C when used for continuous casting of steel) and inexpensive. That is why.

In the mold flux of the present invention, (CaO + SrO ₂ ) / SiO ₂ as the atomic ratio is 1.1 to 3.5 in order to increase the interfacial tension between the molten steel and the molten slag, and to have characteristics that are difficult to be caught in the molten steel. This is because the ratio needs to be 1.1 or more. On the other hand, if the ratio exceeds 3.5, the solidification temperature of the mold flux becomes too high. A more preferable range of the ratio is 1.2 to 2.2.

In the present invention, from the viewpoint of suppressing crystallization of caspidyne, it is more preferable that CaO ₂ / SiO ₂ as an atomic ratio is 1.2 or less after satisfying the other composition ratios. More preferably, CaO ₂ / SiO ₂ as an atomic ratio is 1.0 or less.

  In the mold flux of the present invention, SrO 2 / (CaO + SrO 2) as an atomic ratio is 0.10 to 0.60. If this ratio is less than 0.10, Sr has insufficient action to suppress crystallization of caspodyne. Because. On the other hand, when the ratio exceeds 0.60, the mold flux becomes expensive. A more preferable range of this ratio is 0.22 to 0.55.

  In the mold flux of the present invention, the XRD peak intensity of the caspodyne crystal that precipitates once cooled after being melted is reduced to 90% or less of the XRD peak intensity when the total atomic concentration of SrO is replaced with CaO. It is desirable that the lubricity in the mold is improved by suppressing the crystallization of such caspodyne crystals.

  Here, cooling after melting once means that a sample of 500 g to 2000 g is completely melted at 1350 to 1450 ° C. for 30 to 90 minutes in an Ar atmosphere, and then cooled at a rate of 1 to 5 ° C./min. In the present invention, the temperature is kept at 800 ° C.

  In the present invention, the relative ratio of the XRD diffraction peak intensity is used as an index for evaluating the crystal precipitation suppression state of caspodyne by replacing a part of Ca in the mold flux with Sr. In the present invention, the relative ratio of the XRD diffraction peak intensity is defined as 90% or less, but when the ratio is 60% or less, a clearer improvement in lubricity can be obtained.

  Depending on the type of steel to be cast, it is possible to improve lubricity by suppressing crystallization to such an extent that almost no XRD diffraction peak of caspidine is observed.

  Incidentally, the solidification temperature of the mold flux is also an index of crystallization strength. The solidification temperature is detected as a temperature at which the viscosity rapidly increases due to crystallization in the viscosity measurement in the temperature lowering process. The cooling rate in the temperature lowering process is usually 1 to 5 ° C / min.

  In the mold flux of the present invention, instead of the relative ratio of the XRD diffraction peak intensity, the solidification temperature of the mold flux is lowered by 30 ° C. or more than the solidification temperature when the total atomic concentration of SrO is replaced with CaO. You may let them. Also in this case, the lubricity in the mold is improved by suppressing the crystallization of caspidine crystals. For the purpose of improving lubricity, the lower the solidification temperature, the better. However, the effect of the present invention is expected to be about 200 ° C lower than the solidification temperature when the total atomic concentration of SrO is replaced with CaO. Up to.

  In general, a decrease in XRD diffraction peak intensity and a decrease in solidification temperature occur at the same time, but when caspidine is a secondary crystal instead of the main crystal, even if the XRD diffraction peak intensity of the caspodyne crystal decreases, A decrease in the solidification temperature does not necessarily occur. Even when caspodyne is not a main crystal but a secondary crystal, the suppression of crystallization of caspodyne contributes to the improvement of lubricity.

  In the mold flux of the present invention, the concentration of FeO and MnO is 3 atomic% or less in order to maintain high interfacial tension with molten steel, such as Al and Si, which are deoxidizing elements in molten steel. This is because it is desirable that the lower oxide, which is easily reduced, is as low as possible. In particular, FeO and MnO, which are easily reduced, are desirably 3 atomic% or less. A more preferable range of these concentration upper limit values is 2 atomic% or less.

  In addition, when applying this invention to continuous casting of steel, the object is preferably normal aluminum killed steel or silicon aluminum killed steel. Specifically, the Al concentration in the molten steel is 0.4% by mass or less, more preferably 0.1% by mass or less, and 0.005% by mass or more, and Si in the molten steel is not contained or contained except for inevitable impurities. Even if it is a steel grade of 1 mass% or less.

The reason is as follows. When the Al concentration in the molten steel exceeds the specified upper limit, the reduction reaction of SiO _{2 in} the mold flux by the Al in the molten steel becomes intense, and the interface between the molten slag and molten steel produced by melting the mold flux on the molten steel surface. This is because the tension (interfacial tension) is lowered and the mold flux is easily caught in the molten steel.

  On the other hand, when the Al concentration in the molten steel falls below the specified lower limit, the molten steel becomes insufficiently deoxidized, the oxygen concentration in the molten steel increases, and the molten slag and molten steel generated by melting the mold flux on the molten steel surface This is because the interfacial tension (interfacial tension) decreases and the mold flux is easily caught in the molten steel.

On the other hand, when the Si concentration in the molten steel is higher than the specified upper limit, the reduction reaction of SiO _{2 in} the mold flux by Al in the molten steel is suppressed, and the interfacial tension is lowered and the mold flux is entrained in the molten mold flux. This is because the phenomenon hardly occurs and the necessity of the present invention is lost.

The component concentration of the mold flux defined in the present invention will be supplementarily described below.
For example, CaO is obtained by converting all Ca concentrations contained in CaO and CaF ₂ into CaO concentration, Na ₂ O is obtained by converting all Na concentrations contained in NaF and Na ₂ CO ₃ into Na ₂ O concentration, F indicates all F concentrations contained in CaF ₂ and NaF, respectively. The same applies to the other components.

Hereinafter, the effect of the mold flux for continuous casting according to the present invention will be described in detail based on experimental results.
A to D in Table 1 below are examples of mold fluxes that satisfy the claims of the present invention.

  Since the mold fluxes of Examples A to D satisfy the claims of the present invention, both the characteristics that are difficult to be caught in molten steel and the lubricity in the mold are compatible.

  In these Examples A to D, the XRD peak intensity ratio of the caspodyne crystal relative to the comparative example in which all Sr is replaced with Ca is 2 minutes per minute after 1000 g of mold flux previously decarburized is melt-held at 1400 ° C. for 60 minutes. The sample was cooled to 800 ° C. at a cooling rate of 0 ° C. and then cooled to room temperature in a furnace with the power turned off. From the result of X-ray diffraction investigation, the first peak intensity of the caspodyne crystal was determined by comparison.

  The solidification temperature was determined by measuring the viscosity of the mold flux while cooling at a cooling rate of 2 ° C per minute after 1000g of mold flux was melted and held at 1400 ° C for 60 minutes. It calculated | required as temperature to raise (breakpoint).

In Examples A, C and D, it was confirmed that caspidine was the main crystal phase. On the other hand, in Example B, merwinite (Ca ₃ MgO ₆ Si) was the main crystal phase, and caspidine was a secondary crystal phase.

E, F, G, and H in Table 2 below are comparative examples in which Sr in Examples A, B, C, and D are all replaced with Ca, and I is a high SrO concentration and atomic ratio SrO / (CaO + SrO). Is a comparative example in which (CaO + SrO ₂ ) / SiO ₂ is low, and K is a comparative example in which the MnO concentration is high.

  Comparative Examples E to H in Table 2 are examples in which the properties that are difficult to be caught in molten steel are the same as those in Examples A to D, but caspidine is excessively crystallized in the film and the lubricity in the mold is impaired. . In addition, even in the case of mold fluxes that are inferior in lubricity as in Comparative Examples E to H, it is applicable to subperitectic steels that do not require lubricity due to large shrinkage of the solidified shell (because it is difficult to adhere to the mold). Although possible, it is not possible to enjoy the characteristics that are difficult to get caught in molten steel for a wide range of steel types.

  The effect of the present invention can also be obtained in Comparative Example I in which the SrO concentration is high and the atomic ratio SrO / (CaO + SrO) is excessive, but it is not realistic because of high cost.

In Comparative Example J, since (CaO + SrO ₂ ) / SiO ₂ is low, caspidyne hardly crystallizes and has good lubricity, but is easily caught in molten steel.

  In Comparative Example K having a high MnO concentration, MnO is easily reduced by a deoxidizing element such as aluminum in the molten steel. Therefore, the interface between the molten slag and molten steel produced by melting the mold flux on the molten steel surface due to the reduction reaction. The tension (interfacial tension) decreases, and the mold flux is easily caught in the molten steel.

  Next, the case where low mold aluminum killed steel having the following composition was continuously cast at a drawing speed of 1.6 to 1.8 m / min using the mold fluxes of Example C, Comparative Example F, and Comparative Example I will be specifically described below. The effects of the present invention will be described with specific examples.

  The used mold had a cross-sectional size of 250 mm in thickness and 1300 mm in width, and an effective length of the mold was 800 mm. The type of continuous casting machine is a vertical bending die.

  Continuously cast low-carbon aluminum killed steel has a C concentration of 0.02 to 0.05 mass%, an Si concentration of 0.01 to 0.02 mass%, an Mn concentration of 0.3 to 0.6 mass%, and a sol.Al concentration of 0.03 to 0.08 mass%. Fe.

  When the mold flux of Example C was used, very high lubricity was obtained by the effect of suppressing crystallization using SrO of the present invention, and the mold flux entrainment defect in the slab surface layer was also slight.

  On the other hand, when the mold flux of Comparative Example F was used, a slab burn-in warning in the mold occurred frequently and hindered operation. However, the mold flux entrainment defect in the slab surface layer portion in the steady casting portion was as small as in Example A.

  Further, when the mold flux of Comparative Example I was used, although the lubricity in the mold was good, many mold flux entrainment defects occurred in the slab surface layer portion.

  Specifically, the occurrence frequency of the slab burn-in alarm in the mold per 100 m casting length is less than 0.01 times for the mold flux of Example C, 1.2 times for the mold flux of Comparative Example F, and the mold of Comparative Example I. The flux was less than 0.1 times.

  Further, the mold flux entrainment defect occurrence frequency was 11 for the mold flux of Comparative Example F and 78 for the mold flux of Comparative Example I when the result of the mold flux of Example C was 10.

  Mold flaw entrainment defects on the slab surface layer were randomly sampled from the steady casting part at 10 locations with a width of 100 mm, a length of 100 mm, and a slab surface of 1 to 10 mm at the center of the slab width direction. The number of spherical inclusions having a size of 80 μm or more was counted by a dissolution extraction method, and evaluated by converting to the density per unit steel weight.

Claims (2)

A mold flux used for continuous casting of high-temperature molten metal,
CaO, SrO, SiO ₂ as the main components, and CaO in (CaO + SrO ₂ ) / SiO ₂ and SrO ₂ / (CaO + SrO 2) converted from all Ca concentrations contained in CaO, CaF ₂ etc. to CaO concentration In this case, (CaO + SrO ₂ ) / SiO ₂ as an atomic ratio is 1.1 to 3.5, and SrO 2 / (CaO 2 + SrO 2) is 0.22 to 0.55 .   The XRD peak intensity of the caspodyne crystal that crystallizes or precipitates when the mold flux according to claim 1 is once melted and then cooled is 90% of the XRD peak intensity when the total atomic concentration of SrO is replaced with CaO. %, Or the solidification temperature of the mold flux is lowered by 30 ° C. or more than the solidification temperature when the total atomic concentration of SrO is replaced with CaO, and the FeO and MnO concentrations are both 3 atomic%. A mold flux for continuous casting characterized by the following: